Correct Answer: D. Carboprost
Carboprost is a synthetic 15-methyl PGF2α analogue specifically designed for abortifacient use. Unlike its parent compound dinoprost (natural PGF2α), carboprost has a methyl group at the 15-position that confers two critical advantages: (1) increased receptor affinity for the myometrial PGF2α receptor, and (2) prolonged half-life (~8 hours vs. 5 minutes for dinoprost), allowing intramuscular dosing. In Indian clinical practice, carboprost is the preferred PGF2α derivative for medical abortion in the second trimester (13–20 weeks) and for management of postpartum hemorrhage refractory to oxytocin. The 15-methyl substitution makes it metabolically resistant to 15-hydroxyprostaglandin dehydrogenase, the enzyme that rapidly inactivates natural prostaglandins. Dinoprostone (PGE2) and dinoprost (PGF2α) are natural or minimally modified prostaglandins with shorter half-lives and less potent myometrial effects, making them less suitable as primary abortifacients. Misoprostol, though synthetic, is a PGE1 analogue, not a PGF2α derivative.
Why the other options are wrong
A. Misoprostol — Misoprostol is a synthetic PGE1 analogue, not a PGF2α derivative. While it is used off-label for medical abortion in India (especially in combination with mifepristone), it is not classified as a PGF2α agent. The question specifically asks for a PGF2α derivative, making this a category error. NBE may trap students who know misoprostol is abortifacient but forget its prostaglandin class. B. Dinoprost — Dinoprost is the natural PGF2α, not a synthetic derivative. Although it is a legitimate abortifacient, its extremely short half-life (~5 minutes) due to rapid metabolism by 15-hydroxyprostaglandin dehydrogenase necessitates intravenous infusion, making it impractical for routine clinical use. The question specifies 'synthetic' derivative, and dinoprost is the parent natural compound. This is a common trap for students who conflate 'natural PGF2α' with 'synthetic PGF2α analogue.' C. Dinoprostone — Dinoprostone is PGE2 (natural or semi-synthetic), not a PGF2α derivative. Although it is used as an abortifacient and cervical ripening agent in India, it belongs to the E-series prostaglandins. The question explicitly asks for a PGF2α derivative. Students may confuse dinoprostone with dinoprost due to similar nomenclature, but they are different prostaglandin classes with distinct clinical roles.
High-Yield Facts
- Carboprost is a 15-methyl PGF2α analogue with a half-life of ~8 hours, allowing IM dosing for abortion and PPH.
- 15-methyl substitution confers resistance to 15-hydroxyprostaglandin dehydrogenase, preventing rapid inactivation.
- Dinoprost (natural PGF2α) has a half-life of ~5 minutes and requires IV infusion; not practical for routine use.
- Dinoprostone is PGE2, not PGF2α; used for cervical ripening and abortion but is a different prostaglandin class.
- Misoprostol is a PGE1 analogue; used off-label for medical abortion in India but is not a PGF2α derivative.
- Carboprost dosing: 250 µg IM every 15–90 minutes (max 12 doses) for abortion; 250 µg IM for PPH refractory to oxytocin.
Mnemonics
PGF2α Derivatives for Abortion: 'CARD' Carboprost = synthetic 15-methyl PGF2α (IM, long half-life, abortifacient). Abortifacient = carboprost. Resistant to metabolism = 15-methyl group. Dinoprost = natural PGF2α (IV only, short half-life, impractical). Use this when distinguishing carboprost from dinoprost. Prostaglandin Series Trick: 'E vs. F' E-series (PGE1, PGE2) = Easy to remember as cervical ripening agents (dinoprostone = PGE2). F-series (PGF2α, carboprost) = For uterine contractions and abortion. Carboprost is the synthetic F-series abortifacient. Use when differentiating dinoprostone (E) from carboprost (F).
NBE Trap
NBE pairs 'synthetic' and 'abortifacient' to lure students into choosing misoprostol (which is synthetic and abortifacient but is PGE1, not PGF2α). The question's specificity—'synthetic PGF2α derivative'—is the discriminator that eliminates misoprostol and dinoprost.
Clinical Pearl
In Indian tertiary centers, carboprost is the first-line PGF2α agent for second-trimester medical abortion (13–20 weeks) and for life-threatening postpartum hemorrhage unresponsive to oxytocin and ergot alkaloids. Its IM route and longer half-life make it far more practical than dinoprost in resource-limited settings.
_Reference: KD Tripathi Pharmacology Ch. 20 (Autacoids & Prostaglandins); Harrison Ch. 6 (Abortion & Contraception)_