title: Effects of raloxifene after tamoxifen on breast and endometrial tumor growth in athymic mice.
abstract: BACKGROUND
  In patients with early-stage breast cancer, 5 years of treatment with the selective estrogen receptor modulator (SERM) tamoxifen reduces breast cancer recurrence and mortality, whereas more than 5 years of tamoxifen does not further reduce breast cancer recurrence and doubles the risk of endometrial cancer. We evaluated the effects on tumor growth of raloxifene, another SERM, after tamoxifen treatment in mouse models of breast and endometrial cancers.
  
  
  METHODS
  Athymic, ovariectomized mice were bitransplanted with tumors derived from human breast cancer and endometrial cancer cells that either were tamoxifen-naive or had been exposed to tamoxifen for short (6 months) or long (>5 years) terms. The effects of raloxifene (two dose levels) and tamoxifen on tumor growth in the presence and absence of low-dose estrogen were evaluated. All statistical tests were two-sided.
  
  
  RESULTS
  Raloxifene was less effective than tamoxifen in blocking the stimulatory effects of low-dose estrogen on the growth of tamoxifen-naive breast (P<.001) and endometrial (P =.001) tumors. Raloxifene and tamoxifen had similar inhibitory effects on the growth of short-term tamoxifen-exposed breast tumors. Raloxifene and tamoxifen had similar stimulatory effects on the growth of breast and endometrial tumors that had been exposed to at least 5 years of tamoxifen. However, neither drug blocked the stimulatory effects of estrogen on the growth of these tumors. Raloxifene was less effective than tamoxifen (P<.001) in blocking the stimulatory effects of estrogen on endometrial tumors that had been exposed to tamoxifen in the past.
  
  
  CONCLUSIONS
  Raloxifene and tamoxifen had similar effects on these mouse models of tamoxifen-naive and tamoxifen-resistant breast and endometrial cancer. Treatment with raloxifene following 5 years of adjuvant tamoxifen may not further decrease breast cancer recurrence and may increase endometrial cancer incidence.
authors.affiliation: Division of Hematology/Oncology, Northwestern University, Chicago, IL 60611, USA.
authors.forename: Ruth M
authors.initials: RM
authors.lastname: O'Regan
authors.forename: Csaba
authors.initials: C
authors.lastname: Gajdos
authors.forename: Rita C
authors.initials: RC
authors.lastname: Dardes
authors.forename: Alex
authors.initials: A
authors.lastname: De Los Reyes
authors.forename: Woochan
authors.initials: W
authors.lastname: Park
authors.forename: Alfred W
authors.initials: AW
authors.lastname: Rademaker
authors.forename: V Craig
authors.initials: VC
authors.lastname: Jordan
mesh_terms: D000818:Animals; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use*; D016889:Endometrial Neoplasms / Q000188:drug therapy* / Q000473:pathology / Q000517:prevention & control; D005260:Female; D006801:Humans; D008325:Mammary Neoplasms, Experimental / Q000188:drug therapy* / Q000473:pathology / Q000517:prevention & control; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D020849:Raloxifene Hydrochloride / Q000008:administration & dosage*; D020845:Selective Estrogen Receptor Modulators / Q000008:administration & dosage*; D013629:Tamoxifen / Q000008:administration & dosage*; D014183:Transplantation, Heterologous; D014407:Tumor Cells, Cultured
chemical_list: D020845:Selective Estrogen Receptor Modulators; D013629:Tamoxifen; D020849:Raloxifene Hydrochloride