Selective in vitro inhibition of human MOLT-4 T lymphoblasts by the novel purine nucleoside phosphorylase inhibitor, CI-972

Purine nucleoside phosphorylases: properties, functions, and clinical aspects

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. This review updates the properties of the enzymes from eukaryotes and a wide range of prokaryotes, including a tentative classification of the enzymes from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificities. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiometry of binding; order of ligand binding and release; and its possible relevance to the complex kinetics exhibited with some substrates. Mutations responsible for PNP deficiency are described, as well as clinical methods, including gene therapy, for corrections of this usually fatal disease. Substrate discrimination between enzymes from different sources is also being profited from for development of tumour-directed gene therapy. Detailed accounts are presented of design of potent inhibitors, largely nucleosides and acyclonucleosides, their phosphates and phosphonates, particularly of the human erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applications, such as prevention of host-versus-graft response in organ transplantations. Methods of assay of PNP activity are reviewed. Also described are applications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enzymes for assays of orthophosphate in biological systems in the micromolar and submicromolar ranges, and for coupled assays of other enzyme systems.

A purine nucleoside phosphorylase (PNP) inhibitor induces apoptosis via caspase-3-like protease activity in MOLT-4 T cells

Children with congenital homozygous deficiency of purine nucleoside phosphorylase (PNP) have abnormalities in purine metabolism that result in T-cell selective immune deficiency. The mechanism of action for cell death has been attributed to intracellular accumulation of dGTP, a potent inhibitor of ribonucleotide reductase and subsequently DNA synthesis, in thymocytes and T-cells but not B-cells. However, the mode of cell death has not been determined to be either necrosis or apoptosis. To examine the involvement of apoptosis in T-cells following PNP inhibition, MOLT-4 cells, a human T cell leukemia cell line, were co-treated with the PNP inhibitor, CI-1000 (2-amino 3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]-pyrimidin-4-one HCl), and 2'-deoxyguanosine (dGuo) which resulted in a concentration-dependent loss of cell viability (trypan blue) and inhibition of tritiated thymidine ([3H]-TdR) uptake. Staining of cells with the DNA dye Hoechst 33,258 showed nuclear morphology characteristic of apoptosis. Western blots (24 h lysates) were probed with antibodies against several proteins implicated in apoptosis. Anti-PARP revealed the presence of an 85 kD PARP breakdown product while, anti-alpha-spectrin revealed the accumulation a 120 kD breakdown product, both suggestive of CPP32 cleavage (caspase-3; an ICE-like cysteine protease). Western blots also detected the loss of the intact 32 kD caspase-3 isoform, a biochemical event associated with caspase-3 activation. Corresponding fluorometric activity assays detected a marked increase in caspase-3-like activity using the substrate Ac-DEVD-MCA. Lastly, a pan caspase inhibitor (Z-D-DCB) and 2'-deoxycytidine (dCyd), which is known to prevent dGTP accumulation following PNP inhibition, were able to prevent cell death and all indicators of caspase-3-like activity in MOLT-4 cells co-treated with dGuo and CI-1000. In summary, we provided several lines of evidence for the role of apoptosis and the contribution of caspase-3-like proteases in T-cell death following PNP inhibition.

Crystal structure of calf spleen purine nucleoside phosphorylase in a complex with hypoxanthine at 2.15 A resolution

Trimeric calf spleen purine nucleoside phosphorylase has been complexed with hypoxanthine via phosphorolysis of inosine in the presence of phosphate. The resulting, "Michaelis" complex (three hypoxanthine molecules per trimer), presumed to be formed under these conditions, crystallized in the cubic space group P2(1)3, with unit cell dimension a = 94.11 A and one monomer in the asymmetric crystal unit; the biologically active trimer is located on the crystallographic 3-fold axis. High-resolution X-ray diffraction data were collected using synchrotron radiation (EMBL outstation, Hamburg, c/o DESY). The crystal structure has been determined by molecular replacement and refined at 2.15 A resolution to an R-value of 0.18. In the hypoxanthine binding site, a cis-peptide bond between Asn243 and Lys244 is observed. Side-chains of GIu201 and Asn243, as well as one integral water molecule located in the base binding site, form hydrogen bonds with the hypoxanthine N-1 H, N-7 H and O-6. A second water molecule links the base positions N-3 and N-9 with an adjacent pocket, which presumably is the phosphate-binding site. This pocket is filled completely by a cluster of six water molecules. Hence all possible donor/acceptor-positions of hypoxanthine are saturated by hydrogen-bonding to protein side-chains or integral water molecules. Purine nucleoside phosphorylase isolated form human tissues is a primary target for chemotherapeutic intervention, and the more stable calf enzyme has similar physico-chemical and kinetic properties, as well as response to inhibitors. Hence the high-resolution structure presented here may serve for design of inhibitors with potential pharmacological applications.

In vitro lymphotoxicity and selective T cell immunotoxicity of high doses of acyclovir and its derivatives in mice

The antiviral drug acyclovir [9-(2-hydroxyethoxymethyl)guanine (ACV)], its 7-isomer (7-ACV) and its two derivatives: N2-acetyl ACV (ac-ACV) and N2,O-diacetyl ACV (diac-ACV) were examined for their potential in vitro lymphotoxicity and in vivo immunotoxicity in mice. In vitro lymphotoxicity of ACV and its acetylated derivatives was low, whereas the 7-ACV isomer enhanced the in vitro cell proliferation in PHA-stimulated cultures. Addition of 2'-deoxyguanosine (dGuo) did not exhibit any inhibitory potential of ACV. However, reduction in the absolute number of CD3+, CD8+, and CD25+ cells, but not Ig+ cells, was noted at high concentrations of ACV and its derivatives, suggesting a selective T cell cytotoxicity. Similarly, the in vivo exposure revealed selective T cell immunotoxicity of ACV and its derivatives since the reduced number of Thy 1.2+ and CD8+ cells was not accompanied with any marked changes in the Ig+ population. The CD4+/CD8+ ratio was affected both in vitro and in vivo by high concentrations of ACV.

Blockade of nucleoside degradation in monkey whole blood in vitro by CI-1000, a purine nucleoside phosphorylase (PNP) inhibitor

Purine nucleosides HxR or GdR (2.5 micrograms/mL blood) were added to EDTA-treated cynomolgus monkey whole blood in vitro, alone or with the PNP inhibitor CI-1000 (1 microgram/mL), mixed, and the concentration of nucleosides remaining in plasma followed as a function of time. The half-lives of GdR and HxR in control blood were 1.2 and < 1 min, respectively, and were extended to 17.8 and 39.8 min, respectively, by coaddition of CI-1000. In contrast, a structural analog of CI-1000, CI-972, when tested in parallel at 1 microgram/mL, had markedly less effect on the breakdown of either nucleoside. The ability of CI-1000 to retard nucleoside breakdown in blood in vitro may be a predictor of in vivo activity, and can be viewed as an early and essential biochemical consequence of PNP inhibition culminating in immunosuppression.

Evidence for a pathway independent from 2'-deoxyguanosine and reversible by IL-2 by which purine nucleoside phosphorylase inhibitors block T-cell proliferation

Patients with homozygous deficiency of purine nucleoside phosphorylase (PNP) present with a T-cell selective immune deficiency. To elucidate the potential use of PNP inhibitors in the therapy of cutaneous T-cell lymphomas (CTCLs) the authors studied the effects of CI-1000 (formerly PD141955-2) and CI-972 on a T-cell line MyLa established from a patient with mycosis fungoides. Both PNP inhibitors had significant, dose-dependent, inhibitory effects on the proliferation of the T-cell line. CI-1000 (ED50: 3.7 microM) was approximately six-fold more potent in blocking 3H-thymidine uptake than CI-972 (ED50: 22.5 microM). The inhibitory effect of either substance could not be increased by addition of deoxyguanosine. Flow cytometric analysis revealed that both PNP inhibitors caused a block in the S-phase of the cell cycle. The inhibitory effect on proliferation was reversible partially by addition of IL-2. When testing proliferation inhibition of both substances on an IL-2-dependent T-cell line (SeAx), their inhibitory effects were reduced significantly. These data document a mechanism of action of the PNP inhibitors independent of deoxyguanosine and partially reversible by IL-2. The authors' observations suggest the potential use of PNP inhibitors in the therapy of cutaneous T-cell lymphomas and provide evidence for a pathway independent from deoxyguanosine by which PNP inhibitors might function in T cells.

PD 141955 and CI-972: 9-deazaguanine analog purine nucleoside phosphorylase inhibitors. I. Suppression of the human mixed lymphocyte reaction (MLR)

Inhibitors of purine nucleoside phosphorylase (PNP) are of interest as potential T-cell-selective immunosuppressive agents and for other uses. PD 141955 (9-deaza-9-(3-thienylmethyl)guanine; 2-amino-3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]pyrimidin -4-one) is 12- to 100-fold more potent than CI-972 (8-amino-9-deaza-9-(3-thienylmethyl)guanine; 2,6-diamino-3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]pyrim idin-4- one) in PNP enzyme inhibition assays. In the human MLR, PD 141955 has IC50s of 2.8 and 12.8 microM in the presence and absence, respectively, of 15 microM GdR (means from 10 assays), while the IC50s of CI-972 tested in parallel are > 30 microM. Concentration-dependent accumulation of dGTP occurs in PD 141955-treated MLRs under conditions in which CI-972 lacks detectable activity. Thus, consistent with its greater PNP inhibitory activity in a cell free system, PD 141955 is significantly more potent than CI-972 in its ability to suppress the MLR.

PD 141955 and CI-972: 9-deazaguanine analog purine nucleoside phosphorylase inhibitors. II. Effects on nucleoside catabolism in human and rat blood in vitro

Patients with deficiency in purine nucleoside phosphorylase (PNP) have elevated levels of the PNP substrates inosine, guanosine, and (rarely) 2'-deoxyguanosine (GdR) in their plasma and urine. GdR is critical because it serves as a precursor of dGTP, which blocks T-cell replication, thus leading to T-cell-selective immune dysfunction. We adapted these findings to the study of PNP inhibitors in human and rat blood in vitro. Blood was spiked with GdR (2.5 micrograms/ml) and the effects of PD 141955 (9-deaza-9-(3-thienylmethyl)guanine; 2-amino-3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]pyrimidin -4-one) and CI-972 (8-amino-9-deaza-9-(3-thienylmethyl)guanine; 2,6-diamino-3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]pyrim idin-4- one) on GdR catabolism were determined. GdR was metabolized 89 times faster in human blood than in rat blood (half-life = 12.0 +/- 1.4 s in human blood). When PD 141955 (1 microgram/ml) was added to human blood before spiking, the GdR half-life increased to > 60 min. In contrast, CI-972 (1 microgram/ml) extended the GdR half-life to 7.2 +/- 1.7 min. Both PD 141955 and CI-972 at 1 microgram/ml significantly retarded GdR catabolism from rat blood.

Comparative in vitro and in vivo activities of two 9-deazaguanine analog inhibitors of purine nucleoside phosphorylase, CI-972 and PD 141955

An in-parallel comparison is presented of the in vitro and in vivo properties of two 9-deazaguanine analog inhibitors of purine nucleoside phosphorylase (PNP), CI-972 [8-amino-9-deaza-9-(3-thienylmethyl)guanine] and PD 141955 [9-deaza-9-(3-thienylmethyl)guanine] (published Ki values of 0.83-8.0 and 0.08 microM, respectively). Despite structural similarities, PD 141955 was considerably more potent and active in all systems studied. The respective IC50 values for inhibition of MOLT-4 cell growth in the absence and presence of 10 microM 2'-deoxyguanosine (GdR) were greater than 50 and 5.06 microM for CI-972 and 15.4 and 0.061 microM for PD 141955. PD 141955 induced accumulation of dGTP in GdR-treated MOLT-4 and CEM cells at log-lower concentrations than were required of CI-972, and the magnitude of dGTP accumulation in PD 141955-treated T cell cultures was markedly greater (e.g. 366 vs 100 pmol/10(6) CEM cells at 10 microM). PD 141955 administered orally produced a dose-dependent elevation of plasma inosine and guanosine in rats over a broad concentration range. Mean plasma inosine concentrations following a 150 mg/kg p.o. dose peaked at 6.21 and 13.2 microM in CI-972 and PD 141955-treated rats, respectively. Low levels of inosine were detectable at 50 micrograms/kg following oral administration of PD 141955.