Abstract 574: AGS62P1, a novel site-specific antibody drug conjugate targeting FLT3 exhibits potent anti-tumor activity regardless of FLT3 kinase activation status

Abstract

FLT3 is a member of the class III receptor tyrosine kinase family that includes C-KIT, C-FMS and platelet derived growth factor receptor (PDGFR). FLT3 is primarily expressed in early myeloid and lymphoid progenitors and plays an important role in their proliferation and differentiation. In human leukemia, FLT3 is expressed on 70-90% acute myeloid leukemia (AML) and most B-acute lymphoblastic leukemia (B-ALL). FLT3 genetic aberrations are commonly detected in patients with AML. The most common aberration is internal tandem duplication (ITD), which occurs in 25-30% of AML patients and causes constitutive activation of FLT3. Point mutation in codon D835 of the FLT3 tyrosine kinase domain is reported in 7-10% of AML patients and also causes constitutive activation of the receptor. FLT3 small molecule inhibitors targeting the kinase domain are predominantly active against FLT3 activated AML. The restricted normal tissue expression profile and higher differential in leukemic specimens makes FLT3 amenable to antibody-based therapeutics, requiring only target expression independent of kinase activation status. Therefore, development of an antibody-drug conjugate (ADC) may provide a therapeutic alternative for AML patients. Here, we report the development of the first FLT3specific ADC, AGS62P1, employing site-specific conjugation using the non-natural amino acid, p-acetyl phenylalanine (pAF). AGS62P1 comprises a human gamma one antibody including an inserted pAF residue in each of the heavy chains. The antibody was conjugated to a potent cytotoxic payload via an oxime bond at the pAF sites, thus creating a nearly homogeneous drug distribution, with approximately 2 drug molecules per antibody. Strong binding affinity (0.1-0.9 nM) and potent in vitro cytotoxic activity (IC50 = 0.2-12 nM) was achieved in AML cell lines. The anti-FLT3 ADC was highly efficacious in AML tumor xenografts, leading to statistically significant tumor growth inhibition of both FLT3 ITD and non-ITD models. Additional characterization of both the antibody and ADC was performed, including ligand receptor interaction, degradation, internalization, and apoptosis. In summary, we have developed a site-specific ADC targeting FLT3 that exhibits potent anti-tumor activity in xenograft models regardless of FLT3 activation status. This drug can potentially offer a new and more versatile approach in targeting FLT3-expressing leukemia through a mechanism independent of FLT3 genetic aberration. Citation Format: Nandini Rudra-Ganguly, Pia M. Challita-Eid, Christine Lowe, Mike Mattie, Sung-Ju Moon, Brian A. Mendelsohn, Monica Leavitt, Cyrus Virata, Alla Verlinsky, Linnette Capo, Mi Sook Chang, Deanna L. Russell, Baljinder Randhawa, Gao Liu, René Hubert, Mary Brodey, Hector Aviña, Chunying Zhang, Joseph D. Abad, Banmeet Anand, Sher Karki, Zili An, Roland Luethy, Fernando Doñate, Daniel S. Pereira, Kendall Morrison, Ingrid B.J. Joseph, David R. Stover. AGS62P1, a novel site-specific antibody drug conjugate targeting FLT3 exhibits potent anti-tumor activity regardless of FLT3 kinase activation status. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 574.

Monoclonal Antibodies in Acute Myeloid Leukemia—Are We There Yet?

“…Despite potent preclinical activity, AGS62P1, a human IgG1κ anti-FLT3 ADC linked to AGL-0182-30 (microtubule disrupting agent), failed to show efficacy in R/R AML (NCT02864290). 72,73 Similarly, the unconjugated anti-FLT3 mAb, LY3012218 (IMC-EB10), failed to show clinical activity in AML leading to trial termination (NCT00887926). 74 Anti-CLL-1 ADC CLL-1 (C-type lectin-like molecule-1), alternatively known as CLEC12A (C-type lectin domain family 12 member A), is a self-surface protein with a key role in the regulation of innate and adaptive immunity.…”

“…Antibody-based therapies targeting FLT3 have promise, but there are limited clinical data. Despite potent preclinical activity, AGS62P1, a human IgG1κ anti-FLT3 ADC linked to AGL-0182-30 (microtubule disrupting agent), failed to show efficacy in R/R AML (NCT02864290) 72,73 . Similarly, the unconjugated anti-FLT3 mAb, LY3012218 (IMC-EB10), failed to show clinical activity in AML leading to trial termination (NCT00887926) 74 …”

Expression of a recombinant FLT3 ligand and its emtansine conjugate as a therapeutic candidate against acute myeloid leukemia cells with FLT3 expression

“…Tyrosine kinase inhibitors targeting FLT3 have been approved by the FDA for the treatment of newly diagnosed adult AML patients [12] and are also being evaluated in clinical trials [13]. Monoclonal antibodies targeting FLT3 are also being evaluated in preclinical studies or clinical trials [14][15][16]. Protein drug conjugates, which can eliminate cancer cells directly via the conjugated drug [17,18], may serve as improved targeted therapy methods for AML.…”