Alternative title: So your friend asks: What does your sister do in lab all day?

In 2007, Andrew “Drew” Speaker brought tuberculosis (TB) into the first-world spotlight when he traveled from Atlanta to Paris and then returned on a flight from Prague to Montreal and back into the United States – all while infected with extensively-drug resistant tuberculosis (XDR-TB). This incident triggered uproar in the United States, and may have had implications in the infections of two Czechs and several Canadians.

However, this is just a tiny, microscopic drop in the bucket in the impact and burden TB and XDR-TB has placed on multiple countries around the world. Currently, Mycobacterium tuberculosis (Mtb), the microbial agent of TB, infects one-third of the world’s population and kills approximately 2 million people per year world wide (WHO, 2000). Although we have antibiotics that are efficient for killing Mtb, the drug regimens are months long and in most cases the antibiotics can cause symptoms more severe than TB. Nonadherence with the lengthy course of drug treatment has contributed greatly to the emergence of multi-drug resistant TB (MDR-TB) strains and extensively-drug resistant TB (XDR-TB). For these reasons, it has been a large focus in the TB research field to find new anti-TB drugs to somehow shorten and/or simplify current TB treatments.

Figure 1: Nature news (2007): TB's toll: Countries in sub-Saharan Africa and Asia have the highest prevalence of tuberculosis.

What are the new steps in anti-TB drug development?

Napier, et al. took a new approach to combating TB, instead of using antibiotics that target TB, why not target host proteins that TB require for survival and persistence? Interestingly, they use the drug imatinib (or Gleevec) to inhibit host tyrosine kinases (TK) that they found are required for TB replication and survival.

Figure 2: The cover image of Cell Host & Microbe, November 17, 2011: Depicts Green Fluorescent Protein (GFP)-expressing Mycobacterium marinum infecting a murine bone marrow-derived macrophage. Cellular actin is stained with phalloidin (in red) and the nucleus is stained with DAPI (in blue). Image by R. Napier, K. Ris-Vicari, and D. Kalman.

What are tyrosine kinases?

Tyrosine kinases are a group of enzymes that transfer a phosphate group from ATP to a protein (picture left, below; Figure 3), and generally serve to regulate cell-to-cell communication and different cellular activities. Additionally, when kinases become mutated, or stuck in the “on” position, this causes unregulated growth of th e cell which can cause cancer – therefore inhibitors of kinases, such as imatinib can be used as cancer treatments.

Imatinib specifically inhibits function of the ABL family TKs , ABL1 and ABL2 TKs have been shown to not only be important in mediating cancer, but also bacterial and viral pathogenesis (Lebeis and Kalman, 2009). To date, it is known that multiple bacterial species, such as Pseudomonas aeruginosa, Shigella flexneri, and Chlamydia trachomatis require host ABL family TKs during entry into the cell – therefore, the question was formulated:

Are ABL family TKs required for Mycobacterium tuberculosis infection?

Yes - Napier, et al. reported that when they infected cell lines lacking ABL family TK’s and cells treated with imatinib, the specific ABL TK inhibitor, Mycobacterium tuberculosis showed defects in entry and survival.

Figure 4: Imatinib Reduces Bacterial Load in Mice Infected with Mycobacterium marinum: Cfu/g (bacterial load) from liver (B) or lungs (C) was determined at 7 days p.i. (D and E) Mice were infected with Mm as in (A–C), but administration of imatinib (100 mg/kg/day) commenced 1 h (D) or 24 h (E) p.i. Each point represents an individual mouse.

Additionally, imatinib reduced bacterial load (picture above) and associated pathologies, including tail lesions seen in infected mice without treatment of imatinib (picture below). These data combined, show that an ABL family TK inhibitor can act as a drug against TB infections. The mechanism of when ABL family TKs are important for TB infections has still yet to be elucidated; however, they speculate these TKs may be important for mediation of cell entry or receptor-mediated trafficking of the phagosome.

Follow-up paper, perhaps?

Figure 5: Imatinib Reduces Tail Lesions in Mice Infected with Mycobacterium marinum: Images of tails from mice infected intravenously and treated with carrier (H2O) or imatinib (100 mg/kg/day) for 7 days beginning 24 h p.i. Each mouse received one injection.

They didn’t stop there:

Next, they found that imatinib acts in synergy with frontline anti-TB antibiotics, such as rifabutin (picture below). This means, when a patient is treated with both drugs combined, the outcome is a faster clearance of TB, and therefore a shorter treatment period. Also, if you clear a TB infection faster, you can cut the time for TB to evolve resistance to antibiotics used – thereby increasing the efficacy and clinical lifespan of the drug.

Figure 6: Imatinib and Antibiotics Act in Synergy to Reduce TB Survival: Mice were administered imatinib (100 mg/kg/day), rifabutin (2.5 mg/kg/day, intraperitoneally), or both drugs together. Cfu (bacterial load) in the spleen were determined at day 7 p.i.

Last but not least, they point out that imatinib may be useful against antibiotic-susceptible, MDR-TB, AND XDR-TB, because it targets the host TKs, not bacterial factors, therefore it is highly unlikely to form resistance to this drug. It will be interesting to know the next step in this research, and to find out when and why blocking ABL TKs during TB infections can block bacterial replication, and how do front-line drugs like rifabutin work in synergy with imatinib.