Sublingual Versus Vaginal Misoprostol for Labor Induction at Term

Misoprostol, a synthetic prostaglandin E1 analog, has been given both orally and vaginally for induction of labor in the third trimester.1 Vaginal misoprostol has been shown to be more efficacious than oral misoprostol in equivalent doses,2 although there is the worry of uterine tachysystole and hyperstimulation with vaginal doses of 50 µg or higher.2-4 The higher efficacy after vaginal administration may be explained by the pharmacokinetics of the drug. Zeiman et al5 showed that the systemic bioavailability of vaginally administered misoprostol is 3 times higher than that after oral administration. Plasma concentrations of its metabolite, misoprostol acid, peak one to two hours after vaginal application as compared with the peak seen 30 minutes following oral administration, and although peak levels are lower with the vaginal route, they are sustained longer and overall exposure to the drug is increased, perhaps because of the presystemic gastrointestinal or hepatic metabolism that occurs with the oral route. An additional explanation for the higher efficacy could be that there is a direct effect on the cervix that initiates the physiologic events that lead to increased uterine contractility.6 However, there seems to be a trend toward patient preference for the oral route. The sublingual route of administration has not been reported in the literature prior to 2001. Since then and partly because of issues relating to patient preference, investigators started studying the sublingual route of administration of misoprostol. In theory, the sublingual route could mimic vaginal administration pharmacokinetically, although there have been no such reported studies on this route of administration.

It is speculated that sublingual misoprostol could combine the higher efficacy of the vaginal route by avoiding gastrointestinal and hepatic metabolism, but it could have a more restrained effect on uterine contractility by avoiding direct effects on both the uterus and cervix. Therefore, in theory, the sublingual route may have lower hyperstimulation rates and would have the advantage of a less invasive administration and lack of restriction of mobility.

Although many studies have been published on the use of sublingual misoprostol for medical abortion in the first and second trimesters, 7-11, only two studies (by the same group) have compared sublingual to oral misoprostol, in different doses.12,13 The 50-µg dose was chosen because it is the dose most commonly used orally and vaginally in various studies reported in the literature.3,14 To the best of our knowledge, no study comparing sublingual to vaginal misoprostol for labor induction at term has been previously published in the literature. Therefore, this study, when completed will provide evidence on the relative effect and safety profile of different routes of administration of misoprostol for labor induction.

The aim of our study is to compare the efficacy of a 50-µg sublingual dose of misoprostol administered at 4-hour intervals with an equivalent dose regimen administered vaginally in women admitted for induction of labor for a medical or obstetric indication at term. In addition, we want to assess the safety profile and patient acceptability of the 2 modes of administration.

The study hypothesis is that the sublingual route of administration of misoprostol is as effective as the vaginal route for induction of labor at term and is more acceptable to patients as compared to vaginal misoprostol.

Inclusion criteria

• Live singleton pregnancy at a gestational age of 36 wks or more with a medical or obstetric indication for induction
• Both nulliparous and multiparous women
• A cephalic presentation
• An unfavorable cervix (Bishop's score less than 8)
• A reassuring fetal heart tracing.

Exclusion criteria

• Rupture of membranes
• Multiple gestation
• Malpresentation (presentation other than cephalic)
• Previous cesarean delivery
• Known contraindications to the use of prostaglandins (e.g. asthma)
• Grandmultiparity (more than 5)
• Significant fetal or maternal concerns that made induction necessary under continuous monitoring (e.g. severe IUGR, severe preeclampsia)