Stem Cell-Delivered Cytosine Deaminase/5-Fluorocytosine and TRAIL Gene Therapy for Castration-Resistant Prostate Cancer: Translational Synthesis and First-in-Human Trial Concept

Castration-resistant prostate cancer (CRPC) is characterised by persistent androgen receptor (AR)-axis activity, therapy-driven resistance, and limited durability of available systemic treatments. Tumour-tropic mesenchymal stem/stromal cells (MSCs), including adipose-derived MSCs (ADSCs), have emerged as promising vehicles for targeted gene therapeutics. This review synthesises our three experimental studies examining stem cell-delivered gene-directed enzyme prodrug therapy (GDEPT) using cytosine deaminase (CD)/5-fluorocytosine (5-FC) and secreted TRAIL in CRPC xenograft models. We performed a comparative analysis of three studies in which hTERT-immortalised human ADSCs were engineered via lentiviral vectors to deliver CD alone, secreted TRAIL alone, or CD+TRAIL in combination, and were administered by intracardiac injection into male nude mice bearing PC3 xenografts. In vitro conversion efficiency, cell viability, apoptosis markers, and in vivo tumour volume endpoints were compared across studies. All three therapeutic platforms demonstrated measurable tumour growth inhibition relative to controls. The CD+TRAIL combination achieved the greatest in vivo efficacy (tumours approximately 26% of control at day 14), compared with CD alone (approximately 71%) or TRAIL paired with irinotecan. Enzymatic conversion of 5-FC to 5-FU exceeded 93% in conditioned medium. Primary translational risks include thrombotic events associated with systemic MSC dosing, tumourigenicity and genotoxicity of hTERT-immortalised, integrating-vector–engineered cells, immunogenicity of xenogeneic CD enzyme, and systemic 5-fluorouracil leakage from flucytosine metabolism. Stem cell-delivered CD/5-FC and TRAIL constitutes a biologically rational, modular strategy for local cytotoxicity and resistance circumvention in CRPC. Successful clinical translation will require resolution of delivery-route feasibility, thrombosis risk mitigation, and a rigorous investigational new drug (IND)-enabling safety package.