By David N. Leff
Nitroimidazopyran is not a word that comes trippingly off the tongue, or readily to mind. Yet it bids fare to become the first drug in 30 years to treat successfully all forms of tuberculosis.
A precursor to this novel compound was discovered seven years ago in India - one of the Third World countries most sorely afflicted with TB. Microbiologists at Ciba-Geigy's Pharmaceutical Research Center in Bombay were testing nitroimidazofuran chemicals for their possible radiosensitizing properties - enhancing the efficacy of radiation therapy in treating cancer. Instead, they found by screening that their molecule showed antitubercular activity.
The Indian scientists reported in the February 1993 issue of the journal Antimicrobial Agents and Chemotherapy that "the in vitro activity of [their compound] against M. tuberculosis was comparable to those of isoniazid and rifampin," the world's frontline TB antibiotics. Infected mice treated with the candidate drug experienced "significant increase in survival time." But because the agent showed cancer-causing mutagenicity, they did not pursue it. However, their report concluded, "Further investigations of this compound are warranted."
Organic chemist William Baker took them up on it. He is senior vice president for research and preclinical development at PathoGenesis Corp. in Seattle.
"Having read the article," Baker told BioWorld Today, "I phoned Dr. Ashtekar in Bombay for information on their Ciba-Geigy anti-TB-drug library. But shortly after my discussion with the senior author," he added, "Ciba-Geigy closed down their Indian facility completely.
"It occurred to me," Baker recounted, "that their molecule could be modified, so we then spent the rest of the 1990s developing 328 chemical variants to the nitroimidazopyrans [NAPs], specifically our proprietary lead compound, PA-824."
Today's issue of Nature, dated June 22, 2000, reports, for the first time, on the fortunes to date of that agent under the title: "A small-molecule nitroimidazopyran drug candidate for the treatment of tuberculosis."
"Key to this discovery," Baker observed, "was a mutant recombinant strain of M. tuberculosis that we engineered to express firefly luciferase. This allowed us to screen compounds in animals very quickly and efficiently, to come up with the one we did."
Drug Checks Out In Mice, Guinea Pigs
"TB will grow in a mouse," he observed, "primarily in its lungs and spleen. So the in vivo experiments involved infection followed by daily drug treatment for 10 days. Then we harvested their lungs and spleen," Baker recounted, "and counted the bacterial colonies. If the bug was alive and growing, it was producing luciferase, because the gene's turned on and replication takes place. And if the bug isn't growing it's not expressing luciferase. That's a very rapid assay. Traditional TB assays take about a month. Our system gets an answer in about seven days.
"We started the program in '93," Baker narrated, "and on June 6 of '95 had our first successful mouse protection experiments performed." The co-authors tested their drug first in mice, then in guinea pigs. Mice infected with the TB pathogen by intravenous injection prior to oral doses of the drug for 10 days showed "reductions of mycobacterial burden in both spleen and lung tissues, comparable to that of isoniazid."
The Nature paper reports similar therapeutic results in guinea pigs infected with the bug by aerosol inhalation, and then received the PA-824 antibiotic in their drinking water, laced with piqa colada mix for palatability.
Still experimental, PA-824 is the first effective new class of anti-TB drugs in over 30 years. The last one, Baker recalled, "Rifampin, was introduced in the U.S. in 1963 or '64. It's one of the frontline drugs for treatment of tuberculosis. To account for the 30-year hiatus, he pointed out that during that period, "TB largely disappeared in the First World countries. But it's really never left the planet. Certainly the drug discovery focus of pharma and science in general was more traditional organisms - staph, strep, and so forth.
"The AIDS epidemic," he continued, "caused a resurgence of TB in the early '90s, late '80s, and got other companies, including ourselves, interested in the area. But it's still largely a disease of the Third World. TB is more prevalent, for example, in India and Southeast Asia, and the prevalence is the result of just the numbers of people known to be infected. Even more are thought to harbor the bacterium latently, but with no active disease."
It's in the artful-dodgy nature of M. tuberculosis to hide away in its human victim's macrophage cells. There the bug lurks inactive for months, years or the better part of a lifetime, before flaring up in a virulent, often lethal, replicative infection. PA-824 kills its target lung, spleen or other cells in both the latent and active phases. "The drug acts by two mechanisms: protein synthesis in addition to cell-wall biosynthesis," Baker explained. "Maybe inhibiting protein synthesis by the drug could be responsible for extracellular bacterial killing in the replicating phase, and the cell wall might be a target for the static, intracellular latent phase. But it's not clear at this point."
Signs Of Multidrug-Resistance Resistance
As to whether this new family of NAP drugs will be drug-resistance resistant, Baker pointed out that "it's well known throughout the history of microbiology that bugs will eventually become resistant to any antibiotic. You can attest to the stubborn longevity of isoniazid. But used alone it most certainly generates resistance. So that's why drug cocktails are used, especially for TB. But PA-824 does work well against multidrug-resistant M. tuberculosis strains."
Among the Nature paper's co-authors is biochemist Clifton Barry, chief of the TB research section at the National Institute of Allergy and Infectious Diseases (NIAID) in Rockville, Md. "To facilitate TB drug development," Barry said in a press statement, "NIAID and PathoGenesis established a research collaboration focused on developing a new class of anti-TB agents that attack the protective cell wall of M. tuberculosis. The company's expertise in chemistry and molecular biology meshed well with our expertise in cell wall biochemistry."
NIAID's director, Anthony Fauci, made the point, "Because TB hits hardest in impoverished regions where people cannot afford drug treatment, pharmaceutical companies have been reluctant to invest in research on new drugs. Public-private collaborations like this can effectively accelerate research on new ways to treat and prevent these diseases."
Baker guesstimates that it will take from five to eight years to get FDA approval for PA-824. "The company's position with this particular compound," he said, "is looking for partnerships to help us develop it. We have some leads, including biotech companies."