Pilot Study of Thrice-Weekly Low-Dose Rituximab (RTX) in Chronic Lymphocytic Leukemia (CLL): Decreased CD20 Loss Via “Shaving” and a Novel Strategy for Enhanced Therapeutic Targeting.

Treatment of CLL patients with RTX 375 mg/m² induces 90% loss of CD20 from circulating cells via “shaving” of RTX-CD20 complexes from B cells, presumably by tissue macrophages (Kennedy, et al, J Immunol, 2004). To reduce CD20 loss and potentially enhance leukemic cell clearance, we investigated frequent low-dose RTX for CLL. Following informed consent, patients were treated 3 times/wk with RTX 20 or 60 mg/m² x 2 wks (1 pt) or 4 wks (11 pts) with frequent blood sampling during and post-RTX. 9/12 were previously treated (1–4 prior therapies); only 2 had received prior RTX. Cytogenetics by FISH analysis: del 17p13 (6 pts), del 13q14 (5 pts) and trisomy 12 (1 pt).

Results: In each initial infusion, the first 30 mg of RTX caused a 3 to >10-fold decrease in leukemic cells in 11/12 patients. 60 mg/m²group (n=6): Two hrs after the 1st RTX infusion, cells expressed <20% of baseline CD20 levels; these levels further decreased with subsequent RTX treatments. 2/6 achieved normal lymphocyte counts. One patient achieved CR, ongoing at 9+ mo.; 5 had stable adenopathy and/or splenomegaly (SD) for 3–8 mo.; 3 required subsequent therapy. 20 mg/m²group (n=6): Two hrs after the 1st RTX infusion, cell surface CD20 levels were at ≥50% of initial values (4/6 pts) and were preserved throughout RTX therapy in 5/6 patients. 3/6 achieved normal lymphocyte counts with subsequent RTX infusions. One patient achieved CR lasting 8 mo. There was 1 PR, 3 SD and 1 with no response; 4 required subsequent therapy. The 2 patients with highest cell surface CD20 expression achieved CR, despite the presence of the del 17p13 in both cases. RTX infusion activated Complement (C); large amounts of the C3 activation product C3dg were deposited on targeted cells. However, 70% of deposited C3dg was demonstrable on circulating cells which had lost CD20 and persisted in the circulation.

Toxicity: Treatments were well tolerated, with grade 1–2 infusion reactions and transient neutropenia and thrombocytopenia during initial infusions.

Conclusions: Thrice-weekly low-dose RTX is clinically active and can induce remissions in patients with CLL. Low-dose RTX may promote enhanced cytotoxic attack and clearance of circulating CLL cells via CD20 target preservation; 20 mg/m² dosing better preserves CD20 levels via decreased shaving vs 60 mg/m². Our findings suggest that C3dg can serve as a molecular marker for circulating CLL cells which have lost CD20 due to shaving during RTX therapy. These results support further investigation of thrice-weekly low-dose RTX as a novel strategy for improved therapeutic targeting of RTX in CLL.