Silence Therapeutics has spoken up about its phase 1 polycythemia vera (PV) clinical trial, providing an early look at how its siRNA prospect may match up to rivals from Incyte and Takeda.
PV patients undergo regular blood draws, a procedure known as a phlebotomy, to reduce the number of blood cells and decrease blood volume. At baseline, all subjects in Silence’s trial had undergone at least three phlebotomies in the past six months or five phlebotomies in the past year. In total, the participants underwent 59 blood draws in the six months before starting the study.
The data set features 16 patients split across three doses of divesiran. Silence enrolled participants with levels of hematocrit above and below the threshold it has identified as increasing the risk of thrombotic and cardiovascular events.
None of the participants with well-controlled hematocrit at baseline underwent a phlebotomy during the follow-up period, which ranged from four to 34 weeks in the data set. Two patients on the middle dose had one blood draw each. The patients who needed phlebotomies had the highest hematocrit at baseline.
Silence also showed how levels of hematocrit changed over time in the first two dose cohorts. Levels trended down in the cohorts and fell below the risk threshold identified by Silence. However, the cohorts are small, featuring 12 patients in total, and the error bars are wide. Silence saw a bigger change at the higher of the two doses, leading it to say the data suggest a potential dose response.
No patients had drug-related serious adverse events or withdrew because of divesiran side effects. The results position Silence to continue toward the conclusion of the study, enrollment in which is scheduled to wrap up this month, but leave questions about the competitiveness of divesiran unanswered.
Incyte’s Jakafi and PharmaEssentia’s Besremi are already approved in the indication, although both leave room for improvement. Takeda is leading the charge to deliver a better PV drug. Having paid Protagonist Therapeutics $300 million upfront early this year, the Japanese drugmaker has rights to the phase 3 PV prospect rusfertide.
Protagonist’s molecule is an injectable peptide mimetic of the master iron regulatory hormone hepcidin. Divesiran is designed to inhibit expression of a target involved in hepcidin expression. The two molecules represent different ways of achieving the same goal, namely raising hepcidin to lower the production of red blood cells.