Design, synthesis and evaluation of potential MTA prodrugs for the treatment of MTAP-deficient tumors in combination with 5-FU/6-TG/antifolates
Creators: name: Ranade, Ashwini
Publisher: name: No Publisher Supplied
Year: 2018

description: Many solid tumors and hematologic malignancies are characterized by a deficiency of the enzyme methylthioadenosine phosphorylase (MTAP). MTAP cleaves its natural substrate MTA which is generated during polyamine biosynthesis to adenine via a salvage pathway for AMP production. MTAP deficient cells are unable to salvage adenine from MTA. The difference in the lack of expression of MTAP in tumor cells compared to normal healthy cells has been explored in a therapeutic strategy for selectively killing tumor cells. Antimetabolites such as 5-FU, 6-TG or antifolates disrupt DNA replication and inhibit de-novo purine synthesis through the release of cytotoxic nucleotides generated via phosphorylation using phosphoribosyl-1-pyrophosphate (PRPP). Unfortunately, one major drawback of these cytotoxic nucleotides is that they produce harmful effects on the growth and proliferation of normal cells. To eliminate or reduce the cytotoxicity of these chemotherapeutic drugs and thus increase their therapeutic index, administration of MTA or MTA analogs in conjugation with antimetabolites could prevent damage to healthy MTAP competent cells. This is because adenine generated in this process by MTAP in healthy cells will block the conversion of 5-FU/6-TG to their cytotoxic nucleotides by competing for the rate-limiting pools of PRPP. Since no adenine is produced in tumor cells due to lack of MTAP, PRPP is present in sufficient levels and the co-administered drug can be readily converted to its toxic metabolite. Thus, a high degree of selectivity can be achieved. There are several MTA prodrugs discussed in the literature for delivering MTA to protect MTAP competent cells and proof of concept exists through in vitro studies emphasizing the protective effects of MTA in the presence of antimetabolites. However, there is still no successful clinical trial reported to have co-administered MTA or its analog with an antimetabolite. Also, the optimum dose of MTA that can rescue normal MTAP competent cells without compromising the ability of antimetabolites to selectively kill MTAP deficient tumor cells remains to be identified. In this thesis work, prodrugs of MTA were designed to be activated by the carboxylesterases to release MTA. We explored two types of prodrugs, namely the N-(alkyloxy)carbonyl-MTA derivatives and N-[(acyloxy)alkyloxy]carbonyl-MTA derivatives. The N-(alkyloxy)carbonyl-MTA prodrugs were stable at physiological pH and showed longer activation half-lives when tested in mouse liver microsomes but failed to be activated in human liver microsomes. Conversely, the N-[(acyloxy)alkyloxy]carbonyl-MTA prodrugs showed shorter half-lives ranging from a few minutes to a few hours in the presence of mouse and human liver microsomes but suffered from an inherent instability at physiological pH. As expected, the hydrolytic susceptibility of the ester group in N-[(acyloxy)alkyloxy]carbonyl-MTA prodrugs decreased with increasing steric hindrance around the ester bond by replacement with bulkier alkyl groups. Therefore, the N-[(acyloxy)alkyloxy]carbonyl-MTA prodrugs show promise and further modifications can be made to increase stability at physiological pH.