MDR1 MESSENGER-RNA EXPRESSION DIFFERS BETWEEN GRADE-III ASTROCYTOMAS AND GLIOBLASTOMAS

104 N-2-frozen samples from 33 astrocytic tumors previously untreated with cytotoxic drugs have been analysed for the expression of p-glycop rotein transcripts (mdr1) by RT-PCR using beta(2)-microglobulin as an internal control, A remarkable variation was observed even within a si ngle tumor in 50% of the cases. Nevertheless, a difference became Visi ble between the groups of anaplastic astrocytomas and glioblastomas. W hile 78% of the grade 3 astrocytomas contained at least a minimum of 1 sample with a very low mdr1 expression, this was the case only in 23% of the glioblastomas. This supports an earlier observation revealing a positive correlation between tumor grading and the tumor cell fracti on stained with the monoclonal antibody JSB1, On the other hand, no ma jor differences were found between the histological groups when the sa mples with the highest mdr1 expression were selected to represent the individual tumors. Those samples are less informative, They might be d erived from tumor regions in which capillaries deliver a larger fracti on of the total mRNA pool, No induction of mdr1 was observed in some e arly astrocytoma or glioblastoma cell cultures even after administrati on of high concentrations of the drugs ACNU and VM26, often used in gl ioma therapy.