FAPI Molecular Imaging for Diagnosis of the CMS4 Unfavorable Colorectal Cancer Subtype

Colorectal cancer (CRC) is the 3rd most common cancer worldwide and accounts for \~14,000 new diagnoses and \~5,000 deaths in the Netherlands yearly (1.9 million and 935 thousand on a global level). Large scale transcriptional profiling of primary CRC tumors has revealed the presence of four distinct consensus molecular subtypes (CMSs). The CMS4 subtype is associated with a poor prognosis, especially in early CRC, and may benefit less from several standard systemic treatments (e.g. oxaliplatin, 5-fluorouracil, cetuximab), while being relatively sensitive to irinotecan. This is relevant as in the metastatic setting often the first choice first-line systemic therapy regimen is oxaliplatin and not irinotecan-based. Furthermore, tumor cells can acquire a CMS4 phenotype following exposure to chemotherapy, which may contribute to therapy resistance. CMS4 accounts for \~25% of all early-stage CRC patients and is more prevalent in advanced disease stages (\~40% in stage IV CRC). Currently available CMS4 diagnostic tests require tumor tissue samples. The interpretation of biopsy-based CMS4 diagnosis is however complicated by large intra- and inter-lesion heterogeneity of CMS4 status. Extensive biopsy protocols could address the problem of CMS4 heterogeneity but are challenging in routine clinical practice. The development of CMS4-targeted therapy strategies therefore requires a more robust and clinically applicable diagnostic test for comprehensive quantitative assessment of CMS4 status of all lesions - primary and metastatic - in individual cancer patients. A promising solution for such a diagnostic test is to use a radiotracer that enables the quantitative assessment of CMS4 in vivo by whole body molecular imaging. This technique is particularly suited to assess biomarkers with heterogeneous expression: for diagnostic purposes, as a companion diagnostic for (targeted) therapies, or as part of a 'theranostic' strategy where patient selection using the diagnostic radiotracer is followed by treatment with the same tracer labeled to a therapeutic compound. Radiolabeled fibroblast activating protein inhibitor (FAPI) is an emerging diagnostic radiotracer that allows the comprehensive whole-body, whole-tumor assessment of fibroblast activation protein (FAP) expression in humans with a very low background uptake also at frequent CRC metastatic sites including the liver. FAP is an excellent candidate molecular imaging target for CMS4, as it is highly expressed on cancer-associated fibroblasts (CAF) that are abundantly present in this CRC subtype. Indeed, the investigators found that FAP gene-expression measured in tumor biopsies - as a single marker - accurately discriminates CMS4 from other CRC subtypes (area under the receiver operating characteristic curve (AUROC): 0.91; 95% confidence interval (CI): 0.90-0.93). The FoCus study will aim to take a next step by relating in vivo assessed FAP protein-expression by \[18F\]-ALF-FAPI-74 positron emission tomography (PET) / computed tomography (CT) to CMS4 status in patients eligible for colorectal liver metastatectomy as a first proof of concept. Ultimately this will contribute to the development of a diagnostic tool for the comprehensive assessment of CMS4 load in patients with (metastatic) CRC by using \[18F\]-ALF-FAPI-74 PET/CT molecular imaging, to guide CMS4 subtype-directed therapy decisions.