Abstract 3578: Using a novel [68Ga]-radiolabeled peptide to detect cell surface expression of calreticulin in pancreatic adenocarcinoma
Authors: Guenter Rachael , Ducharme Maxwell , Herring Brendon , Rao Tejeshwar C. , Montes Odalyz , McCaw Tyler , Chen Herbert , Lapi Suzanne E. , Larimer Benjamin , Rose J. Bart 
Published: April 2023

Background: Pancreatic ductal adenocarcinoma (PDAC) is the 3rd leading cause of cancer related death with a 5-year survival rate at 11%. New systemic therapies for patients with PDAC are desperately needed. Upregulation or exposure of a new protein on the tumor cell surface can serve as a therapeutic or diagnostic target. Here, we highlight our recently developed strategy to induce localization of the reticular protein calreticulin (CALR) to the cell surface in PDAC cells and its subsequent detection using a novel radiolabeled peptide.

Methods: Surface translocation of CALR was detected by flow cytometry, western blot, and total internal reflection fluorescence (TIRF) microscopy in PDAC cells treated with either doxorubicin or gemcitabine. The radionuclide-binding chelator ‘DOTA’ was covalently linked to a CALR-specific peptide ‘KLGFFKR’ and then labeled with 68Ga. Samples were analyzed on HPLC with an average radiolabeling efficiency of 93%. Mice bearing Panc02 allografts were treated with doxorubicin for 24h, injected with ~3 MBq (5 μg) of radiopeptide, and sacrificed after 1 hour to determine biodistribution.

Results: Using flow cytometry, we found that treating PDAC cells with doxorubicin or gemcitabine increased both the total number and the median fluorescence intensity of surface staining for live cells expressing CALR. When membrane proteins were isolated from PDAC cells treated with doxorubicin at various time points, a peak in surface CALR protein was detected at 30 minutes with persistent expression lasting for 24 hours. TIRF microscopy showed that Panc02 cells treated with doxorubicin had approximately a 2-fold higher surface CALR expression as detected by enhanced membrane fluorescence compared to controls. In vivo, our novel [68Ga]-CALR peptide showed rapid clearance through the kidneys with no significant uptake in vital organs (n=4). In Panc02 allograft-bearing mice treated intratumorally with either vehicle or doxorubicin, biodistribution analysis after radiopeptide injection showed a significant increase in radiopeptide uptake in the treated tumors (n=6, p &lt; 0.05).

Conclusions: CALR is translocated to the cell surface in PDAC cells, where it can subsequently be targeted by a novel radiopeptide agent. Future studies are needed to determine if induced CALR can be targeted for therapeutic effect.

Citation Format: Rachael Guenter, Maxwell Ducharme, Brendon Herring, Tejeshwar C. Rao, Odalyz Montes, Tyler McCaw, Herbert Chen, Suzanne E. Lapi, Benjamin Larimer, J. Bart Rose. Using a novel [68Ga]-radiolabeled peptide to detect cell surface expression of calreticulin in pancreatic adenocarcinoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3578.