N-Myristoylglutamic acid derivative of 3′-fluoro-3′- deoxythymidine as an organogel

Article abstract of DOI:10.1016/j.tetlet.2012.07.101

Designing microbicidal gels of anti-HIV drugs for local application to prevent HIV infection is a subject of major interest. 3′-Fluoro-3′- deoxythymidine (FLT), a nucleoside reverse transcriptase inhibitor (NRTI), was conjugated with a N-myristoylglutamate scaffold. The conjugate showed gelation at 1% (w/w) in different organic solvents, such as toluene, dichloromethane, and chloroform. The gels were opaque and stable at room temperature. The results indicate that myristoyl glutamate derivative of FLT can form an organogel. The gel could have potential application as a topical anti-HIV microbicidal agent.

Full text of DOI:10.1016/j.tetlet.2012.07.101

Tetrahedron Letters 53 (2012) 5335–5337
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
N-Myristoylglutamic acid derivative of 30-fluoro-30-deoxythymidine
as an organogel
⇑
Bhupender S. Chhikara, Rakesh Tiwari, Keykavous Parang
Department of Biomedical and Pharmaceutical Sciences, College of Pharmacy, University of Rhode Island, 41 Lower College Road, Kingston, RI 02881, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Designing microbicidal gels of anti-HIV drugs for local application to prevent HIV infection is a subject of
major interest. 30-Fluoro-30-deoxythymidine (FLT), a nucleoside reverse transcriptase inhibitor (NRTI),
was conjugated with a N-myristoylglutamate scaffold. The conjugate showed gelation at 1% (w/w) in dif-
ferent organic solvents, such as toluene, dichloromethane, and chloroform. The gels were opaque and sta-
ble at room temperature. The results indicate that myristoyl glutamate derivative of FLT can form an
organogel. The gel could have potential application as a topical anti-HIV microbicidal agent.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 21 June 2012
Revised 19 July 2012
Accepted 24 July 2012
Available online 31 July 2012
Keywords:
Amino acid
Anti-HIV drugs
FLT
Glutamate
Nucleoside
NRTI
Organogel
Peptide
Reverse transcriptase
Nucleoside analogues have been widely used for the treatment
of different cancers1–3 and human immunodeficiency virus (HIV)
infection.4 Nucleoside reverse transcriptase inhibitors (NRTIs) are
the main class of anti-HIV drugs. Reverse transcriptase is an en-
zyme involved in RNA to DNA conversion in the HIV-infected cells.5
NRTIs are used in combination with other drugs in highly active
antiretroviral therapy (HAART).
HIV is mainly transmitted through sexual interactions. There is
an urgent need to develop safe and effective preventative
strategies.6 Microbicides are topically applied agents that prevent
or reduce transmission of infectious diseases, in particular HIV
infection.7 The topical microbicides are expected to be biocompat-
ible, broad-spectrum, potent nontoxic, lacking detergent-type
membrane toxicity, being harmless to the mucosal microflora,
and to display broad-spectrum activity against drug-resistant HIV.
NRTIs are very polar compounds and may not be easily applica-
ble as topical microbicides. Thus, it is important to convert the
drug into a suitable form for topical applications. The gel form of
drugs or drugs incorporated in gels8 are one of the most convenient
forms that have found application in diverse fields, including drug
delivery.9
of self-assembled fibers. A number of organogelators have been re-
ported for drug delivery application, such as lecithin, glyceryl fatty
acid esters, poly(ethylene), N-lauroyl-glutamic acid di-n-butylam-
ide, and N-stearoyl alanine methyl ester, mostly for dermal and
transdermal formulations.9 Herein, we report the gel formation
by conjugation of a model nucleoside analogue, 30-fluoro-20,30-
dideoxythymidine (FLT) as a NRTI,10 and a lipophilic myristoylated
glutamic acid as organogelator. To the best of our knowledge, this
is the first report of designing a gel by synthesizing lipophilic
nucleoside-glutamic acid derivatives.
The glutamic acid–nucleoside conjugate derivative was synthe-
sized starting from Glu(OtBu)-OH (1) (Scheme 1). The myristoyl
group was coupled to 1 by reaction with myristic anhydride in
the presence of N,N-diisopropylethylamine (DIPEA) to yield myri-
stoylated glutamic acid (N-My-Glu(OtBu)-OH, 2). Conjugation of
2 with FLT in the presence of 1,1,3,3-tetramethyluronium hexa-
fluorophosphate (HBTU), DIPEA, and hydroxybenzotriazole (HOBt)
afforded N-My-Glu(OtBu)-OFLT (3). HOBt was used to protect the
racemization of glutamic acid. Deprotection of tBu in 3 was
accomplished in the presence of TFA/DCM (95:5 v/v) to yield
My-Glu(OH)-OFLT (4). The direct hydrolysis of myristoylated t-
butylglutamic acid (2) with 95% TFA solution in DCM (v/v) gave
N-myristoylglutamic acid (5, N-My-Glu-OH).
Organogels are semi-solid formulations with an organic liquid
phase which is trapped by a three-dimensional network composed
The gelation properties of the synthesized derivatives 4 and 5
were evaluated by dissolving them in different solvents at 1% w/
w ratio, three times repeated treatment of heating the mixture to
⇑
Corresponding author. Tel.: +1 401 874 4471; fax: +1 401 874 5787.
E-mail addresses: kparang@gmail.com, kparang@uri.edu (K. Parang).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.101

5336
B. S. Chhikara et al. / Tetrahedron Letters 53 (2012) 5335–5337
O
N
HN
O
O
O
O
O
NH
12
H2N
O
OH
O
OH
O
F
(CH3(CH2)12CO)2O
DIPEA, DMF
FLT, DIPEA
HBTU, HOBt, DMF
NH
12
O
O
O
O
3
1
2
O
O
TFA in DCM
(95% v/v)
TFA in DCM
(95% v/v)
O
O
N
O
NH
12
NH
O
OH
O
O
NH
12
O
O
O
OH
5
F
4
O
OH
Scheme 1. Synthesis of N-myristoylglutamic acid derivative of FLT (4) and N-myristoylglutamic acid (5).
Table 1
45 °C and sonication in a water bath sonicator for 5 min, followed
by keeping the solution stable at room temperature for overnight.
The gelation of compounds 4 and 5 was evaluated in different
solvents by a similar procedure and under similar conditions. As
shown in Figure 1, N-myristoylated glutamate derivative of FLT 4
formed the white opaque gel in dichloromethane and toluene
while the control N-myristoylated glutamic acid 5 did not form
any gel under similar conditions. These data suggest that the pres-
ence of FLT is required for gel formation. The details of the gel for-
mation in different solvents are shown in Table 1.
Furthermore, UV studies were conducted to find the solution to
gel and gel to solution phase transition temperature in CHCl3 solu-
tion. The cooling scan of the solution of compound 4 in chloroform
from 50 to 5 °C followed by heating scan from 5 to 50 °C was mon-
itored by change in UV absorbance at 265 nm (Fig. 2). The cooling
scan graph on extrapolation of change on absorption pattern with
temperature change gave the phase transition between 30 and
35 °C. The graph pattern was different during the heating scan
and showed comparatively higher absorption or non-transmit-
tance of light probably due to the presence of opaque gel form.
There was a significant change in absorption graph from 35 to
Gel formation by compounds 4 and 5 in different solvents
Compd
Solvent
Gel formation
Gel appearance
4
4
5
5
4
4
4
5
4
5
4
5
4
CH2Cl2
CHCl3
CH2Cl2
Methanol
Methanol
Toluene
Xylene(s)
Toluene
Hexane
Hexane
Water (DMSO 1% v/v)
Water
Ethanol
Ya
Y
N
N
N
Y
Opaque, puff white
Opaque, puff white
Precipitate
—
—
Opaque, puff white
Opaque, puff white
—
—
Y
N
N
N
Nb
N
N
—
Precipitate
—
—
a
Y = gel formed.
N = no gel.
b
40 °C, indicative of phase transition in this range. These data sug-
gest the formation of low molecular weight organogel that is
stabilized by weak inter-chain interactions. In non-aqueous condi-
tions, the major attractive forces are possibly hydrogen bonding
between glutamate amides, hydrogen bonding between nucleo-
sides, and van der Waals interactions between alkyl chains of myr-
istoyl groups. These data are consistent with previously reported
gelation of single-walled carbon nanotubes functionalized with
long fatty acyl chain,11 suggesting the potential contribution of
van der Waals interactions between alkyl chains for gel formation.
An organogel of 4 was generated by dissolving the gelator in tol-
uene and heating. Upon cooling, the affinity between the gelator
and organic solvent decreased leading to self-assembly into solid
aggregates of gelator held through intermolecular interactions.
Transmission electron microscopy (TEM) (Fig. 3) showed growth
into fibers with 200–470 nanometers in width and up to several
micrometers in length without breakage. Multidimensional growth
pattern suggests a robust morphology.
Figure 1. Gelation of compounds 4 and 5 in different solvents (1% w/w). (A) 4 in
water (dissolved in DMSO and then added to water (water/DMSO 100:1 v/v); (B) 4
in methanol; (C) 4 in CH2Cl2; (D) 4 in toluene; (E) 5 in toluene; (F) 5 in CH2Cl2.
A number of organogel formulations have been used in drug
delivery. For example, lecithin has been used for the formulation

B. S. Chhikara et al. / Tetrahedron Letters 53 (2012) 5335–5337
5337
tration because this strategy provides a non-invasive mode of
administration minimizing the effects of organic solvents. Trans-
dermal delivery offers net advantages over oral administration in
terms of lowered systemic side effects associated with organic sol-
vents. Gel components could be chosen according to their compat-
ibility with intended applications, such as using non-toxic solvents
for pharmaceutical applications. It is envisioned that organogel for-
mation by 4 provides preliminary results for the potential use of
nucleoside analogue organogels as topical anti-HIV microbicide
applications. Optimization of the solvents to other nontoxic sol-
vents and formulations are required to make these organogels
appropriate for pharmaceutical applications. One major advantage
of this organogel is that the active nucleoside pharmaceutical com-
ponent is a part of gelator and does not need to be mixed with an-
other drug, avoiding problems associated with the uncontrolled
release of drugs from the formulation. FLT is expected to be re-
leased from the conjugate 4 through hydrolysis by cellular esterase
as shown in other ester conjugates of nucleosides.12
In conclusion, the formation of organogels by FLT conjugation
with N-myristoylglutamic acid was investigated. FLT-myristoyl
glutamate conjugate formed organogel in dichloromethane, tolu-
ene, and xylene at 1% w/w. The formed gels were opaque and re-
mained stable at room temperature. This method can be used for
the gelation of other NRTIs conjugated with N-myristoylglutamic
acid as the organogelator. This organogel NRTI can be used for
the potential application as a topical anti-HIV microbicide. This
strategy presents major advantages as drug delivery formulations,
such as ease of preparation and route of administration.
Figure 2. UV thermal graph for the compound 4 in CHCl3 with absorbance at
265 nm. (a) Cooling graph from 50 to 5 °C; (b) heating graph from 5 to 50 °C. The
reading is average of three readings for respective graph.
Acknowledgments
We thank the US National Science Foundation, Grant Number
CHE 0748555 for the financial support and National Center for Re-
search Resources, NIH, and Grant Number 1 P20 RR16457 for spon-
soring the core facility.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.07.
101.
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Figure 3. TEM of the organogel of 4 in toluene.
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of diclofenac, piroxicam, and indomethacin.7 Most of these formu-
lations are used for transdermal, rectal, or subcutaneous adminis-