Synthesis and biological evaluation of Phorbazole D analogues identify IO-5a as a promoter of hUC-MSC chondrogenic differentiation.

Mesenchymal stem cells (MSCs) are a promising cell source for osteoarthritis and intervertebral disc degeneration, but their application is limited by inefficient and unstable chondrogenic differentiation. Here, nine Phorbazole D analogues were synthesized and evaluated for promoting chondrogenesis in human umbilical cord mesenchymal stem cells (hUC-MSCs) under TGF-β/dexamethasone based induction conditions. Among the tested analogues, IO-5a was selected for further evaluation based on its comparatively higher pro-chondrogenic activity and no detectable cytotoxicity. Image-based Alcian blue analysis showed that the positive staining area increased from approximately 17% in the control group to approximately 45% after IO-5a treatment. RT-qPCR further showed that 10 µM IO-5a upregulated COL2A1, SOX9, ACAN, and COMP by approximately 2.6-fold, 1.1-fold, 1.35-fold, and 1.4-fold, respectively. Immunofluorescence, western blotting, and 3D pellet culture analyses supported increased cartilage associated marker expression and cartilage like matrix deposition. Preliminary structure-activity relationship analysis suggested that retention of the A-ring phenolic hydroxyl group and favorable A-/C-ring substituent matching may contribute to activity. In silico target prediction and molecular docking nominated PTPN1 (PTP1B) as a potential candidate protein, although this predicted interaction requires further experimental validation. Collectively, IO-5a represents a bioactive Phorbazole D analogue with chondrogenesis promoting activity in hUC-MSCs.