The development of estrogen and progestin radiopharmaceuticals for imaging breast cancer

Background: The presence of receptors for estrogens and progestins in many breast tumors provides a means for imaging these tumors using positron emission tomography (PET) with appropriate fluorine-18 labeled estrogen and progestin radipharmaceuticals. In this context, the estrogen analog 16a-[¹⁸F]fluoroestradiol (FES) has already proven to be an effective imaging agent for estrogen receptor-positive tumors. Methods: Clinical studies comparing FES images with those based on the metabolic probe 2-[¹⁸F]fluoro-2-deoxyglcose (FDG) in patients before and after tamoxifen hormone therapy are underway. Several fluorine-18 labeled progestins have been prepared, and efforts are underway to develop methods for labeling steroid receptor imaging agents with the widely available radionuclide technetium-99m, using both pendant and integrated approaches. Results and Conclusion: Breast tumor imaging with FES and FDG shows an interesting relationship between tumor metabolic response (assessed with FDG) and tumor estrogen receptor levels (assessed with FES). The fluorine-18 labeled progestins show excellent target tissue selective distribution in experimental animals and are ready for imaging studies in humans. The development of steroids labeled with technetium-99m poses special challenges because of the metallic nature of this radioisotope.