Effects of raloxifene after tamoxifen on breast and endometrial tumor growth in athymic mice.
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.
O'Regan
Ruth M
RM
Division of Hematology/Oncology, Northwestern University, Chicago, IL 60611, USA.
Gajdos
Csaba
C
Dardes
Rita C
RC
De Los Reyes
Alex
A
Park
Woochan
W
Rademaker
Alfred W
AW
Jordan
V Craig
VC
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
D020845:Selective Estrogen Receptor Modulators; D013629:Tamoxifen; D020849:Raloxifene Hydrochloride