Mechanistic study of potentiation of chemotherapy by a haloenol lactone derivative in vitro

The objective of this study was to understand the biochemical mechanisms by which a haloenol lactone (HEL) derivative potentiates cisplatin-induced cytotoxicity in vitro. HEL was originally designed and synthesized as a site-directed inactivator of glutathione S-transferase π isozyme (GST-π). Over-expression of GST-π has been found to be associated with chemotherapy resistance. A concentration-dependent GST inhibition was assessed after UOK130 cells were exposed to HEL at concentrations of 10 and 20 μM. Potentiated cytotoxicity was evaluated by treatment of UOK130 cells with a selection of alkylating agents in the presence or absence of HEL. Intracellular glutathione (GSH) was determined after exposure to HEL. Protective effect of GSH was examined by co-treatment with GSH ester in UOK130 cells exposed with a combination of cisplatin and HEL. Multiple resistance-associated protein (MRP) 1–3 activity was assayed by determining the rate of 3H-LTC4 and 3H-E217βG through the MRPs into recombinant membrane vesicles. Exposure of HEL at 10 and 20 μM caused 28 and 41% of inhibition of cellular GST activity. Cytotoxicity of cisplatin, chlorambucil, and melphalan was enhanced 1.8–2.7-fold by HEL at 10 μM. No significant protection effect by GSH ester exposure was observed on cisplatin toxicity co-treated with HEL. HEL was found to inhibit MRP1, MRP2, and MRP3 with IC50 of 1.30, 28.2, and 3.66 μM, respectively. Haloenol lactone showed inhibitory effect on GST-π and MRP1-3 (selective inhibition of MRP1 and MRP3), and it was also found to deplete intracellular GSH.