Inhibition of Human Purine Nucleoside Phosphorylase

Allopurinol and oxypurinol differ in their strength and mechanisms of inhibition of xanthine oxidoreductase

Xanthine oxidoreductase is a metalloenzyme that catalyzes the final steps in purine metabolism by converting hypoxanthine to xanthine and then uric acid. Allopurinol, an analog of hypoxanthine, is widely used as an antigout drug, as xanthine oxidoreductase-mediated metabolism of allopurinol to oxypurinol leads to oxypurinol rotation in the enzyme active site and reduction of the molybdenum Mo(VI) active center to Mo(IV), inhibiting subsequent urate production. However, when oxypurinol is administered directly to a mouse model of hyperuricemia, it yields a weaker urate-lowering effect than allopurinol. To better understand its mechanism of inhibition and inform patient dosing strategies, we performed kinetic and structural analyses of the inhibitory activity of oxypurinol. Our results demonstrated that oxypurinol was less effective than allopurinol both in vivo and in vitro. We show that upon reoxidation to Mo(VI), oxypurinol binding is greatly weakened, and reduction by xanthine, hypoxanthine, or allopurinol is required for reformation of the inhibitor-enzyme complex. In addition, we show oxypurinol only weakly inhibits the conversion of hypoxanthine to xanthine and is therefore unlikely to affect the feedback inhibition of de novo purine synthesis. Furthermore, we observed weak allosteric inhibition of purine nucleoside phosphorylase by oxypurinol which has potentially adverse effects for patients. Considering these results, we propose the single-dose method currently used to treat hyperuricemia can result in unnecessarily high levels of allopurinol. While the short half-life of allopurinol in blood suggests that oxypurinol is responsible for enzyme inhibition, we anticipate multiple, smaller doses of allopurinol would reduce the total allopurinol patient load.

Predicting target-ligand interactions with graph convolutional networks for interpretable pharmaceutical discovery

Drug Discovery is an active research area that demands great investments and generates low returns due to its inherent complexity and great costs. To identify potential therapeutic candidates more effectively, we propose protein–ligand with adversarial augmentations network (PLA-Net), a deep learning-based approach to predict target–ligand interactions. PLA-Net consists of a two-module deep graph convolutional network that considers ligands’ and targets’ most relevant chemical information, successfully combining them to find their binding capability. Moreover, we generate adversarial data augmentations that preserve relevant biological backgrounds and improve the interpretability of our model, highlighting the relevant substructures of the ligands reported to interact with the protein targets. Our experiments demonstrate that the joint ligand–target information and the adversarial augmentations significantly increase the interaction prediction performance. PLA-Net achieves 86.52% in mean average precision for 102 target proteins with perfect performance for 30 of them, in a curated version of actives as decoys dataset. Lastly, we accurately predict pharmacologically-relevant molecules when screening the ligands of ChEMBL and drug repurposing Hub datasets with the perfect-scoring targets.

Crystal structure of Escherichia coli purine nucleoside phosphorylase in complex with 7-deazahypoxanthine

Purine nucleoside phosphorylases (EC 2.4.2.1; PNPs) reversibly catalyze the phosphorolytic cleavage of glycosidic bonds in purine nucleosides to generate ribose 1-phosphate and a free purine base, and are key enzymes in the salvage pathway of purine biosynthesis. They also catalyze the transfer of pentosyl groups between purine bases (the transglycosylation reaction) and are widely used for the synthesis of biologically important analogues of natural nucleosides, including a number of anticancer and antiviral drugs. Potent inhibitors of PNPs are used in chemotherapeutic applications. The detailed study of the binding of purine bases and their derivatives in the active site of PNPs is of particular interest in order to understand the mechanism of enzyme action and for the development of new enzyme inhibitors. Here, it is shown that 7-deazahypoxanthine (7DHX) is a noncompetitive inhibitor of the phosphorolysis of inosine by recombinant Escherichia coli PNP (EcPNP) with an inhibition constant Ki of 0.13 mM. A crystal of EcPNP in complex with 7DHX was obtained in microgravity by the counter-diffusion technique and the three-dimensional structure of the EcPNP-7DHX complex was solved by molecular replacement at 2.51 Å resolution using an X-ray data set collected at the SPring-8 synchrotron-radiation facility, Japan. The crystals belonged to space group P6122, with unit-cell parameters a = b = 120.370, c = 238.971 Å, and contained three subunits of the hexameric enzyme molecule in the asymmetric unit. The 7DHX molecule was located with full occupancy in the active site of each of the three crystallographically independent enzyme subunits. The position of 7DHX overlapped with the positions occupied by purine bases in similar PNP complexes. However, the orientation of the 7DHX molecule differs from those of other bases: it is rotated by ∼180° relative to other bases. The peculiarities of the arrangement of 7DHX in the EcPNP active site are discussed.

Forodesine: review of preclinical and clinical data

Purine nucleoside phosphorylase (PNP) is an important catalytic enzyme in the purine salvage pathway; its deficiency is associated with T-cell lymphopenia and with humoral deficiency. This clinical observation led to the investigation of PNP inhibitors and their possible clinical application in the management of hematologic malignancies, notably those of T-cell lineage. Forodesine is the most potent of the PNP inhibitors. Its effect appears to be linked to increased 2 -deoxyguanosine levels in plasma, which in turn is converted to 2 -deoxyguanosine triphosphate in target cells and disrupts DNA synthesis. Several preclinical studies have shown forodesine's effect against lymphocytes in vitro and in vivo, and these findings have led to several Phase I/II studies in patients with lymphoid neoplasms. Early clinical trials show that forodesine has promise as a single agent for the treatment of relapsed/refractory hematologic malignancies, and combination therapies might be warranted to improve clinical results.

Influence of bone marrow stromal microenvironment on forodesine-induced responses in CLL primary cells

Forodesine, a purine nucleoside phosphorylase inhibitor, displays in vitro activity in chronic lymphocytic leukemia (CLL) cells in presence of dGuo, which is the basis for an ongoing clinical trial in patients with fludarabine-refractory CLL. Initial clinical data indicate forodesine has significant activity on circulating CLL cells, but less activity in clearing CLL cells from tissues such as marrow. In tissue microenvironments, lymphocytes interact with accessory stromal cells that provide survival and drug-resistance signals, which may account for residual disease. Therefore, we investigated the impact of marrow stromal cells (MSCs) on forodesine-induced response in CLL lymphocytes. We demonstrate that spontaneous and forodesine-induced apoptosis of CLL cells was significantly inhibited by human and murine MSCs. Forodesine-promoted dGuo triphosphate (dGTP) accumulation and GTP and ATP depletion in CLL cells was inhibited by MSCs, providing a mechanism for resistance. Also, MSCs rescued CLL cells from forodesine-induced RNA- and protein-synthesis inhibition and stabilized and increased Mcl-1 transcript and protein levels. Conversely, MSC viability was not affected by forodesine and dGuo. Collectively, MSC-induced biochemical changes antagonized forodesine-induced CLL cell apoptosis. This provides a biochemical mechanism for MSC-derived resistance to forodesine and emphasizes the need to move toward combinations with agents that interfere with the microenvironment's protective role for improving current therapeutic efforts.

Pharmacology and mechanism of action of forodesine, a T-cell targeted agent

Purine nucleoside phosphorylase (PNP) was recognized more than 30 years ago as a potential target for the treatment of patients with T-cell malignancies when an inherited deficiency of PNP was reported to be associated with a profound T-cell lymphopenia. The biochemical basis for this T-cell deficiency was subsequently shown to be related to the accumulation of plasma 2'-deoxyguanosine (dGuo) and intracellular dGuo triphosphate (dGTP). These observations have led to a search for PNP inhibitors that would be useful clinically in the management of T cell-derived malignancies. The most potent inhibitor of PNP described to date is forodesine, a rationally designed, transition-state analogue inhibitor. The preclinical and clinical pharmacology of forodesine showed its effectiveness in inhibiting PNP and augmenting dGuo levels in plasma. Increased dGTP concentrations in leukemia cells of different lineages provides strong support for the potential use of this agent in the treatment of patients with hematologic malignancies of both T- and B-cell origin.

Novel purine nucleoside analogues for T-cell-lineage acute lymphoblastic leukaemia and lymphoma

Purine nucleoside phosphorylase (PNP) deficiency is a rare, inherited immunodeficiency disorder in which the specific molecular defect was identified. Clinically, a lack of PNP manifests as profound T-cell deficiency with minor or variable changes in the humoral system. Biochemically, the absence of PNP results in an increase in plasma deoxyguanosine (dGuo) and a T-cell-specific increase in intracellular deoxyguanosine triphosphate (dGTP). This observation has been the impetus for the search for either inhibitors of the enzyme or PNP-resistant dGuo analogues as potential anti-T-cell-lineage agents over the past 30 years. Forodesine (an inhibitor of PNP) and nelarabine (a PNP-resistant dGuo analogue) proved to be T-cell selective when tested in clinic. This review summarises the preclinical, clinical and pharmacokinetic investigations with these novel agents.

A proof-of-principle pharmacokinetic, pharmacodynamic, and clinical study with purine nucleoside phosphorylase inhibitor immucillin-H (BCX-1777, forodesine)

The discovery of purine nucleoside phosphorylase (PNP) deficiency and T lymphocytopenia suggested that inhibition of this enzyme could serve as a therapeutic target. Inhibitors of PNP failed until structure-based synthesis of immucillin-H (BCX-1777, forodesine), a transition-state analog of PNP. The picomolar potency for PNP, T cell-selective cytotoxicity, and animal studies provided the rationale for use of forodesine in T-cell malignancies. Five patients were treated with an intravenous infusion of forodesine (40 mg/m2) on day 1; treatment continued on day 2; forodesine was administered every 12 hours for an additional 8 doses. Plasma and cellular pharmacokinetics and pharmaco-dynamics were investigated. Median peak level of forodesine (5.4 microM) was achieved at the end of infusion. This level was sufficient to increase plasma 2'-deoxyguanosine (dGuo) concentrations in all patients. Intracellular deoxyguanosine triphosphate (dGTP) increased by 2- to 40-fold in 4 of 5 patients (8 of 9 courses) and correlated with antileukemia activity in 4 patients. However, objective responses were not observed. This was the first clinical study in humans to demonstrate the plasma pharmacokinetics and the pharmacodynamic effectiveness of the PNP inhibitor, forodesine; however, regrowth of leukemia cells in the blood and marrow after course 1 suggested that a different therapeutic schedule should be considered for future studies.

Selectivity of febuxostat, a novel non-purine inhibitor of xanthine oxidase/xanthine dehydrogenase

The purine analogue, allopurinol, has been in clinical use for more than 30 years as an inhibitor of xanthine oxidase (XO) in the treatment of hyperuricemia and gout. As consequences of structural similarities to purine compounds, however, allopurinol, its major active product, oxypurinol, and their respective metabolites inhibit other enzymes involved in purine and pyrimidine metabolism. Febuxostat (TEI-6720, TMX-67) is a potent, non-purine inhibitor of XO, currently under clinical evaluation for the treatment of hyperuricemia and gout. In this study, we investigated the effects of febuxostat on several enzymes in purine and pyrimidine metabolism and characterized the mechanism of febuxostat inhibition of XO activity. Febuxostat displayed potent mixed-type inhibition of the activity of purified bovine milk XO, with Ki and Ki' values of 0.6 and 3.1 nM respectively, indicating inhibition of both the oxidized and reduced forms of XO. In contrast, at concentrations up to 100 muM, febuxostat had no significant effects on the activities of the following enzymes of purine and pyrimidine metabolism: guanine deaminase, hypoxanthine-guanine phosphoribosyltransferase, purine nucleoside phosphorylase, orotate phosphoribosyltransferase and orotidine-5'-monophosphate decarboxylase. These results demonstrate that febuxostat is a potent non-purine, selective inhibitor of XO, and could be useful for the treatment of hyperuricemia and gout.

Xanthosine and xanthine. Substrate properties with purine nucleoside phosphorylases, and relevance to other enzyme systems

Substrate properties of xanthine (Xan) and xanthosine (Xao) for purine nucleoside phosphorylases (PNP) of mammalian origin have been reported previously, but only at a single arbitrarily selected pH and with no kinetic constants. Additionally, studies have not taken into account the fact that, at physiological pH, Xao (pKa = 5.7) is a monoanion, while Xan (pKa = 7.7) is an equilibrium mixture of the neutral and monoanionic forms. Furthermore the monoanionic forms, unlike those of guanosine (Guo) and inosine (Ino), and guanine (Gua) and hypoxanthine (Hx), are still 6-oxopurines. The optimum pH for PNP from human erythrocytes and calf spleen with both Xao and Xan is in the range 5-6, whereas those with Guo and Gua, and Ino and Hx, are in the range 7-8. The pH-dependence of substrate properties of Xao and Xan points to both neutral and anionic forms as substrates, with a marked preference for the neutral species. Both neutral and anionic forms of 6-thioxanthine (pKa = 6.5 +/- 0.1), but not of 2-thioxanthine (pKa = 5.9 +/- 0.1), are weaker substrates. Phosphorolysis of Xao to Xan by calf spleen PNP at pH 5.7 levels off at 83% conversion, due to equilibrium with the reverse synthetic pathway (equilibrium constant 0.05), and not by product inhibition. Replacement of Pi by arsenate led to complete arsenolysis of Xao. Kinetic parameters are reported for the phosphorolytic and reverse synthetic pathways at several selected pH values. Phosphorolysis of 200 micro m Xao by the human enzyme at pH 5.7 is inhibited by Guo (IC50 = 10 +/- 2 micro m), Hx (IC50 = 7 +/- 1 micro m) and Gua (IC50 = 4.0 +/- 0.2 micro m). With Gua, inhibition was shown to be competitive, with Ki = 2.0 +/- 0.3 micro m. By contrast, Xao and its products of phosphorolysis (Xan and R1P), were poor inhibitors of phosphorolysis of Guo, and Xan did not inhibit the reverse reaction with Gua. Possible modes of binding of the neutral and anionic forms of Xan and Xao by mammalian PNPs are proposed. Attention is directed to the fact that the structural properties of the neutral and ionic forms of XMP, Xao and Xan are also of key importance in many other enzyme systems, such as IMP dehydrogenase, some nucleic acid polymerases, biosynthesis of caffeine and phosphoribosyltransferases.

Purine nucleoside phosphorylase inhibitor BCX-1777 (Immucillin-H)--a novel potent and orally active immunosuppressive agent

Patients with purine nucleoside phosphorylase (PNP) deficiency present a selective T-cell immunodeficiency. Inhibitors of PNP are, therefore, of interest as potential T-cell selective immunosuppressive agents. BCX-1777 is a potent inhibitor of PNP from various species including human, mouse, rat, monkey and dog, with IC50 values ranging from 0.48 to 1.57 nM. BCX-1777, in the presence of 2'-deoxyguanosine (dGuo, 3-10 microM), inhibits human lymphocyte proliferation activated by various agents such as interleukin-2 (IL-2), mixed lymphocyte reaction (MLR) and phytohemagglutinin (PHA) (IC50 values < 0.1-0.38 microM). BCX-1777 is a 10-100-fold more potent inhibitor of human lymphocyte proliferation than other known PNP inhibitors like PD141955 and BCX-34. Nucleotide analysis of human lymphocytes indicate that inhibition of proliferation by BCX-1777 correlates with dGTP levels in the cells. BCX-1777 has excellent oral bioavailability (63%) in mice. At a single dose of 10 mg/kg in mice, BCX-1777 elevates dGuo to approximately 5 microM. BCX-1777 was not effective in mouse T-cell models such as delayed type hypersensitivity (DTH) and splenomegaly because mouse T-cells do not accumulate dGTP as do human T-cells. However, in the human peripheral blood lymphocyte severe combined immunodeficiency (hu-PBL-SCID) mouse model, BCX-1777 was effective in prolonging the life span 2-fold or more. This is the first known example of a PNP inhibitor that elevates dGuo in mice similar to the levels observed in PNP-deficient patients. Furthermore, these dGuo levels are also required for in vitro T-cell inhibition by BCX-1777. Thus, BCX-1777 represents a novel class of selective immunosuppressive agents that could have therapeutic utility in various T-cell disorders.

Interaction of Escherichia coli purine nucleoside phosphorylase (PNP) with the cationic and zwitterionic forms of the fluorescent substrate N(7)-methylguanosine

Steady-state and time-resolved fluorescence spectroscopy, and enzyme kinetics, were applied to study the reaction of purine nucleoside phosphorylase (PNP) from Escherichia coli with its substrate N(7)-methylguanosine (m7Guo), which consists of an equilibrium mixture of cationic and zwitterionic forms (pK(a)=7.0), each with characteristic absorption and fluorescence spectra, over the pH range 6-9, where absorption and intrinsic fluorescence of the enzyme are virtually unchanged. The pH-dependence of kinetic constants for phosphorolysis of m7Guo were studied under condition where the population of the zwitterion varied from 10% to 100%. This demonstrated that, whereas the zwitterion is a 3- to 6-fold poorer substrate, if at all, than the cation for the mammalian enzymes, both ionic species are almost equally good substrates for E. coli PNP. The imidazole-ring-opened form of m7Guo is neither a substrate nor an inhibitor of phosphorolysis. Enzyme fluorescence quenching, and concomitant changes in absorption and fluorescence spectra of the two ionic species of m7Guo on binding, showed that both forms are bound by the enzyme, the affinity of the zwitterion being 3-fold lower than that of the cation. Binding of m7Guo is bimodal, i.e., an increase in ligand concentration leads to a decrease in the association constant of the enzyme-ligand complex, typical for negative cooperativity of enzyme-ligand binding, with a Hill constant <1. This is in striking contrast to interaction of the enzyme with the parent Guo, for which the association constant is independent of concentration. The weakly fluorescent N(7)-methylguanine (m7Gua), the product of phosphorolysis of m7Guo, is a competitive non-substrate inhibitor of phosphorolysis (K(i)=8+/-2 microM) and exhibits negative cooperativity on binding to the enzyme at pH 6.9. Quenching of enzyme emission by the ligands is a static process, inasmuch as the mean excited-state lifetime, <tau>=2.7 ns, is unchanged in the presence of the ligands, and the constants K(SV) may therefore be considered as the association constants for the enzyme-ligand complexes. In the pH range 9.5-11 there is an instantaneous reversible decrease in PNP emission of approximately 15%, corresponding to one of the six tyrosine residues per subunit readily accessible to solvent, and OH- ions. Relevance of the overall results to the mechanism of phosphorolysis, and binding of substrates/inhibitors is discussed.

Effect of mycophenolate mofetil on erythropoiesis in stable renal transplant patients is correlated with mycophenolic acid trough levels

BACKGROUND

Both mycophenolate mofetil (MMF) and azathioprine (AZA) are immunosuppressive drugs that inhibit purine synthesis. In theory, MMF selectively inhibits lymphocyte proliferation, while AZA has well-known effects on red blood cells and thrombocytes as well. In renal transplant recipients we replaced CsA therapy by MMF in an attempt to reduce the immunosuppressive load 1 year after kidney transplantation. During this study we observed the effect of MMF on haematological parameters such as haemoglobin (Hb), leukocytes, and thrombocytes.

METHODS

One year after kidney transplantation 26 stable patients were converted from cyclosporin A (CsA) to MMF (2 g/day). Thereafter, these patients were tapered twice in their MMF dose from 2 g to 1.5 g (4 months after conversion) and from 1.5 to 1 g (8 months after conversion) per day. The Hb levels, leukocyte and thrombocyte counts, and mycophenolic acid (MPA) trough levels were routinely measured.

RESULTS

After conversion from CsA to MMF not only creatinine levels and the number of leukocytes, but also the haemoglobin (Hb) level significantly decreased in 21/26 patients (P=0.0004). In eight patients the Hb level dropped more than 1 mmol/l (=1.61 g/dl). Only in two of eight patients was an explanation for blood loss found. The effect on Hb level did not ameliorate after the first MMF dose reduction to 1.5 g/day. After tapering the MMF dose to 1 g/day, the Hb approached the pre-conversion level. Not only the MMF dose but also the mycophenolic acid (MPA) trough level correlated with the Hb level.

CONCLUSIONS

After conversion from CsA to MMF 1 year after kidney transplantation, a decrease in Hb level and leukocyte count was observed. The MPA trough level correlated also with the Hb level. The effect on the Hb level was reversible after dose reduction. This finding suggests that MMF exerts a negative effect on erythropoietic cells.

Recent advances in immunosuppressants

In recent years, a large number of structurally diverse immunosuppressants have been discovered that are effective for the treatment of organ transplantation. Some of them are undergoing clinical trials and may soon enter into routine clinical practice. These compounds are either chemical entities obtained from natural sources/synthetic means or biomaterials such as monoclonal antibodies/gene products/proteins. They have been found to interfere at different stages of T cell activation and proliferation, and can be identified as inhibitors of nucleotide synthesis, growth factor signal transduction and differentiation. Newer strategies involving combination of new agents with traditional immunosuppressants, monoclonal antibodies and gene therapy offer enormous potential, not only for the investigation of mechanisms pertaining to graft rejection, but also for its therapeutic prevention.

Depletion of phosphate in active muscle fibers probes actomyosin states within the powerstroke

Variation in the concentration of orthophosphate (Pi) in actively contracting, chemically skinned muscle fibers has proved to be a useful probe of actomyosin interaction. Previous studies have shown that isometric tension (Po) decreases linearly in the logarithm of [Pi] for [Pi] > or = 200 microM. This result can be explained in terms of cross-bridge models in which the release of Pi is involved in the transition from a weakly bound, low-force actin x myosin x ADP x Pi state to a strongly bound, high-force, actin x myosin x ADP state. The 200 microM minimum [Pi] examined results from an inability to buffer the intrafiber, diffusive buildup of Pi resulting from the fiber ATPase. In the present study, we overcome this limitation by employing the enzyme purine nucleoside phosphorylase with substrate 7-methylguanosine to reduce the calculated internal [Pi] in contracting rabbit psoas fibers to < 5 microM. At 10 degrees C we find that Po continues to increase as the [Pi] decreases for [Pi] > or = 100 microM. Below this [Pi], Po is approximately constant. These results indicate that the free energy drop in the cross-bridge powerstroke is approximately 9 kT. This value is shown to be consistent with observations of muscle efficiency at physiological temperatures.

Purine nucleoside phosphorylase. 2. Catalytic mechanism

X-ray crystallography, molecular modeling, and site-directed mutagenesis were used to delineate the catalytic mechanism of purine nucleoside phosphorylase (PNP). PNP catalyzes the reversible phosphorolysis of purine nucleosides to the corresponding purine base and ribose 1-phosphate using a substrate-assisted catalytic mechanism. The proposed transition state (TS) features an oxocarbenium ion that is stabilized by the cosubstrate phosphate dianion which itself functions as part of a catalytic triad (Glu89-His86-PO4=). Participation of phosphate in the TS accounts for the poor hydrolytic activity of PNP and is likely to be the mechanistic feature that differentiates phosphorylases from glycosidases. The proposed PNP TS also entails a hydrogen bond between N7 and a highly conserved Asn. Hydrogen bond donation to N7 in the TS stabilizes the negative charge that accumulates on the purine ring during glycosidic bond cleavage. Kinetic studies using N7-modified analogs provided additional support for the hydrogen bond. Crystallographic studies of 13 human PNP-ligand complexes indicated that PNP uses a ligand-induced conformational change to position Asn243 and other key residues in the active site for catalysis. These studies also indicated that purine nucleosides bind to PNP with a nonstandard glycosidic torsion angle (+anticlinal) and an uncommon sugar pucker (C4'-endo). Single point energy calculations predicted the binding conformation to enhance phosphorolysis through ligand strain. Structural data also suggested that purine binding precedes ribose 1-phosphate binding in the synthetic direction whereas the order of substrate binding was less clear for phosphorolysis. Conservation of the catalytically important residues across nucleoside phosphorylases with specificity for 6-oxopurine nucleosides provided further support for the proposed catalytic mechanism.

Nicotinamide riboside, an unusual, non-typical, substrate of purified purine-nucleoside phosphorylases

Nicotinamide 1-beta-D-riboside (Nir), the cationic, reducible moiety of the coenzyme NAD+, has been confirmed as an unusual substrate for purified purine-nucleoside phosphorylase (PNP) from a mammalian source (calf spleen). It is also a substrate of the enzyme from Escherichia coli. The Km values at pH 7, 1.48 mM and 0.62 mM, respectively, were 1-2 orders of magnitude higher than for the natural substrate inosine, but the Vmax values were comparable, 96% and 35% that for Ino. The pseudo first-order rate constants, Vmax/Km, were 1.1% and 2.5% for the calf spleen and E. coli enzymes. The aglycon, nicotinamide, was neither a substrate nor an inhibitor of PNP. Nir was a weak inhibitor of inosine phosphorolysis catalyzed by both enzymes, with Ki values close to the Km for its phosphorolysis, consistent with simple competitive inhibition; this was further confirmed by Dixon plots. Phosphorolysis of the fluorescent positively charged substrate 7-methylguanosine was also inhibited in a competitive manner by both Ino and Nir. Phosphorolysis of Nir by both enzymes was inhibited competitively by several specific inhibitors of calf spleen and E. coli PNP, with Ki values similar to those for inhibition of other natural substrates. The pH dependence of the kinetic constants for the phosphorolysis of Nir and of a variety of other substrates, was extensively investigated, particularly in the alkaline pH range, where Nir exhibited abnormally high substrate activity relative to the reduced reaction rates of both enzymes towards other anionic or neutral substrates. The overall results are discussed in relation to present concepts regarding binding and phosphorolysis of substrates by PNP based on crystallographic data of enzyme-inhibitor complexes, and current studies on enzymatic and nonenzymatic mechanisms of the cleavage of the Nir glycosidic bond.

In vivo and in vitro pharmacologic activity of the purine nucleoside phosphorylase inhibitor BCX-34: the role of GTP and dGTP

BCX-34 inhibits RBC PNP in vitro from humans, rats, and mice with IC50S ranging from 5 to 36 nM. BCX-34 also, in the presence but not in the absence of deoxyguanosine, inhibits human CCRF-CEM T-cell proliferation with an IC50 of 0.57 microM but not rat or mouse T-cell proliferation up to 30 microM. Inhibition of human T-cell proliferation is accompanied by an accumulation of intracellular dGTP with an associated reduction in GTP. These nucleotide changes do not occur in BC16A mouse T-cells and explain why proliferation is not inhibited by PNP inhibitors in this case. Reduction in intracellular GTP is not essential for the antiproliferative action of BCX-34. Oral bioavailability of BCX-34 in rats is 76%. BCX-34 is orally active in elevating plasma inosine in rats (2-fold at 30 mg/kg), in suppressing ex vivo RBC PNP activity in rats (98% at 3 h. 100 mg/kg), and in suppressing ex vivo skin PNP in mice (39% at 3 h, 100 mg/kg). The results demonstrate that BCX-34 inhibits human PNP and T-cell proliferation, is orally bioavailable in rodents, and pharmacologically active in vivo in rodents after oral dosing with no apparent side effects or toxicity. BCX-34 may, therefore, be useful in treating human T-cell proliferative inflammatory disorders.

Purine nucleoside phosphorylase: inhibition by purine N(7)- and N(9)-acyclonucleosides; and substrate properties of 7-beta-D-ribofuranosylguanine and 7-beta-D-ribofuranosylhypoxanthine

A series of 10 N(7)- and N(9)-acyclonucleosides of guanine and 8-substituted guanines (8-Br, 8-SH and 8-NH2), and two N(7)-acyclonucleosides of hypoxanthine, were tested for their ability to inhibit purine nucleoside phosphorylase (PNP) (E.C. 2.4.2.1) from human erythrocytes and rabbit kidney. The acyclic chains contained a nitrogen in place of a carbon at the 3', 4' or 5' position and, in one case, an ether oxygen at the 2' position. Most striking was the finding that one of the N(7)-acyclonucleoside analogues, 7-[(1,3-dihydroxypropyl-2)amino]ethylguanine, proved to be a 3-fold more effective inhibitor than its corresponding N(9) counterpart, with Ki = 5 vs 14 microM for the human enzyme and 0.7 vs 2.3 microM for the rabbit enzyme. Both analogues, as well as the others examined, inhibited phosphorolysis competitively with respect to nucleoside substrates (inosine with the human enzyme and guanosine with the rabbit enzyme). The foregoing logically led to the finding that the 7-beta-D-ribosides of guanine (N7Guo) and hypoxanthine (N7Ino) were weak substrates of PNP from human erythrocytes, calf spleen and E. coli. With the human enzyme the pseudo-first-order rate constants (Vmax/Km) for phosphorolysis of N7Guo and N7Ino were 0.08 and 0.02% that for Ino. The Michaelis constants (Km) for N7Guo were 27 (calf PNP), 108 (human PNP) and 450 microM (E. coli PNP). For N7Ino the corresponding Km values were 1.52, 1.26 and 0.64 mM. Four previously well-characterized N(9)-acyclonucleoside inhibitors of calf spleen PNP were found to inhibit phosphorolysis of N7Ino by the same enzyme 2-10-fold more effectively than the parent Ino. The overall results, along with the known excellent substrate properties of N(7)-alkyl- Guo and Ino (Bzowska et al. J Biol Chem 263, 9212-9217, 1988), were examined in relation to present concepts regarding binding of substrates and inhibitors at the active site(s) of these enzymes.

Acyclonucleoside analogue inhibitors of mammalian purine nucleoside phosphorylase

A series of about 60 purine acyclonucleosides, most with guanine as the aglycone and a 4-carbon chain as the acyclic moiety, was examined for ability to inhibit purine nucleoside phosphorylase from human erythrocytes and calf spleen. Compounds with shorter and longer acyclic chains were less effective inhibitors. Synthetic procedures are described. About 25 of the analogues were competitive inhibitors (relative to inosine or 7-methylguanosine as substrates) with Ki values in the range of 2 to 100 microM. The more potent ones (Ki 2-5 microM) included guanine as the aglycone, with various substituents at C(2') of the acyclic chain and hydroxyls at C(3') and C(4'). In one instance, 9-(2-fluoro-3,4-dihydroxybutyl)guanine, the (+)erythro enantiomer was 10-fold more effective than its (-) counterpart (2.5 microM vs 27 microM). Replacement of guanine by 8-bromo- or 8-aminoguanine enhanced affinity for the enzyme by an order of magnitude or more; 7-deazaacyclovir was also 10-fold more effective than acyclovir. With some of the inhibitors, Ki (human)/Ki (calf) varied over the range 0.4 to 4, reflecting differences between the two enzymes; nonetheless, the much more stable, and commercially available, calf spleen enzyme is recommended for preliminary screening of potential inhibitors of the human or other unstable enzymes. The overall results provide useful indications for the synthesis of potentially more potent inhibitors of the enzyme, by simultaneous modifications of the aglycone and the acyclic chains.

Erythrocyte nucleotide stability and plasma hypoxanthine concentrations: improved ATP stability with short-term storage at room temperature

We have measured erythrocyte nucleotide concentrations at timed intervals over 24 h in heparinised blood stored at 4 degrees C, room temperature, or 37 degrees C. The objective was to determine whether the grossly altered NAD concentrations found in the erythrocytes of patients with two different inherited purine disorders could be related to altered stability or turnover rates. An unexpected finding was the improved stability of all erythrocyte nucleotides in blood stored at room temperature compared with 4 degrees C. Not only was the breakdown of ATP greater at 4 degrees C compared with room temperature, higher hypoxanthine concentrations were present in the plasma associated with a fictitious increment in inosine. NAD and NADP, by contrast, showed remarkable stability in both control and patient erythrocytes, irrespective of their original value. Although these studies failed to establish an explanation for the altered NAD levels in the patients, the superior ATP stability in blood stored at room temperature in the erythrocytes from both patients and controls suggests that current practices of storing blood on ice for short-term studies require re-evaluation.

A reversed phase high performance liquid chromatography approach in determining total red blood cell concentrations of 6-thioguanine, 6-mercaptopurine, methylthioguanine, and methylmercaptopurine in a patient receiving thiopurine therapy

A reversed phase high performance liquid chromatographic procedure was developed to quantify 6-thioguanine, 6-mercaptopurine, methylthioguanine, and methylmercaptopurine in red blood cells. The free base of each thiopurine was liberated from the respective nucleoside and nucleotide moiety by acid hydrolysis, which allowed for a determination of the total thiopurine present. 6-Thioguanine and 6-mercaptopurine were analyzed on an octadecylsilane column using methanol + 20 mM sodium phosphate (15:85), pH 7.5, containing 0.07% tetrabutylammonium chloride. Detection was by potassium permanganate oxidation and fluorescence detection at 290 nm excitation and 400 nm emission. Methylmercaptopurine and methylthioguanine were analyzed on a cyanopropylsilane column using methanol + 40 mM sodium phosphate (18:82), pH 2.7, and then ultraviolet absorption at 314 nm and 290 nm, respectively. The method was used to quantify the four primary thiopurines present in red blood cells of an acute lymphoblastic leukemia patient. The procedure may be a therapeutic monitoring technique that quantifies the cytotoxic drug burden in patients receiving azathioprine or 6-mercaptopurine therapy.

Properties of two unusual, and fluorescent, substrates of purine-nucleoside phosphorylase: 7-methylguanosine and 7-methylinosine

The properties of two unusual substrates of calf spleen purine-nucleoside phosphorylase (purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1), 7-methylguanosine and 7-methylinosine, are described. The corresponding bases, 7-methylguanine and 7-methylhypoxanthine, are neither substrates in the reverse, synthetic reaction, nor inhibitors of the phosphorolysis reaction. Both nucleosides exhibit fluorescence, which disappears on cleavage of the glycosidic bond, providing a new convenient procedure for continuous fluorimetric assay of enzymatic activity. For 7-methylguanosine at neutral pH and 25 degrees C, Vmax = 3.3 mumol/min per unit enzyme and Km = 14.7 microM, so that Vmax/Km = 22 X 10(-2)/min per unit as compared to 8 X 10(-2) for the commonly used substrate inosine. The permissible initial substrate concentration range is 5-100 microM. Enzyme activity may also be monitored spectrophotometrically. For 7-methylinosine, Vmax/Km is much lower, 2.4 X 10(-2), but its 10-fold higher fluorescence partially compensates for this, and permits the use of initial substrate concentrations in the range 1-500 microM. At neutral pH both substrates are mixtures of cationic and zwitterionic forms. Measurements of pH-dependence of kinetic constants indicated that the cationic forms are the preferred substrates, whereas the monoanion of inosine appears to be almost as good a substrate as the neutral form. With 7-methylguanosine as substrate, and monitoring of activity fluorimetrically and spectrophotometrically, inhibition constants were measured for several known inhibitors, and the results compared with those obtained with inosine as substrate, and with results reported for the enzyme from other sources.

An overview of the role of nucleosides in chemotherapy

A wide variety of nucleosides have been synthesized which interfere with the functions of natural nucleosides at many different loci. These analogues may act as substrates or inhibitors of nucleoside-metabolizing enzymes or may bind to cell membrane receptors. Because of quantitative and qualitative differences in the enzymes and receptors of various tissues and species, the nucleoside analogues have found important uses as antitumor, antiviral, antiparasitic and immunomodulating agents.

Lack of inhibition of purine nucleoside phosphorylase and adenosine deaminase in patients treated with azathioprine

Deficiency of the purine salvage enzymes purine nucleoside phosphorylase (PNP) and adenosine deaminase (ADA) are known causes of immunodeficiency. Evidence for inhibition of these enzymes was sought in 16 patients on azathioprine therapy by testing for deoxyguanosine (PNP deficiency) and deoxyadenosine (ADA deficiency) in urine using a novel phosphorescence method. These abnormal nucleosides were not found in urine of azathioprine treated patients or in 30 normal controls but were easily detected in urine from proven cases of PNP and ADA deficiency suggesting lack of in vivo inhibition of PNP and ADA by azathioprine.

Severe allopurinol toxicity. Description and guidelines for prevention in patients with renal insufficiency

A life-threatening toxicity syndrome consisting of an erythematous, desquamative skin rash, fever, hepatitis, eosinophilia, and worsening renal function in 78 patients receiving allopurinol is described. In a majority of cases, the development of this syndrome was associated with the use of standard (200 to 400 mg per day) doses of allopurinol in patients with renal insufficiency. In pharmacologic studies, it was demonstrated that the renal clearance of the major metabolite of allopurinol, oxipurinol, is directly proportional to the renal clearance of creatinine (oxipurinol clearance = 0.22 X creatinine clearance -2.87). An inverse linear relation was noted between the serum oxipurinol half-life and the renal creatinine clearance [( serum oxipurinol half-life in hours]-1 = 0.00034 X creatinine clearance in milliliters per minute + 0.0045). Long-term use of 300 mg per day of allopurinol was found to result in elevated steady-state serum oxipurinol concentrations in patients with renal insufficiency (serum oxipurinol concentration in micromoles per liter = -2.5 X creatinine clearance in milliliters per minute + 326). Avoidance of allopurinol or use of reduced doses in patients with renal insufficiency according to proposed guidelines should be adequate to inhibit uric acid production in most patients and may reduce the incidence of life-threatening allopurinol toxicity.

Purine nucleoside synthesis, an efficient method employing nucleoside phosphorylases

An improved method for the enzymatic synthesis of purine nucleosides is described. Pyrimidine nucleosides were used as pentosyl donors and two phosphorylases were used as catalysts. One of the enzymes, either uridine phosphorylase (Urd Pase) or thymidine phosphorylase (dThd Pase), catalyzed the phosphorolysis of the pentosyl donor. The other enzyme, purine nucleoside phosphorylase (PN Pase), catalyzed the synthesis of the product nucleoside by utilizing the pentose 1-phosphate ester generated from the phosphorolysis of the pyrimidine nucleoside. Urd Pase, dThd Pase, and PN Pase were separated from each other in extracts of Escherichia coli by titration with calcium phosphate gel. Each enzyme was further purified by ion-exchange chromatography. Factors that affect the stability of these catalysts were studied. The pH optima for the stability of Urd Pase, dThd Pase, and PN Pase were 7.6, 6.5, and 7.4, respectively. The order of relative heat stability was Urd Pase greater than PN Pase greater than dThd Pase. The stability of each enzyme increased with increasing enzyme concentration. This dependence was strongest with dThd Pase and weakest with Urd Pase. Of the substrates tested, the most potent stabilizers of Urd Pase, dThd Pase, and PN Pase were uridine, 2'-deoxyribose 1-phosphate, and ribose 1-phosphate, respectively. Some general guidelines for optimization of yields are given. In a model reaction, optimal product formation was obtained at low phosphate concentrations. As examples of the efficiency of the method, the 2'-deoxyribonucleoside of 6-(dimethylamino)purine and the ribonucleoside of 2-amino-6-chloropurine were prepared in yields of 81 and 76%, respectively.

Thyroid purine nucleoside phosphorylase. II. Kinetic model by alternate substrate and inhibition studies

Nucleoside analog inhibition studies have been conducted on thyroidal purine nucleoside phosphorylase (purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) which catalyzed an ordered bi-bi type mechanism where the first substrate is inorganic phosphate and the last product is ribose 1-phosphate. Heterocyclic- and carbohydrate-modified nucleoside inhibitors demonstrate mixed type inhibition suggesting such analogs show an affinity (Ki) for the free enzyme. A kinetic model is proposed which supports the observed inhibition patterns. These studies together with alternate substrate studies indicate that nucleoside binding requires a functional group capable of hydrogen bonding at the 6-position of the purine ring and that the orientation of the bound substrate may be syn. Proper geometry of the phosphate is dependent upon the 3'-substituent to the orientated below the furanose ring. The 5'-hydroxyl group is required for substrate activity. The proposed rate limiting step of the phosphorylase mechanism is the enzymatic protonation of the 7-N position of the nucleoside.

The quantitative determination of metabolites of 6-mercaptopurine in biological materials. III. The determination of 14C-labeled 6-thiopurines in L5178Y cell extracts using high-pressure liquid cation-exchange chromatography

A method is presented for the separation of 6-thiopurine bases and ribonucleosides, of sulphate anions and of common purine bases and oxidized purines by means of high-pressure liquid cation-exchange chromatography using a 0.18 X 100 cm column, filled with Beckman M71 resin, and eluted with 0.4M ammonium formate, pH 4.6, at a linear flow velocity of 5.2 cm/min at 50 degrees C. The method has been applied to the separation and quantitative determination of 14C-labeled 6-mercaptopurine metabolites in HClO4 extracts of L5178Y murine lymphoma cells. Distribution patterns of 14C radioactivity within the cells after a 24 h incubation period with (8-14C)-labeled 6-mercaptopurine have been established. The indentification of 6-mercaptopurine metabolites, such as 6-thioxanthosine ribonucleotide, 6-thioinosinic acid, 6-thioguanylic acid, 6-methylthioinosinic acid, and 6-thiouric acid, after the digestion of the extracts with alkaline phosphatase has been confirmed using the behaviour of each compound in enzymatic peak-shifting analyses with purine nucleoside phosphorylase and the corresponding elution volumes of 6-thiopurine bases and ribonucleosides as proofs. According to the specific radioactivity of the (8-14C)-labeled 6-mercaptopurine batch, the amounts of the various 6-mercaptopurine metabolites in about 6% of the total HClO4 extract of 1.6 . 10(8) labeled cells have quantitatively been determined as 1--130 pmol. The intracellular concentration of 6-thiopurines was determined at 1.4 . 10(-5)mol/1.

Purine excretion in complete adenine phosphoribosyltransferase deficiency: effect of diet and allopurinol therapy

1. Abnormal amounts of adenine, 8-hydroxyadenine and 2,8-dihydroxyadenine are found in the urine of homozygotes for APRTase deficiency and are diagnostic of this condition. 2. The renal complication is due to the excessive amounts of 2,8-dihydroxyadenine excreted since it is removed by allopurinol which blocks 2,8-dihydroxyadenine formation. 3. Uric acid metabolism and the excretion of the other minor purine bases is normal, at least in childhood, in homozygotes for APRTase deficiency. 4. Patients with the defect appear to be very sensitive to dietary purine. At least some of the adenine metabolites may have a dietary origin.