Development of effective anti-cancer agents targeting drug-resistant malignancies

Abstract

Rapid development of multidrug resistance (MDR) against current therapies is a major barrier in the treatment of cancer. Over-expression of anti-apoptotic Bcl-2 proteins has been implicated in development of cancer and drug resistance. Hence, our research has focused on developing anti-apoptotic Bcl-2 inhibitors that can eliminate drug-resistant cancer cells. HA 14-1 was the first small-molecule reported to inhibit anti-apoptotic Bcl-2 proteins and selectively eliminated cancer cells over-expressing anti-apoptotic Bcl-2 proteins. However, previous work in our lab revealed that HA 14-1 is unstable and decomposes within 15 min in cell culture medium at 37°C. Therefore, the main aim of this thesis was to develop a stable, more potent analog of HA 14-1, which would retain its beneficial properties. In the current work we were able to design a stable analog of HA 14-1 (sHA 14-1) by removing the cyano group from HA 14-1. Although sHA 14-1 had beneficial properties like HA 14-1, it was 2-fold less potent in activity. Further SAR studies carried out to improve the activity of sHA 14-1 led to the discovery of CXL017, which was ~27 fold more potent than sHA 14-1 and ~12 fold more potent than HA 14-1. Biological studies on CXL017 revealed that CXL017 causes cell death via apoptosis characterized by caspase-3/7 activation and PARP cleavage. Furthermore, CXL017 selectively eliminated drug-resistant cancer cells both in vitro and in vivo. These interesting results, prompted us to investigate the effects of long-term exposure to CXL017 in drug-resistant cancer cells. Our results show that drug-resistant cells do not develop stable resistance to CXL017 upon prolonged exposure for 6 months. In addition, drug-resistant cells exposed to CXL017 for 4 months develop collateral sensitivity towards a variety of standard therapies. Mechanistic investigations suggests that the re-sensitization observed in drug-resistant cells exposed to CXL017 is caused due to the down-regulation of Mcl-1 and a decrease in ER calcium content in these cell lines. Taken together, the results demonstrated in this work show that CXL017 is a more potent stable analog of HA 14-1, which merits further investigation for its potential as a drug candidate for the treatment of MDR cancers.