Absorption
Famotidine is incompletely absorbed. The bioavailability of oral doses is 40 to 45%. Bioavailability may be slightly increased by food, or slightly decreased by antacids; however, these effects are of no clinical consequence.
Peak famotidine plasma levels occur in 1 to 3 hours. Plasma levels after multiple dosages are similar to those after single doses.
Distribution
Fifteen to 20% of famotidine in plasma is protein bound.
Elimination
Metabolism:
Famotidine undergoes minimal first-pass metabolism. Twenty-five to 30% of an oral dose was recovered in the urine as unchanged compound. The only metabolite identified in humans is the S-oxide.
Excretion:
Famotidine has an elimination half-life of 2.5 to 3.5 hours. Famotidine is eliminated by renal (65 to 70%) and metabolic (30 to 35%) routes. Renal clearance is 250 to 450 mL/minute, indicating some tubular excretion.
Specific Populations
Pediatric Patients:
Infants from birth to 12 Months
After a single oral dose administration of 0.5 mg/kg orally in patients from birth to 12 months, the bioavailability is approximately 42%.
The AUC increased 1.4-fold after single oral dose of 1 mg/kg compared to 0.5 mg/kg and 2.7-fold after multiple oral doses of 1 mg/kg compared to 0.5 mg/kg.
Plasma clearance is reduced and elimination half-life is prolonged in pediatric patients from birth to 3 months of age compared to older pediatric patients. Following intravenous administration of 0.5 mg/kg, CL
Total was 0.13 ± 0.06 L/hr/kg, 0.21 ± 0.06 L/hr/kg, and 0.49 ± 0.17 L/hr/kg in pediatric patients <1 month of age, <3 months of age, and >3 to 12 months of age, respectively. Elimination half-life was 10.5 hours, 8.1 hours, and 4.5 hours in pediatric patients <1 month of age, <3 months of age, and >3 to 12 months of age, respectively.
Patients 11 Years to 15 Years
The mean bioavailability in 8 pediatric patients was 50% compared to adult values of 42% to 49%.
Pharmacokinetic parameters in pediatrics 11 years to 15 years is compared to infants from birth to 12 months in Table 4.
Table 4: Mean Pharmacokinetic Parameters Following a Single Oral Dose of 0.5 mg/kg in Infants and Pediatric Patients
| Infants from Birth to 12 Months (N=5)
| Pediatric Patients 11 Years to 15 Years (N=8)
|
AUC
0 to ∞ (ng*hr/mL)
a
| 645 ± 249
| 580 ± 60
|
C
max (ng/mL)
| 79.2
| 97.3
|
T
max (hr)
b
| 2.0 (1.0, 4.1)
c
| 2.3 (2.1, 2.9)
d
|
T
1/2(hr)
| 5.82
| 2.13
|
a arithmetic mean ± S.D.
b median
c observed minimum and maximum values
d reported minimum and maximum values
Patients with Renal Impairment:
In adult patients with severe renal impairment (creatinine clearance less than 30 mL/minute), the systemic exposure (AUC) of famotidine increased at least 5-fold. In adult patients with moderate renal impairment (creatinine clearance between 30 to 60 mL/minute), the AUC of famotidine increased at least 2-fold
[see DOSAGE AND ADMINISTRATION (2.3), USE IN SPECIFIC POPULATIONS (8.6)].
Drug Interaction Studies
Human Organic Anion Transporter (OAT) 1 and 3:
In vitro studies indicate that famotidine is a substrate for OAT1 and OAT3. Following coadministration of probenecid (1500 mg), an inhibitor of OAT1 and OAT3, with a single oral 20 mg dose of famotidine in 8 healthy subjects, the serum AUC
0 to 10h of famotidine increased from 424 to 768 ng•hr/mL and the maximum serum concentration (C
max) increased from 73 to 113 ng/mL. Renal clearance, urinary excretion rate and amount of famotidine excreted unchanged in urine were decreased. The clinical relevance of this interaction is unknown.
Multidrug and Toxin Extrusion Protein 1 (MATE-1):
An in vitro study showed that famotidine is an inhibitor of MATE-1. However, no clinically significant interaction with metformin, a substrate for MATE-1, was observed.
CYP1A2:
Famotidine is a weak CYP1A2 inhibitor.