F2 – Similarity Factor

An f2 parameter is commonly used to establish similarity of two dissolution profiles. The formula and procedure to obtain f2 value is described in one of the publications.

In short, two profiles are considered identical when f2=100. An average difference of 10% at all measured time points results in a f2 value of 50. A public standard of f2 value between 50-100 to indicate similarity between two dissolution profiles. Another way of saying this is that on average if difference at each sampling time is 10% or less then f2 value will be between 50 and 100. Therefore, a quick way to establish similarly of two profiles is to see if differences in dissolution results at each sampling time are less than 10%.

In general, there is not much literature or debate available reflecting its relevance or link to the assessment of the compared products for their qualities or release characteristics in vitro (dissolution) or in vivo (bioequivalence). At best, the parameter (f2) appears to be a rule-of-thumb type gauge, and perhaps appears to add some extra burden for evaluating and reporting dissolution results without real gain in data analysis.

There has been some discussion on the tightness of the standard, which may result in rejection of products of similar drug release characteristics and suggestion appears to have been made to lower limit from 50 to say 30.

It also appears that this limit of 50-100 range for f2 (or a difference of less than 10% in dissolution results) may be in conflict with the commonly accepted pharmacopeial (e.g. USP) standards, where lot to lot testing (without formulation/manufacturing differences) the acceptable deviation is significantly higher than 10%. However, f2 which in general is suggested for evaluating products with formulation/manufacturing differences (for product developments and/or alterations), the allowance should be greater, thus wider range with a lower cut-off value than 50 may be more appropriate.

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