Polyethylene glycol derivatization of the non-active ion in active pharmaceutical ingredient ionic liquids enhances transdermal delivery

Article abstract of DOI:10.1039/c6nj03709g

We report the synthesis of four salts composed of the salicylate anion ([Sal]^?) paired with tributylammonium ([HN₄₄₄]⁺), choline ([Cho]⁺), 1-methylpyrrolidinium ([HMPyrr]⁺), and triethylene glycol monomethyl ether tributylammonium ([mPEG₃N₄₄₄]⁺) cations. Three of the synthesized salts (room temperature liquids [mPEG₃N₄₄₄][Sal] and [Cho][Sal], and a supercooled liquid [HN₄₄₄][Sal]) belong to the category of ionic liquids (ILs), and one salt (solid [HMPyrr][Sal]) was a crystalline solid. ILs in their neat form were studied for membrane transport through a silicon membrane, and exhibited higher transport compared to a control experiment with sodium salicylate dissolved in mPEG₃OH as solvent, but lower membrane transport compared to salicylic acid dissolved in mPEG₃OH. The ‘PEGylated’ IL, [mPEG₃N₄₄₄][Sal], crossed the membrane with an ca. ～2.5-fold faster rate than that of any of the non-PEGylated ILs. This work demonstrates not only that API-ILs can eliminate the use of a solvent vehicle during application and notably transport through a membrane as opposed to a higher melting crystalline salt, but also that the membrane transport can be further enhanced by PEGylation of the counter ions.

Full text of DOI:10.1039/c6nj03709g

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DOI: 10.1039/C6NJ03709G.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Polyethylene Glycol Derivatization of the Non-active Ion in Active Pharmaceutical
Ingredient Ionic Liquids Enhances Transdermal Delivery
Oleksandra Zavgorodnya,1 Julia L. Shamshina,2,3 Max Mittenthal,1,‡ Parker D. McCrary,1,§
Giovanni P. Rachiero,2 Hatem M. Titi,2 and Robin D. Rogers1,2,*
1
2
Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487, USA
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, QC,
H3A 0B8, Canada
3
525 Solutions, Inc., 720 2nd Street, Tuscaloosa, AL 35401, USA
Abstract
We report the synthesis of four salts composed of the salicylate anion ([Sal]ꢀ) paired with
tributylammonium ([HN444]+), choline ([Cho]+), 1ꢀmethylpyrrolidinium ([HMPyrr]+), and
triethylene glycol monomethyl ether tributylammonium ([mPEG3N444]+) cations. Three of the
synthesized salts (room temperature liquids [mPEG3N444][Sal] and [Cho][Sal], and a supercooled
liquid [HN444][Sal]) belong to the category of ionic liquids (ILs), and one salt (solid
[HMPyrr][Sal]) was a crystalline solid. ILs in their neat form were studied for the membrane
transport through a silicon membrane, and exhibited higher transport compared to a control
experiment with sodium salicylate dissolved in mPEG3OH as solvent, but lower membrane
transport compared to salicylic acid dissolved in mPEG3OH. The ‘PEGylated’ IL,
[mPEG3N444][Sal], crossed the membrane with an ca. ~2.5ꢀfold faster rate than that of any of the
nonꢀPEGylated ILs. This work demonstrates not only that APIꢀILs can eliminate the use of a
solvent vehicle during application and notably transport through a membrane as opposed to a
higher melting crystalline salt, but also that the membrane transport can be further enhanced by
PEGylation of the counter ions.
* Corresponding author: robin.rogers@mcgill.ca
‡
Current Address: The University of Alabama, Department of Chemical & Biological Engineering,
Tuscaloosa, AL USA 35487ꢀ0203, mmittenthal@crimson.ua.edu
Current Address: The University of Alabama School of Law, 101 Paul W Bryant Dr., Tuscaloosa, AL
§
USA 35401, parker.mccrary@law.ua.edu
1

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Introduction
Transdermal delivery of active pharmaceutical ingredients (APIs) is a challenging task due to
the low permeability of the top layer of skin, the stratum corneum.1ꢀ3 Yet, drug transport via the
skin is a very attractive option to achieve localized delivery, minimize side effects associated
with systemic therapy, and increase patient compliance.4 A variety of chemical and physical
methods have been developed to deliver drugs through the skin, including formulating drugs
with penetration enhancers such as dimethylsulfoxide (DMSO), alcohols, glycols and
surfactants,5,6 and ionoꢀ and sonophoresis.7,8 However, formulations with penetration enhancers
often require high concentrations of actives, which can cause severe skin irritation or damage.5,9
Ionoꢀ and sonophoresis have been shown to increase skin permeability, but require the use of a
power source and do not provide sustained longꢀterm delivery.10 These limitations have spurred
research to develop other strategies to achieve longꢀterm transdermal delivery of drugs.
Ionic liquids (ILs, salts with melting point of less than 100 °C11) can be efficient solvents to
increase solubility12ꢀ15 and transport of poorly soluble drugs through the skin.12,14,15 For example,
transdermal delivery of acyclovir showed a 3ꢀ4 fold increase in transport in the presence of
dimethylimidazolium dimethylphosphate ([C1mim][(MeO)2PO2]) compared to a commercially
available acyclovir topical cream with polyethylene glycol as a base formulation.12 In another
study, a 2ꢀfold increase was reported for hexylpyridinium chloride ([HPyr][Cl])ꢀassisted caffeine
transport relative to formulation without [HPyr][Cl].14
In addition to the use of ILs as solvents, the application of ILs in pharma has also been
extended to the synthesis of active pharmaceutical ingredients in the form of ILs (APIsꢀILs),
which are salts with at least one pharmaceutically active ion that have a melting point preferably
below body temperature or even liquid at ambient temperatures.16 As compared to crystalline
compounds, application of drugs as APIꢀILs can eliminate many issues associated with the solid
form of drugs such as polymorphism and poor solubility which often diminish drug
bioavailability and therapeutic efficiency and lower thermal stability.17ꢀ One example is
sulfasalazine formulated as the cholinium sulfasalazine APIꢀIL that showed improved aqueous
solubility of sulfasalazine by a factor of ca. 4000.26 Furthermore, the toxicological and
biopharmaceutical properties of APIꢀILs can be tuned by an appropriate choice of the counter
ion.32ꢀ34 For example, APIꢀILs of ampicillin, aspirin, ibuprofen and salicylic acid were paired
with counterꢀions that provided control over cytotoxicity or increased antibacterial or antifungal
2

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activity in addition to improving solubility and stability as compared to their commercial salts.36ꢀ
40
Despite significant progress in designing and preparing APIꢀILs, their potential applications
for transdermal drug delivery have been rarely investigated,41,42 perhaps because salts are
generally thought to be poorly penetrating in comparison to their neutral analogs. Similarly,
transdermal drug delivery of ‘liquid coꢀcrystals’ or deep eutectics43 have not been extensively
studied, although a reported example demonstrated that deep eutectics with strong intermolecular
interactions, such as a hydrogen bonded acidꢀbase pair, exhibited increased membrane transport
in comparison to ILs with weaker interꢀion interactions or traditional salts.
Finding ion pairs with the capability for strong interꢀion interactions could be an excellent
approach to design APIꢀILs while maintaining the drug in its ionized form.23,44 This strategy was
first explored when salicylic acid was paired with alkylamines in ethanolꢀpropylene glycol
solutions.44 Later, Stoimenovski and MacFarlane adopted this approach to increase membrane
transport
of
salicylates.23
Increased
permeation
(~10ꢀtimes)
of
[Prolinium
ethylester][Ibuprofenate] IL across pig skin as compared to ibuprofen free acid in PBS/ethanol
solution was shown recently.42 Very recently Wang et. al.43 have shown that dual–active deep
eutectics can form strongly hydrogenꢀbonded complexes (liquid coꢀcrystals) similar to ion
pairing, but without formal charges. These liquid coꢀcrystals can remain intact in some solutions
allowing for the simultaneous transport of both components. A commercial formulation has also
been reported, the EtodolacꢀLidocaine Topical Patch (RXꢀ7EAT or ‘Etoreat patch’), which is a
transdermal delivery system containing the nonꢀsteroidal antiꢀinflammatory drug (NSAID)
etodolac and the painꢀreliever lidocaine.45,46 In vitro studies showed that this system displayed
ca. 9.3ꢀfold increased skin permeation compared to a system containing only etodolac.46
Drug transport is regulated not only by the degree of ionization of the API but also by the
presence of any penetration enhancers. Polypropylene and polyethylene glycols (PPG and PEG,
respectively) are widely used as penetration enhancers in transdermal and cosmetic formulations
due to their low toxicity.47ꢀ49 For example, addition of PPG in the transporting media has been
reported to change the transport rate of salicylic acid and lidocaine23,29 It has been shown that the
presence of glycols as solvents in APIꢀIL had a strong solvation effect which reduced the ion
pairing and decreased drug transport.23 We thus wondered if the problems associated with skin
penetration could be overcome by pairing the API with a PEGylated counter ion.50ꢀ52 We
3

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hypothesized that the penetration enhancing effect of PEG could be harnessed by pairing a low
molecular weight (MW) PEGylated ion with an ionic API without the realization of the solvation
effect typically seen with neutral PEG solvents.
To test our hypothesis, we set out to prepare APIꢀILs by pairing salicylic acid with an amine
derivative based on triethylene glycol monomethyl ether (mPEG3OH). In order to eliminate the
size effect of PEG on membrane permeability,53 the relatively short PEG, mPEG3OH, was
selected for our studies. Salicylic acid (HSal), an antiꢀinflammatory agent frequently used in
topical cosmetic and medical formulations,54 was chosen as the model API with wellꢀknown
physiological and pharmaceutical properties that is known to form stable ILs.55 Amines that were
chosen for pairing with salicylic acid included two tertiary amines (acyclic tributylamine (N444)
and cyclic Nꢀmethylpyrrolidine (MPyrr)), and two quaternary ammoniums (PEGylated
tributylammonium ([mPEG3N444]+) and choline ([Cho]+)) to compare the effect of cation
structure on membrane transport of the API. Finally, we also compared transport of the
synthesized compounds with the transport of fully ionized sodium salicylate ([Na][Sal]) and
neutral HSal dissolved in mPEG3OH as solvent.
Results and discussion
Synthesis and Characterization of ILs
We have previously shown that a variety of ILs could be prepared using the salicylate anion,
([Sal]ꢀ) by metathesis of corresponding ammonium halides with sodium salicylate in aqueous
environments.55 Previously prepared compounds included quaternary aprotic ammonium salts
(e.g., cetylpyridinium salicylate, benzethonium salicylate, benzalkonium salicylate), as well as
quaternary protic ammonium salts (e.g., lidocainium salicylate, procainium salicylate,
hexetidinium salicylate, procainamidium salicylate, tramadolium salicylate),55 and we
hypothesized that this methodology would also be suitable for the cations selected in our work.
To address the effect of ionization, cation structure, and size on the membrane transport, we
chose to use quaternary cationic N,NꢀdibutylꢀNꢀ(2ꢀ(2ꢀ(2ꢀmethoxyethoxy)ethoxy)ethyl)butanꢀ1ꢀ
aminium 1 ([mPEG3N444]+), tributylamine 2 (N444), choline 3 ([Cho]+), and 1ꢀmethylpyrrolidine
4 (MPyrr) (Figure 1). The quaternary cationic [mPEG3N444]+ 1 was synthesized from mPEG3OH
as the ammonium bromide salt [mPEG3N444][Br], 1[Br] (Scheme 1), using slightly modified
4

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previously published protocols.56ꢀ58 Tributylamine 2, choline 3 (as the hydroxide), and 1ꢀ
methylpyrrolidine 4 were commercially available.
Figure 1. Molecules and ions used in this study.
Scheme 1. Synthetic pathway for synthesis of 1[Br].
The ILs in this study were prepared using two methods, metathesis (Scheme 2, Method A).
or acidꢀbase neutralization (Scheme 2, Method B). A metathesis reaction between silver
salicylate, obtained from salicylic acid and silver nitrate AgNO3, and the quaternary ammonium
bromide 1[Br] was used for the synthesis of compound 1[Sal]. Acidꢀbase reactions of salicylic
acid with the free amines (2 and 4) and choline hydroxide (3) were used for preparation of
2[Sal], 4[Sal], and 3[Sal]. All synthetic and characterization data for all compounds are provided
in the Experimental section.
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Scheme 2. Synthetic pathways for synthesis of [mPEG3N444][Sal] (1[Sal]), [HN444][Sal] (2[Sal]),
[Cho][Sal] (3[Sal]), and [HMPyrr][Sal] (4[Sal]).
Both approaches resulted in the desired compounds (Table 1). All three compounds obtained
via the acidꢀbase route (2[Sal] ꢀ 4[Sal]) were isolated in quantitative yields. Compound 1[Sal]
was obtained by metathesis in only moderate yield (80%). The formation of all compounds
1[Sal]-4[Sal] and 1:1 stoichiometry were proven using Nuclear Magnetic Resonance (1H and 13C
NMR) spectroscopy, while ionization was assessed using Fourier Transform InfraꢀRed (FTꢀIR)
spectroscopy, which is discussed below. All isolated compounds were obtained with purities
>90% (determined using 1H NMR integration, see Experimental for the method and Table 1 for
1
values). Water content was also estimated using H NMR spectra (through the integration of the
water peak in DMSOꢀd6 located at 3.33 ppm) and found to be below 3%, except for the highly
hydrophilic 3[Sal], which was observed to contain 8% water as isolated.
6

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Table 1: Physical properties of the synthesized compounds, compared to that of salicylic acid
and sodium salicylate ([Na][Sal]).
T5%onset
Yield
(%)
mp
(oC)
Purity
(%)
Compound
Structure
Appearance
or Td
(oC)
[mPEG3N444][Sal]
Amber color
liquid
[a]
80
−
162
90[b]
(1[Sal])
[HN444][Sal]
Clear colorless
liquid[c]
100
34[c]
110
193
99[b]
(2[Sal])
[Cho][Sal]
Clear colorless
liquid
[a]
100
100
−
90[b]
99[b]
(3[Sal])
[HMPyrr][Sal]
1st: 113
2nd: 187
Crystalline solid
112
(4[Sal])
158ꢀ
161[d]
HSal
Crystalline solid
Crystalline solid
n/a
n/a
16259
99[d]
99[d]
[Na][Sal]
275[e]
>275[e]
[a]
[b]
No melting/glass transitions were observed in the ranges tested (ꢀ50 to 75 ºC). Purity determined
[c]
using NMR spectra.
The compound exhibited supercooled liquid behavior:60 isolated as a colorless
liquid and crystallized on cooling during DSC. [d] Value provided by manufacturer. [e] Sodium salicylate is
o
o
reported to decompose on melting: the onset has been measured at 275 C when heated at 10 C/min
under an N2 atmosphere (ambient pressure), see reference [61].
Thermal transitions and melting points (mp) were evaluated by differential scanning
calorimetry (DSC, Table 1), and compared to commercial HSal and Na[Sal]. Compounds 1[Sal]
and 3[Sal] were liquids at room temperature with no mp observed at the investigated temperature
range (ꢀ50 to 75 ºC). Compound 2[Sal], which was isolated as a colorless liquid, solidified upon
cooling during DSC and showed melting point at 34 ºC. (The compound is identified in Table 1
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o
as a supercooled liquid.) The solid 4[Sal] was a crystalline solid with mp of 112 C; a single
crystal was isolated upon slow evaporation of the methanol.
Analysis of thermal stability for the new salts by thermal gravimetric analysis (TGA) showed
a singleꢀstep decomposition for three of the synthesized compounds (Table 1 and Figure S11),
1[Sal] ꢀ 3[Sal], while 4[Sal] demonstrated a twoꢀstep decomposition with rather a subtle
o
transition between two steps; all four compounds decomposed above 100 C. We attribute the
lowest thermal stability to supercooled liquid 2[Sal] (T5%onset = 110 °C). Crystalline 4[Sal] (T1ꢀ
5%onset = 113 °C and T2ꢀ5%onset = 187 °C, for the first and second step decomposition, respectively)
was the next one, in order of increasing thermal stability. The decomposition temperature for the
compound 1[Sal] was the same as that reported for salicylic acid (T5%onset = 162 °C).59 Finally,
the choline compound, 3[Sal], was the most thermally stable of all prepared compounds, with
T5%onset of 193 °C. The inorganic salt, [Na][Sal] was even more stable, with decomposition on
melting at 275 °C.61
A single crystal, suitable for a single crystal Xꢀray diffraction (SCXRD) analysis, was
obtained for 4[Sal] and its crystal structure determined (Figure 2). The Nꢀmethylpyrrolidinium
salicylate crystallized in the monoclinic space group, P21/n with one cation and anion in the
asymmetric unit. The structure clearly indicates proton transfer and in the solid state the cations
and anions are held together by strong chargeꢀassisted hydrogen bonds (N–H⋯O = 2.698(1) Å;
Table S1). Intramolecular hydrogen bonds between the hydroxyl group and the carboxylate of
the salicylate (O–H⋯O = 2.528(1) Å) were also observed. Further details of this structure and its
crystal packing are provided in the ESI (Figures S9a and S10).
Figure 2. ORTEP (50% probability thermal ellipsoids) diagram of Nꢀmethylpyrrolidinium
salicylate ([HMPyrr][Sal]) (4[Sal]). Hydrogen bonds are depicted by dashed lines.
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1H NMR and FTꢀIR spectroscopies were used to assess the ionization of the systems. In the
1H NMR spectra of the compounds 1[Sal] - 4[Sal] the acidic proton (ꢀCOOH peak, ca. 12.0
ppm) was absent indicating full ionization. Supporting this observation, the aromatic protons in
1
compounds 1[Sal] - 4[Sal] were shifted upfield when compared with the H NMR spectrum of
free HSal, due to the increase in electron density of the ring upon the formation of the salicylate
anion. However, since the acidic proton may be difficult to assess due to a rapid proton
exchange, secondary confirmation was sought using FTꢀIR.
FTꢀIR spectra were recorded for neat 1[Sal], 2[Sal], 3[Sal] and compared with FTꢀIR spectra
of free HSal, and fully ionized [Na][Sal] (Figures S12 – S14). The crystalline [HMPyrr][Sal]
(4[Sal]) was excluded since its state of ionization was already known from the crystal structure.
In the FTꢀIR, the carbonyl (C=O) band is the most indicative related to ionization of salicylic
acid. The protonated HSal shows the carbonyl vibration (C=O) at 1654 cmꢀ1, while the
carboxylate (COOꢀ) displays two bands at 1580 cmꢀ1 (asymmetric) and 1389 cm−1 (symmetric).
The presence of asymmetric bands for the COOꢀ group at 1580, 1575, and 1581 cmꢀ1 and
symmetric ones at 1375, 1373, and 1377 for neat 1[Sal], 2[Sal], and 3[Sal], respectively, clearly
confirmed the complete ionization, and values were in agreement with those for [Na][Sal]
(asymmetric at 1589 cmꢀ1 and symmetric at 1376 cmꢀ1)62 (Figures S12 – S14).
Membrane Transport
The main advantage of ILs that are liquids at body temperature or below is that they can be
applied directly to the skin without use of a solvent thus minimizing issues of quick formulation
drying and drug solidification. Therefore, the transport of liquid 1[Sal], 3[Sal] and superꢀcooled
liquid 2[Sal], in the amounts of 0.1 ꢀ 0.3 mmol were studied neat, while solid [Na][Sal] (16 mg,
0.1 mmol) and HSal (14 mg, 0.1 mmol) were dissolved in mPEG3OH (ca. 15ꢀ20 µL). The
crystalline [HMPyrr][Sal] (4[Sal]) was excluded from this study.
The membrane transport was investigated at 37 °C using a Franz diffusion cell system (see
ESI for diagrammatic representation) via a procedure we have described earlier.43 A clean, dried
receiver chamber was filled with PBS as the receiving media, and stirred constantly, while
placed in a temperature controlled water bath at 37 ºC, for 15 min to equilibrate. The synthetic
silicone membrane (1.77 cm2 area, 0.254 mm thickness) for skin barrier simulation, imitating the
stratum corneum,63 was mounted between the donor and receiver chambers of the Franz cell.
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Neat ILs and solutions of HSal and [Na][Sal] in mPEG3OH were spread onto the preꢀweighed
airꢀdried membrane so that they covered the entire membrane surface and the cell was sealed
with Parafilm to minimize water absorption from the air. Aliquots were taken from the receiving
media every hour (replacing the aliquots with a fresh PBS solution to keep the volume constant),
and the amount of transported salicylate in the receiving media was monitored by Ultravioletꢀ
visible (UVꢀvis) spectrophotometry using a preꢀmade calibration curve based on concentration of
salicylates. The transport of free acid, HSal, was also monitored as salicylate in the receiving
media due to its deprotonation at the basic pH of the PBS buffer (7.4), which is well above the
pKa value of the acid (3.0).
Transport of the sodium salt, [Na][Sal], across the membrane (Figure 3, red circles) was
below the detection limit of our UVꢀvis method (<0.01 mol/L). By contrast, the neutral HSal
readily permeated the membrane at a rate of 3.0 mol%/h, where mol% indicates the percent of
applied dose. The total after 8 h (or cumulative) transport of HSal was ~24.5 mol% of applied
dose (Figure 3, aqua stars). Such differences between transport of neutral and ionized forms of
salicylic acid are in agreement with the theory that ionic species cannot easily permeate the
membrane or overcome the skin barrier.2,23
Figure 3. Left: Total (cumulative) transport of neat ILs (1[Sal] ꢀ 3[Sal]) and [Na][Sal] and HSal
dissolved in mPEG3OH, through the silicon membrane after 8 h, presented as a molar percentage
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of the applied dose (mol%) vs. time. Right: Expanded view of the linear fit of the transport
curves. ILs [mPEG3N444][Sal] (1[Sal]) (red circles), [HN444][Sal] (2[Sal]) (black triangles up),
and [Cho][Sal] (3[Sal]) (blue triangles down) were applied neat, [Na][Sal] (green diamonds) and
HSal (aqua stars) were dissolved in mPEG3OH and applied as solutions. Permeation of [Na][Sal]
was below the detection limit.
Despite being fully ionized, all three ILs 1[Sal], 2[Sal], and 3[Sal] were able to detectably
permeate the membrane in the first hour (Figure 3). The transport rates, calculated from the
linear portion of the curves, were 0.57, 0.23 and 0.20 mol%/h for 1[Sal], 2[Sal], and 3[Sal] ILs,
respectively. The total amount of 1[Sal], 2[Sal], and 3[Sal] ILs that crossed the membrane after
8 h was ~5.06, 1.94 and 1.73 mol% of the applied dose. Although 1[Sal], 2[Sal], and 3[Sal] ILs
permeated the membrane at a much slower rate as compared to neutral HSal, all of them showed
significant transport as compared to total absence of permeation in the case of high melting
crystalline [Na][Sal] salt dissolved in mPEG3OH.
Considering that both [Na][Sal] and ILs are fully ionized, the phenomenon is somewhat
surprising and might be attributed to strong ionꢀpairing in the ILs similar to that observed in the
solid state of 4[Sal] rather than ions being ‘free’. It was previously shown that ibuprofen in the
form of a fully ionized salt, ephedrinium ibuprofenate [Eph][Ibu], crossed the membrane at ~1
mol% of applied dose after 9 h of transport, while sodium ibuprofenate had essentially zero
membrane permeation.43 Compared to neutral ibuprofen, [Eph][Ibu] crossed the membrane much
more slowly (~11ꢀfold slower).
Enhanced transport due to strong ionꢀpairing between anion and cation was also reported for
ammonium ibuprofenates64 and alkylammonium salicylates.44 In these studies, not only the
ionization state, but also the size of the compounds transporting across the membrane were found
to significantly influence the membrane transport. Furthermore, a deep eutectic hydrogenꢀbonded
complex of lidocaine (Lid·Ibu) transported across the membrane, albeit slower compared to
individual transport of either the free base or free acid precursors.43 Here, the amount of
transported neutral ibuprofen and lidocaine was ~12 and 18 mol% of the applied dose,
respectively, while Lid·Ibu had ca. 5.8 mol% transported after 9 h. Such a significant difference
in transport rate was attributed to the larger size of the Lid·Ibu complex as compared to the free
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acid precursor.65 Therefore, we believe that the slower membrane transport of 1[Sal], 2[Sal], and
3[Sal] ILs than HSal is due to both the ionized state and increased size due to ionꢀpairing.
Among the synthesized ILs, the IL with the PEGylated cation (1[Sal]) crossed the membrane
much faster than either 2[Sal] or 3[Sal]; the cumulative transport of 1[Sal] based on applied
dosage was 5.1 mol% after 8 h, which is ~2.5ꢀfold higher than the cumulative transport seen for
the 2[Sal] and 3[Sal] ILs (Figure 3). The length of the alkyl chain on the cation in ionꢀpairs
could be a part of the reason as it has been shown to regulate the skin transport of salicylic acid
paired with tertiary amines.44 In general, tertiary amines with longer alkyl chains and therefore
increased lipophilicity demonstrated enhanced transport although not that significant, which
could explain the small difference in transport observed between 2[Sal] and 3[Sal] ILs, but not
the 2.5ꢀfold increase seen for 1[Sal] with the PEGylated cation. Based on the previously reported
enhanced drug transport of the PEGylated prodrug of indomethacin derivatives where
lipophilic/hydrophilic balance was controlled by number of polyoxyethylene groups in PEGꢀ
chains,50 we suggest that the higher transport observed for 1[Sal] as compared to 2[Sal] and
3[Sal] is due to increased hydrophilicity of the ILs when the counterꢀion is PEGꢀappended as
compared to 2[Sal].
Conclusions
In summary, we have demonstrated the preparation of fully ionized salts from salicylic acid,
including three ILs (one with PEGylated cation) and a crystalline solid. All synthesized ILs were
fully ionized and had high thermal stability with degradation temperature above 100 ºC. Being
liquids at room temperature, the 1[Sal], 2[Sal], and 3[Sal] directly permeated the membrane
without the need of a solvent as opposed to crystalline HSal and [Na][Sal]. The ILs (1[Sal],
2[Sal], 3[Sal]) were able to cross the membrane at a faster rate (0.57, 0.23, and 0.20 mol%/h,
respectively) than the ionized commercial salt (below the quantification level), but lower than
neutral salicylic acid (3.0 mol%/h). We also showed that choice of cation plays a significant role
in the efficiency of the membrane transport and introducing PEGylated moieties into the
structure of tertiary amines can enhance the transport by ~2.5ꢀfold as compared to PEGꢀfree
cations. We believe that this work opens new prospective in using PEGylated APIꢀILs for drug
delivery.
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EXPERIMENTAL
1.1 Materials
Salicylic acid (Acros Organics, Morris Planes, NJ), sodium salicylate (anhydrous, >99 %,
SigmaꢀAldrich, St. Louis, MO), Nꢀmethylpyrrolidine (Alfa Aesar, Ward Hill, MA), sodium azide
(NaN3, triphenylphosphine (PPh3, Alfa Aesar, Ward Hill, MA), bromine (Br2, Alfa Aesar, Ward
Hill, MA), silver nitrate (AgNO3, Alfa Aesar, Ward Hill, MA), choline hydroxide (46 wt %
aqueous, SigmaꢀAldrich, St. Louis, MO), triethylene glycol monomethyl ether (mPEG3OH,
SigmaꢀAldrich, St. Louis, MO), tributylamine (SigmaꢀAldrich, St. Louis, MO), were used as
received. Anhydrous ethanol, methanol, dichloromethane, and diethyl ether (Fischer Scientific,
Hampton, NH) were used without any further purification. Deꢀionized water (DIꢀH2O) was
obtained from Culligan Water Systems (Chattanooga, TN) at 17.38 MΩ and used as received.
Phosphate buffer saline (PBS) tablets were purchased from SigmaꢀAldrich (St. Louis, MO) and
dissolved in 200 mL of deionized water prior each transport study.
1.2
Methods
NMR. Spectra were recorded on Bruker Avance Spectrometer Bruker/Magnex UltraShield
500 MHz magnet (Madison, WI) or a Bruker Spectrospin DRX 360 MHz UltrashieldTM
1
spectrometer (Madison, WI) with chemical shifts reported in δ (ppm) unless noted. H (500
MHz) and 13C (125 MHz) were collected in a neat insert using DMSOꢀd6 containing 1% v/v
TMS in an external capillary. Solid samples were dissolved at 25 °C in DMSOꢀd6 containing
TMS as an internal reference. The purity of the products was evaluated considering the formula:
[Σ I(product)/Σ I(total)] × 100 where I represents the relative area of the detected signals.
FT-IR. Spectra were collected using a Bruker αꢀFTIR by direct measurement via attenuated
total reflection of the neat samples on a diamond crystal. All the measurements were recorded in
400ꢀ4000 cmꢀ1 region.
SCXRD. Single crystal Xꢀray diffraction data was collected using a Bruker diffractometer
equipped with a PLATFORM 3ꢀcircle goniometer and an Apex II CCD area detector (Bruker
AXS, Madison, WI) and graphiteꢀmonochromatic MoꢀKα radiation. The crystal was coated with
Paratone oil and cooled to 196 K using an Oxford NꢀHelix cryostat (Oxford Cryosystems,
Oxford, UK). Apex2 software suite (AXScale and SAINT, version 2010; Bruker AXS, Inc.:
Madison, WI) was used for data collection, reduction, and unit cell assignment. The crystal
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structure was solved by direct methods and refined by full matrix least squares (SHELXTL66 and
SHELXL67 packages). All nonꢀhydrogen atoms were refined anisotropically, hydrogen atoms
bonded to strong hydrogen bond donors were found from the difference map, and their
coordinates were refined freely. All other hydrogen atoms were placed in calculated positions
and allowed to ride on the carrier atoms. All hydrogen atom thermal parameters were constrained
to ride on the carrier atom.
SCXRD Data for 4[Sal]. C12H17NO3, M =223.26 g/mol: monoclinic, space group P21/n (no.
14), a = 11.655(5) Å, b = 9.012(4) Å, c = 12.686(5) Å, β = 115.981(4)°, V = 1197.9(8) Å3, Z = 4,
T = 196(2) K, ꢀ(MoKα) = 0.089 mmꢀ1, ρcalc = 1.238 g/cm3, 27642 reflections measured (6.012° ≤
2θ ≤ 70.01°), 5227 unique (Rint = 0.0352, Rsigma = 0.0268) which were used in all calculations.
The final R1 was 0.0507 (I > 2σ(I)) and wR2 was 0.1518 (all data).
DSC. Thermal transitions for compounds 1[Sal] – 3[Sal] were measured on a MettlerꢀToledo
TGA/DSC 1, Star DSC unit (Columbus, OH, USA) under a stream of nitrogen. Samples (2ꢀ5
mg) were placed in closed aluminum pan perforated with a pinꢀhole. A typical cycle consisted of
initially heating the samples from 25 to 75 °C at a heating rate of 5 °C/min, followed by a 15 min
isotherm, then cooling at a rate of 5 °C/min to ꢀ50 °C, again followed by a final 15 min isotherm
at ꢀ50 °C. The cycle was repeated twice to ensure consistency of the observed thermal
transitions. The value was analyzed using STARe Thermal Analysis Software provided by
MettlerꢀToledo.
Thermal transitions for the compound 4[Sal] were measured on a TA Instrumentꢀ2000 DSC
unit (New Castle, DE, USA) under a stream of nitrogen. Sample (2.4 mg) was placed into T Zero
pans (Part # 901683.901), covered with Hermetic aluminum lid (Part # 900794.901) and sealed
using press (T Zero Sample Press Kit, New Castle, DE, USA). A typical cycle consisted of
initially heating the samples from 25 to 120 °C at a heating rate of 5 °C/min, followed by a 5 min
isotherm, then cooling at a rate of 5 °C/min to ꢀ60 °C, again followed by a final 5 min isotherm
at ꢀ60 °C. The cycle was repeated twice to ensure consistency of the observed thermal
transitions. The value was analyzed using TA Universal Analysis Software by TA Instruments.
TGA. Data were collected on a MettlerꢀToledo (Columbus, OH) TGA/DSC 1. Samples in
the range 2.0 to 15.0 mg were analyzed on a 70 µL alumina crucible with lid that was removed
by the sample changer during measurement. All the measurements were done using argon as a
purge gas. Samples were heated from room temperature to 800 °C and measured in the dynamic
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heating regime, using a constant heating ramp of 5 °C minꢀ1 with a 30 min isotherm at 75 °C in
order to remove excess volatiles or residual solvents. Decomposition temperatures are reported
as 5 wt% mass loss (T5%oncet).
1.3
Syntheses
[mPEG3N444][Sal] (1[Sal]).
Step 1: mPEG3Br.
In the first step, to obtain mPEG3Br from mPEG3OH,56,57,68 a 200 mL round bottom flask
was charged with mPEG3OH (173 mmol, 28.41 g) and anhydrous Et2O (100 mL). PPh3 (176
mmol, 46.16 g) and Br2 (177 mmol, 9.10 mL) were slowly added to the flask using addition
funnel, and the resulting reaction mixture was stirred. After 1 h, a the precipitated solid formed
which was filtered and discarded. The filtrate was evaporated by rotary evaporation at moderate
heating (Tbath = 30 °C) to afford a light yellow liquid, mPEG3Br. The resultant mPEG3Br was
purified by silica gel column chromatography (SiO2, height = 10 cm, diameter = 5 cm) using
Et2O (375 mL) as eluent. The fractions containing the products (determined by fraction analysis
through thin layer chromatography (TLC) using permanganate stain for visualization) were
collected, combined, and the solvent was removed by rotary evaporation. After that, the oil was
kept at high vacuum, to remove residual Et2O solvent. The isolated purified oil, mPEG3Br, was
used directly in the next step.
Step 2: [mPEG3N444][Br].
In the second step, to obtain [mPEG3N444][Br] from mPEG3Br, a slightly modified procedure
based on references [56,57,68] was used. A twoꢀnecked 500 mL round bottom flask equipped
with a magnetic stirrer, reflux condenser, and a positive flow of nitrogen was charged with
tributylamine N444 (40 mmol, 7.41 g). mPEG3Br from the previous step (20 mmol, 4.88 g) and
anhydrous EtOH (300 mL) were added slowly at once, under stirring. The solution was heated in
an oil bath (83 °C) for 12 h. (During the course of reaction, a second layer was formed, and
appeared darker as the reaction progressed.) After 12 h, the oil bath was removed, stirring
stopped, and the solution was cooled to room temperature. After stirring ceased, two liquid
phases separated. The layers were separated using a separatory funnel, bottom layer containing
excess of N444 discarded, and the top layer collected, and evaporated on the rotary evaporator.
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The formed [mPEG3N444][Br] oil was further dried using an oil pump vacuum to give
[mPEG3N444][Br] as a dark red and extremely viscous oil. This product was used in the next step
without further purification.
Step 3: [Ag][Sal].
To obtain [Ag][Sal] for the following metathesis reaction with [mPEG3N444][Br], a 20 mL
vial was charged with AgNO3 (10 mmol, 1.70 g) and then dissolved in a minimum amount of DI
water (7.5 mL). [Na][Sal] (10 mmol, 1.60 g) as a saturated aqueous solution was added to
AgNO3 under stirring. Upon addition, an offꢀwhite solid immediately precipitated. After 1 h, the
solid was filtered out and dried under high vacuum to afford [Ag][Sal] as a white solid in
quantitative yield.
Step 4: [mPEG3N444][Sal].
A 20 mL vial was charged with [mPEG3N444][Br] (3.3 mmol, 1.35 g) and dissolved in MeOH
(15 mL). Then [Ag][Sal] (6.6 mmol, 1.61 g) was added under stirring whilst the vial was
wrapped into aluminum foil to prevent light oxidation. After two weeks, the solution was filtered
to get rid of insoluble AgBr byproduct and the liquid was dried under high vacuum, to give
[mPEG3N444][Sal] as a highly viscous dark amber liquid (3.23 mmol, 1.51 g, 98%). The total
3
yield in four steps: 80.2 %. 1H NMR (neat) δ ppm: 7.34 (d, 1H, JHH = 6.7 Hz), 6.68ꢀ6.66 (m,
3
1H,), 6.21 (d, 1H, 3JHH = 7.8 Hz), 6.14 (t, 1H, JHH = 6.1 Hz), 3.22ꢀ2.69 (m, 23H), 0.98 (dt, 6H,
13
3JHH = 0.97 Hz, 0.49), 0.71 (m, 6H,), 0.31 (app t, 9H); C NMR (neat) δ ppm: 172.51, 162.31,
131.85, 130.05, 119.41, 116.30, 71.23, 69.53, 63.95, 58.45, 57.79, 23.09, 19.50, 18.92, 13.00,
12.90. FTꢀIR (cmꢀ1): 2957 (m), 2872 (m), 2124 (w), 1633 (m), 1558 (m), 1480 (m), 1453 (m),
1374 (m), 1289 (m), 1250 (m), 1196 (w), 1136 (m), 1104 (m), 1023 (m), 885 (m), 803 (m), 758
°
(m), 701 (m), 666 (m), 564 (w), 540 (w). TGA: T5%onset = 162 C. DSC: No transitions were
observed.
[HN444][Sal] (2[Sal]).
A 20 mL vial was charged with N444 (10 mmol, 1.85 g) in MeOH (15 mL). Salicylic acid (10
mmol, 1.38 g) was added portionꢀwise, under stirring. Upon addition, the solution became warm.
After 12 h of stirring at room temperature, the solvent was removed by rotary evaporation and
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the residue kept under high vacuum to afford [HN444][Sal] as a nonꢀviscous, clear liquid in
quantitative yield. 1H NMR (neat) δ ppm: 7.69 (dd, 1H, 3JHH = 7.1 Hz, 4JHH = 1.9 Hz), 7.19ꢀ7.16
(m, 1H), 6.67ꢀ6.62 (m, 2H), 2.98 (app t, 6H), 1.63ꢀ1.56 (m, 6H), 1.32 (sextet, 6H, 3JHH = 7.4 Hz),
3
0.91 (t, JHH = 7.1 Hz, 9H); 13C NMR (neat) δ ppm: 172.66, 162.81, 132.28, 130.53, 120.08,
116.90, 116.33, 52.09, 25.70, 19.96, 14.00. FTꢀIR (cmꢀ1): 2957 (m), 2930 (m), 2872 (m), 1628
(m), 1590 (m), 1572 (m), 1483 (m), 1452 (m), 1374 (m), 1291 (m), 1257 (m), 1218 (m), 1137
(m), 1106 (w), 1027 (m), 925 (w), 902 (w), 865 (m), 804 (m), 758 (s), 738 (m), 703 (m), 664
°
(m), 581 (w), 561 (w), 537 (w). TGA: T5%onset = 110 C. DSC: Isolated as a colorless liquid, but
crystallized on cooling during DSC, Tm = 34 °C.
[Cho][Sal] (3[Sal]).
A 20 mL vial was charged with salicylic acid (5 mmol, 0.69 g), in MeOH (10 mL) under
stirring. Choline hydroxide (5 mmol, 0.61 g, 1.5 mL solution) as 40% solution in MeOH was
added dropwise using a syringe. After that the reaction was allowed to stir at room temperature
for 12 h. At the end of 12 h, the solvent was removed by rotary evaporation the residue kept
under high vacuum to afford [Cho][Sal] as a nonꢀviscous, clear liquid in quantitative yield.1H
3
3
NMR (neat) δ ppm: 7.98 (bs), 7.18 (d, 1H, JHH = 7.1 Hz), 6.52 (t, 1H, JHH = 7.3 Hz), 6.11 (d,
1H, 3JHH = 8.0 Hz), 6.05 (t, 1H, 3JHH = 7.0 Hz), 3.14 (s, 2H), 2.57 (s, 2H), 2.25 (s, 9H); 13C NMR
(neat) δ ppm: 173.06, 161.20, 132.88, 130.06, 118.37, 117.71, 116.06, 67.04, 55.00, 52.88,
40.52, 40.35, 40.19, 40.02, 39.85, 39.69, 39.52. FTꢀIR (cmꢀ1): 3225 (b), 3028 (w), 2854 (w),
1625 (m), 1578 (m), 1482 (m), 1453 (m), 1414 (w), 1397 (m), 1323 (m), 1291 (m), 1250 (m),
1136 (m), 1084 (m), 1027 (w), 951 (m), 856 (m), 807 (m), 760 (m), 705 (m), 662 (m), 565 (m),
536 (m). TGA: T5%onset = 193 °C. DSC: No transitions were observed.
[HMPyrr][Sal] (4[Sal]).
Nꢀmethylpyrrolidine (10 mmol, 0.87 g) was mixed in equimolar amounts with salicylic acid
(10 mmol, 1.02 g) overnight in MeOH (~ 15 mL). The reaction was left with stirring under room
temperature overnight. After the reaction, MeOH was allowed to slowly evaporate out of the
solution (2 days), leaving behind an orange, crystalline solid in quantitative yield. 1H NMR (500
MHz, DMSOꢀd6) δ ppm: 7.73 (dd, 1H, J = 7.6, 1.8 Hz), 7.21 (ddd, 1H, J = 8.2, 7.2, 1.9 Hz), 6.76
– 6.64 (m, 2H), 3.23 (s, 4H), 2.81 (s, 3H), 2.06 – 1.90 (m, 4H). 13C NMR (126 MHz, DMSOꢀd6)
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δ 173.33, 162.54, 132.50, 130.63, 119.80, 117.19, 116.37, 54.87, 44.88, 24.31, 23.46. FTꢀIR (cmꢀ
1): 3003 (b), 2474 (b), 1625 (m), 1587 (m), 1570 (m), 1482 (m), 1453 (m), 1379 (m), 1328 (m),
1296 (m), 1249 (m), 1221 (w), 1155 (w), 1139 (w), 1072 (w), 1027 (w), 1010 (w), 856 (m), 806
(m), 762 (m), 705 (m), 665 (m), 566 (m), 538 (m). TGA: T1ꢀ5% onset = 113 °C, T2ꢀ5% onset = 187 °C.
DSC: Tm = 112 °C. A single crystal was isolated upon slow evaporation of the methanol.
1.4
Membrane transport measurements
Experiments were conducted using a temperatureꢀcontrolled Franzꢀtype diffusion cell (see
ESI for depiction) according to our previously established protocols.43 Silicone sheets with the
thickness of 0.254 mm (0.01 in) (Specialty Manufacturing Inc., Saginaw MI) were cut into
circular membranes with a surface area of 1.77 cm2 and allowed to soak overnight in 95% EtOH.
After soaking, the membranes were air–dried, weighted and transporting compounds with
desired number of mmol were placed on top of the membrane prior to mount it between the
donor and receiver chambers. The receiving chamber was filled with 12.0 mL of degassed PBS
buffer (pH = 7.4) and kept at 37 °C under constant stirring. The assembled Franz cell was placed
into the diffusion system equipped with a water bath and thermostat and allowed to equilibrate at
this temperature for 10 min before inserting the membrane between donor and receiver chamber.
The liquid samples of ILs were applied neat at 0.1ꢀ0.3 mmol, while [Na][Sal] (16 mg, 0.1
mmol) and HSal (14 mg, 0.1 mmol) were dissolved in mPEG3OH (15ꢀ20 µL) and were applied
onto the membrane to cover the surface area as salt solution in mPEG3OH. Parafilm was used to
seal the donor chamber to minimize water absorption from atmosphere. Samples (0.5 mL) of the
receiving phase were taken for 8 h at 1 h intervals. After each sampling, 0.5 mL of fresh PBS
buffer was added to the receiving chamber to maintain constant volume.
The sample aliquot was diluted with 1.5 mL PBS buffer (pH = 7.4) and analyzed by UVꢀVis
spectroscopy using a fourꢀsided quartz cuvette in a PerkinꢀElmer (Waltham, MA) Lambda XLS
UltravioletꢀVisible Spectrophotometer (Waltham, MA, USA) at 296 nm. Resultant concentration
was backꢀcalculated from the obtained data against a predetermined calibration curve to
determine the concentrations. Calibration curves were obtained by the analyzing individual
compounds five times each with five to eight different solutions of known concentrations (0 to 4
x 10ꢀ4 M).
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For calibration curves, PBS buffer (pH = 7.4) was prepared from commercially available
tablets (SigmaꢀAldrich) by dissolving a single tablet in 200 mL of deionized water followed by
neat IL dissolution (by weight) to form stock solution. The series of standard dilutions were
prepared from stock solutions to obtain a series of known concentrations. The absorbance at
about 296 nm (salicylate) was selected for analysis and the plots of concentration against
absorbance were fit to a straight line passing through the origin (correlation coefficients of R2 of
0.99 were obtained). HSal was monitored at 296 nm (as salicylate) due to full ionization of
carboxylic groups at pH = 7.4.
The transport rate was calculated from the linear portion of the permeation curve according
to a previously reported procedure.69 The steady state transport across the membrane was
calculated as a slope from linear portion of the transport curves (without passing through the
origin) using Origin Pro 8 Software.
Acknowledgements
MSM was supported by The Department of Chemistry at The University of Alabama Research
Experiences for Undergraduates (REU) program. PDM was supported by the Department of
Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship
(NDSEG) Program. This research was undertaken, in part, thanks to funding from the Canada
Excellence Research Chairs Program.
Conflict of Interest Statement (COI)
Dr. Robin D. Rogers has partial ownership of 525 Solutions, Inc. and is a named inventor on
related patent applications. Dr. Julia Shamshina is a consultant of 525 Solutions, Inc. The
University of Alabama and McGill University maintain approved Conflict of Interest
Management Plans. The authors have no other relevant affiliations or financial involvement with
any organization or entity with a financial interest in or financial conflict with the subject matter
or materials discussed in the manuscript. No writing assistance was utilized in the production of
this manuscript.
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REFERENCES
1
2
B. W. Barry, Eur. J. Pharm. Sci. 2001, 14, 101ꢀ114.
P. H. Stahl, C. G. Wermuth, Eds. Handbook of Pharmaceutical Salts: Properties,
Selection, and Use, VHCA and WileyꢀVCH, 2008.
3
4
5
6
M. R. Prausnitz, R. Langer, Nature Biotechnol. 2008, 26, 1261ꢀ1268.
C. A. Heyneman, C. LawlessꢀLiday, G. C. Wall, Drugs 2000, 60, 555ꢀ574.
A. C. Williams, B. W. Barry, Adv. Drug. Deliv. Rev. 2004, 56, 603ꢀ618.
C. M. Schoellhammer, D. Blankschtein, R. Langer, Expert Opin. Drug Deliv. 2014, 11,
393ꢀ407.
7
8
Y. N. Kalia, A. Naik, J. Garrison, R. H. Guy, Adv. Drug Delivery Rev. 2004, 56, 619ꢀ685.
C. M. Schoellhammer, S. Srinivasan, R. Barman, S. H. Mo, B. E. Polat, R. Langer, D.
Blankschtein, J. Control. Release 2015, 202, 93ꢀ100.
9
M. I. Afouna, S. C. Mehta, A.ꢀH. Ghanem, W. I. Higuchi, E. R. Kern, E. D. Clerco, H. H.
ElꢀShattawy, J. Pharm. Sci. 1998, 87, 917ꢀ922.
10
11
12
B. E. Polat, D. Hart, R. Langer, D. Blankschtein, J. Control. Release 2011, 152, 330ꢀ348.
R. D. Rogers, K. R. Seddon, Science 2003, 302, 792ꢀ793.
M. Moniruzzaman, Y. Tahara, M. Tamura, N. Kamiya, M. Goto, Chem. Commun. 2010,
46, 1452ꢀ1454.
13
14
W. Jin, Q. Yang, Z. Zhang, Z. Bao, Q. Ren, Y. Yang, H. Xing, Chem. Commun. 2015,
51, 13170ꢀ13171.
D. Dobler, T. Schmidts, C. Zinecker, P. Schlupp, J. Schäfer, F. Runkel, AAPS
PharmSciTech 2016, 17, 923ꢀ931.
15
16
D. Dobler, T. Schmidts, I. Klingenhofer, F. Runkel, Int. J. Pharm. 2013, 441, 620ꢀ627.
W. L. Hough, M. Smiglak, H. Rodríguez, R. P. Swatloski, S. K. Spear, D. T. Daly, J.
Pernak, J. E. Grisel, R. D. Carliss, M. D. Soutullo, J. H. Davis Jr., R. D. Rogers, New J.
Chem. 2007, 31, 1429ꢀ1436.
17
18
19
A. Balk, U. Holzgrabe, L. Meinel, Eur. J. Pharm. Biopharm. 2015, 94, 291ꢀ304.
L. Yu, S. M. Reutzel, G. A. Stephenson, Pharm. Sci. Technol. Today 1998, 1, 118ꢀ127.
N. Adawiyah, M. Moniruzzaman, S. Hawatulaila, M. Goto, Med. Chem. Commun. 2016,
7, 1881ꢀ1897.
20
21
22
S. Domingos, V. André, S. Quaresma, I. C. B. Martins, M. F. Minas da Piedade, M. T.
Maria Teresa Duarte, J. Pharm. Pharmacol. 2015, 67, 830ꢀ846.
R. Ferraz, L.C. Branco, C. Prudêncio, J. P. Noronha, Ž. Petrovski, ChemMedChem 2011,
6, 975ꢀ985.
J. L. Shamshina, P. S. Barber, R. D. Rogers, Expert Opin Drug Deliv. 2013, 10, 1367ꢀ
1381.
23
24
J. Stoimenovski, D. R. MacFarlane, Chem. Commun. 2011, 47, 11429ꢀ11431.
J. Stoimenovski, D. R. MacFarlane, K. Bica, R. D. Rogers, Pharm. Res. 2010, 27, 521ꢀ
526.
25
26
J. M. M. Araújo, C. Florindo, A. B. Pereiro, N. S. M. Vieira, A. A. Matias, C. M. M.
Duarte, L. P. N. Rebelo, I. M. Marrucho, RSC Adv. 2014, 4, 28126ꢀ28132.
M. Shadid, G. Gurau, J. L. Shamshina, B.ꢀC. Chuang, S. Hailu, E. Guan, S. K.
Chowdhury, J.ꢀT. Wu, S. A. A. Rizvi, R. J. Griffin, R. D. Rogers, Med. Chem. Comm.
2015, 6, 1837ꢀ1841.
20

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27
O. A. Cojocaru, K. Bica, G. Gurau, A. Narita, P. D. McCrary, J. L. Shamshina, P. S.
Barber, R. D. Rogers, Med. Chem. Commun. 2013, 4, 559ꢀ563.
C. Valenta, U. Siman, M. Kratzel, J. Hadgraft, Int. J. Pharm. 2000, 197, 77ꢀ85.
Y. Shahzad, L. J. Waters, C. Barber, Colloids Surf. A 2014, 458, 96ꢀ100.
C. Florindo, J. M. M. Araújo, F. Alves, C. Matos, R. Ferraz, C. Prudêncio, J. P. Noronha,
Ž. Petrovski, L. Branco, L. P. N. Rebelo, I. M. Marrucho, Int. J. Pharm. 2013, 456, 553ꢀ
559.
28
29
30
31
32
33
34
35
36
37
38
I. M. Marrucho, L. C. Branco, L. P. N. Rebelo, Annu. Rev. Chem. Biomol. Eng. 2014. 5,
527–46
M. Vraneš, A. Tot, S. JovanovićꢀŠanta, M. Karaman, S. Dŏzić, K. Tešanović, V. Kojić,
S. Gadžurić, RSC Adv. 2016, 6, 96289ꢀ96295.
Y. Sahbaz, H. D. Williams, T.ꢀH. Nguyen, J. Saunders, L. Ford, S. A. Charman, P. J.
Scammells, C. J. H. Porter, Mol. Pharmaceutics 2015, 12, 1980−1991.
P. C. A. G. Pinto, D. M. G. P. Ribeiro, A. M. O. Azevedo, V. D. Justina, E. Cunha, K.
Bica, M. Vasiloiu, S. Reis, M. L. M. F. S. Saraiva, New J. Chem. 2013, 37, 4095ꢀ4102.
A. R. Dias, J. CostaꢀRodrigues, M. H. Fernandes, R. Ferraz, C. Prudêncio,
ChemMedChem 2016, 11, 1ꢀ9.
C. P. Frizzo, K. Wust, A. Z. Tier, T. S. Beck, L. V. Rodrigues, R. A. Vaucher, L. P.
Bolzan, S. Terra, F. Soares, M. A. P. Martins, RSC Adv. 2016, 6, 100476ꢀ100486.
R. Ferraz, V. Teixeira, D. Rodrigues, R. Fernandes, C. Prudêncio, J. Paulo Noronha, Ž.
Petrovski, L. C. Branco, RSC Adv. 2014, 4, 4301ꢀ4307.
R. Ferraz, L. C. Branco, I. M. Marrucho, J. M. M. Araújo, L. P. N. Rebelo, M. Nunes da
Ponte, C. Prudêncio, J. P. Noronha, Ž. Petrovski, Med. Chem. Commun. 2012, 3, 494ꢀ
497.
39
40
K. S. Egorova, M. M. Seitkalieva, A. V. Posvyatenko, V. N. Khrustalev, V. P. Ananikov,
ACS Med. Chem. Lett. 2015, 6, 1099ꢀ1104.
M. R. Cole, M. Li, B. ElꢀZahab, M. E. Janes, D. Hayes, I. M. Warner, Chem. Biol. Drug
Des. 2011, 78, 33ꢀ41.
41
42
43
H. J. Park, M. R. Prauznitz, AIChe J. 2015, 61, 2732ꢀ2738.
S.ꢀY. Furukawa, G. Hattori, S. Sakai, N. Kamiya, RSC Adv. 2016, 6, 87753ꢀ87755.
H. Wang, G. Gurau, J. Shamshina, O. A. Cojocaru, J. Janikowski, D. R. MacFarlane, J.
H. Davis Jr., R. D. Rogers, Chem. Sci. 2014, 5, 3449–3456.
44
45
S. A. Megwa, S. E. Cross, H. A. E. Benson, M. S. Roberts, J. Pharm. Pharmacol. 2000,
52, 919ꢀ928.
ClinicalTrials.gov Identifier: NCT01198834, titled “MRXꢀ7EAT EtodolacꢀLidocaine
Topical
Patch
in
the
Treatment
of
Ankle
Sprains”,
https://clinicaltrials.gov/ct2/show/NCT01198834, last accessed on 11ꢀ14ꢀ16.
46
47
Y. Miwa, H. Hamamoto, T. Ishida, Eur. J. Pharm. Biopharm. 2016, 102, 92ꢀ100.
R. M. Watkinson, R. H. Guy, J. Hadgraft, M. E. Lane, Skin Pharmacol. Phys. 2009, 22,
225ꢀ230.
48
49
50
P. P. Sarpotdar, J. L. Gaskill, R. P. Giannini, J. Pharm. Sci. 1986, 75, 26ꢀ28.
J. M. Harris, R. B. Chess, Nat. Rev. Drug Discov. 2003, 2, 214ꢀ221.
P. Bonina, L. Montenegro, P. De Caprariis, F. Palagiano, G. Trapani, G. Liso, J. Control.
Release 1995, 34, 223ꢀ232.
21

New Journal of Chemistry
Page 22 of 23
View Article Online
DOI: 10.1039/C6NJ03709G
51
F. P. Bonina, C. Puglia, T. Barbuzzi, P. de Caprariis, F. Palagiano, M. G. Rimoli, A.
Saija, Eur. J. Pharm. Sci. 2001, 14, 123ꢀ134.
52
53
54
55
D. D. N’Da, J. C. Breytenbach, J. Pharm. Pharmacol. 2009, 61, 721ꢀ731.
J. Liaw, J. R. Robinson, Int. J. Pharm. 1992, 88, 125ꢀ140.
F. P. Schwarb, B. Gabard, T. Rufli, C. Surber, Dermatology 1999, 198, 44ꢀ51.
K. Bica, C. Rijksen, M. Nieuwenhuyzen, R. D. Rogers, Phys. Chem. Chem. Phys. 2010,
12, 2011ꢀ2017.
56
57
58
M. B. Harney, R. R. Pant, P. A. Fulmer, J. H. Wynne, ACS Appl. Mater. Inter. 2009, 1,
39ꢀ41.
P. D. McCrary, P. A. Beasley, G. Gurau, A. Narita, P. S. Barber, O. A. Cojocaru, R. D.
Rogers, New J. Chem. 2013, 37, 2196ꢀ2202.
J. F. B. Pereira, K. A. Kurnia, M. G. Freire, J. A. P. Coutinho, R. D. Rogers,
ChemPhysChem 2015, 16, 2219ꢀ2225.
59
60
K. Bica, R. D. Rogers, Chem. Commun. 2010, 46, 1215ꢀ1217.
U. Bengtzelius, W. Gotze, A. Sjolander, J. Phys. C: Solid State Phys. 1984, 17, 5915ꢀ
5934.
61
62
63
P. Taepaiboon, U. Rungsardthong, P. Supaphol, Nanotechnology 2006, 17, 2317–2329.
G. E. Dunn, R. S. McDonald, Can. J. Chem. 1969, 47, 4577ꢀ4588.
V. Tantishaiyakui, N. Phadoongsombut, W. Wongpuwarak, J. Thungtiwachgul, D.
Faroongsarng, K. Wiwattanawongsa, Y. Rojanasakul, Int. J. Pharm. 2004, 283, 111ꢀ116.
V. Sarveiya, J. F. Templeton, H. A. E. Benson, J. Pharm. Pharmacol. 2004, 56, 717ꢀ724.
S. J. Lee, S. W. Kim, J. Control. Release 1987, 6, 3ꢀ13.
64
65
66
G. M. Sheldrick, (2001) SHELXTL, structure determination software suite, v.6.10.
Bruker AXS Inc, Madison, WI, USA.
67
68
69
G. M. Sheldrick, Acta Crystallogr. C. Struct. Chem. 2015, 71, 3ꢀ8.
M. D. Green, J.ꢀH. Choi, K. I. Winey, T. E. Long, Macromolecules 2012, 45, 4749ꢀ4757.
Y. Yang, R. M. Pearson, O. Lee, C.ꢀW. Lee Jr., R. T. Chatterton S. A. Khan, S. Hong,
Adv. Funct. Mater. 2014, 24, 2442ꢀ2449.
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Introducing PEGylated moieties into the counterion structure of API-ILs can significantly enhance
the transport through a membrane without a solvent.