# PharmPK Discussion - Infusion stopped at 24 hrs

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• On 16 May 2009 at 22:39:45, Pharmacologist (pharmacologists.at.gmail.com) sent the message
`Dear All,I would like to know, in case of an infusion stopped at 24 hrs:1. what would be the cmax2.For calculating Vd, we would need the extrapolated value back to they axis intercept?Cordially,Gupta`
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• On 18 May 2009 at 09:54:58, David Bourne (david.-a-.boomer.org) sent the message
`The following message was posted to: PharmPKIt is depend upon the t1/2 of your drug. if your drug has short half-life then my be the plasma concentration is reaching steady state andthe level will be your the maximum plasma concentration produce by therate of infusion will be the Cpss and Vd will be simply calculatedfrom the following equationR0 = CPss.Vd.KelIf the t1/2 is long then the maximum plasma concentration produce bythe rate of infusion will be calculated from the intercept.`
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• On 18 May 2009 at 21:58:41, Pharmacologist (pharmacologists.at.gmail.com) sent the message
`Dear David,The infusion is of Methotrexate, which has a t1/2 of 3-15 hours, andhas a three phase elimination. The infusion was stopped at 24 hours,and samples were collected at 24, 36, 48, 72 hrs post infusion. Whenthe values are plotted on semi log curve, it is a straight line, whichshows a one compartment model. Can  drug llike methotrexate, withthree phase elimination (though first phase of distribution is quiteshort) give a one compartmental model of distribution? How is the drugbehavior in this case, as the results seem to a monoexponential decayakin to IV bolus dose?!Cordially,Gupta[Once the first and second faster phases have completed five half-lives the last phase is all that is left and you will get a mono-exponential decline. Play with the applet athttp://www.boomer.org/c/p4/ja/Fig1966/Fig1966.htmlset the slow k0 to 0.1 (or less, it doesn't like 0) and try increasingthe Duration (you will also need to change the y-axis using the setaxis button) - db]`
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• On 19 May 2009 at 14:41:14, "Graham Mould" (gmould.-a-.gcpl.co.uk) sent the message
`The following message was posted to: PharmPKHi Gupta,We did some work many years ago on Methotrexate following 24 hoursinfusion.Mould GP, Aherne GW, Morris BA, Teale JD and Marks V. Radioimmunoassayofdrugs and its clinical application. European Journal of DrugMetabolism andPharmacokinetics,(1977),  4,  171-184.Aherne GW, Piall E, Marks V, Mould GP and White WF.  Prolongation andenhancement of serum methotrexate concentrations by probenecid. BritishMedical Journal,  (1978),  1,  1097-1099.When we measured over the first 24 hours after stopping the infusion,thekinetic profile was biexponential and in the paper above we postulatedonthe '3rd phase'.GrahamGraham Mould, PhDConsultant PharmacistEmail: gmould.-at-.gcpl.co.ukWeb:  www.xpertwitness.co.ukXPC ServicesSurrey Technology CentreSurrey Research Park, Occam RoadGuildford, Surrey GU2 7YG`
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• On 20 May 2009 at 05:33:14, Pharmacologist (pharmacologists.aaa.gmail.com) sent the message
`Hi,I want to know how to calculate cmax, tmax and AUC in this case wheninfusion of methotrexate has been stopped at 24 hrs...Cordially,Gupta`
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