Mirabegron impurity 4

Mirabegron impurity 4 is a synthetic byproduct formed during the alkylation step of Mirabegron production. It retains the benzimidazole-piperazine core but incorporates a methylthio group at the C-5 position of the benzimidazole ring, substituting the native methoxy moiety. This substitution introduces a sulfur-based hydrophobic interaction site, altering the compound's metabolic stability and receptor binding affinity. The structure includes a piperazine-linked phenyl ring with a sulfonyl bridge, distinguishing it from the parent drug's sulfonylamine linkage. It functions as an HPLC reference standard for quantifying process-derived impurities in Mirabegron bulk drug substance.