Thermal stability of disodium and calcium phosphomycin and the effects of the excipients evaluated by thermal analysis.

In the present study, Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) are simultaneously applied to determine the thermal properties of two antibiotic salts, disodium and calcium phosphomycin, used either pure or in association with several excipients. This study was carried out kinetically as well by mathematical elaboration of the TG curves performed according to an isothermal procedure applied at different fixed temperatures. Kinetic parameters showing agreement with those produced by the isothermal method were also obtained by means of a non-isothermal method using a dynamic TG curve alone. The main aims of the work were to provide reliable kinetic parameters (kinetic constant k, activation energy E(a) and pre-exponential factor A) to evaluate the thermal stability of phosphomycin salts in the presence and absence of the excipients generally contained in the phosphomycin-based pharmaceutical forms available on the market, and to obtain information concerning compatibility towards the active components. These kinetic parameters were then used to extrapolate shelf-life and half-life values at room temperature for pure active components in the solid state and for their pharmaceutical derivatives.