LC-UV method development and validation for the investigational anticancer agent imexon and identification of its degradation products

Imexon (4-imino-1,3-diazabicyclo[3,1,0]-hexan-2-one) is a member of the class of 2-cyanoaziridine derivatives, which have been of interest as immunomodulators and anticancer agents since the late 1970s. The pharmaceutical development of imexon necessitated the availability of an assay for the quantification and purity determination of imexon active pharmaceutical ingredient (API) and the drug in its pharmaceutical dosage form. A liquid chromatographic (LC) method with ultraviolet (UV) detection was developed, using a reverse phase column with phosphate buffer (pH 6; 50 mM) as mobile phase and UV detection at 230 nm. Although retention capacity for imexon was small (capacity factor of 0.5), the method was found to be linear over the concentration range of interest of 1.0-25 μg/mL, precise, accurate, and stability-indicating. Moreover, the use of LC-mass spectrometry (MS) and on-line photodiode array (PDA) detection enabled us to propose structures for four degradation products. Two of these products were also found as impurities in the API. The degradation products, including chloro- and hydroxy-derivatised products were shown to arise from nucleophilic reactions with the activated aziridine moiety of imexon. The developed LC-UV method was found suitable for the pharmaceutical quality control of imexon API and the drug in its pharmaceutical dosage form.