Potent 1,3,4-trisubstituted pyrrolidine CCR5 receptor antagonists: Effects of fused heterocycles on antiviral activity and pharmacokinetic properties

Article abstract of DOI:10.1016/j.bmcl.2005.02.030

A series of 1,3,4-trisubstituted pyrrolidine CCR5 receptor antagonists containing a variety of fused heterocycles at the 4-position of the piperidine side chain has been discovered, which are orally bioavailable with potent anti-HIV activity.

Full text of DOI:10.1016/j.bmcl.2005.02.030

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2129–2134
Potent 1,3,4-trisubstituted pyrrolidine CCR5 receptor
antagonists: effects of fused heterocycles on antiviral activity
and pharmacokinetic properties
Dooseop Kim,a,* Liping Wang,a Jeffrey J. Hale,a Christopher L. Lynch,a,ꢀ
Richard J. Budhu,a Malcolm MacCoss,a Sander G. Mills,a Lorraine Malkowitz,b
Sandra L. Gould,b Julie A. DeMartino,b Martin S. Springer,b Daria Hazuda,c
Michael Miller,c Joseph Kessler,c Renee C. Hrin,c Gwen Carver,c Anthony Carella,c
Karen Henry,c Janet Lineberger,c William A. Schleif c and Emilio A. Eminic,ꢁ
aDepartment of Medicinal Chemistry, Merck Research Laboratories, RY800-206, POBox 2000, Rahway, NJ 07065, USA
bDepartment of Immunology Research, Merck Research Laboratories, RY800-206, POBox 2000, Rahway, NJ 07065, USA
cDepartment of Antiviral Research, Merck Research Laboratories, West Point, PA 19486, USA
Received 12 November 2004; revised 8 February 2005; accepted 9 February 2005
Available online 10 March 2005
Abstract—A series of 1,3,4-trisubstituted pyrrolidine CCR5 receptor antagonists containing a variety of fused heterocycles at the 4-
position of the piperidine side chain has been discovered, which are orally bioavailable with potent anti-HIV activity.
Ó 2005 Elsevier Ltd. All rights reserved.
The discovery of the b-chemokine receptor CCR5 as a
co-receptor for HIV-1 infection has spurred recent in-
tense research efforts directed toward the development
of CCR5 antagonists as potential anti-HIV agents.1
Furthermore, recent human genetic evidence supports
CCR5 as a therapeutic target. Individual homozygous
for a 32-base pair deletion in the gene for CCR5 lack
this receptor on cell surfaces and are highly resistant
to HIV-1 infection, while infected heterozygous individ-
uals show significantly delayed progression to AIDS.2
acidic functionality at the N-position as potent CCR5
receptor antagonists with excellent anti-HIV activity in
vitro.4e We have demonstrated that this class of zwitter-
ionic pyrrolidines can display excellent antiviral activity,
although with modest pharmacokinetic profiles, as
exemplified by 1 (Fig. 1).4
Encouraged by these results, our efforts focused on mod-
ification of the phenylpropyl side chain in compound 1.
Of particular interest was the replacement of the phenyl-
propyl side chain with metabolically more stable hetero-
cycles. The beneficial effect of heterocycles at this site on
antiviral activity and pharmacokinetic properties has al-
ready been demonstrated.5 Thus, we further developed
the SAR of this series with a variety of fused heterocy-
cles, which were expected to give more versatile position-
ing of nitrogen atoms around the rings. The effect of the
spacer length between the fused heterocycle and the
piperidine moiety on antiviral activity and pharmaco-
kinetic properties was also examined herein (Fig. 1).
Progress has already been made in identifying potent
small-molecule CCR5 antagonists for use as therapeutic
agents for the treatment of HIV-1.3 Several CCR5
antagonists are currently under investigation as poten-
tial drugs. Recent reports from these laboratories have
described 1,3,4-trisubstituted pyrrolidines containing
Keywords: CCR5; HIV-1; Antiviral; Fused heterocycles.
*
Corresponding author. Tel.: +1 732 594 4264; fax: +1 732 594 9545;
e-mail: dooseop_kim@merck.com
ꢀ Present address: Department R4N6, Abbot Laboratories, Abbot
Park, IL 60064, USA.
All of the pyrrolidine acid derived compounds in this
paper were prepared by coupling pyrrolidine aldehydes
through a reductive amination strategy with piperidines
as reported earlier.4 The synthesis of the pyrrolidine
ꢁ Present address: International AIDSVaccine Initiative, New York,
NY 10038, USA.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.030

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D. Kim et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2129–2134
Figure 1. 1,3,4-Trisubstituted pyrrolidine CCR5 antagonists.
aldehydes 15 has been reported previously.4e Benzimid-
azole was chosen as the first fused heterocycle for exami-
nation and the requisite piperidine moieties bearing
benzimidazoles with different spacer lengths were syn-
thesized as described in Scheme 1.
Scheme 1. Reagents and conditions: (a) 1,2-phenylenediamine,
NaBH(OAc)3, molecular sieves, CH2Cl2; (b) CH(OMe)3, concd HCl,
D; (c) 10% Pd/C, NH4þ HCOOꢀ, MeOH; (d) TMS–CHN2, LDA,
DIPEA; (e) (EtO)2POCH2CO2Et, NaHMDS, THF; (f) 10% Pd/C, H2
42 psi, EtOH; (g) LAH, THF; (h) Swern oxidation; (i) NaBH(OAc)3,
molecular sieves, CH2Cl2; (j) anisole, TFA.
Reductive amination of N-benzylpiperidone 3 with 1,2-
phenylenediamine followed by cyclization of 4 with
trimethylorthoformate and debenzylation afforded the
desired piperidine 5. Treatment of N-benzylpiperidone
3 with TMS–diazomethane gave aldehyde 6,6 which
was converted to ester 9 by the Horner–Emmons–Wittig
reaction. Hydrogenation of 9 followed by LAH reduc-
tion and subsequent Swern oxidation of the resulting
alcohol gave the two carbon extended aldehyde 10.
Swern oxidation of the commercially available alcohol
12 afforded aldehyde 13. Aldehydes 6, 10, and 13 were
converted to 8, 11, and 14, respectively, following the
same sequence as in the synthesis of 5.
Next, our synthetic efforts focused on other fused het-
erocycles, which were directly attached to the piperidine
moiety. The synthesis of the two sets of regioisomers,
imidazopyridines (56 and 57)7 and quinazolinones (60
and 61), are summarized in Scheme 2. Reductive amina-
tion of N-benzylpiperidone 3 with 1,2-diaminopyridine
afforded 17. Subsequent cyclization followed by debenz-
ylation afforded imidazopyridine 18. Displacement of
the chlorine of 19 with 20 followed by reduction and
the same sequence as above gave imidazopyridine 22,
the regioisomer of 18. For the synthesis of quinazo-
linone analogs, acid 23 was coupled with 20 to afford
24 and displacement of the fluorine of 26 with 20 gave
27. Cyclization of both 24 and 27 following the same se-
quence as above gave quinazolinones 25 and 28,
respectively.
Scheme 2. Reagents and conditions: (a) 3, NaBH(OAc)3, molecular
sieves, CH2Cl2; (b) CH(OMe)3, concd HCl, D; (c) 10% Pd/C,
NHþ4 HCOOꢀ, MeOH; (d) DIPEA, THF; (e) SnCl2, concd HCl; (f)
20, HOBT, EDC, CH2Cl2; (g) 20, Na2CO3, DMS O.
Scheme 3. Amide coupling of 29 with acid 30 followed
by cyclization in PPA at elevated temperature gave the
The synthesis of fused heterocyclic analogs containing a
nitrogen atom at the ring junction is described in

D. Kim et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2129–2134
2131
Scheme 3. Reagents and conditions: (a) HOBT, EDC, CH2Cl2; (b) PPA, 120 °C, 18 h; (c) 45% KOH, EtOH; (d) Ph3PCl2, Et3N, CH3CN; (e) TFA; (f)
Swern oxidation; (g) C6H5N(CH3)3BrÆBr2, THF, 0 °C, 45 min, 50%; (h) EtOH, reflux; (i) DCC, DMAP, NHMeOMe; (j) MeMgBr/ether; (k) LDA/
TMSCl, then NBS.
desired imidazopyridine 31.8 Hydrazinopyridine 33 was
coupled with acid 34, then cyclized to give triazolopyri-
dine 36, according to a literature procedure.9 While
bromoaldehyde 38 afforded the fused heterocycle 39
with a variety of amino-substituted heterocycles 43–46,
bromoketone 415b gave the regioisomer 42.10
pounds was evaluated in a 48 h single-cycle infection as-
say using HIV-1 (BAL) and utilizing HeLa Magi cells
expressing both CXCR4 and CCR5 as previously de-
scribed.12 The CCR5 affinity and antiviral data for these
compounds are presented in Tables 1–4. Table 1 shows
the results for the benzimidazole analogs incorporating
For the synthesis of compound 68, bromo-pyrazolopyr-
imidine 47, prepared according to a known procedure,11
was coupled with the vinyltin intermediate 48 to give 49,
which was reduced, then deprotected to give the target
50 as shown in Scheme 4.
Table 1. Effect of spacer length on CCR5 antagonist activity and
antiviral activity of benzimidazole analogs
Coupling of these piperidine intermediates (Schemes 1–
4) with aldehyde followed by deprotection as described
in Scheme 1 afforded the target compounds in Tables
1–4. Since previous work demonstrated that the cyclo-
hexyl and cyclobutyl methylene (R2) in structure 2 were
essentially equivalent, our new piperidines were evalu-
ated in either series.4e In addition, improved pharmaco-
kinetics and activity of some 3-fluorophenyl analogs
over phenyl analog (R3 = F vs R3 = H in structure 2)
led us to incorporate the 3-fluorophenyl group into the
compounds shown in Tables 3 and 4.4f,g,5b The synthe-
sized compounds were evaluated for CCR5 binding
affinity in an assay that tested their ability to displace
125I-MIP-1a from the human CCR5 receptor expressed
on CHO cell membranes.4a The IC90 of selected com-
Compd
n
CCR5a IC50 (nM)
HeLab IC90 (nM)
51
52
53
54
0
1
2
3
2.5
6.3
2.6
0.84
11/100
100/>300
100/>300
1.2/33
a The data are an average of three independent titrations having cal-
culated standard errors below 15%. The assay-to-assay variation was
generally 2-fold. See Ref. 4a for assay protocol.
b IC90 values obtained in the HeLa cell antiinfectivity assay versus
BAL in the absence and presence of 50% of human serum.14
Scheme 4. Reagents and conditions: (a) Pd(II)(Ph3P)2Cl2, 1,4-dioxane; (b) H2, 10% Pd/C, EtOH/EtOAc; (c) TFA, CH2Cl2.

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D. Kim et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2129–2134
Table 2. CCR5 antagonist activity and antiviral activity of fused
heterocyclic analogs
Table 3. CCR5 antagonist activity and antiviral activity of fused
heterocyclic analogs containing a nitrogen atom at the ring junction
Compd
R
CCR5a IC50 (nM)
(HeLab, IC90 (nM))
Compd
R
CCR5a IC50 (nM)
(HeLab, IC90 (nM))
62
13 (>300/>300)
0.42 (1.2/33)
51
2.5 (11/100)
63 (ND)c
63
64
55
12 (100/>300)
56
57
58
59
23 (ND)
65
66
67
1.0 (11/100)
3.5 (33/100)
3.0 (11/300)
2.8 (33/ND)
10 (ND)
8.2 (100/ND)
68
7.0 (>300/>300)
60
61
12 (ND)
69
70
71
65 (ND)c
100 (ND)
a See footnote a of Table 1.
b See footnote b of Table 1.
c ND = not determined.
29 (300/300)
200 (ND)
a See footnote a of Table 1.
b See footnote b of Table 1.
c ND = not determined.
various spacer lengths. Compound 54 (n = 3) showed
excellent CCR5 antagonist activity (IC50 = 0.84 nM) as
well as antiviral activity comparable to compound 1.
Interestingly, compound 51 without the spacer (n = 0)
was found to possess the second best CCR5 and antivi-
ral activity in this series, albeit less potent than com-
pound 54. Unfortunately, these two potent compounds
(51 and 54) showed negligible oral bioavailability
(<2%) in the rat.13
able to benzimidazole 51. Imidazopyridine 57 was ca.
8-fold more potent than its regioisomer 56. Incorpora-
tion of a carbonyl into benzimidazole 51 resulted in sub-
stantial loss in activity (60, 61).
More promising results were obtained with the fused
heterocycle series containing a nitrogen at the ring junc-
tion (Table 3). Imidazo[1,2-a]pyridine 63, the most po-
tent CCR5 binding compound in this series, also
showed an increase in the antiviral activity, compared
to benzimidazole 51 and was the same as 54 (which con-
tains the 3-carbon spacer). It was notable that the
Encouraged by the results for 51, fused heterocyclic sys-
tems directly attached to the piperidine moiety were
investigated to further improve the CCR5 activity and
PK profiles (Table 2). Among the seven new heterocyclic
analogs in Table 2, only imidazo[4,5-b]pyridine analog
57 showed CCR5 affinity and antiviral activity compar-

D. Kim et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2129–2134
2133
Table 4. CCR5 antagonist activity, antiviral activity and pharmacokinetic parameters of imidazo[1,2-a]pyridine analogs13
Compd
R
CCR5a IC50 (nM)
HeLab (BAL; IC90
)
AUC (Norm)po (lM h kg/mg)
Clp (mL/min/kg)
t1/2 (h)
%F
63
72
73
74
75
76
77
78
H
0.42
0.80
0.35
0.89
0.50
0.30
0.82
0.60
1.2/33
0.032
0.071
0.053
0.524
0.576
0.660
0.415
0.178
38
83
2.1
1.6
2.5
0.7
0.4
0.8
0.8
0.2
4
19
11
26
39
38
26
61
6-Ph
7-Et
11/300
0.13/11
1.2/100
33/>300
3.7/300
11/300
100/300
67
16
6-Cl
6,8-Di-Cl
6-F
20
19
6-CF3
6-NO2
18
120
a See footnote a of Table 1.
b See footnote b of Table 1.
imidazopyridine 63 was ca. 30-fold more potent than its
regioisomer 62. Incorporation of an additional nitrogen
atom at various positions around the imidazopyridine
ring resulted in a gradual loss in CCR5 activity (64–
67). Regioisomers derived from bromoketone 41 exhib-
ited decreased CCR5 activity compared to their corre-
sponding analogs (69–71), suggesting that the angle
between heterocycles and the piperidine ring plays an
important role in the CCR5 binding. Pyrazolopyrimi-
dine analog 68 showed disappointing antiviral activity
despite its CCR5 potency (IC50 = 7 nM). Although the
most potent imidazo[1,2-a]pyridine analog 63 lacked
oral bioavailability (%F = 4), attractive features exhib-
ited by the series, the extremely high CCR5 antagonist
activity and antiviral activity, led us to further develop
the SAR of this imidazo[1,2-a]pyridine series. As ex-
pected, most imidazo[1,2-a]pyridine analogs showed
subnanomolar CCR5 activity as well as excellent antivi-
ral activity (Table 4). Analogs with a phenyl-, ethyl-, or
nitro-substituent (72, 73, and 78) suffered from rapid
clearance, whereas analogs with halogens or a CF3
group (74–77) showed somewhat lower clearance in
the rat.12 These four analogs (74–77) showed reasonable
oral exposure and good oral bioavailability in the range
between 26% and 39%, albeit with relatively short half-
lives (<1 h). We had previously seen some off-target
activity (L-type Ca2+ ion channel activity being repre-
sentative) in pyrrolidine-based CCR5 antagonists.4e,5c
Selected compounds from the current series (63, 74,
and 77) were examined, with the results showing im-
proved selectivity (IC50 or Ki > 10 lM) against the L-
type Ca2+ channel over compound 1 (IC50 = 2.7 lM)
as well as some related pyrazolylpiperidine ana-
logs.4e,5b,c While compound 63 showed an IC50 of
2.7 lM in the hERG K+ assay, improved selectivity
was observed with trifluoromethyl substituted com-
pound 77 (IC50 > 10 lM).
antiviral activity comparable to 1. Although the rat
PK profiles of most of these compounds in these series
were not adequate, analogs 74–77 exhibited promising
PK profiles in the rat. Reports on further improvement
of the pharmacokinetic profiles of pyrrolidine-derived
CCR5 antagonists utilizing a different heterocyclic series
will follow in due course.
Acknowledgements
The authors would like to thank Drs. Paul E. Finke and
Charles G. Caldwell for helpful suggestions and
discussions.
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