Dicarboxylic acid esters as transdermal permeation enhancers: Effects of chain number and geometric isomers

Article abstract of DOI:10.1016/j.bmcl.2008.11.083

A series of transdermal permeation enhancers based on dicarboxylic acid esters was studied. Single-chain amphiphiles were markedly more effective than the double-chain ones. Monododecyl maleate, that is a cis derivative, was a more potent enhancer than its trans isomer, while the activity of succinates strongly depended on the donor vehicle. No difference between diastereoisomeric tartaric and meso-tartaric acid derivatives was found.

Full text of DOI:10.1016/j.bmcl.2008.11.083

Bioorganic & Medicinal Chemistry Letters 19 (2009) 344–347
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Dicarboxylic acid esters as transdermal permeation enhancers:
Effects of chain number and geometric isomers
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Michal Novotny, Alexandr Hrabálek, Barbora Janušová, Jakub Novotny, Katerina Vávrová
Centre for New Antivirals and Antineoplastics, Charles University in Prague, Faculty of Pharmacy in Hradec Králové, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of transdermal permeation enhancers based on dicarboxylic acid esters was studied. Single-chain
amphiphiles were markedly more effective than the double-chain ones. Monododecyl maleate, that is a
cis derivative, was a more potent enhancer than its trans isomer, while the activity of succinates strongly
depended on the donor vehicle. No difference between diastereoisomeric tartaric and meso-tartaric acid
derivatives was found.
Received 16 October 2008
Revised 20 November 2008
Accepted 21 November 2008
Available online 27 November 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Transdermal drug delivery
Skin penetration enhancers
Structure–activity relationships
Transdermal drug delivery offers numerous advantages over the
conventional routes of administration; however, poor permeation
of most drugs across the skin barrier constitutes a major limitation
of this methodology. One of the approaches to promote drug per-
meation through the skin is the use of permeation enhancers.1
The most promising enhancers, possessing high activity together
with low toxicity, are amphiphilic compounds containing a polar
head and a lipophilic chain. These molecules are capable of insert-
ing themselves into the stratum corneum ceramide-rich lipid
lamellae and disrupting their tight packing.2
The structure–activity relationships of the enhancer chain(s)
are relatively well described.3 For example, the optimum chain
length is approximately 10-12C,4 cis-unsaturation in the chain in-
creases activity5 and the effect of branching or cyclization is usu-
ally negative, dependent on its position and extent.6–8 The
properties of the polar head have been less studied. We have pre-
viously reported the structure–activity relationships of a series of
permeation enhancers based on amino acid-based ceramide ana-
logues suggesting that the hydrogen bonding ability is inversely re-
lated to the enhancing potency.9,10 The most potent enhancer was
a maleic acid derivative 12GM12 (Fig. 1).
tial clinical use, they serve as tools to study the structure-enhanc-
ing activity relationships.
The double- (M and S) and single-chain (MOH and SOH)
enhancers derived from maleic and succinic acids were prepared
from the corresponding anhydrides and dodecanol under acid
catalysis in toluene11 or without solvent12 (Scheme 1). F was pre-
pared by esterification of fumaric acid (Scheme 2).13 The monod-
odecyl ester FOH was obtained by iodine-catalyzed
isomerization14 of MOH since attempts to prepare it from fumaric
acid resulted in a formation of a symmetrical diester F. The most
convenient procedure for the synthesis of the unsymmetrical es-
ters M1 and S1 employed partial re-esterification of the corre-
sponding dimethyl esters (Schemes 1 and 2). This procedure was
Figure 1. Transdermal permeation enhancer 12GM12.
The aim of this work was to prepare a series of structurally sim-
plified single- and double-chain amphiphiles based on maleic, fu-
maric, succinic, tartaric, and meso-tartaric acids in order to study
the role of geometric isomers, chain number and hydrogen-bond-
ing groups in their transdermal permeation-enhancing activity. It
should be noted that these compounds are not intended for poten-
Scheme 1. Reagents and conditions: (i) C12H25OH, H+, toluene, reflux, 4 h; (ii) 1—
MeOH, H+, reflux, 3 h, 2—C12H25OH, H+, reflux, 3 h; (iii) C12H25OH, H+, 130–140 °C,
10 min.
* Corresponding author. Tel.: +420 495 067497; fax: +420 495 067166.
E-mail address: katerina.vavrova@faf.cuni.cz (K. Vávrová).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.11.083

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M. Novotny et al. / Bioorg. Med. Chem. Lett. 19 (2009) 344–347
bonding ability as observed previously10 but requires a certain
optimum.
Fumaric acid derivative F even decreased the skin permeability
with ER = 0.6, 0.1, and 0.7 from an aqueous, PG, and IPM suspen-
sion, respectively (Fig. 2). Although the differences were not signif-
icant, similar trans double bond-containing amphiphiles may serve
as a lead for designing potential permeation retardants. These
compounds are currently widely discussed for their potential to
prevent absorption of hazardous substances through the skin.15
Single-chain compounds. The removal of one dodecyl chain re-
sulted in a markedly increased activity (Fig. 2). The most active
compound was MOH, which increased the flux of theophylline 26
times from the PG vehicle. The higher activity of the monoesters
compared to the double-chain compounds cannot only be caused
by the free carboxyl group; for example, lauric acid enhanced the-
ophylline permeation only twice under the same conditions.7
Moreover, the dodecyl methyl ester M1 showed significant activi-
ties as well. To our knowledge, this is the first report of such an ef-
fect. The possible explanation is that these double-chain
amphiphiles may incorporate into the stratum corneum lipid bilay-
ers without causing any significant disturbance. On the other hand,
their single-chain counterparts possess a polar head of the same
size but only one chain, which may prevent tight association of
the chains via hydrophobic interactions. Interestingly, the activity
of the enhancers in lipophilic IPM was negligible except for MOH
and FOH showing ER = 7.5 and 4.5, respectively.
Geometric isomers. The higher permeation-enhancing activity of
the single-chain compounds allowed for a comparison of the cis (Z)
and trans (E) derivatives. The activities of the dodecyl methyl esters
M1 and F1 were comparable. However, monoester MOH, that is a
cis derivative, was significantly more active than the corresponding
trans isomer FOH in all of the evaluated donor vehicles (Fig. 2).
These monoesters differ in acidity but not to such an extent as
the parent maleic and fumaric acid—the dissociation constants of
MOH and FOH are 4.87 and 4.40, respectively.16 Moreover, the dif-
ferent acidity cannot fully explain the difference in activity of the
monoesters, since the less acidic SOH is inactive in aqueous and
IPM vehicle but its activity is similar to MOH in PG.
Thus, the most likely explanation for the observed difference in
permeation-enhancing activity of MOH and FOH is a different pack-
ing of the geometric isomers. Such behavior was described in unsat-
urated fatty acids with a double bond situated approximately in the
center of the hydrophobic chain. For example, oleic acid was much
more effective enhancer than elaidic acid, its trans isomer.5,17 This
was attributed to the presence of a ‘kink’ in the chain caused by
cis double bond18 leading to disruption of the stratum corneum li-
pid packing or phase separation. Although the double bond in
MOH and FOH was situated in their polar head and not in the hydro-
phobic chains where the influence on the lipid packing is obvious,
the difference in spatial arrangement may explain the higher activ-
ity of the maleate as well. Generally, the straighter-shaped trans
isomers pack better than the U-shaped cis isomers as reflected,
for example by their higher melting points (for example, mp of
FOH and MOH were 82 and 58 °C, respectively, see Supporting
Information). The shape of the polar head of the cis isomer does
not allow for close packing of its hydrophobic chain with those of
the stratum corneum lipids. That means that the intermolecular
forces between the hydrophobic chains are not as effective so the
drug can cross the lipid matrix more easily. The lesser ability of
trans isomers to promote skin permeability seen in this study is fur-
ther supported by the fact that the natural skin barrier constituents,
that is sphingosine and 6-hydroxysphingosine-type ceramides,
contain trans double bond in close proximity to their polar head
too. Moreover, this C4-trans double bond was found to promote clo-
ser packing of ceramide in monomolecular films at the argon–buf-
fer interface relative to comparable saturated species.19
Scheme 2. Reagents and conditions: (i) C12H25OH, H+, toluene, reflux, 4 h; (ii)
C12H25OH, DCC, DMAP, 0 °C to rt, 24 h; (iii) I2, 150 °C, 1 h.
not successful in case of F1, where a preferential formation of F was
observed. Thus, the asymmetric diester F1 was prepared via carbo-
diimide coupling of methyl fumarate with dodecanol (Scheme 2).
The tartaric and meso-tartaric acid esters, T and mT, respec-
tively, were prepared by esterification of the pertinent dicarboxylic
acid. For the preparation of the acetonides AT and AmT, either T or
mT reacted with 2,2-dimethoxypropane under acid catalysis. The
analogous carbonates CT and CmT were synthesized from T and
mT, respectively, with dimethylcarbonate and sodium metal. The
monoester TOH was prepared by partial hydrolysis of T. This pro-
cedure was not successful in case of mTOH; thus it was prepared
by syn dihydroxy addition to MOH (Scheme 3).
The transdermal permeation-enhancing activity of the prepared
compounds was evaluated in vitro using Franz diffusion cell and
porcine skin. Theophylline was selected as a model permeant of
medium lipophilicity and to allow for the comparison with our
previous results. All the prepared enhancers were evaluated in
three donor vehicles of different polarity: water, 60% propylene
glycol (PG), and isopropyl myristate (IPM). Theophylline was pres-
ent at its maximum thermodynamic activity in all the donor sam-
ples either with or without the enhancer. The potencies of the
individual enhancers are reported as the enhancement ratio values
(ER), that is a ratio of the flux of theophylline through the skin with
and without the studied enhancer.
Double-chain compounds. The activity of the first series, that is
maleic, fumaric, and succinic acid derivatives is outlined in Fig-
ure 2. The didodecyl esters M, F, and S, that is the double-chain
enhancers, did not increase theophylline permeation from either
vehicle. This finding was rather surprising as the previously de-
scribed maleic acid derivative 12GM12 was a relatively potent en-
hancer.10 The difference between M and 12GM12 is an absence of
glycyl linker. This suggests that the permeation-enhancing activity
is not simply inversely related to the polar head size and hydrogen
Scheme 3. Reagents and conditions: (i) C12H25OH, H+, 100 °C, 2 h; (ii)
(CH3O)2C(CH3)2, H+, CH2Cl2, rt, 2 h; (iii) (CH3O)2CO, Na, toluene, 90 °C, 3 h; (iv) 1—
OHÀ/H2O, 2—H+; (v) 1—KMnO4/acetone/H2O, 2—Na2SO3, H+.

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346
Figure 2. Transdermal permeation-enhancing activity of the maleates, fumarates, and succinates in (A) aqueous, (B) PG, and (C) IPM donor vehicle. Data are presented as the
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mean SEM (n = 4–12, 2–6 donors), indicates statistically significant difference against control (p < 0.5).
Figure 3. Enhancing activity of the tartaric and meso-tartaric acid derivatives in (A) aqueous, (B) PG, and (C) IPM donor vehicle. Data are presented as the mean SEM (n = 4–
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11, 2–4 donors), indicates statistically significant difference against control (p < 0.5).
Double vs. single bond. The activity of the succinate SOH, that is
the single bond counterpart of the above compounds with unre-
stricted rotation about the C2–C3 bond, was highly dependent on
the donor vehicle. While its behavior was close to that of MOH in
PG (ER of SOH = 24), it was inactive in the aqueous and IPM vehicle
(Fig. 2). Such synergic action of solvent-type permeation enhancers
such as PG with the amphiphilic ones has been observed previ-
ously20,21 and may be explained by a combination of different
mechanisms of action, for example increased partitioning plus lipid
fluidization.
1M0508 and a Research Project MSM0021620822), and the Charles
University (Project 79607/2007/B-CH/FaF).
Supplementary data
Synthetic procedures, compounds characterization, skin perme-
ation experiments, HPLC analysis, and data treatment. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.bmcl.2008.11.083.
Tartrates. The activity of the tartaric and meso-tartaric acid
derivatives is outlined in Figure 3. Some of these compounds
showed significant enhancement but the activity of this series
was generally low. Furthermore, no modification of the structure
including masking the hydroxyls by acetonide or a cyclic carbonate
or removal of one chain resulted in a significant improvement.
Furthermore, no significant difference was found between dia-
stereomeric tartarates T, CT, AT, and TOH and the corresponding
meso-tartaric acid derivatives mT, CmT, AmT, and mTOH. This is
consistent with our previous studies showing no stereoselectivity
in the action of amino acid-based enhancers.22,23
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Acknowledgments
This work was supported by the Ministry of Education of the
Czech Republic (the Centre for New Antivirals and Antineoplastics

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Pharm. Res. in press. doi:10.1007/s11095-008-9780-3.