Pharmacokinetics and tolerability of single escalating doses of gabapentin enacarbil: a randomized-sequence, double-blind, placebo-controlled crossover study in healthy volunteers

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Anticancer Effect of Chlorambucil Enhanced by Chiral Phthalidyl Promoiety

Phthalidyl promoiety has been used in several drugs, but they were all marketed in racemic form. The pharmaceutical effects of each enantiomer have not been clearly demonstrated. In this project, an anticancer chemotherapy drug, chlorambucil, was modified as enantiopure phthalidyl prodrugs. The enantiomers, together with phthalidyl unit and their racemic mixture, were then subject to the in vivo bioactivity tests against B16F10 melanoma cells. It was found that proper chirality within the promoiety had noticeably better in vivo pharmacological effects than the parent drug, the enantiomer and racemic mixture. This merit perhaps could be extended from the phthalidyl prodrugs to other chirality containing prodrugs.

Interindividual Variability in the Bioavailability of Gabapentin Enacarbil Extended Release in Healthy Adults: An Analysis of Data From 6 Phase I Studies

BACKGROUND

The absorption and bioavailability of oral gabapentin are associated with a high degree of interindividual variability. Gabapentin enacarbil, a prodrug of gabapentin, is well absorbed and provides sustained, dose-proportional exposure to gabapentin. The aim of this analysis was to describe the interindividual variability in the bioavailability of gabapentin after gabapentin enacarbil administration in healthy subjects.

METHODS

Gabapentin pharmacokinetic (PK) parameters after an oral dose of gabapentin enacarbil 1200 mg (2 600-mg tablets) were compared across 6 phase I studies in healthy adults (n = 12 per study). The distribution of bioavailability values was assessed in all studies.

RESULTS

The mean PK parameters of gabapentin were consistent across the trials: maximum concentration range: 6.4-7.9 μg/mL, time to maximum concentration range: 5.2-8.2 hours, area under the plasma-concentration curve extrapolated from time 0 to infinity or at steady state range: 70.8-109.4 μg·h/mL, and bioavailability range: 64.8%-82.9%. Overall, the mean bioavailability was 74.1% (SD, 14.1; coefficient of variation, 19.1%). Individual bioavailability across all studies ranged from 42% to 100%.

CONCLUSIONS

Gabapentin PK after gabapentin enacarbil administration was consistent across studies, with low interindividual variability in bioavailability. Gabapentin enacarbil may provide more consistent and predictable exposure to gabapentin than oral gabapentin formulations.

Nighttime Agitation and Restless Legs Syndrome in Persons With Alzheimer's Disease: Study Protocol for a Double-Blind, Placebo-Controlled, Randomized Trial (NightRest)

Nighttime agitation is a prevalent symptom in persons with Alzheimer's disease (AD). Effective treatments are absent due to our limited knowledge of its etiology. We hypothesized that restless legs syndrome (RLS), a common neurological sensorimotor disorder of uncomfortable leg sensations that appear at night and interfere with sleep, might be a cause for nighttime agitation in persons with AD. RLS is infrequently identified in persons with AD because traditional diagnosis is dependent on patients answering complex questions about their symptoms. With a validated observational tool for RLS diagnosis, the Behavioral Indicators Test-Restless Legs, we aim to diagnose RLS and determine the effect of gabapentin enacarbil (GEn) compared to placebo on nighttime agitation, sleep, antipsychotic medications, and the mechanism for these effects. We hypothesize that frequency of RLS behaviors will mediate the relationship between GEn and nighttime agitation. This study is an 8-week, double-blind, placebo-controlled, randomized pilot clinical trial, followed by an 8-week open-label trial, that is being conducted in long-term care settings and private homes. The results of this study may shift, personalize, and improve standards of care for treatment of nighttime agitation; reduce aggression and other nighttime agitation behaviors; and improve sleep.

TARGETS

Persons with AD with nighttime agitation potentially caused by RLS.

INTERVENTION DESCRIPTION

Diagnose RLS and determine the effect of GEn.

MECHANISMS OF ACTION

The frequency of RLS behaviors will mediate the relationship between GEn and nighttime agitation.

OUTCOMES

Determine the effect of GEn on nighttime agitation, sleep, and antipsychotic medications.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT03082755 (Date of registration March 6, 2017). [Research in Gerontological Nursing, 13(6), 280-288.].

Pharmacokinetics of immediate release, extended release, and gastric retentive gabapentin formulations in healthy adults

Objective: Gabapentin immediate release (GBP-IR), gabapentin gastric retentive (GBP-GR), and the prodrug gabapentin enacarbil extended release formulation (GEn) have been approved for management of postherpetic neuralgia (PHN) in adults. This is the first pharmacokinetic (PK) comparison of all three formulations using FDA-recommended doses for PHN. Materials: This study compared the steady-state PK of GBP-IR 600 mg t.i.d., GBP-GR 1,800 mg q.d., and GEn 600 mg b.i.d. in healthy adults. Methods: The open-label study consisted of a 3-day lead-in of escalating doses of GBP-IR, 5 days of treatment with each formulation (GPB-IR, GPB-GR, and GEn), and a 7-day taper period on 600 mg GEn q.d.. Plasma concentrations were collected on day 5 for each formulation. PK parameters were estimated from plasma concentration data. Results: 14 healthy subjects (7 men, 7 women; mean (SD) age, 46.8 (7.60) years; mean (SD) body mass index, 26.7 (1.7) kg/m²) received all doses and completed the study. GBP-GR resulted in substantially (~ 4-fold) higher peak-to-trough ratio and percent fluctuation compared to GEn. GEn resulted in more sustained and less fluctuating daily exposure relative to GBP-IR, particularly at the end of 24 hours of dosing. In contrast, gabapentin fluctuation from GBP-IR consisted of 3 distinct peaks. After dose normalization, gabapentin exposure with GEn was ~ 2.2-fold and ~ 1.4-fold higher compared to GBP-GR and GBP-IR, respectively. All treatments were well tolerated. Conclusion: GEn requires less frequent dosing compared with GBP-IR and fluctuates less with sustained gabapentin exposure throughout the day. These PK differences may have clinically relevant implications.

Use of α2δ Ligands for Restless Legs Syndrome/Willis Ekbom Disease

Restless legs syndrome is a common neurological condition affecting a substantial portion of the population. It can be an idiopathic disorder, or one that is secondary to another cause. Given that the underlying pathophysiology of restless legs syndrome is not well understood, several drug classes have been studied for symptom control. While dopamine agonists have long been the mainstay of first-line treatment for restless legs syndrome, recently, the α2δ ligands have been increasingly used. These agents have proven both efficacious and safe in a number of clinical trials. Additionally, compared with the dopamine agonists, they have been associated with less augmentation, a phenomenon whereby symptoms emerge earlier in the day, become more severe, and may spread to areas of the body previously unaffected. Newer clinical guidelines for restless legs syndrome are increasingly recommending the α2δ ligands as a logical first-choice medication for patients needing drug therapy for symptom control.

Gabapentin Enacarbil: A Review in Restless Legs Syndrome

Gabapentin enacarbil is an extended-release prodrug of gabapentin that is approved in the USA (Horizant(®)) and Japan (Regnite(®)) for the treatment of moderate to severe primary restless legs syndrome (RLS) in adults [featured indication]. This article summarizes pharmacological, efficacy and tolerability data relevant to the use of oral gabapentin enacarbil in this indication. In double-blind, multicentre trials, treatment with gabapentin enacarbil 600 mg/day for 12 weeks significantly improved the symptoms of moderate to severe primary RLS in adults. Gabapentin enacarbil also significantly improved RLS pain scores and generally improved sleep and mood outcomes. These data are supported by retrospective pooled analyses of three of these trials (XP081, PIVOT RLS I and PIVOT RLS II), with gabapentin enacarbil generally improving symptoms irrespective of disease severity, associated sleep disturbance or prior dopamine agonist use. Responses to gabapentin enacarbil were sustained in longer-term trials, with lower relapse rates in gabapentin enacarbil than placebo recipients in a longer-term maintenance study. Overall, in short and longer-term trials, relatively few patients discontinued treatment, adverse events were mostly mild to moderate in severity, and somnolence/sedation and dizziness were the most commonly reported adverse events. Notably, there were no reports of augmentation or QT-interval prolongation. Gabapentin enacarbil is an important agent for the treatment of adults with moderate to severe primary RLS.

Can Chronic Pain Patients Be Adequately Treated Using Generic Pain Medications to the Exclusion of Brand-Name Ones?

According to the Food and Drug Administration (FDA) reports, approximately 8 in 10 prescriptions filled in the United States are for generic medications, with an expectation that this number will increase over the next few years. The impetus for this emphasis on generics is the cost disparity between them and brand-name products. The use of FDA-approved generic drugs saved 158 billion dollars in 2010 alone. In the current health care climate, there is continually increasing pressure for prescribers to write for generic alternative medications, occasionally at the expense of best clinical practices. This creates a conflict wherein both physicians and patients may find brand-name medications clinically superior but nevertheless choose generic ones. The issue of generic versus brand medications is a key component of the discussion of health payers, physicians and their patients. This review evaluates some of the important medications in the armamentarium of pain physicians that are frequently used in the management of chronic pain, and that are currently at the forefront of this issue, including Opana (oxymorphone; Endo Pharmaceuticals, Inc., Malvern, PA), Gralise (gabapentin; Depomed, Newark, CA), and Horizant (gabapentin enacarbil; XenoPort, Santa Clara, CA) that are each available in generic forms as well. We also discuss the use of Lyrica (pregabalin; Pfizer, New York, NY), which is currently unavailable as generic medication, and Cymbalta (duloxetine; Eli Lilly, Indianapolis, IN), which has been recently FDA approved to be available in a generic form. It is clear that the use of generic medications results in large financial savings for the cost of prescriptions on a national scale. However, cost-analysis is only part of the equation when treating chronic pain patients and undervalues the relationships of enhanced compliance due to single-daily dosing and stable and reliable pharmacokinetics associated with extended-duration preparations using either retentive technologies or delayed absorption strategies. Medications given to chronic pain patients should be individualized to best serve analgesic needs and assure patient safety primarily, based on high levels of scientific and economic evidence. Decisions regarding utilization should not be made based solely on limited or faulty assessments of cost-benefit analyses.

Modified-Release Formulations of Second-Generation Antiepileptic Drugs: Pharmacokinetic and Clinical Aspects

Modified-release or extended-release (XR) formulations are used to decrease the frequency of dosing for drugs with rapid elimination, to improve convenience and adherence. Use of a modified-release formulation can decrease the peak to trough fluctuations in serum concentrations and theoretically improve the therapeutic benefit of the drug, by decreasing adverse events associated with the higher peak concentrations. Once-daily formulations of lamotrigine (Lamictal XR(®)), levetiracetam (Keppra XR(®)), oxcarbazepine (Oxtellar XR(®), Apydan(®) extent) and topiramate (Qudexy XR™, Trokendi XR™) are approved for the treatment of focal and/or generalized onset seizures. Other seizure medications have been approved for non-epileptic symptoms. Gabapentin XR (Gralise(®)) is approved for the treatment of post-herpetic neuralgias. Gabapentin enacarbil XR (Horizant(®)) is a prodrug of gabapentin and is indicated for treatment of post-herpetic neuralgia and restless leg syndrome, a novel indication. For all but Qudexy XR™, the tablets/capsules must be swallowed whole, without cutting, crushing or chewing, in order to maintain the XR properties of the formulation. Qudexy XR™ can be swallowed intact or the capsules can be opened and sprinkled onto soft food for those with swallowing difficulties, for example, children and the elderly. The bioavailability of Gralise(®) and Horizant(®) is significantly affected by food, specifically fat content, and should be taken with a meal to maximize absorption. Overall, the primary advantage of the newly released XR formulations is the once-daily dosing to improve convenience and adherence, with very limited data suggesting improved tolerability.

Restless legs syndrome: clinical presentation diagnosis and treatment

Restless legs syndrome (RLS) is a circadian disorder of sensory-motor integration that may be related to genetically determined dysregulation of iron transport across the blood-brain barrier. Dopamine agonists (DAs) have been considered the first-line therapy, but with the growing appreciation of problems associated with long-term treatment, particularly augmentation and impulse control disorder, alpha-2-delta drugs, such as gabapentin, are now considered the first line of treatment in patients with troublesome RLS. Opioids can be considered as an alternative therapy, particularly in patients with DA-related augmentation. In more severe cases, a combination therapy may be required. Intravenous iron therapy may be considered on those patients with refractory RLS.

The efficacy of gabapentin in children of partial seizures and the blood levels

AIM

To evaluate the long-term efficacy of gabapentin (GBP) and usefulness of measurement of the blood level for the observation of patients that have partial seizures.

METHODS

Thirty patients (20 effective cases and 10 ineffective cases) treated with GBP for the localization related epilepsy had their peak blood levels of GBP. The levels were measured seven time points, one, 6, 12, 18, 24, 30, and 36month after the start of medication. The efficacy of GBP was evaluated at one month after the initiation of medication and every year for 3years, based on the R Ratio and the degree of improvement for the paroxysmal strength and length.

RESULTS

GBP levels were higher in the effective cases than the levels in the ineffective cases 6months after and 1year after the initiation of medication (p<0.05). The level 6months after the start in the effective cases was 5.429±2.384μg/ml (mean±SD), and 5.837±3.217μg/ml after 1year. The cases that were effective for 1year maintained approximately the same efficacy for 3years after the initiation of medication, but there was no correlation between the level and the R Ratio, paroxysmal strength and length.

CONCLUSIONS

No precise definition of the therapeutic range was recognized because of no correlation between GBP level and the improvement of clinical manifestations. We recommend the GBP optimal range that is established the range within 3-8μg/ml (mean; 5μg/ml) as therapeutic target without the side effect.

Gabapentin enacarbil for the treatment of moderate to severe primary restless legs syndrome (Willis-Ekbom disease): 600 or 1,200 mg dose?

Gabapentin enacarbil is a prodrug of the anticonvulsant gabapentin. The efficacy and safety of gabapentin enacarbil for the treatment of moderate to severe primary restless legs syndrome (RLS) has been evaluated in several clinical trials in the United States and Japan. Although most clinical trials assessed gabapentin enacarbil at doses greater than 600 mg/day and demonstrated the overall safety and efficacy (defined as improvements in the coprimary endpoints of the international RLS rating scale [IRLS] total score and Clinical Global Impression-Improvement response), the US Food and Drug Administration approved the 600 mg once-daily dosage because doses higher than 600 mg/day were considered to provide no additional benefits and were associated with higher rates of adverse events, such as somnolence and dizziness. Nonetheless, the results of clinical trials and post hoc meta-analyses have indicated that the 1,200 mg once-daily dosage was the most validated gabapentin enacarbil treatment for not only subjective RLS symptoms but also severe sleep disturbance associated with RLS. A Japanese dose-finding study showed that 900 mg/day, the intermediate dose between 600 and 1,200 mg, failed to show a significant improvement in IRLS total score, probably because many of the patients who discontinued treatment did so early, suggesting that a half-landing dose may cause more adverse effects than favorable ones in some RLS patients early in the treatment. Gabapentin enacarbil may have two distinct therapeutic doses for the treatment of RLS: 600 mg/day or lower doses for the treatment of subjective RLS symptoms and 1,200 mg/day or higher doses for the treatment of both subjective RLS symptoms and associated problems such as severe sleep disturbances.

Gabapentin enacarbil extended release for the treatment of postherpetic neuralgia in adults

The development of biomedical technology is allowing refinement of drug therapies in order to improve medication profiles and benefit patients. Gabapentin (Gp) is a medication licensed globally for various indications, including postherpetic neuralgia. It has a pharmacokinetic profile which has been suggested may limit its clinical effects and reduce medication compliance. In 2012, the US Food and Drug Administration licensed a novel preparation which aims to circumvent these limitations. Gp enacarbil is a prodrug of Gp, which is additionally prepared in an extended release preparation. The resulting compound has an improved absorption profile and a reduced dosing frequency in comparison to immediate release Gp. An absence of comparative data, however, limits the direct evaluation of the medication to both immediate release and other extended release preparations available on the market. Additionally, no data are currently available addressing efficacy, tolerability, or side effects with other first line treatments of postherpetic neuralgia. Additional experimental data should be sought to clarify the position of Gp enacarbil, both within postherpetic neuralgia treatment protocols and in relation to the increasing numbers of gabapentinoids available.

QTc prolongation by antiepileptic drugs and the risk of torsade de pointes in patients with epilepsy

Sudden unexpected death in epilepsy (SUDEP) is the most common epilepsy-related cause of death. While the precise pathophysiological mechanisms underlying SUDEP are still uncertain, impaired cardiac function including seizure-induced arrhythmias has received increased attention. In addition, the potential role of antiepileptic drugs has been suggested. While the preponderance of clinical data would suggest that use of most antiepileptic drugs does not pose excessive additional risk of QT prolongation, available data also do not provide sufficient evidence that these drugs are entirely free of risk in all patients. In particular, the potential for these medications, either alone or in combination, to prolong the QT interval should be considered. This review will discuss mechanisms for drug-induced QT prolongation and its relationship to potentially fatal arrhythmias such as torsades de pointes.

Gabapentin enacarbil in Japanese patients with restless legs syndrome: a 12-week, randomized, double-blind, placebo-controlled, parallel-group study

OBJECTIVE

Gabapentin enacarbil (GEn) was effective and well-tolerated for the treatment of restless legs syndrome (RLS) in North American studies. However, no placebo-controlled studies of GEn have been performed in Asian patients with RLS. Therefore, we investigated the efficacy and safety of GEn in Japanese patients with RLS to determine the optimal dosage.

RESEARCH DESIGN AND METHODS

Outpatients with RLS (International Restless Legs Syndrome Rating Scale (IRLS) scores ≥15) were randomized (n = 474) and treated (n = 469) in a double-blind manner with once-daily placebo (n = 116), 600 (n = 120), 900 (n = 119) or 1200 (n = 114) mg GEn for 12 weeks.

CLINICAL TRIAL REGISTRATION

NCT00530530 (ClinicalTrials.gov)

MAIN OUTCOME MEASURE

The primary outcome was the change in IRLS score. Secondary outcomes included Investigator (ICGI)- and Patient (PCGI)-rated Clinical Global Impression and adverse events.

RESULTS

The mean change in IRLS score from baseline to the final observation was -8.96 for placebo versus -11.10, -10.28 and -11.38 for 600, 900 and 1200 mg GEn. Williams' multiple comparison test showed that only 1200 mg GEn was superior to placebo (p = 0.011). However, in post hoc mixed-effects models with repeated measures, which accounted for the time-course of changes in IRLS, the placebo-adjusted changes were -2.31, -1.92 and -2.31 for 600, 900 and 1200 mg GEn. ICGI and PCGI response rates were significantly greater for all three GEn doses versus placebo (all p ≤ 0.014). Adverse events, including somnolence, dizziness and nasopharyngitis, were frequent but of mild-to-moderate severity. However, there was a tendency toward a dose-dependent increase in the incidence of adverse events.

CONCLUSIONS

GEn is effective and well-tolerated for the treatment of RLS in Japanese patients. All three doses produced improvements in IRLS compared with placebo; 600 mg GEn is a suitable target dose. However, our analysis possibly introduced positive bias by assuming that symptoms improve after discontinuation.

Gabapentin enacarbil: in patients with restless legs syndrome

Oral gabapentin enacarbil is approved in adult patients for the treatment of moderate to severe primary restless legs syndrome (RLS) [featured indication] and the management of postherpetic neuralgia. In the 12-week Patient Improvements in Vital Outcomes following Treatment (PIVOT) RLS I and II trials in adult patients with moderate to severe primary RLS (n > 500 total evaluable), once-daily gabapentin enacarbil 600 or 1,200 mg significantly improved mean International Restless Legs Scale (IRLS) total scores compared with placebo, with significantly higher investigator-rated Clinical Global Impression-Improvement (CGI-I) responder rates in gabapentin enacarbil groups than in placebo groups. Improvements in other sleep outcomes (assessed using various scales) also generally favoured gabapentin enacarbil treatment. These data are supported by results from a polysomnography, crossover (two 4-week treatment periods) trial (n > 100 evaluable). Improvements in RLS symptoms with gabapentin enacarbil were maintained in a 52-week extension study of clinical trials, including PIVOT RLS I and II. The longer-term efficacy of gabapentin enacarbil in patients with moderate to severe RLS was also demonstrated in the 36-week PIVOT RLS Maintenance study and a 52-week noncomparative study conducted in Japan. Gabapentin enacarbil was generally well tolerated in adult patients with RLS participating in short- and longer-term clinical trials. The most common treatment-emergent adverse events were somnolence/sedation and dizziness. Most adverse events were of mild to moderate severity, with relatively few patients discontinuing treatment because of an adverse event.

Randomized, double-blind, placebo-controlled, phase II trial of gabapentin enacarbil for migraine prophylaxis

Objective

The objective of this article is to evaluate the efficacy and safety of gabapentin enacarbil (GEn) for migraine prophylaxis.

Methods

In this randomized, double-blind, parallel-group study, patients with International Headache Society-defined migraine who met criteria suggesting the need for prophylactic therapy were randomized 2:1:2:2:1 to one of the following five groups, designated according to target daily dose of study medication during the 20-week treatment period: placebo, GEn 1200 mg, GEn 1800 mg, GEn 2400 mg, or GEn 3000 mg.

Results

The intent-to-treat population included 523 patients ( n = 128 placebo, n = 66 GEn 1200 mg, n = 134 GEn 1800 mg, n = 133 GEn 2400 mg, n = 62 GEn 3000 mg). No statistically significant difference between active treatment (the average of 1800 mg and 2400 mg treatment groups) and placebo was found for change from baseline in the number of migraine headache days during the last four weeks of treatment prior to taper (the primary endpoint). Results of analyses of the primary endpoint using the per protocol population, analyses using imputation methods different from those of the primary analysis, and nonparametric analyses were consistent with the primary analysis in showing no difference between active treatment and placebo. The pattern of results was similar for the secondary efficacy endpoints. Pharmacokinetic data demonstrate that patients had adequate estimated exposure to GEn. The adverse event profile of GEn was consistent with that in previous studies.

Conclusion

GEn did not significantly differ from placebo for migraine headache prophylaxis. A high placebo effect should be considered when interpreting these results.

Review of the treatment of restless legs syndrome: focus on gabapentin enacarbil

The FDA approved gabapentin enacarbil in 2011 as the first non-dopaminergic agent for the treatment of restless legs syndrome (RLS) symptoms. Although gabapentin enacarbil is a pro-drug of gabapentin, its pharmacokinetics differ. Absorption of gabapentin enacarbil is more predictable, and inter-patient variability in bioavailability is lower than that of gabapentin. Studies have demonstrated superiority of gabapentin enacarbil compared to placebo. Comparisons to currently available RLS treatments are lacking, but clinical trials demonstrate comparable improvement in RLS symptoms to the dopamine agonists ropinirole and pramipexole, which are usually considered first-line therapy for daily RLS symptoms. Gabapentin enacarbil was well tolerated in clinical trials. The role of the drug in RLS treatment remains undefined, although it will likely be used as an alternative for refractory RLS when other treatments have failed. Additionally, gabapentin enacarbil may be recommended for patients with daily RLS symptoms that are less intense or are associated with pain as an alternative to dopamine agonists.

ADMET considerations for restless leg syndrome drug treatments

INTRODUCTION

Restless legs syndrome (RLS) is a common neurological disorder that might impair nocturnal rest causing decreased alertness, depressed mood, reduced job performance, and poor quality of life. In patients affected by severe RLS, a pharmacological treatment is mandatory.

AREAS COVERED

The present review is based on a search using PubMed from 1994 to 2012. It is focused on the Absorption, Distribution, Metabolism, Elimination and Toxicology (ADMET) characteristics of the most used medications for RLS. In particular, the ADMET characteristics of dopaminergic agents, anticonvulsants able to improve neuropathic pain, and iron were discussed.

EXPERT OPINION

Clinical trials have showed that non-ergolic dopamine agonists are efficacious and safe for patients affected by moderate to severe idiopathic RLS. However, no head-to-head study has compared the long-term effects of the three dopamine agonists approved by the FDA for RLS (ropinirole, pramipexole, and rotigotine). Moreover, further studies should investigate the extended-release formulation of ropinirole and pramipexole in RLS patients affected by all day long distressing symptoms. A standardized treatment for symptomatic forms of RLS is lacking. Randomized, placebo-controlled trials should be performed at least in RLS patients with peripheral neuropathic and chronic kidney disease. Concerning RLS due to iron deficiency, a head-to-head study comparing efficacy, safety and compliance of oral iron versus intravenous one seems to be needed.

Role of gabapentin enacarbil XR in restless legs syndrome

Gabapentin enacarbil XR is a new extended-release formulation which attempts to overcome the reduced efficacy of shorter-acting gabapentin, with sustained delivery over a 24-hour period. It is a gabapentin prodrug which is efficiently and rapidly converted to gabapentin during active transport throughout the length of the intestine via high-capacity monocarboxylate type 1 nutrient transporters unlike its predecessor, which is absorbed via low-capacity transporters largely confined to the upper intestinal region. Its lack of saturable absorption allows for dose-proportional absorption and hence increased bioavailability. Several clinical trials addressing its efficacy in moderate to severe restless legs syndrome (RLS) demonstrate improvements in the International RLS Rating Scale after a 2-week to 3-month period. Open-label studies of 52 weeks' duration showed maintenance of symptom reduction with once-daily administration of the extended-release formulation. The most commonly reported treatment-emergent adverse effects were somnolence and dizziness. Although the incidence of emergent adverse effects is high, it is comparable with that of gabapentin. No studies thus far have documented augmentation as an issue, unlike that observed with most dopaminergic agents. In addition, both dopamine precursors and agonists have not been shown to increase slow wave sleep or improve overall sleep architecture consistently despite improvement in the periodic leg movement index, in contrast with gabapentin enacarbil. Presently, gabapentin enacarbil has not been approved by the Therapeutic Goods Administration or Medsafe for use in RLS. The cost of this medication may also be a potential barrier for many patients. Future comparative efficacy studies with gabapentin, first-line dopaminergic agents, rotigotine, being the other once daily RLS medication, and pregabalin, the structural analog of gabapentin, will be necessary.

Clinical pharmacokinetics of gabapentin after administration of gabapentin enacarbil extended-release tablets in patients with varying degrees of renal function using data from an open-label, single-dose pharmacokinetic study

BACKGROUND

Gabapentin enacarbil, a transported acyloxyalkylcarbamate prodrug of gabapentin, provides predictable and dose-proportional gabapentin exposure (AUC). Gabapentin is cleared via renal excretion, and its elimination is proportional to creatinine clearance (CrCL); CrCL can, therefore, be used as a predictor of gabapentin renal clearance. Gabapentin produced from hydrolysis of gabapentin enacarbil is also eliminated via the renal clearance pathway. It was, therefore, anticipated that the pharmacokinetics of gabapentin derived from gabapentin enacarbil would also be affected by renal function.

OBJECTIVE

The objective of this study was to describe a population pharmacokinetic analysis of gabapentin enacarbil in patients with varying degrees of renal function, using data from an open-label study of gabapentin enacarbil in patients with renal impairment (XenoPort, Inc. protocol XP066), to determine whether dosage adjustments are necessary in patients with renal impairment.

METHODS

Men and women >18 years of age with a body mass index ≤34 kg/m(2) and who were, in general, healthy with the exception of renal impairment were enrolled All patients received a single 600-mg gabapentin enacarbil extended-release tablet under fed conditions. After dosing, plasma, urine, and dialysate samples were analyzed. Safety profile evaluations included adverse events, vital signs, ECGs, and laboratory values. Pharmacokinetic data were compared with those from Phase I-III studies in subjects with normal renal function to evaluate the relationship between gabapentin oral clearance (CL/F) and CrCL.

RESULTS

Fifteen patients (11 men and 4 women) were enrolled. One patient had moderate renal impairment (CrCL 30-59 mL/min), 7 patients had severe renal impairment (CrCL <30 mL/min), and 7 patients had end-stage renal disease (CrCL <15 mL/min). Ten patients were white, 4 were African American, and 1 was American Indian or Alaskan Native. Their mean (range) age was 55 (28-76) years, weight was 85.6 (62-134) kg, and body mass index was 28.3 (22-34) kg/m(2). Mean maximum plasma gabapentin concentration was 5.77 μg/mL in patients with moderate and severe renal impairment, and 5.59 μg/mL in patients with end-stage renal disease who were undergoing hemodialysis. Based on the population pharmacokinetic analysis, gabapentin CL/F after administration of gabapentin enacarbil was proportionally related to CrCL, with an approximately 1.6-fold decrease in CL/F for every 2-fold decrease in CrCL. The most frequent adverse event was dizziness (4 of 15 patients). Other adverse events that were assessed as possibly or probably related to treatment were defecation urgency, extremity pain, feeling of relaxation, and muscle weakness; each occurred in 1 patient only. All events were mild or moderate and resolved without sequelae.

CONCLUSIONS

The data suggest that dosage adjustment for gabapentin enacarbil is necessary in patients with impaired renal function. Gabapentin enacarbil, 600 mg, seemed to be well tolerated in this small selected population.

A randomized, double-blind, placebo-controlled study to assess the efficacy and tolerability of gabapentin enacarbil in subjects with restless legs syndrome

STUDY OBJECTIVE

To evaluate the efficacy and tolerability of gabapentin enacarbil (GEn) 1200 mg or 600 mg compared with placebo in subjects with moderate-to-severe primary restless legs syndrome (RLS).

METHODS

This 12-week, multicenter, double-blind, placebo-controlled study randomized subjects (1:1:1) to GEn 1200 mg, 600 mg, or placebo. Co-primary endpoints: mean change from baseline in International Restless Legs Scale (IRLS) total score and proportion of responders (rated as "very much" or "much" improved) on the investigator-rated Clinical Global Impression-Improvement scale (CGI-I) at Week 12 LOCF for GEn 1200 mg compared with placebo. Secondary endpoints included GEn 600 mg compared with placebo on the IRLS and CGI-I at Week 12 LOCF and subjective measures for sleep. Safety and tolerability assessments included adverse events.

RESULTS

325 subjects were randomized (GEn 1200 mg = 113; 600 mg = 115; placebo = 97). GEn 1200 mg significantly improved mean [SD] IRLS total score at Week 12 LOCF (baseline: 23.2 [5.32]; Week 12: 10.2 [8.03]) compared with placebo (baseline: 23.8 [4.58]; Week 12: 14.0 [7.87]; adjusted mean treatment difference [AMTD]: -3.5; p = 0.0015), and significantly more GEn 1200 mg-treated (77.5%) than placebo-treated (44.8%) subjects were CGI-I responders (p < 0.0001). Similar significant results were observed with GEn 600 mg for IRLS (AMTD: -4.3; p < 0.0001) and CGI-I (72.8% compared with 44.8%; p < 0.0001). GEn also significantly improved sleep outcomes (Post-Sleep Questionnaire, Pittsburgh Sleep Diary and Medical Outcomes Sleep Scale) compared with placebo. The most commonly reported adverse events were somnolence (GEn 1200 mg = 18.0%; 600 mg = 21.7%; placebo = 2.1%) and dizziness (GEn 1200 mg = 24.3%; 600 mg = 10.4%; placebo = 5.2%). Dizziness increased with increased dose and led to discontinuation in 2 subjects (GEn 1200 mg, n = 1; GEn 600 mg, n = 1). Somnolence led to discontinuation in 3 subjects (GEn 600 mg).

CONCLUSIONS

GEn 1200 mg and 600 mg significantly improve RLS symptoms and sleep disturbance compared with placebo and are generally well tolerated.

Restless legs syndrome: a comprehensive overview on its epidemiology, risk factors, and treatment

PURPOSES

Restless legs syndrome (RLS) is underdiagnosed and poorly understood by clinicians and the general public alike; accordingly, a broad literature review with information most relevant to general practice is needed to help dispel misconceptions and improve level of care.

METHODS

Specifically, this review comprehensively provides an epidemiological analysis of RLS and examines the risk factors and treatment options for RLS by compiling the findings of past RLS studies. These RLS studies were identified through a retrospective PubMed search. The epidemiological analysis was conducted by calculating a weighted mean average of all the relevant general population RLS prevalence studies, separated into geographical/racial categories.

RESULTS

A comprehensive analysis of RLS epidemiological studies finds the prevalence rate of RLS to be 5-15% in the general population with 2.5% of adults having symptoms severe enough to require medical intervention. Some of the risk factors for RLS include female gender, pregnancy, low iron levels, lower socioeconomic status, poor health, elderly age, comorbidity with Parkinson's disease, positive family history of RLS, and comorbidity with psychiatric disorders. A wide array of treatment options exist for RLS including pharmacological and nonpharmacologic interventions.

CONCLUSIONS

Clinicians' understanding of RLS enigma has recently improved due to the increased intensity of RLS research over the past decade. This review summarizes the current findings in the RLS field as well as providing guidelines for future RLS-related research.

The use of gabapentin enacarbil in the treatment of restless legs syndrome

Restless legs syndrome (RLS) is a common sleep-related neurological disorder that is characterized by the urge to move, worsening at rest, improvement with activity, and worsening in the evening and night. Dopamine agonists are usually the first-line therapy. Other agents including benzodiazepines, narcotics, and anticonvulsants have been used to treat RLS. Gabapentin has been shown to improve RLS in a small number of clinical studies, but is limited by its short half-life and variable bioavailability. Gabapentin enacarbil is a novel prodrug of gabapentin designed to overcome these pharmacokinetic limitations. In vitro and in vivo studies have demonstrated that gabapentin enacarbil has improved absorption, bioavailability and pharmacokinetics compared with gabapentin. Phase II and III studies have demonstrated that gabapentin enacarbil is generally well tolerated and is useful in the treatment of RLS.

Randomized polysomnography study of gabapentin enacarbil in subjects with restless legs syndrome

We assessed the efficacy and tolerability of gabapentin enacarbil in the treatment of moderate to severe primary restless legs syndrome and associated sleep disturbance. This was a multicenter, randomized, double-blind, placebo-controlled, 2-period crossover polysomnography study of gabapentin enacarbil 1200 mg or placebo taken once daily. Subjects were randomized 1:1 to a sequence of gabapentin enacarbil:placebo or placebo:gabapentin enacarbil, receiving each treatment for 4 weeks. The primary end point was the mean change from baseline at weeks 4 and 10 (4/10) last observation carried forward in wake time during sleep. The key secondary end point was the mean change from baseline at weeks 4/10 last observation carried forward in periodic limb movements associated with arousal per hour of sleep. Tolerability assessments included adverse events. One hundred thirty-six subjects were randomized (gabapentin enacarbil:placebo, 67; placebo:gabapentin enacarbil, 69), and 114 (gabapentin enacarbil:placebo, 53; placebo:gabapentin enacarbil, 61) completed the study. Gabapentin enacarbil 1200 mg significantly reduced wake time during sleep (mean change from baseline [adjusted mean treatment difference]: -26.0 minutes; P < .0001) and periodic limb movements associated with arousal per hour of sleep (adjusted mean treatment difference: -3.1 periodic limb movements with arousal/hour; P = .002) compared with placebo at weeks 4/10 last observation carried forward. The most commonly reported adverse events were dizziness (gabapentin enacarbil 20%, placebo 2%) and somnolence (gabapentin enacarbil 13%, placebo 2%). Gabapentin enacarbil 1200 mg once daily significantly reduces restless legs syndrome-associated sleep disturbance and periodic limb movements associated with arousal per hour of sleep and is generally well tolerated in adults with moderate to severe primary restless legs syndrome.

Synthesis and in vitro evaluation of gabapentin prodrugs that target the human apical sodium-dependent bile acid transporter (hASBT)

Gabapentin is a zwitterionic drug that exhibits low and variable oral absorption at therapeutic doses. The human apical sodium-dependent bile acid transporter (hASBT; SLC10A2) is a potential prodrug target to increase oral drug absorption. The objective was to evaluate several bile acid conjugates of gabapentin as potential prodrugs that target hASBT. Five analogues were synthesized and varied in ionic nature and the presence or absence of glutamic acid linker between the bile acid and drug. Analogues were evaluated for their inhibition and uptake properties using stably transfected hASBT-MDCK cells. The two monoanionic conjugates were potent hASBT substrates, with high affinity (K(m) of 16.3 and 5.99 μM) and high capacity (V(max) of 0.656 and 0.842 pmol/cm(2) /s). The dianionic conjugate inhibited hASBT with moderate potency but was not a substrate. The two monoanionic conjugates were catalytically degraded in Caco-2 homogenate and rat liver microsomes. Each yielded gabapentin from prodrug. These two conjugates are novel prodrugs of gabapentin and illustrate prodrugs that can be designed to target hASBT.

Gabapentin enacarbil - clinical efficacy in restless legs syndrome

Restless legs syndrome (RLS) is a sleep-related movement disorder commonly involving an unpleasant urge to move the limbs, typically the legs. Dopaminergic agents represent the first-line therapy for RLS; however, long-term use of such drugs results in worsening symptoms due to "augmentation" or other adverse events. Gabapentin, an analog of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), is an anticonvulsant/analgesic agent. Gabapentin is only mildly effective in relieving RLS symptoms, perhaps a result of its poor absorption from the gastrointestinal (GI) tract. Gabapentin enacarbil is a prodrug of gabapentin specifically designed to enhance absorption via the GI tract, and hence provide improved circulating levels of gabapentin on metabolism. Clinical trials to date have demonstrated favorable safety and (compared to traditional gabapentin) improved pharmacokinetics and efficacy in treating RLS symptoms. Thus, gabapentin enacarbil may prove to be a useful drug in treating RLS. An application of gabapentin enacarbil for treatment of RLS is currently pending with FDA for approval.

Gabapentin Enacarbil: Pharmacotherapy of Restless Legs Syndrome

Restless Legs Syndrome (RLS) is a prevalent sleep-associated movement disorder greatly affecting patients’ quality of life (QoL). Several drugs can be used to control this condition although the first-line dopamine agents often cause adverse effects. Non-dopaminergic drugs such as oral gabapentin (GBP) have been more recently advocated. Despite ameliorating RLS symptoms, GBP’s pharmacokinetic limitations restrict its overall effectiveness. A novel specifically designed prodrug, gabapentin enacarbil (GE), has demonstrated successful RLS alleviation with a superior pharmacokinetic profile. This review aims to examine the efficacy and tolerability of both GBP and GE as pharmacotherapy for RLS. Despite some heterogeneity and limitations across research methodologies, GE appears to be a potential RLS therapy superior to GBP and other dopaminergic agents.