Action of 1-isonicotinyl-2-palmitoyl hydrazine against the Mycobacterium avium complex and enhancement of its activity by m-fluorophenylalanine

Preparation and Antitubercular Activities of Palindromic Hydrazinecarbothioamides and Carbonothioic Dihydrazides

Background:

With approximately one-third of the world’s population infected, tuberculosis continues to be a global public health crisis. The rise of strains that are unusually virulent or highly resistant to current drugs is a cause of special concern, prompting research into new classes of compounds, as well as the re-evaluation of known chemotherapeutic agents.

Objectives:

The antimycobacterial activities associated with some recently-reported thiocarbonyl compounds kindled our interest in the synthesis of substituted hydrazinecarbothioamides (3) and carbonothioic dihydrazides (4), with the aim of investigating their potential in antitubercular drug design and discovery.

Methods:

In the present study, the title compounds 3 and 4 were prepared by the condensation of hydrazines with isothiocyanates in reactions readily controlled by stoichiometry, temperature and solvent. The compounds were assessed against Mycobacterium bovis BCG in Kirby-Bauer disc diffusion, and minimum inhibitory concentrations were determined against the virulent strain M. tuberculosis Erdman.

Results:

The chemical structures of these thermally stable compounds were determined by IR, 1HNMR, 13C-NMR, high-resolution mass spectrometry and elemental analysis. In the Kirby-Bauer disc diffusion assay, some of the compounds showed substantial diameters of inhibition against BCG. In some cases, the zones of inhibition were so large that no growth at all was observed on the assay plates. Against M. tuberculosis Erdman, several of the compounds showed significant activities. Compound 3h was the most active, demonstrating a minimum inhibitory concentration of 0.5 µg/mL.

Conclusion:

We found that the title compounds may be prepared conveniently in excellent purity and good yields. They are readily identified on the basis of their characteristic spectra. Some members of this class showed significant activities against mycobacteria. We conclude that further work will be warranted in exploring the antitubercular properties of these compounds.

Antimicrobial resistance in Mycobacterium tuberculosis: mechanistic and evolutionary perspectives

Antibiotic-resistant Mycobacterium tuberculosis strains are threatening progress in containing the global tuberculosis epidemic. Mycobacterium tuberculosis is intrinsically resistant to many antibiotics, limiting the number of compounds available for treatment. This intrinsic resistance is due to a number of mechanisms including a thick, waxy, hydrophobic cell envelope and the presence of drug degrading and modifying enzymes. Resistance to the drugs which are active against M. tuberculosis is, in the absence of horizontally transferred resistance determinants, conferred by chromosomal mutations. These chromosomal mutations may confer drug resistance via modification or overexpression of the drug target, as well as by prevention of prodrug activation. Drug resistance mutations may have pleiotropic effects leading to a reduction in the bacterium's fitness, quantifiable e.g. by a reduction in the in vitro growth rate. Secondary so-called compensatory mutations, not involved in conferring resistance, can ameliorate the fitness cost by interacting epistatically with the resistance mutation. Although the genetic diversity of M. tuberculosis is low compared to other pathogenic bacteria, the strain genetic background has been demonstrated to influence multiple aspects in the evolution of drug resistance. The rate of resistance evolution and the fitness costs of drug resistance mutations may vary as a function of the genetic background.

Synthesis of readily available fluorophenylalanine derivatives and investigation of their biological activity

A series of thirty novel N-acetylated fluorophenylalanine-based aromatic amides and esters was synthesized using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide or phosphorus trichloride in pyridine. They were characterized by spectral methods and screened against various microbes (Mycobacterium tuberculosis, non-tuberculous mycobacteria, other bacteria, fungi), for their inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) and cytotoxicity. All amino acids derivatives revealed a moderate inhibition of both cholinesterases with IC₅₀ values for AChE and BChE of 57.88-130.75µM and 8.25-289.0µM, respectively. Some derivatives were comparable or superior to rivastigmine, an established drug. Phenyl 2-acetamido-3-(4-fluorophenyl)propanoate was identified as the selective and most potent inhibitor of BChE. The esterification and amidation of parent acids led to an improved BChE inhibition. The esters are better inhibitors of BChE than the amides. The introduction of NO₂ and CH₃ groups into aniline ring and CF₃ moiety in phenol is translated into lower IC₅₀ values. Seven compounds showed selectivity index higher than 10 for at least one cholinesterase. Especially the esters exhibited a mild activity against Gram-positive bacteria, mycobacteria and several fungal strains with minimum inhibitory concentrations starting from 125µM. The highest susceptibility was recorded for Trichophyton mentagrophytes fungus.

Designing α-helical peptides with enhanced synergism and selectivity against Mycobacterium smegmatis: Discerning the role of hydrophobicity and helicity

Recently, we reported on a series of short amphipathic α-helical peptides, comprising the backbone sequence (LLKK)2, with the ability to kill susceptible and drug-resistant Mycobacterium tuberculosis. In this study, the effect of key physicochemical parameters including hydrophobicity and helicity of α-helical peptides on anti-mycobacterial activity and synergism with rifampicin was investigated. The most hydrophobic analogue, W(LLKK)2W, displayed low selectivity against mycobacteria while peptides with intermediate hydrophobicity were shown to be equally active, yet significantly less toxic. Furthermore, proline substitution impeded the formation of stable amphipathic structures, rendering P(LLKK)2P as one of the least active analogues. Terminal capping with isoleucine was found to promote α-helical folding and the resultant peptide demonstrated the highest selectivity and minimal cytotoxicity against mammalian macrophages. Flow cytometric analysis revealed that enhancements in hydrophobicity and α-helicity increased the rate and extent of peptide-mediated membrane permeabilization. This finding corroborated the hypothesis that synergism between the peptides and rifampicin was likely mediated via peptide-induced pore formation. The rapid, concentration-dependent membrane depolarization, leakage of intracellular ATP and calcein release from PE/PG LUVs supported the membrane-lytic mechanism of action of the peptides. Together, these findings suggest that hydrophobicity and α-helicity significantly impact anti-mycobacterial activity and optimization of both parameters is necessary to develop synthetic analogues with superior selectivity indices and enhanced synergistic potential with conventional antibiotics.

STATEMENT OF SIGNIFICANCE

There is an urgent clinical need for the discovery of new antimicrobials, effective not just for drug susceptible, but also rapidly emerging drug-resistant TB. Recently, we reported on a series of short amphipathic α-helical peptides, comprising the backbone sequence (LLKK)2, with the ability to kill susceptible and drug-resistant M. tuberculosis. In this study, we evaluated a series of synthetic α-helical (LLKK)2 peptides over a range of hydrophobicities for their activity against mycobacteria and provide the first report on the modulating effect of hydrophobicity and α-helicity on the antimicrobial mechanisms of synthetic AMPs and their synergism with first-line antibiotics. These findings demonstrate the applicability of strategies employed here for the rational design of AMPs with the aim of improving cell selectivity and synergistic interactions when co-administered with first-line antibiotics in the fight against drug-resistant tuberculosis.

Colistin as a potentiator of anti-TB drug activity against Mycobacterium tuberculosis

OBJECTIVES

The mycobacterial cell wall is an effective permeability barrier that limits intracellular concentrations of anti-TB drugs and hampers the success of treatment. We hypothesized that colistin might enhance the efficacy of anti-TB drugs by increasing mycobacterial cell wall permeability. In this study, we investigated the additional effect of colistin on the activity of anti-TB drugs against Mycobacterium tuberculosis in vitro.

METHODS

The concentration-dependent and time-dependent killing activity of isoniazid, rifampicin or amikacin alone or in combination with colistin against M. tuberculosis H37Rv was determined. Mycobacterial populations with both high and low metabolic activity were studied, and these were characterized by increasing or steady levels of ATP, respectively.

RESULTS

With exposure to a single drug, striking differences in anti-TB drug activity were observed when the two mycobacterial populations were compared. The addition of colistin to isoniazid and amikacin resulted in sterilization of the mycobacterial load, but only in the M. tuberculosis population with high metabolic activity. The emergence of isoniazid and amikacin resistance was completely prevented by the addition of colistin.

CONCLUSIONS

The results of this study emphasize the importance of investigating mycobacterial populations with both high and low metabolic activity when evaluating the efficacy of anti-TB drugs in vitro. This is the first study showing that colistin potentiates the activity of isoniazid and amikacin against M. tuberculosis and prevents the emergence of resistance to anti-TB drugs. These results form the basis for further studies on the applicability of colistin as a potentiator of anti-TB drugs.

Cationic amphipathic D-enantiomeric antimicrobial peptides with in vitro and ex vivo activity against drug-resistant Mycobacterium tuberculosis

Tuberculosis (TB) is the leading cause of bacterial death worldwide. Due to the emergence of multi-drug resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB), and the persistence of latent infections, a safe and effective TB therapy is highly sought after. Antimicrobial peptides (AMPs) have therapeutic potential against infectious diseases and have the ability to target microbial pathogens within eukaryotic cells. In the present study, we investigated the activity of a family of six AMPs containing all-D amino acids (D-LAK peptides) against MDR and XDR clinical strains of Mycobacterium tuberculosis (Mtb) both in vitro and, using THP-1 cells as a macrophage model, cultured ex vivo. All the D-LAK peptides successfully inhibited the growth of Mtb in vitro and were similarly effective against MDR and XDR strains. D-LAK peptides effectively broke down the heavy clumping of mycobacteria in broth culture, consistent with a 'detergent-like effect' that could reduce the hydrophobic interactions between the highly lipidic cell walls of the mycobacteria, preventing bacteria cell aggregation. Furthermore, though not able to eradicate the intracellular mycobacteria, D-LAK peptides substantially inhibited the intracellular growth of drug-resistant Mtb clinical isolates at concentrations that were well tolerated by THP-1 cells. Finally, combining D-LAK peptide with isoniazid could enhance the anti-TB efficacy. D-LAK peptide, particularly D-LAK120-A, was effective as an adjunct agent at non-toxic concentration to potentiate the efficacy of isoniazid against drug-resistant Mtb in vitro, possibly by facilitating the access of isoniazid into the mycobacteria by increasing the surface permeability of the pathogen.

Preparation and antitubercular activities in vitro and in vivo of novel Schiff bases of isoniazid

Structural modification of the frontline antitubercular isonicotinic acid hydrazide (INH) provides lipophilic adaptations (3-46) of the drug in which the hydrazine moiety of the parent compound has been chemically blocked from the deactivating process of N(2)-acetylation by N-arylaminoacetyl transferases. As a class, these compounds show high levels of activity against Mycobacterium tuberculosis in vitro and in tuberculosis-infected macrophages. They provide strong protection in tuberculosis-infected mice and have low toxicity. With some representatives of this class achieving early peak plasma concentrations approximately three orders of magnitude above minimum inhibitory concentration, they may serve as tools for improving our understanding of INH-based treatment modalities, particularly for those patients chronically underdosed in conventional INH therapy.

Isoniazid-related copper(II) and nickel(II) complexes with antimycobacterial in vitro activity. Part 9

Isonicotinoylhydrazones 1, obtained by the primary antituberculous agent Isoniazid, have been used as monoanionic ligands (L) to prepare copper(II) 2 and nickel(II) 3 octahedral complexes of stoichiometry [MeL2(H2O)2]. Their antimycobacterial in vitro activity was evaluated against Mycobacterium tuberculosis H37Rv in comparison with the ligands. Complexes 2a, 2b, 2f, 3b, 3d and 3g displayed MIC values < or = 0.2 microg/mL.

Synergistic activities of antituberculous drugs with cerulenin and trans-cinnamic acid against Mycobacterium tuberculosis

The recent upsurge in the incidence of tuberculosis with significant emergence of multidrug-resistant cases has focused on the priority of discovering effective new drugs and on the strategies to augment the potential of existing drugs against Mycobacterium tuberculosis. In the present study, we investigated cerulenin and trans-cinnamic acid, which have recently been shown to augment the activity of various antibiotics against Mycobacterium avium [Antimicrob. Agents Chemother. 38 (1994) 2287-2295], to enhance the activity of isoniazid, rifampin, ofloxacin, amikacin and clofazimine against M. tuberculosis. The synergy observed was compared with identical combinations using ethambutol, a cell wall-inhibiting drug used in standard antituberculous chemotherapy. The results showed that ethambutol resulted in synergistic activity in 12/30 drug combinations, as compared to 15/36 for cerulenin and 101/18 for trans-cinnamic acid. This increase in drug activity was even observed with drug-resistant isolates. Use of novel antimicrobials and understanding of their mechanisms of action may be an effective strategy to determine previously undescribed targets for future drug development.

Mechanisms involved in the intrinsic isoniazid resistance of Mycobacterium avium

Isoniazid (INH), which acts by inhibiting mycolic acid biosynthesis, is very potent against the tuberculous mycobacteria. It is about 100-fold less effective against Mycobacterium avium. This difference has often been attributed to a decreased permeability of the cell wall. We measured the rate of conversion of radiolabelled INH to 4-pyridylmethanol by whole cells and cell-free extracts and estimated the permeability barrier imposed by the cell wall to INH influx in Mycobacterium tuberculosis and M. avium. There was no significant difference in the relative permeability to INH between these two species. However, the total conversion rate in M. tuberculosis was found to be four times greater. Examination of in vitro-generated mutants revealed that the major resistance mechanism for both species is loss of the catalase-peroxidase KatG. Analysis of lipid and protein biosynthetic profiles demonstrated that the molecular target of activated INH was identical for both species. M. avium, however, formed colonies at INH concentrations inhibitory for mycolic acid biosynthesis. These mycolate-deficient M. avium exhibited altered colony morphologies, modified cell wall ultrastructure and were 10-fold more sensitive to treatment with hydrophobic antibiotics, such as rifampin. These findings may significantly impact the design of new therapeutic regimens for the treatment of infections with atypical mycobacteria.

Multidrug-resistant Mycobacterium tuberculosis: molecular perspectives

Multidrug-resistant strains of Mycobacterium tuberculosis seriously threaten tuberculosis (TB) control and prevention efforts. Molecular studies of the mechanism of action of antitubercular drugs have elucidated the genetic basis of drug resistance in M. tuberculosis. Drug resistance in M. tuberculosis is attributed primarily to the accumulation of mutations in the drug target genes; these mutations lead either to an altered target (e.g., RNA polymerase and catalase-peroxidase in rifampicin and isoniazid resistance, respectively) or to a change in titration of the drug (e.g., InhA in isoniazid resistance). Development of specific mechanism-based inhibitors and techniques to rapidly detect multidrug resistance will require further studies addressing the drug and drug-target interaction.

Mycobacterial cell wall: structure and role in natural resistance to antibiotics

Mycobacteria show a high degree of intrinsic resistance to most antibiotics and chemotherapeutic agents. The low permeability of the mycobacterial cell wall, with its unusual structure, is now known to be a major factor in this resistance. Thus hydrophilic agents cross the cell wall slowly because the mycobacterial porin is inefficient in allowing the permeation of solutes and exists in low concentration. Lipophilic agents are presumably slowed down by the lipid bilayer which is of unusually low fluidity and abnormal thickness. Nevertheless, the cell wall barrier alone cannot produce significant levels of drug resistance, which requires synergistic contribution from a second factor, such as the enzymatic inactivation of drugs.

Experimental drugs and combination therapy

The worldwide increase in tuberculosis, the additional problem of increasing multiple drug resistance (MDR) and the primary resistance of Mycobacterium avium requires new strategies in drug development and in therapy. The reason for development of MDR is manifold. One important factor is the change in cell wall construction which limits the penetration of the drug to the target receptor. This is supported by the observation that within a class of tuberculostatic drugs (identical mode of action) the more lipophilic derivative is more effective. In addition, it has been shown that mycobacteria within macrophages are able to synthesize additional multilamellar cell wall components. Several possibilities exist to overcome MDR. Besides improving the permeation properties of drugs, the development of synergistic drug combinations based on their special mode of action is a promising approach. This is illustrated with the highly synergistic combination of newly developed hydrazones and thiacetazone respectively with rifampicin. Chance combinations which may even lead to antagonism have to be avoided. Examples of antagonistic behavior of the combinations clofazimine-dapsone and ofloxacin-rifampicin are discussed. An optimization procedure has been developed based on the determination of the specific resistance of patient-derived mycobacteria against single drugs and their combinations. With its use, an individual optimal treatment becomes feasible. Preliminary clinical experience is encouraging.

Potential drug targets for Mycobacterium avium defined by radiometric drug-inhibitor combination techniques

Previously established radiometric techniques were used to assess the effectiveness of combined antimicrobial drug-inhibitory drug (drug-inhibitor) treatment on two clinical isolates of the Mycobacterium avium complex representing three colony variants: smooth opaque (dome) (SmO), smooth transparent (SmT), and rough (Rg). All variants were identified as members of the M. avium complex; however, only the SmT colony type of strain 373 possessed characteristic serovar-specific glycopeptidolipid (GPL) antigens. MICs, determined radiometrically, of drugs with the potential to inhibit the biosynthesis of GPL antigens or other cell envelope constituents were similar for all strains. These drugs included cerulenin, N-carbamyl-DL-phenylalanine, N-carbamyl-L-isoleucine, trans-cinnamic acid, ethambutol, 1-fluoro-1-deoxy-beta-D-glucose, 2-deoxy-D-glucose, and m-fluoro-phenylalanine. The MICs of the antimicrobial drugs amikacin, sparfloxacin, and clarithromycin varied, but overall the MICs for the SmO variant were the lowest. Radiometric assessment of drug-inhibitor combinations by using established x/y determinations revealed enhanced activity when either ethambutol or cerulenin were used in combination with all antimicrobial agents for all variants except the Rg variant of strain 424, for which ethambutol was not effective. Enhanced activity with amino acid analogs was observed with the Rg colony variants of strains 373 and 424. Two potential sites for drug targeting were identified: fatty acid synthesis, for all strains assayed, and peptide biosynthesis, particularly for Rg colony variants that possess previously identified phenylalanine-containing lipopeptides as potential targets for future drug development.

Single and double drug susceptibility testing of Mycobacterium avium complex and mycobacteria other than the tubercle (MOTT) bacilli by a micro-dilution broth minimum inhibitory concentration (MIC) method

SETTING

The incidence of Mycobacterioses is increasing annually, especially in patients with AIDS. There is no clear correlation between in vitro drug susceptibility testing of mycobacteria other than the tubercle (MOTT) bacilli and the in vivo response. Although in vitro, MOTT bacilli appear resistant, some patients respond to treatment possibly as a result of a synergistic action between the drugs being used.

OBJECTIVE

To produce a simple method to determine which individual drugs or combinations of drugs will be effective in killing the causative organism.

DESIGN

A broth microdilution method, using microtitre plates, was used to determine the minimum inhibitory concentration (MIC) of the drugs alone and then in combination.

RESULTS

It was found that the MIC values of the test drugs varied between but were constant within each of the species. Mycobacterium xenopi, M. malmoense and M. kansasii showed a large amount of susceptibility while M. avium complex, M. fortuitum and M. chelonae were limited in their response. These results were reproducible. The test was also easy to perform. The drug combination studies showed that each strain of the M. avium complex tested exhibited synergy between different combinations of drugs.

CONCLUSION

This method, therefore, can be used to indicate individual or combinations of drugs that will or will not act upon the Mycobacterium species isolated. The method was also very rapid, giving a result within 7 days.

Activities of fluoroquinolone, macrolide, and aminoglycoside drugs combined with inhibitors of glycosylation and fatty acid and peptide biosynthesis against Mycobacterium avium

Smooth- and rough-colony variants of Mycobacterium avium serovar 4 were treated with three classes of drugs. The drugs were chosen for their potential inhibitory effects on the biosynthesis of the cell envelope-associated serovar-specific glycopeptidolipid antigens. Growth was monitored radiometrically with a BACTEC 460-TB instrument, and MICs were determined for each drug. Both variants were then treated with inhibitory drugs in combination with antimicrobial agents that have demonstrated effectiveness against M. avium. No growth inhibition was observed with 6-fluoro-6-deoxy-D-glucose or avidin. Inhibitors of glycosylation, i.e., 2-deoxy-D-glucose, bacitracin, and ethambutol, were inhibitory to smooth- and rough-colony variants, whereas drugs that inhibit peptide synthesis, i.e., N-carbamyl-L-isoleucine and m-fluoro-phenylalanine, were more inhibitory for the rough-colony variant. Cerulenin, which affects fatty acid synthesis, was inhibitory for both variants, but it appeared to be more effective at inhibiting the growth of the smooth-colony variant at equivalent concentrations. Generally, when inhibitors of glycosylation were used with sparfloxacin and amikacin, a synergistic effect was observed for only the smooth variant. When drugs that affect peptide synthesis were used in combination with amikacin, a synergistic effect was observed for the rough variant, and when cerulenin was used in combination with sparfloxacin or amikacin, a synergistic effect was observed for both variants. Lipid analysis revealed that although the rough variant lacks the serovar-specific glycopeptidolipid antigens, it does possess a group of phenylalanine-isoleucine-containing lipopeptides that may explain its different susceptibility patterns to m-fluoro-phenylalanine and N-carbamyl-L-isoleucine. The significance of these results is discussed with reference to various components in the cell envelope and their importance in cell wall permeability.

Spectrum of drugs against atypical mycobacteria: how valid is the current practice of drug susceptibility testing and the choice of drugs?

The in vitro activity of 13 drugs against 552 clinical isolates of atypical mycobacteria representing 12 species was performed in 7H11 agar medium at the National Reference Laboratory for Mycobacteria, using the 1% proportion method. All the species tested were resistant to isoniazid and pyrazinamide. In general, clofazimine and D-cycloserine showed the widest spectrum of activity except in the case of Mycobactrium fortuitum and M. chelonei which were resistant to both drugs, and the M. szulgai and M. terrae complex which was resistant to D-cycloserine. The next broad-spectrum drug was ethionamide, followed by ansamycin, rifampin, capreomycin, kanamycin, streptomycin and ethambutol. Among the fluoroquinolones, both ciprofloxacin and ofloxacin were active against M. xenopi, M. gordonae and M. fortuitum whereas M. kansasii and M. gastri were sensitive to ofloxacin only. When the species were listed in respect of the number of drugs to which they were susceptible (less than 10% of resistant strains), they were classified as follows; 7/13 drugs for M. kansasii, M. gastri and M. xenopi; 6/13 for M. gordonae; 5/13 for M. marinum; 3/13 for M. szulgai; 2/13 for M. fortuitum; 1/13 for the M. avium, M. scrofulaceum, M. simiae, and M. terrae complex, and none of the 13 in the case of M. chelonei. These results are discussed in relation to the multiple drug resistance of atypical mycobacteria. We conclude that the critical concentrations of drugs established for M. tuberculosis are not appropriate for atypical mycobacteria.

Activity of clarithromycin compared with those of other drugs against Mycobacterium paratuberculosis and further enhancement of its extracellular and intracellular activities by ethambutol

Radiometric MICs of clarithromycin, a new macrolide drug, were determined against five mycobactin-dependent strains of Mycobacterium paratuberculosis (including two Crohn's disease clinical isolates) and compared with those of other drugs which included rifampin, ethambutol, amikacin, ofloxacin, ciprofloxacin, and sparfloxacin. Among the drugs screened, clarithromycin was the drug for which MICs were lowest against the five strains tested. As MICs were significantly below the reported Cmax levels (about 4 micrograms/ml), the intracellular activity of clarithromycin against the type strain of M. paratuberculosis maintained in cultured macrophages was screened. Clarithromycin was able to kill the initial inoculum by more than 1 log within 7 days, and this activity was further potentiated by ethambutol. Extracellular drug combination screened by using sublethal concentrations of the drugs showed that ethambutol was able to enhance clarithromycin activity in three out of four M. paratuberculosis strains instead of only one out of four strains (or none in the case of ofloxacin) when associated with other drugs. These results suggest that clarithromycin may be fruitful to treat human disease in which M. paratuberculosis may be etiologically involved.

Antimycobacterial spectrum of sparfloxacin and its activities alone and in association with other drugs against Mycobacterium avium complex growing extracellularly and intracellularly in murine and human macrophages

The MICs and MBCs of the new difluorinated quinolone drug sparfloxacin against type strains belonging to 21 species of mycobacteria were screened. The MICs and MBCs were within the range of 0.1 to 2.0 and 0.1 to 4.0 micrograms/ml, respectively (with an MBC/MIC ratio of 1 to 2), and against 18 of the 21 species tested, the drug showed significant bactericidal activity (at least 99% killing or more of the initial inoculum added) at concentrations well within the reported peak concentrations in serum (Cmax) in humans. MICs of sparfloxacin for 7 of 10 Mycobacterium avium complex strains were below the Cmax, with MBC/MIC ratios within the range of 2 to 4. Enhancement of its activity by ethambutol, rifampin, amikacin, and clarithromycin (which were used at sublethal concentrations) assessed by using BACTEC radiometry revealed that its activity was further enhanced in 2 of 10 strains by rifampin and in 7 of 10 strains by ethambutol. The bactericidal effects of various drugs used alone as well as two-drug combinations used at Cmax levels were also screened against four strains of M. avium complex growing intracellularly in two different macrophage systems, namely, mouse bone marrow-derived macrophages and peripheral blood monocyte-derived human macrophages. Our results showed a satisfactory correlation between the extracellular and intracellular drug activity data.

In vitro activity of the new difluorinated quinolone sparfloxacin (AT-4140) against Mycobacterium tuberculosis compared with activities of ofloxacin and ciprofloxacin

MICs of the new fluoroquinolone drugs ofloxacin, ciprofloxacin, and sparfloxacin (AT-4140) for 10 strains of Mycobacterium tuberculosis were determined by using both a BACTEC radiometric method and testing on solid 7H11 agar medium. Radiometric MICs by 7H12 broth testing ranged from 0.5 to 1.0, 0.25 to 0.5, and 0.1 to 0.2 microgram/ml for ofloxacin, ciprofloxacin, and sparfloxacin respectively, whereas MICs in solid medium ranged from 0.5 to 1.0, 0.5 to 1.0, and 0.2 to 0.5 microgram/ml, respectively. The bactericidal action of the quinolones compared with their reported peak concentrations in human serum showed that sparfloxacin is the most bactericidal, followed by ciprofloxacin and ofloxacin. Our results suggest the potential of the new difluorinated quinolone sparfloxacin for use against the tubercle bacillus and indicate that further determination of its antimycobacterial spectrum and intracellular efficacy may be fruitful.

A new insight into the mycobacterial cell envelope architecture by the localization of antigens in ultrathin sections

In an attempt to have a better insight into the mycobacterial cell envelope architecture, various subcellular fractions of Mycobacterium avium were prepared and characterized chemically and ultrastructurally. The various fractions corresponding to the mycobacterial "capsular material", outer layer, cell wall skeleton, cytoplasmic membrane, and cytosol as well as intact bacteria were then used to raise antisera in rabbits. The antisera so raised were then used to immunolabel the intact bacteria prior to embedding in epon. In parallel studies, bacteria were processed by a novel gelatin-uranyl acetate-low temperature Lowicryl HM20 embedding which preserved mycobacterial antigens, permitting to immunolabel antigens on ultrathin sections. Immunolabelling of epon-embedded intact bacteria showed that in the tripartite structure of the bacterial cell envelope, the middle electron-transparent layer acted as a barrier, not permitting the antibodies to penetrate into deeper structures. Immunolabelling of ultrathin sections showed that mycobacteria were surrounded by a "capsule" containing specific surface antigens with a glycocalyx-like topography, and that the intermediate electron transparent layer which separated the surface amphiphils from the inner arabinogalactan-peptidoglycan layer, was a virtual no man's land as it only seldom contained a single gold particle irrespective of the various antisera used. Furthermore, location of various layers in the cell envelope of M. avium using antisera raised against the subcellular fractions prepared was in agreement with chemical and ultrastructural data. A cell envelope model compatible with chemical, ultrastructural and immunolabelling data is proposed and its validity discussed.

Extracellular and intracellular activities of clarithromycin used alone and in association with ethambutol and rifampin against Mycobacterium avium complex

Mycobacterium avium complex bacteria are opportunistic human pathogens, and their chemotherapy remains a challenge since these organisms are resistant to a majority of routine antituberculous drugs. Recently, a wide range of new macrolide antibiotics has been developed, among which the drug clarithromycin appears to have a selective action against M. avium bacteria. In the present study, we have investigated the action of clarithromycin alone (MIC and MBC determinations) and in association with the routine antimycobacterial drugs ethambutol and rifampin at sublethal concentrations (1 micrograms/ml; below concentrations obtainable in human serum) against M. avium. Our viable count data showed that clarithromycin was bactericidal against all 10 strains of M. avium studied and that its activity was enhanced by ethambutol (in 8 of 9 strains) and rifampin (in 3 of 9 strains). The use of all three drugs in association resulted in higher bactericidal effects than found with any of the drugs used alone or in two-drug combinations in seven of nine strains. The bactericidal effects of various drugs used alone and in combination at concentrations obtainable in human serum were investigated against the type strain ATCC 15769 by using 7H9 broth and BACTEC radiometry (extracellular action) and a J-774 macrophage cell line (intracellular action). A good agreement between the extracellular and intracellular activities was found. Electron microscopy using a ruthenium red cytochemical staining of the bacteria showed that clarithromycin disorganized the outer wall layer and the cytoplasmic membrane in the mycobacterial cell envelope and resulted in formation of large vacuoles inside the cytoplasm, with solubilization of ribosomal structures and consequent plasmolysis. Its association with ethambutol and rifampin resulted in more drastic alterations in the bacterial morphology than were seen with any of the drugs used alone, leading to the removal of the bacterial outer layer, homogenization of cytoplasm, complete cell lysis, and formation of ghosts.