Abstract GS6-05: Gain-of-function kinase library screen identifies FGFR1 amplification as a mechanism of resistance to antiestrogens and CDK4/6 inhibitors in ER+ breast cancer

Background: CDK4/6 inhibitors have been approved in combination with endocrine therapy for treatment of ER+ metastatic breast cancer. The goal of this study was to discover mechanisms of resistance to ER antagonists alone and in combination with CDK4/6 inhibitors.

Results: To achieve this goal, we used lentiviral vectors to individually express 559 human kinase open reading frames (ORFs) in ER+ MCF7 human breast cancer cells treated with fulvestrant ± the CDK4/6 inhibitor ribociclib. In MCF7 cells treated with fulvestrant alone or with ribociclib, we identified 15 and 17 kinases associated with resistance, respectively. Ten of these kinases overlapped in both groups. In a secondary screen, MCF7 cells were stably transduced with V5-tagged lentiviruses expressing the positive 'hits' for treatment with fulvestrant/ribociclib. Five of 17 kinases (FGFR1, FRK, HCK, FGR, CRKL) were confirmed to induce resistance to fulvestrant/palbociclib and fulvestrant/ribociblib. Survey of TCGA for copy number alterations and/or expression of these 5 genes showed only FGFR1 to be amplified/overexpressed in ˜15% of ER+ breast cancers. Experiments in vitro showed that ER+/FGFR1-amplified (amp) MDA-134, CAMA-1 and HCC1500 human breast cancer cells and MCF7 cells stably transduced with FGFR1 were relatively resistant to estrogen deprivation, fulvestrant and fulvestrant/palbociclib compared to non-FGFR1 amp MCF7 cells. This resistance was abrogated by treatment with the FGFR tyrosine kinase inhibitor (TKI) lucitanib. Treatment with fulvestrant or palbociclib alone modestly delayed growth of ER+/FGFR1-amp breast cancer patient-derived xenografts (PDX) established in nude mice. However, addition of the FGFR TKI erdafitinib to fulvestrant/palbociclib resulted in marked PDX regression in all mice without associated toxicity and a complete cell cycle arrest measured by Ki67. Treatment of FGFR-amp cells with FGF-2 strongly induced CCND1 (cyclin D1) expression. Downregulation of CCND1 with CCND1 RNAi oligonucleotides restored sensitivity of FGFR1-amp cells to fulvestrant/palbociclib, thus phenocopying the effect of FGFR TKIs. Conversely, overexpression of CCND1 in MCF7 cells induced resistance to estrogen deprivation and to fulvestrant ± palbociclib. Finally, we examined next gen sequencing of cell free tumor DNA by Guardant360 in 34 patients before and after progression on CDK4/6 inhibitor. In 10/34 (29%) post-progression specimens, we detected alterations in the FGFR pathway: FGFR1 amplification (n=7), FGFR1 N546K (n=1), FGFR2 N549K (n=1), and FGFR2 V395D (n=1) activating mutations.

Conclusions: These data suggest aberrant FGFR signaling is a mechanism of resistance to anti-ER therapies ± CDK4/6 inhibitors. We posit overexpression of cyclin D1 induced by both FGFR signaling and ER transcription plays a role in drug resistance. Based on these findings we propose ER+/FGFR1 amplified breast cancers are endocrine resistant and should be candidates for treatment with combinations of ER and FGFR antagonists. Accordingly, we have initiated a phase Ib trial of fulvestrant, palbociclib and erdafitinib in patients with antiestrogen resistant ER+/HER2-negative breast cancer with FGFR1-4 amplification.

Citation Format: Formisano L, Lu Y, Jansen VM, Bauer JA, Hanker A, Gonzalez Ericsson P, Lee K-M, Nixon MJ, Guerrero-Zotano AL, Schwarz LJ, Sanders M, Sudhan D, Dugger TC, Cruz MR, Behdad A, Cristofanilli M, Bardia A, O'Shaughnessy J, Mayer IA, Arteaga CL. Gain-of-function kinase library screen identifies FGFR1 amplification as a mechanism of resistance to antiestrogens and CDK4/6 inhibitors in ER+ breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr GS6-05.

Abstract P6-12-09: Pan-HER, an antibody mixture with antitumor activity against drug-resistant HER2-overexpressing breast cancers with high ERBB ligand expression

Background: Amplification/overexpression of ERBB receptors and/or ligands has been associated with resistance to anti-HER2 therapies. Pan-HER is a mixture of six antibodies targeting each of the ERBB receptors, EGFR, HER2 and HER3, with synergistic pairs of antibodies. Each pair of antibodies simultaneously blocks ligand binding and/or induces target degradation, thus preventing compensatory mechanisms to anti-ERBB therapies. We examined the antitumor activity of Pan-HER against drug-sensitive and -resistant HER2+ breast cancer cells and xenografts.

Results: Pan-HER exhibited potent growth inhibitory activity against a panel of HER2+ breast cancer cells (BT474, MDA-453, MDA-361, SUM190, HCC1954, UACC893 and SKBR3). Growth inhibition was associated with internalization and degradation of EGFR, HER2 and HER3. Pan-HER was superior to the combination of trastuzumab/pertuzumab (TP) against HER2+/PIK3CA mutant MDA-361, HCC1954, UACC893 and MDA-453 cells. We next compared the effect of Pan-HER against BT474, HCC1954 and MDA-361 xenografts established in nude mice to that of trastuzumab/lapatinib (TL), TP and T-DM1. All treatments were effective across the panel of xenografts. In mice with MDA-361 tumors, Pan-HER and TP were superior to TL. Immunoblot analysis showed significant downregulation of EGFR, HER2 and HER3 only in tumors treated with Pan-HER. After a complete response, treatment was discontinued. Among mice with BT474 xenografts treated with TP, TL and T-DM1, 25-50% of mice exhibited a tumor recurrence within 50 weeks of follow-up, while no recurrences were registered in mice treated with Pan-HER. Tumors recurring after TP and T-DM1 expressed significantly higher HER3 and P-HER3 protein levels and NRG1 mRNA levels. HCC1954 xenografts recurring after T-DM1 also overexpressed NRG1 mRNA compared to tumors before therapy.

We next examined the effect of Pan-HER against trastuzumab-resistant HR6 (BT474) cells (Ritter et al. CCR 2007) and HCC1954 and UACC893 cells with acquired resistance to T-DM1 (TDR; IC50 >5-, >6- and 600-fold in HR6, UACC893-TDR and HCC1954-TDR cells, respectively, vs. parental cells). All T-DM1-resistant cells expressed significantly higher HER3 and P-HER3 protein levels and NRG1 mRNA and protein levels. Treatment with the HER3 neutralizing antibody LJM716 resensitized HR6 and HCC1954-TDR cells to T-DM1, suggesting a causal association between the NRG1-HER3 axis and drug resistance. Mice with HR6 tumors were treated with Pan-HER, TL, TP and T-DM1. Only Pan-HER arrested HR6 tumor growth and downregulated EGFR, HER2, HER3, P-HER3 and P-AKT. Finally, HCC1954-TDR tumors rapidly grew in vivo despite treatment with T-DM1. Administration of Pan-HER to mice bearing HCC1954-TDR xenografts growing in the presence of T-DM1, induced rapid tumor regressions.

Conclusions: These data suggest that multitarget therapeutic interventions, such as Pan-HER, which simultaneously remove and/or block all ERBB receptors and ligands, are a feasible and effective approach against HER2-overexpressing cancers both sensitive and resistant to anti-HER2 therapies.

Citation Format: Schwarz LJ, Hutchinson KE, Estrada MV, Sanders ME, Dugger TC, Formisano L, Guerrero AL, Red-Brewer M, Young CD, Lantto J, Pedersen MW, Kragh M, Horak ID, Arteaga CL. Pan-HER, an antibody mixture with antitumor activity against drug-resistant HER2-overexpressing breast cancers with high ERBB ligand expression [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 P6-12-09.

Reversal of tamoxifen resistance of human breast carcinomas in vivo by neutralizing antibodies to transforming growth factor-beta

BACKGROUND

Overexpression of transforming growth factor (TGF)-beta has been reported in human breast carcinomas resistant to antiestrogen tamoxifen, but the role of TGF-beta in this resistant phenotype is unclear. We investigated whether inhibition of TGF-beta2, which is overexpressed in LCC2 tamoxifen-resistant human breast cancer cells, could modify antiestrogen resistance.

METHODS

TGF-beta2 expression was evaluated in LCC2 cells and tamoxifen-sensitive LCC1 cells by northern blot analysis. Secreted TGF-beta activity was quantified by use of an 125I-TGF-beta competitive radioreceptor assay. Sensitivity to tamoxifen was measured in a soft agarose colony-forming assay and in a xenograft model in nude and beige/nude mice. Natural killer (NK) cell cytotoxicity was measured by 51Cr release from LCC1 and LCC2 cell targets coincubated with human peripheral blood mononuclear cells. Decrease in TGF-beta2 expression in LCC2 cells was achieved by treatment with antisense oligodeoxynucleotides and confirmed by TGF-beta2 immunoblot analysis.

RESULTS AND CONCLUSIONS

The proliferative response of LCC2 cells to tamoxifen in vitro was not altered by TGF-beta neutralizing antibodies. However, established LCC2 tumors in nude mice treated with tamoxifen plus TGF-beta antibodies failed to grow, whereas tumors treated with tamoxifen plus a control antibody continued to proliferate. This reversal of tamoxifen resistance by TGF-beta antibodies did not occur in beige/nude mice, which lack NK-cell function, suggesting that immune mechanisms may be involved in the antitumor effects of tamoxifen. Antisense TGF-beta2 oligodeoxynucleotides enhanced the NK sensitivity of LCC2 cells in the presence of tamoxifen. Finally, LCC1 tumors were markedly more sensitive to tamoxifen in NK-active than in NK-deficient mice.

IMPLICATIONS

These data suggest that host NK function mediates, in part, the antitumor effect of tamoxifen and that TGF-beta2 may abrogate this mechanism, thus contributing to tamoxifen resistance.

Blockade of transforming growth factor-beta signaling does not abrogate antiestrogen-induced growth inhibition of human breast carcinoma cells

We have studied the role of autocrine transforming growth factor-beta (TGF-beta) signaling on antiestrogen-mediated growth inhibition of hormone-dependent T47D and MCF-7 human breast carcinoma cells. Tamoxifen treatment increased the secretion of TGF-beta activity into serum-free cell medium and the cellular content of affinity cross-linked type I and III TGF-beta receptors in both cell lines. Anti-pan-TGF-beta antibodies did not block anti-estrogen-induced recruitment in G1 and inhibition of anchorage-dependent and -independent growth of both cell lines. Early passage MCF-7 cells, which exhibit detectable type II TGF-beta receptors at the cell surface and exquisite sensitivity to exogenous TGF-beta1, were transfected with a tetracycline-controllable dominant-negative TGF-betaRII (DeltaRII) construct. Although the TGF-beta1 response was blocked by removal of tetracycline in MCF-7/DeltaRII cells, tamoxifen-mediated suppression of Rb phosphorylation, recruitment in G1, and inhibition of cell proliferation were identical in the presence and absence of tetracycline. TGF-beta1 treatment up-regulated the Cdk inhibitor p21 and induced its association with Cdk2 in MCF-7 cells; these responses were blocked by the DeltaRII transgene product. In MCF-7 cells with a functional TGF-beta signaling pathway, tamoxifen did not up-regulate p21 nor did it induce association of p21 with Cdk2, suggesting alternative mechanisms for antiestrogen-mediated cytostasis. Finally, transfection of late-passage, TGF-beta1 unresponsive MCF-7 cells with high levels of TGF-betaRII restored TGF-beta1-induced growth inhibition but did not enhance tamoxifen response in culture. Taken together these data strongly argue against any role for TGF-beta signaling on tamoxifen-mediated growth inhibition of hormone-dependent breast cancer cells.

The multifunctional role of transforming growth factor (TGF)-beta s on mammary epithelial cell biology

The transforming growth factor-beta s are potent growth inhibitors of normal and transformed breast epithelial cells in culture. In vivo, these peptides modulate the development of the mouse mammary gland. Tissue-specific overexpression of mature TGF-beta 1 in transgenic mice results in mammary gland atrophy and prevention of carcinogen-induced breast tumorigenesis. However, the inhibitory effect of endogenous or exogenous TGF-beta s on established tumor cells is less clear. Several published circumstantial and more direct data argue that, in some cases, the tumor cell TGF-beta s may contribute to the maintenance and/or progression of tumor cells in an intact host by modulating their interaction with host factors. This differential role of the TGF-beta s on mammary cells as determined by their normal or transformed phenotype as well as the biological and clinical implications of these data are discussed.

Epidermal growth factor receptors in human breast carcinoma cells: a potential selective target for transforming growth factor alpha-Pseudomonas exotoxin 40 fusion protein

Epidermal growth factor (EGF) receptors are expressed in high levels by some poor prognosis breast tumors. We have examined the cytotoxic effect of the tumor growth factor alpha (TGF alpha)-delta Cys-Pseudomonas exotoxin (PE40) recombinant fusion protein on normal and tumorigenic human breast epithelial cells in vitro and in vivo. The MDA-468, MDA-231, BT-20, and MCF-7ADR estrogen receptor-negative, EGF receptor-rich breast cancer lines were exquisitely sensitive in vitro to TGF alpha-delta Cys-PE40 with a 50% inhibitory concentration of < or = 0.02 nM. The estrogen receptor-positive, low EGF receptor MCF-7, ZR75-1, and T47D cells were less sensitive to the fusion toxin with a 50% inhibitory concentration of > 0.2 nM. The nontumorigenic cell lines 184, 184A1, and 184B5 were relatively resistant to TGF alpha-delta Cys-PE40 despite exhibiting high levels of EGF receptors. Continuous i.p. administration of TGF alpha-delta Cys-PE40 via an osmotic minipump at a dose of 0.4 microgram/g/day over 7 days inhibited MDA-468, MA-231, and BT-20 but not MCF-7 tumor growth in female athymic mice. Host tissue toxicity was not observed with this dose of TGF alpha-delta Cys-PE40. Mixed MDA-468/MCF-7 tumors were established in nude mice after coinoculation of both cell types in estrogen-supplemented animals. EGF receptor immunohistochemistry and immunoblot procedures indicated that TGF alpha-PE40 eliminated the MDA-468 cells while sparing the adjacent MCF-7 cells. By immunoblot, EGF receptors were consistently more abundant in tumor tissue than in adjacent nontumor tissue from the same mastectomy specimen (n = 7). These data support the notion that EGF receptors can be selectively targeted in human breast cancer cells for the delivery of antitumor agents. Further clinical studies with TGF alpha-delta Cys-PE40 and other chimeric toxins using the same cellular target will address this possibility.

Ligand-like effects induced by anti-c-erbB-2 antibodies do not correlate with and are not required for growth inhibition of human carcinoma cells

The c-erbB-2 gene encodes a M(r) 185,000 tyrosine kinase receptor (p185) with extensive homology to the epidermal growth factor receptor. We have conducted mechanistic studies with several anti-p185 monoclonal antibodies (TAb 250, -255, -257, -260, and -263) directed against the extracellular domain of p185 utilizing the SKBR-3, BT-474, and SKOV-3 cancer cell lines. Several of these antibodies exhibited ligand-mimicking properties: they induced tyrosine phosphorylation of p185; increased the catalytic activity of the receptor substrate phospholipase C-gamma 1; exhibited time- and pH-dependent internalization; induced receptor down-regulation; and increased the turnover of the p185 protein delta 3-fold. However, there was not a universal correlation between the antibody-mediated ligand-like effects and growth inhibition. TAb 250 inhibited BT-474 cells but did not alter p185 phosphotyrosine content or increase receptor turnover in these cells. TAb 260 increased p185 protein turnover but did not affect proliferation of the SKOV-3 cell line. Furthermore, blockade of TAb 250-induced receptor phosphorylation with the tyrosine kinase inhibitor tyrphostin 50864-2 did not abrogate TAb 250-mediated growth inhibition of SKBR-3 cells. These data suggest that ligand-like effects mediated by p185 antibodies are not critical for the growth inhibition of c-erbB-2-overexpressing carcinoma cells.

Evidence for a positive role of transforming growth factor-beta in human breast cancer cell tumorigenesis

To determine the biological role of transforming growth factor-beta (TGF-beta) in mammary carcinomas in vivo, estrogen-dependent MCF-7 cells were transfected with a mouse TGF-beta 1 cDNA. Growth characteristics in culture were not altered in the transfected cells. However, the MCF-7/TGF-beta 1 cells formed tumors in ovariectomized athymic mice in the absence of estrogen supplementation. Daily injections of human recombinant TGF-beta 1 supported tumor formation by wild-type MCF-7 cells in castrated nude mice in the absence of exogenous estradiol. In another approach to the same question, the effect of anti-TGF-beta antibodies on tumor formation by estrogen-independent MDA-231 cells was examined. The 2G7 IgG2b (2G7) antibody, which neutralizes TGF-beta 1, -beta 2, and -beta 3, blocked the formation of MDA-231 tumors at the injection site and lung metastases in nude mice. Inoculation of MDA-231 cells inhibited, while injection of 2G7 increased mouse spleen natural killer (NK) activity. 2G7 did not inhibit MDA-231 tumors and metastases in NK-deficient animals. Finally, medium conditioned by MDA-231 cells inhibited lymphocyte-mediated NK activity; this inhibition was abrogated by 2G7, but not by a control IgG2. These data support a positive role for tumor cell TGF-beta in the maintenance and/or progression of mammary carcinoma cells in an intact host.

Growth stimulation of human breast cancer cells with anti-transforming growth factor beta antibodies: evidence for negative autocrine regulation by transforming growth factor beta

Exogenous TGF beta inhibits the proliferation of human breast cancer cells in vitro. These cells synthesize and secrete TGF beta into their medium predominantly in a latent form. With neutralizing antibodies against native, biologically active TGF beta (278ab and 282ab), we have examined whether HS578T and MDA-231 breast cancer cells utilize their endogenous TGF beta for growth regulation. Low levels of TGF beta activity were detectable in conditioned medium from confluent monolayers of both cell lines in the absence of acid or protease treatment as measured by radioreceptor assay. When added to subconfluent monolayers of the respective cell line, this untreated conditioned medium inhibited DNA synthesis and cell proliferation. This inhibition was blocked by anti-TGF beta antibodies, whereas nonimmune rabbit IgG had no effect. Similar to exogenous TGF beta 1, this conditioned medium induced a dose-dependent increase in steady-state TGF beta 1 mRNA levels when added to subconfluent HS578T cells; this increase was blocked by the 278ab. Consistent with the above, preincubation of either cell line with anti-TGF beta antibodies increased subsequent specific binding of 125I-TGF beta to cell surface receptors without changing binding affinity. Addition of 278ab to quiescent HS578T or MDA-231 cells induced a dose-dependent increase in [3H]thymidine incorporation. Both antibodies stimulated cell proliferation in serum-free medium and anchorage-independent growth of both cell lines. Finally, incubation of HS578T cells with 278ab under serum-free conditions decreased the basal level of TGF beta 1 message expression. These data indicate that cultured human breast cancer cells utilize endogenously produced TGF beta as an autocrine negative growth regulator.