Effect of halofenate and clofibrate on lipid synthesis in rat adipocytes.

The free acids of the plasma lipid-lowering agents, halofenate and clofibrate inhibited the incorporation of radioactive glucose and pyruvate into fatty acids of isolated adipocytes prepared from rat epididymal fat pads. The concentration which inhibited fatty acid synthesis was dependent on the bovine serum albumin concentration in the incubation. The 50 per cent inhibitory concentration of the free acid of halofenate in 1 per cent, 2 percent and 4 per cent albumin was 0.9 mM, 2.3 MM and 4.4 mM, respectively. The potency of clofibrate was also lowered by increasing the albumin concentration. These compounds inhibited the uptake of both [14C]glucose and [14C]pyruvate to the same degree as the incorporation of these substrates into fatty acids. However, the drugs either had no effect on , or stimulated the uptake of palmitate by the cells. Leucine accumulation by the adipocytes was unaffected by halofenate (free acid) and inhibited by clofibrate (free acid). A comparison of these agents with (minus)-hydroxycitrate, kynurenate and cerulenin (inhibitors of ATP-citrate lyase, acetyl CoA carboxylase and fatty acid synthetase, respectively) on the oxidation of pyruvate suggested that they inhibited pyruvate metabolism at or near the enzyme, pyruvate dehydrogenase.

A sensitive assay for phosphatidate phosphohydrolase in mouse liver microsomes.

A technique is described for the assay of phosphatidate phosphohydrolase using 1,2-[9,10-3H]dioleoyl-sn-glycero-3-phosphate as a substrate. This substrate was prepared enzymatically using mouse liver microsomes washed with 0.5 M NaCl, which synthesize minimal amounts of neutral lipids at high enzyme concentrations. Measurement of the product, 1,2-[9,10-3H]dioleoylglycerol, was 10-fold more sensitive than the usual colorimetric assay for inorganic phosphate release. In addition, the assay provides information about the relative contribution of other activities which limit the availability of diacylglycerols for further esterification to triacylglycerols and/or phospholipids.

Stereochemical considerations in the enzymatic phosphorylation and antiviral activity of acyclonucleosides. I. Phosphorylation of 2'-nor-2'-deoxyguanosine.

The antiviral compound 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine (2'-nor-2'-deoxyguanosine, 2'-NDG) is phosphorylated by the HSV-1-induced thymidine kinase to the monophosphate (2'-NDG-MP) and this is further phosphorylated by cellular kinases to the triphosphate (2'-NDG-TP) which is a potent inhibitor of DNA polymerases. Since phosphorylation of 2'-NDG creates a chiral center in the molecule, it was of interest to examine whether both monophosphate enantiomers were produced by the viral thymidine kinase, whether they both could be further phosphorylated by cellular kinases and, if so, whether the respective triphosphates were equally inhibitory to the DNA polymerases. The time course of the phosphorylation by GMP kinase of a chemically synthesized, racemic 2'-NDG-MP was compared to that of a 2'-NDG-MP preparation obtained by enzymatic phosphorylation of 2'-NDG with HSV-1 thymidine kinase. The results indicated that the two enantiomeric monophosphates were phosphorylated by GMP kinase with different rates and that phosphorylation of 2'-NDG by HSV-1 thymidine kinase gave only one of the isomers, whose structure was determined to be S. Both enantiomeric diphosphates were further phosphorylated to the respective triphosphates and it was shown that, in contrast to the triphosphate obtained from the 2'-NDG-MP prepared by viral thymidine kinase which was a potent inhibitor of HSV-1 DNA polymerase, the triphosphate obtained from the slow-reacting R isomer had little or no inhibitory activity against this enzyme.

Enzymatic phosphorylation of the antiherpetic agent 9-[(2,3-dihydroxy-1-propoxy)methyl]guanine.

The antiherpetic agent 9-[(2,3-dihydroxy-1-propoxy)methyl]guanine (iNDG) is phosphorylated by HSV1 thymidine kinase, and its phosphorylated products inhibit DNA polymerase activity. iNDG exists in two enantiomeric forms, each with a primary and a secondary hydroxyl; thus, a number of possibilities for preferential phosphorylation exist, which were explored in this study. HSV1 thymidine kinase phosphorylates the primary hydroxyl of both the R and the S isomers of iNDG. This was established by comparison with analogues in which either the primary or the secondary hydroxyl was replaced by fluorine or hydrogen and also by a study of the NMR spectrum of the monophosphate. GMP kinase phosphorylates the R and the S monophosphates to the respective diphosphates. Further phosphorylation, however, is much more efficient with the S than with the R isomer. Furthermore, (S)-iNDG triphosphate is a more potent inhibitor of HSV1 DNA polymerase than (R)-iNDG triphosphate. These differences in the biochemical specificities of the two isomers account for the observed higher antiviral potency of (S)-iNDG as compared to that of (R)-iNDG.

3-Hydroxy-3-methylglutaryl-coenzyme a reductase inhibitors. 7. Modification of the hexahydronaphthalene moiety of simvastatin: 5-oxygenated and 5-oxa derivatives.

Modification of the hexahydronaphthalene ring 5-position in simvastatin 2a via oxygenation and oxa replacement afforded two series of derivatives which were evaluated in vitro for inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and acutely in vivo for oral effectiveness as inhibitors of cholesterogenesis in the rat. Of the compounds selected for further biological evaluation, the 6 beta-methyl-5-oxa 10 and 5 alpha-hydroxy 16 derivatives of 3,4,4a,5-tetrahydro 2a, as well as, the 6 beta-epimer 14 of 16 proved orally active as hypocholesterolemic agents in cholestyramine-primed dogs. Subsequent acute oral metabolism studies in dogs demonstrated that compounds 14 and 16 evoke lower peak plasma drug activity and area-under-the-curve values than does compound 10 and led to the selection of 14 and 16 for toxicological evaluation.

Efficacy of 2'-nor-cyclicGMP in treatment of experimental herpes virus infections.

9-[(2-Hydroxy-1,3,2-dioxaphosphorinan-5-yl)oxymethyl]guanine P-oxide (2'-nor-cGMP), the cyclic phosphate of 2'-nor-deoxyguanosine (2'-NDG) was synthesized by phosphorylation of 2'-NDG and evaluated for antiherpetic activity in cell cultures and in animal protection studies. 2'-nor-cGMP was effective in cell culture against both thymidine kinase deficient and wild-type herpes simplex virus type 1 strains and also against herpes simplex virus type 2. The anti-herpes activity of 2'-nor-cGMP against thymidine kinase deficient HSV-1 was confirmed by animal protection studies. Also, in comparative cell culture protection studies, the ED50 (microM) of 2'-nor-cGMP was approximately 10-fold lower than that of 2'-NDG against three strains of varicella zoster virus. In addition, 2'-nor-cGMP was effective orally in preventing HSV-1 orofacial infection and HSV-2 genital infection of mice. Topical therapeutic applications of 2'-nor-cGMP prevented orofacial HSV-1 lesion development in mice and development of HSV-2 genital lesions in guinea pigs. Subcutaneous application of 2'-nor-cGMP to intracerebral HSV-1 challenged weanling mice significantly prolonged survival. These studies indicate that 2'-nor-cGMP is not dependent on viral thymidine kinase for its antiviral activity and is highly effective in preventing experimental HSV infections.

Effect of halofenate and clofibrate on growth and lipid synthesis in Saccharomyces cerevisiae.

Halofenate-free acid (HFA) inhibited the growth of Saccharomyces cerevisiae by 50% at a concentration of 0.34 mm. This inhibitory effect was prevented by addition of either oleate or acetate, but not by pyruvate. When cell growth was supported by oleate, HFA inhibited the incorporation of radioactive carbon from glucose-U-(14)C or pyruvate-2-(14)C into fatty acids and sterols. The incorporation of radioactive carbon into fatty acids and sterols from acetate-2-(14)C was unaffected by the compound. When cell growth was supported by either oleate or acetate, HFA inhibited the conversion of pyruvate-1-(14)C to (14)CO(2). These results suggest that HFA inhibits the conversion of pyruvate to acetate in yeast. Partially purified yeast pyruvate dehydrogenase was inhibited 50% by 5.5 mm HFA; however, the concentration required for 50% inhibition was considerably reduced when the enzyme was preincubated with the compound at room temperature. In a similar manner, the hypolipidemic agent clofibrate-free acid inhibited the growth of yeast by 50% at 3.0 mm. This inhibition was also prevented by acetate and not by pyruvate. In addition, clofibrate-free acid inhibited partially purified pyruvate dehydrogenase by 50% at a concentration of 37.0 mm.

Synthesis and structure determination of [8(-13)C]guanosine 5'-diphosphate.

A combined chemical and enzymatic synthesis of [8(-13)C]guanosine 5'-diphosphate (GDP) from H13COOH is described. About 35 mg nucleotide was obtained from 500 mg H13COOH. Analysis of the [8(-13)C]GDP by negative ion fast atom bombardment mass spectrometry and by 13C NMR confirmed that one atom of 13C was incorporated at the 8-position of the guanine ring at 90 +/- 10% enrichment. The chemical shift of the C(8) was 140.2 ppm downfield from internal trimethylsilylpropionate at neutral pH and room temperature, with a spin-spin coupling 1J(13C(8)-H(8] of 217 Hz and a 3J(13C(8)-H(1'] of 3.9 Hz.

Biochemical genetic studies of cycloheximide resistance in Neurospora crassa.

Genetic analysis of a number of cycloheximide-resistant mutants of Neurospora crassa has shown that resistance is controlled by several genes. Two of these appear to be located on linkage group V. Resistance to the antibiotic is dominant in wild-type-mutant heterokaryons. Two types of cycloheximide-resistant mutants were isolated: one type exhibited colonial morphology only when grown in the presence of cycloheximide and the other type maintained normal morphology even at high concentrations of the antibiotic. Reconstitution experiments with supernatant solutions and 80S monosomes prepared from wild-type and resistant mutant strains indicated that the property of cycloheximide resistance most likely is associated with the ribosomes. No electrophoretic or serological differences were found between the ribosomal proteins of the wild-type and resistant mutants.

Low-dose simvastatin is a well-tolerated and efficacious cholesterol-lowering agent in ciclosporin-treated kidney transplant recipients: double-blind, randomized, placebo-controlled study in 40 patients.

The high prevalence of hypercholesterolemia in kidney transplant recipients probably contributes to the high cardiovascular mortality in these patients. Except for diet, there is no generally recommended cholesterol-lowering treatment. We conducted a double-blind, randomized, placebo-controlled study with low-dose simvastatin in 40 ciclosporin (CS)-treated kidney transplant recipients during 16 weeks, focusing on side effects and dose finding. In the simvastatin group, the mean serum total and LDL cholesterol concentrations decreased by 23 and 33%, respectively, and the mean serum HDL cholesterol concentration increased by 12%, after 4 weeks of treatment with simvastatin 10 mg daily. Increasing the dose to 20 mg daily in a few patients only resulted in marginal further reductions of the serum cholesterol concentrations at the expense of doubling the plasma simvastatin 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitory activity concentrations. The differences between the changes in the serum cholesterol concentrations in the simvastatin group and the negligible changes in the placebo group were statistically significant. There was no case of proximal myopathy and the serum creatine kinase concentrations did not differ between treatment groups. In conclusion, low-dose simvastatin appears to be a well tolerated and efficacious cholesterol-lowering treatment in CS-treated kidney transplant recipients. Simvastatin 10 mg daily seems to be the most suitable dose for the majority of these patients.

Tissue selectivity of the cholesterol-lowering agents lovastatin, simvastatin and pravastatin in rats in vivo.

Tissue selectivity of lovastatin, simvastatin and pravastatin was determined in male rats. Peak levels of active drug were found in all tissues examined between 0.5 and 2 hours after oral administration. The area under the curve describing 24 hour exposure of the tissues to drug indicated that the drugs were preferentially concentrated in the liver. However, the concentration of pravastatin was approximately 50% that of either lovastatin or simvastatin in the liver and 3-6 times higher in peripheral tissues. These studies demonstrate that the hydrophobic prodrugs, lovastatin and simvastatin show greater selectivity than the hydrophilic agent pravastatin towards the liver which is the target organ for inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

Comparison of the modes of antiviral action of 2'-nor-deoxyguanosine and its cyclic phosphate, 2'-nor-cyclic GMP.

The metabolisms of 9-(1,3-dihydroxy-2-propoxymethyl)guanine (2'NDG) and its cyclic phosphate, 9-[(2-hydroxy-1,3,2-dioxophosphorinan-5-yl) oxymethyl]guanine P-oxide (2'-nor-cGMP), were compared in cultures of primary rabbit kidney cells infected with herpes simplex virus type 1 (HSV-1). 2'-Nor-cGMP was taken up by the cells essentially intact, after which it was opened to the acyclic monophosphate and phosphorylated further, ultimately to the triphosphate. Formation of the triphosphate was independent of HSV thymidine kinase expression, unlike what is observed with 2'NDG. In addition, there was a direct correlation between the antiviral activity of 2'NDG and the level of triphosphate formed in HSV-1-infected cells, whereas such a correlation was absent with 2'-nor-cGMP. In vivo experiments indicated that only a small percentage of free 2'NDG was formed in the bloodstream of mice after oral administration of 2'-nor-cGMP. Incubation of 2'-nor-cGMP with crude extracts of HSV-1-infected or uninfected HeLa cells resulted in the direct production of 2'NDG triphosphate. The possibility that the triphosphate of 2'NDG produced from 2'-nor-cGMP was the enantiomer of the triphosphate made from 2'NDG by viral and cellular kinases was investigated and disproved. Taken together, these data indicate that (i) 2'-nor-cGMP does not act simply as a prodrug of 2'NDG, (ii) 2'-nor-cGMP does not require viral thymidine kinase for its activity, and (iii) 2'-nor-cGMP may have an additional, triphosphate-independent mode of action.

A comparison of the antiviral agents 2'-nor-2'-deoxyguanosine and acyclovir: uptake and phosphorylation in tissue culture and kinetics of in vitro inhibition of viral and cellular DNA polymerases by their respective triphosphates.

A comparative study was conducted between the antiherpetic agents 2'-nor-2'-deoxyguanosine (2'NDG) and acyclovir (ACV) with respect to 1) the relative rates of uptake and phosphorylation to the "active" triphosphate species in tissue culture and 2) the in vitro inhibition of viral and cellular DNA polymerases by their respective triphosphates. The results indicated that a) six hours after HSV1 infection of primary rabbit kidney cells there was seven times more 2'NDG-triphosphate in the cells than ACV triphosphate; b) the relative rate of triphosphate formation in HSV1-infected versus uninfected cells was 4.5 times higher for 2'NDG than for ACV and c) the triphosphate of 2'NDG (2'NDG-TP) was a more selective inhibitor of the viral compared to the cellular DNA alpha-polymerase than the triphosphate of ACV (ACV-TP). The Km/Ki ratios for 2'NDG-TP and ACV-TP (in the competitive inhibition of dGTP) were 3.10 and 1.37, respectively, for the highly purified HSV1 polymerase; and 0.05 and 1.11, respectively, for the partially-purified HeLa alpha-polymerase. Neither triphosphate inhibited the HeLa DNA beta-polymerase to any significant extent. These results are in line with the findings [Ashton et al. (1982), Biochem. Biophys. Res. Commun. 108, 1716-1721] that 2'NDG has superior in vivo antiherpetic activity compared to ACV without apparent toxicity.

2'-Nor-cGMP: a seco-cyclic nucleotide with powerful anti-DNA-viral activity.

As part of our study of antiherpetic acyclonucleosides, we synthesized a cyclic GMP analog, 9-[(2-hydroxy-1,3,2-dioxaphosphorinan-5-yl)oxymethyl]guanine P-oxide, sodium salt (2'-nor-cGMP), and discovered its potent and broad spectrum anti-DNA-viral activities. 2'-Nor-cGMP inhibits the replication of many DNA viruses, including herpes simplex virus, human cytomegalovirus, vaccinia, SV40, and adenovirus, but does not inhibit RNA viruses. In plaque reduction studies this potent antiviral agent is also approximately 10-fold more potent than 9-(1,3-dihydroxy-2-propoxymethyl)guanine (2'NDG) against varicella-zoster virus and inhibits cell transformation by bovine papilloma virus. Unlike 2'NDG, the potent activity of 2'-nor-cGMP against herpes virus is not dependent upon the action of virus-specified thymidine kinase. Intercellular metabolism of 2'-nor-cGMP produced small amounts of 2'NDG triphosphate which were insufficient to account for the antiviral activity observed, implying that this potent anti-DNA-viral agent operates by a mechanism different from that of known acyclonucleosides.

Toxicity of the HMG-coenzyme A reductase inhibitor, lovastatin, to rabbits.

Lovastatin, a specific inhibitor of the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase, has been shown to be highly effective in lowering serum cholesterol in animals and humans and thus represents a promising approach to the treatment and prevention of cardiovascular disease. During the preclinical safety assessment of lovastatin, oral doses that were tolerated by dogs, rats and mice were found to be lethal to rabbits in subacute studies. Postmortem findings in rabbits consisted of centrilobular hepatic necrosis, frequently accompanied by renal tubular necrosis and occasionally gallbladder necrosis. The liver lesions were associated with up to 300-fold elevations in serum aspartate and alanine aminotransferase activities, whereas the kidney lesions resulted in accumulations of serum urea nitrogen and creatinine. The organ damage was preceded by a progressive decline in food consumption and loss of body weight. All histopathological and serum biochemical changes induced by lovastatin were completely prevented by coadministration of mevalonate, the product of the inhibited HMG-CoA reductase enzyme. In addition, administration of mevalonate after the onset of lovastatin-induced hepatotoxicity effectively reversed the toxicity despite continued drug treatment. These findings indicated that the toxicity of high doses of lovastatin to rabbits is a consequence of a highly exaggerated pharmacologic action in blocking mevalonate synthesis. However, supplementation of lovastatin-treated rabbits with oral doses of the major product of mevalonate metabolism, cholesterol, paradoxically enhanced the liver and kidney damage, which suggested that the toxicity of lovastatin stemmed from depletion of a nonsterol metabolite(s) of mevalonate critical for cell viability.

Farnesol-derived dicarboxylic acids in the urine of animals treated with zaragozic acid A or with farnesol.

Farnesyl diphosphate, the substrate for squalene synthase, accumulates in the presence of zaragozic acid A, a squalene synthase inhibitor. A possible metabolic fate for farnesyl diphosphate is its conversion to farnesol, then to farnesoic acid, and finally to farnesol-derived dicarboxylic acids (FDDCAs) which would then be excreted in the urine. Seven dicarboxylic acids were isolated by high performance liquid chromatography (HPLC) from urine of either rats or dogs treated with zaragozic acid A or rats fed farnesol. Their structures were determined by nuclear magnetic resonance analysis. Two 12-carbon, four 10-carbon, and one 7-carbon FDDCA were identified. The profile of urinary dicarboxylic acids from rats fed farnesol was virtually identical to that produced by treating with zaragozic acid A, establishing that these dicarboxylic acids are farnesol-derived. By feeding [1-14C]farnesol and comparing the mass of the dicarboxylic acids produced with the ultraviolet absorption of the HPLC peaks, a method to quantitate the ultraviolet-absorbing FDDCAs was devised. When rats were treated with zaragozic acid A, large amounts of FDDCAs were excreted in the urine. The high level of FDDCAs that were found suggests that their synthesis is the major metabolic fate for carbon diverted from cholesterol synthesis by a squalene synthase inhibitor. A metabolic pathway is proposed to explain the production of each of these FDDCAs.

HMG-CoA reductase inhibitor-induced myopathy in the rat: cyclosporine A interaction and mechanism studies.

Recent clinical evidence indicates a potential for skeletal muscle toxicity after therapy with HMG-CoA reductase inhibitors (HMGRIs) in man. Although the incidence of drug-induced skeletal muscle toxicity is very low (0.1-0.2%) with monotherapy, it may increase following concomitant drug therapy with the immunosuppressant, cyclosporine A (CsA), and possibly with certain other hypolipidemic agents. In the Sprague-Dawley rat, very high, pharmacologically comparable dosages (150-1200 mg/kg/day) of structurally similar HMGRIs (lovastatin, simvastatin, pravastatin and L-647, 318) produced dose-related increases in the incidence and severity of skeletal muscle degeneration. Physical signs included inappetence, decreased activity, loss of body weight, localized alopecia and mortality. To evaluate the interaction between HMGRIs and CsA, a rat model of CsA-induced cholestasis was developed. In this 2-week model, the skeletal muscle toxicity of the HMGRIs was clearly potentiated by CsA (10 mg/kg/day). Doses of HMGRIs which did not produce skeletal muscle toxicity when given alone caused between 75 and 100% incidence of myopathy (very slight to marked skeletal muscle degeneration) when CsA was coadministered. Typical light microscopic changes included myofiber necrosis with interstitial edema and inflammatory infiltration in areas of acute injury. Histochemical characterization of the muscle lesion indicated that type 2B fibers (primarily glycolytic white fibers) were most sensitive to this toxicity but that, with prolonged administration, all fiber types were ultimately affected. Results of pharmacokinetic studies in rats treated with various HMGRIs +/- CsA indicated that coadministration of CsA alters the disposition of these compounds, resulting in increased systemic exposure (e.g., increased area under the plasma drug concentration vs. time curve-AUC) and consequent (up to 13-fold) increases in skeletal muscle drug levels. Evaluation of the potential interaction between the HMGRI, lovastatin and CsA at the level of hepatic microsomal metabolism indicated that CsA did not inhibit the metabolism of lovastatin in isolated microsomes from female rats. In light of the above findings, it appears that HMGRI-induced myopathy is a class effect in the rat, which is potentiated by CsA as the result of altered clearance and resultant increased tissue exposure. Cholestasis associated with CsA and HMGRIs may form the basis for decreased elimination and the resultant elevated systemic exposure. Furthermore, this toxicity is muscle fiber-selective and may be associated with impaired skeletal muscle energy metabolism.

Studies on the mechanism of simvastatin-induced thyroid hypertrophy and follicular cell adenoma in the rat.

Female Sprague-Dawley rats were treated with either simvastatin (a novel competitive inhibitor of HMG CoA reductase) or phenobarbital (positive control) to ascertain the possible relationship between the effects of simvastatin on hepatic metabolism and the thyroid hypertrophy and follicular cell adenomas which it produces in this strain of rat. The test compounds were administered orally at doses of 100 mg/kg (divided doses at 50 mg/kg, b.i.d.). (This dose of simvastatin represents approximately 250 times the human dose.) After 5 weeks of treatment, either simvastatin or phenobarbital produced significant increases (35% and 39% above control, respectively) in serum thyroid stimulating hormone (TSH), a significant increase (39% and 120% above control, respectively) in the systemic clearance of 125I-thyroxine, and slight decreases in serum thyroxine levels. Statistically significant increases in liver and thyroid weights were associated with phenobarbital treatment. With simvastatin, increased liver weights occurred. At the microscopic level, thyroid hypertrophy was observed in all phenobarbital-treated rats and to a lesser degree in most simvastatin-treated animals. Simvastatin did not markedly alter liver microsomal enzyme activities with the exception of the anticipated induction of HMG CoA reductase (which increased approximately 4.4-fold). Conversely, phenobarbital produced large increases in liver microsomal enzymes, including glucuronosyl transferase, but did not affect the activity of HMG CoA reductase. Therefore, the increased clearance of thyroxine in simvastatin-treated female rats was not associated with enzyme induction but may have been related to the increase in functional liver mass produced by this compound at this dose.(ABSTRACT TRUNCATED AT 250 WORDS)

9-([2-hydroxy-1-(hydroxymethyl)ethoxy]methyl)guanine: a selective inhibitor of herpes group virus replication.

9-([2-Hydroxy-1-(hydroxymethyl)ethoxy]methyl)guanine (2'-nor-2'-deoxyguanosine; 2'NDG) selectively inhibits the replication of herpes group viruses. In cell culture studies 2'NDG was at least 10-fold more potent than acyclovir (ACV) in inhibition of human cytomegalovirus replication and Epstein-Barr virus-induced lymphocyte transformation and was about as effective as ACV in inhibition of herpes simplex viruses 1 and 2 and varicella zoster virus. Orally administered 2'NDG was 6- to 50-fold more efficacious than ACV in treating systemic or local HSV-1 infection or HSV-2 intravaginal infection in mice. The mode of action of 2'NDG appears to involve phosphorylation by herpes simplex virus thymidine kinase and subsequent phosphorylations by cellular kinases to produce 2'NDG triphosphate, which is a potent inhibitor of herpes virus DNA polymerase. Compared to ACV, 2'NDG was a more efficient substrate for HSV-1 thymidine kinase (Vmax/Km for 2'NDG 30-fold higher than that of ACV), whereas 2'NDG monophosphate is a more efficient substrate for GMP kinase (Vmax/Km for 2'NDG monophosphate 492-fold higher than that for ACV monophosphate). The combined effect is more rapid production of the inhibitory triphosphate from 2'NDG than from ACV.