The oral selective estrogen receptor degrader (SERD) LSZ102 plus either ribociclib or alpelisib shows manageable safety and encouraging clinical activity in heavily pretreated estrogen receptor (ER)–positive breast cancer patients who progressed after prior endocrine therapy, according to interim results of an open-label phase 1/1b study.
The effects seen in the study, which is the first to report on an oral SERD in combination with both CDK4/6 and PI3Ka inhibitors, occurred regardless of ESR1 and PIK3CA mutations, said Komal Jhaveri, MD, of Memorial Sloan Kettering Cancer Center in New York.
the results at the European Society of Medical Oncology: Breast Cancer virtual meeting.
The overall response rate (ORR) among 78 patients enrolled in an LSZ102 monotherapy arm (arm A) was 1.3%, and the progression-free survival (PFS) was 1.8 months. The clinical benefit rate (CBR) was 9.1%.
Among 76 patients enrolled in an LSZ102+ribociclib arm (arm B), the ORR was 15.8%, the PFS was 6.2 months, and the CBR was 35.5%.
Among the 39 patients enrolled in an LSZ102+alpelisib arm (arm C), the ORR was 5.4%, the PFS was 3.5 months, and the CBR was 18.9%.
After the data cutoff, one additional partial response (PR) was reported in arm C, Dr. Jhaveri said, noting that two of three confirmed responses were in known PIKC3A-mutant patients.
Study participants were aged 18 years and older with a confirmed diagnosis of ER-positive breast cancer and good performance status, as well as evidence of progression after endocrine therapy for metastatic disease or evidence of progression while on therapy or within 12 months from the end of adjuvant therapy.
“For all arms, prior fulvestrant, CDK46 inhibitor, or chemotherapy were allowed. For arm C, patients with or without PIK3C were eligible, and no prior treatment with PIK3, mTOR, or AKT inhibitors was allowed,” Dr. Jhaveri said.
Dosing in the LSZ102 monotherapy arm ranged from 200 to 900 mg. Arm B patients received LSZ102 at doses of 200-600 mg and ribociclib at doses of 200-600 mg. Both continuous ribociclib and 3 weeks on/1 week off dosing were evaluated. Arm C patients received LSZ102 at doses of 300-450 mg and alpelisib at 200-300 mg.
The recommended expansion doses were 450 mg daily of LSZ102 for arm A and 450 mg LSZ102 with 400 mg of daily ribociclib for arm B. For arm C, they were 300 mg LSZ102 with 250 mg of alpelisib daily.
Of note, two of three patients with a PR in arm C had received 300 mg LSZ102 and 300 mg alpelisib, Dr. Jhaveri said.
Arm A and arm B results were presented at the San Antonio Breast Cancer Symposium inand , respectively. The current report updates those findings and presents arm C data for the first time, Dr. Jhaveri said.
LSZ102 was relatively well-tolerated as a single agent and in combination with ribociclib and alpelisib, according to Dr. Jhaveri. The most frequent adverse events were gastrointestinal toxicities, including nausea, vomiting, diarrhea, and decreased appetite, which occurred across all arms.
Neutropenia and aspartate aminotransferase abnormalities, including grade 3 cases, were reported in arm B and were most likely driven by the ribociclib, Dr. Jhaveri said. Grade 3 hypoglycemia and skin rash commonly occurred in arm C, most likely driven by the alpelisib.
Five dose-limiting toxicities occurred in four patients in arm A, three occurred in two patients in arm B, and seven occurred in seven patients in arm C.
Paired biopsies collected at the time of screening and at day 15 of cycle 1 showed consistent down-regulation of ER protein levels across arms.
“No substantial dose-dependent down-regulation of the ER was observed with increasing doses of LSZ,” Dr. Jhaveri said.
Circulating tumor DNA (ctDNA) analysis showed that the dominant mutations across the arms were ESR1, PIK3CA, and TP53. These were not shown to correlate with response and were not enriched upon progression in patients with matched baseline and end-of-treatment samples, she noted.
An exploratory analysis, conducted in “a preliminary attempt to correlate clinical activity with specific mutations,” showed that, in arms B and C, respectively, ORR, CBR, and PFS weren’t correlated with the presence or absence of ESR1 and PIK3CA mutations, respectively, or the absence of detectable ctDNA from baseline samples, Dr. Jhaveri said.
“While numerically higher responses and better CBR were seen in patients with undetectable ctDNA at baseline, no statistically significant difference in any of these outcomes was observed in arms B and C,” she said.
In arm C, the numbers were small at the time of data cutoff, but incoming data suggest relatively enhanced activity of the LSZ102 plus alpelisib combination in PIKC3A-mutant patients, she noted.
“We know that inhibiting ER signaling is the mainstay of treatment for ER-positive breast cancer,” Dr. Jhaveri explained, adding that aromatase inhibitors, estrogen receptor modulators, and SERDs are important classes of antiestrogenic agents, but fulvestrant is the only approved SERD. These are effective, but many patients develop resistance, she said.
“Proposed mechanisms for endocrine resistance include activation of the cell-cycle and cell-survival signaling pathways, or of the PI3K-AKT-mTOR pathway,” Dr. Jhaveri said. “To that end, ribociclib, a CDK46 inhibitor plus fulvestrant improved survival compared to fulvestrant alone in patients with ER-positive metastatic breast cancer.”
More recently, the PI3K inhibitor alpelisib plus fulvestrant also nearly doubled PFS vs. fulvestrant alone in PIKC3A-mutant, ER-positive metastatic breast cancer, which led to the approval of the combination in the United States.
Another mechanism of endocrine resistance includes acquisition of activating mutations in the estrogen receptor gene itself that allow tumors to survive and proliferate without depending on estrogen.
EGFR mutations appear to predict resistance to aromatase inhibitor therapies, but not outcomes in patients treated with fulvestrant. However, fulvestrant, which is delivered by intramuscular injection, has its own limitations, Dr. Jhaveri said.
“LSZ102 is a novel SERD that could achieve higher exposure than fulvestrant, leading to enhanced efficacy,” she said, noting that it was shown in preclinical models to have activity and to be synergistic in combination with ribociclib and alpelisib, forming the basis for the current study.
Invited discussant, Saverio Cinieri, MD, of Ospedale Antonio Perrino, Brindisi, Italy, said the study “elegantly demonstrated that estrogen receptor protein is down-regulated by LSZ102; [that] the genomic landscape of heavily pretreated patients is dominated by mutations in ESR1, PIK3CA, and TP53; [that] common mutations do not correlate with response and are not enriched on progression; [and that] ctDNA analysis at baseline shows similar outcomes with LSZ plus ribociclib or alpelisib, regardless of mutational status.”
LSZ102 is one of four new-generation SERDs in early-phase studies, he said, concluding that “in the COVID-19 era, the use of oral therapies will be even more necessary to limit access to the hospital.”
Dr. Cinieri also said that overcoming the limitations “of a molecule like the intramuscularly administered fulvestrant goes in this direction,” and that “the clinical efficacy and the biomolecular profile of LSZ102 seems to be able to meet these real needs.”
This study was funded by Novartis. Dr. Jhaveri reported advisory and consultancy roles and/or research grants or other funding to her institution from Novartis, ADC Therapeutics, Pfizer, and numerous other pharmaceutical and biotechnology companies. Dr. Cinieri reported relationships with Lily Oncology, Pfizer, Roche, AstraZeneca, Amgen, Novartis, including honoraria, grant and research support to his institution, advisory board participation, and scientific meeting support.
SOURCE: Jhaveri K et al. ESMO Breast Cancer, .