Table 1.

Approved targeted drugs for breast cancer.

Target	Drugs	Mechanism	References
ER	Tamoxifen	Competitively inhibits the binding of estradiol to ER, resulting in a reduction in DNA synthesis and cellular response to estrogen	[10]
	Fulvestrant	Binds competitively to ER, resulting in ER deformation and decreased estrogen binding	[11]
	Toremifene	Chemically related to tamoxifen, binds competitively to ER	[12]
Aromatase	Anastrozole	Selectively binds to and reversibly inhibits the enzyme aromatase, which catalyzes the final step in estrogen biosynthesis and may result in growth inhibition of estrogen-dependent breast cancer cells	[13]
	Exemestane	Binds irreversibly to and inhibits aromatase	[14]
	Letrozole	Selectively and reversibly inhibits aromatase	[15]
HER2	Trastuzumab	Binds to HER2 on the tumor cell surface, induces an antibody-dependent cell-mediated cytotoxicity against tumor cells that overexpress HER2	[16]
	Pertuzumab	Binds to the dimerization domain of the HER2, therefore prevents the activation of HER signaling pathways, resulting in tumor cell apoptosis	[9]
	Ado-trastuzu-mab emtansine	The maytansinoid DM conjugated to the HER2-targeting transtuzumab is released and binds to tubulin, thereby inhibiting cell division and the proliferation of cancer cells that overexpress HER2	[17]
EGFR, HER2	Lapatinib	Selectively inhibits both EGFR and HER2 tyrosine kinases	[18]
	Neratinib maleate	Binds to and inhibits both HER2 and EGFR	[19]
mTOR	Everolimus	Binds to the immunophilin FKBP-12 to generate an immunosuppressive complex that binds to and inhibits the activation of the mammalian Target of Rapamycin (mTOR)	[20]
CDK4/6	Palbociclib	Selectively inhibits CDK4 and CDK6, thereby inhibiting Rb protein phosphorylation, which suppresses DNA replication and decreases tumor cell proliferation	[21]
	Ribociclib	Specifically inhibits CDK4/6	[22]
	Abemaciclib	Specifically inhibits CDK4/6	[23]
PARP	Olaparib	Selectively binds to and inhibits PARP and PARP-mediated repair of single strand DNA breaks	[24]

ER, Estrogen receptor; HER2, Human epidermal growth factor receptor 2; EGFR, epithelial growth factor receptor; FKBP-12, FK Binding Protein-12; CDK4/6, Cyclin-dependent kinase 4 and 6; Rb, retinoblastoma.