Table 1. Competition by Selected Histamine Antagonists for Specific Radioligand Binding to H₁–H₃ Receptors^a.

	equilibrium inhibition constants of drugs at guinea pig brain H1–H3 receptors sub-types (Ki, nM ± SEM)
test drugs	H1 receptors	H2 receptors	H3 receptors
clemastine	0.23 ± 0.1	143 ± 33	4 015 ± 1 617
pyrilamineb	0.7 ± 0.1	8 612 ± 1 275	9 820 ± 1 098
emedastine	1.2 ± 0.1	39 860 ± 7 453	14 498 ± 2 257
ketotifenb	1.2 ± 0.1	1 122 ± 127	2 458 ± 203
chlorpheniramine	1.4 ± 0.3	7 980 ± 649	3 103 ± 198
brompheniramine	9.9 ± 0.9	5 350 ± 247	5 750 ± 1994
diphenhydramine	11.9 ± 2.9	1 595 ± 141	31 480 ± 12 020
pheniramineb	32.3 ± 2.8	14 475 ± 939	10 190 ± 1 190
olopatadine	36.0 ± 5.7	153 983 ± 94 313	137 980 ± 28 603
antazolineb	39.3 ± 3.4	40 850 ± 3 794	35 295 ± 8 380
levocabastineb	52.6 ± 9.9	27 075 ± 4 996	9 506 ± 5 825

^aSelected antihistamines were evaluated for their ability to displace [³H]-pyrilamine, [³H]-tiotidine, and [³H]-N-methyl-histamine from guinea pig brain H₁-, H₂-, and H₃-receptors, respectively.

^bDrugs marketed for treatment of AC around 1992/1994.^27,29