Treatment of Relapsed/Refractory CLL Patients With PI3Kδ Inhibitor and Anti-CD20 Antibody Rapidly Decreases Tumor Burden but Could Induce Resistance.

Multi-drug combination strategy targeting three different molecules involved in different pathways to overcome single-agent resistance in relapsed/refractory CLL patients. The clinical trial utilized 1)a therapeutic anti-CD20 monoclonal antibody (mAb), ublituximab, which destroys CLL cells by an antibody-dependent cellular phagocytosis (ADCP) mechanism; 2)a B cell receptor (BCR) signaling inhibitor, umbralisib, which blocks PI3Kẟ; 3)and an anti-apoptosis inhibitor, venetoclax, which blocks cell survival promoted by BCL-2 that stops mitochondria from initiating apoptosis. Our correlative study focused on the first two treatments prior to venetoclax addition. We found that patients respond to anti-CD20 antibody and BCR signaling inhibition with rapid reductions in CLL cell counts and CD20 levels. Standard high dose (375 mg/m2) anti-CD20 antibody treatment significantly decreased CLL surface CD20 levels, potentially limiting treatment efficacy. Anti-CD20 antibody plus B cell receptor signaling inhibition reduced CLL cell counts and lymph node tumors, enabling BCL-2 inhibitor treatment to avoid tumor lysis syndrome.