Pharmacokinetics of saquinavir, atazanavir, and ritonavir in a twice-daily boosted double-protease inhibitor regimen

Key Pharmacokinetic Essentials of Fixed-Dosed Combination Products: Case Studies and Perspectives

Fixed-dose combinations are gaining popularity because they provide convenience while enhancing patient compliance. Literature examples suggest that many fixed-dose combinations are being rationalized and investigated for their potential utility in therapy. This article provides an introspection into the pharmacokinetic essentials that need to be considered prior to implementing a fixed-dose combination strategy. While the drug-drug interaction potential is an important question for the two drugs in a fixed-dose combination, the occurrence of a drug-drug interaction in itself is not a negative outcome for the proposed fixed-dose combination. However, the magnitude of a drug-drug interaction may require a re-assessment of the doses of the two drugs in a fixed-dose combination. Several case studies are provided and discussed to provide a broad perspective on the topic along with a representative framework and strategy on the development of fixed-dose combinations using key pharmacokinetic parameters.

Revealing the metabolic sites of atazanavir in human by parallel administrations of D-atazanavir analogs

Atazanavir (Reyataz(®)) is an important member of the HIV protease inhibitor class. Because of the complexity of its chemical structure, metabolite identification and structural elucidation face serious challenges. So far, only seven non-conjugated metabolites in human plasma have been reported, and their structural elucidation is not complete, especially for the major metabolites produced by oxidations. To probe the exact sites of metabolism and to elucidate the relationship among in vivo metabolites of atazanavir, we designed and performed two sets of experiments. The first set of experiments was to determine atazanavir metabolites in human plasma by LC-MS, from which more than a dozen metabolites were discovered, including seven new ones that have not been reported. The second set involved deuterium labeling on potential metabolic sites to generate D-atazanavir analogs. D-atazanavir analogs were dosed to human in parallel with atazanavir. Metabolites of D-atazanavir were identified by the same LC-MS method, and the results were compared with those of atazanavir. A metabolite structure can be readily elucidated by comparing the results of the analogs and the pathway by which the metabolite is formed can be proposed with confidence. Experimental results demonstrated that oxidation is the most common metabolic pathway of atazanavir, resulting in the formation of six metabolites of monooxidation (M1, M2, M7, M8, M13, and M14) and four of dioxidation (M15, M16, M17, and M18). The second metabolic pathway is hydrolysis, and the third is N-dealkylation. Metabolites produced by hydrolysis include M3, M4, and M19. Metabolites formed by N-dealkylation are M5, M6a, and M6b.

Identification and structural elucidation of in vitro metabolites of atazanavir by HPLC and tandem mass spectrometry

Atazanavir (marketed as Reyataz®) is an important member of the human immunodeficiency virus protease inhibitor class. LC-UV-MS(n) experiments were designed to identify metabolites of atazanavir after incubations in human hepatocytes. Five major (M1-M5) and seven minor (M7-M12) metabolites were identified. The most abundant metabolite, M1, was formed by a mono-oxidation on the t-butyl group at the non-prime side. The second most abundant metabolite, M2, was also a mono-oxidation product, which has not yet been definitively identified. Metabolites, M3 and M4, were structural isomers, which were apparently formed by oxidative carbamate hydrolysis. The structure of M5 comprises the non-prime side of atazanavir which contains a pyridinyl-benzyl group. Metabolite M6a was formed by the cleavage of the pyridinyl-benzyl side chain, as evidenced by the formation of the corresponding metabolic product, the pyridinyl-benzoic acid (M6b). Mono-oxidation also occurred on the pyridinyl-benzyl group to produce the low abundance metabolite M8. Oxidation of the terminal methyl groups produced M9 and M10, respectively, which have low chemical stability. Trace-level metabolites of di-oxidations, M11 and M12, were also detected, but the complexity of the molecule precluded identification of the second oxidation site. To our knowledge, metabolites M6b and M8 have not been reported.

Multidrug resistance: a clinical approach

PURPOSE OF REVIEW

New antiretroviral agents have recently become available within existing and new drug classes, increasing treatment options for patients with multidrug-resistant virus. This review discusses the challenges that these new agents pose for the management of treatment-experienced patients.

RECENT FINDINGS

Recent studies of the efficacy and safety of new antiretroviral drugs illustrate that drug regimens containing new agents are well tolerated and can suppress viremia in even the most drug-resistant patients. The goal of any new regimen should therefore be suppression of plasma HIV RNA levels to less than 50 copies/ml, even in treatment-experienced patients. Patients should be given a regimen with at least two, or preferably three, fully active drugs after careful consideration of their treatment and adherence history, current and prior genotype tests, comorbidities, and concomitant medications. Newer and more tolerable agents also offer the possibility of regimen simplification among patients with multidrug-resistant HIV who are virologically suppressed.

SUMMARY

Clinicians must optimize the pairing and sequencing of recently available antiretroviral agents. Future studies should continue to investigate the optimal use of new agents in order to further improve long-term treatment efficacy in patients with multidrug-resistant HIV infection.

Concentration-dependent effects and intracellular accumulation of HIV protease inhibitors in cultured CD4 T cells and primary human lymphocytes

BACKGROUND

The intracellular and plasma concentrations of HIV protease inhibitors (HPIs) vary widely in vivo. It is unclear whether there is a concentration-dependent effect of HPIs such that at increasing concentration they may either block their own efflux (leading to 'autoboosting') or influx (leading to saturability/decreased intracellular accumulation).

METHOD

The effects of various concentrations (0-30 microM) of lopinavir, saquinavir, ritonavir and atazanavir on the accumulation of [(14)C]lopinavir, [(3)H]saquinavir, [(3)H]ritonavir and [(3)H]atazanavir, respectively, were investigated in CEM(parental), CEM(VBL) [P-glycoprotein (ABCB1) overexpressing], CEM(E1000) (MRP1 overexpressing) and in peripheral blood mononuclear cells (PBMCs). We also investigated the effects of inhibitors of ABCB1/ABCG2 (tariquidar), ABCC (MK571) and ABCC1/2 (frusemide), singly and in combination with HPIs, on cellular accumulation.

RESULTS

In all the cell lines, with increasing concentration of lopinavir, saquinavir and ritonavir, there was a significant increase in the cellular accumulation of [(14)C]lopinavir, [(3)H]saquinavir and [(3)H]ritonavir. Tariquidar, MK571 and frusemide (alone and in combination with lopinavir, saquinavir and ritonavir) significantly increased the accumulation of [(14)C]lopinavir, [(3)H]saquinavir and [(3)H]ritonavir. Ritonavir (alone or in combination with tariquidar) decreased the intracellular accumulation of [(3)H]ritonavir in PBMCs. Atazanavir decreased the accumulation of [(3)H]atazanavir in a concentration-dependent manner in all of the cells tested.

CONCLUSIONS

There are complex and variable drug-specific rather than class-specific effects of the HPIs on their own accumulation.

Atazanavir: a review of its use in the management of HIV-1 infection

Atazanavir (Reyataz), a protease inhibitor (PI), is approved in many countries for use as a component of antiretroviral therapy (ART) regimens for the treatment of adult, and in some countries in paediatric, patients with HIV-1 infection. ART regimens containing ritonavir-boosted atazanavir improved virological and immunological markers in adult patients with HIV-1 infection, and had similar efficacy to regimens containing lopinavir/ritonavir in treatment-naive and treatment-experienced patients. In addition, unboosted atazanavir was noninferior to ritonavir-boosted atazanavir in treatment-naive patients. Atazanavir is administered once daily and has a low capsule burden. Atazanavir, whether unboosted or boosted, was generally well tolerated and appeared to be associated with less marked metabolic effects, including less alteration of lipid levels, than other PIs. These properties mean that boosted atazanavir, and unboosted atazanavir in patients unable to tolerate ritonavir, continues to have a role as a component of ART regimens in patients with HIV-1 infection.

Once-daily treatment with saquinavir mesylate (2000 mg) and ritonavir (100 mg) together with a fixed-dose combination of abacavir/lamivudine (600/300 mg) or tenofovir/emtricitabine (245/200 mg) in HIV-1-infected patients

OBJECTIVES

To investigate the feasibility and pharmacokinetics of a once-daily regimen of 2000 mg saquinavir mesylate boosted with 100 mg ritonavir.

PATIENTS AND METHODS

Patients successfully treated with 1000 mg saquinavir boosted with 100 mg ritonavir twice daily together with two nucleoside or nucleotide reverse transcriptase inhibitors [N(t)RTIs] who were switched to 2000 mg saquinavir with 100 mg ritonavir once daily with unchanged N(t)RTI therapy were analysed. CD4 cells, HIV-RNA PCR and metabolic parameters were compared between baseline and 3, 6, 9 and 12 months after the switch. Saquinavir and ritonavir drug levels were measured before and a median of 3 weeks after switching from twice to once daily at 0, 1, 2, 4, 6, 9, 12 and 24 h after intake of the medication. The area under the serum concentration-time curve from 0 to 24 h (AUC(0-24)) was calculated using the trapezoidal rule.

RESULTS

Eighteen patients (16 males, median age of 41 years) with a median CD4 cell count of 464 cells/mm(3) were analysed. HIV-RNA PCR remained <500 copies/mL for all patients. After switching from 100 mg twice daily to 100 mg once daily, the AUC(0-24) for ritonavir decreased significantly [21 874 to 10 267 ng.h/mL, geometric mean ratio (GMR) = 0.47; P < 0.001], whereas the AUC(0-24) for saquinavir decreased only marginally from 35 000 to 34 490 ng.h/mL (GMR = 0.99; P = 0.426). The CD4 cell count and the fasting metabolic parameters remained unchanged.

CONCLUSIONS

Once-daily treatment with ritonavir-boosted saquinavir was well tolerated and resulted in similar saquinavir drug exposure despite much lower ritonavir concentrations when compared with a twice-daily dosing schedule.

Cytochrome P450 3A inhibition by atazanavir and ritonavir, but not demography or drug formulation, influences saquinavir population pharmacokinetics in human immunodeficiency virus type 1-infected adults

Inadequate concentrations of the human immunodeficiency virus (HIV) protease inhibitor saquinavir jeopardize individual therapy success or produce side effects despite treatment according to the current guidelines. We performed a population pharmacokinetic analysis with NONMEM and determined that the steady-state pharmacokinetics of saquinavir in 136 HIV type 1-infected adults was modulated by a decrease in saquinavir CL following coadministration of the cytochrome P450 3A inhibitors ritonavir and atazanavir. In contrast, age, sex, weight, pregnancy, and the pharmaceutical formulation exerted only minor, nonsignificant effects.

Population pharmacokinetics of ritonavir-boosted atazanavir in HIV-infected patients and healthy volunteers

Objectives

The aim of this study was to develop and validate a population pharmacokinetic model to: (i) describe ritonavir-boosted atazanavir concentrations (300/100 mg once daily) and identify important covariates; and (ii) evaluate the predictive performance of the model for lower, unlicensed atazanavir doses (150 and 200 mg once daily) boosted with ritonavir (100 mg once daily).

Methods

Non-linear mixed effects modelling was applied to determine atazanavir pharmacokinetic parameters, inter-individual variability (IIV) and residual error. Covariates potentially related to atazanavir pharmacokinetics were explored. The final model was assessed by means of a visual predictive check for 300/100, 200/100 and 150/100 mg once daily.

Results

Forty-six individuals were included (30 HIV-infected). A one-compartment model with first-order absorption and lag-time best described the data. Final estimates of apparent oral clearance (CL/F), volume of distribution (V/F) and absorption rate constant [relative standard error (%) and IIV (%)] were 7.7 L/h (5, 29), 103 L (13, 48) and 3.4 h⁻¹ (34, 154); a lag-time of 0.96 h (1) was determined. Ritonavir area under the curve (AUC_0–24) was the only significant covariate. Overall, 94%–97% of observed concentrations were within the 95% prediction intervals for all three regimens.

Conclusions

A population pharmacokinetic model for ritonavir-boosted atazanavir has been developed and validated. Ritonavir AUC_0–24 was significantly associated with atazanavir CL/F. The model was used to investigate other, particularly lower, ritonavir-boosted atazanavir dosing strategies.

[Pharmacology, pharmacokinetic features and interactions of atazanavir]

Atazanavir (ATV) is an HIV protease inhibitor (IP) with a high in vitro activity against HIV-1, that demonstrates a high additive activity in the presence of other antiretrovirals and a synergic activity with other PI. Oral absorption is greater than 68%, maximum concentration (C(max)) being reached approximately 2 to 3 h after its administration. Its absorption is dependent on gastric pH, its administration being recommended after meals. The pharmacokinetics (PK) of ATV are non-linear; that is to say, its plasma concentrations (C(p)) do not increase in proportion to the dose. ATV is approximately 86% bound to plasma proteins. Its entry into the cerebrospinal fluid, semen or genital secretions varies but is generally less than 10-20%. Its passage across the placenta, measured as the mean of the ratios between the C(p) in umbilical cord and maternal blood, is 0.13. ATV is metabolised by oxidation by cytochrome P450 enzymes, subsequently being eliminated by the bile duct in the free or glucuronide form (80%) and by the urine. ATV is a weak competitive inhibitor of CYP3A4 and a strong inhibitor of uridine diphosphate-glucuronosyltransferase 1A1, which is the cause of the frequent high plasma bilirubin after its administration and of its pharmacological interactions.

Decrease of atazanavir and lopinavir plasma concentrations in a boosted double human immunodeficiency virus protease inhibitor salvage regimen

The human immunodeficiency virus protease inhibitor combination of atazanavir (ATV)-lopinavir-ritonavir was reported to exhibit a mutual pharmacoenhancement of plasma lopinavir and ATV concentrations which may be beneficial for salvage patients. We identified 17 patients in our pharmacokinetic database taking this combination and found conflicting results. Plasma concentrations of both ATV and lopinavir were modestly, although not significantly, decreased when the drugs were coadministered. Therefore, patients should be selected carefully for this regimen and frequent clinical and therapeutic drug monitoring is strongly advised.

Baseline CD4 cell count and outcome of pegylated interferon plus ribavirin therapy in HIV/hepatitis C virus-coinfected patients

OBJECTIVE

When to start hepatitis C treatment in HIV/hepatitis C virus (HCV)-coinfected patients remains unresolved. Our objective was to determine if a baseline CD4 count >/=350 cells/mm predicts a sustained HCV response to pegylated interferon plus ribavirin.

METHODS

We conducted a multicenter cohort study of HIV/HCV-coinfected patients treated for HIV in hospitals in Nice, Tourcoing, and Marseille (France). Sustained viral response (SVR) was defined as undetectable HCV RNA 24 weeks after treatment. The relation between CD4 cell count and SVR was examined separately for patients with HCV genotype 1 or non-1.

RESULTS

One hundred seventy-five patients were included. In patients with HCV genotype 1, the rate of SVR was 13% and was not related to baseline CD4 cell count (odds ratio [OR] = 1.0, 95% confidence interval [CI]: 0.1 to 9.3). In patients with HCV genotype non-1, the rate of SVR was 46% and was not significantly increased by a baseline CD4 count >/=350 cells/mm (OR = 1.8, 95% CI: 0.6 to 5.9).

CONCLUSIONS

Higher CD4 cell count at treatment initiation with pegylated interferon plus ribavirin did not improve treatment success probability, regardless of HCV genotype.