Positron Emission Tomography-Based Immunoimaging for Cancer Patient Stratification: Toward a More Holistic Approach.

Immunotherapy with immune checkpoint inhibitors (ICIs) changed the treatment management in several solid metastatic tumors with very poor prognosis, in particular in melanoma stage IV since its introduction in 2011. However, it is not yet fully understood why some patients respond to ICIs and others not, and it is also unclear why melanomas are the most sensitive tumors to ICI treatment. Selection criteria for patient stratification are needed and several approaches are under evaluation. These include the PD-L1 expression in the tumor samples, assessment of the tumor mutational burden as well as radiological and molecular imaging techniques, such as positron emission tomography (PET)-CT and PET-MRI with 18F-fluorodeoxyglucose (FDG) or novel radiopharmaceuticals. In the near future, a more holistic approach based on the combination of imaging data and sequencing data and the development of radiogenomic signatures will be needed for a better characterization of immunotherapy response and selection of patients who will benefit from ICI therapy alone or in combination with chemotherapy.

Gallium-68-DOTA-albumin as a PET blood-pool marker: experimental evaluation in vivo.

Investigations into tumor angiogenesis and antiangiogenic treatment have renewed interest in tumor perfusion. To image tumor blood-pool by PET, suitable tracers are not generally available. In this experimental study, we characterized a 68Ga-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) conjugate of rat serum albumin (68Ga-DOTA-RSA) in vivo using a generator-produced isotope. Biodistribution was determined in ACI rats after intravenous administration of 3-6 MBq of 68Ga-DOTA-RSA. Three ACI rats were imaged over 1 h by dynamic PET after intravenous administration of 15-25 MBq of 68Ga-DOTA-RSA while the blood-pool activity was recorded simultaneously in a closed extracorporeal loop (ECL) between the carotid artery and the jugular vein. Time-activity curves (TACs) were obtained from volume of interest (VOI) analysis and from the ECL data. Stability and metabolites in plasma and urine were analyzed by size exclusion HPLC (SE-HPLC) 1 h after intravenous injection of 67Ga-DOTA-RSA. Blood radioactivity decreased by 10% and 18% from 10 to 60 min p.i. by biodistribution and PET or ECL, respectively. Tissue sampling between 10 and 60 min p.i. showed slight increases in the uptake of spleen, myocardium, kidney and skeletal muscle while hepatic accretion remained unchanged. Total urinary excretion after 60 min amounted to 9% of the injected dose. HPLC demonstrated a single urinary metabolite corresponding in size to gallium-labeled DOTA. 68Ga-DOTA-RSA is a blood-pool tracer whose physical and biological half-life is well suited for PET. Our findings support clinical imaging using 68Ga-DOTA-labeled human serum albumin (HSA). The generator-produced label makes 68Ga-DOTA-labeled albumin continuously available even to centers lacking an in-house cyclotron.