Zohreh Varasteh

Cardiac transthyretin amyloidosis is an infiltrative cardiomyopathy with high mortality. To date, there are no specific biomarkers to directly assess disease activity and response to specific treatments. Our aim was to evaluate scintigraphic changes after treatment with the transthyretin stabilizer tafamidis. Methods: We included patients who had undergone ^99mTc-3,3-diphosphono-1,2-propanodicarboxylic acid (^99mTc-DPD) scintigraphy before tafamidis initiation and after at least 9 mo. Tracer activity was assessed visually and quantitatively as SUV_max. Results: The study included 14 patients who were on tafamidis for 44 ± 14 mo. We observed regression of Perugini grade in 5 patients, unchanged grade in 9 patients, and regression of mean heart–to–contralateral-lung ratio (P = 0.015) and SUV_max (P = 0.005). There were no changes in N-terminal pro-B-type natriuretic peptide or echocardiographic measures. Conclusion: Treatment with tafamidis results in regression of myocardial ^99mTc-DPD uptake. ^99mTc-DPD scintigraphy may provide useful imaging biomarkers to assess response to treatment. Visual Abstract

The cardiac bioavailability of peptide drugs that inhibit harmful intracellular protein–protein interactions in cardiovascular diseases remains a challenging task in drug development. This study investigates whether a non-specific cell-targeted peptide drug is available in a timely manner at its intended biological destination, the heart, using a combined stepwise nuclear molecular imaging approach. An octapeptide (heart8P) was covalently coupled with the trans-activator of transcription (TAT) protein transduction domain residues 48–59 of human immunodeficiency virus-1 (TAT-heart8P) for efficient internalization into mammalian cells. The pharmacokinetics of TAT-heart8P were evaluated in dogs and rats. The cellular internalization of TAT-heart8P-Cy(5.5) was examined on cardiomyocytes. The real-time cardiac delivery of ⁶⁸Ga-NODAGA-TAT-heart8P was tested in mice under physiological and pathological conditions. Pharmacokinetic studies of TAT-heart8P in dogs and rats revealed a fast blood clearance, high tissue distribution, and high extraction by the liver. TAT-heart-8P-Cy(5.5) was rapidly internalized in mouse and human cardiomyocytes. Correspondingly, organ uptake of hydrophilic ⁶⁸Ga-NODAGA-TAT-heart8P occurred rapidly after injection with an initial cardiac bioavailability already 10 min post-injection. The saturable cardiac uptake was revailed by the pre-injection of the unlabeled compound. The cardiac uptake of ⁶⁸Ga-NODAGA-TAT-heart8P did not change in a model of cell membrane toxicity. This study provides a sequential stepwise workflow to evaluate the cardiac delivery of a hydrophilic, non-specific cell-targeting peptide. ⁶⁸Ga-NODAGA-TAT-heart8P showed rapid accumulation in the target tissue early after injection. The implementation of PET/CT radionuclide-based imaging methodology as a means to assess effective and temporal cardiac uptake represents a useful and critical application in drug development and pharmacological research and can be extended to the evaluation of comparable drug candidates.

Summary: Fibroblast activation protein (FAP) is frequently expressed in the tumor stroma, whereas expression by normal organs is highly restricted. Despite these promising features, FAP-targeted therapies have shown limited success so far. FAP imaging offers new opportunities to select patients for FAP-targeted therapies and monitor tumor response. See related article by Lee et al., p. 5330

Introduction and Objectives: Wound healing after myocardial infarction (MI) is a dynamic and complex multiple phase process, and a coordinated cellular response is required for proper scar formation. The current paradigm suggests that pro-inflammatory monocytes infiltrate the MI zone during the initial pro-inflammatory phase and differentiate into inflammatory macrophages, and then switch their phenotypes to anti-inflammatory during the reparative phase. Visualization of the reparative phase post-MI is of great interest because it may reveal delayed resolution of inflammation, which in turn predicts adverse cardiac remodeling. Imaging of anti-inflammatory macrophages may also be used to assess therapy approaches aiming to modulate the inflammatory response in order to limit MI size. Reparative macrophages can be distinguished from inflammatory macrophages by the surface marker mannose receptor (MR, CD206). In this study we evaluated the feasibility of ⁶⁸Ga-NOTA-anti-MMR Nb for imaging of MR on alternatively activated macrophages in murine MI models.Methods: Wildtype and MR-knockout mice and Wistar rats were subjected to MI via permanent ligation of the left coronary artery. Non-operated or sham-operated animals were used as controls. MR expression kinetics on cardiac macrophages was measured in mice using flow cytometry. PET/CT scans were performed 1 h after intravenous injection of ⁶⁸Ga-NOTA-anti-MMR Nb. Mice and rats were euthanized and hearts harvested for ex vivo PET/MRI, autoradiography, and staining. As a non-targeting negative control, ⁶⁸Ga-NOTA-BCII10 was used.Results: In vivo-PET/CT scans showed focal radioactivity signals in the infarcted myocardium for ⁶⁸Ga-NOTA-anti-MMR Nb which were confirmed by ex vivo-PET/MRI scans. In autoradiography images, augmented uptake of the tracer was observed in infarcts, as verified by the histochemistry analysis. Immunofluorescence staining demonstrated the presence and co-localization of CD206- and CD68-positive cells, in accordance to infarct zone. No in vivo or ex vivo signal was observed in the animals injected with control Nb or in the sham-operated animals. ⁶⁸Ga-NOTA-anti-MMR Nb uptake in the infarcts of MR-knockout mice was negligibly low, confirming the specificity of ⁶⁸Ga-NOTA-anti-MMR Nb to MR.Conclusion: This exploratory study highlights the potential of ⁶⁸Ga-NOTA-anti-MMR Nb to image MR-positive macrophages that are known to play a pivotal role in wound healing that follows acute MI.

The role of molecular imaging technologies in detecting, evaluating, and monitoring cardiovascular disease and their treatment is expanding rapidly. Gradually replacing the conventional anatomical or physiological approaches, molecular imaging strategies using biologically targeted markers provide unique insight into pathobiological processes at molecular and cellular levels and allow for cardiovascular disease evaluation and individualized therapy. This review paper will discuss currently available and developing molecular-based single-photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging strategies to evaluate post-infarction cardiac remodeling. These approaches include potential targeted methods of evaluating critical biological processes, such as inflammation, angiogenesis, and scar formation.

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.

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are promising targets for molecular imaging of prostate cancer (PCa) lesions. Due to the heterogenic overexpression of PSMA and GRPR in PCa, a heterodimeric radiotracer with the ability to bind to both targets could be beneficial. Recently, our group reported the novel heterodimer BQ7800 consisting of a urea-based PSMA inhibitor, the peptide-based GRPR antagonist RM26 and NOTA chelator. The study reported herein, aimed to improve the affinity of BQ7800 towards PSMA by changing the composition of the two linkers connecting the PSMA- and GRPR-targeting motifs. Three novel heterodimeric analogues were synthesized by incorporation of phenylalanine in the functional linker of the PSMA-binding motif and/or shortening the PEG-linker coupled to RM26. The heterodimers were labeled with indium-111 and evaluated in vitro. In the competitive binding assay, BQ7812, featuring phenylalanine and shorter PEG-linker, demonstrated a nine-fold improved affinity towards PSMA. In the in vivo biodistribution study of [¹¹¹In]In-BQ7812 in PC3-pip tumor-bearing mice (PSMA and GRPR positive), the activity uptake was two-fold higher in the tumor and three-fold higher in kidneys than for [¹¹¹In]In-BQ7800. Herein, we showed that the affinity of a bispecific PSMA/GRPR heterodimer towards PSMA could be improved by linker modification.