IntroductionRifampicin is an inducer of the hepatic mixed function oxygenase enzymes involved in drug metabolism.' 2 Needle biopsy specimens of the liver from patients receiving rifampicin show an increase in cytochrome P4503 and proliferation of smooth endoplasmic reticulum in the hepatocytes.4 Rifampicin induces its own metabolism' during continuous treatment as demonstrated by a shortened half life. In addition, rifampicin can accelerate the elimination of the contraceptive pill,6 leading to menstrual disturbance and unwanted pregnancy. Tolbutamide, hexobarbitone,7 and oral anticoagulants8 are other drugs whose metabolism is similarly affected. Deterioration in renal9 and allograft function'0 associated with concomitant rifampicin treatment may be cited as indirect evidence of the effect of this drug on steroid metabolism.We report on two patients with respiratory disease in whom rifampicin affected metabolism of steroids; and we describe a study of the effects of rifampicin on the pharmacokinetics of prednisolone in seven patients.

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To investigate the hypothesis that renal secretion of penicillins is enhanced in cystic fibrosis the maximal tubular secretion rate (Tmax) of ticarcillin and the serum concentration of ticarcillin at half-maximal secretion rate (TC50) were determined in patients with cystic fibrosis (n = 6) and control subjects (n = 6). Each subject received three consecutive constant-rate intravenous infusions of ticarcillin (4, 13, and 70 mg/kg/hr; 2 1/2 hours each) simultaneously with a constant-rate (30 mg/kg/hr) infusion of insulin. Urine samples were collected at 1/2-hour intervals and serum samples at the midpoint of the urine collections. Ticarcillin and inulin concentrations in serum and urine were determined by high-performance liquid chromatographic and a spectrophotometric method, respectively. Ticarcillin serum protein binding was determined by ultrafiltration. Steady-state ticarcillin serum concentrations were achieved at all three infusion rates. The TC50 was significantly lower (p < 0.05) in patients with cystic fibrosis (33.7 +/- 12.2 micrograms/ml) compared with that in control subjects (77.6 +/- 38.4 micrograms/ml). In contrast, the Tmax was similar (cystic fibrosis, 0.25 +/- 0.12 mg/min/kg; control, 0.22 +/- 0.14 mg/min/kg; p > 0.05). These data indicate that renal clearance of penicillins is enhanced in cystic fibrosis because of greater affinity of the renal secretory system for these drugs.