An estradiol-porphyrin conjugate selectively localizes into estrogen receptor-positive breast cancer cells

A conjugate of a C(11)-beta-derivative of estradiol and an asymmetric tetraphenylporphyrin was synthesized to study its potential selective uptake by breast cancer cells naturally over-expressing the nuclear receptor for estrogen (ER). Competitive radioligand binding assays of this conjugate with recombinant ER showed that the conjugate bound to ER in a dose-dependent manner with an EC50 of 274 nM, compared with 1 nM for estradiol, the natural ligand. Cellular uptake studies with ER-positive MCF-7 and ER-negative HS578t human breast cancer cells revealed that, the conjugate was taken up by MCF-7 cells in a dose-dependent manner, which was obliterated by co-incubation with a large excess of estradiol. On the other hand there was very little uptake of the un-conjugated porphyrin by MCF-7 and Hs578t cells. HS578t cells also showed insignificant uptake of the conjugate under the conditions of our experiment. These results strongly suggested that specific interaction between the endogenous ER in MCF-7 cells and the estrogen part of the conjugate enabled these cells to selectively internalize the conjugate over the un-conjugated porphyrin. Therefore, ER-binding conjugates of estradiol and porphyrins could potentially be used for ER-targeted photodynamic therapy of hormone-sensitive cancers of breast, ovary, gonads etc.