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Objectives Belimumab (BEL) is a B-cell modulating anti-BLyS mAb approved for SLE (2011) and LN (2020). BLISS-LN’s unique design (104-week registrational Phase 3 study; NCT01639339) investigating kidney outcomes with BEL allowed ISN/RPS class III or IV ± V or pure class V, and cyclophosphamide (CYC) or mycophenolate mofetil (MMF) standard therapy (ST). To mitigate challenges of comparing LN trial outcomes due to design differences and better align the BLISS-LN population with other B-cell therapy LN trials (REGENCY), we recently reported a 14.9% Week 104 complete renal response (CRR) treatment difference in favor of BEL vs placebo (PBO) in a BLISS-LN subgroup restricted to class III or IV ± V (excluding pure class V) receiving only MMF ST. We now present further MMF subgroup outcomes.Methods This BLISS-LN MMF subgroup post hoc analysis reports for BEL vs PBO at Week 104: urine protein/creatinine ratio (uPCR) <0.5 responders, changes from baseline in estimated glomerular filtration rate (eGFR) and in biomarkers (uPCR; positive at baseline: anti-dsDNA and anti-C1q; low at baseline: C3/C4); and over Week 104: time to kidney-related event or death. Data are descriptive.Results MMF subgroup cohorts (BEL=135; PBO=136) had similar baseline uPCR and eGFR ( table 1). Greater improvements in uPCR <0.5 responders, eGFR and biomarkers, and lower risk of kidney-related events or death were noted with BEL vs PBO in MMF subgroup (table 1). uPCR <0.5 responders in MMF subgroup showed greater benefits vs BLISS-LN overall population, while other endpoints showed consistent results with BLISS-LN overall population (table 1).Abstract PO:11:297 Table 1uPCR responders, observed change from baseline to Week 104 biomarkers, and time to kidney-related event or death through Week 104 in the MMF subgroup and overall population (while on treatment)Conclusions Clinical trial differences make comparing outcomes challenging. Our findings build on BEL’s extensive evidence and underline kidney function preservation in an MMF subgroup similar to other Phase 3 LN trial populations. This and the ongoing OBSErve-LN study ( NCT06527872) provide further kidney function maintenance data to support LN treatment decisions.Funding GSK (BEL114054, NCT01639339)Original presentation: ASN 2025