(P 170) GASTRIN RELEASING PEPTIDE RECEPTOR EXPRESSION IN GASTROINTESTINAL STROMAL TUMORS (GIST)

Objective: There are few treatment options for patients with advanced or metastasized gastrointestinal stromal tumors (GIST) that lack a tumor mutation sensitive to tyrosine kinase inhibitors (TKI) or have developed TKI resistance. Gastrin releasing peptide receptor (GRPR) theranostics using an antagonist to the GRPR has shown promise in both breast and prostate cancer, and may be a viable option in advanced GIST. We studied the expression of the GRPR in a large cohort of GIST patients.

Methods: The GRPR expression was studied with immunohistochemistry in two separate TissueMicroArrays from patients treated at Sahlgrenska University Hospital in the pre- (1983-2001) and post- (2014-2020) TKI era, respectively. In total, 205 tumor samples were evaluated as low/none or moderate/high expression of the GRPR and correlated to clinical characteristics and survival.

Results: In total, 80% of the tumor samples had a moderate or high expression of the GRPR irrespective of gender, age, tumor location and risk group according to the modified NIH risk criteria. Neoadjuvant treatment with TKI was correlated to low/none GRPR expression (p=0.04). In patients without neoadjuvant treatment who were operated with curative intent, GRPR expression was not correlated to a worse outcome.

Conclusion: This is the first study that elucidates the GRPR expression with immunohistochemistry in a large cohort of GIST tumors. Our results demonstrate that most GIST tumors have a moderate to high expression of the receptor which indicates that GRPR theranostics could be a feasible treatment option for TKI resistant GIST. Interestingly, tumors that were pretreated with TKI seemed to have a downregulated expression of the GRPR which needs to be studied further.