Stability studies of ionised and non-ionised 3,4-diaminopyridine: hypothesis of degradation pathways and chemical structure of degradation products.

3,4-Diaminopyridine is used to treat some symptoms met in Lambert-Eaton myasthenia syndrome. It was shown efficient to reduce a form of variable muscle weakness and fatigability typical of the disease and correlated to a block of acetylcholine release. In France, 3,4-diaminopyridine is nowadays given to patients under capsules form and the status of hospital preparation. Whatever the diluant used in the formulation, the stability period could not exceed 12 months. Preliminary studies were made on a salt form in order to test the influence of various stress factors and determine if there is interaction between them. From this study, the most influent stress condition, presence of hydrogen peroxide, was selected and a comparative study was performed to compare the stability of molecular and salt species. Solutions of each species were exposed to 5 or 15% of hydrogen peroxide and analyzed at 8, 24, 72 and 216 h of degradation by HPLC-UV. Fractions of detected impurities were purified and collected by semi-preparative HPLC-UV and analyzed by HPLC-UV-ESI-MS and IR spectroscopy in order to determine their structure hypotheses. Theses experiments demonstrate that the salt species were more stable under oxidative stress condition than molecular species. The two main degradation products were collected and identified as 4-amino, 3-nitropyridine and 3,4-diaminopyridine-N-oxide when the molecular form was degraded whereas only 4-amino, 3-nitropyridine was found in less quantity in the salt solutions. Nitrogen pyridine and pyridine amine could not easily be oxidized by hydrogen peroxide in salt comparatively to molecular species due to the lone pair of electron engaged in a bound with hydrogen in the first case and by resonance change of the pyridine in the second case. This modification of structure promoted different pathways of degradation for the salt form which are more dependent of energy. Owing to the better stability of the salt species, a new pharmaceutical form containing it was developed to assess its stability under ICH standard conditions allowing an industrial manufacture of this drug.

Determination of A 3,4-diaminopyridine in plasma by liquid chromatography with electrochemical detection using solid-phase extraction.

In order to quantify a small amount of a drug, 3,4-diaminopyridine (3,4-DAP), in animal plasma samples, an analytical method was developed. It involved an extraction of 3,4-DAP and phenylephrine, used as internal standard (IS), from plasma with solid-phase extraction (SPE) on C18 cartridges. This analytical method is a hyphenated technique based on high-performance liquid chromatography with electrochemical detection (HPLC-EC) whose purpose is to obtain first a sensitive method and second a satisfying separation between 3,4-DAP and phenylephrine. The analytical method is accurate, specific, and linear between 10 and 500 g of 3,4-DAP per litre. The recovery of 3,4-DAP is estimated at 70.8% with a 95% confidence interval of (66.0 -75.6%). Intermediate precision was evaluated on three quality control samples; the intra-day precision was estimated at 13.5, 9.1, 7.8% and the inter-day precision at 17.9, 8.4, 9.3%. The limit of quantification of the method was evaluated at 10 g l-1. First toxicokinetic parameters determined on dogs plasma samples after one 3,4-DAP oral administration of 1 mg kg-1 were: Cmax=395.7 microg l-1; Tmax =15 min; t1/2=113.6 min; Clearance/F=16.8 ml kg-1 min-1 and Vd/F=2.7 l kg -1.