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asian journal of pharmaceutical sciences 11 (2016) 337–348
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Review PEGylation in anti-cancer therapy: An overview
Prajna Mishra a, Bismita Nayak b, R.K. Dey a,*
a Centre for Applied Chemistry, Central University of Jharkhand, Ranchi 835 205, Jharkhand, India b Department of Life Science, National Institute of Technology, Rourkela 769 008, Odisha, India
ARTICLE INFO ABSTRACT
Article history: Advanced drug delivery systems using poly(ethylene glycol) (PEG) is an important devel- Received 1 July 2015 opment in anti-cancer therapy. PEGylation has the ability to enhance the retention time of Received in revised form 20 August the therapeutics like proteins, enzymes small molecular drugs, liposomes and nanoparticles 2015 by protecting them against various degrading mechanisms active inside a tissue or cell, which Accepted 26 August 2015 consequently improves their therapeutic potential. PEGylation effectively alters the phar- Available online 14 September 2015 macokinetics (PK) of a variety of drugs and dramatically improves the pharmaceutical values; recent development of which includes fabrication of stimuli-sensitive polymers/smart poly- Keywords: mers and polymeric micelles to cope of with the pathophysiological environment of targeted PEG site with less toxic effects and more effectiveness. This overview discusses PEGylation in- PEGylation volving proteins, enzymes, low molecular weight drugs, liposomes and nanoparticles that Targeted drug delivery has been developed, clinically tried for anti-cancer therapy during the last decade. Polymers © 2016 The Authors. Production and hosting by Elsevier B.V. on behalf of Shenyang Phar- maceutical University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
reliable method for its ability to conjugate with protein, 1. Introduction enzymes, nanoparticles, liposomes and low molecular weight drugs. In this regard, polyethylene glycol (PEG), a water- Cancer is one of the leading causes of death worldwide. Me- soluble and biocompatible polymer, is the most commonly used tastases are the primary cause of death from cancer. Cancer non-ionic polymer in the field of polymer-based drug deliv- cells proliferate at much faster rate than the normal cells. The ery [1]. Passive targeting with PEGs in combination with active available traditional cancer chemotherapy is not essentially se- targeting (entry into the tumor cell via ligand receptor, antigen lective as it depends on the kinetics of the cell growth. Targeted antibody interaction) delivery systems has been effectively em- cancer therapies are expected to be more effective and ben- ployed to achieve better therapeutic index of anti-cancer drugs. eficiary in comparison to available conventional treatment Further, with the introduction of stimuli-responsive chemi- procedures. cal moiety in PEGylated prodrugs, the sensitiveness of the drug The last few decades of research in the particular area are molecule toward the pathophysiological environment of tumor focused on exploring the treatment of cancer at its molecu- cells in vivo evolved as a new era of site-specific targeted drug lar level. This will be helpful in developing better therapeutics. delivery system. Hence this overview deals with the develop- Polymer therapeutics is establishing as an innovative and ment and recent advancement in various aspects of PEGylation
* Corresponding author. Centre for Applied Chemistry, Central University of Jharkhand, Ranchi 835 205, Jharkhand, India. Tel.: +91 8987727378; fax: +91-671-2306624. E-mail address: [email protected]; [email protected] (R.K. Dey). Peer review under responsibility of Shenyang Pharmaceutical University. http://dx.doi.org/10.1016/j.ajps.2015.08.011 1818-0876/© 2016 The Authors. Production and hosting by Elsevier B.V. on behalf of Shenyang Pharmaceutical University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 338 asian journal of pharmaceutical sciences 11 (2016) 337–348
in cancer treatment and its future prospective in a compre- Irreversibly conjugating PEGs had some adverse effects on hensive way. the specific biological activity of many therapeutics. Thus, to minimize the loss of activity, a reversible (or releasable prodrug) PEGylation concept has been formulated. Reversible PEGylation 2. PEGylation and its significance concept deals with attachment of drugs to PEG derivatives through cleavable linkages (Fig. 1). The release of drug occurs The technique of covalently attaching polyethylene glycol (PEG) by therapeutic agents through enzymatic, hydrolytic cleav- to a given molecule is known as “PEGylation” and is now a well- age or reduction in vivo at a predetermined kinetic rate over established method in the field of targeted drug delivery a time period [10]. systems. The general structure of monomethoxy PEG (mPEG) The objective of most PEG conjugation techniques aims at can be represented as CH3O–(CH2-CH2O)n–CH2-CH2–OH. At the increasing the circulation half-life without affecting activity. beginning of PEG chemistry, in the late1970s, Professor Frank It is to be noted that the distinct advancement in the PEG con- Davis and his colleagues had shown that the immunological jugation processes and diversity in the nature of the PEGs used properties as well as the stability of bovine serum albumin and for the conjugation has attributed to the increased demand for bovine liver catalase can be successfully altered by cova- PEGylated pharmaceutical products. lently linking them to methoxy PEG (mPEG) [2,3] using cyanuric PEGylation enhances the therapeutic efficacy of the drugs chloride as an activating agent. The process of PEGylation can by bringing in several advantageous modifications over the non- be extended to liposomes, peptides, carbohydrates, enzymes, PEGylated products. The systematic classification is illustrated antibody fragments, nucleotides, small organic molecules and in Fig. 2. Increase in the serum half-life of the conjugate is the even to different nanoparticle formulations [4–8]. mPEG is the major way of enhancing therapeutic potential of the PEGylated most useful unit for polypeptide modification [9].Nowseveral conjugate. PEGylation prolongs the circulation time of conju- derivatives of PEG molecules are available that vary in mo- gated therapeutics by increasing its hydrophilicity and reducing lecular weight and structure, such as linear, branched, PEG the rate of glomerular filtration [11,12]. Few factors such as pro- dendrimers and more recently multi-arm PEGS [9]. The first step tection from reticuloendothelial cells, proteolytic enzymes and of PEGylation is to activate PEG by conjugating a functional de- decreased formation of neutralizing antibodies against the rivative of PEG at one or both the terminals of PEG chain. protein by masking antigenic sites by formation of a protec- PEGylation conjugation techniques can be classified into two tive hydrophilic shield are the key components of PEG molecule categories: i) first-generation random PEGylation, and ii) second- that attributes to the improved pharmacokinetic profile (PK) generation site-specific PEGylation. Thanks to the second of the conjugates [13–16]. It has also been reported that generation PEGylation processes that resulted in well-defined PEGylation increases the absorption half-life of subcutane- conjugated products with improved product profiles over those ously administered agents and is associated with a decreased obtained through non-specific random conjugations. volume of distribution [11]. PEG is a non-biodegradable polymer
PEG SPACER/LINKER Drug PEG Linker Drug
(PEG-Prodrug)
Fig. 1 – PEG-prodrug.
Fig. 2 – Significance of PEGylated therapeutics. asian journal of pharmaceutical sciences 11 (2016) 337–348 339
that puts limits on its use. It has been shown that PEGs (up reasons, PEGylation is considered to be one of the best methods to molecular weight 20 kDa) is primarily excreted through the for passive targeting of anticancer therapeutics. renal system, whereas higher molecular weight PEG chains get The concept of active targeting of drugs is based on the idea eliminated by fecal excretion [17]. PEGylation proved to be the of conjugating drug molecules to targeting entities (antibod- most promising approach for increasing the serum half-life of ies, ligands, etc.) for specific interaction with the structures the conjugated therapeutics, which is related to enhance- present on the cell surface for targeted delivery of the anti- ment of efficacy of the conjugate. However, PEGylation imposes cancer agent [25,26]. The fate of the pro-drug is dictated by certain disadvantages on liposomes, especially for the deliv- the targeting molecule and the linker molecule present on the ery of genes and nucleic acids in anti-cancer therapy as its pro-drug. The targeting moiety essentially decides the type of surface hydrophilic shield reduces the cellular uptake and im- cancer cell for the act of therapeutics. Further, depending on proves the stability of the lipid envelope, and the process results the linker molecule, the drug gets entry into the tumor cell by in poor endosomal escape via membrane fusion and degra- either of the two ways: (i) receptor-mediated internalization dation of cargos in lysosomes [18,19]. Hence the use of PEG in of the whole pro-drug by endocytosis and subsequent degra- gene and nucleic acid delivery to cancer cell is referred to as dation by endosomal/lysosomal pathway (Fig. 4), or (ii) receptor- “PEG dilemma” [20]. The issue can be efficiently addressed by independent internalization of the drug into targeted cells after designing pH-sensitive and tumor-specific targeted PEGylated extracellular cleavage of the pro-drug (Fig. 5) [27]. PEGylated therapeutics [21–23]. pro-drugs can be efficiently conjugated to targeting moieties by different conjugation chemistry in order to achieve the goal 2.1. Role of PEGylation in passive and active targeting of of active targeting. The targeted delivery of the PEGylated drugs drugs at the desired site causes high bioavailability and low sys- temic toxicity. Passive targeting drug delivery technique, as illustrated in Fig. 3, mostly depends upon the concentration gradient between the intracellular and extracellular space, created due to high con- centration of the drug in the tumor area [24]. PEG conjugates 3. PEGylated proteins in anti-cancer therapy takes the advantage of enhanced permeation and retention (EPR) effect executed by the tumors and gets accumulated in PEGylation of proteins is a well-established method in the phar- the pathophysiological environment of tumor vessels through maceutical field, but the significance of PEGylated peptides and leaky vasculature and poor lymphatic drainage. However, this proteins for anti-cancer therapy has only been realized in the effect cannot be studied with low molecular weight drugs that last several years as more and more PEG conjugates make it freely extravagate causing systemic toxicity, and this is a size- to late-phase clinical trials. Enzymes, monoclonal antibodies dependent effect. PEGylation increases the solubility, size, and cytokines are the three major class of proteins used in an- molecular mass and serum stability of the drugs. For all these ticancer therapy or as adjuvant therapy (Table 1).
Fig.3–Aschematic illustration of passive targeting with acid-sensitive PEG-prodrugs that cleave in the extracellular space. 340 asian journal of pharmaceutical sciences 11 (2016) 337–348
Fig.4–Aschematic illustration of receptor mediated internalization and endosomal/lysosomal degradation of the pro-drug during active targeting.
Fig.5–Aschematic illustration of Active Targeting with acid sensitive PEG- prodrugs which cleaves in the extracellular space.
3.1. PEGylated monoclonal antibody fragment drawback associated with Fab’s antibody fragment is its short serum half-life as it lacks the Fc region of the antibody that In the field of anti-cancer therapy, monoclonal antibodies rep- limits its potential as a therapeutic agent. Hence, suitable resent the major class of protein therapeutics. Antibodies act PEGylation methods and PEGs are used to ensure minimal loss by binding to the specific antigens/cell surface receptors. This of the antibody-antigen/cell surface receptor interaction keeping task is taken care by the fragment antigen-binding (Fab’) region in view the enhancement of serum half-life. It has been re- on an antibody. Depending upon the receptor and the binding ported that the hinge region cysteine residues on immuno- site on the receptor against which the antibody is designed, globulin G (IgG antibody isotype) Fab’ antibody fragments can it can either activate cellular signaling pathways leading to tolerate attachment of one or two PEG moieties (up to a total apoptosis, cell growth arrest, or block the pathways leading to of 40 kDa molecular weight) with little effect on antigen binding cell growth that eventually causes tumor cell death (apopto- affinity. This process also enables significant increase in the sis). This event is illustrated pictorially in Fig. 6. The major half-life of the circulating plasma antibodies by reducing the asian journal of pharmaceutical sciences 11 (2016) 337–348 341
Table 1 – PEG protein/enzyme conjugates in clinical development as anticancer therapy. Trade name Conjugate Protein/Enzyme FDA approved date/clinical trial status and use
Sylatron™ PEG-interferon α-2b PEG-interferon α-2b March 9, 2011, approved as adjuvant therapy for resected stage III melanoma Pegasys® PEG-interferon α-2a Interferon α-2a Phase I for melanoma and phase II as for chronic myelogenous leukemia Neulasta® PEG-filgrastim G-CSF(granulocyte-colony 2002, to treat neutropenia during chemotherapy stimulating factor) Oncaspar® PEG-asparaginase Asparaginase February 1994, acute lymphoblastic leukemia, and in July 24, 2006 first-line treatment for acute lymphoblastic leukemia
Fig. 6 – (a) IgG full length antibody and (b) PEGylated IgG Fab’ antibody fragment–cell surface receptor interaction and its outcome.
glomerular filtration and lower immunogenicity than the parent chemotherapy protocols either to control or bring improve- IgG [28]. ments in patient conditions. These small secreted proteins The example of use of PEG-antibody fragment angiogen- belong to the immunotherapy category and mobilizes the body’s esis inhibitor (CDP791) is illustrated as follows: CDP791 immune system to fight cancer. The process is illustrated pic- PEGylated diFab antibody fragment antagonizes the effect of torially in Fig. 7. vascular endothelial growth factor receptor-2 (VEGFR-2), which is a prominent angiogenesis stimulatory molecule respon- 3.2.1. PEG-interferon-alpha conjugates sible for tumor progression. The short plasma half-life of the This process is illustrated as PEG-interferon-α2b (PEG-INTRON®/ unmodified CDP791 antibody fragment, which lacks Fc region, Sylatron™) and PEG-interferon-α2a (Pegasys®), which are has a low molecular weight and responsible for low therapeu- discussed as follows: tic index. PEGylation of the cysteine amino acid present at the C-terminus of the native antibody could be able to resolve this 3.2.1.1. PEG-interferon-α2b (PEG-INTRON®/Sylatron™). The issue by reducing its kidney clearance. This is demonstrated PEGylated version of interferon-α2b was synthesized by con- by the clinical studies for patients with colorectal, ovarian, renal jugating interferon-α2b, with a single chain 12 kDa PEG-SC via cancer or other tumors [29]. a urethane bond [30]. It displayed a half-life of 27–37 h with 10-fold lower clearance and minor change in the volume of dis- 3.2. PEGylated cytokines tribution in comparison to native form [31]. Based upon the outcome of clinical studies in the year 2011, the PEGylated drug Cytokines represent another class of protein therapeutics em- peginterferon alfa-2b (PEG-IFN) got FDA approval for adju- ployed mainly as adjuvant therapy in classical anti-cancer vant treatment of melanoma patients with microscopic or gross 342 asian journal of pharmaceutical sciences 11 (2016) 337–348
Fig. 7 – Representing PEGylated cytokines in anticancer therapy. nodal involvement following definitive surgical resection in- improving several of these products [43]. Many depleting cluding complete lymphadenectomy [32]. Sylatron™ is another enzymes are active against tumors. Enzymes’ intrinsic prop- brand name for peginterferon alfa-2b exclusively approved by erty of degrading amino acids is essential for cancer cells FDA for adjuvant therapy in cancer treatment. existence. The fate of the tumor cell is dictated by the differ- ent cellular pathways regulated by the substrate (amino acid) 3.2.1.2. PEG-interferon-α2a (Pegasys®). Another PEGylated in- to be degraded, and the process of degradation is illustrated terferon, Pegasys®, is prepared by mono-PEGylation of in Fig. 8. The normal cells are not affected because the normal interferon-α-2a with an N-hydroxysuccinimide (NHS) acti- cells can synthesize the amino acids for their growth. This situ- vated 40 kDa branched PEG molecule [33]. PEGylation prolonged ation is particularly the most advantageous aspect of using the serum half-life from 3.8 to 65 h, slowing down the clear- depleting enzymes in cancer therapy (Table 1). Therefore, during ance by more than 100-fold. This has also reduced the volume PEGylation procedure, a combination of these enzymes, low of distribution to fivefold with respect to the native interferon- molecular weight (5–10 kDa) PEGs and random amine conju- α2b [31]. PEGASYS® was efficient in improving the patient gation strategies are employed. compliance by enabling once-weekly dosing while maintain- ing acceptable safety, tolerability, and activity profiles in clinical studies [34]. Currently, PEGASYS® is under evaluation as ad- 3.3.1. PEG-arginine depleting enzymes juvant therapy for patients with intermediate and high-risk Arginine is a nonessential amino acid in humans. It has been melanomas [35]. reported that arginine deficiency inhibits tumor growth, an- giogenesis and nitric oxide synthesis [44]. Two types of arginine 3.2.2. PEG-granulocyte colony stimulating factor degrading enzymes, i.e. i) arginine deiminase (ADI) and ii) ar- (PEG-filgrastim) ginase (ARG), which can be utilized as antitumor agents, are PEG-filgrastim was synthesized by conjugating a linear 20 kDa discussed below: mPEG-aldehyde derivative to an N-terminal methionine residue of filgrastim through reductive alkylation under mild acidic con- 3.3.1.1. PEG-arginine deiminase. The PEGylation of arginine ditions [36,37]. It is to be noted that a single dose of PEG- deiminase proved to be a better therapeutic approach for filgrastim per chemotherapy cycle could be able to reduce the anticancer treatment. Among the several PEGylated ADI for- risk of febrile neutropenia significantly with respect to the native mulations the ADI-PEG20000, formulated by conjugating protein (11% vs. 19%) [38–40]. Currently, PEGylated-G-CSF 10–12 chains of 20 kDa PEG with ADI by using the succinimidyl (Pegfilgastrim, Neulasta®) is used as an adjuvant therapy for succinate linker, is proved to be the acceptable one from patients with non-myeloid malignancies receiving myelo- in vivo study results [45]. Clinical studies have shown better ef- suppressive chemotherapy (bone marrow suppression as a side ficacy of ADI PEG 200,000 in terms of antitumor activity and effect of chemotherapy) associated with a 20% risk of febrile tolerability [46,47]. Currently, ADI PEG 200,000 versus placebo neutropenia [41,42]. is under phase III clinical trial for advanced hepato- cellular carcinoma. Further, Phase II for acute myeloid 3.3. PEGylated enzymes in anti-cancer therapy leukemia/non-Hodgkin’s lymphoma and Phase I (for meta- static melanoma in combination with cis-platin; for solid Therapeutic enzymes represent a growing class of bio- tumors in combination with docetaxel) are under trial pharmaceuticals, and PEGylation has played a major role in [48]. asian journal of pharmaceutical sciences 11 (2016) 337–348 343
Fig. 8 – Representing PEGylated depleting enzymes in anticancer therapy.
3.3.1.2. PEG-arginase. The depleting enzyme arginase is an en- proved to be a better treatment option for patients who were dogenous protein expressed in humans. The conjugate, PEG- allergic to the native form of the drug. The U.S. Food and Drug rhArg, has 10 to 12 polymer chains of PEG 5000 per protein Administration granted approval to pegaspargase (Oncaspar, molecule that is covalently attached via a succinamide pro- Enzon Pharmaceuticals, Inc) in July 2006 for the first-line treat- pionic acid (SPA) linker. This conjugate remains in fully active ment of patients with acute lymphoblastic leukemia (ALL) as condition [49]. The PEGylated form executes sufficient cata- a component of a multi-agent chemotherapy regimen [51]. lytic activity at physiological pH with a prolonged plasma half- life of 3 days in comparison to the native form, which has a half-life of several minutes only. Currently, this conjugate is 4. PEGYLATED low molecular weight under phase I/II clinical trials. anticancer drugs
3.3.2. PEG-asparagine depleting enzyme (PEG-L-asparaginase) Various PEGylated low molecular weight anti-cancer drugs are Depletion of asparagine eventually results in leukemic cell currently under development. For example, topoisomerase I in- death. Leukemic cells lack the enzyme asparagine synthe- hibitor camptothecin-based drugs (irinotecan, topotecan, SN38, tase, an enzyme required for asparagine synthesis, and depend exetecan, etc.) is reported to be useful in the treatment of many on the exogenous supply of asparagine for their growth and solid tumors. However, the hydrophobicity of such material survival. Therefore, asparaginase, the depleting enzyme for as- limits their therapeutic efficacy. Few of the examples, listed in paragine, plays a critical role as a therapeutic enzyme in treating Table 2, are discussed below: acute lymphoblastic leukemia (ALL). Oncaspar is a modified form of the enzyme L-asparaginase approved by FDA in 1994. 4.1. PEG-SN38 (EZN-2208) Oncaspar consists of tetrameric enzyme L-asparaginase derived from E. coli, and it is covalently conjugated with approxi- EZN-2208, the product Enzon Pharmaceuticals, Inc, is a mately 69–82 molecules of monomethoxy polyethylene glycol PEGylated SN38 (10-hydroxy-7-ethyl-camptothecin (a deriva- (mpeg), each having molecular weight of 5 kDa [50]. Oncaspar tive of camptothecin)). SN38 is the active moiety of CPT-11
Table 2 – PEGylated low molecular weight drugs/liposomal derivatives/thermo-sensitive conjugates and nanoparticles in clinical development as anticancer therapy. Trade name Conjugate Parent drug FDA approved date/clinical trial status
EZN-2208 PEG-SN38 SN38 (camptothecin Phase II, various cancer derivative) NKTR-102 PEG-irinotecan Irinotecan Phase III, metastatic or locally recurrent breast cancer and Phase II, Irinotecan solid tumor malignancies, including ovarian, colorectal, glioma, small cell and non-small cell lung cancers Doxil (Caelyx) PEG-liposomal doxorubicin Doxorubicin November 1995 for ovarian/breast cancer and Kaposi’s sarcoma ThermoDox PEG-thermosensitive Doxorubicin Phase III, hepatocellular carcinoma liposomal doxorubicin CALLA 01 Pegylated cyclodextrin SiRNA In clinical phase I for solid tumors nanoparticle 344 asian journal of pharmaceutical sciences 11 (2016) 337–348
(Camptosar®, irinotecan) and reported to be a potent 5.1. PEGylated liposomes in anticancer therapy topoisomerase I inhibitor. In this PEGylated product, the 20- OH group of SN38 was selectively coupled with a 4 arm PEG Liposomes are spherical, self-closed structures formed by one of 40 kDa through a glycine spacer to preserve the E ring of or more concentric lipid bilayers with an encapsulated aqueous SN38 in the active lactone form while leaving the drug 10-OH phase in the center and between the bilayers composed of free [52]. PEGylation was able to enhance the solubility of SN38 natural or synthetic lipids [65]. The development of long- by about 1000-fold. In fact, EZN-2208 showed a 207-fold higher circulating liposomes with inclusion of the synthetic polymer exposure to SN38 compared to irinotecan in treated mice [53]. poly-(ethylene glycol) (PEG) in liposome composition could be The conjugate showed promising antitumor activity both in vitro able to solve the issue of low serum half-life associated with and in vivo. However, following phase II trial, Enzon Pharma- liposomes. PEG can be incorporated on the liposomal surface ceuticals, Inc. announced the discontinuance of its EZN-2208 in a number of ways. However, anchoring the polymer in clinical program [54]. the liposomal membrane via a cross-linked lipid, PEG- distearoylphosphatidylethanolamine [DSPE], is reported to be the most widely accepted method [66,67]. Preclinical studies 4.2. PEG-irinotecan (NKTR-102, Etirinotecan pegol) with PEGylated liposomes reported that the cytotoxic agents entrapped in PEGylated liposomes tend to accumulate in tumors NKTR-102 was developed by Nektar Therapeutics as a PEGylated [68]. However, recent preclinical studies of anti-cancer drug en- formulation for the treatment of colorectal cancer and other closed in PEGylated liposomes in rodents and dogs have shown solid tumors using the architecture of new multi-arm PEGs. It the rapid blood clearance of the pegylated drug carrier system is the first long-acting topoisomerase I inhibitor. It is synthe- due to the increased anti-PEG-IgM production [69,70].An sized by covalently conjugating irinotecan to a four-arm PEG example of PEGylated liposomal formulations, PEGylated li- [55]. SN38 (10-hydroxy-7-ethyl-camptothecin, a derivative of posomal doxorubicin (PLD), and most extensively studied, is camptothecin) is the active metabolite of NKTR-102. It is re- discussed below: ported in both Phase I and II studies for NKTR-102 that sustained exposure of the active drug was associated with 5.1.1. Doxil (PEGylated liposomal doxorubicin) promising anti-tumor activity [56–58]. Currently NKTR-102 is DOXIL is the trade name for PEGylated liposomal doxorubi- in Phase III clinical trial for patients with metastatic or locally cin formulated to achieve better drug efficacy for cancer recurrent breast cancer and Phase II clinical trial for patients chemotherapy.This product contains doxorubicin (Adriamycin) with solid tumor malignancies that include ovarian, colorectal, enclosed in an 80–90 nm size uni-lamellar liposome coated with small cell and non-small cell lung cancers [59]. PEG. The modification increases the circulatory half-life of the drug leading to its enhanced bioavailability at the tumor site [71,72]. PEGylated liposomal doxorubicin has fewer side effects on healthy cells than regular doxorubicin [73,74]. PLD has im- proved pharmacokinetic features, such as long circulation time 5. PEGylated nanoparticles in anti-cancer of about 60–90 h for doses in the range of 35–70 mg/m2 in pa- therapy tients with solid tumors. After PLD administration, nearly 100% of the drug in the plasma remains in the encapsulated form. Nanoparticles (NPs) are synthetic materials with dimensions Moreover, in comparison to free doxorubicin PLD, plasma clear- from 1 to 1000 nano-meters. NPs have large payloads, stabil- ance is dramatically slower and its volume of distribution ity and the capacity for multiple, simultaneous applications remains very small, which is roughly equivalent to the intra- due to their unique size and high surface area:volume ratio vascular volume [75–77]. After obtaining approval from FDA, [60]. Despite these advantages, the major drawbacks associ- PEGylated liposomal doxorubicin (PLD) (DOXIL/Caelyx) is cur- ated with NP drug delivery system for clinical studies are rently used to treat Kaposi’s sarcoma and recurrent ovarian associated with short circulating half-life due to uptake by the cancer (Table 2). reticuloendothelial system (RES) for larger NPs, whereas smaller NPs are subjects to tissue extravazations and renal clearance [61]. Liposomes, solid lipids nanoparticles, dendrimers, poly- mers, silicon or carbon materials, and gold and magnetic 6. PEGylated smart polymers in anticancer nanoparticles are examples of nano-carriers that have been therapy studied as drug delivery systems in cancer therapy. There- fore, surface modification of the nanoparticles with PEGs of Smart polymers are defined as polymers that undergo revers- various chain length, shape, density, molecular weight and in- ible large, physical or chemical changes in response to small corporation of different targeting moieties (ligands, antibodies, external changes in the environmental conditions, such as tem- etc.) is emerging as a more promising and technologically ad- perature, pH, light, magnetic or electric field, ionic factors, vanced drug delivery system in anti-cancer therapy [62–64]. biological molecules, etc. Smart polymers show promising ap- There are currently more than 35 US FDA-approved PEGylated plications in the biomedical field as delivery systems of NPs, with a larger number in preclinical studies for both imaging therapeutic agents [78]. Among various smart polymers cur- and therapy [8]. Among several PEGylated nanoparticle for- rently in use in biomedical field of research, the temperature mulations for anticancer therapy, liposomes have been most sensitive systems are the most studied systems. The greater extensively studied. therapeutic index of the targeted drug delivery systems can asian journal of pharmaceutical sciences 11 (2016) 337–348 345
be achieved by adjusting the transition temperature (Tt)ofther- Molecular therapy using small interfering RNA (siRNA) has mally responsive polymers, i.e., between body temperature shown great therapeutic potential for tumors and other dis- (37 °C) and the temperature approved for mild clinical hyper- eases caused by abnormal gene over-expression or mutation. thermia (42 °C) [79]. Within the temperature ranges, these It is a highly specific process for gene silencing. However, naked polymers facilitate tissue accumulation by localizing the ag- molecules of siRNA are vulnerable to premature renal clear- gregation of systemically delivered carriers to the heated tumor ance and nuclease degradation. The negative charge and volume [80,81]. For example, ThermoDox, a temperature- hydrophilicity of siRNA also limit its permeability through cel- sensitive doxorubicin-loaded PEGylated liposome (DPPC), lular and endolysosomal membranes. Therefore, in order to releases encapsulated doxorubicin at elevated tissue tempera- overcome these issues, siRNA requires a carrier system for ef- ture. DPPC has a transition temperature of 41.5 °C, which makes fective delivery [97]. Modification of drug delivery systems with it suitable for temperature-sensitive technology [82]. The tem- PEGs of suitable chain length, molecular weight and percent perature can be achieved by radiofrequency ablation technique. composition was proven to be efficient in overcoming intra- For ThermoDox, the concentration of the drug is up to 25 times cellular and systemic siRNA delivery barriers [98]. CALLA 01, more in the treatment area than IV doxorubicin, and several a nanoparticle formulation (Calando Pharmaceuticals) formu- fold the concentration of other liposomally encapsulated doxo- lated by using cyclodextrin nanoparticles conjugated to rubicins [82,83]. Currently, it is under phase III clinical trial for transferrin and coated with PEG, is the first one to enter under hepatocellular carcinoma (Table 2). phase I clinical trials for solid tumors [44,99], in addition to few more which are currently under development (Table 2).
7. PEGylated polymeric micelles in anti- cancer therapy 9. Conclusions
Polymeric micelles are colloidal dispersions prepared by block- PEGylation offers a great advantage for bioactive molecules in copolymers, consisting of hydrophilic and hydrophobic pharmaceutical and biological applications by way of reduc- monomer units. Self-assembling amphiphilic polymeric mi- ing protein immunogenicity and increased serum half-life of celles represents an efficient drug delivery system for poorly the drugs. This overview highlighted on the use of PEGylated soluble or insoluble drugs [84]. Different varieties of amphi- proteins, low molecular weight drugs and PEG micelles. philic polymeric micelles (i.e. diblock AB type, triblock ABA type PEGylation improves the therapeutic efficacy of a drug by or graft copolymers) can be designed by arranging the mono- passive targeting in a novel way. The process can also be com- meric units in different ways and orders [85,86]. The bined effectively with active targeting and stimuli-responsive hydrophobic block constitute the core and the hydrophilic block targeted therapies for the development of new methodolo- makes the corona of the micelles. The water-soluble PEG blocks gies for the treatment of cancer. It is important to note that with a molecular weight from 1 to 15 kDa are considered as the efficacy of PEGylated drugs depends on overall exposure the most suitable hydrophilic corona-forming blocks [87]. Various and its relationship to the pharmacodynamics of the drug. Mo- preclinical and clinical studies have shown the potential use lecular weight of PEG chain and its structural modifications of PEGylated polymeric micelles with different hydrophobic carries strategic importance for conjugation with drug mol- blocks such as PLGA poly(D,L-lactide-co-glycolide) [88–90], poly ecule for effective PEGylation process. The research in this aspartate [91], γ-benzyl-L-glutamate [92], polyglutamate (Pglu) direction shall be helpful in effective cancer treatment process [93], and poly(D,L-lactic acid) [94] in anti-cancer therapy. It is in near future. to be noted that five different PEGylated polymeric micellar for- mulations, as enlisted in Table 3, are currently under clinical trials for possible anticancer treatment [95,96]. Acknowledgements
The authors wish to acknowledge Dr. Rajakishore Mishra from 8. PEGylated nanoparticles for SiRNA delivery Central University Jharkhand, India, and Dr. Basabi Rana from in anticancer therapy Loyola University Chicago, USA, for their valuable suggestions. The grant in aid from DST, New Delhi (Project No. SR/S1/PC-24/ RNA interference is a natural phenomenon employed to se- 2009) is duly acknowledged. University fellowship to Prajna lectively turn off the genes expressed in some diseases. Mishra for pursuing her research work is acknowledged.
Table 3 – PEGylated polymeric micelles in clinical development as anticancer therapy. Trade name for polymeric micelle Block copolymer Parent drug Clinical trial status
NK105 PEG-P(aspartate) Paclitaxel Paclitaxel Phase II, advanced stomach cancer NK012 PEG-PGlu(SN-38) SN-38 Phase II, breast cancer NC-6300 PEG-P(aspartate) Epirubicin Epirubicin Phase I, breast cancer, stomach cancer, lymphoma NC-6004 PEG-PGlu(cisplatin) Cisplatin Phase I/II, solid tumors Genexol-PM PEG-P(D,L-lactide) Paclitaxel Paclitaxel Phase IV, breast cancer Phase II, pancreatic cancer 346 asian journal of pharmaceutical sciences 11 (2016) 337–348
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