Theophylline is marketed in a number of oral dosage forms. Rapid release tablets generally are rapidly and completely absorbed with F close to 1.0 and ka values above 2 hr-1. The apparent volume of distribution is approximately 0.5 L/Kg (ideal body weight, IBW). Average values of theopylline clearance approximate 0.04 L/Kg/hr (based on IBW). A number of factors can influence this average clearance value. For example; smoking x 1.6, cimetidine co-administration x 0.6, phenytoin co-administration x 1.6, congestive heart failure x 0.5 (depending on status), cystic fibrosis x 1.5, hepatic cirrhosis x 0.5. Considering a 70 Kg (IBW) non-smoker patient the expected V and kel might be 35 L and 0.08 hr-1. For a patient that smokes the kel would be expected to be approximately 0.125 hr-1. Try adjusting the parameter values according to these covariates and adjust the dosing regimen to maintain appropriate therapeutic concentrations.
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