A primate bioassay for the determination of renin inhibitory peptides in serum

The study of renin inhibitory peptides (RIPs) in rodents and primates requires the establishment of a simple, high volume method for determining the concentration of RIPs in serum after intravenous or oral dosing. The human renin inhibition assay useful for rodents is not directly applicable to primates due to inherent production of angiotensin I from the primate serum angiotensinogen and added recombinant human renin. Therefore, a novel approach to analyze the serum concentrations of RIPs in primates is described based on in vitro studies with monkey serum. The procedure involves the inactivation of monkey angiotensinogen and monkey renin by thermal denaturation prior to analysis. Application of this assay was demonstrated by analyzing serum samples from an in vivo study in monkeys using ditekiren (U-71,038), a renin inhibitory peptide, and by validation of the assay and results using a tritium-based radioimmunoassay (RIA) for ditekiren. The minimum detectable limit of ditekiren for both the RIA and the bioassay for primates was 10ng/ml serum. The reported bioassay should be of value for monitoring serum levels of thermostable RIPs from pharmacokinetic, bioavailability, and pharmacodynamic studies in primates as well as in humans.