Fetal mitochondrial heart and skeletal muscle damage in Erythrocebus patas monkeys exposed in utero to 3'-azido-3'-deoxythymidine

3'-azido-3'-deoxythymidine (AZT) is given to pregnant women positive for the human immunodeficiency virus type 1 (HIV-1) to reduce maternal-fetal viral transmission. To explore fetal mitochondrial consequences of this exposure, pregnant Erythrocebus patas monkeys were given daily doses of 1.5 mg (21% of the human daily dose) and 6.0 mg (86% of the human daily dose) of AZT/kg body weight (bw), for the second half of gestation. At term, electron microscopy of fetal cardiac and skeletal muscle showed abnormal and disrupted sarcomeres with myofibrillar loss. Some abnormally shaped mitochondria with disrupted cristae were observed in skeletal muscle myocytes. Oxidative phosphorylation (OXPHOS) enzyme assays showed dose-dependent alterations. At the human-equivalent dose of AZT (6 mg of AZT/kg bw), there was an approximately 85% decrease in the specific activity of NADH dehydrogenase (complex I) and three- to sixfold increases in specific activities of succinate dehydrogenase (complex II) and cytochrome-c oxidase (complex IV). Furthermore, a dose-dependent depletion of mitochondrial DNA levels was observed in both tissues. The data demonstrate that transplacental AZT exposure causes cardiac and skeletal muscle mitochondrial myopathy in the patas monkey fetus.     

Zidovudine (AZT) causes an oxidation of mitochondrial DNA in mouse liver

Zidovudine (3'-azido-2',3'-dideoxythymidine [AZT]) inhibits human immunodeficiency virus replication and delays progression of acquired immune deficiency syndrome. We have recently found that, in muscle, AZT causes oxidative damage to mitochondrial DNA (mtDNA) and other signs of mitochondrial oxidative damage. The aim of this work was to test if AZT causes oxidative damage to liver mtDNA. In our study, an experimental mouse model was used in which mice were administered AZT (10 mg/kg body weight/d) in drinking water. Liver mtDNA of mice treated with AZT had 40% more of the oxidized, mutagenic nucleoside, 8-oxo-7,8-dihydroxy-2'deoxyguanosine (8-oxo-dG) than untreated controls. This oxidative damage to mtDNA is caused by a significant increase (of over 240%) in peroxide production by liver mitochondria from AZT-treated mice, which was prevented by dietary administration with vitamins C and E.     

The effects of AZT and DDI on pre- and postimplantation mammalian embryos: an in vivo and in vitro study.

This study reports the effects of the nucleoside analogs dideoxyinosine (DDI) and 3'-azido-3'-deoxythymidine (AZT) on mammalian embryonic development. When administered to pregnant mice (at concentrations ranging from 10 to 300 mg/kg/day), through all or part of gestation, AZT and DDI did not result in any visible effect on mouse embryos nor did they cause any obvious malformation or defect at birth or during postnatal growth. Similarly, when embryonic or fetal mouse or human cells (from brain, limb buds, or different organ rudiments) were exposed to AZT or DDI in vitro, cytotoxicity was observed only in the mM range, with AZT showing slightly higher cytotoxicity and brain cells appearing slightly more sensitive to both nucleosides. However, even in cultures treated with very high concentrations of AZT or DDI, the reduction in the number of terminally differentiated skeletal myotubes, cardiocytes, neurons, and chondrocytes was similar to the reduction in the total number of cells, indicating that AZT and DDI did not selectively inhibit differentiation of any of the above-mentioned cell types. Finally, preimplantation mouse embryos (at the 2-cell or 4-cell stage), treated in vitro with micromolar concentrations of AZT, were arrested at the 4-cell stage. DDI or other nucleoside analogs tested did not have this effect.     

Pharmacokinetic evaluation of drug interactions with anti-HIV drugs, II: Effect of 2',3'-dideoxyinosine (ddI) on zidovudine kinetics in monkeys.

The pharmacokinetic basis of a drug interaction between zidovudine (AZT) and 2',3'-dideoxyinosine (ddI) was investigated in normal monkeys. Five animals received 20 mg/kg of AZT intragastrically in the absence and presence of ddI. ddI was administered intravenously to produce steady-state ddI plasma concentrations for 30 min. Plasma and urine samples were analyzed for AZT, its major glucuronide metabolite, GAZT, and ddI by high-performance liquid chromatography (HPLC). Resultant AZT and GAZT concentration data were analyzed by noncompartmental methods. Statistical analysis indicated no differences in AZT's apparent total clearance, apparent volume of distribution at steady-state, and elimination half-life due to ddI, however, the mean apparent total clearance decreased from 2.92 to 1.67 L/h/kg, and the mean apparent volume of distribution at steady-state decreased from 5.79 to 3.43 L/kg in the presence of ddI. Incomplete urine collections in most animals prevented conclusions from being made about ddI's effect on renal elimination parameters. Nonetheless, the urinary GAZT/AZT ratio, a parameter not influenced by incomplete urine collection, was significantly reduced in the presence of ddI. Although additional studies will be useful to characterize the full importance of the interaction, there is evidence to suggest that both renal and metabolic elimination of AZT and renal elimination of GAZT may be inhibited by ddI.     

IGF-I but not the IGF-I variant long R(3)IGF-I increases serum IGFBP-3 in adolescent monkeys.

Previous work in rhesus monkeys has shown that both acute or chronic subcutaneous (s.c.) administration of insulin-like growth factor (IGF)-I elevates serum concentrations of IGF binding protein (IGFBP)-3. In order to determine whether an analog of IGF-I, which has a reduced affinity for the IGFBPs, has similar effects, a series of studies using adolescent female rhesus monkeys were conducted. In the first study, an s.c. injection of IGF-I (110 mg/kg;n = 6) significantly elevated serum IGFBP-3 concentrations through 7 h following treatment. In contrast, serum IGFBP-3 decreased throughout the day following an injection of Long R(3)IGF-I (110 mg/kg, s.c., n = 5). However, this decrease was not due to the analog treatment as serum IGFBP-3 also declined in a similar fashion in untreated females (n = 5) sampled on the same schedule. Serum GH levels were acutely suppressed by both IGFs but were not altered in untreated females. In the second study, serum IGFBP-3 were compared between untreated control females (n = 6) and females treated continuously by s.c. infusion with either Long R(3)IGF-I (120 mg/kg/day, s.c.;n = 5) or IGF-I (120 mg/kg/day, s.c.;n = 5) or IGF-I s.c.;n = 4). Serum IGFBP-3 was consistently elevated by IGF-I infusion, whereas levels in analog-treated monkeys were similar to those in control females. Although acute or chronic administration of Long R(3)IGF-I did not elevate serum IGFBP-3, chronic administration of the analog did not block the acute facilitating effects of IGF-I on serum IGFBP-3. The increase in serum IGFBP-3 following an acute injection of IGF-I (110 mg/kg, s.c.) was not significantly different between untreated females and females receiving a constant s.c. infusion of Long R(3)IGF-I. These data indicate either acutely or chronically administered IGF-I but not its analog Long R(3)IGF-I can elevate serum concentrations of IGFBP-3. Although the analog fails to increase serum IGFBP-3, it does not block the facilitating effects of IGF-I on concentrations of this IGFBP. Taken together, these data suggest that the increase in serum IGFBP-3 by exogenous IGF-I may not be a receptor mediated event but may be the result of IGF-I binding to IGFBP-3 and forming the binary and ternary complex, slowing IGFBP-3 degradation.     

Sensitivity of erythroid progenitor colonies to erythropoietin in azidothymidine treated immunodeficient mice

The anaemia induced by 3'-azido-3'dideoxythymidine (AZT) is poorly understood. We have used a murine model of AIDS, infection of female C57BL/6 mice with LP-BM5 murine leukaemia (MuLV) virus, to determine if AZT-induced anaemia is due, in part, to decreased responsiveness of erythropoietic precursors (BFU-e) to erythropoietin (EPO). Mice in the early stage of LP-BM5 MuLV disease were given AZT in their drinking water at 1.0 and 2.5 mg/ml. AZT produced anaemia in both groups, in a dose-dependent fashion. Despite the anaemia, the number of splenic and bone marrow BFU-e in AZT treated mice increased up to five-fold over levels observed in infected untreated animals after 15 d of treatment. Colony formation by splenic and bone marrow BFUe was stimulated at lower concentrations of EPO in mice receiving AZT for 15 d than for infected, untreated mice. By day 30, sensitivity of both splenic and bone marrow BFU-e of treated animals returned to that observed from cells of infected untreated animals. The mean plasma levels of EPO observed in AZT treated mice were appropriate for the degree of anaemia observed when compared with phenylhydrazine (PHZ) treated mice. The numbers of BFU-e and the percentage of bone marrow erythroblasts observed were comparable in AZT and PHZ treated mice with similar degrees of anaemia. However, reticulocytosis was inappropriate for the degree of anaemia observed in AZT treated infected mice. AZT-induced peripheral anaemia in the face of increased numbers of BFU-e and increased levels of plasma EPO suggest a lesion in terminal differentiation.     

Oral bioavailability and anti-simian varicella virus efficacy of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) in monkeys.

BV-araU (1-beta-D-arabinofuranosyl-E-5-[2-bromovinyl]uracil) has potent antiviral activity against varicella zoster virus in cell culture and is undergoing clinical evaluation. In the present study, pharmacokinetic parameters and the efficacy of BV-araU against infection with simian varicella virus (SVV) were evaluated in African green monkeys. Pharmacokinetic parameters were determined by analysis of the BV-araU content of sera obtained after oral and intravenous administration to normal monkeys. Peak serum concentrations showed dose proportionality, with the 0.1 mg/kg dose resulting in a peak serum concentration of 0.05 micrograms/ml, the approximately ED50 value for the SVV inoculum in cell culture. BV-araU administered orally twice daily at 0.1 mg/kg for 10 days starting 48 h after intratracheal SVV infection prevented vesicular rash development and suppressed viremia. Effective therapy could be initiated 96 h after infection. Taken together, these results indicate that BV-araU is effective oral therapy at doses that achieve peak serum levels equivalent to the ED50 for SVV in cell culture.     

Pharmacokinetics of human leptin in mice and rhesus monkeys

OBJECTIVE: The pharmacokinetic characteristics of human leptin were examined in rhesus monkeys and in C57BL/6J mice fed a normal chow or a high-fat diet. DESIGN: For the monkey study, in nine rhesus monkeys (body weight 12.4 +/- 2.4 kg; mean +/- s.d.), recombinant met-human leptin was injected intravenously or subcutaneously (1 mg/kg). For the mouse study, after 6 months of feeding C57BL/6J mice a high-fat diet (body weight 32.9 +/- 3.6 g; n = 8) or a control diet (24.5 +/- 1.2 g; n = 6), recombinant met-human leptin was administered intraperitoneally (10 microg/g). Blood samples were collected for leptin measurement at specific time points after leptin administration. MEASUREMENTS: Plasma leptin concentrations were determined by radioimmunoassay and pharmacokinetic analysis was performed. RESULTS: Disposition of human leptin in rhesus monkeys was biphasic following intravenous administration, with a terminal phase half-life of 96.4 +/- 16.5 min and clearance of 1.8 +/- 0.2 ml/min/kg. Subcutaneously administered leptin was absorbed slowly, perhaps by a zero-order process as leptin levels appeared to plateau and remained elevated throughout the 8 h sampling period. In C57BL/6J mice, the absorption and elimination of human leptin were both first-order following intraperitoneal administration. Pharmacokinetic parameters did not differ between normal-weight mice fed a chow diet and obese mice fed a high-fat diet. The elimination half-life was 47.0 +/- 26.4 min in mice fed a high-fat diet and 49.5 +/- 12.0 min in mice fed a control diet. CONCLUSION: The kinetics of leptin in rhesus monkeys were biphasic and clearance was similar to values previously reported in humans. The estimated half-life was 96.4 min in rhesus monkeys and 49.5 min in normal weight mice. The was no difference in leptin kinetics between high-fat fed and control mice, suggesting that the increased baseline leptin levels in the obese mice are due to increased leptin production and secretion.     

Pharmacokinetics of stavudine in patients with AIDS or AIDS-related complex.

The pharmacokinetics of stavudine (d4T; 2',3'-didehydro-3'-deoxythymidine) were studied in patients with AIDS-related complex or AIDS enrolled in a dose-ranging phase I/II study. Twenty-two patients were studied after the first oral dose of 0.67, 1.33, 2.67, or 4 mg/kg of body weight; 17 of them underwent an additional steady-state pharmacokinetic evaluation after thrice-daily dosing of the above doses. Stavudine absorption was rapid, with mean peak concentrations of 1.2-4.2 mg/L over the four dose levels studied. From 34% to 41% of an oral dose was excreted as unchanged drug in the urine. The mean values for plasma elimination half-life ranged from 1 to 1.6 h. The absolute bioavailability of a 4 mg/kg oral dose exceeded 80%. There was no change in pharmacokinetic parameters measured after the first dose and after chronic dosing. Stavudine is a new dideoxynucleoside with more complete and less variable oral absorption than existing nucleosides used for treatment of human immunodeficiency virus infection.     

Responses of neurologic complications of AIDS to 3'-azido-3'-deoxythymidine and 9-(1,3-dihydroxy-2-propoxymethyl) guanine. I. Clinical features

Fourteen patients with AIDS were treated for 23 neurologic complications: four episodes of acute meningoencephalitis; eight episodes of subacute encephalopathy; two cases of progressive multifocal leukoencephalopathy; and nine cases of polyneuropathy. Nine patients were treated with 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG), one with 3'-azido-3'-deoxythymidine (AZT), and four initially with DHPG directed against cytomegalovirus (CMV) retinitis or encephalitis and subsequently with AZT against human immunodeficiency virus (HIV) encephalopathy. CMV retinitis was a helpful clinical observation indicating neurologic involvement. DHPG produced improvement in two of three cases of acute meningoencephalitis but was ineffective in cases of subacute encephalopathy or neuropathy. AZT therapy resulted in resolution in both of the two treated cases of acute confusional state and in two of the four treated cases of polyradiculoneuropathy with paraparesis but was ineffective in the late stage of subacute encephalopathy. These results suggest that CMV is important in some cases of acute meningoencephalitis, whereas HIV is a dominant pathogen in subacute dementia and polyneuropathy in patients with AIDS. DHPG may be beneficial in the former, whereas AZT appears to be effective in the latter complications.     

Suppression of feline immunodeficiency virus infection in vivo by 9-(2-phosphonomethoxyethyl)adenine.

The acyclic purine nucleoside analogue 9-(2-phosphonomethoxyethyl)adenine [PMEA; formerly referred to as 9-(2-phosphonylmethoxyethyl)adenine] is a potent and selective inhibitor of human immunodeficiency virus replication in vitro and of Moloney murine sarcoma virus-induced tumor formation in mice. In the latter system PMEA has stronger antiretroviral potency and selectivity than 3'-azido-3'-thymidine (AZT). We have now investigated the effect of the drug in cats infected with the feline immunodeficiency virus (FIV). In vitro, PMEA was found to efficiently block FIV replication in feline thymocytes (50% effective dose, 0.6 microM). When administered to cats at doses of 20, 5, or 2 mg/kg per day, PMEA caused a dose-dependent suppression of FIV replication and virus-specific antibody production. Seropositive field cats with signs of opportunistic infection (gingivitis, stomatitis, and diarrhea) showed clinical improvement during PMEA therapy (5 mg/kg per day) and recurrence of the disease after treatment was discontinued. Thus, FIV infection in cats is an excellent model to test the efficacy of selective anti-human immunodeficiency virus agents in vivo.     

The effect of recombinant human granulocyte-macrophage colony-stimulating factor (rGM-CSF) on 3'-azido-3'-deoxythymidine (AZT)-mediated biochemical and cytotoxic effects on normal human myeloid progenitor cells

Administration of 3'-azido-3'-deoxythymidine (AZT) to patients with acquired immunodeficiency syndrome (AIDS) causes significant anemia and neutropenia. The bone marrow cytotoxicity of AZT has been attributed to deoxyribonucleotide pool perturbations that might result in impaired DNA synthesis in normal bone marrow elements. We examined the effect of human recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) on AZT-mediated biochemical perturbations and in vitro growth inhibition of normal bone marrow myeloid progenitor cells. Exposure of nonadherent, bone marrow mononuclear cells (BMMC) to 100 ng/ml of rGM-CSF for 6 h resulted in approximately twofold increments in the mean intracellular deoxycytidine triphosphate (dCTP) and thymidine triphosphate (dTTP) levels. Administration of 10 microM AZT alone to BMMC for 6 h markedly reduced dCTP and dTTP levels and generated significant levels of AZT triphosphate (AZT-TP). Coadministration of rGM-CSF (100 ng/ml) along with AZT (10 microM) partly restored dCTP and dTTP levels and significantly reduced AZT-TP levels. Furthermore, simultaneous exposure of BMMC for 4 h to 100 ng/ml of rGM-CSF reduced the mean DNA incorporation of [3H]AZT (10 microM) from 27.2 to 19.1 pmol/micrograms of DNA. Additionally, the inhibitory effects of AZT (10 microM) on granulocyte-macrophage colony-forming unit (CFU-GM) colony growth were significantly reduced in the presence of 100 ng/ml of rGM-CSF. These in vitro studies suggest that rGM-CSF partly corrects AZT-mediated biochemical perturbations as well as reduces the cytotoxicity of AZT in normal human bone marrow myeloid progenitor cells.     

Human immunodeficiency virus-associated progressive multifocal leukoencephalopathy: apparent response to 3'-azido-3'-deoxythymidine

We present the case of a 26-year-old human immunodeficiency virus-seropositive man who developed progressive multifocal leukoencephalopathy as the initial manifestation of AIDS. He appears to have responded dramatically to therapy with 3'-azido-3'-deoxythymidine (AZT). His neurologic status deteriorated shortly after an AZT dose reduction. He has stabilized since resuming his previous AZT dose. Although it remains unclear whether AZT is useful in the treatment of JC virus infection, we think that all AIDS patients with progressive multifocal leukoencephalopathy should be offered treatment with AZT, especially in light of recent reports describing a possible potentiation of human immunodeficiency virus infection of the central nervous system in this setting.     

In Utero Exposure of Female CD-1 Mice to AZT and/or 3TC: II. Persistence of Functional Alterations in Cardiac Tissue

To delineate temporal changes in the integrity and function of mitochondria/cardiomyocytes in hearts from mice exposed in utero to commonly used nucleoside analogs (NRTIs), CD-1 mice were exposed in utero to 80 mg AZT/kg, 40 mg 3TC/kg, 80 mg AZT/kg plus 40 mg 3TC/kg, or vehicle alone during days 12–18 of gestation and hearts from female mouse offspring were examined at 13 and 26 weeks postpartum. Alterations in cardiac mitochondrial DNA (mtDNA) content, oxidative phosphorylation (OXPHOS) enzyme activities, mtDNA mutations, and echocardiography of NRTI-exposed mice were assessed and compared with findings in vehicle-exposed control mice. A hybrid capture-chemiluminescence assay showed significant twofold increases in mtDNA levels in hearts from AZT- and AZT/3TC-exposed mice at 13 and 26 weeks postpartum, consistent with near doubling in mitochondrial numbers over time compared with vehicle-exposed mice. Echocardiographic measurements at 13 and 26 weeks postpartum indicated progressive thinning of the left ventricular posterior wall in NRTI-exposed mice, relative to controls, with differences becoming statistically significant by 26 weeks. Overall, progressive functional changes occurred in mouse mitochondria and cardiac tissue several months after in utero NRTI exposures; AZT and 3TC acted in concert to cause additive cardiotoxic effects of AZT/3TC compared with either drug alone.     

Postexposure prophylaxis with zidovudine suppresses human immunodeficiency virus type 1 infection in SCID-hu mice in a time-dependent manner

Occupational exposure to the human immunodeficiency virus (HIV) has led to a low but finite incidence of infection among health care providers. In such circumstances, postexposure administration of 3'-azido-3'-deoxythymidine (zidovudine; AZT) might be beneficial. To test this possibility, the SCID-hu mouse (the immunodeficient C.B-17 scid/scid mouse engrafted with human hematolymphoid organs) was treated with AZT at different times after intravenous infection with a standard dose of HIV (known to infect 100% of animals). If given within 2 h, AZT suppressed infection in all animals; if given after 2 days, no suppression was observed. At least in some animals, an AZT-sensitive phase lasted for as long as 36 h. These data support the hypothesis that prompt administration of AZT might be efficacious in suppressing acute HIV infection in humans. Further studies in the SCID-hu mouse might provide insight into treatment protocols of even greater efficacy.     

Pharmacokinetic study of the new cyclosporine-A formulation (Neoral) in adult allogeneic bone marrow transplant recipients

BACKGROUND AND OBJECTIVES: A major problem encountered during oral cyclosporin-A (CsA) administration to prevent acute graft-versus-host-disease (GVHD) after allogeneic bone marrow transplantation (allo-BMT) is its irregular pharmacokinetics. The aim of this study was to evaluate the pharmacokinetics of Neoral, a new water-free microemulsion formulation of CsA. DESIGN AND METHODS: Eighteen patients aged over 18 were enrolled into the study. When able to eat normally after allo-BMT, patients received CsA orally and after 4 days a 12-hour CsA pharmacokinetic profile was constructed. Three patients received Sandimmune 10 mg/kg/day, 5 patients received Neoral 7.5 mg/kg/day and 10 patients Neoral 5 mg/kg/day. CsA concentration was analyzed on whole blood by high-performance liquid chromatography (HPLC). RESULTS: Neoral showed concentration-time profiles characterized by a smooth and faster rise to the Cmax value compared to that produced by Sandimmune. The comparison between pharmacokinetic parameters obtained in patients receiving Neoral 5 mg/kg/day or 7.5 mg/kg/day showed a proportional increase of the AUC (4776+/-1084 vs. 7746+/-2006 ng/mL h) and C(max) (1027+/-203 vs. 1514+/-231 ng/mL). In all patients to whom 7.5 mg/kg/day of Neoral were given, C(trough) levels were always above the threshold of 200 ng/mL. INTERPRETATION AND CONCLUSIONS: Our data suggest that oral administration of Neoral 7.5 mg/kg/day early after allo-BMT may represent an appropriate dose resulting in adequate CsA C(trough) levels without significant renal toxicity.     

Effects of AZT and RNA-protein complex （FA-2-b-β） extracted from Liang Jin mushroom on apoptosis of gastric cancer cells

To investigate the synergistic effects of 3′-azido-3′- deoxythymidine （AZT） and FA-2-b-β extracted from Ling Jin mushroom on apoptosis of gastric cancer cells MKN45 in vitro. METHODS： Ml-I- analysis was made to examine the inhibition rate of MKN45 cells treated with AZT （2.5, 5, 10 and 20 mg/L） and FA-2-b-13 （5, 10, 20 and 40 mg/L） singly and combinatively for 24, 48 and 72 h. Apoptotic effects were evaluated by morphological methods, DNA agarose gel electrophoresis and flow cytometry, respectively. Telomerase activity was estimated by TRAP- ELISA. The mRNA expression of caspase-3 and Bcl-2 were detected by RT-PCR. RESULTS： AZT and FA-2-b-13 could significantly inhibit MKN45 cell proliferation and induce its apoptosis. MKN45 cells were inhibited in dose- and time- dependent manner. The inhibition effect of AZT combined with FA-2- b-β was obviously better than that used singly （0.469 ＋ 0.022 vs 1.075 4- 0.055, P 〈 0.05, 0.325 4- 0.029 vs 0.469 ＋ 0.022 P 〈 0.01）. AZT used singly and combination of FA-2-b-β could decrease the activity of tumor cell telomerase, and AZT has synergistic function with FA- 2-b-β. A certain concentration of AZT could up-regulate the expression of caspase-3 mRNA （r = 0.9969, P 〈 0.01）, which was positively related to apoptosis rate, and could down-regulate the expression of Bcl-2 mRNA, which was negatively related to apoptosis rate （r = 0.926, P 〈 0.01）. Furthermore, the effect of AZT combined with FA-2-b-13 was significantly higher than that used singly. CONCLUSION： Combination of AZT and FA-2-b-β has an obviously synergetic effect in the gastric cancer cells MKN45, which has provided a new approach to the treatment of gastric cancer clinically.     

Changes in drug sensitivity of human immunodeficiency virus type 1 during therapy with azidothymidine, dideoxycytidine, and dideoxyinosine: an in vitro comparative study.

Human immunodeficiency virus type 1 (HIV-1) strains were isolated from nine patients before and after prolonged therapy with either an alternating regimen of 3'-azido-3'-deoxythymidine (AZT) and 2',3'-dideoxycytidine (ddC) (AZT/ddC) or 2',3'-dideoxyinosine (ddI) alone. All strains obtained from four patients who received AZT/ddC for up to 41 mo were highly insensitive to AZT in vitro. Only one strain obtained after AZT/ddC therapy showed reduced susceptibility to ddC in addition to AZT and had previously unreported amino acid substitutions in the viral polymerase-encoding pol region, whereas three other strains had one or more of the five previously reported AZT-related mutations. In five HIV-1 strains from patients who received ddI for up to 29 mo, no appreciable decrease in sensitivity to ddI was detected. Two strains isolated after ddI therapy had no significant amino acid mutations, although three strains had a mutation reportedly associated with ddI administration. These data suggest that HIV-1 develops reduced susceptibility to AZT more readily than to ddC and ddI and/or that the reduced susceptibility to ddC and ddI is modest in degree. Moreover, the present data suggest that an alternating regimen of AZT and ddC does not block the emergence of AZT-insensitive variants. It should be noted, however, that the current results do not provide a basis for concluding that AZT/ddC or ddI is inferior, equivalent, or superior to AZT as therapy of AIDS.