693. Regression of Established Subcutaneous B16 Melanoma Tumors after Intratumoral Delivery of a IL-15 Expressing Plasmid Followed by In Vivo Electroporation

Abstract

Melanoma is a major cancer due to metastasis; however, its initial cutaneous presentation makes it amenable to therapy. In vivo electroporation (E) has been used recently to efficaciously deliver conventional drugs and therapeutic genes to tumors, including melanomas. This study reports on the intratumoral (IT) delivery of DNA plasmids expressing interleukin-15 (IL-15), i.e. pIL-15, to established subcutaneous B16 melanoma tumors in C57BL6 mice. The IL-15 expression plasmid used was optimized for expression and was 80 fold more efficient than standard pcDNA3 based plasmids. IL-15 has been shown to have a pivotal role in the function of memory CD8 T cells and natural killer cells, both of which are critical for tumor immunosurveillance in vivo. In this study, female C57BL6 mice were injected, subcutaneously in the flank, with 1 million B16F10 melanoma cells. Approximately 7 days later, when tumor volumes were approximately 40 cu mm (day 0), mice (n=8/group) were randomized into different groups: untreated (V-P-E-) or treated with 50micrograms of pIL-15 (P) or backbone plasmid (V) with or without E (E+-/E-) (E conditions = 6 pulses 100microsecs 1500 V/cm). Subsequent treatments of the groups were made on days 4 and 7 and tumor volumes and survival among the groups were measured after the treatment regimen. Results at day 35-post treatment showed 0, 25 and 71% complete tumor regression in the V+-E+-, P+-E- and P+-E+- groups respectively. Likewise, at day 35 mice survival for the V+-E+-, P+-E-, P+-E+- and V-P-E- groups were 25, 50, 100 and 0% respectively. In addition, tumor growth, as measured by tumor volume, in mice without complete tumor regression was considerably less in the P+-E+- group (177 cu mm) when compared to the P+-E- (518 cu mm) and V+-E+-(1175 cu mm) groups. In addition, IL-15 expression in tumor lysates was considerably higher (7.5 fold) in the P+-E+- treated animals (21.6 pg/mg tumor) compared to the P+-E- group (2.9pg/mg tumor) demonstrating the efficacy of E+- to significantly enhance the expression of pIL-15 and potentially mediate an enhanced therapeutic effect. These results demonstrate the ability of an IL-15 expressing DNA plasmid to mediate an increase in complete regression of established subcutaneous B16 melanoma tumors when delivered intratumorally along with E. This is the first description of the ability of an IL-15 expressing DNA plasmid to alone mediate complete regression in this tumor type. These results are relevant since B16F10 tumors are characteristically difficult to treat and cure and underscores the potential clinical use of IL-15 for the treatment of malignant tumors when delivered with in vivo E. Mechanistic studies on the effects of the treatments reported here are being pursued and are centering on the previously established role of IL-15 in CD8+- T cell and natural killer cell activity.