(PA-168) Advancing Soluble BCMA Quantification in Immunotherapy: Analytical Validation of ELLA versus ELISA for Predicting Toxicity and Response

Introduction:  Serum soluble B-cell maturation antigen (sBCMA)is a validated biomarker associated with treatment response and toxicity in patients receiving BCMA-directed therapies. Elevated baseline sBCMA has been linked to inferior outcomes following chimeric antigen receptor T-cell (CAR-T) therapy and bispecific T-cell engagers (BiTE), while pre-treatment sBCMA reduction correlates with improved outcomes (Freeman et al., 2025; Lee et al., 2024). Recent data support its role in identifying patients who achieve deep responses with long term disease control (Voorhees et al., ASCO 2025, Abstract 7507). Despite its promise, widespread integration into clinical workflows has been limited by technical constraints of ELISA-based platforms, including high inter-assay variability and limited throughput.

Methods:   

This retrospective study was approved by the IRB at H. Lee Moffitt Cancer Center and an external IRB (Advarra) and conducted in accordance with the Declaration of Helsinki. Patients were included if they received FDA-approved BCMA CAR-T therapy, consented to the biospecimen protocol (Pro00021733) and had baseline pre-lymphodepletion serum sample available.  ELISA assays (R&D Systems) were performed using Cytation3 readers and Gen5 software, with concentrations calculated in GraphPad. A subset of 40 samples underwent parallel testing via ELLA following manufacturers protocol (Protein Simple). Paired sBCMA values were compared using Pearson correlation, linear regression, Bland–Altman analysis, and categorical agreement by clinically relevant thresholds. Residual and agreement analyses assessed reproducibility, bias, and overall assay performance to support clinical translation.

Results:   sBCMA values measured via ELLA and ELISA were strongly correlated (Pearson r = 0.94, p < 0.0001). Linear regression yielded an R² of 0.93, with residuals demonstrating no evidence of heteroscedasticity. Bland–Altman analysis showed a consistent minor bias in ELLA over ELISA (+0.26 log₁₀ units), with 95% of values within agreement limits. At a CAR-T–relevant cutoff (≥100,000 pg/mL), ELLA and ELISA demonstrated substantial concordance (Cohen’s κ = 0.79), with ELLA identifying 4 additional high-risk cases. At a BiTE threshold (≥400,000 pg/mL), agreement improved further (κ = 0.89), with only 2 ELLA-classified highs missed by ELISA. No false negatives were observed in either comparison. ELLA demonstrated superior reproducibility, automation, and reduced hands-on time compared to ELISA.

Conclusions:   This analytical validation supports the clinical use of ELLA for sBCMA quantification. Given sBCMA’s growing utility in predicting both response and toxicity to BCMA-targeted immunotherapies—and its emerging role in identifying patients likely to experience long-term disease control—accurate, scalable measurement is critical. ELLA enables timely, reproducible integration of sBCMA into prospective trials and practice, improving biomarker-guided treatment decisions in cellular therapy.