Ondansetron 4 mg Tablet, Orally Disintegrating, 5-count — NDC 63187-256-05 (Billing 63187-0256-05)

CLINICAL PHARMACOLOGY Pharmacodynamics: Ondansetron is a selective 5-HT 3 receptor antagonist. While its mechanism of action has not been fully characterized, ondansetron is not a dopamine-receptor antagonist. Serotonin receptors of the 5-HT 3 type are present both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone of the area postrema.

It is not certain whether ondansetron's antiemetic action is mediated centrally, peripherally, or in both sites. However, cytotoxic chemotherapy appears to be associated with release of serotonin from the enterochromaffin cells of the small intestine. In humans, urinary 5-HIAA (5-hydroxyindoleacetic acid) excretion increases after cisplatin administration in parallel with the onset of emesis.

The released serotonin may stimulate the vagal afferents through the 5-HT 3 receptors and initiate the vomiting reflex. 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-HT 3 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. Pharmacokinetics: Ondansetron is well absorbed from the gastrointestinal tract and undergoes some first-pass metabolism. Mean bioavailability in healthy subjects, following administration of a single 8-mg tablet, is approximately 56%.

Ondansetron systemic exposure does 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.

Bioavailability is also slightly enhanced by the presence of food but unaffected by antacids. Ondansetron 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, C max , and T ½ of ondansetron was observed.

1 This resulted in a significant increase in clearance. However, on the basis of available data, no dosage adjustment for ondansetron is recommended (see PRECAUTIONS: Drug Interactions). In humans, carmustine, etoposide, and cisplatin do not affect the pharmacokinetics of ondansetron.

Gender 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. Slower clearance in women, a small… [Excerpted — this section continues on DailyMed.]

Pharmacokinetics: Ondansetron is well absorbed from the gastrointestinal tract and undergoes some first-pass metabolism. Mean bioavailability in healthy subjects, following administration of a single 8-mg tablet, is approximately 56%. Ondansetron systemic exposure does 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. Bioavailability is also slightly enhanced by the presence of food but unaffected by antacids.

Ondansetron 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, C max , and T ½ of ondansetron was observed. 1 This resulted in a significant increase in clearance.

However, on the basis of available data, no dosage adjustment for ondansetron is recommended (see PRECAUTIONS: Drug Interactions). In humans, carmustine, etoposide, and cisplatin do not affect the pharmacokinetics of ondansetron. Gender 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. Slower clearance in women, a smaller apparent volume of distribution (adjusted for weight), and higher absolute bioavailability resulted in higher plasma ondansetron levels. These higher plasma levels may in part be explained by differences in body weight between men and women.

It is not known whether these gender-related differences were clinically important. More detailed pharmacokinetic information is contained in Tables 1 and 2 taken from 2 studies. Table 1.

Pharmacokinetics in Normal Volunteers: Single 8-mg Ondansetron Tablet Dose Age-group (years) Mean Weight (kg) n Peak Plasma Concentration (ng/mL) Time of Peak Plasma Concentration (h) Mean Elimination Half-life (h) Systemic Plasma Clearance L/h/kg Absolute Bioavailability 18-40 M F 69 62.7 65 26.2 42.7 2 1.7 3.1 3.5 0.403 0.354 0.483 0.663 61-74 M F 77.5 60.2 66 24.1 52.4 2.1 1.9 4.1 4.9 0.384 0.255 0.585 0.643 ≥ 75 M F 78 67.6 56 37 46.1 2.2 2.1 4.5 6.2 0.277 0.249 0.619 0.747 Table 2. Pharmacokinetics in Normal Volunteers: Single 24-mg Ondansetron Tablet Dose Age-group (years) Mean Weight (kg) n Peak Plasma Concentration (ng/mL) Time of Peak Plasma Concentration (h) Mean Elimination Half-life (h) 18-43 M F 84.1 71.8 8 8 125.8 194.4 1.9 1.6 4.7

5.8A 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 were 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.

In patients with mild-to-moderate hepatic impairment, clearance is reduced 2-fold and mean half-life is increased to 11.6 ho… [Excerpted — this section continues on DailyMed.]

Pharmacodynamics: Ondansetron is a selective 5-HT 3 receptor antagonist. While its mechanism of action has not been fully characterized, ondansetron is not a dopamine-receptor antagonist. Serotonin receptors of the 5-HT 3 type are present both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone of the area postrema.

It is not certain whether ondansetron's antiemetic action is mediated centrally, peripherally, or in both sites. However, cytotoxic chemotherapy appears to be associated with release of serotonin from the enterochromaffin cells of the small intestine. In humans, urinary 5-HIAA (5-hydroxyindoleacetic acid) excretion increases after cisplatin administration in parallel with the onset of emesis.

The released serotonin may stimulate the vagal afferents through the 5-HT 3 receptors and initiate the vomiting reflex. 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-HT 3 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.

CLINICAL TRIALS Chemotherapy-Induced Nausea and Vomiting: Highly Emetogenic Chemotherapy: In 2 randomized, double-blind, monotherapy trials, a single 24-mg ondansetron tablet was superior to a relevant historical placebo control in the prevention of nausea and vomitingassociated with highly emetogenic cancer chemotherapy,including cisplatin ≥50 mg/m 2 . Steroid administration was excluded from these clinical trials. More than 90% of patients receiving a cisplatin dose ≥50 mg/m 2 in thehistorical placebo comparator experienced vomiting in the absence of antiemetic therapy.

The first trial compared oral doses of ondansetron 24 mg once a day, 8 mg twice a day, and 32 mg once a day in 357 adult cancer patients receiving chemotherapy regimens containing cisplatin ≥50 mg/m 2 . A total of 66% of patients in the ondansetron 24-mg once- a-day group, 55% in the ondansetron 8-mg twice-a-day group, and 55% in the ondansetron 32-mg once-a-day group completed the 24-hour study period with 0 emetic episodes and no rescue antiemetic medications, the primary endpoint of efficacy. Each of the 3 treatment groups was shown to be statistically significantly superior to a historical placebo control.

In the same trial, 56% of patients receiving oral ondansetron 24 mg once a day experienced no nausea during the 24-hour study period, compared with 36% of patients in the oral ondansetron 8-mg twice-a-day group ( P = 0.001) and 50% in the oral ondansetron 32-mg once-a-day group. In a second trial, efficacy of the oral ondansetron 24-mg once-a-day regimen in the prevention of nausea and vomiting associated with highly emetogenic cancer chemotherapy, including cisplatin ≥50 mg/m 2 , was confirmed. Moderately Emetogenic Chemotherapy: In 1 double-blind US study in 67 patients, ondansetron tablets 8 mg administered twice a day were significantly more effective than placebo in preventing vomiting induced by cyclophosphamide-based chemotherapy containing doxorubicin.

Treatment response is based on the total number of emetic episodes over the 3-day study period. The results of this study are summarized in Table 3: Table 3. Emetic Episodes: Treatment Response Ondansetron 8-mg b.i.d. ondansetron tablets* Placebo P Value Number of patients 33 34 Treatment response 0 Emetic episodes 1-2 Emetic episodes More than 2 emetic episodes/withdrawn 20 (61%) 6 (18%) 7 (21%) 2 (6%) 8 (24%) 24 (71%) < 0.001 < 0.001 Median number of emetic episodes 0 Undefined † Median time to first emetic episode (h) Undefined ‡ 6.5 * The first dose was administered 30 minutes before the start of emetogenic chemotherapy, with a subsequent dose 8 hours after the first dose.

An 8 mg ondansetron tablet was administered twice a day for 2 days after completion of chemotherapy. † Median undefined since at least 50% of the patients were withdrawn or had more than 2 emetic episodes. ‡ Median undefined since at least 50% of patients did not have any emetic episodes. In 1 double-blind US study in 336 patients, ondansetron tablets 8 mg administered twice a day were as effective as ondansetron tablets 8 mg administered 3 times a day in preventing nausea and vomiting induced by cyclophosphamide-based chemotherapy containing either methotrexate or doxorubicin.

Treatment response is based on the total number of emetic episodes over the 3-day study period. The results of this study are summarized in Table 4: Table 4. Emetic Episodes: Treatment Response Ondansetron 8-mg b.i.d. ondansetron tablets* 8-mg t.i.d. ondansetron tablets † Number of patients 165 171 Treatment response 0 Emetic episodes 1-2 Emetic episodes More than 2 emetic episodes/withdrawn 101 (61%) 16 (10%) 48 (29%) 99 (58%) 17 (10%) 55 (32%) Median number of emetic episodes 0 0 Median time to first emetic episode (h) Undefined ‡ Undefined ‡ Median nausea scores (0-100) § 6 6 * The first dose was administered 30 minutes before the start of emetogenic chemotherapy, with a subsequent dose 8 hours after the first dose.

An 8-mg ondansetron tablet was a… [Excerpted — this section continues on DailyMed.]