Study of Pelabresib add-on to Ruxolitinib in Japanese Adult Patients With Myelofibrosis

This study consists of three periods: screening, treatment, and follow-up. After a screening period of up to 28 days, between three and nine eligible and evaluable participants will be enrolled to receive pelabresib in addition to a stable dose of ruxolitinib. The follow-up phase includes a 30-day safety follow-up and a long-term follow-up. Pelabresib will be administered until one of the following occurs: disease progression, unacceptable toxicity, death, participant decision, or investigator decision. After discontinuation of pelabresib, safety assessments will continue with a safety follow-up visit 30 days after the last dose of pelabresib. Participants will then be contacted for long-term follow-up approximately every 12 weeks after the end of treatment (EOT) for at least three years from the first dose of pelabresib and for at least two years following the last dose of pelabresib, whichever is longer. Long-term follow-up will continue until death, withdrawal from the study, loss to follow-up, or completion of the follow-up period, whichever occurs first. Safety follow-up and long-term follow-up visits will include assessment for leukemic transformation, which will be conducted throughout the study and for up to two years after treatment. The study will continue until all participants complete long-term follow-up or until access to pelabresib is ensured through a post-trial access program or reimbursement of pelabresib in Japan becomes available. Japanese safety confirmation will be determined according to the decision rule. The starting dose is 125 milligrams once daily (QD), and no dose escalation or de-escalation for safety confirmation is planned in this study. Initially, three participants will receive 125 milligrams QD of pelabresib as an add-on to ruxolitinib. The dose-limiting toxicity (DLT) evaluation period for Japanese safety confirmation is 21 days (one cycle).