Regenerative therapies for lumbar degenerative disc diseases: a literature review

This review aimed to summarize the recent advances and challenges in the field of regenerative therapies for lumbar disc degeneration. The current first-line treatment options for symptomatic lumbar disc degeneration cannot modify the disease process or restore the normal structure, composition, and biomechanical function of the degenerated discs. Cell-based therapies tailored to facilitate intervertebral disc (IVD) regeneration have been developed to restore the IVD extracellular matrix or mitigate inflammatory conditions. Human clinical trials on Mesenchymal Stem Cells (MSCs) have reported promising outcomes exhibited by MSCs in reducing pain and improving function. Nucleus pulposus (NP) cells possess unique regenerative capacities. Biomaterials aimed at NP replacement in IVD regeneration, comprising synthetic and biological materials, aim to restore disc height and segmental stability without compromising the annulus fibrosus. Similarly, composite IVD replacements that combine various biomaterial strategies to mimic the native disc structure, including organized annulus fibrosus and NP components, have shown promise. Furthermore, preclinical studies on regenerative medicine therapies that utilize cells, biomaterials, growth factors, platelet-rich plasma (PRP), and biological agents have demonstrated their promise in repairing degenerated lumbar discs. However, these therapies are associated with significant limitations and challenges that hinder their clinical translation. Thus, further studies must be conducted to address these challenges.

Tissue engineering aims to repair and regenerate discs using cells, biomaterials, growth factors, and platelet-rich plasma (PRP).This literature review summarized the recent advances and challenges in the field of regenerative therapies for lumbar disc degeneration.

Disc structure and degeneration
IVDs comprise a central gelatinous nucleus pulposus surrounded by the annulus fibrosus and cartilaginous endplates.The proteoglycan-rich nucleus is capable of absorbing water and resisting compressive load.The collagen fibers constituting the highly organized lamellar annulus fibrosus provide tensile strength (Sakai and Andersson, 2015).The proteoglycan and water content of the discs reduces with aging and injury.In addition, further changes, such as disorganized matrix; loss of collagen organization; ingrowth of nerves and vessels; decreased cell viability; and increased inflammatory cytokine (IL-1β, IL-6, and TNF-α), C-reactive protein levels, and type II collagen levels, have also been reported (Khan et al., 2017;de Queiroz et al., 2016;Stürmer et al., 2005;Wang et al., 2010;Goode et al., 2012) (Figure 1).These molecular changes result in structural breakdown, reduction in disc height, annular fissuring, radial bulging, altered biomechanics, nerve compression, instability, herniation, and lower back pain.

Current treatments
The first-line treatment options for symptomatic LDDD include the administration of medications, such as non-steroidal antiinflammatory drugs, muscle relaxants, and opioids, to alleviate pain (Chaparro et al., 2014;Deyo and Mirza, 2016).Physical therapy aims to strengthen core muscles, thereby improving ergonomics (Hayden et al., 2021).Short-term pain relief can be achieved with epidural steroid injections and intradiscal electrothermal therapy (Deyo and Mirza, 2016).Spinal fusion procedures using cages and screws have been performed to stabilize the affected segments in severe cases; however, this can result in increased stress on adjacent segments (Otsuki et al., 2023;Shimizu et al., 2021).Discectomy is performed to remove herniated nucleus material compressing the nerves (Özer and Demirtaş, 2023;Goparaju et al., 2023).Disc arthroplasty involves the replacement of degenerated discs with artificial implants (van den Eerenbeemt et al., 2010;Siepe et al., 2014;Gornet et al., 2017;Park et al., 2018).Nevertheless, the inability of these approaches to modify the disease process or restore the normal structure, composition, and biomechanical function of the degenerated discs underscores the requirement for developing biological therapies that facilitate disc regeneration.

Cell sources
Cell therapy is an innovative treatment approach that involves infusing living cells into a patient to repair or replace damaged tissue or modify the behavior of local cells (Sakai and Andersson, 2015).Cell-based therapies tailored to facilitate IVD regeneration in patients with IVD degeneration have been developed to restore the IVD extracellular matrix (ECM) or mitigate the inflammatory conditions that are characteristic of disc deterioration (Fujii et al., 2019;Sakai et al., 2022).Measures should be taken to ensure that the transplanted cells thrive within the hostile environment of a degenerating IVD by directly contributing to ECM synthesis and inducing a reparative shift in native cell activity via paracrine signaling or by facilitating the arrival of regenerative cells and preventing the infiltration of harmful cells.This would help achieve successful treatment outcomes.Animal studies have demonstrated the potential of cell transplantation to decelerate or halt degenerative processes in some cases (Hiraishi et al., 2018;Nukaga et al., 2019).Minimally invasive procedures, such as a needle injection under image guidance, are used to introduce the cells into the IVD.The cells can be encapsulated in supportive matrices to enhance retention and functionality.
Mesenchymal stem cells (MSCs) have attracted considerable interest in recent years.MSCs derived from the bone marrow The scheme summarizing the modalities of the regenerative therapies and the pathological conditions of the intervertebral disc degeneration.
Frontiers in Bioengineering and Biotechnology frontiersin.org(Orozco et al., 2011;Elabd et al., 2016;Henriksson et al., 2019;Noriega et al., 2017;Amirdelfan et al., 2021), adipose tissue (Kumar et al., 2017), peripheral blood (Haufe and Mork, 2006), or umbilical cords (Pang et al., 2014;Lewandrowski et al., 2023) exhibit potential for chondrogenic differentiation and high proliferative ability.Furthermore, they can be harvested easily from autologous and allogeneic sources.MSCs exhibit remarkable versatility, including the capacity for immunomodulation.This property could be beneficial in tempering the inflammatory environment of the IVD (Weiss and Dahlke, 2019).Environmental stressors within the IVD can affect the proliferation and differentiation capabilities of MSCs; thus, the adaptability and survival of MSCs within IVDs require further study (Gay et al., 2019).
Human clinical trials have predominantly focused on MSCs owing to the promising outcomes exhibited by MSCs in reducing pain and improving function in patients with IVD degeneration (Table 1).Trials using adipose-and bone marrow-derived MSCs have reported improvement in pain and disability scores (Orozco et al., 2011;Elabd et al., 2016;Henriksson et al., 2019;Noriega et al., 2017;Amirdelfan et al., 2021;Kumar et al., 2017;Pang et al., 2014;Lewandrowski et al., 2023).Moreover, the safety profiles of MSCs are generally favorable, and the incidence of serious adverse events is rare (Amirdelfan et al., 2021), underscoring the potential viability of MSC therapies.
Studies on other types of cells, such as the nucleus pulposus (NP) cells (Mochida et al., 2015;Beall et al., 2021) and chondrocytes from articular or hyaline cartilage (Coric et al., 2013;Tschugg et al., 2017), have elucidated the unique regenerative capacities inherent to cells native to avascular tissues, such as the IVD.However, the limitations associated with accessibility and phenotypic stability limit the widespread application of these treatment strategies.Clinical application of these cells, although limited, suggests their safety and potential efficacy in improving the status of the IVD and patient outcomes.
The treatment outcomes of emerging therapies utilizing lessdefined cell products, such as stromal vascular fraction (SVF) (Comella et al., 2017) and bone marrow concentrate (BMC) (Pettine et al., 2015), vary.These complex mixtures, which comprise various types of cells and bioactive factors, offer a multifaceted approach to tissue regeneration.The precise mechanisms of action and individual contributions of these components are unclear; however, early results suggest a potential for facilitating significant clinical improvement in certain cases (Comella et al., 2017;Pettine et al., 2015).Nevertheless, the heterogeneity of these products hinders standardization and quality control.
Animal experiments have used iPS cells as a cell source and induced differentiation into notochordal cells (Sheyn et al., 2019), nucleus pulposus-like cells (Zhang et al., 2020), or cartilaginous tissue (Kamatani et al., 2022).These cells were transplanted into IVDs subsequently.Their application in human beings is anticipated in the future.

Biomaterials and scaffolds
Biomaterials aimed at NP replacement in IVD regeneration, comprising synthetic and biological materials designed to restore disc height and segmental stability without compromising the annulus fibrosus, are predominantly injectable materials (Iatridis et al., 2013;Mehrkens et al., 2012).The properties of synthetic materials must match the mechanical properties of the native disc to achieve successful outcomes.This ensures the integration of the material with the surrounding structures and the restoration of the motion characteristics without inducing adverse immune responses.Furthermore, durability and minimal wear-debris generation are critical factors affecting clinical viability (Bowles and Setton, 2017;Li et al., 2022).
Biologically based materials can be remodeled by the body.Consequently, a different set of criteria, primarily focusing on the ability to support cell-mediated tissue regeneration, must be satisfied by these materials to achieve successful outcomes.This category comprises materials that serve as cell-delivery vehicles to promote NP tissue regeneration (Bowles and Setton, 2017;Li et al., 2022).

Trade name Classification Polymer Outcomes
NeuDisc ™ (Bertagnoli et al., 2005)   injectable DiscCell ™ (Pelletier et al., 2016)   in situ forming in situ curing polymerized water-in-oil emulsion composite Restoring segmental range of motion in vitro BioDisc ™ (Yuksel et al., 2002)   in  and mechanical functions of the native disc; however, it has exhibited varying degrees of success in mimicking the mechanical properties and achieving integration with the native tissue.
Challenges related to the management of the disc space, integration with the native tissue, and the mechanical environment of the spine have been reported by in vivo animal studies on composite discs.These findings emphasize the requirement for conducting further research to address these limitations that hinder successful clinical translation (Mizuno et al., 2006;Sakai et al., 2006;Zhuang et al., 2011;Bowles et al., 2011;Martin et al., 2014;Ukeba et al., 2021).Techniques to engineer motion segments, including the disc and adjacent bony structures, aim to overcome the challenges associated with integration and improve patient outcomes.
Although promising, the field of IVD regeneration through NP replacement is associated with several limitations associated with material design, biological integration, and mechanical performance.Thus, further studies must be conducted in the future to better understand and overcome these obstacles for successful clinical application.
Injecting GDF-6 into the IVD attenuated inflammatory gene expression and improved disc degeneration in a rabbit puncture model (Miyazaki et al., 2018).Phase 1/2 clinical trials on the intradiscal injection of recombinant human growth and differentiation factor-5 (rhGDF-5) have reported promising results (NCT00813813 and NCT01124006).
PRP, which comprises concentrated autologous platelets and growth factors (Akeda et al., 2019), has shown potential in increasing cell proliferation, matrix production, and disc height in preclinical models.PRP can be classified into four categories, comprising four types of preparation, based on the number of leukocytes and fibrin content (Akeda et al., 2019).Two randomized controlled trials have investigated the effects of PRP.Tuakli-Wosornu et al. reported that injecting PRP into the IVD resulted in significant improvement in lower back pain and function over 8 weeks; moreover, the improvement was maintained at the 1year follow-up visit (Tuakli-Wosornu et al., 2016).Akeda et al. reported that injecting PRP into the IVD resulted in a significant improvement in the disability score at 26 weeks and walking ability scores at 4 and 8 weeks compared with those achieved with corticosteroid injection (Akeda et al., 2022) (Table 1).Optimal formulations and delivery methods continue to be explored.Small molecules and drugs aim to inhibit inflammatory cytokines and MMPs, while also regulating the expression of catabolic/ anabolic genes.
Nuclear factor-κB (NF-κB) decoy and TNF-α inhibitors are biologics that exert anti-inflammatory effects.NF-κB, a transcription factor, regulates the inflammatory cytokine levels.NF-κB decoy, an oligodeoxynucleotide containing the NF-κB binding site that entraps NF-κB subunits, can suppress NF-κB activity.Intradiscal injection of NF-κB can suppress inflammatory gene expression in degenerated discs and restore disc height loss (Kato et al., 2021).Notably, intradiscal injection of a TNF-α inhibitor (etanercept) improved discogenic pain in humans within a 2-month follow-up period in a previous study (Sainoh et al., 2016).In recent years, studies on the multi-strategy synergistic therapies that incorporate cells, biomaterials, and growth factors for intervertebral disc degeneration have also been conducted.Wei et al. reported that TGF-1 was embedded with MSCs in decellularized annulus fibrosus matrix (DAFM) hydrogels reinforced with polyethylene glycol diacrylate (PEGDA) to facilitate the controlled release of TGF-1 while preserving the hydrogel's porous structure.Following infusion into a rat model with AF injury, there was a notable increase in the migration of AF cells to the injury site, which promoted anabolic upregulation (Wei et al., 2022).Sun et al. (2020) utilized 3D printing and electrospinning technology to load TGF-β3, CTGF and bone marrow-derived MSCs onto polydopamine nanoparticles and polycaprolactone scaffolds, respectively, mimicking the structure of AF and achieving mechanical properties similar to those of natural AF in the rodents.Although these studies have demonstrated that multi-strategy combination has great potential for IVD repair, further studies in large animal models are necessary.

Animal models
In vitro and in vivo studies have been conducted using small and large animal models of rodents (such as mice and rats), rabbits; dogs; sheep; goats; pigs; and monkeys; to evaluate regenerative techniques (Poletto et al., 2023).The use of small animal models is a cost-effective approach for screening.Rodents, such as mice and rats, are the most commonly used animal species (54%) in IVD studies (Poletto et al., 2023).However, the physiological characteristics of rodents vary from those of humans.In contrast, the disc size and disc degeneration observed in large animals, such as dogs, sheep, and pigs, is similar to that in humans; nevertheless, replicating the slow, progressive human disc pathology in these animals is difficult.IVD degeneration can be induced via bacterial, chemical, genetic, noninvasive, spontaneous, and surgical methods (Poletto et al., 2023;Oichi et al., 2020).The experimental time points vary depending on the species.The commonly used time points for different species are as follows: rodents, 2 or 4 weeks; rabbits, 4 weeks; sheep, 24 weeks; goats, 12 weeks; pigs, 12 weeks; and monkeys, >104 weeks (Poletto et al., 2023).A single animal model that can recapitulate the entirety of human IVD degeneration remains to be established.However, the results can be interpreted reliably if the limitations of a selected animal species are recognized.

Clinical translation
Preclinical studies have demonstrated the promising results of regenerative disc therapies.Nevertheless, these therapies are associated with significant limitations that hinder their translation into clinical practice.These limitations include insufficient graft retention and integration into the disc space, limited survival and proliferation of the cells within the harsh disc microenvironment, uncontrolled differentiation of stem cells, mechanical instability of the implanted scaffolds, and concerns regarding long-term safety.Furthermore, the regulatory requirements for devices and biologics set forth by the U.S.
Food and Drug Administration also pose hurdles.Clinical implementation of regenerative disc therapies requires optimization of cell sources, biomaterials, growth factors, PRP, gene therapy, mechanical stimulation, and delivery methods.Evaluation of clinically relevant models and personalized regenerative therapies tailored to individual patients can be achieved using biomarkers, advanced imaging modalities, and bioreactors.

Discussion
Preclinical studies on regenerative medicine therapies that utilize cells, biomaterials, growth factors, PRP, and biological agents have demonstrated their promise in repairing degenerated lumbar discs.However, these therapies are associated with significant limitations and challenges that hinder their clinical translation.Thus, further studies must be conducted in the future to address these challenges.Advances in the fields of tissue engineering, biomaterials, stem cells, and biological factors will facilitate regenerative therapies to halt or reverse progressive disc degeneration and improve the clinical outcomes and quality of life of patients with LDDD.

TABLE 1
Cell therapies for degenerative disc disease.

TABLE 2
Clinical and preclinical results of the nucleus pulposus repair or replacement devices.

TABLE 3
Overview of the composite materials with cells.