Abstract LB038: Validation of FAPi PET biodistribution by immunohistochemistry in patients with solid cancers: a prospective exploratory study
Authors: Mona Christine E. , Benz Matthias , Hikmat Firat , Grogan Tristan G. , Lueckerath Katharina , Razmaria Aria A. , Riahi Rana , Slavik Roger , Grigis Mark D. , Carlucci Giuseppe , Kelly Kimberly A. , Czernin Johannes , Dawson David W. , Calais Jeremie 
Published: July 2021

Abstract
Context: Fibroblast activation protein (FAP)-expressing cancer associated fibroblasts (CAFs), a major component of tumor stroma, confer treatment resistance, promote local progression, metastasis and immunosuppression. Because FAP is selectively expressed in the tumor stroma of many cancers, radiolabeled small molecule ligands targeting FAP are being explored for their use as pan-cancer theranostic agents. As a required step for further translation and approval by regulatory agencies FAPi PET imaging must be show to be a reliable biomarker of FAP expression in cancer and non-cancer tissues. The aim of the study is to define the incidence and degree of FAP expression by immunohistochemistry (IHC) across various cancers using Tissue Microarrays (TMAs) and to explore whether gallium-68 FAPi-46 PET image biodistribution faithfully reflects tumor FAP expression from resected tumor and non-tumor specimens. Methods: This was a prospective, exploratory, open-label, single-center imaging trial in cancer patients conducted in 2020. Patients scheduled to undergo surgical resection of the primary tumor and/or metastases were eligible. Patients underwent one whole body 68Ga-FAPI-46 PET/CT scan and subsequently underwent surgical resection of the primary tumor and/or metastasis. The primary endpoint measure was the Correlation of 68Ga-FAPI-46 PET maximum standardized uptake value (SUVmax) with FAP IHC score in cancer and non-cancer tissue. Results: The incidence of FAP expression in TMAs from 14 cancers ranged from 25 to 100% (mean 76.6± 25.3). 15 patients with the following cancer types were included: colorectal (n=4), head and neck (n=3), pancreas (n=2), breast (n=2), stomach (n=1), esophagus (n=2) and uterus (n=1). All 15 patients subsequently underwent surgery after the scan with a mean time interval of16.1 ± 14.4 days (range 1 - 50 days). Tumor resection was not attempted in 2 patients because unresectable. FAPI SUVmax and IHC score were higher in cancer tissue than in normal tissue: mean FAPI SUVmax 7.4±4.6 (range 1.5-15.9) vs 1.6±1.2 (range 0.4-5.1), (p&lt;0.001) and mean FAP IHC score 2.38±0.65 vs 0.54±0.66 (p&lt;0.001), respectively. The FAP IHC score was positively correlated with FAPI SUVmax (pairwise p=0.001, repeated measures correlations r=0.85 (95% CI 0.53-0.95), p&lt;0.001). Conclusions: In this translational study using Tissue Microarrays and a prospective exploratory imaging trial of 15 surgical oncology patients, the FAPI PET uptake and FAP expression per immunohistochemistry were strongly correlated in cancer and non-cancer tissue. The 68Ga-FAPi-46 PET biodistribution across multiple cancers reflects FAP expression as determined by IHC. This translational validation paves the way for large scale prospective trials on the use of 68Ga-FAPi-46 PET/CT as a biomarker and stratification tool for FAP-targeted therapies. Trial Registration: ClinicalTrials.gov Identifier: NCT04147494
Citation Format: Christine E. Mona, Matthias Benz, Firat Hikmat, Tristan G. Grogan, Katharina Lueckerath, Aria A. Razmaria, Rana Riahi, Roger Slavik, Mark D. Grigis, Giuseppe Carlucci, Kimberly A. Kelly, Johannes Czernin, David W. Dawson, Jeremie Calais. Validation of FAPi PET biodistribution by immunohistochemistry in patients with solid cancers: a prospective exploratory study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB038.