ABSTRACT
Despite breakthroughs achieved with cancer checkpoint blockade therapy (CBT), many patients do not respond to anti-programmed cell death-1 (PD-1) due to primary or acquired resistance. Human tumor profiling and preclinical studies in tumor models have recently uncovered transforming growth factor-ß (TGFß) signaling activity as a potential point of intervention to overcome primary resistance to CBT. However, the development of therapies targeting TGFß signaling has been hindered by dose-limiting cardiotoxicities, possibly due to nonselective inhibition of multiple TGFß isoforms. Analysis of mRNA expression data from The Cancer Genome Atlas revealed that TGFΒ1 is the most prevalent TGFß isoform expressed in many types of human tumors, suggesting that TGFß1 may be a key contributor to primary CBT resistance. To test whether selective TGFß1 inhibition is sufficient to overcome CBT resistance, we generated a high-affinity, fully human antibody, SRK-181, that selectively binds to latent TGFß1 and inhibits its activation. Coadministration of SRK-181-mIgG1 and an anti-PD-1 antibody in mice harboring syngeneic tumors refractory to anti-PD-1 treatment induced profound antitumor responses and survival benefit. Specific targeting of TGFß1 was also effective in tumors expressing more than one TGFß isoform. Combined SRK-181-mIgG1 and anti-PD-1 treatment resulted in increased intratumoral CD8+ T cells and decreased immunosuppressive myeloid cells. No cardiac valvulopathy was observed in a 4-week rat toxicology study with SRK-181, suggesting that selectively blocking TGFß1 activation may avoid dose-limiting toxicities previously observed with pan-TGFß inhibitors. These results establish a rationale for exploring selective TGFß1 inhibition to overcome primary resistance to CBT.
