Investigation of imaging the somatostatin receptor by opening the blood-brain barrier with melittin – A feasibility study using positron emission tomography and [⁶⁴Cu]Cu-DOTATATE

DOTATATE is a somatostatin peptide analog used in the clinic to detect somatostatin receptors which are highly expressed on neuroendocrine tumors. Somatostatin receptors are found naturally in the intestines, pancreas, lungs, and brain (mainly cortex). In vivo measurement of the somatostatin receptors in the cortex has been challenging because available tracers cannot cross the blood-brain barrier (BBB) due to their intrinsic polarity. A peptide called melittin, a main component of honeybee venom, has been shown to disrupt plasma membranes and increase the permeability of biological membranes. In this study, we assessed the feasibility of using melittin to facilitate the passage of [⁶⁴Cu]Cu-DOTATATE through the BBB and its binding to somatostatin receptors in the cortex. Evaluation included in vitro autoradiography on Long Evans rat brains to estimate the binding affinity of [⁶⁴Cu]Cu-DOTATATE to the somatostatin receptors in the cortex and an in vivo evaluation of [⁶⁴Cu]Cu-DOTATATE binding in NMRI mice after injection of melittin. This study found an in vitro B_max = 89 ± 4 nM and K_D = 4.5 ± 0.6 nM in the cortex, resulting in a theoretical binding potential (BP) calculated as B_max/K_D ≈ 20, which is believed suitable for in vivo brain PET imaging. However, the in vivo results showed no significant difference between the control and melittin injected mice, indicating that the honeybee venom failed to open the BBB. Additional experiments, potentially involving faster injection rates are required to verify that melittin can increase brain uptake of non-BBB permeable PET tracers. Furthermore, an evaluation of whether a venom with a narrow therapeutic range can be used for clinical purposes needs to be considered.