A 42-year-old woman from Mumbai with a history of MDR-TB treatment for 18 months develops persistent cough and sputum positivity. Repeat drug susceptibility testing shows resistance to isoniazid, rifampicin, fluoroquinolones, and second-line injectable agents (amikacin and capreomycin). She has never received bedaquiline or linezolid. What is the drug of choice to add to her regimen for extensively drug-resistant TB (XDR-TB)?

Explanation

## XDR-TB Treatment and Bedaquiline **Key Point:** XDR-TB (extensively drug-resistant TB) is defined as MDR-TB with additional resistance to a fluoroquinolone AND at least one second-line injectable agent (amikacin, kanamycin, or capreomycin). XDR-TB has poor outcomes with standard MDR-TB regimens and requires newer agents. ### Definition and Epidemiology **XDR-TB criteria:** - Resistant to INH + RIF (MDR-TB) AND - Resistant to a fluoroquinolone (levofloxacin or moxifloxacin) AND - Resistant to at least one second-line injectable (amikacin, kanamycin, or capreomycin) **High-Yield:** XDR-TB has treatment success rates of only 20–30% with conventional MDR-TB regimens. Newer agents (bedaquiline, linezolid, delamanid) are essential to improve outcomes. ### Newer Agents for XDR-TB | Agent | Class | Mechanism | Role in XDR-TB | |---|---|---|---| | **Bedaquiline** | Diarylquinoline | ATP synthase inhibitor | Core agent; excellent bactericidal activity | | **Linezolid** | Oxazolidinone | Protein synthesis inhibitor | Core agent; good lung penetration | | **Delamanid** | Nitro-dihydroquinoxaline | Mycolic acid synthesis inhibitor | Sterilizing agent; emerging role | | Meropenem/Clavulanate | β-lactam/β-lactamase inhibitor | Cell wall synthesis | Adjunctive in XDR-TB | **Clinical Pearl:** Bedaquiline is now a WHO-recommended core agent in XDR-TB regimens. It has shown superior outcomes when combined with linezolid and other agents. The typical XDR-TB regimen includes bedaquiline + linezolid + delamanid ± meropenem/clavulanate + ethionamide/prothionamide. **Mnemonic for XDR-TB agents:** **"BLD-M"** = **B**edaquiline + **L**inezolid + **D**elamanid ± **M**eropenem (the newer backbone for XDR-TB). ### Why Bedaquiline is First Choice in This Case 1. **Novel mechanism:** ATP synthase inhibition — no cross-resistance with first- or second-line drugs 2. **Potent bactericidal activity:** Early and sustained sputum conversion 3. **WHO recommendation:** Core agent in XDR-TB regimens since 2019 4. **No prior exposure:** Patient has not received bedaquiline, so resistance is unlikely **Warning:** Bedaquiline can prolong QT interval; baseline ECG and electrolytes are mandatory. It is also hepatotoxic and requires monitoring of liver function. [cite:WHO TB Guidelines 2019; Harrison 21e Ch 205]