Breast Cancer Clinical Trials 2026: ADCs, Immunotherapy & Targeted Therapies

What DESTINY-Breast04 did in 2022 wasn't just approve a new drug — it created a new patient population. Before trastuzumab deruxtecan, "HER2-low" wasn't a treatment category; IHC 1+ and IHC 2+/ISH-negative tumors were classified as HER2-negative and treated accordingly. Suddenly, more than half of HR+/HER2-negative patients had a targetable designation. That's roughly 350,000 patients per year in the US alone who now have a treatment option they didn't have three years ago. That's the scale of change happening in breast cancer research, and it's happening across subtypes simultaneously — ADC sequencing in HER2+, de-escalation trials for early-stage disease, and mRNA vaccines moving into Phase 3 adjuvant TNBC. This is the most consequential period in breast cancer drug development since trastuzumab itself.

HR+/HER2− Breast Cancer: CDK4/6 Resistance and What Comes Next

Hormone receptor-positive, HER2-negative breast cancer accounts for roughly 70% of all breast cancer diagnoses. CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) combined with endocrine therapy are now standard of care in metastatic HR+ disease, having demonstrated overall survival benefit — most compellingly ribociclib plus letrozole in MONALEESA-2 (OS 63.9 vs. 51.4 months) and abemaciclib in MONARCH-2 and MONARCH-3. Most patients eventually develop resistance, and what happens next is the central question driving 2026 HR+ trials.

The PI3K/AKT pathway is the most validated resistance mechanism. Capivasertib (AZD5363, AstraZeneca), an AKT inhibitor, received FDA approval in 2023 in combination with fulvestrant for PIK3CA/AKT1/PTEN-altered HR+/HER2− metastatic breast cancer after CDK4/6 inhibitor progression — showing PFS improvement of 7.3 vs. 3.1 months versus fulvestrant alone in the CAPItello-291 trial. Trials are now testing capivasertib in earlier treatment lines. Elacestrant (oral SERD, Orion/Menarini), the first approved oral selective estrogen receptor degrader, showed PFS benefit over standard endocrine therapy in ESR1-mutated disease (EMERALD trial: median PFS 3.8 vs. 1.9 months) and is being studied in combination with CDK4/6 inhibitors in Phase 3 trials.

• Next-generation CDK2/4/6 inhibitors addressing palbociclib-resistant disease (CDK4/6 amplification as resistance mechanism)
• Capivasertib in first-line PIK3CA-mutated disease — CAPItello-291 data now being extended to earlier settings
• Elacestrant combinations with CDK4/6 inhibitors — Phase 3 ELEVATE trial ongoing
• T-DXd (trastuzumab deruxtecan) in HER2-low HR+ disease — DESTINY-Breast06 reading out in 2026

HER2+ Breast Cancer: De-escalation and the ADC Era

HER2-positive breast cancer has undergone the most dramatic outcome improvement of any breast cancer subtype since trastuzumab approval in 1998. Early-stage HER2+ disease is now highly curable: the KATHERINE trial showed that T-DM1 (trastuzumab emtansine) as adjuvant therapy after residual disease post-neoadjuvant treatment reduced invasive disease-free survival events by 50% versus trastuzumab alone. The current trial focus has shifted toward two questions: who can receive less treatment without compromising cure rates, and how to sequence ADCs optimally in metastatic disease.

De-escalation trials (ADAPT, PHERGain, COMPASS-HER2) are identifying genomic and pathological markers that predict patients who can be cured with HER2-directed therapy alone, sparing them from chemotherapy toxicity. Trastuzumab deruxtecan (Enhertu) transformed the metastatic HER2+ landscape in DESTINY-Breast03 — PFS 28.8 months versus T-DM1's 6.8 months, an unprecedented separation — and is now being studied as neoadjuvant therapy in early-stage HER2+ disease. HER2-directed bispecific antibodies (zanidatamab, targeting HER2 and HER3 simultaneously) and CAR-T cell approaches are in Phase 1/2.

Triple-Negative Breast Cancer: From Worst Prognosis to Active Pipeline

TNBC — ER-negative, PR-negative, HER2-negative — was historically the subtype with the worst prognosis and fewest treatment options. That has changed significantly since 2020. Three major breakthroughs have restructured TNBC treatment:

• Immunotherapy: Pembrolizumab plus chemotherapy (KEYNOTE-522) is now standard neoadjuvant therapy for Stage II/III TNBC regardless of PD-L1 status, showing pathological complete response rate of 64.8% vs. 51.2% with chemotherapy alone and improved event-free survival. For metastatic disease, pembrolizumab plus chemotherapy is standard for PD-L1 CPS ≥10 (atezolizumab's approval was withdrawn after IMpassion130 successor trial failure).
• Sacituzumab govitecan (Trodelvy, Gilead): TROP2-directed ADC delivering SN-38. ASCENT Phase 3 showed PFS 5.6 vs. 1.7 months and OS 12.1 vs. 6.7 months versus single-agent chemotherapy in metastatic TNBC. Now being tested in the neoadjuvant and adjuvant settings in patients with residual disease after standard therapy.
• PARP inhibitors in BRCA-mutated TNBC: Olaparib and talazoparib for germline BRCA1/2-mutated disease. OlympiAD (olaparib) and EMBRACA (talazoparib) both showed PFS benefit of approximately 3 months versus chemotherapy. Combination PARP inhibitor plus immunotherapy trials are actively recruiting — the hypothesis being that PARP inhibition increases tumor mutational burden and PD-L1 expression, potentially sensitizing BRCA-mutated TNBC to checkpoint inhibitors.
• mRNA-4157/V940 adjuvant vaccine (Moderna/Merck): Personalized neoantigen vaccine plus pembrolizumab in Phase 3 for high-risk TNBC and HR+ breast cancer with residual disease after neoadjuvant therapy. After the 44% reduction in melanoma recurrence data, the breast cancer Phase 3 (mRNA-4157-P201) is among the most watched trials of 2026.

Biomarkers You Need Before Searching for Trials

The molecular complexity of breast cancer means that "breast cancer trial" is an almost meaningless search category without your specific biomarker profile. Before investing time in trial identification, you need:

• ER/PR/HER2 status: The foundational classification. HER2 IHC and ISH results — including whether you're HER2-low (IHC 1+ or 2+/ISH-) — are required for ADC trial eligibility.
• BRCA1/2 germline testing: Required for PARP inhibitor trial eligibility. Germline blood test, not just somatic tumor testing — both are needed if you're considering combination trials.
• PIK3CA mutation status: Activating mutations in PIK3CA (found in ~40% of HR+ breast cancers) predict response to capivasertib and alpelisib. Tumor tissue or liquid biopsy.
• PD-L1 CPS: For metastatic TNBC and some HR+ trials. CPS ≥10 is required for some pembrolizumab indications; CPS ≥1 for others.
• ESR1 mutation status: For elacestrant and next-generation SERD trials in HR+ disease that progressed on aromatase inhibitors. Liquid biopsy is preferred (ESR1 mutations emerge under treatment pressure and may not be in archival tissue).

Comprehensive genomic profiling (Foundation One CDx, Tempus xT) covers most of this in a single test. For liquid biopsy specifically, Guardant360 captures ESR1 mutations and PIK3CA mutations from circulating tumor DNA. Many trials will provide or reimburse required testing at screening — ask the trial coordinator before paying out of pocket.