Cancer chemotherapy and beyond: Current status, drug candidates, associated risks and progress in targeted therapeutics

Cancer is an abnormal state of cells where they undergo uncontrolled proliferation and produce aggressive malignancies that causes millions of deaths every year. With the new understanding of the molecular mechanism(s) of disease progression, our knowledge about the disease is snowballing, leading to the evolution of many new therapeutic regimes and their successive trials. In the past few decades, various combinations of therapies have been proposed and are presently employed in the treatment of diverse cancers. Targeted drug therapy, immunotherapy, and personalized medicines are now largely being employed, which were not common a few years back. The field of cancer discoveries and therapeutics are evolving fast as cancer type-specific biomarkers are progressively being identified and several types of cancers are nowadays undergoing systematic therapies, extending patients' disease-free survival thereafter. Although growing evidence shows that a systematic and targeted approach could be the future of cancer medicine, chemotherapy remains a largely opted therapeutic option despite its known side effects on the patient's physical and psychological health. Chemotherapeutic agents/pharmaceuticals served a great purpose over the past few decades and have remained the frontline choice for advanced-stage malignancies where surgery and/or radiation therapy cannot be prescribed due to specific reasons. The present report succinctly reviews the existing and contemporary advancements in chemotherapy and assesses the status of the enrolled drugs/pharmaceuticals; it also comprehensively discusses the emerging role of specific/targeted therapeutic strategies that are presently being employed to achieve better clinical success/survival rate in cancer patients.


Introduction
Cancer is a heterogeneous and multifactorial disease in which a series of genomic/molecular alterations cause the uncontrolled growth and proliferation of the cells, causing a rapid increase in tissue mass in the affected parts of the body. Under normal conditions, a cell gets signals to die and to supplant the organism with a young and healthier cell. Cancer cells grow using the body's oxygen and supplements, depriving other cells of regular supplements and growth factors. These cells can turn the microenvironment in their favor, deceive the immune system of the body, and can exploit the physiology of other cells to accommodate their needs. 1e6 Some of the biomarkers which are currently been used to detect cancer including human epididymis protein 4 (HE4), carcinoembryonic antigen (CEA), legumain, mesothelin, osteopontin, and vitamin Ebinding plasma protein. A plethora of anti-cancer drugs and natural medicinal compounds have been devised over the years, which can suppress tumor growth through diverse mechanisms. Some of the drugs/compounds work on crucial cellular enzymes, while others may alter cell metabolism. They have also shown the potential to interfere with some critical cellular processes, e.g., programmed cell death/ apoptosis, drug resistance, DNA damage, DNA replication, or immune reactions. These drugs have distinct modes of action and selectivity for multiple cancer types 7e12 ; however, subtle alteration to their chemical structure could abolish their anticancer potency. 13,14 The idea of chemotherapy (utilizing toxic compounds and drugs to destroy cancerous cells) came into existence after the reports of mustard gas killing lymphatic tissues and bone marrow. The effects were later validated in mice using an effective derivative of the gas (nitrogen mustard) that showed effective regression of lymphoma tissues. 15e17 The first patient receiving this nitrogen mustard was forty-eight years old lymphosarcoma patient whose cancer softened and cleared initially. Unfortunately, later he died after relapsing, but this secret military trial at Yale University paved the way for chemicals in treating cancers and developing the field of cancer chemotherapy for treating a variety of cancers. 18e21 Chemotherapy primarily works by circumventing the cancer cells from further growth and division. Cancer cells usually divide and grow at a much-accelerated rate than normal cells and are physiologically possess very high endogenous stress. Therefore, the drugs can destroy them rapidly and more effectively in comparison to other surrounding cells. Some of the promising inhibitor therapies which have been used to treat solid cancers are polyadenosine diphosphate-ribose polymerase inhibitors, angiogenesis inhibitors, histone deacetylase (HDAC) inhibitors, mechanistic target of rapamycin (mTOR) inhibitors, poly (adenosine di-phosphate-ribose) polymerase (PARP) inhibitors, p53/mouse double minute 2 homolog (MDM2) inhibitors, hedgehog pathway blockers, tyrosine kinase inhibitors and proteasome inhibitors. 22,23 Chemotherapy may have different variations, which can affect the target cells in distinct manners. 24 Some of the treatments may directly alter the quality of cellular proteins, rendering them non-functional and affecting major cellular physiological pathways. Major chaperone repressors, autophagy suppressors, or proteasome inhibitors belong to these classes of molecules. 25e31 Other drugs may target some essential hormones and interfere with the body's overall metabolism. 32,33 The 2018 Nobel Prize attributed to immune checkpoint therapy research has highlighted the importance of immunotherapy, which underlies the possibilities of modulating our immune system in our favor to cope up with the cancer-like conditions and fight back against the disease. 24,34 The selection of chemo-preventive drugs or a combination of drugs is mostly based on the type and stage of cancer. The primary objective of these drugs is to neutralize the cancerous cells and ameliorate the stress caused by the tumor's growth. The dosage and timespan of the treatment are two important points of consideration. It has been observed that the medications are given at very high doses causing numerous side effects and harm to other healthy cells. 35 One major challenge is the relapse of the disease. During reoccurrence, it has been widely observed that the cancer cells attain drug resistance following longer exposure to the drugs. Despite enormous applications, most chemotherapies are given to prolong and ease the patient's survival, hence called palliative chemotherapy. 21 However, prolonged exposure to these drugs may have excessively adverse effects on a patient's physical and mental health, making it difficult to continue the ongoing treatment. Oncologists, for the most part, provide these medications with possible time intervals in between, which also gives non-transformed cells a chance to mend. 36 In most cases, a team of specialists, including radiologists, nutritionists, psychologists, and the primary consultant collectively decide the treatment plans that include drug combinations, dosage, duration of cycles, and additional supplements.
This report mainly reviews various aspects of cancer chemotherapy, including various applications of the drugs/ chemotherapeutic agents that are already in practice. We also succinctly discussed various side effects and drawbacks of these treatment plans and further provided an overview of recent advancements in interventional strategies by focusing on targeted drug therapy, immunotherapy, personalized and integrative medicines. Figure 1 depicts the significant intrinsic and extrinsic cellular and molecular events causing the transformation of a normal cell to a cancer cell and its distinct hallmarks.
Cancer chemotherapeutics: what is known so far?
The idea of cancer as a disease was known to Hippocrates, the father of medicine, around 400 BC. He later coined the term 'carcinos' due to the similarity of cancer tissue growths with crabs. The term was later replaced with the Latin word cancer by Celsus. Many ancient pieces of literature have described several kinds of malignancies in humans, and multiple treatment approaches were also in use in different parts of the ancient world, as found in the literature of that era. 37,38 However, the progress of strategies to treat the disease was more or less static until the beginning of the twentieth century. 39 The extract of the plant Colchicum autumnale was used by Greek physicians to dissolve the tumor mass. Interestingly, in the 1930s, the active compound in the extract, colchicine, was reported that can interfere with microtubule assembly and may work as a potential drug. 40 Vincristine and vinblastine are other similar compounds that were medicinally recognized as possible anti-tumor agents a long time ago. 41 The ancient pieces of the literature suggest that the Greek and Roman physicians have contributed enormously to the identification and characterization of disease. Arabic, Indian, and Chinese medicinal archives have also paved the way for multiple medieval and modern approaches to understand and treat the disease. Interestingly, for centuries, most of the approaches remain limited to cauterization, bloodletting, surgery, herbal medicine, etc. 42 In modern onco-medicine, which was evolved during the Second World War, many active compounds were reported following the detailed characterization of the disease at the genetic and molecular level. The initial decades have seen enormous debate on the use of cytotoxic agents as drug candidates; however, advancements were made, and success came in the form of Eli Lily introducing Vinca alkaloids in the 1960s, which was then followed by procarbazine. 43 In the following years, the idea of combination therapy was introduced for acute lymphocytic leukemia and non-Hodgkin's lymphoma patients that were also supplemented with supportive medical care to enhance the quality of life. 21 In the second half of the twentieth century, a series of reports have identified numerous mutations, external stimuli (toxic chemicals, viruses, etc.), metabolic alterations, etc. as the underlying causes of cancer. 44,45 Identification of a plethora of cellular pathways that are affected in various cancer types has given different molecular alternatives to target and develop effective drugs (Fig. 2). 46e48 Several modes of medications have also been introduced and tried for drug administration based on the types and stages of the diagnosed cancer. Here, we discuss the available drug medications, following which we have revisited the chemotherapeutic agents, which have substantially pushed the field forward and extended the lives of millions in the past several decades.
Intravenous (IV) chemotherapy requires drug infusion straightforwardly into the patient's veins that may take from minutes to a few hours. Some medications can also be given either as pills or fluid regularly or at some intervals. 49 Intramuscular shots and intra-abdominal administration are other methods of giving chemotherapeutic medications to target the specific locations of tumor mass. 50 In addition, a new technology called pressurized intraperitoneal aerosol chemotherapy has been developed to efficiently deliver intraperitoneal chemotherapy for end-stage peritoneal metastases patients. 51 Topical treatment methods imply providing drugs through the skin surface. The anti-cancer compounds derived from either natural sources or their synthetic chemical analogs can kill cancer cells or stop their development. 52e54 Notably, various other approaches are emerging to treat the cancer cells, including small molecule inhibitors, 55 a combination of anti-cancer agents and immunotherapy, 56 nanotechnology-based drugs, 57e59 and organo-seleno compounds (Fig. 2). 31 Nevertheless, the selectivity of most drug particles is restricted, and such medications are considered one of the most harmful medications utilized in the treatment. 60,61 Since the initial findings of modern anticancer drug discoveries in the 1940s, continuous efforts are being made to treat this life-threatening disease. Advances in scientific methods and techniques have brought new treatment and diagnostic tools and advanced patient care measures (Fig. 2). 62e69 Treatment plans may have palliation as one strategy that may lead to shrinkage of the obvious tumor, easing side effects, and increasing the life quality of patients. Figure 2 describes various ways of providing anticancer medications to patients depending upon their type and stage of the disease.
Along with primary treatment, adjuvant therapy is also given to the patients for additionally combating the disease through supportive strategies. 70,71 Surgery and radiation can be given to patients to supplement a chemotherapeutic plan to curb the disease by destroying shrouded cancer cells that could not be targeted otherwise. 72 Sometimes adjuvant chemotherapy is given to patients after surgery or radiation or both to lessen the chances of recurrence. 73e75 Adjuvant chemotherapy starts typically within three to five weeks of the surgical removal of a tumor and has distinctive treatment lengths relying upon the disease. Although chemotherapy has significantly improved overall survival, patients experience a wide range of physical and psychological symptoms that impact their quality of life. Symptoms seldom occur in isolation. Chemotherapy-induced hair loss remains greatly feared, with a negative impact on the well-being of many cancer patients. Cancer-and chemotherapy-induced poor appetite is usually the result of taste changes, mouth sores, nausea and vomiting, increased satiety, medication side effects, pain, fatigue, depressed mood, and anxiety.

Basic principles of chemotherapy: years of research, miles to go
The cancer cell metabolic mechanisms functionally overlap with the host cells, so cancer treatment is very challenging. 76,77 Designing drugs or therapeutics mainly focuses on selectivity, which can specifically kill the cancerous cells without affecting the non-cancerous cells (Fig. 2). In a way, the anti-cancer therapy may resemble antimicrobials, but the cancerous cells and the bacterial cells have differences in terms of the metabolic and physiological characters. 78e80 For our immune cells, bacterial cell recognition is straightforward, while our transformed cells can hide cell surface receptors to evade the immune system. 81 Early-stage detection of cancers increases the chances of treatment and survival. Therefore, recognizing and diagnosing those transformed cells remains one of the most challenging tasks in oncological research. At early stages, most known biomarkers remain undetectable, and cancer cells' ability to hide from immune cells freezes our immune responses. 82,83 In a tumor subpopulation, transformed cells differ drastically from the other cells in terms of physiology, metabolism, proliferation rate, etc. 84e86 Besides that, the genetic diversity may pose additional challenges to target all kinds of cells in a particular tissue mass owing to the tumor heterogeneity. 87,88 A combination of drug therapy is applied to address this challenge, and the regime is devised according to the cancer phase and type (Fig. 2). 89,90 In the subsequent section, we provide a comprehensive overview of various chemotherapeutic agents which have served the purpose for several decades in different cancer types. These drugs could be classified in different ways based on their chemical nature/source, molecular target, mechanism/mode of action, or effectiveness in model systems against various malignancies/cancer types (Table 1).

Alkylating agents
One major class of the current drugs belongs to alkylating agents, which can interfere with the formation/linkage of DNA double strands. 91 These drugs can cause it by transferring one alkyl group to the guanidine base in DNA (Fig. 2). Cross-linking of nucleic acids with proteins or peptides can likewise affect the DNA geometry, cause erroneous base pairing and DNA strand breakage, and eventually preventing the cell division and causing irreversible senescence. 92 Mechanistically, alkylating agents in their electrophilic forms interact with cellular DNA and form covalent adducts, underlying their broader utility. Therefore, alkylating drugs are the frontline chemotherapeutic agents given against most of the cancer types, yet the greatest therapeutic value of their inhibition is known for slow-growing cancers. In Table 1, we described the common chemotherapeutic alkylating agents including the altretamine, chlorambucil, platinum drugs (cisplatin, carboplatin, oxaliplatin), doxorubicin, epirubicin, etoposide, mitoxantrone, cyclophosphamide, thiotepa, and busulfan that are in clinical practice and also provided details of their molecular targets, mode of action, and studied model systems in various malignancies/cancertypes.

Antimetabolites
Antimetabolites are the next class of substances that may compete, replace, or inhibit some specific metabolites inside the cells and thus meddle with the cellular metabolism (Fig. 2). 93 These molecules mostly possess a structure similar to a cellular metabolite or enzymeesubstrate, usually identified and processed by an enzyme to meet the cellular needs. 94 Tetrahydrofolates are formed from unreduced dietary folates by the enzyme dihydrofolate reductase. For example, various medications, such as aminopterin, methotrexate (amethopterin), pyrimethamine, trimethoprim, and triamterene, act directly on the production of these metabolites, hindering the enzyme from producing folate insufficiency. 95 Methotrexate, an antifolate drug, is one of the exceptionally successful anticancer medications that represses dihydrofolate reductase (DHFR) and obstructs the transformation of dihydrofolic acid (DHFA) into tetrahydrofolic acid (THFA). 96 This coenzyme activity is indispensable to the cells as they are essential for the nucleotide synthesis pathways. Methotrexate is an anti-inflammatory molecule and is hence used as a medication for many other inflammatory diseases, like rheumatoid arthritis and extreme psoriasis, apart from various cancer types. A detailed description of common antimetabolites is comprehensively provided in Table 1.

Pyrimidine-derived antimetabolites (pyrimidine antagonists) and Vinca alkaloids
Pyrimidine-derived antimetabolites are a group of chemotherapeutic agents that interrupt DNA synthesis and exert their cytotoxicity to cancer cells (Fig. 2). For instance, cytarabine also known as cytosine arabinoside or Ara C interrupts DNA as well as RNA synthesis by displacing the cytosine with itself (that differs merely by its base sugar arabinose). Cytarabine/cytosine arabinoside is enrolled frequently in clinics to treat leukemia or non-Hodgkin's lymphoma. 97 Some other notably pyrimidine-derived antimetabolites include fluorouracil, 5-fluorouracil (5-FU), capecitabine, floxuridine, gemcitabine, decitabine, raltitrexed, and tegafur (Table 1). 98e103 Being as pyrimidine or purine analogs having altered chemical groups, these drugs/compounds induce apoptosis while progressing through the S phase of the cell cycle by their misincorporation to RNA and DNA or by inhibiting the key enzymes required for nucleic acid synthesis. Among these enzymes, DNA polymerases, ribonucleotide reductase, and thymidylate synthetase are the main intracellular targets of the pyrimidine antimetabolites. 5-FU has been a promising pyrimidine analog that largely interrupts DNA and RNA synthesis. 5-FU that dependents on its incorporation to DNA inhibits mitosis and stimulates cell death in the dividing cells.
Vinca alkaloids, primarily extracted from several species under the Vinca genus, are mainly cell division inhibitors, which can interact with microtubular protein tubulin to obstruct its polymerization and meddle with the cytoskeleton, leading to stalling of mitotic division. 104 The chromosomes do not separate efficiently during cell division, leading to metaphase capture. Vincristine and vinblastine are primary examples of this drug class with different degrees and ranges of anti-tumor activity (Table 1). 105,106 Cancers treated with these drugs include acute leukemia, Hodgkin's and non-Hodgkin's lymphoma, rhabdomyosarcoma, Ewing's sarcoma, neuroblastoma, Wilms' tumor, miscellaneous myeloma, chronic leukemia, thyroid cancer, brain tumors, and several blood-related disorders. 107

Insights into the cancer chemotherapy and beyond: A review
Other antineoplastic drugs/agents Besides these three established classes, several other antineoplastic agents are also enrolled in the clinical practice of cancer chemotherapy (Table 1). Given their clinical promises, in the subsequent section, we described the biological activities, targets, and mechanisms of key chemotherapeutic drugs that may belong to one or multiple drug classes and can affect more than one cellular pathway in the cancer cells and induce apoptosis by different mechanisms (Fig. 2). A succinct review of well-known anticancer drugs that are in routine clinical practice is provided as follows.

Paclitaxel/Taxol
It is a complex diterpene taxane ring-containing drug acquired from the bark of the Western yew tree (Taxus brevifolia), which induces cytotoxic activity by a novel system. 108 It increases the polymerization of tubulin, thereby inducing a mechanism inverse to that of the aforementioned alkaloids. 109 It forestalls the microtubule's structural adjustment and depolymerization, bringing an inherent cytotoxic activity to the paclitaxel due to the significance of tubulin-microtubule dynamics. 110 The confirmed antagonistic impacts of paclitaxel are reported on the metastatic ovarian and bosom carcinomas after the disappointment from first-line chemotherapy and backslide cases. 111 It has also shown efficacy against the advanced stage tumors of the head and neck disease, small cell lung cancer, oesophageal adenocarcinoma, and prostate cancer (Table 1). 112,113 Docetaxel is a more potent relative of paclitaxel with a comparable activity. It has been shown to provide benefits against the bosom and ovarian cancers, which get resistant to first-line chemotherapeutic drugs. 114 Unfortunately, many side effects have been reported from the long exposure to these drugs, which mainly include neutropenia, arrhythmia, neuropathy, and cardiovascular breakdown (Table 2). 115,116 Etoposide Etoposide is a semi-synthetic derivative of a plant glycoside called podophyllotoxin. 117,118 It causes DNA damage by altering DNA topoisomerase-II function, and posing G2 cell cycle arrest. 119 Etoposide creates a ternary complex with DNA and the topoisomeraseeII complex, preventing the latter's binding to the DNA strands, while in doing so, it causes damage to DNA strands. 120 Cancer cells depend on this enzyme more than healthy cells, as they divide more quickly; therefore, it causes defects in DNA synthesis that promotes apoptosis of the cancer cells. 121 Etoposide is effective against several aggressive malignancies including Kaposi's sarcoma, Ewing's sarcoma, testicular disease, lung disease, lymphoma, nonlymphocytic leukemia, and glioblastoma multiforme (

Camptothecin and its derivatives
Camptothecin is a long-known topoisomerase inhibitory compound of natural origin. It was initially extracted from the bark of the tree Camptotheca acuminata. 123,124 Camptothecin is a cytotoxic alkaloid comprised of a pentacyclic ring structure containing a pyrrole (3,4-b) quinoline moiety, an S-configured alpha-hydroxy lactone form, and a carboxylate form. 125 In later years, many more effective derivatives of this topoisomerase inhibitor have been synthesized and are effectively being applied in cancer chemotherapy. 126 The most recognized drug of this class, topotecan, is used against the advanced cancers of the ovary and lung. It targets topoisomerase-I, an enzyme responsible for easing the torsional strain in DNA by inducing single-strand breaks during replication. 120 The inhibition of topoisomerase-I by the active compounds may lead to sudden inhibition of DNA replication and transcription, causing growth arrest (Table 1). 127 In the physiological conditions, topotecan remains in equilibrium with its carboxylate structure. 128 Its active lactone can intercalate between DNA bases in the topoisomerase-I cleavage complex. 129 The coupling site of topotecan in the cleavage complex interferes with the topoisomerase-I while reiterating the scratched DNA strand in the wake of releasing the strain. Such intercalation traps the topoisomerase-I in the cleavage complex bound to the DNA. 130 Another wellknown drug of this class is irinotecan that has shown efficacy against late-stage colorectal carcinoma, aggressive growths in cancers of the lung, cervix, ovary, colon, etc. 131,132 Actinomycin D (dactinomycin) A transcription inhibitor chromopeptide molecule actinomycin D (also called dactinomycin) binds to DNA while initiating transcription at initiation complex and inhibits RNA elongation steps. 133 It can intercalate, preferably between guanidine and cytosine, or bind the DNA strand's minor grooves to form a highly stable drug-DNA complex, preventing DNA unwinding and thus stalling the progress of the RNA chain. 134 Dactinomycin was initially obtained from Streptomyces sp., identified as an antibiotic agent, but later its anti-tumor potential was recognized, and thus it was inducted in the chemotherapy as an anticancer antibiotic to treat Ewing's sarcoma, Wilms' tumor, rhabdomyosarcoma, trophoblastic neoplasm, testicular and ovarian cancers, etc (Table 1). 135

Mitomycin C
Mitomycins, a class of natural quinones, are also derived from Streptomyces caespitosus and are known for their transformation and competence, enhancing potential in bacterial cells. It is also a known antineoplastic agent as it slows down fibroblast cell growth in glaucoma. 136,137 These bioreductive alkylating agents (especially mitomycin C derivatives) are also utilized as anti-cancer therapeutics because their structural modification impedes the cell cycle during G1 and S phase transitions. Its reduction to form mitosene can lead to the N-alkylation of DNA bases. 138 It can lead to inhibition of replication by cross-linking at adenine N6 and guanine O6 and N2. 139 It is effectively used in the palliative treatment of non-small cell lung carcinoma and other solid tumors of breast, colorectal, oesophageal, pancreatic, and cervical origins. Porfiromycin is an N-methyl derivative of mitomycin C that is found effective against cancer of head and neck regions (Table 1)

Daunorubicin (rubidomycin)
Daunorubicin causes a break in DNA strands by actuating topoisomerase-II and producing quinone-type free radicals. Daunorubicin, otherwise called daunomycin, is a chemotherapy medicine used to treat cancer growth in acute myeloid leukemia (AML), acute lymphocytic leukemia (ALL), chronic myelogenous leukemia (CML), and Kaposi's sarcoma, etc. 141,142 It is applied by infusion into a vein, and following its intercalation with DNA, it hinders macromolecular biosynthesis. This predominantly obstructs the movement of the enzyme topoisomerase-II, which loosens up the DNA supercoiling. 143 Daunorubicin impedes the topoisomeraseeII complex after it unties the DNA chain for replication and thereby it represses the DNA double helix from resealing and therefore interferes with the replication to proceed (Table 1). 144,145 Bleomycin Another Streptomyces compound with anti-tumor activity bleomycin is mostly used against Hodgkin's lymphoma, ovarian and testicular cancers. It chelates copper or iron, produces superoxide radicals, and intercalates between DNA strands e causes chain scission. 146 Emerging evidence proposes that bleomycin also hinders the consolidation of thymidine into DNA strands, and the cleavage of DNA relies upon oxygen and metal particles in vitro. 147 The specific mechanism of DNA strand scission is uncertain, yet it has been proposed that bleomycin chelates metal particles (principally iron), delivering a pseudoenzyme that responds with oxygen to create superoxide and hydroxide free radicals that divide DNA; however, little myelosuppression is the unique highlight. 148 The drug has also been effective against squamous cell carcinoma of the skin, and cancers of the mouth, head, neck, genitourinary tract, and throat (Table 1).
Lateral damage: Putting multiple organs at risk Aggressive tumors are primarily marked with uncontrolled and extensive cell division that may compromise the normal physiology of other non-transformed cells in the vicinity of affected organs/tissues. In many ways, cancer cells achieve super-specialized metabolic and survival potential with the capacity to tolerate extreme stresses like low oxygen and nutrient supplies. Cancers might start from mutation of one type or the other, but as the disease progresses, 149,150 multiple mutations and genetic abnormalities accumulate that lead to increased tumor heterogeneity. 87 The heterogeneous nature of tumor mass plays a vital role in developing resistance against most of the drugs that we have described here. This leads to a challenge of its kind, leaving doctors with the only option of changing the medication after certain intervals as one type of drug stops acting against tumors. In many cases, the combination therapy that includes more than one drug at a time could be an effective strategy against some form/type of cancers, such as cancers affecting bone marrow (leukemia) and lymph nodes (lymphoma). Clinical trials help a lot in understanding the effects of new combinations and thus have rapidly improved the cancer chemotherapy paradigm. The sustainability of chemotherapy relies upon its continuous effect on cell division and tumor growth. As discussed above, the majority of the treatments work by compromising the stability or integrity of nucleic acids that tend to stall the cell cycle or activate programmed cell death pathways. Other drugs may act by generating extreme proteotoxic stress inside the cells by interfering with the protein folding or degradation machinery, including chaperone, proteasome, and autophagy. 151e154 The idea is to generate cellular stresses so that the cancer cells that are already under compromised physiology may die while other cells can survive. However, it is evident from the above descriptions that most of these drugs act by interfering with one or multiple cellular pathways of extreme importance. 155 Therefore, chemotherapeutic drugs are mostly referred to as cytotoxic agents, causing a plethora of side effects. In most cases, long-term treatments may lead to enormous toxicity causing severe damage or failure of multiple organs. This may lead to enormous physiological and psychological stress to the patients under chemotherapeutic medication. Table 2 summarizes the adverse effects of various chemotherapeutic agents and the current stage of their clinical progress. The side effects of these drugs may depend on many factors and variables, such as type, stage, and location of the tumor, along with age, health, and other clinical aspects of the patients. The most common and readily visible side effects originate from the gut that may lead to nausea, vomiting, constipation, and diarrhea. 156,157 Most of the anticancer drugs can directly interrupt the cell division pathways; therefore, the first line of affected organs may also include those which are formed with continuously dividing cells, such as skin, hair, and intestinal linings. 158,148 Skin rashes, alopecia, and mucositis are the most visible side effects that one can observe in patients undergoing chemotherapy. 157,159 Radiation also contributes significantly to these debilitating changes. A general problem observed in many patients is fatigue that can occur due to multiple factors, including depression, anemia, high doses of radiation therapy, etc. 160 Chemotherapeutic drugs may also directly injure the bone marrow and cause a steep decline in blood cell count, exposing patients to several other types of possible infections. 161 Thrombocytopenia, a condition of excessive bleeding because of lower platelet numbers, is observed more frequently in patients taking alkylating and/or antimetabolites agents. 162 Alkylating agents may further trigger conditions, such as hemorrhagic cystitis leading to microscopic haematuria, nocturia, dysuria, etc. along with mild or intense suprapubic pain. 163 Whereas, antimetabolites can generate enormous toxicity in vital organs, like the liver and kidneys. 164,165 The most devastating of all is their direct impact on cardiovascular health. Different classes of chemotherapeutic drugs have varying impacts on cardiac cells, leading to multiple types of systemic conditions adversely affecting patients' survival. 166 Loss of libido and sexual dysfunctions are other conditions caused due to long-term exposure to these drugs, which affects not only physical well-being but also the patient's psychological health (Table 2). 167,168 These drugs and radiation therapies may have several direct or indirect impacts on cognitive functioning leading to a condition, which is now commonly referred to as chemo brain. 169,170 This is a condition of the inability of someone undergoing chemo-drug therapies to do their routine cognitive functions, which may be related to their memory, cognition, and behavior. 171 The drugs may adversely impact communication, reasoning, learning, problem-solving, memory storage, concentration, following commands, etc. Damage of optic and the ocular motor nerves is also observed in several patients taking antineoplastic drugs that may lead to the weakening of vision as well. 172,173 The majority of chemotherapeutic agents affect these tissues in a dose-dependent manner; though, there are differences in their sensitivity to individuals due to enormous factors such as metabolism efficiency, genetic makeup, other medication, and environmental factors (Table 2). 174 Despite enormous toxicities and side effects prevalent in the public domain, the chemotherapy medicines remained the most immediate and viable therapeutic option in most cases, where we can see the cancer cells but cannot remove them through surgery or source of radiation. To address various above-described detrimental physical and psychological changes, modern-day clinical practices involve a comprehensive team of doctors and specialists for the patients who are undergoing long-term treatments and are more prone to develop severe physiological complications.

Recent advancements and next-generation therapeutic approaches
Since the early 19th century, rigorous investigations and clinical studies were carried out around the world. Several research groups have made landmark discoveries during and after the Second World War. Following the development of a fundamental description of the disease and associated pathways, the focus shifted majorly towards advancing the diagnosis and treatment methods of the disease. Despite enormous progress being made in the second half of the last century, not much could have been achieved in successfully curing most of the cancers. One primary reason behind the failures is our inability to diagnose the oncogenic changes inside the body at the early stages of cancer development. In recent years, the concept of liquid biopsies has emerged that primarily relies on the screening of mutated DNAs, proteins, RNAs, and other genetic markers in the patient's blood samples. 175e177 This practice can help diagnose the disease at a much earlier stage than we do now, using the traditional tissue biopsies and imaging methods. 178 New technological tools, like advanced software and robotic platforms, have also speeded cancer diagnosis and treatment. In this subsequent section taking chemotherapy into account, we compressively reviewed the recent progress made in the targeted drug delivery, personalized medication, and immunotherapy (Fig. 3).

Targeted drug delivery
The major drawback of most chemotherapeutic treatment plans is their non-specificity or inability to ascertain and target cancerous cells directly. Therefore, this lack of specificity poses a major therapeutic limit reflected in the failure of delivering drugs locally to the cancer tissue mass.
In the past few decades, many new approaches have been constituted and tested, and have given hopes for future drug delivery (Fig. 3A). Nanoparticle-based delivery methods, targeted antibodies, aptamer functionalization, and specific drugs like Herceptin (in breast cancer) offered the promise that can harness great success in the upcoming years. 179e181 Cancer cell-directed antibodies are at the forefront of targeted drug delivery methods and have accelerated the hunt for more effective or potentially improvised approaches. 182,183 Different types of nanocarriers and formulations of nano-drugs have also improved the delivery of drug compounds to the cancer cells. 184e186 The most significant advantage of targeted drug deliveries is that the enormous drug-mediated toxicities on other adjacent tissues and organs can be minimized. We can also avoid most of the collateral damage leading to stress and organ failures. However, the effectiveness of many of these delivery methods in patients is still under clinical evaluation. Further details regarding these delivery systems can be accessed in the previously published reports. 187,188 Personalized medication Given the heterogeneity of tumor cells, the cancer of one patient is different from that of another in terms of accumulated genetic abnormalities, its pace and progression, the response to the treatment, and the chances of developing resistance to the administered drugs. 45 The current technologies have made it possible to get insights into this disease heterogeneity or explore molecular variations at the individual level, thereby tailoring the treatment regime as per the personalized needs of every cancer patient. 149,189 This helps in choosing the most appropriate drugs and their dosages for a better outcome in cancer management. 190 For instance, dabrafenib and vemurafenib are directed against BRAF gene mutation, while trastuzumab works well against HER-2 mutated cells. 47,191,192 The idea here is to dive deep into the omics (genomic, transcriptomic, proteomic, metabolomic, etc.) data of the patient to identify the major molecular players fostering the transformed cells, and design the treatments against these clinically actionable targets (Fig. 3B). 155,193,194 Different types of mutations have been observed to be frequent in specific subtypes of cancer. Therefore, measuring and manipulating these driver mutations based on an individual patient is expected to yield better results. Following are some examples of drugs that are potentially effective in patients harbouring some specific type of genomic alteration. A mutation in anaplastic lymphoma kinase (ALK) was reported to drive tumor formation in 5% of the non-smallcell lung cancer population and led to the discovery of ALK blockers such as crizotinib and ceritinib. 195 An oral BRAF inhibitor, namely vemurafenib has been shown to possess a greater affinity for the mutant BRAF V600E than the wildtype BRAF, and therefore was recommended to the patients carrying the above mutation in the tumors. 196 Several potential targets for novel drugs have been identified using high-throughput screening technologies, and various compounds against them have been recently approved or are under investigation. Similarly, alterations in phosphoinositide 3-kinases (PI3K)/protein kinase B (AKT)/mTOR pathway have also been frequently found in many solid tumors. Among these, activation of phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) and loss of phosphatase and tensin homolog (PTEN) mutations are the most common. Basket trials have been conducted to evaluate the effectiveness of adding alpelisib, a specific p110a PIK3CA inhibitor to the hormonal treatment for PIK3CA mutant luminal breast cancer patients. The results emphasize the importance of checking for this molecular alteration in patients and following the tested treatment regime for better anti-tumor response. 193 Next, aberrant changes in the fibroblast growth factors (FGF) signaling pathway have also been associated with the initiation and progression of oncogenic events. Different types of genetic alterations in FGF receptor amplification, mutations, and gene-fusion have been shown to respond variably to certain drugs, and hence demand molecular profiling for the selection of effective drugs. 46,197 Another promising example is of using poly ADP ribose polymerase (PARP) inhibitor olaparib for the treatment of breast cancer susceptibility gene (BRCA)-mutant cancers for better targeting of cancer cells, without much effect on the normal cells. 198,199 On the cytoprotective side, recently a natural alkylated withanolide viz. 2,3-Dihydro-3beta-methoxy withaferin-A was shown to protect normal cells against a variety of stresses enrolled in cancer therapeutics. 200 The idea of precision or personalized medicine is new but has been beneficial for lots of patients; however, the cost of these new generation approaches for genome profiling is still high, making it unaffordable for a large population of the world. 201

Antimicrobial peptides
Advances in cancer therapy have prompted scientists to look for natural tumoricidal chemicals, such as antimicrobial peptides (AMPs), that could be used for cancer treatment. 202e204 In addition to antibacterial action, several AMPs have been shown to have intrinsic anticancer potential. 205,206 These molecules constitute an intriguing new resource for anticancer drugs, and they provide several distinct therapeutic advantages over other anticancer treatments since they do not bind directly to any particular receptor in cancer cells. 207 The polycationic and amphipathic character of AMPs has been postulated as a possible explanation for the preferential interaction of AMPs with cancer cells, as compared to normal cells. 208,209 Membrane disruption has been identified as a critical stage in the microbial killing mechanism of several AMPs. These anticancer effects of AMPs are unaffected by molecular heterogeneity inside a tumor or across tumors, nor are they counteracted by cancer cells' flexibility and alternate survival pathways, as is commonly found in cancer resistance (Fig. 4). 204,209,210 Cathelicidins are members of the antimicrobial peptide family found in granules of vertebrate neutrophils. 211 The only human cathelicidin is the cationic peptide LL-37. Cathelicidins are composed of a highly conserved N-terminal signal peptide including a cathelin domain and a Cterminal cationic antimicrobial peptide. Apart from their antimicrobial activity, cathelicidin-derived antimicrobial peptides exhibit a range of biological roles, including antiviral, antifungal, anticancer, and immunomodulatory properties. 212,213 LL-37 exhibits anticancer activity against oral cancer cells and has been identified as a molecular target for colon cancer. 214 Immunohistochemical staining of colon cancer tissue showed that LL-37 is abundantly expressed in normal colon mucosa but downregulated in colon cancer tissues due to DNA methylation in the LL-37 promoter. 215 In human oral squamous cell carcinoma SAS-H1 cells, the active domain of LL-37 induces mitochondrial depolarization, which results in caspase-independent apoptosis. Furthermore, mice lacking cathelicidin are more susceptible to azoxymethane-induced colon carcinogenesis. 216 Many important advancements have been achieved using antimicrobial peptides against colorectal cancer. 217 Using LL-37 on the surface of magnetic nanoparticles led to a significant reduction in cell viability of colon cancer cell lines, inducing apoptosis at a much higher rate than in the case of LL-37 peptide alone. 218 Fan et al created a biodegradable and injectable thermosensitive hydrogel containing docetaxel and LL-37 polymeric nanoparticles. Intraperitoneal injection of this hydrogel substantially inhibited the development of HCT116 carcinomatosis in a mouse model of colorectal carcinoma. 219 In comparison to existing chemotherapeutic agents, the flexible nature of peptide structures, along with nonreceptor activity, might confer strategic benefits on AMPs for the treatment of colon cancer in the future. 220 New treatment approaches for patients with advanced-stage, unresectable melanomas continue to be of significant clinical interest, as these patients cannot be treated surgically and have only a modest or temporary response to chemotherapy and radiation. In phase 1 clinical trial NCT02225366, intra-tumoral injections of LL-37 are being explored for melanoma patients with cutaneous metastases to determine the most effective dose of LL-37 that could be administered. Patients will receive weekly intra-tumoral injections of LL-37 (starting dose, 250 mg/tumor) every 7 days for up to 8 weeks. Researchers are also interested in understanding whether LL37 can activate the immune system, which could help in the control of the disease. 221 Although AMPs have the potential to develop into a more effective and safer alternative to conventional cancer therapy, future research and development of anticancer peptide therapeutics will most likely focus on the poor pharmacokinetics, the control of selectivity, toxicity, and routes of administration through the establishment of appropriate in vivo models. 222 Immunotherapy Supplementing our immune system by devising immunotherapy strategies is another highly-specific and largely promising approach to address the problem of late-stage cancer therapeutics. 223,224 Several new approaches have been developed in the last decade to modify and train a patient's immune system to search, identify and target the transformed cells inside the body by recognizing multiple cell surface markers. 225 For instance, supplementing the treatment regime with immunotherapy has significantly improved the success rate of the chemotherapeutic drugs in many cancer types, especially the triple-negative breast cancers. 226e228 The immunotherapy drugs are given alone or in combination with chemotherapy drugs to achieve better clinical outcomes. 229 Prophylactic and therapeutic vaccines have been developed to help patients' immune systems towards learning to tackle the transformed cells. 230 Atezolizumab is an example of an anti-cancer immune drug that helps to dissolve hard-to-treat breast cancers. 231e233 Tisagenlecleucel, an FDA-approved drug, treats B-cell lymphoblastic leukemia by effectively engineering the chimeric T cell receptor (CAR-T) of the patient (Fig. 3C). 234 Some treatment plans also include interferons and interleukins to jump-start the fight against cancer cells by boosting the patient's immune system. Moreover, the field of immunotherapy is gradually evolving and is beyond the scope of our present article.

Combination therapy
Combination therapy may offer the advantages of the fast tumor regression of targeted therapies with the long-lasting responses induced by immunotherapy. 235 Immunotherapy may be able to consolidate the dramatic tumor responses achieved with targeted therapy into durable, long-lasting remissions, in which sustained host responses targeting multiple antigens may reduce the risk of developing potentially lethal drug-resistant clones. 236 Combining cancer vaccines with immune checkpoint inhibitors (ICIs) such as those targeting cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1/programmed cell death ligand 1 (PD-1/PD-L1) is one approach to avoid T cell anergy. 237,238 Moreover, T-cell and NK-cell invasion may be facilitated by targeted drugs that modify the tumor endothelium. 239 Gemcitabine, for example, is a deoxycytidine analog that induces tumor cell death, which results in antigen cross-presentation and cross-priming. 240,241 Carboplatin, pemetrexed, and pembrolizumab are now commonly used in the treatment of non-small cell lung cancer as first-line agents. 242 For triple-negative breast cancer, nab-paclitaxel and atezolizumab (anti-PD-L1) were recently authorized based on an improvement in progression-free survival (PFS) over chemotherapy. 243 Adenosine receptor antagonists (ARs) are involved in regulating tumor progression. 244 The four ARs discovered so far, A 1 , A 2A , A 2B and A 3 , belong to the class A family of G protein-coupled receptors (GPCRs). 245 The A 2B R seems more promising as a target in chemotherapeutic interventions, 246 whereas the A 2A R is the main target in the immunotherapeutic approach. 247,248 The existence of dual compounds, i.e., molecules acting on the A 2A R and on the A 2B R, has prompted their use in both immunotherapeutic and chemotherapeutic interventions. 249,250 Epigenetic modulators may have a dual function in the tumor microenvironment, boosting both antigen expression and T cell effector activity. 251,252 ICI-induced reactivation of exhausted T cells is linked with considerable chromatin remodeling, indicating that epigenetic modification may relieve certain patients with exhausted T cell function. 253,254 While many strategies focus on modifying the native tumor microenvironment, an alternative strategy involves the direct infusion of engineered immune cells or cellular receptors into the patient via adoptive cellular therapy with the goal of increasing the number of effector cells that recognize tumor-expressed antigens. 255 Upon reinfusion, lymphocytes grown ex vivo may respond more potently to tumor antigen since they have not been continuously exposed to tolerogenic microenvironmental signals. 256 Tumor infiltrating lymphocytes (TIL) treatment, 257 chimeric antigen receptor (CAR) engineered T-cells, 258,259 and TCRengineered cell therapy 260,261 are the three main forms of adoptive T cell therapy currently being investigated. 257,262,263 Injecting oncolytic viruses directly into the tumor microenvironment is an alternate technique for improving tumor antigen recognition and strengthening T cell responses. 264 Talimogene laherparepvec (T-VEC) is a modified herpes simplex virus that is injected intra-lesionally into the melanoma tumor in individuals who have failed to get their tumor removed. It induces immediate lysis of tumor cells and the production of granulocyte-macrophage colony-stimulating factor (GM-CSF), which serves as a cytokine to stimulate the recruitment, maturation, and activity of antigen presenting cells (APCs). 265 Several studies combining immunotherapy with molecularly targeted treatment, either concurrently or sequentially, have shown hepatotoxicity as a major concern. 266 Combinations of dabrafenib, trametinib, and anti-PD-1 treatment have been associated with a greater risk of grade 3/4 adverse events in melanoma than would be expected with targeted therapy alone. 256 Emerging drugs are beginning to take a more comprehensive perspective of the tumor microenvironment (TME) and are revealing complementary techniques to simultaneously restore and enhance immune activity and promote therapeutic synergy. 267,268 Future research of combined therapy should continue to concentrate on exploring the intricate interplay between targeted drugs and immunotherapy, as well as optimizing parameters such as administration time, dose, and sequencing that may enhance the therapeutic index. 240

Conclusions and future perspectives
Chemotherapy is a largely investigated and clinically assessed approach in cancer therapeutics. In this report, we summarized the present status of chemotherapy and several drugs/agents presently enrolled in the clinical practice to provide an overview of how the field has evolved over the years and has traced the possibilities and druggability of diverse cancers in the clinic. New trends are evolving in the drug discovery paradigm that includes designing new derivatives of old drugs with high efficacy and lower toxicities. 269,270 Several natural compounds isolated from microbial, plants, or natural sources were suggested to hold enormous potential to target stress response pathways by mounting a hyperactive response instigating cancer cell death. 271e274 Natural compounds/agents as chemotherapeutic drugs demonstrated several advantages over synthetic ones, owing to their less cytotoxicity, cost, and straightforward production/extraction process. 273,275,276 Therefore, recognition and attenuation of treatment toxicity in cancer chemotherapy will be a key factor in determining the success of its regimes in the future.
Revised clinical strategies further underlined that combination therapy could emerge as a promising therapeutic approach to treat diverse cancers in the future. Combination strategies comprise an obvious overlap of the chemotherapeutic regimes with the targeted drug delivery, personalized medicine, and immunotherapy. It may not only improve the efficacy of enrolled treatment but can also elicit the responsiveness of certain tumors towards their effective therapeutic targeting in the clinic. Among these, personalized combination therapies that target individual tumor types based on their molecular signatures may offer a great promise. With acquiring more personalized insights in the chemotherapeutic treatments, a greater emphasis will be on individual regimes that could meet a high clinical success. 277 Therefore, chemotherapy beyond its conventional clinical efficacy offers to improve the clinical outcome in combination therapies. Translation of these combined approaches in the clinics is yet to see a complete success and thus warrants more clinical investigations.
Conclusively, clinical practice of cancer chemotherapy has achieved considerable success, however, it still holds scope to improve, particularly in terms of efficacy and safety of enrolled chemotherapeutic regimes. Therefore, it further requires concerted efforts and careful investigations assessing the efficacy of combined therapeutic regimes. In summary, more investigative focus on drug design in individual or combined regimes and rigor on their clinical trials involving careful patient selection and stratification are warranted to achieve the greater therapeutic success of chemotherapeutic regimens in the clinic.