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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5615_Библиотеки_им_академика_М_И_Перельмана.pdf
PEGylated Nanocarriers
forSolubilization
RandaS.H.Mansour, RakeshKumarTekade,
andAla’AdnanAli
Abstract
This chapter is dedicated to explaining the principle of PEGylation in the solubiliza-
tion of nanocarriers for pharmaceutical and biomedical applications, in addition to
the enhancement of the propensity of these carriers to solubilize drugs. The chapter
introduces the general rationale for nanocarrier PEGylation with a specic focus on
the topic of solubilization in addition to the solubility characteristics of
PEG.PEGylated small molecular weight drugs and proteins are briey introduced to
emphasize the stages of pharmaceutical solubilization by PEGylation. Furthermore,
protein PEGylation as a means to solubilize enzymes and proteins for facilitated
nanoparticle production is highlighted. Proposed mechanisms and factors affecting
the drug solubilization propensity of PEGylated nanocarriers are also explored.
Illustrative examples and reviewed applications are included wherever appropriate.
5
Keywords
PEGylated nanocarriers · Aqueous solubility · Organic solvents · Nanocarrier
solubilization · Drug solubilization · PEGylation extent · PEG chain length
R. S. H. Mansour (*)
Faculty of Pharmacy, Zarqa University-Jordan, Zarqa, Jordan
e-mail: rmansour@zu.edu.jo
R. K. Tekade
National Institute of Pharmaceutical Education and Research (NIPER) Ahmedabad, An
Institute of National Importance, Government of India, Department of Pharmaceuticals,
Ministry of Chemicals and Fertilizers, Palaj, Opp. Air force station, Ahmedabad,
Gujarat, India
A. A. Ali
Faculty of Pharmacy, Zarqa University-Jordan, Zarqa, Jordan
Faculty of Pharmacy, University of Jordan, Amman, Jordan
137
138
R. S. H. Mansour et al.
Abbreviations
C Carbon
CAC Critical aggregation concentration
FDA Food and Drug Administration
mPEG Methyl PEG
PAMAM Poly-amidoamine
PEG Polyethylene glycol
PEG-b-PAMA Poly(ethylene glycol)-block-poly((2-N,N-dimethylamino)ethyl
methacrylate)
PEG-PLGA Poly(ethylene glycol)-poly(lactic-co-glycolic acid)
Si Silicon
5.1 Introduction
Polyethylene glycol (PEG), commercially known as Macrogol, is a polyether composed of repeated ethylene glycol units [–(CH2CH2O)n] (D’Souza and Shegokar
2016). It is synthesized via anionic polymerization of ethylene oxide and a hydroxyl
initiator such as water, ethylene glycol, or any diols. Alternatively, it can be obtained
by ring-opening polymerization of epoxyethane. Commercially, PEG is available
with different degrees of polymerization and activated functional groups yielding
variable derivatives that are characterized by their molecular weights ranging from
hundreds to several thousands of Daltons, typically from 200 to 6000 (Abuchowski
etal. 1977; Thomas etal. 2014; Chen etal. 2005). The numerical designation of
PEGs generally indicates their average molecular weight.
PEG is inert, nonimmunogenic, nonantigenic, and biocompatible with a wellestablished human safety prole; additionally, it has excellent hydration capacity
and water solubility. It is approved by the US-FDA for versatile pharmaceutical
applications and classied as generally regarded as a safe (GRAS) polymer
(D’Souza and Shegokar 2016; Bhadra etal. 2002; Mohapatra etal. 2019; Luong
etal. 2016; Suk et al. 2016; Knop etal. 2010). Moreover, PEG is an uncharged,
amphiphilic, exible polymer (Sanchez Armengol etal. 2022; Kulhari etal. 2011).
These properties make it suitable for an extensive range of drug-delivery applications and technologies. For example, it has been used in dermatological products,
tablets, soft gelatin capsules, suppositories, and parenterals (D’Souza and Shegokar
2016). The use of PEG was later extended to conjugate (PEGylate) drugs and pro-
teins (Abuchowski etal. 1977), and this concept was then applied to different types
of nanocarriers, as PEG imparts steric stabilization of colloids with the added
advantage of escaping the immune defense mechanisms of the host as a result of
low immunogenicity (Sanchez Armengol etal. 2022).
5 PEGylated Nanocarriers forSolubilization
Fig. 5.1 Examples of marketed PEGylated bioactives
Fig. 5.2 Some PEGelated polymers for drug delivery. * Yin etal. (2009), ** Tamura etal. (2009),
***
Yang etal. (2007a)
139
5.1.1 Rational forPEGylation ofDrugs/Drugs Nanocarriers
PEG can be used to PEGylate a wide range of bioactive and their carriers, including
small molecular weight drug molecules, peptides and proteins, micro- and nanoparticles (Otsuka etal. 2003; Wattendorf and Merkle 2008; Karakoti etal. 2011; Jokerst
etal. 2011), polymers (Joralemon et al. 2010; Freichels etal. 2011), oligonucleotides (Ravelli etal. 2012), in addition to nanocarriers (Howard etal. 2008). Some
PEGylated bioactives currently marketed (González etal. 2011) are illustrated in
Fig.5.1, whereas Fig.5.2 shows some PEGylated biodegradable polymers that have
been investigated as encapsulating materials for several drugs (Elbert and Hubbell
1996). Concerning nanocarriers, PEGylation proves to be the most successful
method of surface modication (Vllasaliu et al. 2014) to provide enhanced
properties.
PEGylation imparts desirable pharmacokinetic and pharmacodynamics properties to drugs and drug carriers (Sanchez Armengol etal. 2022; Ravelli etal. 2012;
Mirkin and Taton 2000; Hussain etal. 2019; Owens and Peppas 2006; Suk etal.
2016; Karra and Benita 2012; Elbert and Hubbell 1996; Awasthi etal. 2004; Ho and
Gibaldi 2013; D’Souza and Shegokar 2016; Roberts et al. 2002; Vllasaliu et al.
2014; Sweet etal. 2009; Fung etal. 1997; Mozar and Chowdhury 2018), eventually
enhancing pharmaceutical and therapeutic performance, particularly in cancer diagnosis and therapy as shown in Fig.5.3. While this chapter focuses on the solubilization effect exerted by PEG on the PEGylated species, other chapters are devoted to
discussing other results.
140
Fig. 5.3 Enhanced
pharmacodynamic and
pharmacokinetic effects of
PEGylated drugs,
nanocarriers, and drugs
loaded into nanocarriers
R. S. H. Mansour et al.
5.1.2 PEG Solubility Characteristics
PEG is readily soluble in aqueous media. Due to its hydrophilicity, it has high
water binding capacity as each ethylene oxide unit can bind two to three water
molecules, thereby showing a high hydrodynamic volume (Roberts etal. 2002).
This pattern of interaction is unique between PEG and the aqueous environment,
unlike the other closely related polymers such as poly(propylene oxide),
poly(methylene oxide) (Israelachvili 1997), and isomeric polyacetaldehyde
(Harris 1992) as these polymers are considered hydrophobic and water-insoluble
(Harris 1992). The Hildebrand solubility parameter of PEG is 10.3 (calcm−3)
(Bailey 1990).
While all grades of PEG are soluble in water (handbook of ph ex), the extent of
solubility is actually dependent on the chain length (Table5.1). It is observed that
the solubility decreases with increasing molecular weight of PEG (Sanchez
Armengol etal. 2022). PEGs with molecular weight lower than 800Da are liquid at
room temperature and are highly soluble in water, whereas PEGs with molecular
weights 1000–2000 are soft waxy substances. PEGs with molecular weight higher
than 2000 occur as hard crystalline solids with melting points around 63°C (Otsuka
etal. 2003; Chen etal. 2005). Aqueous solutions of high molecular weight grades
may form gels (Aranda-Lara etal. 2021).
O.5
5 PEGylated Nanocarriers forSolubilization
Table 5.1 Aqueous solubility and state of variable PEG grades
PEG grade
PEG 200 Clear liquid Highly soluble
PEG 300 Clear liquid Highly soluble
PEG 400 Clear liquid Highly soluble
PEG 600 Clear liquid Highly soluble
PEG 1000 Semi-cystalline solid 75
PEG 4000 Semi-cystalline solid 55
PEG 10000 Semi-cystalline solid 53
PEG 35000 Semi-cystalline solid 50
a
Gullapalli and Mazzitelli (2015)
b
Wiley-VCH (2012), with permission
State at room temperature
a
Aqueous solubility (g/100g)
141
b
On the other hand, PEG has high solubility in most organic and inorganic solvents (Bhadra etal. 2005; Jain etal. 1996). Liquid PEGs are soluble in acetone,
alcohols, benzene, glycerin, and glycols, whereas the solid ones are soluble in acetone, dichloromethane, ethanol (95%), and methanol, and slightly soluble in aliphatic hydrocarbons and ether, but insoluble in fats, xed oils, and mineral oil
(Aranda-Lara etal. 2021).
5.1.3 PEG asSolubilizer
PEG is known to act as a solubilizer as it enhances the water solubility of hydrophobic moieties. This effect is mediated through different mechanisms depending on
the employed solubilization system.
The high solubility of PEG in both aqueous and organic media gives it the ability
to form a monolayer at interfaces, a typical property of amphiphilic molecules, and
this property is affected by the molecular weight of PEG rather than its concentration (Bhadra etal. 2003; Pasut and Veronese 2012). PEG can decrease the contact
angle of substances. Coating of the gold plate with PEG resulted in a 50° decrease
in its contact angle. Similarly, the PEG-coated glass plate exhibited a 34° reduction.
Thus, PEG can signicantly decrease the contact angle of hydrophobic materials
(Bhadra et al. 2003). These characteristics of PEG make it able to increase the
hydrophilicity of hydrophobic drugs (Pasut and Veronese 2012) and decrease the
polarity of the aqueous solvent, i.e., act as a co-solvent (Nayak and Panigrahi 2012),
thus permitting the aqueous solubilization of hydrophobic drugs.
Upon the use of PEG in preparation for solid dispersions, the solubilization of
the drug is aided by permitting the drug to be trapped in the interstitial spaces of
PEG when the molten PEG is solidied (Chiou and Riegelman 1971) or the formation of metastable crystalline polymorphs of the drug (Pasut and Veronese 2012;
Martinez-Oharriz etal. 1999).
Solubilization by the formation of polymeric or copolymeric PEG micelles also
occurs when macromolecular PEG polymers or copolymers are added above their
corresponding critical aggregation concentration (CAC), as they readily
142
self- assemble to form micellar-like, nanosized, thermodynamically stable systems
with core–shell structures (Hagan etal. 1995; Kazunori etal. 1993; Otsuka etal.
2003). The subsequent effect will facilitate the solubilization of hydrophobic moi-
eties such as drugs, proteins, peptides, and genes (Muralidharan etal. 2014). These
micellar systems can increase the solubilization of hydrophobic drugs (Otsuka etal.
2003). In the case of PEG copolymers, the PEG shell ensures stabilization in an
aqueous medium (Savić etal. 2006). Unlike these systems, surfactant micelles tend
to precipitate the drug upon dilution by the blood, as the surfactant concentration
will drop below its critical micelle concentration.
R. S. H. Mansour et al.
5.1.4 PEGylation ofDrugs/Drugs Nanocarriers
forSolubility Enhancement
Good water solubility is a prerequisite for the therapeutic utility of bioactiveness.
PEG is the most commonly employed polymer to conjugate moieties targeting their
aqueous solubility enhancement within this context. PEGylation is considered a
cornerstone in increasing the aqueous solubility of small molecular weight drugs,
proteins, and nanocarriers.
5.2 Water-Soluble PEGylated Small Molecule Drugs
In this section, some examples of small molecule drug PEGylation for the sake of
drug solubility enhancement are illustrated.
PEGylation of taxol at its 7-OH functionality via a urethane or carbonate linkage produced highly water-soluble taxol derivatives. The solubility of PEG 350
and PEG 750 derivatives was 1.87×10−3 mmol/mL, which is much higher than
the native taxol with a solubility value of 3×102nM.The higher molecular weight
PEG derivatives (2000 and 5000) resulted in a superior solubility of (0.1mmol/
mL), which is approximately 30,000 times that of native taxol (Greenwald
etal. 1995).
The aqueous solubility of the poorly water-soluble paclitaxel was also
enhanced by PEGylation. The estimated solubility of it is 10–20μM. In one
study, esterication with PEG 5000 increased the solubility to 120mg/mL (Lee
etal. 2005). Another study showed that the oral bioavailability of PEGylated
paclitaxel in rats was 3.94- fold higher than that of non-PEGylated paclitaxel
(Choi and Jo 2004).
PEGylation of curcumin was attempted to increase its dissolution rate and solubility. The poor water solubility of curcumin limits its use for treating corneal bacterial infections. For increasing solubility, curcumin was conjugated to PEG 6000.
The solubility of PEGylated curcumin was increased up to 93±3.2% compared to
curcumin (Hanif etal. 2022).
5 PEGylated Nanocarriers forSolubilization
143
5.3 Soluble PEGylated Proteins/Enzymes
It is well known that the attachment of PEG can be utilized to improve water solubility, but it could also enhance the solubility of moieties in organic solvents. Both
effects are helpful for the enhancement of the solubility of proteins/enzymes in both
types of solvents. The latter effect is of particular importance in the enhancement of
enzyme activity in dry organic solvents (Harris 1992). In 1985, Veronese and coworkers were the rst to introduce a surface modication of proteins by PEG
(Veronese etal. 1985). PEGylation of enzymes used in organic solvents was demonstrated in 1986 (Inada etal. 1986).
5.3.1 PEGylated Proteins Soluble Under
Physiological Conditions
Many proteins can be used as therapeutic agents; nevertheless, the utility of some of
these is limited by their poor water solubility. A classic example of PEGylated
water-soluble proteins is the uricase enzyme. Humans lack the enzyme uricase, or
urate oxidase, which can be used to treat gout. Nevertheless, there are some challenges for the practical use of this enzyme, including its insolubility in water. The
rst U.S. patent claiming “water-soluble nonimmunogenic polypeptide compositions” was issued in 1979. It described a PEGylated uricase (Davis etal. 1979). A
water-soluble PEGylated recombinant mammalian uricase possessing sufcient
solubility, in addition to other desirable properties, was developed. The enzyme
PEGylated with PEG-10000 was soluble under physiological conditions to enable
good bioavailability, unlike the native enzyme, which was soluble only at high pH
values (Williams etal. 2003; Sherman etal. 2004).
5.3.2 Organic Solvent–Soluble PEGylated Proteins/Enzymes
In industrial biotechnology, organic solvents are frequently employed in enzymatic
reactions as an alternative to aqueous media. This approach offers valuable advantages
such as enhanced solubilization of hydrophobic substrates, shifting the thermodynamic
equilibrium to favor synthesis over hydrolysis, and suppressing water-dependent side
reactions (Carrea and Riva 2000). The main obstacles to the employment of this
approach are the low stability, solubility, and catalytic activity of the enzymes in organic
media as they tend to denature and precipitate (Stepankova etal. 2013).
Modied enzymes were demonstrated to dissolve in organic solvents (Inada
etal. 1986); consequently, PEGylated enzymes were prepared and exhibited solubility, stability, and catalytic activity in organic media due to both the hydrophilic
and hydrophobic properties of PEGs (Stepankova etal. 2013; Castillo etal. 2006,
2008; Kwon etal. 1999; Hernaiz etal. 1997; Castellanos etal. 2005).
144
Fig. 5.4 Formation of the protein nanoparticles mediated by enhanced solubility of the protein in
organic solvents through PEGylation [(Radi etal. 2016) with permission]
R. S. H. Mansour et al.
A study demonstrated the solubilization of lysozyme in organic solvents by
PEGylation with methyl PEG (mPEG) derivatives. This modication leads to an
increase in the solubility of lysozyme in organic solvents such as dichloromethane
while preserving its native structure. Subsequently, the enhanced solubility in
organic solvents permitted the employment of an emulsion-based solvent evaporation method to form enzyme nanoparticles (Fig.5.4) (Radi etal. 2016).
5.4 Water-Soluble PEGylated Drug Nanocarriers
Due to many favorable characteristics (Md etal. 2019; Ndlovu etal. 2019; Gorain
etal. 2018; Hussain etal. 2017a, b; Tran etal. 2017; Albanese etal. 2012), nanocarriers offer various benets (Gao etal. 2018; Khan etal. 2018; Choudhury et al.
2018; Hussain etal. 2018; Howard etal. 2008; Xia etal. 2013; Shi etal. 2017) in
cancer diagnosis and therapy. Despite their paramount pharmaceutical and therapeutic applications, the signicance of nanocarriers is limited due to their short
plasma half-lives, low solubility, and low biocompatibility, among other reasons.
Surface modication of nanocarriers is usually required to enhance their properties,
and PEG has been proven to have a tremendous benecial inuence on the characteristics of nanocarriers (Vllasaliu etal. 2014; Karra and Benita 2012), including the
enhancement of their solubility and that of the drug they carry (Fig.5.3). Indeed,
among all the polymers tested for surface modication of nanocarriers, PEG and
PEG-copolymers are currently the most ideal, widely recognized, popular, and
effective (Ravelli etal. 2012; Joralemon et al. 2010; Howard etal. 2008; Bhadra
etal. 2002).
5 PEGylated Nanocarriers forSolubilization
145
5.4.1 Water-Soluble PEGylated Silicon Nanocarriers
The uorescent properties of silicon (Si) nanoparticles are interesting for sensing
and tagging applications (Erogbogbo etal. 2008). They have potential use in cellular and assay labeling techniques, as well as in deep-tissue imaging (Alivisatos
2004; Bruchez etal. 1998; Michalet etal. 2005; Pinaud etal. 2006), in addition to a
diverse range of materials and cell culture applications (Sudeep etal. 2008). The
solubility of Si nanoparticles in water is crucial for utilizing them in biological
applications. Surface modication of Si nanoparticles with water-soluble functionalities has been attempted with amines (Alivisatos 2004; Bruchez et al. 1998;
Michalet etal. 2005; Pinaud etal. 2006; Rosso-Vasic etal. 2008; Tilley etal. 2005)
carboxylic acids (He etal. 2009), and poly(acrylic acid) (Li and Ruckenstein 2004).
PEGylation of Si nanoparticles has also been described to render them soluble in
aqueous media as well as organic solvents. Si particles PEGylated by PEG 1100
were successfully prepared and yielded optically clear solutions in many solvents,
including water, methanol, and chloroform. Moreover, these nanoparticles could be
stored as solids and then redispersed readily in the same solvents (Sudeep etal.
2008). In another study, water-soluble Si nanoparticles were obtained through
PEGylation with PEG 2000 (Xu etal. 2015).
5.4.2 Water-Soluble PEGylated Carbon Nanotubes
Like Si nanoparticles, PEGylation of carbon nanotubes (C nanotubes) is necessary
to render them more water-soluble (Ravelli et al. 2012; Zhao et al. 2005;
Balasubramanian and Burghard 2010), permitting their biological and medicinal
applications. C nanotubes are potentially used in targeted drug therapy, chemical
and biological imaging, and sensing (Lacerda etal. 2006; Madani etal. 2011; Elhissi
etal. 2012; Prato etal. 2008; Liu etal. 2007). The low solubility and susceptibility
of C nanotubes in biological media and other obstacles limit their utility (Bottini
et al. 2011; Hong et al. 2006; Liu et al. 2008). Novel water-soluble PEGylated
single- walled C nanotubes were prepared and provided a means of attaching drugs.
The study showed that a specic extent of coverage of PEG chains on nanotubes
was both necessary and sufcient to impart aqueous solubility of the nanotubes
without aggregation, and PEG chains extending into the water were responsible for
the enhanced solubility (Liu etal. 2007). In another study, the suspendability in
water of single-walled C nanotubes PEGylated with PEG 400 was superior to that
of the native nanotubes in water and in water-PEG 400 media. Figure5.5 demonstrates that the nonfunctionalized nanotubes immediately precipitated from the
water solution and formed a dispersion in the water–PEG 400 medium that was
stable for only a few minutes. In contrast, the PEGylated nanotubes produced a
more stable dispersion. The study also conrmed that the higher the PEGylation
degree, the higher the dispersion stability (Ravelli etal. 2013).
Figure 5.6 shows another single-walled C nanotube that was noncovalently functionalized with uorescein-PEG 5000 and exhibited excellent water solubility at
146
Fig. 5.5 Improved suspendability of single-walled C nanotubes by PEGylation [(Ravelli etal.
2013), reproduced with permission]
R. S. H. Mansour et al.
different pH values attributed to PEG chains (Nakayama-Ratchford etal. 2007). In
another attempt, single-walled C nanotubes that were noncovalently functionalized
with phospholipid-PEG 2000 or 5000 were prepared to enhance the aqueous solubility of the nanotubes, and aqueous solubility of 0.84mg/mL was achieved for
PEG 5000 conjugates (Hadidi etal. 2011). C nanotubes covalently functionalized
with PEG 600 were also prepared, resulting in an aqueous solubility of 5.9mg/mL
(Zhao etal. 2005). PEG 800 or PEG 750 monomethyl ether functionalized doublewalled carbon nanotubes exhibited good aqueous solubility and formed stable suspensions in water, whereas the unfunctionalized nanotubes immediately settled in
water, PEG 800, and PEG 750 monomethyl ether. The saturated aqueous concentrations of PEG 800 and PEG 750 monomethyl ether PEGylated nanotubes were
0.86mg/mL (equivalent nanotubes concentration of 0.36mg/mL) and 0.93mg/mL
(equivalent nanotubes concentration of 0.37mg/mL), respectively (Nie etal. 2010).
5.4.3 Water-Soluble PEGylated Metal/Metal
Oxides Nanoparticles
Many factors, including their water instability, hinder the diagnostic and therapeutic
applications of metal nanoparticles. Accordingly, surface modication of these inorganic nanoparticles is crucial to impart the desired properties, including the enhancement of solubility in physiological media (Tilley et al. 2005). PEGylation of
inorganic nanoparticles can modify their properties and overcome the limitations of
their applicability (Otsuka etal. 2003).
PEGylation of gold nanoparticles to provide water solubility has been described
in the literature, such as gold nanoparticles containing a noncovalently bound silicon phthalocyanine (Cheng etal. 2008, 2011a, b; Meyers et al. 2015) and gold