engleski

Utilization of a kinetic isotope effect to decrease decomposition of ceftriaxone in a mixture of D2O/H2O

The labile β-lactam ring of penicillins and other β-lactam antibiotics is characterized by its pronounced susceptibility to various nucleophiles, acid-base reagents, oxidizing agents or even solvents like water and alcohol. However, the major pathways of β-lactams degradation are similar, leading to various breakdown products. The discovery of cephalosporin and demonstration of its improved stability in aqueous solution, as well as its enhanced in vitro activity against penicillin-resistant organisms, were major breakthroughs in the development of β-lactam antibiotics. Although cephalosporins are more stable with respect to hydrolytic degradation than penicillins, they still experience a variety of chemical and enzymatic transformations. The present study was designed to gain insight into the kinetics of cephalosporin degradation, more specifically, ceftriaxone degradation at its therapeutic concentration. As ceftriaxone is one of the most frequently used parenteral antibiotics in treating specific infections, understanding of its degradation mechanisms at the concentration administered parenterally is essential for development of formulations containing this API. Therefore, our study was focused on obtaining information on the rates and mechanisms of ceftriaxone degradation in aqueous environment at the therapeutic concentration of 20 mg/mL. The study was directed towards obtaining kinetic information on the rates and mechanisms of ceftriaxone degradation in water, a mixture of water and deuterium oxide, and deuterium oxide itself at the neutral pH (range of 6.7 to 7.6). Specific ceftriaxone degradation products were observed in aged samples prepared in the examined solvents using HPLC and MS. By comparing the degradation rates in H2O and D2O, the observation of a kinetic isotope effect (KIE) provided some valuable insight as to the nature of the initial ceftriaxone degradation. This result, in combination with an investigation of the reaction order for the degradation using the method of initial rates, highlighted the important contribution of the formation of a previously unreported dimer-type species. Computational analysis was utilized to get a molecular insight into chemical processes governing the ceftriaxone degradation and to rationalize the stabilizing effect of replacing H2O with D2O. For that purpose, molecular dynamics simulations in both explicit solvents were carried out, together with a range of mechanistic DFT calculations to obtain the underlying kinetic and thermodynamic parameters of the most prevailing reactions in solution. In doing so, focus was on two dominant processes, (i) the opening of the β-lactam ring following the hydrolytic cleavage of its C–N amide bond, and (ii) the breaking of the C–S bond linking the triazine moiety with the rest of the structure.

ceftriaxone

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