A NEW ASSAY FOR INTRACELLULAR MEASUREMENT OF INOSINE MONOPHOSPHATE DEHYDROGENASE-ACTIVITY - A GUIDE FOR BETTER SELECTION OF PATIENTS FOR ENZYME-TARGETED CHEMOTHERAPY

We developed a new cytochemical assay for identification of cells cont aining inosine monophosphate dehydrogenase (IMPDH) and measurement of tumor cell sensitivity to the escalating dose/schedule of the IMPDH pa ttern-targeting drugs. The assay is based on cytochemical principles f or development of DH activity markers inside morphologically classifie d cells, image analysis for measuring the amount of this marker, and c omputer assistance for data management. The assay was optimized on a h uman leukemia cell line (K562-NS) and reference values for enzyme acti vity were established. Assay specificity was determined with different substrates and enzyme inhibitors. Sensitivity depends on the measurin g instrument (image analyzing system), and the precision of the biolog ical model used for assessment of reference values was high. IMPDH-pos itive malignant cells were found in all specimens obtained from acute leukemia and solid tumor patients (13/13 and 29/29) and in four human tumor cell lines (K562, K562-NS, HL60, and HL60-M). Cells of the K562- NS line were exposed to tiazofurin and ribavirin in conventional assay s for assessment of drug-induced acute and subacute toxicity/sensitivi ty. A reduction of IMPDH activity was recorded at drug concentrations below the range at which cell damage appeared.