Abstract 2887: Bispecific antibody drug conjugates targeting CD7 and CD33 for the treatment of acute myeloid leukemia

Phenotyping of AML patient samples shows that CD33, a myeloid cell marker, and CD7, a lymphoid cell marker, are specifically co-expressed in 25% of AML cases. Development of a bispecific antibody drug conjugate (ADC) dependent on co-expression of CD7 and CD33 may lead to an ADC with better tumor-cell selectivity than the anti-CD33 ADC, gemtuzumab ozogamicin. Anti-CD7 and -CD33 Fabs were expressed in HEK293 cells, and affinities assessed using surface plasmon resonance. Chemically cross-linked anti-CD7, anti-CD33 Bi-Fabs were produced using click chemistry via an individual cysteine residue in each CH1 domain. Bi-Fabs were conjugated to valine-citrulline-monomethylauristatin E (vcMMAE) and maleimidocaproyl monomethylauristatin F (mcMMAF) at native cysteine residues. Bi-Fab drug conjugates were assessed for cytotoxicity against HNT-34 and Kasumi-3 cells (CD7+CD33+), and assessed for selectivity against primary human T cells and myeloid progenitor cells, using monospecific Bi-Fab and gemtuzumab drug conjugates with equivalent drug-antibody ratios as controls. The activity of the lead Bi-Fab, BVX130-mcMMAF, was assessed in a subcutaneous (s.c.) xenograft model in SCID mice implanted with HNT-34 cells. Animals (n=10/group) received BVX-130-mcMMAF (10mg/kg twice weekly i.v. for 4 weeks), cytarabine at its maximum tolerated dose (20mg/kg, 5 days on 2 days off i.p. for 3 weeks), or vehicle control for 4 weeks. The wild-type Fabs had Kd values of 3.6nM (anti-CD7 Fab) and 1.6 nM (anti-CD33 Fab) for their respective antigens. The resulting anti-CD7, anti-CD33 Bi-Fab MMAE conjugate (BVX100-vcMMAE) was highly cytotoxic to HNT-34 and Kasumi-3 cells with IC50 values of 0.20±0.04nM and 0.20±0.03nM respectively, compared to IC50 values of 1.3nM and 1.2nM for gemtuzumab MMAE. BVX100-vcMMAE had 5-fold lower cytotoxicity towards Jurkat cells (CD7+CD33−) and >10-fold margins relative to MV4.11 and SHI-1 (CD7−CD33+) cells whereas corresponding homo-dimeric Bi-Fabs were non-selective. Affinity reduction of the CD7 Fab resulted in Bi-Fab drug conjugates with similar activity and improved selectivity. One such Bi-Fab, BVX130mcMMAF, had no measurable cytotoxicity to resting T-cells, and did not inhibit colony formation in a GM-CFU assay, at concentrations up to 10nM. Treatment of mice carrying s.c. HNT-34 tumors with BVX130-mcMMAF resulted in Tumor Growth Inhibition (TGI) values of 100% (98% regression, p