Clinically, cancer drug resistance to chemotherapy, targeted therapy or immunotherapy remains the main impediment towards curative cancer therapy, which leads directly to treatment failure along with extended hospital stays, increased medical costs and high mortality. Therefore, increasing attention has been paid to nanotechnology-based delivery systems for overcoming drug resistance in cancer. In this respect, novel tumor-targeting nanomedicines offer fairly effective therapeutic strategies for surmounting the various limitations of chemotherapy, targeted therapy and immunotherapy, enabling more precise cancer treatment, more convenient monitoring of treatment agents, as well as surmounting cancer drug resistance, including multidrug resistance (MDR). Nanotechnology-based delivery systems, including liposomes, polymer micelles, nanoparticles (NPs), and DNA nanostructures, enable a large number of properly designed therapeutic nanomedicines. Nanomedicines have paved the way for effective treatment of cancer by rationally designing strategies such as passive targeted drug delivery, active targeted drug delivery, co-delivery of combinatorial agents and multimodal combination therapy, and have broad prospects in overcoming drug resistance. It is believed that nanomedicines will be an attractive strategy for reversing or overcoming cancer drug resistance.
1. Mechanisms in Drug Resistance of Chemotherapy
Nowadays, chemotherapy is still the most widely used strategy for treating cancer; however, the biggest obstacle to this traditional strategy is the development of cancer drug resistance
[43,44,45][1][2][3]. The mechanisms of drug resistance to chemotherapy are extremely complex
[45][3]. Generally, the emergence of chemoresistance may be classified by the following pathways: (1) increased drug efflux by ATP-dependent pumps mediated by transmembrane transporters of the ATP-binding cassette (ABC) superfamily
[46,47,48][4][5][6]; (2) reduced drug uptake mediated by altering specific cellular targets
[49,50,51][7][8][9]; (3) inactivation of apoptotic pathways mediated by high expression of the Bcl-2 antiapoptotic family such as Bcl-2, Mcl-1 and Bcl-XL, which are mainly responsible for the reason why cancer cells can resist apoptosis
[52,53,54][10][11][12]; (4) enhanced DNA repair ability that can contribute to the resistance of cancer by promoting genomic instability and mutation
[55,56,57][13][14][15]; (5) alterations in specific drug targets
[58,59][16][17]; (6) increased drug detoxification mediated by metabolism or biotransformation
[11,60][18][19]. All in all, these resistance mechanisms can allow cancer cells to survive by easily changing different pathways, and ultimately resulting in chemotherapeutic failure.
2. Nanomedicines to Overcome Chemotherapy Resistance
Considering that chemotherapy resistance-related drug efflux proteins mainly reside in the nuclear membranes and blood, but not in the mitochondria
[61,62][20][21], delivering chemotherapy agents into the mitochondria is an emerging strategy to surmount drug resistance to chemotherapy
[63,64,65,66,67,68,69][22][23][24][25][26][27][28]. Yu et al.
[70][29] constructed a weak acid-activated, charge-reversible, triphenylphosphonium (TPP)-based, “shell–core” nanosystem (DOX-PLGA/CPT/PD) for sequential facilitation of tumor accumulation, cellular uptake, mitochondria targeting, intracellular localization and surmounting drug resistance of MCF-7/ADR breast cancer. Firstly, positively charged mitochondrial-targeting lipid-polymer hybrid nanoparticles (PLGA/CPT) were prepared from PLGA and C
18-PEG
2000-TPP (CPT)
[71][30]. Then, DOX was loaded into PLGA/CPT nanoparticles to obtain DOX-PLGA/CPT. Lastly, positively charged PEI-DMMA (PD) shell was wrapped on the surface of positively charged DOX-PLGA/CPT to obtain negatively charged DOX-PLGA/CPT/PD with a diameter of ~150 nm. When DOX-PLGA/CPT/PD was treated at pH 6.5, the hydrolysis of amide in PD occurred, facilitating the elimination of electrostatic interaction between PLGA/CPT and PEI, ultimately resulting in the deshielding of PD to reveal DOX-PLGA/CPT and transformation of the charge from −24 to +19.2 mV. Then, they studied the pharmacokinetics of DOX-PLGA/CPT/PD, and, the results showed that DOX-PLGA/CPT showed significantly slower clearance with a half-life time 15.84 h. After incubation with MCF-7/ADR cells at pH 6.5, DOX-PLGA/CPT/PD showed effective lysosome escape, excellent mitochondrial-targeting capacity and superior cytotoxicity for overcoming DOX resistance by up-regulating the apoptosis-related proteins as well as down-regulating the antiapoptotic protein Bcl-2. Encouraged by the in vitro antitumor effect of DOX-PLGA/CPT/PD, Yu et al. evaluated the in vivo effect in MCF-7/ADR cell-bearing mice. The results show that DOX-PLGA/CPT/PD showed the best inhibitory effect on tumor growth and exhibited the best treatment effect, with a tumor inhibition rate (TIR) of 84.9% with no obvious side effects.
Studies show that the exposure of tumor cells to chemotherapy drugs can result in hypoxia-inducible factor-1 (HIF-1) activation and stabilization
[72[31][32],
73], where HIF-1 plays an important part in drug resistance by regulating multidrug resistance protein (MRP), P-glycoprotein (P-gp), Bcl-2, etc.
[74,75,76][33][34][35]. Moreover, HIF-1 can up-regulate the level of glutathione, which can bind with heavy metal ions, including cisplatin
[77,78][36][37]. Therefore, inhibiting HIF-1 pathways during chemotherapy might be a promising method to circumvent chemo-resistance
[79,80,81,82,83][38][39][40][41][42]. Acriflavine (ACF), a potent HIF-1 inhibitor, has been proven to bind to HIF-1α and thereby impede HIF-1α/β dimerization
[84[43][44],
85], which can be a useful strategy for sensitization of chemotherapy. In this regard, Zhang et al.
[86][45] developed a new type of microporous silica-based co-delivery system (PMONA) to reverse the acquired resistance to cisplatin. Firstly, cisplatin was loaded into the polymeric mPEG-silane functionalized mesoporous silica nanoparticles inner core by reverse microemulsion method, where polymeric mPEG-silane was applied to maintain stability during blood circulation. To achieve tumor-specific glutathione (GSH)-triggered drug release, tetrasulfide bond-bridged organosilica was integrated to obtain the nanoparticles. Finally, ACF was loaded into the inner area of mircopores by electrostatic interactions to obtain ACF-loaded nanoparticles with a diameter of ~45 nm. After internalization by cancer cells, the outer organosilica shell of PMONA could be degraded by intracellular GSH, resulting in nanoparticle disassembly, drug release and synergistic regulation of multiple cancer-related signaling pathways. As shown in an in vitro release experiment, cisplatin and ACF had faster and higher cumulative release rates in a medium containing 10 mM GSH than in a medium containing 10 μM GSH, which confirmed that the tetrasulfide bond in organosilica enabled GSH-responsive disassembly and drug release. After incubation with A459 cells, PMONA exhibited stronger cell cytotoxicity, induced more apoptosis than the single drug-loaded nanoparticles by suppressing HIF-1-related proteins and decreased the level of intracellular GSH. Inspired by the result that ACF strengthens the curative effect of cisplatin in vitro, Zhang et al. assessed the in vivo antitumor effect in A459 cell-bearing mice. The results indicated that PMONA showed the best inhibitory effect on tumor growth and exhibited the best therapeutic effect with limited side effects. Additionally, the immunohistochemical experiment showed that PMONA enhanced cell death and apoptosis in tumor tissues mainly by down-regulating the levels of P-gp, MRP2, HIF-1-activated glutamate-cysteine ligase modifier subunit (GCLM), vascular endothelial growth factor (VEGF) and cystine transporter (xCT). Taken together, these results confirmed that ACF could combat cisplatin-acquired resistance by inhibiting HIF-1 function.
Hyperthermia, a non-invasive treatment strategy, has shown a competitive advantage in reversing drug resistance in cancer by suppressing the expression of drug efflux transporters
[87,88,89,90,91,92][46][47][48][49][50][51]. Therefore, hyperthermia combined with chemotherapy is a hopeful treatment strategy for overcoming chemotherapeutic resistance
[93,94,95,96,97,98][52][53][54][55][56][57]. Huang et al.
[99][58] constructed smart, thermoresponsive, pH low insertion peptide (pHLIP)-modified gold nanocages (DOX@pPGNCs) to realize synergistic thermo-chemotherapy and overcome chemotherapeutic resistance. Firstly, thermoresponsive poly (di (ethylene glycol) methyl ether methacrylate-co-oligo (ethylene glycol) methyl ether methacrylate) (PMEO
2MA-OEGMA) polymer was anchored to gold nanocages to PMEO
2MA-OEGMA-modified gold nanocages, where PMEO
2MA-OEGMA served as a temperature-sensitive gate guard at a lower critical solution temperature of ca. 41.6 °C. In other words, the PMEO
2MA-OEGMA chains extended under 41.6 °C, sealing the pore of gold nanocages to prevent the leakage of drug into the blood; however, once the temperature increased up to 41.6 °C due to the NIR-induced photothermal effects, its chains shrunk, leading to opening of the pores of gold nanocages and fast DOX release. Then, pHLIP was used to decorate PMEO
2MA-OEGMA-modified gold nanocages to obtain pPGNCs, where pHLIP was a good candidate to enhance cancer cell internalization by conformational transition at the weakly acidic tumor microenvironment. Lastly, DOX was loaded into pPGNCs to obtain DOX@pPGNCs with a diameter of ~160 nm and a zeta potential of approximately −20 mV. PMEO
2MA-OEGMA was thermosensitive with a lower critical solution temperature of ca. 41.6 °C. In vitro release experiments indicated that the cumulative release of DOX increased from 3.7 to 20.1% after 5 min of NIR irradiation. More importantly, the rapid release of DOX was consistent under NIR irradiation in another cycle, indicating that PMEO
2MA-OEGMA was a very responsible gatekeeper to precisely control NIR-triggered DOX release from DOX@pPGNCs. Cytotoxicity experiments showed that the antiproliferation ability against MCF-7/ADR cells was strongest in the DOX@pPGNCs and NIR irradiation group at pH 6.5, suggesting that pHLIP could enhance cellular uptake of DOX@pPGNCs under a weak acid tumor microenvironment, and, upon NIR irradiation, DOX@pPGNCs could efficiently achieve synergistic thermo-chemotherapy to overcome cancer resistance. In vivo biodistribution experiments showed that DOX accumulation in tumor site of tumor-bearing mice treated with DOX@ pPGNCs and NIR irradiation was highest, confirming that NIR irradiation-triggered photothermal effects of gold nanocages could further strengthen DOX accumulation. Inspired by the above experimental results, Huang et al. further assessed the in vivo treatment effect in MCF-7/ADR cell-bearing mice. The results indicated that DOX@pPGNCs achieved the strongest antitumor efficacy with a TIR of 97.3%, indicating the highly effective synergistic thermo-chemotherapy in MCF-7/ADR cell-bearing mice.
Studies have shown that tumor cells can develop drug resistance by enhancing DNA repair
[110[59][60][61][62][63][64],
111,112,113,114,115], suggesting that drug resistance owing to DNA repair can be overcome by inhibiting the function of related proteins
[116,117,118,119][65][66][67][68]. Recently, Wang et al.
[120][69] constructed a smart delivery system for overcoming cisplatin-related “cascade drug resistance” (CDR) by mild hyperthermia (43 °C) triggered by NIR. Firstly, hydrophobic photothermal-conjugated polymer and biodegradable amphiphilic polymer were mixed to form F-nanoparticles (F-NPs) with photothermal performance. Secondly, biodegradable amphiphilic polymer and C16-CisPt-Suc (a Pt (IV) prodrug) were mixed to form Pt-nanoparticles (Pt-NPs). Lastly, Pt-NPs and F-NPs were mixed to obtain the mixed nanoparticles (F-Pt-NPs). On the basis of DLS data, the average particle size of F-NPs was 91.0 ± 2.6 nm, while that of the Pt-NPs was 105.1 ± 1.6 nm. In vitro experiments showed that, under the treatment of NIR, mild hyperthermia could efficiently facilitate cellular uptake of drug-resistant A549DDP cells, resulting in enhanced cytotoxicity and surmounting CDR of cisplatin by the consumption of GSH and the reduction of Pt (IV) to Pt (II). More importantly, mild hyperthermia could accelerate the binding of Pt to DNA and promote the formation of irreparable crosslinking of Pt-DNA strands, resulting in the destruction of DNA repair. In vivo experiments showed that, under mild hyperthermia conditions, F-Pt-NPs exhibited the best antitumor effect with a TIR of 94% with few side effects, further indicating that NIR-triggered mild hyperthermia could reverse CDR.
In recent years, substantial evidence has confirmed that drug resistance is closely related to the CSC phenotype
[121,122][70][71]. One proven mechanism of multidrug resistance (MDR) in CSC is the increased expression of ABC transporters
[123][72]. In addition, the CSC phenotype shows increased drug resistance to chemotherapy by modulating many other stem characteristics, including enhanced DNA damage repair capacity and up-regulation of antiapoptotic proteins
[124,125][73][74]. Therefore, eradication of CSCs is an effective strategy to surmounting cancer drug resistance. Shen et al.
[126][75] constructed an alltrans-retinoic acid (ATRA) and camptothecin (CPT) co-loaded nanoplatform (ATRA/CPT-NPs) to surmount chemotherapeutic resistance of both CSCs and bulk tumor cells. Firstly, ROS-responsive nitroimidazole-modified hyaluronic acid-oxalate-CPT conjugate (n-HA-oxa-CPT) was synthesized. Then, n-HA-oxa-CPT assembled into nanoparticles and physically encapsulated ATRA to obtain ATRA/CPT-NPs with a diameter of ~150 nm. Based on the difference levels of ROS between bulk tumor cells and CSCs, ATRA/CPT-NPS could sequentially release ATRA and CPT during the differentiation of CSCs. After uptake by hypoxia CSCs, ATRA was firstly released, which induced CSC differentiation into reduced stemness and chemoresistance, along with increased ROS level. Then, the increased ROS in differentiated CSCs triggered CPT release for enhanced cytotoxicity towards the differentiated cells with decreased drug resistance. On the other hand, after uptake by bulk tumor cells with hypoxia and high ROS, ATRA/CPT-NPS could simultaneously release ATRA and CPT, resulting powerful synergistic anticancer effects. In their study, ATRA/CPT-NPs showed the strongest inhibition efficacy on the orthotopic BCSC-enriched tumor mouse models, suggesting that the differential drug release realized by ATRA/CPT-NPs was very important to strengthen the synergistic efficacy of ATRA-triggered CSC differentiation and CPT-triggered cytotoxic activity for the treatment of poorly differentiated and highly chemo-resistant heterogeneous tumors.
Additionally, a summary of nanomedicines studied for overcoming chemotherapeutic resistance in recent years is displayed in Table 1.
Table 1. Recent advances in nanomedicines for overcoming chemotherapeutic resistance.
Nanoformulation |
Name |
Particle Size |
Payload |
Reversal Mechanism of Drug Resistance |
Cell Line |
Tumor Model |
Reference |
Polymeric micelles |
ACP-Dox and Apa micelles |
104 ± 2 nm |
DOX and apatinib |
Inhibit P-gp activity |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing mice |
[127][76] |
HA-PLGA (PTX and FAK siRNA)-NPs |
232.9 ± 6.9 nm |
PTX and FAK siRNA |
siRNA-mediated silencing of FAK |
HeyA8-MDR and SKOV3-TR cells |
Drug-resistant, patient-derived xenograft (PDX) model |
[128][77] |
ACP-R837 and PPP-DOX |
~110 nm |
R837 and DOX |
Synergistic chemo-immunotherapy |
4T1 cells |
4T1 tumor-bearing mice |
[129][78] |
NC-DOX |
~122 nm |
DOX and IR780 |
Combined chemotherapy/PTT/PDT |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing mice |
[130][79] |
Polymeric nanoparticles |
Dox-Cur-NDs |
55.1 ± 3.0 nm |
DOX and CUR |
Down-regulate the expression of P-gp |
A2780 ADR cells |
A2780 ADR tumor-bearing mice |
[131][80] |
[FeFe]TPP/GEM/FCS NPs |
176.0 ± 17.2 nm |
Gemcitabine and [FeFe]TPP |
Reduce the of function P-gp efflux pump |
T24 cells |
T24 tumor-bearing mice |
[132][81] |
IGU-PLGA-NPs |
199.6 nm |
Iguratimod |
Facilitate BBB penetration and inhibit GSCs proliferation and stemness |
U87 and U251TMZ-R cells |
U87 tumor-bearing mice |
[133][82] |
Liposomes |
rTLM-PEG, PTX liposomes |
/ |
PTX and trichosanthin |
Reverse caspase 9 phosphorylation and induce caspase 3-dependent apoptosis |
A549/T cells |
A549/T tumor-bearing mice |
[134][83] |
PTX/NO/DMA-L |
146.3 ± 0.82 nm |
PTX and DETA NONOate |
NO-mediated down-regulation of P-gp |
A549/T cells |
A549/T tumor-bearing mice |
[135][84] |
CBZ liposomes |
108.53 ± 1.5 nm |
CBZ |
G2/M phase arrest |
MCF-7 and MDA-MB-231 cells |
Female SD rats |
[136][85] |
Lip (Ap-Dox) |
128.6 nm |
Ap-Dox complex |
Bypass the P-gp-mediated drug efflux |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing nude mice |
[137][86] |
(DEX and DTX)-Lip |
74.02 ± 0.41 nm |
DTX and dexamethasone |
Overcome stroma obstacles |
Multidrug-resistant KBv cells and 4 T1 cells |
Multidrug-resistant KBv and metastatic 4 T1 tumor models |
[138][87] |
FPL-DOX/IM |
159 ± 6 nm |
DOX and imatinib |
Inhibit ABC transporter function |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing mice |
[139][88] |
PpIX/Dox liposomes |
55.9 ± 20.9 nm |
DOX and PpIX |
Disrupt the structure of P-gp |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing mice |
[140][89] |
Nanogels |
LNGs-PTX-siRNA |
~100 nm |
PTX and MDR1 siRNA |
Knockdown MDR1 |
DROV cells |
DROV tumor-bearing mice |
[141][90] |
CDDP/DOX-NGs |
~100 nm |
CDDP and DOX |
Combination chemotherapy |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing mice |
[142][91] |
HA/Cis/Dox |
45 ± 9.9 nm |
DOX |
GSH-induced DOX release |
A2780cis cells |
/ |
[143][92] |
SiPT75 |
75.5 ± 19.8 nm |
TPPS |
Elude the drug efflux pumps and retards exocytosis of cells |
A549/DDP cells |
A549/DDP tumor-bearing mice |
[144][93] |
Inorganic nanoparticles |
H-MSNs-DOX/siRNA nanoparticles |
~100 nm |
P-gp siRNA and DOX |
siRNA-mediated silencing of P-gp |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing mice |
[145][94] |
Pt-AuNS |
~85 nm |
Pt |
GSH depletion and GPX4 inactivation |
MCF-7/ADR cells |
MCF-7/ADR tumor-bearing mice |
[146][95] |
FA-GT-MSNs@TPZ |
~60 nm |
TPZ |
Synergistic radio-chemo-photothermal therapy |
Hypoxic SMMC-7721 cells |
SMMC-7721 tumor-bearing mice |
[147][96] |
Hybrid nanoparticles |
SCA4PNPBTZ |
~150 nm |
BTZ and CA4P |
Inhibit the overexpression of BCRP/ABCG2 |
A549 cells |
Human A549 pulmonary adenocarcinoma xenograft model and PDX model of colon cancer |
[148][97] |
cNPs |
286 ± 79 nm |
Afatinib, rapamycin and docetaxel |
Synergistic treatment |
HER2-positive breast cancer cells, EGFR-positive NSCLC cells and SKBR-3/AR cell lines |
HER2-positive breast cancer mouse model |
[149][98] |
4T1-HANG-GNR-DC |
103.1 ± 7.6 nm |
CDDP and DOX |
Synergistic chemo-photothermal therapy |
4T1 cells |
4T1 tumor-bearing mice |
[150][99] |
IR780/DTX-PCEC@RBC |
~150 nm |
IR780 and DTX |
Combination therapy |
MCF-7 cells |
MCF-7 tumor-bearing mice |
[151][100] |
cNC@PDA-PEG |
170.5 ± 1.4 nm |
Paclitaxel/lapatinib |
Combination therapy |
MCF-7/ADR cells |
/ |
[152][101] |
miR497/TP-HENPs |
125 ± 6 nm |
miR497 and triptolide |
Synergically suppress mTOR signaling pathway |
SKOV3-CDDP cells |
SKOV3-CDDP tumor-bearing mice |
[153][102] |