Isoniazid Mediates the CYP2B6*6 Genotype-Dependent Interaction between Efavirenz and Antituberculosis Drug Therapy through Mechanism-Based Inactivation of CYP2A6

Efavirenz is commonly used to treat patients coinfected with human immunodeficiency virus and tuberculosis. Previous clinical studies have observed paradoxically elevated efavirenz plasma concentrations in patients with the CYP2B6*6/*6 genotype (but not the CYP2B6*1/*1 genotype) during coadministration with the commonly used four-drug antituberculosis therapy. This study sought to elucidate the mechanism underlying this genotype-dependent drug-drug interaction. In vitro studies were conducted to determine whether one or more of the antituberculosis drugs (rifampin, isoniazid, pyrazinamide, or ethambutol) potently inhibit efavirenz 8-hydroxylation by CYP2B6 or efavirenz 7-hydroxylation by CYP2A6, the main mechanisms of efavirenz clearance. Time- and concentration-dependent kinetics of inhibition by the antituberculosis drugs were determined using genotyped human liver microsomes (HLMs) and recombinant CYP2A6, CYP2B6.1, and CYP2B6.6 enzymes. Although none of the antituberculosis drugs evaluated at up to 10 times clinical plasma concentrations were found to inhibit efavirenz 8-hydroxylation by HLMs, both rifampin (apparent inhibition constant [K_i] = 368 μM) and pyrazinamide (K_i = 637 μM) showed relatively weak inhibition of efavirenz 7-hydroxylation. Importantly, isoniazid demonstrated potent time-dependent inhibition of efavirenz 7-hydroxylation in both HLMs (inhibitor concentration required for half-maximal inactivation [K_I] = 30 μM; maximal rate constant of inactivation [k_inact] = 0.023 min⁻¹) and recombinant CYP2A6 (K_I = 15 μM; k_inact = 0.024 min⁻¹) and also formed a metabolite intermediate complex consistent with mechanism-based inhibition. Selective inhibition of the CYP2B6.6 allozyme could not be demonstrated for any of the antituberculosis drugs using either recombinant enzymes or CYP2B6*6 genotype HLMs. In conclusion, the results of this study identify isoniazid as the most likely perpetrator of this clinically important drug-drug interaction through mechanism-based inactivation of CYP2A6.     

Sleeping at the margins: a qualitative study of homeless drug users who stay in emergency hostels and shelters

A growing social science literature demonstrates that sleep is not merely a personal matter but also a political problem and a public health issue. Taking this as a point of departure, our article presents an analysis of sleeping practices amongst homeless drug users (HDUs) who make use of emergency hostels and night shelters in England. Data generated by way of qualitative interviews undertaken with 29 men and 11 women reveal that, as we might expect, securing sleep for this group is by no means easy. The strategies they pursue to find places to sleep are described, as are the threats and barriers to their sleeping. Emergency hostels and night shelters can afford a lifeline; providing warmth, water, food and access to support services. But if these are inadequately resourced they can be experienced as volatile environments and inimical to sleeping. It is argued here that although sleep is an essential prerequisite for health, for this population it can, somewhat ironically, be experienced as a risky behaviour. Vulnerable to both physical risks (e.g. inclement weather) and social threats (e.g. abuse and violence), falling asleep can exacerbate exposure to such dangers. These vulnerabilities are compounded by the social position of HDUs who live in socially and physically marginal places. It is this marginality that prevents them from being able to secure sleep that is both restful and restorative.     

Health Status as a Mediating Factor in the Life Change-Academic Performance Relationship

Abstract

This study was designed to determine whether illness during the academic year, when gender and academic readiness are controlled, is the variable accounting for the demonstrated relationship between life change index and subsequent academic performance. A weak though significant inverse association was found between life change and grades: this association persisted even after statistical adjustment for gender, academic readiness, and illness experience. There was no evidence that illness experience mediated the life change-performance relationship. The association was not subject to a threshold effect according to level of life change.     

Pulmonary Atresia with Ventricular Septal Defect and Multifocal Pulmonary Blood Supply: Does an Intensive Interventional Approach Improve the Outcome?

Introduction. Pulmonary atresia with ventricular septal defect (VSD) continues to be associated with significant morbidity and mortality, with significant institutional variation in therapeutic strategies. This study reports a single center experience utilizing an intensive transcatheter approach to promote pulmonary vascular growth. Methods. A retrospective analysis of 20 patients undergoing surgical and transcatheter treatment for pulmonary atresia with VSD between 2002 and 2010. Results. The median age at initial surgical palliation was 6.3 months (8 days to 2.5 years). Eleven patients (group 1) underwent initial surgical palliation without VSD closure and nine patients (group 2) underwent an initial complete repair with fenestrated or complete VSD closure. Group 1 had a smaller Nakata index (54 mm²/m² vs. 134 mm²/m², P= .04) and a smaller absolute native pulmonary artery diameter (2.7 mm vs. 4.5 mm, P= .01) than group 2. Intraoperative angiography was performed in 10 cases to evaluate if early transcatheter intervention was warranted. The median follow‐up during the study period was 2.3 years (1.6 months to 8.3 years). Of the 16 patients who survived the initial early postoperative period, 15 patients (94%) went on to receive surgical (n = 11) and/or interventional (n = 25) catheterization procedures. There was improvement in the mean Nakata index from the initial presurgical evaluation to the most recent catheterization data (38.4 mm²/m² vs. 169.7 mm²/m², P≤ .05). To date, two of 11 (18%) patients in group 1 ultimately underwent surgical VSD closure. Overall mortality was six of 20 (30%) with four deaths in group 1 and two deaths in group 2. There were no procedural deaths. Conclusions. Combining surgical unifocalization procedures with subsequent early and intensive catheter‐based pulmonary artery rehabilitation may improve vascular growth, ultimately rendering many patients suitable for fenestrated VSD closure. Risk stratification, including intraoperative exit angiography, is essential to determine the need for early transcatheter interventions.