Progress in treating medullary thyroid cancer

Medullary thyroid cancer is a rare cancer diagnosed in about 1,200 patients annually in the U.S. Because it’s often a silent disease, many patients are diagnosed at an advanced stage.

Recently, there has been tremendous progress made in treating this disease with targeted therapy. We recently spoke with Mimi Hu, M.D., to learn more. She shared some challenges in treating this disease and advances that have been made just this year, including findings from a clinical trial she presented at the European Society for Medical Oncology (ESMO) Virtual Congress 2020 (Abstract 1913O).

What causes medullary thyroid cancer?

Roughly 25% of patients have an inherited mutation in the RET gene, leading to a hereditary syndrome called Multiple Endocrine Neoplasia type 2A (MEN2A) or MEN2B. These are a constellation of other endocrine tumors, including medullary thyroid cancer and pheochromocytoma, an abnormal tumor of the adrenal glands.

With MEN2A, patients have parathyroid enlargement with unregulated parathyroid hormone secretion, causing high calcium and kidney stones and fractures. Patients with MEN2B don't have the parathyroid issue, but they can have these very striking mucosal neuromas or ganglioneuromas in their lips, tongue or other mucosal surfaces.

We don’t really know how the remaining 75% of patients develop it. There is no known risk factor for medullary thyroid cancer, but we do know certain mutations can lead to the cancer developing. In 40% to 50% of patients with non-hereditary medullary thyroid cancer, it will be caused by an acquired RET mutation in the cells that become cancerous. Another 13% to 15% of patients will have an acquired RAS gene mutation, and in the remainder of patients, we have not identified any mutations.

What are common medullary thyroid cancer symptoms?

In many patients, medullary thyroid cancer can be silent for years before it comes to anyone’s attention in a physical exam or with diagnostic imaging studies performed for other reasons.

If there are any symptoms, a patient might experience diarrhea and/or flushing caused by cancer cells producing proteins that stimulate intestinal activity or lead to dilation of the blood vessels. 

What is standard therapy for medullary thyroid cancer?

The standard of care is surgery – a total thyroidectomy with removal of the lymph nodes in the region. If the disease has already spread to the lymph nodes, it’s unlikely that a patient will be cured by surgery alone, but surgery is important because we don’t want to risk the disease spreading near the airway or esophagus.

For most patients with medullary thyroid cancer, we follow them over time after surgery by monitoring tumor markers, specifically calcitonin and carcinoembryonic antigen (CEA). If those markers rise, we may perform additional imaging to see if the disease is progressing. If the disease is slow-growing, we don’t offer additional therapy because we don’t have any treatments that can cure patients.

How have targeted therapies improved treatment options for medullary thyroid cancer?

In the early 2000s, we started participating in clinical trials with drugs called tyrosine kinase inhibitors, which block proteins important for the cancer cells to grow and survive. A lot of them were targeting a protein called VEGFR, which is important in angiogenesis, or the formation of new blood vessels. Medullary thyroid cancer relies on that process.

Two of these inhibitors gained approval for medullary thyroid cancer – vandetanib in 2011 and cabozantinib in 2012. This was the result of lots of hard work that we and other cancer centers participated in clinical studies. Although these drugs are not curative, they do result in responses in a significant portion of our patients. But there are a number of side effects associated with these medicines, including diarrhea, high blood pressure, skin peeling, mouth sores, weight loss and wound healing problems, amongst others. Experienced physicians can manage these, but most patients do experience some side effects.

How are new RET inhibitors changing the way we treat medullary thyroid cancer?

It has been exciting to be part of the development of a new generation of targeted therapies that specifically block mutant RET, which is implicated in many medullary thyroid cancers. In collaboration with Vivek Subbiah, M.D., and Maria Cabanillas, M.D.,  we have enrolled many patients on clinical trials for the RET inhibitors, pralsetinib and selpercatinib. MD Anderson does see a large number of patients with medullary thyroid cancers, and we were major contributors to these clinical studies.

The RET inhibitor selpercatinib was approved by the Food and Drug Administration in May 2020 for both lung cancers with RET alterations, as well as medullary thyroid cancer with activating RET mutations and other thyroid cancers with RET fusions. 

What is remarkable with these RET inhibitor drugs is that, in addition to the high percentage of responses we see with both drugs, they have a relatively lower side effect profile and are easier for patients to tolerate. I’ve had many patients see their disease symptoms improve in a matter of days, and patients who were previously on vandetanib or cabozantinib comment on how the side effects are much more bearable.

But RET inhibitors still have side effects. The most common ones are high blood pressure and elevated liver enzymes. We also see low white blood cell levels. It’s important to keep track of lab values for these patients and for them to track their symptoms and blood pressure, but they are overall better tolerated than the prior therapies.

Can you tell us about the latest data on the RET inhibitor pralsetinib?

At the 2020 ESMO Virtual Congress, I presented clinical trial data for the RET inhibitor pralsetinib specifically in patients with RET-mutant medullary thyroid cancer. This was recently approved for RET-altered lung cancer and is under new drug evaluation by the FDA for medullary thyroid cancer.

Overall, we saw very nice responses from patients with medullary thyroid cancer. There was a 60% response rate in patients who previously had been treated with vandetanib or cabozantinib. In patients without prior therapies, there was a 74% response rate. We are still evaluating many patients on this clinical trial, but the majority of patients remained on therapy at one year of treatment.

The side effects are also consistent with what I described earlier: it’s well-tolerated overall.

What’s next for medullary thyroid cancer research?

Although these RET inhibitors are promising, they’re not a magic bullet. Some cancers do progress after therapy. There are a new generation of RET inhibitors in early phase clinical development that may be more effective at treating RET-mutant cancers. We may also consider combining a RET inhibitor with other therapies to target the cancer on multiple fronts. Additionally, we are exploring the possibility of combining other therapies for medullary thyroid cancer.

Of course, not every patient has a RET mutation. We see RAS mutations as I mentioned, and we have a major need for therapies that can target mutant RAS.

Immunotherapy has not shown much promise in treating medullary thyroid cancer either, at least not as a single agent. Perhaps by combining immune checkpoint inhibitors with a targeted therapy we could see benefits, but that needs to be investigated. We also need to continue research to understand mechanisms of resistance to understand what causes patients to progress on therapy.

It’s an exciting time to be a physician in the field of medullary thyroid cancer. Over the last 15 years, we have had an explosion of clinical trials for this very rare cancer. I am very lucky to be a clinician in this environment working to improve care for our patients.

Request an appointment at MD Anderson online or by calling 1-877-632-6789.