Calcium antagonism and structure-affinity relationships of terfenadine, a histamine H1 antagonist, and some related compounds.

Calcium channel affinity of terfenadine and its optical isomers was determined by the displacement of [3H]nitrendipine on rat cerebral cortex membranes. Terfenadine showed a pKd of 6.36 +/- 0.03 whereas its R(+)-isomer (VUF4567) had a pKd value of 6.39 +/- 0.03 and the S(-)-isomer (VUF4568) had a pKd of 6.40 +/- 0.04. The same affinity between the enantiomers suggests that the binding domain on the membrane is not sterically restricted towards the part of the molecule in which the chiral centre is present. The characteristics of terfenadine in regulating [3H]nitrendipine binding were similar to those of some other diphenyl-alkylamine type calcium antagonists. It allosterically altered the binding affinity for nitrendipine and acted at the same site linked to the calcium channel as gallopamil. A structure-affinity relationship among a group of terfenadine analogues is discussed.