Carbon Nanomaterials (CNMs) in Cancer Therapy: A Database of CNM-Based Nanocarrier Systems

Carbon nanomaterials (CNMs) are an incredibly versatile class of materials that can be used as scaffolds to construct anticancer nanocarrier systems. The ease of chemical functionalisation, biocompatibility, and intrinsic therapeutic capabilities of many of these nanoparticles can be leveraged to design effective anticancer systems. This article is the first comprehensive review of CNM-based nanocarrier systems that incorporate approved chemotherapy drugs, and many different types of CNMs and chemotherapy agents are discussed. Almost 200 examples of these nanocarrier systems have been analysed and compiled into a database. The entries are organised by anticancer drug type, and the composition, drug loading/release metrics, and experimental results from these systems have been compiled. Our analysis reveals graphene, and particularly graphene oxide (GO), as the most frequently employed CNM, with carbon nanotubes and carbon dots following in popularity. Moreover, the database encompasses various chemotherapeutic agents, with antimicrotubule agents being the most common payload due to their compatibility with CNM surfaces. The benefits of the identified systems are discussed, and the factors affecting their efficacy are detailed.


Introduction
While cancer remains one of the world's leading causes of death, advances in diagnostics and treatment have seen an overall improvement in detection and mortality rates. However, the current treatment approaches are either highly invasive, in the case of surgical operations, or can cause unwanted toxic side effects, as commonly experienced with chemotherapeutic agents and radiotherapy [1,2]. In particular, the effectiveness of chemotherapeutic agents is often limited by their poor aqueous solubility and nonselective nature, resulting in poor bioavailability and the indiscriminate death of both healthy and cancer cells [3]. To overcome these issues, there has been much research into the use of nanocarriers for the targeted and controlled release of anticancer drugs [4], where carbon nanomaterials (CNMs) have emerged in recent years as very promising candidates for this purpose. CNMs are a distinct class of materials that show altered characteristics to those of bulk carbon materials, such as diamond or graphite. They are classified as 0D, 1D, or 2D, according to the number of dimensions they possess which exist on the nanoscale (<100 nm) [5]. The allotropic nature of carbon means that a variety of these materials exists, some examples of which include graphene [1], carbon nanotubes (CNTs) [6], carbon nano-onions (CNOs) [7], nanodiamonds (NDs) [8], and carbon nanohorns [9]. CNMs have garnered widespread attention for their biomedical applications, such as drug delivery and diagnostics, because of their unique and highly desirable physicochemical and mechanical properties, such as size, biocompatibility, high tensile strength, and ease of chemical functionalisation.
By carefully selecting the production method, the particle sizes of CNMs can be precisely controlled, allowing for the creation of particles comparable in size to biomolecules

Drug Loading and Release Metrics
The therapeutic efficacy of a nanocarrier system depends on its ability to absorb and release anticancer drugs; as such, quantitative metrics are needed to measure these systems. Such metrics are used to describe and compare the drug loading and release capabilities of different nanocarrier systems in the database.
The drug loading content (DLC) describes the amount of drug loaded onto the nanocarrier (Equation (1)). It is important to note that whilst most studies use the total mass of the nanocarrier (the CNM base, plus the drug, plus any other components), some studies just use the mass of the CNM itself [25], which leads to artificially higher DLC values. DLC (wt%) = mass of drug bound to nanocarrier total mass of nanocarrier × 100 (1) The drug loading efficiency (DLE), sometimes called the encapsulation or entrapment efficiency, is a measure of the effectiveness of the drug loading process and not a quantitative measure of the drug content (Equation (2)). DLE (wt%) = mass of drug bound to nanocarrier total mass of drug added × 100 (2) The drug release efficiency (DRE) quantifies the cumulative release of a therapeutic agent from the nanocarrier (Equation (3)). This is the total amount of bound drug released throughout the experiment. DRE (wt%) = total mass of drug released mass of drug bound to nanocarrier × 100 (3)

Database of Carbon-Nanomaterial-Based Cancer Therapeutics
Herein, we present a database of CNM-based nanocarrier systems that transport clinically approved anticancer drugs, seen in Table 1 The database includes the composition of the nanocarrier, the in vitro and in vivo biological models the system was tested on, the drug loading and release metrics, and a summary of the experimental results. The database is organized alphabetically by the anticancer drug used in the formulation; an index can be seen in Table 2.   The unloaded nanocarrier is biocompatible, and the use of an aptamer increases uptake and cytotoxicity in breast cancer cells. [26] FA-PEG-bisamine multiwalled carbon nanotube (MWCNT) in vitro: MCF-7 cells 99% DLE, ~90% DRE, with pH-triggered drug release sustained over 900 min This nanocarrier increases circulation time, half-life, and accumulation of 5-FU in target tissues, and this leads to the effective killing of breast cancer cells in vitro. [27] CS/Au/MWCNT in vitro: MCF-7 cells 43% DLC, 59% DRE, with prolonged, sustained drug release Reduced potential side effects and increased efficacy compared to free 5-FU were observed.
A reduction in cancer cell viability was observed at low nanocarrier concentrations. [28] Nanodiamond (ND)-ADH in vitro: MCF-7 and HepG2 cells 88% DLE, 35% DRE, with pH-mediated, sustained drug release This nanocarrier showed potent anticancer effects with low haemolytic toxicity in human blood. [29] Mesoporous carbon nanoframe (mCNF) in vitro: HeLa cells 31% DLC, 80% DRE, with dual pH/NIR-triggered drug release This system displayed excellent photothermal efficiency with the NIR pulse-triggered burst release of 5-FU. The photothermal conversion efficiency of this system was found to be 21%. This synergistic chemo-photothermal therapy combined with photoacoustic imaging capabilities can effectively treat cancer in vitro. [30] PEG-C60 fullerene-alanine in vitro: MCF-7 and BGC-823 cells 1% DLC, with no quantitative drug release data The unloaded nanocarrier displays good biocompatibility and the system is stable in murine serum for over 24 h. This formulation results in the significantly better inhibition of cancer cells compared to free 5-FU. [31] Graphene oxide (GO) in vitro: A549 cells 31% DLC, 35% DRE, with pH-triggered drug release The blank nanocarrier is biocompatible and the loaded system improved the stability of 5-FU. [32] CS-coated Fe3O4-NH2/graphene quantum dot (GQD) nanohybrid in vitro: A549 cells 90% DLC, 84% DRE, with pH-dependent drug release This system has magnetic resonance/fluorescence imaging capabilities and displayed significantly higher cytotoxicity than free 5-FU, whilst the unloaded nanocarrier is biocompatible. [33] HPMC/GO in vitro: Vero, HepG2, and A549 cells No quantitative drug loading/release studies were performed. The blank nanocarrier displays high biocompatibility in normal cells, whilst the drug-loaded system displays a higher antitumour efficacy than free 5-FU. A green synthesis method was used. [34] TAU-GO in vitro: HepG2 cells; in vivo: SD rats 50% DLE, 90% DRE, with pH-triggered 5-FU release This biocompatible nanocarrier improved the circulation time and anticancer efficacy of 5-FU. [35] Carbon dot (CD)-BT in vitro: MCF-7, HeLa, and HEK-296 cells 35% DLE, 81% DRE, with pH-triggered drug release An initial burst of 5-FU is followed by sustained release; this nanocarrier also displays fluorescence imaging capabilities. BT-mediated targeting of cancer cells resulted in high cytotoxicity towards neoplastic cells and increased cellular uptake due to biotin-receptor-mediated endocytosis. The unloaded nanocarrier is biocompatible, and the use of an aptamer increases uptake and cytotoxicity in breast cancer cells. [26] FA-PEG-bis-amine multiwalled carbon nanotube (MWCNT) in vitro: MCF-7 cells 99% DLE,~90% DRE, with pH-triggered drug release sustained over 900 min This nanocarrier increases circulation time, half-life, and accumulation of 5-FU in target tissues, and this leads to the effective killing of breast cancer cells in vitro. [27] CS/Au/MWCNT in vitro: MCF-7 cells 43% DLC, 59% DRE, with prolonged, sustained drug release Reduced potential side effects and increased efficacy compared to free 5-FU were observed. A reduction in cancer cell viability was observed at low nanocarrier concentrations. [28] Nanodiamond (ND)-ADH in vitro: MCF-7 and HepG2 cells 88% DLE, 35% DRE, with pH-mediated, sustained drug release This nanocarrier showed potent anticancer effects with low haemolytic toxicity in human blood. [29] Mesoporous carbon nanoframe (mCNF) in vitro: HeLa cells 31% DLC, 80% DRE, with dual pH/NIR-triggered drug release This system displayed excellent photothermal efficiency with the NIR pulse-triggered burst release of 5-FU. The photothermal conversion efficiency of this system was found to be 21%. This synergistic chemo-photothermal therapy combined with photoacoustic imaging capabilities can effectively treat cancer in vitro. [30] PEG-C 60 fullerene-alanine in vitro: MCF-7 and BGC-823 cells 1% DLC, with no quantitative drug release data The unloaded nanocarrier displays good biocompatibility and the system is stable in murine serum for over 24 h. This formulation results in the significantly better inhibition of cancer cells compared to free 5-FU. [31] Graphene oxide (GO) in vitro: A549 cells 31% DLC, 35% DRE, with pH-triggered drug release The blank nanocarrier is biocompatible and the loaded system improved the stability of 5-FU. [32] CS-coated Fe 3 O 4 -NH 2 /graphene quantum dot (GQD) nanohybrid in vitro: A549 cells 90% DLC, 84% DRE, with pH-dependent drug release This system has magnetic resonance/fluorescence imaging capabilities and displayed significantly higher cytotoxicity than free 5-FU, whilst the unloaded nanocarrier is biocompatible. [33] Pharmaceutics 2023, 15  HPMC/GO in vitro: Vero, HepG2, and A549 cells No quantitative drug loading/release studies were performed. The blank nanocarrier displays high biocompatibility in normal cells, whilst the drug-loaded system displays a higher antitumour efficacy than free 5-FU. A green synthesis method was used. [34] TAU-GO in vitro: HepG2 cells; in vivo: SD rats 50% DLE, 90% DRE, with pH-triggered 5-FU release This biocompatible nanocarrier improved the circulation time and anticancer efficacy of 5-FU. [35] Carbon dot (CD)-BT in vitro: MCF-7, HeLa, and HEK-296 cells 35% DLE, 81% DRE, with pH-triggered drug release An initial burst of 5-FU is followed by sustained release; this nanocarrier also displays fluorescence imaging capabilities. BT-mediated targeting of cancer cells resulted in high cytotoxicity towards neoplastic cells and increased cellular uptake due to biotin-receptor-mediated endocytosis. [36] N-doped mesoporous carbon sphere (NMCS)-DSPE-PEG in vitro: B16F0 cells 38% DLC, 78% DRE, with dual pH/NIR-triggered drug release This nanocarrier produces reactive oxygen species when irradiated with an NIR laser, and the resulting PDT/PTT/chemotherapeutic combination therapy effectively kills melanoma cells much more efficiently than 5-FU alone. [37] Pharmaceutics 2023, 15 This nanocarrier produces reactive oxygen species when irradiated with an NIR laser, and the resulting PDT/PTT/chemotherapeutic combination therapy effectively kills melanoma cells much more efficiently than 5-FU alone. [37] 6-mercaptopurine (6-MP)purine antimetabolite CD-BT in vitro: CHO, MCF-7, and HepG2 cells 5% DLC, 79% DRE, with dual pHand redox-sensitive drug release Comparable anticancer activity to free 6-MP (in cancer cells) with much lower cytotoxicity (in healthy cells). A GSH-sensitive carbonyl vinyl sulphide group was used to bind 6-MP to BT. [38] Anastrozole-aromatase inhibitor GO-Fe nanoparticles in vitro: MCF-7 cells 84% DLE No qualitative drug release data shown. The system displays higher cytotoxicity than the free drug for cancer cells, and it has magnetic properties. [39] Bortezomib-proteasome CD-CuS NPs-MMT7 in vitro: U251 MG cells Synergistic drug delivery and PTT platform that specifically targets cancer cells. No qualitative drug loading/release data shown. This innovative nanocarrier combines immune system evasion capabilities with the enhanced suppression of tumour growth and metastasis to provide excellent control over cancer growth and metastasis. [12] 6-mercaptopurine (6-MP)purine antimetabolite CD-BT in vitro: CHO, MCF-7, and HepG2 cells 5% DLC, 79% DRE, with dual pH-and redox-sensitive drug release Comparable anticancer activity to free 6-MP (in cancer cells) with much lower cytotoxicity (in healthy cells). A GSH-sensitive carbonyl vinyl sulphide group was used to bind 6-MP to BT. [38] Pharmaceutics 2023, 15  This nanocarrier produces reactive oxygen species when irradiated with an NIR laser, and the resulting PDT/PTT/chemotherapeutic combination therapy effectively kills melanoma cells much more efficiently than 5-FU alone. [37] 6-mercaptopurine (6-MP)purine antimetabolite CD-BT in vitro: CHO, MCF-7, and HepG2 cells 5% DLC, 79% DRE, with dual pHand redox-sensitive drug release Comparable anticancer activity to free 6-MP (in cancer cells) with much lower cytotoxicity (in healthy cells). A GSH-sensitive carbonyl vinyl sulphide group was used to bind 6-MP to BT. [38] Anastrozole-aromatase inhibitor GO-Fe nanoparticles in vitro: MCF-7 cells 84% DLE No qualitative drug release data shown. The system displays higher cytotoxicity than the free drug for cancer cells, and it has magnetic properties. [39] Bortezomib-proteasome inhibitor CD-CuS NPs-MMT7 in vitro: U251 MG cells Synergistic drug delivery and PTT platform that specifically targets cancer cells. No qualitative drug loading/release data shown. This innovative nanocarrier combines immune system evasion capabilities with the enhanced suppression of tumour growth and metastasis to provide excellent control over cancer growth and metastasis. [12] Capecitabine-pyrimidine antimetabolite Single-walled carbon nanotube (SWCNT)-FL-FA-NCC in vitro: Caco-2/TC7 cells No quantitative drug loading/release data shown. This nanocarrier is nontoxic and has fluorescence imaging capabilities. The effective targeting of colon cancer cells leads to an increase in anticancer activity compared to the free drug. [40] oxiSWCNT-CS-FA in vitro: COLO320DM and HT29 cells; in vivo: albino rabbits 94% DLE, 89% DRE An increase in cytotoxicity compared to free drug was noticed during in vitro experiments. The capsule formulation of this nanocarrier is exclusively released in the colon in vivo, avoiding premature release in the stomach. Active targeting of cancer cells was achieved via the FA-targeting ligand. [41] Carboplatin (CP)-DNA alkylating agent GO-PAMAM in vitro: hMSC and HeLa cells No quantitative drug loading/release data shown. The 100 nm width GO (unloaded) was found to be the least toxic. This system displayed enhanced anticancer activity compared to free CP, with decreased cytotoxicity. [42] GO-gelatine in vitro: IMR-32 and hMSC cells 99% DLE, with no quantitative drug release data shown This formulation displays effective CP delivery and uptake in vitro, resulting in a higher potency than free CP. Excellent biocompatibility and stability were observed in vitro. [43] Anastrozole-aromatase inhibitor GO-Fe nanoparticles in vitro: MCF-7 cells 84% DLE No qualitative drug release data shown. The system displays higher cytotoxicity than the free drug for cancer cells, and it has magnetic properties. [39] N-doped mesoporous carbon sphere (NMCS)-DSPE-PEG in vitro: B16F0 cells 38% DLC, 78% DRE, with dual pH/NIR-triggered drug release This nanocarrier produces reactive oxygen species when irradiated with an NIR laser, and the resulting PDT/PTT/chemotherapeutic combination therapy effectively kills melanoma cells much more efficiently than 5-FU alone. [37] 6-mercaptopurine (6-MP)purine antimetabolite   CD-BT  in vitro: CHO, MCF-7,  and HepG2 cells   5% DLC, 79% DRE, with dual pHand redox-sensitive drug release Comparable anticancer activity to free 6-MP (in cancer cells) with much lower cytotoxicity (in healthy cells). A GSH-sensitive carbonyl vinyl sulphide group was used to bind 6-MP to BT. [38] Anastrozole-aromatase inhibitor GO-Fe nanoparticles in vitro: MCF-7 cells 84% DLE No qualitative drug release data shown. The system displays higher cytotoxicity than the free drug for cancer cells, and it has magnetic properties. [39] Bortezomib-proteasome inhibitor CD-CuS NPs-MMT7 in vitro: U251 MG cells Synergistic drug delivery and PTT platform that specifically targets cancer cells. No qualitative drug loading/release data shown. This innovative nanocarrier combines immune system evasion capabilities with the enhanced suppression of tumour growth and metastasis to provide excellent control over cancer growth and metastasis. [12] Capecitabine-pyrimidine antimetabolite Single-walled carbon nanotube (SWCNT)-FL-FA-NCC in vitro: Caco-2/TC7 cells No quantitative drug loading/release data shown. This nanocarrier is nontoxic and has fluorescence imaging capabilities. The effective targeting of colon cancer cells leads to an increase in anticancer activity compared to the free drug. [40] oxiSWCNT-CS-FA  Bortezomib-proteasome inhibitor CD-CuS NPs-MMT7 in vitro: U251 MG cells Synergistic drug delivery and PTT platform that specifically targets cancer cells. No qualitative drug loading/release data shown. This innovative nanocarrier combines immune system evasion capabilities with the enhanced suppression of tumour growth and metastasis to provide excellent control over cancer growth and metastasis. [12] Capecitabine-pyrimidine antimetabolite  MWCNT MCF-12A via an ROS-triggered autophagy mechanism.
FA-CDT-C60 fullerene in vitro: HeLa, HeLa-RFP, and A549 cells; in vivo: Danio rerio, both healthy and bearing HeLa tumours 37% DLC, ~80% DRE, with pH-triggered drug release This system displayed increased anticancer effects compared to the free drug alone due to the active targeting of folate-receptor-overexpressing cancer cells and improved cellular uptake. Low toxicity and improved antitumour effects compared to the free drug were also seen in vivo.
[ This nanocarrier displayed similar cytotoxicity to the free drug, with a sustained-release profile. The sponges appear to be more biocompatible than CNTs alone; hence, the blank nanocarrier showed low cytotoxicity, whilst the drug-loaded system displayed strong anticancer effects. [51] oxiMWCNT-HA in vitro: TC-1 and NIH/3T3 cells No quantitative drug loading/release data shown resulting in significantly higher cytotoxic effects in neoplastic cells and lower side effects in healthy cells. [44] Aminated MWCNT in vitro: MDA-MB-23 and MCF-12A 89% DLC, 21% DRE This formulation provided increased cancer cell death compared to free CP and killed cells via an ROS-triggered autophagy mechanism. [45] FA-CDT-C60 fullerene This theragnostic system displayed a significant reduction in anticancer potency compared to free CisP; however, it has imaging capabilities arising from the inclusion of CdSe quantum dots. [55] Chlorambucil-DNA alkylating agent Reduced graphene oxide (rGO)-FA-gelatine in vitro: Siha cells 35% DLC, 82% DRE, with pH-triggered drug release A significant decrease in IC 50 value compared to the free drug was observed. The use of gelatine facilitated sustained drug release. This system is a promising treatment for cervical adenocarcinoma. [52] oxiMWCNT-HA in vitro: TC-1 and NIH/3T3 cells No quantitative drug loading/release data shown This system displayed the selective uptake and targeting of cancer cells over healthy cells, resulting in significantly higher cytotoxic effects in neoplastic cells and lower side effects in healthy cells. [44] Aminated MWCNT in vitro: MDA-MB-23 and MCF-12A 89% DLC, 21% DRE This formulation provided increased cancer cell death compared to free CP and killed cells via an ROS-triggered autophagy mechanism. [45] FA-CDT-C60 fullerene in vitro: HeLa, HeLa-RFP, and A549 cells; in vivo: Danio rerio, both healthy and bearing HeLa tumours 37% DLC, ~80% DRE, with pH-triggered drug release This system displayed increased anticancer effects compared to the free drug alone due to the active targeting of folate-receptor-overexpressing cancer cells and improved cellular uptake. Low toxicity and improved antitumour effects compared to the free drug were also seen in vivo. [46] CS-Fe3O4-GO in vitro: HepG2 and MCF-7 cells 74% DLE, 90% DRE, with pH-triggered drug release A very high amount of CP was released at neutral pH. Despite this, an increase in CP potency and a reduction in systemic toxicity was observed. [47] GO-CS-FA in vitro: LX-2 and SKOV3 cells 14% DLC, ~90% DRE CP release was similar in neutral and acidic environments; hence, this system is unsuitable for pH-triggered drug release via noncovalent drug attachment. The system showed slightly lower cancer cell inhibition than free CP. [48] GO-Fe3O4-PANI  The unloaded nanocarrier is biocompatible, whilst the loaded system exhibits effective cancer cell targeting and internalisation in vitro. This system also has fluorescence imaging capabilities. [77] SWCNT in vitro: PC-3 cells 94% DLE, 95% DRE, with pH-triggered drug release The combination of efficient CUR delivery and SWCNT-mediated PTT successfully inhibited tumour cell growth. This nanocarrier also reduced CUR biodegradation and increased its solubility. [78] SWCNT-PC-PVP in vitro: PC-3 and S180 cells; in vivo: Kunming mice bearing S180 tumours No quantitative drug loading/release data. This biocompatible nanocarrier increased CUR cellular uptake, plasma concentration, and bioavailability. The system overcomes the main barrier to the low anticancer effect of free CUR (low plasma concentration) whilst displaying low in vivo toxicity. This is a combination therapy with the SWCNT-mediated photothermal ablation of cancer cells. [79] oxiND-ADH in vitro: MCF-7 and HepG2 cells 93% DLE, 36% DRE, with pH-triggered, sustained drug release The use of a pH-sensitive amide bond to bind CUR slows release and increases stability, resulting in potent cytotoxicity. [29] Graphene oxide quantum dot (GOQD)-CS-PEG-MUC-1 aptamer in vitro: MCF-7 and HT-2 cells 99% DLC, 64% DRE, with pH-responsive drug release This system effectively targets MUC-1-overexpressing cancer cells whilst displaying photoluminescence imaging and cancer detection abilities. An increase in therapeutic efficacy and cellular uptake compared to free CUR and low haemolysis with human blood was observed with this system. This nanoformulation displayed excellent anticancer activity compared to CUR alone and has no toxic effect on healthy cells. The system is also highly fluorescent when CUR is released. [83] GO-BSA-AS1411 aptamer This simple, low-cost system acts as a contrast agent for magnetic resonance imaging. It also displays improved cytotoxicity, tumour suppression, and reduced side effects compared to free CUR due to effective cancer cell targeting. [76] GQD-GlcN in vitro: MCF-7 cells No quantitative drug loading information, 37% DRE with pH-triggered, sustained drug release The unloaded nanocarrier is biocompatible, whilst the loaded system exhibits effective cancer cell targeting and internalisation in vitro. This system also has fluorescence imaging capabilities. [77] SWCNT in vitro: PC-3 cells 94% DLE, 95% DRE, with pH-triggered drug release The combination of efficient CUR delivery and SWCNT-mediated PTT successfully inhibited tumour cell growth. This nanocarrier also reduced CUR biodegradation and increased its solubility. [78] SWCNT-PC-PVP in vitro: PC-3 and S180 cells; in vivo: Kunming mice bearing S180 tumours No quantitative drug loading/release data. This biocompatible nanocarrier increased CUR cellular uptake, plasma concentration, and bioavailability. The system overcomes the main barrier to the low anticancer effect of free CUR (low plasma concentration) whilst displaying low in vivo toxicity. This is a combination therapy with the SWCNT-mediated photothermal ablation of cancer cells. [79] oxiND-ADH in vitro: MCF-7 and HepG2 cells 93% DLE, 36% DRE, with pH-triggered, sustained drug release The use of a pH-sensitive amide bond to bind CUR slows release and increases stability, resulting in potent cytotoxicity. [29] Graphene oxide quantum dot (GOQD)-CS-PEG-MUC-1 aptamer in vitro: MCF-7 and HT-2 cells 99% DLC, 64% DRE, with pH-responsive drug release This system effectively targets MUC-1-overexpressing cancer cells whilst displaying photoluminescence imaging and cancer detection abilities. An increase in therapeutic efficacy and cellular uptake compared to free CUR and low haemolysis with human blood was observed with this system. [19] CD-PNM in vitro: SH-SY5Y cells No qualitative CUR loading information, 82% DRE This formulation resulted in a 10× enhancement of CUR solubility whilst displaying excellent photophysical properties and low toxicity.
[80] The system displayed dual pH-and temperature-triggered drug release, with an initial burst of drug followed by sustained delivery. [87] Cytarabine-DNA polymerase inhibitor PAA/PEG/CNT/MTX Approximately 80% DRE, with no quantitative drug loading data shown The system displayed dual pH-and temperature-triggered drug release, with an initial burst of drug followed by sustained delivery. [87] Pharmaceutics 2023, 15, x FOR PEER REVIEW 10 of 40 Cyclophosphamide-DNA alkylating agent Approximately 80% DRE, with no quantitative drug loading data shown The system displayed dual pH-and temperature-triggered drug release, with an initial burst of drug followed by sustained delivery. [87] Cytarabine-DNA polymerase inhibitor  Cyclophosphamide-DNA alkylating agent PAA/PEG/CNT/ MTX Approximately 80% DRE, with no quantitative drug loading data shown The system displayed dual pH-and temperature-triggered drug release, with an initial burst of drug followed by sustained delivery. [87] Cytarabine-DNA polymerase inhibitor The potency of dabrafenib was retained, with effective BRAF and HDAC inhibition in human melanoma cells. [89] Dasatinib-tyrosine kinase inhibitor PLA-PGA-PEG-CNT in vitro: U-87 cells 4% DLC, ~65% DRE This nanocarrier system was synthesised via a simple one-pot method and demonstrated improved therapeutic efficacy compared to the free drug in vitro. The drug release profile of this system can be controlled by varying the composition of the polymer coating. [90] Dabrafenib-reversible ATP-competitive kinase inhibitor GO-BSA in vitro: A375, HDF, SKmel28, SKmel23, MelJuSo, MNT-1, and NHEM cells No quantitative drug loading/release data, with pH-triggered drug release The potency of dabrafenib was retained, with effective BRAF and HDAC inhibition in human melanoma cells. [89] Cyclophosphamide-DNA alkylating agent PAA/PEG/CNT/ MTX Approximately 80% DRE, with no quantitative drug loading data shown The system displayed dual pH-and temperature-triggered drug release, with an initial burst of drug followed by sustained delivery. [87] Cytarabine-DNA polymerase inhibitor Specific recognition and targeting of lung cancer cells over other cancer cells was achieved by using the A1 aptamer. This system achieved much higher anticancer efficacy than the free drug. [95] Docetaxel (DTX)-microtubule growth inhibitor GO-PEG in vitro: DU-145 cells No quantitative drug loading or release data provided This system was highly effective at killing prostate cancer cells due to a decrease in IC50 compared to free DTX. The nanocarrier displayed low dispersion stability in biological fluids. [96] RGD-CS-SWCNT in vitro: A549 and MCF-7 cells; in vivo: BALB/c mice inoculated with A549 tumours 32% DLC, 68% DRE, with pH-triggered drug release Significant drug uptake and growth inhibition in A549 cells was observed. The system entered cells via clathrin-and caveolin-mediated endocytosis and displayed strong tumour targeting, growth inhibition, and biosafety in vivo. [97] Oxi-carbon nanohorn (CNH)-PEG-mAb in vitro: MCF-7; in vivo: ICR mice xenografted with H22 tumours 74% DLE, 59% DRE The adsorption of DTX to the nanohorns was achieved via π-π stacking. Prolonged diffusion-controlled DTX release was achieved. The use of mAb resulted in the selective killing of cancer cells in vitro and in vivo and a lower IC50 and no significant side effects compared to free DTX in vivo. This nanocarrier also leveraged the enhanced permeability and retention effect.
[98] The adsorption of DTX to the nanohorns was achieved via π-π stacking. Prolonged diffusion-controlled DTX release was achieved. The use of mAb resulted in the selective killing of cancer cells in vitro and in vivo and a lower IC50 and no significant side effects compared to free DTX in vivo. This nanocarrier also leveraged the enhanced permeability and retention effect. [98] Decitabine-DNA methyltransferase inhibitor A1-GO in vitro: A549, NCI-H157, NCI-H520, NCI-H1299, NCI-H446, MCF-7, and HeLa cells 64% DLE, 75% DRE, with pH-dependent drug release Specific recognition and targeting of lung cancer cells over other cancer cells was achieved by using the A1 aptamer. This system achieved much higher anticancer efficacy than the free drug. [95]  This system achieved an eight-fold increase in the accumulation in tumour tissues compared to the free drug. The synergy between the chemotherapy and photolytic properties of the nanocarrier allowed for deep penetration into tumours and effective treatment in vivo. [104] C 60 fullerene-APA in vitro: MDA MB-231 cells; in vivo: Wistar rats 48% DLE, 96% DRE, with pH-triggered drug release A substantial decrease in haemolysis (human blood) and protein binding (BSA) compared to free DTX was observed. The nanoformulation also increased bioavailability and potency compared to free DTX. Fullerenes display partial P-gp efflux inhibition. The nanocarrier shows specificity for ASGPR-receptor-containing cancer cells whilst retaining DOX therapeutic efficacy. [106] GO-PRM/SA in vitro: MCF-7 cells 29% DLC, 49% DRE, with pH-dependent drug release Protein adsorption in physiological environments was suppressed and the system showed enhanced cytotoxicity compared to GO-DOX alone. [107] Tf/FA/GO/PF68 in vitro: SMMC-7721 and L-02 cells 96% DLC, 55% DRE, with pH-dependent drug release The nanocarrier displayed low toxicity and high specificity due to the synergistic effect of Tf-and FA-targeting ligands on cancer cell targeting. The DOX-loaded nanosystem was not tested on healthy L-02 cells. The nanocarrier shows specificity for ASGPR-receptor-containing cancer cells whilst retaining DOX therapeutic efficacy. [106] GO-PRM/SA in vitro: MCF-7 cells 29% DLC, 49% DRE, with pH-dependent drug release Protein adsorption in physiological environments was suppressed and the system showed enhanced cytotoxicity compared to GO-DOX alone. [107] Tf/FA/GO/PF68 in vitro: SMMC-7721 and L-02 cells 96% DLC, 55% DRE, with pH-dependent drug release The nanocarrier displayed low toxicity and high specificity due to the synergistic effect of Tf-and FA-targeting ligands on cancer cell targeting. The DOX-loaded nanosystem was not tested on healthy L-02 cells.
[108] The co-delivery of DOX and P-gp pump inhibitor (HM) to counteract chemotherapy resistance increased DOX bioavailability and cytotoxicity. The nanosystem could effectively target and treat both drug-and non-drug-resistant tumour models with decreased side effects. FSCNOs also display photothermal capabilities. [119] ATRA-ND This system displayed prolonged retention, enhanced antitumor activity, and enhanced cytotoxicity compared to free EPI. Low systemic toxicity was also seen in vivo. The nanocarrier was also simple and quick to make. This nanocarrier prevents the efflux of EPI by ABC transporters to counter chemoresistance in cancer stem cells. [127] CD-TR-TM This nanocarrier caused a decrease in drug-resistant cancer cell viability. Efficient uptake of nanocarrier via clathrin-dependent endocytosis was the key to its effectiveness, and the system was preferentially consumed by cancer cells. [131] GO in vivo: mice 30% DLC, 80% DRE, with pH-triggered drug release The drug was released in a quick-burst fashion. [132]  Slow and steady delivery of Et was observed at neutral pH, which was accelerated 1.5× upon NIR irradiation. CNHs are also photosensitizers, and the synergistic effect of PTT and Et chemotherapy killed multidrug-resistant cells by combating P-gp-mediated drug efflux. [134] FA-CβCDT-MSCD in vitro: HeLa and HepG2 cells 14% DLC, 25% DRE, with pH-mediated drug release This nanocarrier displayed preferential targeting of folate-receptor-overexpressing cells. The encapsulation of Et in cyclodextrin prevented premature drug release. [135] oxiMWCNT-PEG-Aso This nanocarrier displayed efficient cancer cell uptake via CD44-receptor-mediated endocytosis, resulting in a significant enhancement of the GEF efficacy. The system showed stronger tumour inhibition than the free drug in vivo, with no obvious side effects. [140] HA-PEG-MWCNT in vitro: HT-29 cells; in vivo: SD rats bearing HT-29 tumours 90% DLE, ~85% DRE, with a pH-mediated, sustained drug release profile A reduction in haemolytic activity and remarkable improvement in pharmacokinetic parameters compared to free GEM was observed. This was due to improved cellular internalisation, leading to enhanced anticancer effects.
[141] Slow and steady delivery of Et was observed at neutral pH, which was accelerated 1.5× upon NIR irradiation. CNHs are also photosensitizers, and the synergistic effect of PTT and Et chemotherapy killed multidrug-resistant cells by combating P-gp-mediated drug efflux. [134] FA-CβCDT-MSCD in vitro: HeLa and HepG2 cells 14% DLC, 25% DRE, with pH-mediated drug release This nanocarrier displayed preferential targeting of folate-receptor-overexpressing cells. The encapsulation of Et in cyclodextrin prevented premature drug release. [135] oxiMWCNT-PEG-Aso This nanocarrier displayed efficient cancer cell uptake via CD44-receptor-mediated endocytosis, resulting in a significant enhancement of the GEF efficacy. The system showed stronger tumour inhibition than the free drug in vivo, with no obvious side effects. [140] HA-PEG-MWCNT in vitro: HT-29 cells; in vivo: SD rats bearing HT-29 tumours 90% DLE, ~85% DRE, with a pH-mediated, sustained drug release profile A reduction in haemolytic activity and remarkable improvement in pharmacokinetic parameters compared to free GEM was observed. This was due to improved cellular internalisation, leading to enhanced anticancer effects. [141] Gefitinib (GEF)-tyrosine kinase inhibitor GO-PVP in vitro: PA-1 and IOSE-364 cells 46% DLE, 35% DRE, with GEF release at neutral pH This biocompatible nanocarrier is a combination therapy with quercetin and was found to enter cells via receptor-mediated endocytosis. The synergistic effect of GEF and quercetin results in significant therapeutic efficacy with ovarian cancer cells, higher than that of drugs delivered separately. [137] PEG-CQD-PVA-PLA in vitro: NCI-H522 cells~6 5% DRE, with no quantitative drug loading data shown The PLA microspheres degrade via a hydrolytic reaction in acidic conditions, releasing GEF. The nanosystem delivered the drug directly to lung cancer cells, resulting in a significant decrease in the IC50 value compared to free GEF. [138]  This nanocarrier system accumulates in tumour cells and releases considerable amounts of GEM, resulting in the inhibition of tumour growth and a reduction in GEM side effects. This system also improved the stability of GEM. [142] PEG-Fe3O4@GO@ mSiO2-FA in vitro: A431 cells 14% DLC, 85% DRE, with pH-triggered drug release This system demonstrated enhanced GEM cytotoxicity and cellular uptake. [143] ND-PEI-PAA-PEG-GFLG in vitro: BxPC-3; in vivo: BALB/c nude mice xenografted with BxPC-3 tumours No quantitative drug loading/release studies were performed Significant nanocarrier stability was observed in physiological conditions, with long-term circulation due to PEG attachment and enzyme-sensitive GEM release. This system showed similar anticancer effects in vitro and a significant increase in antitumour effects in vivo compared to free GEM. [8] ND-PEG in vitro: AsPC-1 cells No quantitative drug loading/release data were provided The fluorescent NDs provide imaging and cell-tracking capabilities, and whilst no cytotoxicity enhancement compared to free GEM was seen, the nanocarrier successfully delivered GEM directly to pancreas cancer cells. [144] ND@PHEA-co-POEGMEA in vitro: AsPC-1 cells 7% DLC, ~100% DRE, with pH-triggered drug release GEM was incorporated into HEA polymer and then loaded onto NDs. This resulted in slow, sustained GEM release delivered directly to cancer cells, with a similar IC50 value to free GEM. [145] GO/MMT/CS in vitro: MDA-MB-231 cells 99% DRE, with no qualitative drug loading data shown and pH-triggered drug release GEM intercalated between MMT silicate layers, preventing burst release. The unloaded nanocarrier is nontoxic, and the sustained release of GEM from the system results in the excellent growth inhibition of breast cancer cells. [146] FA-CS/Fe3O4/GO 22% DLC, 83% DRE, with pH-triggered drug release This system was tested in simulated cancer fluid and simulated human blood, and it is also responsive to external magnetic fields.  This system displayed excellent targeted drug delivery, antitumour efficacy, and prolonged animal survival in brain tumour models using intravenous administration coupled with magnetic guidance. A 4.9× increase in drug uptake compared to the free drug was measured in vitro. A 6.5× enhancement in the ability to cross the blood-brain barrier compared to the free drug was seen in vivo. Highly biocompatibility was also seen in vivo. [156] Imatinib-tyrosine kinase inhibitor N-prGO-CMC 74% DLC, 58% DRE, with pH-triggered IM release The drug is bound to the nanocarrier via π-π stacking and hydrogen-bonding interactions.
[ This system displayed excellent targeted drug delivery, antitumour efficacy, and prolonged animal survival in brain tumour models using intravenous administration coupled with magnetic guidance. A 4.9× increase in drug uptake compared to the free drug was measured in vitro. A 6.5× enhancement in the ability to cross the blood-brain barrier compared to the free drug was seen in vivo. Highly biocompatibility was also seen in vivo. [156] Irinotecantopoisomerase I inhibitor This system displayed excellent targeted drug delivery, antitumour efficacy, and prolonged animal survival in brain tumour models using intravenous administration coupled with magnetic guidance. A 4.9× increase in drug uptake compared to the free drug was measured in vitro. A 6.5× enhancement in the ability to cross the blood-brain barrier compared to the free drug was seen in vivo. Highly biocompatibility was also seen in vivo. [156] Pharmaceutics 2023, 15 The drug is released quickly at neutral pH, which could cause toxicity due to premature leakage. This system displayed favourable tumour targeting, cytotoxicity, and accumulation. [25] CMC-GO in vitro: NIH-3T3 and HT-29 cells; in vivo: BALB/c mice and nude mice xenografted with HT-29 tumours 39% DLC, 82% DRE, with pH-triggered drug release This system reduced drug toxicity against healthy cells and facilitated a higher plasma concentration, superior tumour cytotoxicity, and liver cancer metastasis inhibition compared to free METX. [159] Hydroxylated C60 fullerene in vitro: MDA-MB-231 cells; in vivo: Wistar rats No quantitative drug loading data shown, with 85% DRE and pH-sensitive drug release This nanosystem drastically increased plasma half-life and AUC compared to the free drug, resulting in a large reduction in its IC50 value. Enhanced bioavailability, erythrocyte compatibility, protein binding, and haemolysis in human blood compared to free METX were also observed. [23] AF-FA-99m Tc-MWCNT in vitro: A549 and MCF 7 cells; in vivo: New Zealand rabbits and FR+ 33% DLC, >85% DRE, with pH-triggered drug release achieved via a cleavable ester linkage Effective targeting and treatment of folate-receptor-overexpressing cancer cells with reduced side effects and increased efficacy in vivo was observed. This nanocarrier also had fluorescence imaging and radio-tracing capabilities. [ The drug is released quickly at neutral pH, which could cause toxicity due to premature leakage. This system displayed favourable tumour targeting, cytotoxicity, and accumulation. [25] CMC-GO in vitro: NIH-3T3 and HT-29 cells; in vivo: BALB/c mice and nude mice xenografted with HT-29 tumours 39% DLC, 82% DRE, with pH-triggered drug release This system reduced drug toxicity against healthy cells and facilitated a higher plasma concentration, superior tumour cytotoxicity, and liver cancer metastasis inhibition compared to free METX. [159] Hydroxylated C60 fullerene in vitro: MDA-MB-231 cells; in vivo: Wistar rats No quantitative drug loading data shown, with 85% DRE and pH-sensitive drug release This nanosystem drastically increased plasma half-life and AUC compared to the free drug, resulting in a large reduction in its IC50 value. Enhanced bioavailability, erythrocyte compatibility, protein binding, and haemolysis in human blood compared to free METX were also observed. [23] AF-FA-99m Tc-MWCNT in vitro: A549 and MCF 7 cells; in vivo: New Zealand rabbits and FR+ EAT-bearing mice 33% DLC, >85% DRE, with pH-triggered drug release achieved via a cleavable ester linkage Effective targeting and treatment of folate-receptor-overexpressing cancer cells with reduced side effects and increased efficacy in vivo was observed. This nanocarrier also had fluorescence imaging and radio-tracing capabilities. [160] Mitomycin C (MMC)-DNA alkylating agent TAT-graphene in vitro: OCM-1 and ARPE-19 cells 22% DLC, 45% DRE, with pH-triggered drug release; however, the release in acidic and neutral environments was very similar This system could specifically target cancer cells over healthy cells in a co-culture environment. The nanocarrier localised in the cancer cell nuclei, resulting in strong growth suppression. [161]

CD in vitro: MCF-7 cells
Approximately 80% DRE, with no quantitative drug loading information provided and pH-mediated MMC release MMC was bound to the CDs via hydrogen bonding. This nanocarrier showed high affinity towards cancer cell membranes and could effectively enter them and accumulate. This resulted in a significant improvement in anticancer potency compared to free MMC. [162] SWCNT-PEG-CWKG(KWKG)6 in vitro: A549 cells Approximately 80% DRE, with pHmediated MMC release The unloaded nanocarrier showed high biocompatibility whilst the drug-loaded system exhibited similar anticancer efficacy to free MMC. [163] Graphene-BOD-IPY-PEG in vitro: HeLa cells 10% DLC, with no quantitative drug release data given This nanocarrier possesses excellent photothermal conversion efficiency and ROS production capabilities for combination PTT/PDT. The system also has fluorescence and photothermal imaging capabilities and displayed outstanding anticancer effects. [14] GO-Fe2O3-MitP in vitro: A549 cells 19% DLC, 38% DRE, with magneticfield-triggered MTX release MitP grafting improves the drug loading capability of this nanocarrier. Successful targeting and disruption of tumour mitochondria was achieved, causing cell death. The drug is released quickly at neutral pH, which could cause toxicity due to premature leakage. This system displayed favourable tumour targeting, cytotoxicity, and accumulation. This system reduced drug toxicity against healthy cells and facilitated a higher plasma concentration, superior tumour cytotoxicity, and liver cancer metastasis inhibition compared to free METX. [159] Hydroxylated C 60  The drug is released quickly at neutral pH, which could cause toxicity due to premature leakage. This system displayed favourable tumour targeting, cytotoxicity, and accumulation. [25] CMC-GO in vitro: NIH-3T3 and HT-29 cells; in vivo: BALB/c mice and nude mice xenografted with HT-29 tumours 39% DLC, 82% DRE, with pH-triggered drug release This system reduced drug toxicity against healthy cells and facilitated a higher plasma concentration, superior tumour cytotoxicity, and liver cancer metastasis inhibition compared to free METX. [159] Hydroxylated C60 fullerene in vitro: MDA-MB-231 cells; in vivo: Wistar rats No quantitative drug loading data shown, with 85% DRE and pH-sensitive drug release This nanosystem drastically increased plasma half-life and AUC compared to the free drug, resulting in a large reduction in its IC50 value. Enhanced bioavailability, erythrocyte compatibility, protein binding, and haemolysis in human blood compared to free METX were also observed. [23] AF-FA-99m Tc-MWCNT in vitro: A549 and MCF 7 cells; in vivo: New Zealand rabbits and FR+ EAT-bearing mice 33% DLC, >85% DRE, with pH-triggered drug release achieved via a cleavable ester linkage Effective targeting and treatment of folate-receptor-overexpressing cancer cells with reduced side effects and increased efficacy in vivo was observed. This nanocarrier also had fluorescence imaging and radio-tracing capabilities. [160] Mitomycin C (MMC)-DNA alkylating agent TAT-graphene in vitro: OCM-1 and ARPE-19 cells 22% DLC, 45% DRE, with pH-triggered drug release; however, the release in acidic and neutral environments was very similar This system could specifically target cancer cells over healthy cells in a co-culture environment. The nanocarrier localised in the cancer cell nuclei, resulting in strong growth suppression. [161]

CD in vitro: MCF-7 cells
Approximately 80% DRE, with no quantitative drug loading information provided and pH-mediated MMC release MMC was bound to the CDs via hydrogen bonding. This nanocarrier showed high affinity towards cancer cell membranes and could effectively enter them and accumulate. This resulted in a significant improvement in anticancer potency compared to free MMC. [162] SWCNT-PEG-CWKG(KWKG)6 in vitro: A549 cells Approximately 80% DRE, with pHmediated MMC release The unloaded nanocarrier showed high biocompatibility whilst the drug-loaded system exhibited similar anticancer efficacy to free MMC. [163] Graphene-BOD-IPY-PEG in vitro: HeLa cells 10% DLC, with no quantitative drug release data given This nanocarrier possesses excellent photothermal conversion efficiency and ROS production capabilities for combination PTT/PDT. The system also has fluorescence and photothermal imaging capabilities and displayed outstanding anticancer effects. [14] GO-Fe2O3-MitP in vitro: A549 cells 19% DLC, 38% DRE, with magneticfield-triggered MTX release MitP grafting improves the drug loading capability of this nanocarrier. Successful targeting and disruption of tumour mitochondria was achieved, causing cell death. [164] Mitomycin C (MMC)-DNA alkylating agent TAT-graphene in vitro: OCM-1 and ARPE-19 cells 22% DLC, 45% DRE, with pH-triggered drug release; however, the release in acidic and neutral environments was very similar This system could specifically target cancer cells over healthy cells in a co-culture environment. The nanocarrier localised in the cancer cell nuclei, resulting in strong growth suppression. [161]

CD in vitro: MCF-7 cells
Approximately 80% DRE, with no quantitative drug loading information provided and pH-mediated MMC release MMC was bound to the CDs via hydrogen bonding. This nanocarrier showed high affinity towards cancer cell membranes and could effectively enter them and accumulate. This resulted in a significant improvement in anticancer potency compared to free MMC. [162] SWCNT-PEG-CWKG(KWKG) 6 in vitro: A549 cells Approximately 80% DRE, with pH-mediated MMC release The unloaded nanocarrier showed high biocompatibility whilst the drug-loaded system exhibited similar anticancer efficacy to free MMC.
[163] MTX was bound to oxiMWCNTs via electrostatic interactions. This formulation resulted in increased MTX efficacy; however, the system was more toxic to healthy cells than cancerous cells. [165] ND in vitro: MDA-MB-231 and MDA-MB231-ABCG2 cells 87% DLE, 80% DRE, with pH-and soluble-protein-triggered MTX release MTX release was found to be higher in FBS than in water, suggesting that the presence of soluble biological matter increases the DRE. A marked increase in MTX retention and efficacy was observed when using this nanocarrier. [166] oxiSWCNT-PEG-FA in vitro: HeLa cells ~35% DLE, 55% DRE, with pH-mediated, sustained drug release This system selectively targeted cancer cells. [167] EXO-GO-CO-γPGA in vitro: MDA-MB-231 and BEAS-2B cells 73% DLE, 56% DRE, with pH-mediated, sustained drug release This nanocarrier displays excellent cancer cell targetability. The attachment of exosomes was found to improve drug loading, pH response, and biocompatibility. [168] rGO-PEG-SB in vitro: 4T1, CT26, and bone marrow macrophages + DCs harvested from BALB/c mice; in vivo: BALB/c mice with 4T1 tumours 48% DLE, with no quantitative drug release data provided and NIR-triggered drug release The synergistic combination of PTT, chemotherapy, and immunotherapy facilitated the destruction of local primary tumours and distant metastases in an in vivo model. rGO acted as a photosensitizer, whilst the SB immunotherapeutic increased effectiveness of rGO and MTX by TGF-β inhibition. [169] Oxaliplatin (OP)-DNA alkylating agent GO-PNVCL-PGA in vitro: MCF-7 cells 12% DLC, 80% DRE, with pH-and thermal-responsive OP release Improved cytotoxicity compared to free OP against breast cancer cells was observed. The blank nanocarrier is nontoxic. [170] GO-HSA NPs in vitro: HFFF2 61% DLE, ~97% DRE, with pH-mediated, sustained drug release The use of HSA nanoparticles increased nanocarrier biocompatibility. [171] MWCNT-PEG in vitro: HT29 cells 43% DLC, no quantitative drug release data A drastic increase in cytotoxicity towards human bowel cancer cells was observed. [ MTX release was found to be higher in FBS than in water, suggesting that the presence of soluble biological matter increases the DRE. A marked increase in MTX retention and efficacy was observed when using this nanocarrier. [166] oxiSWCNT-PEG-FA in vitro: HeLa cells~3 5% DLE, 55% DRE, with pH-mediated, sustained drug release This system selectively targeted cancer cells. [167] EXO-GO-CO-γPGA in vitro: MDA-MB-231 and BEAS-2B cells 73% DLE, 56% DRE, with pH-mediated, sustained drug release This nanocarrier displays excellent cancer cell targetability. The attachment of exosomes was found to improve drug loading, pH response, and biocompatibility. [168] rGO-PEG-SB in vitro: 4T1, CT26, and bone marrow macrophages + DCs harvested from BALB/c mice; in vivo: BALB/c mice with 4T1 tumours 48% DLE, with no quantitative drug release data provided and NIR-triggered drug release The synergistic combination of PTT, chemotherapy, and immunotherapy facilitated the destruction of local primary tumours and distant metastases in an in vivo model. rGO acted as a photosensitizer, whilst the SB immunotherapeutic increased effectiveness of rGO and MTX by TGF-β inhibition.
[169] Mitoxantrone (MTX)-topoisomerase II inhibitor oxiSWCNT-PEG-FA in vitro: HeLa cells ~35% DLE, 55% DRE, with pH-mediated, sustained drug release This system selectively targeted cancer cells. [167] EXO-GO-CO-γPGA in vitro: MDA-MB-231  and BEAS-2B cells   73% DLE, 56% DRE, with pH-mediated, sustained drug release This nanocarrier displays excellent cancer cell targetability. The attachment of exosomes was found to improve drug loading, pH response, and biocompatibility. [168] rGO-PEG-SB in vitro: 4T1, CT26, and bone marrow macrophages + DCs harvested from BALB/c mice; in vivo: BALB/c mice with 4T1 tumours 48% DLE, with no quantitative drug release data provided and NIR-triggered drug release The synergistic combination of PTT, chemotherapy, and immunotherapy facilitated the destruction of local primary tumours and distant metastases in an in vivo model. rGO acted as a photosensitizer, whilst the SB immunotherapeutic increased effectiveness of rGO and MTX by TGF-β inhibition. [169] Oxaliplatin (OP)-DNA alkylating agent GO-PNVCL-PGA in vitro: MCF-7 cells 12% DLC, 80% DRE, with pH-and thermal-responsive OP release Improved cytotoxicity compared to free OP against breast cancer cells was observed. The blank nanocarrier is nontoxic. [170] GO-HSA NPs in vitro: HFFF2 61% DLE, ~97% DRE, with pH-mediated, sustained drug release The use of HSA nanoparticles increased nanocarrier biocompatibility. [171] MWCNT-PEG  AQ4N) properties. The enhanced hypoxia resulting from PTT bolsters the effects of chemotherapy drugs. This strategy has the benefit of being able to noncovalently attach nonaromatic drug molecules via host-guest complex formation. [173] TAT-BT-PEI-MWCNT in vitro: C6 glioma (cell and tumour spheroid) and CHEM-5 and L02 cells; in vivo: mice bearing C6 tumours 19% DLC, with no quantitative drug release data shown The nanocarrier shows enhanced blood-brain barrier penetration compared to free OX, resulting in a significant decrease in the IC 50 value. The system shows low cytotoxicity towards healthy cells; however, a build-up of cerebrospinal fluid was noticed during treatment of in vivo models. [174] GO-CS-FA in vitro: LX-2 and SKOV3 cells 34% DLC,~80% DRE This nanoformulation shows similar potency to free OX in ovarian cancer cells and good biocompatibility. [48] CD in vitro: L929, HeLa, and HepG2 cells 4% DLC, with redox-sensitive drug release The CDs have multicoloured emission capabilities and high fluorescence stability. This system shows good biocompatibility, bio-imaging, and anticancer effects both in vitro and in vivo.
[175] The nanocarrier shows enhanced blood-brain barrier penetration compared to free OX, resulting in a significant decrease in the IC50 value. The system shows low cytotoxicity towards healthy cells; however, a build-up of cerebrospinal fluid was noticed during treatment of in vivo models. [174] GO-CS-FA in vitro: LX-2 and SKOV3 cells 34% DLC, ~80% DRE This nanoformulation shows similar potency to free OX in ovarian cancer cells and good biocompatibility. [48] CD in vitro: L929, HeLa, and HepG2 cells 4% DLC, with redox-sensitive drug release The CDs have multicoloured emission capabilities and high fluorescence stability. This system shows good biocompatibility, bio-imaging, and anticancer effects both in vitro and in vivo. PTX was attached to GO via π-π stacking and hydrophobic interactions. This system inhibited cancer cell growth in vitro and reduced tumour size in vivo via cell cycle arrest and apoptosis. Highly specific targeting and drug release for folate-receptor-overexpressing cancers was observed. [176] SWCNT/DOA-PEG-FA in vitro: MCF-7 cells; in vivo: athymic nude mice with MFF-7-induced tumours and BALB/c mice No quantitative drug loading or release data were provided This system displayed high specificity, biocompatibility, and efficacy compared to free PTX in vitro. In vivo, studies revealed significant tumour growth inhibition, with no side effects on the blood and major organs of mice observed. [177] PLA composite nanofibers/C70 fullerene in vitro: HepG2 cells No quantitative drug loading information, 72% DRE Control of in vitro PTX release profile was achieved by varying fullerene content. Successful control of cancer cell growth was achieved using this nanoformulation. [178] Rf-MWCNTs in vitro: MCF-7 cells; in vivo: SD rats 82% DLE, 99% DRE Haemolysis in human blood was much lower than free PTX. This system also showed better cytotoxicity than free PTX, with low systemic toxicity, favourable biodistribution, and renal excretion observed in vivo. [179] Au-N-doped carbon nanotube cup (NCNC) in vitro: MDSC cells; in vivo: B16 melanoma cells inoculated into C57BL/6 mice 36% DLE, with no quantitative drug release data shown The PTX-containing NCNCs capped with Au nanoparticles exhibited strong surface-enhanced Raman scattering effects, allowing for extremely sensitive detection. A single injection of nanocarrier solution given to an in vivo model significantly reduced tumour growth and eliminated tumours in ~30% of mice. This system targeted lymphoid tissues surrounding tumours to boost the host immune system response. [180] Graphene-PLA-PEG CNT-PMAA self-assembled micelles in vitro: L929 and HeLa cells 36% DLE, 74% DRE, with pH-triggered drug release The blank nanocarrier is nontoxic, with low haemolysis observed. Higher anticancer activity than free PTX was also noted. The self-assembly of CNT-PMAA is pH-dependant, and at low pH, the nanocarrier disassembles. [182] FA-CD-GOx CNT-PMAA selfassembled micelles in vitro: L929 and HeLa cells 36% DLE, 74% DRE, with pH-triggered drug release The blank nanocarrier is nontoxic, with low haemolysis observed. Higher anticancer activity than free PTX was also noted. The self-assembly of CNT-PMAA is pH-dependant, and at low pH, the nanocarrier disassembles. [182] FA-CD-GOx GOx-induced cancer starvation, which had a synergistic effect with PTX chemotherapy, and resulted in significant cancer cell death. This biocompatible nanocarrier also efficiently targets cancer cells over normal cells. [13] FCDb in vitro: NIH3T3 and B16F10 cells 82% DLC, with no quantitative release data given This nanocarrier targeted cancer cells in a co-culture with healthy cells due to its biotin-targeting ligand. It caused the selective sensing and activation of H2O2, and it also has fluorescence imaging capabilities. [183] Sorafenib-tyrosine kinase inhibitor rGO in vitro: SGC7901 cells No quantitative drug loading/release data A significant increase in cytotoxicity compared to the free drug in gastric cancer cells was observed, with apoptosis being the main mechanism of cell death. [184] CS nanoparticles-FA The blank nanocarrier is nontoxic, with low haemolysis observed. Higher anticancer activity than free PTX was also noted. The self-assembly of CNT-PMAA is pH-dependant, and at low pH, the nanocarrier disassembles. [182] FA-CD-GOx GOx-induced cancer starvation, which had a synergistic effect with PTX chemotherapy, and resulted in significant cancer cell death. This biocompatible nanocarrier also efficiently targets cancer cells over normal cells. [13] FCDb in vitro: NIH3T3 and B16F10 cells 82% DLC, with no quantitative release data given This nanocarrier targeted cancer cells in a co-culture with healthy cells due to its biotin-targeting ligand. It caused the selective sensing and activation of H2O2, and it also has fluorescence imaging capabilities. [183] Sorafenib-tyrosine kinase inhibitor rGO in vitro: SGC7901 cells No quantitative drug loading/release data A significant increase in cytotoxicity compared to the free drug in gastric cancer cells was observed, with apoptosis being the main mechanism of cell death. [184] CS nanoparticles-FA Enhanced cellular uptake, apoptosis, and antitumour activity (compared to free TAM) were observed. This system also has PTT properties. LEN acts as both a dispersant and a potent anticancer agent itself. Combination chemo-PTT results in cancer cell destruction at low drug concentrations. [190]  This haem-compatible formulation reduced the IC50 values and increased the uptake in drug-resistant cells. These favourable properties carried over to in vivo studies, resulting in enhanced efficacy, pharmacokinetics, and biocompatibility. [192] DES-graphene in vitro: MCF-7 cells No quantitative drug loading/release data Nanocarrier possesses acute, selective anticancer activity achieved through intracellular ROS-production-triggering cell cycle arrest. [193] Temozolomide-DNA alkylating agent GO-Fe3O4 in vitro: rat glioma C6 cells 90% DLC and 74% DRE, with pH-mediated drug release The blank nanocarrier is biocompatible, whilst the loaded system showed better inhibitory effects than the free drug in rat glioma cells. This formulation also has strong magnetic properties. [194] rGO in vitro: LN229 cells 84% DLC and 83% DRE, with electrochemically controlled drug release This system retained the anticancer potency of temozolomide. [195] Topotecan-topisomerase I inhibitor GO/TT/CisP/cholesterol-DOX encapsulated in a DSPE-PEG nanocell in vitro: HeLa cells 29% DLC and 40% DRE for TT, with pH-triggered drug release This nanocarrier has fluorescence imaging capabilities and displays remarkably higher anticancer efficacy compared to the free-drug combination due to the synchronised targeting of multiple targets in cancer cells at once. [196] PEI-GO-TT-CisP in vitro: HeLa cells 51% DLC for TT, no quantitative drug release data This nanocarrier has subcellular-organelle-targeting capabilities and can effectively impair the mitochondria of cervical cancer cells, leading to cell death. This resulted in a 4.4× decrease in IC50 compared to the free-drug cocktail. [197] TT/GO-CD/DOX in vitro: HeLa cells 16% DLE and 77% DRE for TT, with sustained, pH-triggered drug release This system demonstrated superior anticancer efficacy compared to free drugs and singledrug-loaded nanocarrier due to the synergistic effect between the drugs and GO. [198] Vinblastine-mitosis inhibitor CQD in vitro: Hela, HGC-27, A549, MCF-7, CF-STTG, and Vero cells; in vivo: NOD-SCID mice carrying A549 tumours 95% DLC, with no qualitative drug release data The cytotoxicity of vinblastine was reduced in normal cells and increased in cancer cells compared to the free drug. Significant inhibition of tumour growth was observed in vivo, with no liver toxicity. The synergistic combination of chemotherapy and PTT allowed for the control of cancer cell growth. [199] Temozolomide-DNA alkylating agent GO-Fe 3 O 4 in vitro: rat glioma C6 cells 90% DLC and 74% DRE, with pH-mediated drug release The blank nanocarrier is biocompatible, whilst the loaded system showed better inhibitory effects than the free drug in rat glioma cells. This formulation also has strong magnetic properties. [194] rGO in vitro: LN229 cells 84% DLC and 83% DRE, with electrochemically controlled drug release This system retained the anticancer potency of temozolomide. [195] Pharmaceutics 2023, 15 This haem-compatible formulation reduced the IC50 values and increased the uptake in drug-resistant cells. These favourable properties carried over to in vivo studies, resulting in enhanced efficacy, pharmacokinetics, and biocompatibility. [192] DES-graphene in vitro: MCF-7 cells No quantitative drug loading/release data Nanocarrier possesses acute, selective anticancer activity achieved through intracellular ROS-production-triggering cell cycle arrest. [193] Temozolomide-DNA alkylating agent GO-Fe3O4 in vitro: rat glioma C6 cells 90% DLC and 74% DRE, with pH-mediated drug release The blank nanocarrier is biocompatible, whilst the loaded system showed better inhibitory effects than the free drug in rat glioma cells. This formulation also has strong magnetic properties. [194] rGO in vitro: LN229 cells 84% DLC and 83% DRE, with electrochemically controlled drug release This system retained the anticancer potency of temozolomide. [195] Topotecan-topisomerase I inhibitor This system demonstrated superior anticancer efficacy compared to free drugs and single-drug-loaded nanocarrier due to the synergistic effect between the drugs and GO. [198]  Temozolomide-DNA alkylating agent rGO in vitro: LN229 cells chemically controlled drug release This system retained the anticancer potency of temozolomide. [195] Topotecan-topisomerase I inhibitor GO/TT/CisP/cholesterol-DOX encapsulated in a DSPE-PEG nanocell in vitro: HeLa cells 29% DLC and 40% DRE for TT, with pH-triggered drug release This nanocarrier has fluorescence imaging capabilities and displays remarkably higher anticancer efficacy compared to the free-drug combination due to the synchronised targeting of multiple targets in cancer cells at once. [196] PEI-GO-TT-CisP in vitro: HeLa cells 51% DLC for TT, no quantitative drug release data This nanocarrier has subcellular-organelle-targeting capabilities and can effectively impair the mitochondria of cervical cancer cells, leading to cell death. This resulted in a 4.4× decrease in IC50 compared to the free-drug cocktail. [197] TT/GO-CD/DOX in vitro: HeLa cells 16% DLE and 77% DRE for TT, with sustained, pH-triggered drug release This system demonstrated superior anticancer efficacy compared to free drugs and singledrug-loaded nanocarrier due to the synergistic effect between the drugs and GO. [198] Vinblastine-mitosis inhibitor CQD in vitro: Hela, HGC-27, A549, MCF-7, CF-STTG, and Vero cells; in vivo: NOD-SCID mice carrying A549 tumours 95% DLC, with no qualitative drug release data The cytotoxicity of vinblastine was reduced in normal cells and increased in cancer cells compared to the free drug. Significant inhibition of tumour growth was observed in vivo, with no liver toxicity. The synergistic combination of chemotherapy and PTT allowed for the control of cancer cell growth. [199] Vinblastine-mitosis inhibitor CQD in vitro: Hela, HGC-27, A549, MCF-7, CF-STTG, and Vero cells; in vivo: NOD-SCID mice carrying A549 tumours 95% DLC, with no qualitative drug release data The cytotoxicity of vinblastine was reduced in normal cells and increased in cancer cells compared to the free drug. Significant inhibition of tumour growth was observed in vivo, with no liver toxicity. The synergistic combination of chemotherapy and PTT allowed for the control of cancer cell growth. [199] Pharmaceutics 2023, 15

Discussion
A total of 38 approved anticancer drugs were used in CNM-based nanocarriers in the literature, a breakdown of which can be seen in Figure 1. The anticancer drugs were further classified according to their mechanism of action, and the prevalence of each class is shown in Figure 1. The classes are as follows: (1) Alkylating agents-these work by adding alkyl groups to DNA, which can lead to DNA strand breaks and inhibit DNA replication; (2) Antimetabolites-these interfere with the synthesis of DNA, RNA, and proteins by mimicking essential cellular metabolites; (3) Natural products-this category consists of chemotherapeutic agents derived from natural sources, such as plants, microorganisms, or marine organisms. These agents often target specific aspects of cell division or DNA replication; (4) Hormone therapies-these target hormone-dependent cancers by interfering with the action of specific hormones or hormone receptors; (5) Antimicrotubule agents-these drugs target the microtubules, which play an essential role in cell division. By disrupting the formation or function of microtubules, these agents can inhibit cell division and lead to cancer cell death; (6) Miscellaneous agents-these include chemotherapeutic drugs that do not fit neatly into any of the other categories. The most diverse class of anticancer drugs used were alkylating agents, which is not surprising, as this class includes many Pt-based drugs, which can be easily complexed with oxidized CNMs. The miscellaneous-agent section contained five tyrosine kinase inhibitors, indicating the popularity of this class of drug. Hormone therapies were the least popular class of chemotherapy agents, with only three entries.
For each of the anticancer drugs in the database, several CNM-based nanocarriers have been investigated for their use in drug delivery. A total of 191 examples of CNMbased nanocarrier systems were found in the literature, many of which displayed higher anticancer efficacy with reduced side effects. As discussed in the introduction, the ease of functionalisation of CNM surfaces (graphene and CNT in particular) offers many different approaches to developing nanocarriers. A huge number of ligands were found to be used for drug delivery in the literature. These included polymers, such as PEG, which offer biocompatibility, water solubility, and reduced aggregation in situ [65,96], and biomolecules, such as folic acid, which enable the active targeting of folate receptors on tumour cells [177]. Other commonly used ligands were fluorescent agents, an example of which is Alexa Fluor, which is used for the fluorescent imaging of tumour cells [160], and peptides and proteins, offering improved bioavailability and stability [84]). Many of these approaches combined in the nanocarriers found in the literature show the complexity and breadth of options available to use. For each of the anticancer drugs in the database, several CNM-based nanocarrie have been investigated for their use in drug delivery. A total of 191 examples of CNM based nanocarrier systems were found in the literature, many of which displayed highe anticancer efficacy with reduced side effects. As discussed in the introduction, the ease o functionalisation of CNM surfaces (graphene and CNT in particular) offers many diffe ent approaches to developing nanocarriers. A huge number of ligands were found to b used for drug delivery in the literature. These included polymers, such as PEG, whic offer biocompatibility, water solubility, and reduced aggregation in situ [65,96], and bio molecules, such as folic acid, which enable the active targeting of folate receptors on tu mour cells [177]. Other commonly used ligands were fluorescent agents, an example o which is Alexa Fluor, which is used for the fluorescent imaging of tumour cells [160], an peptides and proteins, offering improved bioavailability and stability [84]). Many of thes approaches combined in the nanocarriers found in the literature show the complexity an breadth of options available to use.
The numerical analysis of the database shown in Figure 2 reveals that graphene (G in particular) is the most popular class of CNMs to be incorporated into these system likely because it is one of the most well-established carbon nanomaterials. Over the year a catalogue of functionalisation procedures has been developed, allowing for a range o moieties to be attached to the material's surface [201]. The flat, aromatic surface of gra phene lends itself excellently to π-π stacking, which allows for the easy noncovalen stacking of drug molecules. Graphene is also a PTT agent, a property that can be used t bolster the effectiveness of chemotherapy [14]. This is beneficial for the nanocarrier deve oped in [188], where free TAM is more efficacious than the nanocarrier-bound drug bu the PTT potential of rGO makes it an attractive combinatorial therapy. Nonfunctionalise GO and rGO were found to successfully deliver anticancer drugs. GO and rGO showe pH-and NIR-triggered releases in some cases [32,132,148]. This offers a site-specific r lease of the drug, as the pH in tumour cells is typically lower than that in healthy cell and NIR offers similar delivery.
CNTs came second in terms of popularity, which is surprising, as they are the olde and most well-studied class of CNMs. This could be due to their tendency to aggrega into bundles in aqueous solutions, which could affect their biocompatibility. CNTs als do not have much in the way of intrinsic therapeutic or imaging properties; however, the do have large surface areas for drug loading. One example of pristine CNTs shows pH dependent release and PTT [78]. Oxidised MWCNTs were found to be more toxic t healthy cells than cancer cells, which shows a need for further functionalisation [155]. Th lack of control of intracellular accumulation also highlights the need for the attachment o targeting ligands to these systems. The numerical analysis of the database shown in Figure 2 reveals that graphene (GO in particular) is the most popular class of CNMs to be incorporated into these systems, likely because it is one of the most well-established carbon nanomaterials. Over the years, a catalogue of functionalisation procedures has been developed, allowing for a range of moieties to be attached to the material's surface [201]. The flat, aromatic surface of graphene lends itself excellently to π-π stacking, which allows for the easy noncovalent stacking of drug molecules. Graphene is also a PTT agent, a property that can be used to bolster the effectiveness of chemotherapy [14]. This is beneficial for the nanocarrier developed in [188], where free TAM is more efficacious than the nanocarrier-bound drug but the PTT potential of rGO makes it an attractive combinatorial therapy. Nonfunctionalised GO and rGO were found to successfully deliver anticancer drugs. GO and rGO showed pH-and NIR-triggered releases in some cases [32,132,148]. This offers a site-specific release of the drug, as the pH in tumour cells is typically lower than that in healthy cells, and NIR offers similar delivery. Carbon dots, including GQDs, were the third most popular CNM to be used in these systems. The carbon allotropes that make up this new and rising class are tiny, even compared to other CNMs; this allows them to penetrate deep into cells to deliver drug molecules. They are even small enough to cross the blood-brain barrier [202], and their excellent biocompatibility [203] and ease of production in many cases [204] make them excellent nanocarrier scaffolds.
Many types of carbon dots are also water-soluble and have fluorescence imaging capabilities in the pristine form, for dual imaging and drug delivery [175]. Other types of CNMs, such as fullerenes, NDs, CNOs, HMCS, and CNHs, may not be as popular, but they show potential as nanocarriers due to their unique properties. For example, NDs offer improved biocompatibility over other CNMs [114,166], whilst fullerenes and CNOs display ease of functionalisation, narrow PDI, and ease of production [23,59]. In terms of drug moieties, antimicrotubule agents were the most popular payloads for these nanocarriers (Figure 2). This is due to the presence of multiple aromatic rings in these molecules, which facilitate noncovalent attachment to the CNM surface.
Anthracyclines, such as doxorubicin and epirubicin, are particularly popular. Alkylating agents are also quite popular, with platinum-based drugs often being complexed to the surface of the nanomaterial host. This is a particularly popular strategy with highly oxidised materials, such as GO, as the Pt can complex directly with oxygen-containing CNTs came second in terms of popularity, which is surprising, as they are the oldest and most well-studied class of CNMs. This could be due to their tendency to aggregate into bundles in aqueous solutions, which could affect their biocompatibility. CNTs also do not have much in the way of intrinsic therapeutic or imaging properties; however, they do have large surface areas for drug loading. One example of pristine CNTs shows pH-dependent release and PTT [78]. Oxidised MWCNTs were found to be more toxic to healthy cells than cancer cells, which shows a need for further functionalisation [155]. The lack of control of intracellular accumulation also highlights the need for the attachment of targeting ligands to these systems.
Carbon dots, including GQDs, were the third most popular CNM to be used in these systems. The carbon allotropes that make up this new and rising class are tiny, even compared to other CNMs; this allows them to penetrate deep into cells to deliver drug molecules. They are even small enough to cross the blood-brain barrier [202], and their excellent biocompatibility [203] and ease of production in many cases [204] make them excellent nanocarrier scaffolds.
Many types of carbon dots are also water-soluble and have fluorescence imaging capabilities in the pristine form, for dual imaging and drug delivery [175]. Other types of CNMs, such as fullerenes, NDs, CNOs, HMCS, and CNHs, may not be as popular, but they show potential as nanocarriers due to their unique properties. For example, NDs offer improved biocompatibility over other CNMs [114,166], whilst fullerenes and CNOs display ease of functionalisation, narrow PDI, and ease of production [23,59].
In terms of drug moieties, antimicrotubule agents were the most popular payloads for these nanocarriers (Figure 2). This is due to the presence of multiple aromatic rings in these molecules, which facilitate noncovalent attachment to the CNM surface.
Anthracyclines, such as doxorubicin and epirubicin, are particularly popular. Alkylating agents are also quite popular, with platinum-based drugs often being complexed to the surface of the nanomaterial host. This is a particularly popular strategy with highly oxidised materials, such as GO, as the Pt can complex directly with oxygen-containing functional groups. Smaller hydrophilic organic molecules that lack any aromatic rings, such as those in the hormone therapy class, tend to not be so popular in CNM nanocarrier systems due to the lack of noncovalent interactions with the host CNM.
Designing systems that incorporate the intrinsic properties of CNMs allows for additional capabilities without having to chemically modify the material. The fluorescence of CDs and GQDs have proven to be useful for cellular imaging and tracking experiments [36,74,88]. Certain CNMs may also be utilised for killing cancer cells; for example, graphene has been used as a PTT agent, bolstering the effect of traditional chemotherapy [14,148]. Utilising this synergistic approach means that lower amounts of toxic chemotherapy drugs can be given to achieve the same therapeutic effect. The nπ* state of a CNM is essential for its intrinsic photothermal properties, and this state can be modulated by the addition of dopants (such as nitrogen) to the CNM [205]. Strong light absorption is required for a material to display photothermal properties, and a high photothermal conversion efficiency (η) is needed for a nanocarrier to be an effective PTT agent. For example, Forte et al. achieved photothermal-triggered drug release using a carbonised polymer dot-based system with an η value of 67.9% [206].
In general, systems that incorporate combination therapies exhibit some of the strongest anticancer effects due to the synergistic effects of PTT, PDT, single-drug and combination chemotherapy, or immunotherapy [67,129,169,190]. CNMs are the perfect class of nanoparticles for this approach, as they are easy to modify both covalently and noncovalently, with a range of functionalisation approaches available, allowing for the attachment of many different therapeutic agents. This, combined with the intrinsic properties of CNMs, can be leveraged to construct a range of nanocarrier systems.
The DLCs, DLEs, and DREs of nanocarriers are given as the number of entries above and below 50% in Figure 3. This was performed to make the entries more comparable given the differences in the sample sizes. A total of 152 out of 191 nanocarriers found in the literature had DLC, DLE, and/ or DRE data, and where DLC data were given, less than 35% of the nanocarriers were found to display values above 50%. This is similar for all CNMs, and surprisingly, graphene is the lowest, with only 22% of nanocarriers above 50% DLC. A much greater proportion of nanocarriers show DLEs above 50%, with fullerenes showing the lowest percentage of DLE values above 50%, and nanohorns displaying the highest at 100%; however, only one entry was available. This indicates that drug loading is an efficient process. In general, either a DLC or DLE value was given, with the DLE higher than the DLC. attachment of many different therapeutic agents. This, combined with the intrinsic properties of CNMs, can be leveraged to construct a range of nanocarrier systems.
The DLCs, DLEs, and DREs of nanocarriers are given as the number of entries above and below 50% in Figure 3. This was performed to make the entries more comparable given the differences in the sample sizes. A total of 152 out of 191 nanocarriers found in the literature had DLC, DLE, and/ or DRE data, and where DLC data were given, less than 35% of the nanocarriers were found to display values above 50%. This is similar for all CNMs, and surprisingly, graphene is the lowest, with only 22% of nanocarriers above 50% DLC. A much greater proportion of nanocarriers show DLEs above 50%, with fullerenes showing the lowest percentage of DLE values above 50%, and nanohorns displaying the highest at 100%; however, only one entry was available. This indicates that drug loading is an efficient process. In general, either a DLC or DLE value was given, with the DLE higher than the DLC. The DRE, where given, was found to be quite high, on average, for all CNMs, with NDs displaying the lowest values; however, the sample size for this material was small compared to those for graphene and CNTs. Graphene-based nanocarriers incorporating Fe3O4 showed particularly high loading and release: [47,85,193]. GO-COOH also displayed high loading and release properties [133]. A lot of CNT-based systems with very high loading and release were observed; for example, the FA-PEG-bis-amine MWCNT system displayed 99% DLE and 90% DRE [27], SWCNTs showed 94% DLE and 93% DRE [78], and CNT-PEG displayed 95% DLC and 100% DRE [186]. The DRE, where given, was found to be quite high, on average, for all CNMs, with NDs displaying the lowest values; however, the sample size for this material was small compared to those for graphene and CNTs. Graphene-based nanocarriers incorporating Fe 3 O 4 showed particularly high loading and release: [47,85,193]. GO-COOH also displayed high loading and release properties [133]. A lot of CNT-based systems with very high loading and release were observed; for example, the FA-PEG-bis-amine MWCNT system displayed 99% DLE and 90% DRE [27], SWCNTs showed 94% DLE and 93% DRE [78], and CNT-PEG displayed 95% DLC and 100% DRE [186].
The biocompatibility of these formulations must be further investigated, as certain CNMs are known to be toxic [207]. On the one hand, in the case of CNTs, the pristine form is toxic in mice and is dependent on the types of CNTs present [208]. On the other hand, pristine fullerenes such as C60 show no apparent toxicity, whereas some functionalised derivatives are highly toxic [209]. However, as previously discussed, the breadth of functionalisation methods and biocompatible ligands available to modify the surface chemistries of CNMs offers a variety of routes for overcoming this issue. The leakage of a drug at physiological pH is another issue that must be addressed in many systems, as toxic side effects are induced in vivo when the drug is released at neutral pH.

Conclusions and Future Directions
Overall, CNMs are incredibly versatile materials that can be used as both the foundation of nanocarrier systems and as therapeutic agents themselves. These systems can be designed to detect, image, and treat a range of tumours, from colorectal, brain, breast, liver, and stomach cancers. CNTs and graphene (GO in particular) were by far the most popular CNMs used in these systems due to their small size, high surface area, and ease of functionalisation. Other CNMs, such as carbon dots, are also growing in popularity due to their unique properties. A huge range of molecules, such as targeting ligands, fluorophores, dispersants, and drugs, can be easily attached to CNM surfaces, allowing for the construction of complex nanosystems.
Extensive in vivo biological work needs to be undertaken to fully understand the toxicity of these systems towards animals, and to overcome the regulatory hurdles needed to move these treatments into clinical trials.
The trend of designing theragnostic systems that incorporate the intrinsic properties of CNMs (such as PTT and fluorescence imaging) will likely be seen more in the future, as it allows for additional capabilities without damaging the CNM itself. Nanocarriers that leverage combinations of different therapies displayed the most potent anticancer effects, and therefore these systems will likely grow in popularity in the coming years.