**13. Executive summary**

Breast cancer is still a significant public health problem, even though it was first reported more than 3500 years ago. This is particularly true in light of most societies' substantial and harmful lifestyle changes. At both the epidemiological and molecular levels, breast cancer is diverse. Many significant breast cancer risk factors have been discovered by clinical and epidemiological data, including age, family history, early menarche, and medical history; variables that are intangible or beyond our control. However, about 70% of breast cancers nowadays are caused by risk factors that may be altered or avoided. Obesity, lack of exercise, smoking, drinking, and nutrition, as well as other variables that may have a detrimental impact on a woman's hormonal environment, are among them. These important rate-limiting measures in the battle against breast cancer should not be ignored. As discussed in this review, significant advances in cancer biology have led to significant advancements in cancer early detection, therapy, and prevention in recent years. The growing emphasis on personalised treatment, as well as the combination of targeted and immunological therapies with current therapeutic techniques, holds potential for the cure of breast cancer. Drug resistance in breast cancer is a complicated clinical condition caused by a variety of molecular changes. Because chemotherapy is often used in conjunction with targeted treatments for the ER+ or HER2+ subtypes in clinical practice, targeted therapy-induced resistance may lead to chemo-resistance and vice versa. Treatment methods and therapeutics must be specially developed to address each distinct resistance mechanism in various clinical circumstances in response to every particular resistance mechanism. Early clinical trials are looking for drugs that target each route individually. Clinical studies

#### *The Future Perspectives of Drug Repurposing and Treatment for the Drug Resistant Breast… DOI: http://dx.doi.org/10.5772/intechopen.100143*

investigating tailored medication delivery methods are also underway in the meanwhile. These therapeutic agents may enter cells through receptor-mediated endocytosis, thereby bypassing typical drug resistance mechanisms such as drug efflux pumps, cell surface docking site mutations, and so on, allowing them to overcome drug resistance. The heterogeneity of breast cancer cells, on the other hand, poses major difficulties in terms of treatment response and may be a contributing factor in drug resistance. Tamoxifen, a selective oestrogen receptor modulator, is claimed to be used as a therapy for all stages of oestrogen receptor (ER)-positive breast cancer in men and women, thanks to its low risk of adverse effects. Notwithstanding major investments in prevention and treatment, breast cancer remains the primary cause of cancer mortality in women throughout the world. The existing therapeutic options are both expensive and have serious negative effects.

Drug repurposing, or finding new applications for existing therapies, has arisen as an unique drug development strategy. Repositioning existing, off-patent noncancer medicines with established targets into newer indications is like repurposing outdated weaponry for a new war. The process of medication repurposing has been made easier thanks to developments in genomics, proteomics, and information computational biology. The repositioning method not only speeds up the medication development process, but it also results in more effective, less expensive, and safer medicines with fewer/known adverse effects. Alkylating compounds, anthracyclins, antimetabolites, CDK4/6 inhibitors, aromatase inhibitors, mTOR inhibitors, and mitotic inhibitors have all been repurposed for breast cancer therapy in the recent decade.
