INTERFERON SUPPRESSES ERYTHROMYCIN METABOLISM IN RATS AND HUMAN-SUBJECTS

Interferon down-regulates expression of cytochrome P-450 3A in male rats. This study explored the hypothesis that interferon therefore decreases the metabolism of drugs catalyzed by cytochrome P-450 3A. Initial experiments in male rats used microsomal erythromycin N-demethylase activity as a probe for cytochrome P-450 3A catalytic activity. After administration of rat interferon-gamma, erythromycin metabolism was impaired (53% of control; p < 0.01). This change correlated with the decline in cytochrome P-450 3A-dependent androstenedione 6beta-hydroxylase activity, indicating that the decrease in erythromycin N-demethylase activity could be attributed to interferon-mediated suppression of cytochrome P-450 3A. We then used the [C-14]N-methyl erythromycin breath test to assess the activity of hepatic cytochrome P-450 3A in rats and human subjects before and after a single dose of interferon. In rats, rat interferon-gamma decreased erythromycin metabolism to 57% of control (p < 0.005). In the human study, six patients with chronic active hepatitis C and four healthy controls were examined 20 to 26 hr after receiving a subcutaneous injection of human interferon-alpha2b. Interferon produced a small decrease (median = 15%; range = 3% to 35%) in erythromycin metabolism (p < 0.05), as determined by 2-hr excretion of (CO2)-C-14 in the breath. Thus interferon-mediated suppression of cytochrome P-450 3A is less strong in human subjects than in male rats. Comparison of our data about the effect of interferon on cytochrome P-450 3A activity with earlier observations of a major impairment of theophylline clearance in human beings suggests that in human subjects interferon may have a more important effect on other drug-metabolizing enzymes, such as those of the cytochrome P-450 IA subfamily. However, interferon may produce clinically relevant impairment in the elimination of drugs that are substrates for cytochrome P-450 3A, should such agents have a narrow therapeutic index.