[dielske]

Dear all,

what is the potency of antibiotics and when do you need to use it ?
(I did find some info about this in wikipedia, but find it hard to relate this with making stock solutions of antibiotics)

I, for example, have never used it and always made stock solutions according to what they told me. For ex: 100µg/mg kanamycine stock solution.

I have never wondered about potency untill I saw a post on another website about this.

"I need to do the antibiotic susceptibility testing in P. aerugninosa with sulfamethoxazole and trimethoprim. Both antibiotics have been bought from Sigma-Aldrich, Singapore. The potency values are not listed for both the antibiotics. How should I go about for calculating the weight required to prepare the stock solution using this formula wt = (volume * concentration)/(Potency). "

So I wonder: what is this potency and should you use it all the time to calculate stock solutions etc or not?

(for ex. kanamycine 50µl/ml stock solution, I simply add 50µg in 1ml and there it is.... I never wondered about about potency, nor did anyone ever tell me something about this).

[rjb]

Dielske,

Potency is a bit of a wooly definition, and can relate to number of possibilities... Most probably its used to define the minimum inhibitory concentration (MIC) of a particular antibiotic against a specific microbeastie. It can be measured using either Kirby-Bauer or E tests (http://apps.who.int/phint/en/p/docf/), both of which measure the size of the zone of inhibition around an impregnated disk or antibiotic gradient paper. This is then used against a table (or in the case of E test read directly off the paper) to determine MIC. The trouble is, MIC is a measure of the concentration at which visible growth inhibition occurs and not at which all of your microbeasties are killed, so you may need to also consider the minimum bactericidal conc. (MBC) which measures the conc at which 99.9% of the population are killed... also a measure of potency. If you want to go the whole hog, another measure of potency is mutant prevention conc. (MPC) all of which can be defined as potency!

Do you need to use the potency value? Well that depends upon what you're doing... If what you're making up does what its supposed to do, then the answer is probably not!

Kind Regards

R

[dielske]

ok thanks for your reply.

So when do you really use it? You only use it when you cant to calculate what the minimum amount is needed for example to kill a certain bacteria?
Eg: I need to use 10µg kanamycine/ml to make sure the bacterium is dead, must I then use the potency of kanamycine to calculate the true "power" of kanamycine?

Or ?

[rjb]

Dielske,

Sort of... All the MIC and MBC really states is the conc. you need to make up of your antimicrobial to suspend growth or knock out 99.9% of a specific bacteria. I think perhaps the original post that you spotted maybe confused matters a bit...

I'm not entirely sure what work you are involved in, but I suspect that you are using Kanamycin to select against un-transformed microorganisms... perhaps? In most cases, protocols exist for this sort of work, but if you are designing a new transformation protocol, MIC and MBC values might be of some use. You could use these to select an antibiotic concentration based on MIC/MBC high enough to kill off untransformed organisms whilst avoiding concentrations that were so high they would knock out the bacteria you're selecting for.

Clinically, MIC/MBC values are used by clinicians to determine the most effective antimicrobial against a specific bacterial infection and allow them to steer clear of antibiotics that require doses which may cause adverse symptoms... A nice example is vancomycin.

Not sure if I answered your question...

Kind Regards

R

[dielske]

Its getting complicated now

Could you give a practical example of when you use the potency?

Because I am starting to think potency is nothing more then an MIC value?

For some reason I was thinking that potency ment how active a drug was. For ex: if it says 800µg/mg potency, that it means that only 800µg of the 1mg really works. (or is this a too simple vision?)
Another example I saw somewhere: "Kanamycin Sulfate has a potency equivalent to not less than 750 μg of
kanamycin (C18H36N4O11) per mg, calculated on the dried basis". I always "translated" this as: kanamycine sulfate works for only 75% of kanamycine itself.. (its not as good as kanamycine itself , because it has a potency of 750 (0,75) compared with kanamycine itself that is 1). Or am I completely wrong here?


and yes that post I got from another forum makes me very confused since I need to make antibiotics in order to check for transformants (as you allready mentioned). The thing is: I never really wondered about antibiotics and the concentrations: I just used existing literature to make my stock solutions. But due to that post, I am starting to wonder about what potency is and how they get to those stock solutions.

[rjb]

Ah ha!

I think we have an answer... Although antibiotic potency can be used to refer to MIC/MBC, it has been used on a couple of web-sites (e.g. new drug info) and in the original post to express the activity of (in this case) an antibiotic in salt form.
 
In the same way that many nitrogen containing drugs such as cocaine, quinine etc. come in freebase and salts forms, the same is sort of true for a number of antibiotics, like streptomycin and kanamycine which come in sulphate and other derivative forms. What the potency (or activity) value really tells you, is how much of that salt is active and how much is inactive sulphate or hydrochloride or whatever else... Kanamycin has a rmm of 484 gmol-1 and the H2SO4 component a rmm of 98 gmol-1, hence the inactive sulphate component represents about 17% of the total mass of the antibiotic. The manufacturers then pop in a catch all activity of not less than X% to account for other inactive components etc. This is something that you might need to take into account in making up stock solutions when your recipe doesn't explicity specify the sulphate form...

Hope this helps a bit.

Kind Regards

R



  

[dielske]

Whats rmm? I dont know for what it stands. I know what 484 gmol-1 means and I understand what you are saying, but I just dont know what rmm means?
mm= molar mass? But r = ?

And how do you calculate that 17% ? if its 484  gmol-1 and the inactive part of the drug (the sulphate etc) is 98 of that 484 gmol-1 , then shouldnt this be 20% or is there another reason to get in the end that 17% ?


Anyway: I think I get it, in most cases it just means (or they want to say) that off the product they sell only X% is really doing what you want it to do.

Stupid and simply example: if I would sell 100grams that are made out of 40% pure (working) antibiotic and 60% "water" (or whatever that is used), I could state that the potency is 40%.

Is this correct?

I think I am getting confused about something that is just not word to be confused about.. its just a misunderstanding or maybe misuse of some terminology.

[rjb]

Dielske,

RMM is relative molecular mass... The reason that in my calculation my end value was ~17% was because although active kanamycin has a RMM of 484g mol-1, kanamycin sulphate includes the inactive sulphate (98g mol-1) hence has a RMM of 582g mol-1.

Your 100 grams/60% water analogy is a good one and makes perfect sense...

Kind Regards

R

[dielske]

Ok I get it now I think.


I got confused on something I understood allready...

thanks for the answers.