according to data presented from Abstract 1240 at the European Society for Medical Oncology (ESMO) Breast Cancer annual congress.
Patritumab deruxtecan (HER3-DXd) has previously demonstrated an acceptable safety profile and antitumor activity in phase I studies involving heavily pretreated patients with metastatic breast cancer and various levels of HER3 protein expression, said Mafalda Oliveira, MD, of the Vall d’Hebron University Hospital and Vall d’Hebron Institute of Oncology in Barcelona.
Antibody-drug conjugates (ADCs) are a combination of a monoclonal antibody chemically linked to a drug, as defined by the National Cancer Institute. ADCs work by binding to receptors or proteins and selectively delivering cytotoxic drugs to the site of a tumor.
Dr. Oliveira presented results from part B the of SOLTI TOT-HER3 trial, a window-of-opportunity trial that evaluated the effect of a single dose of HER3-Dxd in patients with treatment-naive HR+/HER2– early breast cancer.
In such trials, patients receive one or more new compounds between the time of cancer diagnosis and standard treatment. Biological and clinical activity from part A of the SOLTI TOT-HER3 trial were presented at last year’s ESMO Breast Cancer Congress, Dr. Oliveira said.
In the current study, Dr. Oliveira and colleagues recruited 37 women with HR+/HER2– early breast cancer, including 20 who were hormone receptor–positive and 17 who had triple negative breast cancer (TNBC). The age of the participants ranged from 30 to 81 years, with a median age of 51 years; 54% were premenopausal. The mean tumor size was 21 mm, with a range of 10-81 mm.
Distinct from part A of the SOLTI TOT-HER3 trial, part B included a subset of patients with TNBC to assess preliminary efficacy in this subtype, Dr. Oliveira noted.
All patients in part B received a single dose of 5.6 mg/kg of HER3-DXd. The primary outcome was the variation in the tumor cellularity and tumor-infiltrating lymphocyte (CelTIL) score at baseline and after 21 days via breast ultrasound.
At day 21, the total CelTIL score increased by a significant mean difference of 9.4 points after a single dose; the mean differences for TNBC and HR+/HER2– patients, were 17.9 points and 2.2 points, respectively, Dr. Oliveira said. The overall response rate was 32% (35% in TNBC patients and 30% in HR+/HER2– patients) and was significantly associated with the absolute change in CelTIL (area under the curve = 0.693; P = .049).
In a subtype analysis, a statistically significant change in CelTIL was observed between paired samples overall (P = .046) and in TNBC (P = .016), but not in HR+ (P = .793).
Baseline levels of ERBB3 (also known as human epidermal growth factor receptor type 3, or HER3) were not associated with changes in CelTIL or in overall response rate.
HER3-DXd induced high expression of immune-related genes (such as PD1, CD8, and CD19), and suppressed proliferation-related genes, she said.
A total of 31 patients (84%) reported any adverse events. Of these, the most common were nausea, fatigue, alopecia, diarrhea, constipation, and vomiting, and one patient experienced grade 3 treatment-related nausea. No interstitial lung disease events were reported during the study, and the incidence of hematological and hepatic toxicity was lower with the lower dose in part B, compared with the 6.5 mg/kg dose used in part A, Dr. Oliveira noted.
To further validate the findings of the current study and assess the activity of HER3-DXd in early breast cancer, Dr. Oliveira and colleagues are conducting a neoadjuvant phase II trial known as SOLTI-2103 VALENTINE. In this study, they are testing six cycles of HER3-DXd at a 5.6 mg/kg dose in HR+/HER2– breast cancer patients, she said.
During a question-and-answer session, Dr. Oliveira was asked whether CelTIL is the best endpoint for assessing HER3-DXd. Finding the best endpoint is always a challenge when conducting window-of-opportunity trials, she said. The CelTIL score has been correlated with pathologic complete response (pCR), as well as with disease-free survival and overall survival, she added.