Eribulin (Figure 2) is a synthetic ketone analog of the macrocyclic chemotherapeutically active halichondrin B derived from the sea sponge Halichondria okadai. Eribulin is a potent mitotic inhibitor with a unique mechanism of action as an inhibitor of microtubule dynamics. Eribulin received its FDA approval in 2010 to manage the metastatic breast cancer in patients who have received at least two prior chemotherapy regimens for late-stage disease [28].

Everolimus (Figure 2), a derivative of the natural macrocyclic lactone sirolimus, is an orally bioavailable inhibitor of mTOR. On 20 July 2012, the U.S. FDA approved Everolimus for the treatment of post-menopausal women with advanced HR+/HER2− breast cancer. It is worth mentioning that Everolimus has been approved also for tuberous sclerosis complex-associated partial-onset seizures (in 2018), progressive, nonfunctional gastrointestinal and lung neuroendocrine tumors (in 2016), and advanced pancreatic neuroendocrine tumors (in 2011), as well as being the first approved pediatric-specific dosage form for the management of a rare pediatric brain tumor called subependymal giant cell astrocytoma [29,30,31].

Neratinib (Figure 2), a 4-anilinoquinoline-based orally bioavailable kinase inhibitor, is an irreversible pan-ErbB inhibitor of EGFR, HER2, and HER4 targeting the intracellular domain, which results in reduced phosphorylation and downstream pathways activation. Neratinib has been recently FDA- and EMA-approved for the extended adjuvant treatment of early stage HER2-positive BC [32].

Palbociclib (Figure 2) is a pyrido[2,3-d]pyrimidin-7-one derivative which acts as a selective CDK4/6 inhibitor. On 3 February 2015, the FDA granted accelerated approval of Palbociclib (in combination with letrozole) for postmenopausal women with advanced breast cancer, then in 2016 Palbociclib was approved for treatment of HR+/HER2− metastatic BC. Notably, Palbociclib was the first-in-class CDK 4/6 inhibitor approved by the FDA [33].

Ribociclib (Figure 2) is a pyrrolo[2,3-d]pyrimidine-based potent selective inhibitor of cyclin-dependent kinases (CDKs) 4 and 6. In 2017, the FDA approved Ribociclib for treatment of patients with HR+/HER2− metastatic BC. One year later, it was additionally approved for HR+/HER2− advanced breast cancer [34].

Tucatinib (Figure 2) is an orally bioavailable HER2 tyrosine kinase inhibitor. On 17 April 2020, the FDA granted an approval for tucatinib, in combination with Trastuzumab and capecitabine, for the management of patients with advanced unresectable or metastatic HER2-positive BC [35].

Anastrozole (Figure 2) is a reversible, nonsteroidal inhibitor of the aromatase enzyme that is taken orally. Anastrozole is an essential anticancer drug that is indicated as an adjuvant remedy in the treatment of HR+ BC in postmenopausal women at early stages [36].

Ixabepilone (Figure 2), a semisynthetic analog of epothilone B, stabilizes the microtubules which are essential for cell division via suppression of the dynamic of microtubules (aβ-II and aβ-II). Hence, it arrests the cell cycle in the G2-M phase, induces tumor cell apoptosis and inhibits tumor cell proliferation. In 2007, Ixabepilone was granted an FDA approval for the treatment of patients with locally advanced or aggressive metastatic BC [37].

Fulvestrant (Figure 2) is a synthetic estrogen receptor and an aromatase inhibitor. It decreases the amount of estrogen in the BC cells through binding to estrogen receptors which leads to estrogen receptor deformation [38].

Lapatinib (Figure 2), a synthetic quinazoline derivative, is an orally active reversible ErbB1 and ErbB2 tyrosine kinase receptor inhibitor that possesses antineoplastic activity towards breast cancer [39].

Pertuzumab is a recombinant humanized monoclonal antibody that targets HER2-positive BC which can inhibit the proliferation of human tumor cells [30]. The most common side effects of Pertuzumab include hair loss, low white blood cell count, rash, diarrhea, fatigue, nausea, and peripheral neuropathy (tingling in hands and feet and numbness) [40].

Alpelisib (Figure 2) is a phosphatidylinositol 3-kinase (PI3K) inhibitor that impedes the growth of tumor cells. Alpelisib is used in combination with fulvestrant (Faslodex^®) for treatment of postmenopausal women with a certain type of metastatic BC [41].

Talazoparib (Talzenna^®, Figure 2) belongs to a class of drugs called PARP inhibitors used for treatment of local advanced or metastatic HER2-negative BC women with a BRCA1 or BRCA2 mutation [42].