Non-FDG PET in oncology

Imaging of Uveal Melanoma—Current Standard and Methods in Development

Simple Summary

Uveal melanoma is the most prevalent intraocular tumor in adults, derived from melanocytes; the liver is the most common site of its metastases. Due to troublesome tumor localization, different imaging techniques are utilized in diagnostics, i.e., fundus imaging (FI), ultrasonography (US), optical coherence tomography (OCT), single-photon emission computed tomography (SPECT), positron emission tomography/computed tomography (PET/CT), magnetic resonance imaging (MRI), fundus fluorescein angiography (FFA), indocyanine green angiography (ICGA), or fundus autofluorescence (FAF). Specialists eagerly use these techniques, but sometimes the precision and quality of the obtained images are imperfect, raising diagnostic doubts and prompting the search for new ones. In addition to analyzing the currently utilized methods, this review also introduces experimental techniques that may be adapted to clinical practice in the future. Moreover, we raise the topic and present a perspective for personalized medicine in uveal melanoma treatment.

Abstract

Uveal melanoma is the most common primary intraocular malignancy in adults, characterized by an insidious onset and poor prognosis strongly associated with tumor size and the presence of distant metastases, most commonly in the liver. Contrary to most tumor identification, a biopsy followed by a pathological exam is used only in certain cases. Therefore, an early and noninvasive diagnosis is essential to enhance patients’ chances for early treatment. We reviewed imaging modalities currently used in the diagnostics of uveal melanoma, including fundus imaging, ultrasonography (US), optical coherence tomography (OCT), single-photon emission computed tomography (SPECT), fundus fluorescein angiography (FFA), indocyanine green angiography (ICGA), fundus autofluorescence (FAF), as well as positron emission tomography/computed tomography (PET/CT) or magnetic resonance imaging (MRI). The principle of imaging techniques is briefly explained, along with their role in the diagnostic process and a summary of their advantages and limitations. Further, the experimental data and the advancements in imaging modalities are explained. We describe UM imaging innovations, show their current usage and development, and explain the possibilities of utilizing such modalities to diagnose uveal melanoma in the future.

CD38 as a PET Imaging Target in Lung Cancer

Daratumumab (Darzalex, Janssen Biotech) is a clinically approved antibody targeting CD38 for the treatment of multiple myeloma. However, CD38 is also expressed by other cancer cell types, including lung cancer, where its expression or absence may offer prognostic value. We therefore developed a PET tracer based upon daratumumab for tracking CD38 expression, utilizing murine models of non-small cell lung cancer to verify its specificity. Daratumumab was prepared for radiolabeling with ⁸⁹Zr (t_1/2 = 78.4 h) through conjugation with desferrioxamine (Df). Western blot, flow cytometry, and saturation binding assays were utilized to characterize CD38 expression and binding of daratumumab to three non-small cell lung cancer cell lines: A549, H460, and H358. Murine xenograft models of the cell lines were also generated for further in vivo studies. Longitudinal PET imaging was performed following injection of ⁸⁹Zr-Df-daratumumab out to 120 h postinjection, and nonspecific uptake was also evaluated through the injection of a radiolabeled control IgG antibody in A549 mice, ⁸⁹Zr-Df-IgG. Ex vivo biodistribution and histological analyses were also performed after the terminal imaging time point at 120 h postinjection. Through cellular studies, A549 cells were found to express higher levels of CD38 than the H460 or H358 cell lines. PET imaging and ex vivo biodistribution studies verified in vitro trends, with A549 tumor uptake peaking at 8.1 ± 1.2%ID/g at 120 h postinjection according to PET analysis, and H460 and H358 at lower levels at the same time point (6.7 ± 0.7%ID/g and 5.1 ± 0.4%ID/g, respectively; n = 3 or 4). Injection of a nonspecific radiolabeled IgG into A549 tumor-bearing mice also demonstrated lower tracer uptake of 4.4 ± 1.3%ID/g at 120 h. Immunofluorescent staining of tumor tissues showed higher staining levels present in A549 tissues over H460 and H358. Thus, ⁸⁹Zr-Df-daratumumab is able to image CD38-expressing tissues in vivo using PET, as verified through the exploration of non-small cell lung cancer models in this study. This agent therefore holds potential to image CD38 in other malignancies and aid in patient stratification and elucidation of the biodistribution of CD38.

Characterization of Physiologic (18)F FSPG Uptake in Healthy Volunteers

Purpose To evaluate the normal biodistribution and kinetics of (S)-4-(3-[18F]fluoropropyl)-l-glutamic acid ((18)F FSPG) in healthy volunteers and to compare (18)F FSPG mean and maximum standardized uptake values (SUVmean and SUVmax, respectively) with those of (18)F fluorodeoxyglucose (FDG) across a variety of organs. Materials and Methods This protocol was reviewed and approved by all appropriate regulatory authorities. An 8-mCi (±10%) dose of (18)F FSPG was given to five subjects (three women, two men), and seven whole-body positron emission tomography (PET) scans were performed 5, 10, 20, 30, 45, 150, and 240 minutes after injection. Regions of interest were analyzed on the resultant (18)F FSPG images to evaluate the kinetics of this radiotracer. The images obtained 45 minutes after injection were used to measure SUVmean and SUVmax in additional regions of the body. These values were compared with similar values obtained with (18)F FDG PET published previously. Descriptive statistics, including average and standard deviation across the five subjects, were used. (18)F FSPG SUVmean and SUVmax were compared. Results On the (18)F FSPG images obtained 45 minutes after injection, there was only low-grade background activity in the majority of analyzed regions. Prominent activity was seen throughout the pancreas. Clearance of the radiotracer through the kidneys and collection in the bladder also were seen. SUV quantification shows notable differences between (18)F FSPG and (18)F FDG in the pancreas ((18)F FSPG SUVmean, 8.2; (18)F FDG SUVmean, 1.3), stomach ((18)F FSPG SUVmax, 3.6; (18)F FDG SUVmax, 1.6), and brain ((18)F FSPG SUVmean, 0.08; (18)F FDG SUVmean, 7.8). The kinetic data showed rapid clearance of the radiotracer from the blood pool and most organs, except the pancreas. Conclusion (18)F FSPG is a PET radiopharmaceutical characterized by rapid clearance from most healthy tissues, except the pancreas and kidneys. A consistent biodistribution pattern was observed with low background uptake. The physiologic uptake of this new radiotracer throughout the body is described in more detail, which is important for improved interpretative accuracy and understanding potential clinical applications. (©) RSNA, 2016.