P1′ oxadiazole protease inhibitors with excellent activity against native and protease inhibitor-resistant HIV-1

Article abstract of DOI:10.1016/j.bmcl.2004.06.092

HIV-1 protease inhibitors bearing 1,3,4-oxadiazoles at the P1′ position are highly active against native and PI-resistant HIV-1. HIV-1 protease inhibitors (PI's) bearing 1,3,4-oxadiazoles at the P1′ position were prepared by a novel method involving the diastereoselective installation of a carboxylic acid and conversion to the P1′ heterocycle. The compounds are picomolar inhibitors of native HIV-1 protease, with most of the compounds maintaining excellent antiviral activity against a panel of PI-resistant strains.

Full text of DOI:10.1016/j.bmcl.2004.06.092

Bioorganic & Medicinal Chemistry Letters 14 (2004) 4651–4654
P10 oxadiazole protease inhibitors with excellent activity
against native and protease inhibitor-resistant HIV-1
a
a
b
Ronald M. Kim,a, Elizabeth A. Rouse, Kevin T. Chapman, William A. Schleif,
David B. Olsen,c Mark Stahlhut,c Carrie A. Rutkowski,c
Emilio A. Eminib and James R. Tataa
*
aDepartment of Basic Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
bDepartment of Virus and Cell Biology, Merck Research Laboratories, West Point, PA 19486, USA
cDepartment of Biological Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
Received 6 May 2004; revised 29 June 2004; accepted 30 June 2004
Available online 24 July 2004
Abstract—HIV-1 protease inhibitors (PIÕs) bearing 1,3,4-oxadiazoles at the P10 position were prepared by a novel method involving
the diastereoselective installation of a carboxylic acid and conversion to the P10 heterocycle. The compounds are picomolar inhib-
itors of native HIV-1 protease, with most of the compounds maintaining excellent antiviral activity against a panel of PI-resistant
strains.
Ó 2004 Elsevier Ltd. All rights reserved.
P1'
For nearly a decade, HIV-1 protease inhibitors (PIÕs)
have served as a cornerstone of multi-drug antiretroviral
P1
therapy.1 With increased frequency and duration of
P3
N
OH
treatment, however, the rate of resistance toward anti-
retroviral agents, including PIÕs, has risen alarmingly,
fueling the search for next-generation drugs with broad
efficacy against PI-resistant mutants.2 Previous reports
from our group have described the replacement of the
P3, P2, P10, and P30 subsites of indinavir (Fig. 1) to pro-
vide compounds with improved pharmacokinetic pro-
files and greatly increased potency against native HIV-
1 and a broad spectrum of clinically derived PI-resistant
strains.3 More recently, we have replaced the P10 phenyl
ring of indinavir and related analogs with pyridyl groups
to provide compounds with excellent potency against
wild-type (wt) and PI-resistant HIV-1.4 Transposition
of the pyridyl group from P3 to P10 was also shown to
improve inhibitory profiles against cytochrome P-450
isoforms. Continuing this line of exploration, we de-
scribe herein the preparation and in vitro activity of a
series of HIV protease inhibitors containing 1,3,4-oxa-
diazoles at the P10 position. The compounds were syn-
OH
H
N
O
N
N
O
HN
P3'
P2
Figure 1. Binding sites of indinavir (12).
thesized by
a
novel approach involving the
stereospecific introduction of a P10 carboxylic acid,
which can be used to install a variety of heterocycles
at that position. All of the compounds prepared are
picomolar inhibitors of native HIV-1 protease. A subset
of compounds was tested against enzyme obtained from
highly PI-resistant HIV-1 variant V-18, with most main-
taining picomolar activity. The oxadiazoles were also
screened for their ability to inhibit the viral spread of
native HIV-1 and several clinically isolated PI-resistant
strains5 in MT4 human T-lymphoid cells,6 with most
of the compounds displaying nanomolar activity (IC95)
against the entire panel.
Keywords: Protease inhibitor; HIV.
*
Corresponding author. Tel.: +1-7325944847; fax: +1-7325949464;
e-mail: ron_kim@merck.com
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.06.092

4652
R. M. Kim et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4651–4654
their polarity, flexibility toward diversification, and to
Ar
O
further explore the transposition of a heterocyclic nitro-
gen from P3 to P10. The arylheterocycle P3 substituents
were developed previously in our group and have been
shown to significantly improve the activity of indinavir
analogs against native and PI-resistant HIV-1.3c,4a
O
allyl-Br
Ar
N
N
X
LHMDS
X
O
O
1
2
Scheme 1.
Synthesis of representative 1,3,4-oxadiazole 18 is de-
picted in Scheme 3. Carboxylic acid 7 was converted
to hydrazide 8 using typical peptide coupling conditions.
Due to the presence of the acetonide, heterocycle forma-
tion under nonacidic conditions was required. To that
end, reaction of the hydrazide with the appropriate tri-
methoxy or triethoxy orthoester8 afforded the oxadiaz-
ole 9 in good yield. The final product was assembled
using established procedures,7a,3d involving stereoselec-
tive conversion of the allyl group to the epoxide 10 via
the iodohydrin, followed by reaction with the appropri-
ate piperazine to provide aminoalcohol 11. Removal of
the acetonide using TFA in methylene chloride provided
protease inhibitor 18.9 All products were isolated by re-
verse-phase LC and characterized by 1H NMR and LC–
MS (ESI).
A key step in the synthesis of indinavir and related ana-
logs is the diastereoselective allylation of a chiral phenyl-
propionamide 1, which takes place alpha to the carbonyl
group to provide intermediate 2 (Scheme 1).7 We have
found with some P10 heterocycle analogs, however, that
allylation proceeds alpha to the heterocycle, giving rise
to the undesired regioisomer. To circumvent this prob-
lem, we settled on the strategy shown in Scheme 2,
involving diastereoselective allylation of TBS-protected
hydroxybutyramide 5, followed by TBS deprotection
to afford alcohol 6 and oxidation to the carboxylic acid
7, which can be utilized to generate a variety of P10 het-
erocycles. We chose to prepare 1,3,4-oxadiazoles due to
O
OH
O
OH
O
O
OMe
O
b
H
Cl
H2N
N
N
MeO
MeO
O
O
O
O
O
O
a
c, d
4 (73 %)
3 (89 %)
O
O
HO
O
O
Br
N
N
N
TBSO
HO
O
O
O
O
O
O
e, f
g
6 (90 %)
5 (63 %)
7 (82 %)
Scheme 2. Reagents and conditions: (a) Et3N, CH2Cl2, 0°C; (b) TsOH, CH2Cl2, 16h; (c) NaBH4, MeOH, 0°C; (d) TBS–Cl, imidazole, CH2Cl2;
(e) LHMDS, THF, ꢀ20 to 0°C; (f) TBAF, THF; (g) PDC, DMF, 4h.
Me
N
O
N
O
MeO
MeO
MeO
N
O
Me
H2N
O
N
N
O
O
O
H
N
N
N
7
b
a
c, d
O
O
O
O
O
O
8 (94 %)
9 (86 %)
10 (67 %)
Me
N
NH
NH
O
N
O
N
N
O
CF3
O
Cl
N
OH
O
Cl
18 (17 % from 10)
N
N
NH
CF3
N
f
e
O
O
O
11
Scheme 3. Reagents and conditions: (a) hydrazine, EDC, HOBt, CH2Cl2/DMF; (b) neat, 115°C, 24h; (c) NIS, NaHCO3, EtOAc/H2O, 16h;
(d) NaOMe/MeOH, EtOAc, 15min; (e) t-amyl alcohol, 75°C, 16h; (f) 10% TFA/CH2Cl2, 15min.

R. M. Kim et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4651–4654
Table 1. Enzyme inhibition and intiviral activity of HIV protease inhibitors
4653
N
N
R2
OH
O
R1
N
O
OH
H
N
N
O
O
NH
CF3
HIV-1 protease IC50 (nM)
Entry
R1
R2
Viral spread CIC95 (nM)a
V-18 4X K-60
400
NL4-3
0.60
V-18
61
NL4-3
50
Q-60
12
Indinavir
>1000
>1000
>1000
13
14
15
16
H
0.024
0.027
0.016
0.020
0.065
0.19
0.19
125
125
16
125
63
31
31
16
16
125
125
16
31
31
Me
Pr
O
63
63
68
16
N
N
Ph
68
68
F
17
18
19
20
H
<0.015
0.016
0.043
0.060
0.12
125
250
68
68
63
63
31
16
63
31
31
68
68
16
Me
Pr
O
<0.015
<0.015
63
125
68
16
16
68
Ph
0.18
Cl
21
22
23
24
H
0.015
<0.015
<0.015
0.019
0.099
63
63
125
63
31
31
63
31
31
31
16
63
Me
Pr
O
O
68
15.6
125
250
31
125
68
68
Ph
Cl
25
26
27
28
H
0.24
0.40
0.32
0.48
7.4
15
Me
Pr
>1000
500
125
>1000
>1000
>1000
>1000
500
250
>1000
500
250
>1000
500
500
MeO
N
Ph
N
a The < and > values denote the lower and upper concentrations tested in our assays.
The oxadiazoles were evaluated for their ability to inhi-
bit native HIV-1 protease (NL4-3). As shown in Table 1,
all of the compounds prepared inhibit the enzyme with
picomolar activity (IC50) and are more potent than indi-
navir. Five compounds (17, 19, 20, 22, and 23) exhibit
activities below the limit of the assay (IC50 <15pM).
Increasing polarity at the P3 position results in de-
creased activity, as seen somewhat in fluorophenyl oxad-
iazoles 13–16, and more so in methoxypyridyl
derivatives 25–28. Excepting the limit of the assay, the
oxadiazole substituents do not strongly influence po-
tency, with all the oxadiazoles within a given series being
of similar activity.
in activity against V-18 protease compared to native en-
zyme, compared to a 100-fold loss by indinavir. P3
methoxypyridyl derivatives 25 and 26 lose most activity
against V-18, being 30-fold less active than against na-
tive protease.
The compounds were evaluated for their ability to inhi-
bit the spread of native HIV-1 and clinically obtained
PI-resistant strains5 in MT4 human T-lymphoid cells
in culture.6,10 Listed in Table 1 are the compound con-
centrations required to inhibit infectious viral spread
by 95% relative to untreated virus control culture
(CIC95) in the presence of 10% fetal bovine serum. De-
spite their similar protease inhibitory activities, oxadiaz-
oles with a given P3 substituent differ greatly in their
ability to block the spread of wt virus, with propyl
and phenyl oxadiazoles being significantly more potent
than their 5-Me and 5-H counterparts. This difference
may reflect greater cell penetration of the more hydro-
phobic oxadiazoles, as has been reported previously
with indinavir analogs,11 though this possibility has
yet to be examined.
A subset of compounds was also tested against HIV-1
protease obtained from highly PI-resistant clinical iso-
late V-18. In all cases, the oxadiazoles are less potent
against the mutant protease than against native enzyme,
though compounds with a terminal phenyl group at P3
maintain excellent activity (IC50 < 0.2nM). Interestingly,
the oxadiazole substituents more strongly modulate
activity against the mutant protease, with the more po-
lar 5-H oxadiazoles 13 and 17 being better inhibitors
than their corresponding propyl and phenyl derivatives.
In addition, the 5-H and 5-Me oxadiazoles (with the
exception of 25 and 26) show only a 3–10-fold decrease
In contrast to indinavir, which is ineffective at blocking
the spread of PI-resistant strains, all of the oxadiazoles
tested, with the exception of P3 methoxypyridyl analogs

4654
R. M. Kim et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4651–4654
C.; Jin, L.; Lin, J. H.; Emini, E. A.; Tata, J. R. Bioorg.
Med. Chem. Lett. 2003, 13, 2573; (d) Duffy, J. L.; Kevin,
N. J.; Kirk, B. A.; Chapman, K. T.; Schleif, W. A.; Olsen,
D. B.; Stahlhut, M.; Rutkowski, C. A.; Kuo, L. C.; Jin, L.;
Lin, J. H.; Emini, E. A.; Tata, J. R. Bioorg. Med. Chem.
Lett. 2002, 12, 2423; (e) Duffy, J. L.; Rano, T. A.; Kevin,
N. J.; Chapman, K. T.; Schleif, W. A.; Olsen, D. B.;
Stahlhut, M.; Rutkowski, C. A.; Kuo, L. C.; Jin, L.; Lin,
J.; Emini, E. A.; Tata, J. R. Bioorg. Med. Chem. Lett.
2003, 13, 2569.
26–28, maintain nanomolar activity against the entire
panel of resistant variants. As with native HIV-1, pro-
pyl, and phenyl oxadiazoles are generally more active
against PI-resistant strains than the corresponding Me
and H analogs, though the trend is much less pro-
nounced than is seen against wt virus. The decreased
sensitivity of variant V-18 toward the oxadiazole substit-
uents may in part reflect a balance between greater pro-
tease activity of the less hydrophobic oxadiazoles and
superior cell penetration of the more hydrophobic oxadi-
azoles. Significantly, in all but one case, 5-H oxadiazoles
13, 17, and 21 and 5-methyl oxadiazoles 14, 18, and 22
are at least as effective at inhibiting the spread of all
the PI-resistant strains tested as blocking the spread of
native virus. Such is the case even with variant V-18,
despite greater intrinsic activity of the compounds
against native HIV-1 protease compared to V-18-de-
rived enzyme.
4. (a) Duffy, J. L.; Kirk, B. A.; Kevin, N. J.; Chapman, K. T.;
Schleif, W. A.; Olsen, D. B.; Stahlhut, M.; Rutkowski, C.
A.; Kuo, L. C.; Jin, L.; Lin, J. H.; Emini, E. A.; Tata, J. R.
Bioorg. Med. Chem. Lett. 2003, 13, 3323; (b) Kevin, N. J.;
Duffy, J. L.; Kirk, B. A.; Chapman, K. T.; Schleif, W. A.;
Olsen, D. B.; Stahlhut, M.; Rutkowski, C. A.; Kuo, L. C.;
Jin, L.; Lin, J. H.; Emini, E. A.; Tata, J. R. Bioorg. Med.
Chem. Lett. 2003, 13, 4027.
5. For the genotypes of the viral isolates and their suscep-
tibility to protease inhibitors, see: (a) Olsen, D. B.;
Stahlhut, M. W.; Rutkowski, C. A.; Schock, H. B.;
vanOlden, A. L.; Kuo, L. C. J. Biol. Chem. 1999, 274,
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D. J.; Laird, D.; Quintero, J. C.; Rhodes, A.; Robbins, H.
L.; Roth, E.; Shivaprakash, M.; Yang, T.; Chodakewitz, J.
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McDaniel, S. L.; Darke, P. L.; Zugay, J.; Quintero, J. C.;
Bluhy, O. M.; Roth, E.; Sardana, V. V.; Schlabach, A. J.;
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The P10 oxadiazoles described herein generally show
excellent activity against both native and PI-resistant
HIV-1. Markedly different trends are observed, how-
ever, against wild-type and mutant strains. While oxadi-
azoles with the same P3 substituent show similar
activities against native HIV-1 protease, their ability to
block the spread of wt virus in the cell-based assay vary
greatly. Conversely, oxadiazoles within a series display a
wider range of activity against mutant V-18 protease
than against native enzyme, though differences in their
ability to block the spread of PI-resistant strains are gen-
erally less pronounced than is seen against wt virus.
Notably, compounds were prepared, which are more
effective at blocking the spread of a panel of PI-resistant
HIV-1 variants and native HIV-1. The differing
responses of native and PI-resistant HIV-1 toward the
oxadiazoles implicate the potential importance of the
P10 position for developing next-generation PIÕs of high
potency and wide spectrum.
8. Ainsworth, C. J. Am. Chem. Soc. 1955, 77, 1148.
9. Data for compound 18: 1H NMR (500MHz, CDCl3): d
9.33 (t, J=6.4Hz, 1H); 7.55 (m, 2H); 7.42 (m, 2H); 7.28 (s,
1H); 7.13 (d, J=8.0Hz, 1H); 7.10 (t, J=7.8Hz, 1H); 6.83
(d, J=8.2Hz, 1H); 6.77 (m, 1H); 5.25 (dd, J=8.2, 4.1Hz,
1H); 4.26 (dd, J=7.7, 4.6Hz, 1H); 4.20 (m, 1H); 4.14 (d,
J=11.7Hz); 3.82 (m, 1H); 3.72 (m, 1H); 3.35 (s, 1H); 3.16–
3.23 (m, 2H); 3.05 (d, J=11.7Hz, 1H); 2.92 (m, 1H); 2.81
(d, J=4.1Hz, 1H); 2.62–2.74 (m, 4H); 2.46 (d, J=3.2Hz,
1H); 2.42 (s, 3H); 1.87 (t, J=11.3Hz, 1H); 1.73 (m, 1H);
1.62 (s, 3H); 1.61 (s, 3H); 1.47 (m, 1H). MS (ESI) 812.5
(M+23); 790.5 (M+1).
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1996, 271, 31957.
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Lin, J. H.; Chen, I.-W.; Holloway, M. K.; Anderson, P. S.;
Huff, J. R. Bioorg. Med. Chem. Lett. 1994, 4, 2769.
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