having oxo groups directly attached to the heterocyclic ring, e.g. hypoxanthine, guanine, acyclovir

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Medicinal preparations containing organic active ingredients > Heterocyclic compounds > having nitrogen as a ring hetero atom, e.g. guanethidine or rifamycins > having six-membered rings with two nitrogen atoms as the only ring heteroatoms, e.g. piperazine (A61K31/48 takes precedence ) > Pyrimidines; Hydrogenated pyrimidines, e.g. trimethoprim > ortho- or peri-condensed with heterocyclic rings > Purines, e.g. adenine > having oxo groups directly attached to the heterocyclic ring, e.g. hypoxanthine, guanine, acyclovir