Stem cells in managing glaucoma

Abstract

Glaucoma is a chronic neurodegenerative condition of the visual system characterized by the progressive death of retinal ganglion cells (RGCs), resulting in irreversible vision impairment. Although current therapies effectively lower intraocular pressure, they do not directly target the fundamental neuronal loss that leads to prolonged vision deterioration. Stem cell-based therapies have emerged as a promising regenerative approach, aiming to preserve neural tissue, alter pathological microenvironments, and potentially restore impaired visual circuitry. Recent preclinical and early translational studies suggest that embryonic, induced pluripotent, and mesenchymal stem cells can enhance the lifespan of RGCs via trophic support, immunomodulation, and modulation of glial responses. Progress in pluripotent stem cell differentiation has facilitated the creation of retinal progenitors and RGC-like cells that can partially integrate into experimental animals. Nonetheless, significant obstacles - such as axonal regeneration, functional connection, immunological responses, and long-term safety - must be addressed. Ongoing advancements in stem cell engineering, biomaterials, and delivery methods indicate that regenerative therapies may ultimately augment or surpass pressure-lowering strategies, creating new opportunities for disease modification and visual restoration in glaucoma.

Key Words: Stem cells; Retinal ganglion cells; Glaucoma; Neuroprotection; Induced pluripotent stem cells; Regenerative medicine; Mesenchymal stem cells

Core Tip: Glaucoma is widely acknowledged as a chronic neurodegenerative condition characterized by irreversible loss of retinal ganglion cells, leading to progressive visual deterioration despite good management of intraocular pressure. Stem cell-based approaches present an innovative regenerative framework by delivering neurotrophic support, immunomodulation, and possible cellular replacement. Experimental studies indicate that embryonic, induced pluripotent, and mesenchymal stem cells can extend the survival of retinal ganglion cells and alter the glaucomatous milieu. Despite significant obstacles, stem cell technologies may eventually enhance or transform existing glaucoma treatment protocols.

INTRODUCTION

Glaucoma is a leading cause of irreversible blindness worldwide and a major public health concern due to its chronic, progressive course. It is characterized by a distinctive optic neuropathy associated with degeneration of retinal ganglion cells (RGCs) and their axons, resulting in irreversible visual field loss and, in advanced stages, visual impairment. Although intraocular pressure (IOP)-lowering therapies can slow disease progression, there is currently no cure and no proven definitive neuroprotective treatment in routine clinical practice. It is traditionally characterized by a distinctive optic neuropathy associated with degeneration of RGCs and their axons, leading to irreversible visual field loss and ultimately visual impairment[1,2]. Despite high IOP being the primary modifiable risk factor, clinical and epidemiological investigations have shown that glaucomatous damage may persist even with effective IOP reduction[3]. Additionally, a notable subset of patients develops normal-tension glaucoma, highlighting that pressure-independent processes significantly contribute to the etiology of the disease[4]. These observations have gradually transformed the conceptual framework of glaucoma from a solely pressure-driven condition to that of a multifaceted neurodegenerative disease affecting the visual system. Consequently, therapeutic techniques that target IOP are widely acknowledged as required yet inadequate for sustained disease management.

At the cellular and molecular level, glaucomatous neurodegeneration is propelled by a convergence of pathogenic mechanisms akin to those seen in other central nervous system diseases. Experimental models and clinical investigations have identified mitochondrial malfunction, decreased axonal transport, glutamate excitotoxicity, oxidative stress, calcium dysregulation, and chronic neuroinflammation as key drivers of RGC death[5]. These pathways are exacerbated by reactive glial responses from astrocytes and microglia, which foster a detrimental milieu that accelerates neuronal damage[6]. Neurodegenerative alterations in glaucoma are not limited to the retina; they also extend along the optic nerve and into the central visual pathways, including the lateral geniculate nucleus and visual cortex[7].

In this context, regenerative medicine has been proposed as a promising strategy to address the irreversible neuronal loss underlying glaucomatous vision impairment. Among regenerative approaches, stem cell-based therapies have attracted considerable interest due to biological properties that extend beyond direct cell replacement. In preclinical models of optic nerve injury and glaucoma, embryonic stem cell (ESC)-derived, induced pluripotent stem cell (iPSC)-derived, and mesenchymal stem cells (MSCs) have been shown to exert paracrine effects, including the secretion of neurotrophic factors, modulation of inflammatory responses, and influence on glial cell behavior[8,9]. The paracrine and immunomodulatory effects appear to significantly contribute to RGC survival and the maintenance of optic nerve structure, even in the absence of long-term engraftment or synaptic integration[10]. Progress in pluripotent stem cell differentiation has enabled the production of retinal progenitors and RGC-like cells, thereby enhancing the potential for future cell-replacement therapies[11]. Nonetheless, significant biological and translational obstacles persist, including axonal regeneration over long distances, functional connectivity with central targets, immunological compatibility, and long-term safety. Consequently, stem cell-based methodologies currently represent an emerging therapeutic frontier between neuroprotection and genuine neural regeneration in glaucoma.

LITERATURE REVIEW

This narrative review has comprehensively synthesized existing knowledge on stem cell-based therapy approaches for glaucoma by critically evaluating the literature indexed in prominent biomedical databases, such as PubMed, Scopus, and Web of Science. The search strategy included studies on stem cell biology, mechanisms of neuroprotection in RGCs, regenerative and reparative methods, and novel translational platforms, such as therapies utilizing extracellular vesicles and stem cell-derived secretomes.

Recent experimental investigations, preclinical models, and translational research have been emphasized for their mechanistic insights, safety and feasibility evaluations, and exploration of possible therapeutic applicability for glaucomatous neurodegeneration. Emphasis has been placed on research that elucidates the biological mechanisms of neuroprotection, immunomodulation, and tissue regeneration, alongside investigations into delivery methodologies, biomaterial integration, and enduring therapeutic obstacles.

Further pertinent papers have been discovered via manual examination of the reference lists of significant articles to guarantee thorough coverage of the domain. This review adopts a narrative approach, focusing on a balanced and integrated synthesis of the most relevant evidence instead of a formal systematic or quantitative meta-analysis. This methodology has for the incorporation of varied study designs and conceptual progress, thereby offering a comprehensive and therapeutically relevant viewpoint on the present status and prospective developments of stem cell-based therapy in glaucoma.

Regenerative treatments for glaucoma: A neurodegenerative disorder

Glaucoma is currently recognized as a neurodegenerative condition of the visual system, exhibiting essential biochemical characteristics similar to those of Alzheimer’s disease and Parkinson’s disease. The gradual degeneration of RGCs, the retina’s exclusive output neurons, characterizes this paradigm; their axons constitute the optic nerve. Experimental evidence indicates that RGC degeneration in glaucoma is a gradual, cumulative process influenced by persistent cellular stress and diminished neural resilience[12]. Axonal injury at the lamina cribrosa is believed to occur prior to, resulting in retrograde degeneration and impairment of axonal transport well before cell death is evident[13]. The temporal separation between axonal dysfunction and neuronal death creates a crucial therapeutic window for regenerative or neuroprotective interventions to significantly influence disease progression. Thus, interventions focused on maintaining axonal integrity and neuronal survival are increasingly regarded as vital adjuncts to pressure-reducing therapies. Neuroinflammation is crucial in exacerbating glaucomatous damage and further solidifies the neurodegenerative framework of the condition. Activated microglia and astrocytes have been regularly identified in both experimental models and human glaucomatous tissue, where they facilitate the release of pro-inflammatory cytokines, reactive oxygen species, and complement factors[14]. Although glial activation may initially serve as an adaptive response to injury, prolonged dysregulation creates a toxic milieu that exacerbates neuronal death. Complement-mediated synaptic pruning, a mechanism well characterized in neurodegeneration of the central nervous system, has also been associated with early synaptic loss in glaucoma, thereby linking glaucoma to more extensive neurodegenerative processes[15]. The findings indicate that glaucomatous damage arises not just from mechanical or vascular stress but also from active immune-mediated pathways that compromise neuronal survival. This complexity limits the effectiveness of medications that target only IOP or a single biochemical pathway. Mitochondrial dysfunction and compromised energy metabolism are further convergent processes that enhance RGCs’ susceptibility to glaucoma. RGCs have significantly elevated metabolic demands owing to their elongated, unmyelinated axons in the retina and optic nerve head, making them particularly vulnerable to disturbances in mitochondrial function[16]. Research using animal models has revealed altered mitochondrial dynamics, reduced ATP production, and increased susceptibility to oxidative stress in glaucomatous eyes[17]. These metabolic dysfunctions impede axonal transport and synaptic preservation, thereby accelerating neurodegeneration even in the absence of a prolonged increase in IOP. Mitochondrial dysfunction is not readily reversible with standard therapies, underscoring the need for regenerative techniques to restore or enhance neuronal bioenergetics. Stem cell-based methodologies, through trophic and metabolic support, may provide a mechanism to address these deficiencies at a fundamental level. Regenerative therapies aim to reestablish a conducive environment for neuronal preservation, augment innate survival mechanisms, and potentially replace missing cellular elements. Stem cells are uniquely equipped to perform these functions owing to their capacity to interact dynamically with damaged tissue, to secrete a diverse range of neurotrophic and anti-inflammatory substances, and to modulate resident glial cells[18]. This systemic method of action closely corresponds with the complex etiology of glaucoma. Consequently, regenerative therapies based on stem cell biology are increasingly viewed not as speculative supplements but as logical, mechanism-based approaches to altering the progression of glaucomatous neurodegeneration.

Sources of stem cells for glaucoma treatment

ESCs were among the first stem cell groups examined for retinal repair owing to their limitless proliferative capacity and pluripotent differentiation potential. In experimental contexts, ESCs can be guided toward brain and retinal lineages, including retinal progenitors and RGC-like phenotypes, via sequential manipulation of developmental signaling pathways[19]. These characteristics render ESCs theoretically appealing for cell-replacement approaches aimed at restoring depleted RGC populations in advanced glaucoma. Preclinical studies indicate that ESC-derived neural cells can survive intraocular transplantation and express markers indicative of neuronal development, suggesting a degree of lineage commitment within the retinal milieu[20]. Nonetheless, ethical concerns, the potential for teratoma development, and the necessity for rigorous regulation of differentiation status have considerably curtailed interest in ESC-based strategies for glaucoma. Consequently, while ESCs have been pivotal in demonstrating the feasibility of retinal differentiation, their direct clinical application remains limited.

iPSCs have emerged as a formidable alternative to ESCs, offering pluripotency without the ethical and immunological issues associated with embryonic origins. iPSCs can be derived from adult somatic cells and subsequently differentiated into retinal progenitors or RGC-like cells using established techniques that recapitulate essential elements of retinal development[21]. Crucially, patient-specific iPSCs offer a foundation for autologous or human leukocyte antigen-matched therapy, potentially diminishing immunological rejection post-transplantation. In glaucoma models, iPSC-derived neural cells have been shown to survive in the inner retina and release neurotrophic factors that promote endogenous RGC survival[22]. Notwithstanding these advancements, the functional integration of iPSC-derived RGCs into existing retinal circuitry and the regeneration of long-distance axonal projections toward central visual destinations remain significant unresolved hurdles. As a result, iPSC-based approaches are currently seen as potential tools for disease modeling and neuroprotection, with cell replacement as a long-term goal.

MSCs are the most thoroughly investigated stem cell population in experimental glaucoma and are presently the nearest to therapeutic application. MSCs can be isolated from multiple adult tissues, including bone marrow, adipose tissue, and umbilical cord, and are characterized by significant immunomodulatory and paracrine functions rather than by pluripotent differentiation potential[23]. In animal models of glaucoma and optic nerve injury, intravitreal delivery of MSCs has demonstrated considerable improvements in RGC survival, maintenance of optic nerve architecture, and reduction in glial activation[9]. The benefits are primarily mediated by the release of neurotrophic factors, including brain-derived neurotrophic factor, ciliary neurotrophic factor, and nerve growth factor, and by the control of inflammatory signaling pathways[24]. Significantly, MSCs do not require prolonged engraftment or differentiation into retinal neurons to exert their beneficial effects, thereby alleviating safety concerns about unregulated proliferation. However, variability in MSC source, production techniques, and distribution systems generates heterogeneity that challenges standardization and regulatory approval.

In addition to whole-cell transplantation, there is a growing focus on stem cell-derived secretomes and extracellular vesicles as cell-free therapeutic options. Exosomes generated by MSCs contain a diverse array of proteins, lipids, and microRNAs that can influence neuronal viability, inflammation, and glial responses[25]. In experimental glaucoma models, administration of MSC-derived exosomes has demonstrated neuroprotective effects comparable to those of cell transplantation, including improved RGC survival and functional preservation[26]. The data indicate that paracrine signaling, rather than direct cellular replacement, may be the primary mode of action for stem cell therapy in glaucoma. Cell-free methodologies may offer advantages in safety, scalability, and regulatory compliance. A crucial distinction in the advancing field of regenerative therapies for glaucoma is between whole-cell therapies and cell-free approaches using extracellular vesicles or secretome products. Whole-cell transplantation offers the potential benefit of enduring biological activity, as transplanted cells can adaptively respond to local environmental stimuli and continuously release neurotrophic and immunomodulatory substances. This technique is intrinsically linked to increased complexity of safety monitoring, unpredictability in cellular behavior, risks of uncontrolled proliferation or differentiation, and difficulties connected with long-term engraftment.

Conversely, cell-free therapies utilizing extracellular vesicles offer a more controlled and potentially safer approach, as they mitigate hazards associated with the persistence of viable cells while maintaining the advantageous paracrine effects of stem cells. From a production standpoint, extracellular vesicles may offer benefits in scalability, storage, and standardisation; however, issues persist regarding characterisation, potency evaluation, and batch uniformity. The biological efficacy of vesicle-based therapeutics may be intrinsically time-restricted, perhaps requiring recurrent delivery to sustain therapeutic effects.

From a translational perspective, cell-free methodologies may offer a more viable route to early clinical implementation owing to their enhanced safety profile and regulatory feasibility. At the same time, whole-cell therapies may continue to play a role in advanced regenerative strategies that require prolonged biological interactions. A comprehensive development approach will necessitate simultaneous exploration of both platforms, with meticulous consideration of their distinct benefits, drawbacks, and clinical applications.

As research progresses, the comparative assessment of whole-cell vs secretome-based approaches will be essential for identifying the most effective and clinically viable stem cell-derived therapies for glaucomatous neurodegeneration. The principal stem cell platforms explored for glaucoma, along with their translational status, are summarized in Table 1.

Table 1 Stem cell-based strategies investigated for glaucoma management.

Stem cell source	Primary mechanism of action	Experimental evidence level	Translational status
MSCs	Neurotrophic support, immunomodulation, glial modulation	Robust preclinical evidence	Early translational studies
MSC-derived extracellular vesicles	Paracrine neuroprotection, microRNA-mediated signaling	Strong preclinical evidence	Preclinical development
iPSCs	RGC-like differentiation, disease modeling	Preclinical proof-of-concept	Experimental
ESCs	Retinal progenitor and neuronal differentiation	Early preclinical studies	Limited by safety and ethics

All listed evidence levels are based on preclinical models. None of these stem cell approaches has demonstrated clinical efficacy in human glaucoma therapy, according to the current literature. MSCs: Mesenchymal stem cells; iPSCs: Induced pluripotent stem cells; RGCs: Retinal ganglion cells; ESCs: Embryonic stem cells.

From a translational perspective, MSCs presently constitute the most viable platform for early clinical development focused on glaucoma, as their primary therapeutic effect seems to rely on paracrine neuroprotection and immunomodulation rather than exact structural integration into retinal circuitry. This aspect is especially pertinent in glaucoma, where maintaining viable RGCs and stabilizing the retinal milieu are more readily achievable therapeutic objectives than complete cellular replacement. MSCs provide extensive preclinical expertise, well-established manufacturing processes, and a relatively advantageous safety profile compared to pluripotent cell sources.

Conversely, iPSCs present a promising long-term strategy for personalized disease modeling and prospective cell-replacement therapies; however, their translational applicability is hindered by persistent issues related to lineage stability, tumorigenic potential, incomplete functional integration, and the failure of newly generated RGC-like cells to restore long-distance axonal connectivity with central visual targets. ESCs have demonstrated critical proof of principle for retinal differentiation, but ethical limitations and safety issues have curtailed their immediate clinical use. Based on currently available evidence, MSCs or their secretome-derived products are the most appropriate cell source for imminent clinical application in glaucoma. In contrast, pluripotent stem cell platforms should be viewed as enabling technologies for future regenerative reconstruction rather than primary therapeutic options.

Mechanisms of regeneration and neuroprotection

The majority of stem cell-based approaches currently under investigation for glaucoma focus on neuroprotection rather than neuronal replacement, and this distinction is crucial for assessing preclinical efficacy claims. In experimental settings, MSCs reliably enhance RGC survival, even when the transplanted cells fail to integrate into retinal circuitry, thereby providing robust evidence for a paracrine mechanism of action[27]. A primary process involves the release of neurotrophic factors that enhance RGC resilience and inhibit apoptotic pathways, including brain-derived neurotrophic factor, ciliary neurotrophic factor, and other supporting cytokines[28]. This trophic support is especially pertinent in glaucoma, where disruptions in axonal transport and mitochondrial stress diminish the supply of intrinsic survival signals to RGC somas[29]. In addition to trophic signaling, transplanted stem cells may affect extracellular matrix remodeling, vascular support, and metabolic balance, therefore stabilizing the inner retinal environment in ways that are challenging to replicate with single-target pharmacological treatments. Significantly, these neuroprotective mechanisms are conceptually consistent with glaucoma as a multifaceted neurodegenerative condition rather than a problem driven by a single predominant pathway[30]. Immunomodulation is a significant mechanism by which stem cell-based therapies may mitigate glaucomatous neurodegeneration, given the recognized role of chronic neuroinflammation and glial reactivity in RGC loss. Microglial activation, complement signaling, and astrocytic remodeling might exacerbate injury through prolonged cytokine release and synaptic instability, transforming the retinal environment into one of chronic toxicity rather than repair[31].

Complement-dependent synapse elimination, associated with early glaucomatous dysfunction, suggests that neuronal loss may precede inflammatory alterations in retinal connectivity and communication[32]. MSCs and other stem cell populations can inhibit pro-inflammatory signaling and skew immune responses towards a more regulatory phenotype, thereby mitigating secondary damage cascades that impact RGCs and their axons[33]. Concurrently, substances produced by stem cells may influence the responses of Müller cells and astrocytes, thereby reducing gliosis and limiting maladaptive scarring that could compromise neuronal survival. Their cumulative results support the view that stem cell therapies may function as “microenvironmental therapies”, aiming to modulate inflammatory and glial conditions to protect existing neurons rather than reconstruct the system from scratch.

A growing body of research indicates that many of the advantages attributed to stem cell transplantation can be replicated by cell-free products, particularly extracellular vesicles and exosomes derived from MSCs. Exosomes carry an intricate chemical payload, comprising proteins and microRNAs, that modulate neuronal survival pathways, inhibit inflammatory signaling, and affect glial behavior without necessitating the engraftment of living cells[25]. In models of optic nerve injury and glaucoma-related degeneration, MSC-derived exosomes have been shown to enhance RGC survival and functional preservation, thereby supporting the concept that secreted vesicular signaling serves as a primary therapeutic mediator[26]. This method is appealing because it may mitigate risks associated with cell transplantation, such as ectopic proliferation, unwanted differentiation, and inconsistent engraftment dynamics. Concurrently, exosome medicines raise novel translational questions about dose, biodistribution, manufacturing consistency, and potency, all of which require standardization for clinical use. Nonetheless, the transition from whole-cell transplantation to specified stem cell-derived biologics represents a significant advancement in the field, potentially enhancing repeatability and regulatory viability while preserving neuroprotective benefits.

Unlike neuroprotection, genuine regeneration in glaucoma necessitates replacing damaged RGCs and, importantly, restoring long-distance axonal projections via the optic nerve to central visual destinations, accompanied by appropriate synaptic integration. Progress in iPSC development has enabled the generation of RGC-like cells that express essential lineage markers, providing a viable foundation for prospective cell-replacement therapies[34]. Nonetheless, the biological challenges are significant: Adult mammalian RGCs exhibit a limited intrinsic capacity for axon regeneration, and the optic nerve environment following injury is actively inhibitory due to myelin-associated inhibitors and glial scarring[35]. Significant research beyond the stem cell domain has shown that altering intrinsic growth mechanisms - such as by deleting phosphatase and tensin homolog - can improve optic nerve regeneration, highlighting that effective replacement will likely require a combination of strategies that integrate cell therapy with growth-permissive reprogramming[36]. Consequently, although stem cells may ultimately aid in structural regeneration, the immediate translational potential in glaucoma is more convincingly associated with neuroprotective and microenvironmental-altering effects than with circuit reconstruction. This distinction is clinically significant, as it establishes realistic therapeutic objectives for early human trials and guides future research toward combinatorial strategies that facilitate functional reconnection.

Biomaterials, strategies for delivery, and modulation of the microenvironment

The therapeutic effectiveness of stem cell delivery strategies is significantly influenced by the route of administration, as the route dictates cell viability, biodistribution, and interactions with host retinal tissue. Intravitreal injection is the predominant delivery technique in experimental glaucoma models because of its direct access to the inner retina, home to RGCs, and its familiarity in clinical practice. Research employing intravitreal administration of MSCs has shown considerable neuroprotection for RGCs; however, prolonged engraftment is generally restricted, with transplanted cells frequently remaining localized to the vitreous cavity or the inner limiting membrane[9]. Subretinal administration, although it provides closer proximity to neural tissue, is less appropriate for glaucoma because it predominantly targets the photoreceptor and retinal pigment epithelial layers rather than the ganglion cell layer. Periocular and suprachoroidal methods have been investigated; however, their efficacy in protecting RGCs remains incompletely characterized. Optimizing delivery channels is crucial to achieving translational success[37-40].

Biomaterial scaffolds and supporting matrices have become crucial adjuncts to enhance stem cell survival and functional efficacy post-transplantation. Injectable hydrogels, biodegradable polymers, and materials that imitate the extracellular matrix have been engineered to offer mechanical support, enhance cell viability, and facilitate the regulated release of trophic substances in the retinal milieu[41]. In experimental models of optic nerve injury, the introduction of stem cells supplemented by biomaterials has demonstrated improved cell retention and extended neuroprotective benefits compared to sole cell injection[42]. These scaffolds may serve as platforms for combinatorial therapy, facilitating the co-delivery of stem cells with neurotrophic substances, anti-inflammatory agents, or axon growth-promoting chemicals. The prospective role of hydrogels and scaffolds beyond mere structural support. In glaucoma-focused regeneration approaches, biomaterials may augment neuroprotection by enhancing intraocular retention of transplanted cells, safeguarding them from immediate mechanical loss, and facilitating prolonged local exposure to trophic and anti-inflammatory agents. They may also establish a conducive microenvironment that enhances cell viability, mitigates shear-induced damage during injection, and facilitates the simultaneous delivery of stem cells alongside neurotrophic factors, anti-fibrotic medicines, or axon-guidance molecules. In this setting, biomaterials may serve as biological amplifiers of paracrine therapy rather than only as passive transporters.

Simultaneously, biomaterials pose additional risks that require thorough evaluation before clinical application. Hydrogels or scaffolds may induce sterile inflammation, foreign-body reactions, fibrosis, altered vitreoretinal interface dynamics, or localized mechanical stress on fragile retinal structures, contingent upon their composition, degradation profile, and ocular location. In the absence of typical immunological rejection, material-associated inflammatory activation may undermine therapeutic efficacy or intensify pre-existing retinal stress. Additional problems encompass varied degradation rates, opacification, interference with ocular imaging or visual function, and lot-to-lot variability in composite goods that integrate cells with synthetic matrices. Consequently, biomaterial-assisted therapy for glaucoma must be assessed for efficacy, immunobiocompatibility, optical tolerability, degradation behavior, and surgical repeatability. Biomaterials must be meticulously designed to prevent negative inflammatory reactions or mechanical disruption of retinal structure. The incorporation of biomaterials into stem cell-based methodologies constitutes a promising yet emerging component of regenerative glaucoma treatment. Modifying the glaucomatous microenvironment is widely acknowledged as essential for successful stem cell-mediated neuroprotection and regeneration.

Novel delivery paradigms increasingly prioritize cell-free methods, particularly stem cell-derived extracellular vesicles, to circumvent certain logistical and safety issues associated with live-cell transplantation. Extracellular vesicles can be delivered intravitreally and readily diffuse within the inner retina, where they provide neuroprotective and immunomodulatory benefits without concerns about uncontrolled proliferation or ectopic differentiation[25,43]. The method of administration is a critical factor in therapeutic efficacy, as it affects cell viability, access to target tissue, biodistribution, procedural risk, and the likelihood of achieving a lasting biological effect. Intravitreal injection is now the most logical method for glaucoma-targeted neuroprotective treatments, as it delivers the therapeutic agent in proximity to the inner retina and RGC layer. It also benefits from extensive clinical experience, as intravitreal injections are currently standard in medical retina therapy. Nonetheless, intravitreal administration is not without constraints. Transplanted cells may become entrapped at the vitreoretinal interface or at the inner limiting membrane, thereby diminishing effective tissue interaction. Additionally, complications such as inflammation, vitreous opacity, epiretinal membrane formation, elevated IOP, infection, or traction-related alterations must be contemplated, especially in the context of anticipated repeated dosing.

Subretinal administration facilitates proximity to neural retinal tissue and may enhance the viability of transplanted cells within a more organized microenvironment; however, it is less appropriate for glaucoma, as the target tissue in this condition is the RGC layer rather than the photoreceptor-retinal pigment epithelium interface. Moreover, subretinal administration is physically invasive and entails procedural hazards including retinal detachment, retinal tears, subretinal fibrosis, localized bleeding, and iatrogenic injury to pre-existing damaged ocular tissues. Consequently, subretinal injection may be more pertinent to retinal degenerative illnesses than to glaucoma, except in specific experimental regeneration contexts.

The periocular, episcleral, and suprachoroidal routes have attracted attention as minimally invasive alternatives that may reduce the need for direct intraocular intervention while enabling prolonged diffusion of soluble medicinal agents. These methodologies may be particularly pertinent for cell-free formulations or biomaterial-assisted sustained release systems. Nonetheless, their efficacy in transporting active drugs to the inner retina remains inadequately characterized, and inconsistent tissue penetration may diminish repeatability. The route selection must align with the specific therapeutic goal: Intravitreal administration is currently optimal for inner retinal neuroprotection. In contrast, alternative routes may become more useful as delivery technologies advance and biomaterial-assisted depot systems improve. A significant translational barrier is the lack of consistent dose protocols for glaucoma-targeted stem cell treatment. In preclinical studies, the number of transplanted cells per eye, the concentration of the final suspension, the timing of administration relative to injury induction, and the intervals between repeat treatments have varied significantly, complicating direct comparisons and hindering the establishment of an evidence-based therapeutic dosage. The existing literature does not endorse a specific cell count per eye or a verified retreatment protocol for therapeutic application. The lack of harmonization is particularly significant in glaucoma, a chronic condition in which a temporary biological effect may not suffice for enduring neuroprotection.

Consequently, further translational studies should assess not only efficacy but also determine dose-ranging thresholds, the maximal tolerable intraocular load, the persistence of paracrine activity, and the probable need for repeated administration over time. Comparable considerations apply to extracellular vesicle-derived products, for which dosage may need to be determined based on particle count, protein concentration, or biological efficacy rather than solely on the amount administered. A clinically significant development pathway for glaucoma requires incorporating pharmacodynamic monitoring, safety-oriented dose escalation, and longitudinal evaluation of treatment adherence. From a manufacturing and regulatory standpoint, vesicle-based medicines may offer greater consistency, scalability, and quality control than whole-cell products. Nonetheless, issues about dose, stability, and long-term retention persist, and it remains uncertain whether recurrent administration would be necessary to sustain therapeutic efficacy. Advances in delivery methods will be pivotal to translating stem cell-based therapies from experimental potential to clinical application in glaucoma by integrating optimal routes, biomaterials, and microenvironmental manipulation.

Immunogenicity, safety, and prolonged risks

Safety problems constitute a primary obstacle to the practical translation of stem cell-based therapies for glaucoma, especially due to the chronic nature of the disease and the anticipated long-term therapy effects. A significant concern is the viability and behavior of transplanted cells in the ocular environment, as unchecked proliferation or improper differentiation may lead to adverse outcomes, such as epiretinal membrane formation or intraocular inflammation. Despite MSCs being regarded as having a positive safety profile owing to their limited proliferative capacity, variations in cell source, culture conditions, and passage number can affect biological behavior and associated risks[44]. Products derived from pluripotent stem cells, such as ESCs and iPSCs, pose additional risks, including residual pluripotency and teratoma formation, if differentiation is not fully achieved[45]. These dangers require rigorous production controls and sustained monitoring measures in any forthcoming clinical application.

Immunogenicity is a vital safety aspect, especially with recurrent or prolonged exposure to stem cell-derived products. The eye is frequently characterized as an immune-privileged region; however, this privilege is relative and may be compromised in pathological conditions such as glaucoma, which is associated with chronic inflammation and disruption of the blood-retinal barrier[46,47]. Allogeneic stem cell transplantation may elicit immunological responses that compromise cell viability or induce inflammatory damage to host retinal tissue. Autologous iPSC-derived products are not immunologically inert, as reprogramming and differentiation can modify antigen expression and provoke immune recognition[48]. These findings emphasize the need for immunological evaluation in preclinical models and suggest the potential need for temporary immunosuppression or immune-evasive engineering approaches in forthcoming therapeutics.

Long-term safety encompasses the durability and reversibility of therapeutic benefits, which are especially important in a gradually progressive condition like glaucoma. In contrast to pharmaceutical treatments, which can be discontinued if adverse effects occur, stem cell-based interventions may have endured or irreversible biological effects. For instance, prolonged release of neurotrophic factors could potentially modify synaptic equilibrium, glial activity, or vascular permeability in unforeseen ways over time. Furthermore, the long-term outcomes of transplanted cells or delivered extracellular vesicles within the retinal milieu remain inadequately elucidated, particularly beyond the durations typically investigated in animal studies. This ambiguity underscores the necessity for prolonged follow-up durations and comprehensive post-treatment surveillance in any human application[49]. Cell-free methodologies, including the use of stem cell-derived extracellular vesicles, have been proposed to address certain safety concerns associated with live cell transplantation. Extracellular vesicle-based therapies may offer a more manageable and perhaps safer option by mitigating the risks associated with cell proliferation and differentiation[25]. Nonetheless, these methodologies provide inherent risks, such as unintended biological effects, accumulation with successive dosage, and difficulties in establishing consistent potency and release parameters. Regulatory bodies have asserted that biological complexity does not diminish oversight requirements, and stringent safety evaluations are obligatory irrespective of the distribution format. An equally important translational issue is the requirement for rigorous manufacturing standardization under good manufacturing practice conditions. Clinical-grade stem cell products are not interchangeable with research-grade preparations because they require controlled donor selection, tissue sourcing, culture conditions, expansion protocols, passage limits, cryopreservation procedures, sterility assurance, and validated release testing for therapeutic consistency. MSCs exhibit added complexity due to donor diversity, variations in tissue origin, and the observation that phenotypic resemblance does not inherently indicate comparable biological potency. Stringent regulation of differentiation efficiency, residual undifferentiated cell load, genomic stability, and lineage fidelity is crucial to mitigate tumorigenic risk and product heterogeneity in pluripotent stem cell-derived products.

Quality control frameworks must encompass identity and stability testing, as well as potency assays demonstrating biologically significant neuroprotective or immunomodulatory activities. Comparable issues pertain to extracellular vesicle-based products, for which characterization standards are not fully defined and may encompass particle sizing, cargo analysis, sterility testing, and functional bioassays. From a regulatory standpoint, therapies for glaucoma utilizing stem cells and secretomes will likely be assessed as advanced biological medicinal products, requiring comprehensive preclinical safety data, consistent manufacturing processes, product traceability, and long-term monitoring strategies. Consequently, clinical translation relies not solely on biological efficacy but also on the development of scalable, auditable, and regulatory-compliant production methods. Ultimately, evaluating safety, immunogenicity, and long-term risk will be as essential as establishing efficacy in ascertaining whether stem cell-based therapies can progress from experimental research to recognized elements of glaucoma management.

Limitations

Notwithstanding persuasive preclinical evidence supporting stem cell-based strategies for glaucomatous neurodegeneration, the transition to clinical application remains in a nascent, prudent phase. Numerous limitations must be acknowledged when evaluating the existing evidence on stem cell-based treatments for glaucoma. The majority of existing data originates from preclinical investigations utilizing animal models that only partially replicate the intricacy and chronic nature of human glaucomatous neurodegeneration. Variations in ocular anatomy, immunological responses, disease progression, and lifespan limit the direct applicability of experimental findings to clinical practice. Secondly, significant variation exists across research on stem cell source, differentiation status, administration method, dosage, and outcome measures, which complicates comparisons and synthesis across studies. This heterogeneity indicates the absence of established techniques and accepted benchmarks. Hypothesized mechanisms, such as neuroprotection and immunomodulation, are derived from experimental models and have not been experimentally corroborated in human glaucomatous tissue. These underscore the necessity for careful interpretation of translational assertions. This review employs a narrative format rather than a systematic or meta-analytic approach, which inevitably introduces the possibility of selection bias. Moreover, publication bias favoring favorable experimental outcomes may inflate the perceived therapeutic efficacy. As more extensive and meticulously structured clinical trials are conducted, forthcoming evaluations will be better equipped to enhance assessments of efficacy, safety, and therapeutic relevance.

Future perspectives

Advancements in stem cell-based therapeutics for glaucoma will rely on overcoming biological and translational hurdles via integrated, multidisciplinary approaches. Progress in stem cell engineering, encompassing refined differentiation protocols and genetic stabilization methods, is anticipated to increase the reliability and safety of cell products designed for neural applications[50]. Simultaneously, combinatorial strategies that integrate stem cell therapies with molecular interventions aimed at axon growth, mitochondrial function, or inflammatory signaling may be essential for significant neuroregeneration[51]. These efforts indicate an increasing acknowledgment that no singular intervention is expected to address the complex characteristics of glaucomatous neurodegeneration.

Cell-free therapeutics originating from stem cells, notably extracellular vesicles and modified secretomes, signify a particularly promising avenue for future research. These methodologies may preserve the numerous neuroprotective and immunomodulatory advantages of stem cells while enhancing safety, scalability, and regulatory compliance[25]. Enhancing dosing regimens, delivery frequency, and cargo composition will be crucial for converting these items into clinically effective medicines. Simultaneously, advancements in ocular medication delivery and biomaterials may improve tissue targeting and extend therapeutic efficacy. Future clinical studies must emphasize a stringent research design, including appropriate control groups, validated structural and functional objectives, and sufficiently long follow-up periods to detect disease-modifying effects. The development of biomarkers that predict responses to regenerative therapies may enhance patient stratification and increase trial efficiency. The incorporation of stem cell-based approaches into glaucoma treatment will likely progress gradually, initially serving as supplementary neuroprotective therapies rather than as independent substitutes for pressure-reducing procedures[52]. Moreover, a vital question for future clinical advancement is whether neuroprotection resulting from stem cells can be sustained for a significant period in a chronic condition like glaucoma. Numerous preclinical investigations have shown beneficial structural or functional effects over brief observation periods; however, the durability of these effects remains questionable. This issue is especially significant when the therapeutic method is predominantly paracrine, as the biological efficacy may wane over time despite an initial response. Consequently, continuous dosage, sustained-release biomaterials, or successive delivery of cell-derived vesicles may be necessary to maintain therapeutic efficacy over prolonged durations.

Recent literature has highlighted the intricacies involved in converting stem cell-based therapies from experimental contexts to clinical practice, in addition to the previously mentioned obstacles. Zuo et al[53] conducted a thorough assessment that identified significant issues with manufacturing standardization, scalability, regulatory control, and variability in treatment effects among studies. These variables underscore the imperative for standardized methods, stringent quality control systems, and reliable potency testing to ensure uniformity across clinical-grade products. Furthermore, the authors have highlighted that discrepancies in cell source, expansion methodologies, and delivery approaches continue to pose a significant challenge to the establishment of universally applicable treatment frameworks. These factors underscore the need for coordinated translational initiatives to connect promising preclinical discoveries with reliable clinical application in chronic conditions such as glaucoma.

The need for retreatment also raises practical and safety concerns. Repeated intraocular injections may increase cumulative risks, including inflammation, interface alterations, and treatment burden, while continuous exposure to allogeneic biological products may modify immune responsiveness. Consequently, subsequent research must ascertain not only the efficacy of stem cell-based therapies but also their duration of effectiveness, the potential for retreatment to restore efficacy, and the patient demographics most likely to benefit from a maintenance approach.

CONCLUSION

Stem cell-based therapies for glaucoma have progressed from theoretical regenerative methods to more advanced platforms designed to maintain RGC activity and alter the pathological microenvironment. Current research indicates that paracrine-mediated neuroprotection is the most readily attainable therapeutic goal, especially by the application of MSCs or their secretome-derived products. Successful clinical translation will hinge on overcoming many fundamental obstacles, including standardizing doses, optimizing delivery routes, demonstrating long-term safety, and developing reproducible manufacturing methods in accordance with regulatory standards. Simultaneously, advancements in extracellular vesicle technologies and biomaterial-assisted delivery systems may enable more regulated and scalable treatment approaches. The most probable therapeutic trajectory will likely adopt a phased approach, commencing with cell-free or mesenchymal-based neuroprotective therapy and gradually progressing to more complex regenerative procedures as biological and technological barriers are overcome. A collaborative approach encompassing ophthalmology, bioengineering, and translational science is crucial for converting these promising ideas into clinically successful glaucoma medicines.