HMR 1556 is a chromanol derivative and is an IKs blocker (IC50s of 10.5 nM and 34 nM in canine and guinea pig left ventricular myocytes, respectively).
AR-A 2 is a selective 5-HT1B receptor antagonist, with a high affinity to guinea pig cortex 5HT1B/1D and recombinant guinea pig 5-HT1B receptors (Ki: 0.24 and 0.47 nM) and with 10-fold lower affinity to guinea pig 5-HT1D receptor (Ki: 5 nM).