Drug interactions in the management of HIV infection: an update

A Central Auditory Test reveals differences between drug treatment regimens in adults living with HIV

OBJECTIVE

This exploratory study examined whether central auditory tests show differences between people living with HIV (PLWH) treated with two predominant antiretroviral drug therapy (ART) regimens.

DESIGN

Cross-sectional.

STUDY SAMPLE

253 PLWH (mean age 39.8 years) from the Shanghai Public Health Clinical Centre, China.

METHODS

The Hearing in Noise Test speech reception threshold (SRT) assessed central auditory function and the Montreal Cognitive Assessment (MoCA) assessed cognition. The relationship between ART regimen and SRT was evaluated with multivariable linear regression incorporating age, HIV duration, and peripheral hearing ability. Multivariable logistic regression was used to ascertain if SRT and ART regimen predicted MoCA impairment.

RESULTS

The two predominant ART regimens differed by one drug (zidovudine or tenofovir). Participants taking the zidovudine-containing regimen had poorer SRT performance (p=.012) independent of age and hearing thresholds. MoCA scores did not differ between drug regimens, but a negative relationship was found between SRT and MoCA impairment (p=.048).

CONCLUSIONS

ART regimens differed in their association with central auditory test performance likely reflecting neurocognitive changes in PLWH taking the zidovudine-containing regimen. Central auditory test performance also marginally predicted cognitive impairment, supporting further assessment of central auditory tests to detect neurocognitive deficits in PLWH.

Managing antiretroviral therapy in the elderly HIV patient

Introduction: Owing to more effective and less toxic antiretroviral therapy (ART), people living with HIV (PLWH) live longer, a phenomenon expected to grow in the next decades. With advancing age, effectively treated PLWH experience not only a heightened risk for non-infective comorbidities and multimorbidity, but also for geriatric syndromes and frailty. In addition, older adults living with HIV (OALWH) have a higher prevalence of so-called iatrogenic triad described as polypharmacy (PP), potentially inappropriate medication use, and drug-drug interactions. Areas covered: This review will focus the management of ART in OALWH. We will discuss iatrogenic triad and best way to address PP. Special focus will be given to pharmacokinetic and pharmacodynamic aspects of ART in the elderly, evaluation of ART toxicities, and specific ART strategies commonly used in this population. Expert commentary: Research should be focused on recruiting more OALWH, frail individuals in particular, into the clinical trials and specific geriatric outcome need to be considered together with traditional viroimmunological outcomes.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

Plasma concentrations of efavirenz and nevirapine among HIV-infected patients with immunological failure attending a tertiary hospital in North-western Tanzania

BACKGROUND

Sub-therapeutic and supra-therapeutic plasma concentrations of antriretrovirals are the significant causes of treatment failure and toxicity respectively among HIV-infected patients. We conducted this study to determine the pattern of efavirenz and nevirapine plasma drug concentrations among adult HIV-infected patients with immunological failure attending at a tertiary hospital in North-western Tanzania.

MATERIALS AND METHODS

A cross-sectional study was conducted among adult HIV-infected patients with immunological failure who have been on either efavirenz or nevirapine based antiretroviral regimen for more than 6 months. Patients were serially enrolled through routine Care and Treatment Clinic (CTC) activities. Plasma drug concentrations for efavirenz and nevirapine were determined by high performance liquid chromatography (HPLC) and Gas Chromatography (GC) respectively. Demographic, clinical and laboratory data such as viral load and CD4 counts were collected. Data analysis was done using STATA 12.

RESULTS

Of the 152 patients with immunological failure enrolled, the sub-therapeutic, therapeutic and supra-therapeutic plasma antiretroviral drug concentrations were found in 43/152 (28.3%), 76/152 (50.0%) and 33/152 (21.7%) respectively. Half of the patients were outside therapeutic window with either sub-therapeutic or supra-therapeutic plasma ARV drug concentrations. There was a significant difference in distribution of ARV adherence (p-value<0.001), NRTI backbone (p-value = 0.039), HIV stage (p-value = 0.026) and viral load (p-value = 0.007) within sub-therapeutic, therapeutic and supra-therapeutic ARV plasma drug concentrations.

CONCLUSION

There is a wide inter-individual variability of plasma ARV concentrations among HIV patients with immunological failure, with a large proportion of patients being outside therapeutic window. This variability is significant based on ARV adherence, NRTI backbone, viral load and HIV stage. Routine therapeutic drug monitoring (TDM) could assist identifying these patients early and making timely correction to avoid virological failure, poor immunological outcome and prevent associated drug toxicities. Nonetheless, ARV adherence should be strictly emphasized on HIV patients with immunological failure.

Efavirenz and rifampicin in the South African context: is there a need to dose-increase efavirenz with concurrent rifampicin therapy?

BACKGROUND

Increasing efavirenz (EFV) dose from 600 mg to 800 mg daily has been suggested with concomitant rifampicin (RFN), as induction of cytochrome P450 isoenzymes may reduce EFV plasma concentrations.

METHODS

Individuals from the CIPRA-South Africa cohort taking EFV-based antiretroviral therapy with concomitant tuberculosis (TB) were dosed with either increased (800 mg) or standard (600 mg) dose EFV during TB treatment. After TB therapy, all individuals took 600 mg EFV. Two mid-dosing interval EFV concentrations were determined from each individual: after 4 weeks of concomitant EFV and RFN therapy, and ≥4 weeks after TB therapy completion. Mid-dosing interval EFV concentrations were compared within individuals using the Wilcoxon signed-rank test.

RESULTS

Paired samples were collected from 72 individuals. Overall, 45 (63%) were women and median weight was 59 kg (IQR 52-67). At antiretroviral therapy start, median CD4(+) T-cell count was 114 cells/mm(3) (IQR 37-165), median viral load was 5.5 log (IQR 5.1-5.9). A total of 38 (53%) individuals took 800 mg EFV during TB treatment and 34 (47%) took 600 mg. EFV concentrations in the 800 mg group were higher with RFN (2.9 mg/l [IQR 1.8-5.6]) than without (2.1 mg/l [IQR 1.4-3.0]; P=0.0003). There was no significant difference in EFV concentrations with RFN (2.4 mg/l [IQR 1.2-5.1]) or without (2.2 mg/l [IQR 1.4-3.7]) in the 600 mg group. There was no increase in EFV-linked adverse effects in either group. The proportion of virologically suppressed individuals at 48 weeks was similar in both groups.

CONCLUSIONS

EFV concentrations were significantly increased in the EFV 800 mg group on RFN. There was no significant decrease in EFV concentrations when on RFN in the 600 mg group. Dose escalation of EFV 600 mg to 800 mg is not required during concomitant TB therapy in South Africa.

Prevalence of potential drug-drug interactions involving antiretroviral drugs in a large Kenyan cohort

Background

Clinically significant drug-drug interactions (CSDIs) involving antiretrovirals are frequent and under-recognized in developed countries, but data are lacking for developing countries.

Methodology and Principal Findings

To investigate the prevalence of CSDIs between antiretrovirals and coadministered drugs, we surveyed prescriptions dispensed in a large HIV clinic in Kenya. Of 1040 consecutive patients screened, 996 were eligible for inclusion. CSDIs were defined as ‘major’ (capable of causing severe or permanent damage, contraindicated, avoid or not recommended by the manufacturer, or requiring dose modification) or ‘moderate’ (manufacturers advise caution, or close monitoring, or capable of causing clinical deterioration). A total of 334 patients (33.5%) were at risk for a CSDI, potentially lowering antiretroviral drug concentrations in 120 (12%) patients. Major interactions most frequently involved rifampicin (12.4%, mostly with efavirenz) and azoles (2.7%) whereas moderate interactions were frequently azoles (13%), steroids (11%), and antimalarials (3%). Multivariable analyses suggested that patients at risk for CSDIs had lower CD4 counts (P = 0.006) and baseline weight (P = 0.023) and WHO Stage 3 or 4 disease (P≤0.007). Risk for CSDIs was not associated with particular regimens, although only 116 (11.6%) patients were receiving WHO second line regimens.

Conclusions

One in three patients receiving antiretrovirals in our programme were at risk of CSDIs. Strategies need to be urgently developed to avoid important drug interactions, to identify early markers of toxicity and to manage unavoidable interactions safely in order to reduce risk of harm, and to maximize the effectiveness of mass antiretroviral deployment in Africa.

Virologic and immunologic effectiveness of tipranavir/ritonavir (TPV/r)- versus darunavir/ritonavir (DRV/r)-based regimens in clinical practice

BACKGROUND

Although both tipranavir and darunavir are important options for the management of patients with multidrug resistant HIV, there are at present no studies comparing the effectiveness and safety of these 2 antiretroviral drugs in this population of patients.

OBJECTIVE

To compare the effectiveness and safety of ritonavir (TPV/r)- and darunavir/ritonavir (DRV/ r)-based therapies in treatment-experienced patients (n = 38 and 47, respectively).

METHODS

Multicenter, retrospective cohort study.

RESULTS

The median baseline viral load and CD4 count were 4.7 copies/mL (interquartile range [IQR] 4.3, 5.2) and 168 cells/mm( 3) (IQR 80, 252) for TPV/r patients and 4.7 copies/mL (IQR 3.7, 5.1) and 171 cells/mm(3) (IQR 92, 290) for DRV/r patients. The median number of years on antiretroviral therapy (ART) prior to starting DRV/r or TPV/r were 12.7 (10.2-15.5) and 10.5 (8.4-12.6), respectively (P < .01). Current raltegravir (RAL) use (odds ratio [OR] 5.53, 95% CI 1.08-28.34) was significantly associated with virologic suppression at week 24 in multivariable logistic regression models, whereas the use of TPV/r was not significantly associated with virologic suppression compared to DRV/r (OR 0.93, 95% CI 0.27-3.18, P = .91).

CONCLUSION

No significant difference was observed between DRV/r and TPV/r in terms of virologic suppression.

Drug interactions of new antiretroviral drugs

A systematic review was made of the drug interactions of new antiretroviral drugs. In order to do this a search was made in Pubmed to find articles published from January 2007 to September 2009 and the full-text articles which contained information about new antiretroviral drugs were selected. This search was then complemented with information from the technical specifications of the drugs and consultations made on webpages specialized in antiretroviral interactions: www.interaccioneshiv.com and www.hiv-druginteractions.org. The information about the possible interactions of new antiretroviral drugs with one another and with the therapeutic groups which are most widely used in patients infected with the human immunodeficiency virus was analyzed.

Improving the self-report of HIV antiretroviral medication adherence: is the glass half full or half empty?

Self-reports are the most widely used method for measuring antiretroviral adherence. The association between self-reports and viral loads has been repeatedly demonstrated, but this association does not address how well self-reports measure actual medication-taking behaviors. Understanding adherence self-reports requires studying the science of memory and the reporting of behaviors. In the first section of this review, we discuss research in cognitive psychology that pertains to adherence self-reports, focusing primarily on studies that examine cognitive processes respondents use to answer survey questions. In the second section, we review recent articles examining the relationship between self-reports and objective measures of adherence, highlighting the strength of associations and key methodologic issues. We conclude with key questions for future research and methodologic recommendations.

Clinical management of drug interaction with antiretroviral agents

PURPOSE OF REVIEW

Combination antiretroviral therapy has improved the morbidity and mortality of HIV-infected patients worldwide. As patients live longer, management of HIV infection extends to treatment of a wide spectrum of co-morbid conditions. Pharmacokinetic interactions are common among antiretroviral drugs when they are used in combination and along with treatments for other conditions. This review discusses the clinical significance of drug interactions among antiretroviral drugs and other medications, resources to use in assessing drug interaction potential, and some key principles to follow when managing patients prescribed potentially interacting drugs.

RECENT FINDINGS

Targeted pharmacokinetic drug interaction studies and extrapolations on the basis of potential mechanism of interactions provide an initial basis for recommendations regarding use of certain drug combinations. Some unexpected interactions have emerged in the literature through case reports in which untoward effects were observed.

SUMMARY

Management of patients on multiple drug therapy can be a challenge. The key to safe and effective therapy relies on the clinician's vigilance in their ongoing assessment of interaction potential among drugs prescribed to each patient, the significance for such interactions, the need for modification to therapy, and close follow up to assess safety and toxicity.

Drug-Drug Interaction Between Itraconazole and the Protease Inhibitor Lopinavir/Ritonavir

Objective:

To report the results of therapeutic drug monitoring of lopinavir/ritonavir and itraconazole concentrations in an HIV-infected male who was treated for histoplasmosis.

Case Summary:

A 34-year-old HIV-infected man who had recently initiated efavirenz-based antiretroviral therapy was diagnosed with disseminated Histoplasma capsulatum infection. In the hospital, lopinavir/ritonavir 400 mg/100 mg twice daily replaced efavirenz to avoid etavirenz-itraconazole interactions. After 14 days of liposomal amphotericin B therapy, itraconazole solution was initiated at 150 mg twice daily for 3 days, followed by 200 mg daily. Prior to itraconazole initiation, lopinavir trough concentration was 7.4 mg/L. The lopinavir trough concentration 15 days later, after 14 days of itraconazole, was 6.8 mg/L. An itraconazole concentration measured 2 hours post-dose on day 15 of oral therapy was 1.9 pg/mL. After 2 weeks of liposomal amphotericin, urine Histoplasma antigen was 27.23 ng/mL; after 5 months of oral itraconazole therapy, it decreased to 5.24 ng/mL. Plasma HIV RNA decreased 4.26 log10 in 5 months to less than 40 copies/mL The patient has demonstrated marked clinical improvement.

Discussion:

In this case, dosing recommendations of itraconazole 200 mg daily with lopinavir/ritonavir were appropriate. Lopinavir trough concentrations were not significantly different following the addition of itraconazole and were above the minimum target of 1 mg/L in treatment-naive patients. The itraconazole concentration was above the recommended concentration of at least 1 μg/mL.

Conclusions:

The dose of itraconazole was reduced to 200 mg daily as recommended by current guidelines, and therapeutic drug monitoring of both itraconazole and lopinavir concentrations confirmed that no further dosage adjustments were necessary.

The same but different: autologous hematopoietic stem cell transplantation for patients with lymphoma and HIV infection

In an earlier era, high-dose therapies were thought to be contraindicated in HIV-infected patients. Patients with HIV fared somewhat better with reduced-dose lymphoma therapies and salvage of relapsed patients was rarely possible. With more than a decade of effective antiretroviral therapy, full-dose lymphoma therapies have become standard, and high-dose therapy with autologous hematopoietic stem cell rescue for those who fail frontline therapy or who are judged to have very high risk disease has been pursued with very encouraging results. Transplant-related mortality is less than 5%. With prophylaxis for pneumocystis and herpesvirus infections, deaths due to opportunistic infections are distinctly unusual. Most deaths have been associated with veno-occlusive disease or lymphoma progression. There is no need for quarantine of patients or special isolation procedures. Most patients with responsive lymphoma remain lymphoma free several years after high-dose therapy. CD4(+) cell count and HIV load seem not to be adversely affected in the long term. Much like diabetes, HIV infection should be regarded as a problem that requires special attention during high-dose therapy rather than a contraindication to high-dose therapy in patients with lymphoma who would otherwise be judged transplant candidates.

Pharmacologic consideration for the use of antiretroviral agents in the elderly

The prevalence of human immunodeficiency virus (HIV) infection among people older than 50 years is increasing. HIV-infected patients require lifelong treatment with antiretroviral agents to suppress viral replication and maintain immune function. The use of antiretroviral agents in the elderly can be complicated by multiple chronic comorbidities and coadministered non-HIV medications. The pharmacokinetics of antiretroviral agents may be altered due to age-related decrements in hepatic and renal function. The elderly may be more sensitive than younger people to antiretroviral drug toxicity. A better understanding of the pharmacokinetics of antiretroviral agents in the elderly is of importance for the successful management of complex antiretroviral regimens in this population.

Methadone metabolism and clearance are induced by nelfinavir despite inhibition of cytochrome P4503A (CYP3A) activity

BACKGROUND

Methadone plasma concentrations are decreased by nelfinavir. Methadone clearance and the drug interactions have been attributed to CYP3A4, but actual mechanisms of methadone clearance and the nelfinavir interaction are unknown. We assessed nelfinavir effects on methadone pharmacokinetics and pharmacodynamics, intestinal and hepatic CYP3A4/5 activity, and intestinal P-glycoprotein transport activity. CYP3A4/5 and transporters were assessed using alfentanil and fexofenadine, respectively.

METHODS

Twelve healthy HIV-negative volunteers underwent a sequential crossover. On three consecutive days they received oral alfentanil plus fexofenadine, intravenous alfentanil, and intravenous plus oral methadone. This was repeated after nelfinavir. Plasma and urine analytes were measured by mass spectrometry. Opioid effects were measured by pupil diameter change (miosis).

RESULTS

Nelfinavir decreased intravenous and oral methadone plasma concentrations 40-50%. Systemic clearance, hepatic clearance, and hepatic extraction all increased 1.6- and 2-fold, respectively, for R- and S-methadone; apparent oral clearance increased 1.7- and 1.9-fold. Nelfinavir stereoselectively increased (S>R) methadone metabolism and metabolite formation clearance, and methadone renal clearance. Methadone bioavailability and P-glycoprotein activity were minimally affected. Nelfinavir decreased alfentanil systemic and apparent oral clearances 50 and 76%, respectively. Nelfinavir appeared to shift the methadone plasma concentration-effect (miosis) curve leftward and upward.

CONCLUSIONS

Nelfinavir induced methadone clearance by increasing renal clearance, and more so by stereoselectively increasing hepatic metabolism, extraction and clearance. Induction occurred despite 50% inhibition of hepatic CYP3A4/5 activity and more than 75% inhibition of first-pass CYP3A4/5 activity, suggesting little or no role for CYP3A in clinical methadone disposition. Nelfinavir may alter methadone pharmacodynamics, increasing clinical effects.

Methadone pharmacokinetics are independent of cytochrome P4503A (CYP3A) activity and gastrointestinal drug transport: insights from methadone interactions with ritonavir/indinavir

BACKGROUND

Methadone clearance is highly variable, and drug interactions are problematic. Both have been attributed to CYP3A, but actual mechanisms are unknown. Drug interactions can provide such mechanistic information. Ritonavir/indinavir, one of the earliest protease inhibitor combinations, may inhibit CYP3A. We assessed ritonavir/indinavir effects on methadone pharmacokinetics and pharmacodynamics, intestinal and hepatic CYP3A activity, and intestinal transporters (P-glycoprotein) activity. CYP3A and transporters were assessed with alfentanil and fexofenadine, respectively.

METHODS

Twelve healthy human immunodeficiency virus-negative volunteers underwent a sequential three-part crossover. On three consecutive days, they received oral alfentanil/fexofenadine, intravenous alfentanil, and intravenous plus oral (deuterium-labeled) methadone, repeated after acute (3 days) and steady-state (2 weeks) ritonavir/indinavir. Plasma and urine analytes were measured by mass spectrometry. Opioid effects were assessed by miosis.

RESULTS

Alfentanil apparent oral clearance was inhibited more than 97% by both acute and steady-state ritonavir/indinavir, and systemic clearance was inhibited more than 90% due to diminished hepatic and intestinal extraction. Ritonavir/indinavir increased fexofenadine area under the plasma concentration-time curve four- to five-fold, suggesting significant inhibition of gastrointestinal P-glycoprotein. Ritonavir/indinavir slightly increased methadone N-demethylation, but it had no significant effects on methadone plasma concentrations or on systemic or apparent oral clearance, renal clearance, hepatic extraction or clearance, or bioavailability. Ritonavir/indinavir had no significant effects on methadone plasma concentration-effect relationships.

CONCLUSIONS

Inhibition of both hepatic and intestinal CYP3A activity is responsible for ritonavir/indinavir drug interactions. Methadone disposition was unchanged, despite profound inhibition of CYP3A activity, suggesting little or no role for CYP3A in clinical methadone metabolism and clearance. Methadone bioavailability was unchanged, despite inhibition of gastrointestinal P-glycoprotein activity, suggesting that this transporter does not limit methadone intestinal absorption.

[Utility of atazanavir in special populations]

ART (antiretroviral therapy) currently continues to indefinitely prolong the survival of patients who live with HIV. Due to this, we are increasingly faced with specific problems that previously did not have time to develop or did not have the importance that they have now, and which are related to the existence of special patient populations. Examples of situations that characterise these special populations are: co-infection with hepatitis virus B and/or C, pregnancy, lipodystrophy, cardiovascular risk, renal failure, treatment of children and adolescents, immigrant health care, and the management of patients receiving methadone replacement therapy, among others. In this article we review the role that Atazanavir (ATV) can play in the ART of patients in the situations mentioned, except those that are already dealt with specifically in other articles, such as co-infection by hepatitis virus B and/or C.

Practical therapeutic drug management in HIV-infected patients: use of population pharmacokinetic models supplemented by individualized Bayesian dose optimization

Individualized, model-based, target-oriented optimal concentration-controlled dosing of HIV medications can be beneficial to patients for whom there are limited dosing guidelines, such as children, adolescents, or patients with altered physiologic function. Barriers to this approach include lack of training, expertise, and access to appropriate software to assist the clinician. The authors present 4 illustrative clinical cases of HIV-infected patients whose therapy was optimized using population pharmacokinetic models (here generated from published studies) and supplemented by individualized Bayesian adaptive control of dosage regimens as implemented in the MM-USCPACK software. These 4 cases illustrate how clinicians can maximize therapeutic success in (1) patients with reduced drug clearance, (2) young adolescents transitioning to adult physiology, (3) patients with dose-dependent toxicity, and (4) adolescents with limited therapeutic options.

Rapid clinical induction of hepatic cytochrome P4502B6 activity by ritonavir

Ritonavir is the most potent and efficacious inhibitor of cytochrome P4503A (CYP3A), and it is used accordingly for the pharmacoenhancement of other antiretrovirals. Paradoxically, ritonavir induces the clinical metabolism and clearance of many drugs. The mechanism by which ritonavir inhibits and induces clinical drug metabolism is unknown. Ritonavir induces CYP2B6 in human hepatocytes. This investigation tested the hypothesis that ritonavir induces human CYP2B6 in vivo. Thirteen healthy human immunodeficiency virus-negative volunteers underwent a three-way sequential crossover protocol, receiving racemic bupropion after nothing (control), 3 days of treatment with ritonavir, and 2.5 weeks of treatment with ritonavir (400 mg twice a day). Stereoselective bupropion hydroxylation was used as an in vivo probe for CYP2B6 activity. Plasma and urine (R)- and (S)-bupropion and (R,R)- and (S,S)-hydroxybupropion concentrations were measured by liquid chromatography-mass spectrometry. Racemic, (R)-, and (S)-bupropion plasma ratios of the area under the concentration-time curve from 0 h to infinity (AUC(0-infinity)) (ritonavir/control) were significantly reduced to 0.84, 0.86, and 0.80, respectively, after 3 days of ritonavir treatment and to 0.67, 0.69, and 0.60 after steady-state ritonavir treatment. Apparent oral clearances for racemic, (R)-, and (S)-bupropion all were significantly increased by 1.2-fold after 3 days of ritonavir treatment and by 1.4-, 1.7-, and 1.5-fold after steady-state ritonavir treatment. The plasma (S,S)-hydroxybupropion/(S)-bupropion AUC(0-72) ratio was significantly increased by ritonavir. Formation clearances of both (R,R)- and (S,S)-hydroxybupropion were increased 1.8-fold after 3 days of ritonavir treatment and 2.1-fold after steady-state ritonavir treatment. These results show that ritonavir induces human CYP2B6 activity. Induction is rapid, occurring after only 3 days of ritonavir, and is sustained for at least 2 weeks. The ritonavir induction of CYP2B6 activity may have significant implications for drug interactions and clarify previously unexplained interactions.