Shu-Di Yang,Bao-Wei Cui,Jing-Cheng Song,Yue Gao

1Suzhou Polytechnic Institute of Agriculture,Suzhou 215008,China.

Abstract

Keywords:Polymeric micelles,Carrier,Anticancer,Targeted

Background

It is known that cancer is a severe health menace to human beings and experts have predicted that more people will die from cancer than acquired immune deficiency syndrome,tuberculosis and malaria.According to the latest statistics,there are more than 3 million cases of cancer and some 9,000 new cases each day on average in our country [1].During the clinical treatment,a conservative method of chemotherapy with serious side effects is still utilized.Therefore,it is urgent to explore novel and effective therapies for the treatment of cancer.Fortunately,a series of new drug delivery systems,such as nanoparticles,micelles,liposomes and vesicles,have been developed and become a hot research topic.The progress and application of polymer micelles with targeting function have become the focus of this review.

As a new type of carrier,polymer micelles can increase drug solubility and improve bioavailability.In addition,they possess excellent tissue permeability,staying longer in vivo and reaching tumor sites effectively.More importantly,they have a low critical micelle concentration and better stability.With the above advantages,polymer micelles in pharmacy have been more extensively researched.

The latest advances in polymer micelles as pharmaceutical carriers for solubilizing,slow-release and targeting of insoluble drugs are reviewed.

The polymeric micelles

The size of polymeric micelles is generally less than 200 nm.According to different morphologies,they can be divided into spherical ones,rod-like ones,and tubular ones,among which the spherical ones are used most commonly [2].Polymeric micelles are nanocarriers with a special core-shell structure formed by the self-assembly of amphiphilic polymers in the aqueous phase.Polymer micelles can increase the solubility of the drugs as well as the bioavailability.They can also enhance the stability of the drugs while reducing the toxic side effects.By increasing the enhanced permeability and retention effect or reinforcing the activity by receptor-ligand binding,passive targeting can be achieved [3,4].

According to the polymer source,micelles can be categorized into natural ones and synthetic ones.Commonly used natural polymer micelles are mainly cellulose derivatives,dextran derivatives,starch derivatives,casein,etc.Synthetic polymer polyethylene glycol (PEG) derivative and other materials can be further grouped into amphiphilic polymer micelles,polyionic complex micelles and metal complex micelles according to the different forces between polymer molecules [5].

Drug packaging is mainly divided into physical storage and chemical packaging.The insoluble drugs can be contained in the micelle by a suitable preparation method.Physical encapsulation refers to the combination of drugs and polymer micelles;chemical encapsulation forms covalent bonds,such as drugspecific groups and polymer hydrophobic nuclei,to form amide bonds.In that the physical packaging method can retain the original active group of drugs,its application is more extensive [2].Physical packaging methods mainly include dialysis method,oil/water microemulsion method and direct dissolution method.In dialysis method,drugs and polymer are firstly dissolved in organic solvents such as ethanol,dimethyl formamide,etc.,which are removed later by stirring and dialysis.The solvent (e.g.,water) is dialyzed out of the organic solvent to obtain a polymer micelle.When the good solvent is replaced by a selective solvent,the hydrophobic segments of the copolymer are combined into a micellar core and the insoluble drugs are contained in the nucleus.In oil/water microemulsion method,polymer solution and drugs are simultaneously dissolved in the oil/water emulsion mixed with volatile solvents (such as chloroform,tetrahydrofuran,acetone,mixed solvents,etc.) and then the organic solvent is evaporated.With the evaporation of organic solvents,the micelle-drug binding system is formed [2].

Conventional micelles as carriers of anticancer drugs

In recent years,polymer micelle technology has been widely concerned.Nanoparticles,composed by amphipathic lipids encapsulate drugs,is only 20–50 nm in particle size.They can improve the biodegradability of drugs and reduce rejection,and also serve as a delivery carrier for insoluble drugs.For instance,Samyang’s taxol micelles (Genexol-PM) have been listed in South Korea and some Asian countries.During the development and research period,biodegradable micelle polymer preparation technology was engaged,which can dissolve hydrophobic compounds,so toxicity refrains from increasing with the rise of drug dosage.Compared with the traditional paclitaxel,Genexol-PM was characterized by low toxicity and high efficiency,showing higher tolerance to human,and exhibiting a better anti-tumor effect.

綜上所述，隨著社會經濟的快速發(fā)展，建筑工程技術水平不斷提高，傳統(tǒng)施工管理模式無法滿足時代發(fā)展需求。此種背景下，工程建設中實施全過程造價管控顯得尤為重要。全過程造價管控主要是將技術與經濟有機結合的一種措施，以此分析項目造價控制，從而在項目決策、設計、施工及竣工結算等整個過程中貫穿工程造價控制，為工程造價構建科學而完整的控制管理方案，全面控制建筑項目總投資，為建筑企業(yè)創(chuàng)造更高的投資效益。

Stimulating responsive polymer micelles as carriers of anticancer drugs

PH-sensitive.Various environmental-responsive polymer micelles dissolve,swell or collapse under internal stimuli such as pH,saccharides,reducing potentials,lysosomal enzymes,or external stimuli such as temperature,magnetic field,ultrasound,light,thereby enhancing the release of drugs in a specific location or a specific time [6].There is a pH gradient in the internal environment,in which the pH of human physiological environment is 7.4;the endosome pH is 5.5–6.0;the lysosome pH is 4.5–5.0.Generally,the pH of normal tissues and organs is higher than that of tumor tissues (＜ 6.5).The designed pH-responsive nanocarriers are used to enhance drug release at the tumor site.These pH-sensitive polymeric carriers are usually based on some pH-sensitive chemical bonds or chemical groups such as acetals or ketals,amide bonds,amine groups and hydrazone bonds [7,8].

Luo et al.have reported the pH-sensitive tertiary amine-containing PCL/PDEA/PEG comb polymeric micelles and used them for the encapsulation and release of camptothecin [9].Also Huang et al.have reported the use of pH-sensitive polycarbonate micelles for the controlled release of an anticancer drug,paclitaxel,consisting of a PEG shell and an acetalcontaining polycarbonate core layer [10].The achieved results found that the micelles were stable at pH 7.4,but at pH 5.0,the acetal in the hydrophobic nucleus hydrolyzed,making the micellar nucleus hydrophilic and swollen,thus releasing anticancer drugs rapidly.

Reduction-sensitive.The concentration of intracellular glutathione (GSH) is over 200 times than that of extracellular one (0.5–10 mmol/L vs 2–20 μmol/L).GSH/oxidized glutathione is the main electrical pair that determines the antioxidant capacity of animal cells and the difference between intracellular and intracellular GSH is the main basis for the delivery of reduction-sensitive targeted drugs.In the intracellular reduction conditions,the disulfide bond will break quickly within a few minutes to several hours,thereby promoting the active release of various drugs within the cell [11,12].Due to lower levels of GSH in the blood circulation,the disulfide bonds are stable in normal cells and do no function.However,the higher concentration of GSH in tumor cells cleave the disulfide bonds into sulfhydryl groups,causing the destruction of the micelle structure,thereby effectively and quickly releasing the drugs [13].

Xu et al.have reported the release of doxorubicin from micelles of polycaprolactone-polyphosphate containing disulfide bonds in the main chain [14].In the presence of 10 mM GSH,the drug-loaded micelles were released from adriamycin in 4 days;whereas in the absence of GSH,the drug-loaded micelles released only 40% doxorubicin in 4 days.Zhang et al.also reported a biodegradable polymer micelle which is reductionsensitive and modified by polyethylene glycol for the rapid release of doxorubicin [15].The micelles were very stable in water but rapidly dissociated and aggregated in a 10 mM dithiothreitol reduction environment,releasing doxorubicin in a large amount within 12 hours.

Double and multiple-sensitive.To further improve the behavior of drug release and enhance its anti-tumor effect,increasing attention has been paid to the dual and multiple sensitive polymer micellar systems,such as pH/temperature,pH/reduction,pH/magnetic field,temperature/reduction,dual pH,temperature/magnetic field,temperature/enzyme,temperature/pH/reduction,and temperature/pH/magnetic field [16].What should be highlighted is that these responses can take place simultaneously in the same place.But under different settings,they also occurred in different parts.

Chen et al.have recently reported a pH and reduced dual-sensitive degradable polymer polyethylene glycolthio-poly (2,4,6-trimethoxybenzylacetal pentaerythritol) [17,18].Carbonate (PEG-SS-PTMBPEC) micelles were used for the higher efficient intracellular release of anticancer drugs.The hydrophobic segments of the degradable polymer micelles contain a large number of pH-sensitive acetal groups.Under the acidic conditions of the endosomes of the cells,the acetal bonds can quickly degrade and release the drug,and then the intracellular GSH.The S-S bonds in the polymer break quickly,therefore the anti-cancer drugs can be completely released to kill cancer cells effectively.

Polymeric micelles with targeting function

Targeted drug delivery system can be targeted to transport to the lesion site,and there is little toxicity in non-target organs,tissues and cells,so as to improve efficacy while reducing the toxicity of anti-cancer drugs,the purpose of which is to achieve targeted release of drugs at the lesion site [18].Additionally,the introduced specific targeting groups enable polymer micelles to actively target the lesion site,and these smart micelles can play a greater part in the treatment of cancer.

Folic acid as targeting molecules

Folic acid has become the most commonly used ligand in tumor targeting system because of its low molecular weight,small volume,good stability,no immunogenicity,good biocompatibility,and affinity with its receptor.It has been proved that the affinity and recognition of the folate receptor over-expressed tumor cells can be enhanced by the attachment of folic acid molecules on the hydrophilic surface of the nanocomposite shell,thus the cytotoxicity of the tumor cells is significantly increased [19].

Carbohydrates as targeting molecules

Galactose residues,mannitol residues,and amino mannose derivatives are commonly used as carbohydrate ligand.The micellar targeting properties of the synthesized micelles are also different,depending on the attached glycosyl groups.Asialoglycoprotein receptor,also known as hepatocyte galactose receptor,can be specifically bound to the lactose residue,so lactose can be attached to the micellar shell as a ligand.The beam can target hepatocytes [22].

The Lac-TPCS2 modified micelles (Lac-TPCS2) with pH-sensitive properties have been prepared by our group recently.The micelles are used as carriers to encapsulate the harmless alkali harmine (Lac-TPCS2) HM),a good indicator of the targeting effect of the carrier,to achieve the purpose of slow release of drugs to reduce side effects,while harmine in the liver to obtain effective delivery of liver tumors,enhancing the harmine harm liver cancer effect [23].

Polypeptide as targeting molecules

In previous research,active targeting is divided into targeting tumor cells and targeting tumor vascular endothelial cells [24].Receptors on the surface of endothelial cells,such as integrin,play an important role in vascular remodeling,regeneration and cell proliferation,and these receptors are over-expressed on the surface of tumor endothelial cells.The RGD polypeptide consisting of arginine,glycine and aspartate can specifically target integrins,the targeting of which can be optimized by adjusting the sequence structure of RGD.Furthermore,RGD exists in many extracellular matrices.Therefore,the inclusion of RGD peptide on the surface of nano-carriers,such as polymer micelles,can enable the drug carrier to actively target the tumor vascular endothelial cells and enrich the drug in tumor blood vessels.Yuan et al.have prepared RGDmediated micelles of N-succinylated chitosan palmitic acid,and made a bioluminescent luc-A549 cell model for lung cancer targeted therapy [25].

Proteins as targeting molecules

Active protein-targeting molecules are generally endogenous ligands for tumor cell surface receptors.For example,low-density lipoprotein (LDL) is advantageous because of endogenous degradability,nontoxicity,compatibility with cell membranes,and capacity to escape from reticuloendothelial system [26].The uptake of LDL is mediated by LDL receptor,an integral plasma membrane glycoprotein abundantly expressed in tumors that recognize ApoB-100 [27,28].The active Lys residues of ApoB can be properly modified to retain the binding capability to LDL receptor [29,30].Zhu et al.have isolated LDL from human plasma and incubated with cholesterol-modified multidrug-resistant siRNA to form a complex [27].The multidrug resistance gene delivery system was established by using the chitosan derivatives carrier advantage,the passive targeting property of the nanoparticles,as well as the active targeting characteristic of the LDL,to achieve targeted delivery of genes and chemotherapeutic drugs.

Stepwise pH/reduction-responsive polymeric

In the systemic circulation,the biological distribution,delivery and release of nanocarriers are closely related to their physical and chemical properties,such as carrier particle size,zeta potential,solubility and chemical structure,etc [31,32].Generally speaking,nanoparticles with positively charged form,are easily taken up by tumor cells but they are unstable in the blood circulation;they tend to adsorb proteins with negatively charged form,and their particle size is increased,which leads to the quick elimination by the reticuloendothelial system.Meanwhile,the increased particle size also weakens the enhanced permeability and retention effect of nanoparticles and declines passive targeting.However,negatively charged nanoparticles with a long half-life in vivo keep stable in the blood circulation but are not easily taken up by tumor cells.To resolve this contradiction,researchers have developed nanoparticles with charge reversal at the tumor site,which maintain a negative charge and stay stable in the blood circulation.After they reach the tumor tissue,under the stimulation of the extracellular pH of the tumor (extracellular pH,pHe 6.5–7.0),the surface charge of these nanoparticles reverses from negative to positive and cellular uptake is increased [33].It is reported that the amide bond formed by dimethyl maleic anhydride and primary amines is pHe responsive and can be broken at the extracellular pH of the tumor to achieve charge reversal [34].

Research status of polymeric micelles in controlled drug delivery system

Polymer micelle delivery system is a new type of drug controlled-release system which has been developed and popularized in recent years.Under simulated microenvironments,compared with other drug carriers,polymeric micelles are found to be better to carry and release drugs and require low drug solubility [35].Even for drugs without specific reactive groups,they can be better wrapped with polymeric micelles.In addition,by controlling the hydrophobic segments to ensure the micellar particle size between 10 and 100 nm,the glomerular filtration can be avoided,thereby improving the permeation of tumor tissue.The drug delivery mechanism can be divided into electrostatic adsorption,physical encapsulation and covalent binding.The release process includes immediate release and sustained release.

According to the researches,many effective gene drugs have been discovered and developed,but the gene carrier with good performance is still lacking.The new polymer micelles can be the effective carriers of gene drugs [36].For example,the gene is combined with poly-L-lysine to form a complex,which is then encapsulated with neutral or negative polymer micelles to achieve gene delivery [37].

As a new drug carrier,polymeric micelles have shown great potential in many fields.Their unique advantages are as follows:they enjoy high stability and good biocompatibility;they can increase the dissolution of insoluble drugs to reduce side effects;they tend to improve drug efficacy with passive and active targeting.Although the polymer micellar drug delivery system has made breakthrough progress in chemotherapy,such as the successfully listed Genexol-PM,and a variety of polymer micellar chemotherapeutic agents in clinical research,other aspects including stability in vivo circulatory,the impact of pharmacokinetics,tumor cell targeting,etc.also entail further investigation [38].With the deepening of research as well as the progress of new materials and new technology,it is believed that in the near future,new polymer micellar carrier technology will be more widely used in cancer treatment.

Research status of polymer micelles in tumor targeted therapy system

With the dramatic increase in the incidence of cancer and mortality,malignant tumors have become a major killer of human beings.Therefore,tumor treatment has become a global problem to be solved.However,concerning the treatment of cancer,the effect of drug treatment is not sufficient.Currently,various chemotherapeutic drugs are poor in water solubility and non-specifically distributed in vivo,leading to huge side effects on the normal tissues and organs.What is worse,multidrug resistance also overwhelmingly reduces its efficacy.The biodegradable polymer micelles with core-shell structure have been widely used in the delivery of drugs due to numerous advantages.At present,the biodegradable polymer micelles research department is committed to developing targeted,safe and effective delivery of drugs to overcome some of the existing obstacles inside and outside the cell.

Nano-carrier,to some extent,can reduce the initial shortcomings of chemotherapy drugs,but the cells with a series of mechanisms to produce multidrug resistance refused to die.At present,an effective strategy to overcome multidrug resistance is to use RNA interference technology,which is regarded as a key mechanism for cell regulation of gene expression.However,siRNA is still faced great challenges in the delivery process,including the large molecular weight and strong electronegativity.Worse still,it is difficult to pass through the cell membrane;its half-life is short,easy to be subject to nuclease degradation;in the cell it is difficult to escape the endosomes,greatly reducing its transfection efficiency [39,40].The biodegradable polymer micelles are superior to cationic liposomes in chemical modification,possessing physical stability and biosafety and showing great potential in siRNA delivery.

Conclusion

It is known that the targeting and therapeutic effects of polymer micelles are limited owing to the complexity of the tumor microenvironment and the endogenous signal outside or inside the tumor cell is difficult to control,which leads to the unsatisfactory therapy effect in clinic.Therefore,it is still a long way to further investigate the safety,biocompatibility and degradability of polymer micelles and improve the individualized treatment methods.