Tiagabine: a novel antiepileptic drug

Synthesis and Characterization of Novel Methyl (3)5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates

Series of methyl 3- and 5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates were developed and regioselectively synthesized as novel heterocyclic amino acids in their N-Boc protected ester form for achiral and chiral building blocks. In the first stage of the synthesis, piperidine-4-carboxylic and (R)- and (S)-piperidine-3-carboxylic acids were converted to the corresponding β-keto esters, which were then treated with N,N-dimethylformamide dimethyl acetal. The subsequent reaction of β-enamine diketones with various N-mono-substituted hydrazines afforded the target 5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates as major products, and tautomeric NH-pyrazoles prepared from hydrazine hydrate were further N-alkylated with alkyl halides to give 3-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates. The structures of the novel heterocyclic compounds were confirmed by ¹H-, ¹³C-, and ¹⁵N-NMR spectroscopy and HRMS investigation.

Pharmacokinetic of antiepileptic drugs in patients with hepatic or renal impairment

Many factors influence choice of antiepileptic drugs (AEDs), including efficacy of the drug for the indication (epilepsy, neuropathic pain, affective disorder, migraine), tolerability, and toxicity. The first-generation AEDs and some newer AEDs are predominately eliminated by hepatic metabolism. Other recent AEDs are eliminated by renal excretion of unchanged drug or a combination of hepatic metabolism and renal excretion. The effect of renal and hepatic disease on the dosing will depend on the fraction of the AED eliminated by hepatic and/or renal excretion, the metabolic isozymes involved, as well as the extent of protein binding, if therapeutic drug monitoring is used. For drugs that are eliminated by renal excretion, methods of estimating creatinine clearance can be used to determine dose adjustments. For drugs eliminated by hepatic metabolism, there are no specific markers of liver function that can be used to provide guidance in dosage adjustments. Based on studies with probe drugs, the hepatic metabolic enzymes are differentially affected depending on the cause and severity of hepatic disease, which can aid in predicting dose adjustment when clinical data are not available. Several AEDs are also associated with laboratory markers of mild hepatic dysfunction and, rarely, more severe hepatic injury. In contrast, the risk of renal injury from AEDs is generally low. In general, co-morbid hepatic or renal diseases influence the decision for the selection of an AED. For some patients dosing changes to their existing AEDs may be appropriate. For others, a change to another AED may be a better option.

Increase in tiagabine serum concentration with coadministration of gemfibrozil

OBJECTIVE

To report a case of possible acute tiagabine toxicity secondary to administration of gemfibrozil.

CASE SUMMARY

A 39-year-old male was taking tiagabine 16 mg orally 3 times per day and carbamazepine 500 mg orally twice per day for complex partial seizures secondary to mesial temporal sclerosis. He was found to have type IV hypertriglyceridemia and was prescribed gemfibrozil. Because he reported severe confusion and altered consciousness shortly after a single 600-mg dose of gemfibrozil, he was admitted for controlled challenge with that drug. A single 300-mg dose of gemfibrozil resulted in lightheadedness and led to a 59% and 75% increase in total tiagabine serum concentrations at 2 and 5 hours, respectively, without significant change in baseline carbamazepine concentrations.

DISCUSSION

This is the first report of an interaction between the widely used antihyperlipidemic drug gemfibrozil and tiagabine. Since tiagabine, which was originally developed as an antiepileptic medication, is now being used widely for a variety of other indications such as anxiety and depression, there is an increased risk for clinically significant interactions with gemfibrozil.

CONCLUSIONS

Increased total and unbound tiagabine concentrations following a single 300-mg dose of gemfibrozil and reproduction of clinical symptoms with gemfibrozil rechallenge suggests the toxicity our patient experienced was due to a pharmacokinetic drug interaction. Use of the Horn Drug Interaction Probability Scale showed a probable interaction between gemfibrozil and tiagabine.

Use of second-generation antiepileptic drugs in the pediatric population

Epilepsy is common in the pediatric population. Nine second-generation antiepileptic drugs have been approved in the US for use in epilepsy over the past 15 years: felbamate, gabapentin, lamotrigine, topiramate, tiagabine, levetiracetam, oxcarbazepine, zonisamide, and pregabalin. Their use in pediatric patients is fairly widespread, despite most of these agents not having US FDA indications for use. Felbamate and gabapentin were the first two second-generation antiepileptic drugs to be approved in the US. Felbamate use has been limited because of the occurrence of hepatotoxicity and aplastic anemia. Although gabapentin is a fairly well tolerated antiepileptic drug, its use has also been limited as a result of inconsistent efficacy and concern about seizure exacerbation. Lamotrigine and topiramate are broad-spectrum antiepileptic drugs with efficacy in a wide variety of seizure types. Both agents have some tolerability concerns: rash with lamotrigine and neuropsychiatric events with topiramate. There are very little data on tiagabine use in children, but this agent appears to be effective and to have a good tolerability profile. Levetiracetam is a second-generation antiepileptic agent that is available intravenously. Considering its good efficacy, fast onset of action, and low incidence of serious adverse effects, its use in the acute setting could potentially increase. Oxcarbazepine and zonisamide have been relatively well studied in pediatric seizure patients, including use as monotherapy. Both agents have demonstrated good efficacy and tolerability for patients as young as 1 month old. Vigabatrin and rufinamide are currently not available in the US, but have been shown to have some success in other countries. Pregabalin is the newest antiepileptic agent, but lacks pediatric data currently.

Tiagabine is neuroprotective in the N171-82Q and R6/2 mouse models of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by chorea, incoordination, and shortened life-span, and by huntingtin inclusions and neurodegeneration. We previously screened the 1040 FDA-approved compounds from the NINDS compound library and found that a compound, nipecotic acid, significantly reduced mutant huntingtin aggregations and blocked cell toxicity in an inducible cell model of HD. Because nipecotic acid does not cross the blood-brain barrier (BBB), we studied its analogue, tiagabine, which is able to cross the BBB, in both N171-82Q and R6/2 transgenic mouse models of HD. Tiagabine was administered intraperitoneally at 2 and 5 mg/kg daily in HD mice. We found that tiagabine extended survival, improved motor performance, and attenuated brain atrophy and neurodegeneration in N171-82Q HD mice. These beneficial effects were further confirmed in R6/2 HD mice. The levels of tiagabine at effective doses in mouse serum are comparable to the levels in human patients treated with tiagabine. These results suggest that tiagabine may have beneficial effects in the treatment of HD. Because tiagabine is an FDA-approved drug, it may be a promising candidate for future clinical trials for the treatment of HD.

Application of TDM, pharmacogenomics and biomarkers for neurological disease pharmacotherapy: focus on antiepileptic drugs

Anticonvulsants, or antiepileptic drugs (AEDs), are a vital tool in the therapeutic management of epilepsy patients. However, many AEDs are commonly used in the management of nonepileptic conditions, such as chronic pain, migraine headaches and psychiatric disorders. It is well documented that serum drug levels are an important data tool for the management of patients taking these drugs. As we move toward the personalized optimization of pharmacotherapy, drug level data will not be sufficient. This article will review tools for therapeutic drug management of AEDs including pharmacogenetics and biomarkers, in addition to traditional serum drug levels.

Antiepileptic action induced by a combination of vigabatrin and tiagabine

Vigabatrin, an inhibitor of GABA breakdown by GABA transaminase and of GABA transporter isoform 1 (GAT1), and tiagabine, a highly specific inhibitor of GAT1, have successfully been applied in the treatment of epilepsy. We investigated the effects of individual and combined application of these drugs on GAT1 expressed in Xenopus oocytes, and examined the effects on epileptiform discharges in the CA3 area of brain slices of genetically epileptic El and control ddY mice, and on the occurrence of seizures in El mice. Simultaneous application of vigabatrin and tiagabine inhibited epileptiform discharges induced by high-K+ solution in the brain slices in an antagonistic fashion. The degree of inhibition by tiagabine after pre-treatment with vigabatrin was additive in ddY mice and synergistic in El mice. In Mg2+-free solution, co-treatment by the two drugs produced additive inhibition in slices from both mouse strains, but pre-treatment with vigabatrin produced synergistic inhibition in slices only from ddY mice. In the slices from El mice, a combination of drugs resulted in additive effects in both co- and pre-treatment by the drugs. Although these drugs are also effective in vivo at suppressing seizure occurrence in El mice, the combined application does not show synergistic effects, but rather is antagonistic under the experimental conditions in this particular variant of epilepsy. The synergistic inhibition of epileptiform discharges in brain slices may, in part, have originated from the complex interaction with GAT1. In experiments on the GAT1 expressed in oocytes it could be demonstrated that synergistic inhibition occurs only at low concentration (0.1 nM) of vigabatrin. This illustrates that the oocytes may form a powerful test system for drug screening and investigation of complex drug interactions. These results present a novel interpretation of synergistic inhibition of certain epileptic discharges using vigabatrin and another drug, and that for successful synergistic treatment of epilepsies carefully designed timed dosage regimens are essential.

GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.

Tiagabine in treatment refractory bipolar disorder: a clinical case series

OBJECTIVES

Anticonvulsants have provided major treatment advances for patients with bipolar disorder. Many of these drugs, including several with proven efficacy in bipolar mania or depression, enhance the activity of the gamma-amino butyric acid (GABA) neurotransmitter system. A new anticonvulsant, tiagabine, has selective GABAergic activity and is approved for patients with partial epilepsy. Few reports of its potential effectiveness in bipolar disorder, however, have been published. We sought to evaluate the effectiveness of tiagabine added to ongoing medication regimens in patients with bipolar disorder inadequately responsive to or intolerant of usual treatments.

METHODS

Seventeen treatment-refractory patients participating in the Stanley Foundation Bipolar Network (SFBN) long-term follow-up study were offered open treatment with add-on tiagabine after discussion of the risks, benefits, other treatment options and giving informed consent. Patients' clinical symptoms and somatic complaints were closely monitored with SFBN longitudinal and cross-sectional ratings. Four patients discontinued low-dose tiagabine prior to the second visit and were excluded from data analysis.

RESULTS

Thirteen patients received a mean of 38 days of treatment at a mean dose of 8.7 mg/day of tiagabine. On the Clinical Global Impression Scale for Bipolar Disorder Overall category, three (23%) patients showed much or very much improvement and 10 (77%) patients showed no change or worsening. Three significant adverse events were noted, including two presumptive seizures.

CONCLUSIONS

Open add-on tiagabine for treatment-refractory patients with bipolar disorder demonstrated limited efficacy with the majority of patients showing no change or worsening of clinical symptoms. In addition, patients experienced serious side-effects attributed as likely due to the medication, which resolved without lasting consequence when tiagabine was discontinued.

Seizures in HIV-seropositive individuals: epidemiology and treatment

Seizures are a relatively common occurrence in patients with HIV infection. They may be a result of HIV infection of the CNS or a manifestation of an opportunistic infection. Because seizures are likely to recur in patients infected with HIV and because they are a poor prognostic indicator, it is generally recommended that all HIV-seropositive patients experiencing a first seizure without a recognisable and reversible cause be treated. Clinicians faced with treating seizures in HIV-seropositive patients often encounter a therapeutic dilemma since few data exist in this area. In selecting appropriate anticonvulsant therapy, clinicians must consider both therapy-compromising drug-drug and drug-disease interactions. Ideal anticonvulsants for this setting are those that do not effect viral replication, have limited protein binding and have no effects on the cytochrome P450 system, such as gabapentin, topiramate and tiagabine. Unless the benefits outweigh the risks, valproic acid (sodium valproate) should be avoided as it has been shown to stimulate HIV replication. Since few data exist, controlled trials examining pharmacokinetic and pharmacodynamic interactions between anticonvulsants and antiretrovirals are needed. Until such time, clinicians caring for these patients should examine existing data carefully and employ vigilant monitoring.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

The new generation of GABA enhancers. Potential in the treatment of epilepsy

gamma-Aminobutyric acid (GABA) is considered to be the major inhibitory neurotransmitter in the brain and loss of GABA inhibition has been clearly implicated in epileptogenesis. GABA interacts with 3 types of receptor: GABAA, GABAB and GABAC. The GABAA receptor has provided an excellent target for the development of drugs with an anticonvulsant action. Some clinically useful anticonvulsants, such as the benzodiazepines and barbiturates and possibly valproic acid (sodium valproate), act at this receptor. In recent years 4 new anticonvulsants, namely vigabatrin, tiagabine, gabapentin and topiramate, with a mechanism of action considered to be primarily via an effect on GABA, have been licensed. Vigabatrin elevates brain GABA levels by inhibiting the enzyme GABA transaminase which is responsible for intracellular GABA catabolism. In contrast, tiagabine elevates synaptic GABA levels by inhibiting the GABA uptake transporter, GAT1, and preventing the uptake of GABA into neurons and glia. Gabapentin, a cyclic analogue of GABA, acts by enhancing GABA synthesis and also by decreasing neuronal calcium influx via a specific subunit of voltage-dependent calcium channels. Topiramate acts, in part, via an action on a novel site of the GABAA receptor. Although these drugs are useful in some patients, overall, they have proven to be disappointing as they have had little impact on the prognosis of patients with intractable epilepsy. Despite this, additional GABA enhancing anticonvulsants are presently under development. Ganaxolone, retigabine and pregabalin may prove to have a more advantageous therapeutic profile than the presently licensed GABA enhancing drugs. This anticipation is based on 2 characteristics. First, they act by hitherto unique mechanisms of action in enhancing GABA-induced neuronal inhibition. Secondly, they act on additional antiepileptogenic mechanisms. Finally, CGP 36742, a GABAB receptor antagonist, may prove to be particularly useful in the management of primary generalised absence seizures. The exact impact of these new GABA-enhancing drugs in the treatment of epilepsy will have to await their licensing and a period of postmarketing surveillance. As to clarification of their role in the management of epilepsy, this will have to await further clinical trials, particularly direct comparative trials with other anticonvulsants.

Treatment of painful sensory neuropathy with tiagabine: a pilot study

To evaluate the effect of tiagabine hydrochloride in painful neuropathy in a pilot, open-label study. Painful neuropathy is characterized by preferential involvement of small sensory and autonomic fibers. Tiagabine increases gamma-aminobutyric acid and might enhance the central pain-control mechanisms. Seventeen patients (10 men, 7 women; mean age 51.4 +/- 7.7 y) with chronic painful neuropathy (>6 months) were enrolled in this study. Week 0: All pain medications were discontinued. Weeks 1-4: Dose of tiagabine was increased weekly by 4 mg orally up to 16 mg in week 4. Quantitative sensory testing for vibration, cooling, and heat-pain, and quantitative sudomotor axon reflex test (QSART) were done at week 0 and week 4. The McGill Pain Questionnaire was administered weekly. Nine patients completed the study; 8 patients discontinued the treatment. Baseline pain intensity was 6.2 +/- 3.1 on the McGill Pain Questionnaire scale (0-10 range). Low doses (4-8 mg) of tiagabine reduced pain symptoms by 16-38%, improving surface pain (37.5%), skin sensitivity (32.8%), burning (38.6%), cold (25.4%) and pain sharpness (29%; p <0.03). Dull and deep pain did not improve. Quantitative sensory testing abnormalities diminished with treatment (p <0.02). Autonomic test results did not change. This pilot study evaluated the potential of tiagabine hydrochloride (Gabitril) in treatment of painful sensory neuropathy. Pain symptoms and quantitative sensory test results improved with treatment, especially at low doses of tiagabine (4-8 mg). Higher doses (12-16 mg) were associated with increased number of adverse events. Tiagabine may have potential benefits for treatment of painful neuropathy; however, assessment of its efficacy in a larger study is needed.

Attenuation of malonate toxicity in primary mesencephalic cultures using the GABA transport blocker, NO-711

Cultured rat mesencephalic neurons were used to assess the effects of gamma-aminobutyric acid (GABA) transport blockers on toxicity caused by malonate, a reversible, competitive inhibitor of succinate dehydrogenase. Previous studies utilizing an ex vivo chick retinal preparation have shown that GABA release and cell swelling are early consequences of acute energy impairment and that GABA transport blockers attenuate this toxicity. The present results demonstrate that the nonsubstrate GABA transport blocker, NO-711 (1 nM-1 microM), dose-dependently protected cultured mesencephalic dopamine (DA) and GABA neurons from malonate-induced toxicity. Similar protection was demonstrated with nipecotic acid (1 mM) and SKF89976A (100 nM), substrate and nonsubstrate GABA transport blockers, respectively. These compounds by themselves produced no signs of toxicity, although nipecotic acid caused a long-term decrease in GABA uptake not associated with toxicity. Compounds which decrease intracellular reactive oxygen species (ROS) are protective in this model, but NO-711 did not prevent the rise in intracellular ROS induced by malonate, indicating its protective effects were downstream of ROS production. Supplementation of malonate treated cultures with the GABA(A) agonist, muscimol (10 microM), increased the toxicity toward the DA and GABA neuron populations. Antagonists at the GABA(A) and glycine receptors provided partial protection to both the GABA and DA neurons. These findings suggest that the GABA transporter, GABA(A), and/or glycine channels contribute to cell damage associated with energy impairment in this model.

Safety of long-term treatment with tiagabine

The aim of this study was to evaluate the safety of long-term treatment with tiagabine. We reviewed the case report forms of patients with refractory partial epilepsy who took tiagabine for longer than 6 months in two long-term studies. We classified all adverse events based on severity and persistence, and recorded the dose at onset of each adverse event. We then divided patients into those treated for 6-12 months, 12-24 months and > 24 months. We compared the adverse event profile and change in seizure frequency among the three groups. Forty-two patients took tiagabine for longer than 6 months. The mean duration of treatment was 22.6 months. The mean monthly seizure frequency was 12.7 at baseline and 8.1 at study termination (36% decrease). The most common adverse events were: tiredness (56%), headache (46%), dizziness (44%), visual symptoms (blurring, difficulty focusing, diplopia) (39%), altered mentation (32%), and tremor (31%). The adverse event profile was comparable among the three groups. Seizure frequency was significantly more improved in the > 24 months group. Long-term treatment with tiagabine is well tolerated. The most important predictor of long-term therapy with tiagabine was the degree of seizure improvement.

Neuroprotective activity of tiagabine in a focal embolic model of cerebral ischemia

Gamma aminobutyric acid (GABA) agonists have been shown to have neuroprotective effects when used after focal or global cerebral ischemia. In this study, we evaluated the neuroprotective effects of a GABA re-uptake inhibitory agent, tiagabine, on focal ischemic brain injury in an embolic model in rats. Tiagabine, injected at 1 h after embolization, significantly reduced brain infarction volume, measured with 2,3,5-triphenyltetrazolium chloride (TTC) histological assay. There were varying degrees of neuroprotective effect exhibited in the other experimental groups however this did not reach significance. These results suggest that tiagabine is neuroprotective when administrated at an early period after the ischemic brain injury.

Neurotransmitter transporters: fruitful targets for CNS drug discovery

More than 20 members have been identified in the neurotransmitter transporter family. These include the cell surface re-uptake mechanisms for monoamine and amino acid neurotransmitters and vesicular transporter mechanisms involved in neurotransmitter storage. The norepinephrine and serotonin re-uptake transporters are key targets for antidepressant drugs. Clinically effective antidepressants include those with selectivity for either NE or serotonin uptake, and compounds with mixed actions. The dopamine transporter plays a key role in mediating the actions of cocaine and the amphetamines and in conferring selectivity on dopamine neurotoxins. The only clinically used compound to come so far from research on amino acid transporters is the antiepileptic drug tiagabine, a GABA uptake inhibitor.

Age-dependence of the anticonvulsant effects of the GABA uptake inhibitor tiagabine in vitro

Epileptic syndromes frequently start at childhood and therefore it is crucial to test new anticonvulsants at immature stages of the nervous system. We compared the effects of the gamma-aminobutyric acid (GABA) uptake inhibitor tiagabine [(R)-N-(4, 4-bis(3-methyl-2-thienyl)but)3-en-1-yl nipecotic acid] on low-Mg(2+)-induced epileptic discharges in brain slices from rat pups (p 5-8) and juvenile animals (p 15-20). In tissue from rat pups, tiagabine slightly reduced epileptiform activity in hippocampal area CA1 but had no effect in the entorhinal cortex. In juvenile rats, epileptiform discharges were unaffected in CA1 but suppressed by 60% in the entorhinal cortex. While tiagabine increases its efficacy with age, in-situ hybridisation and PCR analysis show that mRNA coding for the neuronal GABA-transporter GAT-1 is already present at p 5. We therefore conclude that the increasing efficacy of tiagabine during ontogenesis is due to functional maturation of GABAergic synapses rather than to up-regulation of GAT-1 expression.