New renin inhibitor VTP-27999 alters renin immunoreactivity and does not unfold prorenin

Renin inhibitors like aliskiren not only block renin but also bind prorenin, thereby inducing a conformational change (like the change induced by acid) allowing its recognition in a renin-specific assay. Consequently, aliskiren can be used to measure prorenin. VTP-27999 is a new renin inhibitor with an aliskiren-like IC50 and t1/2, and a much higher bioavailability. This study addressed (pro)renin changes during treatment of volunteers with VTP-27999 or aliskiren. Both drugs increased renin immunoreactivity. Treatment of plasma samples from aliskiren-treated subjects with excess aliskiren yielded higher renin immunoreactivity levels, confirming the presence of prorenin. Unexpectedly, this approach did not work in VTP-27999-treated subjects, although an assay detecting the prosegment revealed that their blood still contained prorenin. Subsequent in vitro analysis showed that VTP-27999 increased renin immunoreactivity for a given amount of renin by ≥ 30% but did not unfold prorenin. Yet, it did bind to acid-activated, intact prorenin and then again increased immunoreactivity in a renin assay. However, no such increase in immunoreactivity was seen when measuring acid-activated prorenin bound to VTP-27999 with a prosegment-directed assay. The VTP-27999-induced rises in renin immunoreactivity could be competitively prevented by aliskiren, and antibody displacement studies revealed a higher affinity of the active site-directed antibodies in the presence of VTP-27999. In conclusion, VTP-27999 increases renin immunoreactivity in renin immunoassays because it affects the affinity of the active site-directed antibody. Combined with its lack of effect on prorenin, these data show that VTP-27999 differs from aliskiren. The clinical relevance of these results needs to be established.