Primary Plasma Cells Expressing CD52 Are Efficiently Targeted In Vivo by Alemtuzumab.

Salvage therapy options for multiple myeloma (MM) patients relapsing following autologous stem cell transplantation remain limited. New treatments targeting the malignant plasma cell (PCs) are therefore needed. Alemtuzumab is a humanized monoclonal antibody to CD52 capable of destroying CD52+ cells by antibody-mediated cellular cytotoxicity and complement fixation. In order to evaluate the therapeutic potential of alemtuzumab for MM patients, we initially examined CD52 expression on primary malignant marrow PCs as well as a panel of MM continuous cell lines (RPMI-8226, KMS-11, OPM-2, U-266, LP-1, ARH-77). In addition, the anti-myeloma activity of alemtuzumab was evaluated in vivo in a xenotransplant model of MM in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. PCs were enriched from the marrow of MM patients (n = 40) according to CD138 expression. By using an immunomagnetic technique (Miltenyi Biotec, Germany, EU), highly purified CD138+ cells (median purity = 93%; median recovery = 55%) were obtained and further characterized by 3-color (CD138/CD45/CD52) flow cytometry. Expression of CD52 on CD138-enriched cells was detected in 28/40 (70%) of MM patients, with a median of 95% PCs expressing CD52. Detection of CD52 was equally evident on CD138+CD45+ and CD138+CD45− PCs (P ≥ 0.05). Similarly to what observed for primary CD138+ cells, MM cell lines showed a rather heterogeneous CD52 expression, ranging from dim (KMS-11, OPM-2) to bright positivity (LP-1, ARH-77). Both on primary PCs and MM cell lines, expression of CD52 mRNA by quantitative PCR analysis strongly correlated with CD52 antigen detection by flow cytometry. The in vivo activity of alemtuzumab was evaluated in a xenotransplant model of MM in NOD/SCID mice. Mice were inoculated intravenously with KMS-11 cells (0.5 x 10⁵ per mice) and were treated with alemtuzumab (3 x 1 mg/mouse, subcutaneously, days 2, 5, 7). All placebo-treated mice (n = 15) died, whereas mice treated with alemtuzumab (n = 15) showed a significant prolongation of median survival (P ≤0.009 by log rank test) and 27% of them were alive and well at 120 days. No mice experienced any apparent treatment-related toxicity. According to these data, we conclude that: (1) CD52 is expressed on the plasma cells of a significant proportion of MM patients; (2) alemtuzumab has a strong antitumor activity in vivo on CD52-positive MM cell lines; (3) alemtuzumab might have therapeutic potential in a subset of MM patients.