Silence stops enrollment in blood cancer arm of RNAi trial over 'recruitment challenges'

Silence Therapeutics’ stuttering effort to generate clinical data on SLN124 in myelodysplastic syndrome (MDS) has conked out. Facing recruitment challenges, the RNAi specialist has decided to stop enrollment in the MDS arm of an early-phase clinical trial to focus on thalassemia and polycythemia vera.

SLN124 is designed to temporarily silence TMPRSS6, a gene that stops the liver from making hepcidin. As a deficiency of hepcidin, a hormone that controls iron levels, is a driver of both MDS and thalassemia, Silence targeted both indications when it began testing SLN124 in patients 11 months ago. COVID-19 affected the study, but as recently as last month, Silence was still targeting a third-quarter MDS data drop.

Now, Silence is pulling back from MDS, discontinuing enrollment in that side of the study in the face of “recruitment challenges” and a belief it can “derive the most value in the near term” in thalassemia and polycythemia vera.

The decision sets back Silence’s pursuit of an indication that represents a sizable slice of the opportunity for SLN124. Polycythemia vera, a rare disease in which blood becomes thicker, is the most prevalent of the initial indications targeted by Silence, affecting around 150,000 people in the U.S, but MDS is next in line with 70,000 U.S. patients. Thalassemia trails well behind in third place.

Silence suffered earlier setbacks in its attempt to study SLN124 in MDS. The biotech filed to run a phase 1 trial in patients with MDS and thalassemia late in 2019 only to withdraw the study before enrolling a single patient in response to the pandemic.

Early last year, Silence tried again, posting a new study on and going on to start the trial in April. By August, the study had hit turbulence, with Silence unable to activate some key sites because of surges in COVID-19 cases, forcing it to delay the delivery of data from the second half of 2021 to the summer of 2022. 

With the thalassemia arm fully enrolled, Silence is now aiming to present top-line data on those patients in the third quarter of 2022. The MDS results were due around the same time before Silence rethought its plans.