N-Alkylated Monoterpene Indole Alkaloid Derivatives as Chemotherapy Adjuvants That Overcome P-glycoprotein-Mediated Multidrug Resistance in Cancer Cells, Spheroids and Patient-Derived Colorectal Cancer Organoids.

Chemotherapy efficacy is often limited by multidrug resistance (MDR), largely driven by P-glycoprotein (P-gp). This study aimed to evaluate previously prepared N-alkylated dregamine and tabernaemontanine derivatives as potential adjuvants to overcome P-gp-mediated chemoresistance. Twenty-one N-alkylated derivatives (1-21) were tested for cytotoxicity and chemosensitizing activity in sensitive and doxorubicin (DOX)-resistant ovarian (A2780, A2780DOX) and breast cancer cell lines (MCF-7, MCF-7R), as well as in nonmalignant kidney epithelial cells (hRPTEC). P-gp modulation was assessed using rhodamine 123 accumulation and Pgp-Glo Assay Systems kit. The activity of the most promising compound was further evaluated in 3D monocellular and multicellular spheroids, patient-derived colorectal cancer organoids (PDOs), and peripheral blood mononuclear cells (PBMCs). Computational docking was performed to support mechanistic insights. The derivatives exhibited moderate cytotoxicity toward cancer cells and low toxicity toward hRPTEC. All compounds sensitized resistant cell lines, with the most active ones reducing DOX IC50 by >20-fold at 10 μM and >8-fold at 1 μM. For most derivatives, increased accumulation of rhodamine 123 confirmed modulation of P-gp efflux activity. N-(4-methoxybenzyl) dregamine (compound 11) showed the strongest potential, enhancing DOX sensitivity >7-fold in monocellular and >4-fold in multicellular spheroids. In combination with DOX, compound 11 also increased the sensitivity of colorectal cancer PDOs, thereby reducing their viability from 70% to 30%. No increased toxicity of DOX was observed in PBMCs. N-alkylated monoterpene-indole alkaloids, particularly compound 11, effectively overcome P-gp-mediated chemoresistance without inducing cytotoxicity in healthy cells and represent promising candidates for further in vivo evaluation.