Abstract 4017: Development and application of an immunohistochemistry-based assay for evaluating functional and accessible folate receptor expression in vivo
Authors: Chu Haiyan , Shillingford Jonathan M. , Parker Nikki , Nelson Melissa , Vaughn Jeremy F. , Felten Albert , Lu Yingjuan , Reddy Joseph A. , Vlahov Iontcho R. , Leamon Christopher P. 
Published: July 2017

Abstract
With the development of molecularly-targeted therapeutics, it is critical to have a reliable method for predicting response to therapy of patients with target-expressing diseases. Folate receptor (FR) is overexpressed in a number of disease states while it is low in most healthy individuals with the exception of the kidney. FR-targeted small molecule drug conjugates (SMDCs) are currently under development and have shown promising pre-clinical and clinical results. Two premises that a patient would respond to the folate-drug conjugates are: 1). FR protein expressed in diseased tissues is functionally competent for folate binding and 2). functional FR is accessible to intravenously-infused SMDCs. Although anti-FR antibody-based immunohistochemical (IHC) assays are being developed as companion diagnostic strategies for antibody-drug conjugate (ADC) therapies, it fails to address the functionality and accessibility of FR in an in vivo context.
To circumvent these limitations, we have developed an FR-targeted, crosslinkable small molecule reporter conjugate (SMRC), FRRC, which contains three modules: 1. a high-affinity folate ligand that binds with FR on diseased cells; 2. a small hapten, FITC, which can be detected by anti-FITC antibodies; and 3. an amino acid spacer in between these two modules that can crosslink FRRC to FR in-situ during formalin fixation. After intravenous (i.v.) injection, FRRC would dock to the accessible and functional receptors in vivo. By processing biopsied tissues and performing anti-FITC IHC staining, we can evaluate the cellular localization and relative abundance of functional FR in heterogeneous tissues.
To evaluate our design, we synthesized a compound we refer to as FRRC and examined its properties both in vitro and in vivo. By testing FRRC in cell lines, we found that it can bind to FR and is detected in formalin-fixed cells by anti-FITC immunostaining. To validate this assay in vivo, FRRC was injected via the tail vein into mice bearing FR-positive KB tumor xenografts and its specific accumulation and kinetics in tumor and other tissues were evaluated by performing anti-FITC IHC staining on formalin-fixed paraffin embedded tissues. Our results were shown to be in agreement with previous folate-based functional FR imaging and bio-distribution studies. In contrast, EC17, which lacks the crosslinkable spacer module, showed significantly reduced binding in in vitro and in vivo assays. Owing to its modular design, additional SMRCs with different ligands and small haptens (rhodamine, DNP) have also been designed to assess the functional binding of other receptor and membrane-expressed proteins.
In conclusion, our assay is an effective tool for evaluating functional and accessible receptor expression in vitro and in vivo, and has the potential to be useful in patient or disease selection for our SMDC therapeutics.
Citation Format: Haiyan Chu, Jonathan M. Shillingford, Nikki Parker, Melissa Nelson, Jeremy F. Vaughn, Albert Felten, Yingjuan Lu, Joseph A. Reddy, Iontcho R. Vlahov, Christopher P. Leamon. Development and application of an immunohistochemistry-based assay for evaluating functional and accessible folate receptor expression in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4017. doi:10.1158/1538-7445.AM2017-4017