The reversible biotransformation of N-acetylprocainamide in the rhesus monkey.

: The specific activity of 14C-N-acetylprocainamide (14C-NAPA) was measured in the 24-h urine of two rhesus monkeys given the labelled drug i.v. Specific activity was found to decrease by 15.8% from the original value. This was attributed to an in vivo biotransformation of NAPA to procainamide (PCA) and reacetylation of PCA back to NAPA before it was eliminated in the urine. The major fraction of NAPA, 72.1% of the dose, was found to be excreted in the urine of monkeys in 48 h.