Emerging TB Treatments Require New Paradigm for Drug Classification

A new paper published today in The Lancet Infectious Diseases calls for a new paradigm in the classification of drug resistance for tuberculosis given the imminent release of new anti-tuberculosis drugs. The authors, Timothy Sullivan of Mount Sinai Hospital and Yanis Ben Amor of the Earth Institute, argue that these new drugs will likely change the standard for tuberculosis care, requiring a new, more nuanced naming system.

The current classification system identifies certain strains of tuberculosis as: 1. multidrug-resistant (resistant to first-line treatments: isoniazid and rifampicin), 2. extensively drug-resistant (resistant to first-line treatments, fluoroquinolone, and any one of three second-line treatments: kanamycin, amikacin, or capreomycin), and 3. totally drug resistant (resistant to all first and second-line treatments).

The increased prevalence of tuberculosis – especially with the emergence of drug-resistant strains – has spurred renewed interest in the development of new drugs and treatment regimens. More than 10 drugs are currently in the development pipeline.

The current system has provided clinical distinctions between different strains of tuberculosis, however, with the release of these new treatments an updated classification system will be required to preserve its clinical relevance. In addition, advancements in drug susceptibility testing will enable a better understanding of the susceptibility of certain strains to individual drugs.

In the paper the authors propose three options for a new classification system:

• change the definition of multidrug-resistant and extensively drug-resistant tuberculosis to reflect resistance to the newer drugs;
• leave the multidrug-resistant and extensively drug-resistant terms unchanged and develop new terms to classify cases of tuberculosis that are resistant to the newer drugs; or
• abandon further attempts to classify patients by broad definitions of drug resistance, if affordable drug susceptibility testing for new and existing drugs becomes available.