Impact of combination therapy with anti-PD-1 blockade and a STAT3 inhibitor on the tumor-infiltrating lymphocyte status

Recently, clinical studies using anti-immune checkpoint molecule antibodies have been successful in solid tumors, such as melanoma and non-small cell lung cancers. However, pancreatic cancers are still intractable and difficult to treat once recurrence or metastasis occurs; thus, novel combined use of immune checkpoint blockade (ICB) with molecular targeted drugs is considered a therapeutic option. Previously, we developed a novel humanized MHC-double knockout (dKO) NOG mouse model and demonstrated that an anti-PD-1 antibody or a STAT3 inhibitor showed anti-tumor effects through an immunological mechanism. In the current study, using a humanized mouse model, we aimed to develop a combination therapy with an anti-PD-1 antibody and a STAT3 inhibitor (STX-0119) for use in vivo against pancreatic cancer. In an in vitro investigation, STX-0119 showed weak to moderate cytotoxic activity against several pancreatic cancer cell lines, which exhibited activated pSTAT3 and weak PD-L1 expression. However, unexpectedly, an in vivo study indicated that the combination of the anti-PD-1 antibody with STX-0119 remarkably reduced the anti-tumor effect and TIL numbers despite the effective anti-tumor activity against pancreatic cancer was produced individually by STX-0119 and the anti-PD-1 antibody. These results suggested that the combination of an anti-PD-1 antibody with specific signal inhibiting drugs should be carefully evaluated to avoid unexpected side effects, and such studies might contribute to the development of an effective combination regimen of ICB with cancer-targeting drugs such as tyrosine kinase inhibitors (TKIs).