Synthesis and antibacterial activity of some ester, amides, and hydrazides of 3-carboxyrifamycin S. Relationship between structure and activity of ansamycins

Synthesis of narrow-spectrum anti-mycobacterial molecules without effect on the diversity of gut microbiota in mice based on the structure of rifampicin

Rifampicin (RIF) is a broad-spectrum antimicrobial agent that is also a first-line drug for treating tuberculosis (TB). Based on the naphthyl ring structure of RIF this study synthesized 16 narrow-spectrum antimicrobial molecules that were specifically anti-Mycobacterium tuberculosis (Mtb). The most potent candidate was 2-((6-hydroxynaphthalen-2-yl) methylene) hydrazine-1-carbothioamide (compound 3c) with minimum inhibitory concentration (MIC) of 1 μg/mL against Mtb. Synergistic anti-Mtb test indicated that none of the combinations of 3c with the major anti-TB drugs are antagonistic. Consistent with RIF, compound 3c induced large amounts of reactive oxygen radicals (ROS) in the cells of Mtb. The killing kinetics of compound 3c and RIF are very similar. Furthermore, molecular docking showed that compound 3c was able to access the RIF binding pocket of the β subunit of Mtb RNA polymerase (RNAP). Experiments in mice showed that compound 3c increased the variety of intestinal flora in mice, while RIF significantly decreased the diversity of intestinal flora in mice. In addition, compound 3c is non-toxic to animal cells with a selection index (SI) much more than 10. The evidence from this study suggests that the further development of 3c could contribute to the development of novel drug for TB treatment.

Synthesis of Acyl Hydrazides via a Radical Chemistry of Azocarboxylic tert-Butyl Esters

A new chemistry of azo compounds, that is, addition of free radicals generated in situ to access various acyl hydrazides, has been developed. The protocol provides a novel strategy for the synthesis of valuable acyl hydrazides. The transformation features mild reaction conditions, good tolerance of functional groups, and a broad substrate scope. In view of the importance of acyl hydrazides in functional materials and medicinal chemistry, this approach would find broad applications.

Inhibition of RNA Polymerase by Rifampicin and Rifamycin-Like Molecules

RNA polymerases (RNAPs) accomplish the first step of gene expression in all living organisms. However, the sequence divergence between bacterial and human RNAPs makes the bacterial RNAP a promising target for antibiotic development. The most clinically important and extensively studied class of antibiotics known to inhibit bacterial RNAP are the rifamycins. For example, rifamycins are a vital element of the current combination therapy for treatment of tuberculosis. Here, we provide an overview of the history of the discovery of rifamycins, their mechanisms of action, the mechanisms of bacterial resistance against them, and progress in their further development.

A Semisynthetic Kanglemycin Shows In Vivo Efficacy against High-Burden Rifampicin Resistant Pathogens

Semisynthetic rifamycin derivatives such as rifampicin (Rif) are first line treatments for tuberculosis and other bacterial infections. Historically, synthetic modifications made to the C-3/C-4 region of the rifamycin naphthalene core, like those seen in Rif, have yielded the biggest improvements in pharmacological properties. However, modifications found in natural product rifamycin congeners occur at other positions in the structure. The kanglemycins (Kangs) are a family of rifamycin congeners with a unique collection of natural modifications including a dimethylsuccinic acid appended to their polyketide backbone. These modifications confer activity against the single most common clinically relevant Rif resistance (Rif^R) mutation in the antibiotic’s target, the bacterial RNA polymerase (RNAP). Here we evaluate the in vivo efficacy of Kang A, the parent compound in the Kang family, in a murine model of bacterial peritonitis/sepsis. We then set out to improve its potency by combining its natural tailoring modifications with semisynthetic derivatizations at either its acid moiety or in the C-3/C-4 region. A collection of C-3/C-4 benzoxazino Kang derivatives exhibit improved activity against wild-type bacteria, and acquire activity against the second most common clinically relevant Rif^R mutation. The semisynthetic analogue 3′-hydroxy-5′-[4-isobutyl-1-piperazinyl] benzoxazino Kang A (Kang KZ) protected mice against infection with either Rif sensitive MRSA or a highly virulent Rif^RStaphylococcus  aureus strain in a neutropenic peritonitis/sepsis model and led to reduced bacterial burdens. The compounds generated in this study may represent promising candidates for treating Rif^R infections.

Rifamycins--obstacles and opportunities

With nearly one-third of the global population infected by Mycobacterium tuberculosis, TB remains a major cause of death (1.7 million in 2006). TB is particularly severe in parts of Asia and Africa where it is often present in AIDS patients. Difficulties in treatment are exacerbated by the 6-9 month treatment times and numerous side effects. There is significant concern about the multi-drug-resistant (MDR) strains of TB (0.5 million MDR-TB cases worldwide in 2006). The rifamycins, long considered a mainstay of TB treatment, were a tremendous breakthrough when they were developed in the 1960's. While the rifamycins display many admirable qualities, they still have a number of shortfalls including: rapid selection of resistant mutants, hepatotoxicity, a flu-like syndrome (especially at higher doses), potent induction of cytochromes P450 (CYP) and inhibition of hepatic transporters. This review of the state-of-the-art regarding rifamycins suggests that it is quite possible to devise improved rifamycin analogs. Studies showing the potential of shortening the duration of treatment if higher doses could be tolerated, also suggest that more potent (or less toxic) rifamycin analogs might accomplish the same end. The improved activity against rifampin-resistant strains by some analogs promises that further work in this area, especially if the information from co-crystal structures with RNA polymerase is applied, should lead to even better analogs. The extensive drug-drug interactions seen with rifampin have already been somewhat ameliorated with rifabutin and rifalazil, and the use of a CYP-induction screening assay should serve to efficiently identify even better analogs. The toxicity due to the flu-like syndrome is an issue that needs effective resolution, particularly for analogs in the rifalazil class. It would be of interest to profile rifalazil and analogs in relation to rifampin, rifapentine, and rifabutin in a variety of screens, particularly those that might relate to hypersensitivity or immunomodulatory processes.