A newer group of drugs, known as selective estrogen receptor down-regulators, or SERDs, have a more potent anti-estrogen effect, involving destabilization of the estrogen receptor, which leads to its degradation.
GW5638 fits somewhere in the middle, acting like a SERM in some tissues and more like a SERD in others, including mammary tissue, where it is a powerful estrogen antagonist. As a consequence, GW 5638 can inhibit the growth of breast cancers that have become resistant to tamoxifen. It may also be more effective than tamoxifen at preventing cancer in women at high risk.
Equally important was learning how the very slight difference between tamoxifen and GW5638 altered the interactions between the estrogen receptor and other molecules that are regulated by the estrogen receptor.
Estrogen, tamoxifen and GW5638 all bind to the estrogen receptor in the same "pocket," but after binding they change the shape, or conformation, of the receptor in different ways. GW5638 pushes one small part of the estrogen receptor, a peptide spiral called helix 12, out of place. By shifting helix 12 to an odd spot, GW5638 disrupts the ways in which several other molecules that normally interact with the estrogen receptor go about their jobs.
These molecules, called coactivators or corepressors, can enhance or repress the effects of estrogen. They are present at different levels in different tissues.
"H12 positioning is essential for these interactions," Greene said. "By changing the conformation of the estrogen receptor, this drug changes the way it interacts with a whole series of related downstream molecules. And those interactions explain why these drugs have different effects in different locations, such as breast, bone or uterus."
This finding opens a new arena for drug design, suggests Greene. Nuclear receptors, such as the estrogen receptor, are major drug targets, accounting for more than 20 percent of all drugs. This fi
Source:University of Chicago Medical Center