High-throughput screening of molecular imaging agents using MicroPET

Abstract

77

Objectives: In vitro screening of molecular imaging agents isolated from chemical libraries can be used to identify new compounds for imaging. However, several promising compounds often remain even after stringent in vitro testing and only the most promising compound is selected for in vivo evaluation. While in vitro screening identifies promising lead compounds, in vitro success does not always correspond to in vivo success. Here we report the implementation of a high-throughput approach for the in vivo evaluation of a large number of ¹⁸F-radiolabeled peptides as molecular imaging agents. Methods: The one-bead-one-compound technique was used to create a library of peptides to target the α_vβ₆ integrin. After screening using a stringent on-bead assay, the identified sequences were subjected to ELISAs against the α_vβ₆ integrin. The sequences with the highest affinity and/or selectivity were chosen for in vivo evaluation. To this end, 4 peptides per day were radiolabeled with 4-[¹⁸F]fluorobenzoic acid (¹⁸F]FBA) using solid-phase radiolabeling techniques.¹ Each peptide was injected concomitantly via a tail vein catheter into 2 nude mice bearing both an α_vβ₆ positive and an α_vβ₆ negative tumor. The mice were positioned side by side on the scanner bed and shared the same heating console, while the anesthesia for each mouse was individually controlled. The mice were scanned for 1 hour following injection, and again at 3 hours for 15 minutes. Following this protocol, 4 different peptides were evaluated in vivo in 2 mice at 2 different time points per day. Results: A total of 43 peptides were radiolabeled with [¹⁸F]FBA and evaluated in vivo over the course of 11 consecutive days. Each peptide was evaluated in 2 mice concomitantly injected and scanned at 2 time points using MicroPET. The 4 most promising compounds identified from the in vivo screen would not have been evaluated based solely on in vitro analysis. Conclusions: The feasibility of using MicroPET for high-throughput in vivo studies was demonstrated. Lead compounds that target the α_vβ₆ integrin were identified from this in vivo screen. ¹Sutcliffe-Goulden JL, et. al.(2002). Rapid solid phase synthesis and biodistribution of 18F-labelled linear peptides. Eur J Nucl Med Mol Imaging;29:754-759.

Research Support (if any): NIH R21CA107792