Units used to express inhibitor activity

[hypervalent_iodine]

I have a question about units. In reading papers around the development of new drug compounds (specifically antifungals), I've noticed that many seem to express and compare values such as IC50's in units of mass / volume (e.g. ug / mL) instead of molarity. I have also found this to be the case when I attend group meetings over in the biology labs I work in. This doesn't make a lot of sense to me, as surely it's more appropriate to compare inhibitory activities of different molecules on a per mole basis rather than per gram. Does anyone know if there's a reason for this, or is it just some sort of historical carryover?

[CharonY]

It is a matter of convenience. Especially in micro labs the compounds in question may not be fully characterized or purity may be unknown.

[hypervalent_iodine]

 

10 minutes ago, CharonY said:

It is a matter of convenience. Especially in micro labs the compounds in question may not be fully characterized or purity may be unknown.

 

Alright, I can understand if you are testing mixtures derived from natural sources, etc., or natural product isolates, that mass/volume is more appropriate. The instances I was coming across  that led me to ask my question were fully characterised and purified compounds, however. In those cases, why is it not the default to use molarity? I have been using molarity myself when determining Ki's, or MIC's, but I am routinely asked what this translates to in ug/mL by my colleagues in the biological sciences as a comparison to other compounds we work with. I don't see mass/volume as a useful comparison in these cases, as surely it is misleading? Worded another way, if compound A is a stronger inhibitor per mole than compound B, but a worse inhibitor per gram, should compound A not still be considered the stronger inhibitor of the two?

[CharonY]

It is poorly normalized, I agree. But unless you work in cell-free systems, reality is even worse. Mic will depend on growth conditions and strain, for example. Due to the overall sloppyness and variability many see added precision as superfluous. It really depends on the precise question. Also, if your lab works with extracts as well as characterized compounds, often the lowest common denominator is used out of convenience.