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Gabitril

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Gabitril

CLINICAL PHARMACOLOGY

Mechanism of Action

The precise mechanism by which tiagabine exerts its antiseizure effect is unknown, although it is believed to be related to its ability, documented in in vitro experiments, to enhance the activity of gamma aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system. These experiments have shown that tiagabine binds to recognition sites associated with the GABA uptake carrier. It is thought that, by this action, tiagabine blocks GABA uptake into presynaptic neurons, permitting more GABA to be available for receptor binding on the surfaces of post-synaptic cells. Inhibition of GABA uptake has been shown for synaptosomes, neuronal cell cultures, and glial cell cultures. In rat-derived hippocampal slices, tiagabine has been shown to prolong GABA-mediated inhibitory post-synaptic potentials. Tiagabine increases the amount of GABA available in the extracellular space of the globus pallidus, ventral palladum, and substantia nigra in rats at the ED50 and ED85 doses for inhibition of pentylenetetrazol (PTZ)- induced tonic seizures. This suggests that tiagabine prevents the propagation of neural impulses that contribute to seizures by a GABA-ergic action.

Tiagabine has shown efficacy in several animal models of seizures. It is effective against the tonic phase of subcutaneous PTZ-induced seizures in mice and rats, seizures induced by the proconvulsant DMCM in mice, audiogenic seizures in genetically epilepsy-prone rats (GEPR), and amygdala-kindled seizures in rats. Tiagabine has little efficacy against maximal electroshock seizures in rats and is only partially effective against subcutaneous PTZ-induced clonic seizures in mice, picrotoxin-induced tonic seizures in the mouse, bicuculline-induced seizures in the rat, and photic seizures in photosensitive baboons. Tiagabine produces a biphasic dose-response curve against PTZ- and DMCM-induced convulsions, with attenuated effectiveness at higher doses.

Based on in vitro binding studies, tiagabine does not significantly inhibit the uptake of dopamine, norepinephrine, serotonin, glutamate, or choline and shows little or no binding to dopamine Dl and D2, muscarinic, serotonin 5HT1A,5HT2, and 5HT3, beta-1 and 2 adrenergic, alpha-1 and alpha-2 adrenergic, histamine H2 and H3, adenosine A1 and A2, opiate µ and K1, NMDA glutamate, and GABAA receptors at 100 µM. It also lacks significant affinity for sodium or calcium channels. Tiagabine binds to histamine H1, serotonin 5HT1B, benzodiazepine, and chloride channel receptors at concentrations 20 to 400 times those inhibiting the uptake of GABA.

Pharmacokinctics

Tiagabine is well absorbed, with food slowing absorption rate but not altering the extent of absorption. The elimination half-life of tiagabine is 7 to 9 hours in normal volunteers. In epilepsy clinical trials, most patients were receiving hepatic enzyme-inducing agents (e.g., carbamazepine, phenytoin, primidone, and phenobarbital). The pharmacokinetic profile in induced patients is significantly different from the non-induced population (see PRECAUTIONS, General, Use in Non-Induced Patients). The systemic clearance of tiagabine in induced patients is approximately 60% greater resulting in considerably lower plasma concentrations and an elimination half-life of 2 to 5 hours. Given this difference in clearance, the systemic exposure after a dose of 32 mg/day in an induced population is expected to be comparable to the systemic exposure after a dose of 12 mg/day in a non-induced population. Similarly, the systemic exposure after a dose of 56 mg/day in an induced population is expected to be comparable to the systemic exposure after a dose of 22 mg/day in a non-induced population.

Absorption and Distribution: Absorption of tiagabine is rapid, with peak plasma concentrations occurring at approximately 45 minutes following an oral dose in the fasting state. Tiagabine is nearly completely absorbed ( > 95%), with an absolute oral bioavailability of about 90%. A high fat meal decreases the rate (mean Tmax was prolonged to 2.5 hours, and mean Cmax was reduced by about 40%) but not the extent (AUC) of tiagabine absorption. In all clinical trials, tiagabine was given with meals.

The pharmacokinetics of tiagabine are linear over the single dose range of 2 to 24 mg. Following multiple dosing, steady state is achieved within 2 days.

Tiagabine is 96% bound to human plasma proteins, mainly to serum albumin and al-acid glycoprotein over the concentration range of 10 ng/mL to 10,000 ng/mL. While the relationship between tiagabine plasma concentrations and clinical response is not currently understood, trough plasma concentrations observed in controlled clinical trials at doses from 30 to 56 mg/day ranged from < 1 ng/mL to 234 ng/mL.

Metabolism and Elimination: Although the metabolism of tiagabine has not been fully elucidated, in vivo and in vitro studies suggest that at least two metabolic pathways for tiagabine have been identified in humans: 1) thiophene ring oxidation leading to the formation of 5-oxo-tiagabine; and 2) glucuronidation. The 5-oxo-tiagabine metabolite does not contribute to the pharmacologic activity of tiagabine.

Based on in vitro data, tiagabine is likely to be metabolized primarily by the 3 A isoform subfamily of hepatic cytochrome P450 (CYP 3 A), although contributions to the metabolism of tiagabine from CYP 1A2, CYP 2D6 or CYP 2C19 have not been excluded.

Approximately 2% of an oral dose of tiagabine is excreted unchanged, with 25% and 63% of the remaining dose excreted into the urine and feces, respectively, primarily as metabolites, at least 2 of which have not been identified. The mean systemic plasma clearance is 109 mL/min (CV = 23%) and the average elimination half-life for tiagabine in healthy subjects ranged from 7 to 9 hours. The elimination half-life decreased by 50 to 65% in hepatic enzyme-induced patients with epilepsy compared to uninduced patients with epilepsy.

A diurnal effect on the pharmacokinetics of tiagabine was observed. Mean steady-state Cmin values were 40% lower in the evening than in the morning. Tiagabine steady-state AUC values were also found to be 15% lower following the evening tiagabine dose compared to the AUC following the morning dose.

Special Populations

Renal Insufficiency: The pharmacokinetics of total and unbound tiagabine were similar in subjects with normal renal function (creatinine clearance > 80 mL/min) and in subjects with mild (creatinine clearance 40 to 80 mL/min), moderate (creatinine clearance 20 to 39 mL/min), or severe (creatinine clearance 5 to 19 mL/min) renal impairment. The pharmacokinetics of total and unbound tiagabine were also unaffected in subjects with renal failure requiring hemodialysis.

Hepatic Insufficiency: In patients with moderate hepatic impairment (Child-Pugh Class B), clearance of unbound tiagabine was reduced by about 60%. Patients with impaired liver function may require reduced initial and maintenance doses of tiagabine and/or longer dosing intervals compared to patients with normal hepatic function (see PRECAUTIONS).

Geriatric: The pharmacokinetic profile of tiagabine was similar in healthy elderly and healthy young adults.

Pediatric: Tiagabine has not been investigated in adequate and well-controlled clinical trials in patients below the age of 12. The apparent clearance and volume of distribution of tiagabine per unit body surface area or per kg were fairly similar in 25 children (age: 3 to 10 years) and in adults taking enzyme-inducing antiepilepsy drugs ([AEDs] e.g., carbamazepine or phenytoin). In children who were taking a non-inducing AED (e.g., valproate), the clearance of tiagabine based upon body weight and body surface area was 2 and 1.5-fold higher, respectively, than in non-induced adults with epilepsy.

Gender, Race and Cigarette Smoking: No specific pharmacokinetic studies were conducted to investigate the effect of gender, race and cigarette smoking on the disposition of tiagabine. Retrospective pharmacokinetic analyses, however, suggest that there is no clinically important difference between the clearance of tiagabine in males and females, when adjusted for body weight. Population pharmacokinetic analyses indicated that tiagabine clearance values were not significantly different in Caucasian (N=463), Black (N=23), or Hispanic (N=17) patients with epilepsy, and that tiagabine clearance values were not significantly affected by tobacco use.

Interactions with other Antiepilepsy Drugs: The clearance of tiagabine is affected by the co-administration of hepatic enzyme-inducing antiepilepsy drugs. Tiagabine is eliminated more rapidly in patients who have been taking hepatic enzyme-inducing drugs, e.g., carbamazepine, phenytoin, primidone and phenobarbital than in patients not receiving such treatment (see PRECAUTIONS: DRUG INTERACTIONS).

Interactions with Other Drugs: See PRECAUTIONS: DRUG INTERACTIONS.

Clinical Studies

The effectiveness of GABITRIL (tiagabine hydrochloride) as adjunctive therapy (added to other antiepilepsy drugs) was examined in three multi-center, double-blind, placebo-controlled, parallel-group, clinical trials in 769 patients with refractory partial seizures who were taking at least one hepatic enzyme-inducing antiepilepsy drug (AED), and two placebo-controlled cross-over studies in 90 patients. In the parallel-group trials, patients had a history of at least six complex partial seizures (Study 1 and Study 2, U.S. studies), or six partial seizures of any type (Study 3, European study), occurring alone or in combination with any other seizure type within the 8-week period preceding the first study visit in spite of receiving one or more AEDs at therapeutic concentrations.

In the first two studies, the primary protocol-specified outcome measure was the median reduction from baseline in the 4-week complex partial seizure (CPS) rates during treatment. In the third study, the protocol-specified primary outcome measure was the proportion of patients achieving a 50% or greater reduction from baseline in the 4-week seizure rate of all partial seizures during treatment. The results given below include data for complex partial seizures and all partial seizures for the intent-to-treat population (all patients who received at least one dose of treatment and at least one seizure evaluation) in each study.

Study 1 was a double-blind, placebo-controlled, parallel-group trial comparing GABITRIL (tiagabine hydrochloride) 16 mg/day, GABITRIL (tiagabine hydrochloride) 32 mg/day, GABITRIL (tiagabine hydrochloride) 56 mg/day, and placebo. Study drug was given as a four times a day regimen. After a prospective Baseline Phase of 12 weeks, patients were randomized to one of the four treatment groups described above. The 16-week Treatment Phase consisted of a 4-week Titration Period, followed by a 12-week Fixed-Dose Period, during which concomitant AED doses were held constant. The primary outcome was assessed for the combined 32 and 56 mg/day groups compared to placebo.

Study 2 was a double-blind, placebo-controlled, parallel-group trial consisting of an 8-week Baseline Phase and a 12-week Treatment Phase, the first 4 weeks of which constituted a Titration Period and the last 8 weeks a Fixed-Dose Period. This study compared GABITRIL (tiagabine hydrochloride) 16 mg BID and 8 mg QID to placebo. The protocol-specified primary outcome measure was assessed separately for each group treated with GABITRIL (tiagabine hydrochloride) .

The following tables display the results of the analyses of these two trials.

Table 1: Median Reduction and Median Percent Reduction from Baseline in 4-Week Seizure Rates in Study 1

    Placebo
(N=91)
GABITRIL (tiagabine hydrochloride)
16 mg/day
(N=61)
GABITRIL (tiagabine hydrochloride)
32 mg/day
(N=87)
GABITRIL (tiagabine hydrochloride)
56 mg/day
(N=56)
Combined 32 and 56 mg/day
(N=143)
Complex Partial Median Reduction 0.6 0.8 2.2* 2.9* 2.6*
Median % Reduction 9% 13% 25% 32% 29%
All Partial Median Reduction 0.2 1.2 2.7* 3.5* 2.9*
Median % Reduction 3% 12% 24% 36% 27%
* p < 0.05
Statistical significance was not assessed for median % reduction.

Table 2: Median Reduction and Median Percent Reduction from Baseline in 4-Week Seizure Rates in Study 2

    Placebo
(N=107)
GABITRIL (tiagabine hydrochloride)
16 mg BID
(N=106)
GABITRIL (tiagabine hydrochloride)
8 mg QID
(N=104)
Complex Partial Median Reduction 0.3 1.6 1.3*
Median % Reduction 4% 22% 15%
All Partial Median Reduction 0.5 1.6 1.3
Median % Reduction 5% 19% 13%
* p < 0.027, necessary for statistical significance due to multiple comparisons.
Statistical significance was not assessed for median % reduction.

Figures 1 to 4 present the proportion of patients (X-axis) whose percent reduction from baseline in the all partial seizure rate was at least as great as that indicated on the Y axis in the three placebo-controlled adjunctive studies (Studies 1, 2, and 3). A positive value on the Y axis indicates an improvement from baseline (i.e., a decrease in seizure rate), while a negative value indicates a worsening from baseline (i.e., an increase in seizure rate). Thus, in a display of this type, the curve for an effective treatment is shifted to the left of the curve for placebo.

Figure 1 indicates that the proportion of patients achieving any particular level of reduction in seizure rate was consistently higher for the combined GABITRIL (tiagabine hydrochloride) 32 mg and 56 mg groups compared to the placebo group in Study 1. For example, Figure 1 indicates that approximately 24% of patients treated with GABITRIL (tiagabine hydrochloride) experienced a 50% or greater reduction, compared to 4% in the placebo group.

Figure 1: Study 1

Proportion of patients achieving any particular level of reduction in seizure rate -  Illustration

Figure 2 also displays the results for Study 1, which was a dose-response study, by treatment group, without combining GABITRIL (tiagabine hydrochloride) dosage groups. Figure 2 indicates a dose-response relationship across the three GABITRIL (tiagabine hydrochloride) groups. The proportion of patients achieving any particular level of reduction in all partial seizure rates was consistently higher as the dose of GABITRIL (tiagabine hydrochloride) was increased. For example, Figure 2 indicates that approximately 4% of patients in the placebo group experienced a 50% or greater reduction in all partial seizure rate, compared to approximately 10% of the GABITRIL (tiagabine hydrochloride) 16 mg/day group, 21% of the GABITRIL (tiagabine hydrochloride) 32 mg/day group, and 30% of the GABITRIL (tiagabine hydrochloride) 56 mg/day group.

Figure 2: Study l

Dose-response relationship across the three GABITRIL groups -  Illustration

Figure 3 indicates that the proportion of patients achieving any particular level of reduction in partial seizure rate was consistently greater in patients taking GABITRIL (tiagabine hydrochloride) than in those taking placebo in Study 2. (Study 2 compared placebo to GABITRIL (tiagabine hydrochloride) 32 mg/day; one of the GABITRIL (tiagabine hydrochloride) groups received 8 mg QID, while the other GABITRIL (tiagabine hydrochloride) group received 16 mg BID). For example, Figure 3 indicates that approximately 7% of patients in the placebo group experienced a 50% or greater reduction in their partial seizure rate, compared to approximately 23% of patients in the GABITRIL (tiagabine hydrochloride) 8 mg QID group and 28% of patients in the GABITRIL (tiagabine hydrochloride) 16 mg BID group.

Figure 3: Study 2

Proportion of patients achieving any particular level of reduction in partial seizure rate -  Illustration

Study 3 was a double-blind, placebo-controlled, parallel-group trial that compared GABITRIL (tiagabine hydrochloride) 10 mg TID (N=77) with placebo (N=77). In this trial, patients were followed prospectively during a 12-week Baseline Phase and then randomized to receive study drug during an 18-week Treatment Phase. During the first 6 weeks of treatment (Titration Period), patients were titrated to 30 mg/day, after which they were maintained on this dose during the 12-week Fixed-Dose Period. The protocol-specified primary outcome measure (proportion of patients who achieved at least a 50% reduction from baseline in partial seizure rate) did not reach statistical significance. However, analyses of the median reduction from baseline in 4-week partial seizure rate (the analyses presented above for Study 1 and Study 2) were performed and showed a statistically significant improvement compared to placebo in all partial and complex partial seizure rates (Table 3):

Table 3: Median Reduction and Median Percent Reduction from Baseline in 4-Week Seizure Rates in Study 3

    Placebo
(N=77)
GABITRIL (tiagabine hydrochloride)
30 mg/day
(N=77)
Complex Partial Median Reduction -0.1 1.3*
Median % Reduction -1% 14%
All Partial Median Reduction -0.5 1.1*
Median % Reduction -7% 11%
* p < 0.05
Statistical significance was not assessed for median % reduction.
N=72 and 75 for placebo and GABITRIL (tiagabine hydrochloride) , respectively.

Figure 4 indicates that the proportion of patients achieving any particular level of reduction in seizure activity was consistently higher in those taking GABITRIL (tiagabine hydrochloride) than those taking placebo in Study 3. For example, Figure 4 indicates that approximately 5% of patients in the placebo group experienced a 50% or greater reduction in their partial seizure rate compared to approximately 10% of patients in the GABITRIL (tiagabine hydrochloride) group.

Figure 4: Study 3

Proportion of patients achieving any particular level of reduction in seizure activity -  Illustration

The two other placebo-controlled trials that examined the effectiveness of GABITRIL (tiagabine hydrochloride) were small cross-over trials (N=46 and 44). Both trials included an open Screening Phase during which patients were titrated to an optimal dose and then treated with this dose for an additional 4 weeks. After this Open Phase, patients were randomized to one of two blinded treatment sequences (GABITRIL (tiagabine hydrochloride) followed by placebo or placebo followed by GABITRIL (tiagabine hydrochloride) ). The Double-Blind Phase consisted of two Treatment Periods, each lasting 7 weeks (with a 3 week washout between periods). The outcome measures were median with-in patient differences between placebo and GABITRIL (tiagabine hydrochloride) Treatment Periods in 4-week complex partial and all partial seizure rates. The reductions in seizure rates were statistically significant in both studies.

Animal Toxicology

In repeat dose toxicology studies, dogs receiving daily oral doses of 5 mg/kg/day or greater experienced unexpected CNS effects throughout the study. These effects occurred acutely and included marked sedation and apparent visual impairment which was characterized by a lack of awareness of objects, failure to fix on and follow moving objects, and absence of a blink reaction. Plasma exposures (AUCs) at 5 mg/kg/day were equal to those in humans receiving the maximum recommended daily human dose of 56 mg/day. The effects were reversible upon cessation of treatment and were not associated with any observed structural abnormality. The implications of these findings for humans are unknown.

Last reviewed on RxList: 11/5/2010
This monograph has been modified to include the generic and brand name in many instances.

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