Anti-CD33 monoclonal antibody, clone XN64 [APC]

Alzheimer's disease (AD) affects nearly 50 million people worldwide, and currently no disease-modifying treatment is available. With continuous failure of anti-amyloid-beta- or tau-based therapies, identification of new targets has become an urgent necessity for AD prevention and therapy. Recently, conventional genetic approaches and computational strategies have converged on immune-inflammatory pathways as key events in the pathogenesis of AD. A number of genes have been highly linked to the onset and development of late-onset sporadic AD, the most common form of AD. Strikingly, most of these genes are involved in microglial biology. Mutations and/or differential expression of microglial receptors such as TREM2, CD33, and CR3 have been strongly associated with an increased risk of developing AD. The mechanistic actions of these risk factors in AD etiology have been actively investigated since they were identified. Whether these genes can be targeted for a disease-modifying treatment is under hot debate. CD33 is one of the top-ranked AD risk genes identified by genome-wide association studies. This review summarizes the recently advanced biology of CD33 and its association with AD. It also provides insights from a drug discovery perspective into the druggability, therapeutic strategies, and challenges to target CD33 for treating this devastating disorder.

The CD33-targeting bispecific T-cell engager (BiTE) AMG 330 proved to be highly efficient in mediating cytolysis of acute myeloid leukemia (AML) cells in vitro and in mouse models. Yet, T-cell activation is correlated with upregulation of programmed cell death-ligand 1 (PD-L1) and other inhibitory checkpoints on AML cells that confer adaptive immune resistance. PD-1 and PD-L1 blocking agents may counteract T-cell dysfunction, however, at the expense of broadly distributed immune-related adverse events (irAEs). We developed a bifunctional checkpoint inhibitory T cell-engaging (CiTE) antibody that combines T-cell redirection to CD33 on AML cells with locally restricted immune checkpoint blockade. This is accomplished by fusing the extracellular domain of PD-1 (PD-1(ex)), which naturally holds a low affinity to PD-L1, to an alpha CD3. alpha CD33 BiTE-like scaffold. By a synergistic effect of checkpoint blockade and avidity-dependent binding, the PD-1(ex) attachment increases T-cell activation (3.3-fold elevation of interferon-g) and leads to efficient and highly selective cytotoxicity against CD33(+)PD-L1(+) cell lines (50% effective concentration 5 2.3-26.9 pM) as well as patient-derived AML cells (n = 8). In a murine xenograft model, the CiTE induces complete AML eradication without initial signs of irAEs as measured by body weight loss. We conclude that our molecule preferentially targets AML cells, whereas high-affinity blockers, such as clinically approved anticancer agents, also address PD-L1(+) non-AML cells. By combining the high efficacy of T-cell engagers with immune checkpoint blockade in a single molecule, we expect to minimize irAEs associated with the systemic application of immune checkpoint inhibitors and suggest high therapeutic potential, particularly for patients with relapsed/refractory AML.