Abstract 1229: The next-generation taxanes, SB-T-1214 and SB-CST-10202, exhibit distinct inhibitory effects on photolabeling of β-tubulin from different eukaryotic sources

Next-generation taxanes are known to be active against taxane-resistant cell lines that overexpress P-glycoprotein (P-gp) and/or βIII-tubulin, both of which are associated with drug resistance. In this study, we examined the effect of two potent next-generation taxanes, SB-T-1214 and SB-CST-10202 (C-seco-taxane), on growth of the ovarian cancer cell line Hey and its drug-resistant daughter cell lines that overexpress different levels of P-gp and express increased levels of βIII-tubulin. We found that SB-T-1214 was ∼6-9-fold more potent than SB-CST-10202 against these drug resistant cells. Since the taxanes bind to both tubulin and P-gp, the relative binding affinity of the taxanes to these two cellular targets may influence the effectiveness of the taxanes in resistant cells. To determine the effects of these two taxanes on binding of Taxol® to P-gp and tubulin, a tritium-labeled Taxol® analog, 2-(m-azidobenzoyl)taxol (2-m-AzTax), was used. We have previously demonstrated that 2-m-AzTax photolabels a peptide (amino acids 217-231) in β-tubulin. To study the relative binding affinities of SB-T-1214 and SB-CST-10202, tubulins from bovine brain (BBT) and chicken erythrocytes (CET) were specifically photolabeled with [3H]2-m-AzTax, in the presence and absence of either Taxol®, Taxotere®, SB-T-1214 or SB-CST-10202. β-tubulin isotype content from BBT and CET is different, the former contains βI-, βII-, βIII- and βIV-tubulins, but the latter has only βVI. Taxol® had a minimal inhibitory effect (∼3%) on BBT, but a strong inhibitory effect (99%) on CET. Taxotere® appeared to have a stronger inhibitory effect than Taxol® on photolabeling (31% and 99% for BBT and CET, respectively). The inhibitory effects elicited by the two next-generation taxanes on photolabeling were distinct for β-tubulins from these two sources. SB-T-1214 and SB-CST-10202 inhibited BBT photolabeling by 38% and 32%, respectively. Interestingly, these two drugs caused a 41% and only 6% inhibition, respectively, on photolabeling of CET that does not contain βIII-tubulin. We are currently analyzing the effect of SB-T-1214 on binding of [3H]2-m-AzTax to different tubulin isotypes resolved by isoelectrofocusing. To study photolabeling of P-gp, plasma membranes from a multidrug resistant (MDR) cell line SKVLB1 that expresses very high levels of P-gp were prepared. SB-T-1214 and SB-CST-10202 inhibited photolabeling of P-gp markedly (> 80% inhibition), suggesting that these two taxanes are good substrates for P-gp. They also caused an increase in steady state [3H]2-m-AzTax accumulation in the MDR cell line SKVLB1. Since drug binding to tubulin and P-gp is the primary event that occurs in the cell following drug administration, our data on taxane binding affinity to the two targets will help in understanding the effectiveness of the next-generation taxanes in taxane-resistance. Citation Format: Chia-Ping H. Yang, Changwei Wang, Iwao Ojima, Susan Band Horwitz. The next-generation taxanes, SB-T-1214 and SB-CST-10202, exhibit distinct inhibitory effects on photolabeling of β-tubulin from different eukaryotic sources. [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 1229.