Mercaptopurine and Allopurinol drug interaction

Mercaptopurine is an antimetabolite that shows drug interaction with Allopurinol, the later is used as prophylactic in case of gout. How this interaction is possible?

Allopurinol inhibits uric acid synthesis from hypoxanthine by inhibition of xanthine oxidase enzyme.

On the other hand, Mercaptopurine is converted into 6-thioinosinic acid by Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) thereby it inhibits actions of Inosine Monophosphate (IMP).

But mercaptopurine is metabolized by xanthine oxidase enzyme to 6-thiouric acid which is inhibited by Allopurinol. That's why this combination may result increased levels of 6-MP resulting in its toxicity. So dose of 6-MP should be adjusted in presence of allopurinol.