The efficacy with which disseminated SL2 and P815 tumors, in syngeneic DBA/2 mice, can be eradicated with low-dose recombinant interleukin-2 (rIL-2) therapy is about equal. Treatment (i.p.) of DBA/2 mice, injected i.p. with SL2 or P81 5 cells on day 0, with rIL-2 (Proleukin) on days 10 to 14 results in a cure rate of 50 to 60% in each case. The in vitro sensitivity of SL2 and P815 cells to cis-diammine-dichloro-platinum [11] (cisplatin) is also comparable, although P815 appears to be slightly more sensitive. In vivo, however, therapy with cisplatin is far less effective against P815 than against SL2. In the DBA/2-SL2 model, at all doses tested, combination therapy with cisplatin (administered on day 2) and rIL-2 (administered on days 10-14) resulted in anti-tumor efficacy greater than that of either drug separately. In contrast, in the DBA/2-P815 model, cisplatin decreased the anti-tumor efficacy of subsequently applied rIL-2 therapy. As the only difference between the 2 tumor models is the. tumor itself, the success of combination therapy with cisplatin and rIL-2 was dependent on tumor characteristics. We suggest that in these 2 tumor models, neither the sensitivity of these tumors to cisplatin nor their growth and dissemination patterns were responsible for the contrasting results of combination therapy in these models. Instead, tumor-dependent immune-modulating effects of cisplatin may be the cause of these effects. These immune-modulating effects may comprise (a) effects of cisplatin on the tumor cells, resulting in changes in their susceptibility to immune effector cells, or changes in their immunogenicity; (b) activating or suppressive effects of cisplatin on immune effector cells; or (c) a combination of these effects. These effects could then either synergize or antagonize with the immune activating properties of rIL-2.
