Metabolic resistance to tight-binding inhibitors of enzymes involved in the de novo pyrimidine pathway Simulation of time-dependent effects. Simulation of time-dependent effects

Abstract

When a tight-binding inhibitor interacts with a target enzyme of a metabolic pathway, the end products of the pathway are depleted and the substrate(s) for the inhibited reaction accumulate. The accumulating substrate(s) can reach a concentration which is sufficiently high that the inhibition is effectively reversed, with restoration of the original flux through the inhibited reaction. We have recently developed a theoretical model for this phenomenon which we have called 'metabolic resistance' [R. I. Christopherson and R. G. Duggleby (1983) This simulation shows that on addition of BMP, there is a sequential depletion of all pyrimidine intermediates between UMP and dCDP and a concomitant accumulation of OMP. Eventually, OMP reaches a new steady-state concentration and the concentrations of the depleted intermediates rise to their original levels. We have simulated the depletion of dCDP at various concentrations of BMP; since dCDP is comitted to DNA synthesis we can integrate the dCDP concentrations over time to calculate the amount of DNA synthesis and thereby predict the delay in cell division which would be elicited by BMP. This form of analysis may help to explain in quantitative terms why inhibitors of nucleic acid biosynthesis have a selective toxicity for rapidly growing tumour cells. -3351.Of the approximately 85 enzymic reactions involved in the biosynthesis of pyrimidine and purine nucleotides via the de novo and salvage pathways and their subsequent polymerizations to nucleic acids, approximately 14 of the enzymes can be inhibited by metabolite analogues [l] which have a selective toxicity for rapidly growing cancer cells because of their more rapid rate of nucleic acid biosynthesis, relative to normal cells. The catalytic mechanisms of many of the enzymes involved in nucleic acid biosynthesis are now well understood which in turn enables rational design of potential tight-binding inhibitors of these pathways, with potential therapeutic value.Abbreviations. PAcAsp, N-phosphonacetyl-L-aspartatc; BMP, 1 -(5'-phospho-P-~-ribofuranosyl)barbituric acid ; FUra, 5-flUOrOuracil ; FdUMP, 5-fluorodeoxyuridine 5'-monophosphate; Cbm-P, carbamoyl phosphate; CbmAsp, N-carbamoyl-L-aspartate; hOro, L-5,6-dihydroorotate; Oro, orotate; P-Rib-PP, 5-phosphoribosyl-ldiphosphate.Enzymes. Carbamoyl phosphate synthetase (EC 6.3.5.5); aspartate transcarbamoylase (EC 2.1.3.2); dihydroorotase (EC 3.5.2.3); orotate phosphoribosyltransferase (EC 2.4.2.10); OMP decarboxylase (EC 4.1.1.23); nucleoside diphosphate kinase (EC 2.7.4.6); CTP synthetase (EC 6.3.4.2); ribonucleotide reductase (EC 1.17.4.1); thymidylate synthetase (EC 2.1 .I .45).Theory and techniques for the quantitative analysis of the interaction of a tight-binding inhibitor with a purified target enzyme in vitro have been established (see [2]). An equally important objective is to relate quantitative information obtained in vitro to the interaction of a tight-binding inhibitor with a target enzyme of a functioning metabolic pathway in a growing cell. In a previous pa...

Potent inhibitors of de novo pyrimidine and purine biosynthesis as chemotherapeutic agents

“…5). The drug should therefore block nucleotide biosynthesis for the time necessary for nucleotide pools to deplete plus the period of deprivation to ensure tumor cell death (58). After this period, the nucleotide pools should recover so that the patient survives.…”