Phaeosphaeride A Isolated from an Endophytic Paraphoma sp. Alleviates ABCG2-mediated Resistance to Mitoxantrone in Breast Cancer Cells

The efficacy of breast cancer chemotherapies is frequently limited by multidrug resistance (MDR), partly through efflux by ABC transporters, including ABCG2. This study evaluated whether phaeosphaeride A (PPA), a fungal metabolite isolated from a <i>Paraphoma</i> sp. endophyte of <i>Ferula xylorhachis</i>, can modulate ABCG2-mediated resistance to mitoxantrone (MTX). The endophyte was cultured and extracted with ethyl acetate, and the extract was purified by column chromatography and HPLC to yield PPA, whose structure was confirmed by NMR and MS analyses. Cytotoxicity of MTX, PPA, and their combination was assessed in MCF-7- and ABCG2-overexpressing MCF-7/MX cells. MTX showed marked differential cytotoxicity (IC₅₀ = 1.6 µM, 95% CI: 1.4 - 1.9 in MCF-7 vs. > 25 µM in MCF-7/MX; p < 0.0001), whereas PPA exhibited comparable activity in both lines (23.2 µM, 95% CI: 18.0 - 30.1 vs. 36.1 µM, 95% CI: 28.7 - 46.0; p = 0.01). Co-treatment with PPA IC₅₀ significantly reduced MTX IC₅₀ to 0.4 µM (95% CI: 0.3 - 0.6) in MCF-7 and 1.9 µM (95% CI: 1.2 - 2.7) in MCF-7/MX, restoring MTX sensitivity in resistant cells to near MCF-7 levels. Flow cytometry showed that PPA (IC₅₀) increased intracellular MTX accumulation with stronger effects in MCF-7/MX cells (p < 0.0001) than in MCF-7 (p < 0.05). In combination with MTX, PPA (IC₅₀) increased Sub-G1 apoptotic populations in both lines.These findings demonstrate that PPA is unlikely to be a substrate of ABCG2 but functionally inhibits ABCG2-mediated efflux, contributing to the restoration of MTX sensitivity, although there may be additional mechanisms involved. PPA could be a promising MDR-reversal agent in ABCG2-driven chemotherapy resistance.