Rat hepatocytes were used to study the toxicity of a new semisynthetic macrolide, roxithromycin, in comparison with erythromycin base and erythromycin estolate. Roxithromycin caused lactate dehydrogenase leakage close to that of erythromycin estolate and higher than erythromycin base after 21 h of exposure to the drugs. This effect was, at least in part, explained by the higher uptake: roxithromycin was two to three times more concentrated by liver cells than erythromycin base. For both roxithromycin and erythromycin base, the uptake depended on time, temperature, and extracellular antibiotic concentration. The accumulated macrolides egressed rapidly when cells were incubated in antibiotic-free medium. No uptake and no loss of accumulated drugs were observed at 4C. After accumulation by hepatocytes, roxithromycin and erythromycin base underwent similar subcellular distribution, mostly concentrating in cytosol and lysosomes. The small amount accumulated in the other particulate fractions followed the order mitochondria >> nuclei > microsomes. Roxithromycin, however, was less concentrated than erythromycin base in the microsomes.Erythromycin base and its derivatives are macrolide antibiotics widely used for the treatment of respiratory tract infections and are a useful alternative to penicillin in allergic patients (18). However, they have two major limitations: erythromycin base is inactivated by gastric juice and erythromycin estolate, though more gastro-resistant, may produce hepatic injury in patients (10) and in experimental models (7,8,27,28). A new semisynthetic macrolide, roxithromycin (formerly RU 28965), recently synthesized by Roussel Uclaf (Paris, France) and which is an ether oxime derivative of erythromycin (14) (Fig. 1), was reported to have similar antibacterial activity (1, 14, 24) but more favorable pharmacokinetic properties than erythromycin base (20,26).Cultured rat hepatocytes, which are known to show a good correlation between the in vitro and in vivo toxicities of macrolides (7,8,27,28), were used to evaluate the hepatotoxicity of the new antibiotic in comparison with the two analogs inducing mild (erythromycin base) and severe (erythromycin estolate) hepatic damage. The results presented in this study show that roxithromycin causes lactate dehydrogenase leakage higher than erythromycin base and close to that of erythromycin estolate, although it does not possess a surface-active component like lauryl sulfate, which seems to be the main cause of erythromycin estolate hepatotoxicity (8).Erythromycin and its derivatives have been described to be concentrated intracellularly by eucaryotic cells to different degrees (13), so different uptake of roxithromycin and erythromycin base by hepatocytes might explain their different toxicities for liver cells. To gain an insight into the mechanism of the different hepatotoxicities of the two macrolides, the uptake and subcellular distribution of roxi-* Corresponding author.
