Evaluation of [18F]FAPI-74 PET/CT in healthy dogs and in West Highland white terriers with canine idiopathic pulmonary fibrosis: a pilot study

Canine idiopathic pulmonary fibrosis (CIPF) is a fatal disease affecting primarily West Highland white terriers (WHWT). CIPF remains challenging to diagnose and disease progression is difficult to predict. Recently, fibroblast activation protein (FAP) was identified as a cellular marker of active fibrosis in post-mortem lung biopsies from CIPF-affected WHWTs. Therefore, FAP-targeted imaging using FAP inhibitors (FAPI) may offer a noninvasive means of assessing active fibrosis in canine lungs in vivo. This study aimed to evaluate whether [¹⁸F]FAPI-74 positron emission tomography (PET) combined with computed tomography (CT) can detect FAP expression in the lungs of CIPF-affected WHWTs. This prospective exploratory pilot study included two healthy senior purpose-bred Beagle dogs and two client-owned WHWTs diagnosed with CIPF. In addition to CT, each dog underwent a 90-min dynamic thoracic PET scan following intravenous administration of [¹⁸F]FAPI-74 (median activity 10.3 MBq/kg). In one Beagle and one WHWT, a subsequent static abdominal PET scan was performed to assess biodistribution. PET and CT images were co-registered prior to quantitative uptake analysis. [¹⁸F]FAPI-74 PET was well tolerated in all dogs and compatible with clinical use. [¹⁸F]FAPI-74 uptake was approximately three-fold higher in CIPF-affected lungs than in healthy lungs. Tracer elimination occurred via both urinary and hepatobiliary routes. Moderate uptake was also observed in gastrointestinal organs, potentially reflecting age-related fibrosis, as supported by additional immunohistochemistry. In conclusion, [¹⁸F]FAPI-74 PET/CT enables in vivo detection of active pulmonary fibrosis in CIPF-affected dogs and represents a promising noninvasive tool for detecting and monitoring this fatal disease.