Abstract B126: Potent antigen-specific anti-tumor activity observed with antibody-drug conjugates (ADCs) made using a new class of DNA-crosslinking agents

Abstract

Abstract The clinical support for ADC therapeutics has expanded as more highly-engineered ADCs advance in human clinical testing. Most of the ADCs now in clinical testing contain a tubulin-acting compound (a maytansine or dolastatin derivative) as the cytotoxic agent. While tubulin-acting agents can be effective against many different types of cancers, some cancers are more responsive to DNA-acting agents. To expand the therapeutic potential for ADCs, we sought to develop a new class of cytotoxic agents with a novel, DNA-acting mechanism of action for use with tumor-targeting antibodies. Herein, we report the development of our IGN family of cytotoxic agents. These IGN agents comprise indolino-benzodiazepine dimers that are highly potent by virtue of their ability to alkylate and crosslink DNA. This novel class of compounds demonstrated sequence-selective DNA adduct formation in vitro and cytotoxicity in the picomolar range towards cultured human cancer cells. The intense potency of these compounds, along with their desired aqueous solubility and stability, make them ideally suited for use in ADCs. A lead compound from this class was conjugated to an EpCAM-binding antibody, B38.1, and to a CD33-binding antibody, huMy9-6, through amide bonds. The B38.1-IGN conjugate was highly potent against three different EpCAM-expressing cell lines - COLO 205, LoVo and OVCAR-3 - with IC50 values of 1 pM, 5 pM and 18 pM, respectively. The addition of excess unconjugated B38.1 antibody abolished this cytotoxic effect, demonstrating that the activity of the conjugate is antigen specific. The B38.1-IGN conjugate was considerably less potent towards the antigen-negative Namalwa cell line, with an IC50 value of >1 nM, further demonstrating antigen specificity. Similar potent cytotoxicity was seen with a huMy9-6-IGN conjugate targeting the CD33-positive human promyelocytic leukemia cell line, NB4 (IC50 ∼4pM), in spite of the low antigen expression level (∼10,000 molecules/cell) in this cell line. Of particular interest, the B38.1-IGN conjugate also was potent towards multidrug resistant cancer cells. B38.1-IGN had a IC50 value of 14 pM for COLO 205MDR, a COLO 205 clone engineered to overexpress MDR1 transporter, and 7 pM for HCT-15, an EpCAM-expressing cell line that naturally expresses MDR1. Antibody-IGN conjugates demonstrated dose-dependent activity in multiple human tumor xenograft models in mice, with anti-tumor activity observed at non-toxic doses. The unique mechanism of action of the IGN class of compounds, and the high antigen-specific potency of antibody-IGN conjugates seen in vitro and in vivo, provides a promising new cytotoxic agent for use in the development of new ADCs. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B126.