Binding of medroxyprogesterone acetate in human breast cancer

Human mammary tumor cytosol containing macromolecules which bound 3H-MPA (3H-medroxyprogesterone acetate or 1,2-3H-6 Alpha-methyl-17 alpha-acetoxy-pregn-4-ene-3,20-dione) and 3H-R5020 (6,7-3H-17,21-dimethyl-19-nor-pregna-4,9-diene-3,20-dione) similarly with high affinity (Ka approximately equal to 2 nM-1) and specificity. The progestin-binding components had sedimentation coefficients of about 4S and 7S in sucrose gradients and had approximately the same number of binding sites for Ma and R5020 as revealed by gradient centrifugation and saturation analysis. Among the steroids tested, these components had the highest affinities for progestins and were probably progesterone receptors of human breast cancer. A 4S component of human serum bound 3H-R5020 but not 3H-MPA. With 3H-MPA and 3H-estradiol used as the tracers, the concentrations of progesterone and estrogen receptors have been determined in 236 human breast cancers by saturation analysis. Our results on the receptor content and response of 31 of these tumors to endocrine therapy suggest that progesterone receptor may be a better marker of a hormonally responsive breast cancer.