A new class of antibiotics, the ketolides, is on the horizon for pneumonia and other similar conditions. The new drugs offer an alternative to standard antibiotics and another weapon in the arsenal against infectious agents that are becoming increasingly resistant to known pharmaceuticals.

Medicines called macrolides are a standard treatment for many bacterial infections that cause respiratory diseases. They include such antibiotic warhorses as erythromycin. However, infectious agents such as streptococcus and staphylococcus are growing resistant to marcolides and the primary backup, quinolones.

Ketolides are derived from the macrolides, but they are chemically different, so they will kill bacteria that are resistant to macrolides.

Reports on two varieties of ketolides were presented in Toronto on Wednesday at an infectious diseases meeting sponsored by the American Society for Microbiology.

Aventis Pharmaceuticals filed with the U.S. Food and Drug Administration in March for permission to sell its new ketolide, called Ketek or telithromycin. A similar drug made by Abbott Pharmaceuticals, code named ABT-773, is also in early-stage human testing.

Researchers presented 10 separate Ketek studies at the conference involving nearly 2,500 patients. They were intended to show that once-a-day Ketek is equivalent to other standard antibiotics for treating a variety of bacterial diseases, including pneumonia, bronchitis, sinus infections and sore throats.

Macrolides work by halting bacteria's ability to make new proteins, though some resistant strains can pump a macrolide out of its cell membrane before the macrolide can work. Because the ketolides are chemically different, the resistant bacteria do not spit them out this way.