Abstract 2140: Cabazitaxel is more active than first generation taxanes in ABCB1(+) cell lines due to its higher intracellular accumulation and reduction in ATPase stimulation

Abstract

Abstract The taxanes paclitaxel (Taxol) and docetaxel (Taxotere) have substantial clinical activity in breast, ovarian, lung, and other cancers, but their clinical efficacy is limited by preexisting or acquired drug resistance, such as the expression of the ATP-dependent multidrug resistance (MDR) transporter P-glycoprotein (P-gp). Cabazitaxel (Jevtana), the dimethoxy derivative of docetaxel, was identified because of its activity in taxane-resistant models both in vitro and in vivo. Our studies confirm that cabazitaxel is more active in cell variants that express P-gp, and the objective of this study was to investigate cabazitaxel's affinity for the transporter. Taxane accumulation patterns were studied in ABCB1(+) variants using [14C]-radiolabeled docetaxel and cabazitaxel over a time course up to 1 h. Time points were collected and spun (10,000 x g, 1 min) through Nyosil M20 oil thereby terminating uptake, cell pellets lysed with 2% (w/v) SDS, and counts determined by liquid scintillation, with all measurements normalized to protein content. The kinetics of drug accumulation between the two taxanes is different, with the maximum intracellular drug concentration achieved faster with cabazitaxel (5 min) than docetaxel (15-30 min) in all cell lines. ABCB1(+) variants accumulated less taxane, and these levels could be restored to parental levels in the presence of known P-gp inhibitors (2 μM PSC-833, valspodar). However, the MDR cell models tested accumulated twice as much cabazitaxel than docetaxel under identical experimental conditions. Efflux in drug-free medium confirmed that ABCB1(+) variants retained 2.2x more cabazitaxel than docetaxel. We observed a strong association (r2 = 0.91) between the degree of taxane resistance conferred by P-gp expression and the accumulation differences observed with the two taxanes. Several low P-gp-expressing cell models were not cross-resistant to cabazitaxel while demonstrating resistance to docetaxel. Furthermore, we determined ATPase stimulation in membranes isolated from our MDR cell models as an indirect measure of P-gp activity following taxane treatment. We observed a 1.9x reduction in sodium orthovanadate-sensitive ATPase stimulation resulting from treatment with cabazitaxel than with docetaxel (1 μM for 30 min). Future experiments will include direct photoaffinity labeling of P-gp with [3H]-azido-cabazitaxel and docetaxel, and competition assays with known P-gp substrates. Our studies indicate that the improved activity of cabazitaxel in MDR models might be due to its reduced affinity for P-gp compared to docetaxel. Citation Format: George E. Duran, Dietmar Weitz, Dorothée Sémiond, Diego Gianolio, Sandrine Macé, Branimir I. Sikic. Cabazitaxel is more active than first generation taxanes in ABCB1(+) cell lines due to its higher intracellular accumulation and reduction in ATPase stimulation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2140.