The more acidic the urine, the more the drug will be ionized and the less will be reabsorbed. One other aspect is the free fraction of the drug. It may be argued that protein-bound drug does not cross either. The degree of binding effects the extent to which a drug may cross a membrane barrier but his has nothing to do with the ability of the free drug to cross. The inhibition of P-450 causes a low intrinsic clearance, Cl int .
Theophylline is a low extraction drug.
Since less drug is cleared, less drug should be given in chronic treatment. Determine the creatinine clearance for each and compare to the normal value. The average pH of urine is 6.3. Under forced conditions, the pH may range between 4.5 and 7.5. Consider this and the following information to answer questions 3a-3c.