Can Nuclear Imaging of Activated Macrophages with Folic Acid-Based Radiotracers Serve as a Prognostic Means to Identify COVID-19 Patients at Risk?

Folate Receptor Beta Signaling in the Regulation of Macrophage Antimicrobial Immune Response: A Scoping Review

Introduction

Folate, vitamin B9, is a water-soluble vitamin that is essential to cellular proliferation and division. In addition to the reduced folate carrier, eukaryotic cells take up folate through endocytosis mediated by one of two GPI-anchored folate receptors (FRs), FRα or FRβ. Two other isoforms of FR exist, FRγ and FRδ, neither of which support endocytic activities of FR signaling. FRβ is expressed primarily by monocytes and macrophages and highly expressed on activated macrophages. Macrophage expression of FRβ suggests a role for this receptor in modulating function of these immune sentinels, particularly as they engage in inflammatory processes. Despite several studies suggesting that folates can suppress inflammatory responses of macrophages to proinflammatory stimuli, there appears to be a lack of basic research examining the role of FRβ in modulating macrophage responses to microbial sensing. We therefore conducted a scoping review to assess evidence within the published literature addressing the question, "what is known about the extent to which FRβ regulates macrophage responses to sensing, and responding to, microorganisms?".

Methods

As a strategy for the study selection, we queried articles indexed in the research database PubMed and the search engine Google Scholar (up until August 12, 2023), including combinations of the research words: macrophage, folate receptor beta, FOLR2.

Results

We identified 2 relevant articles out of 153 that are worth discussing here, none of which directly addressed our research question.

Conclusion

There is an unmet need to better define the contribution of FRβ to regulating the macrophage response to microbes.

The Clinical Utility of Molecular Imaging in COVID-19: An Update

The novel pathogen severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first discovered in Wuhan, China in late 2019 with Coronavirus disease 2019 (COVID-19) declared a global pandemic in March 2020. Primarily involving the lungs, conventional imaging with chest radiography and CT can play a complementary role to RT-PCR in the initial diagnosis, and also in follow up of select patients. As a broader understanding of the multi-systemic nature of COVID-19 has evolved, a potential role for molecular imaging has developed, that may detect functional changes in advance of standard cross-sectional imaging. In this review, we highlight the evolving role of molecular imaging such as fluorine-18 (18F) fluorodeoxyglucose (FDG) with PET/CT and PET/MRI in the evaluation of both pulmonary and extra-pulmonary COVID-19, ventilation and perfusion scan with SPECT/CT for thromboembolic disease, long term follow-up of COVID-19 infection, and COVID-19 vaccine-related complications.

Molecular imaging as a tool for evaluation of COVID-19 sequelae – A review of literature

Coronavirus disease 2019 (COVID-19) is caused by the novel viral pathogen, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). COVID-19 primarily involves the lungs. Nucleic acid testing based on reverse-transcription polymerase chain reaction of respiratory samples is the current gold standard for the diagnosis of SARS-CoV-2 infection. Imaging modalities have an established role in triaging, diagnosis, evaluation of disease severity, monitoring disease progression, extra-pulmonary involvement, and complications. As our understanding of the disease improves, there has been substantial evidence to highlight its potential for multi-systemic involvement and development of long-term sequelae. Molecular imaging techniques are highly sensitive, allowing non-invasive visualization of physiological or pathological processes at a cellular or molecular level with potential for detection of functional changes earlier than conventional radiological imaging. The purpose of this review article is to highlight the evolving role of molecular imaging in evaluation of COVID-19 sequelae. Though not ideal for diagnosis, the various modalities of molecular imaging play an important role in assessing pulmonary and extra-pulmonary sequelae of COVID-19. Perfusion imaging using single photon emission computed tomography fused with computed tomography (CT) can be utilized as a first-line imaging modality for COVID-19 related pulmonary embolism. ¹⁸F-fluorodeoxyglucose positron emission tomography (PET)/CT is a sensitive tool to detect multi-systemic inflammation, including myocardial and vascular inflammation. PET in conjunction with magnetic resonance imaging helps in better characterization of neurological sequelae of COVID-19. Despite the fact that the majority of published literature is retrospective in nature with limited sample sizes, it is clear that molecular imaging provides additional valuable information (complimentary to anatomical imaging) with semi-quantitative or quantitative parameters to define inflammatory burden and can be used to guide therapeutic strategies and assess response. However, widespread clinical applicability remains a challenge owing to longer image acquisition times and the need for adoption of infection control protocols.

Folate Receptor Beta for Macrophage Imaging in Rheumatoid Arthritis

Non-invasive imaging modalities constitute an increasingly important tool in diagnostic and therapy response monitoring of patients with autoimmune diseases, including rheumatoid arthritis (RA). In particular, macrophage imaging with positron emission tomography (PET) using novel radiotracers based on differential expression of plasma membrane proteins and functioning of cellular processes may be suited for this. Over the past decade, selective expression of folate receptor β (FRβ), a glycosylphosphatidylinositol-anchored plasma membrane protein, on myeloid cells has emerged as an attractive target for macrophage imaging by exploiting the high binding affinity of folate-based PET tracers. This work discusses molecular, biochemical and functional properties of FRβ, describes the preclinical development of a folate-PET tracer and the evaluation of this tracer in a translational model of arthritis for diagnostics and therapy-response monitoring, and finally the first clinical application of the folate-PET tracer in RA patients with active disease. Consequently, folate-based PET tracers hold great promise for macrophage imaging in a variety of (chronic) inflammatory (autoimmune) diseases beyond RA.

Positron emission tomography in the COVID-19 pandemic era

Coronavirus disease 2019 (COVID-19) has become a major public health problem worldwide since its outbreak in 2019. Currently, the spread of COVID-19 is far from over, and various complications have roused increasing awareness of the public, calling for novel techniques to aid at diagnosis and treatment. Based on the principle of molecular imaging, positron emission tomography (PET) is expected to offer pathophysiological alternations of COVID-19 in the molecular/cellular perspectives and facilitate the clinical management of patients. A number of PET-related cases and research have been reported on COVID-19 over the past one year. This article reviews the current studies of PET in the diagnosis and treatment of COVID-19, and discusses potential applications of PET in the development of management strategy for COVID-19 patients in the pandemic era.

FDG PET/CT imaging features and clinical utility in COVID-19

Purpose

To determine the imaging findings and potential clinical utility of FDG PET/CT in patients with laboratory-confirmed COVID-19.

Methods

We performed a single institution retrospective review of patients diagnosed with COVID-19 using real time reverse transcription–polymerase chain reaction (RT-PCR) who underwent FDG PET/CT for routine cancer care between March 1, 2020 to April 30, 2020, during the height of the pandemic in New York City, New York, United States. PET/CT scans were retrospectively reviewed for imaging findings suspicious for COVID-19. For positive scans, PET and CT findings were recorded, including location, FDG avidity (SUV_max) and CT morphology. Patient demographics and COVID-19 specific clinical data were collected and analyzed with respect to PET/CT scan positivity, lung SUV_max, and time interval between PET/CT and RT-PCR.

Results

Thirty-one patients (21 males and 10 females, mean age 57 years ± 16) were evaluated. Thirteen of 31 patients had positive PET/CT scans, yielding a detection rate of 41.9%. Patients with positive scans had significantly higher rates of symptomatic COVID-19 infection (77% vs 28%, p = 0.01) and hospitalizations (46% vs. 0%, p = 0.002) compared to patients with negative scans. Eleven of 13 patients (84.6%) with positive scans had FDG-avid lung findings, with mean lung SUV_max of 5.36. Six of 13 patients (46.2%) had extrapulmonary findings of FDG-avid thoracic lymph nodes. The detection rate was significantly lower when the scan was performed before RT-PCR versus after RT-PCR (18.8% (n = 3/16) vs. 66.7% (n = 10/15), p = 0.009). Lung SUV_max was not associated with COVID-19 symptoms, severity, or disease course.

Conclusion

FDG PET/CT has limited sensitivity for detecting COVID-19 infection. However, a positive PET scan is associated with higher risk of symptomatic infection and hospitalizations, which may be helpful in predicting disease severity.

Imaging Inflammation with Positron Emission Tomography

The impact of inflammation on the outcome of many medical conditions such as cardiovascular diseases, neurological disorders, infections, cancer, and autoimmune diseases has been widely acknowledged. However, in contrast to neurological, oncologic, and cardiovascular disorders, imaging plays a minor role in research and management of inflammation. Imaging can provide insights into individual and temporospatial biology and grade of inflammation which can be of diagnostic, therapeutic, and prognostic value. There is therefore an urgent need to evaluate and understand current approaches and potential applications for imaging of inflammation. This review discusses radiotracers for positron emission tomography (PET) that have been used to image inflammation in cardiovascular diseases and other inflammatory conditions with a special emphasis on radiotracers that have already been successfully applied in clinical settings.

Diagnostic, Prognostic, and Therapeutic Use of Radiopharmaceuticals in the Context of SARS-CoV-2

The coronavirus disease 2019 (COVID-19) outbreak has devastated the healthcare systems and economies of over 200 countries in just a few months. The etiological agent of COVID-19, SARS-CoV-2, is a highly contagious virus that can be transmitted by asymptomatic and symptomatic carriers alike. While in vitro testing techniques have allowed for population-wide screening, prognostic tools are required to assess the disease severity and therapeutic response, contributing to improve the patient clinical outcomes. Moreover, no specific antiviral against COVID-19 exists at the time of publication, severely limiting treatment against the infection. Hence, there is an urgent clinical need for innovative therapeutic strategies that may contribute to manage the COVID-19 outbreak and prevent future pandemics. Herein, we critically examine recent diagnostic, prognostic, and therapeutic advancements for COVID-19 in the field of radiopharmaceuticals. First, we summarize the gold standard techniques used to diagnose COVID-19, including in vitro assays and imaging techniques, and then discuss how radionuclide-based nuclear imaging provides complementary information for prognosis and treatment management of infected patients. Second, we introduce new emerging types of radiotherapies that employ radioimmunoconjugates, which have shown selective cytotoxic response in oncological studies, and critically analyze how these compounds could be used as therapeutic agents against SARS-CoV-2. Finally, this Perspective further discusses the emerging applications of radionuclides to study the behavior of pulmonary SARS-CoV-2 aerosol particles.

Role of 2-[18F]FDG as a Radiopharmaceutical for PET/CT in Patients with COVID-19: A Systematic Review

Some recent studies evaluated the role of fluorine-18 fluorodeoxyglucose (2-[¹⁸F]FDG) as a radiopharmaceutical for positron emission tomography/computed tomography (PET/CT) imaging in patients with Coronavirus Disease (COVID-19). This article aims to perform a systematic review in this setting. A comprehensive computer literature search in PubMed/MEDLINE and Cochrane library databases regarding the role of 2-[¹⁸F]FDG PET/CT in patients with COVID-19 was carried out. This combination of key words was used: (A) “PET” OR “positron emission tomography” AND (B) “COVID” OR “SARS”. Only pertinent original articles were selected; case reports and very small case series were excluded. We have selected 11 original studies of 2-[¹⁸F]FDG PET/CT in patients with COVID-19. Evidence-based data showed first preliminary applications of this diagnostic tool in this clinical setting, with particular regard to the incidental detection of interstitial pneumonia suspected for COVID-19. To date, according to evidence-based data, 2-[¹⁸F]FDG PET/CT cannot substitute or integrate high-resolution CT to diagnose suspicious COVID-19 or for disease monitoring, but it can only be useful to incidentally detect suspicious COVID-19 lesions in patients performing this imaging method for standard oncological and non-oncological indications. Published data about the possible role of 2-[¹⁸F]FDG PET/CT in patients with COVID-19 are increasing, but larger studies are warranted.