Binding characteristics of 9-fluoropropyl-(+)-dihydrotetrabenzazine (AV-133) to the vesicular monoamine transporter type 2 in rats

The vesicular monoamine transporter type 2 (VMAT2) is highly expressed in pancreatic β-cells and thus has been proposed to be a potential target for measuring β-cell mass (BCM) by molecular imaging. C-11- and F-18-labeled tetrabenazine derivatives targeting VMAT2 have shown some promising results as potential biomarkers for BCM. In the present study, we examined the binding characteristics of 9-fluoropropyl-(+)-dihydrotetrabenzazine ([¹⁸F]AV-133), a potential PET tracer for BCM imaging, in rat pancreas and rat brain. Methods: Pancreatic exocrine cells and pancreatic islet cells were isolated and purified from Sprague-Dawley rats. Membrane homogenates, prepared from both pancreatic exocrine and islet cells as well as from brain striatum and hypothalamus regions, were used for in vitro binding studies. In vitro and ex vivo autoradiography studies with [¹⁸F]AV-133 were performed on rat brain and rat pancreas sections. Immunohistochemistry studies were performed to confirm the distribution of VMAT2 on islet β-cells. Results: Excellent binding affinities of [¹⁸F]AV-133 were observed in rat striatum and hypothalamus homogenates with K_d values of 0.19 and 0.25 nM, respectively. In contrast to single-site binding observed in rat striatum homogenates, rat islet cell homogenates showed two saturable binding sites (site A: K_d=6.76 nM, B_max=60 fmol/mg protein; site B: K_d=241 nM, B_max=1500 fmol/mg protein). Rat exocrine pancreas homogenates showed only a single low-affinity binding site (K_d=209 nM), which was similar to site B in islet cells. In vitro autoradiography of [¹⁸F]AV-133 using frozen sections of rat pancreas showed specific labeling of islets, as evidenced by co-localization with anti-insulin antibody. Ex vivo VMAT2 pancreatic autoradiography in the rat, however, was not successful, in contrast to the excellent ex vivo autoradiography of VMAT2 binding sites in the brain. In vivo/ex vivo islet labeling may be complicated by the presence of the low-affinity/high-capacity site B binding in rat pancreas. Conclusions: [¹⁸F]AV-133 is an excellent imaging agent for mapping VMAT2 sites in rat brain and specifically binds rat islet cells in vitro and postmortem. Additional optimization may be required to achieve ex vivo islet β-cell labeling in rats.