A Trial of ALN-6457 in Adult Participants With Symptomatic Prion Disease (PrD)

This study will test a Regeneron drug called ALN-6457 ("study drug") to see if it can help treat Prion Disease (PrD). In PrD, the prion protein folds into an abnormal shape. These abnormal proteins build up in the brain and cause damage to brain cells, leading to problems with thinking, memory, and movement. ALN-6457 is designed to reduce the amount of prion protein made in the brain. ALN-6457 works by targeting the genetic "instructions" that cells use to produce this protein, with the goal of slowing or stopping disease progression. This study is looking at: * What side effects ALN-6457 might cause * How well ALN-6457 works * What is the best dose of ALN-6457 for treating prion disease * How much ALN-6457 is in the blood and Cerebrospinal Fluid (CSF, fluid that surrounds the brain and spinal cord) at different times