Pfizer Inc.'s new investigational microsphere formulation of azithromycin, which is dosed as a one-time-only oral antibiotic, is comparable to other frequently prescribed treatments for some of the most common respiratory tract infections in adult patients, according to the studies presented during the 44th annual Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) in Washington.

Azithromycin has been available in the United States since 1992 under the tradename Zithromax. Pfizer sponsored the studies and has submitted the data for review by the US FDA. The Company is seeking approval for the treatment of adult respiratory tract infections, including acute bacterial exacerbations of chronic bronchitis (AECB) acute bacterial sinusitis (ABS) and community acquired pneumonia (CAP), the company said here.

These three studies showed that with one dose, azithromycin microspheres provides a complete course of therapy for the treatment of AECB, ABS and CAP. Azithromycin microspheres was compared to a seven-day regimen of clarithromycin XL for the treatment of CAP and to seven- and ten-day courses of levofloxacin for the treatment of AECB and ABS, respectively.

"Given the efficacy of a one-time-dose for these common infections and the assured compliance, the hope would be that this type of regimen can help minimize the emergence of antibiotic resistance," Michael Niederman, chairman of the Department of Medicine at Winthrop-University Hospital, Mineola, N.Y. and professor of Medicine at the State University of New York at Stony Brook said adding, "Antibiotic resistance is a growing problem in the treatment of community respiratory tract infections, and one of the factors that contributes to its development is patients taking an incomplete course of therapy."

In two of three multi-centre, randomized, double-blind, double-dummy trials presented, azithromycin microspheres was compared to levofloxacin. One study examined azithromycin microspheres (2.0 g) and levofloxacin (500 mg/day for seven days) in 446 subjects with AECB. The clinical cure rate in the azithromycin microspheres group was comparable (95 per cent) to the levofloxacin group (96 per cent). Chronic bronchitis is characterized by excessive cough and sputum production on most days for at least three months during at least two consecutive years with intermittent acute exacerbations. AECB are usually manifested as an increase in dyspnea (difficult or labored breathing), sputum volume and/or sputum purulence (pus).

In another study presented during the meeting, the new formulation of azithromycin was compared to clarithromycin XL (1 g/day for seven days) in 501 randomized subjects with mild-to-moderate CAP. Treatment with azithromycin microspheres resulted in comparable pathogen eradication rates (91.8 per cent) vs. clarithromycin XL (90.5 per cent). The clinical success rate in the azithromycin microspheres group was 92.6 per cent vs. 94.7 per cent in the clarithromycin XL group. CAP is a lung infection acquired outside of hospitals or extended-care facilities.

"A one-dose-only treatment is a significant advance, giving physicians and patients an option that can effectively treat the most common respiratory tract infections that occur in adults, while also providing an additional benefit regarding noncompliance with therapy," said Joseph Feczko, president of Worldwide Development, Pfizer Inc.

The new azithromycin formulation allows for delayed release of drug, which occurs in the small intestine instead of in the stomach, maintaining tolerability. This, together with the long half-life of azithromycin microspheres, makes it possible to deliver the entire two-gram dose as a single dose regimen, Pfizer says.