Comparison of pharmacokinetic parameters of a polypeptide, the Bowman-Birk protease inhibitor (BBI), and its palmitic acid conjugate

Purpose. The alteration of the pharmacokinetic parameters of the polypeptide BBI through conjugation with palmitic acid was examined. Methods. I-125-BBI or I-125-Pal-BBI was administered iv to 6 week old CF-I mice at a dose of 3 mg/kg. The mice were sacrificed at 5, 10, 20, 60, 120, 240, 360, and 480 min and the total radioactivity was determined for blood and each organ. The blood was analyzed on a Sephadex G-50 size-exclusion column to determine the amount of intact polypeptide present in the blood. From the amount of intact polypeptide at each time point, the pharmacokinetic parameters were determined. Results. By conjugating three palmitic acids to each BBI molecule, the area under the curve (AUG) and mean residence time (MRT) increase by a factor of 10.8 and 2.8, respectively. There was also a difference in the organ distribution between the two treatments; while I-125-BBI was rapidly cleared from the kidneys, I-125-Pal-BBI was predominantly to the liver. Subsequent studies suggested that the binding of the conjugate to non-albumin serum proteins was most, likely the cause of the altered pharmacokinetics. Conclusions, The residence time in the blood and the lipophilicity of BBI were increased upon conjugation with palmitic acid through a reversible disulfide linkage. Pharmacokinetic studies showed an increase in the AUC and a decrease in kidney clearance in palmitic acid conjugates, indicating a potential increase of the therapeutic efficacy of the polypeptide drug.