Research Article: Priority-Setting for Novel Drug Regimens to Treat Tuberculosis: An Epidemiologic Model

Date Published: January 3, 2017

Publisher: Public Library of Science

Author(s): Emily A. Kendall, Sourya Shrestha, Ted Cohen, Eric Nuermberger, Kelly E. Dooley, Lice Gonzalez-Angulo, Gavin J. Churchyard, Payam Nahid, Michael L. Rich, Cathy Bansbach, Thomas Forissier, Christian Lienhardt, David W. Dowdy, Katharina Kranzer

Abstract: BackgroundNovel drug regimens are needed for tuberculosis (TB) treatment. New regimens aim to improve on characteristics such as duration, efficacy, and safety profile, but no single regimen is likely to be ideal in all respects. By linking these regimen characteristics to a novel regimen’s ability to reduce TB incidence and mortality, we sought to prioritize regimen characteristics from a population-level perspective.Methods and FindingsWe developed a dynamic transmission model of multi-strain TB epidemics in hypothetical populations reflective of the epidemiological situations in India (primary analysis), South Africa, the Philippines, and Brazil. We modeled the introduction of various novel rifampicin-susceptible (RS) or rifampicin-resistant (RR) TB regimens that differed on six characteristics, identified in consultation with a team of global experts: (1) efficacy, (2) duration, (3) ease of adherence, (4) medical contraindications, (5) barrier to resistance, and (6) baseline prevalence of resistance to the novel regimen. We compared scale-up of these regimens to a baseline reflective of continued standard of care.For our primary analysis situated in India, our model generated baseline TB incidence and mortality of 157 (95% uncertainty range [UR]: 113–187) and 16 (95% UR: 9–23) per 100,000 per year at the time of novel regimen introduction and RR TB incidence and mortality of 6 (95% UR: 4–10) and 0.6 (95% UR: 0.3–1.1) per 100,000 per year. An optimal RS TB regimen was projected to reduce 10-y TB incidence and mortality in the India-like scenario by 12% (95% UR: 6%–20%) and 11% (95% UR: 6%–20%), respectively, compared to current-care projections. An optimal RR TB regimen reduced RR TB incidence by an estimated 32% (95% UR: 18%–46%) and RR TB mortality by 30% (95% UR: 18%–44%). Efficacy was the greatest determinant of impact; compared to a novel regimen meeting all minimal targets only, increasing RS TB treatment efficacy from 94% to 99% reduced TB mortality by 6% (95% UR: 1%–13%, half the impact of a fully optimized regimen), and increasing the efficacy against RR TB from 76% to 94% lowered RR TB mortality by 13% (95% UR: 6%–23%). Reducing treatment duration or improving ease of adherence had smaller but still substantial impact: shortening RS TB treatment duration from 6 to 2 mo lowered TB mortality by 3% (95% UR: 1%–6%), and shortening RR TB treatment from 20 to 6 mo reduced RR TB mortality by 8% (95% UR: 4%–13%), while reducing nonadherence to the corresponding regimens by 50% reduced TB and RR TB mortality by 2% (95% UR: 1%–4%) and 6% (95% UR: 3%–10%), respectively. Limitations include sparse data on key model parameters and necessary simplifications to model structure and outcomes.ConclusionsIn designing clinical trials of novel TB regimens, investigators should consider that even small changes in treatment efficacy may have considerable impact on TB-related incidence and mortality. Other regimen improvements may still have important benefits for resource allocation and outcomes such as patient quality of life.

Partial Text: The number of available or prospective drugs for treating tuberculosis (TB) is undergoing a long-overdue expansion. Delamanid and bedaquiline, both recently approved for the treatment of multidrug-resistant (MDR) TB [1,2], are the first novel agents registered for TB treatment in decades. Antibiotic classes such as carbapenems [3] and oxazolidinones [4] are also being repurposed to treat highly resistant TB cases. There is hope that later-generation fluoroquinolones [5], rifamycins [6], and newer drug classes [7,8] could shorten first-line treatment for TB (usually six mo), and in 2016 WHO endorsed a regimen that shortens MDR TB treatment to 9–11 mo [9] from a conventional duration of at least 18–20 mo. Despite these advances, however, many characteristics of TB regimens could be further improved, including not only treatment duration but also tolerability [10,11], efficacy [12,13], drug–drug interactions and medical indications [14,15], and the barrier against acquiring drug resistance while on therapy [16,17].

We created a deterministic compartmental transmission model of a pulmonary TB epidemic in an adult population, similar to prior models with respect to the natural history of TB and HIV [24,25], but incorporating additional structure related to TB treatment and drug-susceptibility phenotypes in order to simultaneously model resistance to rifampicin and to components of novel regimens (Fig 1). Parameters related to novel regimen characteristics (Table 1) were determined through an expert consultation process described below and in S1 Methods.

We used a dynamic transmission model in a series of idealized settings to help prioritize characteristics of novel drug regimens for treating TB. We found that increases in efficacy, for both RS TB and RR TB regimens, have the greatest potential to reduce TB incidence and mortality through direct impacts on treatment outcomes and resulting TB transmission. Shortened duration and improved tolerability may also yield substantial population-level benefits, but these will come in part through facilitating expanded treatment availability or reallocation of resources from treatment to other aspects of TB control. This process of using an epidemiological model, in ongoing consultation with worldwide experts, to help prioritize elements of new drug regimens offers a new approach to inform the development of combination antimicrobial regimens.



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