Efficacy of a novel orally active SERD AZD9496 against hormone dependent post-menopausal breast cancer depends on inhibition of cellular aromatase activity

Treatment of hormone sensitive breast cancer tumors with endocrine therapy such as antiestrogens or aromatase inhibitors has improved the outcome significantly. Studies including our own have shown that downregulation of ERα with pure antiestrogen fulvestrant in combination with aromatase inhibitors may prolong responsiveness of the tumors to endocrine therapy. Fulvestrant has been studied as second line or first line treatment for post-menopausal hormone receptor positive breast cancers as a single agent or in combination with AIs. Studies have also suggested that further escalation of dose may improve benefit. However, dose escalation of fulvestrant, which is administered via intramuscular injection, is difficult due to its poor solubility. To overcome this shortcoming of an injectable drug, a novel orally active antiestrogen, AZD9496 was developed. In addition to being orally active, AZD9496 is designed as a selective ERα downregulator (SERD). In the current study, we compared the effect of AZD9496 and fulvestrant on the growth of MCF-7Ca (human estrogen receptor positive MCF-7 cells stably transfected with human placental aromatase gene) xenografts grown in ovariectomized athymic nude mice. AZD9496 was also compared to fulvestrant in vitro as a single agent or in combination with anastrozole. Our current study shows that AZD9496 is equally effective as fulvestrant at controlling the growth of hormone sensitive human breast cancer tumors. Similar to fulvestrant, AZD9496 inhibits cellular aromatase activity through ERα mediated signaling. However, unlike fulvestrant, combination of AZD9496 with anastrozole did not produce increased tumor inhibition. Our results show that AZD9496 was significantly better at inhibiting cellular aromatase which contributed to its anticancer activity. Next, we measured the effect of AZD9496 on the mouse uterus. Uterine weight of mice treated with AZD9496 was significantly lower than that for mice treated with androstenedione. This reduction in uterine weight was due to AZD9496 mediated inhibition of aromatase activity and not a direct effect on uterine ERα expression. We also observed that anti-cancer efficacy of AZD9496 depended on its ability to inhibit cellular aromatase. These results suggest that AZD9496 may be a better alternative to fulvestrant due to its selectivity for mammary ER and ability to inhibit aromatase in addition of downregulating ERα that can be obtained upon oral administration. As such, AZD9496 may prove to be a better option than fulvestrant for the treatment of hormone sensitive human breast cancer.

Abstract 1014: Characterization of breast cancer cells resistant to letrozole and palbociclib combination therapy and the potential of dinaciclib to treat such resistance

Treatment of estrogen receptor positive (ER+) breast cancer have included single-agent endocrine therapy, such as with aromatase inhibitors (AIs) and, more recently, AIs in combination with the cyclin-dependent kinase 4/6 inhibitor palbociclib. While combination therapy is effective, drug resistance is a well-known phenomenon and breast cancer cells can acquire resistance to aromatase inhibitor and palbociclib individually. In anticipation of patient who may become resistant to combination therapy with aromatase inhibitor (letrozole) and palbociclib, this current study has 1) developed MCF7Ca cells resistant to both letrozole and palbociclib (MCF7Calet+palb); 2) characterized them in comparison to drug naïve (MCF7Ca) and letrozole alone-resistant (LTLTCa) cells; and 3) explored potential treatments. Characterization of MCF7Calet+palb cells by phase contrast microscopy, mammosphere assays, and invasion assays, indicate that they have more basal/mesenchymal morphology, produce more mammospheres (an indicator of cancer stem cells; 150±10 vs. 80±10 in MCF7Ca and 123± 8 in LTLTCa cells), and are more invasive (1200±50 cells vs. 791±15 in MCF7Ca cells). The more aggressive phenotype of MC7Calet+palb correlated with changes in protein expression. Compared to the other cell lines, MCF7Calet+palb expressed significantly lower fold expression levels of cell cycle cyclin D1 (0.2±0.1), phosphorylated CDK9 (0.5±0.1), and phosphoRb and total Rb (0.4±0.2), but higher levels of phosphoCDK1 and CDK2 (1.4±0.1 and 3.1±0.6). Regarding growth pathway proteins, MCF7Calet+palb comparably had increased HER2 and phosphorylated Akt (6.2±1.3 and 3.3±1.2, resp.) as LTLTCa vs. MCF7Ca cells, but had even more reduced levels of ERα (0.04 vs. 0.1±0.4 in LTLTCa and 1±0.01 in MCF7Ca). Cancer stem cell marker breast cancer resistance protein (BCRP) was increased further in MCF7Calet+palb cells (2.11-fold). Based on these results, the effect of dinaciclib, a broader range CDK inhibitor, was studied. In MTT assays, dinaciclib was shown to be effective in inhibiting cell viability in all three cell types. Dinaclicib’s IC50s were 1.4 nM (MC7Ca) 9.5 nM (LTLTCa) and 12.4 nM in MCF7Calet+palb cells, while palbociclib IC50s were 1.5 μM, 11.2 μM, and 118 μM, respectively. Dinaciclib also significantly reduced mammosphere formation 0.01-fold and cell invasion/migration by 0.3-fold vs. untreated. Lastly, preliminary exome sequencing data suggested highly mutated FGFRL1, ARL11, and FCGBP genes, among others, when MCF7Calet+palb cells were compared to MCF7Ca cells. Overall, these results suggest that acquired resistance to combination therapy with letrozole and palbociclib induces genetic, molecular, and phenotypic changes in breast cancer cells, and that dinaciclib may be an effective treatment to either treat and/or prevent this resistance.

Citation Format: Armina Kazi, Emily Shukdinas, Saranya Chumsri, Gauri J. Sabnis. Characterization of breast cancer cells resistant to letrozole and palbociclib combination therapy and the potential of dinaciclib to treat such resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1014.

Abstract 3604: Effect of a new oral SERD AZD9496 on ER mediated signaling in xenograft model of postmenopausal breast cancer

Aromatase inhibitors (AIs) have made significant improvements in the treatment outcomes of patients with breast cancer. However, tumors may eventually acquire resistance. One of the proposed mechanisms of resistance to AIs is overexpression of ERα and cross-talk of ERα with growth factor receptors. Studies including our own have shown that downregulation of ER with fulvestrant may provide benefit in the treatment of AI-resistant breast cancer. Fulvestrant has been employed in the clinic as either first or second line treatment for ER positive breast cancers alone or in combination with AIs. Studies have suggested that further escalation of dose may provide further benefit. However, dose escalation of fulvestrant which is administered via intramuscular injection is difficult due to its poor solubility. To overcome this shortcoming of an injectable drug, a novel orally active SERD (selective estrogen receptor downregulator), AZD9496 was developed. We evaluated the effect of AZD9496 on the growth of hormone sensitive and anastrozole resistant breast cancer tumors using MCF-7Ca xenograft model of postmenopausal breast cancer. Mice bearing xenografts of MCF-7Ca were then treated with fulvestrant (1 mg/d-sc) or AZD9496 (5 mg/kg/d-po), alone or in combination with anastrozole (200μg/d-sc) for 23 weeks. Tumors were measured weekly and growth rate was calculated. AZD9496 was significantly better at inhibiting the growth of tumors compared to control (p<0.001) and anastrozole (p=0.04) while being equally effective as fulvestrant (p>0.99). In the second study, efficacy of AZD9496 was evaluated on against anastrozole resistant MCF-7Ca xenografts. Tumors were treated with anastrozole (200μg/d) for 13 weeks. During this time, the tumors initially regressed but eventually began to grow and had doubled in volume. At this time-point, they were regrouped to receive second line treatment. Single agent AZD9496 was marginally significant compared to continued anastrozole treatment (p=0.07). Nevertheless, second line treatment with AZD9496 was equally effective as fulvestrant (p=0.36). AZD9496 treatment was also equally effective in reducing the expression of ERα protein in MCF-7Ca xenografts as fulvestrant. Next, we measured the effect of AZD9496 on the mouse uterus. Uterine weight of mice treated with AZD9496 was not significantly different from mice that were treated with androstenedione (p=0.99). Furthermore, AZD9496 did not decrease the expression of ERα in the uterus, confirming its selectivity for mammary ERα. AZD9496 treatment was also able to reduce intratumoral aromatase activity. However, it was not due to direct inhibition of the enzyme, but due to reduction in ERα mediated signaling. These results suggest that AZD9496 may be a better alternative to fulvestrant due to its oral bioavailability, selectivity for mammary ER, and ability to reduce aromatase activity while being equally effective as fulvestrant.

Citation Format: Gauri J. Sabnis, Armina Kazi, Amanda Schech, Stephen Yu, Olga Golubeva, Angela Brodie. Effect of a new oral SERD AZD9496 on ER mediated signaling in xenograft model of postmenopausal breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3604. doi:10.1158/1538-7445.AM2017-3604

Abstract P2-09-10: A new oral SERD AZD9496 for treatment of hormone dependent postmenopausal breast cancer

Treatment of hormone sensitive breast tumors with endocrine therapy such as antiestrogens or aromatase inhibitors has improved their clinical outcomes. However, not all tumors respond and the ones that do respond may eventually acquire resistance. One of the proposed mechanisms of resistance to endocrine therapy is overexpression of ERα and cross-talk of ERα with growth factor receptors. Studies including our own have shown that downregulation of ER with pure antiestrogen fulvestrant in combination with AIs may prolong responsiveness of the tumors to endocrine agents. Fulvestrant has been employed as either first or second line treatment for ER positive breast cancers alone or in combination with AIs. Studies have suggested that further escalation of dose may provide further benefit. However, dose escalation of fulvestrant which is administered via intramuscular injection is difficult due to its poor solubility. To overcome this shortcoming of an injectable drug, a novel orally active SERD (selective estrogen receptor downregulator), AZD9496 was developed. In addition to being orally active, AZD9496 is selective for mammary ERα. In the current study, we compared the effect of AZD9496 and fulvestrant on the growth of MCF-7Ca (human estrogen receptor positive MCF-7 cells stably transfected with human placental aromatase gene) xenografts grown in ovariectomized athymic nude mice. Tumors were allowed to form with androstenedione (aromatizable source of estrogen) supplement. When the tumors reached ~250 mm3, mice were grouped such that the mean tumor volumes were not significantly different (p>0.99). Mice bearing xenografts of MCF-7Ca were then treated with fulvestrant (1 mg/d-sc) or AZD9496 (5 mg/kg/d-po), alone or in combination with anastrozole (200μg/d-sc) for 23 weeks. Tumors were measured weekly and growth rate was calculated. AZD9496 was significantly better at inhibiting the growth of tumors compared to control (p<0.001) and anastrozole (p=0.04). AZD9496 was equally effective in inhibiting the growth of MCF-7Ca xenografts as fulvestrant (growth rate, p>0.99 and tumor volume on week 23, p=0.99). In the second study, efficacy of AZD9496 was evaluated on against anastrozole resistant MCF-7Ca xenografts. Tumors were treated with anastrozole (200μg/d) for 13 weeks. During this time, the tumors initially regressed but eventually began to grow and had doubled in volume. At this time-point, they were regrouped to receive second line treatment. Single agent AZD9496 was marginally significant compared to continued anastrozole treatment (p=0.07). Nevertheless, second line treatment with AZD9496 was equally effective as fulvestrant (p=0.36). The combination of anastrozole with AZD9496/fulvestrant was more effective in reducing tumor growth compared to continued anastrozole treatment. Next, we measured the effect of AZD9496 on the mouse uterus. Uterine weight of mice treated with AZD9496 was not significantly different from mice that were treated with androstenedione (p=0.99). These results suggest that AZD9496 was selective for tumor ERα and had no effect on the uterine ERα. These results suggest that AZD9496 may be a better alternative to fulvestrant due to its selectivity for mammary ER and same efficacy as fulvestrant obtained upon oral administration.

Citation Format: Sabnis GJ, Kazi A, Schech A, Yu S, Golubeva O, Weir H, Brodie A. A new oral SERD AZD9496 for treatment of hormone dependent postmenopausal breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-09-10.

Abstract 3490: The effect of novel CYP17 inhibitor galeterone on gonadal and tumor progestogen and androgen levels in SCID mice bearing LNCaP prostate cancer xenografts

Prostate cancer growth is driven by androgen-dependent activation of the androgen receptor (AR). Inhibition of CYP17A1, a cytochrome P450 enzyme responsible for the conversion of progestogens to androgens, is a key strategy in the treatment of prostate cancer. CYP17A1 has both 17α-hydroxylase and 17,20-lyase function. The 17α-hydroxylase catalyzes the conversion of progesterone/pregnenolone to 17α-hydroxyprogesterone/17α-hydroxypregnenolone and the 17,20-lyase converts 17α-hydroxyprogesterone/17α-hydroxypregnenolone to androstenedione (AD)/dehydroepiandrosterone (DHEA). Galeterone, a novel AR degrader and antagonist, is also an inhibitor of CYP17A1 and has been shown to inhibit the growth of prostate cancer cells and tumors. However, the effects of galeterone on steroid levels affected by CYP17A1 inhibition have not been examined in vivo. Male SCID mice bearing LNCaP tumors were randomized to receive vehicle (30% beta-cyclodextran in saline) or 0.15mmol/kg galeterone (either po or sc). Mice were treated twice daily, seven days a week. Route of administration did not alter the efficacy of galeterone, which significantly reduced tumor volume (p = 0.044 and p = 0.049, sc, po). Upon completion of the experiment, tumors, testes, and plasma were collected for analysis of steroid levels. Analysis of intratumoral steroids showed an increase in progesterone (1.7- and 2-fold, sc, po) and a decrease in AD (89% and 77% reduction, sc, po). Similar results were observed in levels of pregnenolone which increased (1.4- and 3.0-fold, sc, po) while DHEA decreased (72% and 17% reduction, sc, po). Interestingly, levels of 17α-hydroxyprenenolone were increased in the sc treatment arm, suggesting selectivity of galeterone for the lyase over the hydroxylase catalytic function. Both routes of administration reduced intratumoral testosterone (97% and 77% reduction, sc, po). Intratesticular androgen levels showed similar trends compared to those in tumors. Following galeterone treatment, the levels of androgen precursors were higher in the testes than in the tumor (pregnenolone increased 2.3- and 1.5-fold sc, po; progesterone elevated 6.4- and 12.8-fold sc, po). Plasma androgen levels varied from those observed in the tumor and testes. However, galeterone treatment reduced the levels of AD (98% and 68% reduction, sc, po) and testosterone (99.6% and 99% reduction, sc, po). Together, these results show for the first time that galeterone specifically targets CYP17A1 in vivo as demonstrated by reduction of its enzymatic products DHEA, AD and testosterone. In addition, a selectivity for the lyase function was observed, as evidenced by greater decreases in DHEA and AD than in 17α-hydroxypregenelone. This is consistent with findings that patients treated with galeterone do not require cortisol replacement therapy and do not show symptoms of mineralocorticoid excess.

Citation Format: Amanda J. Schech, Gauri J. Sabnis, Stephen Yu, Vincent C.O. Njar, Douglas B. Jacoby, Peter Nelson, Ruth Dumpit, Angela M.H. Brodie. The effect of novel CYP17 inhibitor galeterone on gonadal and tumor progestogen and androgen levels in SCID mice bearing LNCaP prostate cancer xenografts. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3490.

Histone deacetylase inhibitor entinostat in combination with a retinoid downregulates HER2 and reduces the tumor initiating cell population in aromatase inhibitor-resistant breast cancer

Resistance to aromatase inhibitors (AIs) involves increased HER2. One mechanism by which HER2 may mediate resistance is through expansion of the tumor initiating cell (TIC) population. This study investigates whether combining all-trans retinoic acid (ATRA) and histone deacetylase inhibitor entinostat (ENT) can inhibit TICs and HER2 in AI-resistant cells and tumors. Modulation of cell viability and HER2 expression were assessed in AI-resistant cells treated with ATRA + ENT. Letrozole-resistant LTLT-Ca cells treated with ATRA + ENT were assayed for changes in TIC characteristics, such as TIC markers (BCRP, ALDH, and BMI-1), side population (SP), and mammosphere formation. Xenograft tumors of MCF-7Ca cells made resistant to letrozole were treated with ATRA, ATRA + letrozole, ATRA + ENT, or ATRA + ENT + letrozole. Resulting tumors were assayed for changes in TIC characteristics. Patient samples taken pre- and post-AI treatment were analyzed for changes in ERα and HER2 protein expression. Treatment with ATRA + ENT reduced HER2 expression and viability (P < 0.001) in AI-resistant cells, as well as decreased SP (P < 0.0001), mammosphere formation (P < 0.01), and expression of TIC molecular markers (P < 0.01) in LTLT-Ca. A reduction in tumor growth rate was observed in mice treated with ENT + ATRA + letrozole when compared to mice treated with single agents (P < 0.0001) or ENT + ATRA (P = 0.02). Decreased TIC characteristics, including mammosphere formation (P < 0.05), were observed in tumors from the triple combination. An increase in HER2 and downregulation in ERα protein expression was observed in patients upon resistance to AI (P < 0.005). These studies indicate that the combination of ATRA and ENT inhibits the TIC population of AI-resistant cells and may be effective in reducing tumor recurrence.

Extending aromatase inhibitor sensitivity in hormone resistant breast cancer

Aromatase inhibitors (AIs) are first-line treatment for ER+ breast cancer. However, despite responses initially, some patients can eventually acquire resistance. Moreover, 25% of all breast cancer patients do not express the estrogen receptor (ERα) and are innately resistance. In tumors of mouse models with acquired AI letrozole resistance, expression of ERα was reduced whereas HER2/growth factor signaling was enhanced. Treatment of mice with trastuzumab (HER2 antibody) reduced HER2/p-MAPK but restored ERα expression. The addition of trastuzumab to letrozole treatment when tumors progressed resulted in significantly longer tumor suppression than these drugs alone. Thus, inhibition of both HER2 and ERα signaling pathways was necessary to overcome resistance. In ERα-negative tumors, the receptor has been shown to be silenced by epigenetic modifications. Treatment of MDA-MB-231 ER-negative tumors with a histone deacetylase inhibitor, entinostat (ENT) increased expression of ERα and also aromatase. When ENT was combined with letrozole, tumor growth rate was markedly reduced compared with control tumors. ENT plus letrozole treatment also prevented the colonization and growth of MDA-MB-231 cells in the lung with significant reduction in visible and microscopic foci. These novel strategies could improve treatment for patients with acquired and innate resistance to AIs.

Abstract 621: Effect of diet-induced obesity on MCF-7Ca xenografts grown in ovariectomized athymic nude mice

Obesity has been identified as one of the risk factor for breast cancer progression. However the mechanisms underlying this link are not completely understood. Specifically, breast cancer patients who are overweight, obese or have excess abdominal fat have increased risk of local or distant (metastasis) recurrence and cancer related death. Furthermore, breast cancer patients gain weight during treatment, especially those who receive hormone depletion therapies, are at increased risk of developing obesity and metabolic syndrome in surviving patients. Importantly, the presence of obesity may influence the resistance of breast cancer to existing treatments, such as aromatase inhibitors (AIs). In an effort to understand the effect of obesity on the growth of hormone dependent breast cancer tumors, we fed ovariectomized athymic nude mice a diet containing 45% kcal fat (45% of the total calories coming from the fat in the diet) and low fat (10% kcal fat) diet. We saw that mice fed with 45%kcal fat diet had a higher rate of tumor growth (p=0.04) compared to mice that were fed with 10% kcal fat containing diet. In addition, the mice fed with 45% kcal fat had higher body weight and also higher level of insulin, as measured by C-peptide ELISA. Next, we examined the effect of insulin on the growth of MCF-7Ca cells in response to E2 or letrozole. When co-treated with 2μM of exogenous insulin (in otherwise serum starved condition), the growth of MCF-7Ca cells was not stimulated by E2 at 1nM. However, at lower does, the combination of insulin with E2 stimulated cell growth more than E2 alone. This suggests that insulin makes MCF-7Ca breast cancer cells hypersensitive to mitogenic effects of E2. Hypersensitivity to E2 is one of the suggested mechanisms of resistance to endocrine therapy. Furthermore, response to anti-proliferative effects of letrozole was also abrogated in presence of insulin (2μM). This was more pronounced at lower dose of letrozole. These results suggest that the presence of insulin makes MCF-7Ca cells less responsive to E2 and letrozole. Furthermore, tumors of mice fed with high-fat diet also had higher activation of MAPK and Akt, suggesting induction of growth factor receptor pathways. Exogenous insulin treatment of MCF-7Ca cells also resulted in reduction of ERα protein levels and increase in p-MAPK and p-AKt. These results suggest that diet-induced obesity may result in acquisition of resistance to letrozole due to development of hyperinsulinemia. We hypothesized that the presence of obesity can augment adaptation of cancer cells and impact the mechanism. This may be the result of a dramatically different hormonal milieu upon development of obesity.

Citation Format: Gauri J. Sabnis, Olga Goloubeva, Preeti Shah, Saranya Chumsri, Angela Brodie. Effect of diet-induced obesity on MCF-7Ca xenografts grown in ovariectomized athymic nude mice. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 621. doi:10.1158/1538-7445.AM2014-621

Abstract P1-07-01: Histone deacetylase inhibitor entinostat reverses epithelial to mesenchymal transition of breast cancer cells by reversing the repression of E-cadherin

Loss of ERa in breast cancer correlates with poor prognosis, increased recurrence rates and higher incidence of metastasis. In our previous studies, we have shown that histone deacetylase (HDAC) inhibitor entinostat (ENT) can upregulate ERα and aromatase in ER-negative cells and tumors, making them sensitive to aromatase inhibitors (AIs). In the current study, we are showing that ENT can also reverse epithelial to mesenchymal transition (EMT), which is considered to be a first step in the process of metastases formation. EMT is characterized by loss of intracellular adhesion (loss of E-cadherin); loss of epithelial markers such as cytokeratins and upregulation of mesenchymal markers such as vimentin; acquisition of fibroblast-like spindle morphology and increased motility. Various carcinomas undergo varying degrees of EMT and capacity to undergo EMT correlates inversely with levels of E-Cadherin. It is widely accepted that loss of E-cadherin is associated with more invasive phenotype. Epigenetic silencing of E-cadherin has been implicated in metastatic cell lines and invasive breast cancers.

Triple negative breast cancer cells such as MDA-MB-231 and Hs578T show a basal phenotype characterized by loss of E-cadherin expression and higher expression of mesenchymal markers such as N-cadherin, vimentin along with transcriptional repressors such as twist and snail.

In this study, we measured the effect of entinostat on the EMT. When MDA-MB-231 and Hs578T cells were treated with ENT, E-cadherin transcription was increased along with reduction in N-cadherin mRNA expression. Similar results were also seen in tumors of MDA-MB-231 and Hs578T xenografts treated with ENT (for 5 weeks and 2 weeks respectively). A dose dependent increase in E-cadherin was seen along with a dose dependent decrease in N-cadherin mRNA. Although, we did not observe any reduction in vimentin protein, phosphorylation of vimentin was increased and vimentin remodeling was changed as seen by immunofluorescence. We performed chromatin immunoprecipitation (ChIP) assay to measure the activation of E-cadherin promoter. Treatment of MDA-MB-231 and Hs578T cells increased the activation of E-cadherin promoter as seen by increased acetyl histones at the promoter region of E-cadherin. Twist and snail are known repressors of E-cadherin gene and we saw that ENT treatment reduced the association of twist and snail with the E-cadherin promoter. ENT was also able to downregulate twist, which may be responsible for reduced twist association with the E-cadherin promoter.

In summary, these findings suggest that HDAC inhibitor ENT can reverse EMT and may help reduce the formation of metastasis.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P1-07-01.

HDAC inhibitor entinostat restores responsiveness of letrozole-resistant MCF-7Ca xenografts to aromatase inhibitors through modulation of Her-2

We previously showed that in innately resistant tumors, silencing of the estrogen receptor (ER) could be reversed by treatment with a histone deacetylase (HDAC) inhibitor, entinostat. Tumors were then responsive to aromatase inhibitor (AI) letrozole. Here, we investigated whether ER in the acquired letrozole-resistant tumors could be restored with entinostat. Ovariectomized athymic mice were inoculated with MCF-7Ca cells, supplemented with androstenedione (Δ(4)A), the aromatizable substrate. When the tumors reached about 300 mm(3), the mice were treated with letrozole. After initial response to letrozole, the tumors eventually became resistant (doubled their initial volume). The mice then were grouped to receive letrozole, exemestane (250 μg/d), entinostat (50 μg/d), or the combination of entinostat with letrozole or exemestane for 26 weeks. The growth rates of tumors of mice treated with the combination of entinostat with letrozole or exemestane were significantly slower than with the single agent (P < 0.05). Analysis of the letrozole-resistant tumors showed entinostat increased ERα expression and aromatase activity but downregulated Her-2, p-Her-2, p-MAPK, and p-Akt. However, the mechanism of action of entinostat in reversing acquired resistance did not involve epigenetic silencing but rather included posttranslational as well as transcriptional modulation of Her-2. Entinostat treatment reduced the association of the Her-2 protein with HSP-90, possibly by reducing the stability of Her-2 protein. In addition, entinostat also reduced Her-2 mRNA levels and its stability. Our results suggest that the HDAC inhibitor may reverse letrozole resistance in cells and tumors by modulating Her-2 expression and activity.

Abstract 1311: Preclinical assessment of an HDAC inhibitor combined with a retinoid in AI resistant breast cancer

Treatment with aromatase inhibitors (AI's) is highly effective against breast cancer in ER positive postmenopausal women. However, some patients eventually become resistant to AIs. Tumor initiating cells (TIC's) represent a subpopulation of tumor cells, which show self-renewal capacity. We are focused towards discovering strategies to reduce the growth of breast cancer TICs, which may result in resistance. We have developed a xenograft model that mimics post-menopausal hormone responsive breast cancer. In this model, aromatase transfected human hormone sensitive MCF-7 cells (MCF-7Ca) are inoculated in ovariectomized athymic nude mice and allowed to grow in presence of Δ4A (aromatizable substrate of estrogen). Using this model, we have established that although AI letrozole provides a longer control over tumor growth, tumors eventually began to grow. In the current study, we investigated the effect of ATRA (All-trans Retinoic acid) (125μg/day, ip) and a histone deacetylase (HDAC) inhibitor entinostat (SNDX- 275) (50μg/day, po) with or without letrozole on letrozole resistant tumors in a xenograft model system. Ovariectomized athymic nude mice bearing xenografts of MCF-7Ca cells were treated with letrozole till they became resistant (15 weeks). At this time, the mice were grouped to receive ATRA, entinostat plus ATRA or the combination of ATRA plus entinostat plus letrozole till week 23. The mice treated with entinostat plus ATRA plus letrozole showed a significant decrease in tumor growth rate compared to mice treated with single agents or entinostat plus ATRA (p&lt;0.0001, p=0.02). On week 20, two mice from each treatment group were euthanized and tumors were harvested. The tumors were digested enzymatically with collagenase and hyaluronidase and freed of debris using centrifugation and filtration. Mammosphere forming ability of TICs in the tumor tissue was measured by seeding 10,000 viable cells from each treated tumors under non-adherent conditions to access the self-renewal capacity. The combination of ATRA plus ENT plus letrozole significantly (p&lt;0.01) reduced number of mammospheres formed compared to single agents alone. Quantitative PCR analysis of tumors cells showed a significant downregulation of the known TIC molecular markers, BCRP, ALDH, BMI-1 and Nanog compared to letrozole treated tumors. Similar results were also obtained when LTLT-Ca (long term letrozole treated MCF-7Ca) cells were treated with ATRA and entinostat in combination with letrozole and then seeded (5000 cells) in non-adherent conditions. Combination of ATRA plus entinostat plus letrozole significantly (p&lt;0.0001) reduced mammosphere-forming ability of the LTLT-Ca cells. Overall, these studies indicate that the combination of ATRA, entinostat and letrozole is effective in reducing tumor recurrence in letrozole resistant tumors.

Citation Format: Preeti Shah, Gauri J. Sabnis, Olga Goloubeva, Armina Kazi, Amanda Schech, Rabia Gilani, Yael Gau, Saranya Chumsri, Angela Brodie. Preclinical assessment of an HDAC inhibitor combined with a retinoid in AI resistant breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1311. doi:10.1158/1538-7445.AM2013-1311

Abstract 2919: Effect of selumetinib and AZD8055 on the growth of anastrozole resistant tumors

Despite significant improvement in the treatment outcome of hormone responsive post-menopausal breast cancer, some eventually acquire resistance to AIs. Using our MCF-7Ca xenograft model, we observed that although, AI anastrozole inhibited tumor growth, tumors eventually began to grow. Our previous data shows that anastrozole resistant tumors upregulate growth factor receptor pathways as they adapt to grow in the low estrogen environment. In the current study, we investigated the effect of inhibiting the growth factor receptor pathways with two signal transduction inhibitors. We treated the mice with anastrozole resistant tumors with selumetinib (AZD6244, ARRY-142866); an MEK 1/2 inhibitor and AZD8055; a dual mTORC1 and mTORC2 inhibitor, alone or in combination with anastrozole. MCF-7Ca cells were inoculated sc into ovariectomized athymic nude mice supplemented with androstenedione (100 μg/d), the substrate for aromatase conversion to estrogen. Once the tumors reached a size of ∼300mm3, the mice were assigned to one of two treatment groups so that mean tumor volumes were not significantly different (p=0.98) at the start of treatment: control, anastrozole (200μg/d). All animals continued to be supplemented with androstenedione (≥4A). The tumors in the anastrozole group doubled after 6 weeks. At week 6, the animals were regrouped into six groups such that the mean tumor volumes were not significantly different (p=0.18). The mice received the following treatments (i) anastrozole, (ii) anastrozole withdrawal (≥4A alone), (iii) selumetinib (25mg/kg/d, bid, po), (iv) selumetinib + anastrozole, (v) AZD8055 (20mg/kg/d, po), (vi) AZD8055 + anastrozole (n=10 mice/group). The treatments were given for 6 weeks (till week 12) and then the mice were euthanized, tumors were collected and analyzed. Using mixed effects model, the growth rates of tumors (≥i) between different groups of treatments were compared. The tumors of mice treated with selumetinib + anastrozole had significantly lower growth rate than both single agents (p=0.008). The growth rate of tumors of mice treated with AZD8055 + anastrozole was marginally lower than the single agents (p=0.058). Western blotting analysis of the tumors showed that treatment with anastrozole resulted in upregulation of proteins in the growth factor receptor cascade such as p-mTOR, pAkt, pMEK and pMAPK. This was accompanied by downregulation of ERα protein, consistent with previous findings. This suggests that anastrozole resistance results in adaptation of the tumors to growth factor receptor pathway. When the tumors were treated with signal transduction inhibitors such as selumetinib or AZD8055, the respective pathways were effectively inhibited, which resulted in upregulation of ERα. Our results suggest that inhibition of growth factor receptor pathway with selumetinib or AZD8055 can reverse anastrozole resistance.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2919. doi:1538-7445.AM2012-2919

Aromatase inhibitors and xenograft studies

Aromatase inhibitors (AIs) have become the front-line choice for treatment of ER+ breast cancer. Nevertheless, although patients are responsive initially, they may acquire resistance and become unresponsive to further treatment. In addition, approximately 25% of breast cancers do not express the estrogen receptor (ERα) and consequently, are innately resistant to endocrine therapy. We have investigated the mechanisms associated with this lack of treatment response using xenograft models. We found that in cells and tumors that acquired resistance to the AI letrozole therapy, expression of the ER was reduced whereas growth factor signally was enhanced, including a marked increase in HER2 expression. Treatment with trastuzumab (HER2 antibody) resulted in a significant down-regulation of HER2 and p-MAPK as well as restoration of ERα expression. Thus, when trastuzumab was added to letrozole treatment at the time of tumor progression, there was significantly prolonged tumor suppression compared to trastuzumab or letrozole alone. This suggests that inhibition of both HER2 and ERα signaling pathways are required for overcoming resistance and restoring treatment sensitivity. ER negative tumors are innately resistant to endocrine therapy. Repression of the ERα has been found to be due to epigenetic modifications such as increased methylation and histone deacetylation. We found that entinostat (ENT), a histone deacetylase inhibitor (HDACi), activated not only expression of ERα but also aromatase in MDA-MB-231 ER-negative breast cancer cells, resulting in their ability to respond to estrogen and letrozole. Treatment with ENT in combination with letrozole significantly reduced tumor growth rate in xenografts compared to control tumors (p<0.001). ENT plus letrozole treatment also prevented the colonization and growth of MDA-MB-231 cells in the lung with a significant reduction (p<0.03) in both visible and microscopic foci. These results provide a strong indication for possible use of AIs in combination with HDAC inhibitors for the treatment of ER-negative breast cancer.

Abstract 2295: Regulation of Her-2 in long-term letrozole resistant cells by HSP-90

Development of aromatase inhibitors (AIs) has significantly improved the treatment outcome of hormone responsive post-menopausal breast cancer. However, not all tumors respond and some eventually acquire resistance. Using our MCF-7Ca xenograft model, we also observed that although, AI letrozole provides a longer control over tumor growth than tamoxifen, tumors eventually began to grow. A cell line was isolated from these Long-Term Letrozole Treated tumors (LTLT-Ca). These cells and had lower expression of ER and aromatase compared to parental MCF-7Ca cells. On the other hand, Her-2/MAPK pathway was upregulated. Inhibition of Her-2 using trastuzumab resulted in reversal of resistance and restoration of sensitivity to estrogens, antiestrogens and AIs, Histone deacetylase (HDAC) inhibitor, entinostat also reversed AI resistance. Similarly, stopping letrozole treatment also resulted in reversal of letrozole resistance. In all these studies, we found that downregulation of Her-2 was associated with reversal of letrozole resistance.

In order to determine the mechanism of regulation of Her-2 in LTLT-Ca cells, we compared parental MCF-7Ca cells to LTLT-Ca for basal levels of Her-2 protein, protein stability, mRNA and mRNA stability. Cell lysates or mRNA were then collected at various time points after trating with Actinomycin D to inhibit RNA synthesis and cycloheximide to inhibit translation. Although, our previous data shows that letrozole resistant tumors do not have amplification of the Her-2 gene (FISH). LTLT-Ca cells have higher basal level of Her-2 protein and mRNA (4.5-fold higher mRNA; p&lt;0.001). We also found that Her-2 protein in LTLT-Ca cells is more stable and has a longer half-life than MCF-7Ca cells. However, Her-2 mRNA in LTLT-Ca did not have a longer half-life. In LTLT-Ca cells, more association between Her-2 and HSP-90 was observed compared to parental MCF-7Ca cells using IP. When hormone sensitive MCF-7Ca cells were placed in estrogen-deprived conditions, there was increased association of Her-2 with HSP-90. Stability of Her-2 protein was regulated by HSP-90. Inhibition of HSP-90 with HSP-90 inhibitor 17-DMAG downregulated Her-2 and also reduced the half-life of Her-2. Furthermore, 17-DMAG also effectively reduced viability of parental MCF-7Ca and LTLT-Ca cells (in 72 hours) with IC50 of 2.5μM and 1.7μM respectively. In addition, combination of letrozole with HSP-90 inhibitors such as 17-DMAG, HSP-990 and AUY9122 inhibited the growth of both LTLT-Ca and MCF-7Ca cells significantly better than single agent alone.

Our results indicate that HSP-90 plays a role in regulation of Her-2 protein, which modulates resistance to letrozole. This suggests use of HSP-90 inhibitors may inhibit the upregulation of Her-2 and thereby overcome subsequent resistance to aromatase inhibitors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2295. doi:10.1158/1538-7445.AM2011-2295

Functional activation of the estrogen receptor-α and aromatase by the HDAC inhibitor entinostat sensitizes ER-negative tumors to letrozole

Approximately 25% of breast cancers do not express the estrogen receptor-α (ERα) and consequently do not respond to endocrine therapy. In these tumors, ERα repression is often due to epigenetic modifications such as methylation and histone deacetylation. For this reason, we investigated the ability of the histone deacetylase inhibitor entinostat (ENT) to trigger reexpression of ERα and aromatase in breast cancer cells, with the notion that this treatment would restore sensitivity to the aromatase inhibitor (AI) letrozole. ENT treatment of tumor cells increased expression of ERα and aromatase, along with the enzymatic activity of aromatase, in a dose-dependent manner both in vitro and in vivo. Notably, ERα and aromatase upregulation resulted in sensitization of breast cancer cells to estrogen and letrozole. Tumor growth rate was significantly lower in tumor xenografts following treatment with ENT alone and in combination with letrozole than in control tumors (P > 0.001). ENT plus letrozole also prevented lung colonization and growth of tumor cells, with a significant reduction (P > 0.03) in both visible and microscopic foci. Our results show that ENT treatment can be used to restore the letrozole responsiveness of ER-negative tumors. More generally, they provide a strong rationale for immediate clinical evaluation of combinations of histone deacetylase and aromatase inhibitors to treat ER-negative and endocrine-resistant breast cancers.

Abstract PD05-03: HDAC Inhibitor Entinostat Restores Responsiveness of Letrozole Resistant MCF-7Ca Xenografts to AIs through Modulation of Her-2

Development of aromatase inhibitors (AIs) has significantly improved the treatment outcome of hormone responsive post-menopausal breast cancer. However, not all tumors respond and some eventually acquire resistance. We have developed a xenograft model that mimics post-menopausal hormone responsive breast cancer (MCF-7Ca). Results obtained using this model have been confirmed by numerous clinical trials. Using this model, we have established that single agent AI is better than tamoxifen in controlling tumor growth. We also observed that although, AI letrozole provides a longer control over tumor growth, tumors eventually began to grow. A cell line was isolated from these Long-Term Letrozole Treated tumors and designated as LTLT-Ca. These cells and the tumors had lower expression of ER and aromatase compared to parental MCF-7Ca cells. On the other hand, growth factor receptor Her-2 and downstream kinases such as MAPK, Akt were upregulated. Inhibition of Her-2 using trastuzumab resulted in reversal of resistance and restoration of sensitivity to estrogens, antiestrogens and AIs.

In order to restore sensitivity to AIs, we treated letrozole resistant MCF-7Ca tumors to HDAC inhibitor entinostat (ENT). In studies performed with ER negative MDA-MB-231 cells, ENT has upregulated ERα and restored sensitivity to AIs. To examine the effect of this combination on tumors that have acquired resistance to letrozole, we utilized MCF-7Ca xenografts model. We inoculated ovariectomized athymic nude mice MCF-7Ca cells and then tumors were allowed to form in the presence of androstenedione (≥4A), aromatizable substrate for estrogen. When the tumors reached ∼300mm3, the mice were randomized such that the mean tumor volume was not significantly different across the groups (p=0.13). 50 mice were treated with letrozole for total 16 weeks, during this time the tumors regressed, but eventually began to grow. When the tumors had reached double the initial volume, they were randomized again into 5 groups; letrozole continued, exemestane (250µg/day), ENT (50µg/day) or the combination of ENT with letrozole or exemestane. The mice were treated till week 26. The growth rates of tumors of mice treated with the combination of ENT with letrozole or exemestane was significantly better than single agent alone (P&lt;0.05). Tumors were collected at autopsy and analyzed. ENT was able to increase ERα expression and aromatase activity in the letrozole resistant tumors. In addition, ENT also downregulated Her-2, p-Her-2, p-MAPK and p-Akt.

We have shown that upregulation of Her-2 in LTLT-Ca cells is a result of longer half-life of Her-2 protein and not gene amplification. To determine the mechanism of effect of ENT on Her-2 expression, we examined the interaction of Her-2 with HSP-90 in LTLT-Ca cells and effect of ENT on this interaction. We observed that ENT reduced the association of Her-2 protein with HSP-90, possibly reducing the stability of Her-2 protein the LTLT-Ca cells. This reduced interaction was also seen the tumors treated with the combination of ENT plus letrozole or exemestane. Our results suggest that HDAC inhibitor entinostat may modulate Her-2 and result in reverse the acquired resistance of letrozole resistant cells and tumors.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD05-03.

Abstract 4611: Growth inhibitory effects of lapatinib on letrozole resistant breast cancer cells via HER2-AIB1-ERα crosstalk

Aromatase inhibitors (AI), such as letrozole, provide effective endocrine therapy for estrogen receptor positive (ER+) breast cancers, but not all patients respond to AIs and those that do may become resistant. Thus, it is clinically important to determine the factors and signaling mechanisms involved in AI resistance and provide potential therapeutic strategies to overcome and/or prevent it. Previous work from this laboratory comparing long-term letrozole treated MCF-7Ca breast cancer (LTLTCa) cells resistant to growth inhibition by letrozole with their parental letrozole-sensitive MCF-7Ca cells, indicated that AI resistance involves a switch from estrogen (E2)-ERα dependent to HER2/MAPK pathway-dependent growth and that inhibition of HER2/MAPK pathway signaling via trastuzumab can restore ERα levels and letrozole sensitivity. We observed that the upregulation of HER2 is through protein stabilization and not gene amplification. This current study further investigates the factors and mechanism(s) involved in letrozole resistance in LTLTCa cells, specifically steroid receptor coactivator Amplified in Breast Cancer-1 (AIB1/SRC-3). AIB1 is often overexpressed in breast cancer cells, is associated with poor prognosis, and has been linked with both E2-ERα and HER2 signaling. Indeed, western blot analyses showed that LTLTCa cells have 2.8-fold higher AIB1 protein levels than MCF-7Ca cells (p &lt; 0.05, n= 3). In order to determine how AIB1 is involved in the cross talk between HER2 and ERα signaling in LTLTCa cells, the effect of HER1/HER2 kinase inhibitor lapatinib (Lap) on LTLTCa cells. As expected, Lap inhibited cell proliferation of LTLTCa cells in MTT assays (IC50= 1 µM; n= 3). Additional experiments were then conducted comparing vehicle- and 1 µM Lap-treated LTLTCa cells after 24 h (n= 3). Lap had no effect on total HER2 expression, but it did decrease HER2 downstream activation of MAPK and PI3K/Akt pathways, as indicated by decreased protein expression of phosphorylated forms of ERK1/2 by 98.9% (p &lt; 0.001) and of Akt by 55% (p &lt; 0.01), respectively. Concurrently, AIB1 protein levels in LTLTCa cells also decreased by 48.0% (p &lt; 0.05). Lastly, experiments were done to investigate the link between HER2/AIB1/ERα signaling with resistance of LTLTCa cells to letrozole growth inhibition. Compared with vehicle-treated cells, Lap-treated LTLTCa cells had decreased protein expression of cyclin D1 by 65.7% (p &lt; 0.01) and of transporter breast cancer resistance protein (BCRP) by 45.2% (p &lt; 0.05). Cyclin D1 and BCRP are involved in cell proliferation and drug efflux, respectively, and both are known to be regulated by HER2, AIB1, and/or ERα. Overall, these preliminary results suggest that AIB1 plays a role in AI resistance in LTLTCa cells via crosstalk with HER2 and ERα, and that it can be therapeutically targeted by lapatinib.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4611.

Progress in aromatase research and identification of key future directions

The IX International Aromatase Conference focused upon key developments in research related to the aromatase enzyme that had occurred since the last meeting. A session took place at the conclusion of conference discussing key areas for future research and issues currently facing researchers in the field. While significant progress on understanding structural elements of the enzyme and regulatory mechanisms of both the gene and protein provides an excellent basis for development of improved aromatase inhibitors and exploration of the important problem of aromatase inhibitor resistance, significant challenges remain. Increasing the speed with which findings are translated into clinical practice and finding an appropriate balance between basic and translational research were identified as areas which require further attention before the next meeting in 2010.

Combination of HDACi entinostat (SNDX-275) with letrozole provides control over tumor growth in MDA-MB-231 xenograft model

Abstract #6128

It is well established that approximately 75% of human breast cancers are ER+ and therefore treated with endocrine therapy. In a past few decades endocrine therapy has made significant advancements. However, the application of these agents is limited to ER+ cancers since ER- patients are unresponsive to endocrine therapy primarily due to lack of ER expression in the tumor. The main purpose of this project is to determine whether ER- breast cancer tumors that display poor anti-proliferative response to aromatase inhibitor (AI) letrozole, can be sensitized by co-treatment with HDACi entinostat. Based on preliminary studies, we hypothesize that by inhibiting HDAC ER is re-expressed making the cells sensitive to the anti-proliferative effects of AIs. In our previous studies we have shown that HDACi entinostat can revert the ERα repression and upregulate ERα and aromatase in vitro and in vivo. In this study we are showing that this activation of aromatase and re-expression of ERα renders ER- breast cancer tumors (xenografts of MDA-MB-231 cells) responsive to letrozole.&#x2028; MDA-MB-231 xenografts were grown in ovariectomized female nude mice. Mice were inoculated with 2.5X 106 cells per site subcutaneously. When the tumors reached a measurable size ∼150 mm3, the mice were grouped into 6 groups (n=10), such that the mean tumor volumes across the groups was not statistically different (p=0.99). The mice were injected with Δ4A (100 μg/day), Δ4A plus letrozole (10 μg/day), entinostat (2.5 mg/kg/day), entinostat plus Δ4A, entinostat plus Δ4A plus letrozole or vehicle. The mice were injected 5 times a week. The tumors were measured every week with calipers and the tumor volumes were calculated using formula, 4/3 π r12r2. The mice in the entinostat plus Δ4A plus letrozole group had the least tumor growth rate (0.004+0.081), which was lower than entinostat plus Δ4A (0.115+0.079) and Δ4A plus letrozole (0.096+0.080). This data suggests a trend towards improved inhibition of tumor growth with combination of entinostat plus letrozole. The mice were sacrificed on week 9 due to large tumor volumes. The tumors and uteri were excised, cleaned, weighed and stored for additional analysis.&#x2028; In addition, ability of this combination to inhibit migration in vitro was examined by wound healing assay. The combination of entinostat plus letrozole provides superior inhibition of migration (p&lt;0.001) compared to control, entinostat and letrozole alone. This suggests that the combination of entinostat plus letrozole has potential of inhibiting metastatic spread along with tumor growth.&#x2028; These findings indicate that ERα in ER- breast cancer cells is silenced along with aromatase but can be restored with HDACi. Thus activation of silenced ER and intratumoral aromatase by HDACi could open a new avenue for management of ER- advanced breast cancer.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 6128.

Stopping treatment can reverse acquired resistance to letrozole

Using the intratumoral aromatase xenograft model, we have observed that despite long-lasting growth inhibition, tumors eventually begin to grow during continued letrozole treatment. In cells isolated from these long-term letrozole-treated tumors (LTLT-Ca), estrogen receptor-alpha (ERalpha) levels were decreased, whereas signaling proteins in the mitogen-activated protein kinase cascade were up-regulated along with human epidermal growth factor receptor 2 (Her-2). In the current study, we evaluated the effect of discontinuing letrozole treatment on the growth of letrozole-resistant cells and tumors. The cells formed tumors equally well in the absence or presence of letrozole and had similar growth rates. After treatment was discontinued for 6 weeks, letrozole was administered again. Marked tumor regression was observed with this second course of letrozole treatment. Similarly, in MCF-7Ca xenografts, a 6-week break in letrozole treatment prolonged the responsiveness of the tumors to letrozole. To understand the mechanisms of this effect, LTLT-Ca cells were cultured in the absence of letrozole for 16 weeks. The resulting cell line (RLT-Ca) exhibited properties similar to MCF-7Ca cells. The cell growth was inhibited by letrozole and stimulated by estradiol. The expression of phosphorylated mitogen-activated protein kinase (MAPK) was reduced and ERalpha and aromatase levels increased compared with LTLT-Ca cells and were similar to levels in MCF-7Ca cells. These results indicate that discontinuing treatment can reverse letrozole resistance. This could be a beneficial strategy to prolong responsiveness to aromatase inhibitors for patients with breast cancer.

Combination of anastrozole with fulvestrant in the intratumoral aromatase xenograft model

Although the aromatase inhibitor anastrozole has been shown to be very effective in the treatment of hormone-dependent postmenopausal breast cancer, some patients with advanced disease will develop resistance to treatment. To investigate therapeutic strategies to overcome resistance to anastrozole treatment, we have used an intratumoral aromatase model that simulates postmenopausal breast cancer patients with estrogen-dependent tumors. Growth of the tumors in the mice was inhibited by both anastrozole and fulvestrant compared with the control tumors. Nevertheless, tumors had doubled in size at 5 weeks of treatment. We therefore investigated whether switching the original treatments to anastrozole or fulvestrant alone or the combination of anastrozole plus fulvestrant would reduce tumor growth. The results showed that the best strategy to reverse the insensitivity to anastrozole or fulvestrant is to combine the two agents. Additionally, the tumors treated with anastrozole plus fulvestrant from the beginning had only just doubled their size after 14 weeks of treatment, whereas the anastrozole and fulvestrant treatments alone resulted in 9- and 12-fold increases in tumor size, respectively, in the same time period. Anastrozole plus fulvestrant from the beginning or in sequence was associated with down-regulation of signaling proteins involved in the development of hormonal resistance such as insulin-like growth factor type I receptor beta, mitogen-activated protein kinase (MAPK), p-MAPK, AKT, mammalian target of rapamycin (mTOR), p-mTOR, and estrogen receptor alpha compared with tumors treated with anastrozole or fulvestrant alone. These results suggest that blocking the estrogen receptor and aromatase may delay or reverse the development of resistance to aromatase inhibitors in advanced breast cancer patients.

Toremifene-atamestane; alone or in combination: predictions from the preclinical intratumoral aromatase model

Since most breast cancers occur in postmenopausal women and are hormone dependent, we developed a model system that mimics this situation. In this model, tumors of human estrogen receptor (ER) positive breast cancer cells stably transfected with aromatase (Ac-1) are grown in immune-compromised mice. Using this model we have explored a number of therapeutic strategies to maximize the antitumor efficacy of antiestrogens (AEs) and aromatase inhibitors (AIs). This intratumoral aromatase xenograft model has proved accurate in predicting the outcome of several clinical trials. In this current study we compared the effect of an AE toremifene and steroidal AI atamestane, alone or in combination, on growth of hormone-dependent human breast cancer. We have also compared toremifene plus atamestane combination with tamoxifen in this study. The growth of Ac-1 cells was inhibited by tamoxifen, toremifene and atamestane in vitro with IC(50) values of 1.8+/-1.3 microM, 1+/-0.3 microM and 60.4+/-17.2 microM, respectively. The combination of toremifene plus atamestane was found to be better than toremifene or atamestane alone in vitro. The effect of this combination was then studied in vivo using Ac-1 xenografts grown in ovariectomized female SCID mice. The mice were injected with toremifene (1000 microg/day), atamestane (1000 microg/day), tamoxifen (100 microg/day), or the combination of toremifene plus atamestane. In this study, our results indicate that the combination of toremifene plus atamestane was as effective as toremifene or tamoxifen alone but may not provide any additional benefit over toremifene alone or tamoxifen alone.

Role of androgens on MCF-7 breast cancer cell growth and on the inhibitory effect of letrozole

Previous work has shown that androgens inhibit breast cancer cells and tumor growth. On the other hand, androgens can be converted to mitogenic estrogens by aromatase in breast cancer cells. Here, we report that androgens, such as the aromatizable androstenedione and the non-aromatizable 5alpha-dihydrotestosterone, inhibit MCF-7 cell proliferation. This effect is observed only in the absence or at a low concentration of estrogens and is evident in cells with low aromatase activity. Growth of a new aromatase stably transfected MCF-7 cell line (Ac1) was stimulated by conversion of androstenedione into estrogens and was sensitive to aromatase inhibitors. We show that blockade of the androgen receptor (AR) in these cells by the antiandrogen casodex or by the anti-AR small interfering RNA inhibited the antiproliferative effect of dihydrotestosterone and letrozole (aromatase inhibitor). We also show that suppression of the estrogen-induced antiapoptotic protein Bcl-2 may be involved in the antiproliferative effects of androgens and letrozole. These effects can be reversed by casodex. In conclusion, the results suggest that aromatase inhibitors may exert their antiproliferative effect not only by reducing the intracellular production of estrogens but also by unmasking the inhibitory effect of androgens acting via the AR.

The role of growth factor receptor pathways in human breast cancer cells adapted to long-term estrogen deprivation

To study the long-term effects of estrogen deprivation on breast cancer, MCF-7Ca human estrogen receptor-positive breast cancer cells stably transfected with human aromatase gene were cultured in the steroid-depleted medium for 6 to 8 months until they had acquired the ability to grow. Proliferation of these cells (UMB-1Ca) was accompanied by increased expression of human epidermal growth factor receptor 2, increased activation of AKT through phosphorylation at Ser473 and Thr308, and increased invasion compared with parental MCF-7Ca cells. Estrogen receptor expression was also increased 5-fold. Although growth was inhibited by the antiestrogen fulvestrant, the IC50 was 100-fold higher than for parental MCF-7Ca cells. Aromatase inhibitor letrozole also inhibited growth at 10,000-fold higher concentration than required for MCF-7Ca cells, whereas anastrozole, exemestane, formestane, and tamoxifen were ineffective at 100 nmol/L. Growth of UMB-1Ca cells was inhibited by phosphatidylinositol 3-kinase inhibitor wortmannin (IC50 approximately 25 nmol/L) and epidermal growth factor receptor kinase inhibitor gefitinib (ZD 1839; IC50 approximately 10 micromol/L) whereas parental MCF-7Ca cells were insensitive to these agents. Concomitant treatment of UMB-1Ca cells with the signal transduction inhibitors and anastrozole and tamoxifen restored their growth inhibitory effects. These studies show that estrogen deprivation results in up-regulation of growth factor signaling pathways, which leads to a more aggressive and hormone refractory phenotype. Cross-talk between ER and growth factor signaling was evident as inhibition of these pathways could restore estrogen responsiveness to these cells.