Mercaptopurine (6-MP) and Thioguanine (6-TG) resistance

Resistance is associated with 1) an inability to biotransform 6-MP to the corresponding nucleotide because of decreased levels of HGPRT (for example, in Lesch-Nyhan syndrome, in which patients lack this enzyme), 2) increased dephosphorylation, or 3) increased metabolism of the drug to thiouric acid or other metabolites.