Synthesis of an estrogen receptor beta-selective radioligand: 5-[18F]fluoro-(2R,3S)-2,3-bis(4-hydroxyphenyl)pentanenitrile and comparison of in vivo distribution with 16alpha-[18F]fluoro-17beta-estradiol.

Estrogen receptor beta (ERbeta), a less active ER subtype that appears to have a restraining effect on the more active ERalpha, could be a factor that determines the level of estrogen action in certain estrogen target tissues. ERbeta is found in breast cancer, and its levels relative to ERalpha decline with disease progression. Thus, the independent quantification of ERalpha and ERbeta levels in breast cancer by imaging might be predictive of responses to different hormone therapies. To develop an imaging agent for ERbeta, we synthesized a fluoroethyl analogue of DPN (2,3-bis(4-hydroxyphenyl)propanonitrile), a known ERbeta-selective ligand. This analogue, FEDPN (5-fluoro-(2R,3S)-2,3-bis(4-hydroxyphenyl)pentanenitrile), has an 8.3-fold absolute affinity preference for ERbeta. [18F]Fluoride-labeled FEDPN was prepared from a toluenesulfonate precursor, which provided [18F]FEDPN with a specific activity greater than 3100 Ci/mmol after HPLC purification. Biodistribution studies in immature female rats using estradiol as a blocking agent revealed specific uptake of [18F]FEDPN in the uterus and ovaries. Experiments using ERalpha- and ERbeta-knockout mice demonstrated the expected ERalpha-subtype dependence in the tissue uptake of the known 16alpha-[18F]fluoro-17beta-estradiol ([18F]FES), which has a 6.3-fold preference for ERalpha. The tissue uptake of [18F]FEDPN in the ER knockout mice showed some evidence of mediation by ERbeta, but the levels of specific uptake of this agent were relatively modest. Based on our results, imaging of ERalpha can be done effectively with [18F]FES, but imaging of ERbeta will likely require agents with more optimized ERbeta binding affinity and selectivity than [18F]FEDNP.