Nano to rescue: repository of nanocarriers for targeted drug delivery to curb breast cancer

Abstract

Breast cancer is a heterogeneous disease with different intrinsic subtypes. The conventional treatment of surgical resection, chemotherapy, immunotherapy and radiotherapy has not shown significant improvement in the survival rate of breast cancer patients. The therapeutics used cause bystander toxicities deteriorating healthy tissues. The breakthroughs of nanotechnology have been a promising feat in selective targeting of tumor site thus increasing the therapeutic gain. By the application of nanoenabled carriers, nanomedicines ensure targeted delivery, stability, enhanced cellular uptake, biocompatibility and higher apoptotic efficacy. The present review focuses on breakthrough of nanoscale intervention in targeted drug delivery as novel class of therapeutics. Nanoenabled carriers like polymeric and metallic nanoparticles, dendrimers, quantum dots, liposomes, solid lipid nanoparticles, carbon nanotubes, drug-antibody conjugates and exosomes revolutionized the targeted therapeutic delivery approach. These nanoassemblies have shown additional effect of improving the solubility of drugs such as paclitaxel, reducing the dose and toxicity. The present review provides an insight on the different drug conjugates employed/investigated to curb breast cancer using nanocarrier mediated targeted drug delivery. However, identification of appropriate biomarkers to target, clearer insight of the biological processes, batch uniformity, reproducibility, nanomaterial toxicity and stabilities are the hurdles faced by nanodrugs. The potential of nano-therapeutics delivery necessitates the agglomerated efforts of research community to bridge the route of nanodrugs for scale-up, commercialization and clinical applications.

Introduction

Cancer incidence and mortality across the globe are rising at an alarming pace (Behera and Patro 2018). According to The Global Cancer Observatory, 2020, 19.3 million new cancer cases and 10 million cancer deaths had occurred worldwide. It was projected that by 2040, 28.4 million cases of cancer will raise the global cancer burden approximately by 47% from 2020. As per WHO reports, breast cancer is rising with an approx 2.3 million new cases in 2020 with 685,000 deaths globally (Sung et al. 2021). Among the global burden of cancer, female-specific breast cancer has been projected to be more than 24% with higher incidences of associated mortality. It is the most prevalent cancer evident with the 7.8 million females alive with breast cancer in 2020. Breast cancer is a heterogeneous disease with uncontrolled division of the natural body cells (Asghari et al. 2019; Weigelt et al. 2010). The development of breast cancer is a multifactorial process with the cumulation of biological, environmental, genetic and hormonal factors. Exposure of breast tissues to reproductive hormone, obesity, postmenopausal hormone long period exposure, menstrual history, use of hormonal contraceptives, etc., contributes to the risk of breast cancer in females (Daly et al. 2021).

Breast cancer is composed of different intrinsic subtypes with varying genetic make-up and distinct molecular characteristics such as Luminal A, Luminal B, Human epidermal growth factor receptor 2 (HER2)-enriched and Basal-like subtypes (Table 1) (Kolářová et al. 2019). Luminal A-type tumors are usually low-grade invasive ductal carcinomas. They are estrogen/progesterone receptor-positive so endocrine therapy is the preferred treatment option for luminal A breast cancer. The luminal A subtype ensures the best prognosis, followed by the luminal B subtype, while the HER2 positive and basal-like subtypes were associated with the worst prognosis (Jia et al. 2020). Luminal B breast cancer is hormone-receptor positive (estrogen-receptor and/or progesterone-receptor positive), and is either HER2 positive or HER2 negative with high levels of mitotic index marker Ki-67 (Hashmi et al. 2018). HER2-enriched subtype shows a high expression of the HER2 and is associated with proliferation-related genes clusters. Irrespective of the HER2 clinical status, this subtype obtains the best clinical and therapeutic gain by anti-HER2 therapies, with or without chemotherapy (Godoy-Ortiz et al. 2019). Triple-negative breast cancer (TNBC) is a basal-like subtype of breast cancer which does not express estrogen receptor (ER), progesterone receptor (PR) or human epidermal growth factor receptor 2 (HER-2).

Table 1.

Classification of breast cancer intrinsic subtypes (Kolářová et al. 2019)

Intrinsic Subtype	Clinico-pathologic definition	Type of therapy
Luminal A	‘Luminal A’ ER and/or PR positive HER2 negative Ki-67 low (< 14%)	Endocrine therapy alone
Luminal B	‘Luminal B (HER2 negative)’ ER and/or PR positive HER2 negative Ki-67 high	Endocrine ± chemotherapy
HER2 enriched	‘HER2 positive (non luminal)’ HER2 over-expressed or amplified ER and PR present/absent	Anti-HER2 + chemotherapy ± endocrine therapy
Basal-like	‘Triple negative (ductal)’ ER and PR absent HER2 negative	Chemotherapy

Compared to other breast cancer, triple negative breast cancer has a higher 5-year mortality rate after treatment (30%) (Sorolla et al. 2020). It has clinical features with high invasiveness, high metastatic nature and occurrence to younger age group, high relapse rate and poor prognosis. It has been a challenge for the therapeutics due to diminished response to endocrine therapy or HER2 treatment as it lacks ER, PR, and HER2 expression (Malhotra and Emens 2020; Shaikh and Emens 2020; Yin et al. 2020; Nunnery et al. 2021). Due to non-availability of advanced selective therapy for TNBC, the subtype is managed with conventional therapeutics including removal of tumors by surgery, sometimes in combination with chemotherapy and localized radiotherapy (Fig. 1). However, associated bystander toxicities to normal tissues and relapse after treatment has been the limiting factor (Bromma et al. 2020). Employing the targeted approach of drug delivery may substantially increase the therapeutic gain by lowering the damage to normal cells. Further, heterogeneity of the tumor poses the restriction on the use of single therapeutic agent, which can confiscate the whole tumor. So there is an utmost need for cancer prevention and controlled targeted strategies to reduce breast cancer associated morbidity and mortality (Harris et al. 2019).

Fig. 1.

Application of nanocarriers in Breast Cancer: the prevalent approaches for breast cancer management and treatment has two principal components viz. Local and systemic. The local therapy includes cancer cell killing by surgery or radiation therapy, whereas, systemic approach includes chemotherapy, hormonal therapy and preferred targeted therapy

Biological drugs, targeted treatment and gene therapy can potentially reduce the mortality of patients with metastatic breast cancer (Al-Mahmood et al. 2018). Intrinsic limitations of cancer therapies encouraged the intervention of nanotechnology in drug development to enhance the therapeutic efficacy and safety, which is referred to as cancer nanomedicine (Afzal et al. 2021). Plausible application of nanomedicines in drug delivery, diagnosis, imaging and therapeutics has the potential to satisfy the need of cancer diagnostics and therapeutic system (Teles et al. 2018). Nanomaterials target the carcinoma cells by biomimetic method without harming normal cells (Harris et al. 2019). In 2005, the first nanoformulation Abraxane^® got approval as therapeutics for metastatic breast cancer. After that various nanotherapeutics are being utilized with great efficacy. The nano paclitaxel provides superior efficacy and reduced toxicity as compared to free paclitaxel (Gradishar et al. 2005). In a study, 4T1 breast cancer treated with either free or nanoparticle-delivered doxorubicin, demonstrates enhanced efficacy in treatment increased nanoparticle accumulation within the tumor (Brocato et al. 2018). Thus, multiple nanoformulations have been designed and investigated as a part of targeted strategy for breast cancer management.

Targeted strategies: nanoparticles in tumor management

Nanocarriers offer various benefits for targeting tumors by overcoming the bioavailability barrier of chemotherapeutic agents. They can passively accumulate at the tumor site due to the vascular permeability of the tumor. In addition, active targeting by surface modification of ligands adapted to tumor biomarkers improves the site specificity of nanocarriers. Therefore, nanosupports have been widely used in recent years for the delivery of drugs targeting tumors (Pindiprolu et al. 2018). Nanoparticles have the efficiency to carry therapeutic molecules and deliver them to targeted tumor location. It can also serve the purpose of both diagnosis and treatment of breast cancer (Singh et al. 2017). Nanoparticles targets tumors either by passive targeting or by active targeting (Fig. 2). Due to leaky vasculatures, tumor tissues have enhanced permeability and retention, known as EPR effect. Nanoparticles take the advantages of EPR effect and decreased removal from the lymphatic system. It gets accumulated in the tumor site, releases the drug and induces apoptosis, this is called passive targeting. Passive targeting is mainly dependent on the nanoparticle size (40–200 nm). Whereas, in active targeting, protein-based targeting elements like antibodies and RGD (arginylglycylaspartic acid) peptides are added to the nanoparticles to enhance their homing to specific tumor site. Active targeting takes advantage of the Ligand-Receptor affinity. It targets the highly expressed surface receptors on tumor cells with their contrast specific ligand attached to the nanoparticles (Hejmady et al. 2020; Raj et al. 2021). The addition of imaging agent to the nanoparticle provides complete details about its path, binding and release of the drug. The nano sized agents designed for the purpose of both diagnosis and therapy are known as Theranostics (Mohammadpour and Majidzadeh-A 2020). They are drug delivery systems integrated with molecular or targeted imaging agents (Hapuarachchige and Artemov 2020). Varying pH, temperature and hypoxia in tumor cells makes them a probable target for physical targeting using pH, temperature, electromagnetic radiation, ultrasound and mechanical forces. Targeting with these stimulus may induce the alteration of nanoparticles structure or may modulate the rate of drug release form nano-drug conjugate (Arachchige et al. 2015). Nano-drug conjugates of lipid nanoparticles and wortmannin radiosensitizer were able to overcome the poor solubility (increased from 4 to 20 g/l) and chemical instability issues of wortmannin (Klochkov et al. 2021). Doxorubicin (Dox)-loaded PEG@Pt (Polyethylene@Mesoporous platinum nanoparticle) exhibited sustain drug release and high killing efficacy in MCF-7 breast cancer cells (Fu et al. 2020).

Fig. 2.

Passive and active targeting: schematic representation highlights the nanoparticle mediated targeted drug delivery methods. In passive targeting nacarriers take the benefit of leaky vasculatures and deliver the drug while in active targeting, specific ligand-receptor binding affinity is utilized in identifying the receptors present on the cancer cells for selectivity by nanocarriers

Advantages of nano-enabled targeted drug delivery

Nanotechnology allows manipulation in materials with at least one dimension sized from 1 to 100 nm. Nano-based approaches have improved the clinical outcome of clinical management of the cancer. It has also assisted in better understanding the biology of the cells, interaction of metastatic cells with tumor microenvironment (Singh et al. 2017). Nanomaterials are preferred for disease diagnosis and drug delivery because of the several reasons including small size, large surface-to-volume ratio, tunable physicochemical properties, ability to carry heavy drug load (Jawahar et al. 2020), longer circulation time, high uptake and retention (Bromma et al. 2020), tumor-targeting efficacy (S. Li et al. 2020a, b), sustained release of the chemotherapeutic payload (Biancacci et al. 2020), biocompatibility, bioavailability (Sohail et al. 2020), increased circulation time, overcoming multidrug resistance (Majidinia et al. 2020) and escape from p-glycoprotein pump (Sharmiladevi et al. 2021; Swain et al. 2016). The aforesaid Pros have shifted the paradigm of drug development in enhancing the therapeutic efficacy using selective nanocarriers-based targeted drug delivery system.

Nanocarriers

Nanomaterials have the potential to transport chemotherapeutic drugs directly to the target breast tumor using specific ligand. Various drugs are specifically assembled with nanocarriers for targeting to tumors. Doxorubicin (Dox) is the widely used drug linked to nanomaterials for tumor therapy (Avitabile et al. 2018). By modulating size and surface characteristics of nano-drug, its efficacy can be controlled. Investigating the in vivo distribution, toxicity, circulation time, bioavailability, biodegradability, and targeted delivery of the nanosystem to specific site is critical for usage in clinical settings (Raj et al. 2021). Currently, a wide variety of platforms are being investigated as nanocarriers (Fig. 3) for breast cancer treatment, including lipid-based, polymer-based, inorganic and drug-conjugated nanoparticles (Tran et al. 2017).

Fig. 3.

Nanosolutions to breast cancer: nanocarriers are able to perform targeted delivery of drugs, enhanced cytotoxicity, bioavailability. It reduces systemic toxicity and severe side effects. The scheme showcases the different reforms of nanocarriers for targeted drug delivery. Polymer based, phospholipid based, carbon nanotubes, Liposome and Quantum dots are the few platforms being readily investigated for efficient and controlled drug release at the site of tumor

Polymer based nanomedicines

Polymeric nanoparticles and polymeric micelles

Polymeric nanoparticles are formed by the polymerization and self-assembly of the monomer units. Polymeric micelles are the nanocarriers contain core/shell structures having amphiphilic block copolymers. It consist of hydrophilic shell and hydrophobic core (Avramović et al. 2020). The hydrophobic core enables the delivery of water-insoluble drugs to the tumor region. Polymeric micelles exhibit biocompatibility, biodegradability, longer circulation time in the blood and enhanced tumor accumulation (Zhang et al. 2014). Because of their excellent properties, they are being utilized as nanocarriers for the site-specific delivery of many drugs (Table 2). Polymeric nanocarriers were utilized successfully for the delivery of two drugs Noscapine (Nos) and Doxorubicin (Dox) to breast cancer cells. Excellent antitumor activity of Nos NPs alone and in combination with Dox was demonstrated in 4T1 breast cancer cell line and tumor bearing mice model. Combination of both drugs inhibited cancer growth (68.50%) in mice more efficiently than Nos NPs (55.10%) and Dox HCl (32%) alone (Esnaashari et al. 2020). Further, Resveratrol has been investigated for therapeutic applications but poor water solubility has been the barrier for its use in clinical settings. This problem is resolved using Resveratrol loaded polymeric micelles. It enabled theranostic targeting in MCF-7 and MDA-MB-231 human breast cancer cells lines. It also enhanced the uptake efficiency and reduced the viability of tumor cells (Gregoriou et al. 2021). Tumor microenvironment also suffers from hypoxia condition due to insufficient blood supply. 17β-Estradiol (E (2)) is an estrogen receptor (ER) ligand. It was attached on the surface of nanocarriers for targeted delivery of drug to ER positive breast cancer cells. Encapsulation of the anticancer drug doxorubicin in estradiol-conjugated hypoxia-responsive polymeric nanoparticles resulted in higher cytosolic and nuclear internalization with increased cytotoxicity (Mamnoon et al. 2020). Paclitaxel (PTX), an anticancer drug, has a low therapeutic response with less circulation time in blood and imparts off-target effects. Self-assembled sodium caseinate nanomicelles (NaCNs) with PTX loading was utilized for targeted delivery, increased cellular uptake and enhanced cytotoxicity in MCF-7 and MDA-MB-231 cell lines (Rehan et al. 2020). In another study, PTX was encapsulated in core–shell nanoparticles from acrylated palm olein with polyol ester cross-linked copolymers. Surface functionalization of nanoparticles with peptide resulted in higher biodegradability and cytotoxicity of PTX (Tajau et al. 2020). Further, stearoyl gemcitabine (GemC18) also investigated for cancer treatment using polymeric micelles. Interestingly, co-delivery of gemcitabine prodrug along with anti NF-κB siRNA by tri-layer micelles showcased more cytotoxicity than free drug on AsPC-1 (human pancreatic cancer cell line) and MCF-7 cell line (Norouzi et al. 2020).

Table 2.

Polymeric nanoparticles, polymeric micelles and Dendrimers with potential application in breast cancer treatment

Nanoformulations	Component of delivery system	Drug	Therapeutic indication	Conclusion	References
Noscapine and Doxorubicin Loaded Polymeric Nanoparticles	Methoxy polyethylene glycol (mPEG), poly lactic-co-glycolic acid (PLGA)	Noscapine (Nos), Doxorubicin (Dox)	4T1 breast cancer cell line and in mice model	Increased tumor growth inhibition	(Esnaashari et al. 2020)
Resveratrol loaded polymeric micelles	Pluronic F127 block copolymer and Vitamin E-TPGS(Tocopherol polyethylene glycol 1000 succinate)	Resveratrol (res)	Breast cancer cell lines MCF-7 and MDA-MB-231	Reduction in viability of breast cancer cells, higher uptake efficiency	(Gregoriou et al. 2021)
Estradiol-conjugated hypoxia-responsive polymeric nanoparticles	17β-Estradiol (E2)	Doxorubicin (E2-Dox-HRPs)	Spheroid cultures of ER-positive MCF7 cells	Higher cytosolic and nuclear internalization, higher cytotoxicity	(Mamnoon et al. 2020)
Nanostructured and Self-Assembled Caseinate Micelles	Self-assembled sodium caseinate nanomicelles (NaCNs)	Paclitaxel (PTX)	MCF-7 and MDA-MB 231 human breast cancer cell lines	Enhanced cytotoxicity, cellular uptake and in-vivo anti-tumor efficacy in a mouse model of breast cancer	(Rehan et al. 2020)
APO-b-polyol ester cross-linked copolymers as core–shell nanoparticles	Acrylated palm olein (APO) with polyol ester	Paclitaxel	MCF-7 cell lines	Good biode- gradable properties, higher cytotoxicity efficacy	(Tajau et al. 2020)
Co-delivery of gemcitabine prodrug along with anti NF-κB siRNA by tri-layer icelles	Poly (ε-caprolactone), polyethyleneimine and polyethylene glycol (PCL-PEI-PEG) copolymers, anti-nuclear factor-κB (NF-κB) siRNA	4-(N)-stearoyl gemcitabine (GemC18)	Breast and pancreatic cancer cells	Dual functional delivery system showed higher tumor accumulation efficiency and cytotoxicity	(Norouzi et al. 2020)
Trastuzumab (TZ)-grafted dendrimers loaded with neratinib	G4 PAMAM dendrimer, hetero-cross linker, fluorescein isothiocyanate	Trastuzumab (TZ), neratinib	SKBR-3 cells	Higher antiproliferation activity, reducing the associated resistance	(Aleanizy et al. 2020)
Trastuzumab-dendrimer-fluorine drug delivery system	Dendrimer-fluorine drug	Trastuzumab-dendrimer-fluorine drug	MCF-7 cells with Her-2 overexpression	Higher efficiency than trastuzumab alone	(Bartusik-Aebisher et al. 2021)

Dendrimers

Dendrimers are tree like nanomolecules having inner core moiety and radially attached generations that possess chemical functional groups at the exterior terminal surface. This multivalency makes it multifunctional (Madaan et al. 2014). Well-defined structure, high loading capacity and monodisperse nature makes it suitable for nanomedical application as drug carriers (Table 2). Drugs can be encapsulated in inner core or can be complexed on outer surface. High surface-to-volume ratio of dendrimer make its outer surface amenable for chemical association with targeted ligands, antibodies, drugs and imaging agents (Kim et al. 2018). Trastuzumab is the fundamental drug for treatment of Her-2 over-expression breast cancer. Trastuzumab-dendrimer-fluorine drug delivery system efficiently treated three-dimensional breast cancer cell culture (MCF-7 cell line) in bioreactor device (Bartusik-Aebisher et al. 2021). Recently, neratinib, an irreversible inhibitor of the HER-2 receptor tyrosine kinase, has been approved as adjuvant therapy to Trastuzumab. Interestingly, Trastuzumab-targeted neratinib loaded poly-amidoamine dendrimer showed higher antiproliferation activity against SKBR-3 cells compared to neratinib alone (Aleanizy et al. 2020).

Polymer based nanocarriers are utilized for delivery of the active agent. These are used frequently to overcome the poor bioavailability of most drugs by increasing their solubility and permeability across a biological membrane. However, increased risk of particle aggregation and toxicity poses a major disadvantage in using NPs. Only a small number of polymeric nano-medicines are currently FDA approved and used in the clinic. But polymeric nanocarriers are currently undergoing testing in numerous clinical trials including Paclitaxel-based NK105/NanoCarrier^™ (Hare et al. 2017; Mitchell et al. 2021). Genexol-PM is a polymeric NP micelle formulation of paclitaxel that has been approved in South Korea. During preclinical trial of Genexol-PM for non-small cell lung cancer (NSCLC) showed significant toxicities with approximately 5% to 10% of patients dying from treatment-related pulmonary toxicities (Toosi 2014).

Inorganic/metallic based nanomedicines

Metallic nanoparticles

Metallic nanoparticles are nanosized particles having unique characteristics such as surface plasmon resonance and optical properties (Venkatesh 2018). These metal and metal oxide nanoparticles can be synthesized either by physical, chemical or biological methods. It can be modified with various chemical functional groups, which allow them to be conjugated with specific ligands, peptides, antibodies and drugs (Table 3). Metallic nanoparticle-mediated therapies lowered the therapeutic dosage, which improved the efficiency of treatments (Wang et al. 2020). Recently, gold nanoparticles loaded with folate receptor-targeted Celastrol was able to overcome the poor water solubility and untargeted effect of Celastrol. It also resulted in high drug encapsulation, increased drug loading and significant apoptosis in 2D and 3D breast cancer model (Law et al. 2020). In another study, gold nanoparticles were also utilized for co-delivery of doxorubicin and polo-like kinase 1 (PLK1) siRNA. The exhibited reduced IC₅₀ (Half-maximal inhibitory concentration) in SKBR3 cell lines indicated the synergistic effect of combined drug and gene delivery in 2D and 3D breast cultured systems (Shrestha et al. 2020). In a study, biogenic gold nanoparticles were synthesized using bacterial extract (B. licheniformis, MTCC 429). These nanoparticles were loaded with albumin capped 5-fluorouracil (5-FU). The efficacy against MCF-7 and in silico prediction using a GastroPlus^® software were noted. It resulted in ensured hemocompatibility (< 14%), increase in % drug loading, entrapment with increased concentration of the cellular lysate and enhanced drug release (96.64% release within 5 h). GastroPlus^® prediction exhibited improved pharmacokinetic parameters and regional absorption from various segments of human intestine. This approach is very significant and efficacious to control breast cancer when administered transdermally or orally (Mahdi et al. 2020). In another similar study, 5-fluorouracil (5-FU)-loaded biogenic gold nanoparticles were coated by pluronic-based coating (PFGNPs). This nanocomposite exhibited optimum size (175.1 nm), %drug delivery (73.8%), % drug release (DR) (75.7%) and greater cytotoxicity (viability ~ 8.9%) against the colon cancer cell lines than 5-FU solution (~ 24.91%), and less hemocompatibility (Mahdi et al. 2021).

Table 3.

Metallic nanoparticles, quantum dots with potential application in breast cancer treatment

Nanoformulations	Component of delivery system	Drug	Therapeutic indication	Conclusion	References
Metallic NPs Doxorubicin (Dox)-loaded PEG@Pt (PEG@Pt/Dox)	mesoPt-PEG	Doxorubicin (Dox)	MCF-7/ADR cells	Excellent biocompatibility,sustain drug release,high killing efficacy	(Fu et al. 2020)
Integrin α5 (ITGA5) active targeting nanoparticles (uPtDs NPs)	Integrin α5(ITGA5) (uPtDs NPs)	Ultrasmall Pt(II) dot (uPtD) from miriplatin	Triple negative breast cancer	Superior DNA damaging capacity, reduced metastasis	(Li et al. 2021)
Folate receptor-targeted celastrol AuNP (FCA)	(AuNP)-PVP-co-2-dimethylaminoethyl methacrylate	Celastrol	2D and 3D breast cancer model	Good solubility, high encapsulation efficiency and loading content	(Law et al. 2020)
Gold nanoparticles mediated drug-gene combinational therapy	Gold nanoparticles coated with polyethyleneimine	Co-delivery of doxorubicin and polo-like kinase 1 (PLK1) siRNA	2D and 3D cultured systems	Synergistic effect of combined drug and gene delivery	(Shrestha et al. 2020)
Pluronic-Coated Biogenic Gold Nanoparticles	5-fluorouracil (5-FU)-loaded biogenic gold nanoparticles with pluronic-based coating (PFGNPs)	5-fluorouracil (5-FU)	Cell line toxicity, in vitro hemocompatibility, and ex vivo intestinal permeation	High drug loading capacity, sustained delivery, hemocompatibility, improved efficacy, and enhanced permeation profiles, greater cytotoxicity, higher drug release compared with 5-FU solution	(Mahdi et al. 2021)
Albumin capped 5-fluorouracil (5-FU) loaded gold nanoparticles (NPs)	B. licheniformis, MTCC 429, BSA (Bovine serum albumin), Chloroauric acid (HAuCl4), 5-Fluorouracil (5-FU)	5-Fluorouracil (5-FU)	MCF-7 and in silico prediction	Ensured hemocompatibility, higher drug release, more cytotoxic, improved pharmacokinetics parameters and regional absorption	(Mahdi et al. 2020)
Quantum dots
Ultrasmall Ag 2 te quantum dots for imaging and augmented photonic tumor hyperthermia	Ag2Te quantum dots (QDs)	–	4T1 breast tumor animal models	High biocompatibility, high tumor suppression rate	(Dong et al. 2020)
Nanohydrogels (NHGs) and quantum dots (QDs)	InP/ZnS QDs, chitosan (CS) based NHGs, nanosystem, amino-modified MUC-1 aptamer (Ap), sodium oxamate (SO)	Paclitaxel (PTX)	Breast cancer MCF-7 cells	Robust theranostics, induce mitochondria-mediated apoptosis	(Ranjbar-Navazi et al. 2021)
PANI/N-GQD/MO/LDH nanocarrier	MgAl-layered double hydroxide, Mn3O4 nanoparticles, N-graphene quantum dot and poly- aniline (PANI/N-GQD/MO/LDH)	Doxorubicin (DOX)	Human breast cancer cell lines (MCF˗7)	Increase the therapeutic efficacy of DOX, excellent blood compatibility	(Ahmadi-Kashani et al. 2020)
Supra-NP assemblies (SiO2@QDs)	Imidazoline-functionalized silica nanoparticles, ligand-coated QDs, imidazole-functionalized dextran	Tetrameric antibody complexes (TACs)	Mammalian breast cancer cells (SK-BR3) that overexpressed the human epidermal growth factor receptor 2 (HER2)	Enhanced Cellular Imaging	(Darwish et al. 2020)
5-fluorouracil-chitosan-carbon quantum dot- aptamer (5-FU-CS-CQD-Apt) nanoparticle	Chitosan, carbon quantum dot, 5TR1 aptamer	5-Fluorouracil	MCF-7 cells	Drug released faster in acidic pH than physiologic pH, biocompatibility of the nanoparticles, effective cytotoxicity	(Zavareh et al. 2020)
Carbon quantum dots(CQDs)- quinic acid	Nitrogen-doped CQDs, Quinic acid	Gemcitabine	MCF-7 cell line	High tumor accumulation, excellent luminescent properties	(Samimi et al. 2021)

Further, mesoporous platinum (mesoPt) nanoparticles were utilized for loading of drug Doxorubicin. To solve the purpose of biocompatibility MesoPt was tagged with polyethylene glycol (PEG). Doxorubicin (Dox)-loaded PEG@Pt can efficiently enter and deliver Dox into the cytoplasm, thereby inducing killing of Dox-resistant breast cancer cells (MCF-7/ADR) (Fu et al. 2020). Miriplatin is a clinically approved drug for usage in hepatocellular carcinoma. Ultra-small Pt (II) dot (uPtD) containing miriplatin was formed and encapsulated into integrin α5 (ITGA5) to attain active targeting nanoparticles (uPtDs NPs). It was shown to cause severe damages to DNA and suppress triple negative breast cancer lung metastasis (Li et al. 2021).

Quantum dots

Quantum dots (QDs) are the semiconductor nanocrystals that have exceptional photo physical properties. QDs have attained the spotlight due to their multifaceted biological applications (Reshma and Mohanan 2019). It possesses good chemical and photo-stability with high quantum yield and size-tunable light. QDs are good nanomaterials as theranostic platforms for simultaneous sensing, imaging and therapy because they can act as the main nanocarriers or can act as fluorescent tagged imaging agent (Matea et al. 2017) (Table 3). Ultra small Ag₂Te quantum dots were constructed as a theranostic agent with a photonic tumor hyperthermia effect. It showed significantly promising biocompatibility, imaging with negligible toxicity. The fabricated Ag₂Te QDs exhibited a high tumor suppression rate (94.3%) on 4T1 breast tumor animal models (Dong et al. 2020). In another study, supra-nanoparticle assemblies (SiO2@QDs) were synthesized that had multiple QDs around a central silica nanoparticle. Spontaneous affinity interactions were present between the constituent materials, which included imidazoline-functionalized silica nanoparticles, ligand-coated QDs, imidazole-functionalized dextran and tetrameric antibody complexes (TACs). This assembly showed higher sensitivity and excellent optical properties (Darwish et al. 2020).

Although semiconductor quantum dots (QDs) have successfully resolved the problem of photostability and brightness issues of organic fluorophores but the associated intrinsic toxicity, poor water solubility and blinking characteristics still poses a challenge to resolve for widespread application. Graphene quantum dots (GQDs), a latest zero-dimensional (0D) member of the carbon family revolutionized the imaging biomedical applications. It consists the fascinating properties of two-dimensional (2D) graphene as well as astonishing physicochemical characteristics of the QDs that include non-zero band gap and quantum confinement effects (Han et al. 2020). Recently, a multifunctional N-graphene quantum dot was fabricated with Mn₃O₄ nanoparticles, MgAl-layered double hydroxide and polyaniline (PANI/N-GQD/MO/LDH) for doxorubicin delivery in breast cancer cells. This assembly was biocompatible and pH-sensitive. Interestingly, it achieved significant inhibition ratio against human breast cancer cell lines (MCF-7), whereas, viability of human L929 normal cells was unaffected. Excellent blood compatibility and increased therapeutic efficacy of doxorubicin was achieved (Ahmadi-Kashani et al. 2020). In another study, a nanosystem was constructed using quantum dots (QDs) and nanohydrogels (NHG). Nanohydrogel is nanomolecular structure having excellent swelling properties, elasticity, porosity, permeation and mechanical properties. It is synthesized from cross-linked polymers and have high water content that make it suitable for delivering therapeutic agents and drugs (Akram and Hussain 2017). NHG-QDs nanosystem were conjugated with an amino-modified MUC-1 aptamer and loaded with PTX. This nanocomposite further attached with the inhibitor of lactate dehydrogenase and sodium oxamate (Ap-NHG-QDs-PTX-SO) to disrupt glycolysis of MCF-7 cells. Ap-NHG-QDs-PTX-SO nanosystem showed targeted mitochondria-mediated apoptosis and act as a promising theranostic for simultaneous imaging and therapy of breast cancer (Ranjbar-Navazi et al. 2021). Further, carbon quantum dots (CQDs) have the dual capability of nanocarriers and imaging agents. Hydrothermally prepared Nitrogen-doped CQDs were conjugated with Quinic acid for specific targeting to breast cancer cells. Gemcitabine was loaded on this nanosystem against MCF7 cell line. This assembly demonstrated high accumulation in tumor and excellent luminescent properties (Samimi et al. 2021). In another study, chitosan-carbon quantum dot-aptamer was constructed for delivering 5-Fluorouracil to tumor cells. This biocompatible nanoconstruct ensures high drug loading and entrapment efficiency. Sustained drug release, better uptake and high cytotoxicity in MCF-7 breast cancer cell line (Zavareh et al. 2020).

Due to their magnetic, radioactive or plasmonic properties, inorganic NPs are particularly suitable for applications such as diagnostics, imaging and photothermal therapy. Most of the inorganic NPs have good biocompatibility, stability and fulfill niche applications that require properties inaccessible to organic materials. However, they are limited in their clinical application owing to the low solubility and associated toxicity, particularly in formulations employing heavy metals. (Mitchell et al. 2021) These heavy metals induce estrogenic effects and potentially alter the anatomy of the breast. Changes in the anatomy of the breast are linked to an increased susceptibility to the development of cancer. In vitro testing suggested the potential of cadmium as a powerful metal and has been correlated with the proliferation of breast cancer (Umapathi et al. 2021).

Lipid-based nanomedicines

Lipid nanocarriers are gaining limelight for targeted drug delivery due to the nontoxic, biocompatible, biodegradable, high loading capacity, prolonged circulation and tumor accumulation (Patel et al. 2021).

Liposome

Liposomes contain one or more concentric spheres of lipid bilayers with internal cavity capable of carrying aqueous solutions. Hydrophilic dugs can be loaded in internal water compartment and hydrophobic drugs can be attached to lipid bilayers ensuring targeted delivery at tumor location. Liposome-loaded drugs are also protected from external conditions (Verma and Roshan 2015). Many drugs loaded with liposomes have showed targeted approach with enhanced therapeutic efficiency (Table 4).

Table 4.

Liposomes and solid lipid NPs with potential application in breast cancer treatment

Nanoformulations	Component of delivery system	Drug	Therapeutic indication	Conclusion	References
Liposome co-loaded liposomal formulations	PEGylated DSPC(1,2-distearoyl-sn-glycero-3-phosphocholine) liposomes (DSPC)	Doxorubicin, umbelliprenin	Human breast cancer cells (MCF-7, MDA-MB 231, BT-474)	Homogenous lipoformulation, Co-loading increased the toxicity	(Gkionis et al. 2020)
EGFR Targeted Paclitaxel and Piperine Co-loaded Liposomes	TPGS (vit E-PEG 1000-succinate)-coated liposomes, cetuximab (CTX)	Paclitaxel (PTX) and piperine (PIP)	Triple-negative breast (TNBC) cancer cell lines MDA-MB-231	Superior uptake and cytotoxicity	(Burande et al. 2020)
Liposomes coencapsulating DSF and DOX (LipoDSF-DOX)	Disulfiram (DSF)	Doxorubicin (DOX	MCF-7, MDA-MB 231,	Increased DOX intracellular accumulation and cytotoxicity	(Rolle et al. 2020)
HER2 targeted liposomes	Chitosan (CS), anti-HER2 tumor homing peptide (THP)	Capecitabine (CAP)	Tumor bearing mice	Improved the specificity and efficacy of CAP	(Singh et al. 2020)
GEM-DOX-MPLA liposomes	Monophosphoryl lipid A (MPLA)	Doxorubicin (DOX), gemcitabine (GEM)	4T1 tumors	Increased dendritic cell expression of CD86 in the presence of liposomes	(Wu et al. 2021)
Targeted Doxorubicin Liposomes	Saturated (HSPC and DPPC) and unsaturated (POPC and DOPC) lipids, aptamer-labeled liposomal nanoparticle	Doxorubicin	MCF-7 and SKBR-3 cells	Substantial reduction in the dose of DOX and improves the therapeutic benefits	(Chowdhury et al. 2020)
5-fluorouracil-loaded elastic liposome	Carbopol-980 gel of 5-Fluorouracil (5-FU), permeation enhancers (azone, propylene glycol (PG) and lauryl alcohol (LA))	5-Fluorouracil (5-FU)	Investigation in vitro flux value across the abdominal rat skin	Enhancing the drug permeability across the rat skin, efficient therapeutic efficacy	(Hussain et al. 2016)
Solid–lipid nanoparticles
Phenylboronic acid-niclosamide solid lipid nanoparticles (PBA-Niclo-SLN)	Phenylboronic acid (PBA), PBA-associated stearylamine (PBSA)	Niclosamide	TNBC (MDA-MB-231) cells and tumor-bearing mice	Greater antitumor efficacy, significant tumor regression	(Ss Pindiprolu et al. 2020)
Lysine conjugated SLNs	Tripalmitin glyceride and stearic acid as lipid constituents	Epirubicin (EPI)	MCF-7 BC cell line	Controlled release of drug, remarkable anticancer effect on breast cancer cell lines	(Bayat et al. 2021)
Glyceryl Monostearate Based Solid Lipid Nanoparticles	Glyceryl Monostearate (GMS)	Docetaxel	Breast cancer cell lines	Biocompatible, controlled delivery of docetaxel and improved therapeutic outcome	(Rai et al. 2021)
CLEN (Curcumin encapsulated lipidic nanoconstructs	Compritol 888 ATO, Glyceryl Monostearate	Curcumin	Female Wistar Rats	Presence of curcumin in a solubilized form, controlled release of drug
Death receptor-5 (DR5) antibody conjugated solid lipid nanoparticles (DR5-DAPT-SLNs)	Death receptor-5 (DR5) antibody	N-[N-(3,5-Difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT)	MDA-MB231 triple negative breast cancer (TNBC) cells and DMBA induced breast cancer model in mice	Greater tumor regression, higher cytotoxicity	(Pindiprolu et al. 2021)
Transferrin targeted solid lipid nanoparticles	Transferrin, Glyceryl Palmitostearate, Cremophor EL	Tamoxifen citrate	Human breast cancer MCF-7 cells	Qualitative uptake,more cytotoxicity	(Bhagwat et al. 2020)
Nanostructured lipid carriers
Cabazitaxel loaded NLCs	Nanostructured lipid carriers (NLCs)	Cabazitaxel (CBZ)	MDA-MB-468 and MCF-7 cell lines	Higher uptake and apoptosis	(Chand et al. 2021)
RLN-loaded nanostructured lipid carriers (RLN-NLCs)	Compritol® 888 ATO Transcutol® HP	Raloxifene (RLN)	In vitro, in vivo, ex vivo studies	Higher cytotoxicity, improved intestinal permeability, increased oral bioavailability	(Soni et al. 2020)
Targeted doxorubicin NLCs	Compritol®ATO 888, Gelucire®, Cetyl palmitate, Oleic acid	Doxorubicin	MDA-MB-231 cell line	Increased cellular uptake, targeted delivery	(Moraes et al. 2021)
TAM-SFN-NLCs	Precirol® ATO5 and Transcutol® HP	Tamoxifen (TAM) and sulforaphane (SFN)	In vitro, in vivo, ex vivo studies	Oral bioavailability, enhanced intestinal permeability, reduced TAM-associated toxicity	(Mangla et al. 2020)

Capecitabine which is use in management of breast cancer cells suffers the drawbacks of poor bioavailability and off target effects. This problem was resolved by HER2 targeted liposomes loaded with capecitabine utilizing tumor targeting potential of chitosan and anti-HER2 tumor homing peptide. It exhibited greater tumor regression ability in tumor bearing mice and further improved the survival (Singh et al. 2020). For the targeted delivery of doxorubicin to HER2+ breast cancer cells, aptamer-labeled liposomal nanostrucured delivery system was formulated. The formulations showed a high entrapment efficiency of about 88 ± 5%. It showed higher cytotoxicity and uptake of the aptamer-labeled liposomes in both MCF-7 and SKBR-3 cells. This targeted approach also proved effective in substantial reduction of doxorubicin doses with improved targeted therapeutic benefits (Chowdhury et al. 2020). Further, P-glycoprotein, pump that actively eliminates the doxorubicin out of the cellular milieu thereby reducing the efficacy. Disulfiram (DSF) inhibits the formation of P-glycoprotein but it exhibited poor solubility and stability. This problem was addressed by liposomes encapsulating both DSF and DOX (LipoDSF-DOX). In this nanocarrier, DSF was loaded in lipid bilayers and DOX was loaded in aqueous core. It released DSF faster than DOX. It also ensures increased intracellular accumulation of DOX and cytotoxicity in P-glycoprotein-expressing breast cancer cells (Rolle et al. 2020). In another study, doxorubicin-umbelliprenin co-loaded liposomes were constructed using optimized microfluidic set-up. This method allowed fine control over liposome size (100–250 nm), shape and uniformity. Umbelliprenin was used to provide fluidity to lipid biomembranes which produced uniform co-loaded liposomes compared to liposomes without umbelliprenin. This nanoassembly showed high drug loading capacity and toxicity against a panel of human breast cancer cells (MCF-7, MDA-MB 231, BT-474) (Gkionis et al. 2020). Doxorubicin was also co-loaded with gemcitabine (GEM) in a liposome. A clinically used TLR4 adjuvant, monophosphoryl lipid A (MPLA) was fabricated into the lipid bilayers of liposomes loaded with a 1:1 molar ratio of DOX and GEM. In vitro and in vivo studies demonstrated increased dendritic cell expression of CD86 in the presence of this liposomes (Debra Wu et al. 2021). EGFR (epidermal growth factor receptor)-targeted therapeutics can be used to treat EGFR up regulated triple negative breast (TNBC) cancer. TPGS (vit E-PEG 1000-succinate)-coated liposomes were constructed with PTX and piperine (PIP). PTX-PIP co-loaded targeted liposomes had demonstrated higher cytotoxicity and superior uptake in MDA-MB-231 cells. It also proved the synergistic antitumor effect of PIP. Lyophilized liposomes of size range between 189 and 210 nm showed an excellent stability profile (Burande et al. 2020).

Solid lipid NPs (SLN)

To overcome the drawbacks of liposomes like instability, drug leakage and high production costs, solid–lipid nanoparticles are frequently utilized (Maja et al. 2020). SLN possess a solid lipid core for drug loading and peripheral lipid bilayers. It is also stabilized with surfactants and efficiently improves bioavailability of the drug molecules (Shirodkar et al. 2019). SLN have attracted researchers as potential nanocarriers for drug delivery (Table 4). Tamoxifen citrate was loaded into the lipid phase along with surfactant. Transferrin glycoprotein was utilized as the surface attachment ligand for the targeted drug delivery to human breast cancer MCF-7 cells. This transferrin-conjugated SLNs demonstrated sustained drug release, targeting effect and improved therapeutic activities (Bhagwat et al. 2020). In another study, death receptor-5 (DR5) antibody was conjugated to solid lipid nanoparticles for targeted delivery of γ-secretase inhibitor, N-[N-(3, 5-Difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT) to MDA-MB 231 triple negative breast cancer (TNBC) cells. The DR5-DAPT-SLNs showed higher cytotoxicity and greater tumor regression (Pindiprolu et al. 2021).

Glyceryl monostearate-based SLNs was prepared with docetaxel in lipid core for the controlled release of drug at tumor location. This nanoassembly showed stability, compatibility, cytotoxicity and improved therapeutic outcome (Rai et al. 2021). For targeted delivery of niclosamide, phenylboronic acid-modified solid lipid nanoparticles (PBA-Niclo-SLN) were employed for delivering cargo to TNBC (MDA- MB231) cells and tumor-bearing mice. Interestingly, TNBC cells overexpress sialic acid (SA) receptors. Therefore, SA is a potential marker for site-specific delivery of anticancer agents to TNBC. Phenylboronic acid (PBA) has high affinity and selectivity toward SA. This nanoformulation showed greater efficacy towards tumors by causing G0/G1 arrest and apoptosis (Pindiprolu et al. 2020). To improve aqueous solubility and stability of Epirubicin (EPI) in the biological environment, EPI was loaded in the lysine- modified SLNs (L-SLNs). These targeted nanocarriers showed control release of drug and excellent anticancerous activity on MCF-7 breast cancer cell line (Bayat et al. 2021).

Nanostructured lipid carriers (NLCs)

Despite several advantages of SLNs, it suffers from various problems like limited drug loading, drug leakage and crystallization during storage. These disadvantages can be overcome using another type of lipid nanoparticles, the NLC (Shirodkar et al. 2019). NLCs were prepared using mixtures of solid lipid (SL) and liquid lipids (LL) that form an amorphous solid matrix. LL contributes to the creation of an amorphous lattice with considerable imperfections in its crystalline solid matrix. It allows greater loading of drugs to NLCs as compared to SLNs (Haider et al. 2020). Various researchers utilized NLCs as nanocarriers for targeted drug delivery (Table 4). Cabazitaxel-loaded NLCs were constructed for targeted delivery of drug. The drug was entrapped inside the NLCs without forming any chemical bond. It showed higher cellular uptake and cytotoxicity against MDA-MB-468 and MCF-7 cell lines (Chand et al. 2021). Similarly, folic acid targeted NLCs were employed for doxorubicin delivery to breast cancer cells. It achieved ~ 65% of encapsulation efficiency. In vitro studies using MDA-MB-231 cell line revealed an enhanced cellular uptake and reduced risk of deleterious off-target effects of doxorubicin (Moraes et al. 2021). Another drug, Raloxifene (RLN) has been used for its anti-proliferative potential on breast tumor tissue. It suffers from the drawback of poor oral bioavailability. RLN-loaded nanostructured lipid carriers (RLN-NLCs) were constructed from scalable ultrasonication method. Compritol^® 888 ATO was used as a solid lipid and Transcutol^® HP as a liquid lipid. The entrapment efficiency of RLN was high with reduced crystallinity of the drug. These nanocarriers demonstrated higher cytotoxicity in MCF-7 cells and improved intestinal permeability. Further, high oral bioavailability and reduced offsite toxicity was observed in female Wistar rats (Soni et al. 2020). In another similar study, NLCs were constructed to load tamoxifen (TAM) and sulforaphane (SFN) drugs. Optimized TAM-SFN-NLCs exhibited a particle size of 121.9 ± 6.42 nm and zeta potential of -21.2 ± 2.91 mV. This nanoassembly exhibited augmented oral bioavailability with SFN significantly reducing the TAM-associated toxicity in female WISTAR rats (Mangla et al. 2020).

Liposome-based NPs can circulate in the bloodstream for an extended period of time, providing a longer treatment effect. Liposomes can also build up at the site of a tumor, locating and delivering the drug concentration at the target site. Since the approval of Doxil in 1995, an increasing number of nanoformulations using liposomal delivery have been approved or are under investigation (Ventola 2017). The use of doxorubicin is linked to some potentially fatal side effects, including cardiotoxicity. However, despite these advantages, liposome-based NPs are limited by the low drug load and low biodistribution that leads to high absorption in the liver and spleen (Mitchell et al. 2021). The most important side effects of pegylated liposomal doxorubicin (PLD) were dermal toxicity and mucositis, but the percentage of patients with grade III and IV side effects was relatively low (Ansari et al. 2017). Despite the high efficacy of cisplatin, it has its use limited by side effects, mainly nephrotoxicity and neurotoxicity (Franco et al. 2021).

Carbon-based nanomedicine

Carbon-based nanomaterials (CBNs) is a promising clad of NPs for its application in biomedical applications including biosensing, drug delivery, tissue engineering, imaging, diagnosis and cancer therapy (Maiti et al. 2019). Carbon is a flexible molecule with different types of allotropes such as carbon nanotubes, graphene, fullerene, nanodiamonds etc. The different carbon allotropes have attracted escalating attention due to their unique structures and physiochemical properties. Due to their excellent optical activity and considerable multifunctional surface area, they are able to reveal higher drug-loading capacity, improved biocompatibility (Debnath 2021). In Table 5, various carbon based nanocarriers are listed for their application in targeted drug delivery.

Table 5.

Carbon-based nanomaterials, antibody–drug conjugates and exosomes with potential application in breast cancer treatment

Nanoformulations	Component of delivery system	Drug	Therapeutic indication	Conclusion	References
Photothermal therapy with targeted single-walled carbon nanotubes (SWCNTs) and immunostimulation with a checkpoint inhibitor(SWCNT-ANXA5)	Single-walled carbon nanotubes, annexin A5 (ANXA5)	Anti-cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA-4)	EMT6 breast tumors in syngeneic BALB/cJ mice	Increased survival	(McKernan et al. 2021)
Folic acid-conjugated raloxifene hydrochloride carbon nanotube	FA-PEG-CNTs (Folic acid conjugated PEG coated CNTs)	Raloxifene hydrochloride (RLX)	MCF7 breast cancer cells	High drug loading, control release of the drug in the pH-dependent manner	(Jawahar et al. 2020)
Fullerene	Graphene and fullerene	Letrozole	Hormone-sensitive breast cancer	Enhancement of absorption and biological activity	(Almuqrin et al. 2020)
Antibody–drug conjugates
MMAE-trastuzumab antibody–drug conjugates (ADCs)	Monomethyl auristatin E (MMAE)	Trastuzumab	MDA-MB-453 (HER2-positive cells) and HEK-293 (HER2-negative cells)	Higher antitumor activity,potent inhibitor of colony formation	(Yaghoubi et al. 2021)
EGFR-targeted antibody–drug conjugates	Navitoclax	Antibody–drug conjugates (ADC: ABT-414; ABBV-321)	Patient-derived xenograft (PDX) models of TNBC	Reduction in tumor growth, tumor regression	(Zoeller et al. 2020)
Sacituzumab govitecan antibody–drug conjugate	SN-38 (topoisomerase I inhibitor	Sacituzumab govitecan	Metastatic Triple-Negative Breast Cancer	Progression-free and overall survival longer	(Bardia et al. 2021)
Exosomes
Macrophage-derived exosome-coated poly(lactic-co-glycolic acid) nanoplatform	Mesenchymal-epithelial transition factor (c-Met), poly(lactic-co-glycolic acid)	Doxorubicin	Triple-negative breast cancer (TNBC)	Excellent tumor-targeting efficacy, improved cellular uptake, increased apoptosis	(Li et al. 2020a, b)
Exosome-mediated miR-33 transfer	Exosomes isolated from 4T1 breast cancer cells	MicroRNAs (miRNAs) miR-33	4T1 breast cancer cell line	Efficient nanocarrier for miR-33 delivery into macrophages	(Moradi-Chaleshtori et al. 2021)
Mesenchymal Stem Cell-Derived Exosomes	Exosomes isolated from adipose-derived mesenchymal stem cells (ADMSC-exosomes)	miRNA-381	MDA-MB-231 cells	Efficient nanocarriers for RNA-based therapies	(Shojaei et al. 2021)
Hyaluronan decoration of milk exosomes	Milk exosomes (mExo)	Doxorubicin (Dox)	CD44 over-expressed human cancer cell lines (breast cancer MDA-MB-231, MCF-7	Excellent tumor cell-specific drug delivery	(Li et al. 2020a, b)

Carbon nanotubes (CNTs) are graphene sheets rolled into a seamless cylinder. CNTs made from a single graphene sheet results known as single-walled nanotubes (SWNT) while several graphene sheets resulted in multiwalled carbon nanotubes (MWNTs). Pristine CNTs are not soluble but functionalization with different organic groups makes it soluble. High surface area ensures its conjugation to various therapeutic molecules (Elhissi et al. 2012; Rastogi et al. 2014). Carbon Nanotubes have very high tensile strength and highly flexible so that they can be bent without damage (Paliwal et al. 2020). In a recent study, the drug raloxifene hydrochloride was entrapped in carbon nanotubes. Further, the surface of the CNTs was functionalized by folic acid for the targeted delivery of drug to tumor location. This nanoconjugate caused higher cellular internalization and remarkable apoptosis in MCF-7 breast cancer cell (Jawahar et al. 2020). Sometimes targeted single-walled carbon nanotubes with photothermal therapy and immunostimulation pose a greater cytotoxic impact on metastatic breast cancer. For this SWCNT was functionalized with an annexin A5 and loaded with anti-cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA-4). Later, it was administered systemically at a relatively low dose of 1.2 mg/kg, which got accumulated in the tumor tissue region. The nanocomposite exhibited prolonged survival (55%) at 100 days after tumor inoculation (McKernan et al. 2021).

Fullerene is another carbon allotrope with cage-like fused-ring structure having unique properties such as symmetric nature, tensile strength, thermal, photoconductivity and drug loading capabilities. Various surface modifications allows fullerene to be tuned for biomedical applications (Kumar and Raza 2017; Wang et al. 2021). Recently, targeted delivery of drug Letrozole was achieved using fullerene. Letrozole self-assembled with fullerene and graphene sheets and form stable structure. This nanoassembly enhances all properties of molecules like surface-enhanced Raman spectra and non-linear optical properties (Almuqrin et al. 2020). Nanodiamonds are biocompatible, carbon-based drug delivery nanocarriers with outstanding surface properties (Alawdi et al. 2019). Its unique properties like chemical, mechanical and biological properties make it suitable for imaging and drug delivery applications. Nanodiamonds surface chemistry, purity, absorption of drug and its release in biological environment are important parameter which should be considered before its application in drug delivery (Chauhan et al. 2020). Boron doped nanodiamonds had the greater ability to heat the environment when excited at a wavelength of 808 nm. Hyperthermia was able to sensitize tumor cells to chemotherapy and radiation therapy, significantly reducing the duration and intensity of treatment (Vervald et al. 2020). Combinational strategies can be a perfect choice for overcoming tumor heterogeneity and complexity. In a study, nanodiamonds (NDs), protamine (PS), curcumin (Cur), hyaluronic acid (HA) and IR780 was utilized for ternary collaborative Cur/photothermal/photodynamic combination therapy of triple-negative breast cancer (TNBC). The resulting nanoassembly had a uniform size, high drug-loading ability, biocompatibility and excellent colloidal stability (Cui et al. 2021).

There have been many concerns regarding the safety issues of biomedical applications of carbon-based nanomaterials. Carbon-based NPs have demonstrated toxicity and long-term effects in several in vitro and in vivo studies. Oxidative stress, inflammatory responses, interstitial fibrosis, granuloma formation, malignant transformation, and DNA mutations are the underlying mechanism of associated toxicities. Carbon nanotubes have also been observed to induce mesothelioma, a condition associated with asbestos, a naturally occurring carcinogenic mineral fiber. It has been suggested that the toxic effects of carbon nanotubes may be a consequence of shape rather than material, demonstrating that the toxicity of nanoparticles can depend on the morphology of the particles. The concentration, lateral dimension, surface properties, types and presence of functional groups have also been shown to greatly influence their toxicity in the biological environment (Mahor et al. 2021; Ventola 2017).

Advanced nanomedicine

Antibody–drug conjugate (ADC)

Antibody–drug conjugate (ADC) is a conjugate of drug and antibody, in which both are attached by a connector head. Monoclonal antibody act as a carrier and deliver the cytotoxic drug at specific site of action (Zhao et al. 2020) (Table 5). With the occurrence of such conjugates like trastuzumab emtansine [T-DM1] and trastuzumab deruxtecan in action, there is an increased survival rates of patients for nonmetastatic HER2-positive breast cancer (Kunte et al. 2020). Sacituzumab govitecan (IMMU-132) and trastuzumab deruxtecan (DS-8201a) are gaining attention for metastatic breast cancer management (Nagayama et al. 2020). In a randomized, phase 3 trial, sacituzumab govitecan antibody–drug conjugate was compared with single-agent chemotherapy (eribulin, vinorelbine, capecitabine, or gemcitabine) targeting the human trophoblast cell-surface antigen 2 (Trop-2), coupled to SN-38 (topoisomerase I inhibitor). These ADCs resulted in progression-free survival among patients without brain metastases (Bardia et al. 2021). In a different study, Trastuzumab was peptide linked to monomethyl auristatin E (MMAE), which is a microtubule-disrupting agent. MMAE-trastuzumab ADCs was used to target Human Epidermal growth factor Receptor 2 (HER2). This nanoconjugate was studied on HER2-positive cells (MDA-MB-453) and HER2-negative cells (HEK-293). It demonstrated same binding ability and higher antitumor action to MDA-MB-453 as compared to unconjugated trastuzumab (Yaghoubi et al. 2021). Two EGFR-targeted antibodies, ABT-414 and ABBV-321 were conjugated with navitoclax. Navitoclax is an antagonist of the anti-apoptotic BCL-2 and BCL-X(L) proteins. This ADCs exhibited significant tumor regression in the EGFR-expressing patient-derived xenograft (PDX) (Zoeller et al. 2020).

Exosomes

Exosomes are natural nano-vesicles (40–120 nm) that have the ability to transport proteins, mRNAs and microRNAs (miRNAs) between cells. Because of its unique properties like non toxicity and long circulation time make it suitable as potent nanocarriers (Li et al. 2020a, b) (Table 5). Triple-negative breast cancer lacks the effectiveness of therapeutics because of poor targeting, easy removal and high toxicity. Exosomes can be helpful to overcome these problems with low immunogenicity. In addition, macrophage-derived exosomes can mimic the immune cell to target cancer cells. Such exosomes were coated with poly (lactic-co-glycolic acid) and loaded doxorubicin was targeted to mesenchymal-epithelial transition factor (c-Met), which is over expressed by TNBC cells. These engineered exosomes-coated nanoparticles demonstrated enhance cellular uptake and apoptotic efficacy of doxorubicin (Li et al. 2020a, b). Similar to cell-derived exosomes, milk exosomes (mExo) can be employed to targeted drug delivery because of its low cost and biocompatibility. Novel strategy was utilized for directing doxorubicin-loaded mExo to CD44-overexpressing tumor cells. Hyaluronan which is a CD44-specific ligand was decorated onto the phospholipids bilayers of Dox-loaded mExo. The in vitro analysis of this nanoassembly exhibited selective delivery of Dox into over-expressed CD44 tumor cells (Li et al. 2020a, b).

However, chemotherapeutic agents may develop the drug resistance over the period. Exosomes-mediated delivery of miRNA or associated proteins could modulate the chemosensitivity (Mao et al. 2016). MicroRNAs (miRNAs) represent small single-stranded non-coding RNA. Its utility in post-transcriptional gene regulation is well known. It hybridizes to target mRNA(s) and induce translation repression or mRNA degradation (Dasgupta and Chatterjee 2021). These potential exosomes can be used for delivering nucleic acid or peptide-based therapeutics directly to the tumor cells. In a study, exosomes isolated from 4T1 breast cancer cells used to deliver miR-33 into macrophages. It showed polarization in macrophages which is required for regression in tumor growth (Moradi-Chaleshtori et al. 2021). Similarly, in a different study, exosomes were isolated from adipose-derived mesenchymal stem cells. It was used to deliver miR-381 to MDA-MB-231 triple negative breast cancer cells. This nanoconstruct successfully internalized and inhibited proliferation, migration, and caused apoptosis in vitro (Shojaei et al. 2021).

Kadcyla is an antibody–drug conjugate targeted therapy. The emtansine is attached (conjugated) to the Herceptin. Herceptin then carries emtansine to the HER2-positive cancer cells. This way, the emtansine is less toxic to healthy cells and more effective in treating cancer cells. Most common side effects of Kadcyla are fatigue, nausea, bone and joint pain and low blood platelet count. It can also induce toxicities and side effects (rare, 1–2% of people treated with Kadcyla) like hepatotoxicity, embryo-fetal toxicity, pulmonary toxicity, hemorrhage (Priestnall et al. 2020; White et al. 2020).

Outcomes of drug targeting using nanocarriers

Nanocarriers have been successfully used to bypass certain limitations in the delivery of conventional anticancer drugs with increased bioavailability, prolonged circulation time and increased tumor accumulation for higher therapeutic gain against breast cancer. An optimal drug delivery system must completely deliver drugs rapidly in cytoplasm leading to an elevated intracellular drug concentration. High concentration will reduce the limitation of drug to inhibit the proliferation and induce cell death in drug resistant breast cancer cells (Rolle et al. 2020). There are a number of key challenges to breast cancer drug therapy and nanomedicine may be an amenable approach to efficiently tackle these challenges including specificity, access of drugs. Employing strategies such as PEGlyation to extend the circulation time, controlled drug release from nanocarrier, drug solubilization, careful optimization of dosing schedule use of nanocarrier to co-deliver multiple drugs, etc. may (Di Wu et al. 2017).

Several nanocarriers have gained FDA approval for use in cancer patients (Table 6), while many others are in clinical development. The first anticancer nanomedicine approved by the FDA was Doxil^™/Caelyx^™ in 1995, which achieves a differential distribution of doxorubicin as compared to free drug (Dahiya et al. 2021; Qingxin Mu and Hui Wang 2017). As discussed above anti-breast cancer nanodrugs in clinical development can be broadly divided into five main types: liposomes, polymeric conjugates, polymeric micelles, antibody–drug conjugates and others. Although many of them are FDA approved and some of them are still in clinical development phases, are summarized in Table 6 (Hare et al. 2017; Rugo 2021; Ventola 2017). Formulations based on the delivery of NPs have the potential to improve physicochemical characteristics and are effective in targeting anticancer drugs. These NPs help improve the water solubility of anti-cancer drugs, increase the efficiency of drug delivery to tumor sites, and enable targeted delivery of anti-cancer drugs (Farooq et al. 2019; Di Wu et al. 2017).

Table 6.

Nanomedicines in clinical trials or on the commercialization (Hare et al. 2017; Rugo 2021; Ventola 2017)

Nanomedicine types	Drug	Product name/company	Indication	Phase
Liposomes	Paclitaxil	LEP—ETU/Insys EndoTAG-1/MediGene	Breast cancer	Phase II
	Doxorubicin	Myocettm/Teva UK	Metastatic breast cancer	Approved
Polymeric micelles	Paclitaxel	Genexol-PM™/Samyang Biopharmaceuticals	Breast cancer	Approved
	Paclitaxel	NK105/NanoCarrier™	Breast cancer	Phase III
	Docetaxel	Genexol-PM™/Samyang Biopharmaceuticals	Breast cancer	Marketed in South Korea
Polymeric conjugates	Irinotecan	NKTR102 (PEG)/Nektar	Metastatic breast cancer	Phase III
Antibody-drug conjugates	Kadcyla	Ado-trastuzumab emtansine/Genentech	HER2–positive , late-stage breast cancer	Approved
	Enhertu	Trastuzumab deruxtecan/AstraZeneca	Metastatic breast cancer	Approved
	Trodelvy	Sacituzumab govitecan-hziy/ Immunomedics	Metastatic breast cancer	Approved
Other	Paclitaxel	Abraxane™/Celgene	Advanced breast cancer	Approved
	Phospholipid stabilized microbubble	SonoVue/Bracco Imaging	Ultrasound enhancement for breast and other cancers	Approved

In vitro breast cancer models

The effective clinical translation of anticancer nanodrugs requires the development of accurate preclinical models that precisely summarize tumor characteristics including determination of safety, efficacy and biomarkers of response to treatment. In vitro tumor models have potentiated the cancer research but the challenge of recurrence due to metastasis contributes majorly towards cancer associated mortality. The dire need of in vitro tumor models for understanding the progression and treatment of cancer has led to emergence of conventional two-dimensional (2D) cell cultures are used before testing in animal models of cancer. However, in vitro models do not replicate the complexities of tumor microenvironment. The interactions with tumor microenvironment govern the regulation of tumor progression which can be investigated using in vivo models. The gap between the in vitro models and in vivo models is required to be filled to understand the therapeutic efficacy. The three-dimensional in vitro models mimic the in vivo settings to the closest. 3D in vitro models provide a detailed molecular and cellular understanding compared to other 2D invitro cell cultures. Thus, employing 3D in vitro models facilitates the screening and development of new drugs with better understanding of therapeutic efficacy parameters (Boix-Montesinos et al. 2021; Jaganathan et al. 2014; Katt et al. 2016).

Industrial scale up issues of nano delivery systems

Various US FDA approved nanomedicine products are typical examples of the result of aggressive research in nanomedicine and commercially available for the treatment of metastatic breast cancer. For a nanomedicine to reach from bench to clinic multiple factors are required to be optimized including therapeutic needs, market demand, research and development, steps of manufacturing, scalability, clinical trials and regulatory issues, a nanomedicine product arrives on the market. However, nanomedicines still pose many challenges for scale up and commercialization. The launch of new nanomedicine products on the market is preceded by several stages of development. One of the big challenges between the stages of nanomedicine development is industrial scale-up. Laboratory-scale nanomedicine preparations are known to be feasible and well documented by various researchers. At the same time, the large-scale design of nanomedicines has received less consideration during development. Limited experiences and insufficient information on technologies for scaling nano-drugs can hinder the introduction of these nano-drugs on the market (Muthu and Wilson 2012). Several elements are associated with the scaling of a nanomedical product from the counter to the market. For example, the nature of the material and its generally considered safe state (GRAS), the toxicological characteristics associated with the size and shape of the nanoparticles, the in vivo biodegradability of the nanosupports and the balancing of the multicomponent on a large-scale system are some of them. Care should be taken before the selection of materials, solvent, nanoparticle development procedure, cost, and acceptability of the finished product to physicians and patients. During the scaling of the laboratory method, the desired characteristics of the nanoparticles are sometimes lost. In a scalability study of nanoparticles prepared using the emulsion method, it was observed that by increasing the turbine speed and the stirring time, the particle size decreased although the trapping efficiency is not impaired. The choice of the manufacturing method of the nanoparticles is also important to save time when producing pilot batches from a scale-up point of view, for the production of nanoparticles (Paliwal et al. 2014).

Recent patents and clinical trials

Over the past decade, new generations of nanoparticles have emerged that can fulfill additional delivery functions which may allow treatment through new therapeutic modalities. From 2016 to 2019, two nanoparticles administered intravenously were approved by the Food and Drug Administration (FDA) or the European Medicines Agency (EMA). More than 15 new nanoparticle technologies have entered clinical trials. VYXEOS is a combined chemotherapy nanoparticle developed and marketed by Jazz Pharmaceuticals which encapsulates a synergistic molar ratio of cytarabine to daunorubicin of 5: 1. VYXEOS are 100 nm bilamellar liposomes whose lipid membrane consists of desaturated phosphatidylcholine (distearyl glycolesterol: Ratio 7: 2: 1 M). In the pivotal efficacy study (NCT01696084), VYXEOS exhibited significant increase (p value = 0.005) in overall survival of 9.6 months compared to 5.9 months for the drug-free control. Since 2016, the number of clinical studies with VYXEOS has increased from 7 to 21 with most recent studies evaluating the use of VYXEOS in additional patient populations (e.g., children; NCT03826992) and leukemias (e.g., lymphoblastic leukemias; NCT03575325) (Anselmo and Mitragotri 2019). A team of researchers at American University of Sharjah (AUS) have recently obtained a U.S. patent for systems and strategies for targeted carcinoma therapies. The patent describes a therapy platform that uses Herceptin-targeted nanocarriers that encapsulate many anti-neoplastic (anti-cancer) agents. Once the nano-vehicles reach and bind to the breast cancer tissues, the drug is free using ultrasound waves. This can make sure the delivery of high concentrations of the NPS to the unhealthy tissues whereas avoiding its interaction with healthy cells within the body, hence reducing the facet effects of standard chemotherapy.

Conclusions and future prospects

Currently, researches on nanotherapeutics for the treatment of breast cancer are rising at a great pace. Special attention was paid on cellular uptake, biocompatibility and the use of targeting ligands to target large tumor tissues of breast cancer. The use of targeting ligands in nanoparticle formulations increases the effectiveness of the treatment and avoids toxicity to healthy cells. Various advances in nanotechnologies such as polymeric and metallic nanoparticles, nanomicelles, dendrimers, carbon nanotubes, liposomes, antibody drug conjugates, exosomes, etc., have been explored and currently employed for targeted drug delivery to tumor tissue with minimum side effects (Fig. 4). Therefore, nanomedicine based targeted drug delivery approach may be the defining therapeutic drug development strategy for breast and other cancers. Despite these potential results, nanodrugs suffer various challenges such as unexpected toxicities, their effect on the immune system, pharmaceutical stability issues and scalability difficulties. The toxicity exerted by nanomaterials has limited their commercialization and translations from laboratory to bench. Therefore, it is essential to control the toxicity of nanomaterials and to design nanomaterials with intelligent approaches for selective biomedical actions. Here, the rational design of nanomaterials can be oriented towards increasing the absorption power, reducing cellular efflux, reducing toxicity and following an integrated approach via computational biology perspectives (e.g., docking studies). Due to the improved properties and highly regulated characteristics of engineered nanomaterials, more research should be directed towards the development of biomimicry materials using the principles of nanomedicine Strict nano-regulations have closely monitored the commercialization of ways to treat and diagnose nanomaterials. This paves the way for more innovations in various fields for nanomaterial applications. Reproduction of uniform nanoparticles batches is also a huge hurdle which warrants further investigations. Each stage of the nanotherapeutics manufacturing process need to be monitored carefully and improved to ensure the repeatability, stability and efficiency of the synthesis. To achieve this goal a clearer insight of biological processes and progression in nanotechnology will help to develop more biocompatible and effective nanomedicines. Interdisciplinary and holistic approach is the best option for its clinical practice. Development and exploitation of clinically more relevant animal models to optimize nanomedicine properties, dosage regimens and pharmacological combinations with a clinical line of sight to the target disease as it develops in patients. The cumulative efforts of different scientific sectors will bring fecund and more beneficial results from the laboratory to clinic and create a new era of nanomedicine-based treatment for breast cancer.

Fig. 4.

Nanoenabled targeting of breast cancer: various nanocarriers have been developed and investigated to selectively carry the potent drug to the breast cancer cells. The selective targeting helps in enhanced cancer cell killing and reduced toxicity to normal healthy cells leading to increased therapeutic index. The effective doses required for cancer cell killing is lower in targeted approach due to enhanced bioavailability compared to conventional non targeted breast cancer therapy

Author contributions

All authors contributed in conceptualizing and designing of the manuscript. PB, LG and GS contributed in writing the manuscript. DG, NK and RS contributed the advanced nanomedicine section in the manuscript. AS and GS conceptualized and analysed the manuscript.

Funding

Not applicable.

Declarations

Conflict of interest

There are no actual or potential conflicts of interest.