(P 109) NOVEL CANDIDATE BIOMARKERS FOR FLUORESCENCE IMAGE-GUIDED SURGERY IN GASTROINTESTINAL STROMAL TUMORS

Objective: Currently, surgery is the only curative treatment for gastrointestinal stromal tumors (GIST). Preoperative imaging, intraoperative palpation and visual inspection are used to identify tumor tissue. However, these methods are not always sufficient for discrimination between malignant and healthy tissue leading to irradical resections or unnecessary removal of healthy tissue. Fluorescence Image-Guided Surgery (F-IGS), a fluorescent label attached to a preferably tumor specific agent which binds to a biomarker, has the potential to improve tumor identification and cancer surgery outcomes. Approximately 95% of GISTs express KIT (CD117) and/or DOG1. Successful in vivo fluorescence imaging of GIST has been demonstrated using anti-KIT antibodies in mouse models. However, several studies have shown a loss of KIT expression after treatment with imatinib, a tyrosine kinases inhibitor which is often used as neoadjuvant therapy in GISTs, making it potentially less reliable as a target for F-IGS.
The aim of this pre-clinical study is to identify novel suitable biomarkers, besides KIT and DOG1, for developing GIST-specific probes that can be used in F-IGS.

Methods: Multiple resources were combined to select candidate biomarkers. Tissue samples containing GIST tumor with adjacent tissue were retrospectively collected and immunohistochemically stained using antibodies against the identified candidate biomarkers. Expression of the potential biomarkers on the tumor and the adjacent tissue was scored using an adapted version of the Total Immunostaining Score (TIS). This score comprised the intensity of the staining (IS) and the percentage of the total amount of positively stained (PS). To calculate the TIS, both scores were multiplied (PS x IS). The TIS score of the tumor and an adjacent tissue per type of tissue were used to calculate the tumor-to-normal (T/N) ratio to make relative comparisons between tissue types. A target selection scoring system was developed (table 1) where expression pattern of the biomarker, T/N of muscle, mucosa, connective tissue and blood vessels and percentage of biomarker positive GIST tumors were scored.

Results: Five candidate targets were identified based on public databases and articles; CD243, DDR2, PDE3A, PIK3CG and TEM1. The immunohistochemical staining showed that the expression of DDR2 was high, but was non-specific to the tumor as the T/N-ratios were on average below 2. The expression of both CD243 and PDE3A were consistent and specific for the GIST tumor with T/N-ratios on average above 3.8. The T/N-ratios of PIK3CG were comparable to CD243, with exception of the T/N-ratio of mucosa as it scored on average below 2. TEM1 expression was high on the connective tissue located in the submucosa, around muscle and blood vessels but not on the tumor itself making it ineffective for F-IGS. Based on the results of the immunohistochemical staining, CD243, PDE3A and PIK3CG are tested on multiple GIST cell lines by flow cytometry. Results are currently being collected.

Conclusion: PDE3A, CD243 and PIK3CG are promising targets for F-IGS of GIST. Currently, in-vitro models based on GIST cell lines are being developed to evaluate peptides targeting the candidate biomarkers for F-IGS.