Integrated “3+1” Oxorhenium(V) Complexes as Estrogen Mimics Marc B. Skaddan, and John A. Katzenellenbogen* Department of Chemistry, University of Illinois, 600 S. Mathews Avenue, Urbana, Illinois 61801. Received August 19, 1998; Revised Manuscript Received October 7, 1998 The diagnosis and staging of breast cancer could be improved by the development of imaging radiopharmaceuticals that provide a noninvasive determination of the estrogen receptor (ER) status of tumor cells. Toward this goal, we have synthesized a number of integrated “3+1” oxorhenium(V) complexes designed to mimic estradiol and a class of nonsteroidal estrogens, the tetrahydrochrysenes (THC). The monodentate component of the estradiol mimic is a p-hydroxyphenethyl thiol ligand with ethyl substituents at the benzylic and homobenzylic positions. Model complexes of this ligand were easily made, but steric hindrance of the secondary thiol prevented the formation of the complex with the disubstituted ligand. The three “3+1” oxorhenium(V) complexes prepared to mimic the THC class mimics represent the first pyridinedithiol rhenium complexes of their kind to be made. These complexes are quite stable to air and moisture. The target tridentate ligand was prepared from chelidamic acid, and the VT NMR of the rhenium complex displays interesting fluxional behavior. The binding affinities of these complexes for the estrogen receptor are low, and their lipophilicities are rather high. Nevertheless, our findings provide a further refinement of our understanding of ligand structure- binding affinity correlations for the estrogen receptor. INTRODUCTION Metastatic breast cancer is the most prevalent form of cancer of women in the United States and is the number one killer of women ages 40-55 (1). Recent figures show that a woman’s lifetime risk of developing breast cancer is 1 in 8 (2). Early detection is imperative for breast cancer survival, as the 5 year survival rate for localized breast cancer is 96% (3). However, if the cancer has spread regionally, the survival rate drops to 75%, and distant metastases further reduce the survival rate to 20% (3). Clearly, better diagnostic techniques for early detection would increase survival. Breast cancers that retain estrogen receptor (ER) 1 can usually be treated effectively by endocrine therapy, such as the use of the antiestrogen tamoxifen. In fact, about 60% of ER+ patients respond to endocrine therapy, whereas the response of ER- patients is much lower (4). However, only 70% of breast cancers are ER+; so, it is important to be able to determine the ER status of each tumor. Current methods used to determine the ER status of breast tumor cells are invasive and do not fully account for cell heterogeneity. One way to eliminate the in- vasiveness and minimize the heterogeneity problems of current tumor ER assays would be to image breast tumors based on their content of ER, using positron- emission tomography (PET) or single photon emission computed tomography (SPECT). These techniques utilize the emission characteristics of certain radionuclides attached to radiopharmaceuticals that have appropriate distribution properties to image tissues. Considerable advances have been made in the development of steroids labeled with either fluorine-18 for PET imaging (5-16) or iodine-123 for SPECT imaging (5, 17-21) of ER+ breast tumors. However, the radionuclide of choice for breast tumor imaging would be Tc-99m, due to its convenient 6 h half-life and its wide availability. We have been interested in the development of Tc-99m- containing steroid mimics for use as imaging agents for receptor-positive breast tumors. In the “conjugated” or “pendant” design, a Tc-99m containing moiety is tethered to an existing steroid, such as progesterone or estradiol (1)(22-29). Such complexes are generally stable, but their large overall size can greatly affect their physical properties and their affinity for steroid receptors (22, 29). Smaller complexes are produced by the alternative “integrated” approach, in which part of the steroid backbone itself is replaced by the requisite Tc-99m chelate (30-32). One way to incorporate Tc-99m into such integrated complexes is via the “3+1” design, in which a tridentate and monodentate ligand surround an oxometal core (33). The advantage of such a design is the well- established stability of many “3+1” complexes themselves (27-29, 33-35), compared with that of the related 2+2 complexes. In addition, appropriately designed “3+1” complexes lack syn and anti stereoisomers that are often produced in the tetradentate oxorhenium and technetium complexes. We report here the synthesis of several “3+1” oxo- rhenium complexes that are in the series of the estradiol mimic 2 and the tetrahydrochrysene (THC) mimic 4 (Figure 1). The syntheses involve rhenium as the sur- rogate metal for Tc-99m (36). As is illustrated in Figure 1, estradiol mimic 2 would contain a tridentate aliphatic ligand (where the heteroatom X could be a sulfur, oxygen, or alkyl nitrogen), intended to mimic the D-ring of estradiol. The monodentate ligand is aromatic, with a 2 carbon “spacer” separating the phenyl ring from the thiol, and is intended to mimic the A, B, and C rings of estradiol. The THC mimic 4 involves thiophenol as the * To whom correspondence should be addressed. Phone: (217) 333-6310. Fax: (217) 333-7325. E-mail: jkatzene@uiuc.edu. 1 Abbreviations: ER, estrogen receptor; DHP, dihydropyran; MEM, methoxyethoxymethyl; MOPS, 3-morpholinopropane- sulfonic acid; PET, positron emission tomography; RBA, recep- tor-binding affinity; SPECT, single photon emission computer- ized tomography; THC, tetrahydrochrysene. 119 Bioconjugate Chem. 1999, 10, 119-129 10.1021/bc980094q CCC: $18.00 © 1999 American Chemical Society Published on Web 12/11/1998