Elacestrant -d 6 (RAD1901-d 6 ) is a deuterated labeled Elacestrant. Elacestrant is a selective estrogen receptor ( estrogen receptor , ER ) degrader (SERD) with oral activity, with IC 50 values of 48 and 870 nM for ERα and ERβ , respectively. Elacestrant can also effectively inhibit the growth of ER + breast cancer cell lines both in vitro and in vivo.