Horse metabolism and the photocatalytic process as a tool to identify metabolic products formed from dopant substances: the case of sildenafil

Two horses were treated with sildenafil, and its metabolic products were sought in both urine and plasma samples. Prior to this, a simulative laboratory study had been done using a photocatalytic process, to identify all possible main and secondary transformation products, in a clean matrix; these were then sought in the biological samples. The transformation of sildenafil and the formation of intermediate products were evaluated adopting titanium dioxide as photocatalyst. Several products were formed and characterized using the HPLC/HRMS(n) technique. The main intermediates identified in these experimental conditions were the same as the major sildenafil metabolites found in in vivo studies on rats and horses. Concerning horse metabolism, sildenafil and the demethylated product (UK 103,320) were quantified in blood samples. Sildenafil propyloxide, de-ethyl, and demethyl sildenafil, were the main metabolites quantified in urine. Some more oxidized species, already formed in the photocatalytic process, were also found in urine and plasma samples of treated animals. Their formation involved hydroxylation on the aromatic ring, combined oxidation and dihydroxylation, N-demethylation on the pyrazole ring, and hydroxylation. These new findings could be of interest in further metabolism studies.