Plasma and urinary excretion kinetics of oral baclofen in healthy subjects.

Baclofen, a centrally acting muscle relaxant, is used in the treatment of spasticity. Its pharmacokinetics has been derived from plasma and urine data in four healthy subjects, whose renal function was simultaneously measured. After oral administration of a single 40 mg dose, baclofen was mainly excreted unchanged by the kidney, 69 (14)%. The half-life, calculated from extended least squares modelling (ELSMOS) both of plasma and urine data was 6.80 (0.68) h, which is longer than reported in most studies based solely on plasma data. The renal excretion rate constant had the high mean value of 0.35 (0.24) h-1, and the apparent renal clearance of baclofen equalled the creatinine clearance. Passive tubular reabsorption is relatively unimportant, since no dependence was observed on variables urine flow or pH. Although active tubular secretion may contribute to its renal clearance, as shown by the effect of co-administration of probenecid, glomerular filtration appears to be the dominant transport mechanism.