Metastatic triple-positive breast cancer frequently resists treatments. Scientists at the University of Colorado Cancer Center are testing a unique combination of medications to change that.
Growth of breast cancer cells is often propelled by one of three receptors – estrogen receptors (ER), progesterone receptors (PR) or the growth factor receptor called HER2. Treatments exist targeting each of these receptors individually. However, when all three receptors are present – this “triple-positive” breast cancer – blocking any single receptor is not enough. Treatments that block hormonal (estrogen and progesterone) receptors may be not very effective because tumor cells may use HER2 receptor to grow. The drugs that block HER2 receptors may not work as well because the cells will use hormonal receptors to survive. Chemotherapy works against triple-positive breast cancers, however, it has multiple side effects. Previous clinical trials have been largely unsuccessful in defining a well-tolerated targeted drug combination that blocks all avenues for growth of triple-positive breast tumors.
“Under the current guidelines, patients with triple-positive metastatic breast cancer have two options as a first line of treatment and neither is a great option,” says Elena Shagisultanova, MD, PhD, investigator at the CU Cancer Center and assistant professor in the University of Colorado School of Medicine’s Division of Medical Oncology. “One approach is to start an anti-hormonal pill, which is generally non-toxic. However, the response usually lasts only three to four months. The other choice is to start chemotherapy combined with HER-2 targeted agents. This option is effective, but it has multiple side effects.”
Shagisultanova believes she and CU Cancer Center colleagues may have another option: a regimen using three pills, each targeting a different pathway of the disease. The tucatinib, palbocilib and letrozole study combines tucatinib, which inhibits HER2, with letrozole targeting hormone receptors, and the CDK4/6 inhibitor palbociclib.
“We think hormone receptor and HER-2 signals are coming together to help cancer cells resist treatment,” says Shagisultanova. “The CDK4/6 inhibitor palbociclib can block these converging signals in the nucleus. We believe that if we can inhibit the signaling deeper in the tumor cell using this triple blockade, patients will have longer lives and better quality of life.”
Tucatinib, palbociclib and letrozole tend to have different side-effects, leading Shagisultanova to believe the triple combination of targeted agents will be well- tolerated.
Early clinical trials often exclude patients whose cancer has already metastasized to the brain, in large part due to the inability of anti-cancer drugs to penetrate the blood-brain barrier to reach the disease in the central nervous system. However, because tucatinib has proven effective in shrinking HER2-positive breast tumors that have spread to the brain, patients with brain metastases are, in fact, included in the current trial.
“Metastatic disease in the brain is one of the most dangerous complications of triple-positive breast cancer. If we can prevent development of brain metastases, or effectively treat metastatic disease in the brain, it will improve the lives of many patients,” Shagisultanova says.
Shagisultanova is the principal investigator on the multi-institutional tucatinib, palbocilib and letrozole trial. It also is an investigator-initiated trial which allows physician/scientists to test treatments that their hands-on experience in the lab and clinic indicate may offer meaningful results.
“There are many challenges in designing and delivering clinical trials,” says Christopher Lieu, MD, CU Cancer Center’s deputy associate director for clinical research. Lieu also leads CU Cancer Center’s efforts in further developing an Investigator-Initiated Trials Committee.
“We are fortunate at CU Cancer Center to have innovative clinicians who are analyzing data to find novel and innovative strategies to target malignancies that are in serious need of better therapies,” Lieu adds. “Trials like this are critical in moving cancer science forward and finding effective, non-toxic therapies.”
The trial is currently open for enrollment at the University of Colorado Cancer Center and will also be accruing patients at Northwestern University, Chicago, IL; University of Texas Health and Science Center in San Antonio, TX; Stony Brook University, NY; University of Arizona, Tucson, AZ, and University of New Mexico, Albuquerque, NM.
The trial is funded by the Pfizer ASPIRE Award in Breast Cancer Research. Cascadian Therapeutics and Pfizer are providing the study drugs tucatinib and palbociclib.