Review
Prodrug-based nanoparticulate drug delivery strategies for cancer therapy Cong Luo1, Jin Sun2,3, Bingjun Sun1, and Zhonggui He1 1
Department of Pharmaceutics, School of Pharmacy, Shenyang Pharmaceutical University, Shenyang 110016, PR China Department of Biopharmaceutics, School of Pharmacy, Shenyang Pharmaceutical University, Shenyang 110016, PR China 3 Key Laboratory of Drug Delivery Technology and Pharmacokinetics, Tianjin Institute of Pharmaceutical Research, Tianjin 300193, PR China 2
Despite the rapid developments in nanotechnology and biomaterials, the efficient delivery of chemotherapeutic agents is still challenging. Prodrug-based nanoassemblies have many advantages as a potent platform for anticancer drug delivery, such as improved drug availability, high drug loading efficiency, resistance to recrystallization upon encapsulation, and spatially and temporally controllable drug release. In this review, we discuss prodrug-based nanocarriers for cancer therapy, including nanosystems based on polymer-drug conjugates, selfassembling small molecular weight prodrugs and prodrug-encapsulated nanoparticles (NPs). In addition, we discuss new trends in the field of prodrug-based nanoassemblies that enhance the delivery efficiency of anticancer drugs, with special emphasis on smart stimulitriggered drug release, hybrid nanoassemblies, and combination drug therapy. Prodrug and nanoparticulate drug delivery strategies for anticancer drug delivery Although great efforts have been made over the past few decades, there is still a long way to go to win the anticancer war [http://www.who.int/cancer/prevention/en/index.html (accessed on 17 June 2014)]. Intravenous (IV) administration is the leading route to deliver antineoplastic agents in cancer therapy, owing to the immediate and complete bioavailability of drugs. However, most anticancer drugs with high toxicity display a narrow therapeutic window because of their nonspecific distribution in the body, resulting in undesirable side effects and reduced patient compliance. In response to these obstacles, various approaches have been developed, including prodrug and nanoparticulate drug delivery strategies. Prodrugs, which are inactive conjugates metabolized in vivo to release the parent bioactive components, have been widely utilized to improve the efficacy of existing anticancer agents [1]. A successful prodrug strategy can overcome multiple barriers hindering anticancer drug delivery, such as low water-solubility or lipid-solubility, poor stability, serious adverse effects, lack Corresponding authors: Sun, J. ([email protected]); He, Z. ([email protected]). Keywords: cancer therapy; prodrug nanoassemblies; stimuli-responsive; hybrid nanoassemblies; combination therapy. 0165-6147/ ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tips.2014.09.008
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of site specificity, and inefficient cell uptake [1]. In addition, nanocarriers based on the rapid development of nanotechnology and biomaterials have shown distinct advantages in the field of anticancer drugs delivery [2– 4], such as improved drug availability, promoted accumulation of anticancer drugs in tumor tissues via the enhanced permeability and retention (EPR) effect, active targeting to tumor by appropriate modifications, and controllable drug release. IV administration of both drugs and drug-loaded nanocarriers experience five stages that should be taken into consideration [5] (Figure 1): (i) disposition of drugs or nanocarriers in systemic circulation; (ii) passing through blood vessel walls towards tumor lesions; (iii) diffusion throughout the interstitial space of tumor tissues (deep tumor penetration); (iv) the internalization of anticancer drugs or drug-loaded NPs into tumor cells; and (v) the disposition of drugs or nanocarriers within tumor cells, including endosomal escape, drug release, nuclear delivery, and P-glycoprotein (P-gp)-mediated efflux. Therefore, drug delivery from blood vessels into intracellular antitumor targets is a complex process, and any unconquered link may lead to reduced chemotherapy efficiency. Despite advances from conventional prodrug strategies and nanoparticulate drug delivery systems (Nano-DDS), some drawbacks have greatly limited their clinical application. Small molecule prodrugs can be subject to rapid clearance and premature degradation [6], and traditional Nano-DDS also has some shortcomings, such as low drug loading efficiency, a high tendency to crystallization during storage, significant drug leakage in systemic circulation postadministration, and potential toxicity caused by the utilization of a large amount of biomaterials for maximum achievable drug loading. In response to these disadvantages, prodrug-based Nano-DDS, integrating prodrug strategy and nanotechnology into one system, has become a notable trend to facilitate more efficient delivery of anticancer drugs in recent years [7–10]. This article provides a review of prodrug-based nanocarriers for cancer treatment (Figure 2), including nanosystems based on polymer-drug conjugates, self-assembling small molecular weight prodrugs and prodrug-encapsulated NPs. Moreover, we highlight progress in the enhancement of the delivery efficiency of anticancer drugs, with special emphasis on smart stimuli-triggered drug release, hybrid prodrug
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Nano-DDS Ligands
Receptors
(ii) (iv)
(iii)
(v)
Biomaterials
(i)
P-gp
(v)
Drugs
Drug release
(v)
(v)
Endosome Nucleus Normal cells Tumor cells Blood vessel
Microtubules TRENDS in Pharmacological Sciences
Figure 1. General schematics of the delivery process of nanoparticles (NPs) after intravenous administration: (i) circulation in blood; (ii) permeation through the blood vessel wall; (iii) deep tumor penetration; (iv) internalization into tumor cells; and (v) disposition of drugs or nanocarriers within tumor cells. Abbreviations: Nano-DDS, nanoparticulate drug delivery systems; P-gp, P-glycoprotein.
NPs, and combination drug therapy based on prodrug Nano-DDS.
has emerged as one of the most promising prodrug-based Nano-DDS for anticancer drug delivery.
A general overview of prodrug-based Nano-DDS Prodrug-based Nano-DDS can be roughly divided into three types (Figure 3): (i) nanosystems based on polymer-anticancer drug conjugates (Figure 3A), in which drug molecules are covalently conjugated to polymers instead of being noncovalently encapsulated in polymeric nanocarriers; (ii) small molecular weight prodrug Nano-DDS (Figure 3B), which are self-assembled by amphiphilic small molecular weight prodrugs of anticancer agents; and (iii) prodrug-encapsulated nanosystems (Figure 3C), in which prodrugs are encapsulated in nanocarriers (e.g., polymeric NPs, micelles, liposomes, and inorganic NPs) by noncovalent interactions (e.g., hydrophobic forces and electrostatic interactions). Among these, nanosystems based on polymer-anticancer drug conjugates have shown great potential to be applied in clinical cancer treatment, because several nanoassemblies of polymer-drug conjugates have reached clinical trials. In addition, the use of small molecular weight prodrugs to promote self-assembly into nanostructures with impressively high drug loading efficiency
Prodrug Nano-DDS based on polymer-anticancer drug conjugates Various polymers have been utilized in the synthesis of polymer–drug conjugates for cancer therapy, including water-soluble polymer–drug conjugates and self-assembling polymer–drug conjugates. Conventional preparations formulated with water-soluble polymer–drug conjugates (e.g., pHPMA–drug conjugates) [11–14] and Nano-DDS self-assembled from amphiphilic polymer–drug conjugates each have their own advantages. The former has a simpler manufacturing process and easier industrialization, and several water-soluble polymer–drug conjugates have already entered clinical trials [11,12]. However, self-assembling polymer–drug conjugates possess some potential advantages as elaborate nanoformulations, such as enhanced chemical stability due to the protective effect of the nanostructures, facilitated co-delivery of different therapeutic agents in the same vehicle for combinational therapy, and easier further functionalizations on the surface of Nano-DDS for enhanced drug delivery efficiency.
Nanohybrid assemblies
A
Three types Small molecular prodrug Nano-DDS
C B
High polymer prodrug Nano-DDS
Prodrug-based nanoparcles
Emerging trends Smuli-triggered drug release
Prodrug-encapsulated Nano-DDS Combinaon drug therapy
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Figure 2. Different types of prodrug strategies and emerging trends in the field.
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(A)
Key:
(B)
(C)
Polymer-drug conjugate Prodrug
Small molecular prodrug
Polymeric or inorganic carrier material TRENDS in Pharmacological Sciences
Figure 3. Schematic illustration of the established prodrug-based nanoparticular drug delivery systems (Nano-DDS): (A) nanosystems based on polymer-drug conjugates; (B) self-assembling small molecular prodrug nanoparticles; and (C) prodrug-encapsulated polymeric or inorganic Nano-DDS.
According to the structural skeleton of polymers, the most common polymers utilized in the synthesis of polymer– drug conjugates mainly include three types (Figure 4): (i) block copolymers; (ii) dendritic polymers; and (iii) comblike copolymers. The common advantage shared with these polymers is that they have a large number of active functional groups on the polymer skeleton, and anticancer drug moieties are usually coupled onto polymer chains as side groups for high drug-loading efficiency. However, linear polymers are seldom utilized to synthesize self-assembling polymer–drug conjugates due to their limited drug loading capacity. Recently developed prodrug nanosystems based on polymer–drug conjugates for cancer therapy are summarized in Table 1, and what these polymers have in common is that they are biocompatible materials readily modified and widely utilized in IV injections. The most widely used polymers include poly(ethylene glycol) (PEG)
Table 1. List of the recently synthesized self-assembling polymer-drug conjugates applied in cancer therapy Categories Polyethylene glycol (PEG)
PEG block copolymers
Polysaccharides
Others
558
Polymers PEG-40k PEG-5k PEG-2k PEG-polymerized block of camptothecin PEG–block-dendritic polylysine PEG–cyclodextrin copolymer PEG-poly(glutamic acid) copolymer
D-alpha-tocopheryl-co-poly(ethylene glycol) 1000 succinate (TPGS) mPEG–b-P(LA-co-MCC-OH) Bi(PEG-PLA) PEGylated peptide Dendron mPEG–b-PAMAM PEG–b-P(HEMA-co-EGMA) PEG–b-poly[N-(2-hydroxypropyl) methacrylamide-lactate] PEG–poly(aspartic acid) copolymer PEG–block-poly(lactide-co-2-methyl2-carboxyl-propylene carbonate) Poly(ethylene oxide)-block-polyphosphoester PEG–b-poly(acrylic acid) block copolymers mPEG–b-P(LA-co-MCC) Triblock copolymer comprised of mPEG, polycaprolactone and poly-L-lysine PEG-bl-poly(propylene sulfide) Cholesterol-modified hyaluronic acid (HA) PEGylated carboxymethylcellulose Chitosan Dendronized heparin Hyaluronic acid (HA) Pullulan Cholesterol-modified hyaluronic acid (HA) N-urocanyl pullulan Heparin Hyaluronic acid (HA) Cholesterol-modified hyaluronic acid (HA) Chimeric polypeptides (CPs) HPMA copolymer Hyperbranched polyglycerol (hPG) Poly(styrene-co-maleic anhydride) (SMA) derivative Polyisoprene (PI) Poly (squalenyl-methacrylate) (PSqMA) Poly (squalenyl-methacrylate) (PSqMA) Poly(methyl methacrylate) (PMMA) Hyperbranched poly(ether-ester) (HPEE) PEGylated triazine dendrimer Poly(L-g-glutamylglutamine) (PGG)
Drug coupled Camptothecin Hydroxycamptothecin Paclitaxel Camptothecin Camptothecin Camptothecin 7-Ethyl-10-hydroxycamptothecin (SN-38) Cisplatin Cisplatin
Drug loading (wt %) 20.3 18 24 >50 38 10–12 20
Refs [15] [16] [17] [18] [19] [20] [21]
4 –
[22] [23]
Cisplatin Doxorubicin Doxorubicin Doxorubicin Doxorubicin Doxorubicin
1 14 26 12 30–40 45
[24] [25] [26] [27] [28] [29]
Oxaliplatin Paclitaxel
11.7 65
[30] [31]
Paclitaxel Paclitaxel Platinum Tioguanine
42 – 9.8–12 –
[32] [23] [33] [34]
Curcumin Docetaxel Doxorubicin Doxorubicin Doxorubicin Doxorubicin Etoposide Methotrexate Paclitaxel Paclitaxel Salinomycin Doxorubicin Doxorubicin Doxorubicin Doxorubicin Gemcitabine Gemcitabine Gemcitabine Gemcitabine Paclitaxel Paclitaxel Paclitaxel
20 37 7.0–12.6 9 0.5 30 20 17.8 35–39 10–15 20 5 – – 25 10.5–31.2 4.2 2.5–7.2 43.7 4.1–10.7 22 >32
[35] [36,37] [38] [39] [40] [41] [35] [42] [43] [44] [35] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55]
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block copolymers, polyaminoacids, polypeptides, and polysaccharides. It is generally accepted that the stretched PEG in the outer shell plays an important role in avoiding the identification and phagocytosis of Nano-DDS by a mononuclear phagocyte system (MPS), and polypeptides and polysaccharides usually have a large number of active groups (e.g., hydroxyl, amino, and carboxyl) for easier chemical modification and higher drug-conjugating efficiency. Among Nano-DDS, the core-crosslinked nanomicelles with covalently entrapped anticancer drugs represent a specific type of Nano-DDS based on polymer–drug conjugates, in which the polymerization of polymer–drug conjugates is conducted following the fabrication of polymeric micelles [28,56]. Talelli et al. have developed such a nanosystem for doxorubicin delivery, in which methacrylamide– doxorubicin derivatives were covalently incorporated into polymeric micelles composed of (PEG)-b-poly[N-(2-hydroxypropyl) methacrylamide-lactate] by free radical polymerization [28]. This unique Nano-DDS with high drug loading (30–40%, w/w) demonstrated higher cytotoxicity in B16F10 and OVCAR-3 cells compared to a methacrylamide–doxorubicin derivative, and showed a better antitumor activity than free doxorubicin in tumor bearing mice [28]. In addition, block copolymers have been most commonly utilized to synthesize self-assembling polymer–drug conjugates. For example, polyprodrug amphiphiles self-assembled from PEG–b-PCPTM have been developed for camptothecin (CPT) delivery, in which PCPTM is a polymerized block of a CPT prodrug monomer with a reduction-responsive drug release manner [18]. Intriguingly, these block copolymers can self-assemble into four types of distinct nanostructures with extremely high CPT loading content (>50%), including spheres, large compound vesicles, smooth disks, and staggered lamellae [18]. Staggered lamellae showed significant advantages over the other three nanostructures in terms of blood circulation time, cellular uptake, intracellular trafficking, and in vitro cytotoxicities [18]. More importantly, several Nano-DDS based on polymer– anticancer drug conjugates have entered clinical trials, and
the encouraging clinical results indicated that they have great potential to be further developed for clinical cancer treatment in the near future [57,58]. For example, NK012, a prodrug-based nanomicelle with SN-38 conjugated to a PEG-poly(glutamic acid) copolymer, has been evaluated in clinical trials [21,57].A Phase I study of NK012 in adult patients with solid tumors showed that NK012 was well tolerated and demonstrated good antitumor activity against a variety of advanced refractory cancers [57]. SN-38 molecules were released in a dose-dependent manner, and the free SN-38 exhibited a linear pharmacokinetics in the dose range of 2–28 mg/m2 [57]. The recommended Phase II dose of NK012 was determined to be 28 mg/m2 with at least a 3week interval between treatment cycles [57]. Self-assembling small molecular weight prodrugs of chemotherapy agents Distinguished from polymer prodrugs, self-assembling small molecular weight prodrugs refer to conjugates in which the carrier materials usually have low molecular weight and take up a small portion of the nanostructure, and anticancer drugs themselves play an important role in adjusting the hydrophilic–lipophilic balance of the conjugates. In other words, a small molecular weight prodrug is usually synthesized by coupling one drug molecule to another small molecule instead of a high molecular polymer. In the case of the self-assembling polymer–drug conjugates, there are usually a large number of drug molecules conjugated to high molecular weight polymers. Small molecular weight prodrug-based Nano-DDS for cancer therapy are summarized in Table 2. These nanosystems can be summarized as four categories: (i) lipid– drug conjugates (e.g., squalenoylations, cholesteryl, and palmityl prodrugs of anticancer agents); (ii) low molecular weight oligo (ethylene glycol) (OEG)–drug conjugates; (iii) amphiphilic peptidic prodrugs; and (iv) tocopherol succinate–drug conjugates. Among them, squalene-based prodrug Nano-DDS has drawn widespread attention in recent years [69,70,74]. Although the conjugation of hydrophobic
Table 2. List of the recently developed small molecular weight prodrug NPs for cancer therapy Anticancer drugs Camptothecin
Self-assembling small molecular prodrugs Camptothecin–peptide sequence derived from the Tau protein Camptothecin–OEG (short oligomer chain of ethylene glycol)
Drug loading wt % 23–38 40–58
Refs [59] [60]
SN-38
SN38–OEG SN38–OEG
35 36
[61] [62]
Curcumin
Curcumin–OEG
25.3
[63]
Doxorubicin
N-doxorubicina-D-tocopherol succinate Doxorubicin–peptide Doxorubicin–low molecular weight PEG Polyisoprenoyl gemcitabine prodrugs Gemcitabine–squalene (mixed with rhodamine–squalene and biotin–squalene) Gemcitabine–squalene Gemcitabine–squalene–paclitaxel
34 21.7 46 50 34 39 17
[64] [65] [66] [67] [68] [69] [70]
Paclitaxel–1,4-cis,cis-pentadiene-squalene Paclitaxel–terpene (stabilized by PEG–squalene) Folate-adenovirus–ICG02-Glu–paclitaxel Paclitaxel–squalene–gemcitabine
– 82 – >53
[71] [72] [73] [70]
Gemcitabine
Paclitaxel
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Block copolymer prodrug
Dendric polymer prodrug
Comb-like copolymer prodrug TRENDS in Pharmacological Sciences
Figure 4. Various polymer–drug conjugates based on different types of polymers, mainly including block copolymers, dendritic polymers and comb-like copolymers. The red ellipsoids represent drug molecules.
squalene with hydrophilic drug molecules such as nucleoside analogues could self-assemble as Nano-DDS in aqueous solution, the lipophilic conjugates of squalene with hydrophobic drugs could also, surprisingly, endow the prodrugs with self-assembly properties. For example, the squalenoylations of paclitaxel (20 -squalenoyl paclitaxel) have been developed for cancer therapy, and these squalene–paclitaxel conjugates can self-aggregate in water into stable prodrug Nano-DDS [74]. An advantage of small molecular weight prodrug NanoDDS lies in their smaller particle size and higher drug loading efficiency. However, they also have their own limitations, such as short circulation time in the blood due to the lack of a dense shield layer (e.g., long chain PEG) and poor structural stability owing to a relatively higher critical micelle concentration (CMC). Therefore, nanosystems self-assembled from small molecular weight conjugates are usually stabilized by using an amphiphilic block copolymer containing a long PEG chain to achieve both long blood circulation time and a high structural stability. For example, a terpene-based self-assembled
prodrug was developed by conjugating a single terpene unit (MIP) to paclitaxel, and then a squalene derivative of polyethylene glycol (SQ–PEG) was selected as a surface active agent [72]. Surprisingly, impressively high drug loading (82 wt %) was achieved, and the anticancer activity of this novel hybrid nanocarrier (MIP–paclitaxel/SQ–PEG) in a breast cancer murine model was significantly improved when compared to the conventional IV formulation (Taxol1) [72]. Anticancer lipophilic prodrugs noncovalently encapsulated in Nano-DDS Prodrug-encapsulated nanosystems as delivery carriers for anticancer drugs represent a special kind of prodrug nanoassembly, in which lipophilic prodrugs are noncovalently encapsulated in polymeric or inorganic nanocarriers [10]. The recent developments in prodrug-encapsulated nanosystems for anticancer drug delivery are summarized in Table 3. Prodrug-encapsulated Nano-DDS possess the advantages of both prodrug strategy and nanotechnology, including improved drug availability, reduced toxicity, and targeted delivery via the EPR effect. Compared with the above self-assembled prodrug nanoassemblies based on either polymer conjugates or small molecular weight prodrugs, the potential advantage of prodrug-encapsulated Nano-DDS is that a wider range of available biomaterials can be used without the limitation of choosing proper hydrophilic or lipophilic properties for conjugation. However, one main disadvantage of this strategy is its relatively low drug loading capacity due to the high proportion of carrier materials used in prodrug-encapsulated Nano-DDS [10]. Drug molecules with poor water solubility, such as paclitaxel and docetaxel, will certainly benefit from the lipid prodrug strategy for improved lipid solubility and enhanced miscibility within the polymer matrix. For example, the conjugate of paclitaxel with fatty acid is more easily encapsulated in Nano-DDS than the parent drug
Table 3. List of the recently developed prodrug-encapsulated nanosystems as delivery carriers for anticancer drug delivery Parent drugs
Prodrugs
Nanocarriers
Docetaxel
4-(4-methylpiperazin-1-yl) butanoic acid-docetaxel Alkyl silyl ether docetaxel prodrug
Doxorubicin 5-fluorouracil Gemcitabine
Platinum
Dextran–doxorubicin 1-alkylcarbonyloxymethyl prodrugs of 5FU Asymmetric bifunctional silyl ether prodrugs of gemcitabine Paclitaxel oleate Paclitaxel-7-palmitate Paclitaxel-7-carbonyl-cholesterol Platinum (IV) prodrug (PtBz) Adamantane–oxoplatin c,c,t [Pt(NH3)2Cl2(O2C(CH2)8CH3)2] c,t,c[Pt(NH3)2-(O2CCH2CH2CH2CH2CH3)2Cl2] Valproic acid–platinum–valproic acid c,c,t-[PtCl2(NH3)2(OOCCH2CH2CH2CH2CH3)2] c,t,c-[Pt(NH3)2(O2CCH2CH2COOH)(OH)Cl2] trans,trans,trans-[Pt(N3)2(NH3)(py)(O2CCH2CH2COOH)2]
Lipid NPs Poly(lactide-co-glycolide) polymer cylindrical NPs Chitosan NPs PLGA NPs PRINT NPs
PNA (analogues of DNA and RNA)
Oligo-aspartic acid–PNA conjugate [Asp(n)-PNA] Oligo-aspartic acid–PNA conjugate [Asp(n)-PNA]
Paclitaxel
560
Drug loading wt % 40 20–22
[75] [76]
– 47.2 20
[77] [78] [79]
Stealth polymeric NPs PEG-b–PCL micelles Folate–lipid NPs Multi-walled carbon nanotubes Cyclodextrin capped gold NPs PLGA-PEG NPs PLGA-b-PEG NPs PEG-PCL NPs RVb3-targeted PLGA-PEG NPs PLGA-PEG NPs PEG-modified upconversion NPs
3 17–22 – 15–16 – 10 5 4.2 3 5 –
[80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90]
Asp(n)–PNA/PEI complexes Asp(n)–PNA/lipofectamine complexes
– –
[91] [91]
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(B)
(A)
Key:
(C)
Conjugate 1 Drug 1
Drug 2
Conjugate 2 Conjugate 3
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Figure 5. Schematic illustrations of the different combination therapy patterns: (A) two polymer–drug conjugates self-assembled into nanoparticular drug delivery systems (Nano-DDS); (B) one free anticancer drug encapsulated in prodrug NanoDDS containing the polymer conjugates of another drug; and (C) two different drugs conjugated to the same polymer.
due to the enhanced lipid solubility and interrupted planar structure [80]. Ansell et al. reported improved antitumor activity of paclitaxel prodrug-encapsulated Nano-DDS by manipulating the hydrophobicity of paclitaxel–lipid alcohol conjugates, and a range of frequently-used lipid alcohols were utilized as the ‘anchor components’ [92]. In addition, Zhigaltsev et al. synthesized a docetaxel prodrug by introducing ionized groups into the neutral docetaxel molecules, namely 20 -O-acyl conjugate of docetaxel with Nmethylpiperazinyl butanoic acid [75]. The weak-base docetaxel prodrug can be readily loaded into lipid nanocarriers by using pH gradient loading techniques to achieve high drug encapsulation [75]. Stimuli-responsive prodrug-based Nano-DDS Despite the rapid progress in Nano-DDS, high drug loading efficiency and smart stimuli-triggered drug release still remain the popular issues for pharmaceutical scientists. Many researchers endeavor to design stimuli-sensitive
Nano-DDS by applying various specific and sensitive linkers [93,94]. These stimuli applied in prodrug-based NanoDDS could be broadly classified into two groups (Table 4): the internal physiopathologic changes of the tumor microenvironment (e.g., pH, enzymes, reductive environment, etc.) and the external stimulus signals (e.g., light, heat, and magnetic field, etc.). Chemical linkers that are sensitive to internal stimuli include hydrazone bonds, acetal linkage, silyl ether bonds, disulfide bonds, and specific peptides sensitive to certain enzymes overexpressed in tumors. Similar to other stimuli-triggered nanocarriers, stimuliresponsive prodrug-based Nano-DDS have shown significant advantages in reducing side effects and enhancing antitumor activity due to tumor-specific drug release. In addition, stimuli-responsive nanosystems provide a promising strategy for the reversal of multidrug resistance (MDR). The ability of nanocarriers to overcome MDR is expected to function as follows [95]: (i) the Nano-DDS should promote the endosomal escape of anticancer drugs; (ii) anticancer drugs should be released into the cytoplasm quickly and accurately after being internalized; and (iii) the capacity to bypass or inhibit drug efflux transporters (e.g., P-gp) is necessary. For stimuli-responsive Nano-DDS, chemotherapeutic agents can be delivered and released in a temporal and spatial pattern, and promote the accumulation of anticancer drugs at target locations to reach the effective therapeutic concentration [95]. pH-sensitive prodrug Nano-DDS Among the common stimuli-triggered strategies, acidresponsiveness is the most frequently used approach in recent years [97]. pH-responsive polymer conjugates endow prodrug Nano-DDS with acid-triggered drug release, which can covalently anchor anticancer drugs at normal physiological pH in systemic circulation, and release
Table 4. List of the common stimuli-sensitive linkers utilized in the chemical synthesis of anticancer drug conjugates Types pH-sensitive
Sensitive linkers or triggers Acetal bond Hydrazone linkage Hydrazone linkage Hydrazone linkage Hydrazone linkage Hydrazone linkage Hydrazone linkage
Redox-responsive
Enzyme-sensitive Light-sensitive
Heat-sensitive Magnetism-sensitive
Hydrazone linkage Silyl ether bond Disulfide bond Disulfide bond Disulfide bond Disulfide bond Thioether linker Octapeptide (GPLGIAGQ) Peptide (GFLG) Near infrared reflection UVA Chimeric polypeptide with Tt value range of 37–42 8C Magnetite nanocrystals (USPIO)
Drug conjugates PEG-b-poly(acrylic acid) block copolymers–paclitaxel Dendronized heparin–doxorubicin mPEGylated peptide dendron–doxorubicin Pullulan–doxorubicin Poly(styrene-co-maleic anhydride)-adipic dihydrazide–doxorubicin (SMA-AD–DOX) Bi(PEG-PLA)–Pt(IV) Hyperbranched polyglycerol-hydrazone bond–doxorubicin (hPG-hydrazone bond–DOX) PEG–b-p(HEMA-co-GMA-DOX) Alkyl silyl ether docetaxel prodrug PEG–block-dendritic polylysine–camptothecin (PEG-SS–xCPT) Peptide sequence derived from the Tau protein–camptothecin (Tau-SS–CPT) PEG-b-poly(propylene sulfide)–tioguanine (PEG-PPS-SS–TG) camptothecin-polyethylene glycol–camptothecin (CPT-SS-PEG-SS–CPT) OEG-2S–SN38 PEG–GPLGIAGQ–paclitaxel Triblock HPMA copolymer-GFLG–doxorubicin trans,trans,trans-[Pt(N3)2(NH3)(py)(O2CCH2CH2COOH)2] mPEG–block-poly(e-caprolactone)-poly(L-lysine)–platinum (mPEG–b-PCL-b-PLL–Pt) Chimeric polypeptide–paclitaxel (CP–PTX) Magnetic nanomedicine (USPIO/squalene–gemcitabine)
Refs [32] [39] [25] [41] [48] [24] [47] [28] [76] [19] [59] [34] [15] [61] [17] [46] [90] [33] [96] [69]
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Review anticancer drugs at a sudden pH descent in tumor tissues (pH 6.5–7.0) or in the intracellular endosomes (pH 5.0–6.0) of tumor cells [98]. Acid-triggered drug release can be realized by the application of pH-sensitive biodegradable chemical linkers, mainly including acid-labile acetal linkage, hydrazone bonds, and silyl ether bonds. For example, an acetal-linked paclitaxel prodrug micellar nanosystem achieves rapid drug release under the acidic endosome/ lysosome condition inside tumor cells [32]. Paclitaxel was conjugated to PEG-b-poly(acrylic acid) (PEG–PAA) block copolymers using ethyl glycol vinyl ether (EGVE) as an acid-sensitive linker [32]. As a result, highly pH-dependent paclitaxel release was observed in in vitro release studies, with 86.9%, 66.4%, and 29.0% release from the prodrug Nano-DDS at pH 5.0, 6.0, and 7.4, respectively [32]. Reduction-responsive prodrug Nano-DDS Another stimulus used to achieve triggered drug release from prodrug Nano-DDS is the redox gradient existing between the mildly oxidizing extracellular spaces and the reducing intracellular cytoplasm (higher reduction level) [95]. A disulfide bond is the most common chemical linker utilized to bridge polymers and anticancer drugs for reduction-responsive drug release under higher intracellular glutathione (GSH) concentration in tumor cells [19,34]. For example, PEG-block–dendritic polylysineSS–CPT conjugates were recently synthesized, and the hydrophobicity of the linear-dendritic drug conjugates could be tailormade by the number of the conjugated hydrophobic CPT molecules, thereby determining their self-assembled nanostructures (nanospheres or nanorods) [19]. In vitro release experiments showed that none of these nanostructures released any CPT in phosphate buffer solution (PBS) for over 5 days, but CPT could be immediately released in the presence of dithiothreitol (DTT) (a strong reductant similar to GSH) [19]. Moreover, the nanorods of moderate lengths (
Prodrug-based nanoparticulate drug delivery strategies for cancer therapy Cong Luo1, Jin Sun2,3, Bingjun Sun1, and Zhonggui He1 1
Department of Pharmaceutics, School of Pharmacy, Shenyang Pharmaceutical University, Shenyang 110016, PR China Department of Biopharmaceutics, School of Pharmacy, Shenyang Pharmaceutical University, Shenyang 110016, PR China 3 Key Laboratory of Drug Delivery Technology and Pharmacokinetics, Tianjin Institute of Pharmaceutical Research, Tianjin 300193, PR China 2
Despite the rapid developments in nanotechnology and biomaterials, the efficient delivery of chemotherapeutic agents is still challenging. Prodrug-based nanoassemblies have many advantages as a potent platform for anticancer drug delivery, such as improved drug availability, high drug loading efficiency, resistance to recrystallization upon encapsulation, and spatially and temporally controllable drug release. In this review, we discuss prodrug-based nanocarriers for cancer therapy, including nanosystems based on polymer-drug conjugates, selfassembling small molecular weight prodrugs and prodrug-encapsulated nanoparticles (NPs). In addition, we discuss new trends in the field of prodrug-based nanoassemblies that enhance the delivery efficiency of anticancer drugs, with special emphasis on smart stimulitriggered drug release, hybrid nanoassemblies, and combination drug therapy. Prodrug and nanoparticulate drug delivery strategies for anticancer drug delivery Although great efforts have been made over the past few decades, there is still a long way to go to win the anticancer war [http://www.who.int/cancer/prevention/en/index.html (accessed on 17 June 2014)]. Intravenous (IV) administration is the leading route to deliver antineoplastic agents in cancer therapy, owing to the immediate and complete bioavailability of drugs. However, most anticancer drugs with high toxicity display a narrow therapeutic window because of their nonspecific distribution in the body, resulting in undesirable side effects and reduced patient compliance. In response to these obstacles, various approaches have been developed, including prodrug and nanoparticulate drug delivery strategies. Prodrugs, which are inactive conjugates metabolized in vivo to release the parent bioactive components, have been widely utilized to improve the efficacy of existing anticancer agents [1]. A successful prodrug strategy can overcome multiple barriers hindering anticancer drug delivery, such as low water-solubility or lipid-solubility, poor stability, serious adverse effects, lack Corresponding authors: Sun, J. ([email protected]); He, Z. ([email protected]). Keywords: cancer therapy; prodrug nanoassemblies; stimuli-responsive; hybrid nanoassemblies; combination therapy. 0165-6147/ ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tips.2014.09.008
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of site specificity, and inefficient cell uptake [1]. In addition, nanocarriers based on the rapid development of nanotechnology and biomaterials have shown distinct advantages in the field of anticancer drugs delivery [2– 4], such as improved drug availability, promoted accumulation of anticancer drugs in tumor tissues via the enhanced permeability and retention (EPR) effect, active targeting to tumor by appropriate modifications, and controllable drug release. IV administration of both drugs and drug-loaded nanocarriers experience five stages that should be taken into consideration [5] (Figure 1): (i) disposition of drugs or nanocarriers in systemic circulation; (ii) passing through blood vessel walls towards tumor lesions; (iii) diffusion throughout the interstitial space of tumor tissues (deep tumor penetration); (iv) the internalization of anticancer drugs or drug-loaded NPs into tumor cells; and (v) the disposition of drugs or nanocarriers within tumor cells, including endosomal escape, drug release, nuclear delivery, and P-glycoprotein (P-gp)-mediated efflux. Therefore, drug delivery from blood vessels into intracellular antitumor targets is a complex process, and any unconquered link may lead to reduced chemotherapy efficiency. Despite advances from conventional prodrug strategies and nanoparticulate drug delivery systems (Nano-DDS), some drawbacks have greatly limited their clinical application. Small molecule prodrugs can be subject to rapid clearance and premature degradation [6], and traditional Nano-DDS also has some shortcomings, such as low drug loading efficiency, a high tendency to crystallization during storage, significant drug leakage in systemic circulation postadministration, and potential toxicity caused by the utilization of a large amount of biomaterials for maximum achievable drug loading. In response to these disadvantages, prodrug-based Nano-DDS, integrating prodrug strategy and nanotechnology into one system, has become a notable trend to facilitate more efficient delivery of anticancer drugs in recent years [7–10]. This article provides a review of prodrug-based nanocarriers for cancer treatment (Figure 2), including nanosystems based on polymer-drug conjugates, self-assembling small molecular weight prodrugs and prodrug-encapsulated NPs. Moreover, we highlight progress in the enhancement of the delivery efficiency of anticancer drugs, with special emphasis on smart stimuli-triggered drug release, hybrid prodrug
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Nano-DDS Ligands
Receptors
(ii) (iv)
(iii)
(v)
Biomaterials
(i)
P-gp
(v)
Drugs
Drug release
(v)
(v)
Endosome Nucleus Normal cells Tumor cells Blood vessel
Microtubules TRENDS in Pharmacological Sciences
Figure 1. General schematics of the delivery process of nanoparticles (NPs) after intravenous administration: (i) circulation in blood; (ii) permeation through the blood vessel wall; (iii) deep tumor penetration; (iv) internalization into tumor cells; and (v) disposition of drugs or nanocarriers within tumor cells. Abbreviations: Nano-DDS, nanoparticulate drug delivery systems; P-gp, P-glycoprotein.
NPs, and combination drug therapy based on prodrug Nano-DDS.
has emerged as one of the most promising prodrug-based Nano-DDS for anticancer drug delivery.
A general overview of prodrug-based Nano-DDS Prodrug-based Nano-DDS can be roughly divided into three types (Figure 3): (i) nanosystems based on polymer-anticancer drug conjugates (Figure 3A), in which drug molecules are covalently conjugated to polymers instead of being noncovalently encapsulated in polymeric nanocarriers; (ii) small molecular weight prodrug Nano-DDS (Figure 3B), which are self-assembled by amphiphilic small molecular weight prodrugs of anticancer agents; and (iii) prodrug-encapsulated nanosystems (Figure 3C), in which prodrugs are encapsulated in nanocarriers (e.g., polymeric NPs, micelles, liposomes, and inorganic NPs) by noncovalent interactions (e.g., hydrophobic forces and electrostatic interactions). Among these, nanosystems based on polymer-anticancer drug conjugates have shown great potential to be applied in clinical cancer treatment, because several nanoassemblies of polymer-drug conjugates have reached clinical trials. In addition, the use of small molecular weight prodrugs to promote self-assembly into nanostructures with impressively high drug loading efficiency
Prodrug Nano-DDS based on polymer-anticancer drug conjugates Various polymers have been utilized in the synthesis of polymer–drug conjugates for cancer therapy, including water-soluble polymer–drug conjugates and self-assembling polymer–drug conjugates. Conventional preparations formulated with water-soluble polymer–drug conjugates (e.g., pHPMA–drug conjugates) [11–14] and Nano-DDS self-assembled from amphiphilic polymer–drug conjugates each have their own advantages. The former has a simpler manufacturing process and easier industrialization, and several water-soluble polymer–drug conjugates have already entered clinical trials [11,12]. However, self-assembling polymer–drug conjugates possess some potential advantages as elaborate nanoformulations, such as enhanced chemical stability due to the protective effect of the nanostructures, facilitated co-delivery of different therapeutic agents in the same vehicle for combinational therapy, and easier further functionalizations on the surface of Nano-DDS for enhanced drug delivery efficiency.
Nanohybrid assemblies
A
Three types Small molecular prodrug Nano-DDS
C B
High polymer prodrug Nano-DDS
Prodrug-based nanoparcles
Emerging trends Smuli-triggered drug release
Prodrug-encapsulated Nano-DDS Combinaon drug therapy
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Figure 2. Different types of prodrug strategies and emerging trends in the field.
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(A)
Key:
(B)
(C)
Polymer-drug conjugate Prodrug
Small molecular prodrug
Polymeric or inorganic carrier material TRENDS in Pharmacological Sciences
Figure 3. Schematic illustration of the established prodrug-based nanoparticular drug delivery systems (Nano-DDS): (A) nanosystems based on polymer-drug conjugates; (B) self-assembling small molecular prodrug nanoparticles; and (C) prodrug-encapsulated polymeric or inorganic Nano-DDS.
According to the structural skeleton of polymers, the most common polymers utilized in the synthesis of polymer– drug conjugates mainly include three types (Figure 4): (i) block copolymers; (ii) dendritic polymers; and (iii) comblike copolymers. The common advantage shared with these polymers is that they have a large number of active functional groups on the polymer skeleton, and anticancer drug moieties are usually coupled onto polymer chains as side groups for high drug-loading efficiency. However, linear polymers are seldom utilized to synthesize self-assembling polymer–drug conjugates due to their limited drug loading capacity. Recently developed prodrug nanosystems based on polymer–drug conjugates for cancer therapy are summarized in Table 1, and what these polymers have in common is that they are biocompatible materials readily modified and widely utilized in IV injections. The most widely used polymers include poly(ethylene glycol) (PEG)
Table 1. List of the recently synthesized self-assembling polymer-drug conjugates applied in cancer therapy Categories Polyethylene glycol (PEG)
PEG block copolymers
Polysaccharides
Others
558
Polymers PEG-40k PEG-5k PEG-2k PEG-polymerized block of camptothecin PEG–block-dendritic polylysine PEG–cyclodextrin copolymer PEG-poly(glutamic acid) copolymer
D-alpha-tocopheryl-co-poly(ethylene glycol) 1000 succinate (TPGS) mPEG–b-P(LA-co-MCC-OH) Bi(PEG-PLA) PEGylated peptide Dendron mPEG–b-PAMAM PEG–b-P(HEMA-co-EGMA) PEG–b-poly[N-(2-hydroxypropyl) methacrylamide-lactate] PEG–poly(aspartic acid) copolymer PEG–block-poly(lactide-co-2-methyl2-carboxyl-propylene carbonate) Poly(ethylene oxide)-block-polyphosphoester PEG–b-poly(acrylic acid) block copolymers mPEG–b-P(LA-co-MCC) Triblock copolymer comprised of mPEG, polycaprolactone and poly-L-lysine PEG-bl-poly(propylene sulfide) Cholesterol-modified hyaluronic acid (HA) PEGylated carboxymethylcellulose Chitosan Dendronized heparin Hyaluronic acid (HA) Pullulan Cholesterol-modified hyaluronic acid (HA) N-urocanyl pullulan Heparin Hyaluronic acid (HA) Cholesterol-modified hyaluronic acid (HA) Chimeric polypeptides (CPs) HPMA copolymer Hyperbranched polyglycerol (hPG) Poly(styrene-co-maleic anhydride) (SMA) derivative Polyisoprene (PI) Poly (squalenyl-methacrylate) (PSqMA) Poly (squalenyl-methacrylate) (PSqMA) Poly(methyl methacrylate) (PMMA) Hyperbranched poly(ether-ester) (HPEE) PEGylated triazine dendrimer Poly(L-g-glutamylglutamine) (PGG)
Drug coupled Camptothecin Hydroxycamptothecin Paclitaxel Camptothecin Camptothecin Camptothecin 7-Ethyl-10-hydroxycamptothecin (SN-38) Cisplatin Cisplatin
Drug loading (wt %) 20.3 18 24 >50 38 10–12 20
Refs [15] [16] [17] [18] [19] [20] [21]
4 –
[22] [23]
Cisplatin Doxorubicin Doxorubicin Doxorubicin Doxorubicin Doxorubicin
1 14 26 12 30–40 45
[24] [25] [26] [27] [28] [29]
Oxaliplatin Paclitaxel
11.7 65
[30] [31]
Paclitaxel Paclitaxel Platinum Tioguanine
42 – 9.8–12 –
[32] [23] [33] [34]
Curcumin Docetaxel Doxorubicin Doxorubicin Doxorubicin Doxorubicin Etoposide Methotrexate Paclitaxel Paclitaxel Salinomycin Doxorubicin Doxorubicin Doxorubicin Doxorubicin Gemcitabine Gemcitabine Gemcitabine Gemcitabine Paclitaxel Paclitaxel Paclitaxel
20 37 7.0–12.6 9 0.5 30 20 17.8 35–39 10–15 20 5 – – 25 10.5–31.2 4.2 2.5–7.2 43.7 4.1–10.7 22 >32
[35] [36,37] [38] [39] [40] [41] [35] [42] [43] [44] [35] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55]
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block copolymers, polyaminoacids, polypeptides, and polysaccharides. It is generally accepted that the stretched PEG in the outer shell plays an important role in avoiding the identification and phagocytosis of Nano-DDS by a mononuclear phagocyte system (MPS), and polypeptides and polysaccharides usually have a large number of active groups (e.g., hydroxyl, amino, and carboxyl) for easier chemical modification and higher drug-conjugating efficiency. Among Nano-DDS, the core-crosslinked nanomicelles with covalently entrapped anticancer drugs represent a specific type of Nano-DDS based on polymer–drug conjugates, in which the polymerization of polymer–drug conjugates is conducted following the fabrication of polymeric micelles [28,56]. Talelli et al. have developed such a nanosystem for doxorubicin delivery, in which methacrylamide– doxorubicin derivatives were covalently incorporated into polymeric micelles composed of (PEG)-b-poly[N-(2-hydroxypropyl) methacrylamide-lactate] by free radical polymerization [28]. This unique Nano-DDS with high drug loading (30–40%, w/w) demonstrated higher cytotoxicity in B16F10 and OVCAR-3 cells compared to a methacrylamide–doxorubicin derivative, and showed a better antitumor activity than free doxorubicin in tumor bearing mice [28]. In addition, block copolymers have been most commonly utilized to synthesize self-assembling polymer–drug conjugates. For example, polyprodrug amphiphiles self-assembled from PEG–b-PCPTM have been developed for camptothecin (CPT) delivery, in which PCPTM is a polymerized block of a CPT prodrug monomer with a reduction-responsive drug release manner [18]. Intriguingly, these block copolymers can self-assemble into four types of distinct nanostructures with extremely high CPT loading content (>50%), including spheres, large compound vesicles, smooth disks, and staggered lamellae [18]. Staggered lamellae showed significant advantages over the other three nanostructures in terms of blood circulation time, cellular uptake, intracellular trafficking, and in vitro cytotoxicities [18]. More importantly, several Nano-DDS based on polymer– anticancer drug conjugates have entered clinical trials, and
the encouraging clinical results indicated that they have great potential to be further developed for clinical cancer treatment in the near future [57,58]. For example, NK012, a prodrug-based nanomicelle with SN-38 conjugated to a PEG-poly(glutamic acid) copolymer, has been evaluated in clinical trials [21,57].A Phase I study of NK012 in adult patients with solid tumors showed that NK012 was well tolerated and demonstrated good antitumor activity against a variety of advanced refractory cancers [57]. SN-38 molecules were released in a dose-dependent manner, and the free SN-38 exhibited a linear pharmacokinetics in the dose range of 2–28 mg/m2 [57]. The recommended Phase II dose of NK012 was determined to be 28 mg/m2 with at least a 3week interval between treatment cycles [57]. Self-assembling small molecular weight prodrugs of chemotherapy agents Distinguished from polymer prodrugs, self-assembling small molecular weight prodrugs refer to conjugates in which the carrier materials usually have low molecular weight and take up a small portion of the nanostructure, and anticancer drugs themselves play an important role in adjusting the hydrophilic–lipophilic balance of the conjugates. In other words, a small molecular weight prodrug is usually synthesized by coupling one drug molecule to another small molecule instead of a high molecular polymer. In the case of the self-assembling polymer–drug conjugates, there are usually a large number of drug molecules conjugated to high molecular weight polymers. Small molecular weight prodrug-based Nano-DDS for cancer therapy are summarized in Table 2. These nanosystems can be summarized as four categories: (i) lipid– drug conjugates (e.g., squalenoylations, cholesteryl, and palmityl prodrugs of anticancer agents); (ii) low molecular weight oligo (ethylene glycol) (OEG)–drug conjugates; (iii) amphiphilic peptidic prodrugs; and (iv) tocopherol succinate–drug conjugates. Among them, squalene-based prodrug Nano-DDS has drawn widespread attention in recent years [69,70,74]. Although the conjugation of hydrophobic
Table 2. List of the recently developed small molecular weight prodrug NPs for cancer therapy Anticancer drugs Camptothecin
Self-assembling small molecular prodrugs Camptothecin–peptide sequence derived from the Tau protein Camptothecin–OEG (short oligomer chain of ethylene glycol)
Drug loading wt % 23–38 40–58
Refs [59] [60]
SN-38
SN38–OEG SN38–OEG
35 36
[61] [62]
Curcumin
Curcumin–OEG
25.3
[63]
Doxorubicin
N-doxorubicina-D-tocopherol succinate Doxorubicin–peptide Doxorubicin–low molecular weight PEG Polyisoprenoyl gemcitabine prodrugs Gemcitabine–squalene (mixed with rhodamine–squalene and biotin–squalene) Gemcitabine–squalene Gemcitabine–squalene–paclitaxel
34 21.7 46 50 34 39 17
[64] [65] [66] [67] [68] [69] [70]
Paclitaxel–1,4-cis,cis-pentadiene-squalene Paclitaxel–terpene (stabilized by PEG–squalene) Folate-adenovirus–ICG02-Glu–paclitaxel Paclitaxel–squalene–gemcitabine
– 82 – >53
[71] [72] [73] [70]
Gemcitabine
Paclitaxel
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Block copolymer prodrug
Dendric polymer prodrug
Comb-like copolymer prodrug TRENDS in Pharmacological Sciences
Figure 4. Various polymer–drug conjugates based on different types of polymers, mainly including block copolymers, dendritic polymers and comb-like copolymers. The red ellipsoids represent drug molecules.
squalene with hydrophilic drug molecules such as nucleoside analogues could self-assemble as Nano-DDS in aqueous solution, the lipophilic conjugates of squalene with hydrophobic drugs could also, surprisingly, endow the prodrugs with self-assembly properties. For example, the squalenoylations of paclitaxel (20 -squalenoyl paclitaxel) have been developed for cancer therapy, and these squalene–paclitaxel conjugates can self-aggregate in water into stable prodrug Nano-DDS [74]. An advantage of small molecular weight prodrug NanoDDS lies in their smaller particle size and higher drug loading efficiency. However, they also have their own limitations, such as short circulation time in the blood due to the lack of a dense shield layer (e.g., long chain PEG) and poor structural stability owing to a relatively higher critical micelle concentration (CMC). Therefore, nanosystems self-assembled from small molecular weight conjugates are usually stabilized by using an amphiphilic block copolymer containing a long PEG chain to achieve both long blood circulation time and a high structural stability. For example, a terpene-based self-assembled
prodrug was developed by conjugating a single terpene unit (MIP) to paclitaxel, and then a squalene derivative of polyethylene glycol (SQ–PEG) was selected as a surface active agent [72]. Surprisingly, impressively high drug loading (82 wt %) was achieved, and the anticancer activity of this novel hybrid nanocarrier (MIP–paclitaxel/SQ–PEG) in a breast cancer murine model was significantly improved when compared to the conventional IV formulation (Taxol1) [72]. Anticancer lipophilic prodrugs noncovalently encapsulated in Nano-DDS Prodrug-encapsulated nanosystems as delivery carriers for anticancer drugs represent a special kind of prodrug nanoassembly, in which lipophilic prodrugs are noncovalently encapsulated in polymeric or inorganic nanocarriers [10]. The recent developments in prodrug-encapsulated nanosystems for anticancer drug delivery are summarized in Table 3. Prodrug-encapsulated Nano-DDS possess the advantages of both prodrug strategy and nanotechnology, including improved drug availability, reduced toxicity, and targeted delivery via the EPR effect. Compared with the above self-assembled prodrug nanoassemblies based on either polymer conjugates or small molecular weight prodrugs, the potential advantage of prodrug-encapsulated Nano-DDS is that a wider range of available biomaterials can be used without the limitation of choosing proper hydrophilic or lipophilic properties for conjugation. However, one main disadvantage of this strategy is its relatively low drug loading capacity due to the high proportion of carrier materials used in prodrug-encapsulated Nano-DDS [10]. Drug molecules with poor water solubility, such as paclitaxel and docetaxel, will certainly benefit from the lipid prodrug strategy for improved lipid solubility and enhanced miscibility within the polymer matrix. For example, the conjugate of paclitaxel with fatty acid is more easily encapsulated in Nano-DDS than the parent drug
Table 3. List of the recently developed prodrug-encapsulated nanosystems as delivery carriers for anticancer drug delivery Parent drugs
Prodrugs
Nanocarriers
Docetaxel
4-(4-methylpiperazin-1-yl) butanoic acid-docetaxel Alkyl silyl ether docetaxel prodrug
Doxorubicin 5-fluorouracil Gemcitabine
Platinum
Dextran–doxorubicin 1-alkylcarbonyloxymethyl prodrugs of 5FU Asymmetric bifunctional silyl ether prodrugs of gemcitabine Paclitaxel oleate Paclitaxel-7-palmitate Paclitaxel-7-carbonyl-cholesterol Platinum (IV) prodrug (PtBz) Adamantane–oxoplatin c,c,t [Pt(NH3)2Cl2(O2C(CH2)8CH3)2] c,t,c[Pt(NH3)2-(O2CCH2CH2CH2CH2CH3)2Cl2] Valproic acid–platinum–valproic acid c,c,t-[PtCl2(NH3)2(OOCCH2CH2CH2CH2CH3)2] c,t,c-[Pt(NH3)2(O2CCH2CH2COOH)(OH)Cl2] trans,trans,trans-[Pt(N3)2(NH3)(py)(O2CCH2CH2COOH)2]
Lipid NPs Poly(lactide-co-glycolide) polymer cylindrical NPs Chitosan NPs PLGA NPs PRINT NPs
PNA (analogues of DNA and RNA)
Oligo-aspartic acid–PNA conjugate [Asp(n)-PNA] Oligo-aspartic acid–PNA conjugate [Asp(n)-PNA]
Paclitaxel
560
Drug loading wt % 40 20–22
[75] [76]
– 47.2 20
[77] [78] [79]
Stealth polymeric NPs PEG-b–PCL micelles Folate–lipid NPs Multi-walled carbon nanotubes Cyclodextrin capped gold NPs PLGA-PEG NPs PLGA-b-PEG NPs PEG-PCL NPs RVb3-targeted PLGA-PEG NPs PLGA-PEG NPs PEG-modified upconversion NPs
3 17–22 – 15–16 – 10 5 4.2 3 5 –
[80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90]
Asp(n)–PNA/PEI complexes Asp(n)–PNA/lipofectamine complexes
– –
[91] [91]
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(B)
(A)
Key:
(C)
Conjugate 1 Drug 1
Drug 2
Conjugate 2 Conjugate 3
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Figure 5. Schematic illustrations of the different combination therapy patterns: (A) two polymer–drug conjugates self-assembled into nanoparticular drug delivery systems (Nano-DDS); (B) one free anticancer drug encapsulated in prodrug NanoDDS containing the polymer conjugates of another drug; and (C) two different drugs conjugated to the same polymer.
due to the enhanced lipid solubility and interrupted planar structure [80]. Ansell et al. reported improved antitumor activity of paclitaxel prodrug-encapsulated Nano-DDS by manipulating the hydrophobicity of paclitaxel–lipid alcohol conjugates, and a range of frequently-used lipid alcohols were utilized as the ‘anchor components’ [92]. In addition, Zhigaltsev et al. synthesized a docetaxel prodrug by introducing ionized groups into the neutral docetaxel molecules, namely 20 -O-acyl conjugate of docetaxel with Nmethylpiperazinyl butanoic acid [75]. The weak-base docetaxel prodrug can be readily loaded into lipid nanocarriers by using pH gradient loading techniques to achieve high drug encapsulation [75]. Stimuli-responsive prodrug-based Nano-DDS Despite the rapid progress in Nano-DDS, high drug loading efficiency and smart stimuli-triggered drug release still remain the popular issues for pharmaceutical scientists. Many researchers endeavor to design stimuli-sensitive
Nano-DDS by applying various specific and sensitive linkers [93,94]. These stimuli applied in prodrug-based NanoDDS could be broadly classified into two groups (Table 4): the internal physiopathologic changes of the tumor microenvironment (e.g., pH, enzymes, reductive environment, etc.) and the external stimulus signals (e.g., light, heat, and magnetic field, etc.). Chemical linkers that are sensitive to internal stimuli include hydrazone bonds, acetal linkage, silyl ether bonds, disulfide bonds, and specific peptides sensitive to certain enzymes overexpressed in tumors. Similar to other stimuli-triggered nanocarriers, stimuliresponsive prodrug-based Nano-DDS have shown significant advantages in reducing side effects and enhancing antitumor activity due to tumor-specific drug release. In addition, stimuli-responsive nanosystems provide a promising strategy for the reversal of multidrug resistance (MDR). The ability of nanocarriers to overcome MDR is expected to function as follows [95]: (i) the Nano-DDS should promote the endosomal escape of anticancer drugs; (ii) anticancer drugs should be released into the cytoplasm quickly and accurately after being internalized; and (iii) the capacity to bypass or inhibit drug efflux transporters (e.g., P-gp) is necessary. For stimuli-responsive Nano-DDS, chemotherapeutic agents can be delivered and released in a temporal and spatial pattern, and promote the accumulation of anticancer drugs at target locations to reach the effective therapeutic concentration [95]. pH-sensitive prodrug Nano-DDS Among the common stimuli-triggered strategies, acidresponsiveness is the most frequently used approach in recent years [97]. pH-responsive polymer conjugates endow prodrug Nano-DDS with acid-triggered drug release, which can covalently anchor anticancer drugs at normal physiological pH in systemic circulation, and release
Table 4. List of the common stimuli-sensitive linkers utilized in the chemical synthesis of anticancer drug conjugates Types pH-sensitive
Sensitive linkers or triggers Acetal bond Hydrazone linkage Hydrazone linkage Hydrazone linkage Hydrazone linkage Hydrazone linkage Hydrazone linkage
Redox-responsive
Enzyme-sensitive Light-sensitive
Heat-sensitive Magnetism-sensitive
Hydrazone linkage Silyl ether bond Disulfide bond Disulfide bond Disulfide bond Disulfide bond Thioether linker Octapeptide (GPLGIAGQ) Peptide (GFLG) Near infrared reflection UVA Chimeric polypeptide with Tt value range of 37–42 8C Magnetite nanocrystals (USPIO)
Drug conjugates PEG-b-poly(acrylic acid) block copolymers–paclitaxel Dendronized heparin–doxorubicin mPEGylated peptide dendron–doxorubicin Pullulan–doxorubicin Poly(styrene-co-maleic anhydride)-adipic dihydrazide–doxorubicin (SMA-AD–DOX) Bi(PEG-PLA)–Pt(IV) Hyperbranched polyglycerol-hydrazone bond–doxorubicin (hPG-hydrazone bond–DOX) PEG–b-p(HEMA-co-GMA-DOX) Alkyl silyl ether docetaxel prodrug PEG–block-dendritic polylysine–camptothecin (PEG-SS–xCPT) Peptide sequence derived from the Tau protein–camptothecin (Tau-SS–CPT) PEG-b-poly(propylene sulfide)–tioguanine (PEG-PPS-SS–TG) camptothecin-polyethylene glycol–camptothecin (CPT-SS-PEG-SS–CPT) OEG-2S–SN38 PEG–GPLGIAGQ–paclitaxel Triblock HPMA copolymer-GFLG–doxorubicin trans,trans,trans-[Pt(N3)2(NH3)(py)(O2CCH2CH2COOH)2] mPEG–block-poly(e-caprolactone)-poly(L-lysine)–platinum (mPEG–b-PCL-b-PLL–Pt) Chimeric polypeptide–paclitaxel (CP–PTX) Magnetic nanomedicine (USPIO/squalene–gemcitabine)
Refs [32] [39] [25] [41] [48] [24] [47] [28] [76] [19] [59] [34] [15] [61] [17] [46] [90] [33] [96] [69]
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Review anticancer drugs at a sudden pH descent in tumor tissues (pH 6.5–7.0) or in the intracellular endosomes (pH 5.0–6.0) of tumor cells [98]. Acid-triggered drug release can be realized by the application of pH-sensitive biodegradable chemical linkers, mainly including acid-labile acetal linkage, hydrazone bonds, and silyl ether bonds. For example, an acetal-linked paclitaxel prodrug micellar nanosystem achieves rapid drug release under the acidic endosome/ lysosome condition inside tumor cells [32]. Paclitaxel was conjugated to PEG-b-poly(acrylic acid) (PEG–PAA) block copolymers using ethyl glycol vinyl ether (EGVE) as an acid-sensitive linker [32]. As a result, highly pH-dependent paclitaxel release was observed in in vitro release studies, with 86.9%, 66.4%, and 29.0% release from the prodrug Nano-DDS at pH 5.0, 6.0, and 7.4, respectively [32]. Reduction-responsive prodrug Nano-DDS Another stimulus used to achieve triggered drug release from prodrug Nano-DDS is the redox gradient existing between the mildly oxidizing extracellular spaces and the reducing intracellular cytoplasm (higher reduction level) [95]. A disulfide bond is the most common chemical linker utilized to bridge polymers and anticancer drugs for reduction-responsive drug release under higher intracellular glutathione (GSH) concentration in tumor cells [19,34]. For example, PEG-block–dendritic polylysineSS–CPT conjugates were recently synthesized, and the hydrophobicity of the linear-dendritic drug conjugates could be tailormade by the number of the conjugated hydrophobic CPT molecules, thereby determining their self-assembled nanostructures (nanospheres or nanorods) [19]. In vitro release experiments showed that none of these nanostructures released any CPT in phosphate buffer solution (PBS) for over 5 days, but CPT could be immediately released in the presence of dithiothreitol (DTT) (a strong reductant similar to GSH) [19]. Moreover, the nanorods of moderate lengths (
