Published OnlineFirst July 7, 2020; DOI: 10.1158/1078-0432.CCR-20-1358 CLINICAL CANCER RESEARCH | PRECISION MEDICINE AND IMAGING Imaging Fibroblast Activation Protein Alpha Improves Diagnosis of Metastatic Prostate Cancer with Positron Emission Tomography A C Hallie M. Hintz1, Joseph P. Gallant1, Donald J. Vander Griend2, Ilsa M. Coleman3, Peter S. Nelson3, and Aaron M. LeBeau1 ABSTRACT ◥ Purpose: Metastatic castration-resistant prostate cancer Results: Analysis of patient data documented FAP overexpres- (mCRPC) is a lethal, heterogeneous disease with few therapeutic sion in metastatic disease across tumor subtypes. PET imaging with strategies that significantly prolong survival. Innovative therapies [89Zr]Zr-B12 IgG demonstrated high tumor uptake and long-term for mCRPC are needed; however, the development of new retention of the probe in the preclinical models examined. FAP- therapies relies on accurate imaging to assess metastasis and positive stroma tumor uptake of [89Zr]Zr-B12 IgG was 5-fold higher monitor response. Standard imaging modalities for prostate than the isotype control with mean %ID/cc of 34.13 Æ 1.99 versus cancer require improvement and there remains a need for 6.12 Æ 2.03 (n ¼ 3/group; P ¼ 0.0006) at 72 hours. Ex vivo selective and sensitive imaging probes that can be widely used biodistribution corroborated these results documenting rapid blood in patients with mCRPC. clearance by 24 hours and high tumor uptake of [89Zr]Zr-B12 IgG Experimental Design: We evaluated the transmembrane prote- by 72 hours. ase fibroblast activation protein alpha (FAP) as a targetable cell Conclusions: Our study reveals FAP as a target for imaging surface antigen for mCRPC. Genomic and IHC analyses were the tumor microenvironment of prostate cancer. Validation of performed to investigate FAP expression in prostate cancer. Our [89Zr]Zr-B12 IgG as a selective imaging probe for FAP- FAP-targeted antibody imaging probe, [89Zr]Zr-B12 IgG, was expressing tumors presents a new approach for noninvasive evaluated by PET/CT imaging in preclinical prostate cancer models. PET/CT imaging of mCRPC. Introduction reactive stroma in prostate cancer. CAFs in the localized prostate tumor microenvironment are known to play an essential role in Malignant cells are surrounded by a complex and supportive tumor tumorigenesis (5–8). Furthermore, FAP-positive CAFs induce epithe- microenvironment that consists of immune cells, extracellular matrix, lial–mesenchymal transition–driven gain of cancer stemness, inva- vasculature, and fibroblasts (1). Cancer-associated fibroblasts (CAF) siveness, and development of distant metastases (9, 10). Recruitment of are the major cell type in the reactive stroma and are known to promote reactive stroma is essential for metastatic tumor survival and tumorigenesis and metastasis (2). Fibroblast activation protein alpha growth (11–13); however, not much has been published on the role (FAP) is a transmembrane serine protease expressed by CAFs in the of FAP or CAFs in prostate cancer metastasis. microenvironment of epithelial tumors (3). Meta-analysis of FAP Prostate cancer that has metastasized to the bone and visceral tissues expression and clinical outcomes in solid cancers indicate that patients is highly lethal with 5-year survival rates remaining at 30%. The with FAP overexpression are at higher risk of tumor invasion, lymph development of new therapeutics for mCRPC is dependent on accurate node metastasis, and decreased overall survival (4). Thus, FAP has imaging modalities for patient staging and evaluating treatment been gaining momentum as the next pan-cancer target for diagnostic response. The current standard for imaging disease burden is bone and therapeutic development given its absence in normal adult tissue, scintigraphy with 99mTc methyl diphosphonate ([99mTc]Tc-MDP). This upregulated expression in activated fibroblasts, and localization to the imaging modality only detects bone lesions and is plagued by poor tumor microenvironment. Although FAP expression has been anno- sensitivity (14, 15). PET imaging agents used in conjunction with either tated in multiple solid cancers, its presence in metastatic castration- CT or MRI have had various degrees of success imaging prostate resistant prostate cancer (mCRPC) is still largely uncharacterized. Few cancer (16). The commonly used PET radiotracer studies have been performed to accurately define and examine the 18 18 [ F]fluorodeoxyglucose ([ F]FDG) is of little utility (17, 18) while [18F]sodium fluoride (Na[18F]F) has documented improved sensitivity 1 over bone scans, but it cannot detect visceral lesions (15, 19). Department of Pharmacology, University of Minnesota Medical School, Min- 18 fl neapolis, Minnesota. 2Department of Pathology and Surgery, University of [ F] uciclovine was recently approved by the FDA for detecting Illinois at Chicago, Chicago, Illinois. 3Division of Human Biology and Clinical biochemical recurrence; however, its utility in mCRPC has not been Research, Fred Hutchinson Cancer Research Center, Seattle, Washington. investigated (20, 21). Prostate-specificmembraneantigen(PSMA) 89 68 H.M. Hintz and J.P. Gallant contributed equally to this article. targeted probes such as, [ Zr]Zr-J591 and [ Ga]Ga-PSMA-11, have met with success in the clinic, but loss of PSMA expression is known to Corresponding Author: Aaron M. LeBeau, University of Minnesota, 312 Church – Street SE, Minneapolis, MN 55455-0217. Phone: 612-301-7231; Fax: 612-625- occur in neuroendocrine, androgen receptor negative prostate cancer, 8408; E-mail: [email protected] rendering the probes ineffective (22). Therefore, standard imaging modalities for prostate cancer leave room for improvement and warrant Clin Cancer Res 2020;XX:XX–XX the discovery and validation of new antigens in prostate cancer. doi: 10.1158/1078-0432.CCR-20-1358 The localization of FAP to the tumor microenvironment and its Ó2020 American Association for Cancer Research. association with aggressive phenotypes make it a compelling imaging AACRJournals.org | OF1 Downloaded from clincancerres.aacrjournals.org on September 25, 2021. © 2020 American Association for Cancer Research. Published OnlineFirst July 7, 2020; DOI: 10.1158/1078-0432.CCR-20-1358 Hintz et al. Antibody production Translational Relevance For antibody expression, the heavy chain (342 bp) and light chain Targeted imaging probes for measuring disease burden and (324 bp) variable domains of the B12 sequence were cloned separately therapeutic response are dependent on the detection of unique into pFUSEss human IgG1 expression vectors (pFUSEss-CHIg-hG1 antigens expressed by the tumor. Men who fail standard-of-care and pFUSE2ss-CLIg-hk, Invitrogen) and cotransfected into HEK293T therapies and progress to metastatic castration-resistant prostate cells. The serum was collected after 72 hours, filtered through a cancer (mCRPC) are left with few therapeutic options. The current 0.45-mm filter, and purified using a 5 mL HiTrap Protein A HP imaging standards for prostate cancer are limited, which hinders column (GE Healthcare). The column was equilibrated with the development of new and urgently needed therapies. Our study 20 mmol/L sodium phosphate at pH 6.8. The serum was loaded onto shows fibroblast activation protein alpha (FAP) is a relevant target the column, washed with equilibration buffer for 10 column volumes, for the PET/CT imaging of mCRPC. FAP is emerging as the next and bound protein was eluted with 0.1 mol/L citric acid at pH 3.5. pan-cancer target given its upregulated expression in cancer- Eluted IgG was collected, concentrated using a 50-kDa centrifugal filter associated fibroblasts and localization to the tumor microenviron- (Millipore), and buffer exchanged into PBS using a Sephadex G-25 ment. Our FAP-targeted, radiolabeled probe was validated in PD-10 desalting column (GE Healthcare). The purity of the final B12 preclinical imaging studies and demonstrated to be highly selective IgG was analyzed by reduced and nonreduced SDS-PAGE, and protein for FAP-expressing tumors. This study suggests [89Zr]Zr-B12 IgG concentration was measured on the basis of absorbance at 280 nm is an ideal candidate for noninvasive PET/CT imaging in mCRPC, using a NanoDrop One UV – Vis Spectrophotometer (Thermo Fisher and more importantly, it represents a platform technology for the Scientific). development of theranostics targeting FAP. Genomic analysis RNA sequencing (RNA-seq) data were extracted from previously published studies of individuals with primary prostate cancer (24) or and therapeutic target for solid tumors including prostate cancer. mCRPC bone and soft tissue tumor biopsies (25). Capture of clinical Previously, we discovered and characterized a mAb targeting the samples was part of standard-of-care approaches for mCRPC that extracellular domain of FAP, termed B12 IgG. Here, we describe the included treatment with abiraterone acetate or enzalutamide followed development of [89Zr]Zr-B12 IgG, a FAP-targeted probe for the by taxane. RNA-seq fragments per kilobase million (FPKM) values detection of prostate cancer by PET/CT imaging. Genomic and IHC were obtained using the previously described methods (26). Micro- analysis revealed overexpression of FAP in metastatic disease com- array data were extracted from previously published studies of a set of pared with primary prostate tumors. In addition, FAP expression was metastatic tumors from men with CRPC (27). The dataset is available found to be uniform across metastatic disease regardless of genomic in the Gene Expression