Indinavir: a review of its use in the management of HIV infection

Drug design with a new transition state analog of the hydrated carbonyl: silicon-based inhibitors of the HIV protease

BACKGROUND

Silicon is the element most similar to carbon, and bioactive organosilanes have therefore been of longstanding interest. Design of bioactive organosilanes has often involved a systematic replacement of a bioactive molecule's stable carbon atoms with silicon. Silanediols, which are best known as unstable precursors of the robust and ubiquitous silicone polymers, have the potential to mimic an unstable carbon, the hydrated carbonyl. As a bioisostere of the tetrahedral intermediate of amide hydrolysis, a silanediol could act as a transition state analog inhibitor of protease enzymes.

RESULTS

Silanediol analogs of a carbinol-based inhibitor of the HIV protease were prepared as single enantiomers, with up to six stereogenic centers. As inhibitors of this aspartic protease, the silanediols were nearly equivalent to both their carbinol analogs and indinavir, a current treatment for AIDS, with low nanomolar K(i) values. IC(90) data from a cell culture assay mirrored the K(i) data, demonstrating that the silanediols can also cross cell membranes and deliver their antiviral effects.

CONCLUSIONS

In their first evaluation as inhibitors of an aspartic protease, silanediol peptidomimetics have been found to be nearly as potent as currently available pharmaceutical agents, in enzyme and cell protection assays. These neutral, cell-permeable transition state analogs therefore provide a novel foundation for the design of therapeutic agents.

Delavirdine: clinical pharmacokinetics and drug interactions

Delavirdine, a non-nucleoside reverse transcriptase inhibitor (NNRTI), is a potent and specific inhibitor of HIV-1 reverse transcriptase. The approved therapeutic regimen for delavirdine is 400mg 3 times daily in combination with appropriate antiretroviral agents; however, a dose of 600mg twice daily appears to provide similar systemic exposure. The steady-state pharmacokinetics of delavirdine are not appreciably affected by food. Delavirdine undergoes extensive metabolism by cytochrome P450 (CYP) with little urinary excretion of unchanged drug. Metabolic drug interactions between delavirdine and nucleoside reverse transcriptase inhibitors are unlikely as their metabolic pathways differ; delavirdine has no effect on the pharmacokinetics of zidovudine. Concomitant use of CYP inducers, such as rifampicin (rifampin), rifabutin, phenytoin, phenobarbital or carbamazepine, should be avoided since delavirdine plasma concentrations are significantly lowered. Reduction in gastric acidity (pH > 3) decreases the extent of delavirdine absorption, so administration of antacids and the buffered formulations of didanosine should be separated from that of delavirdine by at least 1 hour. Delavirdine, unlike other currently available NNRTI agents, is an inhibitor rather than an inducer of CYP isozymes. Consequently, the drug interaction profile and rationale for combining delavirdine with other antiretroviral agents is unique among the current NNRTI agents. Delavirdine inhibits the CYP3A4-mediated metabolism of HIV protease inhibitors and thereby increases systemic exposure to protease inhibitors. The ability of delavirdine to enhance the pharmacokinetic profiles of protease inhibitors may permit the use of simplified administration regimens. Combining delavirdine and indinavir removes the food restrictions during indinavir administration. Furthermore, the superior virological response observed in antiretroviral regimens containing delavirdine and protease inhibitors has been attributed to the favourable pharmacokinetic interactions and the introduction of a new drug class in NNRTI-naïve therapy-experienced patients. Pharmacokinetic drug interactions are an important consideration in selecting an HIV treatment regimen, due to the multiplicity of drugs that are coadministered and the varying direction and magnitude of interaction that can occur. Considerations for utilising delavirdine in a treatment regimen are different than for other NNRTI agents due to the unique drug interaction profile of delavirdine.

Hyperlipidemia and inhibitors of HIV protease

HIV protease inhibitors have been successfully incorporated into therapy for patients with HIV. These otherwise efficacious treatments present with multiple metabolic side-effects and body habitus changes known as the lipodystrophy syndrome. Direct associations of the lipid abnormalities with protease inhibitor use have been described, and ongoing studies are focused on describing mechanisms for future intervention. Mechanisms based on the molecular identity of the protease inhibitor target with human proteins, interference with aspects critical to lipoprotein production, and interference with adipocyte differentiation have been described. This review highlights the complexities of this syndrome, and discusses putative mechanisms whereby protease inhibitors cause hyperlipidemia.

Plasma population pharmacokinetics and penetration into cerebrospinal fluid of indinavir in combination with zidovudine and lamivudine in HIV-1-infected patients

OBJECTIVES

To evaluate plasma population pharmacokinetics and penetration into cerebrospinal fluid (CSF) by indinavir (IDV) in HIV-infected individuals receiving IDV, zidovudine and lamivudine.

METHODS

Plasma population pharmacokinetic analysis was performed on 805 IDV plasma values from 171 patients, using a non-linear mixed-effects modeling approach. CSF data from 19 patients were analyzed using an individual approach.

RESULTS

Mean individual Bayesian estimates for oral clearance (CL) and volume of distribution (V) by the final model that incorporated interoccasion variability were 0.75 l/h per kg [coefficient of variation (CV) 54.8%] and 1.74 l/kg (CV 82.7%), respectively. Mean model-predicted plasma IDV level at 8 h, maximal level, area under the plasma level-time curve up to 8 h and plasma half-life were 0.42 micromol/l (CV 57.5%), 9.51 micromol/l (CV 47.3%), 29.56 micromol/l x h (CV 46.9%) and 1.50 h (CV 20.9%), respectively. The mean IDV CSF level was 0.11 micromol/l (CV 49.7%) and the mean CSF:plasma concentration ratio was 0.017.

CONCLUSIONS

Population estimates of pharmacokinetic parameters of IDV and its CSF penetration were in excellent agreement with previously reported data from individual analyses. Intraindividual interoccasion variability of IDV pharmacokinetics was estimated to be of similar order of magnitude to its interindividual variability, which may affect response to long-term antiretroviral therapy involving IDV. CSF levels of IDV exceeded its in vitro 95% inhibitory concentration of HIV replication. Given that CSF is virtually free of protein, viral suppression in the central nervous system should be achievable with an IDV-containing regimen.

Antiretroviral therapy 2000

As we enter the new millennium, there have been dramatic improvements in the care of patients with HIV infection. These have prolonged life and decreased morbidity and mortality. There are fourteen currently available antiretrovirals approved in the United States for the treatment of this infection. The medications, including their pharmacokinetic properties, side effects, and dosing are reviewed. In addition, the current approach to the use of these medicines is discussed.

Saquinavir soft-gel capsule: an updated review of its use in the management of HIV infection

Saquinavir is a potent and highly selective HIV protease inhibitor. Initially formulated as a hard-gel capsule (HGC), saquinavir was the first protease inhibitor available commercially for the treatment of patients with HIV infection. The limited oral bioavailability of saquinavir HGC has been improved significantly with the introduction of a soft-gel capsule (SGC) formulation. Saquinavir SGC displays greater than dose-proportional pharmacokinetics and mean area under the plasma concentration-time curve (AUC) values are 8- to 10-fold higher with saquinavir SGC 1200 mg 3 times daily than with the HGC formulation 600 mg 3 times daily, the recommended dosages of the 2 formulations. In combination with other protease inhibitors (particularly "low dose" ritonavir), the oral bioavailability of saquinavir (as either the HGC or SGC formulation) is markedly increased, allowing for reduced dosing frequency and/or dosage. The efficacy and tolerability of once- or twice-daily saquinavir SGC/"low dose" ritonavir combinations are currently being evaluated in patients with HIV infection. Data (up to 48 weeks) from noncomparative and comparative clinical trials evaluating saquinavir SGC-containing combination regimens in adult patients with HIV infection, support and strengthen the clinical efficacy profile of the drug that was demonstrated in initial trials. In antiretroviral therapy-naive and -experienced patients, saquinavir SGC combined with > or =2 nucleoside reverse transcriptase inhibitors (NRTIs), or nelfinavir, or nelfinavir plus 2 NRTIs or nonnucleoside reverse transcriptase inhibitors (NNRTIs), markedly improved immunological and virological surrogate markers (increased mean CD4+ cell counts and decreased mean plasma HIV RNA levels) of HIV infection. Saquinavir SGC demonstrated a trend to greater antiviral efficacy (measured by improvements in surrogate markers) than the HGC formulation (not statistically significant); a significantly greater proportion of patients treated with saquinavir SGC had plasma HIV RNA levels <400 copies/ml than patients receiving the HGC formulation. In the first direct comparison of 2 protease inhibitors, saquinavir SGC plus 2 NRTIs demonstrated similar antiviral efficacy to indinavir plus 2 NRTIs in patients with HIV infection (almost all of whom were antiretroviral therapy-naive); at 24 weeks, a significantly greater increase in CD4+ cell count from baseline was obtained in the saquinavir SGC group compared with the indinavir group, although this difference was not apparent at week 32. Triple therapy with saquinavir SGC plus 2 NRTIs was as effective as nelfinavir-containing triple therapy, or quadruple therapy (saquinavir SGC plus 2 NRTIs plus nelfinavir) in markedly suppressing HIV RNA levels in antiretroviral therapy-experienced or -naive patients. Saquinavir SGC is generally well tolerated. Gastrointestinal disturbances (generally nausea, diarrhoea, abdominal pain, vomiting and dyspepsia of moderate or greater intensity) are the most common adverse events associated with saquinavir SGC-containing therapy. In comparative trials, saquinavir SGC-containing therapy was as well tolerated as indinavir- and nelfinavir-containing therapy; although there were no statistical differences between treatment groups, the incidence of diarrhoea was lower in patients receiving saquinavir SGC compared with nelfinavir, saquinavir SGC plus nelfinavir (all combined with 2 NRTIs) or saquinavir SGC plus nelfinavir without additional therapy. Compared with the HGC formulation, saquinavir SGC appears to be associated with a higher overall incidence of adverse events.

CONCLUSIONS

Clinical trial data have shown that as part of triple or quadruple combination therapy, saquinavir SGC is an effective and generally well tolerated protease inhibitor in antiretroviral therapy-naive or -experienced patients with HIV infection. (ABSTRACT TRUNCATED)

Nelfinavir: an update on its use in HIV infection

Nelfinavir is one of several currently available protease inhibitors used to limit viral replication and improve immune function in HIV-infected individuals. It is administered in combination with other antiretroviral agents. Nelfinavir has been evaluated as first-line therapy with nucleoside reverse transcriptase inhibitors (NRTIs) in treatment-naive patients, or as an additional antiretroviral agent in protease inhibitor-naive patients already receiving NRTIs. These studies have shown good efficacy in terms of HIV viral load reduction and increased CD4+ cell counts. When used in combination with NRTIs, nelfinavir 1250 mg twice daily produced similar results to 750 mg 3 times daily. The more convenient twice-daily dosage schedule, which is now approved in the US, may be beneficial in improving patient adherence to therapy. Nelfinavir has also been used successfully in combination with non-nucleoside reverse transcriptase inhibitors and/or other protease inhibitors, with or without NRTIs. Resistance to nelfinavir has been observed in vitro and in clinical isolates from patients experiencing insufficient or waning viral suppression during treatment. Nelfinavir primarily selects for the D30N mutation, which is not seen with other protease inhibitors, and alone does not cause resistance to other protease inhibitors in vitro. Several studies have shown that patients who experience virological failure while receiving nelfinavir can respond to salvage therapy with other protease inhibitors. Diarrhoea is the most frequent adverse event in patients receiving nelfinavir-based combination therapy, but was generally mild and resulted in minimal discontinuation of therapy in clinical trials. Diarrhoea can usually be controlled with drugs that slow gastrointestinal motility. Metabolic disturbances associated with protease inhibitor use (hypercholesterolaemia, hyperglycaemia and lipodystrophy) have also been reported with nelfinavir. Nelfinavir is associated with a number of clinically significant drug interactions and coadministration of some drugs (e.g. astemizole, cisapride, triazolam) is contraindicated. Coadministration of nelfinavir with other protease inhibitors generally resulted in favourable pharmacokinetic interactions (usually increased area under the concentration-time curve for both drugs).

CONCLUSION

Nelfinavir, in combination with reverse transcriptase inhibitors and/or other protease inhibitors, is effective in limiting HIV replication and increasing CD4+ cell counts in HIV-infected adults and children. The convenience of its dosage administration, the low incidence of adverse events, and the potential for salvage therapies indicate that nelfinavir (as part of combined antiretroviral therapy regimens) should be considered as a first-line option in protease inhibitor-naive patients and in those unable to tolerate other protease inhibitors.