Approaches to soft drug analogues of dihydrofolate reductase inhibitors Design and synthesis

Abstract: The main objective of the research described in this thesis has been the design and synthesis of inhibitors of the enzyme dihydrofolate reductase (DHFR) intended for local administration and devoid of systemic side-effects. The blocking of the enzymatic activity of DHFR is a key element in the treatment of many diseases, including cancer, bacterial and protozoal infections, and also opportunistic infections associated with AIDS (Pneumocystis carinii pneumonia, PCP). Recent research indicates that the enzyme also is involved in various autoimmune diseases, e.g., rheumatoid arthritis, inflammatory bowel diseases and psoriasis. Many useful antifolates have been developed to date although problems remain with toxicity and selectivity, e.g., the well-established, classical antifolate methotrexate exerts a high activity but also high toxicity. The new antifolates described herein were designed to retain the pharmacophore of methotrexate, but encompassing an ester group, so that they also would serve as substrates for the endogenous hydrolytic enzymes, e.g., esterases. Such antifolates would optimally comprise good examples of soft drugs because they in a controlled fashion would be rapidly and predictably metabolized to non-toxic metabolites after having exerted their biological effect at the site of administration.A preliminary screening of a large series of simpler aromatic esters as model compounds in a biological assay consisting of esterases from different sources was performed. The structural features of the least reactive ester were substituted for the methyleneamino bridge in methotrexate to produce analogues that were chemically stable but potential substrates for DHFR as well as for the esterases.The new inhibitor showed desirable activity towards rat liver DHFR, being only eight times less potent then methotrexate. Furthermore, the derived metabolites were found to be poor substrates for the same enzyme. The new compound showed good activity in a mice colitis model in vivo, but a pharmacokinetic study revealed that the half-life of the new compound was similar to methotrexate. A series of compounds characterized by a high lipophilicity and thus expected to provide better esterase substrates were designed and synthesized. One of these analogues in which three methoxy groups were substituted for the glutamic residue of methotrexate exhibited favorable pharmacokinetics. This compound is structurally similar to another potent DHFR inhibitor, trimetrexate, used in the therapy of PCP (vide supra). The new inhibitor that undergoes a fast metabolism in vivo is suitable as a model to further investigate the soft drug concept.