Receptor-dependent antiproliferative effects of corticosteroids in radiation-induced fibrosarcomas and implications for sequential therapy.

Competitive binding studies with [3H]dexamethasone and Scatchard analysis demonstrated a single class of high-affinity, low-capacity glucocorticoid receptor sites in 105,000 x g cytosols from radiation-induced fibrosarcomas. In vivo, both dexamethasone (DEX) and methylprednisolone treatments resulted in dose-dependent inhibition of tumor growth and cell proliferation. Changes in the sensitivity of the clonogenic cell population to 3 mM hydroxyurea were used to assess changes in the clonogenic cell proliferation during and after treatments with DEX or methylprednisolone. Neither methylprednisolone nor DEX given every 12 hr for three doses resulted in significant cell kill in the clonogenic fraction. However, changes in the hydroxyurea sensitivity of the clonogenic population after cessation of DEX treatments indicated G1 cell cycle progression delay with transient enrichment of S-phase clonogenic cells 24 to 48 hr after cessation of DEX treatments. The duration of the DEX-induced progression delay and the timing of maximal S-phase cellularity after DEX was directly correlated with the level of glucocorticoid receptors in the treated tumors. Using regrowth delay to assess the efficacy of kinetically directed sequential chemotherapy, the effectiveness of vincristine, given after DEX, was highly sequence dependent, with the most effective treatment interval being coincident with maximal S-phase clonogenic fraction. Other studies indicated that the effectiveness of cyclophosphamide could also be increased by time sequencing after DEX.