Abstract 3029: FAP-mediated tumor accumulation of a T-cell agonistic FAP/4-1BB DARPin drug candidate analyzed by SPECT/CT and quantitative biodistribution

Immunomodulating agents have revolutionized anti-cancer therapy. However, monotherapy is often not sufficient, and development of combination treatments is hampered by cumulative toxicity. In an attempt to overcome this challenge, a tumor-restricted agonistic 4-1BB/FAP DARPin drug candidate, which induces T-cell co-stimulation only when clustered by binding to fibroblast activation protein alpha (FAP) expressing cells, has been developed. FAP is a type II membrane-bound glycoprotein abundantly expressed in the stroma of many solid tumors by cancer-associated fibroblasts. As shown previously using in vitro and in vivo models (HT-29), co-stimulation induced by a FAP-targeted 4-1BB agonistic DARPin molecule leads to enhanced activation and expansion of CD8+ T-cells. To support clinical development of the drug candidate, tumor localization and accumulation were studied by whole-body SPECT/CT imaging and quantitative biodistribution using Indium-111 labeled DARPin molecules in a human colorectal adenocarcinoma (HT-29) xenograft model in CD1 nude mice. Immunohistochemical staining of tumor stroma confirmed local expression of FAP. Labeled 4-1BB/FAP DARPin molecules specifically accumulated in FAP-expressing tumor in vivo. SPECT/CT imaging and biodistribution revealed a maximum tumor accumulation of around 15% of the injected dose per gram of tissue around 72 h post injection. High tumor/blood ratios were observed one week post injection because the activity in the blood decreased according to the expected serum half-life of 26 h, determined in separate pharmacokinetic studies in BALB/c mice following single dose intravenous bolus injections. Based on the decrease of radioactivity in the tumor, a tumor residence half-life of approximately 4 days was calculated, indicating an extended tumor retention potentially due to FAP binding. No accumulation was observed in the muscle tissue that was choosen as a rather weakly-perfused control tissue. Taken together, FAP-targeting of a 4-1BB agonist DARPin molecule resulted in expected high tumor accumulation and retention compared to an untargeted version of the molecule, both relevant observations for further preclinical and clinical studies. These findings suggest that tumor-targeting via FAP has the potential to induce T-cell activation restricted to the tumor site, and thereby reducing toxicities caused by systemic 4-1BB activation. In conclusion, immunostimulatory drugs with tumor-targeted activity may have the potential to circumvent current limitations of immunotherapy and allow safe and effective use, in particular in combination therapy. Citation Format: Christian Reichen, Ralph Bessey, Christine DePasquale, Stefan Imobersteg, Martin Behe, Alain Blanc, Roger Schibli, Alexander Link, Laurent Juglair, Joanna Taylor, Patricia Schildknecht, Julia Hepp, Elmar vom Baur, Hong Ji, Christof Zitt, Victor Levitsky, Keith M. Dawson, Michael T. Stumpp, Dan Snell. FAP-mediated tumor accumulation of a T-cell agonistic FAP/4-1BB DARPin drug candidate analyzed by SPECT/CT and quantitative biodistribution [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3029.