ZRX1, the first EGFR inhibitor-capecitabine based combi-molecule, requires carboxylesterase-mediated hydrolysis for optimal activity

Capecitabine, an orally available prodrug of 5-FU, requires activation by carboxylesterase (CES) enzymes present in the liver to generate 5′-deoxy-5-flurocytidine ribose (5′-DFCR). The deamination of the latter by cytidine deaminase gives 5′-deoxy-5-fluorouridine ribose (5′-DFUR). Finally, the conversion of 5′-DFUR to the cytotoxic drug 5-FU, occurs primarily in the tumour and is catalyzed by thymidine phosphorylase (TP). Accordingly, it was surmised that events associated with an increase of TP levels should enhance the potency of capecitabine and its metabolites. EGFR inhibition was found to be one such event. The observed synergy between gefitinib and 5′-DFUR has inspired the design of single molecules capable of acting as prodrugs of both an EGFR inhibitor and 5-FU. Here, we report on the synthesis and characterization of one such molecule, ZRX1, that consists of an acetylated 5′-DFCR moiety linked to a quinazoline inhibitor of EGFR through an alkyl dicarbamate spacer that requires CES activation to generate the two active metabolites. Our results showed that ZRX1 was ineffective as an intact molecule. However, when CES was present, ZRX1 induced an increase in EGFR inhibition, TP expression, DNA damage and apoptosis. ZRX1 was, at least, 3-fold more potent than capecitabine and 5′-DFUR and recapitulated the effects of the combination treatments. LC-MS analysis showed that in the presence of CES, ZRX1 is metabolized into a mixture of bioactive quinazoline derivatives and 5′-DFCR derived metabolites. Our results in toto, suggest that capecitabine-based EGFR targeting combi-molecules of the same type than ZRX1, have the potential to induce stronger growth inhibitory potency than capecitabine, 5′-DFUR or single EGFR inhibitors and equivalent potency when compared with combinations of EGFR inhibitors + 5′-DFUR.