Gamithromycin, the active pharmaceutical ingredient of ZACTRAN is a novel 7a-azalide that has been developed for treatment and prevention of bovine respiratory disease (BRD). The compound belongs to the 15-membered semi-synthetic macrolide antibiotics of the azalide sub-class with a uniquely positioned alkylated nitrogen at the 7a-position of the lactone ring. Macrolide antibiotics in general have bacteriostatic action by inhibiting bacterial RNA dependent protein biosynthesis, but can also be bactericidal. They reversibly bind to 23S ribosomal RNA in the 50S-subunit of prokaryotic ribosomes and prevent protein elongation during the translocation process. In vitro data show that gamithromycin has both bactericidal and bacteriostatic actions at least at the higher concentrations found in lung tissue. The broad spectrum antimicrobial activity of gamithromycin includes Mannheimia haemolytica, Pasteurella multocida and Histophilus somni, the bacterial pathogens most commonly associated with BRD. Gamithromycin administered subcutaneously is well absorbed and fully bioavailable, and safe for the target animal receiving doses up to five times the recommended dose at 30 mg ⁄ kg body weight. Following s.c. administration, gamithromycin is extensively and rapidly distributed to lung tissue, the site of respiratory infection.