Abstract C162: Antibody-Drug Conjugates (ADCs) with novel IGN DNA-alkylating agents display potent antigen-specific activity against hematologic and solid tumor xenograft models.
Authors: Whiteman Kathleen , Johnson Holly , Wilhelm Alan , Miller Michael , Li Wei , Reid Emily , Archer Katie , Fishkin Nathan , Dandeneau Andre , Maloney Erin , Pinkas Jan , Chari Ravi 
Published: August 2014

Abstract
The clinical benefit demonstrated by brentuximab vedotin and ado-trastuzumab emtansine has attracted considerable interest in ADCs. Most ADCs in the clinic, including these two approved ones, utilize a tubulin-acting cytotoxic agent. Although many cancers are sensitive to tubulin agents, some cancers are more responsive to DNA-acting agents. We have developed a new class of highly potent cytotoxic agents, IGNs, for use in ADCs which consist of indolino-benzodiazepine dimers with a novel DNA-alkylating mechanism of action. The intense in vitro potency (in the picomolar range) of these IGNs, along with desired characteristics such as aqueous solubility and stability, make them well-suited for use in ADCs.
A series of IGN-ADCs was prepared using di-imine (DNA-crosslinking and -alkylating) and mono-imine (DNA-alkylating only) versions of the IGNs, attached using noncleavable and cleavable linkers. The tolerability of IGN-ADCs in mice was found to be dependent upon a) linker format: conjugates with cleavable linkers were better tolerated than those with noncleavable linkers; and b) mechanism of action: conjugates with IGNs with DNA-crosslinking ability (di-imine IGNs) had delayed toxicity that was absent with the DNA-alkylating only (mono-imine) IGN. We found that the cleavable linker format avoids liver toxicity, which was the dose-limiting toxicity of IGN-ADCs with non-cleavable linkers. This absence of liver toxicity resulted in a 4-fold increase in the maximally tolerated dose for the cleavable linker ADC compared with the noncleavable linker design (40 mg/kg and 10 mg/kg conjugate dose, respectively). Thus, IGN-ADCs utilizing a cleavable linker with a mono-imine IGN demonstrated greatly improved tolerability.
The in vivo efficacy of ADCs with the selected IGN, 3m (mono-imine) attached using a cleavable linker was evaluated in xenograft models of both hematologic and solid tumor indications. A CD33-targeting IGN-ADC, anti-CD33-3m, was highly active against acute myeloid leukemia xenografts, with a minimal efficacious dose (MED) of 0.6 mg/kg (conjugate dose), while a non-targeting control was inactive. A folate receptor α-targeting IGN-ADC, anti-FRα-3m, and an epidermal growth factor receptor-targeting IGN-ADC, anti-EGFR-3m, were evaluated in non-small cell lung cancer models and found to be highly active and antigen-specific, with MEDs of 1.6 mg/kg. Anti-CD56-3m was highly active and antigen-specific against small-cell lung cancer xenografts, with a MED of 2.9 mg/kg.
The desired safety profile (absence of delayed toxicity and liver toxicity) combined with the potent antigen-specific activity of these IGN 3m-ADCs (mono-imine IGN, cleavable linker) against various cancer types supports DNA-acting ADCs with the necessary therapeutic index can now be achieved.
Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C162.
Citation Format: Kathleen Whiteman, Holly Johnson, Alan Wilhelm, Michael Miller, Wei Li, Emily Reid, Katie Archer, Nathan Fishkin, Andre Dandeneau, Erin Maloney, Jan Pinkas, Ravi Chari. Antibody-Drug Conjugates (ADCs) with novel IGN DNA-alkylating agents display potent antigen-specific activity against hematologic and solid tumor xenograft models. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C162.