Evaluation of rifalazil in long-term treatment regimens for tuberculosis in mice

Optimization of Benzoxazinorifamycins to Minimize hPXR Activation for the Treatment of Tuberculosis and HIV Coinfection

Tuberculosis (TB) is one of the most significant world health problems, responsible for 1.5 M deaths in 2020, and yet, current treatments rely largely on 40 year old paradigms. Although the rifamycins (RIFs), best exemplified by the drug rifampin (RMP), represent a well-studied and therapeutically effective chemotype that targets the bacterial RNA polymerase (RNAP), these agents still suffer from serious drawbacks including the following: 3-9 month treatment times; cytochrome P450 (Cyp450) induction [particularly problematic for human immunodeficiency virus-Mycobacterium tuberculosis (MTB) co-infection]; and the existence of RIF-resistant (RIF^R) MTB strains. We have utilized a structure-based drug design approach to synthesize and test 15 benzoxazinorifamycins (bxRIFs), congeners of the clinical candidate rifalazil, to minimize human pregnane X receptor (hPXR) activation while improving potency against MTB. We have determined the compounds' activation of the hPXR [responsible for inducing Cyp450 3A4 (CYP3A4)]. Compound IC₅₀s have been determined against the wild-type and the most prevalent RIF^R (β-S450L) mutant MTB RNAPs. We have also determined their bactericidal activity against "normal" replicating MTB and a model for non-replicating, persister MTB. We have identified a minimal substitution and have probed larger substitutions that exhibit negligible hPXR activation (1.2-fold over the dimethyl sulfoxide control), many of which are 5- to 10-fold more potent against RNAPs and MTB than RMP. Importantly, we have analogues that are essentially equipotent against replicating MTB and non-replicating persister MTB, a property that is correlated with faster kill rates and may lead to shorter treatment durations. This work provides a proof of principle that the ansamycin core remains an attractive and effective scaffold for novel and dramatically improved RIFs.

Structure-based design of novel benzoxazinorifamycins with potent binding affinity to wild-type and rifampin-resistant mutant Mycobacterium tuberculosis RNA polymerases

By utilization of three-dimensional structure information of rifamycins bound to RNA polymerase (RNAP) and the human pregnane X receptor (hPXR), representative examples (2b-d) of a novel subclass of benzoxazinorifamycins have been synthesized. Relative to rifalazil (2a), these analogues generally display superior affinity toward wild-type and Rif-resistant mutants of the Mycobacterium tuberculosis RNAP but lowered antitubercular activity in cell culture under both aerobic and anaerobic conditions. Lowered affinity toward hPXR for some of the analogues is also observed, suggesting a potential for reduced Cyp450 induction activity. Mouse and human microsomal studies of analogue 2b show it to have excellent metabolic stability. Mouse pharmacokinetics in plasma and lung show accumulation of 2b but with a half-life suggesting nonoptimal pharmacokinetics. These studies demonstrate proof of principle for this subclass of rifamycins and support further expansion of structure-activity relationships (SARs) toward uncovering analogues with development potential.

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.

Management of multidrug-resistant tuberculosis: Update 2007

Multidrug-resistant tuberculosis (MDR-TB) with bacillary resistance to at least isoniazid and rifampicin in vitro is a worldwide phenomenon. Hot spots of the disease are found scattered in different continents. Prevention of its development through good tuberculosis control programmes operating under the directly observed therapy, short-course (DOTS) strategy is of paramount importance. However, with established MDR-TB, treatment with alternative and specific chemotherapy is necessary to achieve a beneficial outcome. Such an approach on a programme basis is currently known as the 'DOTS-Plus' strategy. Second-line (reserve) drugs utilized in the treatment of MDR-TB are generally less potent and more toxic, perhaps with the notable exceptions of some fluoroquinolones and injectable agents. Surgery has a distinct adjunctive role for the management of MDR-TB in selected patients. The emergence of extensively drug-resistant tuberculosis (XDR-TB), that is, MDR-TB with additional bacillary resistance to the fluoroquinolones and injectables, has provided a very alarming challenge to global health, as the disease currently has a low cure rate and high mortality. In order to combat XDR-TB, strengthening of DOTS and DOTS-Plus programmes is mandatory, especially in the face of surging HIV infection. Furthermore, more attention needs to be focused on developing new drugs with potent bactericidal and sterilizing activities and low side-effects, and above all, drugs that are affordable for communities worldwide.

Drug therapy of experimental tuberculosis (TB): improved outcome by combining SQ109, a new diamine antibiotic, with existing TB drugs

Substitution of the new diamine antibiotic SQ109 for ethambutol in a mouse model of chronic tuberculosis (TB) improved efficacy of combination drug therapy with first-line TB drugs rifampin and isoniazid, with or without pyrazinamide: at 8 weeks, lung bacteria were 1.5 log10 lower in SQ109-containing regimens.

Rapid, simple in vivo screen for new drugs active against Mycobacterium tuberculosis

We evaluated the use of a simple and easy-to-obtain potential marker of tuberculosis (TB) drug efficacy, body weight, and correlated weight loss or gain with the number of CFU of Mycobacterium tuberculosis in lungs and spleens of infected mice. C3H mice were infected intravenously with 10(6) CFU of virulent M. tuberculosis H37Rv, and body weight was evaluated for several weeks after infection. At day 20, infected untreated mice consistently lost more than 25% of their body weight. Chemotherapy with selected orally active anti-TB drugs was initiated 7 days following infection and continued for 13 days. Drugs that were administered daily by gavage included isoniazid (INH), ethambutol (EMB), rifampin (RIF), and moxifloxacin (MXF). At the most effective doses, each of these drugs inhibited bacterial growth and abolished infection-induced body weight loss. Chemotherapy with 1/10 the standard dose of INH determined in accepted long-term murine models of TB also prevented body weight loss, while chemotherapy with 1/10 the standard dose of RIF did not. With only 2 weeks of chemotherapy, we observed a good reverse correlation between CFU in lung or spleen and body weight of mice. The simple measurement of weight in TB-infected drug-treated mice required only a weight balance, and go/no-go drug efficacy data was available on day 20 without the necessity of prolonged drug treatment and long (3 weeks or more) in vitro culture times to obtain organ CFU values.

Gatifloxacin and ethionamide as the foundation for therapy of tuberculosis

The use of gatifloxacin (GAT) in combination with ethionamide (ETA) with or without pyrazinamide (PZA) for a 12-week treatment period followed by an 8-week observation period was evaluated in a model of tuberculosis in mice. Mice treated with GAT at 300 mg/kg of body weight in combination with ETA (25 mg/kg) for 5 days per week had sterile lungs, whereas mice treated with GAT (100 mg/kg) and ETA (25 mg/kg) had about 10 CFU/lung; however, there was regrowth of the organisms in both groups at the end of the observation period. When PZA (450 mg/kg 5 days per week) was added to the high-dose GAT-ETA regimen, no viable mycobacteria were present after the 8-week observation period. GAT in combination with ETA and PZA has great promise for the treatment of tuberculosis.

Recent developments in the treatment of tuberculosis

The history of chemotherapy of tuberculosis commenced in 1944 with the discovery of streptomycin. Currently, short-course chemotherapy comprising rifampicin, isoniazid, pyrazinamide and ethambutol/streptomycin administered under directly observed settings for 6 months (initially all four drugs followed by the former two drugs), constitutes the cornerstone treatment for pulmonary tuberculosis. Multi-drug resistant tuberculosis requires alternative chemotherapy, ideally in the form of individualised regimens, for management. To improve on the duration of chemotherapy for drug-susceptible tuberculosis and to achieve better treatment for multi-drug resistant tuberculosis as well as latent tuberculosis infection, there arises a genuine need for new drugs. The quest for new agents is, however, impeded by obstacles. Hopefully, tackling these through collaborative public-private partnerships on an international scale will lead to a fruitful outcome.

Evaluation of rifalazil in a combination treatment regimen as an alternative to isoniazid-rifampin therapy in a mouse tuberculosis model

The newer rifamycin, rifalazil (RLZ) (previously known as KRM-1648), has been shown in prior experiments to be a highly potent drug against Mycobacterium tuberculosis. In this report, we studied the efficacy of RLZ in combination with pyrazinamide (PZA) and ethambutol (EMB) in a long-term in vivo experiment and compared their activity with the isoniazid (INH)-rifampin (RIF) combination which is presently used in the clinic. Combinations of RLZ with PZA alone or with both PZA and EMB were both found to have sterilizing activities comparable to that of the INH-RIF combination but significantly better activity with respect to relapse of infection. These results suggest that RLZ, or other agents with similar activity, could be combined with available agents to act as a potential alternative drug regimen to the currently used INH-RIF combination.