In this study, researchers will learn more about a drug called felzartamab in people who have received a kidney transplant and then developed antibody-mediated rejection (AMR) or microvascular inflammation (MVI). AMR happens when the body's immune system creates donor-specific antibodies (DSAs) that attack the transplanted kidney. In late AMR, this typically happens more than 6 months after the kidney transplant. MVI is a condition where the small blood vessels in the transplanted kidney become inflamed. MVI can happen with or without DSAs. Both AMR and MVI can cause the transplanted kidney to stop working properly.

Two earlier studies looked at felzartamab in kidney transplant recipients. Study 299AR301 (TRANSCEND) (NCT06685757) included participants with AMR. Study 299AR201 (TRANSPIRE) (NCT07219043) included participants with MVI.

This study, 299AR302-299AR301 LTE, is a long-term extension of both of these "parent" studies. Participants who join this study will have the opportunity to receive felzartamab for up to 4 more years after completing their parent study.

The goal of this study is to learn more about the long-term safety and effects of felzartamab in people with kidney transplants. 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 one of the parent studies 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 their 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 their 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 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; and to evaluate pharmacokinetics (PK) and immunogenicity of felzartamab.

Key Inclusion Criteria:

• Have completed the parent study Week 52 visit or will be completing Week 52 visit procedures (for participants who sign consent before reaching the Week 52 visit).

• Have received at least one dose of felzartamab in the parent studies, and had the following disposition status in the parent study:

  • Ongoing Treatment: Did not discontinue felzartamab treatment in the parent study and received a dose at the Week 44 visit.
  • Treatment Interrupted: Did not discontinue felzartamab treatment in the parent study and did not receive a dose at the Week 44 visit.
  • Off Treatment, On Study: Discontinued felzartamab treatment in the parent study and were not withdrawn from the study.

Participants who discontinued study treatment prior to receiving any doses of felzartamab in the parent study (i.e., those in the placebo group who discontinued before receiving felzartamab) are not eligible for enrollment in this substudy.

• For participants enrolling into this study who have not discontinued felzartamab treatment in the parent study only: The Investigator has determined that the participant could benefit from continued felzartamab treatment.

Key Exclusion Criteria:

• Met a treatment discontinuation criterion in the parent study but treatment was not discontinued (for example, because the criterion was met after the last dose of felzartamab in the parent study).

Note: Other protocol-defined inclusion/exclusion criteria apply.

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/