Piperazine N-substituted naphthyridines, pyridothienopyrimidines and pyridothienotriazines: new antiprotozoals active against Philasterides dicentrarchi.

New antiprotozoals active against Philasterides dicentrarchi, the causative agent of scuticociliatosis in farmed turbot and Black Sea bass-bream, have been synthesised and tested. The most active compounds posses a piperazine ring, generally N-bonded to the heterocycle, and are the 1,8-naphthyridines, 2f and 5o, the pyridothienopyrimidine (7), and the pyridothienotriazines, 8, 9, 12d, 12f, 12h, 12m and 12k. Pyridothienotriazine (12k) presents the same activity (Lethal Dose, LD=0.8/1.5 mg L(-1)) as the well-known antiparasitics niclosamide and oxyclozanide.