Famotidine is incompletely absorbed. The bioavailability of oral doses is 40-45%. Famotidine tablets and famotidine for oral suspension are bioequivalent. Bioavailability may be slightly increased by food, or slightly decreased by antacids; however, these effects are of no clinical consequence. Famotidine undergoes minimal first-pass metabolism. After oral doses, peak plasma levels occur in 1-3 hours. Plasma levels after multiple doses are similar to those after single doses. Fifteen to 20% of famotidine in plasma is protein bound. Famotidine has an elimination half-life of 2.5-3.5 hours. Famotidine is eliminated by renal (65-70%) and metabolic (30-35%) routes. Renal clearance is 250-450 mL/min, indicating some tubular excretion. Twenty-five to 30% of an oral dose and 65-70% of an intravenous dose are recovered in the urine as unchanged compound. The only metabolite identified in man is the S-oxide.
There is a close relationship between creatinine clearance values and the elimination half-life of famotidine. In patients with severe renal insufficiency, i.e., creatinine clearance less than 10 mL/min, the elimination half-life of famotidine may exceed 20 hours and adjustment of dose or dosing intervals in moderate and severe renal insufficiency may be necessary (see PRECAUTIONS, DOSAGE AND ADMINISTRATION).
In elderly patients, there are no clinically significant age-related changes in the pharmacokinetics of famotidine. However, in elderly patients with decreased renal function, the clearance of the drug may be decreased (see PRECAUTIONS, Geriatric Use).
Table 6 presents pharmacokinetic data from clinical trials and a published study in pediatric patients (<1 year of age; N=27) given famotidine I.V. 0.5 mg/kg and from published studies of small numbers of pediatric patients (1-15 years of age) given famotidine intravenously. Areas under the curve (AUCs) are normalized to a dose of 0.5 mg/kg I.V. for pediatric patients 1-15 years of age and compared with an extrapolated 40 mg intravenous dose in adults (extrapolation based on results obtained with a 20 mg I.V. adult dose).
Table 6 Pharmacokinetic Parameters of Intravenous Famotidine |
Age (N=number of patients) | Area Under the Curve (AUC) (ng-hr/mL) | Total Clearance (Cl) (L/hr/kg) | Volume of Distribution (Vd) (L/kg) | Elimination Half-life (T1/2) (hours) |
0-1 monthc (N=10) | NA | 0.13 ± 0.06 | 1.4 ± 0.4 | 10.5 ± 5.4 |
0-3 months (N=6) | 2688 ± 847 | 0.21 ± 0.06 | 1.8 ± 0.3 | 8.1 ± 3.5 |
>3–12 monthsd (N=11) | 1160 ± 474 | 0.49 ± 0.17 | 2.3 ± 0.7 | 4.5 ± 1.1 |
1-11 yrs (N=20) | 1089 ± 834 | 0.54 ± 0.34 | 2.07 ± 1.49 | 3.38 ± 2.60 |
11-15 yrs (N=6) | 1140 ± 320 | 0.48 ± 0.14 | 1.5 ± 0.4 | 2.3 ± 0.4 |
Adult (N=16) | 1726 | 0.39 ± 0.14 | 1.3 ± 0.2 | |
Plasma clearance is reduced and elimination half-life is prolonged in pediatric patients 0-3 months of age compared to older pediatric patients. The pharmacokinetic parameters for pediatric patients, ages >3 months-15 years, are comparable to those obtained for adults.
Bioavailability studies of 8 pediatric patients (11-15 years of age) showed a mean oral bioavailability of 0.5 compared to adult values of 0.42 to 0.49. Oral doses of 0.5 mg/kg achieved AUCs of 645 ± 249 ng-hr/mL and 580 ± 60 ng-hr/mL in pediatric patients <1 year of age (N=5) and in pediatric patients 11-15 years of age, respectively, compared to 482 ± 181 ng-hr/mL in adults treated with 40 mg orally.