Utilization of In Vitro, In Vivo and In Silico Tools to Evaluate the pH-Dependent Absorption of a BCS Class II Compound and Identify a pH-Effect Mitigating Strategy.

PURPOSE: To describe a stepwise approach to evaluate the pH effect for a weakly basic drug by in vitro, in vivo and in silico techniques and identify a viable mitigation strategy that addresses the risk. METHODS: Clinical studies included assessment of the pH effect with famotidine. In vitro dissolution was evaluated in various biorelevant media and in a pH-shift test. PK studies in dogs were conducted under pentagastrin or famotidine pre-treatment and GastroPlus was employed to model human and dog PK data and simulate the performance in human. RESULTS: Clinical data indicated considerable pH dependent absorption of the drug when dosed in the presence of H2-antagonists. In vitro dissolution and in vivo dog data confirmed that the observed pH effect was due to reduced dissolution rate and lower solubility at increased gastric and intestinal pH. A salt form was identified to overcome the effect by providing fast dissolution and prolonged supersaturation. GastroPlus simulations predicted a mitigation of the pH effect by the salt. CONCLUSIONS: The drug exhibited a strong pH-effect in humans. The in vitro, in vivo and modeling approach provides a systematic workflow to evaluate the risk of a new drug and identify a strategy able to mitigate the risk.

Preparation, characterization and in vivo conversion of new water-soluble sulfenamide prodrugs of carbamazepine.

Improved synthetic methods are reported for the preparation of sulfenamide derivatives of carbamazepine (CBZ) for evaluation as prodrugs. These sulfenamide prodrugs were designed to rapidly release CBZ in vivo by cleavage of the sulfenamide bond by chemical reaction with glutathione and other sulfhydryl compounds. Physicochemical characterization and in vivo conversion of a new prodrug of CBZ was evaluated to further establish the proof of concept of the sulfenamide prodrug approach.

Formation of reactive impurities in aqueous and neat polyethylene glycol 400 and effects of antioxidants and oxidation inducers.

A 2,4-dinitrophenylhydrazine (DNPH) precolumn derivatization high-performance liquid chromatography-ultraviolet detection (HPLC-UV) method was developed to quantify levels of formaldehyde and acetaldehyde in polyethylene glycol (PEG) solutions. Formic acid and acetic acid were quantified by HPLC-UV. Samples of neat and aqueous PEG 400 solutions were monitored at 40°C and 50°C to determine effects of excipient source, water content, pH, and trace levels of hydrogen peroxide or iron metal on the formation of reactive impurities. The effects of antioxidants were also evaluated. Formic acid was the major degradation product in nearly all cases. The presence of water increased the rate of formation of all impurities, especially formic acid as did the presence of hydrogen peroxide and trace metals. Acidic pH increased the formation of acetaldehyde and acetic acid. A distribution of unidentified degradation products formed in neat PEG 400 disappeared upon addition of HCl with corresponding increase of formic acid, indicating they were likely to be PEG-formyl esters. Other unidentified degradation products reacted with DNPH to form a distribution of derivatized products likely to be PEG aldehydes. Antioxidants butylated hydroxyanisole, butylated hydroxytoluene, propyl gallate d-alpha tocopheryl polyethylene glycol-1000 succinate, and sodium metabisulfite were effective in limiting reactive impurity formation, whereas ascorbic acid and acetic acid were not.

In vitro and in vivo evaluation of a novel water-soluble N-glycyl prodrug (N-GLY-CBZ) of carbamazepine.

The synthesis and characterization of N-glycyl-carbamazepine (N-Gly-CBZ), an N-acyl urea derivative of carbamazepine (CBZ) designed to act as a prodrug and convert to CBZ and glycine in vivo by enzymatic cleavage of the glycyl-urea bond was recently reported. The rate and extent of conversion of N-Gly-CBZ to CBZ in a whole animal model is reported here along with supporting in vitro data. Pharmacokinetic parameters were determined for N-Gly-CBZ and CBZ following IV and oral administration of N-Gly-CBZ and CBZ control to rats using a crossover design. The in vivo elimination of N-Gly-CBZ following IV administration in rats was biphasic in nature with a t(1/2) of about 1.1 min, which was very similar to the t(1/2) for appearance of CBZ. The mean value for the relative AUC ratio for CBZ from N-Gly-CBZ and CBZ from a cyclodextrin solution showed that N-Gly-CBZ delivered a (± SD) 98 ± 16% (± SD) equivalent dose of CBZ in six rats. The results of the IV dosing pharmacokinetics investigation were consistent with N-Gly-CBZ acting as a prodrug with rapid and complete conversion to CBZ in vivo. The overall absolute oral bioavailability of CBZ from N-Gly-CBZ was determined to be 41 ± 14% in three rats. The relative oral bioavailability of CBZ from N-Gly-CBZ compared to an oral CBZ control was 1.72 ± 0.54. That is, the prodrug, N-Gly-CBZ, demonstrated superior oral bioavailability of CBZ over the CBZ control, which was likely due to its greater aqueous solubility.

Preparation and physicochemical characterization of a novel water-soluble prodrug of carbamazepine.

N-acyl-urea derivatives of carbamazepine (CBZ) were synthesized through the reactions of iminostilbene with acyl-isocyanates to form N-glycyl-carbamazepine (N-Gly-CBZ, after a deprotection step) or N-acetyl-carbamazepine (N-acetyl-CBZ). N-Gly-CBZ was isolated as its water-soluble HCl salt and was designed to act as a prodrug and convert to CBZ and glycine in vivo by enzymatic cleavage of the acyl-urea bond. The stability pH-rate profiles for N-Gly-CBZ and N-acetyl-CBZ were determined. The stability of N-Gly-CBZ was found to range over four orders of magnitude with its greatest stability at pH 3-4 and a t(90) value of 5.9 day at pH 4 at 25 degrees C. From the fit of the pH rate profile two pK(a) values were estimated to be 7.2 (terminal amine) and 10.0 (imide), which were independently verified using UV-visible spectroscopic analysis. The solubility of N-Gly-CBZ in aqueous solution was determined in the range of pH 5.5-7.5. The intrinsic solubility of the neutral form of the prodrug was found to be 4.4 mg/mL, and the solubility of the prodrug increased exponentially (log linear) as pH was decreased below its pK(a1) value. N-Gly-CBZ was found to have an aqueous solubility in excess of 50 mg/mL at pH 4. The presence of N-Gly-CBZ was found to increase the aqueous solubility of CBZ, a degradation product. CBZ showed an 8.6-fold greater solubility in an aqueous solution containing 23 mg/mL of N-Gly-CBZ than in water alone. The solubilization of CBZ by N-Gly-CBZ was investigated by examining the diffusion coefficients of the predominant species in D(2)O and was found to be more consistent with stacking complex formation than micelle formation. The stability of N-Gly-CBZ makes a ready-to-use parenteral formulation impractical, but a freeze-dried preparation for reconstitution appears to be feasible.