Measurement of excretion characteristics of theophylline and its major metabolites.

Analysis of urinary excretion after administration of 320 mg of theophylline to six normal volunteers appears to indicate the occurrence of capacity limited formation of most of the theophylline metabolites and non-linear renal excretion of theophylline. The renal clearance is elevated at high concentrations where the metabolic clearance is reduced. Even though the individual process of elimination is non-linear, the compensating relationship appears to yield a constant total elimination clearance. The implication of these results in chronic therapy and dose adjustment is discussed.