The results of a Salk Institute-led study have found that tandem therapy using an FDA-approved anticancer drug trametinib, plus a drug candidate, entinostat, which is currently in clinical trials, results in fewer and smaller tumors in genetically engineered mouse models (GEMMs) carrying non-small cell lung cancers (NSCLCs) that harbor an LKB1 genetic mutation.

“For NSCLC cases with the LKB1 mutation, standard chemotherapy and immunotherapy treatments are not effective,” said research lead Reuben Shaw, PhD, who is a director of Salk’s Cancer Center. “Our findings demonstrate there is a way to target these cases using drugs that are FDA-approved or already in clinical trials, so this work could easily be used for a clinical trial in humans.”

Shaw, together with former postdoctoral fellow Lillian Eichner, PhD, who is now an assistant professor at Northwestern University, and colleagues, reported on their findings in Science Advances, in a paper titled, “HDAC3 is critical in tumor development and therapeutic resistance in Kras-mutant non–small cell lung cancer,” in which they concluded, “We found that the combination of entinostat plus trametinib treatment elicits therapeutic benefit in the Kras/LKB1 GEMM.”

Targeted, or personalized anticancer strategies have “begun to prove themselves” as successful treatments for cancer types that harbor specific genetic and molecular patterns. And while many of these targeted therapies are highly effective, they aren’t available for all cancers, including, NSCLCs that have an LKB1 genetic mutation. “… only a small subset of tumor types have targeted therapies currently available, as such agents only exist for a limited number of oncogenic drivers,” the authors wrote.

This means that there are no effective targeted treatments yet on the market for the roughly 20% of all NSCLCs that have a mutated liver kinase B1 (LKB1) tumor suppressor. To create a therapy that could target the LKB1 mutation, the researchers looked at histone deacetylases (HDACs). These proteins are associated with tumor growth and cancer metastasis, and demonstrate characteristic overexpression in solid tumors. But while several HDAC-inhibitor drugs are already approved for use against specific forms of lymphoma, as the researchers further noted, “… efficacy of HDAC inhibitors in solid tumors has been “disappointingly limited … Despite the fact that HDAC inhibitors are already in the clinic, little analysis of the disruption of the four class I HDACs has been performed in genetically engineered tumor models in mice that might help narrow down which are most important in different tumor contexts in vivo.

Based on previous findings connecting the LKB1 gene to three other HDACs that all rely on HDAC3, the team started their research by conducting a genetic analysis of HDAC3 in mouse models of NSCLC, and discovered an unexpectedly critical role for HDAC3. “Using GEMMs, we found that HDAC3 is required for lung tumor growth in vivo,” they noted. After establishing that HDAC3 was critical for the growth of the difficult-to-treat LKB1-mutant tumors, the researchers next examined whether pharmacologically blocking HDAC3 could give a similarly potent effect.

The team was curious about testing two drugs, entinostat (Ent), which is an HDAC inhibitor that is in clinical trials and is known to target HDAC1 and HDAC3, and the FDA-approved drug trametinib (Trant), a mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor. Tumors often become quickly resistant to trametinib, but cotreatment with a drug that inhibits a protein downstream of HDAC3 helps reduce this resistance. Because that protein relies on HDAC3, the researchers believed that a drug—such as entinostat—that targets HDAC3 would help manage trametinib resistance.  “… recent evidence suggests that cooperation between trametinib and HDAC inhibition may be emerging as a general principal across different tumor types,” they commented.

The team first tested their dual treatment approach in lung cancer cells, and then progressed to evaluate therapy in genetically engineered mice, including the KL model of NSCLC. They assessed variable treatment regimens in animals carrying LKB1-mutated lung cancer for 42 days, and found that mice given both entinostat and trametinib had 79% less tumor volume and 63% fewer tumors in their lungs than the untreated mice. In contrast, giving mice either drug alone had no effect on tumor burden. “ … neither entinostat nor trametinib alone affected tumor burden compared to vehicle control, but the Ent + Tram drug combination elicited significantly reduced tumor burden compared to all other treatment groups,” the team noted. “The Ent + Tram group contained smaller and fewer tumors than other treatment groups.” Additionally, the investigators confirmed that entinostat was a viable treatment option in cases where a tumor was resistant to trametinib.

“We thought the whole HDAC enzyme class was directly linked to the cause of LKB1 mutant lung cancer. But we didn’t know the specific role of HDAC3 in lung tumor growth,” said first and co-corresponding author Eichner. “We’ve now shown that HDAC3 is essential in lung cancer, and that it is a druggable vulnerability in therapeutic resistance.” Reporting on their results, the team stated, “These data identified that entinostat and trametinib, which are both clinically viable drugs that do not elicit efficacy as single agent treatments for lung cancer, impart therapeutic efficacy in the KL GEMM model when administered simultaneously.” The findings, they added, “… motivate further exploration of the role of HDAC3 in epithelial tumors and resistance to targeted therapies.

The results may lead to clinical trials to test the new regimen in humans, since entinostat is already in clinical trials and trametinib is approved by FDA. Importantly, Shaw, who is the holder of the William R. Brody Chair, sees this discovery as transformative for cancers beyond NSCLC, with potential applications in lymphoma, melanoma, and pancreatic cancer. “Our lab has committed years to this project, taking small and meaningful steps toward these findings. This is truly a success story for how basic discovery science can lead to therapeutic solutions in the not-so-distant future.”

Eichner added, “My independent laboratory is fortunate to be part of the Lurie Cancer Center at the Feinberg School of Medicine at Northwestern University, which is very supportive of translational research. We hope that this environment will facilitate the initiation of a clinical trial based on these findings.”