The folate receptor (FR) is overexpressed on epithelial cancers (FR-alpha) and on activated, but not resting, macrophages (FR-beta) involved in a variety of inflammatory and autoimmune diseases. Therefore, folate-based radiopharmaceuticals have the potential to be used as imaging agents of FR-positive tumor and inflammatory cells. In this study SPECT/CT of FR-positive malignant and normal tissues and organs in mice was performed using an improved organometallic (99m)Tc-radiofolate.
Methods: The (99m)Tc radiolabeling of the histidine-folate was performed using the tricarbonyl technique described earlier for preparation of other organometallic radiofolates. Nude male mice with FR-positive KB tumor xenografts were used. Biodistribution studies were performed 1, 4, and 24 h after injection of the (99m)Tc-His-folate (1.5 MBq/mouse). Images were acquired with a dedicated small-animal SPECT/CT camera 24 h after injection of the radiofolate (500 MBq/mouse). Ex vivo autoradiography was performed on tumors and FR-positive normal tissues. Adjacent sections were used for in vitro autoradiography of FRs after decay of the injected radioactivity.
Results: The SPECT/CT studies revealed accumulation of the radiotracer in FR-positive KB tumor xenografts and kidneys as reported previously. At the same time, specific uptake of the radiofolate in normal tissues-that is, salivary glands and choroid plexus-could be visualized with SPECT. FR-specific accumulation in these tissues and organs was confirmed by coinjection of excess folic acid, which resulted in a complete blockade of radiofolate uptake. In addition, ex vivo and in vitro autoradiography of these organs and tissues confirmed FR expression and displayed radioactivity distribution patterns almost identical to those found on SPECT images. In biodistribution studies we found a high tumor uptake (4.29 +/- 0.67 %ID/g [percentage of the injected dose per gram of tissue], 4 h after injection) that was almost completely retained over time (3.51 +/- 0.37 %ID/g, 24 h after injection).
Conclusion: The novel (99m)Tc-histidine-folate showed improved in vivo characteristics compared with other organometallic radiofolates that allowed imaging of FR-positive malignant (KB tumor xenografts) and kidneys. For the first time, to our knowledge, specific tracer uptake in salivary glands and the choroid plexus could be visualized using a high-resolution animal SPECT/CT camera.