MANAGEMENT OF NONMATRIX INTERFERING PEAKS IN A CHIRAL HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC ASSAY PRODUCED BY SOLID-PHASE EXTRACTION OF RAT PLASMA

PD 146923, under evaluation as an alkylating radiosensitizing drug, co ntains one chiral center and one chemically reactive aziridine ring. A method was developed to evaluate possible in vivo enantiomeric invers ion of PD 146923 in rat plasma. Normal-phase chiral HPLC was necessary to separate the enantiomers, but a typical aqueous-based solid-phase extraction (SPE) was needed to isolate the analytes from plasma. SPE a t higher analyte concentrations removed all interfering peaks and gave acceptable recoveries. However, peaks (A-G) from seven new components interfering with analyte detection at lower concentrations were produ ced by SPE. The interfering peaks overlapped each other, so some were not observed until other, more intense interfering peaks had been mana ged. The low separation efficiency of the chiral column precluded mana gement of interfering peaks by modifying chromatographic parameters. C hemical reactivity of the analytes forced the use of mild conditions f or management of interfering peaks. Peaks A-F were: (A) water from the SPE cartridge; (B) SPE sorbent endcapping; (C, E and F) nonvolatile s alts of the SPE elution acid reacting with bases from the injection so lvent or with unidentified bases from the SPE cartridge; (D and G) ana lyte degradation products. This study identifies the nonmatrix peaks c oeluting with the analytes, and describes how an aqueous-based SPE met hod was developed for isolating these very polar, highly reactive anal ytes in plasma for separation in a normal-phase chiral HPLC assay. Add itionally, B, C, E or probably are present in many other solid-phase e xtractions, but are not observed because of polarity or solubility pro perties.