A 55-year-old man with BPH and hypertension is started on tamsulosin. Unlike prazosin, tamsulosin produces minimal first-dose hypotension. The pharmacological basis for this selectivity is:
- A Tamsulosin undergoes extensive first-pass metabolism, limiting systemic bioavailability
- B Tamsulosin has additional alpha-2 agonist activity that compensates for the fall in BP
- C Tamsulosin is a selective alpha-1A and alpha-1D antagonist, with much lower affinity for vascular alpha-1B receptors ✓
- D Tamsulosin is a partial agonist at alpha-1 receptors in blood vessels
Correct answer: C. Tamsulosin is a selective alpha-1A and alpha-1D antagonist, with much lower affinity for vascular alpha-1B receptors
Explanation
Tamsulosin preferentially blocks alpha-1A receptors (predominant in prostate/urethra) and alpha-1D receptors (bladder), but has very low affinity for alpha-1B receptors which mediate vasoconstriction in blood vessels. Prazosin is non-selective among alpha-1 subtypes, causing significant vascular relaxation and first-dose hypotension. Tamsulosin's bioavailability is actually high (~100%); it has no alpha-2 agonism or partial agonism at vascular sites.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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