Detection and modelling of stroma rich carcinomas

In the 20th century, cancer research primarily focused on understanding cancer cells themselves, aiming to identify mutations responsible for malignant transformation and targeting them with therapies. However, recent decades have seen a paradigm shift: cancer is now understood as a complex ecosystem in which malignant cells interact with their surrounding environment, known as the tumour microenvironment (TME). This microenvironment includes various cell types and structural molecules that support tumour growth. A key component of this environment is the tumour stroma, comprising cancer-associated fibroblasts (CAFs), blood vessels, and connective tissue.

Stroma rich carcinomas, such as the Consensus Molecular Subtype 4 (CMS4) in colorectal cancer, are more prone to metastasize and respond poorly to conventional anti-cancer therapies which results in a overall worse survival. Current diagnostic methods for identifying stroma-rich tumours rely on tissue analysis, which is limited by sampling bias due to the uneven distribution of tumour stroma, and more specifically CAFs, within tumours and between primary and metastatic sites. Molecular imaging of CAF-specific markers offers a non-invasive, whole-body approach to overcome this limitation. In this thesis we focused on imaging two CAF markers: platelet-derived growth factor receptor beta (PDGFRB) and fibroblast activation protein (FAP).

In peritoneal metastasis of colorectal cancer, PDGFRB was highly expressed. Two tracers—a nanobody and a bicyclic peptide—were developed to target PDGFRB, but both require further optimization to improve tumour-specific binding. Furthermore, we identified FAP as an excellent single gene identifier of the colorectal cancer CMS4 subtype and a FAP-targeting PET tracer (FAPI), developed in Germany, shows great potential to detect the CMS4 subtype in vivo. A clinical study (FoCus) is underway to evaluate FAPI-PET’s diagnostic potential in CMS4 patients.
Based on FAP expression data, we identified six clinical scenarios where FAPI-PET could add value, including detection of peritoneal metastases, monitoring treatment response, and potentially predicting resistance to immune checkpoint inhibitors—which tend to be less effective in stroma rich cancers. FAPI-PET may also improve preoperative assessment in cholangiocarcinoma. We observed strong FAP expression in primary cholangiocarcinoma tumours and lymph node metastases, suggesting FAPI imaging could guide surgical decisions. However, false positives in unaffected lymph nodes present a potential challenge that future studies must address.

While accurate imaging is important, ultimately we must develop treatments targeting the tumour stroma. To investigate CAF–cancer cell interactions, we created a co-culture model combining colorectal cancer organoids with CAFs. This model recapitulated features of stromal tumours and revealed that CAFs induce a more aggressive, CMS4-like cancer cell phenotype. The system also revealed strong immunosuppressive effects, relevant for understanding treatment resistance.

In conclusion, this research highlights the essential role of CAFs in cancer biology and introduces innovative tools—molecular imaging and co-culture models—that enable deeper insight into the tumour microenvironment and support the development of more effective, personalized therapies.