Himbacine-derived thrombin receptor antagonists: C7-aminomethyl and C9a-hydroxy analogues of vorapaxar

Article abstract of DOI:10.1021/ml400452v

We have synthesized several C₇-aminomethyl analogues of vorapaxar that are potent PAR-1 antagonists. Many of these analogues showed excellent in vitro binding affinity and pharmacokinetics profile in rats. Compound 6a from this series showed excellent PAR-1 activity (K_i = 5 nM). We have also synthesized a C_9a-hydroxy analogue of vorapaxar, which showed very good PAR-1 affinity (K_i = 19.5 nM) along with excellent rat pharmacokinetic profile and ex vivo efficacy in the cynomolgus monkey.

Full text of DOI:10.1021/ml400452v

Letter
pubs.acs.org/acsmedchemlett
Himbacine-Derived Thrombin Receptor Antagonists:
C7‑Aminomethyl and C9a-Hydroxy Analogues of Vorapaxar
Mariappan V. Chelliah,* Samuel Chackalamannil,† Yan Xia,‡ William J. Greenlee,§ Ho-Sam Ahn,
Stan Kurowski, George Boykow, Yunsheng Hsieh, and Madhu Chintala
Merck Research Laboratories, 2015 Galloping Hill Road, Kenilworth, New Jersey 07033-1300, United States
S
* Supporting Information
ABSTRACT: We have synthesized several C7-aminomethyl
analogues of vorapaxar that are potent PAR-1 antagonists.
Many of these analogues showed excellent in vitro binding
affinity and pharmacokinetics profile in rats. Compound 6a
from this series showed excellent PAR-1 activity (Ki = 5 nM).
We have also synthesized a C9a-hydroxy analogue of vorapaxar,
which showed very good PAR-1 affinity (Ki = 19.5 nM) along
with excellent rat pharmacokinetic profile and ex vivo efficacy
in the cynomolgus monkey.
KEYWORDS: Himbacine, vorapaxar, PAR-1 antagonist, thrombin receptor, platelet aggregation
ardiovascular disease is a major cause of death in both
atherothrombotic events, there was a significant reduction in
Cdeveloped and developing countries. A large number of overall ischemic events with vorapaxar when added to
background antiplatelet therapy.19,20 Thus, treatment of
these deaths is associated with platelet mediated atherothrom-
botic events.1,2 When an atherosclerotic rupture occurs in the
atherothrombotic events with antagonists of the PAR-1
receptor has been validated by the results of the phase-III
clinical trial of vorapaxar.
vascular endothelium, it leads to the adhesion of platelets to the
site of injury, which subsequently form a platelet rich thrombus.
Though the formation of thrombus plays a vital role in
hemostasis, the formation of thrombi often occludes the
coronary artery leading to acute coronary syndrome such as
unstable angina and myocardial infarction. Thus, antiplatelet
agents play a vital role in treating patients prone to
atherothrombotic events. Aspirin and clopidogrel are two
widely used antiplatelet agents for people afflicted by acute
coronary syndrome. Aspirin works by the antagonism of
thromboxane A2 (TXA2) mediated platelet activation, whereas
clopidogrel works by the antagonism of the predominant ADP
receptor P2Y12. Though platelet activation is initiated by several
factors, thrombin is the most potent activator of platelets. This
activation is mediated by thrombin action on two GPCR
receptors located on the surface of the platelets called thrombin
receptors (also known as protease activated receptors; PAR-1
and PAR-4).3−9 Among these two receptors, PAR-1 plays a
major role in primate platelet activation. Agents that antagonize
the activation of this PAR-1 receptor can be expected to be
potent antiplatelet agents.
We have published a series of himbacine-derived antiplatelet
agents that are potent antagonists of PAR-1 receptor.10−16 Our
systematic optimization of the himbacine based hit led to the
discovery of vorapaxar (SCH530348), a potent PAR-1
antagonist.14 Vorapaxar showed excellent ex vivo platelet
aggregation inhibition in a preclinical monkey efficacy
model.17 In the phase-II clinical study, it met the primary
end point of absence of TIMI major and minor bleeding.18 In
the phase-III clinical trial for the secondary prevention of
In our lead optimization of the himbacine based thrombin
receptor antagonists, we replaced the C7 carbon of the tricyclic
himbacine scaffold with nitrogen, which resulted in the
discovery of 1 (Figure 1).12 This heterohimbacine analogue 1
maintained the excellent antithrombotic activity while solving
the critical enzyme induction and clearance issues encountered
in the corresponding carbocyclic series.12 When compound 1
was dosed in cynomolgus monkey, with 20% PEG-HPBCD as
the dosing vehicle, it showed excellent ex vivo efficacy in the
platelet aggregation inhibition assay. Unfortunately, this
compound showed poor aqueous solubility (<2 μM) and the
ex vivo efficacy was markedly reduced when the dosing vehicle
was changed from 20% PEG-HPBCD to 0.4% methylcellulose.
In the subsequent SAR study of this series, nitrogen atom of the
C-ring was moved exocyclic to the ring to give the C7-amino
substituted tricyclic himbacine analogues, which led to the
discovery of vorapaxar.14 In this letter, we disclose the variation
of this C-ring exocyclic amine to give the homologated
analogues 6a−j where a methylene linker is introduced
between the amino group and the C-ring along with the
carboxamide analogues 9a−h. We also synthesized the C9a-
hydroxy analogue 11 of vorapaxar. This analogue was prepared
in reflection of a recent disclosure of novel nor seco himbacine
analogues as potent PAR-1 antagonists.16 In the nor seco series,
Received: November 6, 2013
Accepted: December 18, 2013
Published: December 18, 2013
© 2013 American Chemical Society
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Scheme 1. Synthesis of C7-Aminomethyl Carbamates and
a
Carboxamides
Figure 1. Himbacine-derived thrombin receptor antagonists.
we observed a dramatic improvement in the rat plasma level by
the introduction of a hydroxy functionality at the carbon alpha
to the lactone carbonyl group. We wanted to see whether a
similar improvement in pharmacokinetic profile will result for
vorapaxar by the introduction of a similar C9a-hydroxy group.
Herein we describe the synthesis and the PAR-1 activity results
of these analogues.
The synthesis of the C7-aminomethyl analogues 6a−g
(Scheme 1) starts with the known ketone 2,14 which was
subjected to a Wittig reaction followed by the hydrolysis of the
resulting vinyl ether to give the aldehyde 3. Reduction of this
aldehyde gave the alcohol 4, which was mesylated, and the
mesylate was subsequently displaced with sodium azide to give
the azide intermediate, which was reduced to the amine 5 using
trimethyl phosphine. This amine was subsequently treated with
chloroformates, acid chlorides, sulfonyl chlorides, and iso-
cyanates to give the corresponding carbamates, sulfonamides,
and ureas 6a−j. The preparation carboxamide analogues 9a−h
started with the ketone 7,11 which was converted to the
carboxylic acid 8. Coupling of this acid with amines under
standard condition gave the amide analogues 9a−h.
a
t
Reagents and conditions: (a) Ph3P+CH2OCH3 Cl−, BuOK, THF;
(b) HCl in dioxane/water, rt; (c) sodium borohydride, THF−MeOH;
(d) methanesulfonyl chloride, Et3N; (e) sodium azide, DMSO, 65 °C;
(f) Me3P, H2O; (g) ROCOCl, RCOCl, RSO2Cl, RNCO; (h) TFA−
DCM (for compounds 6e−g only); (i) NaClO2, H2O2, NaH2PO4; (j)
RNH2, HATU, Et3N.
a
Scheme 2. Synthesis of C9a-Hydroxy Analogue of Vorapaxar
Synthesis of the C9a-hydroxy analogue of vorapaxar 11 is
described in Scheme 2. The amino functionality of the
carbamate group was protected with di-tert-butyldicarbonate,
and the resultant product was treated with lithium bis-
(trimethylsilyl)amide to generate the C9a-enolate. This enolate
was stirred under an oxygen atmosphere to introduce the C9a-
hydroxy functionality. Deprotection of the tert-butyl carbamate
group under acidic conditions gave the target 11.
The above synthesized analogues were evaluated in the in
vitro binding assay using PAR-1 receptors isolated from human
platelets and [3H]haTRAP as the radioligand.21 Table 1
presents the PAR-1 binding affinity for the C7-aminomethyl
derivatives, and Table 2 presents the affinity for the
carboxamide analogues. Binding affinity for compound 11 is
presented in Scheme 2. Both the C7-hydroxymethyl analogue 4
and the unsubstituted aminomethyl analogue 5 showed good
binding affinity. The methyl carbamate 6a (Ki = 5 nM) showed
excellent binding affinity, while the ethylcarbamate 6b (Ki = 14
nM) was 3-fold less active. Both the acetamide 6c and the
propionamide 6d analogues showed very good affinity. Amides
a
Reagents and conditions: (a) (Boc)2O, Et3N, DMAP, CH3CN, 60
°C; (b) LHMDS, THF then oxygen; (c) TFA−DCM, 0 °C to rt.
6e−g containing amino groups indicate that polar groups are
tolerated at this position. The amide derivative of piperidine-4-
carboxylic acid 6g showed very good affinity, though the
corresponding analogues of alanine (6e) and proline (6f)
analogues showed 2-fold less affinity. The methane sulfonamide
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Table 1. Binding and PK Data for Compounds 4, 5, and 6a−j
Table 2. Binding Data for Compounds 9a−h
a
n = 2 or more.
indicating that the introduction of the hydroxyl group at the C9a
position is tolerated in this series
Selected analogues were evaluated in the rat pharmacokinetic
assay at an oral dose of 10 mg/kg, and the plasma levels were
analyzed up to 6 h. Several of these analogues showed good
plasma levels. While the ethyl carbamate analogue 6b showed
good rat plasma level (AUC = 1213 ng·h/mL), the
corresponding methyl carbamate analogues 6a showed about
4-fold higher plasma level (AUC = 4863 ng·h/mL). Compared
with the acetamide 6c, the propionamide analogue 6d showed
2-fold increase in plasma level (AUC = 7728 ng·h/mL).
Analogue 6g showed very poor plasma level (AUC = 5 ng·h/
mL) indicating that the polar amino group severely affects the
absorption. While methane sulfonamide 6h showed excellent
plasma level, the N-ethyl urea analogue 6j showed a low level of
plasma. Rat plasma level for the C9a-hydroxy analogue 11 was
excellent (AUC = 7431 ng·h/mL) as indicated in Scheme 2.
Compared with vorapaxar (AUC = 3064 ng·h/mL), analogue
11 showed a 2-fold increase in rat plasma level. This
observation is in parallel to the nor seco himbacine analogues
where introduction of the hydroxy group at alpha to the
carbonyl group showed increased plasma level.16
a
a
n = 2 or more. AUC from 0 to 6 h in ng·h/mL and at 10 mg/kg oral
dose (0.4% methylcellulose).
(6h), ethane sulfonamide (6i), and N-ethyl urea (6j) analogues
showed good affinity.
The carboxamide analogues 9a−c containing N-methyl, N-
ethyl, and N-isopropyl amides showed good affinity. However,
introduction of polar group reduces the affinity as indicated by
the amino ethanol amide analogue 9d. Amides of 4-amino-
pyridine and 3-aminopyridines 9e−f showed considerable
reduction in affinity. Both analogues of 3-hydroxy pyrrolidine
(9g−h) showed reduced affinity indicating that polar groups at
this position reduces the affinity. Also, the C9a-hydroxy
analogue of vorapaxar, 11, showed good affinity (Scheme 2)
We evaluated compounds 11 and 6a in the ex vivo platelet
aggregation inhibition assay in cynomolgus monkey (Figure 2).
The compound was given as an oral dose and blood was drawn
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ABBREVIATIONS
■
PAR-1, protease activated receptor-1; TXA2, thromboxane A2;
PEG-HPBCD, poly(ethylene glycol)−hydroxypropyl-beta-cy-
clodextrin; haTRAP, high affinity thrombin receptor-activating
peptide; ADP, adenosine diphosphate receptor; GPCR, G
protein coupled receptor; PK, pharmacokinetics; DMAP, 4-
dimethylaminopyridine; LHMDS, lithium bis(trimethylsilyl)
amide; DCM, dichloromethane; HATU, 1-[bis-
(dimethylamino) methylene]-1H-1,2,3-triazolo[4,5-b]-
pyridinium 3-oxid hexafluorophosphate
Figure 2. Ex vivo platelet aggregation inhibition in cynomolgus
monkey following a single oral dose (1 mg/kg in 20% PEG-HPBCD)
for 11 and 6a.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details for the preparation of compounds 6a, 9a,
and 11 is provided. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*(M.V.C.) E-mail: mchelliah@aol.com.
■
Present Addresses
†S.C.: Ernest Mario School of Pharmacy, Rutgers, The State
University of New Jersey, Piscataway, New Jersey 08854−8020,
United States.
‡Y.X.: School of Environmental and Municipal Engineering,
Qingdao Technological University, 11 Fushun Road, Qingdao,
Shandong 266033, China.
§W.J.G.: MedChem Discovery Consulting, LLC, 115 Herrick
Avenue, Teaneck, New Jersey 07666, United States.
Notes
The authors declare no competing financial interest.
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Active Himbacine-Based Thrombin Receptor Antagonist (SCH
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ACKNOWLEDGMENTS
■
We acknowledge the support of Drs. John Piwinski, Catherine
Strader, Birendra Pramanik, Pradip Das, Tze-Ming Chan, Jesse
Wong, Jianshe Kong, and Richard Morrison.
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