Data from Preclinical Evaluation of the Hsp70 Peptide Tracer TPP-PEG<sub>24</sub>-DFO[<sup>89</sup>Zr] for Tumor-Specific PET/CT Imaging

Abstract

<div>Abstract<p>High precision <i>in vivo</i> PET/CT imaging of solid tumors improves diagnostic credibility and clinical outcome of patients. An epitope of the oligomerization domain of Hsp70 is exclusively exposed on the membrane of a large variety of tumor types, but not on normal cells, and thus provides a universal tumor-specific target. Here we developed a novel PET tracer TPP-PEG<sub>24</sub>-DFO[<sup>89</sup>Zr] based on the tumor cell–penetrating peptide probe TPP, which specifically recognizes membrane Hsp70 (mHsp70) on tumor cells. The implemented PEG<sub>24</sub> moiety supported tracer stability and improved biodistribution characteristics <i>in vivo</i>. The <i>K</i><sub>d</sub> of the tracer ranged in the low nanomolar range (18.9 ± 11.3 nmol/L). Fluorescein isothiocyanate (FITC)-labeled derivatives TPP-[FITC] and TPP-PEG<sub>24</sub>-[FITC] revealed comparable and specific binding to mHsp70-positive 4T1, 4T1<sup>+</sup>, a derivative of the 4T1 cell line sorted for high Hsp70 expression, and CT26 tumor cells, but not to mHsp70-negative normal fibroblasts. The rapid internalization kinetics of mHsp70 into the cytosol and the favorable biodistribution of the peptide-based tracer TPP-PEG<sub>24</sub>-DFO[<sup>89</sup>Zr] <i>in vivo</i> enabled a tumor-specific accumulation with a high tumor-to-background contrast and renal body clearance. The tumor-specific enrichment of the tracer in 4T1<sup>+</sup> (6.2 ± 1.1%ID/g), 4T1 (4.3 ± 0.7%ID/g), and CT26 (2.6 ± 0.6%ID/g) mouse tumors with very high, high, and intermediate mHsp70 densities, respectively, reflected mHsp70 expression profiles of the different tumor types, whereas benign mHsp70-negative fibroblastic hyperplasia showed no tracer accumulation (0.2 ± 0.03%ID/g). The ability of our chemically optimized peptide-based tracer TPP-PEG<sub>24</sub>-DFO[<sup>89</sup>Zr] to detect mHsp70 <i>in vivo</i> suggests its broad applicability in targeting and imaging with high specificity for any tumor type that exhibits surface expression of Hsp70.</p><p><b>Significance:</b> A novel peptide-based PET tracer against the oligomerization domain of Hsp70 has potential for universal tumor-specific imaging <i>in vivo</i> across many tumor type. <i>Cancer Res; 78(21); 6268–81. ©2018 AACR</i>.</p></div>