Transkarbams as transdermal permeation enhancers: Effects of ester position and ammonium carbamate formation

Article abstract of DOI:10.1016/j.bmcl.2010.03.077

Transkarbam 12, an ammonium carbamate formed by the reaction of dodecyl 6-aminohexanoate with carbon dioxide, is a highly active, broad-spectrum, nontoxic, and nonirritant transdermal permeation enhancer. It probably acts by a dual mechanism: a part of its activity is associated with the carbamic acid salt and/or its decomposition in the acidic stratum corneum. The ammonium ester thereby released is an active enhancer species as well, and its activity highly depends on the position of the ester group.

Full text of DOI:10.1016/j.bmcl.2010.03.077

Bioorganic & Medicinal Chemistry Letters 20 (2010) 2726–2728
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Transkarbams as transdermal permeation enhancers: Effects of ester
position and ammonium carbamate formation
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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, 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:
Transkarbam 12, an ammonium carbamate formed by the reaction of dodecyl 6-aminohexanoate with
carbon dioxide, is a highly active, broad-spectrum, nontoxic, and nonirritant transdermal permeation
enhancer. It probably acts by a dual mechanism: a part of its activity is associated with the carbamic acid
salt and/or its decomposition in the acidic stratum corneum. The ammonium ester thereby released is an
active enhancer species as well, and its activity highly depends on the position of the ester group.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 10 February 2010
Revised 11 March 2010
Accepted 19 March 2010
Available online 25 March 2010
Keywords:
Transdermal drug delivery
Permeation enhancers
Ammonium carbamate
Structure–activity relationships
Transdermal delivery of drugs through the skin to the systemic
circulation offers numerous advantages over the conventional
routes of drug administration.1 A disadvantage to development
however, stems from the fact that the skin is a remarkably efficient
barrier, designed to keep ‘our insides in and the outsides out’.2 To
overcome this limitation, substances temporarily diminishing the
barrier properties of the skin known as permeation enhancers or
penetration/absorption promoters have been used.2–8 These chem-
icals interact with constituents of the major skin barrier, stratum
corneum, to promote drug flux. Although hundreds of compounds
have been evaluated as permeation enhancers, to-date their use in
transdermal formulations is limited since the mechanisms of ac-
tion of these agents are seldom defined.2
Transkarbam 12 (Fig. 1) is an ammonium carbamate formed by
the reaction of 6-aminohexanoic acid dodecyl ester with carbon
dioxide.9 This amphiphilic compound is a highly potent broad-
spectrum enhancer with low toxicity and minimal skin irrita-
tion.9,10 In our effort to obtain enhancers with higher activity
and/or to elucidate the mechanism of action of Transkarbam 12,
numerous synthetic modifications have been studied.
The most unusual structural feature of this enhancer is the ammo-
nium carbamate polar head. It was suggested to be essential be-
cause the parent amino ester was inactive.9 However, its
importance for the mechanism of action of transkarbams is still
unknown. In general, salts of carbamic acids are unstable in an
acidic environment releasing carbon dioxide and the respective
ammonium salt17 (Fig. 1). It was suggested that such carbon diox-
ide release can take place in the acidic stratum corneum, and may
be connected with the mechanism of action of transkarbams.9,14 In
accordance with this hypothesis, stable carbon dioxide derivatives
of similar structure were inactive.18
The purpose of this study was to evaluate the most important
structural features of Transkarbam 12 further. Since the ester
group was found necessary for the activity of this enhancer, we
aimed to prepare and study a series of Transkarbam 12 isomers
with varying distance of the ester group from the carbamate polar
The structural requirements are given in Figure 1. The hydro-
phobic chain(s) should be 10–12 carbons long and linear because
both branching and cyclization had negative effects on the enhanc-
ing activity.11–13 The linking chain between the nitrogen and ester
carbonyl should be flexible.14 The ester group is important as well;
its replacement by carbonate, carbamate, amide, ketone, and
methylene group led to a marked decrease or loss of activity.15,16
* Corresponding author. Tel.: +420 495 067497; fax: +420 495 067166.
E-mail address: katerina.vavrova@faf.cuni.cz (K. Vávrová).
Figure 1. Transkarbam 12, its decomposition in an acidic environment, and the
structural features important for its transdermal permeation-enhancing activity.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.03.077

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M. Novotny et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2726–2728
experiment either with or without the enhancers. For further
experimental details, see our previous reports.21,22
The transdermal flux values of theophylline either without
(control sample) or with the respective enhancers are shown in
Figure 2.
Ammonium carbamates versus ammonium chlorides. The first aim
of this study was to compare the activities of the ammonium chlo-
rides 1 and transkarbams 2, that is, the corresponding two-chain
ammonium carbamates. Similar experiment was already
performed; however, it was a comparison of Transkarbam 12
(2d) with a free base of 1d, not the ammonium salt. In that study,
the free base showed no activity.9 However, when studying the ef-
fects of 2d on adefovir permeation, the maximum activity was
found at pH 4.10 At such pH value, only 1d but not the carbamate
2d could be present in the donor sample. Thus, we aimed at revis-
iting the activity of 1d by performing a direct comparison with 2d.
Figure 2 shows that the carbamates 2 were more active enhanc-
ers than the ammonium chlorides 1. Nevertheless, compounds 1d–
f were not inactive as assumed before. Because all the donor sam-
ples containing compounds 1 were prepared under nitrogen and
the donor compartment of the Franz cell was filled with nitrogen,
the reaction with carbon dioxide and partial formation of the car-
bamates 2 in the donor samples of 1 was excluded. Thus, the
ammonium salt formed upon decomposition of the carbamate is
an active enhancer species. The previous lack of activity of the free
base of 1d may be explained by an intramolecular aminolysis of
the ester by the primary aminogroup yielding caprolactam and
dodecanol.
To exclude the possibility that the higher activities of the carba-
mates 2 compared to 1 were caused only by a different pH of the
donor samples and that it may be drug-specific, another experi-
ment was performed. Two drugs, theophylline and hydrocortisone,
were used, and the donor samples containing 1d or 2d were care-
fully adjusted at pH 7.0. The effects on the flux of these two drugs
are shown in Figure 3.
These results confirmed that the ammonium chlorides 1 pos-
sess enhancing activity, but they do not reach that of the carba-
mates 2. Thus, a part of the permeation-enhancing potency of
carbamates 2 can be attributed to the ammonium carbamate polar
head and a part to the simple ammonium ester that is probably
released from the carbamate in the stratum corneum.
Scheme 1. Synthesis of Transkarbam 12 (2d) and its isomers 2a–c and 2e–h.
Reagents and conditions: (i) HCl, rt, 1h; (ii) (1) SOCl2, rt, 1 h, (2) CH3(CH2)yOH,
CHCl3, 60 °C, 2 h; (iii) (1) potassium phthalimide, DMF, 40 °C, 4 h, (2) N2H4.H2O,
EtOH, reflux, 4 h, (3) HCl(g), Et2O, 30 min; (iv) (1) Et3N, H2O/Et2O, 15 min, (2) CO2,
rt, 15 min.
head 2a–h (Scheme 1). Moreover, the activities of these ammo-
nium carbamate salts were compared to those of the respective
ammonium chlorides 1a–h to confirm the suggestion that the car-
bamate polar head was essential for the action of Transkarbam 12.
The target ammonium carbamates 2a–h were prepared from
the respective amino acids or bromo acid (Scheme 1). The amino
acids were protonized and converted to their acyl chlorides, which
immediately reacted with the corresponding alcohols to form the
esters 1a–f and 1h.13,19 10-Aminodecanoic acid ester was prepared
from a bromo acid via acyl chloride. The bromoester was then sub-
jected to the Gabriel synthesis with subsequent hydrazinolysis20 to
yield the amino ester, which was stored and characterized as its
hydrochloride 1g. The ammonium chlorides 1a–h were alkalized
and reacted with carbon dioxide to yield the ammonium carba-
mates 2a–h.13,19
The effects of the prepared enhancers on the permeation of a
model drug theophylline through porcine ear skin were evaluated
in vitro using Franz diffusion cells. The donor samples contained 5%
(w/v) theophylline with/without 1% (w/v) enhancer in propylene
glycol/water 6:4 (v/v). At this concentration, maximum thermody-
namic activity of the drug was maintained throughout the
It should be noted that although the ammonium carbamates are
decomposed in an acidic environment and at elevated tempera-
Figure 2. Effects of ammonium chlorides 1a–h and the corresponding ammonium
carbamates 2a–h on the in vitro transdermal flux of theophylline. Mean SEM,
n = 5–20. *Indicates statistically significant difference against control sample (i.e.,
without enhancer), +indicates statistically significant difference against the corre-
sponding ammonium chloride.
Figure 3. Effects of the ammonium chloride 1d and the corresponding ammonium
carbamate 2d on the in vitro transdermal flux of theophylline (A) and hydrocor-
tisone (B) at pH 7.0. Mean SEM, n = 5–14. *Indicates statistically significant
difference against control sample (i.e., without enhancer), +indicates statistically
significant difference against 1d.

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This hypothesis that both the polar groups are positioned with-
in the polar layer of the lipid membranes and probably close to
each other agrees well the previously suggested formation of an
intramolecular hydrogen bond between the ester carbonyl and
N–H of transkarbams.15,16 Such interaction would lead to a ‘cyclic’
polar head similar to Azone, which is a well described permeation
enhancer having a dodecyl chain attached to a seven-membered
azepan-2-one ring.25,26
Another possible explanation of these results, that is, that trans-
karbams are only pro-enhancers releasing the respective fatty alco-
hols, was excluded previously.10,13
In conclusion, transkarbams, which are highly potent ammo-
nium carbamate transdermal permeation enhancers, probably act
by a dual mechanism. First, a part of their activity is associated
with the carbamic acid salt and/or with its decomposition in the
acidic stratum corneum. Consequently, the ammonium esters
thereby released possess transdermal permeation-enhancing
activity as well. Further investigation on the mechanism of action
of these unusual enhancers is in progress.
Figure 4. Effects of the ammonium chloride 1d and the corresponding ammonium
carbamate 2d on the in vitro permeation profiles of theophylline (A) and
hydrocortisone (B) at pH 7.0.
Acknowledgements
tures, they are relatively stable at neutral or slightly alkaline pH or,
more precisely, equilibrium between carbamate, bicarbonate and
carbonate is formed in aqueous solutions.17 Thus, the ammonium
carbamate concentration in the donor sample did not significantly
change over time as reflected by stable flux values (and the ratios
of flux values of the carbamates and ammonium salts) during the
assay. To illustrate this, Figure 4 shows the permeation profiles
of theophylline (Fig. 4A) and hydrocortisone (Fig. 4B) in the pres-
ence of 1d and 2d.
This work was supported by the Ministry of Education of the
Czech Republic (Centre for New Antivirals and Antineoplastics
1M0508 and a Research Project MSM0021620822).
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