Comparative in vitro evaluation of dirithromycin tested against recent clinical isolates of Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae, including effects of medium supplements and test conditions on MIC results

The use of macrolides for treatment of respiratory complaints has been comp licated by susceptibility test conditions that adversely effect the in vitr o test results aim perceived potencies of these compounds. Dirithromycin wa s studied as to ifs in vitro activity compared to other macrolides as well as the effects that environmental incubation variations and inoculum concen trations may have on susceptibility results. Dirithromycin was less active than other macrolides tested (azithromycin, clarithromycin, erythromycin) a gainst Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella cata rrhalis with MIC90 values of 16, 32, and 1 mu g/ml, respectively; an activi ty that was most similar to roxithromycin. This reduced activity may be com pensated by the superior pharmacokinetic properties that dirithromycin poss esses compared to other members in its class. Method variation studies show that incubation in CO2 environments increase the MIC values for all macrol ide compounds and dirithromycin was most effected by pH changes in three in vitro methods tested (Etest [AB BIODISK, Solna, Sweden] broth microdilutio n, and disk diffusion). Variations in inoculum concentration had minimal ef fect on dirithromycin potency. In addition the variability (lack of reprodu cibility) of the test results with dirithromycin recur not significant. Dir ithromycin is an alternative therapeutic choice among macrolide compounds f or treatment of community-acquired respiratory infections caused by various streptococci, Legionella pneumophilia, Mycoplasma pneumoniae and M. catarr halis, and also possesses a modest in vitro potency versus H. influenzae co upled with excellent pharmacokinetic properties. In vitro tests with dirith romycin will continue to be problematic for H. influenzae because of the ad verse effects of recommended CO2 incubation for some standardized methods o r commercial products (Etest). (C) 1999 Elsevier Science Inc.