Nelfinavir. A review of its therapeutic efficacy in HIV infection

Nelfinavir is a selective inhibitor of HIV protease, the enzyme responsible for post-translational processing of HIV propeptides. In the presence of the drug, immature, noninfectious virus particles are produced. Nelfinavir in combination with nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors and/or other protease inhibitors profoundly suppresses viral replication. Plasma HIV RNA levels (viral loan) rapidly fall below the limit of detection (LOD; usually 400 or 500 copies/ml in the majority of patients. When used in combination with NRTIs, nelfinavir 1250mg twice daily produced similar results to 3-times-daily nelfinavir at a range of total daily dosages. In an ongoing study > 70% of adults receiving a nelfinavir based combination regimen had plasma HIV RNA levels below the LOD (< 400 copies/ml) after 84 weeks. In addition, 73% of paediatric patients receiving nelfinavir plus at least 1 new NRTI had viral loads below the LOD (< 400 copies/ml) after 34 weeks. Furthermore, CD4+ cell counts generally increased in conjunction with reductions in viral load. Combination therapy with nelfinavir and saquinavir results in higher saquinavir plasma concentrations, make twice-daily administration of saquinavir feasible and may delay the emergence of resistant viral strains. A unique mutation at codon 30 (D30N) of the protease gene confers resistance to nelfinavir, but HIV with D30N mutation remains fully susceptible to indinavir, ritonavir and saquinavir in vitro. Nonetheless, in clinical use, significant cross-resistance is seen with all currently available protease inhibitors. Diarrhoea is the most frequently reported adverse event in patients receiving nelfinavir-based combination therapy and has been reported in up to 32% of nelfinavir recipients in randomised trials. Diarrhoea is generally of mild to moderate severity and does not result in weight loss. Rash, nausea, headache and asthenia were each reported in < or = 5% of patients. Approximately 5% of patients enrolled in an expanded access programme in the US discontinued nelfinavir because of adverse events. Nelfinavir is metabolised by the cytochrome P450 system. Several clinically significant pharmacokinetic drug interactions between nelfinavir and other drugs (i.e. ketoconazole, rifabutin, rifabutin, rifampicin), including other protease inhibitors (i.e. indinavir, ritonavir, saquinavir) have been documented. As with other available protease inhibitors, hyperglycaemia, hyperlipidaemia and abnormal fat distribution have been reported, albeit infrequently, in association with nelfinavir.

CONCLUSION

Nelfinavir-based combination regimens are well tolerated and produce profound and prolonged suppression of HIV replication in adult and paediatric patients. Hence, nelfinavir is suitable for inclusion in antiretroviral regimens for initial therapy for HIV infection and, alternatively, in regimens for patients unable to tolerate other protease inhibitors.

Characterization of the selectivity and mechanism of human cytochrome P450 inhibition by the human immunodeficiency virus-protease inhibitor nelfinavir mesylate

In vitro studies with human liver microsomes and P450 probe substrates were performed to characterize selectivity and mechanism of cytochrome P450 inhibition by nelfinavir mesylate. At therapeutic concentrations (steady-state plasma concentrations approximately 4 microM), nelfinavir was found to be a competitive inhibitor of only testosterone 6beta-hydroxylase (CYP3A4) with a Ki concentration of 4. 8 microM. At supratherapeutic concentrations, nelfinavir competitively inhibited dextromethorphan O-demethylase (CYP2D6), S-mephenytoin 4-hydroxylase (CYP2C19), and phenacetin O-deethylase (CYP1A2) with Ki concentrations of 68, 126, and 190 microM, respectively. Nelfinavir did not appreciably inhibit tolbutamide 4-hydroxylase (CYP2C9), paclitaxel 6alpha-hydroxylase (CYP2C8), or chlorzoxaxone 6beta-hydroxylase (CYP2E1) activities. The inhibitory potency of nelfinavir toward CYP3A4 suggested the possibility of in vivo inhibition of this isoform, whereas in vivo inhibition of other P450s was considered unlikely. In a one-sequence crossover study in 12 healthy volunteers, nelfinavir inhibited the elimination of the CYP3A substrate terfenadine and the carboxylate metabolite of terfenadine. The 24-hr urinary recoveries of 6beta-hydroxycortisol were reduced by an average of 27% during nelfinavir treatment, consistent with CYP3A inhibition by nelfinavir. Inhibition of CYP3A4 by nelfinavir in vitro was NADPH-dependent requiring the catalytic formation of a metabolite or a metabolic intermediate. The catechol metabolite of nelfinavir (M3) was considered unlikely to be responsible for inhibition as the addition of catechol O-methyl transferase, S-adenosyl methionine, and ascorbic acid to the preincubation mixture did not protect against the loss of testosterone 6beta-hydroxylase activity. Also, the addition of M3 to human liver microsomes did not inhibit CYP3A4. Although incubations with nelfinavir showed a time- and concentration-dependent loss of CYP3A4 activity, the partial or complete recovery of enzyme activity upon dialysis indicated that inhibition was reversible. Microsomal incubations with nelfinavir and NADPH did not result in a loss of spectral P450 content compared with the NADPH control. Glutathione, N-acetylcysteine, and catalase did not attenuate CYP3A4 inhibition by nelfinavir. Collectively, these results suggest that the probable mechanism for CYP3A4 inhibition by nelfinavir is a transient metabolic intermediate or stable metabolite that coordinates tightly but reversibly to the heme moiety of the P450.

A pharmacokinetic-pharmacodynamic model of chemotherapy of human immunodeficiency virus infection that relates development of drug resistance to treatment intensity

RNA viruses, including retroviruses, have mutation rates that are about 100 times higher than those of DNA viruses, bacteria, or eukaryotes, so that resistance to AIDS drugs emerges very rapidly. This has been shown to limit the effectiveness of the treatment of AIDS by reverse transcriptase inhibitors, such as zidovudine (AZT) and resistance to the new class of HIV aspartyl protease inhibitors has already been reported. The technique of pharmacokinetic-pharmacodynamic simulation has now been used to predict ways of delaying the development of resistance to these two classes of antiretroviral agents. A model is described that includes pharmacokinetic, pharmacodynamic, and cytokinetic equations, and expressions describing effects if the HIV on the immune system and destruction of virally infected cells by cellular immunity. The model predicted that the degree of viral drug resistance in relation to the sustainable blood level of drug would be the major determinant of response duration. Early treatment was consistently superior to late treatment, both with a drug that caused cumulative toxicity and with a drug that did not. Making reasonable assumptions about the likely degree of viral resistance, in conjunction with typical blood levels achievable for reverse transcriptase inhibitors or aspartyl protease inhibitors led to predicted response durations of several months to a few years, despite the rapid mutation rate of HIV. Preliminary studies of combination chemotherapy showed that predicted response durations were greater than for monotherapy, though less than the sum of responses to the individual drugs. Strategies for delaying the development of resistance include early treatment, combination chemotherapy, and developing novel agents with a high ratio of plasma level to antiviral efficacy.

Viracept (nelfinavir mesylate, AG1343): a potent, orally bioavailable inhibitor of HIV-1 protease

Using a combination of iterative structure-based design and an analysis of oral pharmacokinetics and antiviral activity, AG1343 (Viracept, nelfinavir mesylate), a nonpeptidic inhibitor of HIV-1 protease, was identified. AG1343 is a potent enzyme inhibitor (Ki = 2 nM) and antiviral agent (HIV-1 ED50 = 14 nM). An X-ray cocrystal structure of the enzyme-AG1343 complex reveals how the novel thiophenyl ether and phenol-amide substituents of the inhibitor interact with the S1 and S2 subsites of HIV-1 protease, respectively. In vivo studies indicate that AG1343 is well absorbed orally in a variety of species and possesses favorable pharmacokinetic properties in humans. AG1343 (Viracept) has recently been approved for marketing for the treatment of AIDS.

Activities of the human immunodeficiency virus type 1 (HIV-1) protease inhibitor nelfinavir mesylate in combination with reverse transcriptase and protease inhibitors against acute HIV-1 infection in vitro

Nelfinavir mesylate (formerly AG1343) is a potent and selective, nonpeptidic inhibitor of human immunodeficiency virus type 1 (HIV-1) protease that was discovered by protein structure-based design methodologies. We evaluated the antiviral and cytotoxic effects of two-drug combinations of nelfinavir with the clinically approved antiretroviral therapeutics zidovudine (ZDV), lamivudine (3TC), dideoxycytidine (ddC; zalcitabine), stavudine (d4T), didanosine (ddI), indinavir, saquinavir, and ritonavir and a three-drug combination of nelfinavir with ZDV and 3TC against an acute HIV-1 strain RF infection of CEM-SS cells in vitro. Quantitative assessment of drug interaction was evaluated by a universal response surface approach (W. R. Greco, G. Bravo, and J. C. Parsons, Pharm. Rev. 47:331-385, 1995) and by the method of M. N. Prichard and C. Shipman (Antiviral Res. 14:181-206, 1990). Both analytical methods yielded similar results and showed that the two-drug combinations of nelfinavir with the reverse transcriptase inhibitors ZDV, 3TC, ddI, d4T, and ddC and the three-drug combination with ZDV and 3TC resulted in additive to statistically significant synergistic interactions. In a similar manner, the combination of nelfinavir with the three protease inhibitors resulted in additive (ritonavir and saquinavir) to slightly antagonistic (indinavir) interactions. In all combinations, minimal cellular cytotoxicity was observed with any drug alone and in combination. These results suggest that administration of combinations of the appropriate doses of nelfinavir with other currently approved antiretroviral therapeutic agents in vivo may result in enhanced antiviral activity with no associated increase in cellular cytotoxicity.

Molecular recognition of the inhibitor AG-1343 by HIV-1 protease: conformationally flexible docking by evolutionary programming

BACKGROUND

An important prerequisite for computational structure-based drug design is prediction of the structures of ligand-protein complexes that have not yet been experimentally determined by X-ray crystallography or NMR. For this task, docking of rigid ligands is inadequate because it assumes knowledge of the conformation of the bound ligand. Docking of flexible ligands would be desirable, but requires one to search an enormous conformational space. We set out to develop a strategy for flexible docking by combining a simple model of ligand-protein interactions for molecular recognition with an evolutionary programming search technique.

RESULTS

We have developed an intermolecular energy function that incorporates steric and hydrogen-bonding terms. The parameters in this function were obtained by docking in three different protein systems. The effectiveness of this method was demonstrated by conformationally flexible docking of the inhibitor AG-1343, a potential new drug against AIDS, into HIV-1 protease. For this molecule, which has nine rotatable bonds, the crystal structure was reproduced within 1.5 A root-mean-square deviation 34 times in 100 simulations, each requiring eight minutes on a Silicon Graphics R4400 workstation. The energy function correctly evaluates the crystal structure as the global energy minimum.

CONCLUSIONS

We believe that a solution of the docking problem may be achieved by matching a simple model of molecular recognition with an efficient search procedure. The necessary ingredients of a molecular recognition model include only steric and hydrogen-bond interaction terms. Although these terms are not necessarily sufficient to predict binding affinity, they describe ligand-protein interactions faithfully enough to enable a docking program to predict the structure of the bound ligand. This docking strategy thus provides an important tool for the interdisciplinary field of rational drug design.