Progestin radiopharmaceuticals labeled with technetium and rhenium: synthesis, binding affinity, and in vivo distribution of a new progestin N2S2-metal conjugate.

Abstract

We have prepared and evaluated three metal conjugates of a 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 99Tc-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 (Kd) of the 99mTc-MAMA' conjugate was 0.97 nM. As is typical for 11 beta-substituted progestins, these conjugates also have substantial binding affinity for glucocorticoid receptors. In tissue distribution studies in immature female rats, the progestin-99mTc-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-99mTc-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.