# PharmPK Discussion - Interpretation of Emax

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• On 17 Sep 2004 at 12:22:41, Batul Parta (batulparta.aaa.yahoo.com) sent the message
`Dear everybody,I am trying to learn more the indirect response modelsand have a couple of questions. What does an Emaxreally mean in such a model? Emax is used in thevariations of the model where the kin or kout isincreased. Does this Emax have the same meaning as aHill model, i.e. maximum effect? Or is it percentagechanges? The reason I ask is that in the other twovariants of the indirect response models, the maximumvalue the Imax can get is 1, meaning a 100% (total)decreae in kin or kout. If Imax is in %, so is Emax,right? Or am I missing anything here?I have looked at a couple of articles and the valuesgiven for Emax do not seem to be either the maximum or%change. It is very confusing.Thank you for your kind help.B Parta`
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• On 18 Sep 2004 at 09:57:05, Nick Holford (n.holford.-at-.auckland.ac.nz) sent the message
`Batul,The interpretation of Emax depends primarily on the form of the Emaxmodel. It has nothing to do with the the use of an indirect response ora direct response model. The meaning of Emax will be the same for bothkinds of response model.There are 2 variants of the Emax model:1. E=Emax*C/(EC50+C) ; the standard Emax modelThe interpretation of E (and Emax) is in the units of the responsebeing predicted e.g. mm Hg, if you were measuring blood pressure.2. E=1-Imax*C/(IC50+C) ; the inhibitory Emax modelImax and IC50 are exactly equivalent to Emax and EC50 but it oftenconvenient to use different symbols to indicate that this is referringto an inhibitory effect of the drug.The interpretation of E (and Imax) is a fractional change from anominal response of 1 in the absence of drug (C=0). Note this is NOT apercentage -- it is a fraction of 1. Imax has a practical upper limitof 1. If it is 1 then this means the response can be completelyinhibited by the drug. You may of course wish to multiply by 100 inorder to convert E and Imax to % of the nominal response in the absenceof drug.Both variants can be extended to the sigmoid Emax form (i.e. 'Hillmodel') without any change in the meaning of the parameters.NickNick Holford, Dept Pharmacology & Clinical PharmacologyUniversity of Auckland, 85 Park Rd, Private Bag 92019, Auckland, NewZealandemail:n.holford.-a-.auckland.ac.nz tel:+64(9)373-7599x86730 fax:373-7556http://www.health.auckland.ac.nz/pharmacology/staff/nholford/`
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• On 17 Sep 2004 at 19:15:32, "mathangi" (mathangi.aaa.msn.com) sent the message
`Hello,In the case of indirect response models, Smax/Imax represent maximumfractional stimulation/inhibition of the bioflux, i.e, kin or kout fromitsbasal value.  They are unitless. Imax values range from 0 to 1 and Smaxisany value greater than 0.But in the normal Hill function model, Emax is the maximum effect andhasunits of effect.Hope it is clearThanksMathangi`
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• On 21 Sep 2004 at 09:45:16, "Jadhav, Pravin *" (JadhavP.-at-.cder.fda.gov) sent the message
`For more info and discussion please check these messages:http://www.boomer.org/pkin/PK00/PK2000111.htmlhttp://www.boomer.org/pkin/PK01/PK2001138.htmlThanks,PravinPravin Jadhav[Thank you for using the archive but these links may change in thefuture. More likely with the current year references. It is better toquote the search page (http://www.boomer.org/pkin/simple.html) and thesearch terms used, thanks - db]`
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