Biological properties of ER 42859, a novel erythromycin derivative.

The antimicrobial activity of a new semi-synthetic oral erythromycin derivative, ER 42859, was evaluated in vitro and in vivo in comparison with erythromycin, spiramycin, josamycin, oleandomycin and the newer semi-synthetic derivatives flurithromycin, roxithromycin and A-56268. MIC values of ER 42859 were superior to those of roxithromycin, oleandomycin, josamycin and spiramycin but generally 2-fold poorer than those of erythromycin. The activity equalled that of erythromycin against Haemophilus influenzae and was superior to that of roxithromycin and A-56268 against this organism. MIC values of the compound were greatly influenced by pH due to the dibasic nature of the molecule. ER 42859 had markedly superior activity to erythromycin, spiramycin, josamycin, oleandomycin and flurithromycin against experimental infections in mice and similar activity to roxithromycin and A-56268. Blood and tissue levels were high and prolonged in rodents. In volunteers, blood levels were prolonged but inferior to those of erythromycin.