VARIATION IN COMEDONAL ANTIBIOTIC CONCENTRATIONS FOLLOWING APPLICATION OF TOPICAL TETRACYCLINE FOR ACNE-VULGARIS

A miniaturized sensitive bioassay was used to detect tetracycline in o pen comedones following topical twice daily application of 0.22% tetra cycline hydrochloride for a minimum of 4 weeks to the facial skin of p atients with mild to moderate acne. The lower limit of detection was 4 .8 +/- 0.8 ng per comedone or per 10 mu l. Using this method, 111 of 1 55 open comedones from 15 patients were found to contain a detectable amount of tetracycline, ranging from 1.8 to 156.9 ng per comedone, and between 4.5 and 1140.1 ng per mg comedonal material. There was a sign ificant effect of comedone weight on tetracycline content, with smalle r comedones containing proportionately more tetracycline. The Spearman rank correlation coefficient was -0.5619 (P < 0.001). All 111 comedon es in which tetracycline was detected contained sufficient drug to inh ibit fully antibiotic-sensitive propionibacteria. However, conditions favourable to the selection and overgrowth of highly tetracycline-resi stant strains (MIC greater than or equal to 32 mu g/ml) prevailed in a t least 18.7% (29 of 155) of the comedones tested.