Uridine reverses the toxicity of 3'-azido-3'-deoxythymidine in normal human granulocyte-macrophage progenitor cells in vitro without impairment of antiretroviral activity

Fighting nature with nature: antiviral compounds that target retroviruses

The human immunodeficiency virus (HIV) is a type of lentivirus that targets the human immune system and leads to acquired immunodeficiency syndrome (AIDS) at a later stage. Up to 2021, there are millions still living with HIV and many have lost their lives. To date, many anti-HIV compounds have been discovered in living organisms, especially plants and marine sponges. However, no treatment can offer a complete cure, but only suppressing it with a life-long medication, known as combined antiretroviral therapy (cART) or highly active antiretroviral therapy (HAART) which are often associated with various adverse effects. Also, it takes many years for a discovered compound to be approved for clinical use. Thus, by employing advanced technologies such as automation, conducting systematic screening and testing protocols may boost the discovery and development of potent and curative therapeutics for HIV infection/AIDS. In this review, we aim to summarize the antiretroviral therapies/compounds and their associated drawbacks since the discovery of azidothymidine. Additionally, we aim to provide an updated analysis of the most recent discoveries of promising antiretroviral candidates, along with an exploration of the current limitations within antiretroviral research. Finally, we intend to glean insightful perspectives and propose future research directions in this crucial area of study.

Comparison of Cytotoxicity of Mononucleoside Phosphotriester Derivatives Bearing Biolabile Phosphate Protecting Groups in Normal Human Bone Marrow Progenitor Cells

The effects of three mononucleoside phosphotriester derivatives of 3′-azido-2′,3′-dideoxythymidine (AZT) which incorporate biolabile phosphate protecting groups, namely S-acetyl-2-thioethyl (MeSATE), S-(2-hydroxyethylsulfidyl)-2-thioethyl (DTE), and pivaloyloxymethyl (POM) were studied and compared to their nucleoside parent in human myeloid colony-forming cells. Moreover, the relative antiviral potency of these three pronucleotides were determined in primary human peripheral blood mononuclear cells infected with human immunodeficiency virus type 1. The results indicate that the SATE and DTE pro-moieties, as well as their degradation products, do not induce additional toxicity. The bis(MeSATE) phosphotriester derivative of AZT emerged as the most selective inhibitor with an in-vitro therapeutic index of the same order of magnitude as observed for AZT. This study has been extended to the corresponding bis(MeSATE) and bis(DTE) phosphotriester derivatives of 2′,3′-dideoxyuridine (ddU).

Comparison of the Mechanism of Toxicity of 3′-Azido-3′-Deoxythymidine and 3′-Fluoro-3′-Deoxythymidine in Human Lymphocytes

The thymidine analogue 3′-azido-3′-deoxythymidine (AZT), whilst a useful drug for the treatment of acquired immunodeficiency syndrome, produces toxic side-effects which can be severe and can interfere with therapy. The toxic mechanism of AZT is unknown. We have investigated the relationship between the phosphorylation and effect on natural dNTP pools of 3′-azido-3′-deoxythymidine and the closely related 3′-fluoro-3′-deoxythymidine and their toxicity in human lymphocytes. We attempted to reduce the toxicity by co-administration of natural nucleosides.

The toxicity of 3′-fluoro-3′-deoxythymidine could be reduced with thymidine or deoxyuridine five- and 10-fold, respectively. The toxicity of 3′-azido-3′-deoxythymidine could be reduced twofold with cytidine or uridine but was increased by all other nucleosides, including thymidine. Neither analogue caused significant changes in the dNTP pools at cytotoxic concentrations; the effect of the nucleosides in reducing toxicity was not owing to replacement of a depleted dNTP. Thymidine reduced the phosphorylation of 3′-azido-3′-deoxythymidine and 3′-fluoro-3′-deoxythymidine 6 and 17 times, respectively.

3′-azido-3′-deoxythymidine and 3′-fluoro-3′-deoxythymidine appear to have different mechanisms of toxicity. The toxic mechanism of 3′-fluoro-3′-deoxythymidine is probably inhibition of cellular DNA synthesis by the triphosphate. The toxicity of 3′-azido-3′-deoxythymidine in lymphocytes does not appear to be directly related to the amounts of the phosphorylated forms. The mechanism may be interference with RNA metabolism or precursors, perhaps by the nucleoside.

Studies on the Reversal of Azidothymidine Toxicity in Human Lymphocytes by Cytidine and Uridine

The toxicity of 3′-azidothymidine (AZT) in human lymphocytes has been shown previously to be reversed by co-incubation with the ribonucleosides cytidine or uridine. In the present paper, the effects of 3′-azidothymidine and cytidine/uridine, both alone and in combination, were studied upon key processes in lymphocytes in order to discover more about the mechanism of toxicity reversal.

In these experiments 3′-azidothymidine had only minor effects on the ribonucleoside triphosphate pools. Cytidine increased the CTP pool, and uridine the UTP pool. Co-incubation with AZT caused similar changes to incubation with cytidine or uridine alone. Toxicity reversal was not linked to replacement of deficient ribonucleoside triphosphate pools.

3′-Azidothymidine caused the excretion of thymidine from lymphocytes. Incubation with cytidine and uridine increased the intracellular cytidine and uridine pools, respectively. Co-incubation with 3′-azidothymidine increased still further the intracellular cytidine and uridine pools. Cytidine and uridine did not affect the intracellular 3′-azidothymidine pool.

The toxicity of 3′-azidothymidine was increased by co-incubation with the bases adenine, guanine, hypoxanthine, and uracil, but not by dihydrouracil, thymine, or xanthine.

Anti-HIV cytotoxicity enzyme inhibition and molecular docking studies of quinoline based chalcones as potential non-nucleoside reverse transcriptase inhibitors (NNRT)

A series of fourteen (A1 - A14) qunioline based chalcones were screened for reverse transcriptase inhibitors (RT) and found potentially active against RT. Bioassay, theoretical and dockings studies with RT (the enzyme required for reverse transcription of viral RNA) results showed that the type and positions of the substituents seemed to be critical for their inhibition against RT. The bromo and chloro substituted chalcone displayed high degree of inhibition against RT. The A4 andA6 showed high interaction with RT, contributing high free binding energy (ΔG -9.30 and -9.13kcal) and RT inhibition value (IC50 0.10μg/ml and 0.11μg/ml).

Synthesis, docking, and biological studies of phenanthrene β-diketo acids as novel HIV-1 integrase inhibitors

In the present study we report the synthesis of halogen-substituted phenanthrene β-diketo acids as new HIV-1 integrase inhibitors. The target phenanthrenes were obtained using both standard thermal- and microwave-assisted synthesis. 4-(6-Chlorophenanthren-2-yl)-2,4-dioxobutanoic acid (18) was the most active compound of the series, inhibiting both 3'-end processing (3'-P) and strand transfer (ST) with IC50 values of 5 and 1.3 μM, respectively. Docking studies revealed two predominant binding modes that were distinct from the binding modes of raltegravir and elvitegravir, and suggest a novel binding region in the IN active site. Moreover, these compounds are predicted not to interact significantly with some of the key amino acids (Q148 and N155) implicated in viral resistance. Therefore, this series of compounds can further be investigated for a possible chemotype to circumvent resistance to clinical HIV-1 IN inhibitors.

Role of pyrimidine depletion in the mitochondrial cardiotoxicity of nucleoside analogue reverse transcriptase inhibitors

OBJECTIVE

Long-term antiretroviral treatment with nucleoside analogue reverse transcriptase inhibitors (NRTI) may result in a cardiomyopathy due to mitochondrial DNA (mtDNA) depletion. An intact mitochondrial function is required for the synthesis of intramyocardial pyrimidine nucleotides, which in turn are building blocks of mtDNA. We investigated if NRTI-related cardiomyopathy can be prevented with pyrimidine precursors.

METHODS

Mice were fed with zidovudine or zalcitabine with or without simultaneous Mitocnol, a dietary supplement with high uridine bioavailability. Myocardia were examined after 9 weeks.

RESULTS

Both NRTI induced a cardiomyopathy with mitochondrial enlargement, a disrupted cristal architecture on electron microscopy and diminished myocardial mtDNA copy numbers. The myocardial mtDNA-encoded cytochrome c-oxidase I subunit was impaired more profoundly than the nucleus-encoded cytochrome c-oxidase IV subunit. The myocardial formation of reactive oxygen species and mtDNA mutations was enhanced in zidovudine and zalcitabine treated animals. Mitocnol attenuated or normalized all myocardial pathology when given with both NRTI, but by itself had no intrinsic effects and no apparent adverse effects.

CONCLUSIONS

Zidovudine and zalcitabine induce a mitochondrial cardiomyopathy, which is antagonized with uridine supplementation, implicating pyrimidine pool depletion in its pathogenesis. Pyrimidine pool replenishment may be exploited clinically because uridine is well tolerated.

The effect of thymidine on the antibacterial and antiviral activity of zidovudine

The effect of thymidine and deoxyadenosine on the antiviral and antibacterial effect of zidovudine was studied in human immunodeficiency virus type 1 (HIV-1) Escherichia coli and Salmonella typhimurium. In quantitative assays, 10 μg mL−1 thymidine was shown to increase the 50% inhibitory concentration (IC50) of zidovudine for HIV-1 by approximately 100-fold and to reduce zidovudine (1 μm)-induced protection of C8166 cells from 204 to 0·18 log syncytial-forming units. Thymidine also antagonized the antibacterial effect of zidovudine for two E. coli and three S. typhimurium species in a dose-dependent manner; 10 μg mL of thymidine increased the minimum inhibitory concentration of zidovudine for E. coli strains by 10–40-fold and for S. typhimurium strains by three-fold. Deoxyadenosine reduced the minimum inhibitory concentration of zidovudine against all five bacterial strains but had no effect on the IC50 of zidovudine for HIV-1, nor did it significantly reverse the antagonism of the antibacterial and antiviral activity of thymidine. The induction of the SOS response in E. coli was reversed in a dose-dependent manner by thymidine while the presence of deoxyadenosine increased induction of the SOS response by zidovudine at suboptimal concentrations.

Synthesis, biological evaluation and 3D-QSAR studies of 3-keto salicylic acid chalcones and related amides as novel HIV-1 integrase inhibitors

HIV-1 integrase is one of the three most important enzymes required for viral replication and is therefore an attractive target for anti retroviral therapy. We herein report the design and synthesis of 3-keto salicylic acid chalcone derivatives as novel HIV-1 integrase inhibitors. The most active compound, 5-bromo-2-hydroxy-3-[3-(2,3,6-trichlorophenyl)acryloyl]benzoic acid (25) was selectively active against integrase strand transfer, with an IC(50) of 3.7 μM. While most of the compounds exhibited strand transfer selectivity, a few were nonselective, such as 5-bromo-3-[3-(4-bromophenyl)acryloyl]-2-hydroxybenzoic acid (15), which was active against both 3'-processing and strand transfer with IC(50) values of 11±4 and 5±2 μM, respectively. The compounds also inhibited HIV replication with potencies comparable with their integrase inhibitory potencies. Thus, 5-bromo-2-hydroxy-3-[3-(2,3,6-trichlorophenyl)acryloyl]benzoic acid (25) and 5-bromo-3-[3-(4-bromophenyl)acryloyl]-2-hydroxybenzoic acid (15) inhibited HIV-1 replication with EC(50) values of 7.3 and 8.7 μM, respectively. A PHASE pharmacophore hypothesis was developed and validated by 3D-QSAR, which gave a predictive r(2) of 0.57 for an external test set of ten compounds. Phamacophore derived molecular alignments were used for CoMFA and CoMSIA 3D-QSAR modeling. CoMSIA afforded the best model with q(2) and r(2) values of 0.54 and 0.94, respectively. This model predicted all the ten compounds of the test set within 0.56 log units of the actual pIC(50) values; and can be used to guide the rational design of more potent novel 3-keto salicylic acid integrase inhibitors.

Enhanced uridine bioavailability following administration of a triacetyluridine-rich nutritional supplement

Background

Uridine is a therapy for hereditary orotic aciduria and is being investigated in other disorders caused by mitochondrial dysfunction, including toxicities resulting from treatment with nucleoside reverse transcriptase inhibitors in HIV. Historically, the use of uridine as a therapeutic agent has been limited by poor bioavailability. A food supplement containing nucleosides, NucleomaxX®, has been reported to raise plasma uridine to supraphysiologic levels.

Methodology/Principal Findings

Single- and multi-dose PK studies following NucleomaxX® were compared to single-dose PK studies of equimolar doses of pure uridine in healthy human volunteers. Product analysis documented that more than 90% of the nucleoside component of NucleomaxX® is in the form of triacetyluridine (TAU). Single and repeated dosing with NucleomaxX® resulted in peak plasma uridine concentrations 1–2 hours later of 150.9±39.3 µM and 161.4±31.5 µM, respectively, levels known to ameliorate mitochondrial toxicity in vitro. C_max and AUC were four-fold higher after a single dose of NucleomaxX® than after uridine. No adverse effects of either treatment were observed.

Conclusions/Significance

NucleomaxX®, containing predominantly TAU, has significantly greater bioavailability than pure uridine in human subjects and may be useful in the management of mitochondrial toxicity.

Uridine supplementation in the treatment of HIV lipoatrophy: results of ACTG 5229

BACKGROUND

Lipoatrophy is prevalent on thymidine nucleoside reverse transcriptase inhibitors (tNRTIs). A pilot trial showed that uridine (NucleomaxX) increased limb fat.

METHODS

A5229 was a multicenter trial in which HIV-infected individuals with lipoatrophy on tNRTI regimens were randomized to NucleomaxX or placebo. Primary endpoint was change in limb fat from baseline to week 48. The study was powered to detect 400-g difference between arms at week 48. A stratified Wilcoxon rank-sum test was used to assess between-arm differences.

RESULTS

The 165 participants were 91% men, 62% white; median age 49 years, CD4 cell count 506 cells/μl, and limb fat 3037 g; 81% had HIV-1 RNA 50 copies/ml or less; 76% were on zidovudine (ZDV). Baseline characteristics were similar between groups. Only 59% completed 48 weeks of treatment; however, only three participants (one on uridine) discontinued due to toxicity (diarrhea). In intent to treat, there was no difference for changes in limb fat between treatments at week 24 or week 48. On as-treated analysis, uridine resulted in an increase in %limb fat vs. placebo (3.4 vs. -0.8%, P = 0.01) at week 24 but not at week 48 (1.8 vs. 3.8%, P = 0.93). Similar results were seen when limiting the analysis to patients with at least 80% adherence. The results were not related to severity of lipoatrophy or type of tNRTI. No changes were found in facial anthropometrics, fasting lipids, trunk fat, CD4 cell count, or HIV RNA.

CONCLUSIONS

We found a modest transient improvement in limb fat after 24 weeks of uridine. The lack of sustained efficacy at week 48 was not due to changes in adherence or reduction in sample size. Uridine was well tolerated and did not impair virologic control.

Uridine supplementation antagonizes zidovudine-induced mitochondrial myopathy and hyperlactatemia in mice

OBJECTIVE

Zidovudine is an antiretroviral nucleoside analog reverse transcriptase inhibitor that induces mitochondrial myopathy by interfering with the replication of mitochondrial DNA (mtDNA). Because zidovudine inhibits thymidine kinases, the mechanism of mtDNA depletion may be related to an impairment of the de novo synthesis of pyrimidine nucleotides, which are required building blocks of mtDNA. This study was undertaken to determine whether mitochondrial myopathy is a class effect of antiretroviral nucleoside analogs, and whether the muscle disease can be prevented by treatment with uridine as a pyrimidine nucleotide precursor.

METHODS

BALB/c mice were treated with zidovudine or zalcitabine. Some of the mice were cotreated with mitocnol, a dietary supplement with high uridine bioavailability. Mice hind limb muscles were examined after 10 weeks.

RESULTS

Zidovudine induced muscle fiber thinning, myocellular fat deposition, and abnormalities of mitochondrial ultrastructure. In mice treated with zidovudine, organelles contained low mtDNA copy numbers and reduced cytochrome c oxidase activity. The expression of the mtDNA-encoded cytochrome c oxidase I subunit, but not of nucleus-encoded mitochondrial proteins, was impaired. Zidovudine also increased the levels of myocellular reactive oxygen species and blood lactate. Uridine supplementation attenuated or normalized all pathologic abnormalities and had no intrinsic effects. Zalcitabine did not elicit muscle toxicity.

CONCLUSION

Our findings indicate that zidovudine, but not zalcitabine, induces mitochondrial myopathy, which is substantially antagonized by uridine supplementation. These results provide proof of the importance of pyrimidine pools in the pathogenesis of zidovudine myopathy. Since uridine supplementation is tolerated well by humans, this treatment strategy should be investigated in clinical trials.

Pyrimidine nucleoside depletion sensitizes to the mitochondrial hepatotoxicity of the reverse transcriptase inhibitor stavudine

Stavudine is a hepatotoxic antiretroviral nucleoside analogue that also inhibits the replication of mitochondrial DNA (mtDNA). To elucidate the mechanism and consequences of mtDNA depletion, we treated HepG2 cells with stavudine and either redoxal, an inhibitor of de novo pyrimidine synthesis, or uridine, from which pyrimidine pools are salvaged. Compared with treatment with stavudine alone, co-treatment with redoxal accelerated mtDNA depletion, impaired cell division, and activated caspase 3. These adverse effects were completely abrogated by uridine. Intracellular ATP levels were unaffected. Transcriptosome profiling demonstrated that redoxal and stavudine acted synergistically to induce CDKN2A and p21, indicating cell cycle arrest in G1, as well as genes involved in intrinsic and extrinsic apoptosis. Moreover, redoxal and stavudine showed synergistic interaction in the up-regulation of transcripts encoded by mtDNA and the induction of nuclear transcripts participating in energy metabolism, mitochondrial biogenesis, oxidative stress, and DNA repair. Genes involved in nucleotide metabolism were also synergistically up-regulated by both agents; this effect was completely antagonized by uridine. Thus, pyrimidine depletion sensitizes cells to stavudine-mediated mtDNA depletion and enhances secondary cell toxicity. Our results indicate that drugs that diminish pyrimidine pools should be avoided in stavudine-treated human immunodeficiency virus patients. Uridine supplementation reverses this toxicity and, because of its good tolerability, has potential clinical value for the treatment of side effects associated with pyrimidine depletion.

Mitochondrial Toxicity of Tenofovir, Emtricitabine and Abacavir Alone and in Combination with Additional Nucleoside Reverse Transcriptase Inhibitors

Background

Some nucleoside/nucleotide reverse transcriptase inhibitor (NRTI) combinations cause additive or synergistic interactions in vitro and in vivo.

Methods

We evaluated the mitochondrial toxicity of tenofovir (TFV), emtricitabine (FTC) and abacavir as carbovir (CBV) alone, with each other, and in combination with additional NRTIs. HepG2 human hepatoma cells were incubated with TFV, FTC, CBV, didanosine (ddI), stavudine (d4T), lamivudine (3TC) and zidovudine (AZT) at concentrations equivalent to 1 and 10x clinical steady-state peak plasma levels (Cmax). NRTIs were also used in double and triple combinations. Cell growth, lactate production, intracellular lipids, mtDNA and the mtDNA-encoded respiratory chain subunit II of cytochrome c oxidase (COXII) were monitored for 25 days.

Results

TFV and 3TC had no or minimal toxicity. FTC moderately reduced hepatocyte proliferation independent of effects on mtDNA. ddI and d4T induced a time- and dose-dependent loss of mtDNA and COXII, decreased cell growth and increased levels of lactate and intracellular lipids. CBV and AZT strongly impaired hepatocyte proliferation and increased lactate and lipid production, but did not induce mtDNA depletion. The dual combination of TFV plus 3TC had only minimal toxicity; TFV plus FTC slightly reduced cell proliferation without affecting mitochondrial parameters. All other combinations exhibited more pronounced adverse effects on mitochondrial endpoints. Toxic effects on mitochondrial parameters were observed in all combinations with ddI, d4T, AZT or CBV. TFV and 3TC both attenuated ddI-related cytotoxicity, but worsened the effects of CBV and AZT.

Conclusions

The data demonstrate unpredicted interactions between NRTIs with respect to toxicological endpoints and provide an argument against the liberal use of NRTI cocktails without first obtaining data from clinical trials.

Uridine supplementation in HIV lipoatrophy: pilot trial on safety and effect on mitochondrial indices

OBJECTIVES

Uridine abrogates mitochondrial toxicities of nucleoside reverse transcriptase inhibitor in adipocyte cell culture. We aim to study the effect of uridine supplementation on human adipocyte mitochondrial DNA (mtDNA) levels in subjects with human immunodeficiency (HIV) lipoatrophy.

METHODS

Sixteen patients with lipoatrophy on stavudine-containing antiretroviral therapy were enrolled, and received NucleomaxX, a dietary supplement with a high bioavailability of uridine (36 g TID every other day for 16 weeks). Patients were then followed off-uridine for another 16 weeks. Highly active antiretroviral therapy remained unchanged during the trial.

RESULTS

Fourteen patients completed the study. Two subjects dropped out before week 4 for study-unrelated reasons. No adverse events were noted throughout the study. HIV-1 RNA, CD4 counts, liver enzymes and hemoglobin remained unchanged. Body mass index, lactate, lipids, insulin and homeostasis model assessment of insulin resistance were unaltered. Fat and peripheral blood and mononuclear cell mtDNA levels did not correlate with each other and exhibited no changes throughout the study. Lipoatrophy scores by patients and physician improved significantly at weeks 16 and 32 compared to study entry.

CONCLUSION

In this pilot study, NucleomaxX was safe, well tolerated without apparent deleterious effect on HIV indices. In contrast to in vitro data, NucleomaxX did not lead to changes in fat or blood mtDNA levels.

Uridine supplementation antagonizes zalcitabine-induced microvesicular steatohepatitis in mice

Zalcitabine is an antiretroviral nucleoside analogue that exhibits long-term toxicity to hepatocytes by interfering with the replication of mitochondrial DNA (mtDNA). Uridine antagonizes this effect in vitro. In the present study we investigate the mechanisms of zalcitabine-induced hepatotoxicity in mice and explore therapeutic outcomes with oral uridine supplementation. BalbC mice (7 weeks of age, 9 mice in each group) were fed 0.36 mg/kg/d of zalcitabine (corresponding to human dosing adapted for body surface), or 13 mg/kg/d of zalcitabine. Both zalcitabine groups were treated with or without Mitocnol (0.34 g/kg/d), a dietary supplement with high bioavailability of uridine. Liver histology and mitochondrial functions were assessed after 15 weeks. One mouse exposed to high dose zalcitabine died at 19 weeks of age. Zalcitabine induced a dose dependent microvesicular steatohepatitis with abundant mitochondria. The organelles were enlarged and contained disrupted cristae. Terminal transferase dUTP nick end labeling (TUNEL) assays showed frequent hepatocyte apoptosis. mtDNA was depleted in liver tissue, cytochrome c-oxidase but not succinate dehydrogenase activities were decreased, superoxide and malondialdehyde were elevated. The expression of COX I, an mtDNA-encoded respiratory chain subunit was reduced, whereas COX IV, a nucleus-encoded subunit was preserved. Uridine supplementation normalized or attenuated all toxic abnormalities in both zalcitabine groups, but had no effects when given without zalcitabine. Uridine supplementation was without apparent side effects.

CONCLUSION

Zalcitabine induces mtDNA-depletion in murine liver with consequent respiratory chain dysfunction, up-regulated synthesis of reactive oxygen species and microvesicular steatohepatitis. Uridine supplementation attenuates this mitochondrial hepatotoxicity without apparent intrinsic effects.

Uridine Supplementation for the treatment of Antiretroviral Therapy-Associated Lipoatrophy: A Randomized, Double-Blind, Placebo-Controlled Trial

Background

Highly active antiretroviral therapy (HAART) is associated with loss of subcutaneous fat (lipoatrophy) presumably due to mitochondrial toxicity of nucleoside reverse transcriptase inhibitors. In vitro, uridine abrogates thymidine analogue-induced toxicity in adipocytes.

Methods

A total of 20 patients with HAART-associated lipoatrophy were randomized to receive either a dietary uridine supplement (36 g three times a day for 10 consecutive days/month) or placebo, for 3 months. Body composition was measured using dual energy X-ray absorptiometry, magnetic resonance imaging and proton spectroscopy. Data are mean ± standard error of mean.

Results

The mean increases in limb fat (880 ±140 versus 230 ±270 g; P<0.05), intra-abdominal fat (210 ±80 versus -80 ±70 cm3; P<0.05) and total body fat (1,920 ±240 versus 240 ±520 g; P<0.01) were significantly greater in the uridine than in the placebo group. Within the uridine group, the changes from baseline to 3 months were statistically significant in total limb fat ( P<0.001), intra-abdominal fat ( P<0.05) and total body fat ( P<0.001). The proportion of limb fat to total fat increased from 18% to 25% ( P<0.05) in the uridine group. Liver fat content and lean body mass remained unchanged in both groups. High-density lipoprotein-cholesterol concentrations decreased in the uridine and increased in the placebo group, whereas fasting serum insulin concentrations did not change. Uridine supplementation was well tolerated and the virological effect of HAART was not affected.

Conclusion

Uridine supplementation significantly and predominantly increased subcutaneous fat mass in lipoatrophic HIV-infected patients during unchanged HAART.

Uridine Abrogates the Adverse Effects of Antiretroviral Pyrimidine Analogues on Adipose Cell Functions

Objectives

Side effects of antiretroviral treatment such as lipoatrophy have been mainly attributed to mitochondrial toxicity of nucleoside reverse transcriptase inhibitors (NRTIs). We assessed whether uridine can abrogate the adverse effects of NRTIs on adipocyte functions.

Methods

3T3-F442A preadipocytes were exposed to stavudine (d4T; 10 μM), zidovudine (ZDV; 1 μM), zalcitabine (ddC; 0.2 μM) or didanosine (ddI; 10 μM) in the absence or presence of uridine 21 days prior to and 7 days after induction of differentiation. Then, lipid accumulation (oil red staining), apoptosis (flow cytometry, PARP-cleavage), mitochondrial mass (Mitotracker) and DNA (mtDNA), cytochrome c oxidase (COX) subunits and mitochondrial membrane potential (JC-1) were quantified.

Results

Whereas ddI had no effects, d4T, ZDV and ddC significantly decreased cellular lipid accumulation (by 32%, 46% and 24%, respectively), increased apoptosis and induced mitochondrial depolarization. d4T, ZDV and ddC decreased adipocyte mtDNA (by 64%, 53% and 46%, respectively) and reduced the mtDNA encoded COX II subunit. Uridine (200 μM) had no intrinsic effect, but prevented all adverse effects of d4T, ZDV and ddC on adipocyte morphology, lipid staining, apoptosis, mtDNA depletion (partial prevention with ZDV), mitochondrial mass and membrane potential. The effects of uridine were concentration-dependent. Uridine also fully reverted established d4T toxicities despite continued d4T exposure.

Conclusions

Uridine supplementation protects adipocytes from the adverse effects of d4T, ZDV and ddC on lipid accumulation, cell survival and mitochondrial functions, suggesting that the toxic effects could be linked to intracellular depletion of uridine or its metabolites. Uridine is an interesting candidate in the prevention of NRTI-induced lipoatrophy in vivo.

5-(Phenylthio)acyclouridine: a powerful enhancer of oral uridine bioavailability: relevance to chemotherapy with 5-fluorouracil and other uridine rescue regimens

PURPOSE

The purpose of this investigation was to evaluate the effectiveness of oral 5-(phenylthio)acyclouridine (PTAU) in improving the pharmacokinetics and bioavailability of oral uridine. PTAU is a potent and specific inhibitor of uridine phosphorylase (UrdPase, EC 2.4.2.3), the enzyme responsible for uridine catabolism. This compound was designed as a lipophilic inhibitor in order to facilitate its access to the liver and intestine, the main organs involved in uridine catabolism. PTAU is fully absorbed after oral administration with 100% oral bioavailability.

METHODS

Uridine (330, 660 or 1320 mg/kg) and/or PTAU (30, 45, 60, 120, 240 or 480 mg/kg) were orally administered to mice. The plasma levels of uridine, its catabolite uracil, and PTAU were measured using HPLC, and pharmacokinetic analysis was performed.

RESULTS

Oral PTAU up to 480 mg/kg per day is not toxic to mice. Oral PTAU at 30, 45, 60, 120 and 240 mg/kg has a prolonged plasma half-life of 2-3 h, and peak plasma PTAU concentrations (C(max)) of 41, 51, 74, 126 and 161 microM with AUCs of 70, 99, 122, 173 and 225 micromol h/l, respectively. Coadministration of uridine with PTAU did not have a significant effect on the pharmacokinetic parameters of plasma PTAU at any of the doses tested. Coadministration of PTAU (30, 45, 60 and 120 or 240 mg/kg) with uridine (330, 660 or 1320 mg/kg) elevated the concentration of plasma uridine over that following the same dose of uridine alone, a result of reduced metabolic clearance of uridine as evidenced by decreased plasma exposure (C(max) and AUC) to uracil. Plasma uridine was elevated with the increase of uridine dose at each PTAU dose tested and no plateau was reached. Coadministration of PTAU at 30, 45, 60, 120 and 240 mg/kg improved the low oral bioavailability (7.7%) of uridine administered at 1320 mg/kg by 4.3-, 5.9-, 9.9-, 11.7- and 12.5-fold, respectively, and reduced the AUC of plasma uracil (1227.8 micromol h/l) by 5.7-, 6.8-, 8.2-, 6.3-, and 6.9-fold, respectively. Similar results were observed when PTAU was coadministered with lower doses of uridine. Oral PTAU at 30, 45, 60, 120 and 240 mg/kg improved the oral bioavailability of 330 mg/kg uridine by 1.7-, 2.4-, 2.6-, 5.2- and 4.3- fold, and that of 660 mg/kg uridine by 2.3-, 2.7-, 3.3-, 4.6- and 6.7-fold, respectively.

CONCLUSION

The excellent pharmacokinetic properties of PTAU, and its extraordinary effectiveness in improving the oral bioavailability of uridine, could be useful to rescue or protect from host toxicities of 5-fluorouracil and various chemotherapeutic pyrimidine analogues used in the treatment of cancer and AIDS, as well as in the management of medical disorders that are remedied by the administration of uridine including CNS disorders (e.g. Huntington's disease, bipolar disorder), liver diseases, diabetic neuropathy, cardiac damage, various autoimmune diseases, and transplant rejection.

Ex vivo zidovudine (AZT) treatment of CD34+ bone marrow progenitors causes decreased steady state mitochondrial DNA (mtDNA) and increased lactate production

Haematopoietic suppression is one of the dose-limiting side effects of chronic zidovudine (AZT) therapy. We tested the hypothesis that AZT would reduce mitochondrial DNA (mtDNA) content in haematopoietic progenitors causing impaired haematopoiesis and mitochondrial dysfunction. We studied the effects of AZT 0-50 microM in vitro, on normal human CD34+ haematopoietic progenitor cells cultured ex vivo for up to 12 days. The mean AZT IC50 for granulocyte (phenotype CD15+/CD14-) and erythroid (phenotype glycophorin+/CD45-) cell proliferation was 2.5 microM (SD+/-0.7) and 0.023 microM (SD+/-0.005), respectively. In myeloid-rich cell cultures, the mean lactate content of the media, compared to untreated controls, increased by 86% (SD+/-23) at 10 microM AZT and in erythroid-rich cultures it increased by 134% (SD+/-24) in the presence of 0.5 microM AZT. In myeloid-rich cultures the AZT IC50 for the reduction in the mitochondrial/nuclear DNA content ratio was 5.6 microM, whereas in erythroid rich cultures this AZT IC50 was < 0.0005 microM. AZT produced concentration-dependent inhibition of CD34+ progenitor proliferation into both myeloid and erythroid lineages; erythropoiesis was more sensitive than myelopoiesis. Concurrently, AZT reduced steady state mtDNA content, while increasing lactate production. These findings support the hypothesis that mtDNA is one of the intracellular targets involved in the pathogenesis of AZT-associated bone marrow progenitor cell toxicity.

Uridine in the Prevention and Treatment of Nrti-Related Mitochondrial Toxicity

Long-term side effects of antiretroviral therapy are attributed to the mitochondrial (mt) toxicity of nucleoside analogue reverse transcriptase inhibitors (NRTIs) and their ability to deplete mtDNA. Studies in hepatocytes suggest that uridine is able to prevent and treat mtDNA depletion by pyrimidine NRTIs [zalcitabine (ddC) and stavudine (d4T)] and to fully abrogate hepatocyte death, elevated lactate production and intracellular steatosis. Uridine was also found to improve the liver and haematopoietic toxicities of zidovudine (AZT), which are unrelated to mtDNA depletion, and to prevent neuronal cell death induced by ddC. Most recently, uridine was found to prevent the onset of a lipoatrophic phenotype (reduced intracellular lipids, increased apoptosis, mtDNA depletion and mt depolarization) in adipocytes incubated long-term with d4T and AZT. Various steps of mt nucleoside utilization may be involved in the protective effect, but competition of uridine metabolites with NRTIs at polymerase y or other enzymes is a plausible explanation. Pharmacokinetic studies suggest that uridine serum levels can be safely increased in humans to achieve concentrations which are protective in vitro (50–200 μM). Uridine was not found to interfere with the antiretroviral activity of NRTIs. Mitocnol, a sugar cane extract which effectively increases uridine in human serum, was beneficial in individual HIV patients with mt toxicity and is now being tested in placebo-controlled randomized trials. Until these data become available, the risk-benefit calculation of using uridine should be individualized. The current safety data justify the closely monitored use of uridine in individuals who suffer from mt toxicity but who cannot be switched to less toxic NRTIs.

Liquid chromatographic method for the determination of uridine in human serum

To evaluate uridine levels in humans we developed a very sensitive and specific high-performance liquid chromatographic method for the determination of uridine in serum. We use techniques which are available in a standard analytical laboratory. Chromatographic analysis was carried out on a Phenomenex Aqua C18 5 micro 125A column protected by a guard cartridge system. Potassium dihydrogen phosphate buffer-acetonitrile was used as an eluent and oxypurinol as the internal standard. All sample preparation steps were done at 4 degrees C and the autosampler was cooled down to 4 degrees C. The calibration curve was linear throughout the calibration range from 0.25 to 100 micromol/l. This method was primarily established to evaluate uridine serum levels in patients with HIV infection since patients on highly active antiretroviral therapy (HAART) might develop metabolic disturbances that could lead to severe and fatal lactic acidosis due to mitochondrial toxicity. It is suggested that a limited or inadequate uridine supply is at least in part responsible for the onset of such deterioration.

Rapid determination of rat plasma uridine levels by HPLC-ESI-MS utilizing the Captiva plates for sample preparation

A rapid, accurate and precise HPLC-ESI-MS method for the determination of rat plasma uridine concentrations was developed and is described here. Sample preparation involves methanol precipitation of plasma proteins in a 96-well Captiva protein precipitation filter plate. A clear extract is drawn through the filter plate with vacuum, followed by evaporation of the extract and subsequent reconstitution prior to chromatography on a reversed-phase column with an aqueous mobile phase [0.1% (v/v) glacial acetic acid]. Detection was accomplished by positive-ion electrospray ionization mass spectrometry. A calibration curve ranging in concentration from 0.78 to 25 microM was constructed by best-fit, 1/x weighting linear regression analysis of the calibration standard concentrations vs peak height ratios of analyte with internal standard. The correlation coefficient was >0.995. The overall assay accuracy as shown by the back-calculated concentrations of the calibration curve ranged from 96.6 to 103% with RSD ranging from 4.5 to 20%. While this assay method was developed for the determination of uridine in rat plasma, it could be readily adapted for determination of uridine in plasma from other species, such as human.

Uridine Abrogates Mitochondrial Toxicity Related to Nucleoside Analogue Reverse Transcriptase Inhibitors in Hepg2 Cells

Objective

To assess in vitro if uridine may be suitable to prevent or treat mitochondrial toxicity related to nucleoside analogue reverse transcriptase inhibitors (NRTIs).

Methods

Human HepG2-hepatocytes were exposed to NRTIs with or without uridine for 25 days. Cell growth, lactate production, intracellular lipids, mitochondrial DNA (mtDNA) and the ratio between the respiratory chain components COX II (mtDNA-encoded) and COX IV (nuclear-encoded) were measured.

Results

HepG2 cells exposed to zalcitabine (177 nM) without uridine developed a severe depletion of mtDNA (to 8% of wild-type mtDNA levels), resulting in a decline of cell proliferation and COX II levels, with increased lactate and lipid accumulation. Uridine fully abrogated the adverse effects of zalcitabine on hepatocyte proliferation and normalized lactate synthesis, intracellular lipids and COX II levels by adjusting mtDNA levels to about 65% of NRTI-unexposed control cells. This effect was dose-dependent, with a maximum at 200 μM of uridine. Uridine also rapidly and fully restored cell function when added to cells with established mitochondrial dysfunction (zalcitabine for 15 days) despite continued zalcitabine exposure. Uridine also normalized cell proliferation in HepG2 cells exposed to 36 μM of stavudine and protected HepG2-cells exposed to 7 μM of zidovudine + 8 μM of lamivudine (pyrimidine analogues), but failed to improve cell function or mtDNA in cells exposed to 11.8 or 118 μM of didanosine (a purine analogue).

Conclusions

The pyrimidine precursor uridine may attenuate the mitochondrial toxicity of antiretroviral pyrimidine NRTIs in vitro, and its supplementation may represent a promising strategy in the prevention or treatment of mitochondrial toxicities in HIV-infected patients.

Antiviral L-nucleosides specific for hepatitis B virus infection

A unique series of simple "unnatural" nucleosides has been discovered to inhibit hepatitis B virus (HBV) replication. Through structure-activity analysis it was found that the 3'-OH group of the beta-L-2'-deoxyribose of the beta-L-2'-deoxynucleoside confers specific antihepadnavirus activity. The unsubstituted nucleosides beta-L-2'-deoxycytidine, beta-L-thymidine, and beta-L-2'-deoxyadenosine had the most potent, selective, and specific antiviral activity against HBV replication. Human DNA polymerases (alpha, beta, and gamma) and mitochondrial function were not affected. In the woodchuck model of chronic HBV infection, viral load was reduced by as much as 10(8) genome equivalents/ml of serum and there was no drug-related toxicity. In addition, the decline in woodchuck hepatitis virus surface antigen paralleled the decrease in viral load. These investigational drugs, used alone or in combination, are expected to offer new therapeutic options for patients with chronic HBV infection.

Mechanism of action of 1-beta-D-2,6-diaminopurine dioxolane, a prodrug of the human immunodeficiency virus type 1 inhibitor 1-beta-D-dioxolane guanosine

(-)-beta-D-2,6-Diaminopurine dioxolane (DAPD), is a nucleoside reverse transcriptase (RT) inhibitor with activity against human immunodeficiency virus type 1 (HIV-1). DAPD, which was designed as a water-soluble prodrug, is deaminated by adenosine deaminase to give (-)-beta-D-dioxolane guanine (DXG). By using calf adenosine deaminase a K(m) value of 15 +/- 0.7 microM was determined for DAPD, which was similar to the K(m) value for adenosine. However, the k(cat) for DAPD was 540-fold slower than the k(cat) for adenosine. In CEM cells and peripheral blood mononuclear cells exposed to DAPD or DXG, only the 5'-triphosphate of DXG (DXG-TP) was detected. DXG-TP is a potent alternative substrate inhibitor of HIV-1 RT. Rapid transient kinetic studies show the efficiency of incorporation for DXG-TP to be lower than that measured for the natural substrate, 2'-deoxyguanosine 5'-triphosphate. DXG-TP is a weak inhibitor of human DNA polymerases alpha and beta. Against the large subunit of human DNA polymerase gamma a K(i) value of 4.3 +/- 0.4 microM was determined for DXG-TP. DXG showed little or no cytotoxicity and no mitochondrial toxicity at the concentrations tested.

5-phenylthioacyclouridine: a potent and specific inhibitor of uridine phosphorylase

5-Phenylthioacyclouridine (PTAU or 1-[(2-hydroxyethoxy)methyl]-5-phenylthiouracil) was synthesized as a highly specific and potent inhibitor of uridine phosphorylase (UrdPase, EC 2.4.2.3). PTAU has inhibition constant (K(is)) values of 248 and 353 nM towards UrdPase from mouse and human livers, respectively. PTAU was neither an inhibitor nor a substrate for thymidine phosphorylase (EC 2.4.2.4), uridine-cytidine kinase (EC 2. 7.1.48), thymidine kinase (EC 2.7.1.21), dihydrouracil dehydrogenase (EC 1.3.1.2), orotate phosphoribosyltransferase (EC 2.4.2.10), or orotidine 5'-monophosphate decarboxylase (EC 4.1.2.23), the enzymes that could utilize the substrate (uridine or thymidine) or products (uracil or thymine) of UrdPase. Different isomers of 5-tolylthiouracil also were synthesized and tested as inhibitors of UrdPase. The meta-substituted isomer was 3- to 4-fold more potent as an inhibitor of UrdPase than the para- or ortho-substituted isomers. These data indicate that the hydrophobic pocket in the active site of UrdPase adjacent to the 5-position of the pyrimidine ring can accommodate the meta-substituted 5-phenyluracils better than the other isomers, leading to improved inhibition. Therefore, it is anticipated that the potency of PTAU can be increased further by the addition of certain hydrophobic groups at the meta position of the phenyl ring. PTAU has potential usefulness in the therapy of cancer and AIDS as well as other pathological and physiological disorders that can be remedied by the administration of uridine.

Safety assessment, in vitro and in vivo, and pharmacokinetics of emivirine, a potent and selective nonnucleoside reverse transcriptase inhibitor of human immunodeficiency virus type 1

Emivirine (EMV), formerly known as MKC-442, is 6-benzyl-1-(ethoxymethyl)-5-isopropyl-uracil, a novel nonnucleoside reverse transcriptase inhibitor that displays potent and selective anti-human immunodeficiency virus type 1 (HIV-1) activity in vivo. EMV showed little or no toxicity towards human mitochondria or human bone marrow progenitor cells. Pharmacokinetics were linear for both rats and monkeys, and oral absorption was 68% in rats. Whole-body autoradiography showed widespread distribution in tissue 30 min after rats were given an oral dose of [(14)C]EMV at 10 mg/kg of body weight. In rats given an oral dose of 250 mg/kg, there were equal levels of EMV in the plasma and the brain. In vitro experiments using liver microsomes demonstrated that the metabolism of EMV by human microsomes is approximately a third of that encountered with rat and monkey microsomes. In 1-month, 3-month, and chronic toxicology experiments (6 months with rats and 1 year with cynomolgus monkeys), toxicity was limited to readily reversible effects on the kidney consisting of vacuolation of kidney tubular epithelial cells and mild increases in blood urea nitrogen. Liver weights increased at the higher doses in rats and monkeys and were attributed to the induction of drug-metabolizing enzymes. EMV tested negative for genotoxic activity, and except for decreased feed consumption at the high dose (160 mg/kg/day), with resultant decreases in maternal and fetal body weights, EMV produced no adverse effects in a complete range of reproductive toxicology experiments performed on rats and rabbits. These results support the clinical development of EMV as a treatment for HIV-1 infection in adult and pediatric patient populations.

Azidothymidine induces apoptosis in mouse myeloma cell line Sp2/0

Azidothymidine (AZT), which has been extensively used as an antiviral agent in the treatment of AIDS, showed strong inhibition of growth of Sp2/0 cells in vitro. AZT-treated cells showed a decrease in viability in a dose-dependent manner. AZT specifically induced typical apoptotic cell death with DNA double-strand cleavage and subsequent formation of apoptotic bodies. The induction of DNA double-strand cleavage into the oligonucleosomal ladder by AZT was protected in the presence of thymidine or uridine. An increase in endonuclease activity from nuclear extract of AZT-treated cells was observed. The enzyme activity was found to be Ca(2+)-and Mg(2+)-dependent and was inhibited by zinc acetate. A marked enhancement of PARP activity was observed in AZT-treated cells. These observations show that AZT can trigger both morphological and biochemical changes typical of apoptosis in the mouse myeloma cell line Sp2/0.

The protective role of zinc and N-acetylcysteine in modulating zidovudine induced hematopoietic toxicity

The role of zinc and N-acetylcysteine (NAC) has been investigated in protecting the hematopoietic progenitor cells from zidovudine (AZT)-induced toxicity. Murine bone marrow progenitor cells (BMPC, 1x10(6)) were exposed to various concentrations (0.1-50 microM) of AZT in the presence and absence of zinc acetate (100 microM) or NAC (100 microM). The cell survival was determined by the colony forming assays of erythroid (CFU-E) and granulocytic (CFU-GM) lineage. The IC50 values of AZT in the presence of zinc were increased approximately 3-fold (from 3.0 to 9.5 microM) in the CFU-E assay and 7-fold (from 4.3 to 28.8 microM) in the CFU-GM assay whereas in the presence of NAC, the IC50 values were increased by 2- and 4-fold, respectively. To delineate the mechanism of significant protection of BMPC by zinc, the mRNA levels of metallothionein (MT) were monitored by using a 31-mer cDNA probe. Zinc produced a concentration-dependent increase in the MT mRNA levels in BMPC. These results suggest that zinc and NAC dietary supplementation can be conveniently used to reduce AZT-induced bone marrow toxicity.

5-(m-Benzyloxybenzyl)barbituric acid acyclonucleoside, a uridine phosphorylase inhibitor, and 2',3',5'-tri-O-acetyluridine, a prodrug of uridine, as modulators of plasma uridine concentration. Implications for chemotherapy

5-(m-Benzyloxybenzyl)barbituric acid acyclonucleoside (BBBA), the most potent inhibitor known of uridine phosphorylase (UrdPase, EC 2.4.2.3), the enzyme responsible for uridine catabolism, and 2',3',5'-tri-O-acetyluridine (TAU), a prodrug of uridine, were used to investigate the possibility of improving the bioavailability of oral uridine in mice. Oral BBBA administered at 30, 60, 120, and 240 mg/kg increased the concentration of plasma uridine (2.6 +/- 0.7 microM) by 3.2-, 4.6-, 5.4-, and 7.2-fold, respectively. After administration of 120 and 240 mg/kg BBBA, plasma uridine concentration remained 3- and 6-fold, respectively, higher than the plasma concentration at zero time (C0) for over 8 hr. On the other hand, BBBA did not change the concentration of plasma uracil. TAU was far more superior than uridine in improving the bioavailability of plasma uridine. The relative bioavailability of plasma uridine released from oral TAU (53%) was 7-fold higher than that (7.7%) obtained by oral uridine. Oral TAU at 460, 1000, and 2000 mg/kg achieved area under the curve (AUC) values of plasma uridine of 82, 288, and 754 mumol.hr/L, respectively. Coadministration of BBBA with uridine or TAU further improved the bioavailability of plasma uridine resulting from the administration of either alone and reduced the Cmax and AUC of plasma uracil. Coadministration of BBBA at 30, 60, and 120 mg/kg improved the relative bioavailability of uridine released from 2000 mg/kg TAU (53%) by 1.7-, 2.7-, and 3.9-fold, respectively, while coadministration of the same doses of BBBA with an equimolar dose of uridine (1320 mg/kg) increased the relative bioavailability of oral uridine (7.7%) by 4.1-, 5.3-, and 7.8-fold, respectively. Moreover, the AUC and Cmax of plasma uridine after BBBA (120 mg/kg) coadministration with TAU were 3.5- and 11.5-fold, respectively, higher than those obtained from coadministration of BBBA with an equimolar dose of uridine. The exceptional effectiveness of the BBBA plus TAU combination in elevating and sustaining high plasma uridine concentration can be useful in the management of medical disorders that are remedied by administration of uridine as well as to rescue or protect from host-toxicities of various chemotherapeutic pyrimidine analogues.

Effect of recombinant human hemoglobin on human bone marrow progenitor cells: protection and reversal of 3'-azido-3'-deoxythymidine-induced toxicity

Long-term therapy of AIDS patients with 3'-azido-3'-deoxythymidine (AZT) is limited by hematopoietic toxicity. While the mechanism(s) of this toxicity remain elusive, various strategies are being developed to reduce these toxic effects including combination therapy with non-myelotoxic anti-human immunodeficiency virus (HIV) drugs and/or administration of protective or rescue agents, such as cytokines and growth factors. Using a physiologically relevant human CD34+ bone marrow cell liquid culture system, a crosslinked human recombinant hemoglobin (rHb), currently in Phase II clinical trials, was investigated for effects on hematopoiesis and for its potential in protecting or reversing AZT-induced hematopoietic toxicity. These investigations demonstrated that 0.01, 0.1, or 1 microM human rHb did not affect the proliferation of erythroid or myeloid lineage cells. A concentration of 1 microM rHb partially protected erythroid lineage cells from an inhibition of proliferation induced by 0.1 and 1 microM AZT. Inhibition of proliferation of cells previously exposed to AZT was not reversed at this concentration. These data suggest that human rHb may be of benefit in reducing the toxic effects of AZT in the bone marrow of AIDS patients.

Selective protection of toxicity of 2',3'-dideoxypyrimidine nucleoside analogs by beta-D-uridine in human granulocyte-macrophage progenitor cells

beta-D-Uridine protected human granulocyte-macrophage lineage cells in both semi-solid (granulocyte-macrophage colony-forming units, CFU-GM) and liquid cultures against the toxic effects of 3'-azido-3'-deoxythymidine (AZT), 3'-fluoro-3'-deoxythymidine (FLT) and a combination of AZT and FLT, without impairment of the activities of these respective drugs against human immunodeficiency virus (HIV) replication. In addition, beta-D-uridine also protected human CFU-GM against toxicity of the in vivo AZT metabolite, 3'-amino-3'-deoxythymidine (AMT). Beta-L-uridine and alpha-D-uridine, two stereoisomers of the natural form, and the base uracil, were unable to protect cells against either AZT or FLT toxicity, whereas beta-D-uridine-5'-bis(SATE)phosphotriester, a prodrug of beta-D-uridine-5'-monophosphate, successfully protected cells against AZT toxic effects, suggesting that beta-D-uridine needs to be metabolized to its nucleotides to exert a pharmacological effect. These data suggest in addition that AZT, FLT and AMT share a common target site(s) of toxicity involved in myelosuppression.

Protection and rescue from 2',3'-dideoxypyrimidine nucleoside analog toxicity by hemin in human bone marrow progenitor cells

Long-term therapy of AIDS patients with 3'-azido-3'-deoxythymidine (AZT) remains of concern because of resulting hematopoietic toxicity. While the mechanism(s) of this toxicity remains elusive, alternative strategies are being developed to reduce these toxic effects, including combination therapy with nonmyelotoxic antihuman immunodeficiency virus drugs and/or administration of protective or rescue agents, including cytokines and growth factors. By using a particularly relevant human CD34+ liquid culture system, the unique profiles of dideoxynucleoside (ddN) toxicities to both proliferation and differentiation were demonstrated, with decreased potencies in the order of 3'-fluoro-3'-deoxythymidine (FLT) = 3'-amino-3'-deoxythymidine (AMT) = 2',3'-dideoxycytidine > AZT for inhibition of proliferation and in the order of FLT = AMT > AZT >> 2',3'-dideoxycytidine for inhibition of hemoglobin synthesis. Hemin selectively protected erythroid-lineage human burst-forming unit-erythroid cells from AZT- and AMT-induced inhibition but had no effect on FLT toxicity under similar conditions. Myeloid-lineage human CFU-granulocyte-macrophages were also not protected by hemin against all three ddN analogs. The simultaneous exposure of cells to hemin and AZT resulted in a complete protection of both cell proliferation and hemoglobin synthesis. In contrast, in reversal studies only the inhibition of the percentage of hemoglobin-synthesizing cells returned to control levels, but the inhibition of proliferation of cells previously exposed to AZT was not reversed by hemin. These studies further define the unique and multifactorial mechanism(s) of ddN-induced toxic effects during hematopoietic development of pluripotent stem cells and suggest that the use of hemin could be beneficial in alleviating the toxicity of certain ddN analogs.

Biochemical modulation of iododeoxyuridine by N6-[4-(morpholinosulfonyl)benzyl]-N6-methyl-2,6-diaminobenz[cd]indole glucuronate (AG-331) leading to enhanced cytotoxicity

Inhibition of thymidylate synthase (TS) may increase incorporation of thymidine analogues into DNA, leading to increased inhibition of colony formation in tumor cells. We have reported previously that TS inhibition by N-(5-[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6,-ylmethyl)-N -methylamino]-2 - thenoyl)-L-glutamic acid (ICI D1694 or Tomudex), a folate-based TS inhibitor, increases the cytotoxicity of iododeoxyuridine (IdUrd), a thymidine analogue, in MGH-U1 human bladder and HCT-8 human colon cancer cells. N6-[4-(Morpholinosulfonyl)benzyl]-N6-methyl-2,6-diaminobenz[ cd]-indole glucuronate (AG-331) differs from ICI D1694 in that it is a de novo designed lipophilic TS inhibitor, it does not require a specific carrier for cellular uptake, and it does not undergo intracellular polyglutamation. Exposure of MGH-U1 cells to 5 microM AG-331 for 24 hr decreased clonogenic survival by 30%, but almost completely inhibited TS activity. IdUrd is a cytotoxic thymidine analogue, with IC50 and IC90 values after 24-hr exposures in MGH-U1 cells of 13 and 81 microM, respectively. The combination of IdUrd and AG-331 resulted in an enhanced antitumor effect, as compared with the effect of either agent alone. The cytotoxic IC50 of IdUrd decreased from 13 to 1.5 microM, and the IC90 decreased from 81 to 5 microM with the addition of 5 microM AG-331. Biochemical studies of the combination revealed that pretreating MGH-U1 cells with 5 microM AG-331 increased IdUrd incorporation into cellular DNA by 3.8-fold. This increased incorporation was associated with a greater proportion of DNA single-strand breaks than observed with either agent alone, and the combination of 5 microM AG-331 plus IdUrd produced up to a 2.5-fold increase in DNA single-strand breaks as compared with IdUrd alone. The effects of AG-331, IdUrd, and the combination of IdUrd and AG-331 on the colony-forming ability of normal human bone marrow CFU-GM cells was determined as a measure of myelosuppression. The combination of IdUrd and AG-331, at the same concentrations as those used in the MGH-U1 cells, produced a wider therapeutic index relative to that of IdUrd alone, and the therapeutic index for the combination was 6.5, as compared with 4.0 for IdUrd plus ICI D1694 in previous studies from this laboratory. These observations suggest that the combination of IdUrd and AG-331 may enhance antitumor effects with minimal myelosuppression in vivo.

Antiviral drugs from the nucleoside analog family block volume-activated chloride channels

BACKGROUND

The antiviral drugs AZT and acyclovir are generally used in the treatment of infections with human immunodeficiency virus (HIV) and herpes simplex virus (HSV). These substances are known to impede virus replication by premature nucleic acid chain termination. It is not yet clear, however, if this is the sole mechanism responsible for the antiviral and/or the numerous side effects observed in patients treated with these agents. We investigated the swelling-induced chloride current in fibroblasts, which we demonstrated is closely related or identical to a cloned epithelial chloride channel, ICln: This chloride channel can be blocked by nucleotides.

MATERIALS AND METHODS

Electrophysiological, fluorescence optical, and volume measurements were made to determine the effect of nucleoside analogs on the swelling-dependent chloride current (ICl) in NIH 3T3 fibroblasts and in human T cell lymphoma (H9) cells and the cAMP-dependent chloride current in CaCo cells.

RESULTS

AZT and acyclovir block the swelling-dependent chloride current and the chloride flux in fibroblasts, and the regulatory volume decrease (RVD) and ICl in H9 cells. This immediate effect can be substantially reduced by the simultaneous incubation of the cells with thymidine-5'-diphosphate (TDP) or uridine, both of which are by themselves unable to affect ICl.

CONCLUSIONS

We show here a novel molecular mechanism by which antiviral drugs of the nucleoside analog family could lead to impairments of the kidney, bone marrow, gastrointestinal, and neuronal functions, and how these side effects could possibly be restricted by the presence of TDP or uridine.

Effects of 5-benzylacyclouridine, an inhibitor of uridine phosphorylase, on the pharmacokinetics of uridine in rhesus monkeys: implications for chemotherapy

The effects of subcutaneous administration of 5-benzylacyclouridine (BAU), a uridine phosphorylase (UrdPase, EC 2.4.2.3) inhibitor, on uridine concentration in plasma and urine were evaluated in rhesus monkeys. Administration of BAU at 50, 100 and 250 mg/kg increased the plasma uridine baseline concentration 1.5-, 2.9-, and 3.2-fold, respectively. The basis for this moderate perturbation of plasma uridine by BAU was investigated using a tracer dose of 500 microCi 3H-uridine. Administration of 3H-uridine alone led to its rapid catabolism to uracil and dihydrouracil. Administration of 83.3 mg/kg BAU with 500 microCi 3H-uridine resulted in a 2.5-fold enhancement of 3H-uridine plasma levels and a substantial decrease in the plasma levels of uridine catabolites, suggesting inhibition of UrdPase activity by BAU in rhesus monkeys. Coadministration of 83.3 mg/kg BAU with 83.3 mg/kg uridine also reduced the plasma concentration of uracil and dihydrouracil, but it did not increase plasma uridine concentration above that of control animals receiving 83.3 mg/kg uridine alone. In animals receiving uridine alone at 83.3 or 25 mg/kg, approximately 10% of the administered dose was recovered in the urine within 6 h, with unchanged uridine being the major component. In contrast, administration of 83.3 mg/kg BAU increased the excretion of unchanged uridine to more than 32% of the total dose administered, even when the urinary excretion ratio of uracil to uridine was reduced ten-fold. Administration of multiple doses (three times per day) of BAU alone (83.3 mg/kg) or in the presence of uridine (83.3 mg/kg) did not enhance plasma uridine concentration further. In addition, uridine pharmacokinetics were associated with a time-dependent relationship as evidenced by an increased total plasma clearance, renal clearance and volume of distribution, resulting in a substantial decrease in uridine peak concentration with time. These results indicate that administration of BAU inhibits UrdPase activity in rhesus monkeys as manifested by decreased uracil and dihydrouracil plasma levels, as well as a lower urinary excretion ratio of uracil to uridine, as compared to control animals. However, plasma levels of unchanged uridine were not substantially enhanced by BAU in spite of the large increase in urinary excretion of unchanged uridine. This phenomenon was also observed when uridine was coadministered with BAU, suggesting that plasma uridine concentration in monkeys may be strongly regulated by the renal system as evidenced by the "spillover" of excess plasma uridine into urine. In addition, the pharmacokinetics of uridine were dose-independent, but time-dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

5-Benzylbarbituric acid derivatives, potent and specific inhibitors of uridine phosphorylase

5-Benzylbarbituric acid derivatives were synthesized as a series of new, specific, and potent inhibitors of uridine phosphorylase. Among these, 5-(m-benzyloxy)benzyl-1-[(2-hydroxyethoxy)methyl] barbituric acid (5-benzyloxybenzylbarbituric acid acyclonucleoside, BBBA) was found to be the most potent with Ki values of 1.1 +/- 0.2 and 2.6 +/- 0.3 nM with uridine phosphorylase from human and mouse livers, respectively. BBBA exhibited competitive inhibition with uridine phosphorylase from both human and mouse livers. The 5-benzylbarbituric acid derivatives are specific inhibitors of uridine phosphorylase, as they had no effect on thymidine phosphorylase (EC 2.4.2.4), thymidine kinase (EC 2.7.1.21), uridine-cytidine kinase (EC 2.7.1.48), orotate phosphoribosyltransferase (EC 2.4.2.10), orotidine 5'-monophosphate decarboxylase (EC 4.1.2.23), and dihydrouracil dehydrogenase (EC 1.3.1.2). These compounds are more potent, easier to synthesize, and have better water solubility than their uracil counterparts as inhibitors of uridine phosphorylase. Furthermore, the 5-benzylbarbituric acids were found to be better inhibitors of human uridine phosphorylase than the murine enzyme, whereas the reverse holds true for the 5-benzyluracil derivatives. The 5-benzylbarbituric acid derivatives have potential usefulness in the therapy of cancer and AIDS, as well as other pathological and physiological disorders.