Prepidil (Dinoprostone Vaginal) - Description and Clinical Pharmacology

For Endocervical Use

DESCRIPTION

PREPIDIL Gel contains dinoprostone as the naturally occurring form of prostaglandin E₂ (PGE₂) and is designated chemically as (5Z, 11a, 13E, 15S)-11,15-Dihydroxy-9-oxo-prosta-5,13-dien-1-oic acid. The molecular formula is C₂₀H₃₂O₅ and the molecular weight is 352.5. Dinoprostone occurs as a white to off-white crystalline powder with a melting point within the range of 65° to 69°C. It is soluble in ethanol, in 25% ethanol in water, and in water to the extent of 130 mg/100 mL. The active constituent of PREPIDIL Gel is dinoprostone 0.5 mg/3 g (2.5 mL gel); other constituents are colloidal silicon dioxide NF (240 mg/3 g) and triacetin USP (2760 mg/3 g).

The structural formula is represented below:

CLINICAL PHARMACOLOGY

PREPIDIL Gel (dinoprostone) administered endocervically may stimulate the myometrium of the gravid uterus to contract in a manner similar to contractions seen in the term uterus during labor. Whether or not this action results from a direct effect of dinoprostone on the myometrium has not been determined. Dinoprostone is also capable of stimulating smooth muscle of the gastrointestinal tract in humans. This activity may be responsible for the vomiting and/or diarrhea that is occasionally seen when dinoprostone is used for preinduction cervical ripening.

In laboratory animals, and also in humans, large doses of dinoprostone can lower blood pressure, probably as a result of its effect on smooth muscle of the vascular system. With the doses of dinoprostone used for cervical ripening this effect has not been seen. In laboratory animals, and also in humans, dinoprostone can elevate body temperature; however, with the dosing used for cervical ripening this effect has not been seen.

In addition to an oxytocic effect, there is evidence suggesting that this agent has a local cervical effect in initiating softening, effacement, and dilation. These changes, referred to as cervical ripening, occur spontaneously as the normal pregnancy progresses toward term and allow evacuation of uterine contents by decreasing cervical resistance at the same time that myometrial activity increases. While not completely understood, biochemical changes within the cervix during natural cervical ripening are similar to those following PGE₂-induced ripening. Further, it has been shown that these changes can take place independent of myometrial activity; however, it is quite likely that PGE₂ administered endocervically produces effacement and softening by combined contraction-inducing and cervical-ripening properties. There is evidence to suggest that the changes that take place within the cervix are due to collagen degradation resulting from collagenase secretion as a response, at least in part, to PGE₂.

Using an unvalidated assay, the following information was determined. When PREPIDIL Gel was administered endocervically to women undergoing preinduction ripening, results from measurement of plasma levels of the metabolite 13,14-dihydro-15-keto-PGE₂ (DHK-PGE₂) showed that PGE₂ was relatively rapidly absorbed and the T_max was 0.5 to 0.75 hours. Plasma mean C_max for gel-treated subjects was 433 ± 51 pg/mL versus 137 ± 24 pg/mL for untreated controls. In those subjects in which a clinical response was observed, mean C_max was 484 ± 57 pg/mL versus 213 ± 69 pg/mL in nonresponders and 219 ± 92 pg/mL in control subjects who had positive clinical progression toward normal labor. These elevated levels in gel-treated subjects appear to be largely a result of absorption of PGE₂ from the gel rather than from endogenous sources.

PGE₂ is completely metabolized in humans. PGE₂ is extensively metabolized in the lungs, and the resulting metabolites are further metabolized in the liver and kidney. The major route of elimination of the products of PGE₂ metabolism is the kidneys.