Imidazol-1-ylethylindazole voltage-gated sodium channel ligands are neuroprotective during optic neuritis in a mouse model of multiple sclerosis.

A series of imidazol-1-ylethylindazole sodium channel ligands were developed and optimized for sodium channel inhibition and in vitro neuroprotective activity. The molecules exhibited displacement of a radiolabeled sodium channel ligand and selectivity for blockade of the inactivated state of cloned neuronal Nav channels. Metabolically stable analogue 6 was able to protect retinal ganglion cells during optic neuritis in a mouse model of multiple sclerosis.

Introduction

Voltage-gated sodium (Nav) channels are the fundamental generators of electrical impulses (action potentials) in the membranes of excitable cells. A number of clinically used drugs exert their therapeutic effects through Nav. These drugs are primarily used in the treatment of central nervous system (CNS) disorders, such as epilepsy and neuropathic pain, but are increasingly being scrutinized as potential treatments for neurodegenerative conditions occurring acutely, such as in ischemic stroke, or chronically, as in multiple sclerosis (MS).[1] Phenytoin 1 was one of the first antiepileptics developed (Figure 1). It exerts a voltage-dependent and frequency-dependent block of Na+ currents. This selectivity is suggested to be the reason for its lack of significant cognitive side effects.[2] Other first-generation antiepileptics that act on Nav channels include lamotrigine 2,[3] and carbamazepine 3(4) (Figure 1). Recently, lacosamide 4 was approved for use in partial-onset seizures and neuropathic pain. Lacosamide is proposed to act via an unique mechanism of state-dependent blockade of Nav channels: stabilizing channels in a slow inactivated state without effects on fast inactivation.[5]

Figure 1

Compounds with sodium channel modulatory activity.

Nine Nav channel isoforms have been identified, of which, Nav1.1, 1.2, 1.3, and 1.6 are predominately found within the CNS, whereas Nav1.7, 1.8, and 1.9 are located in the peripheral nervous system. Nav1.4 is found within skeletal muscle, and Nav1.5 is found in the heart. The Nav isoforms are also expressed outside their primary tissues.[6] Recently, the first X-ray crystal structure of a voltage-gated sodium channel was solved: initially in the closed state and subsequently in two presumed inactivated states.[7,8] Under normal physiological conditions, the channels interconvert between different functional states: the resting state in which the channel pore is closed, an open state in which the channel pore is open and permeable to Na+, and one or more inactivation states where the pore is open but blocked.[9]

Recent studies have highlighted the importance of Nav1.3, 1.7, 1.8, and 1.9 in various pain syndromes.[10−12] Notably, genetically inherited pain disorders have been associated with mutation of these Nav encoding genes.[13] Clinically, carbamazepine, lamotrigine, and phenytoin are all used to treat neuropathic pain.[14] The Merck, Pfizer, AstraZeneca, and GlaxoSmithKline pharmaceutical groups have all independently reported their own series of 1.3-, 1.7-, and 1.9-selective sodium channel modulators for use in neuropathic pain,[15] exemplified here by the benzazepinone compounds from Merck Pharmaceuticals, such as 5 (Figure 1).[16,17]

Sodium channel blockers can exhibit significant neuroprotective effects.[18] Influx of sodium from the extracellular space during reperfusion plays a major role in excitotoxic neuronal death after cerebral ischemia.[19] Block of Nav channels can attenuate the primary events that occur in an ischemic attack, most likely by blocking Na+ permeability, resulting in reduced ATP depletion and reduced ischemic-induced release of glutamate.[20] A number of drugs, including lamotrigine, BW-1003C87, the local anesthetic lidocaine, and sipatrigine, have all shown neuroprotective effects in animal models of stroke.[21−24] However, as of yet, these results have failed to translate into beneficial effects in clinical trials.[25,26] Thus, there is a need for drugs with a better clinical efficacy and therapeutic index. Treatment of stroke is limited by the fact that administration of treatment in humans only occurs sometime after the onset of the neurodegenerative insult. In contrast, for slowly evolving neurodegenerative conditions, it is possible to treat during or even before individual neurodegenerative episodes occur. Carbamazepine (3) is commonly used to alleviate the symptoms of MS, such as spasms, nerve pain, and trigeminal neuralgia, and lamotrigine has been trialled in neuroprotection.[27] Research into the potential neuroprotective effects of these compounds is gaining increasing momentum because of a better understanding of the role of Nav channels in nerve damage.[28]

To understand the role of individual Nav channel isoforms in neuroprotection, we need to consider their distribution. Although most of the sodium channel variants are expressed at different levels in the CNS, Nav1.6 expression is widespread[29] but is more abundant in nerves of the retina, cerebellum, and cortex tissue.[30] In a mature, myelinated axon, Nav1.6 tends to cluster and is the predominant isoform at the nodes of Ranvier.[31] A significant increase in Nav1.6 density can be seen subsequent to nerve fiber injury in traumatic brain injury and multiple sclerosis.[32−34] Nav1.6 has also been implicated in secondary degeneration of neuronal cells. Mechanical, ischemic, and inflammatory insult can, over time, contribute to this degeneration. Shear and stretch forces, despite not destroying the cells or severing the axons can, nonetheless, produce secondary degeneration in brain tissues.[35] Rapid tetrodotoxin-sensitive calcium overload occurs in stretch-traumatized axons, which suggests that in neuropathologic conditions structural degradation of the axolemmal bilayer can foster a chronically left-shifted Nav channel operation. This points to leaky Nav channels playing an important role in determining neuronal cell survival.[36] Other Nav channel isoforms, namely, Nav1.1[37] and Nav1.3, have also been implicated in traumatic brain injury models.[38] Overall, there is no clear understanding of the specific contributions of each isoform to the neuroprotective effect; we anticipate that investigations of the action of well-characterized compounds will aid in this regard. Previous work by this group designed a class of voltage-dependent sodium channel modulators based on the indazolyloxadiazolyl scaffold.[39] Further development of analogues around this scaffold has now identified a series of imidazolylethylindazoles with good neuroprotective effects. Following assessment of sodium channel isoform selectivity and functional activity, a candidate molecule, CFM6104, 6 (Figure 1), was tested in an in vivo optic neuritis (inflammation of the optic nerve) mouse model of MS and found to prevent the consequent loss of retinal nerve cells.

Results

Design

In our previous investigations, we demonstrated that aminomethyloxadiazolylindazoles had good neuroprotective activity. If the amino group was protected as a Boc group, then the activity was generally lost. However, exploration of alternative groups to the indazole N1/N2 benzyl revealed a Boc-protected intermediate (vide infra) with surprising neuroprotective activity. This molecule exhibited a different SAR profile to the previous series with lower cytochrome P450 inhibition. We therefore carried out a detailed investigation of the neuroprotective activities of this series. Key objectives were to maintain the neuroprotective activity and to improve the metabolic stability to allow evaluation in vivo.

Chemistry

The synthesis of 3-[3-tert-butoxycarbonylaminomethyl-1,2,4-oxadiazol-5-yl]indazole 7 (Scheme 1) has been previously described.[39] This compound was utilized as a key synthetic intermediate for the production of a further series of compounds. The stereoselective synthesis of alkyl N1- and N2-heteroaromatic-substituted compounds (Scheme 1) was carried out using a cesium- or potassium carbonate-mediated alkylation of an alkyl halide-functionalized heteroaromatic compound (N1), 8, or a Mitsunobu reaction with an alcohol-functionalized heteroaromatic compound (N2), 9. Boc removal was performed using trifluoroacetic acid, triisopropylsilane, and water to yield the free amines 10 and 11. These amines were then derivatized to provide the N112 and N213 analogues shown. The benzamides were formed via a coupling reaction between the free amine and R-substituted benzoic acid in the presence of DMF, HATU, and DIPEA at room temperature. This reaction occurred at various efficiencies (9–75%) across the range of substituted oxadiazolylindazoles and substituted benzoic acids. The sulfonamide analogues were formed by reaction of the free amine with R-substituted benzene sulfonyl chloride in the presence of sodium carbonate at room temperature with reaction efficiencies of 24–39%.

Scheme 1

Biological Assays

Activity against sodium channels was assessed using a radioligand binding assay using [3H]sipatrigine as the ligand[39] and by patch-clamp electrophysiology on cloned human channels. The in vitro neuroprotection assay was an adaptation of an original study by Fowler and Li.[40] In this system, rat hippocampal slices are subjected to oxygen-glucose deprivation, and the subsequent progression to cell death is monitored by determining their ATP levels. This paradigm is highly sensitive to blockade of sodium channels and tetrodotoxin. The compounds were also tested for metabolic stability in rat liver microsomes. Nav channel isoform activity was determined using an automated patch-clamp system against hNav1.1–hNav1.8/β3 expressing cells. T-cell immunosuppressive activity was assessed using a simple contact hypersensitivity model that detects T-cell proliferation to oxazolone in lymphnodes.[41−43] In vivo neuroprotection was evaluated in an optic neuritis model[44] using double-transgenic mice expressing both a Thy1-promoter-driven cyan fluorescent protein restricted to retinal ganglion cells and a myelin oligodendrocyte glycoprotein T-cell receptor (TCR).[44] Following injection of Bordetella pertussis toxin, the mice develop subneurological experimental autoimmune encephalomyelitis (EAE) and optic neuritis in the optic nerve, which then leads to retinal ganglion cell loss secondary to axon damage.

Activity against Sodium Channels and Neuroprotection

Initial testing with N1- and N2-functionalized tert-butyl carbamate-protected analogues showed encouraging results in the [3H]sipatrigine binding and neuroprotection assays (Table 1). Lamotrigine, sipatrigine, and tetrodotoxin were included as reference molecules. Benzyl-functionalized molecules 14 and 15, reported in previous work, are shown for comparison.[39] A combination of reasonable potency (20 μM) and >30% neuroprotection identifies active compounds.

Table 1

Sodium Channel Modulatory and Neuroprotective Activity of Boc-Protected Oxadiazolyindazole Analogues

compound	R (isomer)	[3H]sipatrigine binding, IC50 (μM)	neuroprotection at 50 μM (%)
lamotrigine		1.8 ± 0.2a	33b
tetrodotoxin			100c
sipatrigine		0.04 ± 0.001a	93d
14	CH2-Ph (N1)	8.3e	19e
15	CH2-Ph (N2)	5.5e	25e
16	CH2CH2-2-imidazole (N1)	7.4 ± 1.1	56 ± 5.6 (4)
17	CH2CH2-2-imidazole (N2)	15.5 ± 9.2	35 ± 5.3 (3)
18	2-py (N1)	74 ± 20.4	18 ± 7.1 (4)
19	CH2-2-py (N2)	23 ± 5.9	42 ± 3.8 (4)
20	CH2CH2-2-thiophene (N2)	166 ± 49	1 ± 13.4 (4)
21	CH2-4-(5-CH3)-isoxazole (N1)	339 ± 121	39 ± 7.8 (4)
22	CH2-C6H4-N-pyrazole (N1)	617 ± 160	20 ± 16.4 (3)
23	CH2-3,4-benzodioxole (N1)	6 ± 1.0	–3 ± 6.2 (4)

Values reported in literature.[39]

Tested at 30 μM. Results are normalized to the effect of TTX and are based on the mean of at least three experiments.

TTX tested at 1 μM.

Tested at 3 μM.

Reported previously.[39]

Replacement of the benzyl group with substituted and nonsubstituted heteroaromatic groups showed a general decrease in the ability of the compounds to displace the radioactive ligand, with the notable exceptions of the ethylimidazole analogues 16 and 17 (Table 1). Other side chains with weakly basic pyridyl moieties, 18 and 19, demonstrated moderate binding and neuroprotective activity, whereas nonbasic heterocycles 20 and 21 or bulkier groups 22 and 23 displayed weak binding or low neuroprotection.

In the previous series, a free amine group led to a general increase in the neuroprotection by the benzylindazole analogues (e.g., compounds 24 and 25; Table 2). That trend was not observed here. The modest neuroprotection observed for the imidazole and pyridine analogues was removed with free amines such as 26 and 27. This was also the case for the pyridyl–free amines 28 and 29. The neutral side chains ethylthiophene 30, methylisoxazole 31, and substituted benzyls 32 and 33 all showed good binding and modest neuroprotection.

Table 2

Sodium Channel Modulatory and Neuroprotective Activity of Oxadiazolylindazole Analogues

compound	R (isomer)	[3H]sipatrigine binding, IC50 (μM)	neuroprotection at 50 μM (%)
lamotrigine		1.8 ± 0.2a	33b
tetrodotoxin			100c
sipatrigine		0.04 ± 0.001a	93d
24	CH2-Ph (N1)	3.3 ± 0.6e	>99,e 35 ± 9.0
25	CH2-Ph (N2)	7 ± 2.8e	57 ± 15.6 (4)
26	CH2CH2-2-imidazole (N1)	148 ± 120	–2 ± 1.9 (3)
27	CH2CH2-2-imidazole (N2)	14 ± 1.6	8 ± 26.9 (4)
28	2-py (N1)	32 ± 8.2	–11 ± 13.8 (4)
29	CH2-2-py (N2)	>10 000	25 ± 9.4 (4)
30	CH2CH2-2-thiophene (N2)	0.6 ± 0.05	36 ± 5.4 (4)
31	CH2-4-(5-CH3)- Isoxazole (N1)	15 ± 5.4	19 ± 8.6 (4)
32	CH2-C6H4-N-pyrazole (N1)	6.2 ± 3.8	26 ± 9.0 (4)
33	CH2-3,4-benzodioxole (N1)	2.1 ± 0.3	44 ± 2.8 (4)

Values reported in literature.[39]

Tested at 30 μM. Results are normalized to the effect of TTX and are based on the mean of at least three experiments.

TTX tested at 1 μM.

Tested at 3 μM.

Reported previously[39] (values in bold are IC50’s in micromolar).

These results supported the idea that the activity observed for the indazoles substituted by basic heterocycles at the N1 position was not following a similar SAR pattern to the original series and required a lipophilic group at the oxadiazolylmethylamine. When the free amine analogues were functionalized with a benzamide group at the amine moiety, the neuroprotection and the radioligand-displacement ability of some of the analogues was improved, with moderate to good levels of binding observed for N1 and N2 imidazole analogues 6 and 34 (Table 3). Thus, it appeared that a benzamide group could mimic the action of the tert-butyl carbamate. Methylpyridyl analogue 35 showed modest binding, whereas ethylthiophene 36 and methyloxazole 37 were essentially inactive.

Table 3

Sodium Channel Modulatory and Neuroprotective Activity of Benzamide-Functionalized Oxadiazolylindazole Analogues

compound	R (isomer)	[3H]sipatrigine binding, IC50 (μM)	neuroprotection at 50 μM (%)
lamotrigine		1.8 ± 0.2a	33b
tetrodotoxin			100c
sipatrigine		0.04 ± 0.001a	93d
6	CH2CH2-N-imidazole (N1)	13 ± 9	47 ± 15.1 (4)
34e	CH2CH2-N-imidazole (N2)	4.5 ± 0.3	19 ± 1.2
			41 ± 9.4 (4)
35	CH2-2-py (N2)	41 ± 7	58 ± 4.6 (4)
36	CH2CH2-2-thiophene (N2)	>1000	–2 ± 6.2 (4)
37	CH2-4-(5-CH3)-isoxazole (N1)	>2000	19 ± 20.0 (4)

Values reported in literature.[39]

Tested at 30 μM. Results are normalized to the effect of TTX and are based on the mean of at least four experiments.

TTX tested at 1 μM.

Tested at 3 μM. Values in bold are IC50’s in micromolar.

Tested as a mixture of 83% N1 and 17% N2 isomers.

On the basis of these encouraging effects, a new set of N1 and N2 imidazole analogues was designed and synthesized to see if improved binding and neuroprotective activity was achievable. The new set of N1 and N2 ethyl-2-imidazoles (compounds 38–64, Table 4) showed a range of activity, with most showing good ability to displace the radioligand [3H]sipatrigine, and a range of neuroprotective activity, many with moderate to good neuroprotection. In the N1-substituted series, all but one of the 11 new analogues had a binding IC50 < 13 μM, with some compounds having values as low as 1 μM. Similarly, in the N2-substituted series, all but one of the 16 new analogues had a binding IC50 < 17 μM, with some as low as 0.4 μM. Neuroprotection efficacy also showed a general increase, with compound 50 reaching 88% neuroprotection. In the N1 series, the most profound effect was the 2,3-dimethoxy substitution of the benzamide ring, 41, which led to a marked increase in displacement ability (IC50 13 to 7.1 μM) and neuroprotection (46 to 71%). Interestingly, the analogous trisubstitution at the 2,3,4-positions, 38, resulted in a decrease in neuroprotection. These aside, in the N1 series, functionalization the benzamide changed little in terms of activity from the parent, 6 (IC50 range of 1–13 μM and neuroprotection range of 37–52%), with the potential to add larger groups, such as a indole 39 or t-butyl 40, without significant loss of binding or neuroprotection. Compared to the N1 series, the N2 benzamide series had a much greater range of activities, both in binding (0.4–59 μM) and neuroprotection (2–89%). The 2-, 3-, and 4-methoxybenzamides all showed good binding (IC50 < 8 μM) and neuroprotection (54–63%), with the 2,3-dimethoxy-substituted benzamide once again showing the best activity with good binding (IC50 = 5 μM) and excellent neuroprotection (89%, EC50 = 12.3 ± 5.8). The benzamide substituted at the 3 position with OCF3 showed comparable activity with binding (IC50 = 0.8 μM) and neuroprotection (80%). The only other marked effect was an almost total ablation of neuroprotective effect by fluorination at the 2 and 3 positions, despite showing an IC50 of 4 μM in the radioligand-displacement assay. Of all of the sulfonamide compounds in the N1 series, only one improved on the neuroprotection of parent benzamide 6, although many showed better binding. The 2- and 3-substituted sulfonamide showed good binding and modest neuroprotection, but substitution at the 4 position seemed to provide compounds with poor neuroprotective ability, if not binding, such as 57, 59, and 62. In the N2 series, the sulfonamide analogue of 34, 63, showed comparable binding and neuroprotection, but once again, a 4-substituted sulfonamide, 64, had poorer binding and reduced neuroprotection. In fact, 4-substituted sulfonamide and benzamide analogues across the group had reduced capacity for neuroprotection in the hippocampal slice assay when compared to the 2- or 3-substituted or unsubstituted compounds, despite no significant difference in binding. The presence of unfavorable interactions at the 4 positions is indicated, and this may also account for the observation that small substituents such as Cl have a lesser detrimental effect on activity than larger groups. In the limited number of halide-functionalized benzamides and sulfonamides tested, none showed improved neuroprotection.

Table 4

Sodium Channel Modulatory and Neuroprotective Activity of N1- and N2-Propylimidazole-Functionalized Oxadiazolylindazole Analogues

compound	N1/N2 functionalized	R	[3H]sipatrigine binding, IC50 (μM)	neuroprotection at 50 μM (%) or EC50 (bold)
lamotrigine			1.8 ± 0.2a	33b
tetrodotoxin				100c
sipatrigine			0.04 ± 0.001a	93d
38	N1	CO-2,3,4-(OCH3)3Ph	5.3 ± 2.8	37 ± 6.4 (4)
39	N1	CO-4-indole	13 ± 2.0	52 ± 9.9 (4)
40	N1	CO-4-t-BuPh	1.2 ± 0.2	40 ± 34.2 (4)
41	N1	CO-2,3-(OCH3)2Ph	7.1 ± 0.8	69 ± 7.5 (4)
42	N1	CO-3-OCF3Ph	1.5 ± 0.3	37 ± 13.0 (4)
43	N2	CO-4-ClPh	2.2 ± 0.5	42 ± 3.1 (4)
44	N2	CO-2,3-F2Ph	3.6 ± 0.2	2 ± 1.4 (4)
45	N2	CO-4-OCF3Ph	0.7 ± 0.1	31 ± 8.2 (4)
46	N2	CO-2-CH3Ph	2.9 ± 0.5	62 ± 4.4 (4)
47	N2	CO-4-OCH3Ph	6.2 ± 0.7	55 ± 10.7 (4)
48	N2	CO-4-(NHCOCH3)Ph	59 ± 31	–14 ± 3.6 (4)
49	N2	CO-2-naphthalene	0.4 ± 0.2	22 ± 4.4 (4)
50	N2	CO-2,3-(OCH3)2Ph	5.8 ± 1.8	12.3 ± 5.8
				88 ± 7.7 (4)
51	N2	CO-3-OCF3Ph	0.6 ± 0.1	5.15 ± 0.15
				78 ± 4.2 (4)
52	N2	CO-(3-CH3-4-NO2)Ph	1.7 ± 0.2	43 ± 3.1 (4)
53	N2	CO-3,4-benzodioxole	5.0 ± 0.4	27 ± 4.5 (3)
54	N2	CO-3-CH3Ph	2.5 ± 0.2	41 ± 6.9 (3)
55	N2	CO-2-OCH3Ph	7.1 ± 1.2	54 ± 11.3 (4)
56	N2	CO-3-OCH3Ph	5.0 ± 1.1	63 ± 7.6 (4)
57	N1	SO2-4-ClPh	10 ± 7.1	21 ± 7.9 (4)
58	N1	SO2-2,3-(OCH3)2Ph	>500	37 ± 2.5 (4)
59	N1	SO2-4-OCF3Ph	4.9 ± 6.0	0 ± 9.9 (4)
60	N1	SO2-2-CH3Ph	4.9 ± 1.3	57 ± 10.8 (4)
61	N1	SO2-3-CF3Ph	2.1 ± 0.6	38 ± 12.3 (4)
62	N1	SO2-4-t-BuPh	4.4 ± 1.3	16 ± 20.2 (4)
63	N2	SO2-Ph	1.7 ± 0.5	29.5 ± 3.8
				38 ± 14 (4)
64	N2	SO2-4-(NHCOCH3)Ph	17 ± 6.8	25 ± 15 (4)

Values reported in literature.[39]

Tested at 30 μM. Results are normalized to the effect of TTX and are based on the mean of at least three experiments.

TTX tested at 1 μM.

Tested at 3 μM (values in bold are IC50’s in micromolar).

Stability in Rat Liver Microsomes

The N1 and N2 ethyl-2-imidazole-substituted series was examined for metabolic stability in rat liver microsomes. Table 5 shows a select subset of the compounds screened for rate of turnover. When examined, 14 of 21 compounds tested in this series had turnovers of greater than 90% after 40 min. Boc-protected and free amine N2 analogues 17 and 27 showed better metabolic resistance, with turnovers of 70 and 57 min, respectively. Functionalization of the amine with benzamide, 34, again produced an unstable compound. However the N1 analogue, 6, showed better stability with more than half of the original compound left after 40 min. Despite this promising result, none of the further analogues managed to maintain or improve on this stability, with any substitution making the compounds significantly more labile to metabolic degradation (>90%). Five benzamide and sulphonamide analogues that were fluorine- or chlorine-substituted at the 4, 5, and 6 positions on the indazole ring were also tested to see if this modification could arrest the rate of metabolism. All showed >90% metabolism after 40 min (data not shown). The only compound with moderately increased stability was the CO-3-OCF3Ph-substituted benzamide, 42. Similarly, substitution of the benzamide or changing to a sulfonamide did not provide much improvement in the metabolic stability of the N2 analogues of benzamide 34. Interestingly, however, substitution at the 4 position with the amide group, NHCOCH3, in both the sulfonamide 64 and benzamide 34 analogues gave significant improvement in stability, with turnovers of 54 and 45%, respectively, after 40 min. Unfortunately, these 4 position-substituted NHCOCH3 compounds were also some of the most inactive compounds in the neuroprotective assay.

Table 5

Metabolic Stability Measured in a Rat Liver Microsome Assay for N1- and N2-Propylimidazole-Functionalized Oxadiazolylindazole Analogues

compound	turnover at 40 min (%)
17	70
27	57
34	97
6	47
42	78
63	97
64	54
48	45

Nav Channel Isoform Profiling

Compounds 6 and 51 were examined by patch-clamp electrophysiology to determine their effects on sodium channel isoforms (Table 6 and Figure 2). Compound 6 showed preferential activity for the inactivated state (Figure 2A) but little discrimination between subtypes. The block of Nav1.1, 1.3, and 1.6 is implicated in the pharmacological basis of neuroprotection (vide supra), and activity was noted against these isoforms consistent with the compounds’ ability to displace radiolabeled sipatrigine (Table 6). Compound 51 showed good activity in both the radioligand-displacement and neuroprotection assays, and this is reflected in its potent block across the range of isoforms. It blocks many of the isoforms at an order of magnitude lower concentration than 6, with its most potent block against the Nav1.5 sodium channel, an isoform found primarily in cardiac muscle. There also appears to be less functional selectivity compared to 6 (Figure 2B). The potency of 51 against Nav1.1, 1.3, and 1.6 when compared to 6 may account for the observed increase in neuroprotection seen with this compound (Table 6).

Table 6

Summary of Action of Compounds 6 and 51 on Nav1.1–1.8

	tonic block (a)		10 Hz block (b)		inactivated-state block (c)
compound	IC50	na	IC50	n	IC50	n	IC50 b/cb	IC50 a/c
Nav1.1
6	>100		>100		21 ± 6.29	1.71 ± 0.73	>5	>5
51	15.3 ± 5.02	1.09 ± 0.24	8.54 ± 0.72	1.32 ± 0.16	2.55 ± 0.22	2.09 ± 0.32	3.3	6
Nav1.2
6	71 ± 25	1.12 ± 0.51	46 ± 7.42	1.82 ± 0.73	29 ± 1.49	1.48 ± 0.38	1.6	2.4
51	65 ± 22	0.48 ± 0.14	24.1 ± 3.87	0.63 ± 0.14	3.54 ± 1.24	0.6 ± 0.12	6.8	18
Nav1.3
6	>100		>100		22 ± 4.09	0.78 ± 0.15	>5	>5
51	>100		97 ± 72	0.35 ± 0.19	4.76 ± 0.92	0.96 ± 0.19	20	>25
Nav1.4
6	>100		56 ± 16.4	1.21 ± 0.36	24 ± 2.98	1.43 ± 0.27	2.3	>5
51	18.3 ± 1.65	1.23 ± 0.18	10.5 ± 0.47	1.34 ± 0.13	4.58 ± 0.25	1.75 ± 0.12	2.3	4
Nav1.5
6	>100		69 ± 31	1.16 ± 0.54	19 ± 3.06	0.99 ± 0.22	3.6	>5
51	36 ± 19	0.76 ± 0.23	14.8 ± 7.52	0.37 ± 0.13	0.92 ± 0.89	0.49 ± 0.15	16	39
Nav1.6
6	>100		>100		24 ± 7.3	2.45 ± 2.59	>5	>5
51	>100		32.2 ± 25.9	0.42 ± 0.19	3.32 ± 0.53	1.17 ± 0.18	9.7	>30
Nav1.7
6	37 ± 7.0	2.09 ± 1.73	28 ± 1.8	1.88 ± 0.53	14.5 ± 0.73	2.24 ± 0.17	1.9	2.6
51	5.64 ± 0.57	2.59 ± 0.41	4.82 ± 0.33	2.63 ± 0.33	3.02 ± 0.36	2.5 ± 0.1	1.6	1.9
Nav1.8
6	>100		>100		28 ± 0.97	1.42 ± 0.1	>4	>4
51	6.34 ± 1.08	2.14 ± 0.51	5.66 ± 1.47	2.62 ± 1.05	3.77 ± 0.49	2.75 ± 0.66	1.5	1.7

n = the slope of the fit.

The ratio of IC50 a/c and b/c illustrates the degree of functional selectivity for the inactivated state.

Figure 2

Detail of activity of 6 (A) and 51 (B) against the Nav1.6 isoform. Blue triangles are block of the inactivated state, red circles are block of 10 Hz current, and black squares are block of tonic current.

In Vivo Activity in an Optic Neuritis Mouse Model of MS

Compound 6 was selected for detailed in vivo evaluation on the basis of its mix of properties: in vitro sodium channel activity, activity against the neuronal Nav isoforms, functional selectivity, and stability in rat liver microsomes. The influence of 6 was assessed in vivo in wild-type and myelin oligodendrocyte glycoprotein-specific T-cell receptor transgenic mice (Figure 4; 5).[44] First, 6 was examined for T-cell immunosuppressive activity, as the EAE model of MS is well-known to be T-cell-dependent.[45] Rather than use transgenic mice where all T-cells have the same specificity, a contact-hypersensitivity model was used because it is known to be predictive in identifying in vivo immunosuppressive doses for EAE studies.[41−43] Doses of 6 ranging from 5–50 mg/kg were administered intraperitoneally (i.p.) from day −1 to day 2, and the in vivo induced proliferative response was assessed in the draining lymph nodes 3 days (the peak time of T-cell proliferation) after the sensitization to the contact-sensitizer oxazolone on day 0. There was no evidence suggestive of any inhibitory effect of sensitization based on cell numbers within the lymph nodes or based on a per cell proliferative response (Figure 3). Importantly, this study demonstrated that the drug was well-tolerated and there was no apparent toxicity. We next sought to assess whether 6 could exert a neuroprotective effect and limit the amount of axonal and nerve loss that results as a consequence of T-cell autoimmunity within the CNS. Following the development of optic neuritis in our model mice, demyelination was augmented following injection of 250 μg of Z12 rat IgG2b demyelinating antibody on day 14.[44] Retinal ganglion cell survival following treatment of animals with 5 mg/kg i.p. 6 was assessed on day 21 postinduction in retinal flatmounts (Figures 3A,B and 4). It was found that 6 increased ganglion cell survival (P < 0.02) and reduced the mean retinal loss from 914 ± 117 to 1298 ± 77 cells/mm2 compared to 1653 ± 89 cells/mm2 in a group of age-matched control animals without optic neuritis (Figure 4). This study suggests that this class of compounds may have some utility in controlling the accumulation of nerve loss that occurs in diseases such as MS.

Figure 4

Retinal ganglion cells in the retina of mice (A) before or (B) after the development of optic neuritis. (C) Density of RGC. The box represents the 25th and 75th percentiles. The whiskers represent the fifth and 95th percentiles. The solid line represents the median. Differences between vehicle and drug-treated controls were assessed using Student’s t test.

Figure 3

T-cell immunosuppressive activity of 6 in vivo. Mice received epicutaneous application of 25 μL of 2.5% oxazolone (OX) in acetone olive oil (AOO) 4:1 on day 0 on the dorsum of the ear. Animals received daily intraperitoneal (i.p.) injections of either vehicle (DMSO/Cremophor/phosphate buffered saline, 1:1:18) or 6 from day −1 to day 2. On day 3, the draining auricular lymph nodes were removed from three mice and pooled, and a single-cell suspension was made. Lymph node cells were counted, and 5 × 105 cells per well, in triplicate, were cultured overnight at 37 °C. T-cell proliferation was assessed using the CellTiter 96 AQueous Cell Proliferation Assay, which uses a MTS tetrazolium compound that is bioreduced by live cells. The resulting formazan product absorbance is recorded at 490 nM. The results represent the mean response ± SD of triplicate assays.

Discussion

During MS, Nav channels are redistributed along demyelinated nerves, which can help to restore function to some extent but leaves them vulnerable to excessive sodium entry.[18]

Our strategy was to develop compounds with good activity toward sodium channel inhibition, in particular against the neuronal isoforms Nav1.1, 1.3, and 1.6, but, most importantly, which could also display neuroprotective capabilities in an in vitro hippocampal slice assay system. The selection of a candidate molecule for in vivo studies was based on a balanced assessment of properties, principally sodium channel activity, neuroprotection, and in vitro stability. The use of the hippocampal slice model, measuring ATP levels and tuned for sodium channel blockade, was key in ensuring an appropriate functional effect. In general, the series was found to exhibit poor stability in rat liver microsomes in vitro, with 6 being a notable exception that allowed us to test this paradigm in vivo. There are reports on the metabolism of 1,2,4-oxadiazoles,[49] and this may be the site of the metabolic fragility. Studies detailing the pharmacokinetics and localization of 6 to brain and spinal cord lesions in other EAE models are published elsewhere.[50]

Sodium channel blockade with 6 in vivo was found to be neuroprotective, preventing retinal ganglion cell loss in optic neuritis in this refined EAE model.[44] In this case, the mechanism of neuroprotection is unlikely to be due to peripheral immunosuppression that prevents disease from developing.[18,51] There was no evidence of T-cell immunosuppression seen here in a paradigm that can detect immunosuppressive doses of drugs even following a single dose. Furthermore, in our hands, overt immunosuppression does not affect the development of disease in conventional EAE using a similar delivery of sodium channel-blocking compounds including phenytoin, sipatrigine, carbamazepine, and oxcarbazepine,[50] which is in contrast to some studies where immunosuppression of sodium channel blockers delivered in the drinking water was noted.[18,51] This class of agents has consistently been found to be neuroprotective.[49] Neuroprotection with 6 is most likely to occur at the level of the axon or nerve and perhaps through inhibition of microglial activity, which are probably central to the pathogenesis of progressive MS.[45,46]

Furthermore, some microglial responses are mediated via Nav channel activity. Therefore, blockade of sodium channels could result in neuroprotective effects on immune and nonimmune components that may drive progressive nerve loss in MS.[45,46] Neurodegeneration accumulates slowly over time and therefore allows a larger time window for treatment onset compared to acute conditions such as stroke. Lesions in MS can develop throughout the neuroaxis but are common within the optic nerve, where optic neuritis is often an initial sign of disease. The visual pathway is the most accessible part of the human nervous system, and optic neuritis in MS is considered to be an ideal tissue to study the effect of neuroprotective treatments in MS and is being targeted in a number of studies.[47,48] Amiloride and phenytoin are currently in phase II clinical trials for the treatment of optic neuritis in MS (NCT01802489, NCT01451593).[48] We chose to utilize an optic neuritis model for in vivo evaluation of neuroprotection in this study.

Recently, lamotrigine was examined in progressive MS, but it did not appear to inhibit the development of brain atrophy, which was the primary outcome measure, although significant effects of slowing of walking deterioration were noted.[27] There is a clear need to evaluate better tolerated and more effective neuroprotective agents. The chemical series described here may provide a platform for the development of pharmaceutical agents for the treatment of MS and other neurological diseases.

Experimental Section

All compounds were at least 95% pure as assayed by LCMS (electrospray positive). The preparations of analogues 6 and 51 are shown below, and the preparation of the remainder of the analogues is provided in the Supporting Information.

General Procedure for the Synthesis of Benzamide Analogues

To a solution of R-substituted benzoic acid (0.10 g, 0.82 mmol) in DMF (5 mL) were added HATU (0.34 g, 0.9 mmol, 1.1 equiv) and diisopropylethylamine (0.23 g, 0.31 mL, 1.8 mmol, 2.2 equiv), and the solution allowed to stir for half of an hour at room temperature under nitrogen. The amine (0.9 mmol, 1.1 equiv) was then added, and the brown solution was allowed to stir overnight. The DMF was removed in vacuo, and the resulting crude oil was dissolved in ethyl acetate and washed with water, 1 N HCl, 1 N NaHCO3, and brine. The organic phase was then dried over magnesium sulfate, and the solvent was removed in vacuo. The crude oil was purified using flash chromatography, eluting with 10% methanol in dichloromethane.

General Procedure for the Synthesis of Sulfonamide N1 Analogues

To a solution of the appropriate amine (0.22 g, 0.71 mmol, 1 equiv) in water (10 mL) was added R-substituted benzene sulfonyl chloride (0.78 mmol, 1.1 equiv). The solution was allowed to stir, and saturated sodium carbonate(aq) was added gradually to adjust the acidic pH to 8. The solution was then allowed to stir overnight at room temperature. The white precipitate was filtered off, washed with diethyl ether, and dried in vacuo.

N-[[5-[1-(2-Imidazol-1-ylethyl)indazol-3-yl]-1,2,4-oxadiazol-3-yl]methyl]benzamide (6)

Orange crystals (18% yield, 98% purity). 1H NMR (300 MHz, DMSO) δ 9.27 (t, J = 5.7 Hz, 1H, NH), 8.13 (d, J = 8.1 Hz, 1H, ArH), 7.92 (d, J = 6.7 Hz, 2H, ArH), 7.64 (d, J = 8.5 Hz, 1H, ArH), 7.61–7.41 (m, 4H, ArH), 7.41–7.27 (m, 2H, ArH), 7.02 (s, 1H, ArH), 6.71 (s, 1H, ArH), 4.96 (t, J = 5.8 Hz, 2H, CH2), 4.73 (d, J = 5.7 Hz, 2H, CH2), 4.55 (t, J = 5.8 Hz, 2H, CH2). 13C NMR (75 MHz, CDCl3) δ 170.1, 168.9, 166.5, 140.8, 137.2, 133.7, 131.5, 129.3, 128.4, 127.7, 127.4, 127.3, 123.7, 121.5, 120.3, 119.6, 110.5, 49.7, 45.7, 35.3. HRMS (TOF MS ES+) calcd for C22H20N7O2 (MH+), 414.1678; found, 414.1668.

N-[[5-[2-(2-Imidazol-1-ylethyl)indazol-3-yl]-1,2,4-oxadiazol-3-yl]methyl]-2-(trifluoromethoxy)benzamide (51)

1H NMR (300 MHz, CDCl3) δ 8.18 (s, 1H, NH), 8.09 (d, J = 8.4 Hz, 1H, ArH), 7.99 (d, J = 7.7 Hz, 1H, ArH), 7.95 (s, 1H, ArH), 7.81 (d, J = 8.6 Hz, 1H, ArH), 7.54–7.30 (m, 5H, ArH), 6.69 (s, 1H, ArH), 6.30 (s, 1H, ArH), 5.37 (t, J = 5.5 Hz, 2H, CH2), 4.82 (d, J = 5.1 Hz, 2H, CH2), 4.55 (t, J = 5.5 Hz, 2H, CH2).13C NMR (75 MHz, DMSO) δ 237.95, 168.13, 167.26, 164.96, 151.20, 148.34, 147.35, 137.33, 135.81, 130.68, 128.25, 127.08, 126.41, 125.70, 124.15, 122.28, 119.96, 119.79, 119.23, 118.23, 53.29, 45.60, 35.20. HRMS (TOF MS ES+) calcd for C25H30N10O5 (MH+), 498.1501; found, 498.1500.

In Vivo

Animals

Six to eight week old male and female wild-type and double-transgenic C57BL/6-Cg-Tg(Thy1-CFP)23Jrs.Tg (Tcra2D2,Tcrb2D2)1Kuch/J mice[44] were used from stock bred at Queen Mary University of London. Founder mice were originally obtained from Jackson Laboratories, Bar Habor, ME, USA). These transgenic mice express both a H-2Ab-restricted T-cell receptor (Vα3, Vβ11) specific for myelin oligodendrocyte glycoprotein residues 35–55 and a neuronally restricted Thy1 (CD90)-promoter-driven expression of cyan fluorescent protein, which is only present in the retinal ganglion cells within the retina.[44] Mice were maintained on a 12 h light/dark cycle with controlled humidity and temperature and housed as described previously.[52] The reporting of most aspects of the experimental design, which conform with the ARRIVE guidelines, has been described previously.[52] Animals were fed RM-1E diet and water ad libitum. All animal studies were approved by the Queen Mary ethical review panel and the United Kingdom Government Home Office Inspectorate. These studies conformed to the United Kingdom Animals (Scientific Procedures) Act 1986 for the use of animals in research.

In Vivo Local Lymph Node Assay

Mice (n = 3–4/group) received epicutaneous application of 25 μL of acetone/olive oil (4:1, AOO) or 25 μL of 2.5% oxazolone (OX, Sigma-Aldrich, Poole, UK) on day 0 on the dorsum of the ear. On day 3, the draining auricular lymph nodes were removed, and a single-cell suspension was made. Lymph node cells were counted, and 5 × 105 cells/well, in triplicate, were cultured overnight at 37 °C/5% CO2 in 200 μL of RPMI-1640 medium supplemented with 10% heat-inactivated fetal calf serum, l-glutamine, sodium-pyruvate, and antibiotics as described previously.[42] T-cell proliferation was assessed using the CellTiter 96 AQueous Non-Radioactive Cell Proliferation Assay according to the manufacturer’s instructions (Promega, Southhampton, UK). Animals received daily intraperitoneal (i.p.) injections of either DMSO/Cremophor/phosphate buffered saline (1:1:18) or 6 from day −1 to 2.

Optic Neuritis Multiple Sclerosis Mouse Model

Optic neuritis was induced in C57BL/6-Cg-Tg(Thy1-CFP)23Jrs.Tg(Tcra2D2, Tcrb2D2)1Kuch/J transgenic mice. Subclinical EAE in the absence of paralytic disease was induced following administration of 150 ng of Bordetella pertussis toxin (Sigma-Aldrich) in phosphate-buffered saline.[47] Optic neuritis within the optic nerve was augmented following the injection of 250 μg of Z12 rat IgG2b mouse myelin oligodendrocyte-specific demyelinating antibody on day 14.[47] Animals were injected daily from day 0–21 of the experiment with 5 mg/kg i.p. Animals were killed on day 21 postinduction, and eyes were enucleated and immersed in 4% paraformaldehyde (Sigma-Aldrich) in PBS (pH 7.4) overnight. The retinae were dissected, and the cornea, sclera, lens, hyaloid vasculature, and connective tissue were removed in 2× normal-strength PBS. Four radial incisions were cut around the retina, and flatmounts were mounted onto slides and coverslipped with antifade glycerol (CitiFluor Ltd., London, UK). The retinal flatmounts were imaged by fluorescent microscopy, retinal ganglion cell density was calculated by counting cyan fluorescent protein-expressing retinal ganglion cells using stereology software with a fractionator probe, and the number cells in the four quadrants around the optic nerve head was counted.[47] The parametric data was assessed using Student’s t test, incorporating tests for equality of variance using SigmaStat software (Systat Software, Inc., San Jose, CA, USA).