Targeted Depletion of B-Cell Subsets by Anti-CD22 and Anti-CD79bAntibody Drug Conjugates: Illumination of the Mechanism of Action through Pharmacodynamic Biomarkers

Abstract 3923

CD22 and CD79b are cell surface receptors whose expression is limited to B-cells. Both CD22 and CD79b are expressed on NHL and CLL patient B-cells, as well as on relapsed NHL B-cells (Dornan et al., 2009; Polson et al., 2010). In order to develop a target specific therapy for the treatment of CLL and NHL, we generated anti-CD22 and anti-CD79b antibody drug conjugates (ADCs) linked to an auristatin, a potent anti-mitotic drug that disrupts cellular mitosis through inhibition of tubulin polymerization. Preliminary efficacy data have shown that these ADCs have significant activity in preclinical xenograft models of NHL, while minimal activity was observed with the unconjugated antibody (AG Polson et al, 2010). To evaluate the cellular effects and characterize the mechanism of action (MOA) of these ADCs, we have examined the pharmacokinetics and the pharmacodynamic effects in non-human primates.

Substantial B cell depletion was observed after administration of either anti-CD22-ADC or anti-CD79b-ADC to cynomolgus monkeys. By comparison, the extent and duration of B cell depletion was less substantial in animals dosed with unconjugated anti-CD22 or anti-CD79b antibodies. We evaluated several potential mechanisms for the depletion, including antibody opsonization, antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and internalization of drug conjugates which leads to subsequent intracellular release of cytotoxic drug and cell death. Our data from in vitro anti-CD22 studies showed minimal to no ADCC or CDC activity, suggesting that these mechanisms play little to no role in vivo. Consistent with the expected MOA of ADCs, we observed depletion of proliferating splenic follicular germinal center B cells in cynomolgus monkeys following dosing with anti-CD22 ADC. Furthermore, in studies with either anti-CD22-ADC or anti-CD79b-ADC, preferential depletion of proliferating Ki67⁺ B lymphocytes (compared to Ki67^- B lymphocytes) was observed in peripheral blood of ADC-dosed animals, and not in animals dosed with the unconjugated antibody or with vehicle control. This preferential depletion was observed on Days 8 and 15, but was not seen on Day 21 or at subsequent time points, which correlated with the expected serum clearance of the ADC. In contrast, we observed that administration of either ADC resulted in a dose-dependent decrease in circulating non-proliferating B cells one day after dosing. Taken together, these observations agree with the proposed mechanism of action of initial depletion of both proliferating and non-proliferating B cells via antibody-mediated opsonization at early time points after dosing (1-2 days) with subsequent preferential depletion of proliferating B lymphocytes mediated by the auristatin component of the anti-CD22 and anti-CD79b ADCs.

These data support CD22 and CD79b ADCs as promising candidate therapeutics for the treatment of NHL and CLL, as both anti-CD22 and anti-CD79b drug conjugates are capable of targeting Ki-67⁺ (proliferating) B cells in lymphoid tissues and peripheral blood. In CLL, Ki-67⁺ cells in bone marrow have been hypothesized to represent pathogenic ‘stem’ cells, and our preliminary data indicate increased percentages of Ki-67⁺ B cells in the peripheral blood of CLL patients compared to normal healthy adults. As both anti-CD22 and anti-CD79b ADCs specifically deplete proliferating B cells, the development of these ADCs represents an effective way to target proliferating pathogenic B cells in NHL and CLL, and offers a more favorable risk-benefit profile than traditional chemotherapeutic agents.