A 28-year-old woman from Delhi with newly diagnosed tuberculoid leprosy is started on standard MDT containing rifampicin, dapsone, and clofazimine. She is also taking oral contraceptive pills (OCPs) for contraception. After 3 weeks of MDT, 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–OCP Interaction in Leprosy Treatment ### Mechanism of Interaction **Key Point:** Rifampicin is a potent inducer of hepatic cytochrome P450 enzymes (particularly CYP3A4, CYP2C9, and CYP2C19), which dramatically increases the metabolism and clearance of ethinylestradiol and progestins in OCPs. This reduces OCP serum concentrations below therapeutic levels, leading to contraceptive failure. ### Pharmacokinetic Basis 1. **Enzyme induction:** Rifampicin binds to the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), upregulating expression of multiple CYP450 isoforms within 2–3 days of initiation. 2. **Increased metabolism:** Ethinylestradiol and progestin levels drop by 30–50%, sometimes more. 3. **Reduced efficacy:** Breakthrough bleeding and unintended pregnancies occur due to insufficient hormone levels to suppress ovulation. 4. **Onset:** Clinically evident within 2–4 weeks of starting rifampicin. **High-Yield:** Rifampicin is one of the most notorious drug–drug interactors in clinical medicine. It induces metabolism of OCPs, warfarin, theophylline, protease inhibitors, and many other drugs. ### Clinical Management | Strategy | Details | |----------|----------| | **Increase OCP dose** | Use a formulation with ≥50 µg ethinylestradiol (standard pills contain 20–35 µg) | | **Backup contraception** | Recommend condoms or intrauterine device (IUD) during and for 4 weeks after rifampicin completion | | **Alternative contraception** | IUD (copper or levonorgestrel) is unaffected by enzyme induction and is preferred | | **Progestin-only pills** | May have reduced efficacy; not recommended as sole method | | **Injectable/implant** | Depot medroxyprogesterone (Depo-Provera) may be less affected but still requires monitoring | **Clinical Pearl:** The enzyme-inducing effect of rifampicin persists for 1–2 weeks after discontinuation due to slow clearance and continued enzyme expression. Backup contraception should continue for 4 weeks after the last dose of rifampicin. ### Why Other Antileprotics Do Not Cause This Interaction - **Dapsone:** Metabolized by acetylation and glucuronidation; does NOT induce CYP450 enzymes. It may rarely inhibit enzymes but does not reduce OCP efficacy. - **Clofazimine:** Lipophilic antimycobacterial with minimal hepatic metabolism; does not induce or inhibit CYP450. **Mnemonic:** **Rifampicin = Rapid Remover** — Rapidly removes (induces metabolism of) many drugs, including OCPs. ### Other Important Rifampicin Interactions - **Warfarin:** Increased metabolism → reduced anticoagulation; INR monitoring essential. - **Protease inhibitors:** Reduced levels → treatment failure in HIV. - **Corticosteroids:** Increased metabolism → reduced efficacy. - **Antifungals (itraconazole, voriconazole):** Reduced levels → treatment failure.