VPT 421: Veterinary Toxicology (2+0)

ABSORPTION AND FATE

• It is believed that phenothiazine is converted to phenothiazine sulfoxide by cellular enzymes of the intestinal epithelium.
• Excess phenothiazine sulphoxide escapes liver and enters peripheral circulation and is responsible for the photosensitization.
• Some phenothiazine is absorbed intact.
• After absorption, phenothiazine is further oxidized in the liver, primarily to leucophenothiazine and leucothionol, 2 colorless substances which are excreted in the urine.
• Upon further oxidation in the atmosphere, these compounds form the brown-red dyes, phenothiazine and thionol.
• Urine and milk are, therefore, discolored for several days.
• The presence of these dyes is not, however, indicative of toxicosis.
• Constipation in the treated animals causes retention of phenothiazine and, thereby, increases absorption from the intestine, increasing the probability of toxicosis.