Assuming that there is only one metabolite, the rate constant for its formation is the same as the rate constant for loss of the parent. It would be advisable, by the way, to use more than 2 time points to fit the data to the exponential function.

Thanks for the information. We prefer two points as it was the most feasible method in our local settings and mostly agreed by all participants. I chose elimination rate constant for the parent compound and still searching the formation rate constant or perhaps the metabolite parent ratio established formula. there are few metabolites involved in the formation.