Susceptibilities of drugs to nitrosation under simulated gastric conditions.

ABSTRACT

Drugs of differing structures and pharmacological actions have been incubated at 37 degrees C and pH 2.0 under conditions simulating those within the normal fasting stomach. The nitrite concentration (25 microM) was kept as constant as possible for 3 hr in an attempt to mimic its in vivo replenishment from the saliva. The extents of N-nitrosation varied widely, but were less than those observed by Gillatt et al. (Fd Chem. Toxic. 1984, 22, 269) using the WHO Nitrosation Assay Procedure, in which the initial nitrite concentration is 40 mM, 1600 times greater, and the pH (3.0) is close to the optimum for the N-nitrosation of secondary amines. The highest yield of N-nitroso compound was obtained with the benzathine salt of penicillin G whereas some drugs, including hydrochlorothiazide and chlorthalidone, produced no detectable N-nitroso derivative. The degree of N-nitrosation was consistently reduced when the initial nitrite concentration of 25 microM was not replenished during the incubations, underlining the importance of simulating the continuous supply of nitrite from the saliva. In all instances, the reactions of the drugs with nitrous acid were inhibited and, in most cases, completely prevented by the presence of ascorbic acid (125 mg).