Conformational changes in prorenin during renin inhibition in vitro and in vivo

Background Some renin inhibitors induce changes in the conformation of prorenin in vitro and influence the quantification of active renin by immunoradiometric assays. Whether such changes in renin recognition by monoclonal antibodies exist after oral administration of aliskiren, the first orally available renin inhibitor, is not known. Methods Two commercially available immunoradiometric assays (Cisbio and Nichols) were compared to determine immunoreactive active renin concentrations in plasma samples collected in a single oral dose crossover study comparing the renin inhibitor, aliskiren (300 mg), with the angiotensin II antagonist, valsartan (160 mg), in healthy male subjects. Results The addition of aliskiren to plasma samples in vitro, at concentrations of 1-100 mu mol/l, increased active renin immunoreactivity in both the Cisbio and Nichols assays. In the crossover study, the two assays gave similar values for the plasma immunoreactive active renin concentration before treatment and following valsartan administration (intraclass coefficient for agreement between the two assays = 0.92). However, a Bland-Altman plot showed a systematic bias towards higher values (1.75-fold higher; 95% confidence interval = 1.02-3.01) in the Nichols than in the Cisbio assay following aliskiren administration. The difference between the results obtained with the two assays depended on incubation time. Conclusions Depending on incubation conditions, circulating renin inhibitors interfere with the recognition of active renin molecules by the monoclonal antibodies used in commercially available assays. Careful consideration must therefore be given to the methodology used for quantifying immunoreactive plasma active renin when patients are treated with renin inhibitors, to avoid an overestimation of the magnitude of active renin release attributable to conformational changes in plasma prorenin.