Differentiation Therapy Is Potentiated by Chemotherapy and Hyperthermia in Human and Canine Brain Tumor Cells In Vitro

HUMAN GLIOBLASTOMA (U-87MG) and canine glioma (canine brain tumor [CBT]) cell lines were tested in vitro for their therapeutic sensitivity to sequential treatment with differentiating agents and chemotherapy or hyperthermia. Both cell lines responded to the inducer combination dibutyryl adenosine-3′,5′-cyclic monophosphate/sodium butyrate by the formation of cytoplasmic processes detectable within 7 hours and attained approximately 90% morphological differentiation within 2 days of exposure. The clonogenicity of CBT and U-87MG cells gradually decreased after 1 to 7 days of exposure to the inducer combination, but this treatment alone failed to kill the cells. After the removal of the inducers, both lines dedifferentiated and the rate of clonogenesis increased. 1,3-bis-(2-Chloroethyl)-1-nitrosourea administered to CBT and U-87MG cells before or after 3 days of treatment with inducers potentiated the antiproliferative effects of the differentiating agents. Cisplatin administered to U-87MG cells enhanced the antiproliferative effect of the differentiating agents to a greater extent when added before the inducers rather than after differentiation was stimulated. The sequential treatment of CBT cells with a 44°C heat pulse for 30 minutes followed by differentiating agents produced an additive potentiation of cell killing, whereas the reverse sequence did not. Hyperthermia pretreatment at 44°C for 15 minutes or at 42°C for 30 minutes failed to enhance the antiproliferative effects of inducing agents. Pretreatment at 44°C for 30 minutes enhanced the antiproliferative effects of inducers by 23%, whereas 46°C for 30 minutes potentiated cell killing by 38%. This study demonstrates that nontoxic differentiation therapy can potentiate the therapeutic effects of chemotherapy in glial brain tumors and can thereby reduce the concentration of cytotoxic drugs required to achieve effective cell killing. Hyperthermia can be used to potentiate the effects of differentiating agents.