Kevin Elwood

Genetic variation in carboxylesterase genes and susceptibility to isoniazid-induced hepatotoxicity

Treatment of latent tuberculosis infection (LTBI) generally includes isoniazid (INH), a drug that can cause serious hepatotoxicity. Carboxylesterases (CES) are important in the metabolism of a variety of substrates, including xenobiotics. We hypothesized that genetic variation in CES genes expressed in the liver could affect INH-induced hepatotoxicity. Three CES genes are known to be expressed in human liver: CES1, CES2 and CES4. Our aim was to systematically characterize genetic variation in these novel candidate genes and test whether it is associated with this adverse drug reaction. As part of a pilot study, 170 subjects with LTBI who received only INH were recruited, including 23 cases with hepatotoxicity and 147 controls. All exons and the promoters of CES1, CES2 and CES4 were bidirectionally sequenced. A large polymorphic deletion was found to encompass exons 2 to 6 of CES4. No significant association was found. Eleven single-nucleotide polymorphisms (SNPs) in CES1 were in high linkage disequilibrium with each other. One of these SNPs, C(−2)G, alters the translation initiation sequence of CES1 and represents a candidate functional polymorphism. Replication of this possible association in a larger sample set and functional studies will be necessary to determine if this CES1 variant has a role in INH-induced hepatotoxicity.

A statistical method was used for the meta-analysis of tests for latent TB in the absence of a gold standard, combining random-effect and latent-class methods to estimate test accuracy

OBJECTIVE Because of the lack of a gold standard, the diagnostic performance of tests for the detection of latent tuberculosis infection (LTBI) is not known. However, statistical methods can be used to estimate the accuracy from the studies reporting the concordance among the tests. STUDY DESIGN AND SETTING We developed a random-effect latent-class model to estimate performance characteristics of three LTBI diagnostic tests: tuberculin skin test (TST, at 10-mm cutoff), QuantiFERON-TB gold (QFG), and TSPOT-TB from the studies evaluating agreement among the tests. RESULTS Nineteen studies were included. QFG had a sensitivity of 0.642 (95% confidence interval [CI]: 0.593-0.691) and specificity of 0.996 (95% CI: 0.989-1.000), TSPOT-TB had a sensitivity of 0.500 (95% CI: 0.334-0.666) and specificity of 0.906 (95% CI: 0.882-0.929), and TST had a sensitivity of 0.709 (95% CI: 0.658-0.761) and specificity of 0.683 (95% CI: 0.522-0.844). Results were not sensitive to the inclusion of any single study. When only the three studies that reported on TSPOT were removed, estimates for the other two tests varied minimally. CONCLUSIONS Statistical methods can help estimate the accuracy of LTBI tests. Although the specificities were close to their reported values in the literature, the estimates for sensitivities were low; a finding that should be carefully evaluated.

Interferon-gamma release assays for screening of health care workers in low tuberculosis incidence settings: Dynamic patterns and interpretational challenges

There is considerable evidence that both tuberculin skin tests (TSTs) and interferon-gamma release assays (IGRAs) are valid but imperfect for latent tuberculosis (TB) infection (LTBI). Neither test can distinguish LTBI from TB disease and, therefore, have no value for active TB detection (1). Both tests have suboptimal sensitivity in active TB, especially in HIV-infected persons and children (2,3). Both tests appear to correlate well with gradient of exposure (3). While neither IGRAs nor the TST have high accuracy for predicting active TB, the use of IGRAs in some populations might reduce the number of people considered for preventive treatment (4). However, there are key differences between the two tests (5). While the TST has high specificity in Bacille-Calmette-Guerin (BCG) unvaccinated persons, its specificity is lower and variable in those who have received BCG vaccination after infancy or have received multiple BCG vaccinations (6). In contrast, IGRA specificity remains high in BCG-vaccinated and unvaccinated populations (7). From a logistical perspective, IGRAs are more convenient for patients who do not have to return for the reading, and the laboratory readout is more objective than the subjective reading of TST induration. Finally, there is an important difference in terms of cost for the health care system – in general, IGRAs are more expensive to implement than the TST. While many countries have published guidelines on IGRAs (8), the use of IGRAs for routine screening of health care workers (HCWs) remains an area of controversy. The 2005 United States Centers for Disease Control (CDC) guidelines on the QuantiFERON-TB Gold (QFT) assay (Cellestis, Australia) allowed for replacement of the TST with QFT for annual testing of HCWs in the United States (US) (9), and this was expanded to cover both commercial IGRAs in the 2010 update (10). In contrast, the Canadian guidelines on IGRAs have not recommended the use of IGRAs for serial testing of HCWs (11,12). While the performance of IGRAs in serial testing of HCWs was first reported six years ago (13), this topic has received significantly more attention in the past few years, culminating in a recent systematic review (14). In the current issue of the Canadian Respiratory Journal, Joshi et al (15) (pages 84-88) provide useful data on routine implementation of QFT for HCW screening in Arkansas (USA), where QFT replaced the TST in 2008. Joshi et al describe the challenges they faced in implementing the test and raise concerns about high rates of QFT positivity in a setting with very low historical TST conversion rates. They observed high reversion rates on repeat testing of positives and poor short-term reproducibility of positive QFT results. Based on their results, Joshi et al argue for the need for cautious interpretation of QFT results, especially those in the borderline zone around the cut-off. The report by Joshi et al (15) is highly consistent with data that have emerged from many countries (16-25). Table 1 sumarizes the major serial testing studies of IGRAs in HCWs in low and intermediate

A mutation in Mycobacterium tuberculosis rpoB gene confers rifampin resistance in three HIV-TB cases.

Rifampin is a key component of standard short-course first-line therapy against Mycobacterium tuberculosis (MTB). Rifampin monoresistant MTB, previously a rare phenomenon, is now being reported at increasing rates worldwide. We report a mutation in the rpoB region leading to low level rifampin monoresistance in a cluster of HIV-positive patients. All rifampin monoresistant isolates identified from 2004 to 2006 underwent susceptibility confirmation, sequencing of rpoB and genotyping. Three patients were found to have a previously undocumented 3 base pair insertion at codon 525 in the rpoB region. The earliest initial case was infected with fully susceptible MTB. Disease relapse occurred 7 months later with a genotypically identical MTB isolate, showing acquired rifampin monoresistance. MTB isolates from 2 subsequent patients showed primary rifampin monoresistance with an identical genotype to the index case. Patients with rifampin monoresistant MTB tend to have poorer outcomes than those with fully susceptible strains. Risk factors for the development of rifampin monoresistance include co-morbid HIV infection and previously treated tuberculosis. HIV infection has been associated with malabsorption of anti-tuberculous medications leading to sub-therapeutic levels of administered drugs. These factors may have played a role in the development of this previously undocumented mutation.

Four Months of Rifampin or Nine Months of Isoniazid for Latent Tuberculosis in Adults

BACKGROUND A 9‐month regimen of isoniazid can prevent active tuberculosis in persons with latent tuberculosis infection. However, the regimen has been associated with poor adherence rates and with toxic effects. METHODS In an open‐label trial conducted in nine countries, we randomly assigned adults with latent tuberculosis infection to receive treatment with a 4‐month regimen of rifampin or a 9‐month regimen of isoniazid for the prevention of confirmed active tuberculosis within 28 months after randomization. Noninferiority and potential superiority were assessed. Secondary outcomes included clinically diagnosed active tuberculosis, adverse events of grades 3 to 5, and completion of the treatment regimen. Outcomes were adjudicated by independent review panels. RESULTS Among the 3443 patients in the rifampin group, confirmed active tuberculosis developed in 4 and clinically diagnosed active tuberculosis developed in 4 during 7732 person‐years of follow‐up, as compared with 4 and 5 patients, respectively, among 3416 patients in the isoniazid group during 7652 person‐years of follow‐up. The rate differences (rifampin minus isoniazid) were less than 0.01 cases per 100 person‐years (95% confidence interval [CI], ‐0.14 to 0.16) for confirmed active tuberculosis and less than 0.01 cases per 100 person‐years (95% CI, ‐0.23 to 0.22) for confirmed or clinically diagnosed tuberculosis. The upper boundaries of the 95% confidence interval for the rate differences of the confirmed cases and for the confirmed or clinically diagnosed cases of tuberculosis were less than the prespecified noninferiority margin of 0.75 percentage points in cumulative incidence; the rifampin regimen was not superior to the isoniazid regimen. The difference in the treatment‐completion rates was 15.1 percentage points (95% CI, 12.7 to 17.4). The rate differences for adverse events of grade 3 to 5 occurring within 146 days (120% of the 4‐month planned duration of the rifampin regimen) were ‐1.1 percentage points (95% CI, ‐1.9 to ‐0.4) for all events and ‐1.2 percentage points (95% CI, ‐1.7 to ‐0.7) for hepatotoxic events. CONCLUSIONS The 4‐month regimen of rifampin was not inferior to the 9‐month regimen of isoniazid for the prevention of active tuberculosis and was associated with a higher rate of treatment completion and better safety. (Funded by the Canadian Institutes of Health Research and the Australian National Health and Medical Research Council; ClinicalTrials.gov number, NCT00931736.)

Long term follow-up of drug resistant and drug susceptible tuberculosis contacts in a Low incidence setting

BackgroundStudies examining the transmission of multidrug-resistant tuberculosis (MDR-TB) strains have yielded conflicting results.MethodsWe examined transmission of MDR-TB strains using contact tracing data from a low incidence setting. Contacts of MDR-TB cases diagnosed in British Columbia, Canada, from 1990-2008 were identified through a provincial tuberculosis (TB) registry. Tuberculin skin test (TST) results and TB disease incident rates were determined for contacts. For comparison, TB disease incident rates and TST results were measured in close contacts of isoniazid mono-resistant (HMR-TB) and drug susceptible TB (DS-TB) cases.ResultsOf 89 identified close contacts of MDR-TB patients, 5 patients (6%) developed TB disease and 42 (47%) were TST positive. The incidence rate of TB disease (3%, p = 0.31) and TST positivity (49%, p = 0.82) were similar in contacts of HMR-TB cases. Compared with MDR-TB contacts, DS-TB contacts had lower incidence rate of TB disease (2%, p = 0.04) and TST positivity (32%, p < 0.01). All MDR-TB contacts with culture positive TB diagnosed in follow-up were drug-susceptible; three of six HMR-TB contacts with culture positive TB were HMR-TB. Multivariate analysis demonstrated that contact with MDR-TB (adjusted OR 1.72; 95%CI 1.05-2.81) and HMR-TB (adjusted OR 1.99; 95%CI 1.48-2.67) was associated with TST positivity. In addition, adult age, male gender, BCG positivity, source case sputum smear positivity, foreign birth and fewer contacts per source case were significantly associated with TST positivity in the multivariate model.ConclusionContacts of MDR-TB and HMR-TB patients in a low incidence setting show high rates of TST positivity and TB disease but low rates of drug resistance.

Safety and Side Effects of Rifampin versus Isoniazid in Children

BACKGROUND The treatment of latent infection with Mycobacterium tuberculosis is important in children because of their vulnerability to life‐threatening forms of tuberculosis disease. The current standard treatment — 9 months of isoniazid — has been associated with poor adherence and toxic effects, which have hampered the effectiveness of the drug. In adults, treatment with 4 months of rifampin has been shown to be safer and to have higher completion rates than 9 months of isoniazid. METHODS In this multicenter, open‐label trial, we randomly assigned 844 children (<18 years of age) with latent M. tuberculosis infection to receive either 4 months of rifampin or 9 months of isoniazid. The primary outcome was adverse events of grade 1 to 5 that resulted in the permanent discontinuation of a trial drug. Secondary outcomes were treatment adherence, side‐effect profile, and efficacy. Independent review panels whose members were unaware of trial‐group assignments adjudicated all adverse events and progression to active tuberculosis. RESULTS Of the children who underwent randomization, 829 were eligible for inclusion in the modified intention‐to‐treat analysis. A total of 360 of 422 children (85.3%) in the rifampin group completed per‐protocol therapy, as compared with 311 of 407 (76.4%) in the isoniazid group (adjusted difference in the rates of treatment completion, 13.4 percentage points; 95% confidence interval [CI], 7.5 to 19.3). There were no significant between‐group differences in the rates of adverse events, with fewer than 5% of the children in the combined groups with grade 1 or 2 adverse events that were deemed to be possibly related to a trial drug. Active tuberculosis, including 1 case with resistance to isoniazid, was diagnosed in 2 children in the isoniazid group during 542 person‐years of follow‐up, as compared with no cases in the rifampin group during 562 person‐years (rate difference, ‐0.37 cases per 100 person‐years; 95% CI, ‐0.88 to 0.14). CONCLUSIONS Among children under the age of 18 years, treatment with 4 months of rifampin had similar rates of safety and efficacy but a better rate of adherence than 9 months of treatment with isoniazid. (Funded by the Canadian Institutes of Health Research and Conselho Nacional de Pesquisa; ClinicalTrials.gov number, NCT00170209.)

Universal Genotyping for Tuberculosis Prevention Programs: a 5-Year Comparison with On-Request Genotyping

ABSTRACT Prospective universal genotyping of tuberculosis (TB) isolates is used by many laboratories to detect clusters of cases and inform contact investigations. Prior to universal genotyping, most TB prevention programs genotyped isolates on request only, relying on requests from public health professionals whose knowledge of a patients clinical, demographic, and epidemiological characteristics suggested potential transmission. To justify the switch from on-request to universal genotyping—particularly in the public health domain, with its limited resources and competing priorities—it is important to demonstrate the additional benefit provided by a universal genotyping program. We compared the clustering patterns revealed by retrospective 24-locus mycobacterial interspersed repetitive unit–variable-number tandem repeat genotyping of all culture-positive isolates over a 5-year period to the patterns previously established by our genotyping-on-request program in the low-incidence setting of British Columbia, Canada. We found that 23.8% of isolates were requested during the study period, and while requested isolates had increased odds of belonging to a genotype cluster (adjusted odds ratio, 2.3; 95% confidence interval, 1.5 to 3.3), only 54.6% clustered with the requested comparator strain. Universal genotyping revealed 94 clusters ranging in size from 2 to 53 isolates (mean = 5) and involving 432 individuals. On-request genotyping missed 54 (57.4%) of these clusters and 130 (30.1%) clustered individuals. Our results underscore that TB patient networks are complex, with unrecognized linkages between patients, and a prospective province-wide universal genotyping program provides an informative, bias-free tool to explore transmission to a degree not possible with on-request genotyping.