Androgen Receptor Function and Targeted Therapeutics Across Breast Cancer Subtypes

Emily A. Kolyvas; Carlos Caldas; Kathleen Kelly; Saif S. Ahmad

Breast Cancer Res. 2022;24(79)

AR Prognostic Value by Subtype

Luminal Breast Cancer

PAM50 is a commonly used method of classifying intrinsic subtypes of BC using a minimal gene set.^[26] Under the PAM50 classification, BC, which is both ER+ and PgR+, is subclassified as luminal A and B. The luminal A are low proliferating and luminal B are divided into HER2+ and HER2–. The majority of luminal BCs express AR by IHC, and this expression is associated with a favourable prognosis.^[27,28] One meta-analysis shows that in early BC, AR is more likely to be co-expressed in ER+ over ER– tumours (74.8% vs. 31.8%, respectively).^[29] AR may act through genomic signalling interference to reduce the proliferation of BC in the presence of oestrogen. One proposed mechanism for growth inhibition in the ER+ BC subtypes is by competitive binding of AR to the ER binding site on DNA.^[30] A crosstalk between AR and ER has been proposed, and preclinical data suggest AR can antagonize ER signalling, dependent upon the relative levels of each hormone receptor.^[31] Contrastingly, in BCs that do not express ER, AR is able to bind to oestrogen response elements (EREs) on the DNA and stimulate cell proliferation.^[32] A large-scale clinical and gene expression meta-analysis from Bozovic-Spasojevic et al. confirmed the findings shown in previous studies where AR positivity conferred improved disease-free survival and overall survival in ER+ BC.^[33] Further support for this observation was seen in a retrospective study where patients with tumours expressing both AR and ER had a better prognosis than those with either AR or ER positivity.^[32] Additionally, the ratio of the hormone receptors has been suggested as being relevant for prognostic value, with high AR relative to ER proving to be predictive of hormone therapy resistance in early studies.^[34] While several studies support the improved prognostic value of AR in ER+ BC, some propose the activation of signalling pathways that would lead to increased proliferation of BC cells. Steroid receptors including AR can have extranuclear functions involved in cell growth and survival. For instance, AR is able to activate cell proliferation through oestradiol stimulation of the formation of a complex between AR, ER and Src. This complex ultimately activates the PI3K/Akt and MAPK pathways.^[35,36] Disruption of the interaction between AR/Src weakens the formation of this complex and can inhibit proliferation.^[37,38] Similarly, EGF signalling was dependent on the formation of this complex, confirming that the AR/ER/Src association plays a role in cell cycle progression.^[39] Further studies show that hormone therapy resistance occurs in ER+ models with AR overexpression through EGFR.^[40]

HER2-enriched Breast Cancer

AR is expressed in about 60% of HER2+ BC by IHC.^[4] In women with HER2+ BC, findings consistently report a worse prognosis with AR positivity.^[41] The mechanism proposed for this unfavourable prognosis is through AR signalling mediated transcriptional induction of ligands involved in Wnt/β catenin and HER2 signalling pathways. AR stimulated WNT7B activation leads to nuclear localization of β catenin and subsequent interaction of β catenin with AR to increase HER3 expression.^[42] Both Wnt and HER2 signalling pathways have the potential for positive feed-forward activation of AR activity, suggesting an androgen-independent activation of AR in these tumours. This has also been demonstrated in castrate-resistant prostate cancer (CRPC).^[43]

The efficacy of treatment of HER2+ AR+ BC with enzalutamide and trastuzumab (HER2 mAb) is currently under investigation in a phase 2 clinical trial (NCT02091960). In contrast to these findings, however, a gene expression analysis has found that overall survival is in fact better for AR mRNA expression in HER2+ BC. These conflicting results could be attributed to there being only modest overlap between transcriptional and IHC profiles for AR.^[33,44]

Triple-negative Breast Cancer (TNBC)

TNBC is defined as lacking expression of ER, PgR and HER2. This occurs in approximately 15–20% of BCs, but represents a disproportionate rate of mortality, as it is a more aggressive subtype.^[45] Quadruple negative breast cancer (QNBC) is broadly TNBC that also does not express AR.^[46] Patients with early TNBC suitable for surgery are frequently offered adjuvant chemotherapy, if deemed fit enough. However, these patients continue to have overall poorer survival and higher rates of distant metastasis following treatment.^[47] Due to the lack of molecular targets, new therapeutics are needed for this subtype.

TNBC has recently been further categorized based on the gene expression profiles by the Lehmann molecular classification as follows: basal-like (BL1 and BL2), mesenchymal, mesenchymal stem-like, immunomodulatory, and luminal androgen receptor (LAR).^[48] AR positivity is reported between 12 and 50% of TNBC.^[4,44,49] The existence of an AR-expressing subtype of TNBC along with somatic mutations identified in AR-responsive genes in some sequencing has sparked interest in AR as a target for therapy of this aggressive type.^[50,51] These Lehmann subtypes are shown to respond differently to therapies, with the LAR type being less proliferative and less sensitive to chemotherapy than the basal type.^[52,53] Many studies have shown that AR expression in TNBC is associated with lower histologic grade and lower clinical stage.^[54–56] Other groups have shown that a lack of AR expression increases the risk of recurrence and metastasis in TNBC.^[57,58] A retrospective study found AR positivity in TNBC to be associated with improved disease-free survival, while another found LAR TNBC to have higher overall survival.^[59,60] Another recent study stratified patients into distinct TNBC risk groups, finding LAR (AR+, EGFR−) patients to be in a lower-risk group with a better prognosis and likely to benefit most from antiandrogen therapy. Alternatively, AR− and EGFR+ represent the high-risk group that has a worse prognosis and is more likely to benefit from chemotherapy.^[61] The prognostic value of AR in TNBC is still uncertain, with some reports suggesting a positive prognosis with AR expression and others reporting no effect. This is likely variable based on the molecular profile of the cancer as well as the clinical context (Reviewed in Bozovic-Spasojevic et al. meta-analysis).^[33] Understanding the prognostic value of AR under these variable molecular and clinical contexts will be instrumental in using AR-targeting therapies for the treatment of BC.

Apocrine Breast Cancer

Apocrine BCs are not classified as a distinct subtype within the PAM50 classification and they make up only around 1% of all BCs.^[76] Apocrine BCs are ER/PgR– and generally, but not always, HER2−. Because of this and their uncommonness, apocrine BCs are generally grouped with TNBCs despite having distinct morphology. These tumours are characterized by having apocrine cells with oeosinophilic and granular cytoplasm and a low nuclear–cytoplasmic ratio. Additionally, activation of the AR signalling pathway is a prominent feature in apocrine BC.^[77] Apocrine BCs are ER/PgR– and therefore may be either HER2-enriched or, more frequently triple-negative. Importantly not all AR+ but ER/PgR– BCs are apocrine.

Triple-negative apocrine BC frequently carry actionable genomic alterations including alterations in PIK3CA and PTEN.^[78] Approximately 80% of invasive apocrine tumours occur in postmenopausal women.^[79] Due to its relatively low prevalence, it remains unclear whether the clinical features and outcomes of apocrine BC differ significantly from non-apocrine BC (either AR+ or AR–). Recently, an AR+ apocrine cell line model was shown to have an AR-dependent proliferative response to androgens and expression of genes that are normally expressed in ER+ luminal tumours.^[44] Further work showed that in this model AR binds and regulates ER cis-regulatory elements through a FoxA1-dependent mechanism leading to the gene expression profile overlap with ER+ BC.^[80] Apocrine BC's strong association with AR highlights their importance as a model to understand AR signalling in BC, and these tumours should be enriched for clinical trials investigating AR-targeted agents.

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Breast Cancer Res. 2022;24(79) © 2022 BioMed Central, Ltd.
Copyright to this article is held by the author(s), licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original citation.

Table 1. Defining AR positivity across breast cancer samples
References	ER status	HER2 status	Primary/metastatic	Antibody	AR+ definition	AR+ samples (%)	Total
Agrawal et al. [62]	ER+/ER−	±	Primary Only	AR411 DAKO	Not Reported	212 (43)	488
Gonzalez et al. [63]	ER+/ER−	Not reported	Primary & Lymph Node Metastases	AR411 DAKO	≥ 1%	83 (75)	111
Micello et al. [41]	ER− and PgR−	±	Primary Only	AR27 Novocastra	≥ 1% Nuclear	128 (57)	226
Luo et al. [64]	ER− and PgR−	-	Not Explicitly Reported	Not Reported	≥ 1% Nuclear	38 (28)	137
Niemeier et al. [15]	ER+/ER−	±	Not Explicitly Reported	AR441 DAKO	H score > 10	151(80)	189
Castellano et al. [65]	ER+	±	Primary Only	AR411 DAKO	≥ 1%	609 (71)	859
Hu et al. [28]	ER+/ER−	±	Primary Only	AR411 DAKO	≥ 10% Nuclear	1155 (79)	1467
Loibl et al. [66]	ER+/ER−	±	Primary Only	F39.4.1 BioGenex	≥ 1%	358 (53)	673
Park et al. [27]	ER+/ER−	±	Not Explicitly Reported	AR441 Thermo Scientific	≥ 10% Nuclear	541 (58)	931
Yu et al. [67]	ER+/ER−	±	Not Explicitly Reported	AR441 Lab Vision	≥ 10% Cytoplasmic	237 (72)	327
Gucalp et al. [49]	ER− and PgR−	±	Primary and Metastases	AR411 DAKO	≥ 10% Nuclear	51 (12)	424
Honma et al. [68]	ER+/ER−	±	Not Explicitly Reported	AR27 Novocastra	≥ 10% Nuclear	212 (53)	403
Tokunaga et al. [69]	ER+/ER−	±	Primary Only	AR411 DAKO	≥ 75% Nuclear	155 (62)	250
Thike et al. [60]	ER− and PgR−	-	Not Explicitly Reported	AR27 NCL-AR-318	≥ 1% Nuclear	267 (38)	699
Tsang et al. [70]	ER+/ER−	±	Primary Only	AR441 DAKO	≥ 1% Nuclear	549 (48)	1144
Aleskandarany et al. [71]	ER+/ER−	±	Not Explicitly Reported	Sc-816 Santa Cruz Biotech	H score ≥ 190	613 (54)	1141
Bronte et al. [72]	ER+/ER−	±	Primary and Metastases	SP107 Cell Marque Ventana Medical Systems	≥ 1% and ≥ 10%	136 (83) and 131 (80)	164
Candelaria et al. [73]	ER− and PgR−	-	Not Explicitly Reported	AR441 DAKO	≥ 10%	45 (31)	144
Kensler et al. [13]	ER+	±	Not Explicitly Reported	AR441 DAKO	≥ 1% Nuclear	2475(82)	3021
Xiang et al. [74]	ER+/ER−	±	Primary Only	ZA-0554 ZSGB	≥ 10% Nuclear	201 (67)	298
Zhao et al. [75]	ER− and PgR−	–	Not Explicitly Reported	Abcam ab113273	Not Reported	60 (29)	210

Table 2. Completed and ongoing trials assessing the safety and efficacy and targeting androgen receptor in breast cancer
NCT number	Title	BC subtype	AR agent drug class	Therapeutic agent	AR eligibility criteria	Menopausal status	Phase
NCT02463032	Efficacy and safety of GTx024 in patients with ER+/AR+ breast cancer	ER+/AR+	SARM	Enobosarm	Not Defined	Postmenopausal	Phase II
NCT02955394	Preoperative Fulvestrant with or without Enzalutamide in ER+/HER2− breast cancer	ER+/HER2−/AR+	Antiandrogen	Enzalutamide + Fulvestrant	Not Defined	Postmenopausal	Phase II
NCT02910050	Bicalutamide plus aromatase inhibitors in ER+/AR+/HER2− metastatic breast cancer (BETTER)	ER+/HER2−/AR+	Antiandrogen + AI	Bicalutamide + AI	Not Defined	Postmenopausal	Phase II
NCT01616758	Phase II study of GTx024 in women with metastatic breast cancer	ER+ (any AR status)	SARM	Enobosarm	Not Defined	Postmenopausal	Phase II
NCT00755885	Abiraterone acetate in treating postmenopausal women with advanced or metastatic breast cancer	AR+/ER− or ER+ (any AR status)	CYP17-lyase inhibitor	Abiraterone acetate	Not Defined	Postmenopausal	Phase I/II
NCT03207529	Alpelisib and Enzalutamide in treating patients with AR and PTEN+ metastatic breast cancer	HER2−/PTEN+/AR+	Antiandrogen	Enzalutamide + Alpelisib	≥ 1% nuclear staining	All	Phase I*
NCT02580448	CYP17-Lyase and AR inhibitor treatment with Seviteronel trial (CLARITY-01)	TNBC/AR+ or ER+/AR+	CYP17-lyase inhibitor	Seviteronel	Not Defined	Postmenopausal (if ER+)	Phase I/II
NCT02676986	Short-term preoperative treatment with Enzalutamide, alone or in combination with Exemestane in primary breast cancer (ARB)	ER+ and TNBC/AR+	Antiandrogen	Enzalutamide + Exemestane	> 0% nuclear staining	Postmenopausal (if ER+)	Phase II
NCT01990209	Orteronel as monotherapy in patients with metastatic breast cancer (MBC) that expresses AR	ER/PgR+/AR+ or TNBC/AR+	CYP17-lyase inhibitor	Orteronel	> 10% IHC staining	Postmenopausal or Pre with Ovarian Suppression (if ER+)	Phase II
NCT02067741	CR1447 in endocrine responsive-HER2- and TN-AR+ breast cancer	ER/PgR+ (HER2−) or TNBC/AR+	Antiandrogen	CR1447 (4-OH-testosterone)	> 0% IHC staining	Postmenopausal	Phase II
NCT02091960	A study to assess the efficacy and safety of enzalutamide with trastuzumab in patients with HER2+, AR+ metastatic or locally advanced BC	HER2+/AR+	Antiandrogen	Enzalutamide + Trastuzumab	Not Defined	All	Phase II
NCT00468715	Bicalutamide in treating patients with metastatic breast cancer	ER−/PgR−/AR+	Antiandrogen	Bicalutamide	> 10% nuclear staining	All	Phase II
NCT03055312	Bicalutamide in treatment of AR+ metastatic triple-negative breast cancer	TNBC/AR+	Antiandrogen	Bicalutamide	IHC ≥ 10%	All	Phase III
NCT01889238	Safety and efficacy study of enzalutamide in patients with advanced, AR+, triple-negative breast cancer	TNBC/AR+	Antiandrogen	Enzalutamide	Not Defined	All	Phase II
NCT02457910	Taselisib and Enzalutamide in treating patients with AR+ triple-negative metastatic breast cancer	TNBC/AR+	Antiandrogen	Enzalutamide + Taselisib	≥ 10% nuclear staining	All	Phase Ib/II
NCT02605486	Palbociclib in combination with Bicalutamide for the treatment of AR+ metastatic breast cancer	TNBC/AR+	Antiandrogen	Bicalutamide + Palbociclib	≥ 1% nuclear staining	All	Phase I/II
NCT02971761	Pembrolizumab and Enobosarm in treating patients with AR+ metastatic TNBC	TNBC/AR+	SARM	Enobosarm + Pembrolizumab	≥ 50% nuclear staining	All	Phase II
NCT03090165	Ribociclib and Bicalutamide in AR+ TNBC	TNBC/AR+	Antiandrogen	Bicalutamide + Ribociclib	> 0% IHC staining	All	Phase I/II
NCT02353988	AR-inhibitor Bicalutamide in treating patients with TNBC (Arbre)	TNBC/AR+	Antiandrogen	Bicalutamide	> 10% nuclear staining	All	Phase II
NCT02750358	Feasibility study of adjuvant Enzalutamide for the treatment of early-stage AR+ triple-negative breast cancer	TNBC/AR+	Antiandrogen	Enzalutamide	≥ 1% nuclear staining	All	Phase II
NCT03383679	Study on AR- and triple-negative breast cancer (START)	TNBC/AR+	Antiandrogen	Darolutamide + Capecitabine	≥ 10% IHC staining	All	Phase II
NCT02689427	Enzalutamide and paclitaxel before surgery in treating patients with stage I–III AR+ triple-negative breast cancer	TNBC/AR+	Antiandrogen	Enzalutamide + Paclitaxel	≥ 10% nuclear staining	All	Phase II*
NCT05095207	Abemaciclib in combination with Bicalutamide for androgen receptor-positive, HER2-negative metastatic breast cancer	HER2−/AR+	Antiandrogen	Abemaciclib + Bicalutamide	≥ 1% IHC staining	All	Phase Ib/II*
NCT04869943	Efficacy evaluation of Enobosarm monotherapy in treatment of AR+/ER+/HER2− metastatic breast cancer (ARTEST)	ER+/HER2−/AR+	SARM	Enobosarm	≥ 40% nuclear staining	Postmenopausal or Pre with Ovarian Suppression	Phase III*
NCT05065411	Efficacy and safety evaluation of Enobosarm in combo with Abemaciclib in treatment of ER+/HER2− metastatic breast cancer	ER+/HER2−/AR+	SARM	Enobosarm + Abemaciclib	≥ 40% nuclear staining	All	Phase III*
NCT03650894	Nivolumab, Ipilimumab, and Bicalutamide in HER2− breast cancer patients	HER2−/AR+	Antiandrogen	Nivolumab + Ipilimumab + Bicalutamide	Not Defined	All	Phase II*