Table 2. Relative Selectivities of Key Compounds for Histamine Receptor Sub-Types (H₁–H₃)^a.

	relative selectivities for guinea pig brain histamine receptor sub-types
test drug	H1 relative to H2	H1 relative to H3	H3 relative to H2
emedastine	33 217	12 082	3
pyrilamineb	12 303	14 028	<1
chlorpheniramine	5 700	2 216	3
olopatadine	4 277	3 833	<1
antazolineb	1 039	898	1
ketotifenb	935	2 048	<1
clemastine	621	17 456	<1
brompheniramine	540	580	<1
levocabastineb	515	181	3
pheniramineb	448	315	1
diphenhydramine	134	2 645	<1

^aThe Table has been arranged to reflect high to low relative selectivity with focus on the H₁-receptor since that was predominantly involved in mediating the majority of the proinflammatory effects of histamine in AC by enhancing conjunctival vascular permeability and causing itching.

^bDrugs marketed for treatment of AC at the time of our studies, around 1992/1994.^27,29