Studies show that postmenopausal women with breast cancer treated with long-term use of tamoxifen are at an increased risk for uterine cancer, but why this occurs has been somewhat of a mystery.
In pre-clinical findings presented at the 2021 San Antonio Breast Cancer Symposium (SABCS), an international research team not only revealed a novel molecular mechanism responsible for this adverse side effect, known as tamoxifen-associated uterine cancer (TA-UC), they also identified drug inhibitors capable of neutralizing the growth and proliferation of this secondary cancer.
"Our results highlight the mechanistic differences in the early events that lead to the development of tamoxifen-associated uterine cancer," said Kirsten Kubler, MD, PhD, an associate scientist at the Broad Institute of MIT and Harvard, a member of the research staff at Massachusetts General Hospital, and an instructor at Harvard Medical School, who recently joined the faculty of the Berlin Institute of Health at Charite in Germany.
Kubler, who presented findings at the symposium, said the team's results suggest future therapeutic options to prevent TA-UC among those at an increased risk for this disease. "What's unique is that we found a new mechanistic explanation for the development of therapy-related tumors that, to our knowledge, has not been described previously," she noted.
Tamoxifen, discovered in the late 1960s, is the first hormonal drug approved to treat ER+ early-stage breast cancer. The drug-a nonsteroidal anti-estrogen agent-has been credited for prolonging and saving the lives of thousands of women with breast cancer, regardless of menopausal status.
Despite these benefits, research has revealed a potentially serious drawback for tamoxifen's long-term use-an increased 2-7-fold increased risk for uterine cancer following 2-5 years of tamoxifen treatment, with further increased risk after 10 years. The increased risk is particularly noteworthy among women ages 50 years and older.
That said, the researchers underscore the benefits of tamoxifen use far outweigh the risks.
"We want to make sure people understand that tamoxifen is safe to use, and tamoxifen-associated uterine cancer is infrequent," said Rinath Jeselsohn, MD, Assistant Professor of Medicine and Medical Oncology at Dana-Farber Cancer Institute and Harvard Medical School and co-author of the study.
As outlined in her talk, Kubler noted that the increased risk of therapy-driven or secondary cancers commonly arises from cells exposed to drugs, such as chemotherapy, that promote cancer development and progression. The drugs may directly activate a signaling pathway or create driver mutations that set this pathway in motion.
To determine which model made the most sense for the TA-UC patients, Kubler and colleagues needed to understand the molecular mechanisms behind the increased risk. Once an answer was revealed, the team could then examine potential therapeutic approaches to help prevent TA-UC in the future.
As a first step, the researchers examined what genetic differences might exist between TA-UCs versus de novo uterine cancers.
For this part of the study, Kubler and colleagues performed whole exome sequencing on 21 TA-UC samples from older women (median age 70 years) in the TAMARISK (Tamoxifen Associated Malignancies: Aspects of Risk) clinical trial, which was designed to evaluate the incidence of secondary cancers in tamoxifen-treated breast cancer patients. The median duration of tamoxifen use among this cohort was about 2 years, with the majority (14) diagnosed with uterine cancer within the first year.
This data was then compared to a control cohort of 540 de novo uterine cancer samples from postmenopausal women in The Cancer Genome Atlas.
Overall, Kubler and colleagues discovered that the TA-UC tumor samples were genetically similar to the de novo uterine cancer samples, with one profound exception: the TA-UC samples had a lower-than-expected frequency of mutations in two genes in the phosphoinositol-3-kinase (PI3K) pathway, a well-known driver of uterine cancer.
Specifically, the gene PIK3CA was mutated in 14 percent of the TA-UCs versus 48 percent of the de novo uterine cancers, while the gene PIK3R1 was mutated in none of the TA-UCs compared to 31 percent of the de novo uterine cancers.
To verify these findings, Kubler and colleagues interrogated PIK3CA hot spot mutations among 40 TA-UC independent tissue samples from the TAMARISK clinical trial using droplet digital PCR. Again, the incidence of PIK3CA mutations among the TA-UCs was significantly less when compared to de novo cancer samples from the AACR GENIE project-7.5 percent versus 21 percent.
Based on these results, the team hypothesized that tamoxifen might serve as a substitute for the mutant-driven activation of the PI3K pathway. Kubler said this hypothesis was supported by previous studies showing "crosstalk" between the estrogen-receptor (ER) pathway and the PI3K pathway.
"We wanted to know whether tamoxifen can activate this ER/PI3K crosstalk in normal uterine tissue and whether it can be specifically abrogated by PI3 kinase inhibitors," Kubler aid.
To answer these questions, the team examined uterine tissue from three cohorts of in vivo mice: control mice with normal uterine tissue, mice treated with tamoxifen, and another cohort that was treated with the combination of tamoxifen and a PI3 kinase inhibitor (alpelisib).
"We first looked at some of the key targets of the PI3 kinase pathways and found, indeed, that tamoxifen can increase the activation of PI3 kinase pathway that was further abrogated-and significantly abrogated-by the PI3 kinase inhibitor," Kubler said.
Further study using RNAscope-a technique that examines biomarkers in formalin-fixed paraffin-embedded tissue samples-revealed significant increases in insulin-like growth factor 1 (IGF1), a known promoter of cancer development that's been linked to various primary cancers such as breast, colorectal, and prostate cancer. This activity was reversed with the kinase inhibitor.
"Our findings offer a new mechanism of therapy-related cancerogenesis in which a drug activates a signaling pathway that substitutes for the selection of a driver mutation," Kubler said.
"Furthermore, the ability of PI3K inhibitor to reduce cell proliferation in our mouse model raises the possibility that downregulating the PI3K pathway may prevent or significantly reduce TA-UC development, offering a potential future therapeutic and prevention strategy for specific high-risk patients undergoing tamoxifen therapy," the research team wrote in an abstract of their study.
Limitations of the study include a relatively small number of available patient samples, since TA-UC is not a common disease, as well as the fact that samples were from formalin-fixed paraffin-embedded tissue, which introduces challenges related to genomic characterization.
Warren Froelich is a contributing writer.