Raloxifene-stimulated experimental breast cancer with the paradoxical actions of estrogen to promote or prevent tumor growth: a unifying concept in anti-hormone resistance.

We have previously demonstrated that prolonged treatments with raloxifene (RAL) in vitro will result in phase II RAL resistance and RAL-induced tumor growth. Clinical interest prompted us to re-examine RAL resistance in vivo, particularly the effects of long-term treatments (a decade or more) on the evolution of RAL resistance. In this study, we have addressed the question of this being a reproducible phenomenon in wild-type estrogen receptor (ER)-positive human breast cell line MCF-7. MCF-7 cells cultured under estrogen-deprived conditions in the presence of 1 microM RAL for more than a year develop RAL resistance resulting in an independent cell line, MCF7-RAL. The MCF7-RAL cells grow in response to both estradiol E2 and RAL. Fulvestrant (FUL) blocks RAL and E2-mediated growth. Transplantation of MCF7-RAL cells into athymic ovariectomized mice and treatment with physiologic doses of E2 causes early E2-stimulated tumor growth. In contrast, continuous treatment of implanted animals with daily oral RAL (1.5 mg daily) causes growth of small tumors within 15 weeks. Continuous re-transplantation of the tumors growing in RAL-treated mice indicated that RAL stimulated tumor growth. Tumors in the untreated mice did not grow. Bi-transplantation of MCF7-E2 and MCF7-RAL tumors into the opposing mammary fat pads of the same ovariectomized animal demonstrated that MCF7-E2 grew with E2 stimulation and not with RAL. Conversely, MCF7-RAL tumors grew with RAL and not E2, a characteristic of phase II resistance. Established phase II resistance of MCF7-RAL tumors was confirmed following up to 7 years of serial transplantation in RAL-treated athymic mice. The ERalpha was retained in these tumors. The cyclical nature of RAL resistance was confirmed and extended during a 2-year evolution of the resistant phases of the MCF7-RAL tumors. The MCF7-RAL tumors that initially were inhibited by E2 grew in the presence of E2 and subsequently grew with either RAL or E2. RAL remained the major grow stimulus and RAL enhanced E2-stimulated growth. Subsequent transplantation of E2 stimulated tumors and evaluations of the actions of RAL, demonstrated robust E2-stimulated growth that was blocked by RAL. These are the characteristics of the anti-estrogenic actions of RAL on E2-stimulated breast cancer growth with a minor component of phase I RAL resistance. Continuous transplantation of the phase I RAL-stimulated tumors for >8 months causes reversion to phase II resistance. These data and literature reports of the cyclical nature of anti-androgen/androgen responsiveness of prostate cancer growth, illustrate the generality of the evolution of anti-hormonal resistance in sex steroid-sensitive target tissues.

Effects of a new clinically relevant antiestrogen (GW5638) related to tamoxifen on breast and endometrial cancer growth in vivo.

PURPOSE: Cross-resistance is an important issue for the evaluation of new antiestrogens to treat advanced breast cancer patients who have failed tamoxifen therapy. In addition, postmenopausal patients treated with long-term adjuvant tamoxifen show a 3-4-fold increase in the risk of developing endometrial cancer. Consequently, a new second line agent should be more antiestrogenic and less estrogen-like on the uterus, and be effective at controlling the growth of breast cancer after exposure to tamoxifen. The purpose was to evaluate the effects of the new tamoxifen analogue GW5638 on breast and endometrial cancer growth. EXPERIMENTAL DESIGN: Athymic mice were transplanted with an endometrial tumor model (ECC-1 E2) that is responsive to estrogen and has never been exposed to antiestrogen. In addition, we used three breast tumor models: a tamoxifen-naïve tumor (T47D-E2) and two tamoxifen-stimulated tumors (MT2 TAM and MCF-7 TAM LT). The antiestrogen GW5638 (1.5 mg daily), tamoxifen (0.5 mg or 1.5 mg daily), and raloxifene (1.5 mg daily) were given p.o. The pure antiestrogen ICI182,780 (5 mg once a week) was given s.c. Western blots from MCF-7 TAM breast tumors were performed to demonstrate the regulation of estrogen receptor alpha expression by different ligands. RESULTS: Estradiol and GW5638 down-regulated the receptor compared with control. ICI182,780 completely degraded the receptor but tamoxifen had no effect. GW5638 did not promote tumor growth, and was effective in blocking the effects of postmenopausal estradiol on the growth of tamoxifen-naïve breast and endometrial tumors. However, raloxifene did not completely block the effects of postmenopausal estradiol on the growth of tamoxifen-naïve endometrial tumor after 14 weeks. GW5638 and ICI182,780 but not raloxifene were also effective in blocking the tamoxifen-stimulated breast tumor growth in athymic mice. CONCLUSIONS: GW5638 is more effective than raloxifene in blocking the effect of estrogen on tamoxifen-naïve endometrial cancer. More importantly, GW5638, like the pure antiestrogen ICI182,780, is able to block the growth of breast cancer stimulated by tamoxifen differently from raloxifene. GW5638 down-regulates estrogen receptor but does not completely destroy the receptor. Therefore, based on our findings, GW5638 could be developed as a second line agent for advanced breast cancer patients and an important first line agent to evaluate as an adjuvant treatment or chemopreventive.

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.