I figure you could consult a statistician first, write up a protocol, and statistical analysis plan, and do the trial. This is probably not the way what a PK study should be done. Let us assume that you already have a pretty good idea of the effective dose and tolerability because oxypurinol has been extensively studied following administration of the prodrug allopurinol. So the major unknown is what is the PK of oxipurinol. The idea is to understand the drug for use in real patients not healthy subjects.
Real patients take other meds.
So you need to study real patients. There is no rule that says they cannot be taking other meds. If you find big differences between some individuals then you might look closely at their meds to see if there is a clue about an possible intereaction. You will learn about such an interaction only by studying patients - not by studying healthy subjects. But this is fine tuning.