No abstract

Immunotherapy has improved survival for many cancer patients, especially for immunogenic malignancies such as lung cancer. However, even in lung cancer, the response rate for anti-PD1 therapy (for example) does not exceed 20%. Thus, it is critical to identify mechanisms that block the response to immune checkpoint therapies. STK11 (also known as LKB1) is encoded by the serine threonine kinase 11 gene (STK11). Patients harboring tumors with STK11 mutations have reduced infiltrates of cytotoxic T-cells and clinical studies have shown that they respond poorly to anti-PD1 or anti-PDL-1 therapies regardless of PDL-1 status, which otherwise predicts benefit. Herein, we have used gene expression data from a cohort of 442 lung adenocarcinoma patients to identify CX3CL1 (fractalkine) as a gene silenced in STK11 mutant tumors with potential to be a key direct modulator of the immune system relevant to STK11 loss. To further explore this hypothesis, we have edited the STK11 gene in A549 cells back to wild type (STK11 corrected). As predicted, restoration of STK11 function resulted in modest expression of CX3CL1 as measured by Western blotting. Unexpectedly, exposure of STK11 corrected cells to human immune cells isolated from the blood of healthy donors resulted in a 5 to 10-fold increase in CX3CL1 suggesting interactive signaling between tumor and immune cells. Using transwell assays and STK11 corrected A549 cells we find that restoration of functional STK11 increases immune cell migration, adhesion and invasion in vitro. Finally, cytotoxicity assays demonstrate that STK11 corrected A549 cells are 3 to 5-fold more sensitive to immune cell killing. Taken together these results suggest the hypothesis that CX3CL1 loss mediates immune evasion in STK11 mutant lung adenocarcinomas. Future work will further explore the mechanisms underlying this pathway in preclinical models. Citation Format: Eria Eksioglu, Gabriela M. Wright, Trent R. Percy, Kenneth L. Wright, W. Douglas Cress. Loss of CX3CL1 expression mediates immune evasion in STK11 mutated lung adenocarcinomas. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4454.

Theoretically, small molecule CDK4/6 inhibitors (CDK4/6is) represent a logical therapeutic option in non-small cell lung cancers since most of these malignancies have wildtype RB, the key target of CDKs and master regulator of the cell cycle. Unfortunately, CDK4/6is are found to have limited clinical activity as single agents in non-small cell lung cancer. To address this problem and to identify effective CDK4/6i combinations, we screened a library of targeted agents for efficacy in four non-small cell lung cancer lines treated with CDK4/6 inhibitors Palbociclib or Abemaciclib. The pan-PAK (p21-activated kinase) inhibitor PF03758309 emerged as a promising candidate with viability ratios indicating synergy in all 4 cell lines and for both CDK4/6is. It is noteworthy that the PAKs are downstream effectors of small GTPases Rac1 and Cdc42 and are overexpressed in a wide variety of cancers. Individually the compounds primarily induced cell cycle arrest; however, the synergistic combination induced apoptosis, accounting for the synergy. Surprisingly, while the pan-PAK inhibitor PF03758309 synergizes with CDK4/6is, no synergy occurs with group I PAK inhibitors FRAX486 or FRAX597. Cell lines treated only with Ribociclib, FRAX486 or FRAX597 underwent G1/G0 arrest, whereas combination treatment with these compounds predominantly resulted in autophagy. Combining high concentrations of FRAX486, which weakly inhibits PAK4, and Ribociclib, mimics the autophagy and apoptotic effect of PF03758309 combined with Ribociclib. FRAX597, a PAKi that does not inhibit PAK4 did not reduce autophagy in combination with Ribociclib. Our results suggest that a unique combination of PAKs plays a crucial role in the synergy of PAK inhibitors with CDK4/6i. Targeting this unique PAK combination, could greatly improve the efficacy of CDK4/6i and broaden the spectrum of cancer treatment.