The impact of stability and solubility of photodynamic therapies on method development

A well characterised and fully validated bioanalytical method used for measuring a novel porphyrin photodynamic therapy (Compound I) in human plasma was modified in order to enable the measurement of a structurally and metabolically related compound (Compound II) in rat plasma. The modified method was validated, but after initial attempts to implement the method to support routine sample analysis failed, it was suspected that Compound II was unstable in rat plasma. The preparation of plasma Quality Control (QC) samples used during validation to evaluate precision, inaccuracy, stability and the on-going performance of the method, was closely scrutinised because of the limited solubility of this class of compound in aqueous solution. After investigating the solvent composition of standard solutions of Compound II used during the preparation of QCs, together with the subsequent mixing techniques used to distribute the analyte in plasma, the limited solubility of Compound II was eventually shown to be the cause of the stability problem.