Checkpoint inhibitors (ICIs) are novel cancer therapies, modulating the endogenous immune response to cancer. ICI-associated myocarditis (ICIM) is a rare but severe immune-related adverse event (irAE). ICIM is associated with increased cardiac biomarker and unspecific ECG changes. However, classical tools, such as a myocardial biopsy, echocardiography and cardiac MRI have a low sensitivity and are not able to sufficiently identify all patients with ICIM. Therefore, finding the right diagnosis of ICIM is still challenging and specific diagnostic tools are missing.

We aimed to implement ⁶⁸Ga-FAPI PET/CT imaging in detecting ICI-associated myocarditis. In a retrospective study, FAPI PET/CT scans of 26 patients who received ICIs from 01/2017 to 10/2019 were analyzed. Three patients showed clinical manifestations of an irAE, including ICIM with elevated levels of hs-cTnT (175 ng/l, 1771 ng/l and 157 ng/l). To exclude any significant coronary heart disease, cardiac catherization was performed in these patients. Further cardiological assessments revealed ECG abnormalities, lymphocyte infiltration of the myocardium in endomyocardial biopsies or wall motion abnormalities in echocardiography.

We compared tracer enrichment in the heart of patients on ICI-therapy without signs of a cardiac irAE (n=23) to the three patients with an ICIM. ⁶⁸Ga-FAPI PET/CT showed cardiac enrichment of the tracer which was less distinct or absent in patients receiving ICIs without signs of irAE and ICIM (Median SUV myocarditis patients: 1.79 (IQR: 1.65, 1.85), median SUV non-myocarditis patients: 1.15 (IQR: 0.955, 1.52)).

Our data provide first evidence, that ⁶⁸Ga-FAPI PET/CT might be a useful tool to identify ICIM-patients at an early stage. In patients with negative biopsies and normal cardiac MRI, it potentially has an additional diagnostic value to underpin the diagnosis of an ICIM.