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Discovery of CP-866,087, a mu opioid receptor antagonist for the treatmentof alcohol abuse and dependence†

Stanton F. McHardy,‡*a Steven D. Heck,a Sara Guediche,a Monica Kalman,a Martin P. Allen,a Meihua Tu,a

Dianne K. Bryce,b Anne W. Schmidt,b Michelle Vanase-Frawley,b Ernesto Callegari,c Shawn Doran,c

Nicholas J. Grahame,d Stafford McLeanb and Spiros Liras*a

Received 23rd June 2011, Accepted 27th July 2011

DOI: 10.1039/c1md00164g

CP-866,087 (compound 15) is a novel, potent and selective mu opioid receptor antagonist. The design

rationale, synthesis, in vitro and in vivo biological evaluation are reported herein. Preclinical efficacy

data in disease relevant models measuring reduction of alcohol intake are presented, along with

preclinical pharmacokinetic properties which supported the selection of the title compound for clinical

evaluation.

Introduction

Non-selective opioid antagonists, such as naltrexone, naloxone

and nalmafene have demonstrated clinical efficacy in reduction

of ethanol intake.1 The association of opioid receptor antago-

nism and reduction in ethanol consumption has been studied

extensively. It has been postulated that ethanol stimulates the

endogenous opioid pathways that in turn activate the dopami-

nergic signaling of the reward system in the brain.2 Opioid

antagonism interferes with dopaminergic signaling and activa-

tion of the reward system. While clinical efficacy has been

demonstrated with non-selective opioid antagonists, it has not

been determined which receptor subtype is the primary driver for

efficacy. On this basis, we set out to design and synthesize mu

receptor selective antagonists and investigate their potential

utility as therapeutics for alcohol abuse and dependence. Mu

opioid receptors are located on dopaminargic neurons.3 Stimu-

lation of mu receptors augments dopamine function in the CNS

and this is associated with activation of the reward system.

Kappa opioid receptors are also found on dopaminargic

neurons. However, activation of kappa receptors is thought to

inhibit DA function.4 Thus, our research objective was to

aWorldwide Medicinal Chemistry, Pfizer Worldwide Research andDevelopment, Eastern Point Road, Groton, CT, 06340, USA. E-mail:spiros.liras@pfizer.combNeuroscience Biology Pfizer Worldwide Research and Development,Eastern Point Road, Groton, CT, 06340, USAcPharmacokinetics, Dynamics andMetabolism, PfizerWorldwide Researchand Development, Eastern Point Road, Groton, CT, 06340, USAdDepartment of Psychiatry, Indiana University School of Medicine,Indianapolis, IN, 46202, USA

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1md00164g

‡ Current Address: Synthesis and Process Chemistry, SouthwestResearch Institute, 6220 Culebra Rd, San Antonio, Texas, 78238, USA.

This journal is ª The Royal Society of Chemistry 2011

generate a novel and selective mu opioid receptor antagonist with

sufficient pharmacokinetic and disposition characteristics to

provide an once a day therapy for alcohol abuse and dependence.

Results and discussion

Zimmerman, Caroll et al.5 have shown that 4-phenyl piperidines

exhibit potent affinity for the mu receptor. Based on extensive

studies on 4-phenyl piperidines it was determined that ring

systems which display the aromatic ring in an equatorial

conformation deliver mu receptor antagonists (1, Fig. 1). With

this knowledge, Pfizer coworkers determined that the novel 3.1.0

bridged bicycle 2, shown in Fig. 1, was a privileged template

which displayed the aromatic ring in a locked, equatorial-like

configuration. Extensive SAR work suggested that the scaffold

delivered potent mu receptor antagonists.6 Given our interest in

mu opioid antagonists, we were attracted to this scaffold for

several reasons including molecular symmetry, rich SAR which

Fig. 1 4-Phenyl piperidine derived mu opioid receptor antagonists.

Med. Chem. Commun., 2011, 2, 1001–1005 | 1001

Scheme 2 Reagents and conditions: (a) Zn(CN)2, Pd(PPh3)4, 80%; (b)

H2O2, K2CO3, DMSO, 60%; (c) Pd/C, MeOH, NH4CO2H, 80%; (d)

R1CHO, NaBH(OAc)3, (CH2)2Cl2, 70–90%; (e) Pd(OAc)2, BINAP,

benzophenone imine, NaOtBu, HCl,; (f) RSO2Cl, pyridine, 70% for e and

f; (g) H2, Pd/C, MeOH, NH4CO2H, 78%.

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suggested tolerance with amine substitutions and the demon-

strated ability to replace phenols with a methyl sulfonamide

surrogate.

Thus, our first objective was to develop a scalable synthesis

which would enable the expanded parallel investigation of two

points of diversity; (a) N-substitutions and (b) phenol surrogates.

The synthesis plan and its reduction to practice are summarized

in Schemes 1 and 2. The synthesis commenced with the conver-

sion of 3-bromophenyl-propane-1-one to the corresponding

hydrazone 3 with hydrazine in water-methanol at reflux. The

crude hydrazone was treated with MnO2 in dioxane and N-

benzyl-pyrrole at room temperature to afford compound 6, via

the intermediate diastereomeric cycloaddition products 4 and 5.7

Compound 6 was reduced in high yield with BF3 Et2O and

NaBH4 in THF to the desired 3.1.0 bicylic amine 7.

Amine 7 was further elaborated to the corresponding car-

boxamide via a two step sequence which involved first conversion

to a nitrile with Zn(CN)2 and Pd(PPh3)4 and subsequent oxida-

tion with H2O2, K2CO3 in DMSO.8,9 Debenzylation proceeded

smoothly to yield secondary amine 8 which was readily elabo-

rated to 9 to investigate SAR with N-substitutions and the car-

boxamide as a phenol surrogate. Similarly, amine 7 was

converted to a methyl sulfonamide via transformation of the

bromophenyl moiety to an aniline with Pd (OAc)2, BINAP,

benzophenone imine10 followed by acid hydrolysis and subse-

quent treatment with MeSO2Cl and pyridine. Secondary amine

10 was afforded after standard debenzylation conditions. Amine

10 was similarly elaborated to tertiary amine 11 to investigate

SAR with N-substitutions and the methyl sulfonamide as

a phenol surrogate.

As has been shown in the opioid literature, with phenyl

piperidines and aryl morphans, various amine tethers are well

tolerated and yield potent mu receptor ligands with variable

degrees of selectivity.5 The same was realized with amine tethers

for 9 and 11.6 From the early SAR efforts we identified

compound 12 as a promising lead and this compound was

characterized in depth. Compound 12 exhibited appreciable

Scheme 1 Reagents and conditions: (a) Hydrazine hydrate, MeOH,

reflux, 95%; (b) MnO2, dioxane, 1-benzyl-1H-pyrrole-2,5-dione, 35–40%

overall from 3; (c) BF3-Et2O, NaBH4, THF, piperazine, H2O, reflux,

95%.

1002 | Med. Chem. Commun., 2011, 2, 1001–1005

affinity for the mu receptor and good selectivity over the delta

and kappa receptors (Ki of 5 nM, 112 nM, and 56 nM respec-

tively).11,12 Compound 12 resides in lipophilic physicochemical

space with a cLogP of 4.1 and TPSA of 58. Based on its attractive

in vitro profile, compound 12 was evaluated in key ADME and

safety pharmacology screens. In human liver microsomes the

compound showed an intrinsic clearance of 19 ml min�1/kg. As is

characteristic of numerous lipophilic amines the compound

showed considerable metabolism by polymorphic CYP-2D6

(35% enzyme inhibition at 3 mM).13,14 Compound 12 was pre-

dicted to be a modest P-gp efflux substrate based on MDR1

transfectedMDCK cells data (BA/AB of 3.5).15 Additionally, the

compound showed appreciable affinity in a 3H-dofetilide binding

assay which is used as a surrogate for predicting interaction with

the HERG channel (IC50 of 326 nM).16 Consequently, our

attention was focused on devising a uniform strategy to further

optimize ADME properties, reduce interaction with CYP-2D6,

P-gp and reduce affinity in the 3H-dofetilide binding assay

(Table 1).

Table 1 Key in vitrodataandphysicochemical properties for compound12

Mu Ki (nM) 5Delta Ki (nM) 112Kappa Ki (nM) 56Ratio m/d/k 1/22/11cLogP 4.1TPSA 58Dofetilide IC50 (nM) 326HLM (ml min�1 kg�1) 19CYP-2D6% inh @ 3 mM 35MDR BA/AB 3.5

This journal is ª The Royal Society of Chemistry 2011

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We determined that the key design objective was reduction of

lipophilicity. As demonstrated with related systems, reduction of

lipophilicity leads to improvements in microsomal stability.17

Furthermore, we hypothesized that the observed dofetilide

affinity was an outcome of pharmacological promiscuity due to

lipophilicity.18 With respect to CYP-2D6 interaction, it has been

postulated that CYP-2D6 induced oxidation of basic molecules

occurs at a site proximal to basic nitrogen, within 5–7

Angstroms.19 Thus, we targeted to incorporate hydroxyl-

substituted aliphatic side chains to minimize turnover by CYP-

2D6. The incorporation of these tethers was predicted to reduce

lipophilicity which we expected would reduce affinity for dofe-

tilide and benefit microsomal stability. Thus, we envisaged that

the incorporation of hydroxyl groups on the amine alkyl tethers

would serve as a uniform strategy to address all limitations with

compound 12. We aimed to incorporate hydroxyl-substituted side

chains which would preserve the molecular symmetry inherent to

the scaffold and which would present reasonable steric hindrance

to reduce the risk for secondary metabolism. Compounds 13, 14

and 15 all meet the design criteria (Fig. 2).

Fig. 2 Amines 13, 14 and 15.

Amine 13 was prepared with standard reductive alkylation

conditions as reported in Scheme 2. Amines 14 and 15 were

accessed as described in Scheme 3. 1H-Inden-2(3H)-one was

Scheme 3 Reagents and conditions: (a) trimethylsilyl methyl lithium,

CeCl3, TMEDA 50%; (b) 50% aq HF, CH3CN; (c) mCPBA, NaHCO3,

CH2Cl2, 81% over 2 steps; (d) amines 8 and 10, Et3N, EtOH reflux,

60–80%.

This journal is ª The Royal Society of Chemistry 2011

treated with trimethylsilyl methyl lithium and CeCl3 to afford

hydroxy silane 16, which upon treatment with HF in acetonitrile

and subsequent oxidation of the ensuing olefin with mCPBA

yielded epoxide 17.20Compound 17was then reacted with amines

8 and 10 to yield 14 and 15 in good yields.

Compound 13 showed excellent affinity for the mu receptor

(2 nM), modest selectivity over the kappa receptor (16 nM) and

excellent selectivity over the delta receptor (166 nM). Relative to

12, compound 13 resides in less lipophilic space with a cLogP of

3.2 and a TPSA of 78. The increased polarity, as desired, yielded

an increase in microsomal stability as measured by human liver

microsomes (Clint of 9.3 ml min�1 kg�1). In addition, dofetilide

activity was reduced (IC50 of 710 nM). As designed, the

hydroxylation delivered a compound with reduced interaction

with CYP-2D6 (14% inhibition at 3 mM). Similarly, compound

14 exhibited high affinity for the mu receptor (3.5 nM), sug-

gesting the suitability of the primary hydroxamide as a phenol

surrogate. The compound retained good to excellent selectivity

over the kappa and delta receptors (60 and 108 nM respectively).

Compound 14 also resides in less lipophilic space than the

predecessor 12 (cLogP of 2.8, TPSA of 67). Consistent with these

properties the compound exhibited reduced interaction with

CyP-2D6 (17% inhibition at 3 mM), improved stability in human

liver microsomes (Clint of 11 ml min�1 kg�1) and reduced activity

in dofetilide (IC50 of 2.2 mM), all relative to amine 12. Compound

15 exhibited the best affinity and selectivity profile for the mu,

kappa and delta receptors (1, 42, 170 nM respectively). With

a cLogP of 3.1 and a TPSA of 78 the compound resides in

acceptable physicochemical space. Compound 15 showed

reduced interaction with CYP-2D6 (6% inhibition at 3 mM) and

improved stability in human liver microsomes relative to 12 (Clintof 12 ml min�1 kg�1). Additionally, amine 15 displayed reduced

affinity in 3H-dofetilide relative to 12 (IC50 of 1.6 mM). All

compounds were characterized as mu antagonists based on

a GTP-gS assay (Table 2). Similarly, all compounds were found

to be delta and kappa receptor antagonists in the corresponding

GTP-gS assays.

Compounds 13 and 15 were selected for additional in vivo

evaluation based on their overall in vitro profile. These

compounds are differentiated in one important dimension which

merited detailed investigation. Based on the in vitro MDR data

compound 13 was predicted to be an efflux substrate with a BA/

AB ratio of 6.5 and compound 15 was predicted not to be an

Table 2 Physicochemical properties and key in vitro data forcompounds 13, 14 and 15

Compound 13 14 15

Mu Ki (nM) 2.0 3.5 1.0Delta Ki (nM) 166 108 170Kappa Ki (nM) 16 60 42Ratio m/d/k 1/83/8 1/31/17 1/170/42Mu antag. GTP-gS Ki (nM) 0.18 0.16 0.28cLogP 3.3 2.8 3.1TPSA 78 67 78Dofetilide IC50 (nM) 710 2200 1600HLM (ml min�1 kg�1) 9 14 12CYP-2D6% inh @ 3 mM 14 17 6MDR BA/AB 6.5 2.9 1.2

Med. Chem. Commun., 2011, 2, 1001–1005 | 1003

Fig. 3 In vivo efficacy of compound 15 in alcohol preferring rats (P-rats).

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efflux substrate (BA/AB ratio of 1.2). While the compounds are

structurally similar, they differentiate in one important physi-

cochemical parameter (amine pKa). Based on calculations

compound 15 is predicted to be less basic than compound 13

(pKa of 8.9 and 11.1 respectively).21 Compound 15 forms a strong

intra-molecular hydrogen bond in its protonated form, which is

likely to contribute to its relatively lower pKa. This was an

important design feature. We hypothesized that this parameter

was likely to affect interaction with P-gp and thus produce

different outcomes with respect to plasma and CSF concentra-

tions required for the desired in vivo efficacy.

To assess in vivo CNS activity both compounds 13 and 15 were

evaluated in a rodent tail flick assay. In this assay a focused light

source was applied to the tail of a mouse or a rat. The latency to

remove the tail from this moderately painful stimulus was

measured. In rodents, morphine increases this latency. The

increase in latency is interpreted as morphine-induced anti-

nociception which is thought to be mediated by mu receptor

agonism. Pretreatment with 13 antagonized the effect of

morphine with an ED50 of 0.63 mg kg�1, s.c. Similarly,

pretreatment with 15 yielded an ED50 of 0.1 mg kg�1, s.c. The in

vivo efficacy of 15 as compared to 13 translated into a lower

efficacious plasma concentration (1 nM or approximately 1x the

receptor affinity for 15 versus 22 nM or 11x the receptor affinity

for 13). The higher efficacious plasma concentration for

compound 13 was suggestive of an efflux liability as the in vitro

MDR evaluation predicted. The lower efficacious plasma

concentration for 15 over 13 represented a significant advantage

with respect to safety margins. For example, while the HERG

channel IC20 for 15 was determined as 104 nM, the ratio of

HERG IC20 over plasma efficacious concentration was >200.

Alternatively, the HERG IC20 for 13was 320 nM, yet the ratio of

HERG IC20 over plasma efficacious concentration was just 15.

On the basis of the in vitro and in vivo evaluation, compound 15

was subsequently profiled in rodent pharmacokinetic studies and

in disease relevant models for alcohol intake reduction.

In rats, compound 15 was identified as a high clearance

compound (Clb ¼ 120 ml min�1 kg�1) with a substantial volume

of distribution (21.7 l) and an 11 h half life. Importantly, it was

demonstrated that the compound was metabolized predomi-

nantly by CYP enzymes and that the in vivo clearance was

effectively predicted by the in vitro clearance values.22 In addi-

tion, disposition properties for compound 15 were investigated.

Thus, it was demonstrated that 15 readily accessed the central

compartment in rats (brain/plasma ¼ 2, at 1 mg kg�1 s.c. dosing,

0.75 h time-point). Compound 15 achieved in CSF a concentra-

tion of 11.9 nM (1 mg kg�1 s.c. dosing, 0.75 h time-point) which

equaled the free plasma concentration, thus confirming the in

vitro MDR prediction that 15 posed no risk for P-gp efflux

(338 nM total plasma concentration, fu of 0.034, 1 mg kg�1 s.c.

dosing, 0.75 h time-point).

Subsequently, compound 15 was evaluated in a rodent model

of measuring reduction in ethanol intake. There are numerous

animal models for alcoholism. However, only the selectively

bred, alcohol-preferring rats (P-rats) exhibit spontaneous pref-

erence for alcohol.23,24 We employed this genetic model of alco-

holism as our preclinical model for assessing potential for clinical

efficacy. P-rats selectively bred for alcohol preference typically

consume in excess of 5 g kg�1 of EtOH per day. Both female and

1004 | Med. Chem. Commun., 2011, 2, 1001–1005

male P rats exhibit this behavior and the intake of female rats is

stable across the estrous cycle. In this study, female animals were

given free access to food, water and 10% (v/v) alcohol, for 21

days. This was followed by restricted access to alcohol for two

hours/day, starting at lights out. Drug was introduced via s.c.

administration 30 min prior to the start of this limited access

period and the amount alcohol consumed in the drug condition

was recorded and compared to baseline intake. Compound 15

produced a dose dependent decrease in alcohol intake (Fig. 3). A

dose of 5.6 mg kg�1 produced a 42% reduction in alcohol intake

where a dose of 1 mg kg�1 produced a 23% reduction in alcohol

intake. This is comparable to the 21% reduction observed at

a 3 mg kg�1 dose with the clinically efficacious naltrexone. Thus,

compound 15 (CP-866,087) and clinically validated naltrexone

both demonstrated efficacy in a preclinical paradigm measuring

reduction in alcohol intake.

Conclusions

Compound 15 (CP-866,087) was identified as a potent and

selective mu opiate antagonist. The compound was designed with

a uniform strategy of targeted lipophilicity reduction. Specifi-

cally, it was demonstrated that beta or gamma hydroxylation in

a series of aliphatic amines delivered compounds with improved

pharmacokinetic parameters including interaction with

CYP-2D6. The impact of compound basicity on P-gp efflux was

also investigated. Compound 15 demonstrated excellent activity

in preclinical models measuring reduction of ethanol intake. On

the basis of the preclinical efficacy and its pharmacokinetic

performance the compound was selected for clinical evaluation.

Results from the clinical performance of this agent will be

reported in due course.

Notes and references

1 (a) D. J. Drobes, R. F. Anton, S. E. Thomas and K. Voronin Alcohol,Alcohol.: Clin. Exp. Res., 2004, 28, 1362; (b) S. S. O’Malley, AlcoholAlcohol. Suppl., 1996, 31, 77.

2 (a) M. S. Cowen and A. J. Lawrence, Progress Neuro-Psychopharmacol. Biol. Psychiatry, 1999, 23(7), 1171–1212; (b)R. A. Wise, Pharmacol. Ther., 1987, 35, 227; (c) T. S. Shippenbergand H. L. Altshuler, Alcohol (Fayetteville, N.Y.), 1985, 2(2), 197.

3 L. G. Latimer, P. Duffy, Kalivas and W. Peter, J. Pharm. Exp. Ther,1987, 241, 328.

4 A. W. Brunijnzeel, Brain Res. Rev., 2009, 62, 127.

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5 (a) D. M. Zimmerman, J. D. Leander, B. E. Cantrell, J. K. Reel,J. Snoddy, L. G. Mendelsohn, B. G. Johnson and C. H. Mitch, J.Med. Chem., 1993, 36, 2833; (b) J. B. Thomas, S. W. Mascarella,R. B. Rothman, J. S. Partilla, H. Xu, K. B. McCullough,C. M. Dersch, B. E. Cantrell, D. M. Zimmerman and F. I. Carroll,J. Med. Chem., 1998, 41, 1980.

6 B. J. Banks, D. J. Critcher, A. E. Fenwick, D. M. Gethin,S. P. Gibson, U.S. Pat. Appl. Publ. 2003, US 20030207876.

7 W. Nagai and Y. Hirata, J. Org. Chem., 1989, 54, 635.8 D. M. Tschaen, R. Desmond, A. O. King, M. C. Fortin, B. Pipik,S. King and T. R. Verhoeven, Synth. Commun., 1994, 24, 887.

9 L. McMaster and F. B. Langreck, J. Am. Chem. Soc., 1917, 39, 103.10 S. Wagaw, B. H. Yang and S. L. Buchwald, J. Am. Chem. Soc., 1999,

121, 10251.11 S. Liras, S. F. McHardy, M. P. Allen, B. E. Segelstein, S. D. Heck,

D. K. Bryce, A. W. Schmidt, M. Vanase-Frawley, E. Callegari andS. McLean, Bioorg. Med. Chem. Lett., 2010, 20, 503.

12 All compounds gave satisfactory 1HNMR and MS spectral data. (12)1HNMR (CD3OD) d 7.34 (t, J¼ 8.0 Hz, 1H), 7.21–7.27 (m, 3H), 7.10–7.20 (m, 4H), 4.18 (td, J ¼ 6.2, 4.3 Hz, 2H), 3.43 (d, J ¼ 7.0 Hz, 2H),3.22 (dd, J¼ 14.5, 7.1 Hz, 2H), 3.10 (d, J¼ 13.7 Hz, 2H), 2.97 (s, 3H),2.75–2.92 (m, 3H), 2.36–2.44 (m, 2H), 1.85 (q, J¼ 7.2 Hz, 2H), 0.92 (t,J ¼ 7.2 Hz, 3H); MS (M + 1) 411.3. (13): 400 MHZ

1HNMR (CDCl3)d 7.16–7.32 (m, 2H), 6.98–7.09 (m, 3H), 2.94–2.97 (m, 4H), 2.81–2.84(m, 2H), 2.52 (s, 3H), 1.21–1.94 (m, 16H), 0.76 (t, J¼ 3.4 Hz, 3H); MS(M + 1) 421.2. (14) 400 MHZ

1HNMR (CD3OD) d 7.76 (s, 1H), 7.63–7.65 (m, 1H), 7.41–7.44 (m, 1H), 7.31–7.35 (m, 1H), 7.12–7.14 (m,2H), 7.01–7.09 (m, 2H), 3.18–3.28 (m, 2H), 3.05–3.10 (m, 2H), 2.98–3.01 (M, 2H), 2.86–2.89 (m, 2H), 2.73 (s, 2H), 1.94–1.99 (m, 2H),1.81–1.84 (m, 2H), 0.79 (t, J ¼ 7.4 Hz, 3H); MS (M + 1) 377.2. (15)400 MHZ

1HNMR (CDCl3) d 7.10–7.25 (m, 7H), 6.99–7.05 (m, 1H),3.19–3.22 (m, 2H), 3.06–3.09 (m, 2H), 2.96–2.99 (m, 7H), 2.81 (s,2H), 1.83–1.90 (m, 4H), 0.83 (t, J ¼ 7.4 Hz, 3H); MS (M + 1)427.1. For full experimental procedures please see: S. F. McHardy,S. Liras and S. D. Heck, WO 2003035622, 2003.

13 A. S. Kalgutkar, S. Zhou, O. A. Fahmi and T. J. Taylor,DrugMetab.Dispos., 2003, 31, 596.

14 This single point cocktail drug-drug interaction assay determines ifa test compound inhibits the 2D6 metabolism of a known substrate(dextromethorophan). The test compound is incubated (3 mM) for 8min in microsomes and the data are reported as % inhibition ofprobe substrate metabolism.

15 K. M. Giacomini, S.-M. Huang, D. J. Tweedie, L. Z. Benet,K. L. R. Brouwer, X. Chu, A. Dahlin, R. Evers, V. Fischer,K. M. Hillgren, K. A. Hoffmaster, T. Ishikawa, D. Keppler,

This journal is ª The Royal Society of Chemistry 2011

R. Kim, C. A. Lee, M. Niemi, J. W. Polli, Y. Sugiyama,P. W. Swaan, J. A. Ware, S. H. Wright, S. Wah Yee, M. J. Zamek-Gliszczynski and L. Zhang, Nat. Rev. Drug Discovery, 2010, 9, 215.

16 (a) G. J. Diaz, K. Daniell, S. T Leitza, R. L. Martin, Z. Su, J. SMcDermott, B. F. Cox and G. A. Gintant, J. Pharmacol. Toxicol.Methods, 2004, 50, 187; (b) K. Finlayson, L. Turnbull,C. T. January, J. Sharkey and J. S. Kelly, Eur. J. Pharmacol., 2001,430, 147.

17 S. Liras, S. F. McHardy, M. P. Allen, B. E. Segelstein, S. D. Heck,D. K. Bryce, A. W. Schmidt, R. O’Connor, M. Vanase-Frawley,E. Callegari and S. McLean, Med. Chem. Commun., 2011, 2, 413.

18 Jason D. Hughes, Julian Blagg, David A. Price, Simon Bailey, GaryA. DeCrescenzo, Rajesh V. Devraj, Edmund Ellsworth, YvetteM. Fobian, Michael E. Gibbs, Richard W. Gilles, Nigel Greene,Enoch Huang, Teresa Krieger-Burke, Jens Loesel, Travis Wager,Larry Whiteley and Yao. Zhang, Bioorg. Med. Chem. Lett., 2008,18, 4872.

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20 C. R Johnson and B. D. Tait, J. Org. Chem., 1987, 52, 281.21 Conformational search was performed with MacroModel module in

Maestro software package (V. 9.1) (Schrodinger Inc, NYC, NY).The lowest energy conformations for both neutral and protonatedforms were subjected to further calculation. The COSMO chargesof the lowest energy conformations were calculated with theCALCULATE program from COSMOLogic using the BP-TZVP-COSMO method (b–p function and def-TZVP basis set). The pKa

of the simplified analogs of 13 and 15 were calculated based on theCOSMO charge files in COSMOThermX.

22 N. A. Hosea, W. T. Collard, S. Cole, T. S. Maurer, R. X. Fang,H. Jones, S. M Kakar, Y. Nakai, B. J. Smith, R. Webster andK. Beaumont, J. Clin. Pharmacol., 2009, 49, 513.

23 T. K Li, L. Lumeng, W. J. McBride and J. M. Murphy, AlcoholAlcohol Suppl, 1987, 1, 91.

24 Animals were handled and cared for in accordance with the guide forthe care and use of laboratory animals (National Research Council1996) and all procedures were performed with the approval of thePfizer Animal Care committee.

Med. Chem. Commun., 2011, 2, 1001–1005 | 1005