Drug susceptibility testing of Mycobacterium leprae in the BACTEC 460 system

Disease burden and current therapeutical status of leprosy with special emphasis on phytochemicals

BACKGROUND

Leprosy (Hansen's disease) is a neglected tropical disease affecting millions of people globally. The combined formulations of dapsone, rifampicin and clofazimine (multidrug therapy, MDT) is only supportive in the early stage of detection, while "reemergence" is a significant problem. There is still a need to develop newer antileprosy molecules either of natural or (semi)synthetic origin.

OBJECTIVE

The review intends to present the latest developments in the disease prevalence, available therapeutic interventions and the possibility of identifying new molecules from phytoextracts.

METHODS

Literature on the use of plant extracts and their active components to treat leprosy was searched. Selected phytoconstituents were subjected to molecular docking study on both wild and mutant types of the Mycobacterium leprae. Since the M. leprae dihydropteroate synthase (DHPS) is not available in the protein data bank (PDB), it was modelled by the homology model method and validated with the Ramachandran plot along with other bioinformatics approaches. Two mutations were introduced at codons 53 (Thr to Ile) and 55 (Pro to Leu) for docking against twenty-five selected phytoconstituents reported from eight plants that recorded effective anti-leprosy activity. The chemical structure of phytochemicals and the standard dapsone structure were retrieved from the PubChem database and prepared accordingly for docking study with the virtual-screening platform of PyRx-AutoDock 4.1.

RESULTS

Based on the docking score (kcal/mol), most of the phytochemicals exhibited a higher docking score than dapsone. Asiaticoside, an active saponin (-11.3, -11.2 and -11.2 kcal/mol), was proved to be the lead phytochemical against both wild and mutant types DHPS. Some other useful phytoconstituents include echinocystic acid (-9.6, -9.5 and -9.5 kcal/mol), neobavaisoflavone (-9.2, -9.0 and -9.0 kcal/mol), boswellic acid (-8.90, -8.90 and -8.90 kcal/mol), asiatic acid (-8.9, -8.8 and -8.9 kcal/mol), corylifol A (-8.8, 8.0, and -8.0), etc. Overall, the computational predictions support the previously reported active phytoextracts of Centella asiatica (L.) Urban, Albizia amara (Roxb.) Boivin, Boswellia serrata Roxb. and Psoralea corylifolia L. to be effective against leprosy.

CONCLUSION

A very small percentage of well-known plants have been evaluated scientifically for antileprosy activity. Further in vivo experiments are essential to confirm anti-leprosy properties of such useful phytochemicals.

Screening anti-infectious molecules against Mycobacterium ulcerans: A step towards decontaminating environmental specimens

Mycobacterium ulcerans, a non-tuberculous mycobacterium responsible for Buruli ulcer, resides in poorly defined environmental niches in the vicinity of stagnant water. Very few isolates have been confirmed. With a view to culturing M. ulcerans from such contaminated environmental specimens, we tested the in vitro susceptibility of the M. ulcerans CU001 strain co-cultivated with XTC cells to anti-infectious molecules registered in the French pharmacopoeia. We used a standardised concentration to identify molecules that were inactive against M. ulcerans and which could be incorporated into a decontaminating solution. Of 116 tested molecules, 64 (55.1%) molecules were ineffective against M. ulcerans CU001. These included 34 (29.3%) antibiotics, 14 (12%) antivirals, eight (6.8%) antiparasitics, and eight (6.8%) antifungals. This left 52 molecules which were active against M. ulcerans CU001. Three of the inactive antimicrobial molecules (oxytetracycline, polymyxin E and voriconazole) were then selected to prepare a decontamination solution which was shown to respect M. ulcerans CU001 viability. These three antimicrobials could be incorporated into a decontamination solution to potentially isolate and culture M. ulcerans from environmental samples.

[Guidelines for the treatment of Hansen's disease in Japan (third edition)]

ad hoc committee of Japanese Leprosy Association recommends revised standard treatment protocol of leprosy in Japan, which is a modification of World Health Organization's multidrug therapy (WHO/MDT, 2010). For paucibacillary (PB) leprosy, 6 months treatment by rifampicin and dapsone (MDT/PB) is enough. However, for high bacterial load multibacillary (MB) leprosy, 12 months treatment seems insufficient. Thus, (A) For MB with bacterial index (BI) > 3 before treatment, 2 years treatment by rifampicin, dapsone and clofazimine (MDT/MB) is necessary. When BI becomes negative and active lesion is lost within 2 years, no maintenance therapy is necessary. When BI is still positive, one year of MDT/MB is added (3 years in total), followed by maintenance therapy by dapsone and clofazimine until BI negativity and loss of active lesions. (B) For MB with BI < 3 or fresh MB (less than 6 months after the onset of the disease) with BI > 3, 1 year treatment by MDT/MB is necessary. When BI becomes negative and active lesion is lost within one year, no maintenance therapy is necessary. When BI is still positive or active lesion is remaining, additional therapy with MDT/MB for one more year is recommended. Brief summary of diagnosis, purpose of therapy, character of drugs, and prevention of deformity is also described.

Molecular assays for determining Mycobacterium leprae viability in tissues of experimentally infected mice

Background

The inability of Mycobacterium leprae to grow on axenic media has necessitated specialized techniques in order to determine viability of this organism. The purpose of this study was to develop a simple and sensitive molecular assay for determining M. leprae viability directly from infected tissues.

Methodology/Principle Findings

Two M. leprae-specific quantitative reverse transcription PCR (qRT-PCR) assays based on the expression levels of esxA, encoding the ESAT-6 protein, and hsp18, encoding the heat shock 18 kDa protein, were developed and tested using infected footpad (FP) tissues of both immunocompetent and immunocompromised (athymic nu/nu) mice. In addition, the ability of these assays to detect the effects of anti-leprosy drug treatment on M. leprae viability was determined using rifampin and rifapentine, each at 10 mg/kg for 1, 5, or 20 daily doses, in the athymic nu/nu FP model. Molecular enumeration (RLEP PCR) and viability determinations (qRT-PCR) were performed via Taqman methodology on DNA and RNA, respectively, purified from ethanol-fixed FP tissue and compared with conventional enumeration (microscopic counting of acid fast bacilli) and viability assays (radiorespirometry, viability staining) which utilized bacilli freshly harvested from the contralateral FP. Both molecular and conventional assays demonstrated growth and high viability of M. leprae in nu/nu FPs over a 4 month infection period. In contrast, viability was markedly decreased by 8 weeks in immunocompetent mice. Rifapentine significantly reduced bacterial viability after 5 treatments, whereas rifampin required up to 20 treatments for the same efficacy. Neither drug was effective after a single treatment. In addition, host gene expression was monitored with the same RNA preparations.

Conclusions

hsp18 and esxA qRT-PCR are sensitive molecular indicators, reliably detecting viability of M. leprae in tissues without the need for bacterial isolation or immediate processing, making these assays applicable for in vivo drug screening and promising for clinical and field applications.

M. leprae, the causative agent of leprosy, cannot be grown on laboratory culture media. This characteristic, along with its extremely long generation time of 12–14 days, makes the study of the pathogenicity of this organism and the experimental testing of new drugs for the treatment of leprosy extremely difficult. We developed two M. leprae-specific quantitative reverse transcription PCR assays and tested their utility as biological markers of M. leprae viability in tissue specimens. These assays could detect high viability of bacilli growing in immunosuppressed mice as well as the inhibitory effects of anti-leprosy drug treatment, or of the host immune system in immunocompetent mice. The RNA preparations were also successfully used for detection of host gene expression. The application of these assays to various experimental models would benefit characterization of the infection or novel drug screening. Furthermore, because these assays utilize fixed tissues, their potential application to clinical and field settings could enable monitoring of M. leprae viability in conjunction with the host immune response during treatment.

Mycobacterium leprae: genes, pseudogenes and genetic diversity

Leprosy, which has afflicted human populations for millenia, results from infection with Mycobacterium leprae, an unculturable pathogen with an exceptionally long generation time. Considerable insight into the biology and drug resistance of the leprosy bacillus has been obtained from genomics. M. leprae has undergone reductive evolution and pseudogenes now occupy half of its genome. Comparative genomics of four different strains revealed remarkable conservation of the genome (99.995% identity) yet uncovered 215 polymorphic sites, mainly single nucleotide polymorphisms, and a handful of new pseudogenes. Mapping these polymorphisms in a large panel of strains defined 16 single nucleotide polymorphism-subtypes that showed strong geographical associations and helped retrace the evolution of M. leprae.

High efficacy of clofazimine and its synergistic effect with amikacin against rapidly growing mycobacteria

The aim of this study was to determine whether clofazimine, dapsone and cycloserine may be suitable antimicrobial agents for the treatment of infections due to rapidly growing mycobacteria (RGM). The antimicrobial activity of the three drugs against 117 Mycobacterium abscessus isolates, 48 Mycobacterium fortuitum isolates and 20 Mycobacterium chelonae isolates was evaluated based on their broth microdilution minimal inhibitory concentrations (MICs) against the isolates. Clofazimine was highly efficacious against these RGM. The vast majority of M. abscessus, M. fortuitum and M. chelonae isolates (99.1%, 91.7% and 100%, respectively) had clofazimine MICs of <or=1mg/L. MIC(50) values (MIC for 50% of the organisms) of clofazimine against the isolates ranged from 0.25mg/L to 0.5mg/L and MIC(90) values (MIC for 90% of the organisms) ranged from 0.5mg/L to 1.0mg/L. Cycloserine and dapsone had little or no activity against the isolates. The effects of combined application of clofazimine and amikacin on 40 M. abscessus isolates, 48 M. fortuitum isolates and 20 M. chelonae isolates were evaluated. Addition of 0.25x MIC of amikacin for the isolates to clofazimine reduced clofazimine MICs in all of the M. abscessus and M. chelonae isolates and in 48% of the M. fortuitum isolates tested. Clofazimine, either alone or combined with amikacin, may serve as a promising drug for the treatment of RGM infections.

Structure-activity relationships of macrolides against Mycobacterium tuberculosis

Existing 14, 15 and 16-membered macrolide antibiotics, while effective for other bacterial infections, including some mycobacteria, have not demonstrated significant efficacy in tuberculosis. Therefore an attempt was made to optimize this class for activity against Mycobacterium tuberculosis through semisyntheses and bioassay. Approximately 300 macrolides were synthesized and screened for anti-TB activity. Structural modifications on erythromycin were carried out at positions 3, 6, 9, 11, and 12 of the 14-membered lactone ring; as well as at position 4'' of cladinose and position 2' of desosamine. In general, the synthesized macrolides belong to four subclasses: 9-oxime, 11,12-carbamate, 11,12-carbazate, and 6-O-substituted derivatives. Selected compounds were assessed for mammalian cell toxicity and in some cases were further assessed for CYP3A4 inhibition, microsome stability, in vivo tolerance and efficacy. The activity of 11,12-carbamates and carbazates as well as 9-oximes is highly influenced by the nature of the substitution at these positions. For hydrophilic macrolides, lipophilic substitution may result in enhanced potency, presumably by enhanced passive permeation through the cell envelope. This strategy, however, has limitations. Removal of the C-3 cladinose generally reduces the activity. Acetylation at C-2' or 4'' maintains potency of C-9 oximes but dramatically decreases that of 11,12-substituted compounds. Further significant increases in the potency of macrolides for M. tuberculosis may require a strategy for the concurrent reduction of ribosome methylation.

The continuing challenges of leprosy

Leprosy is best understood as two conjoined diseases. The first is a chronic mycobacterial infection that elicits an extraordinary range of cellular immune responses in humans. The second is a peripheral neuropathy that is initiated by the infection and the accompanying immunological events. The infection is curable but not preventable, and leprosy remains a major global health problem, especially in the developing world, publicity to the contrary notwithstanding. Mycobacterium leprae remains noncultivable, and for over a century leprosy has presented major challenges in the fields of microbiology, pathology, immunology, and genetics; it continues to do so today. This review focuses on recent advances in our understanding of M. leprae and the host response to it, especially concerning molecular identification of M. leprae, knowledge of its genome, transcriptome, and proteome, its mechanisms of microbial resistance, and recognition of strains by variable-number tandem repeat analysis. Advances in experimental models include studies in gene knockout mice and the development of molecular techniques to explore the armadillo model. In clinical studies, notable progress has been made concerning the immunology and immunopathology of leprosy, the genetics of human resistance, mechanisms of nerve injury, and chemotherapy. In nearly all of these areas, however, leprosy remains poorly understood compared to other major bacterial diseases.

Biological implications of Mycobacterium leprae gene expression during infection

The genome of Mycobacterium leprae, the etiologic agent of leprosy, has been sequenced and annotated revealing a genome in apparent disarray and in stark contrast to the genome of the related human pathogen, M. tuberculosis. With less than 50% coding capacity of a 3.3-Mb genome and 1,116 pseudogenes, the remaining genes help define the minimal gene set necessary for in vivo survival of this mycobacterial pathogen as well as genes potentially required for infection and pathogenesis seen in leprosy. To identify genes transcribed during infection, we surveyed gene transcripts from M. leprae growing in athymic nude mice using reverse transcriptase-polymerase chain reaction (RT-PCR) and cross-species DNA microarray technologies. Transcripts were detected for 221 open reading frames, which included genes involved in DNA replication, cell division, SecA-dependent protein secretion, energy production, intermediary metabolism, iron transport and storage and genes associated with virulence. These results suggest that M. leprae actively catabolizes fatty acids for energy, produces a large number of secretory proteins, utilizes the full array of sigma factors available, produces several proteins involved in iron transport, storage and regulation in the absence of recognizable genes encoding iron scavengers and transcribes several genes associated with virulence in M. tuberculosis. When transcript levels of 9 of these genes were compared from M. leprae derived from lesions of multibacillary leprosy patients and infected nude mouse foot pad tissue using quantitative real-time RT-PCR, gene transcript levels were comparable for all but one of these genes, supporting the continued use of the foot pad infection model for M. leprae gene expression profiling. Identifying genes associated with growth and survival during infection should lead to a more comprehensive understanding of the ability of M. leprae to cause disease.

Thalidomide in the treatment of leprosy

Leprosy is a chronic infection of the skin and nerves caused by Mycobacterium leprae. Erythema nodosum leprosum (ENL) is a reactive state in lepromatous leprosy. Thalidomide has been used to treat ENL since the 1960s. One of its mechanisms of action is anti-inflammatory through selective inhibition of the pro-inflammatory cytokine TNF-alpha produced by monocytes.

Massive gene decay in the leprosy bacillus

Leprosy, a chronic human neurological disease, results from infection with the obligate intracellular pathogen Mycobacterium leprae, a close relative of the tubercle bacillus. Mycobacterium leprae has the longest doubling time of all known bacteria and has thwarted every effort at culture in the laboratory. Comparing the 3.27-megabase (Mb) genome sequence of an armadillo-derived Indian isolate of the leprosy bacillus with that of Mycobacterium tuberculosis (4.41 Mb) provides clear explanations for these properties and reveals an extreme case of reductive evolution. Less than half of the genome contains functional genes but pseudogenes, with intact counterparts in M. tuberculosis, abound. Genome downsizing and the current mosaic arrangement appear to have resulted from extensive recombination events between dispersed repetitive sequences. Gene deletion and decay have eliminated many important metabolic activities including siderophore production, part of the oxidative and most of the microaerophilic and anaerobic respiratory chains, and numerous catabolic systems and their regulatory circuits.

Microaerophilic conditions promote growth of Mycobacterium genavense

Our studies show that microaerophilic conditions promote the growth of Mycobacterium genavense in semisolid medium. The growth of M. genavense at 2.5 or 5% oxygen was superior to that obtained at 21% oxygen in BACTEC primary cultures (Middlebrook 7H12, pH 6.0, without additives). By using nondecontaminated specimens, it was possible to detect growth with very small inocula (25 bacilli/ml) of 12 different M. genavense strains (from nude mice) within 6 weeks of incubation under low oxygen tension; conversely, with 21% oxygen, no growth of 8 of 12 (66.7%) M. genavense strains was detected (growth index, <10). The same beneficial effect of 2.5 or 5% oxygen was observed in primary cultures of a decontaminated clinical specimen. Low oxygen tension (2.5 or 5%) is recommended for the primary isolation of M. genavense. Microaerophilic cultivation of other atypical mycobacteria, especially slow-growing (e.g., Mycobacterium avium) and difficult-to-grow (e.g., Mycobacterium ulcerans) species, is discussed.

Effects of decontamination methods and culture conditions on viability of Mycobacterium ulcerans in the BACTEC system

We used the BACTEC system to evaluate the effects of several decontamination methods and the addition of antibiotics on the viability of Mycobacterium ulcerans. The effects of polyoxyethylene stearate or egg yolk as supplements were also evaluated to determine their impact on the growth of M. ulcerans. Strains of different geographic origins were subjected to Petroff, reversed Petroff, oxalic acid, and mild HCI treatments. After treatment, the viability of each strain was assessed in the BACTEC system. All of the decontamination methods tested adversely affected bacterial viability. Treatment with mild HCl gave the best results, allowing better growth rates with some strains and causing a delay in growth with others, depending on the geographic origin of the strain. A mixture of polymyxin B, amphotericin B, nalidixic acid, trimethoprim, and azlocillin did not significantly inhibit growth. Supplementing BACTEC medium with egg yolk markedly improved the recovery of M. ulcerans following the use of each of the decontamination methods. Our findings demonstrate a detrimental impact on the viability of M. ulcerans by all of the decontamination methods currently in common use. This explains, at least in part, the difficulty often experienced in cultivating this organism from clinical specimens. Egg yolk should be added to enhance the rate of successful primary cultivation of M. ulcerans in the BACTEC system.

Inhibitory effects of polyoxyethylene stearate, PANTA, and neutral pH on growth of Mycobacterium genavense in BACTEC primary cultures

We report on the influences of polyoxyethylene stearate (POES), PANTA, and pH on primary cultures of Mycobacterium genavense in BACTEC vials. As a model for primary cultures from tissue, seven different strains first isolated from AIDS patients (five from Switzerland and two from the United States) were inoculated into nude mice in order to obtain large amounts of bacilli to test different conditions simultaneously. Our results demonstrate that the size of the inoculum (10[6] acid-fast bacilli/vial), an acid pH (pH 6.0), and the absence of additives (POES and PANTA) significantly (P < 0.001) increased the probability of a successful culture in 1 month, considering growth index (GI) of > or =100 or a GI of > or =999 as criterion of success. In logistic regression analysis, all factors maintained a significant (P < 0.001) independent effect, and no interactions were observed between them. The best conditions for the primary cultures of M. genavense were the use of Middlebrook 7H12 medium at pH 6.0 without any additives.

Effects of essential fatty acid deficiency on prostaglandin E2 production and cell-mediated immunity in a mouse model of leprosy

Results from animal and in vitro studies suggest that essential fatty acid (EFA) deficiency enhances cell-mediated immunity by reducing production of prostaglandins with immunosuppressive actions. However, direct experimental evidence that EFA deficiency enhances T-lymphocyte function in vivo has not been obtained. In this study, athymic (nu/nu) mice were infected in the footpads with Mycobacterium leprae and fed a linoleic acid-free diet. These mice, and infected nu/nu mice on control diets, were given an adoptive transfer of M. leprae-primed, T-cell-enriched lymphocytes. After 2 weeks, M. leprae bacilli were harvested from the recipient mice and bacterial viability was determined by the BACTEC system. M. leprae recovered from recipient mice fed control diets displayed little reduction in metabolic activity. In contrast, M. leprae from recipient mice fed the EFA-deficient (EFAD) diet exhibited markedly reduced viability. In vitro, donor cells from M. leprae-primed mice secreted elevated levels of gamma interferon upon exposure to the bacilli. These cells also exhibited an enhanced proliferative response, which was reduced by exogenous prostaglandin E2 (PGE2). In addition, M. leprae-infected granuloma macrophages (Mphi) from EFAD recipient nu/nu mice secreted significantly less PGE2 than granuloma Mphi from mice on control diets. These data suggest that enhanced levels of Mphi-generated PGE2, induced by M. leprae or its constituents, could act as an endogenous negative modulator of the immune response occurring in the microenvironment of the lepromatous granuloma.

Chemotherapy of lepromatous leprosy: recent developments and prospects for the future

Leprosy is a major debilitating infectious disease, primarily of the developing world. In this paper the current status and future prospects of antimicrobial therapy of the severe anergic lepromatous form of the disease are reviewed. Until the last few years only dapsone, rifampicin, clofazimine and ethionamide have had practical application in its therapy, and only rifampicin was bactericidal. Recently, antibiotics from three different classes have been found to be bactericidal in lepromatous patients: a tetracycline (minocycline), a macrolide (clarithromycin), and several fluoroquinolones (including pefloxacin, ofloxacin and sparfloxacin). Against a background of drug resistance and bacterial persistence, recommendations for multidrug therapy and the means to devise rationally based therapy for the future are discussed.

Clinical trial of clarithromycin for lepromatous leprosy

Clarithromycin was administered to nine previously untreated lepromatous leprosy patients. Patients received two 1,500-mg doses on the first day, followed by 7 days of no treatment, in order to evaluate the potential efficacy of intermittent therapy. Patients then received 1,000 mg daily for 2 weeks followed by 500 mg daily for 9 weeks. The efficacy of therapy was monitored clinically, by changes in morphological index, mouse footpad infectivity, and radiorespirometric activity of Mycobacterium leprae obtained from serial biopsies and by serum levels of phenolic glycolipid I. Clarithromycin was well tolerated, with only minor side effects noted in two patients. Most patients showed reductions in morphological index and radiorespirometry 1 week after the first two doses. Within 3 weeks of starting treatment (total of 17 g of clarithromycin), biopsy-derived M. leprae specimens from all patients had a morphological index of zero, were noninfectious for mice, and had less than 1% of the radiorespirometric activity of pretreatment specimens. Reductions in serum phenolic glycolipid I levels were observed for most patients at 3 weeks. Significant clinical improvement was evident after 4 weeks of treatment. All analyses indicate that clarithromycin is rapidly bactericidal for M. leprae in humans.

Development of an in vitro drug screening system for Mycobacterium leprae based on the determination of the intrabacterial sodium to potassium ratio of individual bacterial organisms

In vitro drug effects on Mycobacterium leprae (M. leprae) in a cell-free system have been monitored by mass spectrometric determination of the ratio of the intrabacterial concentrations of the sodium and potassium ions (Na(+), K(+) ratio) of a limited number of individual bacteria per sample. From the drug-induced increase of the median values of the distributions of the Na(+), K(+) ratio, information on the concentration and time dependence of drug effects as well as on antagonistic or synergistic interactions of drugs has been obtained. Moreover, absolute values for the percentage of killed bacteria (% kill) have been derived from the distribution of the Na(+), K(+) ratios within a bacterial population. For this, the limiting value of the Na(+), K(+) ratio (up to which bacteria are viable) -which had been determined as 0.45 for cultivable bacteria - has been presumed to be valid also for M. leprae. Highest killing rates have been observed for fusidic acid and clarithromycin, followed by rifabutine, rifampin, and clofazimine. Minocycline and dapsone have shown only moderate killing effects and isoniazid and - probably due to the restricted metabolism of M. leprae in a cell-free medium - ofloxacin have been completely inactive. Strong ofloxacin effects, however, have been observed for cultivable mycobacteria and intracellular M. leprae phagocytized by a murine macrophage cell line.

Fusidic acid is highly active against extracellular and intracellular Mycobacterium leprae

The activity of fusidic acid against Mycobacterium leprae was studied in axenic medium and in bacilli residing within mouse peritoneal macrophages. Activity was assessed by subsequent quantitation of bacillary radiorespirometric activity. Significant inhibition in both systems was observed at 0.156 micrograms/ml, and an approximately 50% reduction in activity occurred after exposure to 1.25 to 2.5 micrograms/ml. The excellent human pharmacokinetics and in vitro activity of fusidic acid against the leprosy bacillus warrant a clinical trial of this drug for leprosy.

Effects of activated macrophages on Mycobacterium leprae

Five alternative methods were used to explore in vitro the effects of normal and activated murine macrophages on the metabolic well-being of intracellular Mycobacterium leprae: fluorescein diacetate-ethidium bromide staining, ATP content, synthesis of phenolic glycolipid 1, and two techniques to quantitate oxidation of palmitic acid. In relatively short-term experiments (7 to 10 days), each of these procedures provided strong evidence that activated macrophages exerted a deleterious effect on the leprosy bacillus. These findings appear to confirm the contention that activated macrophages underlie host resistance to clinical leprosy and limitation of M. leprae growth in paucibacillary leprosy.

In vitro activities of aminoglycosides, lincosamides, and rifamycins against Mycobacterium leprae

The in vitro activities of a variety of aminoglycosides, lincosamides, and rifamycins against Mycobacterium leprae were evaluated with the BACTEC 460 system. At 20 micrograms/ml, gentamicin, kanamycin, tobramycin, streptomycin, and amikacin were inactive. Lincomycin was active at 20 micrograms/ml, and clindamycin was active at 0.31 micrograms/ml. Rifamycin SV, rifabutin, and rifampin were active at 3.1, 3.1 to 12.5, and 200 ng/ml, respectively. The in vitro assay correlates well with the in vivo response of M. leprae to antimicrobial agents, with the exception of the aminoglycosides.

Effectiveness of clarithromycin and minocycline alone and in combination against experimental Mycobacterium leprae infection in mice

As determined by the proportional bactericide method, clarithromycin had strong bactericidal activity against Mycobacterium leprae. Clarithromycin was administered to mice by gavage as 20 daily doses at dosages of 12.5 to 50 mg/kg of body weight. At a dosage of 25 mg/kg, minocycline was more active than clarithromycin at a dosage of 50 mg/kg. Additive effects were displayed with the combination of clarithromycin (50 mg/kg) and minocycline (25 mg/kg), both of which were administered daily by gavage, and of clarithromycin and minocycline, both of which were administered daily by gavage at dosages of 25 mg/kg each, with rifampin at a single oral dose of 10 mg/kg.

Comparative in vitro activities of 20 fluoroquinolones against Mycobacterium leprae

The in vitro activities of 20 fluoroquinolones against Mycobacterium leprae were evaluated by using the BACTEC 460 system. M. leprae was incubated in BACTEC 12B medium at 33 degrees C under reduced oxygen for 2 to 3 weeks in the presence of fluoroquinolones at 0.31 to 5 micrograms/ml. Activity was determined by a reduction in 14CO2 evolution compared with that of drug-free controls. Of the commercially available agents, ofloxacin was most active, while enoxacin and norfloxacin were inactive. However, a number of newer fluoroquinolones (AT-4140, OPC-17100, OPC-17066, PD-117596, PD-124816, PD-127391, and WIN-57273), all containing a cyclopropyl group at R-1 and, with the exception of WIN-57273, either a halogen or methyl group at R-8, were more active than ofloxacin in vitro. Further in vivo evaluations of these agents should help determine their potential for use against leprosy.