A 28-year-old woman from Tamil Nadu with borderline tuberculoid leprosy is started on rifampicin, dapsone, and clofazimine as per WHO MDT. She is also on oral contraceptive pills (OCP) for contraception. After 3 weeks, she reports breakthrough bleeding and is concerned about contraceptive failure. Which antileprotic drug is responsible for this interaction, and what is the mechanism?

Explanation

## Rifampicin–Oral Contraceptive Interaction **Key Point:** Rifampicin is a potent inducer of hepatic cytochrome P450 enzymes (especially CYP3A4 and CYP2C9), which dramatically increases the metabolism of ethinylestradiol and progestin, reducing their plasma concentrations and contraceptive efficacy. ### Mechanism of Interaction 1. **Enzyme Induction:** Rifampicin binds to the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), upregulating CYP3A4, CYP2C9, and other Phase I enzymes. 2. **Increased Metabolism:** Ethinylestradiol and progestins are metabolized faster, leading to subtherapeutic plasma levels. 3. **Reduced Efficacy:** Lower hormone levels fail to suppress the hypothalamic–pituitary–ovarian axis, allowing ovulation and breakthrough bleeding. 4. **Clinical Result:** Unintended pregnancy risk increases to ~10% despite OCP use. ### Clinical Presentation **High-Yield:** Breakthrough bleeding (intermenstrual bleeding) is the first sign of OCP failure due to rifampicin induction. The patient may experience: - Irregular menstrual bleeding - Spotting between periods - Risk of unintended pregnancy ### Management Strategies | Approach | Details | |----------|----------| | **Increase OCP dose** | Use higher-dose ethinylestradiol (50 μg) formulations | | **Alternative contraception** | Intrauterine device (IUD), barrier methods, or long-acting reversible contraceptives (LARC) | | **Timing** | Continue increased-dose OCP for duration of rifampicin therapy + 28 days after completion | | **Counseling** | Advise backup contraception (condoms) during and after MDT | **Clinical Pearl:** This interaction is one of the most clinically significant drug–drug interactions in tuberculosis and leprosy treatment. All rifamycins (rifampicin, rifabutin, rifapentine) share this property. ### Why Other Antileprotics Do NOT Cause This - **Dapsone:** Metabolized by acetylation; does not induce CYP450 enzymes. It may be a substrate for CYP3A4 but does not significantly affect OCP metabolism. - **Clofazimine:** Lipophilic drug with slow metabolism; does not induce hepatic enzymes and does not affect OCP efficacy. ### Contrast with Other Enzyme Inducers | Drug | CYP450 Induction | OCP Interaction | |------|------------------|------------------| | **Rifampicin** | **Potent (CYP3A4, 2C9, 2C19)** | **Yes — significant** | | Phenytoin | Potent | Yes | | Carbamazepine | Potent | Yes | | Dapsone | None | No | | Clofazimine | None | No | | Isoniazid | None | No | **Mnemonic: RIPE = Rifampicin Induces Pill Estrogen metabolism** ### Timing of Interaction - **Onset:** 1–2 weeks after rifampicin initiation (as enzyme induction develops) - **Peak effect:** 2–3 weeks - **Duration:** Enzyme induction persists for 1–2 weeks after rifampicin discontinuation (slow return to baseline) **Warning:** Do NOT assume that stopping rifampicin immediately restores OCP efficacy. Backup contraception should be used for ≥28 days after the last dose of rifampicin.