Discovery of L-791,943: a potent, selective, non emetic and orally active phosphodiesterase-4 inhibitor.

Article abstract of DOI:10.1016/S0960-894X(02)00190-7

Structure-activity relationship studies directed toward improving the potency and metabolic stability of CDP-840 (3) resulted in the discovery of L-791,943 (11n) as a potent (HWB TNF-alpha = 0.67 microM) and orally active phosphodiesterase type 4 (PDE4) inhibitor. This compound, which bears a stable bis-difluoromethoxy catechol and a pendant hexafluorocarbinol, exhibited a long half-life in rat and in squirrel monkey. It is well tolerated in ferret with an emetic threshold greater than 30 mg/kg (po) and was found to be active in the ovalbumin-induced bronchoconstriction model in guinea pig and in the ascaris-induced bronchoconstriction models in sheep and squirrel monkey.

Full text of DOI:10.1016/S0960-894X(02)00190-7

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1457–1461
Discovery of L-791,943: A Potent, Selective, Non Emetic
and Orally Active Phosphodiesterase-4 Inhibitor
Daniel Guay,* Pierre Hamel, Marc Blouin, Christine Brideau, Chi Chung Chan,
Nathalie Chauret, Yves Ducharme, Zheng Huang, Mario Girard, Tom R.Jones,
France Laliberte, Paul Masson, Malia McAuliffe, Hanna Piechuta, Jose Silva,
Robert N.Young and Yves Girard
´
Merck Frosst Centre for Therapeutic Research, PO Box 1005, Pointe-Claire-Dorval, Quebec, Canada H9R 4P8
Received 20 December 2001; accepted 4 March 2002
Abstract—Structure–activity relationship studies directed toward improving the potency and metabolic stability of CDP-840 (3)
resulted in the discovery of L-791,943 (11n) as a potent (HWB TNF-a=0.67 mM) and orally active phosphodiesterase type 4
(PDE4) inhibitor.This compound, which bears a stable bis-difluoromethoxy catechol and a pendant hexafluorocarbinol, exhibited
a long half-life in rat and in squirrel monkey.It is well tolerated in ferret with an emetic threshold greater than 30 mg/kg (po) and
was found to be active in the ovalbumin-induced bronchoconstriction model in guinea pig and in the ascaris-induced bronchocon-
striction models in sheep and squirrel monkey. # 2002 Elsevier Science Ltd.All rights reserved.
The broad family of cyclic nucleotide phosphodiesterases
(PDE) represents a number of hydrolases responsible for
the degradation of the second messengers cAMP and
cGMP.1 Cyclic nucleotides are involved in a wide range
of biological responses.Strong evidence suggests that
cAMP plays a central role in regulating the function of
airway smooth muscle,2 inflammatory cells,3 and
immune cells and the cAMP specific PDE4 is the pre-
dominant isoenzyme found in pro-inflammatory cells
associated with a number of airway disorders.4 It is
believed that selective PDE4 inhibitors could be good
therapeutic agents for the treatment of asthma, chronic
obstructive pulmonary disease (COPD), and other
inflammatory conditions and a number of them have
already entered clinical trials.5 However, severe limiting
side effects precluded the development of the first gen-
eration inhibitors such as (R)-rolipram (1) and many
promising candidates have been discontinued.6 The
most advanced PDE4 inhibitor in clinical trials is Cilo-
milast (2, ArifloTM).This compound has a much
improved tolerability profile relative to 1 and is being
primarily developed for the oral treatment of COPD.7
When we began this program, CDP-840 (3) was one of
our lead structures.This compound has been reported to
significantly reduce antigen-induced bronchoconstric-
tion in animal models8 and in asthmatic patients.9
Although this compound is a potent PDE4 inhibitor, it
suffers from extensive metabolism in vitro10 and this
translates into a short half-life in vivo.In the rat, the
major metabolic pathway of CDP-840 is the para-hydro-
xylation on the pendant phenyl group whereas in human
hepatocytes, the major metabolite is the pyridinium
*Corresponding author.Te:l. +1-514-428-8652; fax: +1-514-428-
4900; e-mail: daniel_guay@merck.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd.All rights reserved.
PII: S0960-894X(02)00190-7

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D. Guay et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1457–1461
glucuronide, which is not detected in rat hepatocytes.
Other major sites of metabolism are centered on the
catechol portion of the molecule with hydroxylation,
dealkylation and glucuronide and sulfate conjugation.
We thus performed SAR studies on CDP-840.Our goal
was to modify the structure in order to not only increase
its potency but also to improve its metabolic stability
and pharmacokinetic profile.Herein, we report the
results of this study that lead to the identification of the
highly potent and metabolically stable PDE4 inhibitor
11n (L-791,943).
Many structural modifications of the catechol ring
including joining the two alkoxy groups in a cyclic
fashion and additional ring substitution (cf. 4b, 4c,
Table 1) led to a very significant loss of potency.Steric
constraints limit the size of the R group to one carbon
ethers.A major improvement came with the bis-
difluoromethoxy analogue 4e, this led to an increased
potency for the inhibition of LPS-induced TNF-a in
human whole blood (HWB) (Table 1).13 In addition,
these alkoxy groups were now very stable towards
metabolic degradation in vitro.14 It was also found that
in many cases, the pyridine N-oxide derivatives were
equipotent in our HWB assay (cf. 4e N-oxide).
Throughout this study, compounds have been synthe-
sized as racemates.Our initial synthetic efforts were
directed toward reducing the metabolism at the catechol
portion.It had been demonstrated that introducing a
substituent such as a chlorine atom on the pendant
phenyl group greatly reduces the metabolism of this
moiety10 so we performed modifications on the 4-
chlorophenyl analogue of CDP-840 (4a).The general
route to these compounds is described in Scheme 1.11
Sequential alkylation of an appropriately substituted
3,4-dihydroxybenzaldehyde 5 with alkyl halides in
DMF gave the corresponding derivative 7.Alkylation
using methyl chlorodifluoroacetate at 90 ꢀC provided
the corresponding difluoromethyl ethers.Treatment of
benzaldehydes 7 with 4-chlorophenyl magnesium bro-
mide gave the corresponding alcohols 8.Conversion to
the chloride and alkylation with the potassium enolate
of ethyl 4-pyridyl acetate in the presence of HMPA
produced a diastereomeric mixture of esters 10.Saponi-
fication followed by acidification led to spontaneous
decarboxylation to give derivatives 4.
Having discovered the metabolically stable catechol, we
then modified the substituents on the phenyl group to
further improve the potency.A few hundred analogues
have been synthesized in this series and in general, sub-
stitution in the para position was more tolerant with
respect to the overall activity.These analogues ( 11) were
prepared using a strategy similar to the one described in
Scheme 1 and involve the addition of substituted phenyl
lithium or Grignard reagents to 3,4-bis(difluoro-
methoxy)benzaldehyde 7e derived from 3,4-dihydroxy-
benzaldehyde.11
Table 1. SAR for R and R1 substituents on 4
Compd
X
R1
R2
GST-PDE4A248a HWB (TNF-a)b
IC50 (nM)
IC50 (mM)
3
—
—
6-F
2-F
—
Me c-Pentyl
Me c-Pentyl
Me c-Pentyl
Me c-Pentyl
CHF2 c-Pentyl
CHF2 CHF2
4.3
2.8
13
110
13
11
32
16
12
>25
98
8.7
4.5
3.8
n.d.
n.d.
4a
4b
4c
4d
4e
4e
4f
4g
—
(N-oxide) CHF2 CHF2
—
–CH2–
–CH2CH2–
>1000
772
Scheme 1. Reagents and conditions: (a) RX or ClF2COOMe, Cs2CO3,
DMF; (b) R1X, Cs2CO3, DMF, 54–80% (2 steps); (c) 4-Chlorophenyl
magnesium bromide, THF, À30 to 0 ꢀC, 80–95%; (d) SOCl2, Hunig’s
base, C6H6; (e) KHMDS, HMPA, ethyl 4-pyridylacetate, THF, À30
to 25 ꢀC; (f) NaOH, THF, EtOH, 60 ꢀC; HCl; (g) MMPP, CH2Cl2,
MeOH, 62–86% (4 steps).
aAssayed against human PDE4A isoform using a construct represent-
ing the common region of spliced variants expressed as GST-fusion
protein in Sf9 cells.12 IC50 represent a mean of n=3 or more.
bInhibition of LPS-induced TNF-a in human whole blood.13 IC50
represent a mean of n=3 or more.

D. Guay et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1457–1461
1459
Metallation of bromobenzene derivative 12 with n-butyl
lithium followed by treatment with benzaldehyde 7e
provided secondary alcohol 13 (Scheme 2).Alkylation
of the corresponding chloride with the potassium eno-
late of ethyl 4-pyridylacetate followed by ester hydro-
lysis and decarboxylation upon acidification gave
dihydroxyester 14.Final oxidation and saponification
afforded the analogue 11b.
Starting with silyl ether 15, a similar sequence of events
provided analogue 11h (Scheme 3).Oxidation of the
latter to the corresponding aldehyde allowed for the
preparation of benzoic acid analogue 11a, whereas
amide formation gave 11e.Ketone 11f was obtained
from the same aldehyde after treatment with the cerium
reagent derived from 5-methyl-2-bromopyridine and
MnO2 oxidation of the resulting secondary alcohol.
Finally, compound 11c was prepared starting from 4-
bromo-N-(tert-butyl)benzenesulfonamide using a similar
synthetic strategy.
A number of carbinol derivatives were synthesized
starting from 1,4-dibromobenzene 16 as shown in
Scheme 4.Metal halogen exchange followed by con-
densation with a ketone and protection of the resulting
alcohol afforded a number of bromophenyl fragments
17.Further elaboration as described above gave alcohol
derivatives 11k, 11m, and 11n.Also available from 16
was the bromophenyl intermediate 18.The latter was
subjected to tin–halogen exchange under palladium cat-
alysis to give the tributyltin derivative 19.Esterification
of 3-bromophenyl acetic acid followed by double alkyl-
ation with methyl iodide led to fragment 21.Stille type
coupling of bromide 21 with stannane 19 afforded the
corresponding biaryl derivative 11d after final depro-
Scheme 2. Reagents and conditions: (a) n-BuLi, THF, À78 ꢀC, then
7e, 50%; (b) SOCl2, pyridine, toluene, 0 ꢀC; KHMDS, HMPA, ethyl
4-pyridylacetate, THF, 0 ꢀC; (c) LiOH, THF, MeOH, 65 ꢀC; HCl,
60% (2 steps); (d) MMPP, CH2Cl2, MeOH; (e) NaOH, THF, EtOH.
Scheme 4. Reagents and conditions: (a) n-BuLi, THF, À78 ꢀC,
R1COR2; (b) SEMCl, CH2Cl2, i-Pr2NEt; (c) Mg or n-BuLi, THF, 7e;
(d) SOCl2, pyridine, toluene, 0 ꢀC; KHMDS, HMPA, ethyl 4-pyr-
idylacetate, THF, 0 ꢀC; (e) LiOH, THF, MeOH, 65 ꢀC; HCl; (f)
TBAF, THF or TFA, CH2Cl2, then NaOH, MeOH; (g) MMPP,
CH2Cl2, MeOH; (h) (Bu3Sn)2, Pd(PPh3)4, LiCl, dioxane 65 ꢀC; (i)
TMSCH2CH2OH, EDC, DMAP, CH2Cl2; (j) LDA, CH3I, THF,
À78–0 ꢀC; repeat; (k) PPh3, PdCl2(PPh3), dioxane, 65 ꢀC.
Scheme 3. Reagents and conditions: (a) n-BuLi, THF, À78 ꢀC, then
7e, 65%; (b) SOCl2, toluene, (i-Pr)2NEt, 60%; (c) KHMDS, HMPA,
ethyl 4-pyridylacetate, THF, 0 ꢀC; (d) LiOH, THF, MeOH, 65 ꢀC;
HCl, 76% (2 steps); (e) AcOH, 92%; (f) MMPP, CH2Cl2, MeOH,
93%; (g) MnO2, EtOAc, 87%; (h) NaClO2, NaH2PO4, isobutylene,
83%; (i) SOCl2, CH2Cl2; (j) Me2NH, (i-Pr)2NEt; (k) n-BuLi, 2-bromo-
5 methyl pyridine, THF, À78 ꢀC, CeCl3, 48%; (l) MnO2, 91%.

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D. Guay et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1457–1461
tection and pyridine oxidation.Mitsunobu reaction of
compound 11h with 4-fluorophenol afforded analogue
11g whereas treatment with ethyl isocyanate gave the
carbamate 11i.
aldehyde and acid derivatives.Attempts to improve on
the pharmacokinetic profile by ether (11g) or carbamate
(11i) formation were not very successful.A more
encouraging trend was noticed when the bulkiness and the
nature of the carbinol moiety were modified.Going from
secondary alcohol 11j to tertiary derivatives 11k and
11m increased the in vitro activity, maintained good
potency in HWB and led to significantly improved
pharmacokinetics for these analogues.For example, the
bioavailability in rat of the dimethyl tertiary alcohol
(11k) was 55% whereas that of spirocyclobutyl analo-
gue (11m) was 100%.Unfortunately, the half-lives for
these compounds remained short (i.e., in the order of 1
or 2 h).Finally, we found that blocking the metabolism
of the alcohol moiety by introduction of a bis-tri-
fluoromethyl carbinol resulted in 11n (L-791,943) a
compound with good in vitro activity and much longer
half-life (Table 3).
Summarized in Table 2 are the intrinsic PDE4A potency
and their cellular efficacy in HWB using the inhibition
of TNF-a production as an index.General trends con-
cerning the effect of substituents in the para position of
the pendant phenyl group can be observed.First, the
benzoic acid 11a and the phenyl acetic acid 11b both
exhibit poor activity on the enzyme when compared to
4a.Also, a very significant loss of potency for the inhi-
bition of LPS-induced TNF-a formation in HWB was
observed.This shift in the activity appeared to be linked
to the acid nature and the polarity of the substituent.As
the size of the group was increased, the intrinsic activity
also increased (11c–d).In addition, steric crowding
around the acid functionality (11d) did improve the
HWB potency.However, it was found that neutral
substituents with moderate polarity and lipophilicity
such as amide 11e and ketone 11f were even more
potent in HWB with IC50s around 1 mM.Most interest-
ingly, the simple hydroxymethyl derivative 11h was also
found to be potent with an IC50 of 0.5mM in HWB.
However, low plasma concentration was observed after
oral administration of this compound in rat and the par-
ent drug was rapidly metabolized to the corresponding
The extent of metabolism of L-791,943 (11n) was eval-
uated in vitro in rat hepatocytes and compared to the
data obtained with CDP-840 (3).In our standard incu-
bation conditions, >98% of the parent drug remained
in the case of L-791,943 whereas only 11% of CDP-840
was left intact.These results are in good agreement with
the respective half-lives (>24 h vs less than 30 min) of
these compounds following iv dosing in rats.
The pharmacological profile of L-791,943 (11n) was
evaluated in comparison to CPD-840 (3) and is sum-
marized in Table 3.One of our initial goals to improve
the metabolic stability of CDP-840 was met with
L-791,943 and this is reflected in the longer half-lives
measured in rat and in squirrel monkey.Moreover, an
increase in potency was also achieved with this com-
pound.L-791,943 was active in blocking the ovalbumin-
induced bronchoconstriction in conscious guinea pig by
58% at a dose of 1.0 mg/kg (ip 4 h pretreatment). It also
showed good in vivo activity in the anesthetized squirrel
monkey18 and in the conscious sheep19 models of
ascaris-induced bronchoconstriction.An oral dose of
3 mg/kg, 4 h before ascaris challenge, blocked the early
phase response in the monkey by 96%.In the sheep
model, 85% of the early phase and 95% of the late
phase response were inhibited following an iv dose of
2 mg/kg, 2 h pre-treatment.Finally, the potential for
causing emesis, one of the major drawbacks of typical
PDE4 inhibitors, was also assessed.Ferrets could be dosed
orally with up to 30 mg/kg of L-791,943 with plasma con-
centrations reaching 14 mM without causing emesis.
Table 2. SAR for R substituents on 11
11
R
GST-PDE4A248a HWB (TNF-a)b
IC50 (nM)
IC50 (mM)
11a
11b
11cc
–COOH
–CH2COOH
–SO2NHCO(o-Tol)
24
53
3.1
77
58
75
11d
2.2
6.5
11e
11f
–CONMe2
8.6
7.3
1.0
0.89
11g
0.5
1.9
11h
11i
11jd
11k
–CH2OH
44
4.6
45
0.47
0.41
0.51
0.36
–CH2OCONHEt
–CH(CH3)OH
–C(CH3)2OH
9.6
11le
16
1.7
11m
7.5
0.38
Table 3. In vivo profile of CDP-840 and L-791,943
11n (L-791,943) –C(CF3)2OH
3 (CDP-840)
4.2
4.3
38
0.67
16
18
CPD-840
L-791,943
ArifloTM
Rat t1/2
<0.5 h
<1 h
64%a (5 mg/kg)
>24 h
48 h
96%b (3 mg/kg)
Squirrel monkey t1/2
Efficacy in squirrel
monkey18 (dose)
Efficacy in sheep19
-early/late (dose iv)
Emesis in ferret
aAssayed against human PDE4A isoform using construct representing
the common region of spliced variants expressed as GST-fusion pro-
tein in Sf9 cells.12 IC50 represent a mean of n=3 or more.
bInhibition of LPS-induced TNF-a in human whole blood.13 IC50
represent a mean of n=3 or more.
—
85%/95% (2 mg/kg)
30 mg/kg
>3 mg/kg
cFor the synthesis of this analogue, see ref 15.
dFor the synthesis of this analogue see ref 16.
aCompound administered iv (30 min pretreatment).
bCompound administered po (4 h pretreatment).
eFor the synthesis of this analogue see ref 17.

D. Guay et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1457–1461
1461
In conclusion, the structure activity relationship study
performed in the CDP-840 series with the aim to reduce
metabolism has led to the discovery of L-791,943 (11n),
a highly potent and metabolically stable PDE4 inhi-
bitor.This was achieved by replacement of the alkoxy
groups of the catechol with bis-difluoromethoxy moi-
eties, substitution of the phenyl ring with a bis-tri-
fluoromethyl carbinol and oxidation of the pyridine to
the corresponding pyridine N-oxide.L-791,943 is potent
in vitro and in vivo as demonstrated by the inhibition of
antigen-induced bronchoconstriction in a variety of
models.The compound is well absorbed and is well
tolerated in the ferret.
R.B;. Compton, C.H. Pulm. Pharmacol. Ther. 1999, 12, 131.
(b) Compton, C. H.; Gubb, J.; Cedar, E.; Nieman, R. B.;
Amit, O.; Brambilla, C.; Ayres, J. Am. J. Respir. Crit. Care
Med. 1999, 159, A806.
8. Hughes, B.; Howat, D.; Lisle, H.; Holbrook, M.; James, T.;
Gozzard, N.; Blease, K.; Hughes, P.; Kingaby, R.; Warrellow,
G.; Alexander, R.; Eaton, M.; Perry, M.; Wales, M.; Smith,
B.; Owens, R.; Catterall, C.; Lumb, S.; Russel, A.; Allen, R.;
Merriman, M.; Bloxham, D.; Higgs, G. Br. J. Pharmacol.
1996, 118, 1192.
9. Harbinson, P. L.; MacLeod, D.; Hawksworth, R.; O’Toole,
S.; Sullivan, P. J.; Health, P.; Kilfeather, S.; Page, C. P.;
Costello, J.; Holgate, S. T.; Lee, T. H. Eur. Respir. J. 1997, 10,
1008.
10. Li, C.; Chauret, N.; Trimble, L. A.; Nicoll-Griffith, D. A.;
Silva, J. M.; Macdonald, D.; Perrier, H.; Yergey, J. A.; Par-
ton, T.; Alexander, R. P.; Warrellow, G. J. Drug Metab. Disp.
2001, 29, 232.
References and Notes
11. Guay, D.; Girard, Y.; Ducharme, Y.; Blouin, M.; Hamel,
P.; Girard, M. 1998, US 5,710,170. Chem. Abstr. 1998, 128,
140611.
12.Laliberte , F.; Han, Y.; Govindarajan, A.; Giroux, A.; Liu,
S.; Bobechko, B.; Lario, P.; Bartlett, A.; Gorseth, E.; Gresser,
M.; Huang, Z. Biochemistry 2000, 39, 6449.
1. (a) Yuasa, K.; Kanoh, Y.; Okumura, K.; Omori, K. Eur. J.
Biochem. 2001, 268, 168.(b) Soderling, S.H;. Beavo, J.A.
Curr. Opin. Cell. Biol. 2000, 12, 174.
2.Torphy, T.J. Agents Actions 1998, 23, S37.
3. (a) Kuehl, F. A.; Zanetti, M. E.; Soderman, D. D.; Miller,
D.K;. Ham, E.A.
Souness, J. E.; Aldous, D.; Sargent, C. Immunopharmacol.
2000, 47, 127.
4.(a) Torphy, T. J. Am. J. Respir. Crit. Care Med. 1998, 157,
351.(b) Barnes, P.J. Nature 1999, 402, B31.
5. (a) Huang, Z.; Ducharme, Y.; MacDonald, D.; Robichaud,
A. Curr. Opin. Chem. Biol. 2001, 5, 432.(b) Martin, T.J.
Idrugs 2001, 4, 312. (c) Nell, H.; Louw, C.; Leichtl, S.; Rath-
geb, F.; Neuhauser, M.; Bardin, P. G. Am. J. Respir. Crit.
Care Med. 2000, 161, A200. (d) Timmer, W.; Leclerc, V.; Bir-
raux, G.; Neuhauser, M.; Hatzelmann, A.; Bethke, T.; Wurst,
W. Am. J. Respir. Crit. Care Med. 2000, 161, A505.
Am. Rev. Respir. Dis. 1987, 136, 210.(b)
13. Brideau, C.; Van Staden, C.; Sthyler, A.; Rodger, I. W.;
Chan, C.-C. Br. J. Pharmacol. 1999, 126, 979.
14.For example, in our standard in vitro hepatocyte incu-
bations,10 compounds 4a and 4e were 45% and 15% metabo-
lized respectively.Only 4e N-oxide was formed with 4e.
15.Compound 11c was synthesized starting from 4-bromo-N-
(tert-butyl)benzenesulfonamide and 7e.Final cleavage of the
N-tert-butyl group was effected with TFA in CH2Cl2.Acyla-
tion with o-toluic acid was done using DMAP and EDC in
CH2Cl2.
16.Compound 11j was prepared from analogue 11h by oxi-
dation (MnO2) followed by Grignard addition (MeMgCl).
17.Synthesis of 11l began with cyclopropanation (LDA,
THF; SmI2, CH2I2) and protection (SEMCl, Hunig’s base) of
4- bromo acetophenone.Remaining steps as for 11k.
18. Jones, T. R.; McAuliffe, M.; McFarlane, C. S.; Piechuta,
H.; Macdonald, D.; Rodger, I. W. Can. J. Phys. Pharmacol.
1998, 76, 210.
6.(a) Horowski, R;. Sastre-y-hernandez, M.
Curr. Ther-
apeutic Res. 1985, 38, 23. (b) Robichaud, A.; Tattersall, F. D.;
Choudhury, I.; Rodger, I. W. Neuropharmacology 1999, 38,
289. (c) Robichaud, A.; Savoie, C.; Stamaciou, P. B.; Tatter-
sall, F.D;. Chan, C.C. Neuropharmacology 2001, 40, 262 and
465 (erratum).(d) Barnette, M.S. Prog. Drug Res. 1999, 53,
193.
19. Abraham, W. M.; Ahmed, A.; Cortes, A.; Sielczak, M. W.;
Hinz, W.; Bouska, J.; Lanni, C.; Bell, R. L. Eur. J. Pharmacol.
1992, 217, 119.
7. (a) Torhpy, T. J.; Barnette, M. S.; Underwood, D. C.;
Griswold, D. E.; Christensen, S. B.; Murdoch, R. D.; Nieman,