In-vitro and intracellular activity of rifabutin on drug-susceptible and multiple drug-resistant (MDR) tubercle bacilli

Rifabutin Is Bactericidal against Intracellular and Extracellular Forms of Mycobacterium abscessus

Mycobacterium abscessus is increasingly recognized as an emerging opportunistic pathogen causing severe lung diseases. As it is intrinsically resistant to most conventional antibiotics, there is an unmet medical need for effective treatments. Repurposing of clinically validated pharmaceuticals represents an attractive option for the development of chemotherapeutic alternatives against M. abscessus infections. In this context, rifabutin (RFB) has been shown to be active against M. abscessus and has raised renewed interest in using rifamycins for the treatment of M. abscessus pulmonary diseases. Here, we compared the in vitro and in vivo activity of RFB against the smooth and rough variants of M. abscessus, differing in their susceptibility profiles to several drugs and physiopathologial characteristics. While the activity of RFB is greater against rough strains than in smooth strains in vitro, suggesting a role of the glycopeptidolipid layer in susceptibility to RFB, both variants were equally susceptible to RFB inside human macrophages. RFB treatment also led to a reduction in the number and size of intracellular and extracellular mycobacterial cords. Furthermore, RFB was highly effective in a zebrafish model of infection and protected the infected larvae from M. abscessus-induced killing. This was corroborated by a significant reduction in the overall bacterial burden, as well as decreased numbers of abscesses and cords, two major pathophysiological traits in infected zebrafish. This study indicates that RFB is active against M. abscessus both in vitro and in vivo, further supporting its potential usefulness as part of combination regimens targeting this difficult-to-treat mycobacterium.

Determination of the Critical Concentration of Rifabutin for Susceptibility Testing Using the Proportion Method with Löwenstein-Jensen Medium against Mycobacterium Tuberculosis Isolates

OBJECTIVE

To determine the critical concentration of rifabutin (RFB) for susceptibility testing against Mycobacterium tuberculosis (Mtb) on Löwenstein-Jensen (L-J) medium using the proportion method.

METHODS

We used 47 strains were used to determine the critical concentration of RFB. The microplate antimicrobial assay (MABA) was used as a reference method. We used 160 strains to evaluate its correlation with the classification results derived from the MABA method. We performed antimicrobial susceptibility testing (AST) against RFB and rifampin (RIF) for 2748 other strains using the proportion method on L-J medium.

RESULTS

The determined critical concentration for RFB was 20 μg per mL. Identical classification as susceptible or resistant was observed in 93.8% and 92.5% strains for RFB and RIF, respectively, using the 2 different methods. The cross-resistance ratio between RFB and RIF was 72.7% in the 2748 Mtb strains.

CONCLUSIONS

We determined that a critical concentration of 20 μg per mL RFB was reliable for AST of Mtb.

Rifabutin Acts in Synergy and Is Bactericidal with Frontline Mycobacterium abscessus Antibiotics Clarithromycin and Tigecycline, Suggesting a Potent Treatment Combination

Mycobacterium abscessus is a rapidly emerging mycobacterial pathogen causing dangerous pulmonary infections. Because these bacteria are intrinsically multidrug resistant, treatment options are limited and have questionable efficacy. The current treatment regimen relies on a combination of antibiotics, including clarithromycin paired with amikacin and either imipenem or cefoxitin. Tigecycline may be added when triple therapy is ineffective. We initially screened a library containing the majority of clinically available antibiotics for anti-M. abscessus activity. The screen identified rifabutin, which was then investigated for its interactions with M. abscessus antibiotics used in drug regimens. Combination of rifabutin with either clarithromycin or tigecycline generated synergistic anti-M. abscessus activity, dropping the rifabutin MIC below concentrations found in the lung. Importantly, these combinations generated bactericidal activity. The triple combination of clarithromycin, tigecycline, and rifabutin was also synergistic, and clinically relevant concentrations had a sterilizing effect on M. abscessus cultures. We suggest that combinations including rifabutin should be further investigated for treatment of M. abscessus pulmonary infections.

Rifabutin and rifampin resistance levels and associated rpoB mutations in clinical isolates of Mycobacterium tuberculosis complex

Cross-resistance in rifamycins has been observed in rifampin (RIF)-resistant Mycobacterium tuberculosis complex isolates; some rpoB mutations do not confer broad in vitro rifamycin resistance. We examined 164 isolates, of which 102 were RIF-resistant, for differential resistance between RIF and rifabutin (RFB). A total of 42 unique single mutations or combinations of mutations were detected. The number of unique mutations identified exceeded that reported in any previous study. RFB and RIF MICs up to 8 μg/mL by MGIT 960 were studied; the cut-off values for susceptibility to RIF and RFB were 1 μg/mL and 0.5 μg/mL, respectively. We identified 31 isolates resistant to RIF but susceptible to RFB with the mutations D516V, D516F, 518 deletion, S522L, H526A, H526C, H526G, H526L, and two dual mutations (S522L + K527R and H526S + K527R). Clinical investigations using RFB to treat multidrug-resistant tuberculosis cases harboring those mutations are recommended.

Macrophage Activation by Ursolic and Oleanolic Acids during Mycobacterial Infection

Oleanolic (OA) and ursolic acids (UA) are triterpenes that are abundant in vegetables, fruits and medicinal plants. They have been described as active moieties in medicinal plants used for the treatment of tuberculosis. In this study, we analyzed the effects of these triterpenes on macrophages infected in vitro with Mycobacterium tuberculosis (MTB). We evaluated production of nitric oxide (NO), reactive oxygen species (ROS), and cytokines (TNF-α and TGF-β) as well as expression of cell membrane receptors (TGR5 and CD36) in MTB-infected macrophages following treatment with OA and UA. Triterpenes caused reduced MTB growth in macrophages, stimulated production of NO and ROS in the early phase, stimulated TNF-α, suppressed TGF-β and caused over-expression of CD36 and TGR5 receptors. Thus, our data suggest immunomodulatory properties of OA and UA on MTB infected macrophages. In conclusion, antimycobacterial effects induced by these triterpenes may be attributable to the conversion of macrophages from stage M2 (alternatively activated) to M1 (classically activated).

Molecular detection of rifabutin-susceptible Mycobacterium tuberculosis

Rapid assays are still needed to detect rifabutin (RFB) susceptibility for proper tuberculosis treatment. To assess the use of the GenoType MTBDRplus assay and subsequent rpoB gene sequencing on detection of RFB susceptibility, we analyzed 800 multidrug-resistant Mycobacterium tuberculosis isolates, and 13% (104/800) were RFB susceptible. Of the 104 RFB-susceptible isolates, 71 (68.3%) isolates were rapidly identified using two molecular assays, while the remaining isolates could be determined using conventional drug-susceptibility testing according to the clinician's decision.

Mutation analysis of mycobacterial rpoB genes and rifampin resistance using recombinant Mycobacterium smegmatis

Rifampin is a major drug used to treat leprosy and tuberculosis. The rifampin resistance of Mycobacterium leprae and Mycobacterium tuberculosis results from a mutation in the rpoB gene, encoding the β subunit of RNA polymerase. A method for the molecular determination of rifampin resistance in these two mycobacteria would be clinically valuable, but the relationship between the mutations and susceptibility to rifampin must be clarified before its use. Analyses of mutations responsible for rifampin resistance using clinical isolates present some limitations. Each clinical isolate has its own genetic variations in some loci other than rpoB, which might affect rifampin susceptibility. For this study, we constructed recombinant strains of Mycobacterium smegmatis carrying the M. leprae or M. tuberculosis rpoB gene with or without mutation and disrupted their own rpoB genes on the chromosome. The rifampin and rifabutin susceptibilities of the recombinant bacteria were measured to examine the influence of the mutations. The results confirmed that several mutations detected in clinical isolates of these two pathogenic mycobacteria can confer rifampin resistance, but they also suggested that some mutations detected in M. leprae isolates or rifampin-resistant M. tuberculosis isolates are not involved in rifampin resistance.

Comparison of rifabutin susceptibility and rpoB mutations in multi-drug-resistant Mycobacterium tuberculosis strains by DNA sequencing and the line probe assay

We compared rifabutin susceptibility and rpoB mutations in 98 multi-drug-resistant strains of Mycobacterium tuberculosis (MDR-TB) by DNA sequencing and with a line probe assay using the commercially available INNO-LiPA Rif. TB kit (the LiPA). Our results indicated that rifabutin continues to remain active against MDR-TB strains harboring certain genetic alterations and also that the LiPA might be useful in identifying MDR-TB strains susceptible to rifabutin.

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.

Rapid determination of rifampin resistance in clinical isolates of Mycobacterium tuberculosis by real-time PCR

Real-time PCR was used to determine rifampin resistance in clinical isolates of Mycobacterium tuberculosis. Ninety-six rifampin-resistant isolates and 23 rifampin-susceptible isolates were included in the study. A 305-bp region covering the 81-bp "rifampin resistance-determining region" of rpoB was amplified. Two hybridization probe pairs that covered the most frequent mutation sites in rpoB, codon regions 526 to 531 and 513 to 516, were used. The results obtained by real-time PCR were compared to those obtained by the proportion method. For detection of rifampin resistance, the real-time PCR assay yielded a sensitivity of 92.7% and a specificity of 100%. Real-time PCR is a very rapid method, and it can be especially helpful for the reporting of resistant clinical isolates in a very short period of time.

Incidence of cross resistance between rifampicin and rifabutin in Mycobacterium tuberculosis strains in Izmir, Turkey

Rifabutin is a spiro-piperidyl-rifamycin derived from rifamycin-S. This study was planned to determine the minimal inhibitory concentrations (MICs) of rifabutin and rifampicin against Mycobacterium tuberculosis strains and to investigate cross resistance between two drugs. The agar dilution method was performed for detection of MICs of rifabutin and rifampicin for 112 M. tuberculosis strains obtained from patients with active pulmonary tuberculosis. Cross resistance between rifampicin and rifabutin was detected in 73.1%. Rifabutin MICs for rifampicin resistant strains were three- to four-fold higher than rifabutin-susceptible and/or -resistant strains' MIC values for rifampicin. Despite having 73% cross-resistance with rifampicin in M. tuberculosis isolates, it was concluded that rifabutin might be a valid component of combination therapy in rifampicin resistant tuberculosis cases when the isolate is tested and detected susceptible to rifabutin.

Uso de rifabutina en casos de toxicidad a rifampicina

Rifampicin is an essential drug in the present treatment of tuberculosis. On some occasions, it cannot be used due to severe adverse effects. Rifabutin is a drug of the same family, with a similar activity and difference adverse effects. This study describes the beneficial results of the substitution of rifampicin with rifabutin in patients with severe toxicity.

Effects of continuous or pulsed exposure to rifabutin and sparfloxacin on the intracellular growth of Staphylococcus aureus and Mycobacterium tuberculosis

The time-kinetics of the intracellular bioactivity and intracellular post-antibiotic effect (PAE) of rifabutin and sparfloxacin against Staphylococcus aureus and Mycobacterium tuberculosis, grown in human monocytes, were evaluated. Intracellular bactericidal activity against staphylococci was shown in the presence of extracellular drug concentrations equal or superior to 1/10 plasma Cmax. The bactericidal activity of rifabutin was dependent on both its extracellular concentrations and the exposure time. In contrast, the pattern of the intracellular activity of sparfloxacin was characterized by a minimal concentration dependent killing. Both antibiotics (from 1/10 to the expected lung Cmax) showed intracellular bioactivity against M. tuberculosis H37Ra and H37Rv strains. A long intracellular PAE on staphylococci (>4 hours) was demonstrated when drugs were removed from the infected monocytes after 1 h treatment. Our findings suggest that rifabutin and sparfloxacin may be useful in the treatment of lower respiratory tract infections due to intracellular pathogens.

Bacteriological and molecular analysis of rifampin-resistant Mycobacterium tuberculosis strains isolated in Australia

To develop a better understanding of the epidemiology and molecular biology of rifampin-resistant Mycobacterium tuberculosis strains in Australia, 50 clinical isolates (33 rifampin-resistant and 17 rifampin-sensitive strains) cultured between 1990 and 1997 were analyzed by a number of bacteriological and molecular techniques. Examination of the drug resistance profiles of the 33 rifampin-resistant isolates revealed that 91% were resistant to rifampin in combination with resistance to isoniazid, 88% were resistant to rifampin on first isolation, and 81% showed cross-resistance with rifabutin. On the basis of the demographic data provided for the patients infected with the rifampin-resistant strains, 90% of the patients were born overseas. Of these patients, 64% developed clinical symptoms within 5 years of residence in Australia. On a molecular level, analysis of the rpoB gene revealed that 97% of the rifampin-resistant isolates had missense mutations within a conserved region of the gene, and eight types of missense mutations were detected. Of the 31 rifampin-resistant isolates that were typed by restriction fragment length polymorphism (RFLP) analysis, 28 distinct patterns were obtained by RFLP analysis with IS6110, and three clusters of genetically related isolates were identified. All isolates within the clusters were from patients who were born overseas and who had the same country of origin. The results from this study provide an overview of the current situation of rifampin resistance in Australia and can serve as a basis for continued monitoring of drug-resistant M. tuberculosis strains isolated within the country.

Hypersensitivity reactions to rifampin. Pathogenetic mechanisms, clinical manifestations, management strategies, and review of the anaphylactic-like reactions

Many of the adverse events induced by rifampin have been considered allergic in origin. The flu-like syndrome and other hypersensitivity reactions seem to be caused by immune complexes, although their pathogenetic mechanisms are not fully elucidated. Many cases have been reported of the flu-like syndrome, thrombocytopenia, hemolytic anemia, and renal failure caused by rifampin. In almost all of the patients in whom they were sought, nonreaginic antirifampin antibodies were detected. On the other hand, anaphylactic reactions seem to be IgE-mediated. We have analyzed the 18 reported cases of anaphylactic reactions severe enough to cause marked hypotension. The interval between the onset of treatment and the anaphylactic reaction was highly variable. Most patients presented with prodromes, mainly rash, before the development of anaphylactic symptoms, and, in most cases, the reaction occurred after reexposure to rifampin. Clinical findings include a variety of symptoms, such as fever, exanthem, dyspnea, abdominal pain, and vomiting. Seven of the 9 patients in whom HIV status was known were seropositive, including the only 2 patients who died. We believe that, in case of a non-life-threatening adverse reaction caused by immune complexes, rifampin could be readministered, if necessary, at a more frequent and reduced dose, perhaps with the addition of corticosteroids. In case of anaphylactic reactions the drug should be avoided, although desensitization procedures may be useful. Certain laboratory findings may serve as a clue to predict anaphylactic reactions in patients who have experienced minor adverse events to rifampin. However, the diagnostic value of such findings is not well established and, therefore, patients with previous adverse reactions should be carefully monitored if reexposure to rifampin is essential.

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.