Abstract 2644: Antibody-drug conjugates (ADCs) with a novel DNA-alkylating agent, DGN462, are highly potent in vitro and in vivo against human cancer models
Authors: Whiteman Kathleen , Audette Charlene , Dandeneau Andre , Ellis Megan , Fishkin Nathan , Harvey Lauren , Johnson Holly , Kovtun Yelena , Maloney Erin , Miller Michael , Wilhelm Alan , Chari Ravi 
Published: May 2015

Abstract
Most ADCs in the clinic utilize a tubulin-binding small molecule as the cytotoxic payload. Although many cancers are sensitive to tubulin agents, some are more responsive to DNA-interacting agents. We have developed a new class of highly potent cytotoxic agents, IGNs, for use in ADCs that consists of indolino-benzodiazepine dimers with a novel DNA-alkylating mechanism of action. We found that IGNs containing a di-imine moiety act via DNA-crosslinking and -alkylating, whereas IGNs with a mono-imine induce DNA alkylation only. Although ADCs with either form of IGN were highly active, ADCs containing di-imine IGNs caused delayed toxicity in mice, leading to mortality. The mono-imine IGN, DGN462, was selected as our lead IGN as, when conjugated to an antibody via a cleavable linkage, the resultant ADC had favorable tolerability in mice (maximally tolerated dose of 40 mg/kg; 700 µg/kg DGN462 dose) without delayed toxicity.
The ability of DGN462 to alkylate DNA was demonstrated by measurement of DNA adduct formation with double-stranded DNA and in cells. DGN462 formed covalent DNA adducts through alkylation of the C2- amino group of guanine. Consistent with the activity of other DNA alkylating agents, DGN462-treated cells progressed slowly through S-phase and were arrested in G2/M phase of the cell cycle in a dose dependent manner.
ADCs consisting of ∼3 molecules of DGN462 per antibody, attached via a cleavable linker, had desirable biochemical characteristics, including high monomer content (&gt; 97%), low free drug (&lt; 0.1%) and symmetrical mass distribution profiles. These DGN462 ADCs were highly potent and antigen-specific in vitro against various cell types, including PgP-expressing cells, with IC50 values ranging from 2 pM to 3 nM depending on the target, cell type and antigen density. DGN462 ADCs display bystander activity in vitro, indicating that intracellular processing of conjugate by target cancer cells releases metabolites that can kill antigen-negative cells in direct proximity, which may provide an advantage in solid tumors expressing antigen heterogeneously.
A CD33-targeting DGN462 ADC was highly active against AML xenografts, with a minimal efficacious dose (MED) of 0.6 mg/kg (conjugate dose), while a non-targeting control was inactive. A DGN462 ADC targeting epidermal growth factor receptor was highly active and antigen-specific against a head and neck squamous cell carcinoma model, with a 1.6 mg/kg MED. The intact conjugate half-life was approximately 90 hours in mice, and bioactivity of conjugate was maintained at 72 hours post dosing, indicating that intact conjugate remains active in vivo.
DGN462 ADCs have an acceptable safety profile and potent antigen-specific DNA-alkylating mechanism of action with the potential for activity in tumors with low sensitivity to tubulin agents, heterogeneous antigen expression and/or PgP-mediated drug resistance.
Citation Format: Kathleen Whiteman, Charlene Audette, Andre Dandeneau, Megan Ellis, Nathan Fishkin, Lauren Harvey, Holly Johnson, Yelena Kovtun, Erin Maloney, Michael Miller, Alan Wilhelm, Ravi Chari. Antibody-drug conjugates (ADCs) with a novel DNA-alkylating agent, DGN462, are highly potent in vitro and in vivo against human cancer models. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2644. doi:10.1158/1538-7445.AM2014-2644