Investigation of CA-4 metabolism and related β-lactam analogues in chemoresistant HT-29 colon cancer cells

Drug resistance is a common cause of the failure of chemotherapeutic agents to achieve cytotoxicity responses in human malignant disease. Drug inactivation by metabolism within tumour cells is recognised as an important mechanism of drug resistance. Glucuronidation is a major route for the metabolic inactivation of many drugs and also endogenous substances. Combretastatin-A4 (CA-4) undergoes direct glucuronidation in the presence of UGTs at the meta-hydroxy group of the B-ring and could cause an inherent resistance in HT-29 colon cancer cells. Here, we assessed the strategic deletion of the ring B hydroxyl group to produce CA-4 analogues that are equally effective in cancer cells expressing UGTs as compared to those expressing little or undetectable levels of UGTs, offering a simple solution to overcoming resistance associated with glucuronidation of CA-4. These compounds play a dual role by improving the stability by blocking the isomerisation of the CA-4 olefin bridge and overcoming the resistance in HT-29 colon cancer cells by improving the metabolic stability. The stability of CA-4 and its β-lactam analogue in HT-29 cells in the absence and presence of many different inhibitors of UGT enzymes (propofol, Borneol, bile acids, U0126 and 4-nitrophenol) was examined. Collectively, these data suggest a key role of UGT in mediating the resistance effect of CA-4 in HT-29 cells and provides a rationale to improve the therapeutic efficacy of CA-4 and its related analogues.