Heart failure remains a major source of late morbidity and mortality after myocardial infarction (MI). The temporospatial presence of activated fibroblasts in the injured myocardium predicts the quality of cardiac remodeling after MI. Therefore, monitoring of activated fibroblasts is of great interest for studying cardiac remodeling after MI. Fibroblast activation protein (FAP) expression is upregulated in activated fibroblasts. This study investigated the feasibility of imaging activated fibroblasts with a new 68 Ga-labeled FAP inhibitor (68 Ga-FAPI-04) for PET imaging of fibroblast activation in a preclinical model of MI. Methods: MI and sham-operated rats were scanned with 68 Ga-FAPI-04 PET/CT (1, 3, 6, 14, 23, and 30 d after MI) and with 18 F-FDG (3 d after MI). Dynamic 68 Ga-FAPI-04 PET and blocking studies were performed on MI rats 7 d after coronary ligation. After in vivo scans, the animals were euthanized and their hearts harvested for ex vivo analyses. Cryosections were prepared for autoradiography, hematoxylin and eosin (H&E), and immunofluorescence staining. Results: 68 Ga-FAPI-04 uptake in the injured myocardium peaked on day 6 after coronary ligation. The tracer accumulated intensely in the MI territory, as identified by decreased 18 F-FDG uptake and confirmed by PET/MR and H&E staining. Autoradiography and H&E staining of cross-sections revealed that 68 Ga-FAPI-04 accumulated mainly at the border zone of the infarcted myocardium. In contrast, there was only minimal uptake in the infarct of the blocked rats, comparable to the uptake in the remote noninfarcted myocardium (PET image-derived ratio of infarct uptake to remote uptake: 6 ± 2). Immunofluorescence staining confirmed the presence of FAP-positive myofibroblasts in the injured myocardium. Morphometric analysis of the whole-heart sections demonstrated 3-and 8-fold higher FAP-positive fibroblast density in the border zone than in the infarct center and remote area, respectively. Conclusion: 68 Ga-FAPI-04 represents a promising radiotracer for in vivo imaging of post-MI fibroblast activation. Noninvasive imaging of activated fibroblasts may have significant diagnostic and prognostic value, which could aid clinical management of patients after MI.

a b s t r a c tAntibodies, which can recognize a plethora of possible antigens, have been considered as a paradigm of protein engineering performed by nature itself. Lipocalins constitute a distinct family of proteins with functions in ligand binding and transport that occur in many organisms, including man. Like antibodies, lipocalins exhibit a structurally conserved framework -a b-barrel with an attached ahelix -which supports four structurally hypervariable loops forming a cup-shaped binding site. Thus, lipocalins offer an ideal platform for protein engineering to generate novel binding reagents. Using recombinant/synthetic DNA technology and methods of combinatorial library selection, 'Anticalins' with prescribed target specificities can be easily generated. Anticalins with picomolar affinities have been developed for three classes of ligands having relevance in basic research and/ or medical application: small molecules, peptides, and proteinaceous signalling molecules as well as cell surface receptors. Anticalins derived from human lipocalins have already reached the clinical trial stage. Due to their very small size and simple composition of a single polypeptide chain, which also facilitates the construction of bifunctional fusion proteins, Anticalins promise benefits as a next class of biopharmaceuticals.

Although prostate carcinoma (PCa) is by far the most commonly diagnosed neoplasia in men, corresponding diagnostic and therapeutic modalities have limited efficacy at present. Anticalins comprise a novel class of binding proteins based on a non-immunoglobulin scaffold that can be engineered to specifically address molecular targets of interest. Here we report the selection and characterization of Anticalins that recognize human prostate-specific membrane antigen (PSMA), a membrane-tethered metallopeptidase constituting a disease-related target for imaging and therapy of PCa as well as solid malignancies in general. We used a randomized lipocalin library based on the human lipocalin 2 (Lcn2) scaffold together with phage display and ELISA screening to select PSMA-specific variants. Five Anticalin candidates from the original panning were expressed in Escherichia coli as soluble monomeric proteins, revealing affinities toward PSMA down to the low nanomolar range. Binding characteristics of the most promising candidate were further improved via affinity maturation by applying error-prone PCR followed by selection via phage display as well as bacterial surface display under more stringent conditions. In BIAcore measurements, the dissociation constant of the best Anticalin was determined as ∼500 pM, with a substantially improved dissociation rate compared with the first-generation candidate. Finally, immunofluorescence microscopy revealed specific staining of PSMA-positive tumor cell lines while flow cytometric analysis confirmed the ability of the selected Anticalins to detect PSMA on live cells. Taken together, Anticalins resulting from this study offer a viable alternative to antibody-based PSMA binders for biomedical applications, including in vivo imaging of PCa or neovasculature of solid tumors.

Our purpose was to define a clinically useful lower limit of injected dose for 68 Ga-prostate-specific membrane antigen (PSMA)-11 PET/ CT imaging of prostate cancer. Methods: 68 Ga-PSMA-11 PET/CT was performed on 11 patients. PET was acquired in list mode and reconstructed using a 3-min full acquisition, a 2-min acquisition, and a 1-min acquisition to generate images obtained with three thirds (standard dose), two thirds (low dose), and one third (very low dose) of the injected dose, respectively. Overall image quality (5-point scale) was assessed, and the detectability of PSMA-positive lesions was determined by 3 readers and compared with the reference standard. Results: Image quality declined with decreasing dose (mean score of 4.1 ± 0.4 for the standard dose, 3.4 ± 0.7 for the low dose, and 1.9 ± 0.4 for the very low dose; all P , 0.05). Readers 1, 2, and 3 correctly identified the lesions (n 5 21) at a rate of 100%, 100%, and 95% with the standard dose; 95%, 81%, and 90% with the low dose; and 71%, 76%, and 59% with the very low dose, respectively. Conclusion: 68 Ga-PSMA-11 dose reduction is not feasible without a negative impact on image quality and lesion detectability.

Blocking the interaction of the immune checkpoint molecules programmed cell death protein-1 (PD-1) and its ligand, PD-L1, using specific antibodies has been a major breakthrough for immune oncology.Whole-body PD-L1 expression positron emission tomography (PET) imaging may potentially allow for a better prediction of response to PD-1 targeted therapies. Imaging of PD-L1 expression is feasible by PET with the Adnectin protein 18 F-BMS-986192. However, radiofluorination of proteins, such as BMS-986192 remains complex and labelling yields are low. The goal of this study was therefore the development and preclinical evaluation of a 68 Ga-labeled Adnectin protein ( 68 Ga-BMS-986192) to facilitate clinical trials. Methods 68Ga-labeling of DOTA-conjugated Adnectin (BXA-206362) was carried out in NaOAc-buffer at pH 5.5 (50°C, 15min). In vitro stability in human serum at 37°C was analyzed using Radio-thin layer chromatography (Radio-TLC) and Radio-high performance liquid chromatography (Radio-HPLC). PD-L1 binding assays were performed using the transduced PD-L1 expressing lymphoma cell line U-698-M and wild-type U-698-M cells as negative control. Immunohistochemical staining studies, biodistribution and small animal PET studies of 68 Ga-BMS-986192 were carried out using PD-L1-positive and negative U-698-M-bearing NSG mice. Results 68Ga-BMS-986192 was obtained with quantitative radiochemical yields (RCYs) >97% and with high radiochemical purity (RCP). In vitro stability in human serum was ≥ 95% after 4h of incubation. High and specific binding of 68 Ga-BMS-986192 to human PD-L1-expressing cancer cells was confirmed, which closely correlates with the respective PD-L1 expression level determined by flow cytometry and IHC staining. In vivo, 68 Ga-BMS-986192 uptake was high in PD-L1+ tumors (9.0±2.1%ID/g at 1hp.i.) and kidneys (56.9±9.2% ID/g at 1hp.i.) with negligible uptake in other tissues. PD-L1 negative tumors 68 Ga-BMS-986192 for PD-L1 PET Imaging 3 demonstrated only background uptake of radioactivity (0.6±0.1% ID/g). Co-injection of an excess of unlabelled Adnectin reduced tumor uptake of PD-L1 by more than 80%. Conclusion 68Ga-BMS-986192 enables easy radiosynthesis and shows excellent in vitro and in vivo PD-L1 targeting characteristics. The high tumor uptake combined with low background accumulation at early imaging time points demonstrate the feasibility of 68 Ga-BMS-986192 for imaging of PD-L1 expression in tumors and is encouraging for further clinical applications of PD-L1 ligands.

Introduction The radiohybrid (rh) prostate-specific membrane antigen (PSMA)-targeted ligand [18F]Ga-rhPSMA-7 has previously been clinically assessed and demonstrated promising results for PET-imaging of prostate cancer. The ligand is present as a mixture of four stereoisomers ([18F]Ga-rhPSMA-7.1, − 7.2, − 7.3 and − 7.4) and after a preclinical isomer selection process, [18F]Ga-rhPSMA-7.3 has entered formal clinical trials. Here we report on the establishment of a fully automated production process for large-scale production of [18F]Ga-rhPSMA-7/ -7.3 under GMP conditions (EudraLex). Methods [18F]Fluoride in highly enriched [18O]H2O was retained on a strong anion exchange cartridge, rinsed with anhydrous acetonitrile and subsequently eluted with a solution of [K+ ⊂ 2.2.2]OH− in anhydrous acetonitrile into a reactor containing Ga-rhPSMA ligand and oxalic acid in DMSO. 18F-for-19F isotopic exchange at the Silicon-Fluoride Acceptor (SiFA) was performed at room temperature, followed by dilution with buffer and cartridge-based purification. Optimum process parameters were determined on the laboratory scale and thereafter implemented into an automated synthesis. Data for radiochemical yield (RCY), purity and quality control were analyzed for 243 clinical productions (160 for [18F]Ga-rhPSMA-7; 83 for [18F]Ga-rhPSMA-7.3). Results The automated production of [18F]Ga-rhPSMA-7 and the single isomer [18F]Ga-rhPSMA-7.3 is completed in approx. 16 min with an average RCY of 49.2 ± 8.6% and an excellent reliability of 98.8%. Based on the different starting activities (range: 31–130 GBq, 89 ± 14 GBq) an average molar activity of 291 ± 62 GBq/μmol (range: 50–450 GBq/μmol) was reached for labeling of 150 nmol (231 μg) precursor. Radiochemical purity, as measured by radio-high performance liquid chromatography and radio-thin layer chromatography, was 99.9 ± 0.2% and 97.8 ± 1.0%, respectively. Conclusion This investigation demonstrates that 18F-for-19F isotopic exchange is well suited for the fast, efficient and reliable automated routine production of 18F-labeled PSMA-targeted ligands. Due to its simplicity, speed and robustness the development of further SiFA-based radiopharmaceuticals is highly promising and can be of far-reaching importance for future theranostic concepts.

Purpose PASylation® offers the ability to systematically tune and optimize the pharmacokinetics of protein tracers for molecular imaging. Here we report the first clinical translation of a PASylated Fab fragment (89 Zr•Df-HER2-Fab-PAS 200) for the molecular imaging of tumor-related HER2 expression. Methods A patient with HER2-positive metastatic breast cancer received 37 MBq of 89 Zr•Df-HER2-Fab-PAS 200 at a total mass dose of 70 μg. PET/CT was carried out 6, 24, and 45 h after injection, followed by image analysis of biodistribution, normal organ uptake, and lesion targeting. Results Images show a biodistribution typical for protein tracers, characterized by a prominent blood pool 6 h p.i., which decreased over time. Lesions were detectable as early as 24 h p.i. 89 Zr•Df-HER2-Fab-PAS 200 was tolerated well. Conclusion This study demonstrates that a PASylated Fab tracer shows appropriate blood clearance to allow sensitive visualization of small tumor lesions in a clinical setting.

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