CHICAGO – Metastatic invasive lobular breast cancers (ILC) that are hormone receptor (HR)-positive and HER2-negative have therapeutic outcomes similar to those of invasive ductal cancer (IDC) following treatment with endocrine therapy combined with a CDK4/6 inhibitor, mTOR inhibitor, or PI3K inhibitor, according to a new retrospective analysis of patients treated at MD Anderson Cancer Center.
between the two subtypes, according to Jason A. Mouabbi, MD, who presented the results at the annual meeting of the American Society of Clinical Oncology.
“All the studies that were done were driven by ductal patients, so you can never take conclusions for the lobular patients. We have a big database at MD Anderson, so we can really study a large number of patients and get some signals whether or not patients would benefit from that therapy or not,” said Dr. Mouabbi, a lobular breast cancer specialist at MD Anderson Cancer Center.
The results of the study are important since patients often come to physicians with sophisticated understanding of their disease, he said. Patients with lobular cancer naturally wonder if a therapeutic regimen tested primarily in IDC will benefit them. “For the longest time, we said, ‘we have no data,’ ” said Dr. Mouabbi.
The new study should offer patients and physicians some reassurance. “We found that all of them benefit from it and most importantly, they all benefit from it (with) the same magnitude,” Dr. Mouabbi said.
The researchers analyzed data from 2,971 patients (82% IDC, 14% ILC, 4% mixed) treated between 2010 and 2021. The median age was 50 in all groups. Eighty percent were White, 10% were Hispanic, and 5% were Black. Ninety-nine percent had estrogen receptor (ER) + tumors, and 88% had progesterone positive (PR) + tumors.
A total of 1,895 patients received CDK4/6 inhibitors, 1,027 received everolimus, and 49 received alpelisib. There was no statistically significant difference in overall survival or progression-free survival between the two cancer types in any of the treatment groups.
Despite the similar outcomes, the two conditions remain unique. IDC is a disease of cells from the ducts that deliver milk, while ILC arises in cells that produce milk. Nearly 95% of ILC cases are hormone-positive, compared to 50%-55% of IDC.
So, while existing treatments seem to benefit both groups, there are nonetheless plans to develop therapeutic strategies tailored to lobular cancer.
Dr. Mouabbi’s group has compared molecular profiles of ILC and IDC tumors to better understand how to target them individually. Almost all ILC cancers have a mutation in a gene called CDH1, which leads to loss of an anchoring protein. They believe this causes a unique growth pattern of thin tendrils, rather than the onion-like growths of IDC. A therapy targeting this mutation could provide a specific benefit for lobular breast cancer.
There are other differences: PI3 kinases are mutated in about 60% of ILC tumors, versus about 30% of IDC tumors, and other genes mutated at lower frequencies are also different between the two subtypes. “So there are a lot of (approaches) we are trying to initiate in lobular cancer because we have awareness now that they are different,” Dr. Mouabbi said.
The study received no external funding.