Background Clinically, cryoablation is used to treat certain early stage prostate, liver, and kidney tumors in addition to bone and soft tissue sarcomas. However, for late-stage cancers, ablation is only an auxiliary step before complete resection. This leaves a gap of patients with advanced and inoperable tumors, where up to 90% of all pancreatic, and 80–85% of all prostate cancers are unresectable at diagnosis.1 2 Because cryoablation can release large amounts of antigen, it is uniquely capable of not only treating advanced, unresectable tumors, but also may induce an in situ vaccination response when combined with the appropriate immunotherapy. Previously, our results in single primary tumor models indicated that the addition of interleukin-12 (IL-12) to cryoablation (cryo) improved tumor burden and survival We hypothesized that intratumoral injection of IL-12 after cryo would activate a strong T cell response and induce systemic immunity in bilateral tumor models.
Methods Panc02 cells were purchased from ATCC; MC38 and MB49 cells were acquired from the NIH. Female C57BL/6 mice were purchased from Jackson Laboratory. For primary tumor implantation, 1 × 106 Panc02 cells and 3 × 105 MC38 cells were injected subcutaneously (s.c.) in the right flank. For rechallenge, the same dose of cells was implanted on the left flank of cured mice. For bilateral models, in both the MB49 and MC38 models, 3 × 105 and 1.5 × 105 cells were injected s.c. in the right and left flanks respectively on the same day. For the Panc02 model, 1 × 106 cells were implanted s.c. on both the right and left flanks on the same day. Tumor volume was calculated as 0.5*a*b2 given the perpendicular long (a) and short (b) dimensions. Tumors measuring between 150–300 mm3 were cryoablated with three cycles of freeze/thaw using the Visual-ICETM Cryoablation System (Boston Scientific). The dose of IL-12 was 1 µg/mouse in 1.5% (w/v) chitosan acetate (CS) dissolved in DPBS, and then injected intratumorally within an hour after cryoablation. For the anti-PD-1 and isotype antibodies (BioXCell, clone: RMP1.14). 300µg was injected intraperitoneally every 3 days starting on the day of cryoablation for a total of 4 doses.
Results In the bilateral MB49 mouse bladder cancer model, the median survival for the cryo alone group was 20 days post treatment (p.t.) compared to 23 days p.t. for cryo + CS/IL-12, which was not significant, and 12 days for the untreated control group. In the bilateral Panc02 model, the median survival for both the cryo alone and cryo + CS/IL-12 groups was the same at 20.5 days p.t., compared to 10 days p.t. for the untreated control. In the bilateral MC38 model, the addition of anti-PD-1 to cryo + CS/IL-12 did not significantly improve survival compared to isotype + cryo + CS/IL-12, with a median survival of 24 days p.t. and 16 days p.t. respectively (p=0.53, Log-rank test) (figure 1). However, addition of anti-PD-1 did significantly delay abscopal tumor growth up to 500 mm3 when compared to the isotype + cryo + CS/IL-12 (p=0.0398, Unpaired t test) (figure 2). Finally, the addition of IL-12 worsens memory in the MC38 model, where 100% of rechallenged cryo alone mice were protected (5/5) compared to only 43% protected of the cryo + CS/IL-12 group (3/7).
Conclusions Conclusions: While cryoablation in combination with immunotherapy has the potential to treat advanced, unresectable primary tumors and distant untreated tumors, the addition of a single injection of IL-12 is not enough to induce a strong abscopal effect. Furthermore, it may actually worsen the establishment of effector memory cells. The addition of anti-PD-1 only slows abscopal tumor growth. Future work is needed to understand the mechanism of T cell priming in the context of the post-ablative tumor.
Acknowledgements This work is supported by Boston Scientific, the NC State University Provost’s Fellowship, the NSF Graduate Research Fellowship and startup funds provided by the College of Engineering at NC State University.
van der Geest LGM. et al. Trends in treatment and survival of patients with nonresected, nonmetastatic pancreatic cancer: a population-based study. Cancer Med 2018; 7, 4943–4951.
Takaki H. et al. Thermal ablation and immunomodulation: from preclinical experiments to clinical trials. Diagn. Interv. Imaging 2017; 98:651–659.
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