High CD52 mRNA Expression Is An Adverse Prognostic Factor In Fludarabine-Refractory CLL Treated With Alemtuzumab On The Gcllsg CLL2H Trial

Alemtuzumab is a humanized mAb that targets CD52 and has been widely used for the treatment of chronic lymphocytic leukemia (CLL) patients with refractory disease or loss / mutation of TP53 . The cell-surface marker CD52 is variably expressed on peripheral blood mononuclear cells and shows relatively high expression levels on the surface of both healthy B lymphocytes and on CLL cells, but the biologic function of CD52 has not been sufficiently clarified yet. To elucidate the prognostic value of CD52 expression, we analyzed a large gene expression dataset. Gene expression profiling was performed on Affymetrix Exon 1.0 ST Arrays. Data sets were generated for 75 fludarabine-refractory CLL patients treated within the multicenter phase II CLL2H trial of the GCLLSG (3x30 mg Alemtuzumab s.c. per week, max. 12 weeks). Normalization of the arrays was conducted by using the RMA algorithm. Statistical analysis was performed with the R software version 2.15.1. Patients were grouped into quartiles (Q1-Q4) according to CD52 expression levels ( CD52 low: Q1, n=19; CD52 intermediate: Q2-Q3, n=37; CD52 high: Q4, n=19). Progression free survival (PFS) and overall survival (OS) were markedly different for patients from Q1 and Q4 (median PFS: Q1=413 days vs. Q4=145 days (p=0.0102); median OS: Q1= 879 days vs. Q4=413 days (p=0.0164)). Patients with progressive disease were overrepresented in Q4 (PD 7/18 (38.8%); ORR 5/18 (27.7%)) while rare in Q1 (PD 2/19 (10.5%); ORR 9/19 (47.3%)). Patients with low expression of CD52 had less frequently genetic/biologic and clinical characteristics of aggressive disease (Q1 vs. Q4; TP53 mutation/17p deletion: 47.36% vs. 36.84%; IGHV unmutated: 61.11% vs. 73.68%; leukocyte counts (median): 45.8 G/L vs. 55.4 G/L; thymidine kinase: 22.2 U/L vs. 30.4 U/L; β2-microglobulin: 3.3 mg/l vs. 5.2 mg/l; no palpable lymph nodes: 47.36 % vs. 10.52 %). Mutations of SF3B1 and NOTCH1 were present in 4/17 (23.5%) and in 5/17 (29.4%) cases in Q1. In contrast, cases from Q4 harbored mutations of SF3B1 and NOTCH1 in 2/18 (11.1%) and 1/18 (5.5%). Cases with high CD52 levels showed overexpression (FDR 2) of genes predominantly involved in signaling of the B-cell-, toll-like- and interleukin-receptor ( BTK , SYK , CD79B; TLR7, TLR10, IL27RA, IL2RG, TRAF3IP3, TRAF5 ) . The corresponding gene set overrepresentation analysis on KEGG pathways was significantly enriched for gene sets related to cell-extracellular matrix interaction, focal adhesion, regulation of actin cytoskeleton and cell adhesion molecules as well as associated signaling pathways like WNT- and MAPK-signaling. In conclusion, high CD52 expression is an adverse prognostic factor in refractory CLL treated with Alemtuzumab. Although suggestive, the level of CD52 expression and subsequent antibody binding capacity appear not to determine the effectivity of treatment. The association with genetic and biological characteristics indicates an independent role in the biology of aggressive CLL and highlights a potential function in cell-microenvironmental interactions for CLL cells. Disclosures: Stilgenbauer: Genzyme: Honoraria, Research Funding.