Design and molecular insights of drug-active metabolite based co-amorphous formulation: A case study of toltrazuril-ponazuril co-amorphous

•Drug’s active metabolite was a potential co-former for drug-drug co-amorphous system.•Drug-drug combination with similar chemical structures exhibit homogeneity.•Heterodimer with complementary intermolecular interaction was more stable.•Solid-state stability and crystallization inhibition enhanced spring parachute effect.