Cyclophosphamide is a well-known anticancer agent acting
by means
of DNA alkylation. Associated with its tumor selectivity, it also
possesses a wide spectrum of toxicities. As the requirement of metabolic
activation before cyclophosphamide exerts either its therapeutic or
toxic effects is well recognized, research aiming at elucidating the
pathways that lead to the activation of this drug is of key importance.
This has created the necessity for developing an effective analytical
method for detecting cyclophosphamide and its breakdown products.
In this paper, an Acquity TQ tandem quadrupole mass spectrometer equipped
with electrospray ionization in positive-ion mode was employed for
detecting cyclophosphamide in its protonated form. The full-scan mass
spectrum of cyclophosphamide shows two ion clusters displaying the
characteristic isotopic pattern of two chlorine atoms and assigned
as sodiated cyclophosphamide, [CP + Na]+, and protonated
cyclophosphamide, [CP + H]+ or PCP. With the aid of quantum
mechanical DFT calculation, free energy differences in the gas phase
among PCP protomers were computed with respect to the most stable
protomer being protonated on the 2-oxide oxygen of the 1,3,2-oxazaphosphorine-2-oxide
ring. In addition, the interconversion mechanisms among the different
protomers were also proposed by intercepting the corresponding transition
states in the gas phase. Collision-induced dissociation (CID) of PCP
generated six characteristic product ions. Fragmentation mechanisms
were proposed and supported by computation. The calculated energy
barriers for all of the located transition states were found to be
accessible under the reported experimental conditions.