In vivo evaluation of 7 alpha-[11-(4-[125I]iodophenoxy)undecyl]-17 beta-estradiol: a potential vector for therapy of adrenal and estrogen receptor-positive cancers

Novel 7α-alkoxy-17α-(4'-halophenylethynyl)estradiols as potential SPECT/PET imaging agents for estrogen receptor expressing tumours: synthesis and binding affinity evaluation

In order to develop potential radiolabelled probes for imaging estrogen receptor (ER) positive tumours, we have synthesized and characterized a series of novel 7α-alkoxy-17α-(4'-iodophenylethynyl)estra-1,3,5(10)-triene-3,17β-diols and 7α-alkoxy-17α-(4'-fluorophenylethynyl)estra-1,3,5(10)-triene-3,17β-diols. The fluoro-substituted compounds showed a higher ER binding affinity than the corresponding iodo-derivatives, where 7α-methoxy- and 17α-(4'-fluorophenylethynyl)estra-1,3,5(10)-triene-3,17β-diol showed the highest ER binding affinities (RBA=80.9% and 78.9%, respectively), among the halophenylethynyl compounds studied and should be further explored as potential PET biomarkers for imaging of ER expressing tumours.

Impact on estrogen receptor binding and target tissue uptake of [18F]fluorine substitution at the 16α-position of fulvestrant (faslodex; ICI 182,780)

Fulvestrant (Faslodex; ICI 182,780) is a pure estrogen receptor (ER) antagonist recently approved for the treatment of hormone-sensitive breast cancer in post-menopausal women with disease progression following antiestrogen therapy. Fulvestrant strongly binds to the ER and its mode of action consists of inhibition of ER dimerization leading to a down regulation of ER protein cellular levels. With the aim to develop a probe for positron emission tomography (PET) imaging capable of predicting the potential therapeutic efficacy of selective ER modulators (SERM), we prepared three new 16alpha-[18F]fluoro-fulvestrant derivatives. These new radiopharmaceuticals were evaluated for their binding affinity to the human ERalpha and for their target tissue uptake in immature female rats. Substitution of one of the side-chain F-atoms of fulvestrant for 18F would have led to a product of low specific activity; instead we selected the 16alpha-position for 18F-labeling, which at least in the case of estradiol (ES) is well tolerated by the ER. Radiochemical synthesis proceeds by stereoselective introduction of the [18F]fluoride at the 16-18F-position of fulvestrant via opening of an intermediate O-cyclic sulfate followed by hydrolysis of the protecting methoxymethyl (MOM) ether and sulfate groups. Three analogs with different oxidation states of the side chain sulfur, i.e. sulfide, sulfone or sulfoxide (fulvestrant) were prepared. Introduction of the 16(18)F-fluorine led to a dramatic decrease of the apparent binding affinity for ER, as reported by Wakeling et al. (Cancer Res. 1991;51:3867-73). Likewise, in vivo ER-mediated uterus uptake values in immature female rats were disappointing. Overall, our findings suggest that these new PET radiopharmaceuticals are not suitable as tracers to predict ER(+) breast cancer response to hormonal therapy with selective ER modulators.

Radiolabeled steroidal estrogens in cancer research

It has now been more than 30 years since the laboratory verification of the localization of estrogen in certain animal tissues. Much has been learned since that time regarding the details of this process, including the presence of specific receptors for these hormones in target tissues, the mechanism of ligand binding, the association of the ligand-receptor complex with unique chromatin sequences, and the activation of transcription. A concrete use of this knowledge has been the exploitation of these receptors as a targeting mechanisms for radiopharmaceuticals. This is an exciting area that encompasses both diagnosis and therapy. This review will summarize the in vitro and in vivo data obtained from evaluation of the many compounds that have been examined as radiolabeled receptor ligands, and will also discuss the chemistry necessary for their preparation. In particular, relative binding affinity values for relevant compounds will be tabulated, grouped according to molecular class. For those materials for which biodistributions have been performed, uterine (target), liver (nontarget, clearance), and, when available, tumor tissue uptake values are presented. These data should provide a reminder of what has been accomplished, and should serve as a working reference for those engaged in the pursuit of new candidates for these applications.

Synthesis, receptor binding and tissue distribution of 17 alpha-E[125I]iodovinyl-11 beta-ethyl-estradiol

In this study we prepared and evaluated a derivative of estradiol with an ethyl group at the 11 beta-position and an E-iodovinyl group at the 17 alpha-position. This new ligand binds to the estrogen receptor with an affinity slightly less than estradiol (RBA = 43%) at 0 degree C but much greater (RBA = 890%) at 25 degrees C. The 125I-labeled derivative was obtained by radioiododestannylation of the tri-n-butylstannyl precursor in good radiochemical yield with a specific activity exceeding 1500 Ci/mmol. The tissue distribution in immature female rats was evaluated over a 48 h period to determine uterine uptake and selectivity. Peak uterine uptake at 2 h was 6% ID/g and was significantly greater than that of [3H]estradiol, 2.4% ID/g. Substantial uptake in the uterus was still present at 48 h (2.4% ID/g). Co-administration of estradiol reduced the uptake at 2 and 24 h by 85%. Uterus-to-plasma ratios increased with time, from about 25:1 at 2 h to nearly 90:1 at 48 h. The affinity, ease of radiosynthesis and tissue distribution of the 17 alpha-E-[125I]iodovinyl-11 beta-ethyl-estradiol suggest that further evaluation of this agent as an imaging agent for estrogen-receptor-positive breast cancer is warranted.

Synthesis, tissue distribution in rats and PET studies in baboon brain of no-carrier-added [18F]RU 52461: in vivo evaluation as a brain glucocorticoid receptor radioligand

11,17 beta-Dihydroxy-6-methyl-17 alpha-(3-[18F]fluoro-prop-1- ynyl)androsta-1,4,6-trien-3-one [( 18F]RU 52461), an 18F-analog of RU 28362, was synthesized by bromide displacement with [18F]fluoride in 12-30% overall radiochemical yield (decay-corrected) within 140 min from end of bombardment (EOB). The specific activity was 900-1500 mCi/mumol (33.3-55.5 GBq/mumol) at the end of synthesis (EOS). Biodistribution studies indicated high adrenal and pituitary retention, and uniformly low uptake of [18F]RU 52461 in all other brain regions of the rat. Except for the pituitary, no specific receptor-mediated uptake of [18F]RU 52461 could be demonstrated using saturating doses of unlabeled RU 52461 in rat brain. While no change was observed throughout the brain areas in adrenalectomized rats and in animals coinjected with dexamethasone, when compared to controls. PET studies revealed extremely low levels of radioactivity in baboon brain. Therefore, [18F]RU 52461 does not appear to cross the blood-brain barrier, suggesting that this radiopharmaceutical is not suitable to visualize the brain glucocorticoid binding sites by PET.