Abstract 234: Pre-clinical characterization of 3A4-PL1601, a novel pyrrolobenzodiazepine (PBD) dimer-based antibody-drug conjugate (ADC) directed against KAAG1-expressing malignancies

Kidney-associated antigen 1 (KAAG1) is an 84 amino acid protein encoded by the reverse strand of a housekeeping gene called DCDC2 which was identified from a cDNA library derived from a histocompatibility leukocyte antigen-B7 renal carcinoma cell line as an antigenic peptide presented to cytotoxic T lymphocytes. More recently, KAAG1 has been identified by a sensitive subtractive cloning technology called STAR as a novel tumor-associated antigen expressed in a high percentage of ovarian tumors and triple-negative breast cancers (TNBCs), while it has restricted normal tissue expression. 3A4 is a humanized antibody raised against human KAAG1, which binds to KAAG1 expressed on the surface of cancer cells, rapidly internalizes and co-localizes with LAMP-1, a lysosomal marker. Altogether, KAAG1 represents an attractive target for an antibody-drug conjugate (ADC) approach based on its high and selective expression in various malignancies and its restricted expression in healthy tissues. 3A4-PL1601 is an ADC composed of the 3A4 antibody, site-specifically conjugated using GlycoConnectTM technology to PL1601, which contains HydraspaceTM, a valine-alanine cleavable linker and the PBD dimer cytotoxin SG3199. The purpose of this study was to characterize the in vitro and in vivoanti-tumor activity of 3A4-PL1601 in human cancer cell lines and xenografts models and to determine its safety and tolerability in the cynomolgus monkey. In vitro, the 3A4-PL1601 demonstrated potent cytotoxicity in a panel of human cancer cell lines of different origin and levels of KAAG1, while its potency was strongly reduced in KAAG1-negative cell lines. In vivo, 3A4-PL1601 showed potent and specific anti-tumor activity in the TNBC-derived MDA-MB-231 xenograft compared to the vehicle- and isotype control ADC-treated mice. A single dose of 3A4-PL1601 at 0.6 mg/kg resulted in 4/8 partial responders (PR) and 3/8 complete responders, two of which were tumor-free survivors at the end of the study on day 59, while none of the mice treated with the vehicle or the isotype-control ADC showed any activity. Moreover, 3A4-PL1601 showed potent and dose-dependent anti-tumor activity in the KAAG1-expressing, human renal cell carcinoma-derived SN12C xenograft when tested as single dose at 0.3, 0.6 or 1 mg/kg. At the highest dose tested, 3A4-PL1601 resulted in sustained tumor growth control and 2/8 PRs at the end of the study on day 60. 3A4-PL1601 was stable, well tolerated (MTD at least 0.8 mg/kg) and showe