Recent advances of morphology adaptive nanomaterials for anti-cancer drug delivery

Donging Cheng ,Zengying Qio ,Lixue Xun ,Ho Wng,d,**

a School of Chemistry,Chemical Engineering &Life Science,Wuhan University of Technology,No.122 Luoshi Road,Wuhan,430070,China

b CAS Center for Excellence in Nanoscience,Laboratory for Biomedical Effects of Nanomaterials and Nanosafety,National Center for Nanoscience and Technology(NCNST),Beijing,100190,China

c Department of Breast Surgery,Nation Cancer Center/Cancer Hospital,Chinese Academy of Medical Sciences and Peking Union Medical College,Beijing,100021,China

d Henan Institute of Advanced Technology,Zhengzhou University,Zhengzhou,450001,China

ABSTRACT The morphology of nanomaterials combines the characteristics of their size,shape and spatial structure.Owing to the improved spatiotemporal control of the nanocarriers for anti-cancer drug delivery,the morphology adaptive nanomaterials have received tremendous attention in the field of nanomedicine,while they are able to respond through chemical or physical change.In this review,we summarize the recent advancements of morphology adaptive nanomaterials and their application in anti-cancer drug delivery system,wherein the size,shape and spatial structure of nanocarriers can be changed to adapt the complex living system,thus overcoming the pathological barriers in drug delivery.The overview of the recent examples shows that each step of delivery process can be improved by the morphology adaption,which represents the great potential for clinic translation of morphology adaptive nanomaterials.

1.Introduction

Drug delivery system has been widely developed for improving drug efficacy and reducing the side effects in tumor treatment.Since the desired nano-scaled delivery system can overcome the pathological barrier and realize controlled spatio-temporal drug delivery,many researches focus on the exploitation of nanodrug carriers[1–4].However,unitary and unchangeable nano-drug carriers are far from enough for clinical translation,and many critical problems,such as inefficient tumor accumulation and poor penetration,will compromise the drug efficacy [5–7].Considering the complex physiological environment,nano-drug carriers require subtle construction for adapting their delivery process.Generally,there are some paradoxes for nano-drug carriers to deliver drug to tumor sites:(i) In blood circulation,the surface of nano-drug carriers is often modified by neutral polyethylene glycol (PEG) or negative charge to avoid the clearance by the reticuloendothelial system (RES),whereas the subsequent tumor cellular uptake of carriers is inadequate.(ii) In tumor site,carriers with relatively large size show considerable tumor accumulation based on enhanced permeability and retention(EPR)effect,while the carrier's large size prefers to accumulate at extravasation sites other than penetrate in tumor tissue.To date,various smart nano-drug carriers have been designed for resolving the above conflicts,which possess the transition ability,including adaptive morphology and switchable surface ligand,to overcome the different physiological barriers[8–12].Our group and others have reported enhanced tumor retention by in situ aggregation/assembly of nanomaterials in response to tumor microenvironment,and the delivery efficiency is significantly improved [13–15].

Compared with the surface ligand switch,the morphology adaptive system is more controllable and universal,which is able to be customized for addressing a cascade of delivery obstacle [16,17].Basically,the morphology adaptive nanomaterials with size below 600 nm can extravasate from leaky tumor vasculature [18–20],wherein the nanomaterials with large size are shrunk/dissociated upon tumor-specific responsiveness for improving penetration ability,which lead to more cellular uptake and intracellular release.Alternatively,nanomaterials with small sizes aggregate/assemble in tumor parenchyma to avoid reentering the bloodstream,consequently enhancing tumor retention.Fukumura and coworkers proposed a multistage system that the original 100-nm gelatin nanoparticles shrunk to 10-nm quantum dots responding to the tumor-specific enzymes[21].These shrunken strategies were readily against the tumor interstitial fluid pressure,and 10-nm quantum dots could act as nano-carriers for therapeutics that penetrate deep into the tumor parenchyma.The morphology of nanomaterials also influences the cellular internalization and intracellular release,while the morphology tunability raises the possibility in the spatial control of intracellular drug delivery (Fig.1).In this review,we will comprehensively summarize the recent advances of morphology adaptive nanomaterials in anti-cancer drug delivery system,and the challenges in clinical translation will also be discussed.

Fig.1.Schematic diagram of various types of morphology adaptive nanomaterials and their applications for anti-cancer drug delivery.

2.Morphology adaptive nanomaterials

The pathological barrier and heterogeneity of tumor impair the delivery of anti-cancer drug into the tumor cells,and morphology adaptive nanomaterials show great potential to solve encountering multiple barriers.The variability of nanomaterial morphology significantly depends on their chemical composition,spatial structure and preparation method,wherein nonequilibrium morphologies of nanomaterials correspond to local minimums of free energy of chemical structure [22,23].Thus,various types of smart materials have been utilized to realize the morphology adaption,which contain both organic materials and inorganic materials (Table 1).

Table 1 Various types of morphology adaptive nanomaterials.

2.1.Organic materials

In view of the difference of microenvironment between normal and tumor tissue,many moderate and controllable organic reactions are introduced for design of morphology adaptive nanomaterials,including enzyme-catalyzed reactions [24–26],redox reactions [27–29],acidhydrolysis/protonation reactions [30–32] and so on.Recent researches have demonstrated both the small molecules and polymers can be utilized to overcome the drug delivery shortcoming through tumor specific reaction.

2.1.1.Small molecule materials

Small molecule materials usually exist in states of single molecule and assemblies in aqueous solution,which depend on their amphiphilicity and concentration.The small molecules with strong hydrophilicity or low concentration are stable as single molecule state during blood circulation,and the systemic toxicity of drug can be eliminated through rapid renal clearance [33,34].However,the dynamic permeability of tumor blood vessels leads to inefficient tumor accumulation or internalization of small molecules.In order to improve the delivery efficiency of small molecules,our group reported the assembly-induced retention (AIR) effect of assembled small molecules,which not only enhanced drug retention efficiency in tumor but increased the selectivity (Fig.2A) [35].A hydrophilic small molecule precursor was designed with targeting moiety,responsive moiety and imaging molecules,simultaneously assembling into nanofibers in tumor sites.Through the evaluation of photoacoustic (PA) signal from imaging molecules in tumor,the small molecule precursor showed the sevenfold higher retention concentration compared to the non-responsive group.Moreover,a tumor-selective.

Cascade activatable self-detained system has been reported,and only the X-linked inhibitor of apoptosis protein induced the self-assembly of small molecules in tumor regions (Fig.2B) [36].This tumor induced selective self-assembly shows high-efficiency drug accumulation and enhanced penetration in tumor tissue,resulting in high specificity and sensitivity for bladder cancer treatment.

Small molecules based on peptide (FF) have been extensively studied due to its strong self-organized properties,while intracellular and pericellular self-assembly have been employed for inhibiting cell growth or overcoming the multidrug resistance (Fig.2C) [37].Xu and coworkers reported enzyme-triggered self-assembly of small molecules based on FF,and a series of morphology adaptive small molecules were constructed for tumor or infection treatment [38–40].Alkaline phosphatase (ALP) is overexpressed in some cancer cells,ALP-instructed assembly of peptides selectively inhibit metastatic osteosarcoma,which provides new opportunities for developing morphology adaptive small molecules.

Owing to the high yield,fastness and versatility of click chemistry,the reactions between azide and alkynes group,1,2-thiolamino and cyano group are an effective strategy for developing morphology adaptive materials.Upon the click reaction of small molecule in tumor region,hydrophobic oligomers or polymers are produced to enhance the drug retention and efficacy.Rao et al.employed click reaction between 2-cyanobenzothiazole (CBT) and D-cysteine for synthesizing macromolecules and assembling different nanostructures [41–43].The click reaction process is controlled by intracellular stimuli,such as pH adjustment,reduction or enzyme cleavage,which result in the structure change of the monomers in vivo.Copper-free azide and alkynes reaction,as a classical click reaction,is demonstrated to be effective in vivo regardless of complex environment [44–47].A nanoassembly was constructed on the cancer cell membrane through copper-free click reaction of alkynes modified targeting peptide and azide decorated aggregated peptide,and the resulting nanostructure could specifically disorder the.

Permeability of tumor cell membranes and more doxorubicin(DOX)was uptaken,ultimately obtaining chemosensitization in vivo [48].

Fig.2.(A)Molecular structure of compound 1,which responsively self-assembled into nanofibers in tumor site [35].(B) Illustration of the tumor-selective cascade activatable self-detained system and its molecular design [36].(C) Molecular structures of precursors 1a and 1b,and the enzyme-instructed self-assembly to form nanofibers inside cells [37].

The nanoparticles are formed from small molecules when the concentration is above the critical aggregation concentration (CMC).Due to the EPR effect,nanoparticles can passively accumulate in tumor sites,which attract great attention in anti-cancer drug delivery.Up to now,there are many morphology adaptive nanoparticles formed from small molecules for promoting drug delivery efficiency (Fig.3A) [49–51].Our group presented an autocatalytic morphology transformation platform based on peptide-based prodrug,wherein preformed fibrous nanodrugs in the tumor site served as “seed” fibrils for accelerating transformation of the subsequent i.v.-injected nanoparticles,which accelerated tumor accumulation of drug (Fig.3B) [52].This morphology adaptive strategy offered a novel perspective for targeted delivery strategy in control of chemical kinetics.Under the stimuli in tumor sites,the small molecule reorganizes from nanoparticles for overcoming the delivery drawback.When it comes to large size of nanoparticles,nanoparticles reorganizes into smaller one,while drug penetration and accumulation in tumor are enhanced (Fig.3C) [53].

2.1.2.Polymer materials

Polymer materials with single chain state possess unique attributes in drug delivery system,including prolonged blood circulation and low cellular internalization [54].Chilkoti's group described a method to extrinsically tune polypeptide with thermally toggled self-assembly from unimer to polymeric micelle,and thereby control the intracellular delivery of drug (Fig.4A) [55].These adaptive polymer materials enhanced the selectivity of cellular uptake relying on intrinsic features of biphase,which could deliver many peptide drugs with poor intracellular delivery for targeted cellular uptake.Recently,a microenvironment-triggered self-assembly of polymer-peptide conjugates(PPCs)was reported to improve solid-tumor penetration and anticancer activity of drug(Fig.4B)[56].Once the PPCs arrived at the tumor,the cis-aconitic anhydride was shed off,resulting in the hydrophobicity increase.Then,PPCs could self-assemble into nanoparticles with a size around 100 nm,and the increased size contributed to high internalization efficiency.These adaptive polymer materials enabled higher.

Penetrability and therapeutic efficacy as compared to those of the preassembled nanoparticles,indicating the great potential of morphology transition from single chain into nanoparticles.

Nanoparticles composed of polymer materials deliver the drug into tumor sites through EPR effect,while morphology adaptive ability will also improve the delivery efficiency.Chen's group reported shellstacked polymer nanoparticles with size reduction from 145 to 40 nm for deep tumor penetration of drug(Fig.5A)[57].The shell was stacked via electrostatic interaction of polypeptide,which could be detached at the acidic microenvironment of tumor tissue.Meanwhile,the core of polymer nanoparticles was cross-linked by disulfide bonds,maintaining the small size after shedding of the shell and accelerating intracellular drug release in cells.This size shrunken strategy based on polymer nanoparticles is an ideal and controllable route to deliver drug deeply in tumor tissue.To enhance tumor accumulation of polymer nanoparticles with small size,other strategies,such as morphology transformation and size aggregation,are taken into consideration.A reactive oxygen species (ROS)-triggered morphology transformation of PPCs was presented for cooperative interaction with mitochondria,which enhances the peptide drug anti-tumor efficacy (Fig.5B) [58].

Fig.3.(A)Molecular beacons are able to assemble into spherical or filamentous supramolecular nanobeacons,depending on both the assembly temperature and the incubation time[50].(B)Molecular structure and self-assembly behavior of CPT-LFPR.The fibrils accelerate the subsequent transformation into nanofibers based on autocatalytic growth mechanism and multiple intravenous injections of nanoparticles show the cumulative effect of fibrous prodrugs[52].(C)Scheme of size-tunable DNA-based micellar assembly that overcomes the conflict between the EPR effect and further deep tumor penetration,resulting in remarkable tumor therapy [53].

2.2.Organic-inorganic hybrid materials

Benefiting from intrinsically physicochemical properties and controllable particle size,organic-inorganic hybrid nanomaterials have shown great potential in drug delivery system [59–61].Ryu and coworkers reported an intracellular biomineralization materials that the basic conditions of the mitochondrial matrix(pH=8)induced specific silicification of triphenylphosphonium conjugated trialkoxysilanes(Fig.6A) [62].The effective silicification led to formation of silica particles in cells,which accumulated in mitochondria to enhance the drug targeting and selectivity.Besides,inorganic ions,such as calcium ion,manganese ion,iron ion and so on,are utilized to be biomineralized for inhibiting tumor growth [63,64].Chen et al.explore a twostep method to form nanotheranostics with Mn2+-assistance for cancer phototherapy (Fig.6B) [66].Nanotheranostics constructed within the tumor cells/tissues showed improved phototherapy efficacy,and the process of in situ generation is monitored by activated fluorescence/magnetic resonance (MR)/PA imaging.

The morphology change can also improve the enrichment of inorganic nanoparticles at tumors.Zhang's group reported in vivo aggregation of lanthanide downconversion or upconversion nanoparticles to realize precision bioimaging of tumors.The aggregation of inorganic nanoparticles enhanced the signal-to-noise ratio and realized long-term inspection of nanoprobes,which can also be exploited to deliver the anti-cancer drug for high tumor accumulation (Fig.7A) [67].As above mentioned,in vivo click reactions are introduced into design of morphology adaptive inorganic nanoparticle since the functional groups are readily to be modified on the surface of nanoparticles.Xing and coworkers reported a unique morphology adaptive upconversion nanocrystal that was premodified by functional groups (Fig.7B) [68].In tumor environment,click reaction between CBT and.

D-cysteine triggered by tumor specific enzyme induced the aggregation of upconversion nanocrystals,which displayed unique tumor localization to help multimodality imaging and tumor treatment of drug.Considering strong self-organized properties of peptide (FF),our group designed peptide functionalized gold nanoparticles that specifically aggregated in tumor (Fig.7C) [69].The aggregated gold nanoparticles not only prolonged tumor retention but enhanced photothermal tumor treatment.Therefore,morphology transition protocols in drug delivery based on organic materials are also suitable in inorganic materials.

2.3.Drug delivery of morphology adaptive nanomaterials for tumor therapy

There are many physiological barriers for delivery of anti-cancer drug,such as elimination by mononuclear phagocytic system(MPS)and reticuloendothelial system (RES),limited intratumoral accumulation and distribution in solid tumor,less cellular internalization,and so on.During the delivery process,the morphology of nano-drug carriers plays a critical role for overcoming physiological barriers,which have been demonstrated in many researches [70,71].Obviously,the pharmacokinetic (PK) and pharmacodynamic (PD) properties are affected by the nanostructure of molecules;also,the size of nano-drug carriers has relation to tumor vessel extravasation and cellular internalization.Thus,morphology change responding to tumor environment is an important way to achieve high drug delivery efficiency.

Fig.4.(A)Design and function of an ELPBC nanopeptifier.At physiologic temperature(“off”state,37°C),nanopeptifiers are soluble unimers,displaying a single CPP on their hydrophilic terminus.At conditions of mild clinical hyperthermia(“on”state,42°C),nanopeptifiers self-assemble into spherical micelles,displaying a high density of CPPs on the micelle corona[55].(B)Process of PT-K-CAA hydrolysis,which leads to its self-assembly,and process for the penetration of PT-K-CAA in the tumor microenvironment [56].

2.4.Blood circulation

Blood circulation is an important indicator to design nano-drug carrier,and generally,the prolonged circulation time is in favor of tumor targeting.Although many promising ways,such as PEGylation and biomimetic erythrocyte membrane coating,have been approved to prolong blood circulation time of nanomaterials,their efficacy of tumor treatment is far less than expected.Further analysis demonstrates circulation time is also affected by morphology,and this morphology possibly reduces subsequent drug accumulation,penetration and cellular uptake in tumor,which results in poor drug availability.Thus,morphology adaptive ability presents substantial potential to resolve above situation.Discher and coworkers reported that the blood circulation of.

Fig.5.(A) Nanoparticles was composed of the PEGylated and negatively charged shell,and the positively charged and disulfide-cross-linked polypeptide core through electrostatic interaction.Owing to the tumor-microenvironment-mediated multi-transformation,SNP could lead to a perfect cascade of drug delivery.I)Long circulation.II) Enhanced accumulation.III) Deep penetration.IV) Promoted internalization.V) Accelerated drug release [57].(B) Synthetic route of ROS-sensitive PPCs and mitochondria locational morphology transformation [58].

Fig.6.(A)Structure design of the mitochondria targeting molecule 1 and the control molecule.Time-dependent light scattering intensity of 1(1,10 and 40 mM)in ammonium buffer for 72 h.TEM images showing particle formation in 40 mM and 1 mM molecule 1[62].(B)Schematic illustration showing a molecular model of the Mn2+ ion linking porphyrin ring and two carboxylate radicals of DVDMS molecules.TEM images of MnO2 nanosheet,MnO2/DVDMS+GSH,and MnO2/DVDMS+H2O2/H+ are presented in vitro [66].

PEGylated cylindrical micelles is about ten times longer than that of their spherical counterparts,and the circulation time is even up to one week[72].Therefore,some researchers reported a strategy that fibrous nano-carriers can specifically transform into nanoparticles in tumor sites,which showed greatly improved blood circulation,solid tumor penetration and cellular internalization (Fig.8A) [73].Similarly,nanoparticles with size of 100 nm exhibit longer blood circulation and better tumor accumulation.Because of increased interstitial pressures and decreased the blood perfusion,nanoparticles with large size stuck at the blood-vessel extravasation sites,and little drug penetrates into tumor,let alone enters in tumor cells.So,lots of researchers employ the deep-penetration nanoparticles with small size to aggregate into large size for addressing the poor blood circulation (Fig.8B) [74,75].

Fig.7.(A)Scheme illustration of assembly and NIR laser-regulated disassembly of nanoprobes for stable and accurate NIR-II bioimaging[67].(B)Illustration of the microenvironment-sensitive strategy for covalent cross-linking of peptide-premodified UCNs in tumor areas[68].(C)Schematic illustration of MMP-2-triggered selfassembly of AuNPs@Pep 1/Pep 2 in vitro and in vivo,which exhibits the enhanced retention for improved PTT [69].

Fig.8.(A)Schematic illustration of the in vivo morphology transformation of aza-BODIPY-1 for enhanced long blood circulation and deep tissue penetration[73].(B)Schematic illustration of PAM/Pt@IcLipo targeting tumor vessels and penetrating to the deep tumor tissues(1)extending the circulation time of PAM/Pt in the blood;(2) targeting the tumor blood vessels;(3) being triggered to release the small size PAM/Pt nanoparticles with a size less than 10 nm through NIR laser [74].

Fig.9.(A)Illustration of CAF-instructed self-assembly of nanofibers in situ for enhanced tumor imaging;congo Red(red) staining of a tumor slice and its magnified section;the TEM images of the CAFs and PC-3 co-implant tumor slices [76].(B) Schematic illustration of the hypothesis that retention and cellular uptake of small nanoparticles in tumors can be enhanced by inducing aggregation of nanoparticles in the tumor environment;Representative TEM images of sections of KB tumor tissue after injection with AuNPs for 24 h [80].

Fig.10.(A)Self-assembly and redelivery of MONCs served as ultrasmall nanodrug generators in response to tumor microenvironment;48 h following i.v.Injection,the excised tumors (about volume of 90 mm3) were developed into frost slices,and then the DOX fluorescence distribution in entire tumors was revealed by fluorescence microscope(scale bar:200 μm)[82].(B)Characteristics and delivery process of PTN.(II)Aggregation-enhanced photodynamic effect of PTN.(III) 1O2-triggered size reduction of PTN and accelerated chemodrug release.(IV) Passive tumor-targeting of PTN and deep tumor penetration of size-reduced nanoparticles.(V) Cellular uptake,lysosome accumulation and chemo-drug release of PTN for chemo-photodynamic therapy.Immuno-fluorescence staining images of the intratumor distribution of PTN can be taken with and without light irradiation.Cell nuclei were stained with DAPI (blue),the tumor vessels were stained with FITCtagged CD31 antibody (green),and ICy 5 was represented by red [84].

2.5.Tumor accumulation

Through the targeting effect,nano-drug carriers may selectively accumulate in tumor,wherein anti-cancer drug extravasate from tumor vessel into tumor parenchyma.It should be noted that tumor vessel is featured by dynamic permeability,and the leaky blood vessels in tumor let the carriers back to bloodstream.Thus,in order to enhance the tumor accumulation,aggregation/assembly in tumor is considered as a powerful means to reduce the reflux of nano-drug carriers.Our group reported in situ aggregation/assembly on the surface of cancer-associated fibroblasts (CAFs) in tumor site,while both the selectively and specificity of probe are promoted (Fig.9A) [76].Basically,after it extravasates from tumor vessel,the constituent of carriers is changing under the specific stimuli in tumor,and subsequently bulky nanomaterials are constructed to be retained for improving drug accumulation[77–79].Analogously,Ji and coworkers demonstrated that controlled aggregation in response to tumor acidic microenvironment could enhance the retention and cellular uptake of nanoparticles (Fig.9B)[80].In other situations,the designed nanocarriers cannot be uptaken by cells.After nanocarriers reach to the tumor site,the morphology of them changes into the liable cellular internalization state,significantly increasing the tumor specific accumulation [56,81].

2.6.Solid tumor penetration

Most of malignant tumors are filled with and surrounded by the dense stroma,which contribute to interstitial pressures and decreasing the blood perfusion that hamper drugs attacking the tumor cells.According to the previous reports,the nanoparticles with size below 30 nm showed effective tissue diffusion than the larger ones in tumor parenchyma,and thus many morphology adaptive materials have been presented for deep solid tumor penetration(Fig.10A)[82,83].Yin et al.rationally integrated self-assembly manner and delivery strategy to enhance the photodynamic effect and facilitate deep tumor penetration of the nanodrug (Fig.10B) [84].Upon light irradiation,the generated singlet oxygen by photosensitizer reduced the nanodrug size from 90 to 10 nm,leading to the deep penetration in tumor and acceleration of the drug release into tumor cells.Particularly,inverse morphology change strategy has the similar effect.When carriers with small size arrive at the tumor,they can penetrate and diffuse inside tumor.Under the tumor specific stimuli,carriers transform into bulky nanostructure,which deepen the tumor penetration to some extent [36,56,80].Both opposite strategies have their unique superiority,and it is difficult to evaluate which one is more efficient for solid tumor penetration of drug.

2.7.Cellular internalization

Fig.11.(A) Chemical structures of CBT-Taxol and schematic illustration of intracellular furin-controlled self-assembly of Taxol-NPs for anti-multidrug resistance[85].(B) Proposed mode of the tandem molecular self-assembly in the extra-and intracellular environment of liver cancer cells;Merged confocal laser scanning microscopy images (blue fluorescence from Hoechst 33,342 and yellow fluorescence from NBD represent the nucleus and NBD peptide,respectively) of HepG2 and QGY7703 cells incubated with 1(200 mm)for different periods of time.Scale bars:10 mm.The white dashed line indicates the boundary of a cell.TEM images of cell culture medium for HepG2 collected at the 0.5 h time point and broken HepG2 cells collected at the 4 h time point.Scale bars:100 nm [86].

The physiochemical properties of nanodrug have great impact on cellular internalization.There are many cellular uptake pathways for diverse morphology carriers,such as diffusion,phagocytosis,macropinocytosis,clathrin-mediated endocytosis,caveolin-mediated endocytosis,and translocation.Usually,diffusion and translocation are the two most efficient pathways for cellular internalization while the cell membrane is a natural barrier that large size cannot direct access.Liang and coworkers reported a taxol derivative CBT-Taxol which selfassembled into nanoparticles in cells for overcoming multidrug resistance(Fig.11A)[85].The small molecule CBT-Taxol enters into cells through passive diffusion,and in the furin-overexpressed cells,a condensation reaction triggered by furin cleavage produce hydrophobic oligomers,then oligomers self-assemble into taxol nanoparticles.The free taxol can slowly release from intracellular taxol nanoparticles,which showed high cellular internalization of taxol in cells and efficient anti-multidrug resistance effect.Taking advantage of the difference of tumor and normal cells,morphology adaptive materials can specifically deliver anti-cancer drug into tumor cells while decrease the normal cell uptake (Fig.11B) [86].

2.8.Organelle targeting

Most of anti-cancer drugs exert their pharmaceutical actions at organelle,such as doxorubicin DOX enters into nucleus to intercalate DNA strands and cytotoxic peptide KLAK disrupts mitochondrial membrane,so organelle targeting is necessary for drug delivery.Nanomaterials with unique physical attributes,including size effect and multivalent binding ability,are exploited to improve organelle accumulation and the efficacy of drug [87,88].Ryu et al.reported an organelle-localized self-assembly,in which small molecules consisted of amphiphilic peptide accumulated inside mitochondria and formed a fibrous nanostructure while reached the critical aggregation concentration (Fig.12A) [89].The mitochondria accumulation of molecules was about 500–1000 times higher than that in extracellular space.Furthermore,in order to enter into nucleus,the size of carriers should be small enough to pass through nucleopores,in which the diameter of nuclear pore complex is 20–50 nm.Chen and coworkers developed kinetically-controlled large compound nanoparticles with size of 150 nm,wherein the detachment of PEG shell induced the nanoparticles to dissociate into smaller one(40 nm).The small nanoparticles can actively translocate into nucleus to release drug,leading to 20-fold higher cytotoxicity compared to the native drug in vitro(Fig.12B)[90].

2.9.Local treatment

The localized drug delivery systems have the potential to dramatically deliver drug in tumor sites efficiently,which is an important strategy for minimal toxicity and maximal efficacy,including decreased systemic drug exposure and maintained therapeutic levels.Owing to the extremely large surface,high porosity and great flexibility,fibrous carriers are developed for localized chemotherapy.Moreover,for increasing the bioavailability of anti-cancer drug,many morphology adaptive materials are utilized to deliver the drug into tumor cells.Li's group focused on the localized chemotherapy in which fibers could spontaneously transform into micelles,and the micelles was essential for efficient cellular uptake and intracellular release of drugs(Fig.13A)[91].This in situ micelle formation in response to tumor acidic microenvironment was confirmed by fluorescence light-up analysis,and the sustained micelle liberation from fibers resulted in significantly antitumor activity in vivo tumor treatment.Morphological change from micellar aggregates to fibers or gels is developed for constructing the localized drug deports in tumor (Fig.13B and C) [92,93].Upon the morphology adaption responding to stimuli in tumor,anti-cancer drug could be retained in the fibrous state,which appeared as valuable carriers for formation of localized drug depots,and then slowly released hydrophobic anti-cancer drugs.Finally,due to intrinsic biocompatibility of carriers,morphology change from micelles to assembled fibers showed great potential for localized drug delivery.

Fig.12.(A)Intra-mitochondrial assembly of Mito-FF.The self-assembly process is driven by the increased mitochondrial membrane potential of cancer cells,leading to high mitochondrial accumulation of Mito-FF followed by self-assembly into fibrils.The intra mitochondrial fibrils further induce cellular apoptosis.Construction images showing the side and cross-sectional views of fibrils at different tilt-angles of -60° and+20°(from left to right)(scale bar,100 nm)[89].(B)The dynamic behavior of LNPdePEG-FA passing through multiple barriers from vascular environment to the nucleus.(1)LNPdePEG-FA binds to tumor cells due to the folate-folate receptor affinity,(2) becomes internalized by receptor-mediated endocytosis,(3) dissociates into smaller nanoparticles (SNPs) and detaches from PEG chains upon endo/lysosome acidification.TEM images of ultrathin sections of HeLa cells incubated with LNPdePEG-FA for 6 h,showing that SNP enters the nucleus via nucleopores (indicated by the red arrows) and the nuclear localization of SNP (indicated by the red dashed circles).The pink arrow indicates a nuclear pore complex(NPC) [90].

3.Challenges and perspectives

Most of barriers in drug delivery are correlated with morphology of nanomaterials,including blood circulation,tumor accumulation,cellular uptake and so on,so morphology adaptive nanomaterials have been developed to overcome pathological barriers and improve therapeutic index efficiently in drug delivery system.Up to now,various morphology adaptive systems acquire tremendous achievements,whereas they still have a long way to be put forward for clinical translation.It should be noted that the essential problems of drug delivery system remain unresolved and some principles are under dispute.The reported data analysis of various carriers found that only about 0.7% of the administered dose could be accumulated in the tumor site[94].This extremely low drug delivery efficiency is caused by many factors,such as non-specificity of carriers and inherent pathological characteristics of tumor.Besides,there is obvious distinction in tumor site between humans and rodents,and some researches point out that the EPR effect is extremely heterogeneous in humans [95].Thus,conventional EPR model in rodents is improper to predict tumor accumulation of carriers.Recently,Chan and coworkers reported that up to 97% of nanoparticles actively entered tumors through tumor endothelial cells,and inter-endothelial gaps were not responsible for the transport of nanoparticles into solid tumors [96].This finding questioned the concept of nanoparticles transport through gaps between endothelial cells in the tumour blood vessel,which would probably lead to debates and discussion about the passive tumor accumulation.

Although EPR effect is somewhat controversial,the morphology indeed has an influence on the tumor accumulation and penetration,suggesting the possible another factor affected by morphology,such as blood circulation time and endothelial cellular uptake.This review briefly summarizes recent works of morphology adaptive nanomaterials for drug delivery,and each barrier in drug delivery can be eliminated through a rational formulation.Furthermore,multiple-stage adaptive nanomaterials are recently developed,and considered as a more promising platform,which integrate several activatable strategies and multifunctions to overcome several pathological barriers [97,98].By combining the multiple stimuli in tumor environment,including pH,redox and enzyme,multiple-stage adaptive system may be constructed to break through the limitations in drug delivery.However,the sophisticated preparation and poor repeatability still impede the clinic translation of multiple-stage adaptive system.In order to increase clinical applicability of morphology adaptive nanomaterials,simplification and intelligence should be considered in future researches.Also,new-type morphology adaptive nanomaterials are valuable and rewardable,which may provide a novel insight in precision medicine.The drug release behavior should be controllable,while the anti-cancer drugs are precisely delivered to the tumor cells and released to act on organelles.Finally,morphology adaptive nanomaterials are supposed to be biocompatible and purgeable to avoid systemic toxicity.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig.13.(A)Schematic diagram of the release of HA-drug conjugates from short fibers in response to the slightly acidic tumor matrix(1),followed by self-assembly into hybrid micelles(2);the hybrid micelles are endocytosed into tumor cells via HA mediation(3);in response to intracellular GSH,CPT and TOS-TPP are released to target nuclei and mitochondria(4)for induction of cell-cycle arrest(5)and ROS production.SEM image of MC–T@SF short fibers and typical TEM images of MC–T micelles released from MC–T@SF [91].(B) Schematic illustration of the preparation of D-PPy@PNA nanogels and in situ formation of D-PPy@PNA hydrogels for synergistic photothermo-chemotherapy[92].(C)Schematic showing the in situ sprayed bio-responsive fibrin gel containing aCD47@CaCO3 nanoparticles within the post-surgery tumour bed.aCD47@CaCO3 nanoparticles encapsulated in fibrin scavenge H+ in the surgical wound site and release aCD47,thus promoting both polarization of TAMs to an M1-like phenotype and blockade of the ‘don't eat me’ signal in cancer cells [93].

Acknowledgement

This work was supported by National Natural Science Foundation of China (21704020,21674027,31870998,51573032,81672630 and 51725302).