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

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). CRC is a heterogeneous disease in terms of prognosis, therapy response, and the underlying tumor biology. Large scale transcriptional profiling of primary CRC tumors has revealed the presence of four distinct consensus molecular subtypes (CMSs), which is currently the most robust molecular classification system for CRC. This classification system shows important associations with clinical variables such as primary tumor location, histopathological grade, and prognosis. CMS4 accounts for ~25% of all early-staged CRC patients and is more prevalent in advanced disease stages (~40% in stage IV). Moreover, the CMS4 subtype is characterized by a poor prognosis (e.g. relapse-free survival hazard ratio (HR) of ~1.75 compared to the other subtypes), especially in early CRC, and may benefit less from (often oxaliplatin based) standard systemic treatment regimens. Interestingly, CMS4 CRC seems to benefit more from irinotecan-based chemotherapy regimens. This is relevant as in the metastatic setting often the first choice first-line systemic therapy regimen is oxaliplatin and not an irinotecan-based regimen. Additionally, tumor cells may acquire a CMS4 phenotype following exposure to (fluoropyrimidine-based) chemotherapy, which could contribute to therapy resistance. So far, no effective CMS4-targeted therapies are available for clinical use, although this is an active area of research. For instance, it was recently shown in primary colon cancer that imatinib may have value as CMS-switching (CMS4 to CMS2) drug, characterized by gene expression changes associated with improved survival. Moreover, the MoTriColor study is investigating a TGF-β receptor inhibitor (galunisertib) in combination with capecitabine in patients with advanced chemotherapy resistant CRC with an activated transforming growth factor beta (TGF-β) signature, which is typically activated in the CMS4 CRC subtype (NCT04031872). Finally, as mentioned before, CRC patients with a high CMS4 load could benefit more from an irinotecan-based compared to an oxaliplatin-based treatment regimen.

The genetic signatures upregulated in CMS4 tumors are genes related to TGF-β signaling, epithelial to mesenchymal transition and stromal invasion to which CAFs contribute. In the past decade the effect of the tumor stroma on tumor biology has been studied extensively. The most prominent cell in tumor stroma is the CAF. One of the cell surface markers that distinguish CAFs from normal fibroblasts is FAP.

FAP is expressed during embryogenesis and wound healing. On healthy adult tissues there is almost no expression of FAP. However, FAP is overexpressed on CAFs in the tumor stroma of many different solid tumors. It enables tumor cell migration through extracellular matrix remodeling and has been described to be pro-angiogenic and immunosuppressive in the tumor microenvironment. High FAP expression is associated with a worse overall survival in many epithelial cancers, including CRC. High FAP expression based on IHC of primary CRC tumor samples is associated with poor survival (HR: 1.72, 95% CI: 1.58-9.48, p = 0.009).

In 2018, the first clinical data of a new radiotracer targeting FAP was published. This radiotracer, [68Ga]-FAPI, is based on a quinoline small molecule FAP inhibitor: FAPI. In the last five years, FAPI imaging data of over 3000 cancer patients with many different cancer types were published. Also, multiple publications reported on FAP-targeted radioligands used as last resort therapy with Yttrium-90, Lutetium-177 ([177Lu]) or Samarium-153, with some patients showing stable disease for several months. A first phase I trial investigating [177Lu]-DOTA-FAPI is currently recruiting (NCT04849247). Moreover, another group has recently conducted several first-in-human studies with [177Lu]-radiolabeled FAP inhibitors, further showing the potential of theranostic therapy using FAPI compounds.

Albeit data are still scarce, three studies related tumor FAPI-uptake with FAP-expression by IHC, and all showed a significant positive correlation (r = 0.43-0.94). One study compared this correlation between tissues obtained through resection or biopsy and found the correlation to be stronger following resection. This shows that the sampling method is important due to the heterogeneous expression of FAP within a tumor.