I a2 Bioconjugate Chem. 1994, 5, 182-193 ARTICLES Progestin Radiopharmaceuticals Labeled with Technetium and Rhenium: Synthesis, Binding Affinity, and in Vivo Distribution of a New Progestin N~S2-Metal Conjugate James P. O’Nei1,tJ Kathryn E. Carlson,? Carolyn J. Anderson,$ Michael J. Welch,g and John A. Katzenellenbogen’vt Department of Chemistry, University of Illinois, 600 South Mathews Avenue, Urbana, Illinois 61801, and Division of Radiation Sciences, Washington University Medical School, 600 South Kingshighway, St. Louis, Missouri 63110. Received December 27, 1993’ We have prepared and evaluated three metal conjugatesof a zyxwv progestin-monoamine-monoamide (MAMA’) bisthiol chelate system. These conjugates of rhenium and technetium-99 and -99m, are structural analogs of the bisamino-bisthiol (BAT) conjugates we have described recently, but the MAMA’ chelate, being more polar than the BAT system, gives a conjugate that is much less lipophilic, having an octanol- water partition coefficient that is nearly 80-fold lower. In competitive binding assays, the Re- and ggTc-MAMA’-progestin conjugates bind to the progesterone receptor with affinities greater than that of progesterone itself, and in a direct binding assay, the equilibrium dissociation constant zyx (&) of the 99mTc-MAMA’ conjugate was 0.97 nM. As is typical for llp-substituted progestins, these conjugates also have substantial binding affinity for glucocorticoid receptors. In tissue distribution studies in immature female rats, the pr~gestin-~~~Tc-MAMA’ conjugates show selective uptake for principal target tissue (such as uterus) over that of blood and nontarget tissue (such as muscle); these uptake ratios reach maximum levels of 5 and 4, respectively. Uptake by fat, liver, and kidney is quite high; however, only the uptake in uterus is displaceable upon coinjection of the selective progestin ORG2058. Metabolism studies show that the radioactivity in the uterus is essentially unmetabolized out to 4 h, while liver activity is completely due to metabolites. Other tissues show an intermediate fraction of unmetabolized conjugates that decreases with time. The in vivo behavior of the progestin-WmTc- MAMA’ conjugate is similar to that of the labeled BAT conjugate: its uptake selectivity is somewhat greater than that of the BAT conjugate, but its target tissue uptake is lower. Factors that may be responsible for limiting the target tissue uptake properties of these conjugates are their moderate affinity for progesterone receptor, their substantial binding to glucorticoid receptors, and their large overall molecular size. INTRODUCTION Because of its wide availability, convenient half-life, and appropriate y energy, technetium-99m is frequently the radionuclide of choice in the development of diagnostic imaging agents (1). Nevertheless, there are certain situ- ations in which it is difficult to utilize technetium-99m: being a metal, it requires a chelate system to form stable complexes (1, 2) and other systems of organometallic bonding, while intriguing in structure, are difficult to prepare at the no-carrier-added level, as is required for receptor-based imaging agents (3). Since the technetium complexes are large, generally having molecular weights in excess of 250, their incorporation into small ligands for receptors, such as steroid hormones and neurotransmitters, provides a formidable challenge: (1) metal complex substituents of this size can severely compromise the binding affinity of these receptor ligands, and (2) the bulk of these conjugates can alter the physicochemical proper- ties of receptor ligands to such an extent that their receptor- + University of Illinois. t Current address: Center for Functional Imaging, Lawrence Berkeley Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720. Washington University Medical School. @ Abstract publishedin zyxwvutsrqp Aduance ACS Abstracts, April 1,1994. mediated biodistribution is limited by permeability bar- riers or overwhelmed by nonspecific uptake. In a recent study in which we examined the receptor binding affinity of four progestin-metal conjugates, we reported the preparation of a conjugate I between a progestin related to the Roussel-Uclaf antiprogestin RU486 IV and a bisamine-bisthiol (BAT)-technetium complex that retained high affinity for the progesterone receptor (4). While we could demonstrate nanomolar receptor binding of this conjugate in in vitro assays, its nonspecific binding was high and its tissue distribution in vivo showed high uptake by nontarget tissues (5). We postulated that the poor distribution properties of I might be largely due to the very lipophilic character of the bisamine-bisthiol (BAT) chelate system that was used. To lower lipophilicity, we considered other more polar metal chelate systems, and we have recently described the preparation of a smaller and considerably less lipophilic monoamine-monoamide-bisthiol-metal chelate system I1 (MAMA’) that forms stable, neutral complexes with rhenium and technetium (6). The MAMA‘-metal chelate system is a structural isomer of the well known MAMA system (7, 8). In this present report, we describe the preparation of a conjugate I11 of the RU486-related progestin with this 1043-1802/94/2905-0182$04.50/0 0 1994 American Chemical Society