Stability of Isoniazid in Aqueous Solutions and Plasma

Abstract

Isoniazid is stable for several weeks in aqueous and buffered solutions at pH values below 8 and at low temperature. It is unstable in human and rabbit plasma whether added to pure plasma or present after oral or parenteral administration. Instability is enhanced by increased temperature, but is quite marked at refrigerator temperature. The presence of a factor in blood causes changes in the chemical state of isoniazid so that it no longer reacts with the reagent used in the chemical assay method of Poole and Meyer. The importance of these findings for the evaluation of INH bioassay is discussed.