at ic50 level azelastine was about 18 , 23 , 28 and 425 times as effective as astemizole , ketotifen ( newer histamine h1-receptor antagonists ) , diphenhydramine ( a traditional h1-receptor antagonist ) , and theophylline ( a phosphodiesterase inhibitor ) , respectively .
C0022642
ketotifen
C2919009
h1-receptor antagonist