Introduction

Cancer is a genetic disorder marked by the unregulated proliferation and spread of aberrant cells, which may be triggered by external elements such as tobacco, pathogens, poor nutrition, chemical agents, radiation, and internal factors. Most cancers arise from DNA alterations in proto-oncogenes or tumor-suppressor genes.1 Cancer remains the second most common cause of death, highlighting the urgent need for ongoing research and development of innovative therapies and treatment strategies.2 The mainstay of cancer therapy revolves around chemotherapy, surgery, radiation therapy, and their combinations.3 However, radiation causes acute radiation damage, including skin irritation and adverse effects on adjacent healthy tissues, and tissue-specific stem cells and tumor removal could prove fatal.2 Chemotherapeutics may eliminate rapidly proliferating malignant cells; they may damage normal, fast-dividing cells, including those in the bone marrow, gastrointestinal tract, and hair follicles. The harmful effects of chemotherapy include direct toxicity and indirect toxicity, mediated by liver metabolites, systemic immune suppression, decreased oxygen delivery, and enhanced inflammation.4 Consequently, creative options must be used to provide safer and more targeted cancer therapeutics.4,5 Nanotechnology-based drug delivery systems (DDS) can optimize the delivery process by enhancing drug solubility, blood circulation, and tumor site accumulation, while reducing therapeutic interactions and facilitating controlled release.6 The application of nanotechnology in cancer treatment has shown significant progress in terms of negating the adverse effects like neurotoxicity and tissue damage.7 Nanoparticles (NPs) (particles less than 100 nm in size) have made a profound impact on DDS by shielding drugs from degradation, delivering them precisely to the intended targets, and regulating their release.8 There has been a significant improvement in NP delivery strategy, including efficacy, stability, safety, and the pharmacokinetic profile of the NPs used for chemotherapeutic purposes.9 Cancer-targeting nanocarriers (NCs) are an emerging platform that can be tailored in terms of tumor pathophysiology to enhance therapeutic properties, tumor permeability, and drug retention time. Similarly, using NPs for therapeutic encapsulation makes it possible to reduce the chemotherapy-associated adverse side effects.10 Such nanosystems can increase the therapeutic index and tumor tissue concentrations, enhancing the efficacy of current regimens. Benefits include overcoming solubility and stability challenges, protecting drugs from degradation, enhancing drug distribution and targeting, facilitating sustained drug release, enabling multiple drug delivery, and reducing drug resistance.11 Nanoscale drug delivery systems (NDDS) offer several advantages in targeting cancer, like targeted drug delivery, multifunctional targeting, and tumor selectivity. These systems can provide a controlled and sustained drug release condition, perform multiple functions, and target tumor lesions either passively or actively.9,12 NDDS can extend the drug’s half-life and improve its uptake in the tumor, based on the size and surface properties of NPs. Nanotechnology has transformed drug delivery by enabling medications to enter previously inaccessible target areas of the body, such as transversing the blood-brain barrier (BBB) for neurological disorders/cancers.2 The NPs can release drugs in response to stimuli like light, pH, temperature, electromagnetic waves, magnetism, and ultrasound. Based on the physicochemical properties of drugs, customized nanocarriers can be designed to avoid unwanted clearance by macrophages and the kidneys.7 On the contrary, new technologies, like theranostic materials, combine therapeutic agents with imaging capabilities to monitor real-time treatment responses.13 Additionally, the development of personalized nanocarriers tailored to the molecular profiles of individual tumors enhances targeting and therapeutic efficacy, paving the way for more effective and individualized cancer treatment strategies.14

Many nano-therapeutic drugs have been commercialized or entered into clinical trials in recent years. For example, in 2010, the first clinical trial delivered small interfering RNA (siRNA) to patients with solid cancers using a targeted NP-based system.2 Herein, different types of nanocarriers and details on their composition, preparation method, and functionalization, as well as loaded drugs/active agents, have been reviewed, and the validation of each system in specific cancer-related applications has been highlighted.

Understanding Drug Resistance in Cancer Therapy

Drug resistance is a substantial obstacle to effective cancer treatment, accounting for approximately 90% of chemotherapy-associated failures.15,16 Multi-drug resistance (MDR) is a condition in which cancer cells develop resistance to various anticancer drugs, thereby diminishing the efficacy of the treatment. Higher doses of chemotherapy agents have been used to tackle MDR, but this approach frequently results in increased toxicity and may potentially damage healthy organs and tissues.17,18 This resistance can be classified into intrinsic resistance (present before treatment) and acquired resistance (developed during therapy) and is mediated by key genetic alterations and associated pathways. Oncogenes like KRAS and BRAF activate the MAPK/ERK signaling pathway, promoting cell survival, while HER2 overexpression activates the PI3K/AKT/mTOR pathway, contributing to resistance against therapies like trastuzumab.19,20 Tumor suppressor genes such as TP53 and BRCA1/BRCA2 are critical; mutations in TP53 disrupt apoptotic pathways, and alterations in BRCA genes impair DNA repair, leading to increased genomic instability and resistance to agents like cisplatin. Drug metabolism and efflux mechanisms, such as the overexpression of permeability-glycoprotein (P-gp), reduce the effectiveness of chemotherapeutic agents. Epigenetic modifications, including aberrant DNA methylation by DNA methyltransferases (DNMTs), can silence tumor suppressor genes, while histone modifications may impact drug sensitivity.19,20 Microsatellite instability (MSI) from DNA mismatch repair defects increases mutation rates and affects immunotherapy responses. Gene rearrangements like the BCR-ABL fusion gene activate tyrosine kinase signaling, contributing to resistance in chronic myeloid leukemia (CML). Cancer stem cells (CSCs) exhibit enhanced DNA repair and high drug efflux pump expression, making them resistant to conventional therapies. Alternative pathway activation, such as the MET signaling pathway, allows cancer cells to evade targeted therapies.19,20 There are a number of potential causes of MDR in cancer cells which revealed in Figure 1,21 including overexpression of drug efflux systems (Figures 1A), deregulated apoptosis process, and alterations in cell signaling pathways (Figure 1B–D), DNA repair mechanisms (Figures 1E and F), tumor heterogeneity, genetic and epigenetic modifications in cells (Figure 1I–L).17,21 Generally, the cooperation of these mechanisms leads to drug resistance.22 One example of MDR is seen in patients with metastatic non-small cell lung cancer (NSCLC) who acquire resistance to epidermal growth factor receptor (EGFR) inhibitors.23 Insufficient drug delivery to deep tumors and continuous exposure to sub-lethal doses of cytotoxic drugs are factors that promote drug resistance.17 Also, molecularly-targeted therapies, like tyrosine kinase inhibitors and antibodies, can cause drug resistance. The resistance is imparted by the growth of resistant cancer cell populations, shifts in target expression, and adaptation of immune evasion mechanisms. These therapies may contribute to the selective proliferation of CSCs. CSCs are a small subset of cells within tumors capable of self-renewing, developing new tumors, and resisting traditional cancer treatments. CSCs are crucial in driving cancer progression, heterogeneity development, and relapse. Addressing drug resistance and adequately eradicating CSCs in tumors is crucial to enhancing treatment effectiveness and increasing survival rates.22

Figure 1 An overview of drug resistance mechanisms in cancer cells. (A) Anti-cancer Drug Efflux: Cancer cells depend on various transport mechanisms, including solute carrier (SLC) protein, receptor-mediated, and transport protein-mediated efflux, to expel anticancer drugs, thereby reducing their intracellular concentrations. (B–D) Apoptosis blocking pathways: Reveal the crosstalk between intrinsic and extrinsic through the signaling pathways. In IIA these pathways are involved in regulating apoptosis and their activation promoting apoptosis inhibition; In IIB, the loss-of-function of p53 and inhibition of pro-apoptotic signals, such as increased BCL-2 levels, prevents the formation of the apoptosome, a critical complex for initiating apoptosis; In IIC, the death receptor pathway can trigger apoptosis through receptor-mediated mechanisms by activation of caspases while dysfunction promoting survival signals. (E) DNA Repair Pathway Activation: Increased levels of poly (ADP-ribose) polymerase (PARP) enhance DNA repair mechanisms, allowing cancer cells to survive DNA damage caused by therapies. (F) Decreased Topoisomerase II Activity: Reduced activity of Topoisomerase II contributes to the resistance against drugs that target DNA replication and repair. (G) Drug Metabolism: Anticancer drugs are metabolized within the cell, forming metabolites that autophagosomes can sequester. (H) Alteration of Drug Targets: Oncogenic proteins may be modified, diminishing the effectiveness of targeted therapies. (I-L) Inhibition of Gene Expression: The presence of sponges on microRNA (miRNA) (VA) leads to the degradation of target mRNA; in circular RNAs (circRNAs) and long non-coding RNAs (lncRNAs) resulting in translational repression and leads to inhibiting the expression of pro-apoptotic genes and promoting survival pathways.

Mechanisms of Drug Resistance Drug Efflux Pumps

MDR in tumor cells is primarily caused by the increased expression of drug efflux pumps, specifically adenosine triphosphate (ATP)-binding cassette (ABC) proteins (Figure 1A).18 These proteins, classified into various subfamilies (ABCA, ABCB, ABCC, ABCD, ABCE, ABCF, and ABCG), are crucial in pumping out chemotherapeutic drugs from the cell, reducing their therapeutic concentration. One of the well-known efflux pumps is P-gp, encoded by the MDR1 gene. P-gp is highly expressed in cancer cells and is associated with low therapeutic efficacy, especially in colorectal cancer (CRC). Its overexpression leads to the expulsion of drugs like paclitaxel (PTX) and doxorubicin (DOX), resulting in drug resistance.21,24 Advanced DDS, known as nanomedicines, have been developed to overcome MDR. These systems utilize NPs, nanotubes, micelles, liposomes, and metal nanomaterials to bypass P-gp-mediated drug efflux. NPs can achieve drug release under specific conditions such as pH, hypoxia, and reducibility, allowing drug accumulation in tumors through passive targeting effect, ie, enhanced permeability and retention (EPR) effect or active targeting, ie, ligand-receptor binding (Figure 1A). They can also utilize the endosome-lysosome pathway to transport drugs into cells instead of relying on passive diffusion. Certain substances or groups, such as clathrin, can attach to glycoproteins or lipoproteins on the cell membrane, causing the membrane to curve and form early endosomes. These endosomes can then encapsulate NPs or other cargo and transport them inside the cell. This mechanism, clathrin-mediated endocytosis (CME), has been extensively utilized in reversing MDR.18 Solute carrier (SLC) transporters (Figure 1A) also play a crucial role in cancer drug resistance by regulating the cellular uptake of anticancer drugs and essential nutrients. The expression levels of SLC transporters in tumors and host tissues significantly impact drug efficacy, side effects, and interactions. Downregulation of SLC transporters in cancer cells can lead to ineffective therapy.25 This resistance mechanism has been observed in various malignancies, including multiple myeloma.26 In multiple myeloma, both ABC and SLC transporters contribute to resistance against novel therapies like proteasome inhibitors and immunomodulatory drugs.26 To target the problem of drug efflux pumps, novel efflux pump inhibitors can be screened using small-molecule/natural product/repurposed drug libraries, screening for anti-adaptive molecules, designing peptide-based inhibitors, and applying CRISPR Cas to fine-tune their functions. Among the other interesting approaches are prodrug design, photosensitizers, and photodynamic therapy (PDT) to damage drug efflux pumps, as well as artificial intelligence (AI)-based structural modification of potential drug candidates.

Inhibition of Apoptosis Pathways

Most anticancer treatments aim to induce apoptosis, or programmed cell death, in tumor cells.27 Cell death is primarily regulated by necrosis, apoptosis, and autophagy.28 Apoptosis occurs through two pathways: the intrinsic pathway is controlled by mitochondria and involves B-cell leukemia/lymphoma 2 (BCL-2) family members, caspase-9, and protein kinase B (or Akt) (Figure 1B and C). The extrinsic pathway is initiated by death receptors on the cell surface (Figure 1D). Both of these pathways converge through caspase-3 and 7 activation, resulting in apoptosis. However, there are additional interactions between the pathways. In various cancers, the overexpression of anti-apoptotic proteins, viz, BCL-2 and Akt, and the increased activity of downstream transcription modulators like NF-κB and STAT are observed, making them promising drug targets. Resistance to chemotherapeutics can be associated with the up-regulation of anti-apoptotic (eg, BCL-2, AKT) and down-regulation of pro-apoptotic genes (eg, Bax, Bcl-xL) in tumor cells. An imbalance between anti-apoptotic (such as BCL-2, Mcl-1, and Bcl-xL) and pro-apoptotic proteins (such as Bax, Puma, Noxa, Bak, Bil, and Bid) has been linked to drug resistance in cancer.24,28 Overexpression of BCL-2 can reduce tumor cell death and lead to treatment resistance. Studies have shown that excessive BCL-2 expression can contribute to MDR in lymphoma cells, colon cancer, and gastric cancer. This upregulation of BCL-2 can diminish the apoptosis triggered by chemotherapeutic drugs and decrease the cells’ susceptibility to these drugs. BCL-2 overexpression has also been shown to increase cellular resistance to drugs like cisplatin (CP), docetaxel (DTX), and methotrexate (MTX). Mutations in apoptotic functions can mediate MDR in tumor cells, making them resistant to many chemotherapeutics that induce apoptosis. These mutations also play a crucial role in carcinogenesis and cancer development. For example, the wild-type P53 gene regulates normal cell growth, triggers programmed cell death, and prevents abnormal cell growth (Figure 1E and F).16 However, the mutant P53 gene can render cellular DNA damage irreparable, compromising the link between DNA damage and apoptosis induction.16,28

Reduced Drug Uptake Drug Inactivation

The other drug resistance mechanism may be the drug’s lack of activation or inactivation due to alterations in enzymatic conditions associated with cancer (Figure 1G).29 In vivo, drug activation involves complex interactions between proteins and substances, potentially forming complexes or undergoing partial degradation. Many anticancer medications require metabolic activation for clinical efficacy, yet diminished activation may lead to tumor cell resistance, highlighting the importance of careful drug activation.30 An illustrative instance is the alteration and reduction of phosphorylation occurrences in converting Cytarabine (AraC) to AraC-triphosphate, a substance employed in treating acute myelogenous leukemia (AML).24 AraC does not affect the cancer cells, but its phosphorylated form is lethal in many cancers. Down-regulation or mutations that induce phosphorylation reduce the AraC activity, resulting in drug resistance. Additional examples of drug activations and inactivation mechanisms include the cytochrome P450 (CYP) system, glutathione S-transferase (GST) superfamily, and UDP-glycosyltransferase (UGT) superfamily.31 Synthetic lethality, generation of resistant prodrugs, new nanocarriers, and dual inhibitors can be interesting ways to target drug inactivation.

Alteration of Drug Targets

Targeted therapy utilizes medications that effectively damage cancer cells and specifically target crucial elements like genes, proteins, or the environment of cancer-specific tissues, which impedes cancer growth and enhances overall survival rates. However, the cancer cells can become resistant by modifying these pharmacological targets via genetic mutations or shifts in epigenetic gene expression (Figure 1H).32 An example of resistance caused by modifications to the therapeutic target is the identification and use of estrogen-related receptor inhibitors in breast cancer treatment. Patients with estrogen receptor (ER)-positive breast tumors are often prescribed tamoxifen (TAM), which functions by competing with estrogen for the ligand-binding domain of the ER. However, extended exposure to TAM usually leads to drug resistance. The resistance mechanisms can differ among cases, but two prevalent mechanisms include mutations in the ER gene and reduced expression of the ER.30

Role of Non-Coding RNAs

MicroRNAs (miRNAs) are short non-coding RNA (~21-24 nucleotides) molecules that regulate gene expression (Figure 1I).28,33 They are too small to code for proteins but are involved in controlling the expression of protein-coding genes, including those involved in cancer and drug resistance.28 Altered expression of miRNAs in cancer cells can lead to the development of MDR by misregulating the expression of genes essential for MDR.16,33 Research has shown that miRNAs can amplify tumor-promoting genes or genes related to apoptosis, cellular proliferation, and the cell cycle. Depending on its target tissue, the same miRNA can enhance or hinder chemotherapy resistance. For example, in breast cancer, increased expression of miRNA-21 leads to a decrease in the expression of PTEN protein, impacting the effectiveness of DOX in tumor targeting. Conversely, when PTEN is overexpressed, the expression of miRNA-21 is suppressed, reducing the resistance of breast cancer cells to DOX.24 Recent research has focused on the 3’ half of the miRNA’s involvement in target selectivity and gene regulation, suggesting that nucleotides outside the seed can influence miRNA function and existence, and can be investigated further. Moreover, recent advances in machine learning (ML) can help decipher drug-miRNA interaction, discovery of miRNA therapies, miRNA-miRNA pairings, generating miRNA disease-association models, miRNA treatment prediction, and determining synergy (Figure 1J). Long non-coding RNAs (lncRNAs) (Figure 1K) regulate drug resistance by influencing ATP-binding transporter expression, DNA damage response, epithelial-mesenchymal transition (EMT), apoptosis, and cancer stem cell formation.34 They are involved in resistance to multiple cancer drugs, including platinum drugs, TAM, trastuzumab, 5-fluorouracil (5-FU), PTX, and androgen deprivation therapy across the top five prevalent cancers.35 Along with other non-coding RNAs like microRNAs and circular RNAs, lncRNAs modulate the expression of specific target genes and m6A modification involved in drug resistance mechanisms by regulating RNA stability, localization, and translation.36 Circular RNAs (circRNAs) primarily function as miRNA sponges, regulating gene expression and signaling pathways in drug resistance (Figure 1L).37 They also regulate drug efflux, apoptosis, autophagy, DNA damage repair, immune cells, cytokines, metabolism, and tumor microenvironment (TME) interactions.37 They contribute to chemotherapeutic resistance in non-small cell lung and breast cancer by affecting ATP-binding cassette transporters, inhibiting apoptosis, and promoting autophagy.37,38

Influence of Tumor Microenvironment (TME)

Cancer cells can manipulate their surrounding area by releasing signaling networks that benefit their growth and survival. This interaction with the TME allows cancer cells to adapt and overcome stressful conditions, leading to cancer progression, metastasis, and drug resistance.10 The TME consists of various stromal cells, including tumor-associated macrophages,27 as well as abnormal physiological and biochemical characteristics such as low pH, hypoxia, and high levels of intracellular glutathione (GSH).27 Hypoxia, a condition of low oxygen levels, is commonly experienced by tumors due to uncontrolled cell growth and inadequate blood supply.16 The transcription factor HIF-1α plays a crucial role in hypoxia, as its overexpression has been observed in many human cancers.16 HIF-1α is rapidly degraded under normal oxygen levels, but under hypoxia, it escapes degradation and forms a functional heterodimer with the HIF-1β subunit. This heterodimer enters the nucleus and activates genes related to vascularization, metastasis, and drug resistance. HIF-1α also contributes to chemotherapy resistance in cancer cells.33,39 Nanocarrier-based strategies have been explored to target cancer hypoxia and improve the therapy, including the encapsulation and delivery of hypoxia-activated prodrugs (HAPs) to hypoxic cancers (Figure 2).22 To combat MDR cases, researchers are developing advanced nanocarriers that can bypass efflux pumps like P-gp. These nanoparticles can be engineered to release drugs in response to the acidic environment of tumors, which enhances drug accumulation in resistant cancer cells.40 For example, nanocarriers that encapsulate siRNA targeting P-gp inhibit its expression, thereby increasing the concentration of chemotherapeutics in resistant cells. Furthermore, the development of dual-action nanocarriers that respond to both pH and temperature changes allows for targeted delivery to resistant tumors.41 Combination therapy formulations that deliver both chemotherapeutics and agents inhibiting resistance pathways, such as paclitaxel alongside a P-gp inhibitor, represent promising strategies to overcome MDR in cancer treatment.42

Figure 2 Schematic illustration showing how nanocarriers overcome the cellular barriers for drug delivery. Nanocarriers could deliver drugs into subcellular locations by passing a cascade of barriers, including interface barrier, endocytic pathway barrier, drug resistance, subcellular targeting barrier and controlled drug release.

Targeting Mechanism of Nanocarriers

Engineering drug and gene delivery systems that are able to target affected cells with no influence on healthy cells is a significant challenge in cancer treatment. Understanding the biology of tumors and the interaction of nanocarriers with cancer cells is crucial to tackling therapeutic challenges and developing effective and efficient nanocarrier systems.43 Targeting strategies to deliver drugs and active agents to the desired organs and tissues include passive, active, inverse, ligand-mediated, physical, dual, double, and combination targeting (Figure 3).44

Figure 3 Different strategies of drug targeting. (A) Passive, (B) Active, (C) Inverse, (D) Ligand-mediated, (E) Physical, F) Dual, (G) Double, (H) Combination.

Passive Targeting

Passive targeting is a drug delivery system that targets the drug to the systemic circulation (Figure 3A).45 Nanocarrier-based cancer therapies primarily consist of first-generation nanomedicines that rely on passive targeting. The first-generation nanocarriers regulate biodistribution and pharmacokinetics by modulating the EPR effect, thereby enhancing nanomedicine accumulation in tumor cells.5 While the smaller particles have higher penetrability without leaking into normal vessels, the larger particles are more susceptible to being eliminated by the immune system.2 Solid tumors have inherent abnormalities of tumor vasculature, which can be effectively exploited for targeted delivery of anticancer NP agents. However, large-sized drugs cannot penetrate tight endothelial junctions of normal blood vessels, resulting in a long plasma half-life, which builds up in tumor tissues due to their abnormal vascular nature.44 EPR effect-based passive targeting is inefficient in controlling cytotoxic drug side effects and can negatively affect drug delivery due to cancer heterogeneity and stroma.5

Active Targeting

This strategy includes drug targeting by identifying a particular set of cells and attaching a drug delivery system to their receptors (Figure 3B).45 Active targeting methods incorporate ligands, such as antibodies or small organic molecules, which facilitate the uptake by the targeted cells. These ligands can target surface molecules expressed in the diseased cells, proteins, sugars, lipids in the organs, and molecules in the diseased cells’ microenvironment or the physicochemical environment.46 Ligands on the surface of NPs are selected to target the molecules overexpressed on the surface of cancer cells, allowing them to distinguish between healthy and targeted cells. The interaction between NPs’ ligands and cancer cells’ receptors triggers receptor-mediated endocytosis, enabling the successful release of therapeutic drugs. This active targeting approach is particularly suitable for delivering macromolecular drugs like proteins and siRNAs.2 This way, direct tumors or the mildly acidic TME, the TME’s vascularization, or the tumor nucleus can be targeted.

Inverse Targeting

Inverse targeting of drugs occurs when the reticuloendothelial system (RES) prevents colloidal carrier absorption, suppressing its regular function, which is achieved by pre-injecting blank carriers or macromolecules such as dextran sulfate, facilitating RES saturation and suppressing defense mechanisms, making it a practical method for targeting drugs to the non-RES body organs (Figure 3C).44

Ligand-Mediated Targeting

Ligands are affixed to carrier surfaces, guiding the carrier to designated locations. Colloidal carrier systems may be functionalized with biologically relevant molecular ligands like antibodies, polypeptides, oligosaccharides, viral proteins, and fusogenic residues. These ligands facilitate recognition and specificity, allowing targeted drug delivery (Figure 3D).47

Physical Targeting

This approach employs environmental changes like pH, temperature, light intensity, ionic strength, electric field, and specific stimuli like glucose concentration to locate the drug carrier, making it optimal for tumor targeting and cytosolic delivery (Figure 3E).12

Dual and Multiple Targeting Approaches

The dual targeting mechanism enhances the therapeutic effect by synchronizing carrier molecules with antiviral drugs, eg, enhancing the therapeutic effect when entrapped in a carrier molecule with antiviral activity (Figure 3F).45

The double targeting strategy combines the temporal and spatial methodologies to target specific organs, tissues, cells, or subcellular compartments with a controlled drug release rate to the target site (Figure 3G).12

Combination Targeting

These systems, which include carriers, polymers, and molecularly specific homing devices, provide a direct targeting approach (Figure 3H). 48 Recent studies have explored hybrid targeting strategies that combine passive and active targeting mechanisms to enhance drug delivery to tumors. Nanoparticles can be engineered to exploit the enhanced EPR effect, which allows them to accumulate in tumor tissues due to leaky vasculature.49 Additionally, these nanocarriers can be functionalized with ligands that specifically bind to overexpressed receptors on cancer cells, such as HER2 or folate receptors, enhancing tumor cell uptake.50 Magnetic nanoparticles that can be guided to tumor sites using external magnetic fields further enhance targeting precision, reducing systemic exposure and improving therapeutic outcomes.51,52

Impact of Nanocarrier Physicochemical Properties

Understanding the physicochemical properties of NPs is crucial for improving their concentration and functionality in tumor tissues.53 While advancements have been made in drug nanocarriers, few have been effectively implemented in clinical studies. The properties of these nanocarriers influence their in vivo biological barriers, which can affect the therapeutic index of the cargo and the desired outcome.54 Comprehension of the physicochemical peculiarities of nanocarriers is essential since their size, shape, surface charges, and chemistry affect drug distribution, behavior, and delivery efficiency.54

Shape

The shape of nanocarriers is an important property affecting their behavior in vivo. Different shapes, such as rods or spheres, can impact processes like uptake by macrophages, circulation in the blood, distribution in tissues, and targeting of diseases.54 Factors like the curvature structure of NPs and the time needed for cell membrane wrapping can influence the interactions between nanocarriers and biological systems. For example, rod-shaped NPs have a larger contact area with cell membrane receptors but longer internalization than spherical NPs.53 Gold nanorods have lower uptake efficiency than spherical particles, which decreases as the aspect ratio increases.55 Anisotropic nanocarriers with a non-uniform shape have distinct interactions with cells and drugs. Their increased surface area allows for better drug encapsulation and delivery through localized degradation. Additionally, their larger surface area facilitates binding to specific cells and enhances targeting.56

Size

The size of nanocarriers is a crucial physicochemical characteristic affecting their in vivo behavior since it directly influences the available surface area for interaction with biological environments. Plasma proteins attach to nanocarriers, creating a protein corona, which gives them a unique “biological identity” and determines their interactions with the phagocytes.54 While nanocarriers of less than 5 nm in diameter can be rapidly cleared from the blood, those of more than 15 μm can be accumulated in the liver, spleen, and bone marrow. The optimal size for DDS is currently considered 100–200 nm, as this size allows the EPR effect in tumors and avoids filtration in the spleen.57

Surface Chemistry

Surface chemistry design plays a critical role in the development of nanomaterials for biomedical applications. Research studies have shown that the surface chemistry of polymer-based nanomaterials significantly affects their interactions with cells.46 Surface modification of NPs ensures their stability in physiological environments. However, the reactive nature of NP’s surface can also influence biological responses.58 Surface functionalization of NPs prevents their aggregation and minimizes the non-specific cellular absorption. Additionally, surface chemical modification can reduce the potential hazards of NPs and establish a safe-by-design approach for their future use. Overall, surface chemistry design is essential for optimizing the performance and safety of nanomaterials in biomedical applications.53

Surface Charge

The surface charge of NPs is critical to their interaction with biological systems. Their surface charge influences cellular uptake, while positively charged nanocarriers show better internalization and higher efficiency than negatively charged ones.46 However, the positive surface charge also rapidly removes NDDSs from the bloodstream. The interaction between negatively charged albumin and positively charged NPs is the main factor in removing cationic nanocarriers.59 Surface charge also affects the fate of nanocarriers in vivo by influencing the biodistribution, opsonization process, and plasma protein adsorption. High surface charge densities lead to an acceleration of blood clearance and capture by the RES, while neutral charges promote prolonged blood circulation and reduced clearance by the RES.54

Types of Nanocarriers in Cancer Therapy

Nanocarriers are colloidal particles smaller than 500 nm that are used to encapsulate drugs and biological materials. They can be made from various materials, including (i) organic NPs, such as solid lipid NPs, polymeric micelles (PMs), liposomes, nanoemulsions (NEs), polymeric NPs, and exosomes, (ii) inorganic NPs, such as carbon-based nanomaterials, metals and metal oxides, quantum dots (QDs), and (iii) hybrids (Figure 4). Stimuli-responsive nanocarriers are the third generation of controlled-release DDS, allowing precise drug release within target cancer cells. Their benefits include improving drug pharmacokinetics and biodistribution, enhancing solubility and permeability, minimizing therapeutic doses, and reducing toxicities. At the same time, modifying the properties, such as composition, size, shape, and surface properties of nanocarriers, can protect drugs from premature degradation to overcome MDR in cancer cells. Cancer cell-targeted drug delivery strategies often involve nanocarriers with cancer cell-specific ligands on their surface and mechanisms that cause the release of drugs within the cytoplasm of cancer cells.60 The development of third-generation nanocarriers, including stimuli-responsive and hybrid systems, marks a significant advancement in cancer therapy. These carriers can be designed to release their drug payloads in response to specific triggers such as temperature or light, allowing for controlled drug release at the tumor site.61,62 For instance, light-activated nanocarriers can release DOX upon exposure to specific wavelengths of light, while biodegradable polymeric nanoparticles made from materials like poly(lactic-co-glycolic acid) (PLGA) degrade into non-toxic byproducts, enhancing safety and reducing long-term toxicity.63 Additionally, temperature-sensitive nanocarriers that release drugs in response to hyperthermia enable localized treatment of tumors, providing a targeted approach to cancer therapy.

Figure 4 Classification of nanocarriers for cancer therapy.

Organic Nanocarriers Lipid-Based Nanocarriers (LBNCs)

LBNCs, a class of nanomedicines with FDA approval, offer benefits like easy formulation, self-assembly, biocompatibility, high bioavailability, large payload capacity, and adjustable physicochemical features. They are classified as liposomes, nano-emulsions, and solid lipids.64

Innovations in lipid-based nanocarriers, particularly polyethylene glycol (PEG)ylated liposomes, have improved their circulation time and stability in the bloodstream, enhancing their effectiveness in cancer treatment.65 Enhanced formulations of liposomes, such as Doxil, encapsulate doxorubicin for breast cancer treatment, reducing immunogenicity and improving pharmacokinetics. New developments also involve combination therapy liposomes that co-encapsulate multiple drugs, allowing for synergistic effects against cancer cells.66 Furthermore, lipid nanoparticles specifically designed for the delivery of messenger RNA (mRNA) vaccines or therapies have shown promise in improving stability and cellular uptake, representing a significant advancement in the field of cancer immunotherapy.67

Liposomes

Liposomes, spherical structures made up of phospholipids, are widely used in drug delivery, particularly in cancer treatment. They have several advantages, including compatibility with both hydrophilic and hydrophobic drugs, low toxicity, extended-release potential, and the ability to carry large drug payloads and protect them from the external environment. Because they are biocompatible, biodegradable, and non-immunogenic, liposomes are suitable choices for DDS. However, liposome has limitations, including possible carrier toxicity and challenges in large-scale production and stability.68–70 One of the most successful applications of liposomes is the delivery of chemotherapeutic drugs/agents for cancer treatment. There are several approved and commercially available liposomal formulations for cancer therapy, such as Vyxeos®, Onivyde®, Marqibo®, DepoCyt®, and Doxil®. In the year 2024, the FDA approved the use of liposomal irinotecan as a first-line therapy for pancreatic adenocarcinoma. These liposomal formulations have shown promising results in improving drug delivery and reducing toxicity.69 Akbari et al developed nanoliposomes containing hydrophobic hydroxyurea (HU) for breast cancer cell therapy. The HU-loaded nanoliposomes were 174 nm in size, not significantly larger than blank ones, and showed a rapid release over 5 hours, followed by a sustained release for up to 36 hours. This release pattern was attributed to adsorbed HU on the nanoliposome surface, liposome erosion, and HU diffusion mechanisms. The nanoliposomes also demonstrated high cellular internalization and uptake in breast cancer cells, indicating their affinity for the cells, which led to an increased accumulation of HU.71 To improve the properties of liposomes, researchers have attached PEG to the liposomes and developed PEGylated liposomes,72 which can prolong the blood circulation and protect the drug from clearance by the mononuclear phagocyte system (MPS).73 PEGylated liposomes modified with PEG2000 have enhanced blood retention.74 Some ionizable cationic liposomes have a surface charge of neutrality under physiological conditions after being loaded with nucleic acids.75,76 Altering the composition of liposome head groups, lipid tails, and cholesterol quantity can also affect cellular uptake and tumor penetration in specific cancer types like glioblastoma and triple-negative breast cancer (TNBC).77 Liposomes have emerged as a promising tool in gastric cancer therapy, mainly combined with SATB1 siRNA, CD44 antibodies, and DNA complexes. These formulations facilitate enhanced drug accumulation in tumor-bearing mice, demonstrating superior targeting precision and effective silencing of SATB1 gene expression in CD44+ gastric CSCs. This approach significantly improves the therapeutic efficacy for this challenging cancer type.78 Additionally, Myocet®, a non-PEG-DOX liposome, has been utilized in temozolomide-based chemotherapy for high-grade glioma, establishing the maximum tolerable dose for patients aged 28 to 65 and diagnosed with gliomas who received nanoliposomal irinotecan.79 Furthermore, the combination of PEG-DOX liposomes with lapatinib is the most effective treatment strategy for individuals with HER2-positive breast cancer. In xenograft models, liposomal formulations loaded with DOX and Apache have been reported to elevate DOX levels in tumor tissues, enhancing therapeutic outcomes.80 Liposome nanoparticles, due to their unique structure, have been used in cancer therapy through gene therapy.81 They were applied in gene therapy through encapsulation of siRNA molecules and regulating the responsible genes in the multiplication of cancerous cells through gene modulation and silencing.82 In addition, liposomal nanoparticles helped destroy and kill cancer cells by delivering plasmid DNA- encoding tumor suppressors to tumor sites. Liposomes have also been used to deliver various diagnostic agents, such as64 Cu,14,83 Cisotopes,84 QDs,85 gadolinium (Gd)-based contrast agents,86 etc. Researchers hope that liposomes will soon be used as a simultaneous carrier of therapeutic drugs and diagnostic agents for cancer patients in clinical trials. Clinical trials have shown that liposomal formulations, especially liposomal doxorubicin, effectively treat various cancers like breast, ovarian, and lung cancer. Research highlights its cardiac safety and effectiveness as a standalone treatment and in combination with other therapies.87 Recent advancements include thermosensitive liposomes that release drugs in response to temperature changes, enhancing targeting and reducing side effects.88 In pediatric oncology, the use of liposomes is limited, but optimizing their design could improve drug delivery for children with cancer.89 Innovative approaches, such as dual pH-responsive liposomes, are being explored to release drugs specifically in tumor environments, while combining liposomes with immunotherapy shows potential for enhanced anti-tumor effects.90 The use of liposomal DNA delivery and DNA barcoding can be an efficient method to predict treatment responses.

Solid Lipid Nanoparticles

Solid lipid NPs are physiological fluids (room and body temperature) used to administer colloidal drugs, ranging from 50 to 1000 nm.91 These particles are contained within a drug-encapsulating matrix made of glycerides/fatty acids complex combinations.92 Nanotechnology can enhance patient prognoses. A steroid prepared by cold dilution techniques for chitosan-coated lipid NPs and filled with curcumin (CUR) could inhibit the growth of PANC-1 cells.93 PTX and 5-FU-charged lipid nanocarriers (such as Intaxel®) were used in managing male patients with LivC, shielding the medication from enzymatic deterioration by its plasma buildup and performance.94 Sorafenib (SOR) drug and superparamagnetic iron oxide NPs have also been co-loaded in SLNs for the dual treatment of HepG2 cells.95

Nanoemulsions

NEs are colloidal mixtures of two non-miscible liquids that exhibit thermodynamic instability. They are used as carriers in cancer treatment to deliver drugs to the targeted site. The drug is enclosed in the core of the NEs, which enhances its bioavailability and reduces its undesired action on other cells or tissues. NEs have droplets with diameters ranging from 10 to 200 nm and a protective coating of emulsifier molecules,96 which have several advantages in drug delivery. They have a higher drug solubilization capacity, better thermodynamic stability, long shelf life, rapid onset of action, and reduced inter-subject variability.97 In addition, NEs are non-toxic and non-irritant and permit easy application to the skin and mucous membranes.98 They can be formulated as foams, creams, liquids, and sprays, and if their formulation contains biocompatible surfactants, they can also be orally administered.99 NEs are particularly effective in delivering drugs through the skin, and due to their small size, they can penetrate the rough skin surface and enhance the penetration of active ingredients.98 Several NE delivery systems have been marketed in the past decade for various applications, including ophthalmic treatments for dry eye syndrome and oral immunosuppressive agents.100 In a study by Miranda et al, a NEs containing a novel fucoside derivative of Lapachol was prepared and characterized for intravenous treatment. The NEs with a size of around 190 nm and a zeta potential close to −20 mV showed excellent drug encapsulation efficiency (nearly 100%). The NEs also demonstrated sustained drug release, with approximately 50% of the drug remaining after 24 hours.101 The NE oil-in-water formulation, a toxoid drug associated with omega-3 fatty acids, has demonstrated a 12-fold reduction in the toxoid IC50 of PPT2 cells, resulting in a greater reduction in tumor volume in tumor-bearing mice than Abraxane™.102 Catechin-extract NE has also shown anticancer properties in PC-3 cells.103 DTX-NE and NE lipophilic diffuse methane show enhanced antitumor activity in A-549 cells. Nebulized DTX NE (MRC-5 cells) has succeeded more than normal cells, creating a standard framework for co-encapsulating gemcitabine (GC) and PTX with a ligand targeting the glucose receptor and revealing the potent synergism.104 Clinical studies such as Phase III of PTX intratemporal liposome injection have also shown successful results for lung cancer.105

Nanoscale lipid-based theranostic systems, particularly lipid-based nanoparticles (LNPs), are prospective carriers for cancer theranostic agents to cure different types of tumors. LNPs can be tailored with different targeting moieties and possess the potential to bypass different physiological barriers and improve the pharmacokinetics. Zhang et al106 developed a fluorinated nanoemulsion that significantly improved fluorescence imaging signals and revealed effective diagnosis of specific tumors, facilitating photodynamic therapy. Similarly, Hou et al developed a porphyrin shell-nanoemulsion that stabilized the oil core, resulting in a monodisperse nanostructure for imaging and phototherapy.107 Liang et al developed a theranostic nano platform using smaller (<20 nm) iron oxide loaded with porphyrin-grafted lipid nanoparticles (Fe3O4@PGLNPs), which demonstrated significant photodynamic effects against HT-29 cancer cells in vitro.108 The negligible toxicity, multifunctional capabilities, and flexibility in functionalization of LNPs position them as a vital tool in cancer theranostics, enabling effective treatment strategies across various tumor types.109

Polymer-Based Nanocarriers

Polymeric NPs have gained significant attention in biomaterials and drug delivery due to their unique properties and versatility. These NPs can be synthesized using synthetic and natural polymers, allowing customization and optimization for various biomedical applications.5,110,111 One of the key advantages of using polymeric NPs as drug carriers is their ability to deliver therapeutic agents directly to the target site, such as inflammation regions or the central nervous system, which is particularly important when dealing with potentially dangerous drugs that need to cross the BBB.110 Polymeric NPs can have a core-shell structure, with a hydrophilic shell and a hydrophobic core, effectively protecting the drug from degradation.112,113 They offer a controlled-release potential, allowing for prolonged drug delivery, and can also be manipulated to have different release kinetics, ensuring the drug is released at the desired rate and duration. This is beneficial, especially in the case of chemotherapeutic agents, where site-specific targeting and side effect reduction are crucial. Their distinct features, including drug solubility, stability, and selective accumulation, render them adaptable platforms for delivering therapeutic agents. In addition to drug delivery, polymeric NPs can be used in biomedical applications such as biosensors and catalysis. They can be designed to have different chemical compositions, charges, and physical structures, allowing for customization based on specific requirements.46 One commonly used polymeric NP is PLGA, known for its biocompatibility, biodegradability, and EPR effect.114 PLGA NPs have been extensively studied and utilized as drug carriers.115 For example, Zumaya et al developed a multifunctional PLGA-based nanocarrier system for co-delivering colchicine and purpurin 18 to human cancer cells. They compared the release of colchicine from PEGylated and non-PEGylated PLGA NPs, and the latter nanocarriers exhibited a slower and more sustained release profile.116 Wu-Cheng et al investigated the use of a nitric oxide (NO) conjugate of albumindinitrosyl iron unit, [(NO)2Fe(m-SCH2CH2OH)2Fe(NO)2] (DNIC-1), for oral delivery to activate hippocampal neurogenesis in chronic neuropathy, demonstrating effective mucosal absorption and sustained NO release without toxic byproducts. On the flip side, DNIU [Fe(NO)2]’s binding affinity to the cysteine residue (thiol group) in serum albumin oxidizes DNIC, which leads to the release of NO without toxic peroxynitrite formation.117 Yafang Qin et al, in 2022, developed luteinizing hormone-releasing hormone (LHRH) peptide-conjugated tumor targeting ruthenium (II) complex (Ru1-LHRH) that selectively targets the mitochondria, induces apoptosis by inducing reactive oxygen species (ROS) and activating the Caspase3/7 pathway, while imaging property reveals theranostic capabilities.118 Costagliola di Polidoro et al developed a hyaluronic acid conjugated Angiopep-2 (Thera-ANGcHANPs) for glioma targeting, which enhances imaging and selectively delivers therapeutic agents across the BBB, exhibits significant cytotoxic effects.119 Yan et al introduced a novel synthetic nanocomplex (NC) to overcome MDR cancer, contained synthetic redox-responsive polyethyleneimine (PEI) polymer (disulfide linked PSP), tetrahedral DNAs (TDNs) loaded with DOX drug (PSP/TDNs@DOX) that enhances drug delivery by creating pores-due to the difference in charge- in the cell membrane and facilitate the internalization of DOX while bypassing endocytosis pathways. PSP/TDNs@DOX NC revealed higher therapeutic efficacy in xenograft drug-resistant tumor mouse models, including human breast cancer (MCF-7/R) and ovarian cancer (SKOV3/R), due to its respond to GSH and DNase I to cleave and release DOX.120 Cai et al developed a theranostic system based on a small gadolinium chelate with PTX, linked via a cathepsin B-responsive linker, and covalently attaching a fluorescent probe, pheophorbide, to a branched glycopolymer. Their study demonstrated that this multifunctional system, derived from the glycopolymer prodrug in 4T1 xenograft mice, exhibited improved pharmacokinetics, enhanced tumor accumulation, angiogenesis inhibition, excellent biocompatibility, and significantly reduced gadolinium ion retention post-injection. The system also showed prolonged circulation time, a tumor inhibition rate exceeding 90%, and brighter MRI contrast intensity.121 The Incorporation of targeting ligands, such as antibodies, aptamers, and peptides, enhanced the specificity of these systems. For example, Zhong Yuan Chen et al reported aptamer-dendrimer-functionalized magnetic nano-octahedrons that consist of PAMAM dendrimer, zinc metal doped with superparamagnetic iron oxide, hydrophobic anti-cancer drug DOX, and HSP70/HSP90 siRNAs surface modified by PEG (ZIPPApt: DOX/siHSPs), which combines hyperthermia and as a chemotherapy theranostic system effectively enhanced apoptosis in 4T1 cancer cells.122 Additionally, Hosseini et al recently developed a theranostic dendrimer system to target CRC by attaching the cetuximab monoclonal antibody to PAMAM G4 and labeling it with Lu-177 via DTPA-CHX chelator on the mouse tumor and SW480 cancer cell line by expressing EGFR. Single-photon emission computed tomography (SPECT) images showed that the system caused apoptosis those cells that expressing epidermal growth factor.123

Micelles

Polymeric micelles (PMs) are nanosized molecules with a core-shell structure formed by the self-association of amphiphilic block copolymers in an aqueous solvent.110 They have gained attention as potential nanocarriers for the controlled delivery of hydrophobic anticancer drugs, which can enhance solubility, extend blood circulation time, improve cellular uptake, and passively target tumor cells through EPR.115 The advantages of PMs in drug delivery include biocompatibility, low toxicity, core-shell arrangement, micellar association, morphology, size, and stability.110 Their size (10–100 nm) is critical for the passive targeting of solid tumors, especially poorly vascularized ones. The outer surface of micelles is composed of components that do not react to tissue or blood, allowing them to remain in the blood for a long time without being recognized by phagocytic cells or specific proteins while protecting drug inactivation through the biological environment effect. This longevity is a significant feature of micelles as drug carriers.124 The core-shell structure of PMs consists of amphiphilic polymers, with the hydrophobic part carrying the hydrophobic drugs and the hydrophilic corona imparting hydrophilicity.125 Parameters such as cellular internalization, circulation, renal clearance, and the strength of the interface between the hydrophobic core and external aqueous environment are affected by the stability of micelles. The critical micelle temperature and the critical micelle concentration (CMC) determine the spontaneous assembly of micelles and play a key role in micelle formation. While the CMC micelle is formed at lower concentrations, the nanostructure can be dissolved, l