How to exclude Predose from being identified as Cmax in Phoenix

Phoenix Certara Forums Phoenix WNL basics
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Hi all,

I received a single dose plasma concentration-time dataset (n=50 subjects) that I will be using NCA in Phoenix to generate the standard PK parameters. The compound of interest is endogenous so there is existing concentration prior to dosing (pre-dose). The sampling scheme included pre-dose (0) and upto 12 hours post-dose at predetermined intervals; only 1 predose sample was collected.

Upon reviewing the conc-time datasets, I noticed that there were 2 out of 50 subjects with pre-dose level being higher than the post-dose concentrations. The post-dose concentrations did follow the patten of a drug release profile (increased to peak conc and then decreased overtime).

Question: Is there a way for me to setup Phoenix so to prevent the NCA analysis from identifying the pre-dose as Cmax (and the corresponding Tmax)? I tried to setup the NCA for multi-dose scenario but the predose was still identifed as the Cmax (and Tmax) for the two subjects.

1 Like
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Hi John - an easy way to do this is to use Actual Time, and therefore predose samples would be a time or e.g. -0.25, or -01.667 h. Samples prior to dose are automatically excluded from NCA

https://onlinehelp.certara.com/phoenix/8.6/index.html#t=Phoenix_UserDocs%252FNCA_Object%252FData_checking_and_pre-treatment.htm

I would also recommend creating a table of all pre-dose samples to check that you understand/are happy with the data that is being dropped from the NCA. I would always like to be report positive pre-dose so I can discuss/explain why some of these may have happened and how/whether dropping these from NCA was appropriate. Simon.

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I agree with Simon. Those may be the best two options, especially if you want to specifically exclude pre-dose samples. I would just make sure that you indicate how excluding them affected the PK to proactively provide justification of their exclusion if reviewers ask. Thanks.