The comparative metabolism of pyrantel in five species

The absorption, distribution and excretion of [14C]‐ and [3H]pyrantel tartrate after oral administration to the sheep, ox, pig, dog and rat have been studied. Urinary excretion of radioactivity was greatest in the pig and the dog, accounting for approx. 34 and 40% of the dose respectively. Absorption was also good in the rat as indicated by some excretion of radioactivity in the bile. In the sheep and the ox approximately 60% of dose was excreted unchanged in the faeces, this probably representing unabsorbed rather than biliary‐excreted material. The highest tissue levels of radioactivity were found in the liver. Apart from the excretion of unchanged pyrantel in the faeces of the sheep and the ox, pyrantel was extensively metabolised in vivo into many metabolites. None of these was identified, but degradation experiments showed that the thiophene ring had been extensively biotransformed and that the tetrahydropyrimidine ring was more resistant to metabolic attack.