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Abstract
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 K d of the tracer ranged in the low nanomolar range (18.9 ± 11.3 nmol/L). 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.Significance: A novel peptide-based PET tracer against the oligomerization domain of Hsp70 has potential for universal tumor-specific imaging in vivo across many tumor type.

Author information

Stangl S#1, Tei L#2, De Rose F3, Reder S3, Martinelli J2, Sievert W1, Shevtsov M1,4,5, Öllinger R6, Rad R6, Schwaiger M3, D'Alessandria C#3, Multhoff G7.
1
Radiation Immuno Oncology Group, Center for Translational Cancer Research (TranslaTUM), Campus Klinikum rechts der Isar, Technische Universität München (TUM), Munich, Germany.
2
Dipartimento di Scienze e Innovazione Tecnologica, Università del Piemonte Orientale "A. Avogadro", Alessandria, Italy.
3
Department of Nuclear Medicine, Klinikum rechts der Isar, Technische Universität München (TUM), Munich, Germany.
4
Institute of Cytology of the Russian Academy of Sciences (RAS), St. Petersburg, Russia.
5
Pavlov First Saint Petersburg State Medical University, St. Petersburg, Russia.
6
Medical Department II, Translational Gastroenterological Oncology, Centre for Translational Cancer Research (TranslaTUM), Klinikum rechts der Isar, Technische Universität München, Munich, Germany.
7
Radiation Immuno Oncology Group, Center for Translational Cancer Research (TranslaTUM), Campus Klinikum rechts der Isar, Technische Universität München (TUM), Munich, Germany. Gabriele.Multhoff@tum.de.
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Contributed equally