# Molecular Targets of Minor Cannabinoids in Breast Cancer: In Silico and In Vitro Studies
## 1 Introduction  
Despite the continuous advances in drug development, breast cancer incidence keeps rising, having the main responsibility for all cancer-related death among women worldwide [1, 2]. Due to their great heterogeneity, breast tumors are grouped in different subtypes, with most of them being classified as estrogen receptor-positive (ER\({}^{+}\)) [3, 4, 5]. This subtype overexpresses estrogen receptor alpha (ER\(\alpha\)) and progesterone receptor (PR). Moreover, in these tumors, the enzyme aromatase is overexpressed, resulting in high rates of local estrogen production [4, 6, 7]. The elucidation of these targets was pivotal for the development of endocrine therapies, which prevent estrogen production or signaling, such as aromatase inhibitors (AIs), including Anastrozole (Ana), Letrozole (Let), and Exemestane(Exe), as well as anti-estrogens, such as Tamoxifen and Fulvestrant (Figure 1) [8]. In fact, these drugs are part of the therapeutic strategy applied as first-line therapy, both in pre- and post-menopausal status [8; 9; 10; 11; 12]. Unfortunately, and despite their clinical success, these compounds are not only associated with adverse side effects, but also with the development of endocrine resistance, which compromises their effectiveness [13; 14]. In view of that, alternative therapeutic approaches have been developed to improve treatment, including the introduction of new targeted therapies with novel drugs such as CDK4/6, mTOR, and PI3K inhibitors that are recommended in combination with endocrine therapy [11; 12; 13; 14; 15; 16; 17]. Indeed, the elucidation of the role of additional targets in the development of ER\({}^{+}\) breast cancer represents a new hope for its treatment. One of the targets that has been considered is the androgen receptor (AR), which is expressed in 70-90% of ER\({}^{+}\) breast tumors [13; 18; 19]. This receptor seems to cooperate with ER\(\alpha\) and displays different functions depending on the hormonal and resistance status. In ER\({}^{+}\) breast cancer cases, AR antagonizes ER\(\alpha\) signaling, contributing to the decrease in cell proliferation and tumor growth [20]. On the other hand, in cases where resistance to endocrine therapy develops, and thus ER\(\alpha\) activity is modified, AR switches its function from tumor suppressor to oncogenic in order to promote cell growth and survival. This emphasizes the existence of a hormone-dependent crosstalk between ER\(\alpha\) and AR [21; 22; 23; 24]. Cannabinoids have demonstrated promising therapeutic actions in different pathological conditions, including epilepsy, asthma, sleep disorders, depression, and inflammation, and in the relief of chemotherapy-associated side effects [27; 28; 29]. Notably, various studies have attributed anti-proliferative, anti-angiogenic, and anti-invasive actions to cannabinoids in various cancer types [28; 30; 31; 32; 33; 34; 35; 36; 37]. In fact, the in vitro and in vivo effects of the two best known cannabinoids, \(\Delta^{9}\)-tetrahydrocannabinol (**THC**) and cannabidiol (**CBD**), were already explored in different cancer types, including breast cancers. Usually, the cannabinoids exert anti-tumor effects through the inhibition of cell growth, neovascularization, migration, adhesion, invasion, and metastasis, and also by promoting cell cycle arrest, autophagy, and apoptosis [38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57]. To date, the studies conducted on cannabinoids in breast cancer were mainly performed in triple-negative and HER2\({}^{+}\) tumors [58; 33; 59; 60]. Nevertheless, there is evidence that molecular pathways between cannabinoid receptors and estrogens or  
Figure 1: Chemical structures of the AIs, Ana, Let, and Exe, and the anti-estrogens Tamoxifen and Fulvestrant. The IC50 values described for AIs are relative to aromatase and were evaluated in human placental microsomes [25; 26]. androgens may overlap, which may impact ER\({}^{+}\) breast cancer [60; 61]. In 1997, [PERSON] et al.