Pharmacokinetic analysis and antiepileptic activity of two new isomers of N-valproyl glycinamide

Valproyl glycinamide (TV 1901-VPGD) is a new antiepileptic drug, which is currently undergoing clinical trials. The present study explored the pharmacokinetics and pharmacodynamics (anticonvulsant activity and neurotoxicity) of two new isomers of valproyl glycinamide: valnoctyl glycinamide (VCGD) and diisopropylacetyl (DIGD). Both VCGD and DIGD showed anticonvulsant activity and a safety margin in mice similar to those of VPGD. Following iv administration (556 mg) to six dogs, VCGD had a clearance (Cl) value of 3·8±1·1 L h⁻¹ (mean±SD), a volume of distribution (V_ss) of 15±2 L, and a half-life (t_1/2) of 1·9±0·3 h. DIGD had Cl, V_ss, and t_1/2 values of 10±0·8 L h⁻¹, 19±3 L, and 1·6±0·2 h, respectively. Neither VCGD nor DIGD operated as chemical drug delivery systems (CDDSs) of glycine, valnoctic acid, or diisopropyl acetic acid and both showed antiepileptic profiles different from that of valproic acid (VPA). Both glycinamides were biotransformed to their glycine analogues with similar fractions metabolized ( f_m): 59±5% (VCGD) and 62±15% (DIGD). The two glycine metabolites, valnoctyl glycine (VCGA) and diisopropylacetyl glycine (DIGA), were also administered to the same dogs in order to calculate the above f_m values. Both VCGA and DIGA had higher Cl and lower V_ss values than VCGD and DIGD and therefore their mean t_1/2 values were 0·43±0·02 and 0·30±0·07 h, respectively. VCGA and DIGA were excreted mainly intact in the urine, with fractions excreted unchanged ( f_e) of 60±9 and 55±7%, respectively. The improved pharmacokinetic profile of VCGD and DIGD relative to their glycine analogues may explain the similarity of their anticonvulsant activity to that of valproyl glycinamide. The current study demonstrates the benefit of the structure–pharmacokinetic–pharmacodynamic relationship (SPPR) approach in developing and selecting a potent antiepileptic compound in intact animals based not only on its intrinsic pharmacodynamic activity but also on its improved pharmacokinetic profile. © 1997 John Wiley & Sons, Ltd.