Amoxicillin

Hydrolysis

Description of key information

Amoxicillin is rapidly hydrolysed under mild acidic and basic conditions (Erah et al., 1997; Barschi et al. 2014) by the opening of the ß-lactam ring through the attack of the nucleophile H2O (Andreozzi et al., 2004) and it is not expected to persist in the environment. Amoxicillin penicilloic acid was proposed as the first degradation product obtained from the hydrolyzation of β-lactam ring with further transformation into amoxicillin penicilloic acid and amoxicillin 2',5'-diketopiperazine (Golza et al., 2013 and Pérez-Parada et al. 2011). The substance could be also susceptible of photoinduced transformation (Andreozzi et al., 2004) into amoxicillin-Soxides (Gozlan et al., 2010).

Additional information

Abiotic degradation:

The stability of amoxicillin in aqueous media has been analysed in several publications. Erah et al. (1997) prepared 100 mg/L test item in 0.01M and 0.1M HCL and 0.04M phosphate buffers preheated to 37 ºC. Samples were withdrawn and analysed by HPLC at intervals. The degradation kinetic was found to be pseudo-first order. Amoxicillin was most stable over the pH range 4.0-7.0 (half-life between 176h and 153h respectively) and degraded most rapidly under acidic conditions (half-life of 5.2h at pH 1 and 19h at pH 2). As the article points out, the β-lactam ring of amoxicillin is more susceptible to hydrolytic degradation when the pH deviates significantly from isoelectric point (pH 4.8).

Abiotic degradation of amoxicillin trihydrate has been also studied by several authors.Hydrolysis kinetics at different pH values were analysed by Braschi et al (2013). Buffer solutions at antibiotic concentrations of 100 μM were kept in dart at 25 ºC. The persistence of amoxicillin in water as a function of pH was in the order 5 > 7 > 9 > 3 (half-lives of 46.1, 19.7, 14.4 and 6.2 days respectively), highlighting weak catalytic activity at both acidic and alkaline conditions, due to the susceptibility of the β-lactam ring to nucleophilic attack by hydroxide ions.

Chadha et al. (2003) studied the kinetic of the degradation of amoxicillin trihydrate in aqueous solution using heat conduction microcalorimetry as a function of pH (1-10) and temperature (30-45 ºC). The calorimetrically determined enthalpy of reaction and rate constants gave and estimation of the β-lactam ring opening as the main degradation pathway of test item degradation. As stated by Erah et al. (1997), the β-lactam ring of amoxicillin was more susceptible to hydrolytic degradation when the pH deviated significantly from the isoelectric point (pH 4.8). The degradation followed the Pseudo-first order kinetics. The zwitterionic form of the substance was the most stable species at pH 5-7 with a half-life of 183.58h at pH 5 (30 ºC). The half-life at pH1 was of 4.95h (penalmidic acid formation).

Degradation products:

Although amoxicillin is known to be unstable in environment due to the susceptibility of its β-lactam ring to cleavage by abiotic (chemical degradation) and biotic (enzymic and biological degradation) (Längin et al. 2009), there is no much information available on chemical identification and quantitative detection of amoxicillin’s degradation products. In a publication by Gozlan et al. (2013), the formation of degradation products in aqueous solutions containing 100 μg/mL of amoxicillin trihydrate was studied. Three phosphate buffer solution were examined under controlled artificial environment conditions at pH 5, 7 and 8 and a fourth one at pH 7 containing the bivalent ions Mg2+ and Ca2+, this last one to analyse the possible bivalent ion chelation. In addition, two solutions from natural sources were examined, secondary effluents from a Shafdan municipal wastewater treatment plant (Israel) and groundwater from a farmland at Glil Yam (Israel). The analytical monitoring was performed by HPLC-UV after 3, 6 and 16 days in triplicate. The identified degradation products were amoxicillin penicilloic acid (ADP1/2), amoxicillin penilloic acid (ADP4/5), phenol hydroxypyrazine (ADP6) and amoxicillin 2',5'-diketopiperazine (ADP8/9). Their identity was confirmed by MS. According to the authors, ADP1/2 were the first degradation products obtained from the hydrolyzation of β-lactam ring and the ADP4/5 and ADP8/9 probable stem from those. As the article points out, the ADP6 would be probably obtained followed by further process (not suggested) after the β-lactam ring opening. The quantitative analysis of amoxicillin’s degradation products in the aquatic environment indicated trace amounts of ADP4/5 (0.15 μg/L) and ADP8/9 (0.5 μg/L) in the secondary effluents. ADP1/2 (non-calibrated results) and ADP6 (non-quantitative) were detected in several micrograms per litter in secondary effluents. In the groundwater, only ADP8/9 was detected at 0.03 μg/L.