Higher pretreatment drug concentrations close to a resistance breakpoint for susceptibility were associated with greater relapse risk in TB, based on data from 54 patients who relapsed and 63 who were treated and cured.
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“We postulated that drug-susceptible Mycobacterium tuberculosis might have a graded spectrum of susceptibilities that could be used to determine the risk of relapse,” wrote Roberto Colangeli, PhD, of Rutgers University, Newark, N.J., and his colleagues.
In a study published in the New England Journal of Medicine, the researchers examined pretreatment bacterial isolates from adults with TB who had experienced relapse and those who were cured. Using these isolates, they identified the minimum inhibitory concentration (MIC) – the lowest concentration of the drug that prevents visible bacterial growth in culture – for isoniazid and rifampin.
Overall, after controlling for other potential relapse risk factors, higher pretreatment MIC values for both isoniazid and rifampin were associated with an increased relapse risk. For isoniazid, the average MIC below the breakpoint was 0.0334 mcg/mL for relapsed patients and 0.0286 mcg/mL for cured patients. For rifampin, the average MIC below the breakpoint was 0.0695 mcg/mL for relapsed patients and 0.0453 mcg/mL for cured patients. The higher values for the relapsed versus cured patients were represented by factors of 1.17 and 1.53 for isoniazid and rifampin, respectively.
The average age of the patients was 41 years; 83% were men, and 35% were non-Hispanic white.
The study findings were limited by several factors, including the small sample size, retrospective design, and inability to test MIC values from primary cultures versus subcultures, the researchers wrote. However, the results suggest an impact of MIC values on treatment outcomes, and “additional studies that are performed in larger, well-defined prospective cohorts and that include MIC testing of pretreatment culture isolates will be useful to better validate these findings,”
The study was funded by the National Institute of Allergy and Infectious Diseases. Dr. Colangeli reported no financial conflicts. Dr. Alland disclosed funding from Cepheid and several current and pending patents in the United States and Europe, with some royalties paid to Cepheid.
SOURCE: Colangeli R et al. N Engl J Med. 2018;379:823-33.