Drugs with susceptible sites for free radical induced oxidative transformations: the case of a penicillin

Penicillins, as bactericidal antibiotics, have been widely used to treat infections for several decades. Their structure contains both aromatic and thioether moieties susceptible to free radical oxidation. The ^?OH induced oxidation mechanism of amoxicillin was investigated by pulse radiolysis techniques and by final product analysis performed after steady-state γ-irradiation. The predominant sites of the ^?OH attack are suggested to be the thioether group, initially yielding an ^?OH adduct to the sulfur, and the aromatic ring. This adduct to the sulfur converts to sulfur radical cation, which has three competitive reaction paths: (1) by deprotonation at the adjacent carbon α-(alkylthio)alkyl radicals form, which undergo disproportionation leading presumably to sulfoxide as main product; (2) via the pseudo-Kolbe mechanism it may transform to α-aminoalkyl radicals; (3) the radical cation can be stabilized through intramolecular S.˙.O bond formation. The reaction mechanism suggests the presence of a short-living and a stabilized (via hydrogen bonding) long-living ^?OH adduct to the sulfur. The three-electron bonded dimers of amoxicillin were not formed owing to steric hindrance. Thiyl radicals were also present in equilibrium with α-aminoalkyl radicals. In the presence of dissolved oxygen, aromatic ring hydroxylation occurred along with complex reactions resulting in e.g. oxidation of the methyl groups. The formation of the sulfoxide is especially effective in the presence of dissolved oxygen, under anaerobic condition, however, it is also generated owing to H₂O₂ and α-(alkylthio)alkyl radicals. The thioether moiety appears to be more sensitive to oxidation compared to the aromatic ring in case of amoxicillin.