Age-dependent alteration of the serum-unbound fraction of nicardipine, a calcium-channel blocker, in man.

Abstract

To determine whether the age-dependent increase in the pharmacological effect of calcium-channel blockers is a result of age-dependent alteration of the unbound fraction the drug in serum, the unbound fraction of the nicardipine was investigated in the serum of 38 adults. The unbound concentration of nicardipine in serum to which nicardipine (205.4 ng mL-1) had been added was determined by ultracentrifugation to range from 0.49 to 4.01% (mean +/- s.d., 1.55 +/- 0.78%). Non-glycosylated albumin was most strongly correlated with age (r = 0.901). Total bilirubin was weakly correlated with age whereas levels of alpha-1-acid glycoprotein, triglycerides and glycosylated albumin were not correlated with age. A significant (P < 0.01) linear correlation was obtained between the unbound fraction of nicardipine and parameters such as age, albumin, albumin/globulin ratio, albumin/glycosylated albumin ratio, non-glycosylated albumin and total bilirubin. To assess the relative effect of each variable on the unbound fraction of nicardipine, stepwise multiple linear regression was performed using age and biochemical parameters. The three variables (non-glycosylated albumin, total bilirubin and age) were entered into the regression equation. The results of this study showed that the major ligand of nicardipine in serum was non-glycosylated albumin, which decreased with age. It was, moreover, shown that the serum-unbound concentration of nicardipine increased with age. This finding would be one factor accounting for the increase in the pharmacological effect of nicardipine with age. In addition, our predicted model for the unbound fraction of nicardipine might be useful in determining the appropriate nicardipine dose for the elderly.