Determinants of response to frusemide in normal subjects.

I The pharmacokinetic and diuretic response of frusemide have been investigated in six normal subjects. Frusemide (80 mg) was administered (a) intravenously to unstressed subjects, (b) orally to unstressed subjects, (c) orally to sodium depleted subjects who had received 80 mg oral frusemide 36 h previously followed by a 20 mmol sodium, 160 mmol potassium diet. 2 After i.v. administration, the logarithmic plasma concentration-time curve was biexponen- tial. There was a linear relationship between the frusemide plasma concentration in the 3-phase of elimination and the rate of sodium excretion. Urinary clearance of frusemide was 60% of total plasma clearance, similarly recovery of frusemide in the urine over 36 h was 65% of the dose administered. These observations suggested an extrarenal route of elimination. 3 After oral administration there was also a linear relationship between frusemide plasma concentration and rate of sodium excretion. Oral bioavailability estimated from the ratio of the areas under the plasma concentration-time curve (AUC) and urine recovery over 36 h after i.v. and oral administration was approximately 50%, yet the diuretic response was similar. 4 The AUC of the 1-phase after i.v. administration was similar to the total AUC after oral administration suggesting that response was related to drug present in a tissue pool rather than in plasma. After sodium depletion, there was no change in frusemide kinetics, however the diuretic response decreased. Once again, there was a significant relationship between plasma concentration and rate of sodium excretion. This relationship during the elimination phase after oral administration to sodium depleted subjects was significantly shifted to the right compared to the elimination phase after oral administration to unstressed subjects, suggesting a change in plasma concentration response. 5 In conclusion, the response to frusemide is determined by the concentration of drug in the tissue compartment. This response is modified by factors controlling sodium homeostasis.