[Effects of cysteine compounds on antibiotics I. Effects of L-cysteine and its derivatives on the potency of antibiotics (author's transl)].

Various derivatives of L-cysteine obtained by conversion to an -S-S- bond in the mucoprotein by means of -SH in the chemical structure are widely used as expectorants because they show mucous dissolving action. Recently, there have been reports that L-cysteine derivatives lower the potencies of various antibiotics. Various types of antibiotics and cysteine-type expectorants are often used concomitantly for the treatment of bacterial infections in respiratory tract diseases, and any decrease in the antibiotic potency presents a major therapeutic problem. We investigated the effects of four cysteine derivatives on 12 antibiotics, ampicillin (ABPC), amoxicillin (AMPC), sulbenicillin (SBPC), cefazolin (CEZ), cephalexin (CEX), cephalothin (CET), oxytetracycline (OTC), doxycycline (DOTC), minocycline (MINO), erythromycin (EM), ribostamycin (VSM) and lincomycin (LCM), widely used clinically in vitro with the minimum inhibitory concentration (MIc) obtained by the liquid dilution method as an index. L-Cysteine, acetylcysteine, ethylcysteine and mecysteine lowered the potencies of almost all of the antibiotics at high concentrations (500 mcg/ml), but at low concentrations (12.5 mcg/ml), mecysteine lowered the potencies of only three antibiotics and L-cysteine those of only four antibiotics, while acetylcysteine decreased the potencies of six and ethylcysteine those of seven antibiotics.