Enhancement of ibuprofen solubility and skin permeation by conjugation with l-valine alkyl esters

Article abstract of DOI:10.1039/d0ra00100g

New ibuprofen derivatives were made via conjugation with l-valine alkyl esters (ValOR), where R was changed from an ethyl to a hexyl group. The ionic structure was confirmed using NMR and FTIR. Specific rotation, solubility in commonly used solvents, thermal properties including phase transitions temperatures, and thermal stability were also determined. The ionic structure with a protonated amine group on an l-valine ester and melting points below 100 °C allowed inclusion of these ibuprofen derivatives into the pharmaceutically active protic ionic liquids. The ibuprofen salt solubility in deionised water and two buffer solutions at pH 5.4 and 7.4 were established and compared with the parent acid solubility. The octanol/water (buffer) partition coefficient, permeation through porcine skin, and accumulation in the skin were also measured. Ibuprofen pairing with l-valine alkyl esters [ValOR][IBU], caused higher solubility and a greater drug molecule absorption through biological membranes. log?P was lower for ibuprofen salts than for the acid and it increased with a longer l-valine ester cation alkyl chain. In vitro porcine skin tests showed that ibuprofen salts with a propyl or isopropyl ester in l-valine are particularly relevant for topical application. They provide transport for ibuprofen through the skin at much higher rate than the unmodified acid and a higher permeated ibuprofen concentration, which can improve efficacy. Thus, synthesised ibuprofen derivatives could be used as drug carriers in transdermal systems to provide better drug bioavailability, and they can be also be the source of exogenous l-valine.

Full text of DOI:10.1039/d0ra00100g

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Enhancement of ibuprofen solubility and skin
permeation by conjugation with L-valine alkyl
Cite this: RSC Adv., 2020, 10, 7570
esters†
a
a
a
b
Ewa Janus, * Paula Ossowicz,
Joanna Klebeko,
Anna Nowak,
b
b
b
Wiktoria Duchnik,
Łukasz Kucharski
and Adam Klimowicz
New ibuprofen derivatives were made via conjugation with L-valine alkyl esters (ValOR), where R was
changed from an ethyl to a hexyl group. The ionic structure was confirmed using NMR and FTIR. Specific
rotation, solubility in commonly used solvents, thermal properties including phase transitions
temperatures, and thermal stability were also determined. The ionic structure with a protonated amine
ꢀ
group on an L-valine ester and melting points below 100 C allowed inclusion of these ibuprofen
derivatives into the pharmaceutically active protic ionic liquids. The ibuprofen salt solubility in deionised
water and two buffer solutions at pH 5.4 and 7.4 were established and compared with the parent acid
solubility. The octanol/water (buffer) partition coefficient, permeation through porcine skin, and
accumulation in the skin were also measured. Ibuprofen pairing with L-valine alkyl esters [ValOR][IBU],
caused higher solubility and a greater drug molecule absorption through biological membranes. log P
was lower for ibuprofen salts than for the acid and it increased with a longer L-valine ester cation alkyl
chain. In vitro porcine skin tests showed that ibuprofen salts with a propyl or isopropyl ester in L-valine
are particularly relevant for topical application. They provide transport for ibuprofen through the skin at
much higher rate than the unmodified acid and a higher permeated ibuprofen concentration, which can
improve efficacy. Thus, synthesised ibuprofen derivatives could be used as drug carriers in transdermal
systems to provide better drug bioavailability, and they can be also be the source of exogenous L-valine.
Received 4th January 2020
Accepted 12th February 2020
DOI: 10.1039/d0ra00100g
rsc.li/rsc-advances
solubility was recorded by various authors as the pK
a
(e.g. pK
a
¼
Introduction
4,5
4
.44), was achieved and at a pH above the pK , the solubility
a
Ibuprofen (IBU) [2-(4-isobutylphenyl)-propionic acid] is a repre- increases greatly, which results from ionisation. For example,
ꢀ
sentative non-steroidal anti-inammatory drug (NSAID), and it Potthast et al. showed that solubility of ibuprofen at 37 C was
ꢁ
3
ꢁ3
is commonly used to treat rheumatoid arthritis and osteoar- 0.038 mg cm at pH 1 and it increased two-times (0.084 mg cm
)
ꢁ3
thritis, pain, and fever. The oral delivery route has been the and almost 20-times (0.685 mg cm ) when the pH was changed to
most widely used. To treat pain and fever, it is desirable to 4.5 and 5.5, respectively. However, the experimentally determined
achieve quick relief, which is directly related to the appropriate solubility at pH 6.8 was already 100-times higher (3.37 mg cm ).
ꢁ3
3
1
concentration of ibuprofen in the blood.
The simplest modication of ibuprofen is the formation of its
The absorption rate into the blood depends on the ibuprofen salts. By replacing the acid proton with the sodium cation, solu-
dissolution rate. However, ibuprofen, which is a carboxylic acid, bility in polar solvents, including water, was signicantly increased
6
has a relatively high lipophilicity and shows poor solubility in an compared to parent acid. It had a measurable effect on faster
7–10
aqueous media. Ibuprofen solubility depends on the pH, and it is action and earlier pain relief. The ibuprofen anion can also be
especially low in the acidic environment of the stomach; changes paired with organic cations, such as for the amino acids lysine,
2–4
are slight within the pH range of 1–4. An increase in ibuprofen arginine and polylysine, in which the positive charge is located on
11–14
the nitrogen atom of the amino group.
Ibuprofen arginine or
a
lysine salts, similar to its sodium salt, showed better solubility and
a higher mean maximum plasma or serum ibuprofen concentra-
West Pomeranian University of Technology, Szczecin, Faculty of Chemical Technology
and Engineering, Department of Chemical Organic Technology and Polymeric
Materials, Piast ´o w Ave. 42, 71-065 Szczecin, Poland. E-mail: ejanus@zut.edu.pl; tion in a shorter time.
Tel: +48 91 449 4584
15–18
Additional signicant advantages of
ibuprofen conjugates with amino acids including providing
a source of amino acids, which are nutritional substances and
possess marked activity. It was demonstrated that ibuprofen
arginate retains key functional NSAID effects with similar or
b
Pomeranian Medical University in Szczecin, Department of Cosmetic and
Pharmaceutical Chemistry, Powsta n´ c ´o w Wielkopolskich Ave. 72, 70-111 Szczecin,
Poland
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra00100g
1
7570 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
increased potency compared to sodium salt, and it can also
decrease the risk of cardiovascular side effects.
The aim of this study was to synthesise a series of ibuprofen
derivatives as ion-pairs with a counterion based on L-valine. In
19,20
Formation of ibuprofen ion-pairs using organic bases was addition to the expected effects for better solubility and
also shown to be an effective approach in topical ibuprofen permeation of ibuprofen, such derivatives can be the source of
administration. Dermal delivery overcomes several side effects L-valine. Valine is an essential exogenous amino acid. It has an
of systemic use. It connes the therapeutic effects to the anti-catabolic effect, which means that it inhibits muscle-
affected site and limits systematic absorption to the vascular building protein degradation process. It also triggers gluco-
21
system, thereby avoiding rst-pass hepatic metabolism and neogenesis, i.e. the transformation of non-sugar precursors into
22,23
ibuprofen gastrointestinal toxicity.
However, the highly glucose. Valine is necessary for proper immune and nervous
organised stratum corneum is a crucial barrier, which controls system functions, and it provides the body with energy, reduces
percutaneous absorption, and it thereby limits the achievement fatigue, and has a positive effect on mood. Valine intensies the
of effective therapeutic concentrations. Three penetration secretion of anabolic hormones. It is necessary for the synthesis
routes were considered including intercellular, intracellular, of pantothenic acid, i.e. vitamin B5.
and follicular pathways, and lipid organisation in the intercel-
We designed and characterised ibuprofen salts with L-valine
alkyl esters, in which the alkyl group had different lengths from
24,25
lular space is important for skin barrier properties.
26
Sarveiya et al. reported increased diffusion of ibuprofen ethyl to hexyl. Physicochemical properties, i.e. thermal stability,
alkylamine salts through the PDMS (polydimethylsiloxane) specic rotation, solubility in organic solvents, and water, including
membrane compared to the sodium salt. Recently, various buffer solutions, partition coefficient, and skin penetration were
aromatic, tetra-alkylammonium and tetra-alkylphosphonium determined for salts and compared with those of the parent acid.
counterions were paired with the ibuprofen anion to form ionic
liquids. Most of these salts have a markedly improved solubility
in water compared to the parent acid. Two of these nine salts, i.e.
Experimental
tetrahexylammonium and didecyldimethylammonium, provided Materials
more and faster ibuprofen permeation. All the synthesised
All reagents were commercially available materials and were
ibuprofen ionic liquids showed improved skin permeability
compared to the conventional sodium salt. Furukawa et al.
used without further purication. (R,S)-Ibuprofen (99%) was
27
28
obtained from Acros Organics (Geel, Belgium). L-Valine ($99%)
was provided by Carl Roth (Karlsruhe, Germany). Trimethylsilyl
chloride ($99%) (TMSCl) was purchased from Sigma-Aldrich
combined ibuprofen with proline ethyl ester as the base to form
an ionic liquid, which showed signicant enhancement in the
cumulative amount of drug in the permeation experiments using
(
(
(
Steinheim am Albuch, Germany). Methanol (MeOH), ethanol
EtOH), propan-2-ol (iPrOH), propan-1-ol (PrOH), butan-1-ol
BuOH), pentan-1-ol (AmOH), hexan-1-ol (HexOH), acetic acid,
29
pig skin compared to the acid form. Rogers et al. revealed that
ibuprofen with lidocaine form liquid co-crystals in a deep
eutectic form, and both components can be simultaneously
transported through the model membrane at much higher rates
than the corresponding commercially available crystalline salts,
i.e. lidocaine chloride and ibuprofen sodium.
potassium chloride, sodium chloride, sodium sulfate anhy-
drous, orthophosphoric acid (98%), dimethyl sulfoxide, chlo-
roform, ethyl acetate, diethyl ether, toluene and n-hexane were
high purity provided by Chempur (Gliwice, Poland). Ammo-
nium hydroxide solution 25% (NH $H O) was of analytical
3
2
Scheme 1 Synthesis path and structures of ibuprofen salts with L-valine alkyl esters.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 7570–7584 | 7571

View Article Online
RSC Advances
Paper
grade purchased from StanLab (Lublin, Poland). Acetonitrile 2.01 (m, 1H, H13); 1.74–1.80 (m, 1H, H9); 1.38 (d, 3H, J3,2
¼
(
$99.9%) for HPLC gradient grade and n-octanol ($99%) were 7.1 Hz, H3); 1.18 (t, 3H, J18,17 ¼ 7.1/7.4 Hz, H18); 0.86 (d, 3H,
obtained from Sigma-Aldrich (Steinheim am Albuch, Germany). J12,13 ¼ 6.8 Hz, H12); 0.82 (d, 6H, J
¼ 6.6 Hz, H10); 0.81 (d,
10,9
1
3
Disodium hydrogen phosphate dihydrate ($99%) (Na HPO - 3H, J
¼ 6.9 Hz, H11); C NMR (100 MHz, CDCl )
2
4
11,13
3
$
2H
Pennsylvania, USA) and sodium dihydrogen phosphate anhy- 129.21 (C5); 127.26 (C6); 61.05 (C17); 59.00 (C14); 45.62 (C8);
drous (98%) (NaH PO ) was obtained from Acros Organics 45.06 (C2); 31.51 (C13); 29.71 (C9); 22.41 (C10); 18.79 (C12);
Geel, Belgium). Deuterated chloroform (CDCl (99.8%) 18.50 (C3); 17.29 (C11); 14.23 (C18); FT-IR: n(ATR): 2958;
+0.03% TMSCl) was obtained from Eurisotop (Cheshire, 2930; 2867; 1741; 1606; 1511; 1464; 1416; 1387; 1359; 1328;
2
O) was obtained from Fisher Bioreagents (Pittsburgh, d in ppm: 179.25 (C1); 173.92 (C16); 140.25 (C7); 138.35 (C4);
2
4
(
(
3
)
ꢁ
1
England). Potassium dihydrogen phosphate anhydrous (99%) 1288; 1259; 1226; 1210; 1175; 1114; 1059; 983; 881; 692 cm
;
was provided from Merck (Darmstadt, Germany).
elemental analysis: calc. (%) for C20H33NO4 (351.485): C
Porcine skin was used in the permeation experiments due to 68.34, H 9.46, N 3.99, O 18.21, found C 68.25, H 9.47, N 3.92,
30
its similar permeability to human skin. The fresh abdominal O 18.18.
porcine skin was washed in PBS buffer pH 7.4 several times.
Aer drying, dermatome section skin 0.5 mm thick was
prepared (Humby Dermatome, Surtex Instruments, New Mal-
[
ValOPr][IBU] – L-valine propyl ester ibuprofenate
den, England). Then samples of skin were stored frozen at
ꢀ
ꢁ
20 C not longer than 3 months. This time was safe to keep the
31
barrier properties of skin. Prior to the permeation experi-
ments, the skin samples were thawed at room temperature for
3
0 min. Then the skin was allowed to hydration by phosphate
32–34
buffer pH 7.4.
Skin integrity was evaluated based on skin
electrical impedance measurements (see below). Only undam-
aged skin pieces were used in the permeation experiments.
1
Yield: 95%. H NMR (400 MHz, CDCl
3
) d in ppm: 7.21 (d, 2H, J5,6
¼
8.1 Hz, H5); 7.08 (d, 2H, J6,5 ¼ 8.4 Hz, H6); 5.46 (s, 3H, H15);
.05–4.10 (m, 2H, H17); 3.63–3.67 (q, 1H, H2); 3.39 (d, 1H, J14,13
4.7 Hz, H14); 2.43 (d, 2H, J8,9 ¼ 7.1 Hz, H8); 2.04–2.08 (m, 1H,
H13); 1.81–1.87 (m, 1H, H9); 1.63–1.68 (m, 2H, H18); 1.46 (d,
H, J3,2 ¼ 7.3 Hz, H3); 0.88–0.96 (m, 15H, H10, H11, H12, H19);
4
¼
Synthesis of ibuprofen derivatives
The three-step synthesis of L-valine alkyl esters ibuprofenates
was shown in the Scheme 1. General procedure for L-valine alkyl
3
esters hydrochlorides (ValOR$HCl) synthesis, their neutraliza- 13
3
C NMR (100 MHz, CDCl ) d in ppm: 179.19 (C1); 173.34 (C16);
tion to L-valine alkyl ester (ValOR) together with an identica-
1
40.18 (C7); 138.46 (C4); 129.18 (C5); 127.25 (C6); 68.71 (C17);
8.95 (C14); 45.44 (C8); 45.06 (C2); 30.19 (C13); 30.19 (C9); 22.40
C10); 22.40 (C18); 18.73 (C12); 18.53 (C3); 17.23 (C11); 10.41
C19); FT-IR: n(ATR): 2967; 2933; 2868; 2847; 2690; 2363; 2343;
193; 1744; 1680; 1609; 1548; 1513; 1467; 1414; 1390; 1360;
329; 1321; 1308; 1290; 1260; 1221; 1176; 1153; 1116; 1061; 929;
tion and properties of these intermediates are available in (ESI,
5
S4–S16† for ValOR$HCl; S17–S29 for ValOR).
(
(
General procedure for conjugation of ibuprofen with L-valine
alkyl esters for preparation of [ValOR][IBU].
2
The L-valine alkyl ester (ValOR) was dissolved in diethyl ether
or chloroform and added into equimolar amount of ibuprofen
1
ꢁ1
8
83; 693 cm ; elemental analysis: calc. (%) for C H NO
2
1
35
4
and stirred thoroughly at room temperature for 20 minutes.
(
365.512): C 69.01, H 9.65, N 3.83, O 17.51, found: C 68.80, H
.70, N 3.82, O 17.50.
Then solvent was removed by evaporation under vacuum at
9
ꢀ
3
6
5 C. The obtained ibuprofenate was dried in a vacuum oven at
ꢀ
0 C for 24 h.
[
ValOiPr][IBU] – L-valine isopropyl ester ibuprofenate
1
13
H and C NMR spectra, FTIR spectra, TG and DSC curves
for [ValOR][IBU] are available in ESI (S30–S47†).
[ValOEt][IBU] – L-valine ethyl ester ibuprofenate
1
Yield: 94%. H NMR (400 MHz, CDCl
3
) d in ppm: 7.21 (d, 2H, J5,6
¼
8.1 Hz, H5); 7.07 (d, 2H, J6,5 ¼ 8.1 Hz, H6); 5.59 (s, 3H, H15);
5
.01–5.07 (m, 1H, H17); 3.61–3.66 (q, 1H, H2); 3.36 (d, 1H, J14,13
¼
3.9 Hz, H14); 2.43 (d, 2H, J8,9 ¼ 7.3 Hz, H8); 2.04–2.08 (m, 1H,
1
Yield: 99%. H NMR (400 MHz, CDCl ) d in ppm: 7.2 (d, 2H, H13); 1.81–1.87 (m, 1H, H9); 1.45 (d, 3H, J ¼ 7.3 Hz, H3); 1.24
3
3,2
J5,6 ¼ 8.1 Hz, H5); 7.00 (d, 2H, J ¼ 8.1 Hz, H6); 5.33 (s, 3H, (dd, 6H, J19,17 ¼ 6.2 J
¼ 6.2 Hz, H18, H19); 0.93 (d, 3H, J
6,5
18,17
12,13
H15); 4.06–4.13 (m, 2H, H17); 3.54–3.59 (q, 1H, H2); 3.31 (d, ¼ 7.1 Hz, H12); 0.89 (d, 6H, J10,9 ¼ 6.6 Hz, H10); 0.88 (d, 3H,
1
3
1
H, J14,13 ¼ 4.6 Hz, H14); 2.36 (d, 2H, J8,9 ¼ 7.1 Hz, H8); 1.96–
J
11,13 ¼ 6.9 Hz, H11); C NMR (100 MHz, CDCl
3
) d in ppm:
7572 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
1
1
3
79.17 (C1); 173.34 (C6); 140.18 (C7); 138.46 (C4); 129.18 (C5); d in ppm: 179.19 (C1); 174.33 (C16); 140.33 (C7); 138.21 (C4);
27.25 (C6); 68.71 (C17); 58.95 (C14); 45.73 (C8); 45.06 (C2); 129.23 (C5); 127.25 (C6); 65.19 (C17); 59.12 (C14); 45.51 (C8);
1.47 (C13); 30.19 (C9); 22.40 (C10); 21.78 (C18, C19); 18.73 45.07 (C2); 31.62 (C13); 30.20 (C9); 28.27 (C18); 28.03 (C19);
(C12); 18.53 (C3); 17.23 (C11); FT-IR: n(ATR): 2966; 2953; 2925; 22.42 (C10); 22.27 (C20); 18.89 (C12); 18.46 (C3); 17.24 (C11);
2
1
6
869; 2606; 1742; 1614; 1585; 1509; 1465; 1417; 1386; 1366; 13.96 (C21); FT-IR: n(ATR): 2955; 2930; 2867; 2730; 2694; 2616;
330; 1288; 1228; 1182; 1146; 1106; 1053; 917; 880; 825; 786; 2604; 1739; 1603; 1583; 1541; 1510; 1463; 1417; 1388; 1359;
ꢁ
1
77; 592 cm ; elemental analysis: calc. (%) for C21
H
35NO
4
1321; 1288; 1228; 1228; 1207; 1176; 1168; 1114; 1079; 1057; 971;
ꢁ
1
(
365.512): C 69.01, H 9.65, N 3.83, O 17.51, found: C 69.23, H 881; 692 cm ; elemental analysis: calc. (%) for C23
.65, N 3.86, O 17.45. (393.565): C 70.08, H 9.99, N 3.56, O 16.26, found: C 70.08, H
.99, N 3.74, O 16.25.
H39NO
4
9
9
[ValOBu][IBU] – L-valine butyl ester ibuprofenate
[
ValOHex][IBU] – L-valine hexyl ester ibuprofenate
1
Yield: 94%. H NMR (400 MHz, CDCl
3
) d in ppm: 7.22 (d, 2H, J5,6
¼
8.1 Hz, H5); 7.08 (d, 2H, J6,5 ¼ 8.0 Hz, H6); 5.09 (s, 3H, H15);
.08–4.15 (m, 2H, H17); 3.63–3.68 (q, 1H, H2); 3.38 (d, 1H, J14,13
4.6 Hz, H14); 2.43 (d, 2H, J8,9 ¼ 7.0 Hz, H8); 2.04–2.08 (m, 1H,
H13); 1.80–1.87 (m, 1H, H9); 1.58–1.65 (m, 2H, H18); 1.46 (d,
4
¼
1
Yield: 96%. H NMR (400 MHz, CDCl
3
) d in ppm: 7.21 (d, 2H, J5,6
¼
7.9 Hz, H5); 7.06 (d, 2H, J6,5 ¼ 8.1 Hz, H6); 6.13 (s, 3H, H15);
4
4
.06–4.14 (m, 2H, H17); 3.60–3.64 (q, 1H, H2); 3.40 (d, 1H, J
¼
3
1
H, J3,2 ¼ 7.1 Hz, H3); 1.33–1.42 (m, 2H, H19); 0.88–0.96 (m,
14,13
1
3
.5 Hz, H14); 2.43 (d, 2H, J8,9 ¼ 7.1 Hz, H8); 2.02–2.06 (m, 1H,
5H, H10, H11, H12, H20); C NMR (100 MHz, CDCl )
3
H13); 1.80–1.87 (m, 1H, H9); 1.58–1.64 (m, 2H, H18); 1.44 (d, 3H,
d in ppm: 179.16 (C1); 174.44 (C6); 140.36 (C7); 138.21 (C4);
J
(
3,2 ¼ 7.1 Hz, H3); 1.26–1.36 (m, 6H, H19, H20, H21); 0.87–0.93
1
4
1
29.24 (C5); 127.25 (C6); 64.88 (C7); 59.20 (C14); 45.45 (C8);
5.07 (C2); 31.68 (C13); 30.63 (C18); 30.18 (C9); 22.41 (C10);
9.13 (C19); 18.91 (C12); 18.43 (C3); 17.24 (C11); 13.06 (C20); FT-
13
3
m, 15H, H10, H11, H12, H22); C NMR (100 MHz, CDCl )
d in ppm: 179.22 (C1); 173.89 (C16); 140.14 (C7); 138.54 (C4);
1
4
29.17 (C5); 127.26 (C6); 65.26 (C17); 58.97 (C14); 45.73 (C8);
5.07 (C2); 31.46 (C13); 31.37 (C18); 30.20 (C9); 28.53 (C19); 25.55
IR: n(ATR): 2962; 2933; 2870; 2848; 2691; 2604; 2349; 2313; 2184;
1
1
743; 1606; 1545; 1511; 1466; 1391; 1361; 1320; 1301; 1288;
ꢁ1
(
(
C20); 22.52 (C21); 22.42 (C10); 18.75 (C12); 18.55 (C3); 17.31
C11); 14.00 (C22); FT-IR: n(ATR): 2958; 2930; 2867; 2731; 2165;
260; 1245; 1215; 1176; 1117; 1062; 881; 727; 694 cm
;
elemental analysis: calc. (%) for C H NO (379.539): C 69.62,
2
2
37
4
1
1
741; 1606; 1511; 1464; 1416; 1387; 1359; 1328; 1288; 1259; 1226;
H 9.83, N 3.69, O 16.86, found: C 69.64, H 9.82, N 3.67, O 16.77.
ꢁ
1
210; 1175; 1114; 1059; 983; 881; 692 cm ; elemental analysis:
(405.576): C 70.72, H 10.14, N 3.44, O
5.70, found: C 70.39, H 10.10, N 3.44, O 15.64.
calc. (%) for C24H41NO
4
1
[ValOAm][IBU] – L-valine pentyl ester ibuprofenate
Methods
General analytical methods – NMR, FTIR, UV-VIS, Elemental
analysis, Thermogravimetric (TG) analysis, Differential Scan-
ning Calorimetry (DSC), specic rotation, were presented in ESI
(S2 and S3†).
A Shimadzu, model Nexera-i, LC-2040C 3D PLUS HPLC
system (Kyoto, Japan), equipped with a UV-VIS/DAD detector
and Kinetex® F5 column (2.6 mm; 150 ꢂ 4.6 mm; Phenomenex,
1
Yield: 95%. H NMR (400 MHz, CDCl
3
) d in ppm: 7.21 (d, 2H, J5,6 Torrance, CA, USA) was used in solubility and partition coeffi-
ꢀ
¼
8.1 Hz, H5); 7.07 (d, 2H, J6,5 ¼ 8.1 Hz, H6); 5.36 (s, 3H, H15); cient experiments. Analyses were performed at 30 C under
4
.04–4.14 (m, 2H, H17); 3.62–3.67 (q, 1H, H2); 3.39 (d, 1H, J14,13 isocratic conditions, with mobile phase consisted of water–
3
ꢁ1
¼
4.6 Hz, H14); 2.43 (d, 2H, J8,9 ¼ 7.0 Hz, H8); 2.03–2.07 (m, 1H, acetonitrile mixture (50/50, v/v) and a ow rate of 1 cm min
.
H13); 1.79–1.86 (m, 1H, H9); 1.61–1.65 (m, 2H, H18); 1.46 (d, Detection wavelength was 210 nm. Data acquisition and pro-
3
H, J3,2 ¼ 7.1 Hz, H3); 1.30–1.34 (m, 4H, H19, H20); 0.88–0.95 cessing were performed using a LabSolutions/LC Solution
13
(m, 15H, H10, H11, H12, H21); C NMR (100 MHz, CDCl
3
)
System (soware). Injections were repeated at least three times
RSC Adv., 2020, 10, 7570–7584 | 7573
This journal is © The Royal Society of Chemistry 2020

View Article Online
RSC Advances
Paper
for each sample and the results were averaged. Concentration of
Concentration of the compound dissolved in octanol was
ibuprofen and its salts were calculated on peak area measure- calculated by the formula:
ments using a calibration curve method.
ꢁ
3
c
oct ¼ c
0
ꢁ c
w
[mg dm
]
A Knauer (Berlin, German) liquid chromatograph consisting
of a WellChrom model K1001 pump, model K2600 UV detector,
model EuroChrom 2000 integrator, a Rheodyne model 7125
injector, equipped with UV-VIS detector and Hypersil ODS (C18)
column (5 mm; 125 ꢂ 4.0 mm I.D.; Thermo Scientic™ Wal-
tham, MA, USA) was used in skin permeation experiments for
ꢁ
3
where c
0
is a total concentration [mg dm ], calculated based on
the mass of compound used in the experiment.
Skin electrical impedance
determination of compounds concentration in acceptor uid Skin impedance was measured using an LCR meter 4080
and for measurements of accumulation in the skin. Isocratic (Voltcra LCR 4080, Conrad Electronic, Hirschau, Germany),
elution with mobile phase consisted of acetonitrile – 1% (w/w) which was operated in parallel mode at an alternating frequency
aqueous solution of acetic acid – methanol (45/45/10, v/v/v) at of 120 Hz (error at kU values <0.5%). Skin sample was placed in
3
ꢁ1
1
cm min ow rate was carried out. The signals were moni- Franz diffusion cell. For the measurement, the tips of
ꢀ
tored at 264 nm and the temperature was 25 C. Duplicate measuring probes were immersed in donor and in acceptor
injections (20 mL) were carried out.
36
chamber, both lled with PBS buffer pH 7.4. Skin samples
with determined impedance above 3 kU were used in the
permeation experiments. These values were similar to the
Solubility experiments
37
electrical resistance of human skin reported by others.
Solubility of ibuprofen and salts of ibuprofen with L-valine
esters was evaluated in polar and nonpolar solvents by modied
Vogel's method at the temperature of 25 C. By this method
Skin permeation studies
35
ꢀ
the compound was classied as soluble, partly soluble and The permeation experiments were performed in the Franz
practically insoluble. Dimethyl sulfoxide, ethanol, chloroform, diffusion cells (SES GmbH Analyse Systeme, Bechenheim, Ger-
2
ethyl acetate, diethyl ether, toluene and n-hexane were used as many) with a diffusion area of 1 cm . The volume of the acceptor
3
solvents.
chamber was 8 cm and the volume of the donor chamber was
3
Solubility (saturation concentration) dened as a maximum approximately 2 cm . The acceptor chamber was lled with
quantity of a substance that may be dissolved at given temper- phosphate buffer pH 7.4 (0.1 M). In each diffusion unit
ꢀ
ature in unit of solvent volume was determined for ibuprofen a constant temperature of 32.0 ꢃ 0.5 C was maintained via
and salts of ibuprofen with L-valine alkyl esters in deionised thermostat (VEB MLW Pr
u¨ fger ¨a te-Werk type of 3280, Leipzig,
water and phosphate buffers (pH 5.4 and 7.4). An excess of Germany). The content of the acceptor chamber was stirred with
3
substance was added to 2 cm of water or buffer in screwed vial a magnetic stirring bar at the same speed for all cells. The donor
ꢀ
and was stirred vigorously at 25.00 ꢃ 0.05 C or at 32.00 ꢃ chamber was lled with the solution of a tested compound in
ꢁ3
ꢀ
0
.05 C, for 24 hours. Then mixture was centrifuged at respec- ethanol 70% (v/v). Concentration of compound was 0.01 g cm .
tive temperature, and liquid above the solid was taken, diluted
The undamaged skin pieces, prepared as described in the
and analysed by the HPLC method to determine the concen- Material section, were placed between donor and acceptor
tration of the substance.
chamber of Franz diffusion cells and integrity has been
checked. Following by a 500 mL of the solution of the tested
compound was applied to each skin sample.
Determination of partition coefficient
The experiment was carried for 24 hours. The acceptor uid
The n-octanol/water and n-octanol/phosphate buffer (pH 5.4
and 7.4) partition coefficient was determined for ibuprofen and
salts of ibuprofen with L-valine alkyl esters. For this purpose,
3
was sampled (0.3 cm ) aer 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 8 h and
24 h and the acceptor chamber was relled with the same
volume of the fresh buffer. Experiments were carried out in
triplicate for each compound. The drug concentration in the
acceptor uid was determined using HPLC method. The
1
0 mg of the substance, weighed with an accuracy of 0.01 mg,
3
was added to 5 cm of water (or buffer) saturated with n-octanol
and 5 cm of n-octanol saturated with water. Mixture was
3
ꢁ2
cumulative mass (mg cm ) was calculated based on this
ꢀ
vigorously agitated at 25 C for 3 hours followed by centrifu-
gation at 7500 rpm, at 25 C for 10 min for the better phase
separation. Aer centrifugation aqueous layer was decanted
and analysed by HPLC to determine the concentration of the
compound. The partition coefficient log Pow was calculated in
accordance with the formula:
ꢁ2 ꢁ1
concentration. The ux (in mg cm
h ) of the active drug –
ꢀ
ibuprofen and its derivatives through the skin into acceptor
uid was determined as the slope of the plot of cumulative mass

in the acceptor uid versus time.
Permeation of ibuprofen was also expressed as percentage of
applied dose of ibuprofen in compound.
log Pow ¼ log coct ꢁ log c
w
Accumulation in the skin
ꢁ3
where cw and coct represents concentration [mg dm ] of the Aer 24 hours permeability tests, the donor chamber was
substance dissolved in aqueous layer (water or phosphate disconnected from Franz diffusion cell and each skin sample
buffer) and octanol respectively.
was removed, carefully rinsed in PBS solution at 7.4 pH and
7574 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
dried at the room temperature. The skin sample was weighed, The presence of the protonated amine group in ibuprofen salts
diffusion area cut out and minced using scissors. Next skin with L-valine esters was conrmed by strongly downeld-shied
3
sample was placed in 2 cm methanol and was incubated for N–H signals, which appeared in CDCl at d 5–6 ppm. Integration
2
3
ꢀ
4 h at the temperature of 4 C. Aer this time skin sample was of these signals corresponds to three protons. The results of
homogenised for 3 minutes using a homogenizer (IKA®T18 NMR analysis conrm the ionic structure of the synthesised
digital ULTRA TURRAX, Staufen im Breisgau, Germany). The compounds [ValOR][IBU] and the equimolar ratio between both
homogenate was centrifuged at 3500 rpm for 5 min. The ionic components (ibuprofen anion and L-valine alkyl ester
supernatant was collected and analysed by HPLC.
cation). Additionally, evidence for the ionic structure is the
Accumulation of the ibuprofen in the skin was calculated by carbonyl carbon signal, which is shied about 2 ppm upeld (to
dividing the amount of the drug remaining in the skin by about 179 ppm) in ibuprofen derivatives compared to the
a mass of skin sample and was expressed in mass of ibuprofen chemical shi value for this carbon in the parent ibuprofen
ꢁ1
39–41
per mass of the skin (mg g ). The average and standard devia- (181.16 ppm).
tion of these parameters were calculated from three replicates. The characteristic IR absorption bands for [ValOR][IBU] also
Moreover, accumulation of ibuprofen in the skin was expressed conrmed the ionic structure. The FTIR spectra for ibuprofen, L-
as percentage of applied dose of ibuprofen in the compound in valine ester, L-valine ester hydrochloride, and ester ibuprofenate
the donor chamber.
are presented for comparison in Fig. 2. In FTIR spectra of all
ibuprofen derivatives, the characteristic sharp absorption band
was observed in the range of 1739–1744 cm , which is attrib-
uted to C]O stretching vibrations of the ester carbonyl group in
the L-valine ester part. FTIR spectra of L-valine ester and L-valine
ꢁ1
Statistical analysis
Statistical calculations were done using Statistica 12 PL soware
(StatSo, Polska). Results were presented as an arithmetical
ester hydrochloride also showed this band and conrmed this
mean ꢃ standard deviation (SD). The results were evaluated
using one-way analysis of variance (ANOVA). All determinations
were performed in triplicate (n ¼ 3).
ꢁ1

nding. For ibuprofen, there was a broad band at 1709 cm
,
which is characteristic of the C]O stretching vibrations of the
carboxylic acid group. In ibuprofen derivatives, FTIR spectra of
this band were shied and two bands were observed at
ꢁ
1
approximately 1610 and 1390 cm , which are assigned to the
Results and discussion
Synthesis, identication and characterisation of the [ValOR]
ꢁ
asymmetric v(COO ) and symmetric stretching vibrations
as
ꢁ
v(COO )sym, respectively. This shi and a difference above
[IBU]
ꢁ1
2
00 cm between the frequency values of these two bands
ꢁ
Salts of ibuprofen with L-valine alkyl esters were synthesised conrmed the presence of the carboxylate anion COO and the
42
using the three-step method (Scheme 1). In the rst step, the ionic structure of ibuprofen derivatives. The differences in the
hydrochlorides of L-valine alkyl esters (L-ValOR$HCl) were IR bands for the salts of ibuprofen with various L-valine alkyl
prepared from L-valine and the respective alcohol as starting esters were negligible.
materials in the presence of chlorotrimethylsilane, with
Elemental analysis conrmed the content of individual
a similar method as that used previously for hydrochlorides of elements (C, H, N, O) in synthesised ibuprofen derivatives.
38
amino acid methyl esters. The length of the alkyl chain (R)
Salts of ibuprofen with L-valine alkyl esters [ValOR][IBU] were
ester group was from C2 to C6, and the C3 group was used as white solids at room temperature, which was opposite of the
a straight-chain n-propyl and branched-chain isopropyl group. series of ibuprofen salts with counterions, such as phospho-
Hydrochlorides were puried from by-products, such as hex- nium, imidazolium, ammonium, procaine, lidocaine, or rani-
27
amethyldisilane, trimethylsilyl hydroxide, alkyltrimethylsilyl tidine that were recently reported by Wu et al. which were
ether and obtained with high yields of 89–95%. Then hydro- liquids at ambient temperature. As shown in Table 1, the
chlorides were neutralised with excess aqueous ammonia to the melting point of [ValOR][IBU], which was determined using the
respective amino acid esters (ValOR) with yields of 76–84%, and DSC method as a peak temperature, depended on the alkyl
ꢀ
this was followed by pairing the ester with ibuprofen to form chain R in the ester group and it was between 67.4 and 79.8 C.
[ValOR][IBU]. No by-products were formed in the last synthesis The lowest melting point was for ibuprofen paired with L-valine
stage and the obtained ibuprofenates were characterized by very hexyl ester and the highest melting point was for L-valine propyl
high purity. The structure and purity of the products at each ester. Salts containing butyl and longer chains in the ester
1
13
stage was checked and conrmed using H and C-NMR group of L-valine showed lower melting points than the parent
ꢀ
spectroscopy (attached in ESI†). For example, comparison of acid (77.5 C). Ibuprofen salts give very broad melting peaks
1
the three H NMR spectra for hydrochloride L-valine ethyl ester compared to the sharp melting peak for ibuprofen, which can
and L-valine ethyl ester and its ionic pair with ibuprofen is be caused by the size distribution of the crystallites. All syn-
shown in Fig. 1. The NH
3
protons in hydrochloride L-valine alkyl thesised compounds resulting from a melting point below
ꢀ
ester appear as a somewhat broadened signal at d 8.7–8.9 ppm 100 C and the ionic structure can be classied as ionic
43
in CDCl . The broadening is partially caused by a rapid inter- liquids. DSC curves for ibuprofen salts were recorded in
3
molecular exchange. Aer neutralisation of hydrochloride, the a heating–cooling–heating sequence of the sample and are
NH
2
protons in L-valine alkyl ester appear as a signal at d 1– available in ESI.† In DSC analysis, the exothermic peak of
, which is typical for the primary amine group. crystallisation for each ibuprofen salt, [ValOR][IBU], was
2
ppm in CDCl
3
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 7570–7584 | 7575

View Article Online
RSC Advances
Paper
1
Fig. 1 Comparison of H NMR spectra for hydrochloride L-valine ethyl ester [ValOEt][HCl] (dark blue), ethyl ester of L-valine [ValOEt] (green), and
its ibuprofen salt [ValOEt][IBU] (red) (the amino and protonated amino group are marked in the red square).
Fig. 2 Comparison of FTIR spectra ibuprofen (dark blue), L-valine ethyl ester hydrochloride (green), L-valine ethyl ester [ValOEt] (light blue), and
ꢁ
ꢁ
L-valine ethyl ester ibuprofenate (red) (v(COO )as and v(COO )sym are marked with a yellow and orange line, respectively.
7576 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
a
Table 1 The properties of ibuprofen and its L-valine ester derivatives
The ionic pair of ibuprofen and L-valine hexyl ester [ValOHex]
[IBU] had the highest thermal stability, while [ValOEt][IBU] was
ꢀ
ꢀ
ꢀ
T
Compound
Colour
T
m
/ C
T
c
/ C
T
onset/ C
[a]
l
characterised by the lowest thermal stability (Fig. 3).
All obtained ibuprofen derivatives were optical active
compounds, which resulted from using optically pure L-valine
to synthesise ibuprofen derivatives. Ibuprofen also has the
chiral centre, but we started from the ibuprofen racemate.
Specic rotation was similar for all ibuprofen salts (Table 1) and
it was found to be between +8.867 for [ValOEt][IBU] and +11.852
for [ValOiPr][IBU].
rac-IBU
White
White
White
White
White
White
White
77.51
77.99
79.81
78.01
76.80
73.81
67.35
—
189.8
89.5
109.5
90.2
119.9
129.8
128.9
—
+8.867
+9.760
+11.852
+11.094
+10.076
+8.987
[
[
[
[
[
[
ValOEt][IBU]
ValOPr][IBU]
ValOiPr][IBU]
ValOBu][IBU]
ValOAm][IBU]
ValOHex][IBU]
55.20
48.67
64.02
46.22
41.99
36.51
a
c m
T , crystallisation peak temperature; T , melting point; Tonset, the
onset temperature of the thermal degradation.
Solubility
Solubility is a very important property in drug discovery and
development, and it may be expressed in several ways. We
applied Vogel's methodology to determine the solubility of
observed, while the crystallisation temperature was not deter-
mined for the parent ibuprofen (ESI, S50†). Crystallisation peak
[ValOR][IBU] in solvents with different polarities. Three cate-
ꢀ
temperatures were about 30 C lower than the melting peak
gories of solubility were created based on the amount of
temperatures for the respective ibuprofen salt, but the onset of
melting and crystallisation was observed at a similar tempera-
ture. The evidence that, upon cooling, the sample returns to the
same state as before the melting transition, is shown by the
melting point temperature that was determined in a second
heating mode, which is almost identical to the temperature that
was observed at the rst heating.
Thermal stability of pharmaceutics is a very important useful
property that is necessary to control the formulation method of
the nished drug. The thermal stability of [ValOR][IBU] was
investigated using the TG analysis (Fig. 3). The degradation
3
compound which dissolved in 1 cm of solvent. The compound
was recognised as soluble when more than 100 mg of
3
compound was soluble in 1 cm , as partly soluble when 33–
3
1
00 mg of compound dissolved in 1 cm ; as practically insoluble
3
when less than 33 mg of compound was soluble in 1 cm . The
solubility was established and summarised in Table 2.
Ibuprofen salts [ValOR][IBU] were soluble or partly soluble in
toluene, chloroform, ethanol, dimethyl sulfoxide, diethyl ether,
and ethyl acetate. Moreover, the ability of [ValOR][IBU] to
dissolve in nonpolar hexane increased with elongation of the
alkyl chain R in the L-valine ester group. Ibuprofen and its salts
with ethyl to butyl ester of L-valine were practically insoluble in
hexane. However, salt with an amyl ester of L-valine [ValOAm]
onset temperature for [ValOR][IBU] was in the range of 89.5–
ꢀ
1
28.9 C, depending on the alkyl chain in the ester group of L-
ꢀ
valine, whereas the value for ibuprofen was 189.8 C (Table 1).
Thus, the synthesised ibuprofen derivatives showed lower
thermal stability compared to the acid form of ibuprofen. The
thermal stability increased with the length of the alkyl chain.
[IBU] and with hexyl ester [ValOHex][IBU] were, respectively,
partly soluble and soluble in hexane.
Considering that the different pharmacological solutions
were aqueous solutions, we determined the solubility of the
Fig. 3 The TG curves of ibuprofen and its L-valine ester derivatives green, IBU; red [ValOEt][IBU]; dark blue [ValOiPr][IBU]; light blue [ValOPr][IBU];
purple/rose [ValOBu][IBU]; black [ValOAm][IBU]; and brown [ValOHex][IBU].
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 7570–7584 | 7577

View Article Online
RSC Advances
Paper
ꢀ
a
Table 2 Solubility of ibuprofen and its L-valine ester derivatives in organic solvents at 25 C
Compound
Ethanol
DMSO
Chloroform
Ethyl acetate
Diethyl ether
Toluene
n-Hexane
rac-IBU
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+/ꢁ
+/ꢁ
+/ꢁ
+
+
+
+/ꢁ
+/ꢁ
+/ꢁ
+
+
+
+
+
+
+
+
—
—
—
—
—
+/ꢁ
+
[ValOEt][IBU]
[ValOPr][IBU]
[ValOiPr][IBU]
[ValOBu][IBU]
[ValOAm][IBU]
[ValOHex][IBU]
+
+
a
+”, soluble >100 mg cmꢁ3; “+/ꢁ”, partly soluble 33–100 mg cmꢁ3; “—”, practically insoluble <33 mg cm at 25 C.
ꢁ3
ꢀ
“
ibuprofen salts in deionised water and two buffer solutions at solubility than a branched-chain isopropyl (Table 3). The solu-
ꢁ
3
pH 5.4 and 7.4 and compared with solubility of the parent acid bility in deionised water was 2.382 g IBU dm for [ValOPr][IBU]
ꢁ
3
(
Table 3). The saturation concentration of the salts in the water and 1.957 g IBU dm for [ValOiPr][IBU].
and buffers was also expressed as the concentration of Compared to the water solubility of the known ibuprofen
ibuprofen and they are presented in Table 3. As was shown, the organic salts, the solubility of L-valine ester ibuprofenates was
pairing of ibuprofen with L-valine alkyl esters markedly higher than for tetrahexylammonium, trihexylte-
enhanced ibuprofen's solubility in water and buffer solutions. tradecylphosphonium, and didecyldimetylammonium ibu-
However, better solubility was found for conjugates that had profenates, while was close to lidocaine or procaine
shorter alkyl chains in the L-valine ester moiety. Lower solubility ibuprofenates but lower than for butylmetylimidazolium or
27
for longer hydrocarbon chains is typical and caused by their tetrabutylphosphonium salts.
weaker solvation, and more energy is necessary to overcome the
strength of intermolecular interaction between more tightly
44
packed molecules.
Partition coefficient
The obtained salts were slightly less soluble in pH 5.4 buffer
than in pH 7.4 buffer. In pH 5.4 buffer, the saturation concentra-
The partition coefficient between n-octanol and water, which is
commonly reported as log Po/w, is a key parameter that is used
to predict drug hydrophobicity and partitioning in biological
systems. The octanol–water (or buffer) partition coefficients for
the acidic form of ibuprofen and its salts with L-valine esters are
presented in Table 4. All salts showed a positive log P, but they
were lower compared with that of ibuprofen in the respective
system, which indicated that formation of salts reduced
hydrophobicity. Reduction of the hydrophobic nature was more
signicant for the octanol/water and octanol/buffer systems at
pH 5.4. For example, log P octanol/water and octanol/buffer at
pH 5.4 for [ValOEt][IBU] was, respectively, 2.5-times and 1.5-
times lower than for ibuprofen, for which log P in two systems
were 2.415 and 3.192 respectively. Log P values for ibuprofen
salts in the system octanol/buffer at pH 7.4 were very close to
ꢁ3
ꢁ3
tion was 2.329 g IBU dm for [ValOEt][IBU] to 0.386 g IBU dm
for [ValOHex][IBU], which was almost up 30-times higher than for
ꢁ
3
the unmodied acidic form (0.082 g dm ). Solubility in pH 7.4
ꢁ3
buffer was found to be between 3.055 g IBU dm for [ValOEt][IBU]
ꢁ3
and 0.627 g IBU dm for [ValOHex][IBU], and it was up to seven-
ꢁ
3
times higher than that of the parent acid (0.432 g dm ).
An increase in solubility of salts with increasing pH is in
26
accordance with what is known about sodium ibuprofenate.
In deionised water, the solubility of salts was similar to the
results that were obtained in pH 7.4. It was in the range of
ꢁ
3
3
.252–0.538 g IBU dm , and up to 40-times higher than for
ꢁ
3
unmodied ibuprofen (0.076 g dm ). Moreover, a straight-
chain n-propyl in the L-valine ester group resulted in better
ꢀ
Table 3 Solubility of ibuprofen and its L-valine derivatives in water and phosphate buffers (pH 5.4 and 7.4) at 25 C
Solubility in phosphate buffer
Solubility in water
pH ¼ 5.4
pH ¼ 7.4
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
ꢁ3
Compound
g dm
g IBU dm
g dm
g IBU dm
g dm
g IBU dm
rac-IBU
0.076 ꢃ 0.001
5.542 ꢃ 0.007
4.221 ꢃ 0.033
3.468 ꢃ 0.007
3.125 ꢃ 0.007
1.798 ꢃ 0.019
1.058 ꢃ 0.002
0.076 ꢃ 0.001
3.252 ꢃ 0.007
2.382 ꢃ 0.033
1.957 ꢃ 0.007
1.699 ꢃ 0.007
0.942 ꢃ 0.019
0.538 ꢃ 0.002
0.082 ꢃ 0.001
3.969 ꢃ 0.056
3.057 ꢃ 0.045
2.350 ꢃ 0.027
2.371 ꢃ 0.016
1.380 ꢃ 0.007
0.759 ꢃ 0.006
0.082 ꢃ 0.001
2.329 ꢃ 0.056
1.725 ꢃ 0.045
1.326 ꢃ 0.027
1.289 ꢃ 0.016
0.723 ꢃ 0.007
0.386 ꢃ 0.006
0.432 ꢃ 0.001
5.206 ꢃ 0.044
5.367 ꢃ 0.019
4.998 ꢃ 0.018
3.089 ꢃ 0.015
1.931 ꢃ 0.008
1.233 ꢃ 0.001
0.432 ꢃ 0.001
3.055 ꢃ 0.044
3.029 ꢃ 0.019
2.821 ꢃ 0.018
1.679 ꢃ 0.015
1.012 ꢃ 0.008
0.627 ꢃ 0.001
[ValOEt][IBU]
[ValOPr][IBU]
[ValOiPr][IBU]
[ValOBu][IBU]
[ValOAm][IBU]
[ValOHex][IBU]
7578 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
47–49
log P of acid form of ibuprofen (1.284), which can indicate percutaneous absorption for topically applied chemicals.
similar ionisation of the acid and salts at a higher pH.
Articial membranes models are also developed to eliminate
50
Shorter alkyl chains in the ester group of L-valine provided natural tissue. Luo et al. conducted comparative study of
a decrease in hydrophobicity. The log P in the octanol/water ibuprofen permeation using a skin PAMPA model and silicone
system for ibuprofen salts decreased from 1.883 to 0.992 membrane with human and porcine tissue. They found the
when the alkyl chain was changed from hexyl to ethyl. In the better permeation of ibuprofen through PAMPA than through
octanol/pH 5.4 buffer system, the change in log P was from human or porcine skin. Additionally, the permeation data with
2.880 to 2.023, and in the octanol/pH 7.4 buffer system, log P using PAMPA were comparable to the results through silicone
changed slightly, from 1.279 to 1.026, with decreasing alkyl membrane. Moreover, authors demonstrated that the use of
chain length in valine ester moiety. The L-valine esters ibu- articial membranes was associated with greater repeatability
profenate showed comparable log P values with known ammo- of experimental results than with porcine and human skin. The
nium salts, as follows: [N1,1,10,10][IBU], 1.87; [N6,6,6,6][IBU], 2.47; variable results for natural tissue are caused by their biological
phosphonium [P4,4,4,4][IBU], 1.54; lidocaine salt, 1.49; and imi- greater complexity but are more relevant to real conditions of
27
dazolium [BMIm][IBU], 1.15.
using.
In our studies the penetration of new ibuprofen derivatives
was compared with the penetration of the parent acid. Perme-
ꢀ
Skin penetration
ation through porcine skin at 32 C was studied using the Franz
diffusion cell device. The donor phase was a solution of the
compound that was tested in 70% (v/v) ethanol. Ethanol at
a concentration of 70% was chosen as the donor solvent
because ibuprofen and its salts were freely soluble in it. More-
over, this concentration of ethanol is an optimal vehicle for
Skin penetration of topically applied active compounds is
restricted by the stratum corneum, which is a lipophilic skin
barrier for exogenous compounds. To determine the possibility
of using ibuprofen derivatives in topical and transdermal drug
delivery, in vitro skin penetration studies were conducted with
using an abdomen porcine skin. Because of difficulties in
sourcing human skin as well as ethical issues, the porcine skin
is frequently used for preliminary evaluation of percutaneous
permeation of topically applied drugs. Numerous histopatho-
51,52
transepidermal application of the drug.
A higher ethanol
concentration, above 70%, caused dehydration of the skin and
53
decreased the drug penetration. The acceptor phase was buffer
pH 7.4.
45,46
The cumulative mass of the ibuprofen in the acceptor phase
aer 24 h of permeation (Table 5) was higher in application
onto the skin the ethanol solution of [ValOPr][IBU] (382.35 mg
logical studies conrmed its similarity to human skin.
Thickness of the layers, i.e. basale, spinosum and granulosum
as well as the number of cell layers in stratum corneum is
ꢁ
2
ꢁ2
46
IBU cm ) or [ValOiPr][IBU] (341.20 mg IBU cm ) compared to
comparable for human and porcine skin. In et al. compared
the thickness of abdomen human and porcine skin. The overall
thickness of epidermis was about 60 and 54 mm, thickness of
stratum basale – 9.8 and 10 mm, stratum spinosum – 32.9 and
ꢁ
2
application of the parent acidic form (302.84 mg IBU cm ). For
ValOAm][IBU] (308.96 mg IBU cm ) the cumulative mass was
ꢁ
2
[
similar to that for ibuprofen and for the other salts was the
lower. The lowest cumulative mass was determined for [ValOEt]
34.4 mm, for human and porcine skin, respectively. Whereas the
ꢁ
2
[
IBU] (215.19 mg IBU cm ). However, expressing the perme-
thickness of stratum granulosum was 3.2 mm for both human
and pig skin. The stratum corneum of both species is very
similar and consists of dense atted epithelial cells. The hair
follicle structure of porcine skin is also very similar to that of
human. As in the structure of human skin, the sebaceous
ation as % applied dose of IBU (Table 5), reveals that the
application of each salt gives higher part of permeated
ibuprofen than the use of unmodied ibuprofen. For the etha-
nolic solution of ibuprofen salts, between 7.33% and 13.55% of
ibuprofen in donor phase was permeated to the acceptor phase,
while for acidic form only 6.06% of applied dose.
45,46
glands of porcine skin are associated with the hair follicle.
It
should be added that results of numerous studies suggested
that porcine skin as well as skin obtained from other animals
The ux of ibuprofen and its salts through the skin was
determined from the slope of the plot of cumulative mass in the
acceptor phase over time (ESI, S51 and S52†). The ux was
expressed as the amount of compound or active ibuprofen per
(guinea pig, rats) seem to be also useful models to evaluation of
ꢁ
2
ꢁ1
skin area and time (mg cm
h ). As shown in Fig. 4, improved
Table 4 The n-octanol/(water or phosphate buffer (pH 5.4 and 7.4)
partition coefficient (log P) of ibuprofen and its L-valine esters rate of permeation of ibuprofen by pairing it with ValOPr,
derivatives
Compound
rac-IBU (rac)
ValOiPr, ValOBu, or ValOAm was achieved. These salts showed
the faster permeation of ibuprofen through the skin. For the
log Pwater
log PpH¼5.4
log PpH¼7.4
ꢁ2 ꢁ1
best salt, [ValOPr][IBU], the ux was 18.555 mg IBU cm
h
ꢁ
2
ꢁ1
compared to 13.644 mg IBU cm
h
for unmodied acid. Salts
2.415 ꢃ 0.001
0.992 ꢃ 0.008
1.249 ꢃ 0.005
1.154 ꢃ 0.004
1.520 ꢃ 0.004
1.750 ꢃ 0.003
1.833 ꢃ 0.002
3.192 ꢃ 0.005
2.023 ꢃ 0.001
2.061 ꢃ 0.001
2.186 ꢃ 0.001
2.337 ꢃ 0.001
2.421 ꢃ 0.002
2.880 ꢃ 0.011
1.284 ꢃ 0.006
[
[
[
[
[
[
ValOEt][IBU]
ValOPr][IBU]
ValOiPr][IBU]
ValOBu][IBU]
ValOAm][IBU]
ValOHex][IBU]
1.026 ꢃ 0.001 of ibuprofen with ValOHex and ValOEt showed a lower
1.197 ꢃ 0.003 ibuprofen ux. There are some data about the ux for various
1.172 ꢃ 0.001
1.237 ꢃ 0.012
1.259 ꢃ 0.001
1.279 ꢃ 0.001
ibuprofen salts in the literature, but permeation experimental
conditions were different than those in this study. For example,
27
Wu et al. demonstrated a four–ve-fold increase in ux for
[N
1,1,10,10][IBU] and [N6,6,6,6][IBU] compared to IBU (0.542 mg
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 7570–7584 | 7579

View Article Online
RSC Advances
Paper
Table 5 Permeation as cumulative mass of compound or ibuprofen (IBU) and as % applied dose of IBU, after 24 h permeation from ethanolic
solution into acceptor phase (mean values ꢃ SD with n ¼ 3)
Cumulative mass of compound
Cumulative mass of IBU
ꢁ2
ꢁ2
Compound
(mg cm
)
(mg IBU cm
)
% applied dose of IBU
rac-IBU
302.84 ꢃ 3.63
366.64 ꢃ 8.14
604.51 ꢃ 4.87
677.41 ꢃ 1.05
533.00 ꢃ 1.05
589.31 ꢃ 3.77
302.84 ꢃ 3.63
215.19 ꢃ 8.14
341.20 ꢃ 4.87
382.35 ꢃ 1.05
289.74 ꢃ 1.05
308.96 ꢃ 3.77
6.06
7.33
a
b
b
b
b
a
b
b
b
b
[ValOEt][IBU]
[ValOiPr][IBU]
[ValOPr][IBU]
[ValOBu][IBU]
[ValOAm][IBU]
[ValOHex][IBU]
12.09
13.55
10.66
11.79
9.81
b
b
490.33 ꢃ 26.06
249.46 ꢃ 26.06
a
b
Value is signicantly different from control (ibuprofen) (P < 0.05). Value is signicantly different from control (ibuprofen) (P < 0.001).
ꢁ
2
ꢁ1
cm
h ) using Britton–Robinson buffer at pH 7.4 as an the highest ux from the series of ethylamine salts of
26
26
acceptor and donor solution. Sarveiya et al. measured the ibuprofen. Among the obtained salts, the least lipophilic was
steady-state ux through a PDMS membrane with a buffer at pH [ValOEt][IBU], with log P of 1.026, and the most lipophilic was
7
.0 as the acceptor phase for the ibuprofen triethylammonium [ValOHex][IBU], with log P of 1.279, which provided lower rate
salt, which was over 15-fold higher than for sodium ibu- of permeation of ibuprofen than the parent acid.
profenate. Furukawa et al. reported that the rate of permeation The obtained data for two salts, [ValOPr][IBU] and [ValOiPr]
of ibuprofen-ProOEt through pig skin was ten-times higher than [IBU], were the best for topical use. They provided the highest
for free ibuprofen and variation in skin permeation depends on amount of transported ibuprofen and the highest rate of perme-
28
strong ion-pairing between a cation and an anionic drug.
ation. In practice, this means quickly obtaining the relevant
In Fig. 5, the relationship between the ux and log P in the concentration to overcome pain. The salts used were also a source
octanol/buffer 7.4 system for salts of ibuprofen and unmodied of L-valine, which is an additional advantage of these derivatives.
ibuprofen is presented. The data in Fig. 5 show that there is
a maximum ux for a series of ibuprofen salts when log P is
between 1.172 and 1.197. A similar log P value (1.18) was found
for triethylammonium ibuprofenate, which is characterised by
Skin accumulation
The compound may both penetrate and accumulate in the skin.
Table 6 shows the mass of ibuprofen that accumulated in
ꢁ2
ꢁ1
h ; orange bars refer to
Fig. 4 Flux through the skin for ibuprofen and ibuprofen derivatives (blue bars refer to the flux of compound in mg cm
ꢁ2
ꢁ1
flux of active ibuprofen in mg IBU cm
h ).
7580 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 5 The relationship between flux and the log P in octanol/pH 7.4 system for ibuprofen and its L-valine ester derivatives.
ꢁ
1
porcine skin in 24 hours, expressed in mg IBU g of skin. In
Toxicological aspects of new ibuprofen conjugates are very
most salts applied in donor phase, a lower mass of ibuprofen in important and studies known from the literature for similar
the skin was found compared with applied ibuprofen free acid. structures allow to predict the tendencies of changes in toxicity
The exception was [ValOBu][IBU] for that a slightly higher of synthesized ibuprofen conjugates with valine alkyl esters.
accumulation of ibuprofen per mass of skin compared to the
In general, as the literature data show, pairing a drug with
unmodied acid was measured. However, expressing the accu- hydrophilic cations, gives combinations with decreased log P
mulation as percentage of applied dose of IBU in form of salt or values which results in similar or less toxicity than for the
acid in donor phase, it can be found that all used salts reveal unchanged form of the drug. In contrast, combinations of
increased skin accumulation of IBU. It was observed the regular ibuprofen with lipophilic cations, with increased log P values,
increase of IBU accumulation in the range of 3.5% to 7.0% of proved to be more toxic than ibuprofen, which was the result of
applied dose of IBU with increase of alkyl chain from ethyl to the high cytotoxicity of the cation itself. EC50 values for conju-
butyl in valine moiety. Further elongation of alkyl chain resulted gates with lipophilic cations were two-three orders of magni-
27
in decrease of skin accumulation to about 5.0% applied IBU tude lower than for that with hydrophilic cations. Counterions
dose for [ValOAm][IBU] and [ValOHex][IBU]. All used salts reveal with longer alkyl chains and higher log P values may interact
increased skin accumulation of IBU compared to the use of with cell membrane more extensively and cause more severe
parent acid (3.1% of applied dose).
cytotoxicity.
Toxicity increases with increasing lipophilicity. Hydrophilic
cations reduced the lipophilicity and slightly reduced the cyto-
toxicity. It has been also shown that pairing of salicylic acid with
the ethyl esters of three different amino acids – L-ProOEt, L-
AlaOEt and L-AspOEt, proved less cytotoxicity, determined on
two mammalian cell lines – L929 and HeLa, than that deter-
mined for the native acid. When amino acid cations were
incorporated into toxic salicylic acid, the cytotoxicity
Table 6 Skin accumulation, expressed as mg of IBU per g of skin, and %
applied dose of IBU, after 24 h skin permeation of ibuprofen free acid
and its salts with L-valine esters from ethanolic solution into acceptor
fluid pH 7.4 (mean values ꢃ SD with n ¼ 3)
%
applied
ꢁ1
Compound
Skin accumulation (mg IBU g
)
dose of IBU
54
decreased.
rac-IBU
745.69 ꢃ 61.87
412.19 ꢃ 38.45
3.1
3.5
4.9
5.9
7.0
5.0
4.8
[ValOEt][IBU]
[ValOiPr][IBU]
[ValOPr][IBU]
[ValOBu][IBU]
[ValOAm][IBU]
[ValOHex][IBU]
The alkyl chain length in the ester moiety of L-valine in the
presented ibuprofen conjugates may affect the nal toxicity.
However, taking into account the low values of log P for all salts
(0.992 to 1.883), and lower than for acidic ibuprofen, it seems
that the cytotoxicity should not change in the broad range with
a
630.75 ꢃ 61.87
a
726.42 ꢃ 113.37
b
815.14 ꢃ 58.92
a
565.60 ꢃ 71.09
a
612.86 ꢃ 129.33
54
increase of alkyl chain length. Moshikur et al. demonstrated,
that among four esters of L-proline (Me, Et, Pr and Bu), methyl
and ethyl ester showed similar and the lowest cytotoxicity.
a
b
Value is signicantly different from control (ibuprofen) (P < 0.05).
Value is signicantly different from control (ibuprofen) (P < 0.001).
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 7570–7584 | 7581

View Article Online
RSC Advances
Paper
Toxicity of the propyl ester was only slightly higher than Et and
Me esters, and the highest was for the butyl ester. However, even
for butyl ester of L-proline the IC50 values (in HeLa cells test)
were only about 3-times lower than for the least toxic ethyl ester.
Moreover, Furukawa et al. studies have shown that even at
high concentrations of proline ethyl ester, the viability of L929
cell lines decreases only by 20% and the toxicity of proline ester
is only slightly higher than that of proline itself.
Therefore, ibuprofen conjugates with valine ester counter-
ions, which have been shown to have lower lipophilicity than
native ibuprofen, are expected to have lower toxicity. Due to
a number of properties of valine, which has a positive effect on
the body, but also repair the skin, they can also reduce the
toxicity of ibuprofen.
4 L. R. Shaw, W. J. Irwin, T. J. Grattan and B. R. Conway, The
effect of selected water-soluble excipients on the
dissolution of paracetamol and ibuprofen, Drug Dev. Ind.
Pharm.,
2005,
31,
515–525,
DOI:
10.1080/
28
03639040500215784.
5 R. M. Watkinson, C. Herkenne, R. H. Guy, J. Hadgra,
G. Oliveira and M. E. Lane, Inuence of Ethanol on the
Solubility, Ionization and Permeation Characteristics of
Ibuprofen in Silicone and Human Skin, Skin Pharmacol.
Physiol., 2009, 22, 15–21, DOI: 10.1159/000183922.
6 P. Bustamante, M. A. Pe n˜ a and J. Barra, The modied
extended Hansen method to determine partial solubility
parameters of drugs containing a single hydrogen bonding
group and their sodium derivatives: benzoic acid/Na and
ibuprofen/Na, Int. J. Pharm., 2000, 194(1), 117–124, DOI:
1
0.1016/s0378-5173(99)00374-9.
Conclusion
7
P. Schleier, A. Prochnau, A. M. Schmidt-Westhausen,
H. Peters, J. Becker, T. Latz, J. Jackowski, E. U. Peters,
G. E. Romanos, B. Zahn, J. L u¨ demann, J. Maares and
B. Petersen, Ibuprofen sodium dihydrate, an ibuprofen
formulation with improved absorption characteristics,
provides faster and greater pain relief than ibuprofen acid,
Int. J. Clin. Pharmacol. Ther., 2007, 45(2), 89–97, DOI:
10.5414/cpp45089.
8 P. M. Dewland, S. Reader and P. Berry, Bioavailability of
ibuprofen following oral administration of standard
ibuprofen, sodium ibuprofen or ibuprofen acid
incorporating poloxamer in healthy volunteers, BMC Clin.
Pharmacol., 2009, 9, 19, DOI: 10.1186/1472-6904-9-19.
9 S. E. Nørholt, F. Hallmer, J. Hartlev, L. Pallesen, J. Bloml ¨o f,
E. J. Hansen, N. Fernandes, L. Eriksson and E. M. Pinholt,
Analgesic efficacy with rapidly absorbed ibuprofen sodium
dihydrate in postsurgical dental pain: results from the
randomized QUIKK trial, Int. J. Clin. Pharmacol. Ther.,
2011, 49, 722–729, DOI: 10.5414/cp201553.
We demonstrated that new ionic pairs of ibuprofen with the
biocompatible counterions of L-valine alkyl ester, where the
alkyl chain was between ethyl and hexyl, can enhance ibuprofen
transport through biological membranes. They are charac-
terised by better solubility in water and buffer (pH 5.4 and 7.4)
solutions and a lower partition coefficient compared to the
parent acid, which can have a positive impact on ibuprofen
bioavailability with different administration routes. In vitro
permeation experiments through porcine skin showed the best
ibuprofen permeability for pairs with L-valine propyl and iso-
propyl ester, in which a higher rate of transport and mass of
permeated ibuprofen were achieved. Besides better absorption
of drug by penetrating faster and more completely, the pre-
sented ibuprofen salts with valine ester can provide dietary
supplementation with exogenous L-valine, which is their supe-
riority to the known salts of ibuprofen providing a host of
various advantages. L-Valine has a positive effect on the body's
functions, including skin repairs effect. It provides the body
with energy, reduces fatigue and is necessary for the synthesis of 10 P. Brain, R. Leyva, G. Doyle and D. Kellstein, Onset of
pantothenic acid, which accelerates wound healing.
analgesia and efficacy of ibuprofen sodium in postsurgical
dental pain: a randomized, placebo-controlled study versus
standard ibuprofen, Clin. J. Pain, 2015, 31(5), 444–450,
DOI: 10.1097/ajp.0000000000000142.
Conflicts of interest
There are no conicts of interest to declare.
11 T.
Bruzzese
and
R.
Ferrari,
Salze
von
Phenylalkancarbons ¨a uren mit basischen Aminos ¨a uren,
Salts of phenylalkanecarboxylic acids with basic amino
acids, Ger. Pat., 2508895, 1975.
References
1
2
3
E. M. Laska, A. Sunshine, I. Marrero, N. Olson, C. Siegel and 12 T. Bruzzese and R. Ferrari, Relieving pain and treating
N. McCormick, The correlation between blood levels of
ibuprofen and clinical analgesic response, Clin. Pharmacol.
Ther., 1986, 40(1), 1–7, DOI: 10.1038/clpt.1986.129.
K. A. Levis, M. E. Lane and O. I. Corrigan, Effect of buffer
media composition on the solubility and effective
inammatory conditions in warm-blooded animals, US
Pat., 4279926, 1981.
13 K. C. Kwan, S(+)-ibuprofen-L-amino acid and S(+)-ibuprofen-
D-amino acidas onset-hastened enhanced analgesics, US
Pat., 5200558, 1993.
permeability coefficient of ibuprofen, Int. J. Pharm., 2003, 14 S. Ando and V. J. Stella, Pharmaceutical preparations for
53, 49–59, DOI: 10.1016/s0378-5173(02)00645-2.
taste masking, WO Pat., 03/086293, 2003.
H. Potthast, J. B. Dressman, H. E. Junginger, K. K. Midha, 15 R. Cristofoletti and J. B. Dressman, Dissolution Methods to
2
H. Oeser, V. P. Shah, H. Vogelpoel and D. M. Barends,
Biowaiver Monographs for Immediate Release Solid Oral
Dosage Forms: Ibuprofen, J. Pharm. Sci., 2005, 94(10),
Increasing Discriminatory Power of In Vitro Dissolution
Testing for Ibuprofen Free Acid and Its Salts, J.
Pharmaceut. Sci., 2017, 106, 92–99, DOI: 10.1016/
j.xphs.2016.06.001.
2121–2131, DOI: 10.1002/jps.20444.
7582 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
16 R. A. Moore, S. Derry, S. Straube, J. Ireson-Paine and 28 S. Furukawa, G. Hattori and S. Sakai, Highly efficient and low
P. J. Wiffen, Faster, higher, stronger? Evidence for
formulation and efficacy for ibuprofen in acute pain, Pain,
toxic skin penetrants composed of amino acid ionic liquids,
RSC Adv., 2016, 6, 87753–87755, DOI: 10.1039/c6ra16926k.
29 H. Wang, G. Gurau, J. Shamshina, O. A. Cojocaru,
J. Janikowski, D. R. MacFarlane, J. H. Davis Jr and
R. D. Rogers, Simultaneous membrane transport of two
active pharmaceutical ingredients by charge assisted
hydrogen bond complex formation, Chem. Sci., 2014, 5,
3449–3456, DOI: 10.1039/c4sc01036a.
2014, 155, 14–21, DOI: 10.1016/j.pain.2013.08.013.
1
7 J. T. Legg, L. A. Laurent, R. Leyva and D. Kellstein, Ibuprofen
Sodium Is Absorbed Faster than Standard Ibuprofen Tablets:
Results of Two Open-Label, Randomized, Crossover
Pharmacokinetic Studies, Drugs R&D, 2014, 14, 283–290,
DOI: 10.1007/s40268-014-0070-8.
ˇ ˇ´
8 P. Desjardins, P. Black, M. Papageorge, T. Norwood, 30 B. A. Curıkova, K. Prochazkova, B. Filkova, P. Diblıkova,
´
´
´
´
´
´
1
1
2
2
D. D. Shen, L. Norris and A. Ardia, Ibuprofen arginate
provides effective relief from postoperative dental pain
with a more rapid onset of action than ibuprofen, Eur. J.
Clin. Pharmacol., 2002, 58, 387–394, DOI: 10.1007/s00228-
J. Svoboda, A. Kov ´a ˇc ik, K. V ´a vrov ´a and J. Zbytovsk ´a ,
Simplied stratum corneum model membranes for
studying the effects of permeation enhancers, Int. J.
Pharm., 2017, 534(1–2), 287–296, DOI: 10.1016/
j.ijpharm.2017.10.038.
002-0491-0.
9 B. Ahmetaj-Shala, A. Tesfai, C. Constantinou, R. Leszczynski, 31 M. M. Badran, J. Kuntsche and A. Fahr, Skin penetration
M. V. Chan, H. Gashaw, G. Galaris, S. Mazi, T. D. Warner,
N. S. Kirkby, et al., Pharmacological assessment of on
platelet aggregation and colon cancer cell killing, Biochem.
Biophys. Res. Commun., 2017, 484(4), 762–766, DOI:
enhancement by a microneedle device (Dermaroller®) in
vitro: dependency on needle size and applied formulation,
Eur. J. Pharm. Sci., 2009, 36, 511–523, DOI: 10.1016/
j.ejps.2008.12.008.
1
0.1016/j.bbrc.2017.01.161.
32 A. Haq and B. Michniak-Kohn, Effects of solvents and
penetration enhancers on transdermal delivery of
thymoquinone: permeability and skin deposition study,
Drug Deliv., 2018, 25(1), 1943–1949, DOI: 10.1080/
10717544.2018.1523256.
0 N. S. Kirkby, A. Tesfai, B. Ahmetaj-Shala, H. Gashaw,
W. Sampaio, G. Etelvino, N. M. Leao, R. A. Santos and
J. A. Mitchell, Ibuprofen arginate retains eNOS substrate
activity and reverses endothelial dysfunction: implications
for the COX-2/ADMA axis, FASEB J., 2016, 30(12), 4172– 33 J. Kuntsche, H. Bunjes, A. Fahr, S. Pappinen, S. R ¨o nkk ¨o ,
4
179, DOI: 10.1096/.201600647r.
M. Suhonen and A. Urtti, Interaction of lipid nanoparticles
with human epidermis and an organotypic cell culture
model, Int. J. Pharm., 2008, 354(1–2), 180–195, DOI:
10.1016/j.ijpharm.2007.08.028.
1 I. Tegeder, U. Muth-Selbach, J. Lotsch, G. R u¨ sing, R. Oelkers,
K. Brune, S. Meller, G. R. Kelm, F. S ¨o rgel and G. Geisslinger,
Application of microdialysis for the determination of muscle
and subcutaneous tissue concentrations aer oral and 34 A. Simon, M. I. Amaro, A. M. Healy, L. M. Cabral and V. P. de
topical ibuprofen administration, Clin. Pharmacol. Ther.,
999, 65(4), 357–368, DOI: 10.1016/s0009-9236(99)70128-1.
2 A. Patel, M. Bell, C. O'Connor, An. Inchley, J. Wibawa and
Sousa, Comparative evaluation of rivastigmine permeation
from a transdermal system in the Franz cell using
synthetic membranes and pig ear skin with in vivo–in vitro
correlation, Int. J. Pharm., 2016, 512(1), 234–241, DOI:
10.1016/j.ijpharm.2016.08.052.
1
2
M. E. Lane, Delivery of ibuprofen to the skin, Int. J. Pharm.,
2013, 457, 9–13, DOI: 10.1016/j.ijpharm.2013.09.019.
23 J. Irvine, A. Afrose and N. Islam, Formulation and delivery 35 B. H. Furniss, A. J. Hannaford, P. W. G. Smith and
strategies of ibuprofen: challenges and opportunities, Drug
Dev. Ind. Pharm., 2018, 44(2), 173–183, DOI: 10.1080/
A. R. Tatchell, Vogel's Textbook of Practical Organic
Chemistry, Longman Scientic & Technical, John Wiley
&Sons, Inc., New York, 5th edn, 1989.
03639045.2017.1391838.
4 M. A. Bolzinger, S. Briancon, J. Pelletier and Y. Chevalier, 36 M. Kope ˇc n ´a , M. Mach ´a ˇc ek, E. Prchalov ´a , P. Stepanek,
ˇ ˇ ´
2
2
Penetration of drug through skin,
controlling membrane, Curr. Opin. Colloid Interface Sci.,
012, 17, 156–165, DOI: 10.1016/j.cocis.2012.02.001.
5 A. Baroni, E. Buommino, V. De Gregorio, E. Ruocco,
a
complex rate-
P. Dra ˇs ar, M. Kotora and K. V ´a vrov ´a , Galactosyl
pentadecene reversibly enhances transdermal and topical
drug delivery, Pharm. Res., 2017, 34(10), 2097–2108, DOI:
10.1007/s11095-017-2214-3.
2
V. Ruocco and R. Wolf, Structure and function of the 37 D. J. Davies, R. J. Ward and J. R. Heylings, Multi-species
epidermis related to barrier properties, Clin. Dermatol.,
012, 30, 257–262, DOI: 10.1016/j.clindermatol.2011.08.007.
6 V. Sarveiya, J. F. Templeton and H. A. E. Benson, Ion-pairs of
assessment of electrical resistance as a skin integrity
marker for in vitro percutaneous absorption studies,
Toxicol. In Vitro, 2004, 18(3), 351–358, DOI: 10.1016/
j.tiv.2003.10.004.
2
2
2
ibuprofen: increased membrane diffusion, J. Pharm.
Pharmacol., 2004, 56, 717–724, DOI: 10.1211/0022357023448. 38 J. Li and Y. Sha, A Convenient Synthesis of Amino Acid
7 H. Wu, Z. Deng, B. Zhou, et al., Improved transdermal
Methyl Esters, Molecules, 2008, 13, 1111–1119, DOI:
permeability of ibuprofen by ionic liquid technology:
10.3390/molecules13051111.
Correlation between counterion structure and the 39 P. Ossowicz, E. Janus, G. Schroeder and Z. Rozwadowski,
physicochemical and biological properties, J. Mol. Liq.,
019, 283, 399–409, DOI: 10.1016/j.molliq.2019.03.046.
Spectroscopic Studies of Amino Acid Ionic Liquid-
Supported Schiff Bases, Molecules, 2013, 18, 4986–5004,
DOI: 10.3390/molecules18054986.
2
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 7570–7584 | 7583

View Article Online
RSC Advances
Paper
40 Z. Rozwadowski, Deuterium isotope effects on 13C chemical
species, Pharm. Res., 2006, 23(7), 1517–1524, DOI: 10.1007/
shis of lithium salts of Schiff bases amino acids, J. Mol.
s11095-006-0273-y.
Struct.,
j.molstruc.2005.06.005.
2006,
753,
127–131,
DOI:
10.1016/ 49 C. G ´e ni `e s, E. L. Jamin, L. Debrauwer, D. Zalko, E. N. Person,
J. Eilstein, S. Gr ´e goire, A. Schepky, D. Lange, C. Ellison,
4
4
1 E. Breitmaier and W. Voelter, in Voelter, Monographs in
Modern Chemistry, C NMR Spectroscopy, ed, H. F. Ebel,
Verlag Chemie, Weinheim, 1974.
2 S. Vairam, T. Premkumar and S. Govindarajan, Trimellitate
complexes of divalent transition metals with hydrazinium
A. Roe, S. Salhi, R. Cubberley, N. J. Hewitt, H. Rothe,
M. Klaric, H. Duplan and C. Jacques-Jamin, Comparison of
the metabolism of 10 chemicals in human and pig skin
explants, J. Appl. Toxicol., 2019, 39, 385–397, DOI: 10.1002/
jat.3730.
1
3
cation, J. Therm. Anal. Calorim., 2010, 100, 955–960, DOI: 50 L. Luo, A. Patel, B. Sinko, M. Bell, J. Wibawa, J. Hadgra and
1
0.1007/s10973-009-0459-8.
M. E. Lane, A comparative study of in vitro permeation of
ibuprofen in mammalian skin, the PAMPA model and
silicone membrane, Int. J. Pharm., 2016, 505(1–2), 14–19,
DOI: 10.1016/j.ijpharm.2016.03.043.
4
3 S. Wilkes, A short history of ionic liquids—from molten salts
to neoteric solvents, Green Chem., 2002, 4, 73–80, DOI:
1
0.1039/b110838g.
44 S. Forster, G. Buckton and A. E. Beezer, The importance of 51 A. Zhang, E. C. Jung, H. Zhu, Y. Zou, X. Hui and H. Maibach,
chain length on the wettability and solubility of organic
homologs, Int. J. Pharm., 1991, 72(1), 29–34, DOI: 10.1016/
Vehicle effects on human stratum corneum absorption and
skin penetration, Toxicol. Ind. Health, 2016, 33(5), 416–425,
DOI: 10.1177/0748233716656119.
0378-5173(91)90377-z.
45 U. Jacobi, M. Kaiser, R. Toll, S. Mangelsdorf, H. Audring, 52 A. Rashid, E. T. White, T. Howes, J. D. Litster and
N. Otberg, W. Sterry and J. Lademann, Porcine ear skin: an
in vitro model for human skin, Skin Res. Technol., 2007, 13,
I. Marziano, Effect of Solvent Composition and
Temperature on the Solubility of Ibuprofen in Aqueous
Ethanol, J. Chem. Eng. Data, 2014, 59, 2699–2703, DOI:
10.1021/je400819z.
19–24, DOI: 10.1111/j.1600-0846.2006.00179.x.
4
6 M. K. In, K. C. Richardson, A. Loewa, S. Hedtrich,
S. Kaessmeyer and J. Plendl, Histological and functional 53 R. M. Watkinson, C. Herkenne, R. H. Guy, J. Hadgra,
comparisons of four anatomical regions of porcine skin
with human abdominal skin, Anat., Histol., Embryol., 2019,
G. Oliveira and M. E. Lane, Inuence of ethanol on the
solubility, ionization and permeation characteristics of
ibuprofen in silicone and human skin, Skin Pharmacol.
Physiol., 2009, 22, 15–21, DOI: 10.1159/000183922.
48, 207–217, DOI: 10.1111/ahe.12425.
4
4
7 H. Bando, S. Mohri, F. Yamashita, Y. Takakura and
M. Hashida, Effect of skin metabolism on percutaneous 54 R. M. Moshikur, M. R. Chowdhury, R. Wakabayashi,
penetration of lipophilic drug, J. Pharm. Sci., 1997, 86(6),
59–761, DOI: 10.1021/j960408n.
Y.
Tahara,
M.
Moniruzzaman
and cytotoxicity
and
evaluation
M.
Goto,
of
7
Characterization
8 J. J. Prusakiewicz, C. Ackermann and R. Voorman,
Comparison of skin esterase activities from different
biocompatible amino acid esters used to convert salicylic
acid into ionic liquids, Int. J. Pharm., 2018, 546, 31–38,
DOI: 10.1016/j.ijpharm.2018.05.021.
7584 | RSC Adv., 2020, 10, 7570–7584
This journal is © The Royal Society of Chemistry 2020