The design of 8,8-dimethyl[1,6]naphthyridines as potential anticonvulsant agents

Article abstract of DOI:10.1016/S0960-894X(03)00288-9

Starting from a series of 7-linked tetrahydroisoquinoline derivatives, as exemplified by SB-270664, a new series of 8,8-dimethylnaphthyridine compounds has been identified. SAR studies around these attractive leads have provided compounds such as 12 which display excellent anticonvulsant activity and an encouraging pharmacokinetic profile in vivo.

Full text of DOI:10.1016/S0960-894X(03)00288-9

Bioorganic & Medicinal Chemistry Letters 13 (2003) 1627–1629
The Design of 8,8-Dimethyl[1,6]naphthyridines as Potential
Anticonvulsant Agents
Nigel E. Austin, Michael S. Hadley, John D. Harling, Frank P. Harrington,
Gregor J. Macdonald, Darren J. Mitchell, Graham J. Riley, Tania O. Stean,
Geoffrey Stemp, Sharon C. Stratton, Mervyn Thompson* and Neil Upton
Neurology & GI Centre of Excellence for Drug Discovery, GlaxoSmithKline Research & Development Limited,
New Frontiers Science Park, ThirdAvenue, Harlow, Essex CM19 5AW, UK
Received 5 February 2003; revised 18 March 2003; accepted 19 March 2003
Abstract—Starting from a series of 7-linked tetrahydroisoquinoline derivatives, as exemplified by SB-270664, a new series of
8,8-dimethylnaphthyridine compounds has been identified. SAR studies around these attractive leads have provided compounds
such as 12 which display excellent anticonvulsant activity and an encouraging pharmacokinetic profile in vivo.
# 2003 Elsevier Science Ltd. All rights reserved.
SAR Studies and Discussion
Epilepsy is one of the most prevalent neurological con-
ditions worldwide. Pharmacological therapy remains
the cornerstone of epilepsy treatment, however, refrac-
tory epilepsy (in 20–30% of sufferers) is still a significant
clinical problem despite the discovery of the second
generation of anticonvulsant agents.1,2 Anticonvulsant
treatment failures may result from lack of efficacy and
the occurrence of significant undesired side-effects. The
newer generation of anticonvulsants has taken into
account the shortcomings of existing therapies and
attempted to improve on the currently available treat-
ments using rational drug design.3 One such compound
with a novel mechanism of action, the benzopyran car-
abersat (SB-204269), is currently being progressed for
the treatment of epilepsy and migraine prophylaxis.4
In an earlier paper, the 7-substituted tetra-
hydroisoquinoline (THIQ) SB-270664 was reported as a
promising key lead compound.5 SB-270664 has high
affinity at the [3H]-SB-204269 binding site (pKi 8.9) but
was later found to have high in vivo clearance and poor
oral bioavailability (Fpo<2%) in the rat. In-vitro
metabolism studies in rat liver microsomes and hepato-
cytes indicated that the major routes of metabolism were
hydroxylation of the THIQ benzo ring, N-demethylation
and aromatisation to the isoquinolinium species.6
Consequently, the aim for medicinal chemistry was to
address these issues and improve the overall in vivo
profile and properties of SB-270664. Several cycles of
*Corresponding author. Tel.:+44-1279-622-097; fax:+44-1279-627-
896; e-mail: mervyn_thompson@gsk.com
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00288-9

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N. E. Austin et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1627–1629
target design and SAR analysis provided a series of
8,8-dimethyl[1,6]naphthyridines (1) for further explora-
tion. In this series the gem dimethyl group is incorpo-
rated to prevent aromatisation and replacement of the
THIQ benzo ring by pyridyl represents an attempt to
reduce hydroxylation. This paper describes the proper-
ties and potential of this class of compounds.
although increasing the steric bulk of this group was
shown to have little effect on activity (cf. 5, 7 and 8).
From the data obtained on the N-H derivatives (9–14),
it was apparent that this change had little effect on affi-
nity and the SAR followed the existing pattern.
When examined in vivo in the rat MESTtest at 2 mg/kg
po, compounds 4, 5, 7, 8 and 10–12 showed a good level
of anticonvulsant activity.
Analysis of SAR around the THIQs indicated that the
optimal substitution on the benzamide ring was an
electron withdrawing group at the 3-position and a
bulky lipophilic group at the 4-position. Using this
information a limited number of compounds was pre-
pared with the 8,8-dimethylnaphthyridine template. In
addition to the parent N-Me analogues, the corres-
ponding N-H compounds were also prepared to address
the likely dealkylation previously observed in vivo in the
THIQ series.
From this investigation, a number of compounds was
selected for in vitro and in vivo pharmacokinetic (PK)
profiling. The N-H compounds 10–12 all showed good
oral bioavailability in the rat when dosed at 2 mg/kg
(Fpo>30%). The 3-CF3, 4-OMe analogue 12 had
excellent affinity (pKi 8.7) and exhibited the optimal in
vitro profile. Compound 12 was >100-fold selective
across a range of 5-HTand adrenergic receptors. In
addition 12 exhibited a clean human cytochrome P450
profile and had low intrinsic clearance in liver micro-
somes from rodents, dog and man.10 Following iv/po
administration to the rat, 12 showed low clearance and
high oral bioavailability (CLb 30 mL/min/kg,
Fpo>50%) and was progressed to the rat supraMES
anticonvulsant model.8
Compounds were screened in the [3H]-SB-204269 bind-
ing assay (Table 1). From primary data, it was apparent
that the SAR paralleled that seen in the THIQ series.
The nature of the electron withdrawing group at the
3-position (R1) was crucial for high affinity. The chloro
and bromo analogues, 4 and 5, respectively, showed
equivalent affinity and were around 10-fold more active
than the fluoro derivative, 3. The cyano analogue 6
showed reduced affinity compared with the chloro and
bromo derivatives but the trifluoromethyl 7 was well
tolerated. Lipophilic groups at the 4-position (R1) were
also shown to be a key requirement for high affinity,
In contrast, the corresponding N-Me derivative 7
showed only moderate oral bioavailability in the rat
(Fpo 20%) but had high in vivo clearance (CLb 82
L/min/kg), presumably due to N-demethylation.
Further PK work on 7 and 12 revealed interesting find-
ings. From a rat CNS penetration study, compound 12
was found to have a brain:blood ratio of 0.3:1, while the
corresponding N-Me analogue 7 exhibited a much
higher level of CNS penetration (brain:blood 1.9:1).
This appears to be reflected in the superior performance
of 7 in the rat MESTtest (see Table 1).
Table 1. Biological data for 8,8 dimethylnaphthyridinyl benzamides
Compound 12 was dosed orally at 1, 10 and 30 mg/kg in
the rat supraMES model. Each dose was assessed for
the ability to protect the rat from hindlimb tonic sei-
zures following a single shock (180 mA of 0.3 s duration)
delivered via corneal electrodes of the HSE shocker, 4 h
post dosing. The response to the shock was measured
and compared to control. Analysis by ‘ALLFITrd-
quo;11 produced an ED50 value of 3.9 mg/kg. This figure
is comparable to that obtained with carabersat in the
same model under identical conditions (ED50 of 6.3
mg/kg).
Compd
R1
R2 [3H]SB-204269 Rat MESTb % increase
bindinga
pKi
in seizure threshold
at 2 mg/kg p.o.
2 h post-dose
Carabersatc
2
3
4
5
6
7
8
7.3
6.2
7.3
8.5
8.6
7.3
8.8
8.5
7.2
8.1
8.7
8.7
8.6
8.4
120**
3ns
Nd
380***
210***
110**
570***
310***
Nd
260**
260**
260***
80**
H
Me
3-F, 4-OMe Me
3-Cl, 4-OMe Me
3-Br, 4-OMe Me
3-CN, 4-OMe Me
3-CF3, 4-OMe Me
3-CF3, 4-OEt Me
3-F, 4-OMe
3-Cl, 4-OMe
3-Br, 4-OMe
3-CF3, 4-OMe
3-CF3, 4-OEt
3-CF3, 4-OiPr
9
H
H
H
H
H
H
10
11
12
13
14
Synthesis
Compounds12 were prepared according to Schemes 1
and 2. The synthesis of the 3-amino-8,8-dimethyl-N-
methyl[1,6]naphthyridine template 15 has previously
been reported, albeit in low yield.13 Compounds 2–8
were prepared in high yield from 15 via coupling with
the requisite carbonyl chloride. The corresponding des-
methyl analogues were also accessible by this route via
demethylation with 1-chloroethyl chloroformate (ACE-
Cl)14 but this step proceeded in variable yields and
15*
ns: not significant. Nd: not determined.
aProcedures as detailed in ref 7; all determinations were carried out in
triplicate, sem < Æ0.05.
bProcedures as detailed in ref 8; * p<0.05, **p<0.01, *** p<0.001,
compared to vehicle-treated controls according to analysis following
Lichfield and Wilcoxon in ref 9; doses refer to free base.
cDosed at 2.5 mg/kg po.

N. E. Austin et al. / Bioorg. Med. Chem. Lett. 13 (2003) 1627–1629
1629
Scheme 1. Synthesis of compounds 2–8. Reagents and conditions: (i) ArCOCl, Et3N, CH2Cl2, 6 h, 25 ꢀC (70–94%); (ii) ACE-Cl, CH2Cl2, 18 h,
50 ꢀC, then MeOH, 1–24 h (36–97%).
Scheme 2. Synthesis of compounds 9 to 14. Reagents and conditions: (i) HCHO, BnNH2, EtOH, c.HCl; (ii) 10% Pd/C, H2, (Boc) 2O EtOH; (iii) 3,5-
dinitro-2-pyridone, NH3, MeOH; (iv) 10% Pd/C, H2, EtOH, 4 h (25% from 17); (v) ArCOCl, Et3N, CH2Cl2, 6 h, 25 ꢀC (75–89%); (vi) TFA, CH2Cl2
18 h, 25 ꢀC (89–99%).
2. Upton, N. Trends Pharmacol. Sci. 1994, 15, 456.
3. Hovinga, C. A. Exp. Opin. Invest. Drugs 2002, 11, 1387.
4. Upton, N.; Thompson, M. In Progress in Medicinal Chem-
istry, Vol. 37; King, F. D., Oxford, A. W, Eds.; Elsevier
Science: Oxford, 2000; p 177.
5. Chan, W. N.; Hadley, M. S.; Harling, J. D.; Herdon, H. J.;
Orlek, B. S.; Riley, G. J.; Stead, R. E. A.; Stean, T. O.;
Thompson, M.; Upton, N.; Ward, R. W. Bioorg. Med. Chem.
2000, 8, 2085.
6. Austin, N. E., Jeffrey, P. Unpublished observations.
7. (a) Preliminary results are in: Herdon, H.; Jerman, J.;
Stean, T. O.; Chan, W.; Middlemiss, D. N.; Upton, N. Eur. J.
Pharmacol. 1996, 314, R7. (b) Herdon, H. J.; Jerman, J. C.;
Stean, T. O.; Middlemiss, D. N.; Chan, W. N.; Vong, A. K.;
Evans, J. M.; Thompson, M.; Upton, N. Brit. J. Pharmacol.
1997, 121, 1687.
reproducibility. As it became apparent that the N-H
analogues showed superior pharmacokinetic parameters
to the N-Me derivatives, the synthesis of larger quan-
tities of key compounds became a requirement. The
unpredictable and toxic nature of the demethylation
step meant that an alternative synthetic strategy was
required. Route optimisation work produced an alter-
native intermediate 16 which was prepared via 18 and 3,5-
dinitro-2-pyridone15 (Scheme 2). The overall yield to 9–14
was high (20%) compared with using the original pub-
lished route to such benzamides (<4% over nine steps).
Conclusions
8. Upton, N.; Blackburn, T. P.; Campbell, C. A.; Cooper, D.;
Evans, M. L.; Herdon, H. J.; King, P. D.; Ray, A. M.; Stean,
T. O.; Chan, W. N.; Evans, J. M.; Thompson, M. Brit. J.
Pharmacol. 1997, 121, 1679.
9. Litchfield, J. T.; Wilcoxon, F. J. Pharmacol. Exp. Ther.
1949, 96, 99.
10. For 12, human CYP’s all IC50>35 mM at 1A2, 2C9, 2C19,
2D6 and 3A4). Intrinsic clearance rates in microsomes were
mouse <0.5, rat 1.1, dog <0.5 and human <0.5 mL/min/g liver.
11. De Lean, A.; Munson, P. J.; Rodbard, D. Am. J. Physiol.
235, E97. 1978.
A series of novel 8,8-dimethyl[1,6]naphthyridines has
been prepared with high affinity in the [3H]-SB-204269
binding assay. Compound 1216 has excellent aqueous
solubility (>1 mg/mL) and has been shown to have an
encouraging pharmacokinetic profile and good in vivo
activity in preclinical anticonvulsant models in the rat.
A modified synthesis of the naphthyridine core has also
been developed allowing this particular series to be
amenable to high throughput chemistry and scale-up.
12. All data reported herein reflect characterized samples.
Compounds 2–14 were purified by crystallization as mono-
hydrochlorides. All compounds were >95% purity as mea-
sured using a VG micromass OpenLynx LCMS system and a
Bruker AC 250MHz 1H NMR.
This group of investigational anticonvulsants, from a
new structural class, possesses an improvement in drug
chemical structure, improved pharmacokinetic proper-
ties and as such represents an exciting potential for the
treatment of epilepsy and related disorders.
13. Harling, J. D.; Harrington, F. P.; Thompson, M. Synth.
Commun. 2001, 31, 787.
14. Olofson, R. A.; Martz, J. T.; Senet, J. P.; Piteau, M.;
Malfroot, M. J. Org. Chem. 1984, 49, 2081.
15. Takada, S.; Sasatani, T.; Chomei, N.; Adachi, M.;
Fujishita, T.; Eigyo, M.; Murata, S.; Kawasaki, K.; Matsush-
ita. J. Med. Chem. 1996, 39, 2844.
16. Spectroscopic data for 12. N-(8,8-Dimethyl-5,6,7,8-tetra-
hydro[1,6]naphthyridin-3-yl)-4-methoxy-3-trifluoromethyl-
benzamide, hydrochloride. 1H NMR (250 MHz; CD3OD) [free
base] d:1.37 (6H, s), 3.35 (2H, s), 4.35 (2H, s), 7.25 (1H, d,
J=9 Hz), 8.07 (1H, d, J=2 Hz), 8.15–8.18 (2H, m), 8.70 (1H,
d, J=2 Hz); m/z (API)+: 378.1 (MH+; 100%); aqueous
solubility >1 mg/mL.
Acknowledgements
The authors would like to thank Ms. G. Espasa-Bosch
and Mr. S. P Wright for technical assistance.
References and Notes
1. (a) Nicolson, A.; Leach, J. P. CNS Drugs 2001, 15, 955.
(b) Chadwick, D. Arch Neurol. 1998, 55, 1140.