The absolute oral bioavailability of nizatidine exceeds 70%. Peak plasma concentrations (700 to 1,800 mcg/L for a 150-mg dose and 1,400 to 3,600 mcg/L for a 300-mg dose) occur from 0.5 to 3 hours following the dose. A concentration of 1,000 mcg/L is equivalent to 3 μmol/L; a dose of 300 mg is equivalent to 905 μmoles. Plasma concentrations 12 hours after administration are less than 10 mcg/L. The elimination half-life is 1 to 2 hours, plasma clearance is 40 to 60 L/h, and the volume of distribution is 0.8 to 1.5 L/kg. Because of the short half-life and rapid clearance of nizatidine, accumulation of the drug would not be expected in individuals with normal renal function who take either 300 mg once daily at bedtime or 150 mg twice daily. Nizatidine exhibits dose proportionality over the recommended dose range.
The oral bioavailability of nizatidine is unaffected by concomitant ingestion of propantheline. Antacids consisting of aluminum and magnesium hydroxides with simethicone decrease the absorption of nizatidine by about 10%. With food, the AUC and C
max increase by approximately 10%.
In humans, less than 7% of an oral dose is metabolized as N2-monodesmethylnizatidine, an H
2-receptor antagonist, which is the principal metabolite excreted in the urine. Other likely metabolites are the N2-oxide (less than 5% of the dose) and the S-oxide (less than 6% of the dose).
More than 90% of an oral dose of nizatidine is excreted in the urine within 12 hours. About 60% of an oral dose is excreted as unchanged drug. Renal clearance is about 500 mL/min, which indicates excretion by active tubular secretion. Less than 6% of an administered dose is eliminated in the feces.
Moderate to severe renal impairment significantly prolongs the half-life and decreases the clearance of nizatidine. In individuals who are functionally anephric, the half-life is 3.5 to 11 hours, and the plasma clearance is 7 to 14 L/h. To avoid accumulation of the drug in individuals with clinically significant renal impairment, the amount and/or frequency of doses of nizatidine should be reduced in proportion to the severity of dysfunction
( seeDosage and Administration).
Approximately 35% of nizatidine is bound to plasma protein, mainly to α
1-acid glycoprotein. Warfarin, diazepam, acetaminophen, propantheline, phenobarbital, and propranolol did not affect plasma protein binding of nizatidine in vitro.