Which statement best describes the selectivity of apraclonidine and brimonidine?

• A. All are selective for alpha-1 receptors
• B. Apraclonidine is more selective alpha-2 than brimonidine
• C. Apraclonidine and brimonidine are less selective alpha-2 agonists
• D. Brimonidine is a non-selective beta-blocker

Answer: (C)

Understanding selectivity here means which receptor subtypes these drugs preferentially activate. Apraclonidine and brimonidine are used to lower intraocular pressure by activating alpha-2 adrenergic receptors, which reduces aqueous humor production. They are not alpha-1 selective, and they are not beta-blockers. In practice, apraclonidine has some alpha-1 activity, so it is less selective for alpha-2 than brimonidine. Brimonidine is more selectively alpha-2, but no drug is perfectly exclusive to that subtype. Taken together, both act mainly as alpha-2 agonists, but with less than full alpha-2 selectivity, which is why the statement describing them as less selective alpha-2 agonists best captures their pharmacologic profile.