The effect of ritonavir on the pharmacokinetics of meperidine and normeperidine

Simultaneously Predicting the Pharmacokinetics of CES1-Metabolized Drugs and Their Metabolites Using Physiologically Based Pharmacokinetic Model in Cirrhosis Subjects

Hepatic carboxylesterase 1 (CES1) metabolizes numerous prodrugs into active ingredients or direct-acting drugs into inactive metabolites. We aimed to develop a semi-physiologically based pharmacokinetic (semi-PBPK) model to simultaneously predict the pharmacokinetics of CES1 substrates and their active metabolites in liver cirrhosis (LC) patients. Six prodrugs (enalapril, benazepril, cilazapril, temocapril, perindopril and oseltamivir) and three direct-acting drugs (flumazenil, pethidine and remimazolam) were selected. Parameters such as organ blood flows, plasma-binding protein concentrations, functional liver volume, hepatic enzymatic activity, glomerular filtration rate (GFR) and gastrointestinal transit rate were integrated into the simulation. The pharmacokinetic profiles of these drugs and their active metabolites were simulated for 1000 virtual individuals. The developed semi-PBPK model, after validation in healthy individuals, was extrapolated to LC patients. Most of the observations fell within the 5th and 95th percentiles of simulations from 1000 virtual patients. The estimated AUC and Cmax were within 0.5-2-fold of the observed values. The sensitivity analysis showed that the decreased plasma exposure of active metabolites due to the decreased CES1 was partly attenuated by the decreased GFR. Conclusion: The developed PBPK model successfully predicted the pharmacokinetics of CES1 substrates and their metabolites in healthy individuals and LC patients, facilitating tailored dosing of CES1 substrates in LC patients.

Cytochrome P4503A does not mediate the interaction between methadone and ritonavir-lopinavir

Plasma concentrations of orally administered methadone are reduced by the human immunodeficiency virus protease inhibitor combination ritonavir and lopinavir, but the mechanism is unknown. Methadone metabolism, clearance, and drug interactions have been attributed to CYP3A4, but this remains controversial. This investigation assessed the effects of acute (2 days) and steady-state (2 weeks) ritonavir-lopinavir on intravenous and oral methadone metabolism and clearance, hepatic and intestinal CYP3A4/5 activity (using the probe substrate intravenous and oral alfentanil), and intestinal transporter activity (using oral fexofenadine) in healthy volunteers. Plasma and urine concentrations of methadone and metabolite enantiomers, and other analytes, were determined by mass spectrometry. Acute and chronic ritonavir-lopinavir reduced plasma methadone enantiomer concentrations in half, with an average 2.6- and 1.5-fold induction of systemic and apparent oral methadone clearances. Induction was attributable to stereoselectively increased hepatic methadone N-demethylation, hepatic extraction, and hepatic clearance, and there was a strong correlation between methadone N-demethylation and clearance. Methadone renal clearance was unchanged. Alfentanil's systemic clearance and hepatic extraction, apparent oral clearance, and intestinal extraction were reduced to 25%, 16%, and 35% of control, indicating strong inhibition of hepatic and intestinal CYP3A activities. Ritonavir-lopinavir (acute > chronic) increased fexofenadine exposure, suggesting intestinal P-glycoprotein inhibition. No correlation was found between methadone clearance and CYP3A activity. Acute and steady-state ritonavir-lopinavir stereoselectively induced methadone N-demethylation and clearance, despite significant inhibition of hepatic and intestinal CYP3A activity. Ritonavir-lopinavir inhibited intestinal transporters activity but had no effect on methadone bioavailability. These results do not support a significant role for CYP3A or ritonavir-lopinavir-inhibitable intestinal transporters in single-dose methadone disposition.

Practical Perspectives on the Use of Tipranavir in Combination With Other Medications: Lessons Learned From Pharmacokinetic Studies

Drug-drug interactions are a major practical concern for physicians treating human immunodeficiency virus (HIV) because of the many medications that HIV-positive patients must take. Pharmacokinetic drug interactions can occur at different levels (absorption, distribution, metabolism, excretion) and are difficult to predict. Of all the processes that give rise to drug interactions, metabolism by cytochrome P450 (CYP3A) is the most frequent. Moreover, medications prescribed to HIV-positive patients may also be CYP3A inhibitors and inducers: Tipranavir, in the absence of ritonavir, is a CYP3A inducer, and ritonavir is a CYP3A inhibitor. Fortunately, the drug interactions between tipranavir coadministered with ritonavir and other antiretroviral medications or with other medications commonly used in HIV therapy are well characterized. This review summarizes the pharmacokinetic interactions between tipranavir/ritonavir and 11 other antiretroviral medications and between tipranavir/ritonavir and drugs used to treat opportunistic infections such as fungal infections, antiretroviral-treatment-related conditions such as hyperlipidemia, and side effects such as diarrhea.

Safety of midazolam for sedation of HIV-positive patients undergoing colonoscopy

OBJECTIVES

Because of concerns regarding interactions between midazolam and antiretroviral therapy (ART), alternative sedatives are sometimes used during procedural sedation. Our objective was to compare outcomes in patients on ART who received intravenous (iv) midazolam vs. iv diazepam, a second-line agent, during colonoscopy.

METHODS

We conducted a retrospective analysis of adult HIV-positive patients who underwent colonoscopy over a 3.5-year period. Primary outcomes were sedation duration, nadir systolic blood pressure (SBP), nadir oxygen saturation, abnormal cardiac rhythm, and change in level of consciousness using a standardized scale. We calculated rates of adverse events according to benzodiazepine use and identified risk factors for complications using univariate and multivariate analyses.

RESULTS

We identified 136 patients for this analysis: 70 received midazolam-based sedation and 66 received a diazepam-based regimen. There were no significant differences between the two groups with respect to sedation duration (mean 48.0 vs. 45.7 minutes for the midazolam and diazepam groups, respectively; P = 0.68), nadir SBP (mean 97.0 vs. 101.6 mmHg; P = 0.06), nadir oxygen saturation (mean 94.6 vs. 94.8%; P = 0.72) or rate of abnormal cardiac rhythm (11.4 vs. 19.7%; P = 0.18). More patients in the midazolam group experienced a depressed level of consciousness (91% vs. 74% in the diazepam group; P = 0.0075), but no patient required reversal of sedation or became unresponsive.

CONCLUSIONS

We did not find evidence that patients who received midazolam for procedural sedation had clinical outcomes statistically different from those who received diazepam. These findings should be confirmed in prospective studies or in a randomized controlled trial.

Efflux transporters- and cytochrome P-450-mediated interactions between drugs of abuse and antiretrovirals

Multidrug regimens and corresponding drug interactions cause many adverse reactions and treatment failures. Drug efflux transporters: P-gp, MRP, BCRP in conjunction with metabolizing enzymes (CYPs) are major factors in such interactions. Most effective combination antiretrovirals (ARV) therapy includes a PI or a NNRTI or two NRTI. Coadministration of such ARV may induce efflux transporters and/or CYP3A4 resulting in sub-therapeutic blood levels and therapeutic failure due to reduced absorption and/or increased metabolism. A similar prognosis is true for ARV-compounds and drugs of abuse combinations. Morphine and nicotine enhance CYP3A4 and MDR1 expression in vitro. A 2.5 fold rise of cortisol metabolite was evident in smokers relative to nonsmokers. Altered functions of efflux transporters and CYPs in response to ARV and drugs of abuse may result in altered drug absorption and metabolism. Appropriate in vitro models can be employed to predict such interactions. Influence of genetic polymorphism, SNP and inter-individual variation in drug response has been discussed. Complexity underlying the relationship between efflux transporters and CYP makes it difficult to predict the outcome of HAART as such, particularly when HIV patients taking drugs of abuse do not adhere to HAART regimens. HIV(+) pregnant women on HAART medications, indulging in drugs of abuse, may develop higher viral load due to such interactions and lead to increase in mother to child transmission of HIV. A multidisciplinary approach with clear understanding of mechanism of interactions may allow proper selection of regimens so that desired therapeutic outcome of HAART can be reached without any side effects.

Raltegravir-based HAART regimen in a patient with large B-cell lymphoma

OBJECTIVE

To describe the antiretroviral management of a patient diagnosed simultaneously with HIV/AIDS and diffuse large B-cell lymphoma, focusing on the drug-drug interactions between highly active antiretroviral therapy (HAART) and concomitant cancer chemotherapy.

CASE SUMMARY

A 55-year-old white man was recently diagnosed with HIV/AIDS and presented 1 month later with complaints of nausea, vomiting, abdominal pain, double vision, right eye discomfort/swelling, and a 3.6-kg weight loss. An excisional biopsy of a right inguinal lymph node confirmed a new diagnosis of diffuse large B-cell lymphoma. HAART and a chemotherapeutic regimen, including cyclophosphamide, doxorubicin, vincristine, prednisone (CHOP) with intrathecal methotrexate, was to be initiated. As the potential for multiple drug-drug interactions existed, raltegravir, abacavir, and lamivudine were chosen for the initial HAART regimen. The patient achieved and maintained an undetectable viral load throughout 6 CHOP cycles.

DISCUSSION

HAART improves the chemotherapeutic response in patients with HIV and lymphoma. Multiple drug-drug interactions are possible in patients who are to receive CHOP and HAART. Cyclophosphamide and vincristine are metabolized via the CYP3A4 isoenzyme. Protease inhibitors (PIs) and nonnucleoside reverse transcriptase inhibitors both inhibit and induce CYP3A4, with the potential for altered chemotherapeutic and cytotoxic effects. When PIs are combined with CHOP, mortality is reduced, but increased adverse effects are demonstrated. Raltegravir, an integrase inhibitor, is eliminated via glucuronidation and results in minimal drug-drug interactions. Raltegravir improves virologic and immunologic responses in HAART-naïve patients and thus would be a suitable alternative for preventing chemotherapeutic-HAART interactions.

CONCLUSIONS

There is limited information published regarding the potential for interactions between HAART and cancer chemotherapy. While further research is necessary, it is important for clinicians to consider the potential for drug-drug interactions when designing a HAART regimen concurrently with chemotherapy.

Anaesthetic considerations of the HIV-infected patients

PURPOSE OF REVIEW

Human immunodeficiency virus (HIV) infection is one of the major global health problems. Because the advances in treatment of HIV infection increase the patient's survival, anaesthesiologists may care for these patients during their practice. This article highlights HIV infection and anaesthetic implications.

RECENT FINDINGS

HIV infection is a spectrum of disease varying from asymptomatic to multiple organ involvement. Safe anaesthetic management in HIV-infected patients includes understanding basic knowledge of HIV infection, organ involvement, pharmacology and adverse reactions of antiretroviral agents. There are no specific anaesthetic agents and techniques for HIV-infected patients. Issues on central neural blockade and immunological aspects on HIV infection were discussed. Infection control to prevent transmission of infections to and from HIV-infected patients must be strictly conducted.

SUMMARY

Anaesthesia in HIV-infected patients should be individualized and depend on the status of the patient.

Induction effects of ritonavir: implications for drug interactions

OBJECTIVE

To review the literature on the induction effects of ritonavir on the cytochrome P450 enzyme system and glucuronyl transferase and identify resultant established and potential drug interactions.

DATA SOURCES

Primary literature was identified from MEDLINE (1950-April 2008), EMBASE (1988-April 2008) and International Pharmaceutical Abstracts (1970-April 2008) using the search terms ritonavir, cytochrome P450 enzyme system, enzyme induction, glucuronyl transferase, and drug interactions. Additionally, relevant conference abstracts and references of relevant articles were reviewed.

STUDY SELECTION AND DATA ABSTRACTION

All English-language articles and abstracts identified were reviewed.

DATA SYNTHESIS

Ritonavir is a well-known inhibitor of the metabolism of numerous medications that are substrates of the CYP3A and CYP2D6 pathways. It also exhibits a biphasic, time-dependent effect on P-glycoprotein of inhibition followed by induction. Numerous pharmacokinetic studies suggested that ritonavir induces cytochrome P450 enzymes 3A, 1A2, 2B6, 2C9, and 2C19, as well as glucuronyl transferase. Additionally, several case reports described clinically significant subtherapeutic effects of drugs metabolized by these isoenzymes when coadministered with ritonavir. Both therapeutic and boosting doses of ritonavir appear to induce these enzymes; however, most of the studies of low-dose ritonavir involved a second protease inhibitor such as lopinavir, darunavir, or tipranavir. It is, therefore, difficult to distinguish the relative effects of additional medications unless well-designed, 3-way studies are conducted.

CONCLUSIONS

At both therapeutic and boosting doses, ritonavir exhibits a clinically relevant induction effect on numerous drug-metabolizing enzymes. A decrease or loss of therapeutic effect may be observed when ritonavir is coadministered with medications that are substrates for these enzymes. It is important for clinicians to be aware of drugs potentially impacted by ritonavir therapy to identify and manage these interactions.

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.

Pharmacokinetics and interactions of headache medications, part I: introduction, pharmacokinetics, metabolism and acute treatments

Recent progress in the treatment of primary headaches has made available specific, effective and safe medications for these disorders, which are widely spread among the general population. One of the negative consequences of this undoubtedly positive progress is the risk of drug-drug interactions. This review is the first in a two-part series on pharmacokinetic drug-drug interactions of headache medications. Part I addresses acute treatments. Part II focuses on prophylactic treatments. The overall aim of this series is to increase the awareness of physicians, either primary care providers or specialists, regarding this topic. Pharmacokinetic drug-drug interactions of major severity involving acute medications are a minority among those reported in literature. The main drug combinations to avoid are: i) NSAIDs plus drugs with a narrow therapeutic range (i.e., digoxin, methotrexate, etc.); ii) sumatriptan, rizatriptan or zolmitriptan plus monoamine oxidase inhibitors; iii) substrates and inhibitors of CYP2D6 (i.e., chlorpromazine, metoclopramide, etc.) and -3A4 (i.e., ergot derivatives, eletriptan, etc.), as well as other substrates or inhibitors of the same CYP isoenzymes. The risk of having clinically significant pharmacokinetic drug-drug interactions seems to be limited in patients with low frequency headaches, but could be higher in chronic headache sufferers with medication overuse.

Liver enzyme induction and inhibition: implications for anaesthesia

Recent breakthroughs in molecular biology have enabled a reclassification of drug metabolising enzymes based on their amino acid sequence. This has led to a better understanding of drug metabolism and drug interactions. The majority of these drug metabolising enzymes may be either induced or inhibited by drugs or by extraneous substances including foodstuffs, cigarette smoke and environmental pollutants. Virtually all drugs used in anaesthesia are metabolised by either hepatic phase 1 or phase II enzymes. This review considers the classification of drug metabolising enzymes, explains the mechanisms of enzyme induction and inhibition, and also considers how the action of drugs commonly used by anaesthetists, including opioids and neuromuscular blocking drugs, may be altered by this mechanism.

Med-psych drug-drug interactions update. Antiretrovirals, part III: antiretrovirals and drugs of abuse

The third in a series reviewing the HIV/AIDS antiretroviral drugs, this report summarizes the interactions between antiretrovirals and common drugs of abuse. In an overview format for primary care physicians and psychiatrists, the metabolism and drug interactions in the context of antiretroviral therapy are presented for the following drugs of abuse: alcohol, benzodiazepines, cocaine, GHB (liquid X), ketamine (special K), LSD (acid), MDMA (Ecstasy), opiates, PCP (angel dust), and THC (marijuana).

Antiretrovirals, Part 1: Overview, History, and Focus on Protease Inhibitors

This column is the first in a series on HIV/AIDS antiretroviral drugs. This first review summarizes the history of HIV/AIDS and the development of highly active antiretroviral therapy (HAART) and highlights why it is important for non-HIV specialists to know about these drugs. There are four broad classes of HIV medications used in varying combinations in HAART: the protease inhibitors, nucleoside analogue reverse transcriptase inhibitors, the non-nucleoside reverse transcriptase inhibitors, and cell membrane fusion inhibitors. This paper reviews the mechanism of action, side effects, toxicities, and drug interactions of the protease inhibitors.

St John's wort and imipramine-induced gene expression profiles identify cellular functions relevant to antidepressant action and novel pharmacogenetic candidates for the phenotype of antidepressant treatment response

Both the prototypic tricyclic antidepressant imipramine (IMI) and the herbal product St John's wort (SJW) can be effective in the treatment of major depressive disorder. We studied hypothalamic gene expression in rats treated with SJW or IMI to test the hypothesis that chronic antidepressant treatment by various classes of drugs results in shared patterns of gene expression that may underlie their therapeutic effects. Individual hypothalami were hybridized to individual Affymetrix chips; we studied three arrays per group treatment. We constructed 95% confidence intervals for expression fold change for genes present in at least one treatment condition and we considered genes to be differentially expressed if they had a confidence interval excluding 1 (or -1) and had absolute difference in expression value of 10 or greater. SJW treatment differentially regulated 66 genes and expression sequence tags (ESTs) and IMI treatment differentially regulated 74 genes and ESTs. We found six common transcripts in response to both treatments. The likelihood of this occurring by chance is 1.14 x 10(-23). These transcripts are relevant to two molecular machines, namely the ribosomes and microtubules, and one cellular organelle, the mitochondria. Both treatments also affected different genes that are part of the same cell function processes, such as glycolytic pathways and synaptic function. We identified single-nucleotide polymorphisms in the human orthologs of genes regulated both treatments, as those genes may be novel candidates for pharmacogenetic studies. Our data support the hypothesis that chronic antidepressant treatment by drugs of various classes may result in a common, final pathway of changes in gene expression in a discrete brain region.

In Vitro Interaction of the HIV Protease Inhibitor Ritonavir with Herbal Constituents: Changes in P-gp and CYP3A4 Activity

The purpose of this study was to evaluate in vitro interactions of commercially obtained pure herbal constituents with p-glycoprotein P-gp and cytochrome P-450 3A4 (CYP3A4) activities, which can further modulate the transcellular transport and metabolism kinetics of orally administered drugs. Caco-2 cells grown in the presence of 0.25 micromol/L 1alpha,25-dihydroxy vitamin D3 and multidrug-resistant 1 (MDR1) transfected MDCK cells were used as models to evaluate the effect of purified herbal constituents (quercetin, hypericin, hyperforin from St. John's wort, kaempferol from ginseng, silibinin from milk thistle, and allicin from garlic) on P-gp-mediated efflux of the human immunodeficiency virus (HIV) protease inhibitor ritonavir. In addition, the inhibitory effect of these constituents on CYP3A4-mediated metabolism was determined by using cortisol as a model compound. Silibinin and hyperforin did not significantly alter cellular uptake of H-ritonavir in Caco-2 cells. A similar result was also observed for silibinin when tested in MDR1-MDCK cells. Quercetin, hypericin, and kaempferol exhibited a remarkable inhibition of P-gp-mediated efflux of ritonavir by increasing its cellular uptake in these models. These values were also comparable with the inhibitory effect of quinidine in Caco-2 cells, a well-known inhibitor of P-gp, on ritonavir efflux from Caco-2 cells. Allicin exhibited a concentration-dependent inhibition of ritonavir efflux when tested on MDR1-MDCK cells. There was a significant decrease in the Apical to Basal/Basal to Apical (AP-BL/BL-AP) transport ratio of ritonavir in presence of hypericin, kaempferol, and quercetin. These herbal constituents inhibited the CYP3A4 activity when tested with the Vivid CYP3A4 assay kit, whereas silibinin did not alter cortisol metabolism. Hypericin showed a significant inhibition in reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent metabolism of cortisol with 64.6% of intact drug at the end of a 1-hour study. Similarly, kaempferol and quercetin also caused substantial inhibition of cortisol metabolism with 89.7% and 90.1% of intact cortisol, respectively, compared with 45.9% in the control. Prolonged exposure of quercetin resulted in significant increase of mRNA expression of both MDR1 and CYP3A4 levels in Caco-2 cells. However, hyperforin caused upregulation of CYP3A4 and downregulation of MDR1, whereas the effect of silibinin and kaempferol remained inconclusive on these gene expressions. Hypericin, kaempferol, quercetin, and allicin inhibit the efflux and CYP3A4-mediated metabolism of xenobiotics in vitro. Hence, this study warns against the use of herbal constituents along with prescribed HIV protease inhibitors that are substrates for P-gp and/or CYP3A4.

Drug interactions between antiretroviral drugs and comedicated agents

HIV-infected individuals usually receive a wide variety of drugs in addition to their antiretroviral drug regimen. Since both non-nucleoside reverse transcriptase inhibitors and protease inhibitors are extensively metabolised by the cytochrome P450 system, there is a considerable potential for pharmacokinetic drug interactions when they are administered concomitantly with other drugs metabolised via the same pathway. In addition, protease inhibitors are substrates as well as inhibitors of the drug transporter P-glycoprotein, which also can result in pharmacokinetic drug interactions. The nucleoside reverse transcriptase inhibitors are predominantly excreted by the renal system and may also give rise to interactions. This review will discuss the pharmacokinetics of the different classes of antiretroviral drugs and the mechanisms by which drug interactions can occur. Furthermore, a literature overview of drug interactions is given, including the following items when available: coadministered agent and dosage, type of study that is performed to study the drug interaction, the subjects involved and, if specified, the type of subjects (healthy volunteers, HIV-infected individuals, sex), antiretroviral drug(s) and dosage, interaction mechanism, the effect and if possible the magnitude of interaction, comments, advice on what to do when the interaction occurs or how to avoid it, and references. This discussion of the different mechanisms of drug interactions, and the accompanying overview of data, will assist in providing optimal care to HIV-infected patients.

Pharmacokinetics of ritonavir and delavirdine in human immunodeficiency virus-infected patients

To evaluate the pharmacokinetic effect of adding delavirdine mesylate to the antiretroviral regimens of human immunodeficiency virus (HIV)-infected patients stabilized on a full dosage of ritonavir (600 mg every 12 h), 12 HIV-1-infected subjects had delavirdine mesylate (400 mg every 8 h) added to their current antiretroviral regimens for 21 days. Ritonavir pharmacokinetics were evaluated before (day 7) and after (day 28) the addition of delavirdine, and delavirdine pharmacokinetics were evaluated on day 28. The mean values (+/- standard deviations) for the maximum concentration in serum (C(max)) of ritonavir, the area under the concentration-time curve from 0 to 12 h (AUC(0-12)), and the minimum concentration in serum (C(min)) of ritonavir before the addition of delavirdine were 14.8 +/- 6.7 micro M, 94 +/- 36 micro M. h, and 3.6 +/- 2.1 micro M, respectively. These same parameters were increased to 24.6 +/- 13.9 micro M, 154 +/- 83 micro M. h, and 6.52 +/- 4.85 micro M, respectively, after the addition of delavirdine (P is <0.05 for all comparisons). Delavirdine pharmacokinetic parameters in the presence of ritonavir included a C(max) of 23 +/- 16 micro M, an AUC(0-8) of 114 +/- 75 micro M. h, and a C(min) of 9.1 +/- 7.5 micro M. Therefore, delavirdine increases systemic exposure to ritonavir by 50 to 80% when the drugs are coadministered.

Interactions between recreational drugs and antiretroviral agents

OBJECTIVE

To summarize existing data regarding potential interactions between recreational drugs and drugs commonly used in the management of HIV-positive patients.

DATA SOURCES

Information was obtained via a MEDLINE search (1966-August 2002) using the MeSH headings human immunodeficiency virus, drug interactions, cytochrome P450, medication names commonly prescribed for the management of HIV and related opportunistic infections, and names of commonly used recreational drugs. Abstracts of national and international conferences, review articles, textbooks, and references of all articles were also reviewed.

STUDY SELECTION AND DATA EXTRACTION

Literature on pharmacokinetic interactions was considered for inclusion. Pertinent information was selected and summarized for discussion. In the absence of specific data, prediction of potential clinically significant interactions was based on pharmacokinetic and pharmacodynamic properties.

RESULTS

All protease inhibitors (PIs) and nonnucleoside reverse transcriptase inhibitors are substrates and potent inhibitors or inducers of the cytochrome P450 system. Many classes of recreational drugs, including benzodiazepines, amphetamines, and opioids, are also metabolized by the liver and can potentially interact with antiretrovirals. Controlled interaction studies are often not available, but clinically significant interactions have been observed in a number of case reports. Overdoses secondary to interactions between the "rave" drugs methylenedioxymethamphetamine (MDMA) or gamma-hydroxybutyrate (GHB) and PIs have been reported. PIs, particularly ritonavir, may also inhibit metabolism of amphetamines, ketamine, lysergic acid diethylmide (LSD), and phencyclidine (PCP). Case series and pharmacokinetic studies suggest that nevirapine and efavirenz induce methadone metabolism, which may lead to symptoms of opiate withdrawal. A similar interaction may exist between methadone and the PIs ritonavir and nelfinavir, although the data are less consistent. Opiate metabolism can be inhibited or induced by concomitant PIs, and patients should be monitored for signs of toxicity and/or loss of analgesia. PIs should not be coadministered with midazolam and triazolam, since prolonged sedation may occur.

CONCLUSIONS

Interactions between agents commonly prescribed for patients with HIV and recreational drugs can occur, and may be associated with serious clinical consequences. Clinicians should encourage open dialog with their patients on this topic, to avoid compromising antiretroviral efficacy and increasing the risk of drug toxicity.

Human drug metabolism and the cytochromes P450: application and relevance of in vitro models

The cytochromes P450 (CYPs) constitute a superfamily of hemoprotein enzymes that are responsible for the biotransformation of numerous xenobiotics, including therapeutic agents. Studies of the biochemical and enzymatic properties of these enzymes and their molecular genetics and regulation of gene expression and activity have greatly enhanced our understanding of several aspects of clinical pharmacology such as pharmacokinetic variability, drug toxicity, and drug interactions. This review evaluates the major human hepatic drug-metabolizing CYP enzymes and their clinically relevant substrates, inhibitors, and inducers. Also discussed are the molecular bases and clinical implications of genetic polymorphisms that affect the CYPs. Much of the information on the specificity of substrates and inhibitors of the CYP enzymes is derived from in vitro studies using human liver microsomes and heterologously expressed CYP enzymes. These methods are discussed, and guidelines are provided for designing enzyme kinetic and reaction phenotyping studies using multiple approaches. The strengths, weaknesses, and discrepancies among the different approaches are considered using representative examples. The mathematical models used in predicting the pharmacokinetic clearance of a drug from in vitro estimates of intrinsic clearance and the principles of quantitative in vitro-in vivo scaling of metabolic drug interactions are also discussed.

Drug interactions

The prevalence of undesirable drug interactions is substantial but largely unknown. Although drug interactions are certainly an important cause of drug toxicity, their clinical significance may have been exaggerated. This review presents a brief overview of possible drug interactions.