Tumor Cells Resistant to Alemtuzumab Complement Mediated Cytotoxicity in Patients with High Risk Previously Untreated Early Stage CLL: A Possible Mechanism of Treatment Failure.

The monoclonal antibodies (MoAb) alemtuzumab and rituximab have proven efficacy in the treatment of CLL. In addition, alemtuzumab is effective in patients with defective p53 function responding poorly to purine analogue therapy. The action of both MoAb is not completely understood. Proposed mechanisms include complement dependent cytotoxicity (CDC), antibody dependent cellular cytotoxicity (ADCC), and direct induction of apoptosis of CLL B cells. We have done correlative studies on CLL B cells from patients enrolled in a trial of alemtuzumab and rituximab in “high risk” early stage previously untreated CLL to determine: 1. Role of apoptosis induction and CDC in each MoAb and 2. If the addition of rituximab to alemtuzumab increases their in vitro cytotoxicity.

Patients and Methods: Patients with early stage, previously untreated, high risk CLL are treated with subcutaneous alemtuzumab (dose escalation over 3 days then 30 mg Mon-Wed-Fri for 4 weeks) and rituximab (375 mg/m²/dose weekly from day 8 x 4 doses). High risk disease was defined as one or more of the following features of the CLL B cell clone: (1) 17p13⁻; (2) 11q22⁻; (3) unmutated IgV_H (< 2%) and either CD38+ or ZAP-70+. Blood B lymphocytes collected prior to the start of therapy were tested for response to MoAb in vitro. Cells were cultured at 2 x 10⁶/ml in AIM-V medium using standard conditions. Alemtuzumab and rituximab were used at 20 μg/ml and complement as 10% of 40 CH50 units/ml human serum. The impact of the MoAb was measured by counting viable cells (trypan blue negative) and measuring early apoptosis (annexin V) and cell death (cell membrane permeability to propidium iodide) using flow cytometry at 1 hour, and then daily for 3 days.

Results: Treatment caused rapid resolution of lymphocytosis in all 7 patients and 3 patients were negative for circulating CLL cells using a highly sensitive 3 color flow cytometry (CD5+/CD19+/CD79b-) after therapy. All patients had a clinical response (2 CR, 5 PR). Alemtuzumab and complement were rapidly cytotoxic to most CLL cells. Mean cell viability was 39% (sd: 8%) after 1 hour of incubation. Cytotoxicity was similar in all samples irrespective of FISH defects, IgV_H mutation status, and in vitro resistance to F-ara-A (n = 3). Alemtuzumab was inactive in the absence of complement for all samples. Rituximab alone and together with complement did not induce cytotoxicity or apoptosis. However, the addition of rituximab to alemtuzumab and complement did increase CDC where the number of viable cells was significantly lower at 1, 24, 48, and 72 hours incubation (p = 0.075, 0.047, 0.031, 0.027, respectively, for pairwise comparisons). CLL cells surviving alemtuzumab CDC subsequently had a lower level of apoptosis than control cells, implying a selection for resistant cells. Alemtuzumab CDC on this residual population was not increased at higher concentrations of alemtuzumab or complement. This mechanism of CDC resistance is currently under investigation.

Conclusion: These data suggest that alemtuzumab CDC is an important mechanism of action in patients with CLL. However, alemtuzumab CDC kills only about 61% of CLL cells in vitro, and the surviving cells are more resistant to spontaneous apoptosis. This suggests that cells that survive alemtuzimab CDC contribute to disease progression or relapse. We intend to elucidate the mechanism of this resistance using our in vitro model with the hope that treatment strategies can be deployed to remove this residual CLL B cell clone.