Synergistic inhibition of human immunodeficiency virus replication in vitro by combinations of 3'-azido-3'-deoxythymidine and 3'-fluoro-3'-deoxythymidine

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.

Metabolism of 3′-Azido-3′-Deoxythymidine and 3′-Fluoro-3′-Deoxythymidine in Combination in Human Immunodeficiency Virus Infected Lymphoblastoid Cells

A combination of 3′-azido-3′-deoxythymidine (AZT) with 3′-fluoro-3′-deoxythymidine (FLT) has been shown previously to give synergistic inhibition of human immunodeficiency virus replication and greatly reduced cytotoxicity in vitro. The phosphorylation of the compounds, and their effect upon the natural deoxynucleoside triphosphate pools, were compared in CEM, H9, and HIV-infected H9 lymphoblastoid cells, both for the compounds when used alone and when combined together.

Higher levels of FLT triphosphate than AZT triphosphate, and higher levels of AZT monophosphate than FLT monosphosphate, were formed in all cell types. Both compounds were phosphorylated most efficiently in CEM cells, whereas they were least efficiently phosphorylated in infected H9 cells.

Owing to competition, the phosphorylation of both analogues was reduced when used in combination, compared to the phosphorylation of the separate compounds. The phosphorylation of the separate compounds was therefore at a maximum and was not increased by combining the compounds. The two compounds competed equally with each other for phosphorylation when used at a ratio of AZT: FLT of 5: 1.

Both analogues severely reduced the deoxynucleoside triphosphate pools in uninfected and human immunodeficiency virus-infected H9 cells, but not in CEM cells. The effects of the two compounds were similar to those found when the compounds were combined, and thus H9 cells were shown to be much more sensitive to the effects of the analogues upon deoxynucleoside triphosphate pools than CEM cells were.

Thus the synergistic combination of 3′-azido-3′-deoxythymidine and 3′-fluoro-3′-deoxythymidine was shown to have a similar metabolism and a similar effect upon cellular deoxynucleoside triphosphate pools to the individual compounds.

Synergistic Inhibition of Primary Isolates of Human Immunodeficiency Virus Type 1 by Combinations of 3′-Fluoro-3′-Deoxythymidine and 2′,3′-Dideoxyinosine

We examined the antiviral effect against human immunodeficiency virus type 1 of 3′-fluoro-3′-deoxythymidine and 2′,3′-dideoxyinosine, both alone and in combination. Primary isolates of human immunodeficiency virus were used directly after isolation on peripheral blood mononuclear cells, without culturing on established cell lines, in order to maintain close similarity to the clinical situation. All isolates were synergistically inhibited by combinations of the drugs, though their susceptibility to the separate drugs varied. The selectivity indices of the combinations were similar to that of the individual drugs. The synergistic combination of 3′-fluoro-3′-deoxythymidine and 2′,3′-dideoxyinosine appears promising for the treatment of human immunodeficiency virus infection and deserves further investigation.

Synthesis, in vitro anti-HIV activity, and biological stability of 5'-O-myristoyl analogue derivatives of 3'-fluoro-2',3'-dideoxythymidine (FLT) as potential bifunctional prodrugs of FLT

A group of 5'-O-myristoyl analogue derivatives of FLT (2) were evaluated as potential anti-HIV agents that were designed to serve as prodrugs to FLT. 3'-Fluoro-2',3'-dideoxy-5'-O-(12-methoxydodecanoyl)thymidine (4) (EC50 = 3.8 nM) and 3'-fluoro-2',3'-dideoxy-5'-O-(12-azidododecanoyl)thymidine (8) (EC50 = 2.8 nM) were the most effective anti-HIV-1 agents. There was a linear correlation between Log P and HPLC Log retention time for the 5'-O-FLT esters. The in vitro enzymatic hydrolysis half-life (t1/2), among the group of esters (3-8) in porcine liver esterase, rat plasma and rat brain homogenate was longer for 3'-fluoro-2',3'-dideoxy-5'-O-(myristoyl)thymidine (7), with t1/2 values of 20.3, 4.6 and 17.5 min, respectively.

Drug susceptibility of subtypes A,B,C,D, and E human immunodeficiency virus type 1 primary isolates

We determined the susceptibility to antiviral drugs of clinical isolates of human immunodeficiency virus type 1 (HIV-1) subtypes A, B, C, D, and E. Isolates from treated and untreated patients were tested for sensitivity to zidovudine (ZDV), lamivudine (3TC), didanosine (ddI), nevirapine (NVP), foscarnet (PFA), and ritonavir (RNV). The susceptibility to these different drugs was broadly similar between the different subtypes of HIV-1. Isolates of subtype D showed a tendency toward slightly lower susceptibility to all the antiviral drugs, which could be related to the rapid growth characteristics of these isolates.

Increased activation of the combination of 3'-azido-3'-deoxythymidine and 2'-deoxy-3'-thiacytidine in the presence of hydroxyurea

The intracellular phosphorylation of 3'-azido-3'-deoxythymidine and 2'-deoxy-3'-thiacytidine was increased two- to threefold by the addition of hydroxyurea (HU) to the single drugs or to the two drugs in combination. The ratios of drug triphosphate to competing cellular deoxynucleoside triphosphate were increased two- to threefold for both 3'-azido-3'-deoxythymidine and 2'-deoxy-3'-thiacytidine in the presence of HU. These HU-induced increases in 3'-azido-3'-deoxythymidine and 2'-deoxy-3'-thiacytidine metabolism may further enhance the anti-human immunodeficiency virus activity of these two drugs.

Synergistic inhibition of human immunodeficiency virus isolates (including 3'-azido-3'-deoxythymidine-resistant isolates) by foscarnet in combination with 2',3'-dideoxyinosine or 2',3'-dideoxycytidine

The combinations of foscarnet plus 2',3'-dideoxyinosine and foscarnet plus 2',3'-dideoxycytidine synergistically inhibit the replication of human immunodeficiency virus type 1 isolates, including two 3'-azido-3'-deoxythymidine-resistant isolates. The combination of 2',3'-dideoxyinosine plus 2',3'-dideoxycytidine showed additive inhibition of the majority of the human immunodeficiency virus isolates tested. All three combinations showed pronounced antagonistic cytotoxicity and thus were less toxic to the growth of peripheral blood mononuclear cells than the separate drugs.

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.

Intracellular activation and cytotoxicity of three different combinations of 3'-azido-3'-deoxythymidine and 2',3'-dideoxyinosine

We measured the intracellular activation and cytotoxicity of 3'-azido-3'-deoxythymidine (zidovudine, ZDV) and 2',3'-dideoxyinosine (ddI) when combined at three different clinically relevant combinations of 1:1, 1:10, and 10:1 (ZDV:ddI). The activation of ddI to ddA-TP was increased in all three combinations with ZDV, compared to ddI alone. A maximum twofold increase in ddA-TP was observed, which could not be further increased by raising the concentration of ZDV in the combination. On the other hand, the concentration of ZDV in the combination could be reduced to one-tenth while retaining increased activation of ddI. We also examined the cytotoxicity of these combinations in CEM cells, phytohemagglutinin (PHA)-stimulated and resting human peripheral blood mononuclear cells (PBMCs). CEM cells were the least sensitive overall to the drugs. ZDV showed greater cytotoxicity in stimulated PBMCs than resting PBMCs, whereas the reverse was true for ddI. This could be explained by the different activation pathways of these two drugs. The 1:1 and 10:1 ZDV:ddI combinations showed reduced toxicity compared to the separate drugs. These results indicate that ZDV and ddI need not necessarily be combined together at a ratio of ZDV:ddI of 1:1, but that some alteration in the dosages of ZDV or ddI in patients could be possible without loss of the benefits of combined ZDV:ddI therapy.

Antiviral therapy for human immunodeficiency virus infections

Depending on the stage of their intervention with the viral replicative cycle, human immunodeficiency virus inhibitors could be divided into the following groups: (i) adsorption inhibitors (i.e., CD4 constructs, polysulfates, polysulfonates, polycarboxylates, and polyoxometalates), (ii) fusion inhibitors (i.e., plant lectins, succinylated or aconitylated albumins, and betulinic acid derivatives), (iii) uncoating inhibitors (i.e., bicyclams), (iv) reverse transcription inhibitors acting either competitively with the substrate binding site (i.e., dideoxynucleoside analogs and acyclic nucleoside phosphonates) or allosterically with a nonsubstrate binding site (i.e., non-nucleoside reverse transcriptase inhibitors), (v) integration inhibitors, (vi) DNA replication inhibitors, (vii) transcription inhibitors (i.e., antisense oligodeoxynucleotides and Tat antagonists), (viii) translation inhibitors (i.e., antisense oligodeoxynucleotides and ribozymes), (ix) maturation inhibitors (i.e., protease inhibitors, myristoylation inhibitors, and glycosylation inhibitors), and finally, (x) budding (assembly/release) inhibitors. Current knowledge, including the therapeutic potential, of these various inhibitors is discussed. In view of their potential clinical the utility, the problem of virus-drug resistance and possible strategies to circumvent this problem are also addressed.

Comparison of the sensitivities of primary isolates of HIV type 2 and HIV type 1 to antiviral drugs and drug combinations

The sensitivity of primary isolates of HIV-2 to the antiretroviral drugs 3'-azido-3'-deoxythymidine (AZT), 2',3'-dideoxyinosine (ddI), and 3'-fluoro-3'-deoxythymidine (FLT) was measured in vitro in PBMCs and compared to that of primary isolates of HIV-1. HIV-2 isolates showed a similar sensitivity to the drugs compared to HIV-1 isolates. Both the relative and the absolute potencies of the three drugs were similar for inhibition of HIV-1 or HIV-2 replication. The effect of combinations of the antiviral drugs was also studied. The combinations of AZT plus FLT, AZT plus ddI, and FLT plus ddI showed synergistic inhibition of three primary HIV-2 isolates, similar to that previously shown for primary HIV-1 isolates. These results indicate that primary isolates of HIV-2 from untreated persons show a level of sensitivity to antiviral nucleoside analogs similar to that shown by HIV-1 isolates, and are also synergistically inhibited by drug combinations shown to be synergistic against HIV-1. Therapeutic regimes with nucleoside analogs used clinically against HIV-1 infection may therefore also be similarly useful against infection with HIV-2.

Lack of synergy in the inhibition of HIV-1 reverse transcriptase by combinations of the 5'-triphosphates of various anti-HIV nucleoside analogs

3'-Deoxy-3'-azidothymidine (AZT) has been shown to synergistically inhibit the replication of human immunodeficiency virus type 1 (HIV-1) in cell culture when combined with several other 2',3'-dideoxynucleoside analogs. In an effort to understand the biochemical mechanism of this synergy, we have examined the effect of combinations of the 5'-triphosphate of AZT (AZT-TP) with either ddCTP, ddATP, or the 5'-triphosphate of the carbocyclic analog of 2',3'-didehydro-2',3'-dideoxyguanosine (carbovir) on both the RNA-directed and DNA-directed DNA polymerase activity of HIV-1 reverse transcriptase. Kinetic studies, which evaluated the ability of these combinations to competitively inhibit the enzyme, showed that AZT-TP could not bind to the enzyme with either the RNA or DNA template at the same time as either of the other three inhibitors. None of these analogs could affect the incorporation of another analog into the DNA chain by the HIV-1 reverse transcriptase. These results indicated that synergistic inhibition of the HIV-1 reverse transcriptase is not responsible for the synergistic antiviral activity seen in cell culture with combinations of these nucleoside analogs.

Zidovudine. An update of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy

Zidovudine remains the mainstay in the treatment of patients infected with human immunodeficiency virus (HIV). The drug delays disease progression to acquired immunodeficiency syndrome (AIDS) and to AIDS-related complex (ARC), reduces opportunistic infections, and increases survival in patients with advanced HIV infection. There is evidence to suggest that zidovudine also delays disease progression in patients with mild symptomatic disease. Although one study has shown zidovudine to have no significant beneficial effects on survival or disease progression in patients with asymptomatic HIV infection, several other studies have shown zidovudine to delay disease progression in this patient group. Results from related ongoing studies are awaited with interest. Zidovudine reduces the incidence of AIDS dementia complex (ADC) and appears to prolong survival in these patients, and improves other neurological complications of HIV infection. The drug also appears to enhance the efficacy of interferon-alpha in patients with Kaposi's sarcoma. Although zidovudine is widely used as postexposure prophylaxis following accidental exposure to HIV, its efficacy in preventing seroconversion is unclear. Whether zidovudine prevents vertical transmission also remains to be determined. The overall efficacy of zidovudine in the treatment of children with HIV infection appears similar to that in adults despite more rapid disease progression in younger patients. Zidovudine-resistant isolates can emerge as early as after 2 months' therapy, and primary infection with zidovudine-resistant strains has been documented. Both zidovudine resistance and the syncytium-inducing HIV phenotype appear to be associated with poor clinical outcome. However, zidovudine resistance may revert on drug withdrawal or switching to an alternative therapy. Zidovudine-associated haematotoxicity may be dose-limiting. Nonhaematological adverse events associated with zidovudine therapy are generally mild and usually resolve spontaneously. Dosages of approximately 500 to 600 mg/day appear to be at least as effective as dosages of 1200 to 1500 mg/day and are better tolerated in patients with less advanced disease. However, optimal dosage are unclear. Despite beneficial effects, zidovudine monotherapy is not curative. There is evidence to suggest that the concomitant administration of zidovudine with didanosine or zalcitabine is effective in patients with HIV disease progression despite receiving zidovudine monotherapy, and there is some evidence that concomitant zidovudine plus didanosine therapy is more effective than alternating monotherapy. However, results from studies of combination therapy in asymptomatic patients, and from comparative combination therapy studies are awaited. Cotherapy with agents that augment haematopoiesis allows the continuation of therapeutic zidovudine dosages.(ABSTRACT TRUNCATED AT 400 WORDS)

Antiviral therapy in human immunodeficiency virus infections. Current status (Part II)

Part I of this article reviewed the targets against which anti-HIV drugs can be directed, problems in assessing active compounds (e.g. resistance development and use of surrogate end-points). and nucleoside analogues effective against HIV reverse transcriptase. Intensive research is currently being undertaken in laboratories and hospitals to design and evaluate new inhibitors of HIV. In this work, combining different drugs is one important approach, both to decrease toxicity and to offset the rate of resistance development, which seems to be a major problem associated with therapy directed against the ever-changing HIV. Therapeutic vaccines and immunomodulators are other modalities being actively evaluated against HIV and AIDS, although this effort has not yet yielded any licensed treatment. It appears likely that new antiviral drugs and immunotherapies will be forthcoming during the next 5 years, that they will be used in a variety of combinations, and that the treatment options available for opportunistic infections will increase. These developments should improve the survival and the quality of life of patients with HIV infection.

Effect of resistance on combination chemotherapy for human immunodeficiency virus infection

The effect of 3 drug combinations (AZT/FLT, AZT/ddI and FLT/ddI) upon the replication of AZT-sensitive and -resistant human immunodeficiency virus (HIV) was studied. The 3 combinations synergistically inhibited drug sensitive virus. However, AZT resistant virus showed an altered response to the combinations containing AZT: synergy was replaced by addition or antagonism. Thus, the susceptibility to a drug may affect the synergistic effect of combinations containing that drug. Other drug combinations may not be affected, since the AZT resistant virus retained a synergistic response to the combination of FLT/ddI. The synergistic effect could be regained upon reversion of resistance; a viral isolate taken after cessation of therapy, which had reverted to sensitivity to AZT, regained the synergistic response to drug combinations containing AZT. These results have implications for the use of combination chemotherapy to treat infection with HIV. Drug combinations will be most useful if the virus is sensitive to all components in the combination. It might be preferable to avoid the inclusion in combinations of drugs to which there is resistance, since we saw an antagonistic effect for combinations containing AZT in AZT resistant virus. Other combinations not containing the drug to which resistance has arisen may maintain their synergistic effect and remain good choices. Thus, a strategy of monitoring drug sensitivity and altering the combination therapy accordingly would appear to offer promise for the treatment of HIV infection.

Anti-human immunodeficiency virus effects of zidovudine in combination with double-stranded RNA poly I poly C in T cells and monocytes-macrophages

A comparison of the activity against human immunodeficiency virus 1 of zidovudine (AZT) and poly I poly C double-stranded RNA both alone and in combination in MT4 cells and primary monocyte/macrophage (M/M) cultures was made. The inhibition of the HIV-induced cytopathic effect or reverse transcriptase production by AZT in MT4 cells was not modified by the combination of the two agents. In contrast, AZT inhibition of reverse transcriptase production in the supernatant of M/M cultures was enhanced by the addition of poly I poly C. The inhibitory effect of the drug combination was more marked in M/M than in MT4 cells, indicating that the evaluation of compounds involving the induction of an antiviral state should be tested not only CD4+ T cells but also in monocyte-macrophages.

Loss of synergistic response to combinations containing AZT in AZT-resistant HIV-1

The use of combination chemotherapy for the treatment of HIV-1 infection offers promise for overcoming the problems of toxicity and development of resistance. Primary HIV-1 isolates from three patients before and after treatment with azidothymidine (AZT) were examined for sensitivity to AZT and synergistic response to three different combinations of drugs: AZT+fluorothymidine (FLT), AZT+dideoxyinosine (ddI), and FLT+ddI. All three patients initially harbored AZT-sensitive virus, but posttherapy isolates were resistant to AZT. The pretreatment, AZT-sensitive virus from each patient showed synergistic inhibition by the combinations of AZT+FLT, AZT+ddI, and FLT+ddI. In contrast, the posttreatment, AZT-resistant virus showed only addition or antagonism by the combinations containing AZT, whereas the synergistic response to the combination of FLT+ddI was preserved. Our study argues for early intervention with combination chemotherapy, since after development of resistance, AZT no longer showed synergy but addition or antagonism in combination with other drugs. After resistance to AZT has developed, combination chemotherapy not involving AZT may offer advantages over continued mono- or combination therapy involving AZT.

Three-drug synergistic inhibition of HIV-1 replication in vitro by 3'-fluoro-3'-deoxythymidine, recombinant soluble CD4, and recombinant interferon-alpha

3'-Fluoro-3'deoxythymidine (FLT), recombinant soluble CD4 (CD4), and recombinant interferon-alpha (IFN alpha) were evaluated in two- and three-drug regimens against HIV-1 replication in vitro. Peripheral blood mononuclear cells were studied using p24 antigen production as the virologic endpoints. FLT showed 2.5-fold higher efficacy and a similar selectivity index to zidovudine. Drug interactions were evaluated by the median effect principle and the isobologram technique. FLT, CD4, and interferon alpha at noncytotoxic concentrations inhibited HIV-1 synergistically in two- and three-drug combinations with a combination index smaller than one and dose reduction index greater than one. The three-drug regimen provided greater virus suppression than the two-drug regimen. These results suggest that FLT is an alternative agent to AZT for the treatment of HIV infection either as a single agent or in combination with CD4 and/or interferon-alpha.

Computer-assisted structure-activity correlations of halodideoxynucleoside analogs as potential anti-HIV drugs

Analysis of the structure-anti-HIV activity correlations of halo dideoxynucleosides (ddN's) in the public domain was accomplished through computer substructure searching, retrieval and sorting of in vitro anti-HIV data. In the survey, selectivity index (ratio of cytotoxicity to the potency in inhibiting HIV replication in vitro) was used to rank compounds in congeneric groups. Factors contributing to the anti-HIV activity, e.g. the nature and location of the halogen on the sugar or the base and its stereochemical configuration, could not be generalized for all the halogenated ddN's. Conclusions were drawn for specific classes within the pyrimidine and purine series, with compounds further divided into halo substitutions at the sugar 2',3',4'-positions or in the pyrimidine or purine ring systems. At the 3'-position, only a fluoro substitution enhanced the activity of the dideoxypyrimidine nucleosides. A 2'-ara fluoro substituent increased the activity of purine ddN's but decreased activity of pyrimidine ddN's. Halogenation of the side chain in acyclic adenine and 2,6-diaminopurine nucleosides improved their anti-HIV activity. Halo substitution at the 6- or 2'-ara position of selected purine ddN's resulted in compounds with increased lipophilicity, chemical stability and retention of anti-HIV activity. The number of halodideoxynucleosides tested as anti-HIV reagents suggested that this class of compounds is well studied.

Differential alteration of the anti-HIV-1 effect of phosphorothioate oligonucleotide S-dC28 by AZT, interferon-alpha, and dextran sulfate

The combination of S-dC28 (a phosphorothioate oligodeoxcytidine 28 mer) with AZT, recombinant interferon alpha-A (IFN-alpha A) or dextran sulfate (DS) against replication of human immunodeficiency virus type 1 (HIV-1) were studied in MT4 cells, using both p24 core antigen and reverse transcriptase (RT) assays. Under the standardized conditions, the anti-HIV-1 dose-effect relationships of all test drugs showed sigmoidal curves with the following EC50 values: for the p24 core antigen assay, S-dC28, 0.03 microM; AZT, 0.004 microM; IFN-alpha A, 9.2 U/ml; DS, 0.26 micrograms/ml; for the RT assay, S-dC28, 0.04 microM; AZT, 0.01 microM; IFN-alpha A, 11.6 U/ml; and DS, 0.31 micrograms/ml. A computer software based on the median-effect principle and isobologram techniques were used to quantitatively analyze drug interactions by calculating the combination index (CI) where CI less than 1, = 1, and greater than 1 indicates synergism, additive effect and antagonism, respectively. For p24-ELISA, the interaction of S-dC28 and AZT in combination produced a slight antagonism on HIV-1 replicative inhibition with CI values of 1.29-1.10; for RT assays, at EC50-EC95 levels, the CI values are 1.96-1.11. For p24 core antigen assay, the combination of S-dC28 with IFN-alpha A exhibited a dose-dependent anti-HIV synergism with CI values of 1.15-0.87 at EC75-EC95 levels. The RT assays for the same combination showed a broad synergistic effect with CI values of 0.62-0.60, at EC50-EC95 levels. S-dC28 plus DS showed a nearly additive effect based on both assay methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic combinations and peptides in the inhibition of human immunodeficiency virus

Various combinations of inhibitors of HIV reverse transcriptase were tested for inhibition of HIV replication in order to reveal any potential synergism or antagonism. PFA, a pyrophosphate analogue, gave synergistic inhibition of HIV replication in combination with both of the thymidine analogues AZT and FLT. The combination of PFA and AZT-TP gave only additive or weakly synergistic inhibition in a reverse transcriptase enzyme assay. The combination of AZT and FLT also gave synergistic inhibition of HIV replication, whilst the combination of AZT-TP and FLT-TP gave only additive or weakly synergistic inhibition of reverse transcriptase. Thus, the synergy does not arise from effects on reverse transcriptase alone but must be owing to other, cellular factors, such as effects on nucleoside metabolism or metabolism of the analogues. The results are consistent with the hypothesis that AZT may have an alternative mechanism of inhibition other than inhibition of reverse transcriptase. The diminished cytotoxicity observed in addition to the synergistic inhibition makes these combinations attractive from the point of view of combination chemotherapy. The inhibition of HIV replication by peptides from various parts of the V3 region of gp120 whose sequences were homologous with the tryptase inhibitor trypstatin was tested. Inhibitory activity was displayed by two peptides containing cysteine in their sequence. Antibodies to two peptides containing the two conserved cysteine residues from opposite sides of the neutralizing loop of gp120 were previously associated with protection from vertical transmission of HIV. The V3 region thus seems to be important for the function of gp120 and the transmission of HIV.