Breast cancer is the most common cancer and the most frequent cause of cancer death among women
worldwide. Breast cancer is a complex, heterogeneous disease classified into hormone-receptor-positive, human
epidermal growth factor receptor-2 overexpressing (HER2+) and triple-negative breast cancer (TNBC) based on
histological features. Endocrine therapy, the mainstay of treatment for hormone-responsive breast cancer involves use
of selective estrogen receptor modulators (SERMs), selective estrogen receptor downregulators (SERDs) and
aromatase inhibitors (AIs). Agents that target estrogen receptor (ER) and HER2 such as tamoxifen and trastuzumab
have been the most extensively used therapeutics for breast cancer. Crosstalk between ER and other signalling
networks as well as epigenetic mechanisms have been envisaged to contribute to endocrine therapy resistance. TNBC,
a complex, heterogeneous, aggressive form of breast cancer in which the cells do not express ER, progesterone
receptor or HER2 is refractory to therapy. Several molecular targets are being explored to target TNBC including
androgen receptor, epidermal growth factor receptor (EGFR), poly(ADP-ribose) polymerase (PARP), and vascular
endothelial growth factor (VEGF). Receptors, protein tyrosine kinases, phosphatases, proteases, PI3K/Akt signalling
pathway, microRNAs (miRs) and long noncoding RNAs (lncRNAs) are potential therapeutic targets. miR-based
therapeutic approaches include inhibition of oncomiRs by antisense oligonucleotides, restoration of tumour
suppressors using miR mimics, and chemical modification of miRs. The lnRNAs HOTAIR, SPRY4-IT1, GAS5, and
PANDAR, new players in tumour development and prognosis may have theranostic applications in breast cancer.
Several novel classes of mechanism-based drugs have been designed and synthesised for treatment of breast cancer.
Integration of nucleic acid sequencing studies with mass spectrometry-based peptide sequencing and posttranslational
modifications as well as rational drug design will provide a more comprehensive understanding of the pathophysiology
of breast cancer and help in evolving therapeutic strategies.