Background/Purpose: Tabalumab, a monoclonal antibody neutralizing membrane bound and soluble B cell activating Factor (BAFF), has been shown to reduce the signs and symptoms of rheumatoid arthritis (RA)¹ . To better define tabalumab’s molecular mechanism of action, gene expression profiling of whole blood RNA was performed to characterize individual gene expression signatures and to map the associated pathways involved with BAFF blockage in RA.

Methods: Whole blood RNA was obtained from 158 RA subjects with inadequate response to methotrexate enrolled in a phase 2 randomized trial in which patients received subcutaneous placebo,  1, 3, 10, 30, 60 or 120 mg of tabalumab  every 4 weeks (wks) over 24 wks.  At total of 669 RNA samples were obtained at baseline, wk  1, 4 and 16, and follow up visits at wk 32 and 44. Samples from 30 healthy blood donors controls were analyzed. Affymetrix U133 Plus 2.0 expression arrays were used to measure gene expression. Data were normalized using Robust Multichip Average (RMA) algorithm². Following quality analysis a total of 676 samples were profiled. B cells from peripheral blood were enumerated using flow cytometry. Total serum IgG, A and M, rheumatoid factor (RF) and ACPA were measured using nepholometry. Statistical analyses were performed using a mixed effect model and p-values were adjusted for multiple hypotheses testing using False Discovery Rate (FDR) and Bonferroni techniques. Paired t-tests were used to generate a list of biologically relevant genes whose transcripts exhibited consistent changes in response to tabalumab treatment.

Results: The genes demonstrating significant changes with treatment over time are listed in the table. Many of these genes are represented by multiple probe sets. Most were down regulated . Several of these have been previously reported to be associated with RA susceptibility. The use of gene ontology, pathway and literature mining revealed that the most significant changes were related to B cell development and signaling. Changes in CD19 expression were correlated with CD19 enumeration. Additional genes related to B cell development and signaling were associated with changes in CD19 expression. Consistent with these changes, serum immunoglobulins IgM, IgA decreased significantly from baseline and IgG had a numerical decrease over time.  There were no statistically significant changes in RF or ACPA levels from baseline.

Conclusion:

Using RNA gene expression profiling and pathway analysis, significant gene changes associated with the clinical response to tabalumab were identified. We observed changes in genes related to B cell development and maturation which is consistent with tabalumab’s mechanism of action. These data may provide additional potential targets or biomarkers for future studies in RA.  

1. Genovese et al. Ann Rheum Dis 2011;70(Suppl3):71

2. Irizarry et al, (2003)Nucleic Acids Research2003 31(4):e15

Table  Top candidate genes whose gene expression changes were significant following treatment with tabalumab
Gene	Raw p-value/ Multiplicity Adjusted p-value	Gene	Raw p-value/ Multiplicity Adjusted p-value
T-cell leukemia/lymphoma 1A, TCL1A	5.40E-18  / 2.90E-13	phosphatidic acid phosphatase type 2 domain containing 1B, PPAPDC1B	6.00E-08 / 3.30E-03
MACRO domain containing 2, MACROD2	2.90E-09 / 1.60E-04	like-glycosyltransferase, LARGE	1.70E-10 / 9.10E-06
immunoglobulin heavy constant delta, IGHD	1.90E-10  / 1.00E-05	ATP-binding cassette, sub-family B (MDR/TAP), member 4, ABCB4	3.00E-10  / 1.60E-05
CD72 molecule, CD72	1.70E-09  / 9.20E-05	immunoglobulin heavy constant mu, IGHM	2.10E-09  / 1.10E-04
chronic lymphocytic leukemia up-regulated 1, CLLU1	3.20E-09 / 1.70E-04	sodium channel, voltage-gated, type III, alpha subunit, SCN3A	1.10E-08 / 5.80E-04
Fc receptor-like A, FCRLA	7.4 E-08 / 4 E-08	major histocompatibility complex, class II, DO beta, HLA-DOB	4.30E-08 / 2.30E-03
tetraspan 13, TSPAN13	4.30E-08  / 2.30E-03	CD79a molecule, CD79A	3.20E-07  / 9.20E-04
CD19 molecule, CD19	4.80E-07 / 1.20E-03	family with sequence similarity 129, member C, FAM129C	4.60E-07 / 1.20E-03
Fc fragment of IgE, low affinity II, receptor for (CD23), FCER2	1.10E-06 / 2.30E-03	protein kinase (cAMP-dependent, catalytic) inhibitor gamma, PKIG	7.30E-07 / 1.70E-03
CD200 molecule, CD200	9.80E-07 / 2.20E-03	sorting nexin 22, SNX22	6.70E-07  / 1.70E-03
coronin, actin binding protein, 2B, CORO2B	1.10 E-06 / 4.90 E-03	Fc receptor-like 2, FCRL2	2.40E-06 / 5.30E-03
probe 243780_at near the BAFF Receptor, BAFF-R	2.90E-06  / 5.50E-03	Fc receptor-like 1, FCR1	3.50E-06  / 6.30E-03

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/gene-expression-profiling-and-pathway-changes-associated-with-clinical-response-to-tabalumab-blockade-of-membrane-bound-and-soluble-b-cell-activating-factor-in-rheumatoid-arthritis/