Macrolide resistance mechanisms can be target-based with a change in a 23S ribosomal RNA (rRNA) residue or a mutation in ribosomal protein L4 or L22 affecting the ribosome's interaction with the antibiotic. Alternatively, mono-or dimethylation of A2058 in domain V of the 23S rRNA by an acquired rRNA methyltransferase, the product of an erm (erythromycin ribosome methylation) gene, can interfere with antibiotic binding. Acquired genes encoding efflux pumps, most predominantly mef(A) þ msr(D) in pneumococci/streptococci and msr(A/B) in staphylococci, also mediate resistance. Drug-inactivating mechanisms include phosphorylation of the 2 0 -hydroxyl of the amino sugar found at position C5 by phosphotransferases and hydrolysis of the macrocyclic lactone by esterases. These acquired genes are regulated by either translation or transcription attenuation, largely because cells are less fit when these genes, especially the rRNA methyltransferases, are highly induced or constitutively expressed. The induction of gene expression is cleverly tied to the mechanism of action of macrolides, relying on antibiotic-bound ribosomes stalled at specific sequences of nascent polypeptides to promote transcription or translation of downstream sequences. Macrolide antibiotics are polyketides composed of a 14-, 15-, or 16-membered macrocyclic lactone ring (14-, 15-, and 16-membered) to which several sugars and/or side chains have been attached by the producing organism or as modifications during semisynthesis in the laboratory (Figs. 1 and 2). Newer semisynthetic derivations, like ketolides telithromycin, and solithromycin, have a C3-keto group in place of the C3 cladinose (akin to naturally occurring pikromycin) (Brockmann and Henkel 1950) and an 11,12-cyclic carbamate with an extended alkyl -aryl side chain that increases the affinity of the antibiotic for the ribosome by 10-to 100-fold (Hansen et al. 1999;Dunkle et al. 2010); in the case of solithromycin, a fluorine substituent at C2 provides an additional ribosomal interaction (Llano-Sotelo et al. 2010). Macrolides continue to be important in the therapeutic treatment of community-acquired pneumonia (Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and atypicals Legionella pneumophila, Mycoplasma pneumoniae, Chlamydia pneumoniae), sexually transmitted diseases (Neiserria gonorhoeae, Chlamydia trachomatis, Mycoplasma genitalium), shigellosis, and salmonellosis. With solithromycin heading for a new drug application (NDA) filing in 2016 and having the in vitro potency to treat erythromycin-resistant

We describe novel rifamycin derivatives (new chemical entities [NCEs]) that retain significant activity against a comprehensive collection of Staphylococcus aureus strains that are resistant to rifamycins. This collection of resistant strains contains 21 of the 26 known single-amino-acid alterations in RpoB, the target of rifamycins. Some NCEs also demonstrated a lower frequency of resistance development than rifampin and rifalazil in S. aureus as measured in a resistance emergence test. When assayed for activity against the strongest rifamycin-resistant mutants, several NCEs had MICs of 2 g/ml, in contrast to MICs of rifampin and rifalazil, which were 512 g/ml for the same strains. The properties of these NCEs therefore demonstrate a significant improvement over those of earlier rifamycins, which have been limited primarily to combination therapy due to resistance development, and suggest a potential use of these NCEs for monotherapy in several clinical indications.