Fellow’s research: The antimalarial chloroquine makes existing anti-TB treatment more effective
06 Dec 2019
Prof Amit Singh, Indian Institute of Science, Bangalore
In our study published recently, we identified factors that aid drug tolerance in replicating Mycobacterium tuberculosis (Mtb), the tuberculosis (TB)-causing bacteria. This helped us identify chloroquine, an antimalarial drug, as a potential addition to the existing anti-TB regimens to overcome drug tolerance and disease relapse.
A major challenge to the clinical treatment of TB is the long therapy time (6–9 months) required to clear the infection. A possible reason is the development of drug-tolerant sub-populations in otherwise genetically identical population of Mtb. This form of drug insensitivity is called phenotypic drug tolerance, which is achieved without genetic alterations and represents the greatest hurdle to effective chemotherapy.
Heterogeneity in bacterial population is a likely contributor to phenotypic drug tolerance in the sputum of active TB patients. However, molecular basis of heterogeneity is largely uncharacterized in the context of TB infection in vivo.
When Mtb infects a host (e.g. human body), it is normally engulfed by immune cells, such as macrophages, that promote an oxidative environment unfit for the survival of Mtb. In the present study, we identified that host environment can promote variations in redox physiology of Mtb population that promote tolerance to anti-TB drugs. We mechanistically dissect host and bacterial factors responsible for redox heterogeneity and multi-drug tolerance in Mtb population during infection. We discovered that limited phagosomal acidification inside naïve macrophages facilitates the emergence of a redox-altered, drug-tolerant subpopulation of Mtb. To this end, we used a range of cutting-edge technologies such as redox biosensor, replication clock, flow sorting, intra-phagosomal RNA-sequencing, mass spectrometry, and animal models.
Our study gives rise to new possibilities for managing phenotypic antibiotic resistance. For example, inhibitors of host signal(s) that are sensed by Mtb to generate redox diversity could be exploited to restore phenotypic homogeneity and potentiate the killing activity of existing frontline antibiotics.
On this basis, we reasoned that pharmacological inhibition of phagosomal acidification could prevent development of redox-mediated drug-tolerant phenotype in vivo. We tested our hypothesis by using the antimalarial drug chloroquine (CQ), which is well known to increase vesicular pH. We demonstrated that administering CQ with isoniazid (Inh) or rifampicin (Rif) dramatically reversed the tolerance of Mtb towards these first-line anti-TB drugs. In mice and guinea pigs with chronic Mtb infection, combining CQ with Inh or Rif not only increased the efficacy of the antibiotics in clearing bacterial load and improving disease-related pathology, but also significantly reduced chances of post-treatment relapse.
Pharmacological inhibition of phagosomal acidification can diminish phenotypic drug tolerance during Mtb infection
Since chloroquine is already approved for clinical use and showed no adverse interactions with existing anti-TB drugs, our results provide compelling evidence for its re-positioning into existing anti-TB treatment regimen. Overall, our work provides an elegant proof of how targeting heterogeneity in host-pathogen interactions improves the efficacy of antibiotics against Mycobacterium tuberculosis (Mtb). These findings have immense clinical and public health relevance, considering the emergence of drug-resistant forms of TB.
Targeting redox heterogeneity to counteract drug tolerance in replicating Mycobacterium tuberculosis. Richa Mishra, Sakshi Kohli, Nitish Malhotra, Parijat Bandyopadhyay, Mansi Mehta, Mohamed Husen Munshi, Vasista Adiga, Vijay Kamal Ahuja, Radha K. Shandil, Raju S. Rajmani, Aswin Sai Narain Seshasayee and Amit Singh. Science Translational Medicine. November 2019.