This is a phase II, multicentre, open-label, randomised controlled trial (patients are randomly assigned to one treatment arm or the other) evaluating two treatment strategies (sacituzumab govitecan and trastuzumab deruxtecan in an alternative schema or sacituzumab govitecan alone) in patients with locally advanced or metastatic triple-negative breast cancer.

The goal is to answer the question: Does alternating sacituzumab goveitecan (SG) and trastuzumab deruxtecan (T-DXd) improve survival in patients with HER2-low metastatic triple-negative breast cancer compared to continuing treatment with SG alone?

Patients with locally advanced or metastatic triple-negative breast cancer (mTNBC) have poor survival outcomes

Among mTNBC, about 40% of tumors have a low expression of HER2 (HER2-low; defined as IHC 2+/ISH or IHC 1+). Two antibody-drug conjugates (ADCs) have been approved for HER2-low mTNBC: sacituzumab govitecan (SG) and trastuzumab deruxtecan (T-DXd) that target TROP2 and HER2 at the tumor cell surface, respectively. Each of these ADCs, used as monotherapy, outperformed conventional chemotherapy according to the ASCENT and DESTINY-Breast 04 trials and thereby have become the new second or third line standard of care for HER2-low mTNBC. However, as with other types of treatments, resistance is inescapable mostly due to intra-tumor heterogeneity. In the case of ADCs, resistance mechanisms involve changes in tumor antigen expression, ADC intracellular uptake and processing, and efflux of the ADC cytotoxic payload. Consequently, SG and T-DXd are used sequentially after progression even though the most effective sequence has so far been scarcely investigated and remains to be established

The ASCENT trial was designed to compare the efficacy of SG with chemotherapy in patients with mTNBC and reported significantly greater progression-free and overall survival compared with the physician's choice of chemotherapy. On the other hand, the DESTINY-Breast-04 compared the efficacy of T-DXd with chemotherapy but included just 63 patients with HER2-low mTNBC. Despite a significantly longer progression-free and overall survival were observed compared with the physician's choice of chemotherapy the efficacy analysis of T-DXd in this subpopulation was not a prespecified endpoint of the trial. Even though, collectively these results have contributed to a treatment paradigm shift for mTNBC, the superiority to chemotherapy can only be formally claimed for SG in mTNBC patients and for T-DXd in HER2-low metastatic breast cancer patients

We hypothesize that an upfront alternating SG and T-DXd regimen would have a greater antitumor effect compared with their prescription at progression while limiting the emergence of drug resistance. Hence, the ALTER trial aims to compare the efficacy and safety of an upfront alternating SG and T-DXd regimen with SG alone in HER2-low mTNBC patients

Inclusion Criteria:

• Patient must have signed a written informed consent prior to any trial specific procedures. (Note : When the patient is physically unable to give his/her written consent, a impartial witness of their choice, independent from the investigator or the sponsor, can confirm in signing the patient's consent)
• Men or women ≥ 18 years of age
• Histologically confirmed metastatic or locally advanced and unresectable triple-negative breast cancer, meeting both of the following criteria by local testing:

HER2-low breast cancer, defined as either immunohistochemistry (IHC) 2+ / in situ hybridization (ISH)-negative or IHC 1+ (ISH-negative or untested), on either the primary or any metastatic site

Estrogen receptor (ER) expression <10% and progesterone receptor (PR) expression <10% (Note: In case of bilateral breast cancer, participation in the study is permitted as long as both tumours correspond to a triple-negative breast cancer meeting the above criteria)

• Patient eligible to receive sacituzumab-govitecan and T-Dxd according to their indication
• Eastern Cooperative Oncology Group (ECOG) performance status ≤1
• Women of childbearing potential and male patients must agree to use adequate contraception for the duration of trial participation and up to 7 months after completing treatment for women and up to 4 months for men.

A woman is considered to be of childbearing potential if she is not postmenopausal or has not undergone hysterectomy. Postmenopausal is defined as any of the following:

Age ≥ 60 years Age < 60 years and ≥ 12 continuous months of amenorrhea with no identified cause other than menopause Surgical sterilization (bilateral oophorectomy)

- Adequate organ and bone marrow function within 28 days before enrollment. The most recent results available must be used for all parameters below:

Hemoglobin ≥ 9 g/dL. Red blood cell transfusion is not allowed within 1 week prior to screening assessment

Absolute neutrophil count (ANC) ≥ 1,500/mm³. Granulocyte-colony stimulating factor (G-CSF) administration is not allowed within 1 week prior to screening assessment

Platelet count ≥ 100,000/mm³. Platelet transfusion is not allowed within 1 week prior to registration

Total bilirubin ≤ 1.5 × upper limit of normal (ULN) if no liver metastases, or < 3 × ULN in the presence of documented Gilbert's Syndrome (unconjugated hyperbilirubinemia) or liver metastasis at baseline

Alanine transaminase (ALT) and aspartate aminotransferase (AST) ≤ 3 × ULN, or < 5 × ULN in patients with liver metastasis

Serum albumin ≥ 2.5 g/dL

Creatinine clearance (CrCl) ≥ 30 mL/min, calculated using the Cockcroft-Gault equation: CrCl (mL/min) = [(140 - age in years) × weight in kg] / [72 × serum creatinine in mg/dL] (× 0.85 for females)

• Adequate cardiac function, defined as a left ventricular ejection fraction ≥ 55% estimated by echocardiogram or multigated acquisition scintigraphy
• Women of childbearing potential must have a negative serum or urine pregnancy test done within 7 days before randomization
• Affiliated to the French Social Security System (or equivalent)
• Patient willing and able to comply with the protocol for the duration of the trial including undergoing treatment and scheduled visits, and examinations including follow-up.

Exclusion Criteria:

• Patient previously treated with any ADC targeting HER2 or TROP2
• Patient with uncontrolled or significant cardiovascular disease
• Patients with brain metastases (BM) except for asymptomatic treated BM not requiring ongoing corticosteroid treatment with stable lesions on baseline/screening brain MRI. Patients who require treatment of brain metastases are eligible after 14 days post surgery or radiation, if felt to be clinically stable and not requiring ongoing corticosteroid treatment
• History of (non-infectious) interstitial lung disease (ILD)/pneumonitis that required steroids, or current ILD/pneumonitis, or suspected ILD/pneumonitis that cannot be ruled out by imaging at screening
• Any medical history or condition that per protocol or in the opinion of the investigator is incompatible with the study
• Patients with known allergy or severe hypersensitivity to any of the trial drugs or their excipients
• Other concurrent medical or psychiatric conditions that, in the Investigator's opinion, may cause misleading study interpretation or prevent completion of study procedures and followup examinations
• Patients with any other disease or illness that requires hospitalisation or is incompatible with the trial treatment are not eligible
• Patients enrolled in another therapeutic trial within 30 days of inclusion
• Pregnant or breast-feeding women at the time of randomization or intention to become pregnant during the study and up to 7 months after treatment
• Person deprived of their liberty or under protective custody or guardianship
• Patients unwilling or unable to comply with the medical follow-up required by the trial because of geographic, familial, social, or psychological reasons

Individual Participant Data will not be shared at an individual level. Those data will be part of the study database including all enrolled patients.