Missing time data

Dear all: I am trying to figure out if there is any effect of enzyme genotype on the PK of a drug and its active metabolites. But I don't know if I could use NONMEM to do this with my data. I have a dataset from multiple dose study. Drug was administered orally and only one sample was taken from each patient at steady state. Parent drug and three metabolites concentrations were measured. Each patient was genotyped. Time since last dose was not recorded because the maximum concentration and trough concentration varied only 3%. Could NONMEM be used for this analysis? Any comment will be highly appreciated. Best regards, Huali