Ritonavir was the second agent in the human immunodeficiency virus-1 (HIV-1) protease inhibitor (PI) class that ushered in the era of highly active antiretroviral therapy for HIV infection more than a decade ago. Although these highly active antiretroviral therapy agents offered great promise in reducing the morbidity and mortality of HIV disease, their suboptimal physicochemical and pharmacokinetic properties necessitated high doses and frequent administration and were often poorly tolerated by patients. Upon clinical use it was observed that ritonavir is a potent, irreversible inhibitor of cytochrome P450 isoform 3A (CYP3A) and that the use of ritonavir can lead to substantial increases in circulating concentrations of other drugs subject to metabolism by CYP3A in vivo. Today, ritonavir is almost exclusively used at low, subtherapeutic doses to enhance or "boost" the exposures of concomitantly administered HIV PIs, thus exploiting what was originally thought to be one of the greatest limitations of ritonavir.
Ritonavir-mediated boosting of PI-containing regimens has become a standard of care, as it allows for less frequent dosing intervals and higher trough concentrations, each of which has been shown to be an important factor in improving adherence to therapy, providing coverage against partially resistant viral strains, and ultimately improving treatment outcomes.1,2 Moreover, ritonavir boosting has been critical to the drug development pathway for a number of antiretroviral agents that, despite their compelling in vitro antiviral properties, may not have been suitable drug candidates when administered alone because of pharmacokinetic limitations of rapid clearance (CL) through metabolism by CYP3A.
Although the 600-mg twice-daily dose of ritonavir is approved for antiretroviral treatment, this dose is poorly tolerated due to gastrointestinal adverse events and changes in serum lipids, insulin resistance, and lipoatrophy; thus, it is desirable to use the lowest possible dose to achieve the desired boosting effect. Ritonavir is typically used to boost HIV PIs at doses of 100–200 mg once or twice daily; however, even these doses have been associated with clinical adverse events, laboratory abnormalities, and/or intolerance in some patients.3
Elvitegravir (also known as EVG, GS-9137, and JTK-303) is an HIV-1 integrase inhibitor that selectively and potently inhibits (protein binding–adjusted IC50 = 16 nmol/l) integration of HIV cDNA into the host chromosomal DNA. Elvitegravir undergoes metabolism primarily by CYP3A and also by glucuronidation (uridine glucuronosyltransferase 1A1 and 1A3). Metabolites formed by these pathways are markedly less potent (5- to 38-fold) than the parent drug and are present in plasma at lower concentrations; thus, the metabolites do not contribute to the antiviral activity of elvitegravir. Once-daily administration of elvitegravir with low-dose ritonavir (100 mg) results in inhibition of elvitegravir CYP3A metabolism, as evidenced by a marked enhancement of systemic exposure (area under the plasma concentration–time curve over the dosing interval (AUCtau) increased approximately 20-fold) by a combination of enhanced oral bioavailability and reduced systemic CL.4
In a 10-day monotherapy study in treatment-naive and treatment-experienced HIV patients, elvitegravir was well tolerated and demonstrated potent exposure-dependent antiviral activity (mean reductions up to 2 log10 copies/ml), with its antiviral response best predicted by the plasma concentration at the end of the dosing interval (Cmin/tau).5 Data from a randomized, active-controlled, dose-ranging, phase II study in HIV-infected treatment-experienced patients demonstrated that a once-daily, 125/100-mg elvitegravir/ritonavir dose was superior to a regimen (selected using resistance testing) of ritonavir-boosted comparator PIs and was generally well tolerated.6
There are no clinical data available regarding the detailed dose–response relationship for ritonavir on CYP3A activity or the potential for doses <100 mg to provide boosting. We sought to understand the dose–response relationship for ritonavir's inhibition of CYP3A, its impact on boosting of elvitegravir, and the potential for additional changes to elvitegravir pharmacokinetics when dosed concurrently with other agents that may be metabolized by or inhibit CYP3A. Specifically, we assessed, at steady state, the effect of increasing doses of ritonavir (doses of 20, 50, 100, and 200 mg once daily) on the relative oral exposure of elvitegravir. In addition, in an effort to measure the effect and persistence of ritonavir inhibition on hepatic metabolism—the factor most important for maintaining therapeutic trough concentrations—midazolam, a CYP3A-specific probe drug was administered intravenously to allow for parallel measures of elvitegravir terminal elimination and hepatic CYP3A activity.
Results
Subject demographics
Twenty-four healthy subjects were randomized in the study (Figure 1); 12 subjects were assigned to each of the two treatment sequences, and 21 completed the study (11 subjects in sequence 1 and 10 subjects in sequence 2). Three subjects discontinued the study prematurely; one subject each because of withdrawal of consent, positive pregnancy test, and worsening hypertension while receiving the study drug. The majority of subjects were Caucasian (63%), 25% were black, 8% were Asian, and 4% were native Hawaiian or Pacific Islander. There were more males (62.5%) than females (37.5%). The mean (SD) age at baseline was 29 (9) years, mean (SD) weight at screening was 77.2 (11.5) kg, mean (SD) height at screening was 173.2 (10) cm, and the mean (SD) body mass index was 25.7 (2.6) kg/m2.
Safety
All treatments were generally well tolerated by subjects. The most common adverse events were headache, fatigue, nausea, and diarrhea. Most adverse events in this study were considered by the investigator to be unrelated to the study drug, and most resolved while continuing the study. Three subjects discontinued the study prematurely. One subject (elvitegravir/ritonavir 125/50-mg group) discontinued the study drug because of an adverse event (grade 2 drug-related hypertension). One subject suffered a serious adverse event of fetal death following an on-study pregnancy (identified on day 21 of study); the death was considered possibly related to the study drugs, but the investigator noted that there was no clear-cut etiology. Elvitegravir has not demonstrated toxicities in short-term, chronic, genetic, or reproductive studies that would explain this event (data on file; Gilead Sciences, Foster City, California). A third subject discontinued participation in the study because of withdrawal of consent.
Pharmacokinetics
Ritonavir. Mean (SE) ritonavir steady-state plasma concentration–time profiles over 24 h after oral administration of elvitegravir with increasing doses of ritonavir (20, 50, 100, and 200 mg once daily) are shown in Figure 2. A greater than proportional increase in the plasma exposure of ritonavir, with reductions in intersubject variability, was observed with increasing doses, indicating nonlinear pharmacokinetics, possibly due to autoinhibition of CYP3A metabolism (Table 1). Ritonavir's apparent oral clearance (CL/F) was significantly reduced with increasing dose (P < 0.0001); its CL/F decreased by ~70% when the ritonavir dose was increased from 20 to 50 mg (P < 0.0001). At doses of 100 and 200 mg, ritonavir CL/F reached a nadir of <10% of the CL/F of a 20-mg dose (200 mg vs. 100 mg, P = 0.455). Over a 10-fold dosage range, ritonavir systemic exposure as assessed by AUCtau increased 119-fold.
Midazolam. The single-dose mean (SE) plasma concentration–time profile of midazolam after administration alone on day 1 (baseline) and following its coadministration with elvitegravir plus ritonavir 20, 50, 100, or 200 mg once daily are shown in Figure 3. As shown in Table 2, addition of 20-mg ritonavir substantially reduced midazolam CL by ~66% compared to baseline (P < 0.0001). Relatively smaller reductions in CL were observed with ritonavir dose increases of 20–50 (reduction of 43%, P 
0.0001) and 50–100 mg (reduction of 15%, P = 0.173), reaching a maximum reduction in hepatic CYP3A activity of 82 and 83% from baseline at ritonavir doses of 100 and 200 mg, respectively (P < 0.0001 for both doses). Characterized in terms of systemic exposure, ritonavir increased midazolam exposure (AUC extrapolated to infinity, AUCinf) by a maximum of 6.8-fold. As noted above, the majority of the increase in midazolam exposure was achieved by the 20-mg dose, although systemic ritonavir exposures (AUCtau and maximum observed plasma concentration (Cmax)) were nearly 50- and 126-fold lower than corresponding exposures observed at doses of 100 and 200 mg, respectively.
Figure 3.
Mean (SE) single-dose intravenous midazolam concentration–time profiles.
Full figure and legend (14K)With the 50-mg ritonavir dose, concentrations of 1'-OH midazolam decreased to below the limit of quantitation for most subjects at most time points (data not shown); therefore, no pharmacokinetic analyses were performed.
Elvitegravir. Mean (SE) elvitegravir steady-state plasma concentration–time profiles after oral administration of elvitegravir with increasing doses of ritonavir are shown in Figure 4. The mean (percentage of coefficient of variation (%CV)) pharmacokinetic parameters for elvitegravir are presented in Table 3. Plasma exposures of elvitegravir increased with increasing doses of ritonavir; however, similar to midazolam, the increase was less than proportional to increasing ritonavir dose or exposure. Relative to the 20-mg ritonavir dose, elvitegravir CL/Fs were significantly reduced by 38, 52, and 52% for 50, 100, and 200-mg doses of ritonavir, respectively (P 0.0016 for each comparison). Ritonavir doses of 100 and 200 mg resulted in an ~22% (P = 0.068) and 23% (P = 0.023) reduction in elvitegravir CL/F, respectively, vs. 50 mg; the difference between the 100- and 200-mg doses was 1.2% (P = 0.929).
Figure 4.
Mean (SE) steady-state elvitegravir concentration–time profiles.
Full figure and legend (16K)Concentrations of the CYP3A-formed metabolite M1 were reduced to undetectable levels in most subjects at most time points at the lowest (20 mg) ritonavir dose (data not shown). AUCtau of the uridine glucuronosyltransferase–mediated M4 metabolite remained low and composed <10% of the corresponding elvitegravir exposures. The metabolite-to-parent ratio remained unchanged over the ritonavir dose range tested (data not shown), indicating that increasing doses of ritonavir did not alter the formation or elimination of M4.
The nonlinearity in ritonavir pharmacokinetics and the effect of increasing doses of ritonavir on presystemic and systemic CYP3A activity, assessed by changes in midazolam and elvitegravir CLs (through inclusion of historical CL from an earlier study of elvitegravir administered without ritonavir)7 are illustrated in Figure 5. The lowest dose of ritonavir tested resulted in substantial reduction (approximately three- and ninefold, respectively) in midazolam CL and elvitegravir CL/F (with and without weight normalization). Further increases in the ritonavir dose resulted in more modest additional reductions in midazolam CL and elvitegravir CL/F, indicating that maximum ritonavir-mediated inhibition of both presystemic and systemic CYP3A-mediated metabolism is achieved at ritonavir doses of 50–100 mg.
Figure 5.
Ritonavir, midazolam, and elvitegravir clearance values (mean (SE)) by ritonavir dose. The asterisk represents historical data for elvitegravir.
Full figure and legend (24K)Discussion
Ritonavir, a CYP3A substrate and a potent inhibitor of CYP3A activity, is frequently used to boost HIV PIs. Because it exhibits nonlinear pharmacokinetics and mechanism-based CYP3A inactivation and has the potential to induce background CYP3A activity, it is impossible to predict its dose–response relationship on CYP3A function or optimal dose for boosting.8,9 This is the first study to systematically evaluate the dose–response relationship for ritonavir required for pharmacoenhancement. This study also attempted to understand the effect of increasing doses of ritonavir on elvitegravir pharmacokinetics to be able to provide the optimal dose required for boosting and maintaining adequate elvitegravir exposure, including trough concentrations.
Ritonavir exhibited nonlinear pharmacokinetics, with a 119-fold increase in AUCtau over a 10-fold dose range. In this study, the lowest dose of ritonavir tested (20 mg) was associated with the majority of CYP3A inhibition. The inhibitory potency of ritonavir reflects time-dependent inhibition (mechanism-based inactivation) and reversible inhibition. In vitro data in human liver microsomes indicate that ritonavir exhibits a mean KI (the dissociation constant of the inhibitor for the enzyme) of ~0.17–0.38 
mol/l or 0.123–0.274 g/ml for inhibition of CYP3A activity;9,10 however, ritonavir plasma concentrations observed at the 20-mg dose (Caverage ~0.006 g/ml or 0.008 mol/l and Cmax ~0.02 g/ml or 0.027 mol/l) are several magnitudes lower than these reported KI values. In contrast, the local gut concentration of ritonavir at the 20-mg dose is estimated to be ~80 g/ml (approximate gastrointestinal volume = 250 ml), and the portal vein concentrations (assuming absorption is complete) over the range of absorption rate constants of 0.3–1.16/h observed in the 20-mg data set are similar to or as much as threefold higher than the KI for nearly an hour postdosing. As a rapid inactivator of CYP3A, the results of this study indicate that ritonavir presystemic concentrations attained and their duration of exposure are adequate for efficient inactivation of CYP3A activity in vivo.9,10
Although a ritonavir dose of 20 mg produced substantial reductions in midazolam and elvitegravir CL values, further reductions were achieved by the 50-mg dose; these reductions plateaued at the dose of 100 mg. Exploration of ritonavir dose–response for midazolam CL and elvitegravir CL/F fit well to a simple inhibitory dose–response curve with estimated ED50 values (effective dose of ritonavir resulting in 50% decrease CL) of 12.2 mg for midazolam and of 25.9 mg for elvitegravir (through inclusion of historical AUCtau data from an earlier study of elvitegravir administered without ritonavir).7 Caution should be applied, however, when interpreting the mean effective dose values, as the lowest 20-mg dose of ritonavir resulted in large reductions from baseline in CYP3A activity—greater than the interpolated mean effective dose value. Minor differences in the dose–response relationship for CYP3A activity as measured by midazolam and elvitegravir, with two substrates exhibiting comparable Km (Michaelis constant) values (midazolam ~3–4 mol/l (ref. 11) and elvitegravir ~10 mol/l) may be due to intravenous vs. oral administration and/or to the limited sample size in this study.
Although ritonavir doses of 100–200 mg once or twice daily (b.i.d.) are typically used to boost PIs,12,13,14 the results of this study demonstrate that substantially lower ritonavir doses and dramatically lower systemic exposures than those achieved by the lowest commercially available solid dosage form (100 mg) result in substantial reductions in CYP3A activity. Based on these results, clinical studies of elvitegravir have used a ritonavir dose of 100 mg or simply utilized the inhibition afforded by the ritonavir doses required by the coadministered HIV PIs of interest. Notably, elvitegravir coadministration with boosted darunavir or boosted tipranavir at ritonavir doses of 100 and 200 mg twice daily15,16 did not result in altered elvitegravir pharmacokinetics, a finding consistent with the data from this study.
Overall, the treatments in this study were generally well tolerated by subjects. One subject (elvitegravir/ritonavir 125/200-mg group) experienced a serious adverse event (fetal death following an on-study pregnancy) that was considered to be possibly related to the study drugs, but we note that there was no clear-cut etiology. One subject (elvitegravir/ritonavir 125/50-mg group) with a medical history of hypertension discontinued the study due to a grade 2 drug-related hypertension adverse event. Headache and fatigue were the only adverse events experienced by subjects across all four elvitegravir plus ritonavir treatments.
In conclusion, these data indicate that low doses of ritonavir substantially inhibit presystemic first-pass effect and systemic CL, including that of the HIV integrase inhibitor elvitegravir. Doses of ritonavir above the lowest available dosage (100-mg solid) did not result in additional increases in elvitegravir exposure, suggesting that further substantial CYP3A-mediated inhibitory effects by other CYP3A inhibitors would not be expected. For drugs subject to boosting and dependent on maintaining high trough concentrations, these data provide a critical understanding of the relationship between ritonavir dose and inhibition of CYP3A activity in humans.
Methods
Subjects and study design
The protocol, informed consent forms, and advertisements were submitted by the investigator to a duly constituted Institutional Review Board (Aspire Independent Review Board) for review and approval before study initiation (Northwest Kinetics, Tacoma, Washington). Study GS-US-183-0113 was conducted under a US Investigational New Drug Application and in accordance with recognized international scientific and ethical standards, including but not limited to the International Conference on Harmonisation Guideline for Good Clinical Practice and the Declaration of Helsinki. These standards are consistent with the requirements of the US Code of Federal Regulations Title 21, Part 312 (21CFR312), and the European Community Directive 2001/20/EC. Written informed consent was obtained from each individual who participated in this study, after adequate explanation of the aims, methods, objectives, and potential hazards of the study and before undertaking any study-related procedures.
Twenty-four healthy male and female subjects (nonpregnant, nonlactating) aged from 18 to 45 years old (inclusive) were enrolled in an effort to have 16 evaluable subjects. After the baseline visit and upon confirmation that the subjects met all the inclusion criteria and none of the exclusion criteria, subjects were assigned a subject number at the time of first dose. Subjects were randomized to one of two treatment sequences to be studied within a "leapfrog" study design wherein one sequence received ritonavir doses of 20 and 100 mg (treatments A and C) while the other sequence received ritonavir doses of 50 and 200 mg (treatments B and D) (Figure 1). This design shortened study duration and limited the number of exposures to elvitegravir, midazolam, and ritonavir in individual subjects while facilitating assessments of within-subject and model-based dose–response pharmacodynamic relationships between ritonavir and CYP3A function. Each eligible subject was administered two treatments (i.e., A and C or B and D) over the course of a 21-day treatment phase, followed by a 7-day follow-up phase.
Elvitegravir (125 mg) plus ritonavir was administered once daily for 10 days within each treatment group to achieve steady-state pharmacokinetic conditions. As the lowest commercially available solid dosage form of ritonavir is 100 mg, all ritonavir doses were administered in Ensure (25 ml) as the oral solution to remove potential confounding effects of formulation dissolution or solubility on ritonavir's pharmacokinetics and pharmacodynamics.17 The dosing cup was rinsed with water and the contents swallowed; immediately following, subjects received their elvitegravir dose with 240 ml of water.
A slow intravenous push of midazolam (1 mg over 1 min) was administered to each subject on day 1 to measure baseline CYP3A activity and also 6 h post-elvitegravir/ritonavir dosing on days 11 and 21 to ensure measurable midazolam concentrations throughout the elvitegravir terminal elimination phase. Although midazolam and ritonavir are contraindicated at their therapeutic doses, low doses of midazolam such as used in this study are frequently used in pharmacokinetic studies and are associated with minimal clinical effects in healthy subjects.18,19,20,21 To ensure safety, subjects were confined to an in-patient setting throughout the study, with intensive monitoring performed for at least 2 h after each midazolam dose.
All adverse events and concomitant medications from the screening visit and throughout the study were recorded and reported. Blood samples were drawn, and urine was collected for routine clinical lab testing (hematology, chemistry, and urinalysis panels) and serum pregnancy testing at screening, baseline (day 0), and on days 10 and 20. An electrocardiogram assessment was performed at screening. A full physical exam with assessment of vital signs was performed at screening; a brief physical exam, targeted at new signs and symptoms, and assessment of vital signs was performed at baseline (day 0), on days 10 and 20, and on days 1, 11, and 21.
Bioanalysis
Plasma concentrations of elvitegravir and its primary metabolites (M1 and M4), ritonavir, and midazolam and its primary metabolite 1'-OH-midazolam were determined using validated high-performance liquid chromatography/tandem mass spectrometry bioanalytical assays at Gilead Sciences (for elvitegravir, its metabolites, and ritonavir) or Quest Pharmaceuticals Services (Newark, DE, for midazolam and its 1'-OH-metabolite). Elvitegravir, M1, M4, and ritonavir assay details have been previously published.22,23,24 Briefly, the methods were as follows: elvitegravir, its stable label isotope (internal standard; IS), and ritonavir (GS-9201 as the IS) were loaded on an Empore C8-SD solid-phase extraction column, washed with 0.1% formic acid in water, eluted with 10/90/0.1 water/acetonitrile/formic acid, diluted in 0.1% formic acid in water, and injected on a YMC ODS-AM column and separated using water/acetonitrile/formic acid as the mobile phase. The compounds were detected by tandem mass spectrometry using electrospray ionization in the positive mode and following ion transitions m/z 448 
344 for elvitegravir, m/z 456 344 for the elvitegravir IS, m/z 624 344 for M4, m/z 464 360 for M1, m/z 721 268 for ritonavir, and m/z 632 438 for GS-9201.
Quantitation was based on the peak area ratios of analyte/IS. Due to the wide range of expected concentrations, two sets of overlapping calibration curves and respective quality-control sample pools were developed for elvitegravir and ritonavir. The assay calibration curve was linear from 0.5 to 50 ng/ml (low) and from 20 to 10,000 ng/ml (high) for elvitegravir, from 20 to 1,000 ng/ml for M4 and M1, and from 0.5 to 50 ng/ml (low) and from 5 to 5,000 ng/ml (high) for ritonavir. The interassay accuracy (expressed as % bias) and precision ranged from -13 to 6.7% and 2.1 to 20% for elvitegravir, 4.7 to 14.7% and -4.5 to 1.5% for M4, 3.5 to 10.9% and -5.1 to -3.3% for M1, and -2.8 to 9.4% and 0.0 to 11.6% for ritonavir. The assay calibration curve for midazolam (data on file) was linear from 0.1 to 100 ng/ml, with interassay accuracy (expressed as % bias) and precision that ranged from -2.7 to -1.0% and 2.4 to 7.1%.
Pharmacokinetic analyses
Pharmacokinetic parameters were estimated with noncompartmental methods using the linear up/log down method for AUC estimation (WinNonlin software, Professional Edition, version 5.0.1; Pharsight, Mountain View, CA). Predose sample times of less than time zero were assigned values of zero. Samples below the limit of quantitation of bioanalytical assays that occurred before the first quantifiable concentration was achieved were assigned a concentration of zero to prevent overestimation of initial AUC. Values below the limit of quantitation at all other time points were treated as missing data.
Parameters estimated included AUCtau, AUC up to the last quantifiable concentration (AUClast), and AUC extrapolated to infinity (AUCinf); elimination rate constant (
z) and half-life (T1/2); and concentration at the end of dosing interval (Ctau) and CL/F for elvitegravir, its metabolite, and ritonavir. In addition, Cmax and time to reach maximum concentration (Tmax) were also identified. For midazolam, single-dose pharmacokinetic parameters, including AUClast, AUCinf, Clast, Tlast, 
z, T½, and CL, were assessed.
The pharmacokinetic/pharmacodynamic relationship between ritonavir doses and exposure and hepatic (midazolam) and apparent oral (elvitegravir) CLs as a measure of reduced hepatic CYP3A activity were explored using a simple inhibitory dose–response curve (WinNonlin software, Professional Edition, version 5.0.1; Pharsight).
Statistical analyses
Data from a previous pilot study that assessed the impact of 100-mg ritonavir on elvitegravir pharmacokinetics were used to estimate the within-subject SD used in the sample size and power calculations for this study. Sample size was based on the most variable exposure parameters of interest (Cmax, AUC, and Ctau) for elvitegravir and included an overage to account for potential dropouts. The final sample size provided at least 90% power to conclude equivalent elvitegravir exposures, assuming an expected geometric least square means ratio of treatments of 100% and an equivalence interval of 70–143%.
Pharmacokinetic parameters were summarized by treatment (ritonavir dose) using descriptive statistics. The primary statistical evaluation of dose proportionality over the range of ritonavir doses (20, 50, 100, and 200 mg once daily) for AUCtau, Cmax, and Ctau (elvitegravir, M4, and ritonavir) and AUCinf and AUClast (midazolam) was based on a power model and an analysis of variance model that was fitted using the above-mentioned pharmacokinetic parameters. The population mean slope, along with a corresponding 90% confidence interval, was estimated. Parametric mixed-effects modeling, accounting for paired and unpaired data comparisons, with corresponding 95% confidence intervals of natural-log transformed clearance (CL/F and CL) data were also performed to examine the effects of various ritonavir dose levels on midazolam and elvitegravir CLs.
Conflict Of Interest
The authors are employees of Gilead Sciences, Inc. Gilead Sciences employees potentially own stock and/or hold stock options in the company.
References
- Shulman, N. et al. Virtual inhibitory quotient predicts response to ritonavir boosting of indinavir-based therapy in human immunodeficiency virus-infected patients with ongoing viremia. Antimicrob. Agents Chemother. 46, 3907|[ndash]|3916 (2002). | Article | PubMed | ChemPort |
- Katlama, C. et al. Efficacy of TMC114/r in 3-class experienced patients with limited treatment options: 24-week planned interim analysis of 2 96-week multinational dose-finding trials [abstract 164LB]. In 12th Conference on Retroviruses and Opportunistic Infections, 22|[ndash]|25 February 2005 (Boston, MA, 2005).
- Malan, N. et al. Efficacy and safety of atazanavir-based therapy in antiretroviral naive HIV-1 infected subjects, both with and without ritonavir: 48-week results from AI424-089 [abstract 107LB]. 13th Conference on Retroviruses and Opportunistic Infections, 5|[ndash]|9 February 2006 (Denver, CO, 2006).
- Kearney, B.P. et al. Pharmacokinetics/pharmacodynamics of GS-9137 an HIV integrase inhibitor [oral presentation]. 7th International Workshop on Clinical Pharmacology of HIV Therapy, 20|[ndash]|22 April 2006 (Lisbon, Portugal, 2006).
- DeJesus, E. et al. Antiviral activity, pharmacokinetics, and dose response of the HIV-1 integrase inhibitor GS-9137 (JTK-303) in treatment-naive and treatment-experienced patients. J. Acquir. Immune Defic. Syndr. 43, 1|[ndash]|5 (2006). | Article | PubMed | ChemPort |
- Zolopa, A.R. et al. The HIV integrase inhibitor GS-9137 has potent antiretroviral activity in treatment-experienced patients [oral presentation 143LB]. 14th Conference on Retroviruses and Opportunistic Infections, 25|[ndash]|28 February 2007 (Los Angeles, CA, 2007).
- Kearney, B.P. et al. Pharmacokinetics/pharmacodynamics of GS-9137 an HIV integrase inhibitor in treatment-na|[iuml]|ve and experienced patients [abstract 73]. 7th International Workshop on Clinical Pharmacology of HIV Therapy, 20|[ndash]|22 April 2006 (Lisbon, Portugal, 2006).
- Zhou, S. et al. Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs. Clin. Pharmacokinet. 44, 279|[ndash]|304 (2005). | Article | PubMed | ISI | ChemPort |
- Obach, R.S., Walsky, R.L. & Venkatakrishnan, K. Mechanism-based inactivation of human cytochrome p450 enzymes and the prediction of drug-drug interactions. Drug Metab. Dispos. 35, 246|[ndash]|255 (2007). | Article |
- Ernest, C.S. II, Hall, S.D. & Jones, D.R. Mechanism-based inactivation of CYP3A by HIV protease inhibitors. J. Pharmacol. Exp. Ther. 312, 583|[ndash]|591 (2005). | Article | PubMed | ChemPort |
- Thummel, K.E. et al. Oral first-pass elimination of midazolam involves both gastrointestinal and hepatic CYP3A-mediated metabolism. Clin. Pharmacol. Ther. 59, 491|[ndash]|502 (1996). | Article | PubMed | ChemPort |
- Prezista (darunavir) tablets [US prescribing information]. Raritan, NJ: Tibotec, 2006.
- Aptivus (tipranavir) capsules, 250 mg [US prescribing information]. Ridgefield, CT: Boehringer Ingelheim Pharmaceuticals, 2006.
- KALETRA (lopinavir/ritonavir) tablets (lopinavir/ritonavir) oral solution [US prescribing information]. North Chicago, IL: Abbott Laboratories, 2007.
- Mathias, A., Hinkle, J., Enejosa, J., Piliero, P.J. & Kearney, B.P. Lack of pharmacokinetic interaction between ritonavir-boosted GS-9137 (elvitegravir) and tipranavir/r [poster no. TUPDB06]. 4th International AIDS Society Conference and HIV Pathogenesis, Treatment, and Prevention, 22|[ndash]|25 July 2007 (Sydney, Australia, 2007).
- Mathias, A.A. et al. Lack of pharmacokinetic interaction between ritonavir-boosted GS-9137 (elvitegravir) and darunavir/r [poster no. TUPDB03]. 4th International AIDS Society Conference on HIV Pathogenesis, Treatment, and Prevention, 22|[ndash]|25 July 2007 (Sydney, Australia, 2007).
- NORVIR (ritonavir capsules) soft gelatin; (ritonavir oral solution) [US prescribing information]. North Chicago, IL: Abbott Laboratories, 2006.
- Grasela, D.M., LaCreta, F.P., Kollia, G.D., Randall, D.M. & Uderman, H.D. Open-label, nonrandomized study of the effects of gatifloxacin on the pharmacokinetics of midazolam in healthy male volunteers. Pharmacotherapy 20, 330|[ndash]|335 (2000). | Article | PubMed | ChemPort |
- Phimmasone, S. & Kharasch, E.D. A pilot evaluation of alfentanil-induced miosis as a noninvasive probe for hepatic cytochrome P450 3A4 (CYP3A4) activity in humans. Clin. Pharmacol. Ther. 70, 505|[ndash]|517 (2001). | Article | PubMed | ChemPort |
- U.S. Department of Health and Human Services Food and Drug Administration Center for Drug Evaluation and Research (CDER) and Center for Biologics Evaluation and Research (CBER). In Vivo Drug Metabolism/Drug Interaction Studies|[mdash]|Study Design, Data Analysis, and Recommendations for Dosing and Labeling. Rockville, MD (1999).
- Tsunoda, S.M., Velez, R.L., von Moltke, L.L. & Greenblatt, D.J. Differentiation of intestinal and hepatic cytochrome P450 3A activity with use of midazolam as an in vivo probe: effect of ketoconazole. Clin. Pharmacol. Ther. 66, 461|[ndash]|471 (1999). | Article | PubMed | ChemPort |
- Ramanathan, S., Shen, G., Cheng, A. & Kearney, B.P. Pharmacokinetics of emtricitabine, tenofovir, and GS-9137 following coadministration of emtricitabine/tenofovir disoproxil fumarate and ritonavir-boosted GS-9137. J. Acquir. Immune Defic. Syndr. 45, 274|[ndash]|279 (2007) | PubMed | ChemPort |
- Brosnan-Cook, M., Cronin, T., Wintermute, J. & Kagel, J. Importance of complete automated control of SPE conditions in validated LC/MS/MS assay of GS-9137, metabolites and ritonavir in human plasma (Poster ThP198). ASMS Conference on Mass Spectrometry Indianapolis, IN, 3|[ndash]|7 June 2007.
- Wintermute, J., Brosnan-Cook, M. & Kagel, J. Overcoming autosampler carryover in a validated LC/MS/MS assay of polar, fluorinated GS-9137, its metabolites and ritonavir in human plasma (Poster WP333). ASMS Conference on Mass Spectrometry, Indianapolis, IN, 3|[ndash]|7 June 2007.
Acknowledgments
Financial support for the clinical studies discussed in this manuscript was provided by Gilead Sciences, Inc.
