Profile of SB‐204269, a mechanistically novel anticonvulsant drug, in rat models of focal and generalized epileptic seizures

1 Earlier optimization of structure‐activity relationships in a novel series of 4‐(benzoylamino)‐benzopyrans, led to the discovery of SB‐204269 (trans‐(+)‐6‐acetyl‐4S‐(4‐fluorobenzoylamino)‐3,4‐dihydro‐2,2‐dimethyl‐2H‐benzo[b]pyran‐3R‐ol, hemihydrate), a potent orally‐active anticonvulsant in the mouse maximal electroshock seizure threshold (MEST) test. 2 Studies have now been undertaken to determine the effects of SB‐204269 in a range of seizure models and tests of neurological deficits in rats. In addition, the compound has been evaluated in a series of in vitro mechanistic assays. 3 SB‐204269 proved to be an orally‐effective anticonvulsant agent, at doses (0.1–30 mg kg−1) devoid of overt behavioural depressant properties, in models of both electrically (MEST and maximal electroshock (MES)) and chemically (i.v. pentylenetetrazol (PTZ) infusion)‐evoked tonic extension seizures. However, the compound did not inhibit PTZ‐induced myoclonic seizures at doses up to 30 mg kg−1, p.o. 4 SB‐204269 also selectively reduced focal electrographic seizure activity in an in vitro elevated K+ rat hippocampal slice model at concentrations (0.1–10 μM) that had no effect on normal synaptic activity and neuronal excitability. 5 In all of these seizure models, SB‐204269 was equivalent or better than the clinically established antiepileptic drugs carbamazepine and lamotrigine, in terms of anticonvulsant potency and efficacy. 6 Unlike SB‐204269, the corresponding trans 3S,4R enantiomer, SB‐204268, did not produce marked anticonvulsant effects, an observation in accord with previous findings for other related pairs of trans enantiomers in the benzopyran series. 7 In the rat accelerating rotarod test, a sensitive paradigm for the detection of neurological deficits such as sedation and motor incoordination, SB‐204269 was inactive even at doses as high as 200 mg kg−1, p.o. This was reflected in the excellent therapeutic index (minimum significantly effective dose in the rotarod test/ED50 in the MES test) for SB‐204269 of >31, as compared to equivalent values of only 7 and 13 for carbamazepine and lamotrigine, respectively. 8 At concentrations (10 μM) well above those required to produce anticonvulsant activity in vivo (i.e. 0.1 μM in brain), SB‐204269 did not interact with many of the well known mechanistic targets for established antiepileptic drugs (e.g. Na+ channels or GABAergic neurotransmission). Subsequent studies have shown that the anticonvulsant properties of SB‐204269 are likely to be mediated by a novel stereospecific binding site present in the CNS. 9 The overall efficacy profile in rodent seizure models, together with a minimal liability for inducing neurological impairment and an apparently unique mechanism of action, highlight the therapeutic potential of SB‐204269 for the treatment of refractory partial and generalized tonic‐clonic seizures.