Discovery of olmesartan hexetil: A new potential prodrug of olmesartan

Article abstract of DOI:10.1016/j.bmcl.2012.12.090

Synthesis of a new ester prodrug of olmesartan, olmesartan hexetil (1), is described. It is in vitro stabilities and in vivo pharmacokinetics (PK) were evaluated. It showed high stability in simulated gastric juice, and was rapidly hydrolyzed to olmesartan in rat liver microsomes and rat plasma in vitro. C _max and AUC_last for olmesartan were significantly increased in case of hexetil prodrug, compared with olmesartan medoxomil. Olmesartan hexetil is proposed to be an efficient prodrug of olmesartan with markedly increased oral bioavailability.

Full text of DOI:10.1016/j.bmcl.2012.12.090

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1347–1350
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of olmesartan hexetil: A new potential prodrug
of olmesartan
Mohammed I. El-Gamal a,b,c, Hanan S. Anbar a, Hye Jin Chung a, Hyun-Il Kim d, Young-Jin Cho d,
Bong Sang Lee d, Sun Ahe Lee d, Ji Yun Moon d, Dong Jin Lee d, Dow Kwon d, Won-Jai Choi d,
Hong-Ryeol Jeon d, Chang-Hyun Oh a,b,
⇑
a Biomedical Research Institute, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Republic of Korea
b Department of Biomolecular Science, University of Science and Technology, 113 Gwahangno, Yuseong-gu, Daejeon 305-333, Republic of Korea
c Department of Medicinal Chemistry, Faculty of Pharmacy, University of Mansoura, Mansoura 35516, Egypt
d CTCBIO Inc. 450-34, Noha-ri, Paltan-myeon, Hwaseong-si, Gyeonggi-do 445-913, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of a new ester prodrug of olmesartan, olmesartan hexetil (1), is described. It is in vitro stabil-
ities and in vivo pharmacokinetics (PK) were evaluated. It showed high stability in simulated gastric juice,
and was rapidly hydrolyzed to olmesartan in rat liver microsomes and rat plasma in vitro. Cmax and
AUClast for olmesartan were significantly increased in case of hexetil prodrug, compared with olmesartan
medoxomil. Olmesartan hexetil is proposed to be an efficient prodrug of olmesartan with markedly
increased oral bioavailability.
Received 2 November 2012
Revised 10 December 2012
Accepted 25 December 2012
Available online 4 January 2013
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Antihypertensive
Olmesartan medoxomil
Olmesartan hexetil
Prodrug
Ester
Pharmacokinetics
In the current era of globalization; which is characterized by so
much stress, worry, and hurry; the incidence of cardiovascular dis-
orders has dramatically increased. Hypertension is a disease which
affects an estimated one billion people worldwide.1 The American
Heart Association estimates high blood pressure affects approxi-
mately one in three adults in the United States. Hypertension is a
serious disease with a momentous impact on health and life expec-
tancy. Controlling blood pressure and prevention of its complica-
tions such as coronary heart disease, renal failure, eye damage,
and brain damage (stroke) are the main objectives for the treat-
ment of hypertension.2
The renin–angiotensin–aldosterone system (RAAS) is a very
complex system that plays a pivotal role in the regulation of blood
pressure. Angiotensin II, the primary effector hormone of RAAS, af-
fects the cardiovascular system by influencing the vascular tone,
fluid volume, and electrolyte balance.3,4
tan is an example of ARBs which acts by blocking type 1 angioten-
sin II receptors (AT1-R), leading to prevention of vasoconstriction,
reduction of sodium and water retention, and decrease of cellular
proliferation and hypertrophy.6 In addition to AT1-R blockade,
olmesartan is assumed to exhibit an angiotensin-converting en-
zyme (ACE) inhibitory effect, prevent an increase in angiotensin
II level, and protect cardiovascular remodeling through an increase
in cardiac nitric oxide production and endogenous angiotensin-(1–
7) via over-expression of ACE2.7
The once daily dosing interval of most ARBs helps enhance pa-
tient compliance which may lead to better patient outcomes.5
Olmesartan medoxomil is an ester prodrug of olmesartan that
has shown potent and long-lasting antihypertensive activity after
oral administration.8 Olmesartan medoxomil is rapidly de-esteri-
fied by enzymatic hydrolysis in the intestine, liver, and plasma.
After oral administration of the prodrug, first-pass bioactivation
occurs in the intestine, followed by the portal blood and liver, be-
fore the prodrug reaches the systemic circulation. It was reported
that multiple enzymes are capable of bioactivating olmesartan
medoxomil in human, including plasma albumin,9 and an intesti-
nal and liver hydrolase carboxymethylenebutenolidase homolog
(CMBL).10 High metabolic clearance of intestinal CMBL suggests
that the intestinal bioactivation firstly and predominantly contrib-
utes to the quick onset of drug action after oral administration of
There are many antihypertensive drugs available, many of
which act on the RAAS system. Angiotensin receptor blockers
(ARBs) are a class of antihypertensive agents that are growing in
popularity due to their excellent blood pressure control potential,
low adverse event profile, and high patient tolerability.5 Olmesar-
⇑
Corresponding author. Tel.: +82 2 958 5160; fax: +82 2 958 5189.
E-mail address: choh@kist.re.kr (C.-H. Oh).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.12.090

1348
M. I. El-Gamal et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1347–1350
N
N
OH
OH
+
N
N
H2O
CO2
O
OH
O
O
O
O
O
N
N
N
N
O
Enzymatic Hydrolysis
N
N
N
N
O
H
H
Diacetyl
Olmesartan medoxomil
Ester Prodrug
Olmesartan
Active metabolite
Figure 1. Hydrolysis of olmesartan medoxomil to olmesartan.
the synthesis, in vitro bioconversion, and in vivo PK evaluation of
a new ester prodrug of olmesartan, olmesartan hexetil. The
medoxomil moiety of olmesartan medoxomil was replaced by
hexetil, a more lipophilic moiety, in order to increase the lipophil-
icity of olmesartan ester prodrug, and hence, olmesartan bioavail-
ability. Hexetil promoiety has been reported to increase the oral
bioavailability of candesartan.14 The calculated clogP15 and total
polar surface area (TPSA)16 values of olmesartan medoxomil and
olmesartan hexetil prodrugs are illustrated in Table 1. Olmesartan
hexetil (1) with higher clogP and lower TPSA values compared
with olmesartan medoxomil (i.e., more lipophilic than olmesartan
medoxomil) was synthesized and evaluated as a new potential
prodrug of olmesartan. Our target is to improve the pharmacoki-
netic properties of olmesartan, and hence, the antihypertensive
outcomes. The synthetic and screening protocols are illustrated
in details.
Synthesis of the target compound 1 was carried out according
to the sequence of reactions illustrated in Scheme 1. It was impor-
tant at the beginning to prepare the key carboxylic acid compound,
trityl olmesartan (4). N-alkylation of ethyl 4-(1-hydroxy-1-methyl-
ethyl)-2-propylimidazole-5-carboxylate (2) with 5-(40-bromo-
methyl-biphenyl-2-yl)-1-trityl-1H-tetrazole afforded the ethyl
ester of trityl olmesartan (3). Alkaline hydrolysis of the ethyl ester
moiety of 3 followed by acidification of the formed potassium salt
gave trityl olmesartan (4).17 The target hexetil ester 1 was obtained
Table 1
cLogP and total polar surface area (TPSA) calculations for olmesartan medoxomil and
olmesartan hexetil (1)
Structure
Compd no
R
cLogP TPSA
(A2)
O
O
N
OH
O
O
Olmesartan
medoxomil
N
3.49
5.06
162.18
O
R
O
CH3
O
N
N
N
CH3
O
1
154.37
N
H
olmesartan medoxomil.11 Figure 1 illustrates the enzymatic hydro-
lysis of olmesartan medoxomil into the active metabolite,
olmesartan.9
We reported the synthesis, bioconversion, and PK evaluation of
new prodrugs of olmesartan with higher lipophilicity than olme-
sartan medoxomil. The new ester prodrugs of olmesartan showed
improved PK parameters for olmesartan, compared with olmesar-
tan medoxomil.12,13 These results showed that lipophilic ester pro-
drugs of olmesartan can improve the oral bioavailability and PK
properties of olmesartan. In the present investigation, we report
N
N
N
OH
OH
OH
OH
N
N
N
O
O
O
O
N
b
a
c
O
O
O
O
HN
OH
O
N
N
N
N
N
N
N
N
N
O
N
N
N
Tr
Tr
Tr
2
3
4
5
N
OH
N
O
O
O
O
O
d
N
N
N
N
H
1
Scheme 1. Reagents and conditions: (a) 5-(40-bromomethyl-biphenyl-2-yl)-1-trityl-1H-tetrazole, K2CO3, acetone, DMAc, reflux, 10 h, 75%; (b) (i) KOH, isopropanol, 60 °C, 4 h;
(ii) HCl, workup, 90%; (c) hexetil chloride, K2CO3, KI, DMAc, 70 °C, 2 h, 95%; (d) concd HCl, acetone, H2O, rt, 2 h, 92%.

M. I. El-Gamal et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1347–1350
1349
Table 2
In vitro stabilities of olmesartan medoxomil and compound 1 in simulated gastric juice, rat plasma, and rat liver microsomesa
Compd no
Half-lives (min)
Simulated gastric juice
Rat plasma
Rat liver microsomes
Olmesartan medoxomil
1
>1000
390
0.956
1.54
4.95
2.77
a
Data represent the mean of duplicated experiments.
the reported liquid chromatography-tandem mass spectrometric
method (LC–MS/MS).21 Pharmacokinetic parameters were deter-
mined by a non-compartmental analysis. The total area under
the plasma concentration–time curve from time zero to the last
measured time (AUClast) was calculated by the linear trapezoidal
rule method.22 The mean arterial plasma concentration–time pro-
files of olmesartan after oral administration of olmesartan hexetil
and olmesartan medoxomil in rats are shown in Figure 2, and some
relevant pharmacokinetic parameters of olmesartan are summa-
rized in Table 3. After oral administration of the two compounds,
olmesartan was detectable in plasma from the first blood sampling
time, 30 min. This result suggested that compound 1 was well ab-
sorbed from rat gastrointestinal tract and rapidly converted into
the active form. After administration of prodrugs, the peak plasma
concentrations of olmesartan (Cmax) and AUClast values were signif-
icantly higher (795% and 500% increase, respectively) than those
observed after administration of olmesartan medoxomil. These ef-
fects are possibly due to increased lipophilicity of compound 1 in-
duced by hexetil moiety over olmesartan medoxomil. The results
indicated that introduction of lipophilic promoiety, such as hexetil,
enhanced the oral absorption and systemic exposure level of
olmesartan.
In conclusion, olmesartan hexetil (1), a new ester prodrug of
olmesartan was synthesized. It is in vitro stabilities in simulated
gastric juice, rat plasma, and rat hepatic microsomes were studied.
In addition, the in vivo pharmacokinetic parameters of olmesartan
after it is oral administration were studied and compared with
those after administration of olmesartan medoxomil. Compound
1 showed high stability in simulated gastric juice and rapid hydro-
lysis to the active form in rat plasma and rat liver microsomes, in
addition to improved pharmacokinetic profiles compared with
olmesartan medoxomil. Olmesartan hexetil is proposed to be a
promising prodrug of olmesartan with significantly improved oral
bioavailability.
Figure 2. Mean arterial plasma concentration–time profiles of olmesartan after oral
administration of olmesartan medoxomil (d; n = 4) and olmesartan hexetil (.;
n = 4) at a dose of 20 mg/kg as olmesartan in rats.
Table 3
Pharmacokinetic parameters (mean standard deviation) of olmesartan after oral
administration of prodrugs (20 mg/kg as olmesartan) to male rats
Compd. administered
Cmax
Tmax
AUClast
(ng/mL)a
(h)b
(ng h/mL)c
Olmesartan medoxomil (n = 4) 153.6 71.4
1.0 (0.5–2) 501.9 283.4
1 (n = 4) 1374.7 1122.3 0.9 (0.5–3) 3013.8 1556.6
a
b
c
Cmax: peak plasma concentration.
Tmax: time to reach Cmax, expressed as median (range).
AUClast: total area under the plasma concentration–time curve from time zero
to last measured time.
by esterification of the carboxylic acid group of 4 with hexetil chlo-
ride, and subsequent detritylation using concd HCl.18
The prodrugs intended to be administered orally should be
stable in acidic conditions encountered in the stomach following
oral dosing, and be hydrolyzed to active metabolites in the sys-
temic circulation after absorption. The stability of the target
compound 1 was determined in simulated gastric juice, rat plas-
ma, and rat hepatic microsomes in vitro.19 The chemical or enzy-
matic stabilities of compounds after the incubation are presented
(Table 2). The half-life of compound 1 in simulated gastric juice
was 390 min. In addition, its half-lives in rat hepatic microsomes
and rat plasma were <2.8 min. The results showed that olmesar-
tan hexetil (1) was rapidly hydrolyzed to olmesartan, the active
metabolite, in rat liver microsomes and rat plasma, while it is
hydrolysis rate in simulated gastric juice was slow. These results
suggested that compound 1 can pass through the stomach with
slow degradation and the absorbed molecules from the gastroin-
testinal tract can be rapidly converted into the active form in li-
ver and plasma.
Acknowledgments
This work was supported by Seoul R&BD program (Grant num-
ber PA100015). We would like to thank CTCBIO Inc., Republic of
Korea, for kind contribution to the biological experiments.
References and notes
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Pharmacokinetic studies were conducted to determine whether
the new prodrug of olmesartan 1 was converted into olmesartan
in vivo.20 Olmesartan medoxomil and olmesartan hexetil (1) were
administered orally using a feeding tube to male Sprague–Dawley
rats at a dose of 20 mg/kg as olmesartan. The plasma concentra-
tions of olmesartan were determined by a slight modification of

1350
M. I. El-Gamal et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1347–1350
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128.2, 129.5, 131.0, 131.4, 136.9, 138.7, 141.5, 151.8, 152.3, 155.5, 158.4,
159.7; LC–MS m/z: 618.06 [M+1]+.
19. Stability studies of prodrugs: Stabilities of the newly synthesized ester prodrug,
olmesartan hexetil, and olmesartan medoxomil were measured in simulated
gastric juice, rat plasma, and rat liver microsomes. To determine the
microsomal stability of prodrugs, prodrugs were incubated with rat liver
microsomes in the presence of NADPH. The microsomal reaction mixtures
were containing 1.2 mM NADPH, 0.5 mg/mL (total protein) microsomes,
100 mM phosphate buffer (pH 7.4). Solutions of the prodrugs in acetonitrile
14. Kubo, K.; Kohara, Y.; Yoshimura, Y.; Inada, Y.; Shibouta, Y.; Furukawa, Y.; Kato,
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(100
microsomes reaction mixture with a final concentration of 1
solutions were kept at 37 °C and sampled at 0, 15, 30, 60, and 120 min. The
50 L aliquot of the mixtures was terminated at the above time points by
l
M) were added to simulated gastric juice, rat plasma, or rat liver
lM. The reaction
16. http://www.molinspiration.com/cgi-bin/properties.
17. Ramanjaneyulu, G. S.; Mohan, B.; Ray, P. C.; Sethi, M. K.; Rawat, V. S.; Krishna, Y.
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l
addition of twofold volume of cold acetonitrile containing internal standard.
After centrifugation, the supernatant was collected and analyzed immediately
using LC–MS/MS.
18. Synthesis of the target compound 1: To
a solution of compound 5 (10 g,
11.6 mmol) in acetone (30 mL), concd HCl (30 mL) and water (20 mL) were
added. The reaction mixture was stirred at room temperature for 2 h. The
organic solvent was evaporated under reduced pressure, and the pH of the
remained aqueous solution was adjusted to 4–5 by addition of aqueous
potassium carbonate solution. The aqueous mixture was extracted with ethyl
acetate (3 ꢀ 50 mL), and the combined organic extracts were washed with
brine and dried over anhydrous Na2SO4. After evaporation of the organic
solvent, the residue was purified by column chromatography (silica gel,
hexane/ethyl acetate 5:1 v/v then switching to ethyl acetate) to give the target
product 1 (6.8 g, 92%). mp 113–115 °C; 1H NMR (300 MHz, DMSO-d6) d 0.87 (t,
J = 7.6 Hz, 3H), 1.23–1.82 (m, 21H), 2.59 (t, J = 7.6 Hz, 2H), 4.48–4.51 (m, 1H),
5.14 (s, 1H), 5.42 (s, 2H), 6.75 (q, J = 2.7 Hz, 1H), 6.91 (d, J = 7.9 Hz, 2H), 7.07 (d,
J = 7.9 Hz, 2H), 7.51–7.70 (m, 4H); 13C NMR (100 MHz, DMSO-d6) d 14.1, 19.5,
20.9, 23.3, 25.1, 28.7, 30.0, 30.1, 31.2, 48.4, 70.1, 77.1, 92.1, 116.3, 124.0, 126.1,
20. Pharmacokinetic studies: Olmesartan hexetil (1) and olmesartan medoxomil
were administered orally using a feeding tube to male Sprague–Dawley rats at
a dose of 20 mg/kg as olmesartan. Blood samples were collected via carotid
artery at 0 (to serve as a control), 0.5, 1, 2, 3, 4, 6, 8, and 10 h after oral
administration of each compound. After centrifugation at 3000 rpm for 10 min,
a 200-
lL aliquot of plasma samples were stored at ꢁ80 °C until analysis.
Pharmacokinetic parameters were determined by
a non-compartmental
analysis using WinNonlinÒ (Pharsight Corporation, Mountain View, CA)
program. The AUClast values were calculated by the trapezoidal rule–
extrapolation method.
21. Lee, B. S.; Kang, M. J.; Choi, W. S.; Choi, Y. B.; Kim, H. S.; Lee, S. K.; Lee, J.; Choi, Y.
W. Arch. Pharm. Res. 2009, 32, 1629.
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