ANTITUMOR EFFECTS OF HUMAN RECOMBINANT INTERLEUKIN-1-ALPHA AND ETOPOSIDE AGAINST HUMAN TUMOR-CELLS - MECHANISM FOR SYNERGISM IN-VITRO AND ACTIVITY IN-VIVO

Recombinant human interleukin 1 alpha (rhIL-1 alpha) and etoposide (VP -16) synergize for direct growth inhibition of several human tumor cel l lines in vitro. Our previous studies demonstrated that VP-16 increas ed the number of membrane-associated IL-1 receptors (IL-1Rs) and also enhanced the internalization of receptor-bound rhIL-1 alpha. The purpo ses of this study were to test our hypothess that these events were cr itical to the synergy between rhIL-1 alpha and VP-16, to determine whe ther rhIL-1 alpha and VP-16 synergize to increase superoxide (SO) anio n radical production in vitro since SO anion has been implicated in th e toxic effects of IL-1, and to investigate the antitumor efficacy of the combination against tumors in vivo. A375/C6 melanoma cells and OVC AR-3 ovarian carcinoma cells were tested with IL-1 receptor antagonist (IL-1ra) before exposure to rhIL-1 alpha, VP-16 and rhIL-1 alpha plus VP-16. The synergistic or antagonistic effects were assessed by MTT a ssay. SO production was measured by reduction of cytochrome C. Athymic female mice bearing the A375/C6 melanoma were treated by rhIL-1 alpha , VP-16, and rhIL-1 alpha+VP-16. The antitumor effects were evaluated by quantitating tumor growth and survival time. Pretreatment with the IL-1ra abrogated the synergistic effects of rhIL-1 alpha and VP-16. Th e production of SO radical by A375/C6 cells was increased 2.5 fold by the combination of rhIL-1 alpha and VP-16, and the addition of exogeno us SOD blocked the synergy between rhIL-1 alpha and VP-16. However, wh en A375/SOD15 cells which over-expressed manganese superoxide dismutas e (MnSOD) after MnSOD cDNA transfection were exposed to rhIL-1 alpha a nd VP-16, in vitro antagonism was observed. In vivo studies demonstrat ed that the combination of rhIL-1 alpha and VP-16 delayed tumor growth better than either agent alone, although long-term survival was not i mproved because of substantial toxicity. Our results suggest that the synergistic antitumor effects of IL-1 alpha and VP-16 may be due to IL -1R modulation and increased internalization of IL-1-IL-1R complex by VP-16 treatment, as well as to a subsequent increase in SO anion radic al production from the tumor cells exposed to both drugs. Thus, the co mbination of IL-1 alpha and VP-16 might prove useful for the treatment of malignant disease in vivo, if the increased toxicity can be reduce d or managed.