Clinical trial landscape of cell therapy for spinal cord injury: from integrated practices to future developments

Abstract

Spinal cord injury (SCI) precipitates a multiphasic secondary injury cascade that establishes a hostile, inhibitory microenvironment, rendering the condition refractory to conventional surgical stabilization and rehabilitation. While cell-based therapies offer promise for neural reconstruction, their clinical translation is impeded by protocol heterogeneity and fragmented safety data. To address this, we mapped clinical trials for SCI to quantify patient demographic parameters, identify lineage-specific adverse-event patterns, evaluate objective motor and sensory efficacy outcomes, and analyze methodological trial designs to formulate concrete structural recommendations for future advanced-phase trials. We analyzed clinical trials from the Web of Science Core Collection (SCIE and ESCI) published since 2005. The dataset comprised 116 eligible studies involving patients with SCI receiving cellular therapies with reported safety outcomes. We employed a dual-method approach combining manual extraction of clinical characteristics (demographics, interventions, adverse events, efficacy outcomes) with quantitative data analysis. Statistical associations between therapeutic variables (cell type, route, dosage) and safety profiles were evaluated using Fisher’s exact test. The clinical landscape is predominantly defined by early-phase (Phase 1: 59.5%), single-arm investigations (63.8%) utilizing autologous bone marrow-derived cells. Reflecting a cautious paradigm to minimize severe complications, patient selection frequently targeted the hemodynamically stable chronic phase (58.3%) and thoracic SCI (21.6%). Safety analyses revealed lineage-specific profiles: mesenchymal stromal cells were significantly associated with transient fever (P  = 0.038), whereas intrathecal administration correlated with procedural symptoms such as headache (P < 0.001). The observation of higher systemic adverse event rates in low-dose cohorts was likely confounded by the mandatory concurrent immunosuppressive regimens required for specific allogeneic lineages, rather than the absolute cell dose. Regarding therapeutic efficacy, outcomes were critically influenced by the chronological phase of injury. Patients treated in the acute or subacute phases exhibited higher rates of neurological improvement, though distinguishing this from spontaneous recovery remains challenging, whereas chronic phase interventions demonstrated more limited primary sensorimotor gains. Additionally, intrathecal administration showed an advantage in preserving sensory pathways due to minimized structural disruption, while dose requirements could not be generalized and varied fundamentally based on specific cellular mechanisms of action. While the baseline safety of cellular transplantation for spinal cord injury is established, clinical translation remains hindered by methodological heterogeneity, imprecise patient stratification, and a reliance on single-arm trial designs. To navigate this translational bottleneck, future investigations should adopt multi-tiered methodological frameworks. First, study designs should transition toward controlled protocols, utilizing crossover designs for chronic cohorts and matched historical or synthetic controls for acute and subacute phases. Concurrently, patient selection must evolve from broad clinical grading to advanced stratification. Integrating biomarkers, electrophysiology, and imaging to objectively quantify tissue sparing can better identify responsive subgroups, thereby improving trial efficiency and accelerating clinical translation. Beyond cohort refinement, intervention parameters, specifically dosage and delivery routes, should be individualized according to cell lineage, as adverse events associate more with intrinsic cellular biology and procedural invasiveness. Furthermore, isolating the true therapeutic effect requires the standardization and reporting of confounding variables, such as immunosuppressive regimens and physical rehabilitation. Finally, by separating shorter-term efficacy measurements from long-term safety registries, the field can facilitate a more reliable and objective clinical translation.