Can Targeting Notch Help Tumor Vessels Deliver Immune Attack?

Targeting tumor blood vessels has long been a cornerstone of cancer therapy, but agents that inhibit vascular endothelial growth factor (VEGF) often produce only transient responses, and early inhibitors of the Notch signaling pathway were limited by toxicity. Researchers are now exploring whether reshaping the tumor vasculature could instead enhance immunotherapy.

At the Big Ten Cancer Research Consortium (Big Ten CRC) Summit, CancerNetwork® spoke with Jan Kitajewski, PhD, director of the University of Illinois Cancer Center and head of the Department of Physiology and Biophysics at the University of Illinois Chicago, whose laboratory developed Notch decoys and Notch peptibodies. Kitajewski discussed:

• how Notch-directed agents differ from VEGF inhibition;
• his laboratory's work on specialized blood vessels called high endothelial venules (HEVs), which help deliver immune cells into tumors;
• where vascular-directed strategies may have the greatest potential.

CancerNetwork: Your laboratory pioneered Notch decoys and therapeutic strategies targeting Notch signaling in tumor angiogenesis. How does targeting Notch differ from traditional VEGF pathway inhibition?

Kitajewski: Notch was initially identified as a key pathway that might drive the cancer cell itself. For example, T-cell acute lymphoblastic leukemia is essentially addicted to Notch activation. But our lab and a variety of other labs also recognized that there are critical steps in vascular formation and function, as well as in immune differentiation and function, that involve Notch.

One of the challenges of targeting Notch, and in fact of targeting VEGF, is that both are so globally important for healthy human physiology that there is considerable concern about how effectively and safely you can target them. Early Notch inhibitors, called gamma-secretase inhibitors, had very extensive toxicities. Notch is a cell surface protein that gets clipped by an enzyme called gamma secretase, which allows it to be activated. Chemicals were made to block that activation, but they were so difficult to tolerate, in terms of preserving gastrointestinal, immune, and normal skin function, that they never really made their way into clinical trials.

Subsequently, a variety of companies developed Notch inhibitors. Genentech developed an inhibitor against a Notch ligand called Delta-like 4, and Regeneron did the same. We used a receptor antagonist that we called the Notch decoy, and we now have a modified, patent-protected version that we refer to as Notch peptibodies. These differ from agents that target a specific ligand or receptor because they are broad receptor antagonists. We have shown that they are among the best-tolerated inhibitors while retaining some of the antiangiogenic activity we have seen in the past.

Notch inhibitors aimed directly at cancer cells or directly at angiogenesis were not optimal as single agents. So the field, including my own work, has transitioned to asking which agents we should combine with Notch inhibitors to get greater efficacy. VEGF inhibitors have faced challenges because many tumors become refractory; they figure out different ways to build blood vessels. Notch inhibits blood vessel function in a very different way than VEGF does, so there are still many studies, both in our lab and in the clinical setting, combining VEGF and Notch inhibitors. But we are now focusing quite extensively on whether Notch and VEGF inhibitors can be used to manipulate the vasculature in a way that enhances immuno-oncology.

Your research has also examined the tumor microenvironment and endothelial barriers. How are you working to harness the tumor vasculature to enhance immune attack?

All the agents that work in immuno-oncology depend on the delivery of cytotoxic T cells, natural killer cells, and other immune cell types into the vicinity of the tumor or directly into it. Different tumors have variable vasculature that may or may not allow that to happen, and our lab and others are trying to learn how to maximize blood vessels for delivering immune cells to attack the tumor.

When tumors are detected by the immune system, tumor antigens first travel to the lymph node, which you can think of as a command center that sends immune cells back to attack the tumor. In addition, little mini lymph nodes called tertiary lymphoid structures form around the tumor, and within them is a specialized blood vessel called the high endothelial venule, or HEV. These HEVs and tertiary lymphoid structures create mini command centers around the tumor that deliver immune-attacking cells. Our lab has now pivoted to understanding what it would take to stimulate the formation of those structures.

Studies of breast cancer samples have shown that the presence of tertiary lymphoid structures is predictive of better outcomes with immuno-oncology agents, and a more recent study showed that the presence of HEVs is also predictive of better outcomes. So we have moved our focus to how these blood vessels form.

If you activate Notch, these blood vessels regress. They are by definition venous blood vessels, and back around 1999, I discovered in a mouse model that Notch promotes the formation of arterial vasculature and must be completely shut down to make venous vasculature. When each of us was an embryo forming our first vessels, Notch had to be stimulated to make the arterial system and suppressed to make the venous system. What we discovered is that HEVs, which are veins, need to keep Notch turned off to stay formed and stable. So the Notch inhibitors I developed, the Notch decoys and more recently the Notch peptibodies, are now being tested as a way of maintaining these specialized blood vessels that deliver immune cells into the tumor.

Current antiangiogenic strategies often produce transient responses and face hypoxia-driven escape. Where do you see the greatest therapeutic potential for novel vascular-directed approaches in upcoming cooperative group trials?

As a vascular biologist and tumor biologist, I often think of VEGF as the agent of life. All mammals have a circulatory system, and VEGF starts it, maintains it, regulates it, and changes it. When VEGF-neutralizing agents were first made, there was a lot of fear that they would cause major malfunction of the blood vessels. They are most useful where blood vessels are far too active.

VEGF inhibition has been used in 2 clinical settings, and one has had remarkable success. Inhibiting VEGF is frontline treatment for blindness caused by overgrowth of leaky blood vessels, known as wet macular degeneration, and millions, if not tens of millions, of people have been treated with anti-VEGF agents for it. As you age, vessels in the eye become leakier, which can cause the retina to detach and focal vision to be lost. When you add an anti-VEGF agent to the eye, you can tighten up those vessels, the fluid clears, and focal acuity is restored. Individuals have been on these agents for decades, including my own father. Diabetic retinopathy is a similar case, in which altered glucose levels and other endocrine changes cause fragility and dysfunction of blood vessels, and anti-VEGF therapy has restored function there as well. In ophthalmology, anti-VEGF is the go-to agent, and it has restored vision for an amazing number of people.

That is local administration, however. In tumors, anti-VEGF therapy is given systemically, and it can control the formation and function of blood vessels to a large extent in many settings, but it never completely removes all the blood vessels from a tumor. The challenge is that once you remove the agent, the vessels mostly restore their activity. You never truly eliminate the tumor vessels to a state in which they cannot simply reform. These agents are also not designed to be used continually throughout your life, because they affect blood vessel function elsewhere, including in the kidney and intestine. When you stop them, many, if not most, tumors will re-engage that system and regrow.

What has changed in the field is the search for combinations… Those include immune checkpoint inhibitors or other immune-based therapies combined with VEGF inhibitors.