CLINICAL PHARMACOLOGY
Mechanism of Action
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
Pharmacodynamics
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
Absorption
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 Effect
When 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 Interactions
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 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.
Excretion
Ondansetron and its metabolites are eliminated via the
urine.
Gender Effects
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. 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 |
Elderly
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 Impairment
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 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 Impairment
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.
Protein Binding
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.
Clinical Studies
The clinical efficacy of ondansetron, the active
ingredient of ZUPLENZ, was assessed in clinical trials as described below.
Chemotherapy-Induced Nausea and Vomiting
Highly Emetogenic Chemotherapy
In 2 randomized, double-blind, monotherapy trials, a
single 24 mg ondansetron HCl 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
HCl 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 5.
Table 5: Emetic Episodes: Treatment Response After Ondansetron
HCl Tablets 8 mg Twice A Day
|
Ondansetron Tablet 8 mg twice dailya |
Placebo |
p Value |
Number of patients |
33 |
34 |
|
Treatment response |
|
|
|
0 emetic episodes |
20 (61%) |
2 (6%) |
< 0.001 |
1 -2 emetic episodes |
6 (18%) |
8 (24%) |
|
> 2 emetic episodes/ withdrawn |
7 (21%) |
24 (71%) |
< 0.001 |
Median number of emeticepisodes |
0.0 |
Undefinedb |
|
Number of patients |
33 |
34 |
|
Median time to first emetic episode (h) |
Undefinedc |
6.5 |
|
a 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 HCl tablet was administered twice a
day for 2 days after completion of chemotherapy.
b Median undefined since at least 50% of the patients were withdrawn
or had more than 2 emetic episodes.
c Median undefined since at least 50% of patients did not have any
emetic episodes. |
In 1 double-blind US study in 336 patients, ondansetron
HCl tablets 8 mg administered twice a day were as effective as ondansetron HCl
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 6.
Table 6: Emetic Episodes: Treatment Response After
Ondansetron HCl Tablets 8 mg Twice A Day and Three Times A Day
|
Ondansetron 8 mg twice dailya |
Ondansetron 8 mg three times dailyb |
Number of patients |
165 |
171 |
Treatment response |
|
|
0 emetic episodes |
101 (61%) |
99 (58%) |
1 -2 emetic episodes |
16 (10%) |
17 (10%) |
> 2 emetic episodes/withdrawn |
48 (29%) |
55 (32%) |
Median number of emetic episodes |
0.0 |
0.0 |
Median time to first emetic episode (h) |
Undefinedc |
Undefinedc |
Median nausea scores (0-100)d |
6 |
6 |
a 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 HCl tablet was administered twice a
day for 2 days after completion of chemotherapy.
b The first dose was administered 30 minutes before the start of
emetogenic chemotherapy, with subsequent doses 4 and 8 hours after the first
dose. An 8-mg ondansetron HCl tablet was administered three times daily for 2
days after completion of chemotherapy.
c Median undefined since at least 50% of patients did not have any
emetic episodes.
d Visual analog scale assessment: 0=no nausea, 100=nausea as bad as
it can be. |
Retreatment
In uncontrolled trials, 148 patients receiving
cyclophosphamide-based chemotherapy were re-treated with ondansetron HCl
tablets 8 mg three times daily during subsequent chemotherapy for a total of
396 retreatment courses. No emetic episodes occurred in 314 (79%) of the
re-treatment courses, and only 1 to 2 emetic episodes occurred in 43 (11%) of
the re-treatment courses.
Pediatrics
Three open-label, uncontrolled, foreign trials have been
performed with 182 pediatric patients 4 to 18 years old with cancer who were
given a variety of cisplatin or non-cisplatin regimens. In these foreign
trials, the initial dose of ondansetron HCl injection ranged from 0.04 mg/kg to
0.87 mg/kg for a total dose of 2.16 mg to 12 mg. This was followed by the
administration of ondansetron HCl tablets ranging from 4 mg to 24 mg daily for
3 days. In these studies, 58% of the 170 evaluable patients had a complete
response (no emetic episodes) on day 1. Two studies showed the response rates
for patients less than 12 years of age who received ondansetron HCl tablets 4
mg three times daily to be similar to those in patients 12 to 18 years of age
who received ondansetron HCl tablets 8 mg three times daily. Thus, prevention
of emesis in these pediatric patients was essentially the same as for patients
older than 18 years of age. Overall, ondansetron HCl tablets were tolerated in
these pediatric patients.
Radiation-Induced Nausea and Vomiting
Total Body Irradiation
In a randomized, double-blind
study in 20 patients, ondansetron HCl tablets (8 mg given 1.5 hours before each
fraction of radiotherapy for 4 days) were significantly more effective than
placebo in preventing vomiting induced by total body irradiation. Total body
irradiation consisted of 11 fractions (120 cGy per fraction) over 4 days for a
total of 1,320 cGy. Patients received 3 fractions for 3 days, then 2 fractions
on day 4.
Single High-Dose Fraction
Radiotherapy
Ondansetron was significantly
more effective than metoclopramide with respect to complete control of emesis
(0 emetic episodes) in a double-blind trial in 105 patients receiving single
high-dose radiotherapy (800 to 1,000 cGy) over an anterior or posterior field
size of ≥ 80 cm² to the abdomen. Patients
received the first dose of ondansetron HCl tablets (8 mg) or metoclopramide (10
mg) 1 to 2 hours before radiotherapy. If radiotherapy was given in the morning,
2 additional doses of study treatment were given (1 tablet late afternoon and 1
tablet before bedtime). If radiotherapy was given in the afternoon, patients
took only 1 further tablet that day before bedtime. Patients continued the oral
medication on a three times daily basis for 3 days.
Daily Fractionated Radiotherapy
Ondansetron was significantly
more effective than prochlorperazine with respect to complete control of emesis
(0 emetic episodes) in a double-blind trial in 135 patients receiving a 1- to
4-week course of fractionated radiotherapy (180 cGy doses) over a field size of
> 100 cm² to the abdomen. Patients received the first dose of ondansetron HCl
tablets (8 mg) or prochlorperazine (10 mg) 1 to 2 hours before the patient received
the first daily radiotherapy fraction, with 2 subsequent doses on a three times
a day basis. Patients continued the oral medication on a three times daily
basis on each day of radiotherapy.
Postoperative Nausea and
Vomiting
Surgical patients who received
ondansetron 1 hour before the induction of general balanced anesthesia (barbiturate:
thiopental, methohexital, or thiamylal; opioid: alfentanil, sufentanil,
morphine, or fentanyl; nitrous oxide; neuromuscular blockade:
succinylcholine/curare or gallamine and/or vecuronium, pancuronium, or
atracurium; and supplemental isoflurane or enflurane) were evaluated in 2
double-blind studies (1 US study, 1 foreign) involving 865 patients. Ondansetron
HCl tablets (16 mg) were significantly more effective than placebo in
preventing postoperative nausea and vomiting.
The study populations in all
trials thus far consisted of women undergoing inpatient surgical procedures. No
studies have been performed in males. No controlled clinical study comparing
ondansetron HCl tablets to ondansetron injection has been performed.