Novel approach to pro-drugs of lactones: Water soluble imidate and ortho-ester derivatives of a furanone-based COX-2 selective inhibitor

Article abstract of DOI:10.1016/j.bmcl.2005.03.009

Interest in water soluble COX-2 inhibitors that can be administered intravenously led to the development of novel pro-drugs of a furanone based COX-2 inhibitor 2. Transforming the lactone moiety of the furanone to an imidate or an ortho-ester with a hydro

Full text of DOI:10.1016/j.bmcl.2005.03.009

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2259–2263
Novel approach to pro-drugs of lactones: water soluble imidate
and ortho-ester derivatives of a furanone-based COX-2
selective inhibitor
Steve F. Poon,* Nicholas Stock,* Joseph E. Payne, Angela R. McGuire, Brian Stearns,
Xiaoqing Yang, Weichao Chen, Benito Munoz and Nicholas D. Smith
Department of Medicinal Chemistry, Merck Research Laboratories, MRLSDB2, 3535 General Atomics Court, San Diego,
CA 92121, USA
Received 3 February 2005; revised 1 March 2005; accepted 3 March 2005
Abstract—Interest in water soluble COX-2 inhibitors that can be administered intravenously led to the development of novel pro-
drugs of a furanone based COX-2 inhibitor 2. Transforming the lactone moiety of the furanone to an imidate or an ortho-ester with
a hydrophilic, endogenous appendage resulted in water soluble pro-drugs that converted to the parent drug in vivo.
Ó 2005 Elsevier Ltd. All rights reserved.
There exists a clinical need for the acute treatment of
inflammation where the oral administration of analge-
sics is inadequate. In areas such as post-operative pain1
and stroke,2 rapid administration of selective COX-2
inhibitors have been shown to have a beneficial effect.
As such, there is continued interest in developing water
soluble COX-2 inhibitors for intravenous administra-
tion. Currently there is only one injectable COX-2 inhib-
itor, paracoxib,3 which is awaiting FDA approval. In
addition, other water soluble COX-2 inhibitors have
been reported in the literature.4,5
Cl
O
O
O
O
O
O-Na+
MeO2S
MeO2S
1
2
COX-2 IC50 (HWB*): 1.7 µM
COX-1 IC50 (HWB*): >86 µM
COX-2 IC50 (HWB*): 0.4 µM
COX-1 IC50 (HWB*): >86 µM
*HWB = human whole blood
Figure 1.
Previous attempts at developing water soluble forms of
furanone based COX-2 inhibitors resulted in analogues,
such as compound 1, with lower potency in a COX-2
whole blood functional assay (Fig. 1).6
Our efforts to develop a pro-drug of compound 2 fol-
lowed a set of criteria: To be administered intrave-
nously, the derivatives need to be highly water soluble
and stable in aqueous solution. Additionally we wanted
to incorporate endogenous or nontoxic solubilizing
groups into the pro-drug, which in vivo, would rapidly
convert to the parent drug and not compromise the
safety profile of 2 (Fig. 2).
In contrast, furanone based COX-2 inhibitor, 2, exhibits
excellent potency, selectivity, pharmacokinetic charac-
teristics, and safety profile. The drawback to 2 is that
it is insoluble in aqueous solution.7 We thus sought to
identify water soluble pro-drugs of the inhibitor 2 and
herein we report our progress.
Our initial strategy focused on synthesizing a ring-
opened form of the lactone moiety, where solubilizing
endogenous substituents would be appended onto the
resultant alcohol and acid functionalities (Route A,
Fig. 2). Upon in vivo administration, it was envisioned
that these groups would cleave to regenerate the lactone
Keywords: COX-2; Stroke; Pro-drug; ortho-Ester.
*
Corresponding authors. Tel.: +1 858 202 5438; fax: +1 858 202
5752; e-mail: steve_poon@merck.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.03.009

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S. F. Poon et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2259–2263
endogenous
alcohol
O
O
Route A
O
O
endogenous
ester
O
MeO2S
O
in vivo
conversion
A
O
Route B
MeO2S
R
2
R
R'
endogenous
and solubilizing
N
O
O
O
O
in vivo
conversion
O
O
MeO2S
MeO2S
B
C
Figure 2. Pro-drug strategies.
O
OH
OH
O
O
O
O
O
R1
b
O
a
O
O
O
O
X
R2
O
MeS
MeS
MeS
MeO2S
A
3
4
5
Scheme 1. Reagents and conditions: (a) DBU, toluene, reflux, 24 h; (b) (i) LiAlH4, THF, 0 °C; (ii) DIBAL, THF, À10 °C.
2.8 Due to the thermodynamic stability of the lactone in
2, we attempted to access the open form through a step-
wise reduction–oxidation route (Scheme 1). Reducing
the methyl sulfide derivative 4 (generated from the
known compound 3)9 with LiAlH4 and DIBAL gave
the diol 5. However, further attempted derivatization
of 5 to the desired pro-drug, either through the oxida-
tion of the primary alcohol or capping of the tertiary
alcohol led to extensive decomposition.
We thus turned our attention to converting the lactone
of 2 to an imidate or ortho-ester derivative that incorpo-
rated a water-solubilizing, endogenous moiety (Route B,
Fig. 2). This strategy relies on the premise that imidates
R'
N
O
O
b
O
S
MeO2S
O
O
a
B
O
O
R
b
MeO2S
MeO2S
R
2
6
O
O
O
O
MeO2S
C
Scheme 2. Reagents and conditions: (a) LawessonÕs reagent, toluene, reflux (95%); (b) diol or amine, AgOTf, Et3N, MeCN (50–75%).

S. F. Poon et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2259–2263
2261
and ortho-esters would be stable in aqueous solution yet
suitably labile in vivo, thus effecting rapid release of the
desired inhibitor 2.
ing thiolactone 6 in excellent yield (95%) (Scheme 2).
The imidate and ortho-ester derivatives were then syn-
thesized through a metal ion mediated desulfuriza-
tion–condensation reaction between the thioester and
an amine or diol. Thus, reaction of 6 with AgOTf and
Et3N in the presence of the appropriate nucleophile
gave the corresponding pro-drug B or C in 50–75%
yield.12
Attempted direct conversion of 2 to B or C proved
unsuccessful due to its inherent thermodynamic stabil-
ity. However, previous research demonstrated that
acetals and imidates could be synthesized from acti-
vated thioketones.10,11 Building upon this chemistry,
we successfully synthesized water soluble imidate and
ortho-ester derivatives of lactone 2 via the thiolactone
intermediate 6. Thus, refluxing compound 2 with
LawessonÕs reagent in toluene afforded the correspond-
A series of imidates and ortho-esters were synthesized
using this protocol as shown in Scheme 3 where each
of the pendant groups is a compound endogenous to
the human body.
O
O
N
O
O-Na+
a
O
MeO2S
S
O
7
O
OH
O-K+
MeO2S
N
O
b
6
O
O
O
MeO2S
8
O
OH
O-Na+
c
O
O
O
O
S
O
9
MeO2S
O
O
P
O-
N+
MeO2S
O
O
6
O
d
O
O
O
e
10
MeO2S
O
OH
O
O-Na+
O
O
O
O
f
O
O
O
O
O
MeO2S
12
MeO2S
11
Scheme 3. Reagents and conditions: (a) (i) H2NOH, AgOTf, Et3N, MeCN; (ii) 2-bromoacetic acid, NaOH; 48%; (b) (i) L-serine-OMe, AgOTf, Et3N,
MeCN; (ii) KOTMS, THF; 64%; (c) (i) dibenzyl ascorbate, AgOTf, Et3N, MeCN; (ii) H2, Pd/C, MeOH; 37%; (d) GPC, AgOTf, Et3N, MeCN; 75%;
(e) glycerol, AgOTf, Et3N, MeCN; 80%; (f) succinic anhydride, DMAP, Et3N, CH2Cl2; 55%.

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S. F. Poon et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2259–2263
Table 1.
R
R
R'
N
O
O
O
O
in vivo
O
O
O
O
OR
O
MeO2S
MeO2S
MeO2S
2
Compd
Solubility (mg/mL)a
Stabilityb
Levels of 2 @ 0.5 hc (lM)
Levels of pro-drug @ 0.5 hc (lM)
7
10
>20
>20
>20
<1
Y
<0.1
<0.1
0.1
ND
ND
ND
0.2
1.4d
<0.1
8
Y
9
No
Y
10
11
12
0.2
Y
2.8d
0.4
2
Y
a Solubility is measured in deionized water @ 25 °C.
b 4 mg/mL solution in water. Stability judged @ 24 h by LC–MS.
c Concentration of 2 following a 2 mg/kg i.v. dose of pro-drug in Sprague–Dawley rats (n = 2); measuring at 5, 15, 30, 60 min. Value quoted is @
30 min.
d Dosed in PEG-400 solution.
Thus thiolactone 6 was reacted with hydroxylamine fol-
lowed by 2-bromoacetic acid to give the imidate 7.
Condensation of 6 with L-serine methyl ester and subse-
quent saponification afforded the amino acid imidate
derivative 8. The ascorbic acid derivative 9 was synthe-
sized by condensing thiolactone 6 with dibenzyl ascor-
bate,13 followed by deprotection under catalytic
hydrogenation conditions. Compounds 10 and 11 were
synthesized by the condensation of 6 with glycerol and
glycerylphosphorylcholine (GPC), respectively. Further
derivatization of 11 by acylation with succinic anhydride
furnished compound 12.
Next we examined the physiochemical properties of
compounds 7–12 and their ability to behave as pro-
drugs and release 2 in vivo (Table 1). Compounds 7–
10 exhibited good solubility in water of >10 mg/mL,
while 11 and 12 showed reduced solubility of <1 and
2 mg/mL, respectively. Stability in solution was deter-
mined by HPLC analysis, and all of the pro-drugs were
stable after 24 h in water, with the exception of 9, which
demonstrated rapid decomposition. To ascertain the
rates of conversion of the pro-drugs in vivo, the
compounds were dosed as a saline solution of the salt
form (except for 11, which was dosed in a PEG-400
O
O-Na+
O
O
O
O
O
O
O
in vivo
O
O
MeO2S
MeO2S
2
12
Plasma level after 2mpk iv dose with 12
1.50
1.00
0.50
0.00
12
2
0
0.2
0.4
0.6
0.8
1
Time (hr)
Figure 3. Graph of in vivo conversion of pro-drug 12.

S. F. Poon et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2259–2263
2263
solution) in Sprague–Dawley rats at 2 mg/kg intrave-
References and notes
nously. Plasma levels were measured for levels of the
respective pro-drug and the desired COX-2 inhibitor 2
at 5, 15, 30, and 60 min time points. The plasma levels
measured at 30 min are given below (Table 1).
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Cromlish, W.; Gordon, R.; Grimm, E. L.; Hughes, G.;
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Chem. Lett. 2002, 12, 3317. Note: Compound 2 has been
in Phase 2 clinical trials.
8. Smith, N. D.; Reger, T.; Payne, J.; Zunic, J.; Lorrain, D.;
Correa, L.; Stock, N.; Arruda, J.; Chen, W.; Yang, J.;
Prasit, P.; Munoz, B. Bioorg. Med. Chem. Lett., in press.
9. The sulfone derivative 2 could not be reduced successfully,
therefore sulfide was prepared as described in: Black, C.;
Leger, S.; Prasit, P.; Wang, Z.; Hamel, P.; Han, Y.;
Hughes, G. U.S. Patent 6,057,319.
As can be ascertained from Table 1, imidates 7 and 8
failed to give appreciable amounts of the parent drug
2, yet the levels of each pro-drug dropped rapidly in
vivo. Apparently, in vivo conversion of 7 and 8 was
poor, which suggests that the imidate derivatives are sta-
ble in vivo and are cleared rapidly as the pro-drug in the
rat model.
Gratifyingly, ortho-esters 9–12 demonstrated superior
pharmacokinetic characteristics. Of the compounds that
were dosed at 2 mg/kg, only compounds 11 and 12 con-
verted to yield appreciable levels of the parent drug 2 in
the plasma at 30 min. Pro-drug 12 reached 0.4 lM in the
plasma and the less soluble derivative 11 was measured
at 2.8 lM. In comparison, the COX-2 IC50 concentra-
tion of inhibitor 2 is 0.4 lM in human whole blood.
Figure 3 graphically illustrates the in vivo profile of 12
and demonstrates the desired pro-drug profile; a rapid
drop in concentration of pro-drug as it is converted to
parent COX-2 inhibitor 2. Compound 12, however,
was not completely soluble in water at the desired con-
centration of 4 mg/mL. Nevertheless, 12 clearly demon-
strated proof of concept that ortho-ester based water
soluble pro-drugs of 2 can give significant levels of the
drug in the plasma when dosed intravenously.
10. Pirkle, W. H.; Sowin, T. J. J. Org. Chem. 1987, 52, 3011.
11. Shibuya, I.; Katoh, E.; Gama, Y.; Oishi, A.; Taguchi, Y.;
Tsuchiya, T. Heterocycles 1996, 43, 851.
In conclusion, we have demonstrated a novel strategy
for preparing water soluble pro-drugs of the potent
and selective COX-2 inhibitor 2 by formation of an
ortho-ester from the lactone moiety. This led to water
soluble and stable pro-drugs such as 12, which upon
intravenous administration delivered 2 in vivo.
12. Typical experimental procedure: A flask containing com-
pound 6 (0.355 g, 1.0 mmol), glycerol (0.103 g, 1.1 mmol),
and AgOTf (0.643 g, 2.5 mmol), was charged with 6 mL of
dry AcCN. The solution was stirred vigorously and Et3N
(0.56 mL, 4.0 mmol) was added dropwise to the reaction.
The resulting mixture was filtered through Celite to
remove the silver byproducts, and then concentrated
under pressure. The crude material was then purified on
alumina, yielding an amorphous white solid (80%).
13. Kulkarni, M. G.; Thopate, S. R. Tetrahedron 1996, 52,
1293.
Acknowledgments
The authors would like to thank Mike Gardner and
Merryl Cramer for help with pharmacokinetic studies.