Pharmacokinetic interactions between two human immunodeficiency virus protease inhibitors, ritonavir and saquinavir

Recent advances in nanoformulation development of Ritonavir, a key protease inhibitor used in the treatment of HIV-AIDS

INTRODUCTION

AIDS is one of the world's most serious public health challenges. Protease inhibitors are key components of AIDS treatment regimen. Ritonavir is a well-known protease inhibitor with low aqueous solubility belonging to BCS class II category. Some of the severe adverse effects associated with this drug restricted its use in the treatment of AIDS. However, several attempts were made by researchers in the past to enhance the oral bioavailability of Ritonavir.

AREAS COVERED

The current review mainly focuses on the adverse effects of Ritonavir and recent approaches followed by researchers on the development of nanoformulations of Ritonavir. Further, various patents filed on Ritonavir have also been discussed in the current review.

EXPERT OPINION

Most research on nanoformulation development for Ritonavir is mainly focused on enhancing the solubility and oral bioavailability of the drug. Some of the researchers focused on the lymphatic targeting of the drug in order to bypass the hepatic metabolism of the drug. However, most of the research topics did not cover the toxicity evaluation of the developed formulation. Since the major issue of Ritonavir is not only oral bioavailability but also drug-induced toxicity, this area needs to be considered during the formulation development.

The Mechanism-Based Inactivation of CYP3A4 by Ritonavir: What Mechanism?

Ritonavir is the most potent cytochrome P450 (CYP) 3A4 inhibitor in clinical use and is often applied as a booster for drugs with low oral bioavailability due to CYP3A4-mediated biotransformation, as in the treatment of HIV (e.g., lopinavir/ritonavir) and more recently COVID-19 (Paxlovid or nirmatrelvir/ritonavir). Despite its clinical importance, the exact mechanism of ritonavir-mediated CYP3A4 inactivation is still not fully understood. Nonetheless, ritonavir is clearly a potent mechanism-based inactivator, which irreversibly blocks CYP3A4. Here, we discuss four fundamentally different mechanisms proposed for this irreversible inactivation/inhibition, namely the (I) formation of a metabolic-intermediate complex (MIC), tightly coordinating to the heme group; (II) strong ligation of unmodified ritonavir to the heme iron; (III) heme destruction; and (IV) covalent attachment of a reactive ritonavir intermediate to the CYP3A4 apoprotein. Ritonavir further appears to inactivate CYP3A4 and CYP3A5 with similar potency, which is important since ritonavir is applied in patients of all ethnicities. Although it is currently not possible to conclude what the primary mechanism of action in vivo is, it is unlikely that any of the proposed mechanisms are fundamentally wrong. We, therefore, propose that ritonavir markedly inactivates CYP3A through a mixed set of mechanisms. This functional redundancy may well contribute to its overall inhibitory efficacy.

Clinically Relevant Interactions Between Ritonavir-Boosted Nirmatrelvir and Concomitant Antiseizure Medications: Implications for the Management of COVID-19 in Patients with Epilepsy

Ritonavir-boosted nirmatrelvir (RBN) has been authorized recently in several countries as an orally active anti-SARS-CoV-2 treatment for patients at high risk of progressing to severe COVID-19 disease. Nirmatrelvir is the active component against the SARS-CoV-2 virus, whereas ritonavir, a potent CYP3A inhibitor, is intended to boost the activity of nirmatrelvir by increasing its concentration in plasma to ensure persistence of antiviral concentrations during the 12-hour dosing interval. RBN is involved in many clinically important drug–drug interactions both as perpetrator and as victim, which can complicate its use in patients treated with antiseizure medications (ASMs). Interactions between RBN and ASMs are bidirectional. As perpetrator, RBN may increase the plasma concentration of a number of ASMs that are CYP3A4 substrates, possibly leading to toxicity. As victims, both nirmatrelvir and ritonavir are subject to metabolic induction by concomitant treatment with potent enzyme-inducing ASMs (carbamazepine, phenytoin, phenobarbital and primidone). According to US and European prescribing information, treatment with these ASMs is a contraindication to the use of RBN. Although remdesivir is a valuable alternative to RBN, it may not be readily accessible in some settings due to cost and/or need for intravenous administration. If remdesivir is not an appropriate option, either bebtelovimab or molnupiravir may be considered. However, evidence about the clinical efficacy of bebtelovimab is still limited, and molnupiravir, the only orally active alternative, is deemed to have appreciably lower efficacy than RBN and remdesivir.

Saquinavir: From HIV to COVID-19 and Cancer Treatment

Saquinavir was the first protease inhibitor developed for HIV therapy, and it changed the standard of treatment for this disease to a combination of drugs that ultimately led to increased survival of this otherwise deadly condition. Inhibiting the HIV protease impedes the virus from maturing and replicating. With this in mind, since the start of the COVID-19 outbreak, the research for already approved drugs (mainly antivirals) to repurpose for treatment of this disease has increased. Among the drugs tested, saquinavir showed promise in silico and in vitro in the inhibition of the SARS-CoV-2 main protease (3CLpro). Another field for saquinavir repurposing has been in anticancer treatment, in which it has shown effects in vitro and in vivo in several types of cancer, from Kaposi carcinoma to neuroblastoma, demonstrating cytotoxicity, apoptosis, inhibition of cell invasion, and improvement of radiosensibility of cancer cells. Despite the lack of follow-up in clinical trials for cancer use, there has been a renewed interest in this drug recently due to COVID-19, which shows similar pharmacological pathways and has developed superior in silico models that can be translated to oncologic research. This could help further testing and future approval of saquinavir repurposing for cancer treatment.

Quantitative prediction of pharmacokinetic properties of drugs in humans: Recent advance in in vitro models to predict the impact of efflux transporters in the small intestine and blood-brain barrier

Efflux transport systems are essential to suppress the absorption of xenobiotics from the intestinal lumen and protect the critical tissues at the blood-tissue barriers, such as the blood-brain barrier. The function of drug efflux transport is dominated by various transporters. Accumulated clinical evidences have revealed that genetic variations of the transporters, together with coadministered drugs, affect the expression and/or function of transporters and subsequently the pharmacokinetics of substrate drugs. Thus, in the preclinical stage of drug development, quantitative prediction of the impact of efflux transporters as well as that of uptake transporters and metabolic enzymes on the pharmacokinetics of drugs in humans has been performed using various in vitro experimental tools. Various kinds of human-derived cell systems can be applied to the precise prediction of drug transport in humans. Mathematical modeling consisting of each intrinsic metabolic or transport process enables us to understand the disposition of drugs both at the organ level and at the level of the whole body by integrating a variety of experimental results into model parameters. This review focuses on the role of efflux transporters in the intestinal absorption and brain distribution of drugs, in addition to recent advances in predictive tools and methodologies.

Polycaprolactone and polycaprolactone triol blends to obtain a stable liquid nanotechnological formulation: synthesis, characterization and in vitro - in vivo taste masking evaluation

The use of polymeric blends is a potential strategy to obtain novel nanotechnological formulations aiming at drug delivery systems. Saquinavir, an antiretroviral drug, was chosen as a model drug for the development of new stable liquid formulations with unpleasant taste masking properties. Three formulations containing different polymeric ratios (1:3, 1:1 and 3:1) were prepared and properly characterized by particle size distribution, zeta potential, pH, drug content and encapsulation efficiency measurements. The stability was verified by monitoring the zeta potential, particle size distribution, polydispersity index and drug content by 90 days. The light backscattering analysis was used to early identify possible phenomena of instability in the formulations. The in vitro drug release and saquinavir cytotoxicity were evaluated. The in vitro and in vivo taste masking properties were studied using an electronic tongue and a human sensory panel. All formulations presented nanometric sizes around 200 nm and encapsulation efficiency above 99%. The parameters evaluated for stability remained constant throughout 90 days. The in vitro tests showed a controlled drug release and absence of toxic effects on human T lymphocytes. The electronic tongue experiment showed taste differences for all formulations in comparison to drug solutions, with a more pronounced difference for the formulation with higher polycaprolactone content (3:1). This formulation was chosen for in vivo sensory panel evaluation which results corroborated the electronic tongue experiments. In conclusion, the polymer blend nanoformulation developed herein showed the promising application to incorporate drugs aiming at pharmaceutical taste-masking properties.

Investigating the interaction between nifedipine- and ritonavir-containing antiviral regimens: A physiologically based pharmacokinetic/pharmacodynamic analysis

AIMS

Hypertension is a common comorbidity of patients with COVID-19, SARS or HIV infection. Such patients are often concomitantly treated with antiviral and antihypertensive agents, including ritonavir and nifedipine. Since ritonavir is a strong inhibitor of CYP3A and nifedipine is mainly metabolized via CYP3A, the combination of ritonavir and nifedipine can potentially cause drug-drug interactions. This study provides guidance on nifedipine treatment during and after coadministration with ritonavir-containing regimens, using a physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) analysis.

METHODS

The PBPK/PD models for 3 formations of nifedipine were developed based on the Simcyp nifedipine model and the models were verified using published data. The effects of ritonavir on nifedipine exposure and systolic blood pressure (SBP) were assessed for instant-release, sustained-release and controlled-release formulations in patients. Various nifedipine regimens were investigated when coadministered with or without ritonavir.

RESULTS

PBPK/PD models for 3 formulations of nifedipine were successfully established. The predicted maximum concentration (C_max ), area under plasma concentration-time curve (AUC), maximum reduction in SBP and area under effect-time curve were all within 0.5-2.0-fold of the observed data. Model simulations showed that the inhibitory effect of ritonavir on CYP3A4 increased the C_max of nifedipine 17.92-48.85-fold and the AUC 63.30-84.01-fold at steady state and decreased the SBP by >40 mmHg. Thus, the combination of nifedipine and ritonavir could lead to severe hypotension.

CONCLUSION

Ritonavir significantly affects the pharmacokinetics and antihypertensive effect of nifedipine. It is not recommended for patients to take nifedipine- and ritonavir-containing regimens simultaneously.

PXR-mediated idiosyncratic drug-induced liver injury: mechanistic insights and targeting approaches

INTRODUCTION

The human liver is the center for drug metabolism and detoxification and is, therefore, constantly exposed to toxic chemicals. The loss of liver function as a result of this exposure is referred to as drug-induced liver injury (DILI). The pregnane X receptor (PXR) is the primary regulator of the hepatic drug-clearance system, which plays a critical role in mediating idiosyncratic DILI.

AREAS COVERED

This review is focused on common mechanisms of PXR-mediated DILI and on in vitro and in vivo models developed to predict and assess DILI. It also provides an update on the development of PXR antagonists that may manage PXR-mediated DILI.

EXPERT OPINION

DILI can be caused by many factors, and PXR is clearly linked to DILI. Although emerging data illustrate how PXR mediates DILI and how PXR activity can be modulated, many questions concerning the development of effective PXR modulators remain. Future research should be focused on determining the mechanisms regulating PXR functions in different cellular contexts.

An in vitro evaluation of drugs used in the Kenyan ART program

The majority of anti-HIV drug susceptibility tests have been performed on subtype B HIV-1 strains, since these are the most prevalent in countries designing, testing, and manufacturing the current anti-HIV agents. The increasing global spread of HIV subtype highlights the need to determine the activity of anti-HIV drugs against subtypes of HIV other than subtype B. Furthermore an increasing number of individuals infected with many of the non subtype B virus strains now receive antiretroviral therapy because of rollout programs in developing countries as well as increasing migration to the developed world. The phenotypic susceptibility of two laboratory strains HIV-1JFRL and HIV-1IIIB (representing subtype B) and two clinical isolates HIV-104RTA and HIV-1025RTA (representing subtypes A and D respectively) was determined. The in vitro drug susceptibility testing of the isolates was carried out in C8166 cell line and in peripheral blood mononuclear cells (PBMCs). The study revealed that the drugs used in the Kenyan national ART program inhibited HIV-1 replication in-vitro as their inhibitory concentrations (IC50) compared well with the standard Inhibitory concentration values. The results also suggest a biochemical similarity of the reverse transcriptase (RT) and protease enzymes from these subtypes despite the divergence at the genetic level. The findings suggest that similar clinical benefits of antiviral therapy obtain in persons infected with other subtypes of HIV-1other than subtype B and that the generic drugs used in the national ART program in Kenya are as efficacious as branded drugs in inhibiting HIV replication in vitro despite the limited number of the viruses studied.

In vitro identification of human cytochrome P450 isoforms involved in the metabolism of Geissoschizine methyl ether, an active component of the traditional Japanese medicine Yokukansan

1. Yokukansan (YKS) is a traditional Japanese medicine also called kampo, which has been used to treat neurosis, insomnia, and night crying and peevishness in children. Geissoschizine methyl ether (GM), a major indole alkaloid found in Uncaria hook, has been identified as a major active component of YKS with psychotropic effects. Recently, GM was reported to have a partial agonistic effect on serotonin 5-HT1A receptors. However, there is little published information on GM metabolism in humans, although several studies reported the blood kinetics of GM in rats and humans. In this study, we investigated the GM metabolic pathways and metabolizing enzymes in humans. 2. Using recombinant human cytochrome P450 (CYP) isoforms and polyclonal antibodies to CYP isoforms, we found that GM was metabolized into hydroxylated, dehydrogenated, hydroxylated+dehydrogenated, demethylated and water adduct forms by some CYP isoforms. 3. The relative activity factors in human liver microsomes were calculated to determine the relative contributions of individual CYP isoforms to GM metabolism in human liver microsomes (HLMs). We identified CYP3A4 as the CYP isoform primarily responsible for GM metabolism in human liver microsomes. 4. These findings provide an important basis for understanding the pharmacokinetics and pharmacodynamics of GM and YKS.

Stereoselective inhibition of CYP2C19 and CYP3A4 by fluoxetine and its metabolite: implications for risk assessment of multiple time-dependent inhibitor systems

Recent guidance on drug-drug interaction (DDI) testing recommends evaluation of circulating metabolites. However, there is little consensus on how to quantitatively predict and/or assess the risk of in vivo DDIs by multiple time-dependent inhibitors (TDIs) including metabolites from in vitro data. Fluoxetine was chosen as the model drug to evaluate the role of TDI metabolites in DDI prediction because it is a TDI of both CYP3A4 and CYP2C19 with a circulating N-dealkylated inhibitory metabolite, norfluoxetine. In pooled human liver microsomes, both enantiomers of fluoxetine and norfluoxetine were TDIs of CYP2C19, (S)-norfluoxetine was the most potent inhibitor with time-dependent inhibition affinity constant (KI) of 7 μM, and apparent maximum time-dependent inhibition rate (k(inact,app)) of 0.059 min(-1). Only (S)-fluoxetine and (R)-norfluoxetine were TDIs of CYP3A4, with (R)-norfluoxetine being the most potent (K(I) = 8 μM, and k(inact,app) = 0.011 min(-1)). Based on in-vitro-to-in-vivo predictions, (S)-norfluoxetine plays the most important role in in vivo CYP2C19 DDIs, whereas (R)-norfluoxetine is most important in CYP3A4 DDIs. Comparison of two multiple TDI prediction models demonstrated significant differences between them in in-vitro-to-in-vitro predictions but not in in-vitro-to-in-vivo predictions. Inclusion of all four inhibitors predicted an in vivo decrease in CYP2C19 (95%) and CYP3A4 (60-62%) activity. The results of this study suggest that adequate worst-case risk assessment for in vivo DDIs by multiple TDI systems can be achieved by incorporating time-dependent inhibition by both parent and metabolite via simple addition of the in vivo time-dependent inhibition rate/cytochrome P450 degradation rate constant (λ/k(deg)) values, but quantitative DDI predictions will require a more thorough understanding of TDI mechanisms.

HIV-1 antiretroviral drug therapy

The most significant advance in the medical management of HIV-1 infection has been the treatment of patients with antiviral drugs, which can suppress HIV-1 replication to undetectable levels. The discovery of HIV-1 as the causative agent of AIDS together with an ever-increasing understanding of the virus replication cycle have been instrumental in this effort by providing researchers with the knowledge and tools required to prosecute drug discovery efforts focused on targeted inhibition with specific pharmacological agents. To date, an arsenal of 24 Food and Drug Administration (FDA)-approved drugs are available for treatment of HIV-1 infections. These drugs are distributed into six distinct classes based on their molecular mechanism and resistance profiles: (1) nucleoside-analog reverse transcriptase inhibitors (NNRTIs), (2) non-nucleoside reverse transcriptase inhibitors (NNRTIs), (3) integrase inhibitors, (4) protease inhibitors (PIs), (5) fusion inhibitors, and (6) coreceptor antagonists. In this article, we will review the basic principles of antiretroviral drug therapy, the mode of drug action, and the factors leading to treatment failure (i.e., drug resistance).

Lack of Interaction between Enfuvirtide and Ritonavir or Ritonavir-Boosted Saquinavir in HIV-1-Infected Patients

Enfuvirtide (Fuzeon) is an HIV fusion inhibitor, the first drug in a new class of antiretrovirals. The HIV protease inhibitors ritonavir and saquinavir both inhibit cytochrome P450 (CYP450) isoenzymes, and low-dose ritonavir is often used to boost pharmacokinetic exposure to full-dose protease inhibitors. These two studies were designed to assess whether ritonavir and ritonavir-boosted saquinavir influence the steady-state pharmacokinetics of enfuvirtide. Both studies were single-center, open-label, one-sequence crossover clinical pharmacology studies in 12 HIV-1-infected patients each. Patients received enfuvirtide (90 mg twice daily [bid], subcutaneous injection) for 7 days and either ritonavir (200 mg bid, ritonavir study, orally) or saquinavir/ritonavir (1000/100 mg bid, saquinavir/ritonavir study, orally) for 4 days on days 4 to 7. Serial blood samples were collected up to 24 hours after the morning dose of enfuvirtide on days 3 and 7. Plasma concentrations for enfuvirtide, enfuvirtide metabolite, saquinavir, and ritonavir were measured using validated liquid chromatography tandem mass spectrometry methods. Efficacy and safety were also monitored. Bioequivalence criteria require the 90% confidence interval (CI) for the least squares means (LSM) of C(max) and AUC(12h) to be between 80% and 125%. In the present studies, analysis of variance showed that when coadministered with ritonavir, the ratio of LSM for enfuvirtide was 124% for C(max) (90% confidence interval [CI]: 109%-141%), 122% for AUC(12h) (90% CI: 108%-137%), and 114% for C(trough) (90% CI: 102%-128%). Although the bioequivalence criteria were not met, the increase in enfuvirtide exposure was small (< 25%) and not clinically relevant. When administered with ritonavir-boosted saquinavir, the ratio of LSM for enfuvirtide was 107% for C(max) (90% CI: 94.3%-121%) and 114% for AUC(12h) (90% CI: 105%-124%), which therefore met bioequivalence criteria, and 126% for C(trough) (90% CI: 117%-135%). The pharmacokinetics of enfuvirtide are affected to a small extent when coadministered with ritonavir at a dose of 200 mg bid but not when coadministered with a saquinavir-ritonavir combination (1000/100 mg bid). However, previous clinical studies have shown that such increases in enfuvirtide exposure are not clinically relevant. Thus, no dosage adjustments are warranted when enfuvirtide is coadministered with low-dose ritonavir or saquinavir boosted with a low dose of ritonavir.

The Effects of Once-Daily Saquinavir/Minidose Ritonavir on the Pharmacokinetics of Methadone

Twelve methadone-maintained HIV-negative subjects were given saquinavir/ritonavir (SQV/rtv) 1600 mg/100 mg once daily for 14 days. Pharmacokinetic evaluations of total and unbound methadone enantiomers (R and S) were conducted before and after SQV/rtv. SQV/rtv was well tolerated, with no ACTG Grade 3-4 adverse events, no evidence of sedation, and no changes in methadone dose. For R-methadone (active isomer), C(max), AUC(0-24 h), and C(min) were unchanged, but percent unbound 4 hours after dosing was reduced by 12%. For S-methadone, no differences in pharmacokinetic parameters of total drug were seen, but unbound concentrations were reduced by 15% and 21% at 4 and 24 hours after dosing, respectively. SQV trough concentrations exceeded the anticipated EC(50) (50 ng/mL) in 10/12 subjects, persisting for at least 6 hours after the final dose in 4/6 subjects. Once-daily SQV/rtv in methadone-maintained subjects is safe and not associated with any clinically significant interaction with methadone during 14 days of concomitant administration.

Mechanisms of pharmacokinetic enhancement between ritonavir and saquinavir; micro/small dosing tests using midazolam (CYP3A4), fexofenadine (p-glycoprotein), and pravastatin (OATP1B1) as probe drugs

We investigated the mechanisms of ritonavir-mediated enhancement effect on the pharmacokinetics of saquinavir using in vivo probes for CYP3A4 (midazolam), p-glycoprotein (fexofenadine), and OATP1B1 (pravastatin) following oral micro/small dosing. A cocktail of the drugs (2 mg of saquinavir, 100 µg of each probe) was administered to eight healthy volunteers (phase 1), and then coadministered with 20 mg (phase 2) and 100 mg (phase 3) of ritonavir. Plasma concentrations of the drugs were measured by validated LC-MS/MS methods. The mean plasma AUC0-24 (pg hour/mL) of saquinavir at phases 1, 2, and 3 was 101, 2 540, and 23 900 (P < .01), respectively. The relative area under the plasma concentration-time curve (AUC)0-24 ratios of midazolam and fexofenadine at phases 1, 2, and 3 were 1:5.9:14.7 (P < .01), and 1:1.4:2.2 (P < .01-.05), respectively. In contrast, there was no difference in the pharmacokinetics of pravastatin. Inhibition of intestinal and hepatic CYP3A-mediated metabolism, and intestinal p-glycoprotein-mediated efflux of saquinavir, but not OATP1B1, is involved in the enhancement mechanism. Micro/small dosing is useful for examining the mechanism of drug interactions without safety concern.

Importance of multi-p450 inhibition in drug-drug interactions: evaluation of incidence, inhibition magnitude, and prediction from in vitro data

Drugs that are mainly cleared by a single enzyme are considered more sensitive to drug-drug interactions (DDIs) than drugs cleared by multiple pathways. However, whether this is true when a drug cleared by multiple pathways is coadministered with an inhibitor of multiple P450 enzymes (multi-P450 inhibition) is not known. Mathematically, simultaneous equipotent inhibition of two elimination pathways that each contribute half of the drug clearance is equal to equipotent inhibition of a single pathway that clears the drug. However, simultaneous strong or moderate inhibition of two pathways by a single inhibitor is perceived as an unlikely scenario. The aim of this study was (i) to identify P450 inhibitors currently in clinical use that can inhibit more than one clearance pathway of an object drug in vivo and (ii) to evaluate the magnitude and predictability of DDIs caused by these multi-P450 inhibitors. Multi-P450 inhibitors were identified using the Metabolism and Transport Drug Interaction Database. A total of 38 multi-P450 inhibitors, defined as inhibitors that increased the AUC or decreased the clearance of probes of two or more P450s, were identified. Seventeen (45%) multi-P450 inhibitors were strong inhibitors of at least one P450, and an additional 12 (32%) were moderate inhibitors of one or more P450s. Only one inhibitor (fluvoxamine) was a strong inhibitor of more than one enzyme. Fifteen of the multi-P450 inhibitors also inhibit drug transporters in vivo, but such data are lacking on many of the inhibitors. Inhibition of multiple P450 enzymes by a single inhibitor resulted in significant (>2-fold) clinical DDIs with drugs that are cleared by multiple pathways such as imipramine and diazepam, while strong P450 inhibitors resulted in only weak DDIs with these object drugs. The magnitude of the DDIs between multi-P450 inhibitors and diazepam, imipramine, and omeprazole could be predicted using in vitro data with similar accuracy as probe substrate studies with the same inhibitors. The results of this study suggest that inhibition of multiple clearance pathways in vivo is clinically relevant, and the risk of DDIs with object drugs may be best evaluated in studies using multi-P450 inhibitors.

Risk assessment of mechanism-based inactivation in drug-drug interactions

Drug-drug interactions (DDIs) that occur via mechanism-based inactivation of cytochrome P450 are of serious concern. Although several predictive models have been published, early risk assessment of MBIs is still challenging. For reversible inhibitors, the DDI risk categorization using [I]/K(i) ([I], the inhibitor concentration; K(i), the inhibition constant) is widely used in drug discovery and development. Although a simple and reliable methodology such as [I]/K(i) categorization for reversible inhibitors would be useful for mechanism-based inhibitors (MBIs), comprehensive analysis of an analogous measure reflecting in vitro potency for inactivation has not been reported. The aim of this study was to evaluate whether the term λ/k(deg) (λ, first-order inactivation rate at a given MBI concentration; k(deg), enzyme degradation rate constant) would be useful in the prediction of the in vivo DDI risk of MBIs. Twenty-one MBIs with both in vivo area under the curve (AUC) change of marker substrates and in vitro inactivation parameters were identified in the literature and analyzed. The results of this analysis show that in vivo DDIs with >2-fold change of object drug AUC can be identified with the cutoff value of λ/k(deg) = 1, where unbound steady-state C(max) is used for inhibitor concentration. However, the use of total C(max) led to great overprediction of DDI risk. The risk assessment using λ/k(deg) coupled with unbound C(max) can be useful for the DDI risk evaluation of MBIs in drug discovery and development.

Clinical outcomes and management of mechanism-based inhibition of cytochrome P450 3A4

Mechanism-based inhibition of cytochrome P450 (CYP) 3A4 is characterized by NADPH-, time-, and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYPs to reactive metabolites. Such inhibition of CYP3A4 can be due to the chemical modification of the heme, the protein, or both as a result of covalent binding of modified heme to the protein. The inactivation of CYP3A4 by drugs has important clinical significance as it metabolizes approximately 60% of therapeutic drugs, and its inhibition frequently causes unfavorable drug–drug interactions and toxicity. The clinical outcomes due to CYP3A4 inactivation depend on many factors associated with the enzyme, drugs, and patients. Clinical professionals should adopt proper approaches when using drugs that are mechanism-based CYP3A4 inhibitors. These include early identification of drugs behaving as CYP3A4 inactivators, rational use of such drugs (eg, safe drug combination regimen, dose adjustment, or discontinuation of therapy when toxic drug interactions occur), therapeutic drug monitoring, and predicting the risks for potential drug–drug interactions. A good understanding of CYP3A4 inactivation and proper clinical management are needed by clinical professionals when these drugs are used.

Immunomodulation in female B₆C₃F₁ mice following treatment with the HIV protease inhibitor saquinavir for 28 days by gavage

Saquinavir (SQV) is a protease inhibitor that binds to the protease active site of the human immunodeficiency virus and prevents the cleavage of viral polyproteins resulting in the formation of non-infectious virus particles. The purpose of these studies was to determine the potential effects of SQV on the immune system in female B₆C₃F₁ mice. SQV was administered by gavage twice daily for 28 days at total doses of 300, 600, and 1200 mg/kg/day. No significant differences were observed in body weight, or the weights of spleen, thymus, liver, kidneys, or lungs. Exposure to SQV produced no biologically meaningful changes in hematological parameters. However, a statistically significant increase in the number of T-cells (23%) was observed at the high dose level of SQV. The number of splenic immature T-cells (CD4+CD8+ cells) also showed increases of 46% and 92% at the 600 and 1200 mg/kg dose levels, respectively. The immunoglobulin M antibody-forming cell (AFC) response was significantly increased by 41% when the data were expressed as AFC/10⁶ spleen cells at the 1200 mg/kg dose level. Treatment with SQV had no effects on the mixed leukocyte response. Overall, the activities of natural killer cells and cytotoxic T-cells were not altered in SQV-treated animals when compared to vehicle controls. In addition, exposure to SQV did not affect host resistance in the B16F10 melanoma model. In conclusion, SQV produced an enhancement of the humoral immune response, possibly through modulating T-cell function in female B₆C₃F₁ mice.

Effect of prescribed minimum benefits on the prevalence of possible drug-drug interactions of antiretroviral agents in a section of the private health care sector in South Africa: a 2 year comparative study

Objective

The aim of this study was to determine the impact of prescribed minimum benefits (PMBs) after implementation, on the prevalence of possible drug-drug interactions (DDIs) between antiretrovirals (ARVs) themselves and other drugs on prescriptions claimed in a section of the private health care sector in South Africa.

Setting

A section of the private health care sector in South Africa.

Method

A comparative, retrospective drug-utilisation study was performed using 2004 and 2005 data from a medicine claims database. Possible DDIs found were classified according to Tatro (2005).

Key findings

The data consisted of 43 482 ARV prescriptions claimed during 2004 and 51 613 for 2005. A total of 5 305 882 and 3 606 992 medicine items were claimed during 2004 and 2005 respectively, of which 1.92% were ARVs for 2004 and 3.38% for 2005. Of 18 035 DDIs identified, 83.89% were between ARVs and other medications, and 16.11% were between ARVs themselves for 2004. Of 25 130 DDIs identified for 2005, 92.59% were between ARVs and other medications, and 7.41% were between ARVs themselves.

Conclusions

The decrease in DDIs between ARVs alone for 2005 as compared to 2004 could indicate a possible impact of PMBs on HIV/AIDS, as a chronic disease in which management programmes were introduced to ensure the appropriateness and effectiveness of drugs in HIV/AIDS. It is therefore recommended that further investigations be done on the management of the most important DDIs between ARVs alone and other drugs prescribed on the same prescription.

Drug-drug interaction pattern recognition

BACKGROUND AND OBJECTIVE

Drug-drug interaction (DDI) is an important aspect of drug development, especially for safety. When a drug is used concomitantly with other drug(s), one of the major concerns is the change of exposures, including the rate and extent of drug absorption, distribution, metabolism and elimination. To address the concerns, a common practice is to measure and report the differences between the exposure in the presence and in the absence of concomitant medication (COMED). The area under the plasma concentration versus time curve (AUC), maximum plasma concentration (C(max)) and time to reach the C(max) (t(max)) changes are usually measured in DDI studies. A usual observation is the different extents of changes among AUC, C(max) and t(max), which may raise concerns in certain therapeutic areas or some special agents. The objective of this study was to investigate the variation among changes of AUC, C(max) and t(max) in DDI studies, and its pharmacokinetic manifestation.

DATA SOURCES

Based on a list of DDI results from the literature, with the assumptions that the primary parameters of a drug of interest were altered during a DDI, two sets of simulated data were generated according to a single oral dose, one-compartment model. The first set including 24 cases with different half-lives and absorption constants (k(a)) considered the exposure changes upon independent variation of bioavailability (F), clearance (CL), volume of distribution (V(d)) and k(a) up to 50-fold increases or decreases. The second set considered the exposure changes with simultaneous variation of F, CL, V(d), and k(a) within 5-fold range (increase or decrease) for a case selected from the first set.

STUDY SELECTION, DATA EXTRACTION AND SYNTHESIS

Parameter fold changes (defined in a fashion showing fold increase or fold decreases, including CL fold change, F fold change, V(d) fold change and k(a) fold change) and exposure changes (AUC fold change, C(max) fold change, t(max) fold change and fold change difference [AUC fold change - C(max) fold change]) were used to generate plots demonstrating various relationships between parameter fold changes and exposure changes. Based on the observations that AUC was influenced by CL and F, C(max) was affected by all four parameters, t(max) was mainly determined by CL and k(a), F did little for t(max) and k(a) was unrelated to AUC, a chart was created for DDI pattern recognition.

CONCLUSION

An approach, named DDI pattern recognition, is proposed for didactical purposes. It provides a quick initial estimate for interpreting the DDI results based on the exposure changes. This approach entails the following stages: (i) performing a drug interaction study; (ii) calculating the exposure changes in the presence of COMED compared to those in the absence of COMED, and the fold change difference; (iii) selecting the parameter fold changes that may play important roles in a specific DDI, by estimating their possible ranges; and (iv) interpreting the DDI by integrating all the information available, such as the possible mechanism involved. A quicker and better understanding about the processes, which dominate a DDI, has been achieved using this approach by focusing on integration of all information available and mechanistic interpretation.

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.

How much ritonavir is needed to boost protease inhibitors? Systematic review of 17 dose-ranging pharmacokinetic trials

BACKGROUND

Ritonavir has been evaluated at boosting doses of 50–800 mg daily with seven protease inhibitors: amprenavir, atazanavir, darunavir, indinavir, lopinavir,saquinavir and tipranavir. Minimizing the boosting dose of ritonavir could improve tolerability and lower costs.

METHODS

A MEDLINE search identified 17 phamacokinetic trials using different ritonavir doses with protease inhibitors. The dose of ritonavir used was correlated with plasma levels of each boosted protease inhibitor. For the five pharmacokinetic trials of lopinavir/ritonavir, a meta-analysis was used to estimate the effects of lopinavir dose versus ritonavir dose on lopinavir pharmacokinetics.

RESULTS

Saquinavir, fosamprenavir and darunavir were boosted equally well by lower(50–100 mg) versus higher doses of ritonavir. Indinavir, tipranavir and lopinavir were boosted more by higher ritonavir doses. Data on atazanavir were inconclusive. The ritonavir dose-dependence of boosting effects did not correlate with their bioavailability or their effects on ritonavir plasma levels. Atazanavir and indinavir raised plasma ritonavir levels by 69–72%, whereas saquinavir had no effects on ritonavir. Darunavir,lopinavir, tipranavir and fosamprenavir all lowered ritonavir plasma levels. For the meta-analysis of lopinavir/ritonavir trials, the 200/150 mg twice daily (b.i.d.) dose of lopinavir/ritonavir (one Meltrex 200/50mg tablet and one ritonavir 100mg b.i.d.)showed lopinavir area under the curve and minimum concentration similar to the standard 400/100mg b.i.d. dose.

CONCLUSION

It may be possible to use three protease inhibitors (saquinavir, amprenavir and darunavir) with lower doses of ritonavir. A 200/150 mg b.i.d. dose of lopinavir/ritonavir could lower costs while maintaining very similar lopinavir plasma levels to the standard dose. New pharmaco enhancer drugs may need to be used at different doses to boost different antiretrovirals.

Induction of P-glycoprotein expression and activity by ritonavir in bovine brain microvessel endothelial cells

Extended treatment with human immunodeficiency virus (HIV) protease inhibitors (HPIs) is standard in HIV/AIDS therapy. While these drugs have helped decrease the overall incidence of AIDS defining illnesses, the relative prevalence of HIV/AIDS dementia has increased. HPIs may cause induction of blood-brain barrier (BBB) drug transporters (P-glycoprotein; P-gp) and thereby limit entry of HPIs into brain tissue, increasing the probability that the brain could become an HIV sanctuary site. Using bovine brain microvessel endothelial cells (BMEC) as an in-vitro model of the BBB, the potential for the HIV protease inhibitor ritonavir to cause induction of P-gp activity and expression was examined. BMEC were isolated from fresh cow brain by enzymatic digest and density centrifugation. Primary culture BMEC were co-incubated with ritonavir or vehicle control for 120 h. Quantitative drug accumulation of rhodamine 123 (Rh123) and fluorescence microscopy were used as measures of P-gp activity. P-gp expression was assessed using quantitative Western blotting. Ritonavir decreased Rh123 cell accumulation and increased P-gp immunoreactive protein in a concentration-dependent manner. Fluorescent microscopy mirrored Rh123 quantitative studies. In BMEC pretreated with 30 microM ritonavir, Rh123 accumulation was decreased 40% and immunoreactive P-gp protein increased 2-fold. Collectively, a strong correlation between decreased Rh123 BMEC accumulation and increased P-gp immunoreactive protein was observed (Spearman r2 = 0.77, P < 0.0001). Thus extended exposure of BMEC to ritonavir caused a concentration-dependent increase in P-gp activity and expression. Similar findings may occur at the clinical level with prolonged HIV protease inhibitor use, giving insight into the central nervous system as an HIV sanctuary site and eventual development of HIV dementia.

Concise Review: Clinical Relevance of Drug–Drug and Herb–Drug Interactions Mediated by the ABC Transporter ABCB1 (MDR1, P‐glycoprotein)

The importance of P-glycoprotein (P-gp) in drug-drug interactions is increasingly being identified. P-gp has been reported to affect the pharmacokinetics of numerous structurally and pharmacologically diverse substrate drugs. Furthermore, genetic variability in the multidrug resistance 1 gene influences absorption and tissue distribution of drugs transported. Inhibition or induction of P-gp by coadministered drugs or food as well as herbal constituents may result in pharmacokinetic interactions leading to unexpected toxicities or undertreatment. On the other hand, modulation of P-gp expression and/or activity may be a useful strategy to improve the pharmacological profile of anticancer P-gp substrate drugs. In recent years, the use of complementary and alternative medicine (CAM), like herbs, food, and vitamins, by cancer patients has increased significantly. CAM use substantially increases the risk for interactions with anticancer drugs, especially because of the narrow therapeutic window of these compounds. However, for most CAMs, it is unknown whether they affect metabolizing enzymes and/or drug transporter activity. Clinically relevant interactions are reported between St John's wort or grapefruit juice and anticancer as well as nonanticancer drugs. CAM-drug interactions could explain, at least in part, the large interindividual variation in efficacy and toxicity associated with drug therapy in both cancer and noncancer patients. The study of drug-drug, food-drug, and herb-drug interactions and of genetic factors affecting pharmacokinetics and pharmacodynamics is expected to improve drug safety and will enable individualized drug therapy. Disclosure of potential conflicts of interest is found at the end of this article.

Inhibitory quotient as a prognostic factor of response to a salvage antiretroviral therapy containing ritonavir-boosted saquinavir. The CIVSA Study

BACKGROUND

The addition of a low dose of ritonavir to protease inhibitors (PIs) has become a widespread strategy to improve PI pharmacokinetics. As resistance is a major barrier to long-term suppression, in salvage therapy genotype and/or phenotype scoring is currently used to predict the response. We evaluated the relationship between the saquinavir (SQV) inhibitory quotient (IQ) (virtual and genotypic) and virological response.

METHODS

Eligible patients were on a PI-containing highly active antiretroviral therapy (HAART) regimen excluding SQV and had a viral load >5000 HIV-1 RNA copies/mL. The PI was switched to SQV/ritonavir (RTV) 1000/100 mg twice a day (bid) and the same two backbone nucleoside reverse transcriptase inhibitors (NRTIs) were maintained at least until week 4, when the resistance test results became available. Genotype and virtual phenotype were determined at baseline, while the SQV trough plasma concentration was determined at week 4.

RESULTS

Fifty-three patients were included in the study. Mean baseline viral load and CD4 count were 137,693 copies/mL and 263 cells/microL, respectively, the mean number of previous PIs was 2.3 and the mean number of protease gene mutations (PGMs) was 4.1. Using an on-treatment analysis, at week 16 the mean increase in CD4 count was 70.9 cells/microL, viral load was <200 copies/mL in 17 out of 37 patients (45.9%), and 30 out of 45 patients (66.7%) were considered virological responders (VRs) (viral load <200 copies/mL or viral load declined > or =1 log(10) at week 16). Median virtual phenotype was 1.3 (0.6-6.9). Baseline differences were detected between VR and non-VR populations: the mean numbers of PGMs were 3.2 and 5.8 (P<0.05), the mean numbers of SQV-associated mutations were 2 and 3.8 (P<0.05), and the mean CD4 counts were 365.9 and 184.3 cells/microL (P<0.05), respectively. Mean SQV trough concentrations at week 4 were 1.1 and 1.0 microg/mL (not significant), and mean virtual IQs were 0.7 and 0.1 (P<0.01), respectively. Multivariate analysis showed that baseline PGMs >5 or SQV-associated mutations>5, virtual phenotype, baseline viral load >50,000 copies/mL, and virtual IQ <0.5, but not genotypic IQ, were the variables independently associated with non-VR.

CONCLUSION

In heavily pretreated patients, the use of SQV virtual IQ or alternatively virtual phenotype, as well as PGMs, is a useful tool for the prediction of virological response.

The effects of ritonavir and lopinavir/ritonavir on the pharmacokinetics of a novel CCR5 antagonist, aplaviroc, in healthy subjects

AIMS

This study assessed the effects of the CYP3A inhibitors lopinavir/ritonavir (LPV/r) on the steady-state pharmacokinetics (PK) of aplaviroc (APL), a CYP3A4 substrate, in healthy subjects.

METHODS

In Part 1, APL PK was determined in eight subjects who received a single oral 50-mg APL test dose with/without a single dose of 100 mg ritonavir (RTV). Part 2 was conducted as an open-label, single-sequence, three-period repeat dose study in a cohort of 24 subjects. Subjects received APL 400 mg every 12 h (b.i.d.) for 7 days (Period 1), LPV/r 400/100 mg b.i.d. for 14 days (Period 2) and APL 400 mg + LPV/r 400/100 mg b.i.d. for 7 days (Period 3). All doses were administered with a moderate fat meal. PK sampling occurred on day 7 of Periods 1 and 3 and day 14 of Period 2.

RESULTS

In Part 1, a single RTV dose increased the APL AUC(0-infinity) by 2.1-fold [90% confidence interval (CI) 1.9, 2.4]. Repeat dose coadministration of APL with LPV/r increased APL exposures to a greater extent with the geometric least squares mean ratios (90% CI) being 7.7 (6.4, 9.3), 6.2 (4.8, 8.1) and 7.1 (5.6, 9.0) for the APL AUC, C(max), and C(min), respectively. No change in LPV AUC or C(max) and a small increase in RTV AUC and C(max) (28% and 32%) were observed. The combination of APL and LPV/r was well tolerated and adverse events were mild in severity with self-limiting gastrointestinal complaints most commonly reported.

CONCLUSIONS

Coadministration of APL and LPV/r was well tolerated and resulted in significantly increased APL plasma concentrations.

Clinical and pharmacokinetic data support once-daily low-dose boosted saquinavir (1,200 milligrams saquinavir with 100 milligrams ritonavir) in treatment-naive or limited protease inhibitor-experienced human immunodeficiency virus-infected patients

We evaluated the plasma and intracellular pharmacokinetics, clinical efficacy, and safety of once-daily low-dose boosted saquinavir (SQVr; 1,200 of saquinavir [SQV] with 100 mg of ritonavir) plus two nucleotide reverse transcriptase inhibitors in treatment-naive or limited protease inhibitor (PI)-experienced human immunodeficiency virus (HIV)-infected patients. A prospective study without entry restrictions on the plasma HIV-RNA (VL) or CD4 cell count was carried out. Plasma and intracellular SQV levels were measured by high-performance liquid chromatography. Efficacy was evaluated by an intention-to-treat analysis; treatment failure was defined as virological failure (a VL of >50 copies/ml after 24 weeks or a confirmed rebound to >50 copies/ml) or interruption for any reason. A total of 151 patients were included in the study (106 of them either had never received PI or had no previous virological failure on PIs) and could be characterized as follows: previous C3 stage, 28.9%; injection-drug users, 69.1%; subjects with chronic viral hepatitis, 53%; and subjects with cirrhosis, 10%. The median baseline CD4 level was 184/mul, and the median VL was 4.8 log(10) copies/ml. Median C(max), area under the concentration-time curve from 0 to 24 h, and C(min) plasma and intracellular SQV levels were 3,672 and 10,105 ng/ml, 34,283 and 99,535 ng.h/ml, and 359 and 1,062 ng/ml, respectively. The efficacy as determined by intention to treat at 52 weeks was 69.7% (96% in the on-treatment analysis), with similar results regardless of the baseline VL and CD4 counts. Only five patients had virological failure despite adequate C(min) levels, but with a poor adherence (the only variable related to virological failure). Adverse events caused the withdrawal of the treatment in four patients (2.6%). In conclusion, given the pharmacokinetic profile, efficacy, and tolerability of this regimen, once-daily low-dose SQVr may be considered a treatment option in treatment-naive or limited PI-experienced HIV-infected patients, with the additional benefit of being currently the least-expensive PI-based regimen available.

Clinical Validation of Saquinavir/Ritonavir Genotypic Resistance Score in Protease-Inhibitor-Experienced Patients

Objective

To identify a genotypic score for resistance to saquinavir boosted with ritonavir (SQV/r; 1,000/100 mg twice daily)-based regimens in protease inhibitor (PI)-experienced patients.

Methods

One-hundred and fifty-one PI-experienced patients receiving a SQV/r-containing regimen were enrolled retrospectively. The virological response (VR) was defined as the decrease in HIV RNA at months 3–5. The effect of each mutation in the protease gene on the VR to SQV/r regimen was assessed using non-parametric univariate analyses and then a step-by-step analysis was carried out using a Jonckheere-Tepstra (JT) non-parametric test to retain the group of mutations most strongly associated with VR.

Results

Among the 138 patients with detectable plasma SQV, the median VR was -1.48 [range: -4 to +1.2] log10 copies/ml. Changes at 12 codons were associated with a reduced VR to SQV/r: codons 10, 15, 20, 24, 46, 54, 62, 71, 73, 82, 84 and 90. The JT procedure led to selection of the following genotypic score, 10+15+20+ 24+62+73+82+84+90, as providing the strongest association with VR. In the 35 patients with none of the mutations in this score, the median decrease in HIV RNA was -2.24 log10 copies/ml and it was -1.88 ( n=29), -1.43 ( n=24), -0.52 ( n=30), -0.18 ( n=9), -0.11 ( n=6) and -0.30 ( n=5) log10 copies/ml in those with 1, 2, 3, 4, 5 and 6 mutations, respectively.

Conclusion

With this resistance score to SQV/r, the isolates were classified as having no evidence of resistance (0–2), possible resistance (3) or resistance (≥4) by grouping the number of mutations in samples for which the viral load reduction was similar.

Indinavir protein-free concentrations when used in indinavir/ritonavir combination therapy

OBJECTIVE

To describe the in vivo protein-binding characteristics of indinavir (IDV) in the presence of ritonavir (RTV) relative to total IDV plasma concentrations.

DESIGN

The ACTG protocol 5055 was a multicenter study comparing the safety and pharmacokinetics of IDV/RTV at doses of 800/200 and 400/400 mg twice daily in HIV-infected adults.

METHODS

Forty-four patients underwent a 12-h intensive pharmacokinetic assessment after 2 weeks of therapy. Three plasma samples from 35 patients at Cmax, 6 and 12 h post dose were used to determine the unbound IDV concentrations. Unbound IDV was separated in plasma samples using ultra-filtration and measured using high-performance liquid chromatography with UV detection.

RESULTS

Mean IDV protein-bound fraction across all time points in the 800/200 and 400/400 arm were 53.4 and 51.8%, respectively. In the 800/200 arm, percentage binding at Cmax was 50% compared with 56% at 12 h (P = 0.008). In the 400/400 arm, percentage binding at Cmax was 49% compared with 54% at 12 h (P = 0.008).

CONCLUSIONS

The extent of plasma protein binding of IDV in this study was less than in previously published data with IDV alone. Although IDV concentrations differed across the arms, the percentage of IDV protein binding at all time points was not different between the 800/200 and 400/400 arms. However, the percentage of IDV protein binding at Cmax was significantly lower compared with 12 h in each arm, possibly suggesting that IDV protein binding is concentration-dependent. These data suggest that RTV affects IDV protein-binding characteristics and IDV also exhibits concentration dependent binding when administered with RTV.

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)). Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs. Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.

Development and validation of a population pharmacokinetic model for ritonavir used as a booster or as an antiviral agent in HIV-1-infected patients

AIMS

The aim of this study was to develop and validate a population pharmacokinetic model of ritonavir, used as an antiviral agent or as a booster, in a large patient population and to identify factors influencing its pharmacokinetics.

METHODS

Ambulatory HIV-1-infected patients from the outpatient clinic of the Slotervaart Hospital, Amsterdam, the Netherlands, who were being treated with a ritonavir-containing regimen were included. During regular visits, blood samples were collected for the determination of ritonavir plasma concentrations and several clinical chemistry parameters. Furthermore, complete pharmacokinetic curves were available in some patients. Single and multiple compartment models with zero-order and first-order absorption, with and without absorption lag-time, with linear and nonlinear elimination were tested, using nonlinear mixed effect modelling (NONMEM). Pharmacokinetic parameters and interindividual, interoccasion and residual variability were estimated. In addition, the influence of several factors (e.g. patient characteristics, comedication) on the pharmacokinetics of ritonavir was explored.

RESULTS

From 186 patients 505 ritonavir plasma concentrations at a single time-point and 55 full pharmacokinetic profiles were available, resulting in a database of 1228 plasma ritonavir concentrations. In total 62% of the patients used ritonavir as a booster of their protease inhibitor containing antiretroviral regimen. First order absorption in combination with one-compartment disposition best described the pharmacokinetics of ritonavir. Clearance, volume of distribution and absorption rate constant were 10.5 l h(-1) (95% prediction interval (95% PI) 9.38-11.7), 96.6 l (95% PI 67.2-121) and 0.871 h(-1) (95% PI 0.429-1.47), respectively, with 38.3%, 80.0% and 169% interindividual variability, respectively. The interoccasion variability in the apparent bioavailability was 59.1%. The concomitant use of lopinavir resulted in a 2.7-fold increase in the clearance of ritonavir (P value < 0.001). No patients characteristics influenced the pharmacokinetics of ritonavir.

CONCLUSIONS

The pharmacokinetic parameters of ritonavir were adequately described by our population pharmacokinetic model. Concomitant use of the protease inhibitor lopinavir strongly influenced the pharmacokinetics of ritonavir. The model has been validated and can be used for further investigation of the interaction between ritonavir and other protease inhibitors.

Drug interactions in the management of HIV infection

The availability of antiretroviral therapy has significantly reduced the morbidity and mortality of HIV infection. In addition, improved treatment of opportunistic infections and comorbidities common to patients with HIV is further prolonging the lives of patients. Improvement in the treatment of HIV has led to a significant increase in the number of medications which caregivers are able to utilise to manage HIV/AIDS. Antiretroviral medications, as well as many of the drugs used in the management of opportunistic infections and primary care (e.g., macrolide antibiotics, azole antifungals, cholesterol-lowering medications), are particularly prone to drug interactions. The interpretation of clinically significant interactions is complicated by the rate at which new information on drug metabolism and transport is becoming available. Management of drug interactions in HIV is further confounded by conflicting study results and differences between documented and theoretical inter-actions. The mechanisms and significance of interactions involving antiretrovirals, drugs used for opportunistic infections, and other medications commonly used in HIV patients will be reviewed.

Metabolism and disposition of the HIV-1 protease inhibitor lopinavir (ABT-378) given in combination with ritonavir in rats, dogs, and humans

PURPOSE

The objective of this study was to examine the metabolism and disposition of the HIV protease inhibitor lopinavir in humans and animal models.

METHODS

The plasma protein binding of [14C]lopinavir was examined in vitro via equilibrium dialysis technique. The tissue distribution of radioactivity was examined in rats dosed with [14C]lopinavir in combination with ritonavir. The metabolism and disposition of [14C]lopinavir was examined in rats, dogs, and humans given alone (in rats only) or in combination with ritonavir.

RESULTS

The plasma protein binding of lopinavir was high in all species (97.4-99.7% in human plasma), with a concentration-dependent decrease in binding. Radioactivity was extensively distributed into tissues, except brain, in rats. On oral dosing to rats, ritonavir was found to increase the exposure of lopinavir-derived radioactivity 13-fold. Radioactivity was primarily cleared via the hepato-biliary route in all species (>82% of radioactive dose excreted via fecal route), with urinary route of elimination being significant only in humans (10.4% of radioactive dose). Oxidative metabolites were the predominant components of excreted radioactivity. The predominant site of metabolism was found to be the carbon-4 of the cyclic urea moiety, with subsequent secondary metabolism occurring on the diphenyl core moiety. In all the three species examined, the primary component of plasma radioactivity was unchanged lopinavir (>88%) with small amounts of oxidative metabolites.

CONCLUSIONS

Lopinavir was subject to extensive metabolism in vivo. Co-administered ritonavir markedly enhanced the pharmacokinetics of lopinavir-derived radioactivity in rats, probably due to inhibition of presystemic and systemic metabolism, leading to an increased exposure to this potent HIV protease inhibitor.

Clinically relevant interpretation of genotype and relationship to plasma drug concentrations for resistance to saquinavir-ritonavir in human immunodeficiency virus type 1 protease inhibitor-experienced patients

It has been shown that virological protease inhibitor (PI) resistance mutations present at the initiation of saquinavir (SQV) plus ritonavir (RTV) therapy in PI-experienced patients are the strongest predictors of virological response. But most of the current resistance algorithms are adapted for unboosted SQV regimens. We applied a stepwise methodology for the development and validation of a clinically relevant genotypic resistance score for an SQV (800 mg twice per day [b.i.d.]) plus RTV (100 mg b.i.d.)-containing regimen. PI-experienced patients treated by this regimen achieved a human immunodeficiency virus plasma viral load (VL) of <200 copies/ml at months 3 to 5 for 41.7% of subjects. Adjusted in a multivariate analysis, taking into account all the confounding factors, such as the nucleoside used, five mutations were combined in a resistance score associated with a reduced virological response to an SQV-plus-RTV regimen: L24I, I62V, V82A/F/T/S, I84V, and L90IM. Patients with isolates harboring 0 to 1 mutation among the score achieved -2.20 log10 and -1.23 log10 copies/ml of VL reduction, respectively, while it was -0.27 log10 copies/ml for those with at least two mutations, classifying the isolates as "no evidence of resistance" (0 or 1 mutation) or "resistance " (> or =2 mutations). The minimum concentration in plasma (Cmin) of SQV alone was not associated with the virological response. However, the combination of the SQV Cmin and the genotypic score, expressed as the genotypic inhibitory quotient, was predictive of the virological response, suggesting that the interpretation of SQV concentrations in plasma should be done only in the context of the resistance index provided by viral genotype for PI-experienced patients.

Steady-State Pharmacokinetics of Saquinavir Hard-Gel/Ritonavir/Fosamprenavir in HIV-1???Infected Patients

BACKGROUND

In vitro synergy and complementary resistance profiles provide a strong rationale for combining fosamprenavir with saquinavir as part of a potent double-boosted protease inhibitor regimen. This study evaluated the steady-state pharmacokinetics of saquinavir 1000 mg twice daily (bid) and fosamprenavir 700 mg bid administered with 2 different doses of ritonavir (100 and 200 mg bid) in HIV-1-infected subjects.

METHODS

On day 1, 12-hour pharmacokinetic profiles for saquinavir/ritonavir (1000/100 mg bid) were obtained for 18 subjects. All subjects were receiving ongoing treatment with a saquinavir/ritonavir-containing regimen. Fosamprenavir 700 mg bid was then added to the regimen, and pharmacokinetic sampling was repeated for all 3 agents at day 11. The ritonavir daily dose was then increased to 200 mg bid, and a 3rd pharmacokinetic profile was obtained at day 22.

RESULTS

The coadministration of fosamprenavir 700 mg bid with saquinavir/ritonavir 1000/100 mg bid resulted in a statistically nonsignificant decrease in saquinavir concentrations (by 14, 9, and 24%, for saquinavir area under the concentration-time curve [AUC]0-12, C(max), and C(trough), respectively). This was compensated for by an increased ritonavir dose of 200 mg bid, which resulted in a statistically nonsignificant increase in saquinavir exposure compared with baseline. Amprenavir levels did not appear to be significantly influenced by coadministration of saquinavir with fosamprenavir. Fosamprenavir significantly reduced ritonavir exposure, but the increased ritonavir dose compensated for this interaction.

CONCLUSIONS

Our findings showed that saquinavir/ritonavir/fosamprenavir was well tolerated over the study period. Saquinavir plasma concentrations were slightly lowered by the addition of fosamprenavir to the regimen. However, the addition of a further 100 mg ritonavir bid restored the small and insignificant decrease.

Pharmacokinetic enhancement of protease inhibitor therapy

Combination antiretroviral therapy with two or more protease inhibitors has become the standard of care in the treatment of HIV infection. Dual protein inhibitor (PI) regimens, such as lopinavir/ritonavir, are commonly used as initial PI therapy. As viral resistance increases and the development of mechanistically novel protease inhibitors decreases, clinicians turn to ritonavir-enhanced dual PI therapy to treat salvage patients. Potency of these combination regimens is increased while pill burden, food restrictions and often, side effects are decreased. These clinical advantages result from the enhancement of their pharmacological properties, including alterations in the absorption and metabolism process. Alterations in the absorption and metabolism of protease inhibitors when co-administered with a cytochrome P450 (CYP) enzyme inhibitor, such as low dose ritonavir, are reflected by impressive changes in pharmacokinetic parameters. For example, the addition of ritonavir 100 or 200 mg to saquinavir 1200-1800 mg has been shown to increase saquinavir area under the concentration-time curve (AUC) by approximately 300-800% compared with saquinavir alone. The ability of ritonavir to increase plasma trough concentrations (C(min)) of concomitantly administered PIs is perhaps the greatest clinical benefit of dual or ritonavir-enhanced dual PI therapy since inadequate concentrations of antiretrovirals may support long term antiretroviral resistance. For example, lopinavir 400mg alone in healthy volunteers produced plasma concentrations that briefly exceeded the concentration required to inhibit 50% of viral replication (IC(50)). Yet, when low doses of ritonavir were added, C(min) values were 50- to 100-fold greater than the concentration required to produce 50% of the maximum effect for wild-type HIV (EC(50)). The following manuscript will discuss the rationale for combining protease inhibitors and will review pertinent pharmacokinetic and clinical data on these combination regimens.

Pharmacokinetics of Once-Daily Saquinavir/Ritonavir in HIV-Infected Subjects: Comparison with the Standard Twice-Daily Regimen

Objective

To evaluate the steady-state pharmacokinetics and safety of two once-daily saquinavir/ritonavir (SQV/RTV) regimens, 1600/100 and 2000/100 mg, in HIV-positive patients.

Methods

Eighteen HIV-infected adults treated with the standard twice-daily SQV/RTV 1000/100 mg regimen were enrolled in this open-label, two-phase, crossover pharmacokinetic study. The steady-state pharmacokinetics of SQV administered with 100 mg RTV were investigated following once-daily doses of 1600 mg or 2000 mg or a twice-daily dose of 1000 mg. Plasma drug concentrations were determined by high performance liquid chromatography–tandem mass spectrometry and pharmacokinetic parameters were calculated using a non-compartmental model.

Results

Compared with SQV 1000 mg twice daily, the Cmax of SQV following a 1600 mg and 2000 mg dose increased in a dose-proportional manner [geometric mean (95% CI) 1915 (1656–2850) ng/ml for 1000 mg, 2782 (2249–4330) ng/ml for 1600 mg and 4179 (3429–6105) ng/ml for 2000 mg doses, respectively]. SQV Ctrough values were 539 (453–1011), 106 (76–223) and 231 (75–822) ng/ml, respectively. A SQV Ctrough value greater than 100 ng/ml was achieved in all subjects on the twice-daily regimen, in 9/18 (50%) subjects on the 1600/100 mg once-daily regimen, and in 14/17 (82%) subjects on the 2000/100 mg once-daily regimen. The once-daily regimens were well tolerated, with mild-to-moderate gastrointestinal symptoms being the only events reported by a small number of patients.

Conclusion

This is the first study to evaluate the pharmacokinetics of once-daily SQV/RTV 2000/100 mg in HIV-infected subjects. Our findings suggest that this regimen may be an alternative to twice-daily 1000/100 mg doses and should be further evaluated in efficacy studies. The data indicate that most patients (14/17) on once-daily 2000/100 mg achieve trough concentrations above target values (determined for HIV wild-type) for efficacy of SQV with the use of just 100 mg RTV/day and with good tolerability.

Pharmacodynamics and clinical use of anti-HIV drugs

Antiretroviral drug exposure has been linked to both antiviral efficacy and the development of toxicity and further research in this area is ongoing and necessary. Use of these data may have important implications for TDM of HAART regimens in clinical practice. TDM, in conjunction with an assessment of the patient's viral resistance in the form of an IQ, needs to be examined and validated in large clinical trials.

Contributions of CYP3A4, P-glycoprotein, and Serum Protein Binding to the Intestinal First-Pass Extraction of Saquinavir

Using CYP3A4-expressing Caco-2 cell monolayers, we assessed the roles of CYP3A4-mediated metabolism, P-glycoprotein (P-gp)-mediated efflux, and serum protein binding in determining the extent of the intestinal first-pass extraction (E(i)) of saquinavir. Saquinavir (5-40 microM) was added to the apical compartment of culture inserts. After 3 h, apical and basolateral media and cell scrapings were analyzed for saquinavir and a major CYP3A4-mediated metabolite (M7). The intracellular concentration of saquinavir was estimated from the degree of inhibition of CYP3A4 catalytic activity (midazolam 1'-hydroxylation). Compared with vehicle, the P-gp inhibitor LY335979 (zosuquidar trihydrochloride) (0.5 microM, apical) increased saquinavir cell content and M7 formation rate, but decreased the E(i) by approximately 50% due to a >90% increase in the amount of saquinavir recovered in the basolateral compartment. Compared with LY335779, physiological concentrations of basolateral serum proteins [human serum albumin and alpha1-acid glycoprotein (AAG)] increased saquinavir permeability by a similar degree but decreased the E(i) by approximately 50% due to a marked reduction in M7 formation. Increasing AAG concentration (1.0-2.5 g/l) had no additional effect on permeability or E(i). An estimate of the range of the E(i) of saquinavir (7-60%) was less than has been predicted based on in vitro data (>99%) but was consistent with a clinical study involving grapefruit juice. The incidental finding of greater M7 formation after basolateral compared with apical dosing could not be explained by differences in saquinavir cell content. We conclude that variable intestinal first-pass extraction of saquinavir in human immunodeficiency virus-infected patients could reflect variation in P-gp-mediated efflux and/or CYP3A4-catalyzed metabolism, but not in blood AAG levels.

Inhibitory effect of stiripentol on carbamazepine and saquinavir metabolism in human

AIMS

To characterize the in vitro and in vivo inhibitory effect of stiripentol, a new anticonvulsant, on the metabolism of carbamazepine and saquinavir, which are substrates of CYP3A4.

METHODS

Human liver microsomes and cDNA-expressed CYP enzymes were used for the in vitro experiments. Pharmacokinetic data from epileptic children and healthy adults were used for the carbamazepine and saquinavir in vivo studies, respectively.

RESULTS

Carbamazepine biotransformation to its 10,11-epoxide by human liver microsomes (Vmax = 10.3 nmol min(-1) nmol(-1) P450, apparent Km = 362 microm), cDNA-expressed CYP3A4 (Vmax = 1.17 nmol min(-1) nmol(-1) P450, apparent Km = 119 microm) and CYP2C8 (Vmax = 0.669 nmol min(-1) nmol(-1) P450, apparent Km = 757 microm) was inhibited by stiripentol (IC50 14, 5.1, 37 microM and apparent Ki 3.7, 2.5, 35 microm, respectively). Saquinavir biotransformation to its major metabolite M7 by human liver microsomes (Vmax = 5.7 nmol min(-1) nmol(-1) P450, apparent Km = 0.79 microm) was inhibited by stiripentol (IC50 163 microM, apparent Ki 86 microm). In epileptic children treated with carbamazepine and stiripentol, the plasma concentration ratio of carbamazepine epoxide/carbamazepine was decreased by 65%. The in vivo apparent Ki for stiripentol ranged from 10.5 to 41.4 microm. The pharmacokinetics of saquinavir was not modified by stiripentol in healthy adults. The 95% confidence intervals for the difference for Cmax and AUC of saquinavir between the placebo and stiripentol phase were (-39.8, 39.8) and (-33.2, 112), respectively.

CONCLUSIONS

These results showed that stiripentol was a weak inhibitor of saquinavir metabolism both in vitro and in vivo. In contrast, stiripentol is a potent inhibitor of carbamazepine 10,11-epoxide formation in vitro and in vivo in epileptic patients.