Effects of rifabutin and rifampicin on the pharmacokinetics of ethinylestradiol and norethindrone

U.S. Medical Eligibility Criteria for Contraceptive Use, 2024

The 2024 U.S. Medical Eligibility Criteria for Contraceptive Use (U.S. MEC) comprises recommendations for the use of specific contraceptive methods by persons who have certain characteristics or medical conditions. These recommendations for health care providers were updated by CDC after review of the scientific evidence and a meeting with national experts in Atlanta, Georgia, during January 25-27, 2023. The information in this report replaces the 2016 U.S. MEC (CDC. U.S. Medical Eligibility Criteria for Contraceptive Use, 2016. MMWR 2016:65[No. RR-3]:1-103). Notable updates include 1) the addition of recommendations for persons with chronic kidney disease; 2) revisions to the recommendations for persons with certain characteristics or medical conditions (i.e., breastfeeding, postpartum, postabortion, obesity, surgery, deep venous thrombosis or pulmonary embolism with or without anticoagulant therapy, thrombophilia, superficial venous thrombosis, valvular heart disease, peripartum cardiomyopathy, systemic lupus erythematosus, high risk for HIV infection, cirrhosis, liver tumor, sickle cell disease, solid organ transplantation, and drug interactions with antiretrovirals used for prevention or treatment of HIV infection); and 3) inclusion of new contraceptive methods, including new doses or formulations of combined oral contraceptives, contraceptive patches, vaginal rings, progestin-only pills, levonorgestrel intrauterine devices, and vaginal pH modulator. The recommendations in this report are intended to serve as a source of evidence-based clinical practice guidance for health care providers. The goals of these recommendations are to remove unnecessary medical barriers to accessing and using contraception and to support the provision of person-centered contraceptive counseling and services in a noncoercive manner. Health care providers should always consider the individual clinical circumstances of each person seeking contraceptive services. This report is not intended to be a substitute for professional medical advice for individual patients; when needed, patients should seek advice from their health care providers about contraceptive use.

Lack of CYP3A4 protein induction despite mRNA induction in primary hepatocytes exposed to rifabutin as a possible explanation for its low interaction risk in vivo

Rifampicin is a strong inducer of cytochrome P450 (CYP3A4) and P-glycoprotein (P-gp/ABCB1), leading to profound drug-drug interactions. In contrast, the chemically related rifabutin does not show such pronounced induction properties in vivo. The aim of our study was to conduct a comprehensive analysis of the different induction potentials of rifampicin and rifabutin in primary human hepatocytes and to analyze the mechanism of potential differences. Therefore, we evaluated CYP3A4/ABCB1 mRNA expression (polymerase chain reaction), CYP3A4/P-gp protein expression (immunoaffinity-liquid chromatography-mass spectrometry, IA-LC-MS/MS), CYP3A4 activity (testosterone hydroxylation), and considered intracellular drug uptake after treatment with increasing rifamycin concentrations (0.01-10 µM). Furthermore, rifamycin effects on the protein levels of CYP2C8, CYP2C9, and CYP2C19 were analyzed (IA-LC-MS/MS). Mechanistic analysis included the evaluation of possible suicide CYP3A4 inhibition (IC50 shift assay) and drug impact on translational efficiency (cell-free luminescence assays). Rifabutin accumulated 6- to 15-fold higher in hepatocytes than rifampicin, but induced CYP3A4 mRNA comparably to rifampicin (e. g. rifampicin 61-fold vs. rifabutin 44-fold, 72 h). While rifampicin for example enhanced protein (10 µM: 21-fold) and activity levels considerably (53-fold), rifabutin only slightly increased CYP3A4 protein expression (10 µM: 3.3-fold) or activity (11-fold) compared to rifampicin after 72 h. Both rifamycins similarly influenced expression of other eliminating proteins. A potential CYP3A4 suicide inhibition by a specific rifabutin metabolite or disruption of ribosome function were excluded experimentally. In conclusion, the lack of protein enhancement, could explain rifabutin's weaker induction-related drug-drug interaction risk in vivo.

Physiologically-based pharmacokinetic modeling of prominent oral contraceptive agents and applications in drug-drug interactions

Considerable interest remains across the pharmaceutical industry and regulatory landscape in capabilities to model oral contraceptives (OCs), whether combined (COCs) with ethinyl estradiol (EE) or progestin-only pill. Acceptance of COC drug-drug interaction (DDI) assessment using physiologically-based pharmacokinetic (PBPK) is often limited to the estrogen component (EE), requiring further verification, with extrapolation from EE to progestins discouraged. There is a paucity of published progestin component PBPK models to support the regulatory DDI guidance for industry to evaluate a new chemical entity's (NCE's) DDI potential with COCs. Guidance recommends a clinical interaction study to be considered if an investigational drug is a weak or moderate inducer, or a moderate/strong inhibitor, of CYP3A4. Therefore, availability of validated OC PBPK models within one software platform, will be useful in predicting the DDI potential with NCEs earlier in the clinical development. Thus, this work was focused on developing and validating PBPK models for progestins, DNG, DRSP, LNG, and NET, within Simcyp, and assessing the DDI potential with known CYP3A4 inhibitors (e.g., ketoconazole) and inducers (e.g., rifampicin) with published clinical data. In addition, this work demonstrated confidence in the Simcyp EE model for regulatory and clinical applications by extensive verification in 70+ clinical PK and CYP3A4 interaction studies. The results provide greater capability to prospectively model clinical CYP3A4 DDI with COCs using Simcyp PBPK to interrogate the regulatory decision-tree to contextualize the potential interaction by known perpetrators and NCEs, enabling model-informed decision making, clinical study designs, and delivering potential alternative COC options for women of childbearing potential.

Predicted Pharmacokinetic Interactions Between Hormonal Contraception and Single or Intermittently Dosed Rifampicin

The scale-up of rifampicin-based prevention regimens is an essential part of the global leprosy strategy. Daily rifampicin may reduce the effectiveness of the oral contraceptive pill (OCP), but little is known about the effects of rifampicin at the less frequent dosing intervals used for leprosy prophylaxis. As many women of reproductive age rely on OCP for family planning, evaluating the interaction with less-than-daily rifampicin regimens would enhance the scalability and acceptability of leprosy prophylaxis. Using a semi-mechanistic pharmacokinetic model of rifampicin induction, we simulated predicted changes in OCP clearance when coadministered with varying rifampicin dosing schedules. Rifampicin given as a single dose (600 or 1200 mg) or 600 mg every 4 weeks was not predicted to result in a clinically relevant interaction with OCP, defined as a >25% increase in clearance. Simulations of daily rifampicin were predicted to increase OCP clearance within the range of observed changes previously reported in the literature. Therefore, our findings suggest that OCP efficacy will be maintained when coadministered with rifampicin-based leprosy prophylaxis regimens of 600 mg once, 1200 mg once, and 600 mg every 4 weeks. This work provides reassurance to stakeholders that leprosy prophylaxis can be used with OCP without any additional recommendations for contraception prevention.

The effect of rifampin on serum etonogestrel concentrations and biomarkers of ovulation among contraceptive implant users: A pharmacokinetic and pharmacodynamic study

OBJECTIVES

Rifampin, a strong CYP3A inducer, is the gold standard for evaluating CYP3A-mediated drug-drug interactions. We aimed to evaluate the pharmacokinetic and pharmacodynamic effects of a short course (2 weeks) of rifampin on serum etonogestrel (ENG) concentrations and serologic measures of ovarian activity (endogenous estradiol [E2] and progesterone [P4]) among ENG implant users.

STUDY DESIGN

We enrolled healthy females using ENG implants for 12 to 36 months. We measured baseline serum ENG concentrations using a validated liquid chromatography mass-spectrometry assay and baseline E2 and P4 concentrations using chemiluminescent immunoassays. After 2 weeks of rifampin 600 mg daily, we repeated ENG, E2, and P4 measurements. We compared pre- and post-rifampin serum measurements using paired Wilcoxon signed-rank tests.

RESULTS

Fifteen participants completed all study procedures. Participants' median age was 28.2 years (range 21.8-34.1), median body-mass index was 25.2 kg/m2 (range 18.9-37.3), and median duration of implant use was 22 months (range 12-32). All participants experienced significant decreases from baseline ENG concentrations (median 164.0 pg/mL [range 94.4-265.0]) to post-rifampin measurements (median 47.8 pg/mL [range 24.7-82.8]) (p < 0.001). Serum E2 concentrations also significantly increased with rifampin exposure (median 73 pg/mL vs 202 pg/mL, p = 0.003); increases in serum P4 concentrations were not statistically significant (p = 0.19). Three participants (20%) experienced increased luteal activity, with one presumptively ovulating post-rifampin (P4 = 15.8 ng/mL).

CONCLUSIONS

With only short exposure to a strong CYP3A inducer, ENG implant users experienced clinically significant decreases in serum ENG concentrations that led to changes in biomarkers indicative of waning suppression of ovulation.

IMPLICATIONS

Even a short, 2-week course of treatment with rifampin places etonogestrel contraceptive implant users at risk for decreased contraceptive efficacy. Clinicians should counsel patients using etonogestrel implants considering any duration of rifampin therapy on the need for backup nonhormonal contraception or the use of an intrauterine device to avoid unintended pregnancies.

Evidence for Implementation: Management of TB in HIV and Pregnancy

PURPOSE OF REVIEW

Pregnant people living with HIV (PLWH) are at especially high risk for progression from latent tuberculosis infection (LTBI) to active tuberculosis (TB) disease. Among pregnant PLWH, concurrent TB increases the risk of complications such as preeclampsia, intrauterine fetal-growth restriction, low birth weight, preterm-delivery, perinatal transmission of HIV, and admission to the neonatal intensive care unit. The grave impact of superimposed TB disease on maternal morbidity and mortality among PLWH necessitates clear guidelines for concomitant therapy and an understanding of the pharmacokinetics (PK) and potential drug-drug interactions (DDIs) between antitubercular (anti-TB) agents and antiretroviral therapy (ART) in pregnancy.

RECENT FINDINGS

This review discusses the currently available evidence on the use of anti-TB agents in pregnant PLWH on ART. Pharmacokinetic and safety studies of anti-TB agents during pregnancy and postpartum are limited, and available data on second-line and newer anti-TB agents used in pregnancy suggest that several research gaps exist. DDIs between ART and anti-TB agents can decrease plasma concentration of ART, with the potential for perinatal transmission of HIV. Current recommendations for the treatment of LTBI, drug-susceptible TB, and multidrug-resistant TB (MDR-TB) are derived from observational studies and case reports in pregnant PLWH. While the use of isoniazid, rifamycins, and ethambutol in pregnancy and their DDIs with various ARTs are well-characterized, there is limited data on the use of pyrazinamide and several new and second-line antitubercular drugs in pregnant PLWH. Further research into treatment outcomes, PK, and safety data for anti-TB agent use during pregnancy and postpartum is urgently needed.

Eligibility criteria for Menopausal Hormone Therapy (MHT): a position statement from a consortium of scientific societies for the use of MHT in women with medical conditions. MHT Eligibility Criteria Group

This project aims to develop eligibility criteria for menopausal hormone therapy (MHT). The tool should be similar to those already established for contraception A consortium of scientific societies coordinated by the Spanish Menopause Society met to formulate recommendations for the use of MHT by women with medical conditions based on the best available evidence. The project was developed in two phases. As a first step, we conducted 14 systematic reviews and 32 metanalyses on the safety of MHT (in nine areas: age, time of menopause onset, treatment duration, women with thrombotic risk, women with a personal history of cardiovascular disease, women with metabolic syndrome, women with gastrointestinal diseases, survivors of breast cancer or of other cancers, and women who smoke) and on the most relevant pharmacological interactions with MHT. These systematic reviews and metanalyses helped inform a structured process in which a panel of experts defined the eligibility criteria according to a specific framework, which facilitated the discussion and development process. To unify the proposal, the following eligibility criteria have been defined in accordance with the WHO international nomenclature for the different alternatives for MHT (category 1, no restriction on the use of MHT; category 2, the benefits outweigh the risks; category 3, the risks generally outweigh the benefits; category 4, MHT should not be used). Quality was classified as high, moderate, low or very low, based on several factors (including risk of bias, inaccuracy, inconsistency, lack of directionality and publication bias). When no direct evidence was identified, but plausibility, clinical experience or indirect evidence were available, "Expert opinion" was categorized. For the first time, a set of eligibility criteria, based on clinical evidence and developed according to the most rigorous methodological tools, has been defined. This will provide health professionals with a powerful decision-making tool that can be used to manage menopausal symptoms.

Characterization of JNJ-2482272 [4-(4-Methyl-2-(4-(Trifluoromethyl)Phenyl)Thiazole-5-yl) Pyrimidine-2-Amine] As a Strong Aryl Hydrocarbon Receptor Activator in Rat and Human

[4-(4-Methyl-2-(4-(trifluoromethyl)phenyl)thiazole-5-yl)pyrimidine-2-amine] (JNJ-2482272), under investigation as an anti-inflammatory agent, was orally administered to rats once daily at 60 mg/kg for 6 consecutive days. Despite high plasma exposure after single administration (C_max of 7.1 μM), JNJ-2482272 had plasma concentrations beneath the lower limit of quantification (3 ng/ml) after 6 consecutive days of dosing. To determine if JNJ-2482272 is an autoinducer in rats, plated rat hepatocytes were treated with JNJ-2482272 for 2 days. The major hydroxylated metabolites of JNJ-2482272 were isolated and characterized by mass spectrometry and NMR analyses. Compared with the vehicle-treated cells, a concentration-dependent increase was observed in the formation of phase I- and II-mediated metabolites coinciding with greater expression of cytochrome P450s (P450s) and UDP-glucuronosyltransferases (UGTs) in rat hepatocytes. CYP1A1, CYP1A2, CYP1B1, and UGT1A6 transcripts were predominantly induced, suggesting that JNJ-2482272 is an activator of the aryl hydrocarbon receptor (AhR). In a human AhR reporter assay, JNJ-2482272 demonstrated potent AhR activation with an EC₅₀ value of 0.768 nM, a potency more comparable to the strong AhR activator and toxin 2,3,7,8-tetrachloro-dibenzodioxin than to weaker AhR activators 3-methylcholanthrene, β-naphthoflavone, and omeprazole. In plated human hepatocytes, JNJ-2482272 induced CYP1A1 gene expression with an EC₅₀ of 20.4 nM and increased CYP1A activity >50-fold from basal levels. In human recombinant P450s, JNJ-2482272 was exclusively metabolized by the CYP1 family of enzymes and most rapidly by CYP1A1. The summation of these in vitro findings bridges the in vivo conclusion that JNJ-2482272 is a strong autoinducer in rats and potentially in humans through potent AhR activation. SIGNIFICANCE STATEMENT: Drugs that induce their own metabolism (autoinducers) can lack sustained exposures for pharmacology and safety assessment hindering their development. JNJ-2482272 is demonstrated herein as a strong aryl hydrocarbon receptor (AhR) activator and CYP1A autoinducer, explaining its near complete loss of exposure after repeat administration in rat, which is likely translatable to human (if progressed further) considering its nanomolar potency comparable to "classical" AhR ligands like 2,3,7,8-tetrachloro-dibenzo-dioxin despite bearing a "nonclassical" drug structure.

Drug-Drug Interaction Studies With Oral Contraceptives: Pharmacokinetic/Pharmacodynamic and Study Design Considerations

Oral contraceptives (OCs) are the most widely used form of birth control among women of childbearing potential. Knowledge of potential drug-drug interactions (DDIs) with OCs becomes imperative to provide information on the medication to women of childbearing potential and enable their inclusion in clinical trials, especially if the new molecular entity is a teratogen. Although a number of DDI guidance documents are available, they do not provide recommendations for the design and conduct of OC DDI studies. The evaluation of DDI potential of a new molecular entity and OCs is particularly challenging because of the availability of a wide variety of combinations of hormonal contraceptives, different doses of the ethinyl estradiol, and different metabolic profiles of the progestin component. The aim of this review is to comprehensively discuss factors to be considered such as pharmacokinetics (PK), pharmacodynamics (PD), choice of OC, and study population for the conduct of in vivo OC DDI studies. In this context, metabolic pathways of OCs, the effect of enzyme inhibitors and inducers, the role of sex hormone-binding globulin in the PK of progestins, current evidence on OC DDIs, and the interpretation of PD end points are reviewed. With the emergence of new tools like physiologically based PK modeling, the decision to conduct an in vivo study can be made with much more confidence. This review provides a comprehensive overview of various factors that need to be considered in designing OC DDI studies and recommends PK-based DDI studies with PK end points as adequate measures to establish clinical drug interaction and measurement of PD end points when there is basis for PD interaction.

The Effects of Weak and Strong CYP3A Induction by Rifampicin on the Pharmacokinetics of Five Progestins and Ethinylestradiol Compared to Midazolam

It is known that co‐administration of CYP3A inducers may decrease the effectiveness of oral contraceptives containing progestins as mono‐preparations or combined with ethinylestradiol. In a randomized clinical drug‐drug interaction study, we investigated the effects of CYP3A induction on the pharmacokinetics of commonly used progestins and ethinylestradiol. Rifampicin was used to induce CYP3A. The progestins chosen as victim drugs were levonorgestrel, norethindrone, desogestrel, and dienogest as mono‐products, and drospirenone combined with ethinylestradiol. Postmenopausal women (n = 12–14 per treatment group) received, in fixed sequence, a single dose of the victim drug plus midazolam without rifampicin, with rifampicin 10 mg/day (weak induction), and with rifampicin 600 mg/day (strong induction). The effects on progestin exposure were compared with the effects on midazolam exposure (as a benchmark). Unbound concentrations were evaluated for drugs binding to sex hormone binding globulin. Weak CYP3A induction, as confirmed by a mean decrease in midazolam exposure by 46%, resulted in minor changes in progestin exposure (mean decreases: 15–37%). Strong CYP3A induction, in contrast, resulted in mean decreases by 57–90% (mean decrease in midazolam exposure: 86%). Namely, the magnitude of the observed induction effects varied from weak to strong. Our data might provide an impetus to revisit the currently applied clinical recommendations for oral contraceptives, especially for levonorgestrel and norethindrone‐containing products, and they might give an indication as to which progestin could be used, if requested, by women taking weak CYP3A inducers—although it is acknowledged that the exact exposure‐response relationship for contraceptive efficacy is currently unclear for most progestins.

Quantitative evaluation of hepatic and intestinal induction of CYP3A in clinical practice

This is the first report quantitatively evaluating the clinical induction of CYP3A in the liver and the intestine.To evaluate hepatic induction, we collected literature data on endogenous biomarkers of hepatic CYP3A induction which we then used to calculate the fold-induction (inducer-mediated change in biomarker level). Literature data on decreases in the area under the curve (AUC) of alfentanil, a CYP3A substrate, caused by CYP3A inducers were also collected. We used the hepatic intrinsic clearance of alfentanil to calculate the hepatic induction ratio (inducer-mediated change in intrinsic clearance). For intestinal induction, the intestinal bioavailability (F_g) of alfentanil was used to calculate the intestinal induction ratio. We determined in vivo maximum induction (E_max) and the average unbound plasma concentration (C_av,u) required for half the maximum induction (EC₅₀) for inducers using an E_max model analysis.In our results, fold-induction was comparable to the induction ratio at several inducer concentrations, and almost the maximum induction was achieved by a therapeutic dose. Induction ratios in the intestine were similar to the liver.Our findings suggest that, by knowing only hepatic induction ratios for common inducers, we can quantitatively predict the decreases in the AUC of substrates by CYP3A induction.

ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions

Although numerous studies have addressed the safety and effectiveness of hormonal contraceptive use in healthy women, data regarding women with underlying medical conditions or other special circumstances are limited. The U.S. Medical Eligibility Criteria (USMEC) for Contraceptive Use, 2016 (), which has been endorsed by the American College of Obstetricians and Gynecologists, is a published guideline based on the best available evidence and expert opinion to help health care providers better care for women with chronic medical problems who need contraception. The goal of this Practice Bulletin is to explain how to use the USMEC rating system in clinical practice and to specifically discuss the rationale behind the ratings for various medical conditions. Contraception for women with human immunodeficiency virus (HIV) (); the use of emergency contraception in women with medical coexisting medical conditions, including obesity, (); and the effect of depot medroxyprogesterone acetate (DMPA) on bone health () are addressed in other documents from the American College of Obstetricians and Gynecologists.

Drug interactions between rifamycin antibiotics and hormonal contraception: a systematic review

BACKGROUND

Rifamycin antibiotics are commonly used for treatment of tuberculosis, but may reduce the effectiveness of hormonal contraception (HC).

OBJECTIVES

To determine whether interactions between rifamycins and HC result in decreased effectiveness or increased toxicity of either therapy.

SEARCH STRATEGY

We searched MEDLINE, Embase, Cochrane and clinicaltrials.gov through May 2017.

SELECTION CRITERIA

We included trials, cohort, and case-control studies addressing pregnancy rates, pharmacodynamics or pharmacokinetic (PK) outcomes when HC and rifamycins were administered together versus apart. Of 7291 original records identified, 11 met inclusion criteria after independent review by two authors.

DATA COLLECTION AND ANALYSIS

Two authors independently abstracted study details and assessed study quality using the United States Preventive Services Task Force grading system. Findings are reported descriptively.

MAIN RESULTS

Studies only addressed combined oral contraceptives (COCs) and none reported pregnancy rates. Quality ranged from good to poor. Rifampin increased the frequency of ovulation in two of four studies, and reduced estrogen and/or progestin exposure in five studies. Rifabutin led to smaller PK changes than rifampin in two studies. In one study each, rifaximin and rifalazil did not alter hormone PK.

CONCLUSIONS

No studies evaluated pregnancy risk or non-oral HCs. PK and ovulation outcomes support a clinically concerning drug interaction between COCs and rifampin, and to a lesser extent rifabutin. Data are limited for other rifamycins.

TWEETABLE ABSTRACT

Rifampin and rifabutin reduce systemic exposure of oral contraceptives, but no studies have evaluated pregnancy risk.

Risk-Benefit Assessment of Ethinylestradiol Using a Physiologically Based Pharmacokinetic Modeling Approach

Current formulations of combined oral contraceptives (COC) containing ethinylestradiol (EE) have ≤35 μg due to increased risks of cardiovascular diseases (CVD) with higher doses of EE. Low‐dose formulations however, have resulted in increased incidences of breakthrough bleeding and contraceptive failure, particularly when coadministered with inducers of cytochrome P450 enzymes (CYP). The developed physiologically based pharmacokinetic model quantitatively predicted the effect of CYP3A4 inhibition and induction on the pharmacokinetics of EE. The predicted C_max and AUC ratios when coadministered with voriconazole, fluconazole, rifampicin, and carbamazepine were within 1.25 of the observed data. Based on published clinical data, an AUC_ss value of 1,000 pg/ml.h was selected as the threshold for breakthrough bleeding. Prospective application of the model in simulations of different doses of EE (20 μg, 35 μg, and 50 μg) identified percentages of the population at risk of breakthrough bleeding alone and with varying degrees of CYP modulation.

Lack of Pharmacokinetic Interactions Between Macitentan and a Combined Oral Contraceptive in Healthy Female Subjects

Macitentan, a dual endothelin receptor antagonist used in pulmonary arterial hypertension, induces cytochrome P450 (CYP) 3A at supratherapeutic concentrations in vitro. Most combined oral hormonal contraceptives (OCs) are CYP3A substrates and their efficacy can be affected by CYP3A inducers. This randomized crossover study assessed possible pharmacokinetic (PK) interactions between macitentan and an OC containing ethinyl estradiol and norethindrone (or norethisterone). Twenty-six healthy women received a single oral dose of OC alone (reference) and concomitantly with 10 mg macitentan at steady state (test). No PK interaction was concluded if the 90% confidence intervals (CIs) of geometric mean ratios (GMRs; test/reference) of the peak plasma concentration (Cmax ) and the exposure from 0 to infinity (AUC0 - ∞ ) to the OC components were within the equivalence limits of 0.8 to 1.25. Cmax and AUC0-∞ of the OC were within the equivalence limits. For ethinyl estradiol, GMRs (90%CIs) of Cmax and AUC0-∞ were 0.92 (0.85-0.99) and 0.95 (0.90-0.99). For norethindrone, these values were 1.02 (0.95-1.09) and 1.04 (0.98-1.09), respectively. Overall, study treatments were well tolerated. No major changes from baseline in safety parameters were reported in either treatment. Macitentan does not affect the PK of OCs.

Understanding and preventing drug-drug and drug-gene interactions

Concomitant administration of multiple drugs can lead to unanticipated drug interactions and resultant adverse drug events with their associated costs. A more thorough understanding of the different cytochrome P450 isoenzymes and drug transporters has led to new methods to try to predict and prevent clinically relevant drug interactions. There is also an increased recognition of the need to identify the impact of pharmacogenetic polymorphisms on drug interactions. More stringent regulatory requirements have evolved for industry to classify cytochrome inhibitors and inducers, test the effect of drug interactions in the presence of polymorphic enzymes, and evaluate multiple potentially interacting drugs simultaneously. In clinical practice, drug alert software programs have been developed. This review discusses drug interaction mechanisms and strategies for screening and minimizing exposure to drug interactions. We also provide future perspectives for reducing the risk of clinically significant drug interactions.

Effects of ponesimod, a selective S1P1 receptor modulator, on the pharmacokinetics of a hormonal combination contraceptive

PURPOSE

To determine the effects of steady-state concentrations of the selective S1P1 receptor modulator ponesimod on the pharmacokinetics (PK) of a single dose of a combined oral contraceptive, containing 1 mg norethisterone (NET) and 35 μg ethinyl estradiol (EE) and to investigate the effects on heart rate at different ponesimod doses within an up-titration regimen prior to co-administration of the contraceptive.

METHODS

Twenty-two healthy women (age: 29-60 years) received twice a single oral dose of the combined oral contraceptive, alone or in combination with multiple doses of 40 mg ponesimod attained by an up-titration regimen. Heart rate (HR) effects were assessed on the first day of each up-titration level. PK parameters of NET and EE were determined by non-compartmental analysis.

RESULTS

Geometric mean ratios (ponesimod and contraceptive / contraceptive alone) of Cmax and AUC0-24 of NET were 0.87 (90 % CI: 0.80, 0.94) and 0.84 (90 % CI: 0.76, 0.93), respectively. Geometric mean ratios of Cmax and AUC0-24 of EE were 0.94 (90 % CI: 0.86, 1.03) and 0.95 (90 % CI: 0.89, 1.01), respectively. The maximum mean HR reduction after the first dose of 10 mg ponesimod was 12.4 bpm (SD ± 6.2) at 2.5 h post-dose. On Day 4 (first dose of 20 mg) and Day 7 (first dose of 40 mg) the maximum mean HR reduction was 4.3 bpm (SD ± 5.7) and 1.4 (SD ± 6.4), respectively, at 2.5 h post-dose compared to baseline.

CONCLUSION

No clinically relevant PK interactions between ponesimod and the combined oral contraceptive were observed, therefore, efficacy of hormonal contraceptives is not expected to be affected by concomitant administration of ponesimod. The up-titration regimen showed that HR reductions are diminished upon repeated ponesimod administration.

The pharmacokinetic interaction between an oral contraceptive containing ethinyl estradiol and norethindrone and the HCV protease inhibitor telaprevir

BACKGROUND

Telaprevir is a hepatitis C virus protease inhibitor that is both a substrate and an inhibitor of CYP3A.

STUDY DESIGN

The effect of steady-state telaprevir (administered 750 mg every 8 hours) on the steady-state pharmacokinetics of ethinyl estradiol (EE) and norethindrone (NE) was evaluated in 24 healthy women receiving oral contraceptives (OC) containing 0.5 mg NE and 0.035 mg EE for at least 3 months at the time of screening. This was a phase 1, open-label, single-center, nonrandomized study that included a cycle 1 (OC only for 21 days, followed by no OC for 7 days), cycle 2 (OC plus telaprevir for 21 days, followed by telaprevir alone for 7 days), and a follow-up period.

RESULTS

When administration with or without telaprevir was compared, the least-squares mean ratios (90% confidence limits) for EE were 0.74 (0.68; 0.80) for C(max), 0.67 (0.63; 0.71) for C(min), and 0.72 (0.69; 0.75) for AUC; neither NE nor telaprevir exposure was affected.

CONCLUSIONS

The efficacy of the OC may be compromised by the 26% to 33% reduction in EE exposure. Therefore, alternative methods of nonhormonal contraception should be used when hormonal contraceptives are coadministered with telaprevir and for up to 2 weeks following cessation of telaprevir.

Predictive utility of in vitro rifampin induction data generated in fresh and cryopreserved human hepatocytes, Fa2N-4, and HepaRG cells

Rifampin is a potent inducer of CYP3A4 in vitro and precipitates numerous drug-drug interactions (DDIs) when coadministered with CYP3A4 substrates. In the current study, we have critically assessed reported rifampin in vitro CYP3A4 induction data in Fa2N-4, HepaRG, and cryopreserved or primary human hepatocytes, using either CYP3A4 mRNA or probe substrate metabolism as induction endpoints. An in vivo data base of intravenously administered victim drugs (assuming hepatic induction only) was collated (n = 18) to assess the predictive utility of these in vitro systems and to optimize rifampin in vivo E(max). In addition, the effect of substrate hepatic extraction ratio on prediction accuracy was investigated using prediction boundaries proposed recently (Drug Metab Dispos 39:170-173). Incorporation of hepatic extraction ratio in the prediction model resulted in accurate prediction of 89% of intravenous induction DDIs (n = 18), regardless of the in vitro system or induction endpoint (mRNA or CYP3A4 activity). Effects of in vitro parameters from different cellular systems, and optimized in vivo E(max), on the prediction of 21 oral DDIs were assessed. Use of mRNA data resulted in pronounced overprediction across all systems, with 86 to 100% of DDIs outside the acceptable prediction limits; in contrast, CYP3A4 activity predicted up to 62% of the oral DDIs within limits. Although prediction accuracy of oral DDIs was improved when using intravenous optimized rifampin E(max), >35% of DDIs were incorrectly assigned, suggesting potential differential E(max) between intestine and liver. Implications of the findings and recommendations for prediction of rifampin DDIs are discussed.

Pharmacokinetic interaction study of ritonavir-boosted saquinavir in combination with rifabutin in healthy subjects

The effect of multiple doses of rifabutin (150 mg) on the pharmacokinetics of saquinavir-ritonavir (1,000 mg of saquinavir and 100 mg of ritonavir [1,000/100 mg]) twice daily (BID) was assessed in 25 healthy subjects. Rifabutin reduced the area under the plasma drug concentration-time curve from 0 to 12 h postdose (AUC(0-12)), maximum observed concentration of drug in plasma (C(max)), and minimum observed concentration of drug in plasma at the end of the dosing interval (C(min)) for saquinavir by 13%, 15%, and 9%, respectively, for subjects receiving rifabutin (150 mg) every 3 days with saquinavir-ritonavir BID. No effects of rifabutin on ritonavir AUC(0-12), C(max), and C(min) were observed. No adjustment of the saquinavir-ritonavir dose (1,000/100 mg) BID is required when the drugs are administered in combination with rifabutin. The effect of multiple doses of saquinavir-ritonavir on rifabutin pharmacokinetics was evaluated in two groups of healthy subjects. In group 1 (n = 14), rifabutin (150 mg) was coadministered every 3 days with saquinavir-ritonavir BID. The AUC(0-72) and C(max) of the active moiety (rifabutin plus 25-O-desacetyl-rifabutin) increased by 134% and 130%, respectively, compared with administration of rifabutin (150 mg) once daily alone. Rifabutin exposure increased by 53% for AUC(0-72) and by 86% for C(max). In group 3 (n = 13), rifabutin was coadministered every 4 days with saquinavir-ritonavir BID. The AUC(0-96) and C(max) of the active moiety increased by 60% and 111%, respectively, compared to administration of 150 mg of rifabutin once daily alone. The AUC(0-96) of rifabutin was not affected, and C(max) increased by 68%. Monitoring of neutropenia and liver enzyme levels is recommended for patients receiving rifabutin with saquinavir-ritonavir BID.

Effect of steady-state etravirine on the pharmacokinetics and pharmacodynamics of ethinylestradiol and norethindrone

BACKGROUND

Etravirine, a non-nucleoside reverse transcriptase inhibitor (NNRTI) active against NNRTI-resistant HIV, is an inducer of CYP3A4 and an inhibitor of CYP2C9/19.

STUDY DESIGN

The effect of etravirine on the pharmacokinetics and pharmacodynamics of ethinylestradiol and norethindrone was assessed in 30 HIV-negative females. Following a run-in cycle with ethinylestradiol/norethindrone, the pharmacokinetics of ethinylestradiol and norethindrone was assessed on Day 15 of Cycle 2. Etravirine 200 mg bid was coadministered on Day 1 to Day 15 of Cycle 3, with pharmacokinetic assessments of ethinylestradiol, norethindrone and etravirine on Day 15.

RESULTS

When combined with etravirine, the least-squares means (LSM) ratios (90% confidence interval) for ethinylestradiol AUC(24h), C(max) and C(min) were 1.22 (1.13-1.31), 1.33 (1.21-1.46) and 1.09 (1.01-1.18), respectively, compared to administration alone. LSM ratios for norethindrone parameters were 0.95 (0.90-0.99), 1.05 (0.98-1.12) and 0.78 (0.68-0.90), respectively.

CONCLUSION

These changes are not considered clinically relevant. No loss in contraceptive efficacy is expected when coadministered with etravirine.

Modeling, prediction, and in vitro in vivo correlation of CYP3A4 induction

CYP3A4 induction is not generally considered to be a concern for safety; however, serious therapeutic failures can occur with drugs whose exposure is lower as a result of more rapid metabolic clearance due to induction. Despite the potential therapeutic consequences of induction, little progress has been made in quantitative predictions of CYP3A4 induction-mediated drug-drug interactions (DDIs) from in vitro data. In the present study, predictive models have been developed to facilitate extrapolation of CYP3A4 induction measured in vitro to human clinical DDIs. The following parameters were incorporated into the DDI predictions: 1) EC(50) and E(max) of CYP3A4 induction in primary hepatocytes; 2) fractions unbound of the inducers in human plasma (f(u, p)) and hepatocytes (f(u, hept)); 3) relevant clinical in vivo concentrations of the inducers ([Ind](max, ss)); and 4) fractions of the victim drugs cleared by CYP3A4 (f(m, CYP3A4)). The values for [Ind](max, ss) and f(m, CYP3A4) were obtained from clinical reports of CYP3A4 induction and inhibition, respectively. Exposure differences of the affected drugs in the presence and absence of the six individual inducers (bosentan, carbamazepine, dexamethasone, efavirenz, phenobarbital, and rifampicin) were predicted from the in vitro data and then correlated with those reported clinically (n = 103). The best correlation was observed (R(2) = 0.624 and 0.578 from two hepatocyte donors) when f(u, p) and f(u, hept) were included in the predictions. Factors that could cause over- or underpredictions (potential outliers) of the DDIs were also analyzed. Collectively, these predictive models could add value to the assessment of risks associated with CYP3A4 induction-based DDIs by enabling their determination in the early stages of drug development.

Application and interpretation of hPXR screening data: Validation of reporter signal requirements for prediction of clinically relevant CYP3A4 inducers

A human pregnane X receptor (PXR) reporter-gene assay was established and validated using 19 therapeutic agents known to be clinical CYP3A4 inducers, 5 clinical non-inducers, and 6 known inducers in human hepatocytes. The extent of CYP3A4 induction (measured as RIF ratio in comparison to rifampicin) and EC50 was obtained from the dose-response curve. All of the clinical inducers (19/19) and human hepatocyte inducers (6/6) showed positive responses in the PXR assay. One out of five clinical non-inducers, pioglitazone, also showed a positive response. An additional series of 18 commonly used drugs with no reports of clinical induction was also evaluated as putative negative controls. Sixteen of these were negative (89%), whereas two of these, flutamide and haloperidol showed 16-fold (RIF ratio 0.79) and 10-fold (RIF ratio 0.48) maximal induction, respectively in the reporter-gene system. Flutamide and haloperidol were further demonstrated to cause CYP3A4 induction in human cryopreserved hepatocytes based on testosterone 6beta-hydroxylation activity. The induction potential index calculated based on the maximum RIF ratio, EC50, and in vivo maximum plasma concentration was used to predict the likelihood of CYP3A4 induction in humans. When the induction potential index is greater than 0.08, the compound is likely to cause induction in humans. A high-throughput screening strategy was developed based on the validation results at 1microM and 10microM for the same set of drugs. A RIF ratio of 0.4 was set as more practical screening cut-off to minimize the possibility of generating false positives. Thus, a tiered approach was implemented to use the human PXR reporter-gene assay from early lead optimization to late lead characterization in drug discovery.

Prevalence and predictors of potential drug-drug interactions in Regione Emilia-Romagna, Italy

BACKGROUND AND OBJECTIVE

Drug-drug interactions (DDIs) are preventable medication errors associated with potentially serious adverse events and death. Several studies have examined the prevalence of potential DDIs among ambulatory patients in various countries. Limited recent data on the prevalence of potential DDIs in Italy are available in the medical literature. The objective of this study was to estimate the prevalence of clinically important potential DDIs among the approximately 4 million residents of Regione Emilia-Romagna (RER), Italy, and to examine possible predictors of potential DDI exposure.

METHODS

A retrospective follow-up study of 2004 outpatient prescription data from RER was conducted. A previously published list of clinically important potential DDIs was refined to include only pairs of drugs in which both drugs were reimbursed by the 2004 Italian National Formulary. A potential DDI was defined as the presence of a minimum 5-day overlap in days supply for drugs in an interacting pair. The 1-year period prevalence of each potential DDI was quantified. A logistic regression analysis was conducted to examine patient characteristics as predictors of potential DDIs.

RESULTS AND DISCUSSION

The list of clinically important potential DDIs included 12 drug pairs that could be captured using the RER database. These 12 potential DDIs occurred 8894 times in the RER population in 2004. The most commonly identified potentially interacting medication pairs were warfarin and non-steroidal anti-inflammatory drugs (6824 cases), theophylline/aminophylline and ciprofloxacin/fluvoxamine (930), and warfarin and barbiturates (567). Odds of exposure were highest among those aged 65 years or older, males, and those with more chronic conditions. Odds of exposure increased 1.39 times with each addition of a prescription medication.

CONCLUSION

A substantial number of clinically important potential DDIs were identified, particularly among warfarin users. Awareness of the most prevalent potential DDIs can help practitioners prevent concomitant use of these dangerous medication combinations.

Use of Immortalized Human Hepatocytes to Predict the Magnitude of Clinical Drug-Drug Interactions Caused by CYP3A4 Induction

Cytochrome P4503A4 (CYP3A4) is the principal drug-metabolizing enzyme in human liver. Drug-drug interactions (DDIs) caused by induction of CYP3A4 can result in decreased exposure to coadministered drugs, with potential loss of efficacy. Immortalized hepatocytes (Fa2N-4 cells) have been proposed as a tool to identify CYP3A4 inducers. The purpose of the current studies was to characterize the effect of known inducers on CYP3A4 in Fa2N-4 cells, and to determine whether these in vitro data could reliably project the magnitude of DDIs caused by induction. Twenty-four compounds were chosen for these studies, based on previously published data using primary human hepatocytes. Eighteen compounds had been shown to be positive for induction, and six compounds had been shown to be negative for induction. In Fa2N-4 cells, all 18 positive controls produced greater than 2-fold maximal CYP3A4 induction, and all 6 negative controls produced less than 1.5-fold maximal CYP3A4 induction. Subsequent studies were conducted to determine the relationship between in vitro induction data and in vivo induction response. The approach was to relate in vitro induction data (E(max) and EC(50) values) with efficacious free plasma concentrations to calculate a relative induction score. This score was then correlated with decreases in area under the plasma concentration versus time curve values for coadministered CYP3A4 object drugs (midazolam or ethinylestradiol) from previously published clinical DDI studies. Excellent correlations (r(2) values >0.92) were obtained, suggesting that Fa2N-4 cells can be used for identification of inducers as well as prediction of the magnitude of clinical DDIs.

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.

Tazarotene does not affect the pharmacokinetics and efficacy of a norethindrone/ethinylestradiol oral contraceptive

OBJECTIVE

To determine the pharmacokinetic and pharmacodynamic interaction between oral tazarotene and an oral contraceptive containing norethindrone 1mg and ethinylestradiol 0.035 mg (Ortho-Novum 1/35).

DESIGN

Two separate open-label, parallel-group, single-centre, pharmacokinetic and pharmacodynamic interaction studies.

PARTICIPANTS AND METHODS

Twenty-seven healthy women (age 20-55 years) completed Study I, with a duration of 64 days during three consecutive menstrual cycles. Ortho-Novum 1/35 was taken once daily from study day 0 (first cycle day 1) to day 61 (third cycle day 6), and oral tazarotene 1.1 mg was coadministered daily from study day 34 (second cycle day 7) to day 61. Twenty-nine healthy women (age 20-44 years) completed Study II, with a duration of 75 days during three consecutive menstrual cycles. Ortho-Novum 1/35 was taken once daily from study day 0 (first cycle day 1) to day 74 (third cycle day 19), and oral tazarotene 6 mg was coadministered daily from study day 48 (second cycle day 21) to day 74. In both studies, the pharmacokinetics of tazarotenic acid on study day 61 (third cycle day 6) were evaluated from plasma tazarotenic acid concentrations. Pharmacokinetic parameters of plasma norethindrone and ethinylestradiol were compared before and after tazarotene administration (cycle day 6 of the second and third cycles, respectively). Serum luteinising hormone (LH) and follicle-stimulating hormone (FSH) concentrations were compared before and after tazarotene administration (cycle days 2, 4 and 6 of the second and third cycles, respectively). In Study II, serum progesterone concentrations were also determined on cycle days 18 and 20 of the second and third cycles. Tazarotenic acid was determined by liquid chromatography-tandem mass spectrometry. Ethinylestradiol and norethindrone were determined by gas chromatography-mass spectrometry. LH and FSH were assayed by microparticle enzyme immunoassay in Study I and by double-antibody radioimmunoassay in Study II. Progesterone was determined by solid-phase radioimmunoassay.

RESULTS

In Study I (tazarotene 1.1 mg), the area under the plasma concentration-time curve from zero to 24 hours (AUC24) and the peak concentration in plasma (Cmax) for tazarotenic acid were 121 +/- 27 microg. h/L and 28.9 +/- 9.4 microg/L (mean +/- SD), respectively. In Study II (tazarotene 6 mg), AUC24 and Cmax for tazarotenic acid were 379 +/- 78 microg. h/L and 111 +/- 37 microg/L (mean +/- SD), respectively. In both studies, for both norethindrone and ethinylestradiol, the 90% CIs of AUC24 and Cmax on cycle day 6 before and after tazarotene administration were within the 80-125% boundary. In Study I, the 90% CIs of serum FSH and LH concentrations on cycle day 4 were within the 80-125% boundary. FSH and LH concentrations on cycle day 6 were marginally/partially outside the 80-125% boundary as a result of high variability. However, the mean FSH and LH serum concentrations on cycle day 6 of the third cycle were lower than those of the second cycle. In Study II, the 90% CIs of serum FSH, LH and progesterone concentrations were all within the 80-125% boundary, except for LH on cycle day 2. LH concentrations on cycle day 2 were marginally/partially outside the 80-125% boundary as a result of high variability. However, the mean serum LH concentration on cycle day 2 of the third cycle was lower than that of the second cycle.

CONCLUSIONS

Oral tazarotene up to 6 mg once daily does not affect the pharmacokinetics and efficacy of Ortho-Novum 1/35.

THE INVOLVEMENT OF CYP3A4 AND CYP2C9 IN THE METABOLISM OF 17α-ETHINYLESTRADIOL

The role of specific cytochrome P450 (P450) isoforms in the metabolism of ethinylestradiol (EE) was evaluated. The recombinant human P450 isozymes CYP1A1, CYP1A2, CYP2C9, CYP2C19, and CYP3A4 were found to be capable of catalyzing the metabolism of EE (1 microM). Without exception, the major metabolite was 2-hydroxy-EE. The highest catalytic efficiency (Vmax/Km) was observed with rCYP1A1, followed by rCYP3A4, rCYP2C9, and rCYP1A2. The P450 isoforms 3A4 and 2C9 were shown to play a significant role in the formation of 2-hydroxy-EE in a pool of human liver microsomes by using isoform-specific monoclonal antibodies, in which the inhibition of formation was approximately 54 and 24%, respectively. The involvement of CYP3A4 and CYP2C9 was further confirmed by using selective chemical inhibitors (i.e., ketoconazole and sulfaphenazole). The relative contribution of each P450 isoform to the 2-hydroxylation pathway was obtained from the catalytic efficiency of each isoform normalized by its relative abundance in the same pool of human liver microsomes, as determined by quantitative Western blot analysis. Collectively, these results suggested that multiple P450 isoforms were involved in the oxidative metabolism of EE in human liver microsomes, with CYP3A4 and CYP2C9 as the major contributing enzymes.

The pharmacokinetics of sumatriptan when administered with norethindrone 1 mg/ethinyl estradiol 0.035 mg in healthy volunteers

BACKGROUND

Because the majority of migraineurs are young women in their peak reproductive years, it is important to understand the possible effects on the pharmacokinetics of both medications when sumatriptan is coadministered with an oral contraceptive (OC).

OBJECTIVES

The primary objective of this study was to assess the effect of multiple dosing of the OC norethindrone 1 mg/ethinyl estradiol 0.035 mg (NE/EE) on the single-dose pharmacokinetics of sumatriptan in healthy volunteers. Secondary objectives were to determine the effect of a single dose of sumatriptan on the multiple-dose pharmacokinetics of NE and EE, and to assess the safety and tolerability of the combination.

METHODS

This was an open-label, 1-sequence, crossover study in healthy women who had been receiving NE/EE for at least 3 months. Subjects received 1 cycle of NE/EE, consisting of 21 days of OC and 7 days of placebo. They also received a single dose of sumatriptan 50 mg on the last day of the OC or placebo regimen. Blood samples for the determination of plasma sumatriptan concentrations were collected on days 21 and 28, and blood samples for the determination of plasma NE and EE concentrations were collected on days 20 and 21. Treatments were compared by analysis of variance. Equivalence between treatments was to be concluded if the 90% Cl for the ratio of reference to test means for log(e)-transformed parameters (area under the plasma concentration-time curve [AUCI and maximum measured plasma concentration [C(max)]) for each analyte fell within the interval 0.80 to 1.25.

RESULTS

Twenty-six women (mean age, 29.8 years; age range, 18-44 years; weight range, 52-82 kg) participated in the study. The 90% CI for the ratio of reference to test means for the AUC extrapolated to infinity (AUC(infinity)) of sumatriptan was 1.11 to 1.22, and the 90% CIs for the AUC over the dosing interval at steady state (AUC(tau)) of NE and EE were 0.96 to 1.00 and 0.91 to 0.97, respectively. The 90% CIs for the ratio of reference to test means for the C(max) of sumatriptan, NE, and EE were a respective 1.05 to 1.30, 0.76 to 0.88, and 0.88 to 1.04. Study treatments were well tolerated. Adverse events were mild or moderate, and there were no clinically significant changes in vital signs or laboratory values.

CONCLUSIONS

The extent of absorption (AUC) of sumatriptan, NE, and EE was similar after oral administration of sumatriptan and NE/EE, both alone and in combination. Thus, in the opinion of the study investigators, there were no clinically relevant changes in the AUC of any of the medications when sumatriptan and NE/EE were administered concomitantly compared with administration alone. The results of this study suggest that dose adjustment is not necessary when sumatriptan is administered concomitantly with NE/EE in healthy premenopausal women.

St. John's wort and oral contraceptives: reasons for concern?

St. John's wort (Hypericum perforatum) is one of the best-selling herbal remedies in the United States. It has been implicated as an inducer of the P450 enzyme system, and as such, may cause increased metabolism of certain drugs, including oral contraceptives. Women using oral contraceptives have been warned against using St. John's wort. To date, there are some case reports but little clinical data demonstrating risk of contraceptive failure if they do. This article reviews available data and discusses theoretical reasons for concern about possible drug-herb interactions.

Clinically significant interactions with drugs used in the treatment of tuberculosis

Clinically significant interactions occurring during antituberculous chemotherapy principally involve rifampicin (rifampin), isoniazid and the fluoroquinolones. Such interactions between the antituberculous drugs and coadministered agents are definitely much more important than among antituberculous drugs themselves. These can be associated with consequences even amounting to therapeutic failure or toxicity. Most of the interactions are pharmacokinetic rather than pharmacodynamic in nature. The cytochrome P450 isoform enzymes are responsible for many interactions (especially those involving rifampicin and isoniazid) during drug biotransformation (metabolism) in the liver and/or intestine. Generally, rifampicin is an enzyme inducer and isoniazid acts as an inhibitor. The agents interacting significantly with rifampicin include anticoagulants, anticonvulsants, anti-infectives, cardiovascular therapeutics, contraceptives, glucocorticoids, immunosuppressants, psychotropics, sulphonylureas and theophyllines. Isoniazid interacts principally with anticonvulsants, theophylline, benzodiapines, paracetamol (acetaminophen) and some food. Fluoroquinolones can have absorption disturbance due to a variety of agents, especially the metal cations. Other important interactions of fluoroquinolones result from their enzyme inhibiting potential or pharmacodynamic mechanisms. Geriatric and immunocompromised patients are particularly at risk of drug interactions during treatment of their tuberculosis. Among the latter, patients who are HIV infected constitute the most important group. This is largely because of the advent of new antiretroviral agents such as the HIV protease inhibitors and the non-nucleoside reverse transcriptase inhibitors in the armamenterium of therapy. Compounding the complexity of drug interactions, underlying medical diseases per se may also contribute to or aggravate the scenario. It is imperative for clinicians to be on the alert when treating tuberculosis in patients with difficult co-morbidity requiring polypharmacy. With advancement of knowledge and expertise, it is hoped that therapeutic drug monitoring as a new paradigm of care can enable better management of these drug interactions.

Oral contraceptive efficacy and antibiotic interaction: a myth debunked

The purpose of this study was to review the pharmacokinetic and clinical literature regarding the efficacy of oral contraceptives when used concomitantly with antibiotic therapy. Relevant literature was identified by searching MEDLINE and EMBASE. Other sources were located by consulting the bibliographies of the material collected from MEDLINE and EMBASE. Pharmacokinetic evidence demonstrates that plasma levels of oral contraceptive steroids are unchanged with the concomitant administration of antibiotics, including ampicillin, ciprofloxacin, clarithromycin, doxycycline, metronidazole, ofloxacin, roxithromycin, temafloxacin, and tetracycline. However, reduced steroid levels have been reported in women taking rifampin with oral contraceptives. Clinical reports of contraceptive failure with antibiotic use are retrospective, have multiple potential biases, and are not supported by pharmacokinetic data. Available scientific and pharmacokinetic data do not support the hypothesis that antibiotics (with the exception of rifampin) lower the contraceptive efficacy of oral contraceptives.

Effect of rofecoxib on the pharmacokinetics of chronically administered oral contraceptives in healthy female volunteers

The effect of rofecoxib, a highly selective cyclooxygenase (COX)-2 inhibitor, on the pharmacokinetics of ethinyl estradiol (EE) and norethindrone (NET), two common components of a combination oral contraceptive product, was examined. A double-blind, two-period crossover study was conducted in 18 healthy women who received ORTHO-NOVUM 1/35, a combination of EE (35 microg) and NET (1 mg), concurrently for 14 days with either 175 mg rofecoxib or matching placebo during two consecutive menstrual cycles. Plasma was sampled for EE, NET, sex hormone binding globulin (SHBG), and albumin. The AUC(0-24 h) geometric mean ratio (GMR: rofecoxib/placebo) with corresponding 90% confidence interval (CI) of EE and NET was 1.13 (1.06, 1.19) and 1.18 (1.13, 1.24), respectively. The Cmax GMR of EE and NET was 1.06 (0.98, 1.16) and 1.04 (0.99, 1.09), respectively. In each case, the 90% CIs satisfied the predefined bioequivalence limits of (0.80, 1.25). Measures of SHBG and albumin and routine clinical and laboratory safety parameters showed no clinically meaningful changes. The addition of rofecoxib to the oral contraceptive was not associated with any clinically important changes in EE or NET pharmacokinetics and thus would not be anticipated to influence the efficacy of this contraceptive regimen.

Complementary and alternative therapies to manage HIV-related symptoms

Persons with HIV infection report substantial use of complementary and alternative medical (CAM) therapies for symptom management. Anecdotal reports from patients indicate that CAM approaches are helpful; however, there is limited scientific information on the safety and efficacy of these therapies in the HIV population. The purpose of this review is to critically appraise the scientific evidence for selected CAM therapies that are used by HIV-infected persons to manage three common symptoms: nutritional alterations, pain, and depression.

Induction of drug metabolising enzymes: pharmacokinetic and toxicological consequences in humans

Currently, 5 different main mechanisms of induction are distinguished for drug-metabolising enzymes. The ethanol type of induction is mediated by ligand stabilisation of the enzyme, but the others appear to be mediated by intracellular 'receptors'. These are the aryl hydrocarbon (Ah) receptor, the peroxisome proliferator activated receptor (PPAR), the constitutive androstane receptor (CAR, phenobarbital induction) and the pregnane X receptor [PXR, rifampicin (rifampin) induction]. Enzyme induction has the net effect of increasing protein levels. However, many inducers are also inhibitors of the enzymes they induce, and the inductive effects of a single drug may be mediated by more than one mechanism. Therefore, it appears that every inducer has its own pattern of induction; knowledge of the main mechanism is often not sufficient to predict the extent and time course of induction, but may serve to make the clinician aware of potential dangers. The possible pharmacokinetic consequences of enzyme induction depend on the localisation of the enzyme. They include decreased or absent bioavailability for orally administered drugs, increased hepatic clearance or accelerated formation of reactive metabolites, which is usually related to local toxicity. Although some severe drug-drug interactions are caused by enzyme induction, most of the effects of inducers are not detected in the background of nonspecific variation. For any potent inducer, however, its addition to, or withdrawal from, an existing drug regimen may cause pronounced concentration changes and should be done gradually and with appropriate monitoring of therapeutic efficacy and adverse events. The toxicological consequences of enzyme induction in humans are rare, and appear to be mainly limited to hepatoxicity in ethanol-type induction.

Endocrine disrupting chemicals, phthalic acid and nonylphenol, activate Pregnane X receptor-mediated transcription

Recently, Pregnane X receptor (PXR), a new member of the nuclear receptor superfamily, was shown to mediate the effects of several steroid hormones, such as progesterone, glucocorticoid, pregnenolone, and xenobiotics on cytochrome P450 3A genes (CYP3A) through the specific DNA sequence for CYP3A, suggesting that PXR may play a role in steroid hormone metabolism. In this paper, we demonstrated that phthalic acid and nonylphenol, endocrine-disrupting chemicals (EDCs), stimulated PXR-mediated transcription at concentrations comparable to those at which they activate estrogen receptor-mediated transcription using a transient reporter gene expression assay in COS-7 cells. However, bisphenol A, another EDC, had no effect on PXR-mediated transcription, although this chemical significantly enhanced ER-mediated transcription. In the yeast two-hybrid protein interaction assay, PXR interacted with two nuclear receptor coactivator proteins, steroid hormone receptor coactivator-1 and receptor interacting protein 140, in the presence of phthalic acid or nonylphenol. Thus, EDC-occupied PXR may regulate its specific gene expression through the receptor-coactivator interaction. In contrast, these EDCs had no effect on the interaction between PXR and suppressor for gal 1, a component of proteasome. Finally, the expression of CYP3A1 mRNA in the liver of rats exposed to phthalic acid or nonylphenol markedly increased compared with that in rats treated with estradiol, bisphenol A, or ethanol as assessed by competitive RT-PCR. These data suggest that EDCs may affect endocrine functions by altering steroid hormone metabolism through PXR.