Dear colleagues,
I address to the experts on how to calculate the antiviral activity of compounds in vitro. I always thought that the medium effective conc. of a compound (EC50) – it’s the conc. which diminishes twice infectious virus titer (we are dealing with influenza virus). For instance: if in virus control lgTCID50=6, EC50 corresponds to lgTCID50=3 (drop on 50 %). But one colleague of mine affirms that conc. of viral particles drops twice with lgTCID50 drop on 0,3 (his logic is that log102≈0,3). Besides that it’s unrealistic to catch such subtle drop of infectious titer, a great part of inactive substances or having very poor antiviral activity would turn active and having a good SI (SI=CTD50/EC50). On your opinion, is that way of reasoning valid?
try using this formula
IC50 = (50%-LowInh%)/(HighInh%-LowInh%)x(HighConc-LowConc) + LowConc
where
LowInh% / HighInh%: means % inhibition directly below / above 50% inhibition
LowConc / HighConc: Corresponding concentrations of test compound
I think the person suggesting the 0.3 factor is confusing IC50 with the midpoint parameter of a 4PL curve fit using log scale transforms. IC50 has a specific meaning - it means 50% reduction (as you are calculating it). Even if you are using log-transformed data to calculate IC50's, you should calculate the IC50 as where the curve crosses 50%, not where it crosses the middle of the log-scale curve (which is at about 30%). That makes it different and no longer comparable to IC50's calculated the standard way.
To estimate EC50, serial dilutions of a compound are tested against a fixed dose of virus. For each dilution, number of wells without CPE are recorded. EC50 can be calculated from this data using standard methods.
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