Activity of KRM-1648, a new benzoxazinorifamycin, against Mycobacterium tuberculosis in a murine model.

The activity of KRM-1648 was evaluated in a murine model of tuberculosis. Approximately 10(7) viable Mycobacterium tuberculosis ATCC 35801 organisms were given intravenously to 4-week-old female outbred mice. Treatment was started 1 week postinfection and given by gavage for 4 weeks. Viable-cell counts were determined from homogenates of spleen and lung tissues. The activity of KRM-1648 was compared with those of rifampin and rifabutin at 20 mg/kg of body weight. KRM-1648 was more active than either rifampin or rifabutin against organisms in spleens and lungs. KRM-1648 alone and in combination with either isoniazid, ethambutol, pyrazinamide, or levofloxacin was evaluated. Other treatment groups received isoniazid, ethambutol, pyrazinamide, or levofloxacin as single agents. KRM-1648 was the most active single agent evaluated. KRM-1648-pyrazinamide and KRM-1648-isoniazid were the most active combinations. These combinations were more active than KRM-1648 alone. The promising activity of KRM-1648 in M. tuberculosis-infected mice suggests that it is a good candidate for clinical development as a new antituberculosis agent.     

In vitro and in vivo antibacterial activities of KRM-1648 and KRM-1657, new rifamycin derivatives.

The in vitro and in vivo antibacterial activities of the new rifamycin derivatives KRM-1648 and KRM-1657 were compared with those of rifampin. Rifabutin, ciprofloxacin, and clarithromycin were also tested for reference. The respective MICs of KRM-1648 and KRM-1657 for 90% of the strains tested (MIC90S) were 0.016 and 0.0078 microgram/ml, respectively, for methicillin-susceptible Staphylococcus aureus, 0.016 and 0.0039 microgram/ml, respectively, for methicillin-resistant S. aureus, and 0.0625 and 0.016 microgram/ml, respectively, for methicillin- and quinolone-resistant S. aureus. These MIC90S of KRM-1657 were equal to or 2- to 64-fold lower than those of rifampin. KRM-1648 and KRM-1657 with MIC90S of between 0.002 and 0.078 microgram/ml were 2- to 128-fold more active than rifampin against Staphylococcus epidermidis and Streptococcus species, including Streptococcus pneumoniae and Streptococcus pyogenes. The MIC90S of KRM-1657 for Haemophilus influenzae and Neisseria gonorrhoeae were 0.25 and 0.1 microgram/ml, respectively; KRM-1657 was almost as active as rifampin and was 8- to 16-fold more active than KRM-1648 against these strains. The frequency of occurrence of spontaneous mutations to resistance to KRM-1648 and KRM-1657 was equal to that to rifampin. Against systemic infection with S. aureus in mice, the efficacies of KRM-1648 and KRM-1657 were comparable to that of rifampin.     

In vitro activity of the benzoxazinorifamycin KRM-1648 against drug-susceptible and multidrug-resistant tubercle bacilli.

We investigated the activity of benzoxazinorifamycin (KRM-1648) against several drug-susceptible and multidrug-resistant strains of tubercle bacilli. Since KRM-1648 is a rifamycin derivative, we included some strains of Mycobacterium tuberculosis resistant to rifampin (RIF) among the multidrug-resistant strains. For RIF-susceptible strains, the MIC of KRM-1648 was much lower than that of RIF (MICs of KRM-1648 and RIF at which 90% of strains are inhibited, < or = 0.015 and < or = 0.25 micrograms/ml, respectively). The MBC of KRM-1648 (range, 0.007 to 0.03 microgram/ml) was also much lower than that of RIF (range, 0.5 to 1.0 microgram/ml). Postantibiotic effect studies with KRM-1648 showed a rapid reduction in the CFU counts with an exposure of 24 h or more, and its sterilizing effect was maintained even up to 21 days thereafter. Parallel postantibiotic effect studies with RIF showed a less significant effect with a faster recovery of growth, and RIF failed to sterilize the organisms even after 72 h of exposure. KRM-1648 at 0.125 and 0.25 microgram/ml caused complete inhibition of intracellular growth of M. tuberculosis in J774 A.1 macrophages after 48 h of exposure. After a similar exposure time RIF at a concentration of 0.25 microgram/ml caused complete inhibition of growth, but a concentration of 0.125 microgram/ml caused only a 50% reduction in growth compared with that of controls at day 7. With 24 h of pulsed exposure of the intracellular organisms to 0.25 micrograms of the drugs per ml, KRM-1648 caused complete inhibition of intracellular growth, while RIF caused only moderate inhibition of intracellular growth. These findings suggest that KRM-1648 is a potentially useful drug for the treatment of tuberculosis.     

Activity of KRM-1648 in combination with isoniazid against Mycobacterium tuberculosis in a murine model.

The activity of KRM-1648, alone and in combination with isoniazid, was compared with those of isoniazid, rifampin, and the combination of rifampin plus isoniazid in a murine model of tuberculosis. Four-week-old female CD-1 mice were infected intravenously with approximately 10(7) viable Mycobacterium tuberculosis ATCC 35801 organisms. Treatment was started 1 week postinfection and was given by gavage 5 days per week. The duration of the treatment phase was 12 weeks, with groups of mice sacrificed at 2, 4, 6, 8, and 12 weeks. For the observation phase, additional groups of treated mice were sacrificed at 4, 8, 16, and 24 weeks after the cessation of treatment. Viable cell counts were determined from homogenates of the spleens and the right lungs. KRM-1648 was the most active single agent evaluated and resulted in no detectable CFUs in the spleens and lungs by the end of 6 weeks of treatment. Neither rifampin nor isoniazid reduced cell counts to undetectable levels, even after 12 weeks of treatment. The combination of KRM-1648 plus isoniazid was much more active than rifampin plus isoniazid. KRM-1648 plus isoniazid resulted in the apparent sterilization of organs at 6 months following the cessation of treatment. The promising activity of KRM-1648 may allow for ultrashort-course therapy of tuberculosis, i.e., treatment regimens of 4 months or less.     

In vitro antimicrobial activity of benzoxazinorifamycin, KRM-1648, against Mycobacterium avium complex, determined by the radiometric method.

MICs of a newly developed benzoxazinorifamycin derivative, KRM-1648, for Mycobacterium avium complex (MAC) were determined by the BACTEC 460 TB system and compared with those of other known antimicrobial agents. The radiometric method gave a fast, accurate, and reproducible MIC for each antimicrobial agent. MICs of KRM-1648 for 30 strains of MAC (10 strains each of M. avium isolated from AIDS and non-AIDS patients and of Mycobacterium intracellulare isolated from non-AIDS patients) were measured. The MICs, ranging from 0.004 to 0.0625 microgram/ml, were the lowest of all tested drugs, including rifampin, rifabutin, streptomycin, kanamycin, isoniazid, ethambutol, ofloxacin, ciprofloxacin, sparfloxacin, and clarithromycin. The MICs were 2 to 512 and 1 to 32 times lower than those of rifampin and rifabutin, respectively. With rifampin and ethambutol, there were some differences between the MICs for M. avium isolated from AIDS patients (American) and those for M. avium from non-AIDS patients (Japanese). Moreover, appreciable differences between the MICs of some drugs against M. avium and M. intracellulare isolated from non-AIDS patients were found. Many strains of M. avium were more susceptible to ofloxacin than M. intracellulare, but, conversely, M. avium was more resistant to rifampin, streptomycin, ethambutol, and clarithromycin than M. intracellulare.     

Accumulation of KRM-1648 by Mycobacterium aurum and Mycobacterium tuberculosis

After exposure to 2 mg/L (14)C-labelled KRM-1648 (a new broad-spectrum benzoxazinorifamycin antibiotic) for 5 min, a steady-state concentration of 31.3 +/- 3 ng/mg cells KRM-1648 and 12. 6 +/- 0.3 ng/mg cells KRM-1648 was accumulated by wild-type antibiotic-susceptible Mycobacterium aurum (A+) and Mycobacterium tuberculosis (H37Rv), respectively. However, 2 mg/L KRM-1648 was bactericidal for M. tuberculosis. A steady-state concentration of 3. 7 +/- 0.1 ng/mg cells KRM-1648 was accumulated after exposure to 0.5 mg/L. At pH 4 higher concentrations were accumulated than at pH 7. A sub-inhibitory concentration of ethambutol increased the concentration of KRM-1648 accumulated, but Tween 80 and reserpine had little or no effect.     

Low-dose aerosol infection model for testing drugs for efficacy against Mycobacterium tuberculosis.

As a paradigm for chronic infectious diseases, tuberculosis exhibits a variety of clinical presentations, ranging from primary pulmonary tuberculosis to reactivation tuberculosis and cavitary disease. To date, the animal models used in evaluating chemotherapy of tuberculosis have been high-dose intravenous models that mimic the disseminated forms of the disease. In the present study, we have used a low-dose aerosol exposure model which we feel better reflects newly diagnosed tuberculosis in patients converting to tuberculin positivity. As appropriate examples of chemotherapy, four rifamycins (rifampin, rifabutin, rifapentine, and KRM-1648) were tested, first in an in vitro murine macrophage model and then in the low-dose aerosol infection model, for their activity against Mycobacterium tuberculosis. In both models, KRM-1648 had the highest level of activity of the four compounds. In the infected-lung model, rifabutin, rifapentine, and KRM-1648 all had sterilizing activity when given orally at 5 mg/kg of body weight per day. When given at 2.5 mg/kg/day, KRM-1648 had the highest level of activity of the four drugs, reducing the bacterial load by 2.7 logs over 35 days of therapy.     

In vitro antimycobacterial activities of newly synthesized benzoxazinorifamycins.

Newly synthesized rifamycin derivatives, KRM-1648, KRM-1657, KRM-1668, KRM-1686, and KRM-1687, having the chemical structures of 3'-hydroxy-5'-(4-alkylpiperazinyl)-benzoxazinorifamycins (alkyl residues: isobutyl, propyl, sec-butyl, sec-butyl [R configuration], and sec-butyl [S configuration], respectively), were studied for their in vitro antimycobacterial activities. Representative (KRM-1648) MICs for 90% of the strains tested, determined by the agar dilution method on 7H11 medium, of various pathogenic mycobacteria (9 species, 174 strains) were as follows (in micrograms per milliliter): Mycobacterium tuberculosis (rifampin [RMP]-susceptible strains), less than or equal to 0.0125; M. tuberculosis (RMP-resistant strains), 12.5; M. kansasii, 0.05; M. marinum, less than or equal to 0.0125; M. scrofulaceum, 0.1; M. avium, 1.56; M. intracellulare, 0.1; M. fortuitum, greater than 100; and M. chelonae subsp. abscessus and M. chelonae subsp. chelonae, greater than 100. These values are more than 64 times lower than those of RMP, except for the values against RMP-resistant M. tuberculosis (8 times lower) and those against rapid growers, including M. fortuitum and M. chelonae (the same as those of RMP). The other derivatives had similar levels of in vitro activity against these mycobacteria. When murine peritoneal macrophages in which M. intracellulare was phagocytosed in vitro were cultured in the presence of the benzoxazinorifamycins (1 microgram/ml), much more rapid killing of the organisms ingested in the macrophages was seen compared with when the same amount of RMP was added to the medium. The addition of benzoxazinorifamycins at the concentration of 0.05 micrograms/ml caused more marked suppression of intracellular growth of the organisms compared with addition of RMP. KRM-1648 and KRM-1657 inhibited intracellular growth of M. tuberculosis, and their efficacies were much greater than that of RMP.     

In vitro activity of levofloxacin, singly and in combination with rifamycin analogs, against Mycobacterium leprae.

The in vitro susceptibility of Mycobacterium leprae to levofloxacin was studied by using two biochemical parameters to measure the metabolic activity of the organism. Levofloxacin consistently exhibited twofold greater bactericidal activity than ofloxacin, with the MIC being 0.75 microgram/ml. When combined with one of the three rifamycin analogs, synergism was obtained with KRM-1648 and rifabutin but not with rifampin.     

Pharmacokinetics of KRM-1648, a new benzoxazinorifamycin, in rats and dogs.

The pharmacokinetics of 3'-hydroxy-5'-(4-isobutyl-1-piperazinyl) benzoxazinorifamycin (KRM-1648) in rats and dogs given a single oral dose of 3, 30, or 100 mg/kg of body weight were studied. In the rats, the concentrations of KRM-1648 in plasma, whole blood, and tissues peaked between 2.0 and 24.0 h, with elimination half-lives ranging from 6.2 to 19.5 h. The peak concentrations and the areas under the concentration-versus-time curves (AUC) for whole blood and tissues were 2 to 277 times higher than those for plasma. The high levels of KRM-1648 in tissues were consistent with its large volume of distribution (in excess of 10 liters/kg). A nonlinear increase in peak concentrations and AUCs for plasma, whole blood, and tissues occurred as the dose was increased and was consistent with the dose-dependent decrease in bioavailability. In the dogs, KRM-1648 levels in plasma and whole blood also exhibited a late time to the peak concentration (ranging from 4.0 to 11.2 h), a long elimination half-life (ranging from 15.2 to 24.0 h), and nonlinear kinetics. KRM-1648 exhibited high levels of plasma protein binding (more than 99%) and a high degree of affinity for lipoproteins in the plasma of both animals. After administration of KRM-1648, measurable levels of its metabolites, 25-deacetyl KRM-1648 in rats and 25-deacetyl KRM-1648 and 30-hydroxy KRM-1648 in dogs, were found in the biological samples tested. Thus, KRM-1648 is characterized by a high tissue affinity, a long elimination half-life, and nonlinear pharmacokinetics.     

Activity of KRM-1648 alone or in combination with both ethambutol and kanamycin or clarithromycin against Mycobacterium intracellulare infections in beige mice.

The in vivo activities of KRM-1648 alone or in combination with both ethambutol (EB) and kanamycin (KM) or clarithromycin (CAM) were tested against Mycobacterium intracellulare infections in beige mice. KRM-1648 was more active than rifampin (RFP) when each drug was used alone, and the efficacy of KRM-1648 was similar to those of both kanamycin and clarithromycin. The combination KRM-1648-KM-EB was strongly active and rapidly reduced the numbers of bacilli both in lungs and in spleens compared with RFP-KM-EB. The combination KRM-1648-CAM had greater therapeutic effects than RFP-CAM; this difference in efficacy was more pronounced for lungs than for spleens. These findings suggest that KRM-1648 is a promising candidate for combination therapy with other potent antimycobacterial drugs.     

Therapeutic efficacy of the benzoxazinorifamycin KRM-1648 against experimental Mycobacterium avium infection induced in rabbits.

The therapeutic efficacy of the benzoxazinorifamycin KRM-1648 was studied in an experimental rabbit infection system with avian Mycobacterium avium. The infected rabbits died from Yersin type infections, a peculiar type of experimental bovine tuberculosis characterized by a very rapid course, enlargement of the spleen and liver, and septic infection, 14 to 20 days after bacterial challenge, as evidenced by bacteremia and severe bacterial loads in the visceral organs. Histopathologic studies of the visceral organs of the infected rabbits revealed the development of numerous typical granulomatous lesions. This experimental rabbit infection system, features of which resemble certain features of disseminated M. avium complex infections in AIDS patients, was used to evaluate the therapeutic efficacy of KRM-1648, a newly synthesized benzoxazinorifamycin. KRM-1648 given orally at 25 and 50 mg/kg of body weight reduced the incidence and degree of bacteremia in infected rabbits and protected against subsequent death. Moreover, the drug allowed almost complete recovery of infected rabbits by week 7. KRM-1648 cleared infections in the lungs, liver, spleen, and kidneys and restored histopathologic features of healthy tissue in the visceral organs. KRM-1648 exhibited a more potent therapeutic effect against M. avium infection than rifampin and clarithromycin.     

Bacteriostatic and bactericidal activities of benzoxazinorifamycin KRM-1648 against Mycobacterium tuberculosis and Mycobacterium avium in human macrophages.

Inhibitory and bactericidal activities of KRM-1648 were determined against Mycobacterium tuberculosis and M. avium residing in human monocyte-derived macrophages and extracellular M. tuberculosis and M. avium. MICs and MBCs of KRM-1648 against intracellular and extracellular bacteria were substantially lower than those of rifampin. The MICs and MBCs of either drug against the intracellular bacteria were only twofold lower than or equal to the values found for extracellular bacteria. The prolonged effect of KRM-1648 found in this study is probably associated with high ratios of intracellular accumulation, which were 50- to 100-fold higher than that found for rifampin. Further studies on intracellular distribution of KRM-1648 and on the sites of actual interaction between the drug and bacteria residing in macrophages are necessary, as well as evaluation of combined effects of KRM-1648 with other drugs in long-term macrophage culture experiments.     

In vitro anti-Helicobacter pylori activities of new rifamycin derivatives, KRM-1648 and KRM-1657

The new rifamycin derivatives KRM-1657 and KRM-1648 were evaluated for their in vitro antimicrobial activities against 44 strains of Helicobacter pylori. Although the drugs were not very active against other gram-negative bacteria, the MICs at which 90% of isolates are inhibited for these drugs were lower (0.002 and 0.008 microgram/ml, respectively) than those of amoxicillin and rifampin for H. pylori. Time-kill studies revealed that the bactericidal activities of these agents were due to cell lysis. The results presented here indicate that these new rifamycin derivatives may be useful for the eradication of H. pylori infections.     

Comparative antimycobacterial activities of rifampin, rifapentine, and KRM-1648 against a collection of rifampin-resistant Mycobacterium tuberculosis isolates with known rpoB mutations.

A collection of 24 rifampin-resistant clinical isolates of Mycobacterium tuberculosis with characterized RNA polymerase beta-subunit (rpoB) gene mutations was tested against the antimycobacterial agents rifampin, rifapentine, and KRM-1648 to correlate levels of resistance with specific rpoB genotypes. The results indicate that KRM-1648 is more active in vitro than rifampin and rifapentine, and its ability to overcome rifampin resistance in strains with four different genetic alterations may prove to be useful in understanding structure-function relationships.     

Safety and Bactericidal Activity of Rifalazil in Patients with Pulmonary Tuberculosis

Rifalazil, also known as KRM-1648 or benzoxazinorifamycin, is a new semisynthetic rifamycin with a long half-life of approximately 60 h. Rifalazil has potent bactericidal activity against Mycobacterium tuberculosis in vitro and in animal models of tuberculosis (TB). Prior studies in healthy volunteers showed that once-weekly doses of 25 to 50 mg of rifalazil were well tolerated. In this randomized, open-label, active-controlled phase II clinical trial, 65 subjects with sputum smear-positive pulmonary TB received one of the following regimens for the first 2 weeks of therapy: 16 subjects received isoniazid (INH) (5 mg/kg of body weight) daily; 16 received INH (5 mg/kg) and rifampin (10 mg/kg) daily; 17 received INH (5 mg/kg) daily plus 10 mg of rifalazil once weekly; and 16 received INH (5 mg/kg) daily and 25 mg of rifalazil once weekly. All subjects were then put on 6 months of standard TB therapy. Pretreatment and day 15 sputum CFU of M. tuberculosis were measured to assess the bactericidal activity of each regimen. The number of drug-related adverse experiences was low and not significantly different among treatment arms. A transient decrease in absolute neutrophil count to less than 2,000 cells/mm(3) was detected in 10 to 20% of patients in the rifalazil- and rifampin-containing treatment arms without clinical consequences. Decreases in CFU counts were comparable among the four treatment arms; however, the CFU results were statistically inconclusive due to the variability in the control arms. Acquired drug resistance did not occur in any patient. Studies focused on determining a maximum tolerated dose will help elucidate the full anti-TB effect of rifalazil.     

Activity of KRM 1648 alone or in combination with ethambutol or clarithromycin against Mycobacterium avium in beige mouse model of disseminated infection.

Rifamycins are active against slowly growing mycobacteria, such as Mycobacterium tuberculosis and Mycobacterium kansasii, but the majority of rifamycins thus far investigated both in vitro and in vivo are inactive or have only modest activity against the Mycobacterium avium complex (MAC). We investigated the activity of three doses of the semisynthetic benzoxazinorifamycin KRM 1648, alone or in combination with ethambutol or clarithromycin, in beige mice challenged with the MAC strain 101. Our results show the following. (i) KRM 1648 was significantly effective against MAC infection as determined by the reduction of the number of bacteria in the blood, liver, and spleen when administered at doses of 20 and 40 mg/kg of body weight per day but not at 10 mg/kg/day, compared with untreated controls. (ii) KRM 1648 (40 mg/kg/day) administered in combination with ethambutol (100 mg/kg/day) resulted in significant reduction in bacteremia compared with values for untreated controls (P 0.001), KRM 1648 alone (P = 0.019), and ethambutol alone (P = 0.003). Furthermore, the combination of KRM 1648 and ethambutol was associated with a significant decrease of the number of bacteria in the spleen and the liver compared with values for both untreated controls and each drug alone (P < 0.001 for all comparisons). (iii) KRM 1648 (40 mg/kg/day) administered in combination with clarithromycin (200 mg/kg/day) resulted in a significant decrease of the number of bacteria in the blood and the spleen compared with the number for untreated controls (P < 0.001 for all comparisons). In our experience, using MAC 101 as the challenging organism, KRM 1648 is the first the number of bacteria in the blood and spleen compared with the number for untreated controls (P >0.001 for all comparions). In our experience, using MAC 101 as the challenging organism, KRM 1648 is the first rifamycin with significant activity in vivo against MAC infection in beige mice.     

Chemotherapeutic efficacy of a newly synthesized benzoxazinorifamycin, KRM-1648, against Mycobacterium avium complex infection induced in mice.

Newly synthesized benzoxazinorifamycin, KRM-1648, was studied for its in vivo anti-Mycobacterium avium complex (MAC) activities. When the MICs were determined by the agar dilution method with Middlebrook 7H11 agar medium, KRM-1648 exhibited similarly potent in vitro antimicrobial activities against the MAC isolated from AIDS and non-AIDS patients, indicating possible usefulness of KRM-1648 against AIDS-associated MAC infections. KRM-1648 exhibited potent therapeutic activity against experimental murine infections induced by M. intracellulare N-260 (virulent strain) and N-478, which has much weaker virulence. Similarly, KRM-1648 exhibited an excellent therapeutic efficacy against M. intracellulare infection induced in NK-cell-deficient beige mice (as a plausible model for AIDS-associated MAC infection), in which a much more progressed state of gross lesions and bacterial loads at the sites of infection were observed. When the infected beige mice were killed at weeks 4 and 8, obvious therapeutic efficacy was seen on the basis of reduction in the incidence and degree of lung lesions and bacterial loads in the lungs and spleen with infections due to M. intracellulare N-241, N-256, and N-260. In this case, the efficacy was the highest in N-260 infection, followed by strain N-241. When mice were observed until infection-induced death, survival time of the infected beige mice was found to be prolonged by KRM treatment. However, KRM-1648 was not efficacious in suppressing the progression of pulmonary lesions and the increase in bacterial loads at the sites of infection, including lungs and spleen, at the late phase of infection. This may imply some difficulty with chemotherapy for AIDS-associated MAC infection, even with KRM-1648 treatment, which has excellent in vitro and in vivo anti-MAC activities, as shown in present study.     

Activities of the benzoxazinorifamycin KRM 1648 and ethambutol against Mycobacterium avium complex in vitro and in macrophages.

KRM 1648 is a 4-aminobenzoxazine derivative of rifamycin S with potent in vitro activity against the Mycobacterium avium complex (MAC); the MIC for 90% of 24 MAC isolates from AIDS patients was 0.25 microgram/ml as determined by a radiometric broth macrodilution assay. KRM 1648 was bactericidal for MAC isolates in Middlebrook 7H9 broth, with a reduction in viability of 1 to 4 orders of magnitude over 72 h. In human macrophages, KRM 1648 also was bactericidal, with a reduction of 3 to 4 orders of magnitude in CFU per ml of macrophage lysate at a concentration of 1 microgram/ml; however, the bactericidal activity varied approximately 10-fold among the three MAC serovars tested. In growth medium, ethambutol potentiated the effect of KRM 1648, but this potentiation was modest when tested against MAC in macrophages and also varied between MAC strains. KRM 1648 has potential as an antimycobacterial agent for MAC disease, perhaps in combination with other agents so that the use of lower dosages of KRM 1648 than are needed with other rifamycins may be possible.     

Preclinical testing of the nitroimidazopyran PA-824 for activity against Mycobacterium tuberculosis in a series of in vitro and in vivo models

This study extends earlier reports regarding the in vitro and in vivo efficacies of the nitroimidazopyran PA-824 against Mycobacterium tuberculosis. PA-824 was tested in vitro against a broad panel of multidrug-resistant clinical isolates and was found to be highly active against all isolates (MIC<1 microg/ml). The activity of PA-824 against M. tuberculosis was also assessed grown under conditions of oxygen depletion. PA-824 showed significant activity at 2, 10, and 50 microg/ml, similar to that of metronidazole, in a dose-dependent manner. In a short-course mouse infection model, the efficacy of PA-824 at 50, 100, and 300 mg/kg of body weight formulated in methylcellulose or cyclodextrin/lecithin after nine oral treatments was compared with those of isoniazid, rifampin, and moxifloxacin. PA-824 at 100 mg/kg in cyclodextrin/lecithin was as active as moxifloxacin at 100 mg/kg and isoniazid at 25 mg/kg and was slightly more active than rifampin at 20 mg/kg. Long-term treatment with PA-824 at 100 mg/kg in cyclodextrin/lecithin reduced the bacterial load below 500 CFU in the lungs and spleen. No significant differences in activity between PA-824 and the other single drug treatments tested (isoniazid at 25 mg/kg, rifampin at 10 mg/kg, gatifloxacin at 100 mg/kg, and moxifloxacin at 100 mg/kg) could be observed. In summary, its good activity in in vivo models, as well as its activity against multidrug-resistant M. tuberculosis and against M. tuberculosis isolates in a potentially latent state, makes PA-824 an attractive drug candidate for the therapy of tuberculosis. These data indicate that there is significant potential for effective oral delivery of PA-824 for the treatment of tuberculosis.