Bis(4-hydroxyphenyl)[2-(phenoxysulfonyl)phenyl]methane: isolation and structure elucidation of a novel estrogen from commercial preparations of phenol red (phenolsulfonphthalein)

Commercial preparations of phenolsulfonphthalein (Phenol Red), a pH indicator dye widely added to cell culture media, have weak estrogenic activity that can be accounted for by a minor lipophilic impurity (ca. 0.002%). We have isolated this impurity, determined its structure to be bis(4-hydroxyphenyl)[2-(phenoxysulfonyl)phenyl]methane, and synthesized it from phenolsulfonphthalein. This compound binds to the estrogen receptor with an affinity 50% that of estradiol; it stimulates the proliferation and increases the progesterone receptor content of estrogen-responsive breast cancer cells in vitro, and it stimulates uterine weight gain in rats in vivo, but shows a potency in these assays only 0.1-0.2% that of estradiol. We suggest how this novel estrogen may be generated during the preparation of phenolsulfonphthalein.