Susceptibilities of MDR Mycobacterium tuberculosis isolates to unconventional drugs compared with their reported pharmacokinetic/pharmacodynamic parameters.

Background: The second-line drugs recommended to treat drug-resistant TB are toxic, expensive and difficult to procure. Given increasing resistance, the need for additional anti-TB drugs has become more urgent. But new drugs take time to develop and are expensive. Some commercially available drugs have reported anti-mycobacterial activity but are not routinely used because supporting laboratory and clinical evidence is sparse. Methods: We analysed 217 MDR M. tuberculosis isolates including 153 initial isolates from unique patients and 64 isolates from follow-up specimens during the course of treatment. The resazurin microdilution assay was performed to determine MICs of trimethoprim/sulfamethoxazole, mefloquine, thioridazine, clofazimine, amoxicillin/clavulanate, meropenem/clavulanate, nitazoxanide, linezolid and oxyphenbutazone. Isoniazid was used for validation. We calculated the MIC 50 and MIC 90 as the MICs at which growth of 50% and 90% of isolates was inhibited, respectively. Results: The MIC 50 s, in mg/L, for initial isolates were as follows: trimethoprim/sulfamethoxazole, 0.2/4; mefloquine, 8; thioridazine, 4; clofazimine, 0.25; amoxicillin/clavulanate, 16/8; meropenem/clavulanate, 1/2.5; nitazoxanide, 16; linezolid, 0.25; and oxyphenbutazone, 40. The MIC 90 s, in mg/L, for initial isolates were as follows: trimethoprim/sulfamethoxazole, 0.4/8; mefloquine, 8; thioridazine, 8; clofazimine, 0.5; amoxicillin/clavulanate, 32/16; meropenem/clavulanate, 8/2.5; nitazoxanide, 16; linezolid, 0.25; and oxyphenbutazone, 60. By comparison, the MIC 90 of isoniazid was >4 mg/L, as expected. There was no evidence that previous treatment affected susceptibility to any drug. Conclusions: Most drugs demonstrated efficacy against M. tuberculosis . When these MICs are compared with the published pharmacokinetic/pharmacodynamic profiles of the respective drugs in humans, trimethoprim/sulfamethoxazole, meropenem/clavulanate, linezolid, clofazimine and nitazoxanide appear promising and warrant further clinical investigation.

Bactericidal action of ampicillin/sulbactam against intracellular mycobacteria.

The resistance of mycobacteria to beta-lactam antibiotics is attributed to their ability to synthesize beta-lactamase. In our previous studies, beta-lactam/beta-lactamase-inhibitor combinations suppressed the growth of several mycobacteria in axenic cultures and ampicillin/sulbactam was bactericidal to Mycobacterium tuberculosis H37Rv in vitro, and to Mycobacterium leprae multiplying in mouse foot-pads. Since both these organisms multiply in phagocytic cells in the host, it is important to know whether the drug combination is active against mycobacteria multiplying in macrophages. We tested the action of ampicillin/sulbactam against four potentially pathogenic (to humans or to animals) mycobacteria, M. simiae, M. haemophilum, M. avium, M. microti, when phagocytosed by mouse macrophages. Bacteria were exposed to monolayers of peritoneal macrophages harvested from BALB/c mice. Unphagocytosed bacilli were removed and three concentrations of ampicillin/sulbactam were tested. Optimum activity was observed at 100 mg/l which killed 58-97% of the mycobacteria within macrophages, as determined by the CFU. beta-Lactam/beta-lactamase-inhibitors, especially ampicillin/sulbactam, might provide an effective alternative therapy against infections caused by mycobacteria resistant to other drugs.

Use of beta-lactam/beta-lactamase-inhibitor combinations as antimycobacterial agents.

Mycobacterium tuberculosis and Mycobacterium leprae develop resistance against the drugs used to treat tuberculosis and leprosy, respectively. Now multidrug-resistant tuberculosis is spreading in many countries, especially with the emergence of AIDS. Multidrug treatment is being promoted at present to eradicate leprosy. Since M. leprae may also become multidrug-resistant, new approaches have to be adopted for controlling mycobacterial diseases. Mycobacteria usually synthesize beta-lactamase and are insensitive to beta-lactam antibiotics. M. tuberculosis contains a constitutive beta-lactamase; de-repression of beta-lactamase has been reported in M. leprae. Three different beta-lactam/beta-lactamase-inhibitor combinations (ampicillin/sulbactam, amoxicillin/clavulanate and piperacillin/tazobactam) were used to suppress the growth of several strains of mycobacteria (including M. tuberculosis H37Rv) in vitro. Ampicillin/sulbactam is a potent bactericidal agent against M. leprae multiplying in mouse foot pads. In the present work, ampicillin/sulbactam showed higher activity than the other drug combinations. The beta-lactam/beta-lactamase inhibitors are likely to be effective as rational therapeutic agents against mycobacterial infections.

Unique metabolic properties of Mycobacterium leprae.

My first contact with Dr. Dharmendra was through correspondence. While working for Ph.D., I wrote to him that a section in his book "Notes on Leprosy" was ambiguous. Instead of ignoring the letter, he replied, agreeing to clarify it in the revised edition. I went to work at Carville at the invitation of Dr. Kirchheimer, who had seen my Ph.D. thesis. Dr. Dharmendra visited Carville to receive the Damien-Dutton award and stayed there for a few days. Carville is an isolated place with no public transportation. I used to take him for afternoon drives to the countryside around Carville. He published some of our papers in Leprosy in India and later in Indian Journal of Leprosy. He was very prompt in acknowledging receipt of manuscripts and suggesting any changes to be made. He also reprinted in the Journal several of our papers published elsewhere, and also a lecture I gave at a meeting of the Japanese Leprosy Association. During one of my visits to India, Dr. M. C. Vaidya had arranged a talk by me at the All India Institute of Medical Sciences, New Delhi. At the invitation of Dr. Dharmendra, I visited him in his home. We used to exchange new year cards and letters. He wrote to me about his eye infection and consequent loss of sight in one eye. He asked me to write an editorial for an issue of Indian Journal of Leprosy (January 1989). The last time I met him was during the International Leprosy Congress held in New Delhi.