Reduction of therapeutic efficacy by a second cycle of PEG-IL-2.

The formation of antibodies with interleukin-2 (IL-2)-neutralizing capacity and their possible impact on successful locoregional PEG-IL-2 therapy was studied in the guinea pig Line 10 (L10) tumor model. Previously it was shown in this animal model that intratumoral therapy with polyethylene glycol-modified human recombinant IL-2 (PEG-IL-2) induced tumor regression and eradication of lymph node metastases. Therefore, the putative negative effects of IL-2-neutralizing antibodies can be studied in this model. In two similar experiments, animals were immunized by subcutaneous injections (3x/week for 5 weeks) with PBS/BSA, rIL-2, or PEG-IL-2. One week after the last injection, L10 tumor cells were inoculated contralaterally on the flank. Seven days after tumor transplantation, intratumoral treatment with PBS/BSA or PEG-IL-2 was started. During immunization and subsequent intratumoral therapy, sera were collected from the guinea pigs and tested in the CTLL-16 bioassay for IL-2-inhibitory activity. Intratumoral injections with PBS/BSA after immunization only incidently resulted in cure of tumor and metastases, suggesting that by sensitization treatment alone, the immune system was not sufficiently activated to eradicate a tumor inoculum. PBS/BSA immunization followed by intratumoral PEG-IL-2 therapy resulted in complete cure of primary tumor and lymph node metastases in all treated animals. Immunization with rIL-2 evoked IL-2-inhibitory activity in the serum of all guinea pigs. Nonetheless, six of these 11 rIL-2-immunized animals were completely cured by intratumoral PEG-IL-2 therapy. PEG-IL-2 immunization resulted in IL-2-inhibitory serum activity in three of 12 guinea pigs. The guinea pigs with high IL-2-neutralizing activity in their serum showed progressive growth of tumor and metastases, whereas the other animals were completely cured. The level of IL-2-neutralizing capacity in the sera from rIL-2-pretreated animals was higher than that from PEG-IL-2-pretreated guinea pigs, confirming a reduced immunogenicity of PEG-IL-2 as reported after intravenous administration. Passage of the sera from the immunized guinea pigs over a protein G column drastically reduced IL-2-inhibitory activity, implying IgG-related IL-2 inactivation. In conclusion, IL-2-neutralizing activity, possibly mediated by IgG antibodies, is more readily induced by intradermal rIL-2 administration than by PEG-IL-2 injections. Our data suggest that local treatment of cancer patients with a second cycle of PEG-IL-2 after previous therapy with rIL-2 or PEG-IL-2 may reduce the therapeutic efficacy of PEG-IL-2.