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

The tetrapeptide AcSerAspLysPro (Seraspenide), a hematopoietic inhibitor, may reduce the in vitro toxicity of 3'-azido-3'-deoxythymidine to human hematopoietic progenitors

3'-azido-3'-deoxythymidine (AZT), the main antiviral drug used in AIDS treatment, is known to induce anemia and neutropenia. These effects have been attributed to its toxicity to hematopoietic progenitors. In this report, we present a new approach to reduce AZT hematotoxicity by using an inhibitory factor of the hematopoietic stem cells, the tetrapeptide AcSerAspLysPro (AcSDKP, Seraspenide), which has been shown to increase the survival of mice subjected to high doses of chemotherapy and to block reversibly the cycling of human granulocyte-macrophage colony forming unit (CFU-GM) and burst forming unit erythroid (BFU-E) progenitors. Normal bone marrow mononuclear cells (BMMNC) from 14 subjects were incubated with or without AcSDKP (10(-10) M) for 20 h and with or without AZT (100 microM) for another 2 h. After washing, cells were plated in methylcellulose in the presence of interleukin 3 (IL-3), granulocyte-macrophage colony stimulating factor (GM-CSF) and erythropoietin (EPO). Under these conditions, the preincubation of cells with AcSDKP reduced significantly the toxicity of AZT to both BFU-E and CFU-GM at least in 3 out of 8 and 4 out of 10 cases, respectively. A careful statistical analysis of these observations indicates that AcSDKP may be an efficient factor in preserving progenitors against AZT-induced hematopoietic toxicity.

3'-azido-3'-deoxythymidine (AZT) monophosphate: an inhibitor of exonucleolytic repair of AZT-terminated DNA

A 3'-exonuclease(s) that excised 3'-azido-3'-deoxythymidine (AZT) monophosphate (AZTMP) from the 3' terminus of DNA was partially purified from two human cell lines. AZTMP inhibited the hydrolysis of AZTMP-terminated single-stranded and double-stranded DNA substrates. Thus, high levels of AZTMP might inhibit the exonuclease and trigger the toxicity of AZT by impairing the repair of AZTMP-terminated DNA.

Species-dependent differences in the biochemical effects and metabolism of 5-benzylacyclouridine

The pharmacokinetics and biochemical effects of the uridine phosphorylase (UrdPase) inhibitor 5-benzylacyclouridine (BAU) were investigated in the mouse, rat and monkey. Following i.p. administration of BAU (30 mg/kg) in the mouse and i.v. administration in the rat and monkey, initial BAU plasma half-life values were 36, 36 and 25 min, and the areas under the plasma BAU concentration versus time curves (AUC) were 127, 80 and 76 microM.hr, respectively. Rats were also dosed p.o. and i.v. with BAU at 90 mg/kg, and a comparison of the AUC values showed an oral bioavailability of 70%. Analyses of plasma samples by HPLC indicated that the metabolism of BAU differed in these species. A major BAU metabolite was observed in monkeys. Its concentration was greater than or equal to that of BAU in almost every plasma sample, and its elimination paralleled that of BAU. Urinary recovery of the metabolite was 10-fold higher than the recovery of unchanged drug. The compound was identified as the ether glucuronide of BAU by its UV absorption spectrum, its co-elution with BAU after incubation with beta-glucuronidase, and liquid chromatography/mass spectrum analysis. A different metabolite was detected in rat plasma; its maximum concentration was 15% of the BAU level, and its elution position on the HPLC chromatogram was not affected by the action of beta-glucuronidase. BAU had equivalent potency against UrdPase in liver extracts from the three species, with Ki values of about 0.17 microM. However, the in vivo effects of BAU on plasma uridine concentrations were species dependent. In mice, a 30 mg/kg i.p. dose of BAU increased the plasma uridine concentration to 11 microM from a control level of 1.8 microM. In the rat, a 30 mg/kg i.v. dose of BAU increased plasma uridine to 2.1 from 1.1 microM control levels, and a 300 mg/kg oral dose resulted in a peak plasma uridine concentration of only 6 microM. In the monkey, BAU (30 mg/kg, i.v.) had no effect on plasma uridine despite the presence of 10-100 microM BAU levels in plasma for 1.5 hr. These data show that there are significant differences in the biochemical effects and metabolism of BAU in CD-1 mice, CD rats and cynomolgus monkeys.

Selective inhibition of human immunodeficiency viruses by racemates and enantiomers of cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

2',3'-Dideoxy-5-fluoro-3'-thiacytidine (FTC) has been shown to be a potent and selective compound against human immunodeficiency virus type 1 in acutely infected primary human lymphocytes. FTC is also active against human immunodeficiency virus type 2, simian immunodeficiency virus, and feline immunodeficiency virus in various cell culture systems, including human monocytes. The antiviral activity can be prevented by 2'-deoxycytidine, but not by other natural nucleosides, suggesting that FTC must be phosphorylated to be active and 2'-deoxycytidine kinase is responsible for the phosphorylation. By using chiral columns or enzymatic techniques, the two enantiomers of FTC were separated. The (-)-beta-enantiomer of FTC was about 20-fold more potent than the (+)-beta-enantiomer against human immunodeficiency virus type 1 in peripheral blood mononuclear cells and was also effective in thymidine kinase-deficient CEM cells. Racemic FTC and its enantiomers were nontoxic to human lymphocytes and other cell lines at concentrations of up to 100 microM. Studies with human bone marrow cells indicated that racemic FTC and its (-)-enantiomer had a median inhibitory concentration of > 30 microM. The (+)-enantiomer was significantly more toxic than the (-)-enantiomer to myeloid progenitor cells. The susceptibilities to FTC of pretherapy isolates in comparison with those of posttherapy 3'-azido-3'-deoxythymidine-resistant viruses in human lymphocytes were not substantially different. Similar results were obtained with well-defined 2',3'-dideoxyinosine- and nevirapine-resistant viruses. (-)-FTC-5'-triphosphate competitively inhibited human immunodeficiency virus type 1 reverse transcriptase, with an inhibition constant of 2.9 microM, when a poly(I)n.oligo(dC)19-24 template primer was used. These results suggest that further development of the (-)-Beta-enantiomer of FTC is warranted as an antiviral agent for infections caused by human immunodeficiency viruses.

Tissue-specific expansion of uridine pools in mice. Effects of benzylacyclouridine, dipyridamole and exogenous uridine

The concentration of uridine (Urd) in murine tissues appears to be controlled by Urd catabolism, concentrative Urd transport, and the non-concentrative, facilitated diffusion of Urd. Previous reports document the tissue-specific disruption of these processes, and subsequently intracellular pools of free Urd in mice, by the administration of exogenous Urd (250 mg/kg) or the Urd phosphorylase (EC 2.4.2.3; uracil:ribose-1-phosphate phosphotransferase) inhibitor 5-benzylacyclouridine (BAU) (240 mg/kg). We now report the effect of combinations of BAU (120 mg/kg, p.o.), the nucleoside transport inhibitor dipyridamole (DP) (25 mg/kg, i.p.), and exogenous Urd (250 mg/kg, i.v.) on Urd pools in mice. This dose of BAU increased Urd pools 2- to 6-fold, in a tissue-specific manner, for up to 5 hr. DP increased Urd pools 3-fold in spleen, over a 4-hr period, but did not affect other tissues. Administration of BAU 1 hr prior to exogenous Urd resulted in a 50- to 100-fold expansion of tissue normal after 6 hr. Administration of DP 1 hr prior to exogenous Urd caused a tissue-specific 40- to 100-fold increase in Urd pools which, except in spleen, returned to normal within 2 hr. The marked additive effects of these combinations were in contrast to those obtained following the administration of BAU 1 hr prior to DP. This regimen increased Urd pools from 4- to 9-fold, in a tissue-specific manner. In addition, Urd pools remained elevated for up to 9 hr, except in spleen where the Urd concentration was elevated for up to 15 hr. Analysis of enzyme activities indicated that DP does not enhance the inhibitory effect of BAU against murine liver Urd phosphorylase. However, DP did inhibit plasma clearance of BAU, and this effect may partially explain the apparent synergistic effect of this combination. In spite of the prolonged and dramatic expansion of tissue Urd pools produced by BAU + DP, the total Ura nucleotide content in spleen, gut and colon tumor 38 (CT38) increased by less than 70% over a 12-hr period following administration of this combination. These findings are discussed in light of their biochemical and therapeutic implications.

Differential inhibition of 2'-deoxycytidine salvage as a possible mechanism for potentiation of the anti-human immunodeficiency virus activity of 2',3'-dideoxycytidine by dipyridamole

Dipyridamole, a commonly used coronary vasodilator and antithrombotic drug, was recently shown to potentiate the activity of 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine against the human immunodeficiency virus type 1 (HIV-1) in human monocyte-macrophages in vitro. We report in the present paper that in uninfected monocyte-macrophages dipyridamole significantly inhibits cellular salvage of [3H]deoxycytidine, whereas it does not affect the salvage of [3H]dideoxycytidine. Similar differential inhibition by dipyridamole of the salvage of thymidine, as opposed to 3'-azido-3'-deoxythymidine, was reported previously (G. V. Betageri, J. Szebeni, K. Hung, S. S. Patel, L. M. Wahl, M. Corcoran, and J. N. Weinstein, Biochem. Pharmacol. 40:867-870, 1990). Taken together, these observations suggest that inhibition of the salvage of competing physiological nucleosides may explain or contribute to the potentiating effect of dipyridamole on these antiviral dideoxynucleoside drugs.

3'-Azido-3'-deoxythymidine inhibition of human lymphocyte cytolytic function in vitro

Despite administration of 3'-azido-3'-deoxythymidine (AZT, Zidovudine) to seriously immunocompromised patients, little has been reported regarding effects of AZT on specific immune functions. This study analyzed the in vitro effect of AZT on normal human lymphocyte cytolytic activity. AZT at concentrations up to 100 microM had no effect when added directly to cytotoxicity assays with lymphocyte effector cells and natural killer (NK)-sensitive or NK-resistant target cells. In contrast, addition of AZT to lymphocytes cultured for 4-10 days with interleukin-2 (IL-2) prior to cytotoxicity assays produced a concentration- and time-dependent inhibition; this effect was not mimicked by acyclovir or ganciclovir. Lymphocyte cell numbers and viability were not reduced in parallel to inhibition of cytolytic activity by AZT. Furthermore, AZT inhibition of IL-2-dependent cytolytic activity was not correlated with alterations in lymphocyte cell surface phenotypes by flow cytometry, and lymphocyte culture supernatant levels of interferon-gamma were not reduced by AZT. These results suggest that AZT may selectively inhibit human lymphocyte functions and thus may have implications for long-term therapeutic administration of AZT in chronic immunodeficiency states.

Effects of thymidine and uridine on the phosphorylation of 3'-azido-3'-deoxythymidine (zidovudine) in human mononuclear cells

The effects of thymidine and uridine on the phosphorylation of 3'-azido-3'-deoxythymidine (AZT) were studied in various human mononuclear cell preparations. Thymidine suppressed [3H]AZT phosphorylation in the same concentration range (20 to 100 microM) in which it antagonizes the anti-human immunodeficiency virus activity of AZT. Uridine, in turn, had no influence on AZT phosphorylation, just as it has no effect on the anti-human immunodeficiency virus activity of AZT. These findings are consistent with a close relationship between the inhibition of AZT phosphorylation and the influence of physiological nucleosides on the antiviral activity of AZT.

Antiretroviral activity, biochemistry, and pharmacokinetics of 3'-azido-2',3'-dideoxy-5-methylcytidine

3'-Azido-2',3'-dideoxy-5-methylcytidine (CS-92, AzddMeC) is an antiviral nucleoside analogue structurally related to 3'-azido-3'-deoxythymidine (AZT). CS-92 is a potent and selective inhibitor of HIV-1 reverse transcriptase and HIV-1 replication in human lymphocytes and macrophages. The EC50 for CS-92 in HIV-1-infected human PBM cells was 0.09 microM. In HIV-1-infected human macrophages, the EC50 was 0.006 microM. This compound was also effective against human immunodeficiency virus type 2 in lymphocytes. The replication of Friend murine virus was only weakly inhibited, and no effect was observed against herpes simplex virus type 1 and type 2 and coxsackievirus B4. CS-92 was not toxic to PBM or Vero cells when tested up to 200 microM and was, furthermore, at least 40 times less toxic to granulocyte-macrophage and erythroid precursor cells in vitro than was AZT. The interaction of the 5'-triphosphate of CS-92 with HIV-1 reverse transcriptase indicated competitive inhibition (the inhibition constant, Kis, was 0.0093 microM) with a 30-fold greater affinity for CS-92-TP than for ddCTP. CS-92-TP inhibited HIV-1 reverse transcriptase by 50% at a concentration 6,000-fold lower than that which was required for a similar inhibition of DNA polymerase alpha. Pharmacokinetic studies showed that CS-92 was not deaminated to AZT in rats, but this compound was found to have a half-life of 2.7 hours. In rhesus monkeys, however, a compound with a retention time and ultraviolet spectra characteristics similar to AZT was detected. The mean half-life in rhesus monkeys for CS-92 was 1.52 and 1.74 h after intravenous and oral administration, respectively, and the oral bioavailability was about 21 percent. Additional preclinical studies with CS-92 will determine the ultimate utility of this antiviral agent for the treatment of HIV-1 infections.

Molecular targets for AIDS therapy

The development of antiretroviral therapy against acquired immunodeficiency syndrome (AIDS) has been an intense research effort since the discovery of the causative agent, human immunodeficiency virus (HIV). A large array of drugs and biologic substances can inhibit HIV replication in vitro. Nucleoside analogs--particularly those belonging to the dideoxynucleoside family--can inhibit reverse transcriptase after anabolic phosphorylation. 3'-Azido-2',3'-dideoxythymidine (AZT) was the first such drug tested in individuals with AIDS, and considerable knowledge of structure-activity relations has emerged for this class of drugs. However, virtually every step in the replication of HIV could serve as a target for a new therapeutic intervention. In the future, non-nucleoside-type drugs will likely become more important in the experimental therapy of AIDS, and antiretroviral therapy will exert major effects against the morbidity and mortality caused by HIV.

Effect of dipyridamole on transport and phosphorylation of thymidine and 3'-azido-3'-deoxythymidine in human monocyte/macrophages

Dipyridamole (DPM), a commonly used coronary vasodilator and antithrombotic drug, was shown recently to potentiate the antiviral effect of 3'-azido-3'-deoxythymidine (AZT) in HIV-1 infected human monocyte-derived macrophages (M/M) in vitro. We report in the present study that in uninfected M/M, DPM markedly inhibited cellular uptake of [3H]thymidine (dThd) and its incorporation into the nucleotide pools, particularly the dThd-triphosphate pool. In contrast, DPM did not affect cellular uptake and phosphorylation of [3H]AZT. Since dThd counteracts the phosphorylation and antiviral action of AZT, these findings support the hypothesis that the potentiation of the anti-HIV effect of AZT is due, at least in part, to differential inhibition of nucleoside salvage.

Inhibitory effects of potent inhibitors of human immunodeficiency virus and cytomegalovirus on the growth of human granulocyte-macrophage progenitor cells in vitro

Phosphonoformate, ganciclovir, zidovudine and the novel acyclic nucleotide analogues (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine [(S)-HPMPA] and (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine [(S)-HPMPC] were evaluated for their inhibitory effect on colony forming unit formation by human granulocyte-macrophage progenitor cells (obtained from 11 healthy volunteers) in vitro. The 50% inhibitory dose of zidovudine, (S)-HPMPA, (S)-HPMPC, ganciclovir and phosphonoformate were 10.61, 16.55, 80.88, 41.02 and 668.64 microM, respectively, when the median-effect principle was applied. The bone marrow toxicity of zidovudine used at a low concentration (3.74 microM) was significantly decreased if the drug was combined with (S)-HPMPA, (S)-HPMPC or ganciclovir. If used at higher concentrations (74.90 microM), zidovudine showed increased myelotoxicity in the presence of (S)-HPMPA and ganciclovir, but not (S)-HPMPC.

Activities of 3'-azido-3'-deoxythymidine nucleotide dimers in primary lymphocytes infected with human immunodeficiency virus type 1

The relative antiviral potencies of five nucleotide heterodimers of 3'-azido-3'-deoxythymidine (AZT), 3'-azido-3'-deoxythymidilyl-(5',5')-2'-3'-dideoxy-5'-adenylic acid (AZT-P-ddA), 3'-azido-3'-deoxythymidilyl-(5',5')-2',3'-dideoxy-5'-inosinic acid (AZT-P-ddI), and the corresponding 2-cyanoethyl congeners AZT-P(CyE)-ddA and AZT-P(CyE)-ddI, were determined in primary human peripheral blood mononuclear cells infected with human immunodeficiency virus type 1. The homodimer 3'-azido-3'-deoxythymidilyl-(5',5')-3'-azido-3'-deoxythymidilic acid (AZT-P-AZT) was also included for comparison. The potencies of the compounds were AZT-P-ddA greater than or equal to AZT-P-ddI greater than AZT-P(CyE)-ddA greater than or equal to AZT-P(CyE)-ddI greater than or equal to AZT greater than AZT-P-AZT. Whereas AZT-P-ddA and AZT-P-ddI had in vitro therapeutic indices greater than that of AZT, the homodimer of AZT had a low therapeutic index. AZT-P-ddI exhibited the lowest toxicity in peripheral blood mononuclear, Vero, or CEM cells. Combination studies between AZT and 2',3'-dideoxyinosine (ddI) at nontoxic concentrations indicated a synergistic interaction at a drug ratio of 1:100. At higher ratios (1:500 and 1:1,000), the interactions were synergistic only at concentrations that produced up to 75% virus inhibition. At higher levels of antiviral effects, this combination was antagonistic, as determined by the multiple drug effect analysis method. AZT-P-ddI was about 10-fold less toxic than AZT to human granulocyte-macrophage progenitor cells. However, no significant difference was apparent when the compounds were evaluated against cells of the erythroid lineage. The greater antiviral activity and lower toxicity of this compound could not be attributed to the extracellular decomposition of the dimer in media at physiological temperature and pH. However, in acidic solutions, AZT-P-ddI decomposed in a pH-dependent manner. Advanced preclinical studies with this heterodimer of two clinically effective antiretroviral agents should be considered.

Increased therapeutic efficacy of zidovudine in combination with vitamin E

Antiviral activity and bone marrow toxicity of 3'-azido-3'deoxythymidine (Zidovudine; AZT) was evaluated in the presence of alpha-D-tocopherol acid succinate (ATS) in the MT4 cell line and in murine hematopoietic progenitor cells, respectively. At varying concentrations (.016 to .125 microM) of AZT, addition of ATS (5 to 15 micrograms/ml) showed a dose-dependent increase in anti-HIV activity. The ED90 of AZT in this test system was 0.37 microM, whereas in the presence of ATS (15 micrograms/ml) it was 0.06 microM, thus producing an approximately 6-fold increase in anti-HIV activity. In contrast, in murine bone marrow cells, ATS (4 micrograms/ml) showed significant protection (p less than 0.05) against AZT-induced toxicity as measured by CFU-E and CFU-GM assays. The IC50 values in the presence and absence of ATS for CFU-E were 3.7 and 1.5 microM, whereas for CFU-GM were 6.0 and 2.7 microM, respectively. Overall, these data suggest that AZT in combination with ATS has greater therapeutic efficacy against HIV-1.

Antiviral therapy in human immunodeficiency virus infection

A rapid expansion of our knowledge of drugs that intervene with human immunodeficiency virus (HIV) infection has taken place. This review covers known and potential anti-HIV targets, including receptor blocking agents, membrane stabilisers, reverse transcriptase inhibitors and chain terminators, RNases, agents altering activation, assembly, budding or regulation of transcription and translation, post-transcriptional modifications and other areas. Important or promising agents, such as zidovudine (ZVD; azidothymidine, AZT), dideoxycytidine, dideoxyinosine, foscarnet, interferons, imuthiol, isoprinosine and others that are either on the market or in advanced clinical trials are emphasised. Four years after the discovery of the aetiological agent, the first drug, zidovudine, has been registered. Many questions about this drug remain, however, owing to the haste with which it was developed. An unprecedented number of other compounds are under evaluation, making it difficult to assess the relative merits of the different compounds and thus set priorities for their development. The point has been reached where a better economical and intellectual framework is necessary so that researchers and physicians are not overwhelmed by the difficulties of conducting clinical trials during the epidemic and have a reasonable chance of keeping up with laboratory developments.

Zidovudine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy

Zidovudine (azidothymidine) is a thymidine analogue antiretroviral drug active against human immunodeficiency virus (HIV). In acquired immunodeficiency syndrome (AIDS) and AIDS-related complex (ARC) patients, orally and intravenously administered zidovudine is effective in reducing the incidence of opportunistic infections and neoplasms, increasing helper T lymphocyte numbers, and improving survival rates and quality of life. Adverse effects include serious haematological abnormalities and severe headache, abdominal discomfort, nausea, myalgia and insomnia. In addition, neutropenia and other anaemias frequently limit zidovudine therapy and may result in a need for multiple blood transfusions, dose reductions or withdrawal of the drug. However, despite these problems and the lack of information about some aspects of zidovudine use, zidovudine provides a major hope for HIV-infected patients, and it has rapidly become the standard therapy for improving the quality and duration of the lives of AIDS and ARC patients.

Interaction of the 5'-phosphates of the anti-HIV agents, 3'-azido-3'-deoxythymidine and 3'-azido-2',3'-dideoxyuridine, with thymidylate synthase

A study has been made of the interaction of 3'-azido-3'-deoxythymidine 5'-phosphate (AZTMP) and 3'-azido-2',3'-dideoxy-uridine 5'-phosphate (AZdUMP) with thymidylate synthase. With the enzyme from L1210 cells and the tapeworm Hymenolepis diminuta, AZTMP was a weak inhibitor competitive with respect to dUMP (Ki = 6.3 mM and 0.5 mM); hence cytotoxicity of AZT, in cells in which accumulation of AZTMP is not high, is not due to inhibition of cellular thymidylate synthase. AZdUMP, with the L1210 enzyme, was a weak substrate (competition with dUMP described by apparent Ki = 4.7 mM), excluding conversion of AZdUMP to AZTMP as a source of toxicity of 3'-azido-2',3'-dideoxyuridine. An efficient procedure is described for enzymatic phosphorylation on a preparative scale of dideoxynucleosides.