Which drug was historically used in chronic SIADH because it impairs the action of vasopressin on the collecting duct, effectively inducing nephrogenic diabetes insipidus?

• A Demeclocycline ✓
• B Doxycycline
• C Fluconazole
• D Gentamicin

Explanation

Demeclocycline, a tetracycline derivative, blocks vasopressin-mediated aquaporin-2 expression and insertion in the collecting duct, producing a reversible nephrogenic diabetes insipidus that counteracts water retention in SIADH. It fell out of favor because of nephrotoxicity, photosensitivity and unpredictable onset, and vaptans have largely replaced it. Doxycycline shares the tetracycline class but lacks clinically useful vasopressin antagonism. Fluconazole and gentamicin have no anti-vasopressin action.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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