In animals, the emetic response to cisplatin can be prevented by pretreatment with an inhibitor of serotonin synthesis, bilateral abdominal vagotomy and greater splanchnic nerve section, or pretreatment with a serotonin 5-HT3 receptor antagonist.
In normal volunteers, single intravenous doses of 0.15 mg/kg of ondansetron had no effect on esophageal motility, gastric motility, lower esophageal sphincter pressure, or small intestinal transit time. Multiday administration of ondansetron has been shown to slow colonic transit in normal volunteers. Ondansetron has no effect on plasma prolactin concentrations.
Ondansetron does not alter the respiratory depressant effects produced by alfentanil or the degree of neuromuscular blockade produced by atracurium. Interactions with general or local anesthetics have not been studied.
Ondansetron is well absorbed from the gastrointestinal tract and undergoes some first-pass metabolism. After a single dose of ZUPLENZ (ondansetron) oral soluble film 8 mg under fasting conditions (n=46), the peak plasma concentrations were achieved in 1.3 hours and the mean elimination half-life was 4.6 hours in healthy subjects. The mean (±S.D.) Cmax and AUC were 37.28 (±14.9) ng/mL and 225 (±88.1) ng·h/mL, respectively. In the same study, mean ondansetron Cmax and AUC following administration of 8 mg ZUPLENZ oral soluble film were comparable to those after 8 mg ondansetron ODT (orally disintegrating tablet). The systemic exposure after administration of ZUPLENZ oral soluble film 8 mg with or without water was found to be comparable.
In a study using ondansetron tablets, ondansetron systemic exposure did not increase proportionately to dose. AUC from a 16 mg tablet was 24% greater than predicted from an 8 mg tablet dose. This may reflect some reduction of first-pass metabolism at higher oral doses.
Food EffectWhen administered with a high fat meal, 8 mg ZUPLENZ (ondansetron) oral soluble film's mean time to peak plasma concentration (tmax) was delayed by approximately 1 hour and its AUC remained similar compared to that of under fasted stated.
Metabolism and Drug InteractionsOndansetron is extensively metabolized in humans, with approximately 5% of a radiolabeled dose recovered as the parent compound from the urine. The primary metabolic pathway is hydroxylation on the indole ring followed by subsequent glucuronide or sulfate conjugation. Although some nonconjugated metabolites have pharmacologic activity, these are not found in plasma at concentrations likely to significantly contribute to the biological activity of ondansetron.
In vitro metabolism studies have shown that ondansetron is a substrate for human hepatic cytochrome P- 450 enzymes, including CYP1A2, CYP2D6, and CYP3A4. In terms of overall ondansetron turnover, CYP3A4 played the predominant role. Because of the multiplicity of metabolic enzymes capable of metabolizing ondansetron, it is likely that inhibition or loss of one enzyme (e.g., CYP2D6 genetic deficiency) will be compensated by others and may result in little change in overall rates of ondansetron elimination. Ondansetron elimination may be affected by cytochrome P-450 inducers. In a pharmacokinetic study of 16 epileptic patients maintained chronically on CYP3A4 inducers, carbamazepine, or phenytoin, reduction in AUC, Cmax, and T½ of ondansetron was observed1; this resulted in a significant increase in clearance. However, on the basis of available data, no dosage adjustment for ondansetron is recommended.
In humans, carmustine, etoposide, and cisplatin do not affect the pharmacokinetics of ondansetron.
ExcretionOndansetron and its metabolites are eliminated via the urine.
Gender EffectsGender differences were shown in the disposition of ondansetron given as a single dose. The extent and rate of ondansetron's absorption is greater in women than men. It is not known whether these gender-related differences are clinically important.
Table 4: Mean Pharmacokinetic Parameters by Gender in
Healthy Volunteers After A Single 8 mg
| Gender | Mean Weight (kg) | n | C vmax (ng/mL) | Tmax (h) | T½ (h) | AUC (h•ng/mL) |
| M | 62 | 39 | 35.2 | 1.67 | 4.54 | 207 |
| F | 56.7 | 7 | 49.1 | 1.7 | 5.39 | 323 |
A reduction in clearance and increase in elimination half-life are seen in patients over 75 years of age. In clinical trials with cancer patients, safety and efficacy was similar in patients over 65 years of age and those under 65 years of age; there was an insufficient number of patients over 75 years of age to permit conclusions in that age-group. No dosage adjustment is recommended in the elderly.
Hepatic ImpairmentIn patients with mild-to-moderate hepatic impairment, clearance is reduced 2-fold and mean half-life is increased to 11.6 hours compared to 5.7 hours in healthy subjects. In patients with severe hepatic impairment (Child-Pugh score of 10 or greater)2, clearance is reduced 2-fold to 3-fold and apparent volume of distribution is increased with a resultant increase in half-life to 20 hours. In patients with severe hepatic impairment, a total daily dose of 8 mg should not be exceeded.
Renal ImpairmentDue to the very small contribution (5%) of renal clearance to the overall clearance, renal impairment was not expected to significantly influence the total clearance of ondansetron. However, ondansetron oral mean plasma clearance was reduced by about 50% in patients with severe renal impairment (creatinine clearance < 30 mL/min). This reduction in clearance is variable and was not consistent with an increase in half-life. No reduction in dose or dosing frequency in these patients is warranted.
Protein BindingPlasma protein binding of ondansetron as measured in vitro was 70% to 76% over the concentration range of 10 to 500 ng/mL. Circulating drug also distributes into erythrocytes.
