A Study to Learn More About the Long-Term Safety and Effects of Felzartamab Infusions in Adults With Kidney Transplants Who Have Antibody-Mediated Rejection (AMR)

In this study, researchers will learn more about a drug called felzartamab in people who have received a kidney transplant and then developed a condition called antibody-mediated rejection (AMR). AMR happens when the body's immune system creates antibodies that attack the transplanted kidney. In late AMR, this happens more than 6 months after the kidney transplant. It can lead to serious kidney problems over time.

An earlier study called 299AR301 (TRANSCEND) (NCT06685757) began in 2024 and is investigating felzartamab in participants with AMR. It includes a treatment period of about 1 year. It first compares treatment with felzartamab to placebo for about 6 months and then all participants are given felzartamab to complete the study. This study, 299AR301 LTE, is a long-term extension of the parent study 299AR301. Participants who join this study will have the opportunity to receive felzartamab for up to 4 more years.

The goals of this study are to learn more about the long-term safety and effects of felzartamab in people with AMR. This study is part of a group of studies looking at long-term felzartamab use in people with organ transplants. This study is a substudy of the main study 299AR302.

The main question researchers will answer relate to safety. Namely, how many participants have adverse events during the study and how lab test results change over time. Adverse events are health problems that may or may not be caused by the study drug.

Researchers will perform kidney biopsies to track kidney health. Researchers will also study how felzartamab affects kidney inflammation, kidney function, immune activity, and overall health.

The study will be done as follows:

• Participants who complete the final visit of the treatment period in the parent study can enroll in this study. This includes participants who stopped receiving felzartamab early but still attended their final visits.
• Participants who did not stop receiving felzartamab in the parent study will continue to receive felzartamab for up to 4 more years in this study. Participants may also stop felzartamab during this study at any time.
• Participants who stopped receiving felzartamab in the parent study will only attend study visits for health monitoring- they will not receive felzartamab.
• Felzartamab will be given as an intravenous (IV) infusion, which is a slow injection into a vein using a needle.
• Participants receiving felzartamab may have up to 27 study visits over 200 weeks with an additional safety follow-up visit 4 weeks after their final dose.
• Participants who are not receiving felzartamab may have up to 9 study visits over 200 weeks.

The primary objective of this study is to evaluate the long-term safety of felzartamab. The secondary objectives of this study are to describe the ongoing efficacy of felzartamab on biopsy-proven histologic response (BPHR), microvascular inflammation (MVI) and graft function; to evaluate the ongoing efficacy of felzartamab by monitoring circulating donor-derived cell-free DNA (dd-cfDNA) and to evaluate pharmacokinetics (PK) and immunogenicity of felzartamab.