Potent and specific inhibitors of mammalian phosphoribosylpyrophosphate (PRPP) synthetase

The genetic and functional basis of purine nucleotide feedback-resistant phosphoribosylpyrophosphate synthetase superactivity

The genetic and functional basis of phosphoribosylpyrophosphate synthetase (PRS) superactivity associated with purine nucleotide inhibitor-resistance was studied in six families with this X chromosome-linked purine metabolic and neurodevelopmental disorder. Cloning and sequencing of PRS1 and PRS2 cDNAs, derived from fibroblast total RNA of affected male patients by reverse transcription and PCR amplification, demonstrated that each PRS1 cDNA contained a distinctive single base substitution predicting a corresponding amino acid substitution in the PRS1 isoform. Overall, the array of substitutions encompassed a substantial portion of the translated sequence of PRS1 cDNA. Plasmid-mediated expression of variant PRS1 cDNAs in Escherichia coli BL21 (DE3/pLysS) yielded recombinant mutant PRS1s, which, in each case, displayed a pattern and magnitude of purine nucleoside diphosphate inhibitor-resistance comparable to that found in cells of the respective patient. Kinetic analysis of recombinant mutant PRS1s showed that widely dispersed point mutations in the X chromosome-linked PRPS1 gene encoding the PRS1 isoform result in alteration of the allosteric mechanisms regulating both enzyme inhibition by purine nucleotides and activation by inorganic phosphate. The functional consequences of these mutations provide a tenable basis for the enhanced production of phosphoribosylpyrophosphate, purine nucleotides, and uric acid that are the biochemical hallmarks of PRS superactivity.

Cytotoxicity and metabolism of 4-methoxy-8-(beta-D-ribofuranosylamino)pyrimido[5,4-d]pyrimidine in HCT 116 colon cancer cells

We examined the cytotoxicity, biochemical effects and metabolism of 4-methoxy-8-(beta-D-ribofuranosylamino)pyrimido[5,4-d]pyrimidine (MRPP), a synthetic nucleoside inhibitor of phosphoribosylpyrophosphate synthetase, in HCT 116 human colorectal cancer cells. A 4-hr exposure to 1 and 10 microM MRPP inhibited cell growth over a 72-hr period by 76 and 89%, and inhibited clonogenic capacity by 36 and 65%, respectively. MRPP was avidly metabolized to the 5'-monophosphate derivative (MRPP-MP), and MRPP-MP formation increased with increasing MRPP exposure (microM.hr). MRPP-MP was stable, and the intracellular half-life was in excess of 48 hr. A 4-hr exposure to 10 microM MRPP resulted in significant decreases in ATP, UTP, GTP, CTP, dATP, dTTP, and PRPP pools. Near maximal ribonucleotide triphosphate depletion was achieved with > or = 24 microM.hr MRPP, and growth inhibition as a function of MRPP microM.hr closely reflected the biochemical effects. Ribonucleotide triphosphate pools remained depleted for up to 48 hr after drug removal, apparently as a consequence of the prolonged retention of MRPP-MP. MRPP (10 microM) inhibited the salvage of [3H]guanine, [3H]adenine and [3H]guanosine, and concurrent exposure to MRPP and either 100 microM adenine, hypoxanthine, or guanine did not reverse ATP or GTP depletion. Concurrent exposure to 10 microM MRPP and either 10 microM adenosine, uridine or thymidine was accompanied by repletion of ATP, UTP, and dTTP pools, respectively, but depletion of other nucleotide pools was not corrected. In contrast, 10 microM guanosine did not correct GTP depletion in the presence of MRPP. The combination of 10 microM each of thymidine, uridine, adenosine and guanosine during and following a 24-hr exposure to MRPP provided partial protection against 0.1 or 1 microM MRPP, but did not affect the cytotoxicity associated with 10 microM MRPP. MRPP is a novel antimetabolite that inhibits both de novo and salvage pathways for purine synthesis and de novo pyrimidine synthesis.

Structural mimicry of adenosine by the antitumor agents 4-methoxy- and 4-amino-8-(beta-D-ribofuranosylamino)pyrimido[5,4-d]pyrimidine as viewed by a molecular modeling method

A rationale for the antitumor activity of 4-methoxy- and 4-amino-8-(beta-D-ribofuranosylamino)pyrimido-[5,4-d]pyrimidine (beta-MRPP and beta-ARPP, respectively) was studied by a molecular modeling method. Although these nucleoside analogues are structurally different from adenosine, they act as substrates for adenosine kinase. The molecular modeling method, which considered the three-dimensional structure and atom-based physicochemical properties of the nucleosides to quantify the molecular similarities, showed that certain low-energy conformations of the beta anomers of a series of nucleosides including beta-MRPP, beta-ARPP, and their 4-hydroxy, 4-amino-6-chloro, 4-methylthio-2,6-dichloro, 4,6-diamino, 4-dimethylamino, 4-methylamino, and 4-hydroxy-2,6-dichloro analogues have remarkable structural similarity to adenosine. The method also suggested that the selection of the reference compound adenosine in the structural comparison is of primary importance to gain insight into the observed antitumor activity. The success of the present method led to AM1 (Austin model 1) molecular orbital calculations and experimental studies indicating that the antitumor activity of the alpha anomer of ARPP is probably due to equilibration to the beta anomer. The AM1 calculation of the protonation energy of N5 of pyrimido[5,4-d]pyrimidines, which occupies the same position in space as the N1 of adenosine, gave a direct correlation between the basicity of the nitrogen with a lone pair of electrons and the observed antitumor activity.