Part 3: Design and synthesis of proline-derived α2δ ligands

Article abstract of DOI:10.1016/j.bmcl.2011.04.058

A potent series of substituted (2S,4S)-benzylproline α ₂δ ligands have been designed from the readily available starting material (2S,4R)-hydroxy-l-proline. The ligands have improved pharmacokinetic profile over the (4S)-phenoxyproline derivati

Full text of DOI:10.1016/j.bmcl.2011.04.058

Bioorganic & Medicinal Chemistry Letters 21 (2011) 3771–3773
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Part 3: Design and synthesis of proline-derived
a2d ligands
⇑
David J. Rawson , Delphine Brugier, Anthony Harrison, Jo Hough , Julie Newman, Joe Otterburn,
Graham N. Maw, Jenny Price, Lisa R. Thompson, Paul Turnpenny, Andrew N. Warren
Sandwich Laboratories, Ramsgate Road, Pfizer Global Research and Development, Sandwich, Kent CT13 9NJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 20 April 2011
A potent series of substituted (2S,4S)-benzylproline
available starting material (2S,4R)-hydroxy-L-proline. The ligands have improved pharmacokinetic profile
a2d ligands have been designed from the readily
over the (4S)-phenoxyproline derivatives described previously and have potential for development as
oral agents for the treatment of neuropathic pain. Compound 16 has been progressed to clinical
development.
Keywords:
Gabapentin
Renal clearance
Glomerular filtration
a2d
Ó 2011 Elsevier Ltd. All rights reserved.
Pregabalin (LyricaÒ) (1)1,2 has recently been approved for the
management of neuropathic pain associated with diabetic periphe-
ral neuropathy and post-herpetic neuralgia and is now a leading
medicine in the treatment of neuropathic pain.3 It shows an im-
proved pharmacokinetic profile and increased potency over gaba-
pentin (2)4 and shows linear pharmacokinetics over the
efficacious dose range.5 Mild sedation and dizziness are the main
dose limiting side effects of gabapentin and these effects are
believed to be centrally mediated. In this work we were seeking a
H2N
CO2H
CO2H
H2N
CO2H
N
Cl
H
S
O
CO2H
Cl
N
Pregabalin
Gabapentin
H
(2)
(3)
(1)
(4)
As part of the exploration of the phenoxy prolines described
previously we showed that a single carbon homologation (e.g., 17
in Table 1) was well tolerated. As an extension to this work we also
looked at reversing the oxygen and methylene atoms linking the
aromatic group to the proline to give compounds such as (8).
The synthesis of the 4-phenoxymethyl prolines is outlined in
Scheme 1. Oxidation of the phenyl group of 4-phenyl-pyrroli-
dine-1,2-dicarboxylic acid di-tert-butyl ester with RuCl3 and
sodium periodate8 was followed by reduction of the resulting acid
with borane to furnish 6. A Mitsunobu reaction furnished the
phenyl ether 7 and global deprotection resulted in the formation
of the hydrochloride salt of 8 which showed comparable levels of
once daily oral modulator of
side effects.
a2d devoid of any centrally-mediated
The binding of both gabapentin (1) and pregabalin (2) to a high
affinity binding site, located on the a2d subunit of a voltage-gated
calcium channel, has been reported6 and it is thought that this site
may be involved in the mediation of the pharmacological actions of
these agents.7 Gabapentin and pregabalin bind to this
a2d subunit
with IC50 values of 140 and 80 nM, respectively. In the previous
paper we described how potent (4S)-phenoxyproline derivatives
had been discovered through conformational constraint of a
N-substituted glycine lead (3). The best compound (4) showed
excellent potency but rapid clearance lead to a short half-life in
man.
a2d potency to 4.
The potency of (8) (see Table 1) encouraged us to look at
analogues where the linker was reduced to a single methylene
which would share a similarly increased basic p Ka (vide infra).
The synthesis of these compounds is described in Scheme 2 and
a summary of potency SAR shown in Table 1.
Wittig coupling of the prolinone 12 using 3-fluorobenzyl tri-
phenylphosphonium bromide and a suitable base gave a mixture
of double bond isomers10 which were not separated but directly
hydrogenated using PtO2 (to minimise dehalogenation of the halo-
genated aromatics) to give a 3.8:1 mixture of cis:trans isomers (14).
⇑
Corresponding author. Tel.: +44 1304 648733.
E-mail address: david.rawson@pfizer.com (D.J. Rawson).
Present address: Centre for Therapeutics Discovery, MRC Technology, 1-3
Burtonhole Lane, London NW7 1AD, UK.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.04.058

3772
D. J. Rawson et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3771–3773
Table 1
O
OH
HO
Ar
ii
i
t
t
t
CO2 Bu
X
CO2 Bu
CO2 Bu
N
N
N
CO2H
O
O
O
O
O
N
O
H
5
6
a
Compound
X
O
Ar
a
2d IC50 (nM)
Cl
Cl
O
O
4
4
iii
Cl
Cl
v
CO2tBu
iv
CO2H
N
N
H
8
CH2O
CH2
13
17
48
8
O
7
O
16
17
Scheme 1. Reagents and conditions: (i) NaIO4, RuCl3, H2O/CH3CN, 78%; (ii) BH3,
THF, 41%; (iii) DIAD, PPh3, 3-chlorophenol, THF, 51%; (iv) HCl, dioxan, 35% (5:1 mix
of diastereomers).10
F
Cl
OCH2
HO
18
CH2
108
HO
HO
CO2Me
ii
i
F
N
CO2H
CO2Me
N
N
O
H
H
O
19
20
21
CH2O
CH2
25
8
9
10
11
F
F
F
O
Cl
iii
v
CO2Me
Cl
iv
N
CH2
5
CO2Me
O
CO2Me
Cl
O
N
N
12
O
O
22
23
CH2
CH2
4
O
O
OCF3
14
13
F
27
F
vii
vi
24
CH2
27
F
CO2H
.HCl
CO2H
.HCl
F
15
N
H
N
16
H
3.8:1 mix of diastereomers
25
26
CH2
CH2
28
10
OMe
F
Scheme 2. Reagents and conditions: (i) AcCl, MeOH, 95%; (ii) BOC2O, DCM, NEt3,
97%; (iii) TEMPO, trichloroisocyanuric acid, DCM, 85%; (iv) 3-fluorobenzyl triphen-
ylphosphonium bromide, KOtBu, DCM, 82%; (v) H2, PtO2, toluene/EtOAc, 100%; (vi)
6 M HCl, 91%, 3.8:1 mix of diastereomers;9 (vii) recrystallisation from CH3CN/H2O,
50%, >99:1 mixture of diastereomers.9
F
27
CH2
CH2
272
corresponding benzyl analogues (20 and 16) but the opposite
enantiomer (28) (made using the same method but starting from
(2R)-4-hydroxyprolinol), showed no activity in the binding assay.
The lead compounds were progressed to assess the rat pharma-
cokinetic profile to try and differentiate from the phenoxyproline
series described in the previous paper (Table 2).
28 (2R,4R)
>10,000
F
a
IC50 for displacement of tritiated gabapentin from porcine cerebral cortex
membranes as described in Ref. 6.
The desired cis isomer was predominant with hydrogen delivered
preferentially to the less hindered b-face of the proline. Deprotec-
tion of both the Boc and ester groups was accomplished with 6 M
HCl to give 15. Crystallisation (CH3CN/H2O) gave the desired prod-
uct in >98% diastereomeric excess. This is a six step process from
Table 2
Rat pharmacokinetic profile
Compound
4
17
19
20
16
readily available trans-4-hydroxy-L-proline and the synthesis of
Log D
À0.1
93
81
À0.2
83
35
26
0.4
À0.3
0.6
—
33
0.1
27
18
6
1.6
89
5
16 proceeds in 29% overall yield to give product of >99% diastereo-
meric excess (as determined by chiral stationary phase HPLC).
Potency levels for the benzylprolines were comparable to the
phenoxyprolines (4 and 20) and the meta position was the pre-
ferred site for modulating potency. Chloro was preferred to fluoro
in this position (20 and 16) but this was not just a simple lipophilic
interaction as methoxy was equivalent to ethyl (23 and 25). The
benzyloxy analogues (8 and 19) had comparable potency to the
Human PPB (%)
Rat PPB (%)
68
9
7
Rat Clua,b (ml/min/kg)
Rat T1/2 (h)
41
15
0.4
100
2
1.4
iv only
5
0.5
iv only
—
Rat F (%)
Predicted T1/2 in man
iv only
1
a
Clu = unbound clearance (clearance of free drug).
Unchanged parent was collected in the urine and no metabolites were detected.
b

D. J. Rawson et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3771–3773
3773
The compounds all showed good absorption. In common with
other 2d ligands these compounds exhibited low flux in Caco-2
cell monolayer model of oral absorption (Caco-2 A–B = 5, B–A <1
at 5 and 25
M for 16) consistent with their physicochemistry,11
however the low MW and polarity are features usually associated
with paracellular absorption. Unlike previous 2d ligands, the pro-
line-derived ligands showed a low affinity for the System amino
acid transporter12 (IC50 >25
M) and the high bioavailability ob-
The clearance of free drug of 16 in dog (0.4 ml/min/kg) was sig-
nificantly lower than glomerular filtration rate suggesting that ac-
tive renal reabsorption is occurring. The predicted human half-life
based on scaling of the dog pharmacokinetic profile was estimated
to be 26 h.
This compound has the potential to be a once daily oral treat-
ment for neuropathic pain with minimal centrally mediated side
effects and has been progressed into the clinic.13
a
l
a
L
l
served suggests that gut wall absorption is indeed paracellular
(although other amino acid transporters were not assessed and
can not be discounted).
Acknowledgment
The modulation of basic pKa (pKa = 8.9 for 4 and 10.1 for
16) appeared to be key to altering both the plasma protein
binding (PPB) and the extent of renal secretion in rat. The 4-
benzyl and 4-benzyloxyprolines (16, 19, and 20) consistently
showed reduced unbound clearance (in rat) compared to the
4-oxyproline derivatives (4 and 17). Within a series, increased
lipophilicity correlated with increased unbound clearance. The
unbound clearance (Clu) of 16 and 19 was consistent with
passive renal elimination resulting in improvements in half-life
(in contrast to 4 where active renal secretion results in a short
half-life).
Compound 16 showed very low CNS exposure in rat (CSF:free
plasma = 0.004) and was predicted to show low incidence of cen-
trally-mediated side effects in man.
The potency and low rat clearance of compound 16 encouraged
us to progress it to dog pharmacokinetic profiling (Fig. 1).
We thank the staff of the Structural and Analytical Services
Department, Sandwich, for analytical data.
References and notes
1. Dworkin, R. H.; Kirkpatrick, P. Nat. Rev. Drug Disc. 2005, 4, 455.
2. Zareba, G. Drugs Today 2005, 41, 509.
3. (a) Rogers, L. C.; Armstrong, D. G. Nat. Clin. Pract. Endocrinol. Metab. 2009, 5, 14;
(b) Rice, A. S. C.; Maton, S. Pain 2001, 94, 215.
4. Bryans, J. S.; Wustrow, D. J. Med. Res. Rev. 1999, 19, 149.
5. Guay, D. R. P. Am. J. Geriatr. Pharmacother. 2005, 3, 274.
6. Gee, N. S.; Brown, J. P.; Dissanayake, V. U. K.; Offord, J.; Thurlow, R.; Woodruff,
G. N. J. Biol. Chem. 1996, 271, 5768.
7. Stahl, S. M. J. Clin. Psychiatry 2004, 65, 1033. and references therein.
8. Carlsen, P. H. J.; Katsuki, T.; Martin, V. S.; Sharpless, K. B. J. Org. Chem. 1981, 46, 3936.
9. Determined by 400 MHz 1H NMR.
10. Experimental for Wittig reaction to convert 12 to 13.
To
a solution of m-fluorobenzyl triphenylphosphonium bromide (8.08 g,
0.018 mmol) in anhydrous dichloromethane (200 ml), was added potassium
t-butoxide (1 M in THF, 17.2 ml, 0.017 mmol) dropwise at room temperature
and stirred for 1 h. The mixture was cooled to 0 °C and to this was added a
solution of (2S)-4-oxo-pyrrolidine-1,2-dicarboxylic acid 1-tert butyl ester 2-
methyl ester (3.8 g, 0.016 mmol) in dichloromethane (20 ml) dropwise. The
mixture was warmed to room temperature and stirred for 18 h. The reaction
was quenched with saturated ammonium chloride (100 ml), the aqueous
extracted with dichloromethane (2 Â 100 ml) and the combined organics dried
over magnesium sulfate. The solvent removed by evaporation under reduced
pressure. The residue was purified by flash chromatography on silica gel
eluting with a solvent gradient of heptane/ethyl acetate (4:1) to give the title
compound (3.48 g, 67%) as a colourless oil.
1H NMR (400 MHz, CD3OD) (mixture of geometric isomers, cis and trans):
O = 1.44 (s, 10H), 1.50 (s, 8H), 2.79–2.94 (m, 2H), 3.20–3.37 (m, 2H), 3.66 (d,
3H), 3.72 (d, 3H), 4.20–4.38 (m, 4H), 4.42–4.48 (m, 1H), 4.52–4.60 (m, 1H),
6.42–6.51 (m, 2H), 6.89–7.10 (m, 6H), 7.30–7.40 (m, 2H).
LRMS (APCI): m/z [(M+H)–Boc] 236.
Microanalysis: found: C, 64.46; H, 6.77; N, 4.07. C18H22FN04, requires C, 64.46;
H, 6.61; N, 4.18.
11. Artursson, P. J. Pharm. Sci. 1990, 79, 476.
12. Su, T. Z.; Lunney, E.; Campbell, G.; Oxender, D. L. J. Neurochem. 1995, 64, 2125.
13. Results to be disclosed in future publications.
Figure 1. Dog pharmacokinetic profile following 0.5 mg/kg po dose.