Van der Wenden E M, Van Galen P J, IJzerman A P, Soudijn W
Center for Bio-Pharmaceutical Sciences, Division of Medicinal Chemistry, Leiden, The Netherlands.
Eur J Pharmacol. 1991 Apr 25;206(4):315-23. doi: 10.1016/0922-4106(91)90116-y.
Substitution at the 8-position of 1,3-dipropylxanthines can lead to very potent and selective adenosine A1 antagonists. The xanthine C8-region was investigated in this study, using CAMM (computer-assisted molecular modeling). This region can be divided into two subregions with a considerable overlap in volume: a phenyl region which binds the flat substituents and a cycloalkyl region which binds the other substituents. The 8-phenyl-substituted derivatives bind with an N9-C8-Cl'-C2' dihedral angle of 220 degrees; this dihedral angle is 330 degrees for the 8-cycloalkyl-substituted derivatives. The lower affinity of C8-substituted 7-methyl-1,3-dipropylxanthines can be explained quantitatively with steric hindrance, which C8-substituents experience from the 7-methyl group in these conformations. The substitution pattern determines the affinity for 8-phenyl-substituted compounds for which the energy cost to reach the dihedral angle of 220 degrees is low, but has little influence otherwise. The affinity of the 8-cycloalkyl-1,3-dipropylxanthines is mainly volume dependent, because of a forbidden area near the cycloalkyl region.
