Abstract A215: Antitumor activity of an antibody-drug conjugate targeting 5T4, a putative tumor-initiating cell antigen.

5T4 (also known as TPBG) is a transmembrane, tumor-associated protein that rapidly internalizes. In preclinical models of NSCLC, 5T4 was identified as a marker of undifferentiated tumorigenic cells that express properties of tumor-initiating cells (TICs) and was associated with a highly proliferating (Ki67 positive) cell phenotype. In addition, 5T4 has been associated with a worse clinical outcome in NSCLC (Damelin et al, Cancer Res, 2011). We constructed an ADC with a humanized anti-5T4 Ab (A1) linked to the tubulin inhibitor monomethylauristatin F through the noncleavable maleimidocaproyl (mc) linker attached to cysteine residues (A1mcMMAF) with a DAR of approximately 4 in order to target 5T4 expressing tumor cells. Anti-5T4 A1 Ab has a Kd of 0.9 nM against the 5T4 antigen, binds to a broad range of 5T4 expressing cell lines, and rapidly internalizes (66% internalized within 4h). As an ADC, A1mcMMAF retains similar properties to the unconjugated Ab. In vitro, A1mcMMAF exhibited receptor-density dependent cytotoxic activity against a panel of 5T4 + cell lines with an IC 50 of 5 ng Ab/ml against high 5T4 expressing cells. A1mcMMAF exerts no inhibitory effect on 5T4 negative cells demonstrating it does not exert off-target activity in vitro. In vivo, A1mcMMAF causes long-term (100 days) tumor regressions at 10 mg Ab/kg in MDAMB435/5T4 tumor xenografts (Q4d×2, high 5T4 expression by IHC) and in the NSCLC patient-derived xenograft 37622A model (Q4d×4, high 5T4 expression by IHC though only a portion of the cells express 5T4). Significant anti-tumor activity was demonstrated at 1 mg/kg. Against the low 5T4 expressing H1975 tumor xenograft, A1mcMMAF causes potent tumor regression in spite of its lack of in vitro inhibitory activity against this cell line. Bioimaging of the H1975 tumor demonstrated good penetration of Alexa 750 labeled A1 Ab into the xenograft. A1mcMMAF exhibited no overt toxicities when administered at 10 mg Ab/kg/cycle × 2 to cynomolgus macques and possessed a half-life of 4 to 7 days. The observation that A1mcMMAF regresses H1975 tumor xenografts while demonstrating poor activity against H1975 in vitro demonstrates the attractiveness of targeting 5T4 in the tumor environment and may be explained by 5T49s principal association with proliferating cells in the tumor. It is also interesting that A1mcMMAF was able to completely regress 37622A xenografts since this patient-derived xenograft presents heterogeneous 5T4 expression, i.e. not all cells demonstrate detectable levels of 5T4. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A215.