Thiopurine drug monitoring has become an important issue in treating children with acute lymphoblastic leukaemia (ALL). In this population, a genetic polymorphism causes wide differences in the activity of
thiopurine S-methyletransferase (TPMT)--the rate-limiting
enzyme of the
thiopurine degradation metabolism--leading to the necessity of
drug dose adjustments. It is not yet known if similar differences exist in the
inosine 5'-monophosphate
dehydrogenase (IMPDH; EC 1.1.1.205), the rate-limiting
enzyme of the
thiopurine synthesis. To test this, we established and validated a high-performance liquid chromatographic (HPLC)-based assay to determine the IMPDH
enzyme activity in erythrocytes. The remarkable features of this assay are its simple erythrocyte separation/
haemolysis and assay conditions and a distinct segregation of
xanthosine 5'-monophosphate (
XMP) from the clear supernatant after precipitation. The probes were processed without a time-consuming extraction and heating procedure and the assay demonstrated a good intra- and interday stability as well as a recovery rate of approximately 100%. The IMPDH
enzyme activity was measured in erythrocytes of 75 children with diagnosis of ALL before starting antileukaemic
therapy and their activity compared to those of 35 healthy adult controls. The measured
enzyme activity was wide ranging in both groups. The individual
enzyme activity differences observed in children with ALL might led to differences in the thionucleotide levels in those undergoing the standard
thiopurine dose regimen.