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Background: Gallium-68 positron emission tomography (68Ga-PET) with the two registered somatostatin analogs, [68Ga]Ga-DOTA-Tyr3-octreotide ([68Ga]Ga-DOTA-TOC) and [68Ga]Ga-DOTA-Tyr3-octreotate ([68Ga]Ga-DOTA-TATE), where DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, is routinely used for imaging of somatostatin receptor (SST)-expressing tumors. We investigated copper-61 (61Cu) as an alternative radiometal for PET imaging of SST-expressing tumors. Compared to gallium-68, copper-61 (t1/2=3.33 h, Eß + max = 1.22 MeV) can be produced on a large scale, enables late time point imaging, and has the therapeutic twin copper-67. Herein, DOTA-TOC and 1,4, 7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA)-TOC were labeled with copper-61 and compared with the clinically used [68Ga]Ga-DOTA-TOC. Methods: [61Cu]CuCl2 was produced from an irradiated natural nickel target. DOTA-TOC and NODAGA-TOC were labeled with [61Cu]CuCl2 in ammonium acetate buffer so to achieve a reaction pH of 5-6 and a temperature of 95°C for DOTA-TOC or room temperature for NODAGA-TOC. The radioligands were evaluated head-to-head in vitro using human embryonic kidney (HEK)-SST2 cells (affinity, binding sites, cellular uptake, and efflux) and in vivo using HEK-SST2 xenografts [PET/computed tomography (CT) imaging, biodistribution, and pharmacokinetics] and compared with [68Ga]Ga-DOTATOC, which was prepared using a standard procedure. Dosimetry estimates were made for [61Cu]Cu-NODAGA-TOC. Results: [61Cu]Cu-DOTA-TOC and [61Cu]Cu-NODAGA-TOC were prepared at an apparent molar activity of 25 MBq/nmol with radiochemical purities of ≥96% and ≥98%, respectively. In vitro, both presented a sub-nanomolar affinity for SST2 (IC50 = 0.23 and 0.34 nM, respectively). They were almost entirely internalized upon binding to SST2-expressing cells and had similar efflux rates at 37°C. In vivo, [61Cu]Cu-DOTA-TOC and [61Cu]Cu-NODAGATOC showed the same accumulation in SST2-expressing tumors. However, PET/CT images and biodistribution analyses clearly showed an unfavorable biodistribution for [61Cu]Cu-DOTA-TOC, characterized by accumulation in the liver and the abdomen. [61Cu]Cu-NODAGA-TOC displayed favorable biodistribution, comparable with [68Ga]Ga-DOTA-TOC at 1 h post-injection (p.i.). Notwithstanding, [61Cu]Cu-NODAGA-TOC showed advantages at 4 h p.i., due to the tumor retention and improved tumor-to-non-tumor ratios. The effective dose (2.41 × 10-3 mSv/MBq) of [61Cu]Cu-NODAGA-TOC, but also the dose to the other organs and the kidneys (9.65 × 10-2 mGy/MBq), suggested a favorable safety profile. Conclusion: Somatostatin receptor 61Cu-PET imaging not only matches the performance of 68Ga-PET at 1 h p.i. but has advantages in late-time imaging at 4 h p.i., as it provides improved tumor-to-non-tumor ratios. [61Cu]Cu-NODAGATOC is superior to [61Cu]Cu-DOTA-TOC in vivo. The use of the chelator NODAGA allows quantitative labeling with copper-61 at room temperature and enables the straightforward use of a kit formulation for simple manufacturing in medical centers. [ABSTRACT FROM AUTHOR]

Background: Radiohybrid PSMA-targeted ligands (rhPSMA) have been introduced as a novel platform for theranostic applications. Among a variety of rhPSMA-ligands developed for radioligand therapy, two stereoisomers [177Lu]Lu-rhPSMA-10.1 and -10.2 have been synthesized and initially characterized in preclinical experiments with the aim to provide an optimized binding profile to human serum albumin, a reduction of charge, and thus accelerated kidney excretion, and unaffected or even improved tumor uptake. As both isomers showed similar in vitro characteristics and tumor uptake at 24 h post injection in tumor bearing mice and in order to identify the isomer with the most favorable pharmacokinetics for radioligand therapy, we carried out in-depth biodistribution and dosimetry studies in tumor-bearing and healthy mice. Results: rhPSMA-10.1 and -10.2 were radiolabeled with lutetium-177 according to the established procedures of other DOTA-based PSMA ligands and displayed a high and comparable stability in all buffers and human serum (> 97%, 24 h). Biodistribution studies revealed fast clearance from the blood pool (0.3–0.6%ID/g at 1 h) and other background tissues within 48 h. Distinctive differences were found in the kidneys, where [177Lu]Lu-rhPSMA-10.1 displayed lower initial uptake and faster excretion kinetics compared to [177Lu]Lu-rhPSMA-10.2 expressed by a 1.5-fold and ninefold lower uptake value at 1 h and 24 h in healthy animals, respectively. Tumor uptake was comparable and in the range of 8.6–11.6%ID/g for both isomers over 24 h and was maintained up to 168 h at a level of 2.2 ± 0.8 and 4.1 ± 1.4%ID/g for [177Lu]Lu-rhPSMA-10.1 and [177Lu]Lu-rhPSMA-10.2, respectively. Conclusion: Our preclinical data on biodistribution and dosimetry indicate a more favorable profile of [177Lu]Lu-rhPSMA-10.1 compared to [177Lu]Lu-rhPSMA-10.2 for PSMA-targeted radioligand therapy. [177Lu]Lu-rhPSMA-10.1 shows fast kidney clearance kinetics resulting in excellent tumor-to-organ ratios over a therapy relevant time course. Meanwhile, [177Lu]Lu-rhPSMA-10.1 is currently being investigated in clinical phase I/II studies in patients with mCRPC (NCT05413850), in patients with high-risk localized PC (NCT06066437, Nautilus Trial) and after external beam radiotherapy (NCT06105918). [ABSTRACT FROM AUTHOR]

Background: The lack of facile methods for the specific characterization of malignant thyroid nodules makes the diagnosis of thyroid cancer (TC) challenging. Due to its restricted expression in such nodules, the cell-associated lectin galectin-3 (Gal3) has emerged as a marker for TC with growing interest for in vivo imaging as well as targeted radionuclide therapy. To accelerate translation into clinical application, we have developed a cognate chimeric human antigen-binding fragment (Fab) derived from the rat anti-Gal3 monoclonal antibody M3/38. Methods: The variable immunoglobulin (Ig) light and heavy chain sequences were cloned from the hybridoma cell line, and the corresponding Fab carrying human IgG1/κ constant genes was functionally produced in the periplasm of Escherichia coli and purified to homogeneity. To moderately prolong its plasma half-life and, thus, increase tumor uptake, the recombinant Fab was fused with a long disordered amino acid chain comprising in total 200 Pro, Ala, and Ser residues (PASylation). This novel tracer was subjected to in vitro characterization and in vivo validation by using two thyroid cancer orthotopic murine models. To this end, the αGal3-Fab-PAS200 was conjugated with deferoxamine (Dfo), labeled with 89Zr under mild conditions and tested for binding on TC cell lines. Athymic nude mice were inoculated either with FRO82-1 or with CAL62 tumor cells into the left thyroid lobe. After intravenous injection with ∼3.0 MBq of 89Zr-Dfo-PAS200-Fab, these mice were subjected to positron emission tomography (PET)/computed tomography imaging followed by quantification of tumor accumulation and immunohistochemical analysis. Results: The αGal3-Fab-PAS200 revealed high affinity toward the recombinant Gal3 antigen, with a dissociation constant ≤1 nM as measured via enzyme-linked immunosorbent assay, surface plasmon resonance spectroscopy, and radioactive cell binding assay. The in vivo Gal3-targeting by the 89Zr(IV)-labeled protein tracer, as investigated by immuno-PET, demonstrated highly selective and fast accumulation in orthotopically implanted tumors, with strong contrast images achieved 24 hours postinjection, and no uptake in the tumor-free thyroid lobe, as also confirmed by biodistribution studies. Conclusions: The chimeric αGal3 89Zr-Dfo-PAS200-Fab tracer exhibits selective accumulation in the tumor-bearing thyroid lobe of xenograft mice. Thus, this novel radioactive probe offers potential to change TC management, in addition to current diagnostic procedures, and to reduce unnecessary thyroidectomies. [ABSTRACT FROM AUTHOR]

In this study, we used cytotoxicity assays, proteomic and gene expression analyses to examine the difference in response of A549 cells to two silica particles that differ in physical properties, namely cristobalite (CR) and α-quartz (Min-U-Sil 5, MI). Cytotoxicity assays such as lactate dehydrogenase release, 5-bromo-2′-deoxyuridine incorporation and cellular ATP showed that both silica particles could cause cell death, decreased cell proliferation and metabolism in the A549 human lung epithelial cells. While cytotoxicity assays revealed little difference between CR and MI exposures, proteomic and gene expression analyses unveiled both similar and unique molecular changes in A549 cells. For instance, two-dimensional gel electrophoresis data indicated that the expression of proteins in the cell death (e.g., ALDH1A1, HTRA2 and PRDX6) and cell proliferation (e.g., FSCN1, HNRNPAB and PGK1) pathways were significantly different between the two silica particles. Reverse transcription-polymerase chain reaction data provided additional evidence supporting the proteomic findings. Preliminary assessment of the physical differences between CR and MI suggested that the extent of surface interaction between particles and cells could explain some of the observed biological effects. However, the differential dose-response curves for some other genes and proteins suggest that other physical attributes of particulate matter can also contribute to particulate matter-related cellular toxicity. Our results demonstrated that toxicoproteomic and gene expression analyses are sensitive in distinguishing subtle toxicity differences associated with silica particles of varying physical properties compared to traditional cytotoxicity endpoints. Copyright © 2016 Her Majesty the Queen in Right of Canada. Journal of Applied Toxicology published by John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In this work, we designed, developed, characterized, and investigated a new chelator and its bifunctional derivative for 89Zr labeling and PET-imaging. In a preliminary study, we synthesized two hexadentate chelators named AAZTHAS and AAZTHAG, based on the seven-membered heterocycle AMPED (6-amino-6-methylperhydro-1,4-diazepine) with the aim to increase the rigidity of the 89Zr complex by using N-methyl-N-(hydroxy)succinamide or N-methyl-N-(hydroxy)glutaramide pendant arms attached to the cyclic structure. N-methylhydroxamate groups are the donor groups chosen to efficiently coordinate 89Zr. After in vitro stability tests, we selected the chelator with longer arms, AAZTHAG, as the best complexing agent for 89Zr presenting a stability of 86.4 ± 5.5% in human serum (HS) for at least 72 h. Small animal PET/CT static scans acquired at different time points (up to 24 h) and ex vivo organ distribution studies were then carried out in healthy nude mice (n = 3) to investigate the stability and biodistribution in vivo of this new 89Zr-based complex. High stability in vivo, with low accumulation of free 89Zr in bones and kidneys, was measured. Furthermore, an activated ester functionalized version of AAZTHAG was synthesized to allow the conjugation with biomolecules such as antibodies. The bifunctional chelator was then conjugated to the human anti-HER2 monoclonal antibody Trastuzumab (Tz) as a proof of principle test of conjugation to biologically active molecules. The final 89Zr labeled compound was characterized via radio-HPLC and SDS-PAGE followed by autoradiography, and its stability in different solutions was assessed for at least 4 days. [ABSTRACT FROM AUTHOR]

Background: Gallium-68 positron emission tomography ( 68 Ga-PET) with the two registered somatostatin analogs, [ 68 Ga]Ga-DOTA-Tyr 3 -octreotide ([ 68 Ga]Ga-DOTA-TOC) and [ 68 Ga]Ga-DOTA-Tyr 3 -octreotate ([ 68 Ga]Ga-DOTA-TATE), where DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, is routinely used for imaging of somatostatin receptor (SST)-expressing tumors. We investigated copper-61 ( 61 Cu) as an alternative radiometal for PET imaging of SST-expressing tumors. Compared to gallium-68, copper-61 (t 1/2  = 3.33 h, E β + max  = 1.22 MeV) can be produced on a large scale, enables late time point imaging, and has the therapeutic twin copper-67. Herein, DOTA-TOC and 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA)-TOC were labeled with copper-61 and compared with the clinically used [ 68 Ga]Ga-DOTA-TOC., Methods: [ 61 Cu]CuCl 2 was produced from an irradiated natural nickel target. DOTA-TOC and NODAGA-TOC were labeled with [ 61 Cu]CuCl 2 in ammonium acetate buffer so to achieve a reaction pH of 5-6 and a temperature of 95°C for DOTA-TOC or room temperature for NODAGA-TOC. The radioligands were evaluated head-to-head in vitro using human embryonic kidney (HEK)-SST 2 cells (affinity, binding sites, cellular uptake, and efflux) and in vivo using HEK-SST 2 xenografts [PET/computed tomography (CT) imaging, biodistribution, and pharmacokinetics] and compared with [ 68 Ga]Ga-DOTA-TOC, which was prepared using a standard procedure. Dosimetry estimates were made for [ 61 Cu]Cu-NODAGA-TOC., Results: [ 61 Cu]Cu-DOTA-TOC and [ 61 Cu]Cu-NODAGA-TOC were prepared at an apparent molar activity of 25 MBq/nmol with radiochemical purities of ≥96% and ≥98%, respectively. In vitro , both presented a sub-nanomolar affinity for SST 2 (IC 50  = 0.23 and 0.34 nM, respectively). They were almost entirely internalized upon binding to SST 2 -expressing cells and had similar efflux rates at 37°C. In vivo , [ 61 Cu]Cu-DOTA-TOC and [ 61 Cu]Cu-NODAGA-TOC showed the same accumulation in SST 2 -expressing tumors. However, PET/CT images and biodistribution analyses clearly showed an unfavorable biodistribution for [ 61 Cu]Cu-DOTA-TOC, characterized by accumulation in the liver and the abdomen. [ 61 Cu]Cu-NODAGA-TOC displayed favorable biodistribution, comparable with [ 68 Ga]Ga-DOTA-TOC at 1 h post-injection (p.i.). Notwithstanding, [ 61 Cu]Cu-NODAGA-TOC showed advantages at 4 h p.i., due to the tumor retention and improved tumor-to-non-tumor ratios. The effective dose (2.41 × 10 -3  mSv/MBq) of [ 61 Cu]Cu-NODAGA-TOC, but also the dose to the other organs and the kidneys (9.65 × 10 -2  mGy/MBq), suggested a favorable safety profile., Conclusion: Somatostatin receptor 61 Cu-PET imaging not only matches the performance of 68 Ga-PET at 1 h p.i. but has advantages in late-time imaging at 4 h p.i., as it provides improved tumor-to-non-tumor ratios. [ 61 Cu]Cu-NODAGA-TOC is superior to [ 61 Cu]Cu-DOTA-TOC in vivo . The use of the chelator NODAGA allows quantitative labeling with copper-61 at room temperature and enables the straightforward use of a kit formulation for simple manufacturing in medical centers., Competing Interests: MF acts as a scientific advisor of Nuclidium AG and is co-inventor on patent applications filed by Nuclidium AG and the University of Basel related to 61Cu-labeled tracers. FDR and LJ-T are employees of Nuclidium AG and co-inventors in a series of patent related to 61Cu-labeled tracers. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors declare that this study received funding from Nuclidium AG. The funder had the following involvement in the study: Production of copper-61 and involvement in the study design and project supervision., (© 2024 Basaco Bernabeu, Mansi, Del Pozzo, Gaonkar, McDougall, Johayem, Blagoev, De Rose, Jaafar-Thiel and Fani.)

The demand for PET tracers that target prostate-specific membrane antigen (PSMA) continues to increase. Meeting this demand with approved 68 Ga- and 18 F-labeled PSMA tracers is challenging outside of major urban centers. This is because the short physical half-life of these radionuclides makes it necessary to produce them near their sites of usage. To overcome this challenge, we propose cyclotron-produced 61 Cu for labeling PSMA PET tracers. 61 Cu can be produced on a large scale, and its 3.33-h half-life allows shipping over considerably longer distances than possible for 68 Ga and 18 F. Production of true theranostic twins using 61 Cu and the β - -emitter 67 Cu is also feasible. Methods: PSMA-I&T (DOTAGA-(l-y)fk(sub-KuE)) and its derivative in which the DOTAGA chelator was replaced by NODAGA (NODAGA-(l-y)fk(sub-KuE)), herein reported as DOTAGA-PSMA-I&T and NODAGA-PSMA-I&T, respectively, were labeled with 61 Cu and compared with [ 68 Ga]Ga-DOTAGA-PSMA-I&T, [ 68 Ga]Ga-NODAGA-PSMA-I&T, [ 68 Ga]Ga-PSMA-11, and [ 18 F]PSMA-1007. In vitro (lipophilicity, affinity, cellular uptake, and distribution) and in vivo (PET/CT, biodistribution, and stability) studies were performed in LNCaP cells and xenografts. Human dosimetry estimates were calculated for [ 61 Cu]Cu-NODAGA-PSMA-I&T. First-in-human imaging with [ 61 Cu]Cu-NODAGA-PSMA-I&T was performed in a patient with metastatic prostate cancer. Results: [ 61 Cu]Cu-DOTAGA-PSMA-I&T and [ 61 Cu]Cu-NODAGA-PSMA-I&T were synthesized with radiochemical purity of more than 97%, at an apparent molar activity of 24 MBq/nmol, without purification after labeling. In vitro, natural Cu ( nat Cu)-DOTAGA-PSMA-I&T and nat Cu-NODAGA-PSMA-I&T showed high affinity for PSMA (inhibitory concentration of 50%, 11.2 ± 2.3 and 9.3 ± 1.8 nM, respectively), although lower than the reference nat Ga-PSMA-11 (inhibitory concentration of 50%, 2.4 ± 0.4 nM). Their cellular uptake and distribution were comparable to those of [ 68 Ga]Ga-PSMA-11. In vivo, [ 61 Cu]Cu-NODAGA-PSMA-I&T showed significantly lower uptake in nontargeted tissues than [ 61 Cu]Cu-DOTAGA-PSMA-I&T and higher tumor uptake (14.0 ± 5.0 percentage injected activity per gram of tissue [%IA/g]) than [ 61 Cu]Cu-DOTAGA-PSMA-I&T (6.06 ± 0.25 %IA/g, P = 0.0059), [ 68 Ga]Ga-PSMA-11 (10.2 ± 1.5 %IA/g, P = 0.0972), and [ 18 F]PSMA-1007 (9.70 ± 2.57 %IA/g, P = 0.080) at 1 h after injection. Tumor uptake was also higher for [ 61 Cu]Cu-NODAGA-PSMA-I&T at 4 h after injection (10.7 ± 3.3 %IA/g) than for [ 61 Cu]Cu-DOTAGA-PSMA-I&T (4.88 ± 0.63 %IA/g, P = 0.0014) and [ 18 F]PSMA-1007 (6.28 ± 2.19 %IA/g, P = 0.0145). Tumor-to-nontumor ratios of [ 61 Cu]Cu-NODAGA-PSMA-I&T were superior to those of [ 61 Cu]Cu-DOTAGA-PSMA-I&T and comparable to those of [ 68 Ga]Ga-PSMA-11 and [ 18 F]PSMA-1007 at 1 h after injection and increased significantly between 1 and 4 h after injection in most cases. Human dosimetry estimates for [ 61 Cu]Cu-NODAGA-PSMA-I&T were similar to the ones reported for 18 F-PSMA ligands. First-in-human imaging demonstrated multifocal osseous and hepatic metastases. Conclusion: [ 61 Cu]Cu-NODAGA-PSMA-I&T is a promising PSMA radiotracer that compares favorably with [ 68 Ga]Ga-PSMA-11 and [ 18 F]PSMA-1007, while allowing delayed imaging., (© 2024 by the Society of Nuclear Medicine and Molecular Imaging.)

Rationale: The high expression of Galectin-3 (Gal3) in macrophages of atherosclerotic plaques suggests its participation in atherosclerosis pathogenesis, and raises the possibility to use it as a target to image disease severity in vivo . Here, we explored the feasibility of tracking atherosclerosis by targeting Gal3 expression in plaques of apolipoprotein E knockout (ApoE-KO) mice via PET imaging. Methods: Targeting of Gal3 in M0-, M1- and M2 (M2a/M2c)-polarized macrophages was assessed in vitro using a Gal3-F(ab') 2 mAb labeled with AlexaFluor®488 and 89 Zr- desferrioxamine-thioureyl-phenyl-isothiocyanate (DFO). To visualize plaques in vivo , ApoE-KO mice were injected i.v. with 89 Zr-DFO-Gal3-F(ab') 2 mAb and imaged via PET/CT 48 h post injection. Whole length aortas harvested from euthanized mice were processed for Sudan-IV staining, autoradiography, and immunostaining for Gal3, CD68 and α-SMA expression. To confirm accumulation of the tracer in plaques, ApoE-KO mice were injected i.v. with Cy5.5-Gal3-F(ab') 2 mAb, euthanized 48 h post injection, followed by cryosections of the body and acquisition of fluorescent images. To explore the clinical potential of this imaging modality, immunostaining for Gal3, CD68 and α-SMA expression were carried out in human plaques. Single cell RNA sequencing (scRNA-Seq) analyses were performed to measure LGALS3 (i.e. a synonym for Gal3) gene expression in each macrophage of several subtypes present in murine or human plaques. Results: Preferential binding to M2 macrophages was observed with both AlexaFluor®488-Gal3-F(ab') 2 and 89 Zr-DFO-Gal3-F(ab') 2 mAbs. Focal and specific 89 Zr-DFO-Gal3-F(ab') 2 mAb uptake was detected in plaques of ApoE-KO mice by PET/CT. Autoradiography and immunohistochemical analyses of aortas confirmed the expression of Gal3 within plaques mainly in macrophages. Moreover, a specific fluorescent signal was visualized within the lesions of vascular structures burdened by plaques in mice. Gal3 expression in human plaques showed similar Gal3 expression patterns when compared to their murine counterparts. Conclusions: Our data reveal that 89 Zr-DFO-Gal3-F(ab') 2 mAb PET/CT is a potentially novel tool to image atherosclerotic plaques at different stages of development, allowing knowledge-based tailored individual intervention in clinically significant disease., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)

Positron emission tomography (PET) is a non-invasive molecular imaging technology that is constantly expanding, with a high demand for specific antibody-derived imaging probes. The use of tracers based on temperature-sensitive molecules (i. e. Fab, svFab, nanobodies) is increasing and has led us to design a class of chelators based on the structure of 2-aminomethylpiperidine (AMP) with acetic and/or hydroxybenzyl pendant arms (2-AMPTA, NHB-2-AMPDA, and 2-AMPDA-HB), which were investigated as such for {Al 18 F} 2+ -core chelation efficiency. All the compounds were characterized by HPLC-MS analysis and NMR spectroscopy. The AlF-18 labeling reactions were performed under various conditions (pH/temperature), and the radiolabeled chelates were purified and characterized by radio-TLC and radio-HPLC. The stability of labeled chelates was investigated up to 240 min in human serum (HS), EDTA 5 mM, PBS and 0.9 % NaCl solutions. The in vivo stability of [Al 18 F(2-AMPDA-HB)] - was assessed in healthy nude mice (n=6). Radiochemical yields between 55 % and 81 % were obtained at pH 5 and room temperature. High stability in HS was measured for [Al 18 F(2-AMPDA-HB)] - , with 90 % of F-18 complexed after 120 min. High stability in vivo, rapid hepatobiliary and renal excretion, with low accumulation of free F-18 in bones were measured. Thus, this new Al 18 F-chelator may have a great impact on immuno-PET radiopharmacy, by facilitating the development of new fluorine-18-labeled heat-sensitive biomolecules., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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-PEG 24 -DFO[ 89 Zr] based on the tumor cell-penetrating peptide probe TPP, which specifically recognizes membrane Hsp70 (mHsp70) on tumor cells. The implemented PEG 24 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-PEG 24 -[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-PEG 24 -DFO[ 89 Zr] 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-PEG 24 -DFO[ 89 Zr] 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. Cancer Res; 78(21); 6268-81. ©2018 AACR ., (©2018 American Association for Cancer Research.)