New microbiological assay for determination of caspofungin in the presence of its degradation products and its measurement uncertainty.

Caspofungin is an echinocandin antifungal used in the treatment of invasive fungal infections. Several methods have been reported for the quantitative analysis of echinocandins; however, there is no microbiological assay for determination of caspofungin potency in the presence of its degradation products. This study aimed to develop and validate a microbiological method for quantitative analysis of caspofungin in lyophilized powder, evaluate the stability, and determinate the degradation kinetics of the drug when the finished product is submitted to heat stress. A procedure was established to estimate measurement uncertainty for routine analysis. The validation was performed as recommended in the current official guidelines. The agar diffusion method is based on the inhibitory effect of caspofungin on Candida albicans. Results showed selectivity, linearity, precision, and accuracy of the method. Statistical analysis demonstrated that method is linear (in the range 2.5 to 16 microg/mL, y= 15.73 + 6.4x, r2 = 0.9965), precise (intermediate precision: 2.54%), and accurate (recovery range: 95.01-102.46%). The proposed method allowed evaluation of the thermal stability of the drug at 80 degreesC for 120 min and determination of first order degradation kinetics. The variability of inhibition zone sizes was the most important source of uncertainty at about 87% of the overall uncertainty (103.0+/-1.7%). These results show that the proposed method is applicable to routine laboratory testing, and is sensitive to thermal degradation of caspofungin.