53: T-VEC Treatment Reverses PD-1 Resistance by Altering the Microenvironment of Tumors

T-VEC is approved for patients with recurrent melanoma, however, a recent phase III clinical trial of anti-PD1 + T-VEC for anti-PD1 naive melanoma failed to show an improvement in survival. This preclinical study looked at the effect of T-VEC in a higher risk population of anti-PD1 resistant melanoma.



Methods:

Mice were injected with 3 x 10⁵ B16OVA melanoma tumors and simultaneously treated with anti-PD1 antibody (100ug IP, 3x/week). Implanted tumors that grew on anti-PD1 treatment to a size >25mm² after 14 days of treatment were used as a model of anti-PD1 resistant tumors. 

Mice with PD1 resistant tumors were then treated with continued anti-PD1 monotherapy, anti-PD1 + T-VEC (0.5 x 10⁵pfu x1 dose, then 0.5 x 10⁷pfu x 2 doses), or no further treatment. Tumors were measured 3 times weekly to obtain growth curve and survival data.  Exploration for changes in the tumor microenvironment was performed by tumor harvest after 7 days of T-VEC treatment.  Tumors and draining lymph nodes were processed for flow cytometry.



Results:

As shown in Figure 1, treatment of mouse PD1 resistant tumors with intralesional T-VEC combined with anti-PD1 resulted in improved survival over continued PD1 or no further treatment.

Tumor microenvironment changes were notable for increased CD8 cells within the tumor and dramatic loss of T regulatory cells (including FoxP3⁺CD25⁺ CD4 cells) along with significant increase in Ki67⁺CD8 and Ki67⁺CD4 cells. Draining lymph nodes were grossly enlarged and had significantly increased CD45⁺ cells after treatment with T-VEC. 

In vitro splenocyte stimulation showed increased IFNg signature when treated with anti-PD1 and T-VEC combination therapy when compared to anti-PD1 monotherapy or control. Reimplantation of B16ova cell line in mice with resolution of disease after combination therapy resulted in no tumor growth in 80% of mice, supporting long lasting tumor immunity. These results were confirmed in a separate cell line of oral squamous cell carcinoma, using similar methods.



Conclusions:

These data support the use of local viral therapy to reverse resistance in anti-PD1 refractory tumors and may confer long-term resistance. Further analysis is ongoing to clarify direct or indirect effect on Tregs.