The global tuberculosis crisis urgently demands new, efficacious, orally available drugs with the potential to shorten and simplify the long and complex treatments for drug-sensitive and drug-resistant disease. Clofazimine, a riminophenazine used for many years to treat leprosy, demonstrates efficacy in animal models of tuberculosis via a novel mode of action. However, clofazimine's physicochemical and pharmacokinetic properties contribute to side effects that limit its use; in particular, an extremely long half-life and propensity for tissue accumulation together with clofazimine's dye properties leads to unwelcome skin discoloration. We recently conducted a systematic structure-activity study of more than 500 riminophenazine analogs for antiMycobacterium tuberculosis activity. We describe here the characteristics of 12 prioritized compounds in more detail. The new riminophenazine analogs demonstrated enhanced in vitro activity compared to clofazimine against replicating M. tuberculosis H37Rv, as well as panels of drug-sensitive and drug-resistant clinical isolates. The new compounds demonstrate at least equivalent activity compared to clofazimine against intracellular M. tuberculosis and, in addition, most of them were active against nonreplicating M. tuberculosis. Eleven of these more water-soluble riminophenazine analogs possess shorter half-lives than clofazimine when dosed orally to mice, suggesting that they may accumulate less. Most importantly, the nine compounds that progressed to efficacy testing demonstrated inhibition of bacterial growth in the lungs that is superior to the activity of an equivalent dose of clofazimine when administered orally for 20 days in a murine model of acute tuberculosis. The efficacy of these compounds, along with their decreased potential for accumulation and therefore perhaps also for tissue discoloration, warrants further study. Despite global efforts, tuberculosis (TB) remains responsible for the second greatest number of deaths due to an infectious disease with 1.7 million deaths reported due to TB in 2009 (45). Of particular concern, the increasing prevalence of TB caused by multidrug-resistant (MDR) and extensively drugresistant (XDR) strains of Mycobacterium tuberculosis puts at risk hard-won gains to public health. MDR-TB treatment regimens, where available, comprise multiple expensive drugs with limited efficacy and significant toxicity that must be administered by both oral and parenteral routes for up to 24 months (30). Treatment of drug-sensitive TB is also long and complex, requiring at least 6 months of a four-drug regimen to achieve a stable cure. The first-line TB drugs are poorly tolerated, reducing compliance and increasing the risk of resistance development. New TB drugs that are safe, orally available, have novel modes of action and efficacy sufficient to simplify and shorten the regimen required to cure TB would impact both patients and TB control by providing improved treatment for drug-resistant TB and by providing a faster, more tolerable cure for dr...