Breast Cancer Research Advances: New Drug Trials and Resistance Mechanisms Identified
Recent developments in breast cancer treatment include new clinical trials for advanced disease, findings on CDK4/6 inhibitor sequencing, and discovery of genetic markers explaining drug resistance in HER2-positive breast cancer.
Multiple pharmaceutical companies have launched new clinical trials for breast cancer treatment while researchers have uncovered mechanisms explaining why some aggressive breast cancers resist therapy. On March 16, 2026, Merck Sharp & Dohme LLC announced a phase 3 study to determine if patients receiving patritumab deruxtecan (also known as HER3-DXd and MK-1022) live longer overall or without cancer progression compared to those receiving chemotherapy or trastuzumab deruxtecan.
Bristol-Myers Squibb initiated a phase 2/3 study on March 11, 2026, to assess the efficacy and safety of iza-bren, a bi-specific antibody-drug conjugate against EGFR and HER3 with a topoisomerase inhibitor payload versus treatment of physician's choice (paclitaxel, nab-paclitaxel, carboplatin plus gemcitabine, and capecitabine) for first-line metastatic triple-negative breast cancer (TNBC) or estrogen receptor (ER)-low, human epidermal growth factor receptor 2 (HER2)-negative breast cancer patients who are not candidates for anti-PD(L)1 therapy and endocrine therapies.
On March 04, 2026, Pfizer conducted a clinical study to evaluate the safety and effects of disitamab vedotin for the treatment of people with advanced breast cancer that has spread in the body.
Frontline treatment with a CDK4/6 inhibitor did not improve overall survival in advanced hormone receptor (HR)-positive/HER2-negative breast cancer compared with second-line treatment, according to updated results from a randomized trial published in JAMA Oncology. After almost 5 years of follow-up, patients who received a CDK4/6 inhibitor in the first- or second-line setting had a median OS of 4 years.
The randomized phase III SONIA trial addressed the sequencing question by comparing first- and second-line CDK4/6 inhibitor strategies. Investigators at 74 Dutch centers randomized 1,050 patients with advanced HR-positive/HER2-negative breast cancer to ribociclib, palbociclib, or abemaciclib plus an aromatase inhibitor (AI), followed by fulvestrant at progression, or an AI in first line followed by a CDK4/6 inhibitor plus fulvestrant in second line.
The updated report provided a final OS analysis after a median follow-up of 58.9 months (3 years minimum). The results showed a nearly identical median OS of 47.9 months with first-line CDK4/6 inhibition and 48.1 months with second-line use. A prespecified subgroup analysis also showed no difference in OS between the two strategies.
A post hoc analysis showed a substantial OS benefit with upfront CDK4/6 inhibition in premenopausal patients but no evidence of benefit in the much larger postmenopausal subgroup. Premenopausal patients made up less than 15% of the study population. Grade ≥3 adverse events were higher among patients who received upfront CDK4/6 inhibition.
Previous studies have shown that tumors in premenopausal patients often exhibit more aggressive biology, including a higher prevalence of the luminal B subtype, elevated Ki-67 values, and lower ER expression, compared with postmenopausal patients. This reduced ER dependence, along with enrichment of oncogenic drivers, may explain a reduced response to endocrine monotherapy and greater relative benefits from early CDK4/6 inhibitor use in this population.
Researchers at City St George's, University of London, identified a new molecular signature that explains why some aggressive breast cancers become resistant to treatment. The study, published in the British Journal of Cancer, focused on HER2-positive breast cancer, a subtype that often develops resistance to targeted therapies such as lapatinib.
By layering multiple advanced genetic mapping techniques, scientists uncovered nine red flags or genetic markers that drive this resistance. Seven of these markers — including genes related to cell stress and metabolism — had never been linked to HER2-positive breast cancer before.
Integrated chromatin-accessibility and expression profiling isolated nine regulatory regions/markers associated with acquired lapatinib resistance, including seven previously unlinked genes in HER2-positive breast cancer. Resistant cells demonstrated global decreases in DNA accessibility with localized opening at resistance-driving loci, indicating non-mutational epigenetic reprogramming rather than canonical oncogenic chromatin activation.
The researchers found that these aggressive cells that were made resistant to lapatinib had DNA that was actually closing and becoming tighter. However, in approximately nine key regions, there were nine key regulators where the DNA was actually opening around those particular regions. Rather than classical opening up, those cells had reduced DNA accessibility with near-specific genes that drive resistance.
The study also found differential expression of epigenetic enzymes called H tags, which highlight that there are broad epigenetic regulators that are altered during resistance acquisition. The team discovered that even when treatment appears to be working, resistant cancer cells physically reshape themselves and activate specific hidden genetic pathways to survive and invade healthy tissue.
The breast cancer treatment landscape includes 250+ companies working to develop 300+ pipeline therapies. Leading companies include Tanvex Biopharma, Sichuan Kelun-Biotech Biopharmaceutical, Shanghai Henlius Biotech, Byondis, CSPC Ouyi Pharmaceutical Co., Ltd., Pfizer, Jazz Pharmaceuticals, Biostar Pharma, Inc., Chia Tai Tianqing Pharmaceutical Group, InventisBio, Coherent Biopharma, Shanghai Jiaolian Drug Research and Development Co., Ltd., Ambrx, MediLink Therapeutics (Suzhou) Co., Ltd., Tasly Pharmaceutical Group, Convalife (Shanghai) Co., Ltd., Merck & Co., AstraZeneca, Aclaris Therapeutics, Boehringer Ingelheim, NovaOnco Therapeutics Co., Ltd., Verastem Oncology, Ellipses Pharma, Shenzhen Yangli Pharmaceutical Technology Co., Ltd., TYK Medicine, Ascendis Pharma, ExpreS2ion Biotechnologies, Mersana Therapeutics, Exelixis, Shenzhen Celconta Life Science, Beijing Wehand-Bio Pharmaceutical, VM Oncology, Hinova Pharmaceuticals, OS Therapies, and Syntab Therapeutics.
Promising pipeline therapies include Oraxol, ARV-471, Ribociclib, E7389, Trastuzumab, GM-CSF, Paclitaxel, Gemcitabine/Carboplatin, and Iniparib. Vepdegestrant (ARV-471) from Pfizer/Arvinas is an investigational, orally bioavailable PROteolysis TArgeting Chimera (PROTAC) protein degrader designed to specifically target and degrade the estrogen receptor (ER) for the treatment of patients with ER-positive/HER2-negative breast cancer.