Please see:  http://www.sciencedirect.com/science/article/pii/S0003267006021076

 

A nice table is presented in the article also.

Summary:

None of the antibiotics tested retained its complete activity during a preservation period of 6 months at −20 °C. The potency loss after 6 months of conservation, measured as inhibition zones produced on an agar layer seeded with B. subtilis, ranged from less than 10% to 43%.

The correlation between the diameters of the zones and the number of days of preservation was low in the case of the most stable antibiotics, such as the quinolones, and highest in the case of benzylpenicillin, the least stable antibiotic. For most antibiotics, some of the variation could not be explained by loss of activity due to a longer time of freezing. Keto–enol tautomerism, which occurs between C-11 and C-12 of chlortetracycline [10], might be an explanation for the variation observed with this antibiotic. The equilibrium between the keto- and the enol-form of chlortetracycline in aqueous solutions depends on several factors, including the pH, the type of buffer and the temperature [11]. It is not well known if there is a difference in antimicrobial activity between both tautomers. Still, the variation of results with freshly prepared solutions was low, suggesting that the method is repeatable and suited for stability testing.

The loss of activity ranged from less than 10% for the quinolones to more than 40% for amoxicillin and penicillin G. In short, the potency of the three quinolones tested remained quite constant. Tetracyclines were less stable. Two of the three beta-lactam antibiotics and two cephalosporins showed the highest loss of activity.

It is well known that beta-lactam antibiotics are susceptible to different forms of breakdown in aqueous solutions. According to Boison, incurred samples should be kept at −70 °C [12], and stock solutions of ampicillin may be preserved at −20 °C but better at −76 °C [13]. In our trials, ampicillin was more stable than the other penicillins.





Frank J. Nice, RPh, DPA, CPHP



 





 		 	   		  



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