Discovery of PF-04457845: A highly potent, orally bioavailable, and selective urea FAAH inhibitor

Article abstract of DOI:10.1021/ml100190t

Fatty acid amide hydrolase (FAAH) is an integral membrane serine hydrolase that degrades the fatty acid amide family of signaling lipids, including the endocannabinoid anandamide. Genetic or pharmacological inactivation of FAAH leads to analgesic and anti-inflammatory phenotypes in rodents without showing the undesirable side effects observed with direct cannabinoid receptor agonists, indicating that FAAH may represent an attractive therapeutic target for the treatment of inflammatory pain and other nervous system disorders. Herein, we report the discovery and characterization of a highly efficacious and selective FAAH inhibitor PF-04457845 (23). Compound 23 inhibits FAAH by a covalent, irreversible mechanism involving carbamylation of the active-site serine nucleophile of FAAH with high in vitro potency (k_inact/K_i and IC₅₀ values of 40300 M^-1 s^-1 and 7.2 nM, respectively, for human FAAH). Compound 23 has exquisite selectivity for FAAH relative to other members of the serine hydrolase superfamily as demonstrated by competitive activity-based protein profiling. Oral administration of 23 at 0.1 mg/kg results in efficacy comparable to that of naproxen at 10 mg/kg in a rat model of inflammatory pain. Compound 23 is being evaluated in human clinical trials.

Full text of DOI:10.1021/ml100190t

pubs.acs.org/acsmedchemlett
Discovery of PF-04457845: A Highly Potent, Orally
Bioavailable, and Selective Urea FAAH Inhibitor
Douglas S. Johnson,*,†,§ Cory Stiff,†,§ Scott E. Lazerwith,§ Suzanne R. Kesten,§
Lorraine K. Fay,§ Mark Morris,§ David Beidler,‡,§ Marya B. Liimatta,§ Sarah E. Smith,§
David T. Dudley,§ Nalini Sadagopan,§ Shobha N. Bhattachar,§ Stephen J. Kesten,§
Tyzoon K. Nomanbhoy, Benjamin F. Cravatt,^ and Kay Ahn†,§
Pfizer Worldwide Research and Development, †Groton, Connecticut 06340, United States, ‡Cambridge, Massachusetts 63017,
United States, §Ann Arbor, Michigan 48105, United States, ActivX Biosciences, 11025 North Torrey Pines Road, La Jolla,
California 92037, United States, and ^The Skaggs Institute for Chemical Biology and Department of Chemical Physiology,
The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, United States
ABSTRACT Fatty acid amide hydrolase (FAAH) is an integral membrane serine
hydrolase that degrades the fatty acid amide family of signaling lipids, including the
endocannabinoid anandamide. Genetic or pharmacological inactivation of FAAH
leads to analgesic and anti-inflammatory phenotypes in rodents without showing
the undesirable side effects observed with direct cannabinoid receptor agonists,
indicating that FAAH may represent an attractive therapeutic target for the
treatment of inflammatory pain and other nervous system disorders. Herein, we
report the discoveryand characterization of a highly efficacious and selective FAAH
inhibitor PF-04457845 (23). Compound 23 inhibits FAAH by a covalent, irreversible
mechanism involving carbamylation of the active-site serine nucleophile of FAAH
with high in vitro potency (kinact/Ki and IC50 values of 40300 M-1 s-1 and 7.2 nM,
respectively, for human FAAH). Compound 23 has exquisite selectivity for FAAH
relative to other members of the serine hydrolase superfamily as demonstrated by
competitive activity-based protein profiling. Oral administration of 23 at 0.1 mg/kg
results in efficacy comparable to that of naproxen at 10 mg/kg in a rat model of
inflammatory pain. Compound 23 is being evaluated in human clinical trials.
KEYWORDS FAAH, endocannabinoid, irreversible inhibitor, clinical candidate
atty acid amide hydrolase (FAAH) is an integral mem-
brane enzyme that degrades the fatty acid amide
familyofsignaling lipids, including the endocannabinoid
others have previously reported a series of piperidine/piper-
azine urea FAAH inhibitors (e.g., 3-7).16-21 Mechanistic
studies revealed that PF-750 (4) inhibited FAAH in a time-
dependent manner by covalently modifying the enzyme's
active site nucleophile, despite the inherent stability of the
urea functional group.16 PF-750 was confirmed to be cova-
lently attached to the Ser241 of FAAH through a carbamate
linkage by a X-ray crystal structure of humanized rat (h/r)
FAAH in complex with PF-750.22 Additional medicinal chem-
istry efforts led to the identification of a more potent series of
FAAH inhibitors based on a biaryl ether piperazine/piperi-
dine urea scaffold (e.g., 6 and 7).17,21 The prototype from
this series, PF-3845 (7), inhibits FAAH by covalently modify-
ing the enzyme's active site serine nucleophile as expected
and is selective for FAAH relative to other mammalian serine
hydrolases as demonstrated by activity-based protein profil-
ing (ABPP).17 In addition, several groups have recently
F
anandamide.1-3 FAAH is a serine hydrolase that employs an
unusual serine-serine-lysine catalytic triad to efficiently
hydrolyze both amides and esters.4 Genetic or pharmacologi-
cal inactivation of FAAH leads to elevated endogenous levels of
fatty acid amides5 and a range of analgesic effects in various
animal models of inflammation and chronic pain.6-9 Impor-
tantly, these behavioral phenotypes occur in the absence of the
undesirable side effects observed with direct cannabinoid
receptor agonists, indicating that FAAH may represent an
attractive therapeutic target for the treatment of inflammatory
pain and related conditions.
Several classes of reversible and irreversible covalent
FAAH inhibitors have been reported (Figure 1).8,10 Electro-
philic ketone inhibitors such as OL-135 (1) reversibly form an
enzyme-stabilized hemiketal between the active-site Ser241 and
the electrophilic carbonyl.11 -12 Carbamates such as URB597 (2)
irreversibly inhibit FAAH by carbamylation of Ser241 with the
alcohol group serving as the leaving group.13-15 We and
Received Date: August 11, 2010
Accepted Date: October 15, 2010
Published on Web Date: November 15, 2010
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reported apparent noncovalent FAAH inhibitors with no
Table 1. Human and Rat FAAH Potency (kinact/Ki values) for
Piperidine Spacer Analoguesa
obvious serine-interacting groups (i.e., 8).23
We reasoned that in the case of a target like FAAH, where
inhibition leads to elevated levels of endogenous N-acyl
ethanolamine (NAE) substrates by up to 10-20-fold in
central tissues, an irreversible inhibitor may be beneficial
because the nonequilibrium binding mechanism limits the
competition with high endogenous substrate concentrations
leading to increased biochemical efficiency.24 Furthermore,
the pharmacodynamic effect resulting from covalent inhibi-
tion often outlasts the pharmacokinetics of the inhibitor,
because enzyme activity can be recovered only by synthesis
of new enzyme or, in some cases, by slow hydrolysis of the
covalent enzyme adduct. To minimize the risk associated
with developing a covalent inhibitor, two critical design/
screening principles were emphasized;selectivity to avoid
off target toxicity and potency to achieve as low a dose as
possible.25
In this manuscript, we describe the evolution of a series of
benzylidine piperidine urea FAAH inhibitors with improved
potency and pharmaceutical properties, culminating in the
clinical candidate PF-04457845 (23). Given the lipophilic
nature of the fatty acid amide substrates of FAAH (i.e., clog
P of anandamide = 6.2), it is not surprising that adding
lipophilicity to FAAH inhibitors in the acyl chain binding
region improves potency.26,27 The major challenge is balanc-
ing potency with appropriate physicochemical properties to
identify FAAH inhibitors with high oral bioavailability that are
suitable for clinical development. During the evolution of 23
from 7, we focused on improving the potency without
compromising essential druglike properties.28 One strategy
was to rigidify the conformation of the molecule within the
acyl chain binding domain to reduce the entropic penalty of
binding by having less rotatable bonds and lowering the con-
formational energy of the active conformer.29 Another key
area of investigation was focused on finding optimal places
in the molecule to incorporate polarity for modulating param-
eters such as clog P and pKa, while simultaneously optimizing
lipophilic interactions within the acyl chain binding pocket.
We initially sought to explore piperidine isosteres with
different ring sizes and conformational constraints with the
a Each kinact/Ki value corresponds to an average of at least two
independent determinations ((SD) and was obtained using the method
described in the Supporting Information.
goal of optimally projecting the biaryl ether moiety into the
acyl chain binding channel. Because these ureas inhibit
FAAH irreversibly, the potency was measured as the second
order rate constant kinact/Ki using an enzyme-coupled human
FAAH (hFAAH) and rat FAAH (rFAAH) assay as described
previously.17,22 Unlike IC50 values, kinact/Ki values are inde-
pendent of preincubation times and substrate concentrations
and are considered the best measure of potency for irrevers-
ible inhibitors. We found that incorporating a methylenepi-
peridine into the previous quinoline (i.e., 4)16 and benzothio-
phene (i.e., 5)18 ureaseriesto give analogues9and 10 resulted
in improved FAAH potency, particularly against the rat en-
zyme. Therefore, we made the corresponding methylenepi-
peridine analogue in the PF-3845 biaryl ether piperidine
series (Ta bl e 1). We were gratified to see that methylenepi-
peridine 11 wasapproximately 2-and 4-fold more potent than
the corresponding methylpiperidine 7 for hFAAH and rFAAH,
respectively. Furthermore, 11 was equipotent against the
hFAAH and rFAAH enzymes. We also prepared the corre-
sponding pyrrolidine (12-14) and azetidine (15) analogues,
but these were considerably less active. We do not know the
precise reason why the activity drops off with smaller ring
sizes, but we speculate that the urea is not able to be activated
to the same extent upon binding in the FAAH active site.30 In
addition, we prepared the (E)- and (Z)-3-methylenepiperidine
analogues (16 and 17). The (E)-isomer 16 was approxi-
mately 19-fold less potent than 11 for hFAAH, while the
activity was almost completely abolished in the case of the
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(Z)-isomer 17. On the basis of this data, the 4-methylenepi-
peridine spacer is optimal in this biarylether urea scaffold.
Table 2. Human and Rat FAAH Potency (kinact/Ki Values) and CYP
IC50 Values for Heteroaryl Urea Analoguesa
We next examined the structure-activity relationship
(SAR) of the 3-aminopyridyl portion of the molecule, which
constitutes the leaving group upon carbamylation of the
FAAH active site serine. Our previous work on benzothio-
phene urea FAAH inhibitors indicated that a heteroaryl group
was preferred at this position.18 We were especially inter-
ested in introducing substituents next to the pyridyl nitro-
gen as well as exploring other heterocycle replacements,
because of the potential cytochrome P450 metabolic liability
associated with unsubstituted pyridines. Indeed, com-
pound 11 exhibited moderate CYP2D6 and CYP3A4 inhi-
bition (Table 2). As observed with previous urea FAAH
inhibitors, the 3-aminopyridine analogue (11) was superior
to the 2-aminopyridine (18) and aniline (19) analogues.
Introduction of a methyl (20) or methoxy (21) substituent
at the 6-position of the 3-aminopyridine provided analogues
with slightly reduced potency. The methylpyridine analogue
(20) had less CYP3A4 inhibition, but the CYP2D6 inhibition
was unaffected. We next explored similar heterocycles with
reduced basicity while maintaining the position of the nitro-
gen relative to the 3-aminopyridine. As expected, re-
placement of the pyridine with the less basic pyrazine (22)
or pyridazine (23) removed any potential CYP liabilities.
Unexpectedly, compounds 22 and 23 displayed a 2-fold
higher kinact/Ki value for hFAAH as compared to 11. We also
assessed several 5-membered and 5,6-fused heterocycles
(e.g., 24 and 25). These compounds generally showed
reduced potency for hFAAH but increased potency for
rFAAH. For example, the dimethylisoxazole 24 was 5-fold
less potent for hFAAH but 2-fold more potent for rFAAH as
compared to 23. This highlights the importance of screening
compounds against both the hFAAH and the rFAAH
enzymes.17-18,22 Thus, the benzylidenepiperidine pyrida-
zine urea scaffold appears to be optimal.
We next surveyed the importance of the pyridyl nitrogen
as well as the position and nature of the substituent on the left-
hand side biaryl ether moiety (Table 3 ). Removal of the pyridine
nitrogen (i.e., compound 26) resulted in a 2-fold reduction in
potency for hFAAH. An additional 10-fold loss in potency resulted
from removal of the trifluoromethyl substituent as observed for
compound 27. The trifluoromethyl group at the 5-position of the
2-pyridyl ring makes key van der Waals contacts in the acyl
chain-binding pocket that are important for potency in an
analogous fashion to that described for PF-3845 (7).17 In con-
trast, substituents at the 6-position (34), 4-position (32), or
3-position (33) impair potency, demonstrating that steric
bulk is not accommodated in that region. The position of
the nitrogen in the pyridine ring is also important. Inter-
estingly, the potency of 28 for hFAAH was within 2-fold of
a Each kinact/Ki value corresponds to an average of at least two
independent determinations ((SD) and was obtained using the method
described in the Supporting Information. b Midazolam was used as the
CYP3A4 substrate. c Testosterone was used as the CYP3A4 substrate.
23, but the potency for rFAAH was dramatically reduced.
Finally, several other substituents were shown to be tolerated
at the 5-position (i.e., 29-31), and the trifluoromethyl group
proved to be optimal.
These medicinal chemistry efforts culminated in the dis-
covery of PF-04457845 (compound 23, Table 2), a benzyli-
denepiperidine pyridazine urea that displayed a 3- and 8-fold
higher kinact/Ki value for hFAAH and rFAAH, respectively, as
compared to PF-3845 (7). We also measured FAAH potency
for 23 by determining IC50 values with a 60 min preincuba-
tion time, and the IC50 was 7.2 ( 0.63 nM for hFAAH and
7.4 ( 0.62 nM for rFAAH (see the Supporting Information).
Key features of 23 that contribute to its potency include the
p-trifluoromethyl substituent and the pyridyl nitrogen on the
biaryl ether moiety, the benzylidenepiperidine urea, and the
3-aminopyridazine leaving group. It is worthwhile to note
that not only did the pyridine and pyridazine heterocycles
contribute to the improved potency, but they also served to
lower the lipophilicity and improve the physicochemical
properties (i.e., clog P = 3.9, PSA = 80) and ADME param-
eters (vide supra) for 23.
PF-04457845 (23) was synthesized in six linear steps with
a 40% overall yield from commercially available starting
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Table 3. Human and Rat FAAH Potency (kinact/Ki Values) of Biaryl
Scheme 1. Synthesis of PF-04457845 (23)a
Ether Analoguesa
a Reagents and conditions: (a) K2CO3, DMF, 90-95 ꢀC, 16 h, quant.
(b) SOCl2, CH2Cl2, room temperature, 2 h, quant. (c) P(OEt)3, 130ꢀC, 16 h,
80%. (d) N-Boc-piperidin-4-one, t-BuOK, THF, 69%. (e) HCl, dioxane,
84%. (f) i-Pr2NEt, CH3CN, RT, 16 h, 86%.
labeling intensity in the presence of inhibitor are scored as
targets of the compound. The selectivity of 23 was compared
to that of the carbamate FAAH inhibitor URB597 (2)
(Figure 1). Both 23 and 2 completely inhibited FAAH in
human and mouse membrane proteomes at both 10 and
100 μM with no off targets. However, in soluble proteomes of
liver, the profile of 23 was drastically different than that of 2.
Compound 23 was completely selective as none of the other
FP-reactive serine hydrolases were inhibited even at 100 μM.
In contrast, 2 displayed multiple off targets as it blocked FP
labeling of several additional serine hydrolases, particularly
among FP-labeled proteins migrating between 55 and 65 kDa.
Several off targets were completely inhibited even at 10 μM 2.
Thus, 23 is exquisitely selective for FAAH relative to other
mammalian serine hydrolases. In addition, 23 exhibited a
favorable selectivity profile when screened against a 68 target
CEREP panel (Table S1 in the Supporting Information).
PF-04457845 (23) has an excellent pharmacokinetic pro-
file in rats and dogs as well as in human in vitro assays. The
permeability of 23 (A-B, 15.2 ꢀ 10-6 cm/s; and B-A, 20.2 ꢀ
10-6 cm/s) across MDCK cells was moderate, with no
apparent efflux due to Pgp (B/A ratio is 1.33). Compound
23 has low clearance and moderate volume of distribution in
rats and dogs (Table 4). Compound 23 was rapidly absorbed
following oral administration of a methylcellulose suspen-
sion of the crystalline material with a bioavailability of 88%
in rats and 58% in dogs. Low risk for drug-drug interactions
is anticipated based on in vitro CYP inhibition studies in
human liver microsomes using relevant substrates (Table S2
in the Supporting Information). The potential of 23 to form
reactive metabolites was investigated, and no glutathione
conjugate of 23 was observed from in vitro rat or human
hepatocyte incubations. The half-life of 23 in human liver
microsomal experiments was determined to be 105 min
tested at 0.8 mg/mL protein concentration. The microsomal
protein binding of 23 was 90%. Human blood clearance
based on these in vitro experiments was predicted to be
0.6 mL/min/kg. On the basis of this data, 23 is predicted to be
suitable for once a day oral administration in humans.
a Each kinact/Ki value corresponds to an average of at least two
independent determinations ((SD) and was obtained using the method
described in the Supporting Information.
materials (Scheme 1). The synthesis began with a nucleo-
philic aromatic substitution of 3-(hydroxymethyl)phenol
with 2-chloro-5-(trifluoromethyl)pyridine in the presence of
K2CO3 to form biaryl ether 35. Treatment of benzyl alcohol
35 with thionyl chloride provided the corresponding benzyl
chloride, which was reacted with triethyl phosphite to give
the benzylphosphonate 36. Horner-Wadsworth-Emmons
olefination of 1-Boc-4-piperidone with 36 in the presence of
potassium tert-butoxide followed by removal of the Boc
protecting group using HCl in dioxane afforded the benzyli-
denepiperidine 37 as the hydrochloride salt. Finally, reaction
of 37 with phenyl pyridazin-3-ylcarbamate in the presence
of diisopropylethyl amine provided the urea 23.
We next assessed the selectivity of PF-04457845 (23)
against the serine hydrolase superfamily of enzymes (>200
members in humans) of which FAAH is a member.16-18
Compound 23 was profiled at 10 and 100 μM utilizing a
functional proteomic screen based on competitive activity-
based protein profiling (ABPP)31 in brain membrane and
soluble liver proteomes derived from both human and
mouse. We used a rhodamine-tagged fluorophosphonate
(FP) ABPP probe, which serves as a general activity-based
profiling tool for the serine hydrolase superfamily.32 Serine
hydrolases that show significant reductions in FP probe
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Table 4. Plasma Pharmacokinetic Data Following Administration
of PF-04457845 (23) (iva and pob Dose of 1 mg/kg in Rat and
0.5 mg/kg in Dog)
oral Cmax tmax
CL
Vdss
species
(ng/mL)
(h)
(mL/min/kg)
(L/kg) iv t1/2 (h) % F
rat
256
240
4.0
1.0
3.0
1.3
3.2
2.9
14.3
35.2
88
58
dog
a Intravenous doses were formulated in 5% dimethylacetamide and
95% (40% SBE-cyclodextrin in water). b Oral doses were formulated in
5% (0.1% Tween 80) and 95% (0.5% methylcellulose).
Figure 1. Selectivity profiling of PF-04457845 (23) and URB597
(2) by competitive ABPP. Gel images of proteomes labeled with FP-
rhodamine in the presence or absence of FAAH inhibitors (lane 1,
DMSO; lane 2, 2 at 100 μM; lane 3, 2 at 10 μM; lane 4, 23 at 100 μM;
lane 5, 23 at 10 μM; and lane 6, DMSO). Gel profiles of FP-
rhodamine-labeled mouse and human (A) liver soluble serine
hydrolases and (B) brain membrane serine hydrolases in the
presence or absence of 23 and 2. The band on the gel correspond-
ing to the FAAH enzyme is highlighted by the black arrow. Note
that 2, but not 23, blocks FP-rhodamine labeling of several liver
serine hydrolases (red arrows and bracket). For both A and B,
fluorescent gel images are shown in grayscale.
Figure 2. Antihyperalgesic effects of PF-04457845 (23) in the
CFA model of inflammatory pain in rats. Compound 23, at 0.1 to
10 mg/kg (po), produced a reduction of mechanical allodynia
(hyperalgesia) in rats (black bars). The effect of the nonsteroidal
anti-inflammatory drug naproxen (10 mg/kg, po, hatched bar) is
shown for comparison. The antihyperalgesic responses were deter-
mined at 4 h following drug treatment and were significantly
different for 23 (0.1-10 mg/kg, po) and naproxen (10 mg/kg, po)
ascompared tovehicle-treated groups(p<0.05), n=8 ratspergroup.
We next assessed in vivo efficacy of PF-04457845 (23) in a
rat model of inflammatory pain. Subcutaneous injection of
complete Freund's adjuvant (CFA) into the plantar surface of
the hind paw produced a significant decrease in mechanical
paw withdrawal threshold (PWT) after 5 days postinjection
as previously described (Figure 2).17 Oral administration
of 23 caused a significant inhibition of mechanical allody-
nia measured after 4 h with a minimum effective dose
(MED) of 0.1 mg/kg, which is ∼30-fold lower than PF-3845
(7) (MED of 3 mg/kg). Furthermore, at 0.1 mg/kg (p.o.), 23
inhibited the pain response to a comparable degree as the
nonsteroidal anti-inflammatory drug naproxen at 10 mg/kg
(Figure 2). It has been reported that a near-complete
blockade of FAAH activity is required to maintain elevated
anandamide levels in vivo.14,17 In full agreement with this
observation, the robust in vivo efficacy by 23 was accom-
panied by near complete inhibition of FAAH activity with
concomitant elevations in anandamide and other fatty
acid amides in brain and plasma (details will be described
elsewhere). The degree of inhibition in painresponse by 23
was similar at all efficacious doses likely because near com-
plete FAAH inhibition is necessary to obtain in vivo efficacy,
which was achieved at all doses starting at 0.1 mg/kg.
has excellent pharmacokinetic properties and is suitable for
once a day oral administration. It is efficacious in the CFA
model of inflammatory pain in rats. Compound 23 is currently
being evaluated in human clinical trials with the potential to
treat chronic pain and other nervous system disorders.
SUPPORTING INFORMATION AVAILABLE Experimental
procedures for the synthesis of 11-34, assay protocols, and two
supplementary tables. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author: *E-mail: doug.johnson@pfizer.com.
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NOTE ADDED AFTER ASAP PUBLICATION This paper was
published on the Web on November 15, 2010 with an error in
structure PF-750 (4). The corrected version was reposted on January
19, 2011.
r
2010 American Chemical Society
96
DOI: 10.1021/ml100190t ACS Med. Chem. Lett. 2011, 2, 91–96
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