The quantitation of cyclophosphamide in human blood and urine by mass spectrometry-stable isotope dilution.

The levels of cyclophosphamide in the blood and the urine of patients have been monitored by direct insertion, electron impact mass spectrometry using the principle of stable isotope dilution. When a tetradeuterated analogue of the drug was added to a sample of blood or urine the concentration of cyclophosphamide could be determined from the ratio of the intensities of the signals (M-CH2 C1) characteristic for cyclophosphamide and the tetradeuterated analogue present in the mass spectrum of a chloroform extract. The procedure is highly specific for cyclophosphamide and obviates the need to use radioactively labelled cyclophosphamide for quantitation of the drug in man.