Characterization of high molecular weight forms of peptide drugs in vivo

Studies were undertaken to investigate the site and conversion mechanism of exogenous peptides into high molecular weight (HMW) forms when administered in vivo. Functional nephrectomy reduced the conversion of aprotinin into HMW form in circulation whereas incubation of aprotinin in kidney homogenate in vitro resulted in an increase in HMW component, indicating that the kidney participates in conversion of aprotinin into HMW form. Incubation of aprotinin in rat serum showed that HMW forms can be produced in blood. Analysis of molecular weight of the components produced in vivo and in serum in vitro demonstrated that they consist of heterogeneous mixture and that the components produced in vitro resemble those produced in vivo. Incubation of (Asu1,7)-eel calcitonin, which lacks intramolecular disulfide bonds, in serum did not result in the production of HMW form, whereas incubation of three other peptides having intramolecular disulfide bonds did, suggesting that conversion mechanism of peptide drugs into HMW forms in blood is mainly by thiol-disulfide interchange reaction between peptides and serum protein.