In the present study both experimental and theoretical studi
In the present study, both experimental and theoretical studies were performed to explore the inhibitory mechanisms of indomethacin and its analogues towards GLOI. A remarkable correlation (=0.974) was derived for the four structurally similar NSAIDs and two curcumins between the experimental binding data ) and the theoretically estimated counterparts (Δ) by means of the MM-PBSA method, which suggests that Δ of a GLOI/inhibitor complex can be used to prognosticate the experimental inhibitory affinity towards GLOI of a ligand of similar structural features. Energetic analysis revealed that electrostatic contribution (Δ) plays an important role in their inhibitory mechanisms, which reflects the significance of the coordination bond between the zinc sirreal receptor and ligand.
The aldehyde of pyruvic acid, methylglyoxal, has been the subject of a large number of investigations due to its cytotoxicity. This cytotoxic metabolite is transiently formed in a myriad of metabolic pathways, for example, lipid peroxidation, glycolysis, and DNA metabolism. The removal of this ketoaldehyde from the body occurs through the glyoxalase pathway involving the cofactor glutathione. Two key enzymatic steps are involved in this process (): the isomerization of the hemithioacetal of methylglyoxal and glutathione (GSH) to --lactoylglutathione (by glyoxalase-I) and its subsequent glyoxalase-II catalyzed transformation to -lactate, that is then actively transported out of the cell and oxidized back to pyruvate which enters Kreb’s cycle. This cascade limits the use of α-ketoaldehydes as potential antitumor agents. In 1969, our laboratory was the first to propose the obstruction of the glyoxalase pathway as a means to cause accumulation of methylglyoxal in cancer cells, eventually leading to their death. We subsequently reported the competitive nature of -alkylglutathione analogs toward glyoxalase-I (Glx-I) and the discovery of --bromobenzyl glutathione (PBBG, ) () as a highly potent Glx-I inhibitor. Because of their charged nature, these inhibitors lacked sufficient cell penetration to exhibit antitumor activity. In addition, loss of enzyme inhibition occurred through the rapid hydrolysis of the glu–cys amide bond of the glutathione scaffold by γ-glutamyltranspeptidase (). Years later Lo and Thornalley used ester prodrugs of our PBBG (), which were able to penetrate cell membranes and inhibit the growth of human leukemia 60 (HL-60) cells. One of the shortcomings of these -alkylglutathiones, otherwise-promising Glx-I inhibitors, was their inability to selectively inhibit tumor cells when compared to normal human cells. This suggested that simple competitive inhibitors of glyoxalase are perhaps inadequate tumor-selective agents. This shortcoming was addressed by Creighton and Murthy through the design of hydroxamic acid-based transition state inhibitors (, for example). The rationale behind this design was thus: the hydroxamic acid moiety acts as a close mimic of the enediol(ate) intermediate of the Glx-I catalytic cycle (), thus conferring upon these molecules a high Glx-I inhibitory potency. Incorporation of a thioester linkage into these compounds caused them to resemble the glyoxalase-II (Glx-II) substrate (--lactoylglutathione, ). Since the Glx-II activity is abnormally low in certain types of cancer cells, compared to normal cells, Creighton and Murthy, suggested that the inactivation of compounds like by Glx-II in tumor cells will be very low, thus rendering the desired tumor-selectivity from their reduced ability to hydrolyze these inhibitors. Although these inhibitors were in fact able to restrict the growth of solid tumors in mice when administered as their ester prodrugs, the problem of their breakdown by γ-glutamyltranspeptidase still remained. Recently, we initiated a project to revisit the design and synthesis of these inhibitors with an aim to address various pharmacokinetic, metabolic, and synthetic problems that plague the development of this class of molecules into clinically useful antitumor agents. In this report, we wish to describe the application of our urea-isostere strategy to prevent breakdown of the hydroxamic acid inhibitors by γ-glutamyltranspeptidase. Incorporation of the urea linkage and the hydroxamic acid transition-state mimic into gave the tripeptide (). Here, the urea-isostere was expected to provide metabolic stability to the glyoxalase-I inhibitor along with conservation of glyoxalase-I inhibitory activity. A good body of experimental evidence exists for the presence of a significantly large hydrophobic pocket in the active site of Glx-I, with which the S--bromobenzyl substituent of PBBG () interacts., This led us to conserve that particular pharmacophore. We then set forth to develop an efficient synthetic route to the tripeptide backbone and the thiocarbamate linkage of compound .