Estrogenic potency of oral replacement therapy estimated by the induction of pregnancy zone protein

The induction of a highly estrogen-inducible plasma protein, the pregnancy zone protein/pregnancy associated alpha 2-globulin (PZP/PA alpha 2 G), was used to design a sensitive method to quantify and compare the potency of various estrogens commonly used in clinical practice. Serum PZP levels were followed in 100 post-menopausal women during 6 months' estrogen replacement therapy. Ethinyl estradiol caused more marked changes than the natural estrogens. Conjugated estrogens were more potent than estrone sulphate and estradiol-17 beta which had similar values. Estriol, irrespective of daily dosage, exerted a negative effect. The following sequence of equipotency was calculated: ethinyl estradiol = 100 x conjugated estrogens = 450 x estrone sulphate = 500 x estradiol-17 beta = 650 x estradiol valerate. These data were compared with other estimates of estrogenic potency from the literature. Compared with other clinical methods, PZP induction offers several advantages. Factors such as intestinal absorption, protein binding and intracellular metabolism are included in the net result of an increased protein synthesis which is easily followed in patients' sera during therapy.