NCA analysis using PKNCA - Back extrapolation not possible

[caribap]

Rversion = 4.3.2
I want to perform an NCA analysis for a single dose studies. I have concentration measurements at 5 timepoints from 0.25 to 24h for each subject, and have 3 subjects for each single dose - 1) 1mg/kg IV, 2) 10mg/kg IV and 3) 10mg/kg PO. In the end I want to have a table with pk parameters for the mean between the 3 subjects at each single dose study.

My problem is that PKNCA is not able to understand that I don't have time points before 0.25; in phoenix this is automatically done by back extrapolation. In PKNCA I get different results or no results at all - cant get an AUClast value right.

Note: I am using asnumeric and is.na to make sure my dose, concentration and time are manipulated correctly; I am using read_excel() to read my data and in the beginning I split my data by route creating 2 data frames

Thank you.