Which feature best distinguishes cyclosporine from tacrolimus among calcineurin inhibitors used in transplant immunosuppression?

Explanation

## Structural and Chemical Distinction **Key Point:** Cyclosporine is a cyclic peptide (11 amino acids) derived from the fungus *Tolypocladium inflatum*, while tacrolimus is a macrolide antibiotic (56-membered lactone ring) from *Streptomyces tsukubaensis*. This fundamental structural difference is the most reliable discriminator between these two calcineurin inhibitors. ## Mechanism of Action — Identical Despite Structure Despite their different chemical structures, both drugs: - Bind to immunophilins (cyclosporine → cyclophilin; tacrolimus → FK-binding protein) - Inhibit calcineurin phosphatase - Prevent IL-2 and other cytokine transcription - Achieve the same immunosuppressive endpoint ## Clinical Comparison Table | Feature | Cyclosporine | Tacrolimus | |---------|--------------|------------| | **Chemical class** | Cyclic peptide | Macrolide | | **Potency** | Lower (requires higher doses) | Higher (10–100× more potent) | | **Neurotoxicity** | Less frequent tremor | More frequent tremor, headache | | **Nephrotoxicity** | Dose-dependent | Dose-dependent (similar) | | **Hepatotoxicity** | Mild | Mild (similar) | | **Therapeutic window** | Narrow | Narrow (both require TDM) | | **GI effects** | Gingival hyperplasia, hirsutism | Hyperglycemia, alopecia | **High-Yield:** The structural difference (peptide vs. macrolide) is the textbook discriminator and appears frequently in comparative pharmacology questions. ## Why This Matters Clinically The structural difference underlies: - Different immunophilin binding partners - Different drug interactions (cyclosporine is a CYP3A4 substrate and inhibitor; tacrolimus is also CYP3A4-dependent but with different kinetics) - Different adverse effect profiles (though both cause nephrotoxicity and neurotoxicity) **Clinical Pearl:** In clinical practice, tacrolimus is now preferred over cyclosporine in many transplant centers due to superior efficacy, but both remain in use depending on institutional protocols and patient factors. [cite:Harrison 21e Ch 297]