Application of an interleukin 2 slow delivery system to the immunotherapy of established murine colon 26 adenocarcinoma liver metastases.

We evaluated the antitumor effect of an interleukin 2 (IL-2) slow delivery system, the IL-2 minipellet, using a murine hepatic metastasis model. The IL-2 minipellet consists of atelocollagen derived from natural bovine skin together with 1 x 10(6) units of recombinant IL-2. Administration of the IL-2 minipellet was performed into the spleens of BALB/c mice after translocation of the spleens to the s.c. position. Administration produced detectable serum IL-2 levels for 72 h. The IL-2 minipellet was evaluated for its efficacy against hepatic metastases from colon 26 adenocarcinoma in the BALB/c mice. Both the administration of the IL-2 minipellet alone and its combination with the injection of 5 x 10(7) lymphokine-activated killer cells resulted in significant reductions of the number of metastatic nodules. Moreover, increased survival of mice bearing colon 26 adenocarcinoma was noted in these two treatment groups. To investigate the mechanism of the IL-2 minipellet activity, we tested the lytic potential of splenocytes obtained after administration of the IL-2 minipellet in a 51Cr release assay. Cytotoxicity against YAC-1 cells and colon 26 cells was significantly augmented on Day 2 after minipellet administration. These results demonstrated that local administration of the IL-2 minipellet into the hepatic circulation was extremely effective against metastatic liver cancer.