Drug Metabolism in Drug Design and Development Basic Concepts and Practice

and 14.4b), consistent with the addition of the element ofN-acetylcysteine to
the parent molecule. The MS^2 spectrum ofm/z611 reveals a loss of the element
of water (18 Da) to give a product ion atm/z593 (Fig. 14.4c). The other
fragment ion atm/z482 was ascribed to cleavage adjacent to the thioether
moiety. The product ion atm/z464 resulted from elimination of the element of
water from the fragment ion at m/z482. The MS^3 spectrum of m/z 464
generated product ions atm/z393 (loss of the pyrrolidinyl group) and 273 (loss
of the 4-(2-pyrrolidin-1-ylethoxyl) group) (Fig. 14.4d). Further NMR analysis
of theN-acetylcysteine adductDrevealed that the element ofN-acetylcysteine
was added to a biphenyl hydroquinone metabolite with the same molecular
weight as the parent moleculeCthrough structural rearrangements (Zhang
et al., 2005). These results suggest that compound C is metabolized by
cytochrome P450 in human liver microsomes to form a reactive extended
quinone intermediate, and the proposed bioactivation mechanism of Cis
shown in Fig. 14.3b.

14.1.3 Protocol for Detection of Glutathione or Cyano Adducts Using
Constant Neutral Loss Scanning of Triple Quadrupole Mass Spectrometer

14.1.3.1 Incubation Incubation procedures of test compound in human or
rat liver microsomes in the presence of GSH or NAc or KCN and NADPH

O

HO S

O N

OH

O

HO S OH

O N

O

HO S

O N

OH

NC

CN

O S

O

O

N

H

OH

O

HO S

O N

+

OH

O

HO S

O N

OH

NC

HO S

OH

O

N

H

OH

S

HN

O

OH

O

Compound A

Compound C

Compound B

P450 3A4

P450 3A4

• CN


• CN

Compound D

P450 3A4 NAc

(a)

(b)

FIGURE 14.3 Proposed mechanism for the cytochrome P450 3A4-mediated
bioactivation of dihydrobenzoxathiinsA(a) andC(b).

GLUTATHIONE,N-ACETYLCYSTEINE, AND POTASSIUM CYANIDE 455