title: Transformation of beta-lactam antibacterial agents during aqueous ozonation: reaction pathways and quantitative bioassay of biologically-active oxidation products.
abstract: Reactions of ozone (O3) with the beta-lactam antibiotics penicillin G (PG) and cephalexin (CP) have previously been found to yield products retaining antibacterial activities. These products are unequivocally identified here as the stereoisomeric (R)-sulfoxides of each parent molecule and characterized by a combination of chemical analysis and an antibacterial activity assay. PG-(R)-sulfoxide, which is approximately 15% as potent as PG itself, is formed in approximately 55% yield, whereas CP-(R)-sulfoxide, which is 83% as active as CP, is formed with a maximum 34% yield. PG-(R)-sulfoxide is recalcitrant toward further oxidation by O3, but readily transformed by hydroxyl radical (HO*) (kHO*,app"=7.4x10(9) M(-1) s(-1), pH 7), resulting in quantitative elimination of its antibacterial activity. In contrast, CP-(R)-sulfoxide is degraded by both O3 and HO* (kO3,app"=2.6x10(4) M(-1) s(-1) and kHO*,app"= 7.6x10(9) M(-1) s(-1), pH 7), leading to quantitative elimination of its antibacterial activity. During ozonation of a secondary municipal wastewater effluent sample (pH 8.1, CDOC=4.0 mg/L, [alkalinity]=3.6 mM as HCO3-) spiked with [PG]0=1 microM, PG-(R)-sulfoxide yields did not exceed 0.15 microM for O3 doses up to 100 microM (4.8 mg/L), but reached 0.47 microM with 10-mM t-BuOH added as a HO* scavenger. In contrast, CP-(R)-sulfoxide yields did not exceed 0.1 microM for the same wastewater spiked with [CP]0=1 microM in either the presence or absence of t-BuOH, indicating that CP-(R)-sulfoxide transformation is governed primarily by direct reaction with O3. These findings suggest that, for a given degree of parent compound transformation, PG-(R)-sulfoxide yields would likely be greatest during ozonation of wastewaters characterized by low O3 demands and high HO* scavenging rates, whereas CP-(R)-sulfoxide yields would be less matrix-dependent. In general, complete deactivation of penicillins during wastewater treatment will likely require higher O3 exposures than necessary for deactivation of cephalosporins.
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Duebendorf, Switzerland. doddm@uw.edu
authors.forename: Michael C
authors.initials: MC
authors.lastname: Dodd
authors.forename: Daniel
authors.initials: D
authors.lastname: Rentsch
authors.forename: Heinz P
authors.initials: HP
authors.lastname: Singer
authors.forename: Hans-Peter E
authors.initials: HP
authors.lastname: Kohler
authors.forename: Urs
authors.initials: U
authors.lastname: von Gunten
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry*; D001711:Biotransformation; D002506:Cephalexin / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry; D010084:Oxidation-Reduction; D010126:Ozone / Q000737:chemistry*; D010400:Penicillin G / Q000737:chemistry*; D014865:Waste Disposal, Fluid / Q000379:methods
chemical_list: D000900:Anti-Bacterial Agents; D017665:Hydroxyl Radical; D010126:Ozone; D002506:Cephalexin; D010400:Penicillin G