Pharmacokinetics
Misoprostol is extensively absorbed, and undergoes rapid de-esterification
to its free acid, which is responsible for its clinical activity and, unlike
the parent compound, is detectable in plasma. The alpha side chain undergoes
beta oxidation and the beta side chain undergoes omega oxidation followed by
reduction of the ketone to give prostaglandin F analogs.
In normal volunteers, Cytotec (misoprostol) is rapidly absorbed after oral administration with a Tmax of misoprostol acid of 12 ± 3 minutes and a terminal half-life of 20-40 minutes.
There is high variability of plasma levels of misoprostol acid between and within studies but mean values after single doses show a linear relationship with dose over the range of 200-400 mcg. No accumulation of misoprostol acid was noted in multiple dose studies; plasma steady state was achieved within two days.
Maximum plasma concentrations of misoprostol acid are diminished when the dose
is taken with food and total availability of misoprostol acid is reduced by
use of concomitant antacid. Clinical trials were conducted with concomitant
antacid, however, so this effect does not appear to be clinically important.
| Mean ± SD |
Cmax(pg/ml) |
AUC(0-4)
(pg·hr/ml) |
Tmax(min) |
| Fasting |
811 ± 317 |
417 ± 135 |
14 ± 8 |
| With Antacid |
689 ± 315 |
349 ± 108* |
20 ± 14 |
| With High Fat Breakfast |
303 ± 176* |
373 ± 111 |
64 ± 79* |
| * Comparisons with fasting results statistically significant, p < 0.05. |
After oral administration of radiolabeled misoprostol, about 80% of detected
radioactivity appears in urine. Pharmacokinetic studies in patients with varying
degrees of renal impairment showed an approximate doubling of T1/2,
Cmax, and AUC compared to normals, but no clear correlation between the degree
of impairment and AUC. In subjects over 64 years of age, the AUC for misoprostol
acid is increased. No routine dosage adjustment is recommended in older patients
or patients with renal impairment, but dosage may need to be reduced if the
usual dose is not tolerated.
Drug interaction studies between misoprostol and several nonsteroidal anti-inflammatory drugs showed no effect on the kinetics of ibuprofen or diclofenac, and a 20% decrease in aspirin AUC, not thought to be clinically significant.
Pharmacokinetic studies also showed a lack of drug interaction with antipyrine and propranolol when these drugs were given with misoprostol. Misoprostol given for 1 week had no effect on the steady state pharmacokinetics of diazepam when the two drugs were administered 2 hours apart.
The serum protein binding of misoprostol acid is less than 90% and is concentration-independent in the therapeutic range.
After a single oral dose of misoprostol to nursing mothers, misoprostol acid was excreted in breast milk. The maximum concentration of misoprostol acid in expressed breast milk was achieved within 1 hour after dosing and was 7.6 pg/ml (CV 37%) and 20.9 pg/ml (CV 62%) after single 200 µg and 600 µg misoprostol administration, respectively. The misoprostol acid concentrations in breast milk declined to < 1 pg/ml at 5 hours post-dose.
Pharmacodynamics
Misoprostol has both antisecretory (inhibiting gastric acid secretion) and
(in animals) mucosal protective properties. NSAIDs inhibit prostaglandin synthesis,
and a deficiency of prostaglandins within the gastric mucosa may lead to diminishing
bicarbonate and mucus secretion and may contribute to the mucosal damage caused
by these agents. Misoprostol can increase bicarbonate and mucus production,
but in man this has been shown at doses 200 mcg and above that are also antisecretory.
It is therefore not possible to tell whether the ability of misoprostol to reduce
the risk of gastric ulcer is the result of its antisecretory effect, its mucosal
protective effect, or both.
In vitro studies on canine parietal cells using tritiated misoprostol
acid as the ligand have led to the identification and characterization of specific
prostaglandin receptors. Receptor binding is saturable, reversible, and stereospecific.
The sites have a high affinity for misoprostol, for its acid metabolite, and
for other E type prostaglandins, but not for F or I prostaglandins and other
unrelated compounds, such as histamine or cimetidine. Receptor-site affinity
for misoprostol correlates well with an indirect index of antisecretory activity.
It is likely that these specific receptors allow misoprostol taken with food
to be effective topically, despite the lower serum concentrations attained.
Misoprostol produces a moderate decrease in pepsin concentration during basal conditions, but not during histamine stimulation. It has no significant effect on fasting or postprandial gastrin nor on intrinsic factor output.
Effects on gastric acid secretion
Misoprostol, over the range of 50–200 mcg, inhibits basal and nocturnal gastric
acid secretion, and acid secretion in response to a variety of stimuli, including
meals, histamine, pentagastrin, and coffee. Activity is apparent 30 minutes
after oral administration and persists for at least 3 hours. In general, the
effects of 50 mcg were modest and shorter lived, and only the 200-mcg dose had
substantial effects on nocturnal secretion or on histamine and meal-stimulated
secretion.
Uterine effects
Cytotec has been shown to produce uterine contractions that may endanger pregnancy.
(See Boxed Warnings.)
Other pharmacologic effects
Cytotec does not produce clinically significant effects on serum levels of
prolactin, gonadotropins, thyroid-stimulating hormone, growth hormone, thyroxine,
cortisol, gastrointestinal hormones (somatostatin, gastrin, vasoactive intestinal polypeptide, and motilin), creatinine, or uric acid. Gastric emptying, immunologic
competence, platelet aggregation, pulmonary function, or the cardiovascular
system are not modified by recommended doses of Cytotec.
Clinical studies
In a series of small short-term (about 1 week) placebo-controlled studies
in healthy human volunteers, doses of misoprostol were evaluated for their ability
to reduce the risk of NSAID-induced mucosal injury. Studies of 200 mcg q.i.d.
of misoprostol with tolmetin and naproxen, and of 100 and 200 mcg q.i.d. with
ibuprofen, all showed reduction of the rate of significant endoscopic injury
from about 70–75% on placebo to 10–30% on misoprostol. Doses of 25–200 mcg q.i.d.
reduced aspirin-induced mucosal injury and bleeding.
Reducing the risk of gastric ulcers caused by nonsteroidal anti-inflammatory
drugs (NSAIDs)
Two 12-week, randomized, double-blind trials in osteoarthritic patients who
had gastrointestinal symptoms but no ulcer on endoscopy while taking an NSAID
compared the ability of 200 mcg of Cytotec, 100 mcg of Cytotec, and placebo
to reduce the risk of gastric ulcer (GU) formation. Patients were approximately
equally divided between ibuprofen, piroxicam, and naproxen, and continued this
treatment throughout the 12 weeks. The 200-mcg dose caused a marked, statistically
significant reduction in gastric ulcers in both studies. The lower dose was
somewhat less effective, with a significant result in only one of the studies.
Reduction of Risk of Gastric Ulcers Induced by Ibuprofen,
Piroxicam, or Naproxen [No. of patients with ulcer(s) (%)]
| Therapy |
Therapy Duration |
| 4 weeks |
8 weeks |
12 weeks |
|
| Study No. 1 |
| Cytotec 200 mcg q.i.d. (n=74) |
1 (1.4) |
0 |
0 |
1 (1.4)* |
| Cytotec 100 mcg q.i.d. (n=77) |
3 (3.9) |
1 (1.3) |
1 (1.3) |
5 (6.5)* |
| Placebo (n=76) |
11 (14.5) |
4 (5.3) |
4 (5.3) |
19 (25.0) |
| Study No. 2 |
| Cytotec 200 mcg q.i.d. (n=65) |
1 (1.5) |
1 (1.5) |
0 |
2 (3.1)* |
| Cytotec 100 mcg q.i.d. (n=66) |
2 (3.0) |
2 (3.0) |
1 (1.5) |
5 (7.6) |
| Placebo (n=62) |
6 (9.7) |
2 (3.2) |
3 (4.8) |
11 (17.7) |
| Studies No. 1 & No. 2** |
| Cytotec 200 mcg q.i.d. (n=139) |
2 (1.4) |
1 (0.7) |
0 |
3 (2.2)* |
| Cytotec 100 mcg q.i.d. (n=143) |
5 (3.5) |
3 (2.1) |
2 (1.4) |
10 (7.0)* |
| Placebo (n=138) |
17 (12.3) |
6 (4.3) |
7 (5.1) |
30 (21.7) |
* Statistically significantly different from placebo at the
5% level.
** Combined data from Study No. 1 and Study No. 2. |
In these trials there were no significant differences between Cytotec and placebo
in relief of day or night abdominal pain. No effect of Cytotec in reducing the
risk of duodenal ulcers was demonstrated, but relatively few duodenal lesions
were seen.
In another clinical trial, 239 patients receiving aspirin 650–1300 mg q.i.d. for rheumatoid arthritis who had endoscopic evidence of duodenal and/or gastric inflammation were randomized to misoprostol 200 mcg q.i.d. or placebo for 8 weeks while continuing to receive aspirin. The study evaluated the possible interference of Cytotec on the efficacy of aspirin in these patients with rheumatoid arthritis by analyzing joint tenderness, joint swelling, physician's clinical assessment, patient's assessment, change in ARA classification, change in handgrip strength, change in duration of morning stiffness, patient's assessment of pain at rest, movement, interference with daily activity, and ESR. Cytotec did not interfere with the efficacy of aspirin in these patients with rheumatoid arthritis.
Animal Toxicology
A reversible increase in the number of normal surface gastric epithelial cells
occurred in the dog, rat, and mouse. No such increase has been observed in humans
administered Cytotec for up to 1 year.
An apparent response of the female mouse to Cytotec in long-term studies at 100 to 1000 times the human dose was hyperostosis, mainly of the medulla of sternebrae. Hyperostosis did not occur in long-term studies in the dog and rat and has not been seen in humans treated with Cytotec.
Last updated on RxList: 10/15/2009