Abstract 2887: Bispecific antibody drug conjugates targeting CD7 and CD33 for the treatment of acute myeloid leukemia
Authors: Bethell Richard , Eberlein Cath , Pollard Victoria , Georgiou Theonie , Orphanides George , Mooney Lorraine , Kendrew Jane , Daniels Tiffany 
Published: September 2020

Abstract
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 &gt;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&lt;0.001) and 100% (97% regression, p&lt;0.001) at days 21 and 28 respectively, with three animals tumor-free at the end of dosing. Treatment with cytarabine resulted in TGI of 100% (47% regression, p&lt;0.001) at day 21. No adverse effects were observed in the BVX130-mcMMAF treatment group.
Anti-CD7, anti-CD33 Bi-Fab drug conjugates are highly cytotoxic to CD7+CD33+ cells in vitro and in vivo, and have substantially greater selectivity than monospecific drug conjugates for such cells, supporting the development of bispecific antibody drug conjugates targeting CD7 and CD33 for the treatment of CD7+/CD33+ AML.
Citation Format: Richard Bethell, Cath Eberlein, Victoria Pollard, Theonie Georgiou, George Orphanides, Lorraine Mooney, Jane Kendrew, Tiffany Daniels. Bispecific antibody drug conjugates targeting CD7 and CD33 for the treatment of acute myeloid leukemia [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2887.