Pharmacokinetics of carbamazepine derived from a new tablet formulation

Characterization in Inhibitory Effectiveness of Carbamazepine in Voltage-Gated Na+ and Erg-Mediated K+ Currents in a Mouse Neural Crest-Derived (Neuro-2a) Cell Line

Carbamazepine (CBZ, Tegretol^®) is an anticonvulsant used in the treatment of epilepsy and neuropathic pain; however, several unwanted effects of this drug have been noticed. Therefore, the regulatory actions of CBZ on ionic currents in electrically excitable cells need to be reappraised, although its efficacy in suppressing voltage-gated Na⁺ current (I_Na) has been disclosed. This study was undertaken to explore the modifications produced by CBZ on ionic currents (e.g., I_Na and erg-mediated K⁺ current [I_K(erg)]) measured from Neuro-2a (N2a) cells. In these cells, we found that this drug differentially suppressed the peak (transient, I_Na(T)) and sustained (late, I_Na(L)) components of I_Na in a concentration-dependent manner with effective IC₅₀ of 56 and 18 μM, respectively. The overall current–voltage relationship of I_Na(T) with or without the addition of CBZ remained unchanged; however, the strength (i.e., ∆area) in the window component of I_Na (I_Na(W)) evoked by the short ascending ramp pulse (V_ramp) was overly lessened in the CBZ presence. Tefluthrin (Tef), a synthetic pyrethroid, known to stimulate I_Na, augmented the strength of the voltage-dependent hysteresis (Hys_(V)) of persistent I_Na (I_Na(P)) in response to the isosceles-triangular V_ramp; moreover, further application of CBZ attenuated Tef-mediated accentuation of I_Na(P)’s Hys_(V). With a two-step voltage protocol, the recovery of I_Na(T) inactivation seen in Neuro-2a cells became progressively slowed by adding CBZ; however, the cumulative inhibition of I_Na(T) evoked by pulse train stimulation was enhanced during exposure to this drug. Neuro-2a-cell exposure to CBZ (100 μM), the magnitude of erg-mediated K⁺ current measured throughout the entire voltage-clamp steps applied was mildly inhibited. The docking results regarding the interaction of CBZ and voltage-gate Na⁺ (Na_V) channel predicted the ability of CBZ to bind to some amino-acid residues in Na_V due to the existence of a hydrogen bond or hydrophobic contact. It is conceivable from the current investigations that the I_Na (I_Na(T), I_Na(L), I_Na(W), and I_Na(P)) residing in Neuro-2a cells are susceptible to being suppressed by CBZ, and that its block on I_Na(L) is larger than that on I_Na(T). Collectively, the magnitude and gating of Na_V channels produced by the CBZ presence might have an impact on its anticonvulsant and analgesic effects occurring in vivo.

Biowaiver Monograph for Immediate-Release Solid Oral Dosage Forms: Carbamazepine

Literature relevant to assessing whether BCS-based biowaivers can be applied to immediate release (IR) solid oral dosage forms containing carbamazepine as the single active pharmaceutical ingredient are reviewed. Carbamazepine, which is used for the prophylactic therapy of epilepsy, is a non-ionizable drug that cannot be considered "highly soluble" across the range of pH values usually encountered in the upper gastrointestinal tract. Furthermore, evidence in the open literature suggests that carbamazepine is a BCS Class 2 drug. Nevertheless, the oral absolute bioavailability of carbamazepine lies between 70 and 78% and both in vivo and in vitro data support the classification of carbamazepine as a highly permeable drug. Since the therapeutic and toxic plasma level ranges overlap, carbamazepine is considered to have a narrow therapeutic index. For these reasons, a BCS based biowaiver for IR tablets of carbamazepine cannot be recommended. Interestingly, in nine out of ten studies, USP dissolution conditions (900 mL water with 1% SLS, paddle, 75 rpm) appropriately discriminated among bioinequivalent products and this may be a way forward to predicting whether a given formulation will be bioequivalent to the comparator product.

Validation of the hepatic blood flow rate model for verapamil first-pass metabolism

The bioavailability of a new retard tablet formulation of verapamil was investigated in a randomized cross-over bioequivalence study on 12 healthy subjects. The drug was given in the form of a single 240-mg oral dose of a new retard tablet formulation, or as a standard retard tablet at the same dose to all subjects, followed by a single intravenous (i.v.) dose of 5 mg to 8 of the 12 subjects. Plasma verapamil concentrations were determined by a high performance liquid chromatography (HPLC) method. The bioavailability of the new peroral retard formulation was (20.00 +/- 4.30)% and was in reasonable agreement with that determined for the already registered verapamil retard formulation, i.e. (19.46 +/- 4.02)%, thereby indicating bioequivalence. For the prediction of systemic availability and estimation of the first-pass metabolism, only based on the data for peroral plasma levels, a hepatic blood flow rate limited model was used. In our experience, this model has been found to be extremely useful in providing reasonable estimates of verapamil first-pass effect.

Spline functions in convolutional modeling of verapamil bioavailability and bioequivalence. II: Study in healthy volunteers

The pharmacokinetics of a new verapamil retard tablet formulation have been investigated in a randomized cross-over bioequivalence study on 12 healthy subjects. The drug was given orally at a single new or standard retard tablet dose of 240mg and at a single intravenous dose of 5mg. Plasma verapamil concentrations were determined by HPLC. New retard tablets produced peak plasma verapamil concentrations of 81.34+/-5.69microg/l, time to peak plasma concentrations of 4.91+/-0.89h and an AUC (0-24h) of 1291+/-103.4h x microg/l, with a terminal phase half-life of 55.1+/-14.9h. After intravenous administration verapamil exhibited biphasic elimination kinetics with a terminal plasma half-life of 2.36+/-0.42h and systemic clearance of 34.32+/-5.81 l/h. Bioavailability of the new peroral retard formulation ranged from 19.49+/-4.41% to 67.69+/-11.70%. Absorption rates and amounts were evaluated by means of the spline-convolutional method. Input rates for the new verapamil retard formulation ranged from 0.77+/-0.20mg/h to 5.57+/-1.58mg/h. The cumulative amount of verapamil input was 39.17+/-9.71% for the new retard tablets. All pharmacokinetic parameters for the new verapamil retard tablet formulation, were in reasonable agreement with the data obtained on already registered verapamil retard formulations, indicating their bioequivalence.

Derivation of Laplace transform for the general disposition deconvolution equation in drug metabolism kinetics

This paper presents some very simplified general treatments which will allow workers to derive equations for any linear kinetic metabolic process. This is done through the use of the Laplace transforms. A general equation is presented to describe the disposition function in Laplace operators for the central compartment of a linear n compartment mammillary model with elimination occurring from any of the compartments. Input functions describes IV bolus, zero-order infusion, intramuscular injection or GI absorption. The Laplace transform for the amount of drug or metabolite in the central compartment is given by the products of the input and disposition functions.