Overcoming Resistance
Some patients experience amazing results from new immunotherapy treatments, including unprecedented long-term remission. However, most patients don’t. And that bothers Brent Hanks, MD, PhD.
“There is a lot of room for improvement," Hanks says. “We want to broaden the population that is capable of responding and benefiting from these treatments.”
Hanks is assistant professor of both medicine and pharmacology and cancer biology. In 2019, he received the ASCI Young Physician-Scientist Award from the American Society of Clinical Investigators, one of only 35 such accolades awarded nationwide.
Hanks focuses on understanding how some cancers resist immunotherapy treatments called checkpoint inhibitors. His goal is to design new pharmaceuticals or deploy existing ones to disrupt that resistance.
Immunotherapy works by unleashing the immune system’s cell-killing power on cancer. It’s not easy to do that, because cancer has mechanisms to outsmart the immune system, often by co-opting signals that healthy cells use to persuade the immune system not to attack.
“The tumors have essentially usurped these mechanisms and use them to their advantage,” Hanks says.
Our bodies have dozens of different ways to help the immune system tone down its killer instincts and leave healthy tissue alone. These mechanisms are called checkpoints because they keep the immune system in check. For example, a protein called PD-L1 on the surface of healthy cells interacts with a protein called PD-1 on the immune system’s T cells to send the message: “Hey, I’m friendly. Don’t kill me.
Cancer cells also have PD-L1 proteins, which fool the T cells into passing them by. Pharmaceuticals that prevent this from happening, called anti-PD-1 therapies, essentially remove cancer’s disguise.
In melanoma, anti-PD-1 therapies like pembrolizumab (Keytruda) and nivolumab (Opdivo) work for about 40 percent of patients. The other 60 percent are said to have primary resistance. And among those who initially responded well, some will later develop adaptive resistance and relapse.
Hanks is seeking to discover the exact biological mechanisms that cause the different responses in different patients.
“We feel like understanding the mechanism is actually very important,” Hanks says. “If you are able to understand these mechanisms of resistance then you can reverse those processes.”
Understanding the biological pathways of resistance could also make it possible to identify proteins or other molecules in the blood or tumor that could serve as biomarkers to guide decisions about treatment.
The approach Hanks takes in the lab begins with identifying a possible molecular mechanism that cancer might be using to outsmart the immune system. He’s not just looking for the presence or absence of a particular receptor or cell type, but a progression or cascade of biological signals that leads to resistance.
If the mechanism seems promising in theory, he then studies it in mice that are genetically engineered to develop melanoma. Is there evidence that the mechanism is active in mice that don’t respond well to anti-PD-1 therapy? He can also run experiments in which some mice receive both anti-PD-1 therapy plus another pharmaceutical that disrupts the mechanism of resistance.
If the mouse studies support the theory, the next step is to check for clinical relevance. Hanks turns to a bank of tissue samples from melanoma patients who were treated with anti-PD-1 therapies (see “Crucial Support"). The samples are anonymous, but each is linked to data about how that patient responded to the therapy.
For this part of his work, Hanks collaborates with Andrew Nixon, PhD, professor of medicine. Nixon has been the director of Duke’s Phase I Biomarker Laboratory since 2004. He and his team have the expertise and technology to detect the presence of a wide variety of proteins and other molecular markers in blood and other tissue samples.
“We are undertaking novel approaches to building better assays for molecules that are typically undetectable in blood,” Nixon says. “We work with investigators to try to bring new and better technology to biomarker questions that have not been able to be properly addressed yet.” Nixon and his team in the biomarker lab collaborate with partners nationwide.
In the research he’s doing with Hanks, Nixon analyzes patient blood samples and develops techniques to detect specific molecules associated with one or more pathways of resistance that Hanks is investigating. Nixon then works with the Center for Biostatistics and Bioinformatics to see if there is a statistically relevant correlation between the presence of those biomarker molecules and immunotherapy resistance in the patient.
This painstaking and multi-step process is paying off.
“We’ve identified two bona fide treatment strategies to overcome resistance to anti-PD-1 therapy,” Hanks says.
One of the strategies involves dendritic cells, which are like generals in charge of soldier T cells. Cancer uses a signaling pathway called WNT to convince dendritic cells not to activate cancer-killing T cells. Disrupting the WNT pathway could provide a way to improve the effectiveness of anti-PD-1 therapy.
The other strategy focuses on preventing a buildup of immune-suppressing cells called myeloid-derived suppressor cells (MDSC) in cancer tumors. Hanks and his team believe they have identified a molecular mechanism that leads to the MDSC buildup in response to anti-PD-1 treatment. They have found evidence of the mechanism in both mice and patients that developed adaptive resistance to anti-PD-1 therapy.
Although both of these strategies have been confirmed in mouse models and clinical specimens, they need to be validated in a larger group of patient specimens, and ultimately, clinical trials. “It’s an iterative process, a back and forth,” Hanks says. “We keep searching and keep digging and finding things.”
Hanks is motivated to keep digging by his patients. The science intrigues him, but the clinical relevance drives him. Although he focuses on melanoma in his clinic and in his research, Hanks believes that some of these mechanisms that he has identified can also be applicable to other tumor types.
Crucial Support
Duke alumnus Ross Bierkan decided to support the research of Brent Hanks, MD, PhD, and April Salama, MD, after he was diagnosed with melanoma. Read more.
Learn more about the new DCI Center for Cancer Immunotherapy.