Quantitative analysis of [11C]-erlotinib PET demonstrates specific binding for activating mutations of the EGFR kinase domain

Abstract

Activating mutations of the epidermal growth factor receptor (EGFR) occur in multiple tumor types, including non-small cell lung cancer (NSCLC) and malignant glioma, and have become targets for therapeutic intervention. The determination of EGFR mutation status using a noninvasive, molecular imaging approach has the potential for clinical utility. In this study, we investigated [(11)C]-erlotinib positron emission tomography (PET) imaging as a tool to identify activating mutations of EGFR in both glioma and NSCLC xenografts. Radiotracer specific binding was determined for high and low specific activity (SA) [(11)C]-erlotinib PET scans in mice bearing synchronous human cancer xenografts with different EGFR expression profiles (PC9, HCC827, U87, U87 ΔEGFR, and SW620). Although xenograft immunohistochemistry demonstrated constitutive EGFR phosphorylation, PET scan analysis using the Simplified Reference Tissue Model showed that only kinase domain mutant NSCLC (HCC827 and PC9) had significantly greater binding potentials in high versus low SA scans. Xenografts with undetectable EGFR expression (SW620), possessing wild-type EGFR (U87), and expressing an activating extracellular domain mutation (U87 ΔEGFR) were indistinguishable under both high and low SA scan conditions. The results suggest that [(11)C]-erlotinib is a promising radiotracer that could provide a novel clinical methodology for assessing EGFR and erlotinib interactions in patients with tumors that harbor EGFR-activating kinase domain mutations.

Figures

Figure 1

Xenograft tumor models for [11C]-erlotinib PET imaging. (A) Xenograft IHC using EGFR and tyrosine (Y) 1173 phosphorylation-specific antibodies. Representative images from colon cancer-derived (SW620) and NSCLC-derived (HCC827 and PC9) xenografts are shown. (B) IHC of EGFR-phosphorylated Y1173 in U87 glioma xenografts with and without expression of the extracellular domain-truncated ΔEGFR. (C) Western blot analysis demonstrating constitutive or EGF-induced EGFR Y1173 phosphorylation in U87 cell lines.

Figure 2

[11C]-erlotinib imaging of a mouse bearing bilateral HCC827 and PC9 bilateral flank xenografts. (A) Transmission scan of the mouse used to gain anatomic information for ROI placement. (B) [11C]-erlotinib PET early time image summed from 0 to 10 minutes. PET images are smoothed with a 1.0-mm3 kernel for illustrative purposes. (C) [11C]-erlotinib PET late time image summed from 95 to 120 minutes. (D) ROIs (blue) corresponding to flank xenograft location used for data acquisition and TAC analysis.

Figure 3

[11C]-erlotinib TACs in mice bearing multiple tumor xenografts for mice imaged under high (A and B) or low (C and D) SA conditions. TACs for HCC827 (squares), PC9 (diamonds), and SW620 (triangles) or U87 (asterisks) and U87Δ (closed circles) xenografts with muscle (open circles) used as reference. Data points are SUVs with SD, and overlaid lines depict SRTM curve fits.

Figure 4

Average BP of [11C]-erlotinib determined by SRTM in both high SA (black) and low SA (white) conditions. (A) Comparison of average BPs for SW620, HCC827, and PC9 xenografts. (B) Average BPs for U87 and U87 ΔEGFR tumors. Error bars represent the SD. Significant differences are reported in Results section.