Using a CCK2R targeted near-infrared fluorescent probe to aid in intraoperative identification of loco-regional metastasis of medullary thyroid cancer

Objectives Medullary thyroid cancer (MTC) is a rare form of thyroid cancer arising from the calcitonin-secreting parafollicular cells. Total thyroidectomy with central lymph node dissection is the primary treatment modality. The extent of nodal metastasis and tumor size governs the extension of surgery. Wide surgical interventions are associated with worse prognosis. An intraoperative imaging probe that is able to detect MTC positive tissues would represent an unmet clinical need and allow real time analysis of diseased tissues, providing physicians with an additional level of control, and ultimately aiding to avoid morbidities attributed to extensive neck surgeries. Here we explore the near-infrared fluorescent (NIRF) labeling of a minigastrin (MG) analogue in order to study its applicability in a mouse model of MTC, targeting cholecystokinin B receptors (CCK2R), which are overexpressed in more than 90% of MTC.

Methods The expression of CCK2R in the MTC cell line (TT) was determined by western blot analysis and compared to CCK2R overexpressing cells (AR42J) and CCK2R negative cells (RAW264.7). In vivo imaging of xenograft and orthotopic MTC bearing mice were conducted to study the applicability of the probe in identifying local and regional metastasis. For imaging, 23 nmol of the peptide were injected via tail vein and imaged between 0 h and 72h using an IVIS imaging system.

Results Western blot analysis confirmed that CCK2R is highly expressed in TT. In vivo imaging of an Alexa Fluor 750 labeled MG analogue in xenograft mouse models demonstrated tumor specific uptake with signal to background (STB) ratios of 16.2 at 24 h. The kidneys are the only organs showed significant STB ratios of 13.0 at 24 h.

Conclusions The AF750 labeled MG analogue demonstrated specific uptake in CCK2R expressing tumors, warranting further investigation of the imaging agent.

Research Support Supported by the Imaging and Radiation Sciences Program of MSKCC (TR)