Preclinical evaluation of the Hsp70 peptide tracer TPP-PEG24-DFO[89Zr] for tumor-specific PET/CT imaging

High precision in vivo 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-PEG24-DFO[89Zr] based on the tumor cell penetrating peptide probe TPP, which specifically recognizes membrane Hsp70 (mHsp70) on tumor cells. The implemented PEG24 moiety supported tracer stability and improved biodistribution characteristics in vivo. The KD of the tracer ranged in the low nanomolar range (18.9±11.3nM). Fluorescein-isothiocyanate (FITC)-labeled derivatives TPP-[FITC] and TPP-PEG24-[FITC] revealed comparable and specific binding to mHsp70-positive 4T1, 4T1+, 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-PEG24-DFO[89Zr] in vivo 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+ (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-PEG24-DFO[89Zr] to detect mHsp70 in vivo suggests its broad applicability in targeting and imaging with high specificity for any tumor type that exhibits surface expression of Hsp70.

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