Methotrexate is contraindicated in pregnancy because it acts as which type of teratogen, causing neural tube defects and skeletal abnormalities?

Explanation

## Methotrexate as a Folate Antagonist Teratogen ### Mechanism of Teratogenicity **Key Point:** Methotrexate is a dihydrofolate reductase (DHFR) inhibitor that blocks the conversion of dihydrofolate to tetrahydrofolate (THF), thereby inhibiting one-carbon transfer reactions essential for DNA and RNA synthesis. ### Pathophysiology of MTX Teratogenesis 1. **Folate Antagonism:** MTX competitively inhibits DHFR, reducing the pool of active folate cofactors (THF) 2. **Impaired DNA Synthesis:** Reduced THF availability decreases purine and pyrimidine synthesis 3. **Cellular Proliferation Failure:** Rapidly dividing cells (especially in the developing nervous system and skeletal tissues) are most vulnerable 4. **Teratogenic Effects:** - Neural tube defects (anencephaly, spina bifida) - Skeletal dysplasia (hypoplasia of long bones, vertebral anomalies) - Cleft palate - Growth restriction ### Critical Period **High-Yield:** MTX exposure during the **first trimester (particularly weeks 6–12)** carries the highest teratogenic risk. The risk of congenital anomalies is approximately 10–25% with first-trimester exposure. ### Classification of MTX as a Teratogen | Teratogenic Class | Characteristics | Example | |---|---|---| | **Folate antagonist** | Inhibits folate metabolism; causes neural tube defects | Methotrexate, Trimethoprim | | **Oxidative stress inducer** | Generates free radicals | Phenytoin, Warfarin | | **DNA intercalator** | Inserts into DNA double helix | Doxorubicin, Daunorubicin | | **Receptor antagonist** | Blocks hormone/growth factor signaling | ACE inhibitors, Retinoids | **Clinical Pearl:** Methotrexate is used in the treatment of ectopic pregnancy (medical management) precisely because of its anti-proliferative effects on rapidly dividing cells. However, it is absolutely contraindicated in ongoing intrauterine pregnancy. **Mnemonic:** **FOLATE antagonists = Fetal Organ Loss And Teratogenic Effects** **Warning:** Do not confuse methotrexate's mechanism with other teratogens: - ~~Phenytoin~~ (enzyme inducer, not a direct folate antagonist) - ~~Warfarin~~ (vitamin K antagonist, not a folate antagonist) - ~~Thalidomide~~ (cereblon binding agent, not a folate antagonist) [cite:KD Tripathi 8e Ch 12]