BRAF inhibitor targeted therapies such as vemurafenib (PLX4032) or dabrafenib, block BRAFV600 driver oncogenic mutations and result in high initial response rates and improve survival in patients with melanoma. Although response rate is over 50%, resistance eventually develops. A possible explanation is established tumors escape supported by the immune suppressive tumor-infiltrating myeloid cells (TIMs) including M2 type macrophages and myeloid derived suppressor cells (MDSC).

Our murine melanoma SM1 model that harbors BRAFV600 mutation is an aggressive model, where mice with established tumors have to be sacrificed within two-three weeks. Treatment with PLX4032 has significant anti-tumor effect, but it cannot fully eradicate the tumor. In addition, our prior results show that SM1 is infiltrated by M2-polarized macrophages. By depleting TIMs with the CSF-1R inhibitor, PLX3397, the tumor infiltrating lymphocytes (TILs) is increased. To determine if combined therapy with PLX3397 and PLX4032 improved anti-tumor responses against SM1 tumors, C57BL/6 mice with established subcutaneous SM1 tumors received oral gavage daily with PLX3397 (50mg/kg) and i.p. daily with PLX4032 (100mg/kg). Combined treatment demonstrated superior effects compared to either of single treatment (tumor size on day 12_vehicle: 862.0±17.0 mm2, PLX3397: 439.3±6.9 mm2, PLX4032: 113.7±2.6 mm2, PLX3397+PLX4032: 12.9±4.6 mm2, p<0.05). When SM1 tumors were implanted in immunodeficient NSG mice treated with PLX3397 and PLX4032, the superior anti-tumor effect of the combined group disappeared and its tumor growth overlapped with PLX4032 treated group (tumor size on day 15_vehicle: 1432.9±46.2 mm2, PLX3397: 1374.6±41.4 mm2, PLX4032: 1080.0±30.0 mm2, PLX3397+PLX4032: 1028.3±49.0 mm2, p>0.05). To further confirm that combined treatment effect is mediated by CD8+ T-cells, SM1 tumors were implanted in immunocompetent C57BL/6 mice and treated with PLX3397, PLX4032, and anti-CD8 neutralizing antibody. The anti-tumor response in PLX3397, PLX4032, and anti-CD8 antibody group was greatly diminished (tumor size on day 16_vehicle: 751.6±12.2 mm2, PLX4032: 232.4±8.8 mm2, anti-CD8: 722.8±12.5 mm2, PLX3397+PLX4032: 101.0±7.6 mm2; PLX3397+PLX4032+anti-CD8: 442.0±5.7 mm2, p>0.05).

In conclusion, our data from SM1 in vivo model combining CSF-1R blockade and BRAF oncogenic blockade supports the rationale for testing this combination in patients with advanced melanoma, and suggest that the benefit is mediated by anti-tumor T cells. This combination is now in clinical testing (NCT01826448).

Citation Format: Stephen Mok, Richard Koya, Jennifer Tsoi, Lidia Robert, Jesse Zaretsky, Christopher Tsui, Thomas Graeber, Antoni Ribas. Improving antitumor effects of a BRAF inhibitor with a colony stimulating factor 1 receptor (CSF-1R) inhibitor, PLX3397. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 147. doi:10.1158/1538-7445.AM2014-147