C25-modified rifabutin analogs as a novel medicinal chemistry strategy to overcome drug-resistant tuberculosis - ABSTRACT While tuberculosis (TB) cure rates have reached 85% globally, this progress is threatened by the emergence of drug resistance. Rifampicin-resistant TB (RR-TB) occurs due to acquired mutations in the drug’s target – the beta-subunit (RpoB) of bacterial RNA polymerase (RNAP) – that reduce drug-target binding. Multidrug-resistant TB (MDR-TB), defined as resistance to both rifampicin and isoniazid, has also emerged. MDR/RR-TB cannot be treated with first-line TB drug regimens. Instead, treatment has typically relied on second-line drugs that have worse side effects, require longer treatment times, and are increasingly ineffective due to drug resistance. Just 60% of global MDR/RR-TB cases can be cured with the highest mortality observed among HIV-coinfected patients, who require drugs that are compatible with anti-retroviral therapy (ART). Better drugs are needed to address the global health threat presented by MDR/RR-TB. Recently, we established a medicinal chemistry program to redesign rifamycins for Mycobacterium abscessus lung disease, synthesizing 150 novel C25- modified rifabutin (RFB) analogs. Many of these compounds display attractive pharmacokinetic (PK) properties and significant reductions in cytochrome P450 3A4 (CYP3A4) induction. Unexpectedly, we found that a subset of C25-modified RFB analogs were 10-200 times more potent than RFB against two MDR-TB strains with RpoB S450L and D435V mutations. Given these findings, the major goals of this application are to (i) establish the structure-activity relationship and mechanism of action of MDR-TB active RFB analogs, (ii) demonstrate activity of MDR-TB active RFB analogs against a majority of RpoB mutations encountered in the clinic, and (iii) demonstrate in vivo proof-of-concept against MDR/RR-TB for this compound series. Our team of antimycobacterial drug discovery experts in microbiology and pharmacology at the Center for Discovery and Innovation will leverage longstanding and successful partnerships with external chemistry and structural biology collaborators. We will also use an existing library of C25-modified RFB analogs to de-risk our approach in several critical ways. C25-modified RFB analogs can exploit the RFB-susceptibility of a subset of MDR/RR-TB strains, providing a better baseline for finding potent MDR/RR-TB actives. Since many C25- modified RFB analogs have improved PK properties and reduced CYP3A4 induction, screening them will increase the likelihood of identifying compounds with not just MDR/RR-TB activity but also good PK and ART compatibility. In this way, we will establish the potential of C25-modified RFB analogs as a novel medicinal chemistry strategy to overcome MDR/RR-TB. We will identify a lead compound with enhanced potency against MDR/RR-TB, broad spectrum activity against a range of MDR/RR-TB isolates, reduced CYP3A4 induction, and in vivo proof of concept. These efforts will form the basis for a subsequent lead optimization program and, ultimately, development of a new drug candidate for MDR/RR-TB.
