Clinical pharmacokinetic drug interaction studies of gabapentin enacarbil, a novel transported prodrug of gabapentin, with naproxen and cimetidine

Discovery of Prodrug of MRTX1133 as an Oral Therapy for Cancers with KRASG12D Mutation

Effective oral therapies are urgently required to treat KRASG12D mutant cancers. Therefore, synthesis and screening were performed for 38 prodrugs of MRTX1133 to identify an oral prodrug of MRTX1133, a KRASG12D mutant protein-specific inhibitor. In vitro and in vivo evaluations revealed prodrug 9 as the first orally available KRASG12D inhibitor. Prodrug 9 exhibited improved pharmacokinetic properties for the parent compound in mice and was efficacious in a KRASG12D mutant xenograft mouse tumor model after oral administration.

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.

The System Profile of Renal Drug Transporters in Tubulointerstitial Fibrosis Model and Consequent Effect on Pharmacokinetics

With the widespread clinical use of drug combinations, the incidence of drug–drug interactions (DDI) has significantly increased, accompanied by a variety of adverse reactions. Drug transporters play an important role in the development of DDI by affecting the elimination process of drugs in vivo, especially in the pathological state. Tubulointerstitial fibrosis (TIF) is an inevitable pathway in the progression of chronic kidney disease (CKD) to end-stage renal disease. Here, the dynamic expression changes of eleven drug transporters in TIF kidney have been systematically investigated. Among them, the mRNA expressions of Oat1, Oat2, Oct1, Oct2, Oatp4C1 and Mate1 were down-regulated, while Oat3, Mrp2, Mrp4, Mdr1-α, Bcrp were up-regulated. Pearson correlation analysis was used to analyze the correlation between transporters and Creatinine (Cr), OCT2 and MATE1 showed a strong negative correlation with Cr. In contrast, Mdr1-α exhibited a strong positive correlation with Cr. In addition, the pharmacokinetics of cimetidine, ganciclovir, and digoxin, which were the classical substrates for OCT2, MATE1 and P-glycoprotein (P-gp), respectively, have been studied. These results reveal that changes in serum creatinine can indicate changes in drug transporters in the kidney, and thus affect the pharmacokinetics of its substrates, providing useful information for clinical use.

Population pharmacokinetics of gabapentin in patients with neuropathic pain: Lack of effect of diabetes or glycaemic control

AIMS

Gabapentin (GBP) is widely used to treat neuropathic pain, including diabetic neuropathic pain. Our objective was to evaluate the role of diabetes and glycaemic control on GBP population pharmacokinetics.

METHODS

A clinical trial was conducted in patients with neuropathic pain (n = 29) due to type 2 diabetes (n = 19) or lumbar/cervical disc herniation (n = 10). All participants were treated with a single oral dose GBP. Blood was sampled up to 24 hours after GBP administration. Data were analysed with a population approach using the stochastic approximation expectation maximization algorithm. Weight, body mass index, sex, biomarkers of renal function and diabetes, and genotypes for the main genetic polymorphisms of SLC22A2 (rs316019) and SLC22A4 (rs1050152), the genes encoding the transporters for organic cations OCT2 and OCTN1, were tested as potential covariates.

RESULTS

GBP drug disposition was described by a 1-compartment model with lag-time, first-order absorption and linear elimination. The total clearance was dependent on estimated glomerular filtration rate. Population estimates (between-subject variability in percentage) for lag time, first-order absorption rate, apparent volume of distribution and total clearance were 0.316 h (10.6%), 1.12 h^-1 (10.7%), 140 L (7.7%) and 14.7 L/h (6.97%), respectively. No significant association was observed with hyperglycaemia, glycated haemoglobin, diabetes diagnosis, age, sex, weight, body mass index, SLC22A2 or SLC22A4 genotypes.

CONCLUSION

This population pharmacokinetics model accurately estimated GBP concentrations in patients with neuropathic pain, using estimated glomerular filtrationrate as a covariate for total clearance. The distribution and excretion processes of GBP were not affected by hyperglycaemia or diabetes.

(Carbonyl)oxyalkyl linker-based amino acid prodrugs of the HIV-1 protease inhibitor atazanavir that enhance oral bioavailability and plasma trough concentration

We describe the design, synthesis and pharmacokinetic (PK) evaluation of a series of amino acid-based prodrugs of the HIV-1 protease inhibitor atazanavir (1) derivatized on the pharmacophoric secondary alcohol using a (carbonyl)oxyalkyl linker. Prodrugs of 1 incorporating simple (carbonyl)oxyalkyl-based linkers and a primary amine in the promoiety were found to exhibit low chemical stability. However, chemical stability was improved by modifying the primary amine moiety to a tertiary amine, resulting in a 2-fold enhancement of exposure in rats following oral dosing compared to dosing of the parent drug 1. Further refinement of the linker resulted in the discovery of 22 as a prodrug that delivered the parent 1 to rat plasma with a 5-fold higher AUC and 67-fold higher C₂₄ when compared to oral administration of the parent drug. The PK profile of 22 indicated that plasma levels of this prodrug were higher than that of the parent, providing a more sustained release of 1 in vivo.

Therapeutic drug monitoring of gabapentin in various indications

OBJECTIVES

Gabapentin has been increasingly used in various indications in recent years. Despite variable pharmacokinetics, therapeutic drug monitoring (TDM) is scarcely described in other indications than epilepsy. The aim of the study was to investigate the use and pharmacokinetic variability of gabapentin in epilepsy and non-epilepsy indications and to further evaluate the use of TDM in patients with restless legs syndrome (RLS).

MATERIALS & METHODS

Population-based data from the Norwegian Prescription Database, retrospective TDM data from the section for Clinical Pharmacology, the National Center for Epilepsy, Norway, and prospective observational data on patients with RLS were used.

RESULTS

The use of gabapentin increased by 30% from 2014 to 2017 (32 181 to 42 675 users). TDM data from 120 patients showed a 22-fold pharmacokinetic variability in concentration/dose ratios, and this ratio was elevated in elderly patients (≥65 years). The majority of elderly used gabapentin for non-epilepsy indications. In patients with RLS, intake in the evening/night only was common due to nocturnal symptoms, in contrast to regular dosing regimens in epilepsy. Thus, drug fasting concentrations do not reflect concentrations at the time of required therapeutic effect. TDM was still found useful in most patients to support dosage increase or evaluate adverse effects.

CONCLUSION

Due to extensive pharmacokinetic variability, TDM can benefit patients using gabapentin. Challenges with applying TDM in new indications such as RLS include different dosage regimens and consequently different interpretation of serum concentrations. Thus, TDM should be requested on clear clinical grounds and the service tailored according to the therapeutic indication.

Pharmacogenetics-based population pharmacokinetic analysis of gabapentin in patients with chronic pain: Effect of OCT2 and OCTN1 gene polymorphisms

Gabapentin (GAB) is eliminated unchanged in urine, and organic cation transporters (OCT2 and OCTN1) have been shown to play a role in GAB renal excretion. This prospective clinical study aimed to evaluate the genetic polymorphisms effect on GAB pharmacokinetic (PK) variability using a population pharmacokinetic approach. Data were collected from 53 patients with chronic pain receiving multiple doses of GAB. Patients were genotyped for SLC22A2 c.808G>T and SLC22A4 c.1507C>T polymorphisms. Both polymorphisms' distribution followed the Hardy-Weinberg equilibrium. An one-compartment model with first-order absorption and linear elimination best described the data. The absorption rate constant, volume of distribution, and clearance estimated were 0.44 h^-1 , 86 L, and 17.3 × (estimated glomerular filtration ratio/89.58)^1.04  L/h, respectively. The genetic polymorphism SLC22A4 c.1507C>T did not have a significant influence on GAB absorption, distribution or elimination. Due to the low minor allelic frequency of SLC22A2 c.808G>T, further studies require higher number of participants to confirm its effect on GAB renal elimination. In conclusion, GAB clinical pharmacokinetics are strongly influenced by renal function and absorption process, but not by the OCTN1 (SLC22A4 c.1507C>T) polymorphism.

Low colonic absorption drugs: risks and opportunities in the development of oral extended release products

INTRODUCTION

Currently numerous drugs have been observed with lower colonic absorption than small intestine absorption, which can significantly impact in vivo performance of their oral extended release (ER) products.

AREAS COVERED

We reviewed over 300 publications, patents, book chapters, and commercial reports of drug products from regulatory agencies for low colonic absorption (LCA) drugs and critical findings are discussed. The focuses of this article are (1) current findings on the causes of low colonic absorption to support early assessment of LCA candidates, and (2) current knowledge on successful ER strategies and technical platforms used for LCA drugs in commercial drug products to facilitate oral ER product development.

EXPERT OPINION

Colonic drug absorption is one of the critical considerations in successful development of oral ER products. The root causes of low colonic absorption in many LCA drugs are still unclear. It is recommended to evaluate colonic drug absorption of drug candidate at early stage of oral ER product development. After evaluation, the selection of a formulation platform to develop an oral ER product needs to be carefully considered for LCA drugs. Based on the current commercial oral ER formulation platforms for LCA drugs, compounds are first divided into five types (I-V) and different ER formulation approaches with higher success rate are recommended for each type.

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.

Alpha2delta ligands, gabapentin, pregabalin and mirogabalin: a review of their clinical pharmacology and therapeutic use

INTRODUCTION

The first two alpha₂delta ligands - gabapentin (GBP) and pregabalin (PGB) - were initially synthesized as antiepileptics; however, they were later also found to be useful for the treatment of additional conditions. Areas covered: Relevant publications describing potential underlying mechanisms, clinical pharmacokinetics/pharmacokinetics, and clinical efficacy and safety of these drugs in various disease conditions were searched in PubMed and Scopus and included in this review. Expert commentary: GBP and PGB are effective for the treatment neuropathic pain, fibromyalgia and epilepsy; in addition, they may be useful for the reduction of postoperative pain. PGB is also effective for the treatment of generalized anxiety disorder and GBP for the treatment of restless legs syndrome. GBP may be considered a treatment option for pain associated with Guillain-Barré Syndrome and phantom limb and for the management of uremic pruritus. Mirogabalin (MGB), recently developed, is being investigated for the treatment of peripheral neuropathic pain and fibromyalgia, showing promising results in patients with diabetic peripheral neuropathy. Their most frequent adverse reactions are of neuropsychiatric nature and include fatigue, dizziness, sedation, somnolence, and ataxia; peripheral edema and weight gain are also frequently described. Pharmacokinetic interactions are scarce; however, pharmacodynamic interactions have been described in association with drugs with CNS-depressant effects.

The Complexities of Interpreting Reversible Elevated Serum Creatinine Levels in Drug Development: Does a Correlation with Inhibition of Renal Transporters Exist?

In humans, creatinine is formed by a multistep process in liver and muscle and eliminated via the kidney by a combination of glomerular filtration and active transport. Based on current evidence, creatinine can be taken up into renal proximal tubule cells by the basolaterally localized organic cation transporter 2 (OCT2) and the organic anion transporter 2, and effluxed into the urine by the apically localized multidrug and toxin extrusion protein 1 (MATE1) and MATE2K. Drug-induced elevation of serum creatinine (SCr) and/or reduced creatinine renal clearance is routinely used as a marker for acute kidney injury. Interpretation of elevated SCr can be complex, because such increases can be reversible and explained by inhibition of renal transporters involved in active secretion of creatinine or other secondary factors, such as diet and disease state. Distinction between these possibilities is important from a drug development perspective, as increases in SCr can result in the termination of otherwise efficacious drug candidates. In this review, we discuss the challenges associated with using creatinine as a marker for kidney damage. Furthermore, to evaluate whether reversible changes in SCr can be predicted prospectively based on in vitro transporter inhibition data, an in-depth in vitro-in vivo correlation (IVIVC) analysis was conducted for 16 drugs with in-house and literature in vitro transporter inhibition data for OCT2, MATE1, and MATE2K, as well as total and unbound maximum plasma concentration (Cmax and Cmax,u) data measured in the clinic.

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.

Drug interactions with the newer antiepileptic drugs (AEDs)--Part 2: pharmacokinetic and pharmacodynamic interactions between AEDs and drugs used to treat non-epilepsy disorders

Since antiepileptic drugs (AEDs) are prescribed to treat various non-epilepsy-related disorders in addition to the fact that patients with epilepsy may develop concurrent disorders that will need treatment, the propensity for AEDs to interact with non-AEDs is considerable and indeed can present a difficult clinical problem. The present review details the pharmacokinetic and pharmacodynamic interactions that have been reported to occur with the new AEDs (eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, perampanel, pregabalin, retigabine (ezogabine), rufinamide, stiripentol, tiagabine, topiramate, vigabatrin and zonisamide) and drugs used to treat non-epilepsy disorders. Interaction study details are described, as necessary, so as to allow the reader to take a view as to the possible clinical significance of particular interactions. Pharmacokinetic interactions relate to hepatic enzyme induction or inhibition and involved a variety of drugs including psychoactive drugs, cardioactive drugs, oral contraceptives, antituberculous agents, analgesics and antineoplastic drugs. A total of 68 pharmacokinetic interactions have been described, with lamotrigine (n = 22), topiramate (n = 18) and oxcarbazepine (n = 7) being associated with most, whilst lacosamide, pregabalin, stiripentol and vigabatrin are associated with none. Overall, only three pharmacodynamic interactions have been described and occur with oxcarbazepine, perampanel and pregabalin.

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.

Gabapentin Therapy of Hiccups

OBJECTIVE:

To determine whether gabapentin is effective in the treatment of persistent or intractable hiccups.

DATA SOURCES:

A search of MEDLINE (1966-March 2013) using the MeSH search terms gabapentin, hiccups, and hiccups/drug therapy was performed. Additional databases searched included Web of Science (1945-March 2013) and International Pharmaceutical Abstracts (1970-March 2013) using the text words gabapentin and hiccups. Bibliographies of relevant articles were reviewed for additional citations.

STUDY SELECTION AND DATA EXTRACTION:

All data sources were considered for inclusion. Preference was given for articles written in English, although one abstract in German was used.

DATA SYNTHESIS:

Because of the low incidence of persistent or intractable hiccups, few if any controlled clinical trials are conducted on the efficacy of drug treatment. Therefore, most of the data involve case reports or case series. We evaluated 17 case reports and 2 case series involving gabapentin therapy for persistent or intractable hiccups. Therapeutic outcomes with gabapentin were positive in all cases, with temporal evidence suggesting an effect, but outcomes often were obscured by combination therapy and comorbidities in some cases. Case reports suggest that gabapentin might be useful as a second-line agent in patients undergoing stroke rehabilitation or in the palliative care setting where chlorpromazine adverse effects are undesirable. Gabapentin was very well tolerated, with only a few minor adverse effects.

CONCLUSIONS:

Gabapentin has a similar body of evidence as other pharmacotherapeutic agents used to treat hiccups. Gabapentin is well tolerated and should be considered as a second-line agent in selected patients.

Combined treatment with gabapentin and drugs affecting the renin-angiotensin system against electroconvulsions in mice

Recent experimental data suggest that certain angiotensin-converting enzyme (ACE) inhibitors and angiotensin AT1 receptor antagonists may possess anticonvulsant activity. The purpose of this study was to examine the effects of two ACE inhibitors, captopril and enalapril, and two AT1 receptor antagonists, losartan and telmisartan, on the protective action of gabapentin in the maximal electroshock seizure threshold test in mice. Additionally, the effects of the combined treatment with gabapentin and antihypertensive drugs on memory retention in the passive avoidance task and motor coordination in the chimney test were assessed. All drugs were injected intraperitoneally. Losartan (50mg/kg) significantly increased the convulsive threshold for gabapentin. The other antihypertensive drugs, captopril (50mg/kg), enalapril (30 mg/kg) and telmisartan (30 mg/kg), did not affect the anticonvulsant activity of gabapentin. The observed interaction between gabapentin and losartan could be pharmacokinetic in nature. Losartan increased plasma and total brain concentrations of gabapentin. In the chimney test, losartan (50mg/kg) administered with gabapentin (50mg/kg) caused motor impairment. In the passive avoidance test, memory retention was not affected by the combined treatment with gabapentin and antihypertensive drugs. It is suggested that the use of captopril, enalapril and telmisartan in epileptic patients receiving gabapentin is presumed neutral upon its anticonvulsant action. The utmost caution is advised when combining losartan and gabapentin in clinical practice due to the appearance of pharmacokinetic interactions between losartan and gabapentin as well as motor impairment evoked by these drugs in mice.

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.

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.

Impact of concomitant antacid administration on gabapentin plasma exposure and oral bioavailability in healthy adult subjects

The aim of this open-label, randomized, and 3-period crossover study was to evaluate the influences of concomitant antacid administration on the plasma disposition, intestinal absorption, and urinary excretion of gabapentin in humans. Gabapentin (200 mg) was orally administered alone, with 1 g magnesium oxide (MgO), or with 20 mg omeprazole to 13 healthy adult subjects. Oral bioavailability (BA) of gabapentin was estimated by 24-h urine collection. The C(max), T(max) and AUC(0-∞) of gabapentin + MgO were significantly lower than that of gabapentin alone (by 33%, 36% and 43%, respectively) and gabapentin + omeprazole (by 29%, 46% and 40%, respectively). In contrast, no significant differences were observed in the plasma disposition parameters of gabapentin between the treatments with and without omeprazole. The gabapentin BA in the MgO treatment was significantly lower, by 32% and 39%, compared to the gabapentin alone and with omeprazole treatment, respectively. There was no significant difference in the gabapentin BA between the gabapentin alone and with omeprazole treatment. Concomitant MgO and omeprazole did not affect the renal clearance of gabapentin. In conclusion, concomitant MgO decreased the gabapentin exposure through the reduction of intestinal absorption extent and rate. This reduction may be independent of the suppression of gastrointestinal acidification caused by antacids.

Polypharmacy and food-drug interactions among older persons: a review

Polypharmacy is generally defined as the use of 5 or more prescription medications on a regular basis. The average number of prescribed and over-the-counter medications used by community-dwelling older adults per day in the United States is 6 medications, and the number used by institutionalized older persons is 9 medications. Almost all medications affect nutriture, either directly or indirectly, and nutriture affects drug disposition and effect. This review will highlight the issues surrounding polypharmacy, food-drug interactions, and the consequences of these interactions for the older adult.

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.