Enantioselective retention of 4-aryl-1,4-dihydropyridine calcium-channel blockers on human serum albumin and alpha(1)-acid glycoprotein HPLC columns:Relationships with different scales of lipophilicity

The enantioselective retention of eight 4-aryl-1,4-dihydropyridine (DHP) ca lcium-channel blockers on HPLC stationary phases supporting human serum alb umin (HSA) or alpha(1)-acid glycoprotein (AGP) was investigated. All chiral neutral DHPs were resolved on the AGP column, whereas, on the HSA column, only isradipine showed a split chromatographic peak. Analyses performed on AGP with eluents containing dimethyloctylamine (DMOA) as the displacer demo nstrated that the protein has at least two binding sites for DHPs. The firs t family of binding sites is enantioselective, binds exclusively to the (R) -forms, and presumably interacts competitively with DMOA. The second family of binding sites appears to be non-enantioselective and is affected by a c ooperative interaction with DMOA. For the selected set of DHPs, the lipophi licity scale in octan-1-ol/H2O (log P) was not collinear with log k(w)(IAM) values obtained with immobilized artificial membranes (IAM-HPLC) due to th e inclusion of both neutral and basic congeners. Only for the neutral DHPs did log k(w)(IAM) behave as a better descriptor than log P for retention da ta on HSA and AGP. In fact, the behavior of the basic DHPs amlodipine and n icardipine on both proteins correlated better with the octan-1-ol/H2O log P values. We, therefore, infer that the amphipathic nature of the IAM phase only mimics the interaction of non-ionizable compounds with serum proteins. In contrast, the IAM-HPLC retention data of protonated bases encode additi onal interaction mechanisms that are specific for phospholipids and not inv olved in ligand-protein interactions.