Science Editor

In the Feb. 27, 2009, issue of Science, researchers reported identifying a combination of two FDA-approved drugs that, in cell culture studies, killed the tuberculosis-causing Mycobacterium tuberculosis. The combination regimen was effective under circumstances that mimic the latent state of tuberculosis infection, and against all 13 strains of extensively drug-resistant tuberculosis that the authors tested.

Beta-lactam antibiotics - which share a common structural motif, the beta-lactam ring - is a major class of antibiotics that includes penicillin in its ranks. But those are only effective against certain bacteria. Some bacteria, including M. tuberculosis, produce an enzyme called beta-lactamase that breaks down the central ring of beta-lactam antibiotics.

Beta-lactamase can be inhibited by several compounds, including one known as clavenulate. But it took a comprehensive investigation of multiple combinations of beta-lactamase inhibitors and beta-lactam antibiotics to find a combination effective against M. tuberculosis.

Senior author John Blanchard's laboratory at the Albert Einstein College of Medicine studies "the structure and mechanism of proteins that are critical to the survival of pathogenic organisms."

In research published in 2007 in the journal Biochemistry, scientists found that of the FDA-approved beta-lactamase inhibitors, only clavenulate was able to irreversibly inhibit beta-lactamase in M. tuberculosis. Those findings "really set the stage for the current study," Blanchard told BioWorld Today. Blanchard is professor of biochemistry at the Albert Einstein College of Medicine in New York.

In the studies described in Science, Blanchard and his team used a combination of structural and biochemical studies to determine that in tandem, meropenem and clavenulate have what it takes to vanquish M. tuberculosis.

Meropenem, the antibiotic in the combination, is "a very slow substrate for the enzyme, to the point of being an inhibitor." Beta-lactamase can chomp up 30 molecules of amoxicillin per second. But "that same enzyme takes 12 minutes to hydrolyze a single molecule of meropenem."

Though meropenem itself inhibits beta-lactamase by virtue of the glacial speed with which the enzyme processes the drug, the addition of clavenulate, which inhibits beta-lactamase, is synergistic, having "an enormous effect" on the minimum inhibitory concentration, or the amount of drug necessary to kill M. tuberculosis.

The combination was not only effective against regular tuberculosis - which in itself is enough of a scourge. The combo also may be a useful weapon against drug-resistant tuberculosis, for several different reasons.

First, the researchers directly tested 13 different strains of extensively drug-resistant tuberculosis, and found that the combination was effective against all 13.

More subtly, the researchers found that their combination treatment was effective under conditions which mimic the nonreplicating or latent state of M. tuberculosis.

In any tuberculosis infection, a fraction of bacteria is not dividing and thus not susceptible to the antibiotics currently used to treat tuberculosis, all of which target dividing bacteria.

Those nonreplicating bacteria are the reason that tuberculosis treatment currently needs to last half a year - which in turn sharply reduces treatment compliance. Any drug that targets nonreplicating bacteria could drastically shorten the treatment time, which would improve compliance and help prevent the further spread of drug-resistant tuberculosis. (See BioWorld Today, March 24, 2008.)

Not surprisingly, the authors plan to take their findings to the clinic. Co-author Clifton E. Barry III, of the National Institute of Allergy and Infectious Diseases, is involved with plans for a Phase II study with roughly 100 patients in South Korea by the end of the year.

A separate trial, also due to start this year, will test the potency of the drug combination in a smaller number of TB patients in South Africa, where XDR-TB is particularly common. If the results are successful and funding is available, a trial involving a larger number of XDR-TB patients will be conducted.

As encouraging as they are, the new findings do not signal victory over tuberculosis. Asked what the likelihood is that M. tuberculosis will eventually become resistant to the combination, Blanchard did not mince words: "100 percent," was the prompt answer.

Beyond that, though, predictions are hard to come by. "This is a class of compounds, which have never been used against TB," he said. "So the way in which resistance will ultimately develop, as well as the time frame over which it will develop, are not at all clear.

"Resistance will develop eventually," he said. "How and over which time frame, that's just pure speculation." And given that one-third of the world is infected with M. tuberculosis and extremely drug-resistant tuberculosis is on the rise, any reprieve the medical community can get will be helpful - until the next effective approach rolls around.