The antibiotic sorangicin A inhibits promoter DNA unwinding in a Mycobacterium tuberculosis rifampicin-resistant RNA polymerase.

Rifampicin (Rif) is a first-line therapeutic used to treat the infectious disease tuberculosis (TB), which is caused by the pathogen Mycobacterium tuberculosis (Mtb). The emergence of Rif-resistant (RifR) Mtb presents a need for new antibiotics. Rif targets the enzyme RNA polymerase (RNAP). Sorangicin A (Sor) is an unrelated inhibitor that binds in the Rif-binding pocket of RNAP. Sor inhibits a subset of RifR RNAPs, including the most prevalent clinical RifR RNAP substitution found in Mtb infected patients (S456>L of the ß subunit). Here, we present structural and biochemical data demonstrating that Sor inhibits the wild-type Mtb RNAP by a similar mechanism as Rif: by preventing the translocation of very short RNAs. By contrast, Sor inhibits the RifR S456L enzyme at an earlier step, preventing the transition of a partially unwound promoter DNA intermediate to the fully opened DNA and blocking the template-strand DNA from reaching the active site in the RNAP catalytic center. By defining template-strand blocking as a mechanism for inhibition, we provide a mechanistic drug target in RNAP. Our finding that Sor inhibits the wild-type and mutant RNAPs through different mechanisms prompts future considerations for designing antibiotics against resistant targets. Also, we show that Sor has a better pharmacokinetic profile than Rif, making it a suitable starting molecule to design drugs to be used for the treatment of TB patients with comorbidities who require multiple medications.

Sorangicin A Is Active against Chlamydia in Cell Culture, Explanted Fallopian Tubes, and Topical In Vivo Treatment.

Current treatment of Chlamydia trachomatis using doxycycline and azithromycin introduces detrimental side effects on the host's microbiota. As a potential alternative treatment, the myxobacterial natural product sorangicin A (SorA) blocks the bacterial RNA polymerase. In this study we analyzed the effectiveness of SorA against C. trachomatis in cell culture, and explanted fallopian tubes and systemic and local treatment in mice, providing also pharmacokinetic data on SorA. Potential side effects of SorA on the vaginal and gut microbiome were assessed in mice and against human-derived Lactobacillus species. SorA showed minimal inhibitory concentrations of 80 ng/mL (normoxia) to 120 ng/mL (hypoxia) against C. trachomatis in vitro and was eradicating C. trachomatis at a concentration of 1 µg/mL from fallopian tubes. In vivo, SorA reduced chlamydial shedding by more than 100-fold within the first days of infection by topical application corresponding with vaginal detection of SorA only upon topical treatment, but not after systemic application. SorA changed gut microbial composition during intraperitoneal application only and did neither alter the vaginal microbiota in mice nor affect growth of human-derived lactobacilli. Additional dose escalations and/or pharmaceutical modifications will be needed to optimize application of SorA and to reach sufficient anti-chlamydial activity in vivo.

Natural Products Lysobactin and Sorangicin A Show In Vitro Activity against Mycobacterium abscessus Complex.

The prevalence of lung disease caused by Mycobacterium abscessus is increasing among patients with cystic fibrosis. M. abscessus is a multidrug resistant opportunistic pathogen that is notoriously difficult to treat due to a lack of efficacious therapeutic regimens. Currently, there are no standard regimens, and treatment guidelines are based empirically on drug susceptibility testing. Thus, novel antibiotics are required. Natural products represent a vast pool of biologically active compounds that have a history of being a good source of antibiotics. Here, we screened a library of 517 natural products purified from fermentations of various bacteria, fungi, and plants against M. abscessus ATCC 19977. Lysobactin and sorangicin A were active against the M. abscessus complex and drug resistant clinical isolates. These natural products merit further consideration to be included in the M. abscessus drug pipeline. IMPORTANCE The many thousands of people living with cystic fibrosis are at a greater risk of developing a chronic lung infection caused by Mycobacterium abscessus. Since M. abscessus is clinically resistant to most anti-TB drugs available, treatment options are limited to macrolides. Despite macrolide-based therapies, cure rates for M. abscessus lung infections are 50%. Using an in-house library of curated natural products, we identified lysobactin and sorangicin A as novel scaffolds for the future development of antimicrobials for patients with M. abscessus infections.