Clarithromycin resistance and susceptibility patterns of Mycobacterium avium strains isolated during prophylaxis for disseminated infection in patients with AIDS.

A randomized, placebo-controlled trial was conducted to evaluate the efficacy of clarithromycin in the prevention of disseminated Mycobacterium avium complex (MAC) infection in patients with AIDS; special attention was given to the development of clarithromycin resistance. The median time to documented MAC bacteremia was 199 days for placebo-treated patients, 217 days for clarithromycin-treated patients infected with clarithromycin-susceptible MAC, and 385 days for clarithromycin-treated patients infected with clarithromycin-resistant MAC. Most of the patients with clarithromycin-resistant isolates (91%) had a baseline CD4 T-cell count of < 20/microL, while these low counts occurred in only 25% of patients having clarithromycin-susceptible breakthrough isolates. The emergence of clarithromycin resistance did not affect the total period of survival. Resistance to clarithromycin in breakthrough MAC isolates emerges most likely when the patient is extremely immunodeficient at the time of initiation of the preventative therapy.