Belimumab With Rituximab for Primary Membranous Nephropathy

This trial is a two-part study (Part A and Part B) of adults with primary membranous nephropathy (MN), ages 18-75 inclusive. The study will be conducted at multiple sites in the United States and Canada. Part A: Open-label Phase Part A is an open-label, PK study to compare belimumab exposure between participants who have "low" proteinuria (≥ 4 to \< 8 g/day) and "high" proteinuria (≥ 8 g/day) at Visit -1. Initially Part A planned to enroll 20 individuals with primary MN: 10 individuals with low proteinuria and 10 individuals with high proteinuria. All Part A participants received 200 mg subcutaneous belimumab weekly, the initially approved dose of belimumab in SLE, for 52 doses (weeks 0-51). Trough serum belimumab levels would be obtained weekly following the first 4 doses of belimumab. All participants would receive rituximab 1000 mg IV at weeks 4 and 6, and are followed after the 52 week treatment period on no study medication until week 156. Belimumab trough levels were to be analyzed after all 20 participants received the first 4 doses to compare the belimumab exposure between the low and high proteinuria groups. If the belimumab exposure was not comparable between the high and low proteinuria groups, the belimumab dose would be doubled to 400 mg/weekly for participants with high proteinuria in Part B. Dose determination for participants with high proteinuria in Part B would be made by an adjudication committee comprised of the Protocol Chair, NIAID Medical Monitor, ITN Clinical Trial Physician, and Rho Scientist, in consultation with the belimumab PK expert at GSK. Due in part to the observed imbalance in enrollment between the high and low proteinuria groups, an ad hoc PK analysis was conducted. The serum belimumab trough levels of the first 12 participants (8 with high proteinuria and 4 with low proteinuria) who received the first 4 belimumab doses were analyzed to compare belimumab exposure between the low and high proteinuria groups. The results of the PK analysis were reviewed by the adjudication committee, who determined that the results did not support doubling the dose of belimumab in individuals with high proteinuria nor did it identify a new proteinuria threshold that warranted an increased belimumab dose. The belimumab PK expert at GSK concurred. Thus, enrollment into Part A has been suspended, and all participants in Part B are to receive the same dose of belimumab. All participants currently enrolled in Part A continue to receive belimumab and rituximab as previously planned and are undergoing the safety assessments as presented in Appendix A. All enrolled participants in Part A will be followed until week 156 and will be assessed for the same study endpoints as participants in Part B. Part B: Randomized Phase Part B is a prospective, randomized, phase II, double-blind, placebo-controlled, multicenter clinical trial in adults with primary MN to assess the effectiveness of belimumab and rituximab at treating primary MN. Part B is commencing after the completion of the ad hoc PK analysis, which did not support increasing the belimumab dose in participants with high proteinuria. A total of 58 participants will be randomized into two treatment arms. Randomization will be stratified by low (≥ 4 to \< 8 g/day) and high proteinuria (≥ 8 g/day). Eleven participants who entered Part B prior to protocol version 7.0 were randomized at a 1:1 ratio; 47 participants entering at or after protocol version 7.0 will be randomized at a 3:1 ratio. A total of 41 participants are projected to be enrolled in the belimumab and rituximab arm and 17 participants in the belimumab placebo and rituximab arm. The effectiveness of belimumab with rituximab will be evaluated using a hybrid control arm that combines participants who receive belimumab placebo and rituximab in this study and the subgroup of subjects with primary MN in the MENTOR trial who were anti-PLA2R positive at baseline and received rituximab alone. The randomized arms will also be used to evaluate the safety of adding belimumab to rituximab in primary MN and to support mechanistic studies. Participants randomized to the belimumab and rituximab arm will receive subcutaneous belimumab 400 mg (two 200 mg injections) once weekly from weeks 0-3, and then 200 mg once weekly from weeks 4-51. This dosing regimen is based on the recommended dosing of subcutaneous belimumab for lupus nephritis. Participants randomized to the belimumab placebo and rituximab arm will receive subcutaneous belimumab placebo according to the same dose and schedule. Participants in both arms will receive rituximab 1000 mg IV at weeks 4 and 6. At week 30, participants will be assessed for a response to study treatment. Participants who meet at least two of the following three criteria at week 30 will be considered to have an inadequate response to study treatment and, defined as fulfilling at least two of the following three criteria at week 30, will receive a second course of rituximab (defined as 1000 mg IV given at weeks 34 and 36): * Anti-PLA2R levels is ≥ 25% of baseline * Proteinuria is ≥ 50% of baseline * Serum albumin is \< 2.8 g/dL After the 52 week treatment period, all participants will be followed on no study medication with assessment of the primary endpoint (CR or PR) at week 104. The primary endpoint will be assessed at week 104 because the proteinuric response to treatment is known to lag behind the active treatment period and is recommended to be assessed at least 18 months after the initiation of therapy. There will be a tolerance endpoint at week 156 to determine if treatment with belimumab with rituximab results in a more durable remission compared to rituximab alone, and to assess the rate of relapse after having achieved complete or partial remission.