Toremifene (toremifene citrate; see figure 8), chemically designated as 2-(p-[(Z)-4-chloro-1,2-diphenyl-1-butenyl]phenoxy)-N,N-dimethylethylamine citrate, is a chlorinated derivative of the nonsteroidal triphenylethylene antiestrogen tamoxifen with a chloro substituent at the ethylene side chain producing similar binding affinities to that of tamoxifen. The structure and activity relationship of toremifene is similar to that of tamoxifen, but it has a substantial improvement from the older drug in regards to DNA alkylation. The presence of the added chlorine atom reduces the stability of cations formed from activated allylic metabolites and thus decreases alkylation potential, and indeed toremifene does not display DNA adduct formation in rodent hepatocytes. Toremifene protects against bone loss in ovariectomized rat models and affects bone resorption markers clinically in a similar fashion to tamoxifen. Toremifene undergoes phase I metabolism by microsomal cytochrome P450 enzymes, like tamoxifen, but primarily by the CYP3A4 isoform. Toremifene forms its two major metabolites N-desmethyltoremifene and deaminohydroxy-toremifene (ospemifene) by undergoing N-demethylation and deamination-hydroxylation. N-desmethyltoremifene has similar efficacy as toremifene while 4-hydroxytoremifene has a higher binding affinity to the ER than toremifene. 4-hydroxytoremifene has a role similar to that of 4-hydroxytamoxifen.