Current and future perspectives on functional molecular imaging in nephro-urology: theranostics on the horizon

FAPI-PET in Cardiovascular Disease

PET probes targeting fibroblasts are frequently used for varying applications in oncology. In recent years, the clinical spectrum has been expanded towards cardiovascular medicine, e.g., after myocardial infarction, in aortic stenosis or as a non-invasive read-out of atherosclerosis. We herein provide a brief overview of the current status of this PET radiotracer in the context of cardiovascular disease, including translational and clinical evidence. In addition, we will also briefly discuss future applications, e.g., the use of fibroblast-targeting PET to investigate bilateral organ function along the cardiorenal axis.

Cardiorenal ketone metabolism: a positron emission tomography study in healthy humans

Ketones are alternative energy substrates for the heart and kidney but no studies have investigated their metabolism simultaneously in both organs in humans. The present double tracer positron emission tomography (PET) study evaluated the organ distribution and basal kinetic rates of the radiolabeled ketone, 11C-acetoacetate (11C-AcAc), in the heart and kidney compared to 11C-acetate (11C-Ac), which is a well-validated metabolic radiotracer. Both tracers were highly metabolized by the left ventricle and the renal cortex. In the heart, kinetic rates were similar for both tracers. But in the renal cortex, uptake of 11C-Ac was higher compared to 11C-AcAc, while the reverse was observed for the clearance. Interestingly, infusion of 11C-AcAc led to a significantly delayed release of radioactivity in the renal medulla and pelvis, a phenomenon not observed with 11C-Ac. This suggests an equilibrium of 11C-AcAc with the other ketone, 11C-D-beta-hydroxybutyrate, and a different clearance profile. Overall, this suggests that in the kidney, the absorption and metabolism of 11C-AcAc is different compared to 11C-Ac. This dual tracer PET protocol provides the opportunity to explore the relative importance of ketone metabolism in cardiac and renal diseases, and to improve our mechanistic understanding of new metabolic interventions targeting these two organs.

Advances in Imaging-Based Biomarkers in Renal Cell Carcinoma: A Critical Analysis of the Current Literature

Cross-sectional imaging is the standard diagnostic tool to determine underlying biology in renal masses, which is crucial for subsequent treatment. Currently, standard CT imaging is limited in its ability to differentiate benign from malignant disease. Therefore, various modalities have been investigated to identify imaging-based parameters to improve the noninvasive diagnosis of renal masses and renal cell carcinoma (RCC) subtypes. MRI was reported to predict grading of RCC and to identify RCC subtypes, and has been shown in a small cohort to predict the response to targeted therapy. Dynamic imaging is promising for the staging and diagnosis of RCC. PET/CT radiotracers, such as ¹⁸F-fluorodeoxyglucose (FDG), ¹²⁴I-cG250, radiolabeled prostate-specific membrane antigen (PSMA), and ¹¹C-acetate, have been reported to improve the identification of histology, grading, detection of metastasis, and assessment of response to systemic therapy, and to predict oncological outcomes. Moreover, ⁹⁹Tc-sestamibi and SPECT scans have shown promising results in distinguishing low-grade RCC from benign lesions. Radiomics has been used to further characterize renal masses based on semantic and textural analyses. In preliminary studies, integrated machine learning algorithms using radiomics proved to be more accurate in distinguishing benign from malignant renal masses compared to radiologists' interpretations. Radiomics and radiogenomics are used to complement risk classification models to predict oncological outcomes. Imaging-based biomarkers hold strong potential in RCC, but require standardization and external validation before integration into clinical routines.

Role of PSMA-ligands imaging in Renal Cell Carcinoma management: current status and future perspectives

Background

Renal masses detection is continually increasing worldwide, with Renal Cell Carcinoma (RCC) accounting for approximately 90% of all renal cancers and remaining one of the most aggressive urological malignancies. Despite improvements in cancer management, accurate diagnosis and treatment strategy of RCC by computed tomography (CT) and magnetic resonance imaging (MRI) are still challenging. Prostate-Specific Membrane Antigen (PSMA) is known to be highly expressed on the endothelial cells of the neovasculature of several solid tumors other than prostate cancer, including RCC. In this context, recent preliminary studies reported a promising role for positron emission tomography (PET)/CT with radiolabeled molecules targeting PSMA, in alternative to fluorodeoxyglucose (FDG) in RCC patients.

Purpose

The aim of our review is to provide an updated overview of current evidences and major limitations regarding the use of PSMA PET/CT in RCC.

Methods

A literature search, up to 31 December 2021, was performed using the following electronic databases: PubMed, SCOPUS, Web of Science, and Google Scholar.

Results

The findings of this review suggest that PSMA PET/CT could represent a valid imaging option for diagnosis, staging, and therapy response evaluation in RCC, particularly in clear cell RCC.

Conclusions

Further studies are needed for this “relatively” new imaging modality to consolidate its indications, timing, and practical procedures.

11C-Para-aminobenzoic acid PET imaging of S. aureus and MRSA infection in preclinical models and humans

Tools for noninvasive detection of bacterial pathogens are needed but are not currently available for clinical use. We have previously shown that para-aminobenzoic acid (PABA) rapidly accumulates in a wide range of pathogenic bacteria, motivating the development of related PET radiotracers. In this study, ¹¹C-PABA PET imaging was used to accurately detect and monitor infections due to pyogenic bacteria in multiple clinically relevant animal models. ¹¹C-PABA PET imaging selectively detected infections in muscle, intervertebral discs, and methicillin-resistant Staphylococcus aureus–infected orthopedic implants. In what we believe to be first-in-human studies in healthy participants, ¹¹C-PABA was safe, well-tolerated, and had a favorable biodistribution, with low background activity in the lungs, muscles, and brain. ¹¹C-PABA has the potential for clinical translation to detect and localize a broad range of bacteria.

Kit-based synthesis of 2-deoxy-2-[18F]-fluoro-D-sorbitol for bacterial imaging

Clinically available imaging tools for diagnosing infections rely on structural changes in the affected tissues. They therefore lack specificity and cannot differentiate between oncologic, inflammatory and infectious processes. We have developed 2-deoxy-2-[¹⁸F]fluoro-D-sorbitol (¹⁸F-FDS) as an imaging agent to visualize infections caused by Enterobacterales, which represent the largest group of bacterial pathogens in humans and are responsible for severe infections, often resulting in sepsis or death. A clinical study in 26 prospectively enrolled patients demonstrated that ¹⁸F-FDS positron emission tomography (PET) was safe, and could detect and localize infections due to drug-susceptible or multi-drug-resistant Enterobacterales strains as well as differentiate them from other pathologies (sterile inflammation or cancer). ¹⁸F-FDS is cleared almost exclusively through renal filtration and has also shown potential as a PET agent for functional renal imaging. Since most PET radionuclides have a short half-life, maximal clinical impact will require fast, on-demand synthesis with limited infrastructure and personnel. To meet this demand, we developed a kit-based solid phase method that uses commercially and widely available 2-deoxy-2-[¹⁸F]fluoro-D-glucose as the precursor and allows ¹⁸F-FDS to be produced and purified in one step at room temperature. The ¹⁸F-FDS kit consists of a solid-phase extraction cartridge packed with solid supported borohydride (MP-borohydride), which can be attached to a second cartridge to reduce pH. We evaluated the effects of different solid supported borohydride reagents, cartridge size, starting radioactivity, volumes and flow rates in the radiochemical yield and purity. The optimized protocol can be completed in <30 min and allows the synthesis of ¹⁸F-FDS in >70% radiochemical yield and >90% radiochemical purity.