Clinical pharmacology and pharmacometric analyses of CNS drugs used in the acute management of seizures and spasticity

Abstract

The overall goal of my thesis projects was to characterize the pharmacokinetics, bioavailability and tolerability of CNS drugs used in the acute management of seizure emergencies and spasticity. Two drugs were studied: diazepam and baclofen. Both of these drugs are already approved by the FDA and have been extensively used. For diazepam, two novel formulations were evaluated for intranasal delivery as rescue therapy in seizure emergencies. In the case of baclofen, commercially available intrathecal baclofen solution was administered intravenously to characterize its pharmacokinetics and safety in humans.Diazepam rectal gel (Diastat®) is the only FDA-approved product indicated for acute repetitive seizures. Despite its proven efficacy, most older children and adults object to this route of administration. As a result, many patients do not realize the benefit of a therapy that can improve outcomes and decrease healthcare costs. Intranasal administration of benzodiazepines offers a potential alternative. The primary objective of this study was to compare the bioavailability and pharmacokinetics of two novel intranasal diazepam formulations versus intravenous administration in healthy volunteers. Twenty-four healthy volunteers were randomized into an open-label, three-way crossover study. Ten mg doses of two investigational intranasal diazepam formulations (solution, suspension) and a 5 mg intravenous dose of commercially available diazepam injectable, USP were given. A two-week washout period separated treatments. Plasma samples for diazepam analysis were collected pre-dose and at regular intervals up to 240 hours post-dose. diazepam concentration-time data were analyzed using a non-compartmental pharmacokinetics approach. Exposure following administration of diazepam intranasal solution (absolute bioavailability - 97%) was greater than the intranasal suspension (absolute bioavailability- 67%). Both investigational intranasal formulations were well tolerated. The results of this pilot study indicate that development of an intranasal diazepam formulation with high bioavailability, reasonable variability, and good tolerability is feasible. Further, a PK model was developed and simulation studies were performed to optimize future bioequivalence trials. Absorption characteristics of rectal and nasal diazepam formulations were compared using the deconvolution analysis.An additional identified problem that this thesis work aims to address is management of baclofen withdrawal. The current recommended management strategies for baclofen withdrawal are inadequate and availability of intravenous baclofen would permit rapid attainment of drug concentrations in plasma as well as accurate and precise dose titration, thus allowing for prevention and expeditious treatment of withdrawal symptoms and reduced risk of adverse effects. The objective was to characterize baclofen pharmacokinetics and safety given orally and intravenously. Twelve healthy subjects were enrolled in a randomized, open-label, crossover study and received single doses of baclofen: 3 or 5 mg given IV and 5 or 10 mg taken orally with a 48-hr washout. Blood samples for baclofen analysis were collected pre-dose and at regular intervals up to 24 hours post-dose. Clinical response was assessed by sedation scores, ataxia and nystagmus. Mean absolute bioavailability of oral baclofen was 74%. Dose-adjusted areas under the curve (AUC) between the oral and IV arms were statistically different (p=0.0024), while AUC variability was similar (co-efficient of variation: 18-24%). Adverse effects were mild in severity and not related to either dose or route of administration. Three and 5 mg IV doses of baclofen were well tolerated. Seventy-four percent oral bioavailability indicates that smaller doses of IV baclofen are needed to attain comparable total drug exposures. Both intranasal diazepam and intravenous baclofen hold promise in management of seizure emergencies and baclofen withdrawal, respectively. Results from studies described in this thesis will inform the design of subsequent clinical studies that are needed for market approval.