Relationship between platelet and brain GABA transaminase inhibition by single and multiple doses of vigabatrin in rats

Association Between Patient Age and Gabapentin Serum Concentration-to-Dose Ratio

OBJECTIVES

To evaluate the association between patient age and gabapentin (GBP) concentration-to-dose ratio by a multivariate analysis.

METHODS

The association between patient age and the trough steady-state serum concentration of gabapentin (GBP) normalized to 1 mg/kg body weight or concentration-to-dose ratio (CDR) was retrospectively assessed by analysis of covariance. Potential confounding factors considered were GBP dosage, the number of GBP doses per day, and the presence of concomitant antiepileptic drugs (AEDs). Concentrations that had been measured in predose "trough" samples collected from 66 patients, aged 5-84 years, with partial seizures or neuropathic pain chronically receiving GBP BID (n = 21) or TID (n = 45), alone (n = 15) or in combination with other AEDs (n = 51) were used in this retrospective analysis.

RESULTS

Average GBP CDR was 0.23 +/- 0.18 (mean +/- SD). The GBP CDR increased with age (r2 = 0.46, P < 0.001), and the correlation was improved when only samples from patients taking GBP BID were separately considered (r2 = 0.68, P < 0.001). The ratio was lower in the 10 children younger than 11 years of age (0.07 +/- 0.05) than in 8 adolescents aged 12 to 18 years (0.14 +/- 0.04), lower than in 35 adults aged 19 to 65 years (0.22 +/- 0.13), and lower than in 13 patients older than 65 years of age (0.45 +/- 0.20) by 1-way analysis of variance (F = 19.4, P < 0.001). Analysis of covariance showed a significant influence on GBP CDR of patient age (P < 0.001) and the number of GBP daily doses (P < 0.01), but GBP daily dosage or concomitant AEDs had no significant influence on the ratio.

CONCLUSIONS

In this retrospective study of a small, select group of patients, (1) the GBP CDR increased significantly with age when potential confounding factors such as GBP dosage, number of GBP doses per day, and concomitant AEDs were considered by analysis of covariance, and (2) patients older than 65 years, even without any known renal disease, may have double GBP CDR than younger adults and, therefore, may need half of the GBP dose per body weight to achieve a similar concentration.

Time course of the GABAergic effects of vigabatrin: is the time course of brain GABA related to platelet GABA-transaminase inhibition?

PURPOSE

To analyze the time course of the effects of vigabatrin (VGB) on brain gamma-aminobutyric acid (GABA), and its relation with 4-aminobutyrate-2-ketoglutarate amino-transferase (GABA-T) in brain and platelets.

METHODS

Blood and brain samples were collected at 4, 24, 48, and 72 h after a single dose and after 3 and 8 days of treatment with 200 mg/kg of VGB in rats.

RESULTS

Time courses of the GABAergic effects of VGB were different after single and multiple doses: with multiple doses, the inhibition of brain GABA-T was quicker and longer, the inhibition of platelet GABA-T was greater and longer, the increase in brain GABA was greater, and recovery began earlier. After pooling the data obtained at 4, 24, 48, and 72 h, we observed a power correlation between the increase in brain GABA in individual rats as percentage of the control and both the inhibition of brain GABA-T after a single dose of VGB (r = -0.40; p < 0.05), and the inhibition of platelet GABA-T after 3 days (r = -0.48; p < 0.01) and 8 days of treatment (r = -0.53; p < 0.01). When all data after single and multiple doses were pooled, the increase in brain GABA correlated better with the inhibition of GABA-T in platelets (r = -0.62; p < 0.001) than in brain (r = -0.38; p < 0.001). Platelet GABA-T correlated with brain GABA at 4 h (r = -0.64; p < 0.001) and 24 h (r = -0.66; p < 0.001) but not at 48 and 72 h.

CONCLUSIONS

Platelet GABA-T reflects the time course of the increase in brain GABA better than does brain GABA-T after multiple doses of VGB in rats.

Effects of increasing doses of vigabatrin on platelet gamma-aminobutyric acid-transaminase and brain gamma-aminobutyric acid in rats

We report the relationship of GABA-transaminase inhibition in platelets and brain with the increase in brain gamma-aminobutyric acid (GABA), as percents of the control, at 24 h after single and after 3 and 8 days of treatment with increasing doses (1, 3, 10, 30, 100 and 300 mg kg(-1) day(-1) of vigabatrin in rats. The inhibition of GABA-transaminase in platelets correlated at least as well as that in brain with the increase in brain GABA after 3 days (r = - 0.87 vs. r = -0.78), and 8 days of treatment (r = -0.77 vs. r = -0.74), and when the data of single and multiple doses were pooled (r = -0.77 vs. r = -0.75). The correlation between platelet GABA-transaminase and brain GABA fitted to a power curve, the increase in brain GABA being significant only when platelet GABA-transaminase was inhibited to less than 50% of the control. Our results suggest that platelet GABA-transaminase could be a peripheral marker of the effect of vigabatrin on brain GABA in rats.

Vigabatrin serum concentration to dosage ratio: influence of age and associated antiepileptic drugs

The relationship between the ratio of vigabatrin concentration to dosage (VGB C/D) and both patient age and the presence of other antiepileptic drugs (AEDs) was analyzed retrospectively by bivariate and multivariate methods in 179 patients with epilepsy (114 children and 65 adults). Of the 179 patients, 33 received VGB alone (30 children and 3 adults) and 146 received VGB with other AEDs (84 children and 62 adults). Vigabatrin trough steady-state serum concentration correlated better with VGB dosage in milligrams per kilogram than the dosage in milligrams in children (r = 0.62 vs. r = 0.17, P < 0.001) but not in adults (r = 0.51 vs. r = 0.49, NS). The correlation between milligrams per kilogram and serum concentration of VGB was greater in children on monotherapy (r = 0.83) than in those on polytherapy (r = 0.46). Vigabatrin C/D ratio increased significantly with age (r = 0.51, P < 0.001), being lower in children than in adults both by Student's t-test (0.087 +/- 0.039 vs. 0.128 +/- 0.057, mean +/- SD, P < 0.001) and by two-way analysis of variance when controlling for other AEDs (P < 0.001). Inducing AEDs seemed to increase VGB C/D ratio in the bivariate tests, but this influence decreased and even disappeared if patient age was considered in the multivariate analysis. However, the increase in VGB C/D ratio with VPA serum concentration (r = 0.46, P < 0.001) was confirmed by multiple regression including age (P < 0.001). Intrapatient variability of VGB C/D ratio was 29 +/- 18%. It was concluded that trough steady state VGB serum concentration may be more predictable in children based on the milligrams per kilogram dosage than on the milligram dosage, and that the influence of patient age should be considered if the VGB C/D ratio is used to estimate patient compliance.

Gamma-vinyl GABA (vigabatrin): relationship between dosage, plasma concentrations, platelet GABA-transaminase inhibition, and seizure reduction in epileptic children

The relationship between vigabatrin gamma-vinyl GABA (GVG, vigabatrin) daily dosage or steady-state plasma concentrations (CSS), platelet GABA-transaminase (GABA-T) inhibition, and seizure reduction were studied in 16 children with refractory epilepsy. After 2 months of observation and 1 month of single-blind add-on placebo, a fixed GVG dosage was added for 2 months. The dosage was then adjusted in two 2-month periods each, based on the patient's clinical response. In the fixed-dose period, GVG dosages of 56.8 mg/kg/day and CSS of 8.1 mg/L reduced GABA-T activity from 13.9 to 5.1 pmol/min/mg protein (p less than 0.001) and that of seizures from 51.4 to 22.3 seizures per month (p less than 0.01). Seizure reduction was correlated with dosage (r = 0.83, p less than 0.001), but not with CSS or with platelet GABA-T inhibition. After the GVG dose-adjustment periods, in which dosages of 84.4 mg/kg/day and CSS of 10.6 mg/L were reached, only a slight reduction was observed in both GABA-T activity (from 5.1 to 4.9 pmol/min/mg protein) and seizures (from 22.3 to 18.1 seizures per month). In GVG-responsive patients (excluding placebo-sensitive and GVG-resistant patients), a greater reduction of seizures was achieved (from 17.0 to 7.1 seizures per month, p less than 0.05), which was not accompanied by greater inhibition of GABA-T. GVG treatment in children should be started with a dosage of 50 mg/kg/day, increased to 75 or even 100 mg/kg/day when a partial response is observed. If seizures do not improve or if they become worse, the patient should be considered resistant and GVG should be discontinued.

Effects of single and multiple increasing doses of vigabatrin on brain GABA metabolism and correlation with vigabatrin plasma concentration

The effects of increasing (50-1600 mg/kg/day) doses of vigabatrin (GVG) both as single doses and after 8 or 28 days of treatment have been studied in 19 groups of 10 adult Wistar rats. The parameters studied were brain gamma-aminobutyric acid-transaminase (GABA-T) activity, GABA concentration and L-glutamate decarboxylase (GAD) activity. Single increasing doses of GVG progressively inhibited GABA-T activity, but a residual activity of about 40% was observed with the highest doses. GABA concentration increased in a dose-dependent manner but a ceiling was not reached. GAD activity was slightly inhibited at low doses and stimulated at high ones. When treatment was continued for 8 days, more marked effects of GVG on GABA-T and GABA, a more severe toxicity and higher GVG plasma concentrations were observed. GAD was inhibited instead of stimulated by high GVG doses. After 28 days of treatment the effects of GVG on GABA-T and GABA were similar to those after 8 days. However, toxic effects decreased and lower GVG plasma concentrations were found.

IN CONCLUSION

(a) the more marked brain GABAergic effects observed after 8 days of treatment with GVG may explain the greater anticonvulsant effects observed by others in animals, and (b) GVG plasma concentrations correlate well with changes in brain GABA-T and GABA, and may partly explain changes in the effects of GVG related to the length of treatment.