Determination of ansamycin MICs for Mycobacterium avium complex in liquid medium by radiometric and conventional methods

The combined effect of clarithromycin and rifabutin against Mycobacterium avium multiplying either within human monocyte-derived macrophages or extracellularly in a liquid medium was additive: both MICs and MBCs were twofold lower for the combination than they were for each drug alone. Prolonged exposure for 4 weeks of M. avium-infected macrophages to combined concentrations that were only twofold greater than the MICs resulted in a 100-fold decrease in the number of viable bacteria, while in the drug-free controls a 100-fold or greater increase in comparison with the initial viable counts took place. Comparison of this effect with the results of the prolonged exposure to each drug alone suggested that under these experimental conditions rifabutin enhanced the antimicrobial activity of clarithromycin against intracellular bacteria. At the same time, inhibition of intracellular growth by a 2-h pulsed exposure of the infected macrophages to the combination of the two drugs was not different from the effect induced by clarithromycin alone. In conclusion, clarithromycin played the major role in the antimicrobial activity of the tested combination, while rifabutin may have enhanced this effect during a prolonged exposure of the intracellular bacteria to these two agents.Combinations of clarithromycin with a variety of other drugs have been investigated in vitro in cell-free culture media and macrophages (3,7,9,10,16,17). The overall analyses of these data indicated that the interaction in these combinations tested against Mycobacterium avium was additive rather than synergistic (2). The effect of this combination for the treatment and prevention of disseminated M. avium infection in patients with AIDS is under investigation. The aim of the present study was to evaluate the activity of clarithromycin in combination with rifabutin against M. avium in a cell-free culture medium and human monocyte-derived macrophages. The study included an evaluation of the antimicrobial activity during prolonged exposure and the effect of a drug combination given to infected macrophages by pulsed exposure.MATERUILS AND METHODS Antimicrobial agents. Rifabutin (Adria Laboratories, Pharmacia, Columbus, Ohio) was dissolved first in methanol (640 jig/ml) and was then diluted further in distilled water to make appropriate working solutions. Clarithromycin (Abbott Laboratories, Abbott Park, Ill.) was first dissolved in dimethyl sulfoxide (1,280 jig/ml) and was then diluted further in phosphate buffer (pH 6.5).Test strains. Three strains identified as M. avium by the Gen-Probe (San Diego, Calif.) technique were used for the studies. These strains, isolated from blood samples obtained from patients with AIDS, were used in our previous studies (8,12,13). They were preserved in frozen 7H9 broth culture aliquots at -70°C. For each experiment, a subculture in 7H9 broth was made from a frozen aliquot.Antimicrobial activity against extracellular bacteria. The MIC was defined as the lowest drug concentration that inhibited more than 99% of the bacteria...

Section: Discussionsupporting

confidence: 82%

mentioning

confidence: 99%

Effects of clarithromycin and rifabutin alone and in combination on intracellular and extracellular replication of Mycobacterium avium

Mor

Vanderkolk

Mezo

et al. 1994

“…In this context, we investigated the therapeutic potential of an antituberculous agent, capreomycin (4,14), which is used as a secondary-line drug against Mycobacterium tuberculosis infections because of its toxicity and which has only been tested against MAC in in vitro studies. (25,27,40,42,47). The MICs of the two forms of capreomycin for the MAC strain used in vivo were determined.…”

mentioning

confidence: 99%

Pharmacokinetics, toxicity, and efficacy of liposomal capreomycin in disseminated Mycobacterium avium beige mouse model

Conte¹,

Gallou²,

Potel³

et al. 1994

Capreomycin was incorporated into multilamellar vesicles of pure dipalmitoylphosphatidylcholine. The pharmacokinetics and nephrotoxicity of capreomycin in the free and liposomal forms were studied in normal mice. The efficacies of the two forms were evaluated by using the Mycobacterium avium complex beige mouse model. Approximately 107 viable M. avium cells were injected intravenously. Seven days later, treatment with either liposomal or free capreomycin at 60 or 120 mg/kg of body weight was administered daily for 5 days. Mice were sacrificed 5 days after the end of treatment, and the viable bacteria in liver, spleen, lungs, and blood were counted. After 5 days of treatment with dosages of 60 or 120 mg/kg/day, the level of blood urea nitrogen increased in the group treated with free capreomycin but not in the group treated with liposomal capreomycin. After intravenous injection of 120 mg/kg, liposomes enhanced the diffusion of capreomycin in the spleen, lungs, and kidneys and increased the half-life in serum. The 120-mg/kg dose of liposomal capreomycin significantly reduced the number of viable mycobacteria in the liver, spleen, and blood compared with those in the controls. Although these results are promising, further studies are needed to assess the efficacy of liposomal capreomycin for the treatment of M. avium complex infections.Since the appearance of the human immunodeficiency virus (HIV), the incidence of Mycobacterium avium complex (MAC) infections has increased considerably and has involved between 18 and 50% of HIV-infected patients (17,24,33). Treatment is difficult because of the weakened immune defense system of the host and the resistance of MAC strains to many classic antituberculous agents. Moreover, the phenotypes of resistance of these strains are very heterogeneous (25,26,49).Experimental infection models involving animal or human macrophage cultures and acute or chronic infections in the mouse have been developed to assess the activities of antiinfectious agents against MAC (32). Numerous studies have been performed either in vitro (25,28,36,49) or on macrophages (46) and in animal models (6,21,34,38,39) with classic or more recent antituberculous agents (ciprofloxacin, sparfloxacin, azithromycin, and clarithromycin) alone or in combi-

“…These methods can give information about the inhibitory activity of antimicrobial agents but do not give information about their bactericidal action. The recently developed broth tube dilution and radiometric techniques for mycobacteria also yield information about inhibition of growth but not about killing (2,8). Consequently, most of the information that is available concerning the in vitro susceptibility of M. avium complex isolates to rifabutine and clofazimine is based only on the ability of these antimicrobial agents to inhibit the growth of this organism.…”

mentioning

confidence: 99%

“…This organism is resistant to most of the antimicrobial agents that are used in the treatment of Mycobacterium tuberculosis infections (10,14). Two antimicrobial agents which show good in vitro activity against most strains of M. avium are rifabutine (LM-427; Ansamycin; Farmitalia Carlo Erba Research Laboratories, Milan, Italy) and clofazimine (2,7,8,14). Although clinical trials involving the use of these antimicrobial agents in the treatment of M. avium infection in patients with AIDS are not complete, early results of their efficacy have been disappointing (1,13).…”

mentioning

confidence: 99%

Therapeutic implications of inhibition versus killing of Mycobacterium avium complex by antimicrobial agents

Yajko¹,

Nassos²,

Hadley³

1987

Patients with the acquired immune deficiency syndrome (AIDS) with disseminated Mycobacterium avium infection have responded poorly to treatment with rifabutine (Ansamycin) and clofazimine, in spite of the good in vitro response of M. avium to these antimicrobial agents. We compared the ability of these and other antimicrobial agents to kill versus the ability to inhibit the growth of strains of the M. avium complex isolated from patients with AIDS. Killing curve experiments showed that the concentrations of rifabutine and clofazimine needed to kill two log units of M. avium are at least 32 times greater than the concentrations needed to inhibit growth. Little or no killing occurred at concentrations of these antimicrobial agents that are achievable in serum. In contrast, five of seven strains tested were killed by ciprofloxacin at concentrations that can be achieved in serum. Ciprofloxacin should be studied further for possible use in the treatment of M. avium infections.