ONDANSETRON: Package Insert and Label Information

ONDANSETRON — ondansetron hydrochloride tablet, film coated
STAT Rx USA LLC

The active ingredient in ondansetron tablets USP is ondansetron hydrochloride (HCl) USP as the dihydrate, the racemic form of ondansetron and a selective blocking agent of the serotonin 5-HT3 receptor type. Chemically it is (±) 1, 2, 3, 9-tetrahydro-9-methyl-3-[(2-methyl-1H-imidazol-1-yl)methyl]-4H-carbazol-4-one, monohydrochloride, dihydrate. It has the following structural formula:

Ondansetron HCl Structural Formula
(click image for full-size original)

The empirical formula is C18 H19 N3 O•HCl•2H2 O, representing a molecular weight of 365.9.

Ondansetron HCl USP dihydrate is a white to off-white powder that is soluble in water and normal saline.

The active ingredient in ondansetron orally disintegrating tablets USP is ondansetron base, the racemic form of ondansetron, and a selective blocking agent of the serotonin 5-HT3 receptor type. Chemically it is (±) 1, 2, 3, 9-tetrahydro-9-methyl-3-[(2-methyl-1H-imidazol-1-yl)methyl]-4H-carbazol-4-one. It has the following structural formula:

Structural Formula of Ondansetron Base

The empirical formula is C18 H19 N3 O representing a molecular weight of 293.4.

Each 4-mg ondansetron tablet USP for oral administration contains ondansetron HCl USP dihydrate equivalent to 4 mg of ondansetron. Each 8-mg ondansetron tablet USP for oral administration contains ondansetron HCl USP dihydrate equivalent to 8 mg of ondansetron. Each 24-mg ondansetron tablet USP for oral administration contains ondansetron HCl USP dihydrate equivalent to 24 mg of ondansetron. Each tablet also contains the inactive ingredients colloidal silicon dioxide, hypromellose, iron oxide yellow (8 mg tablet only), iron oxide red (24 mg tablet only), lactose monohydrate, magnesium stearate, microcrystalline cellulose, pregelatinized starch, titanium dioxide and triacetin.

Each 4-mg ondansetron orally disintegrating tablet USP for oral administration contains 4 mg ondansetron base. Each 8-mg ondansetron orally disintegrating tablet USP for oral administration contains 8 mg ondansetron base. Each ondansetron orally disintegrating tablet USP also contains the inactive ingredients aspartame, colloidal silicon dioxide, crospovidone, magnesium stearate, mannitol, sodium stearyl fumarate and strawberry flavor. Ondansetron orally disintegrating tablets USP are an orally administered formulation of ondansetron which rapidly disintegrates on the tongue and does not require water to aid dissolution or swallowing. This product disintegrates in approximately 60 seconds.

CLINICAL PHARMACOLOGY

Pharmacodynamics:

Ondansetron is a selective 5-HT3 receptor antagonist. While its mechanism of action has not been fully characterized, ondansetron is not a dopamine-receptor antagonist. Serotonin receptors of the 5-HT3 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-HT3 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-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.

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, Cmax , and T½ 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 EliminationHalf-life (h) Systemic Plasma Clearance L/h/kg Absolute Bioavailability

18-40 M

69

6

26.2

2

3.1

0.403

0.483

61-74 M

77.5

6

24.1

2.1

4.1

0.384

0.585

≥75 M

78

5

37

2.2

4.5

0.277

0.619

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

84.1

8

125.8

1.9

4.7

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 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 hours compared to 5.7 hours in normals. In patients with severe hepatic impairment (Child-Pugh2 score of 10 or greater), 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.

Due 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.

Plasma 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.

Four-and 8-mg doses of ondansetron orally disintegrating tablets are bioequivalent to corresponding doses of ondansetron tablets and may be used interchangeably. One 24-mg ondansetron tablet is bioequivalent to and interchangeable with three 8-mg ondansetron hydrochloride tablets.

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 vomiting associated with highly emetogenic cancer chemotherapy, including cisplatin ≥50 mg/m2. Steroid administration was excluded from these clinical trials. More than 90% of patients receiving a cisplatin dose ≥50 mg/m2 in the historical 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/m2. 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/m2 , 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.

Placebo P Value
Number of patients 33 34

Treatment response

0 Emetic episodes

1-2 Emetic episodes

20 (61%)

6 (18%)

2 (6%)

8 (24%)

<0.001

Median number of emetic episodes 0 Undefined
Median time to first emetic episode (h) Undefined 6.5

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
*
Median undefined since at least 50% of patients did not have any emetic episodes.
Ondansetron

8-mg b.i.d.

8-mg t.i.d.

Number of patients 165 171

Treatment response

0 Emetic episodes

1-2 Emetic episodes

101 (61%)

16 (10%)

99 (58%)

17 (10%)

Median number of emetic episodes 0 0
Median time to first emetic episode (h) Undefined * Undefined *
Median nausea scores (0-100) 6 6
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