STUDIES ON PROGESTERONE RECEPTORS IN HUMAN-BREAST CARCINOMAS - USE OF NATURAL AND SYNTHETIC LIGANDS

Progesterone receptors were measured in human breast carcinomas using both (3H)progesterone in the presence of excess non-radioactive cortisol and tritiated synthetic progestins which are not bound with high affinity by corticosteroid-binding-globulin. At concentrations of receptor above 10 fmole/mg protein good agreement was found between the amount of progesterone and the synthetic progestins R5020 and ORG 2058 bound. In 2 27 tumors low levels of progesterone binders were present but no specific binders for the synthetic progestins were detected. In all the tumors without specific progesterone binders, receptors for both R5020 and ORG 2058 were also absent. The Kd using the different ligands were in the order R5020 < ORG 2058 < progesterone in the presence of glycerol < progesterone in the absence of glycerol. Competition experiments using excess non-radioactive steroids suggest that both progesterone and the synthetic progestins bind to identical sites. It is concluded that neither (3H)progesterone nor (3H)ORG 2058 offers any advantage over R5020 in the assay of progesterone receptors in human breast tumors. © 1979.