Anticoccidial and tolerance studies in the chicken with two 6-amino-9-(substituted benzyl)purines

Initial assays of 6-amino-9-(2-chloro-6-fluorobenzyl) purine (MK-302) and 6-amino-9-(2,6-dichlorobenzyl)purine (coded L-628,914) showed potential as anticoccidial agents on the basis of broad-spectrum activity and safety. In battery efficacy studies, dietary levels of 60 to 70 p.p.m. and above MK-302 and 45 to 60 p.p.m. L-628,914 proved to have excellent broad-spectrum anticoccidial activity in chickens given heavy exposure to virulent field isolates of coccidia. Eight-week floor-pen tolerance trials showed that the maximum tolerated diet concentration (MTC) of MK-302 was approximately 95 p.p.m. while the MTC of L-628,914 was approximately 60 p.p.m. Dietary relationships (p.p.m. MK-302:p.p.m. L-628,914 for equivalent effects) derived from the efficacy and tolerance results were 1.2:1 and 1.6:1 respectively and clearly demonstrated a higher therapeutic ratio for MK-302.