ADAPT-HER2-IV will address question of optimal neoadjuvant therapy in patients with less advanced -HER2+ EBC.

ADAPT-HER2-IV is planned as a superiority trial to demonstrate higher pCR rates in both clinically relevant subgroups of low-intermediate risk HER2+ EBC. Moreover, it aims to demonstrate excellent survival in patients treated by T-DXd (with the use of standard chemotherapy at investigator´s decision restricted only to patients with substantial residual tumour burden after T-DXd-treatment).

As the ADAPT-trials have clearly shown, pCR after 12 weeks of therapy, independent of the specific de-escalated neoadjuvant regimen and independent of further use of systemic chemotherapy, is an independent predictor of excellent prognosis4,19, also in patients treated by an antibody-drug conjugate alone (T-DM1), or in those receiving pertuzumab+trastuzumab+/-weekly paclitaxel.

In contrast to the adjuvant setting, none of the neoadjuvant trials so far has focused on HER2+ patients with a low-intermediate risk profile (e.g., node-negative patients with cT1-2 tumours). The ADAPT-HER2-IV trial aims to close this evidence gap.

Since there is some uncertainty about the optimal treatment duration in intermediate- to high-risk HER2+ EBC (e.g., tumour size >3 cm), we recommend using a longer 18-week taxane-based treatment (+/- carboplatin, at investigator´s decision) due to a large body of evidence for taxane + carboplatin combinations in patients in locally advanced stages.

Antibody-drug conjugates appear to be ideal candidate drugs for a "de-escalated" treatment due to their favourable safety (reduced alopecia, polyneuropathy rates, etc.) and a high efficacy profile (e.g., comparable pCR rates after 18 weeks of T-DM1 and taxane+pertuzumab+trastuzumab in the PREDIX HER2 trial20). Similarly to the classical chemotherapy landscape, optimal duration of antibody-drug conjugate-based neoadjuvant therapy remains unclear. pCR rates of around 40% to 60% were observed after 12 and 18 weeks of T-DM1 treatment (+/-pertuzumab) in the ADAPT TP, KRISTINE and PREDIX HER2 trials in HR+/HER2+ disease21,22. Moreover, long-term survival seem to be comparable between T-DM1+pertuzumab and older chemotherapy-containing regimens (docetaxel+carboplatin+trastuzumab+pertuzumab) despite of higher local progression rates and lower pCR in one study22.

Trastuzumab-deruxtecan (T-DXd) has shown promising activity in a small cohort of metastatic patients, including both HER2+ and HER2-low BC, pre-treated with several lines of therapy. Doi et al. reported overall response rates (ORR) of 58% and a disease control rate of 100% with overall survival at 12 months at in HER2+ disease pre-treated by T-DM1+/-pertuzumab in a late line setting23. T-DXd-therapy was associated with a manageable safety profile. Recently, clearly higher efficacy of T-DXd vs. T-DM1 was shown in second line metastatic breast cancer (MBC) in the DESTINY-03 trial24. Median progression free survival was not reached in T-DM1-arm vs. 6.8 months in the T-DXd-arm. This effect was independent of hormone receptor status, prior pertuzumab treatment, visceral metastases, number of prior therapy lines and presence of brain metastases. ORR was doubled (34.2 vs. 79.7%), favouring the T-DXd arm.

Inclusion Criteria:

Patients eligible for inclusion in this study must meet all the following criteria:

1. Female patients with invasive, untreated HER2+ breast cancer (as assessed by local pathology) maximum 6 weeks before registration (standard-of-care diagnostic biopsy according to current AGO guidelines) 2. Age ≥18 years 3a. Cohort 1: low- to intermediate-risk for recurrence as per investigator´s decision (recommendation: cT1c - cT2 (1 - ≤3cm) AND cN0; cT1a/b, cN0 excluded), OR 3b. Cohort 2: intermediate- to high-risk for recurrence as per investigator´s decision (recommendation: cT2 (>3 - ≤5cm), cN0) 3c. Cohort 3: intermediate- to high-risk for recurrence as per investigator´s decision, (recommendation: clinical stage II (cT2, cN0); cT1c, cN0 only if neoadjuvant treatment intended) 4. Written informed consent 5. LVEF ≥ 50% within 28 days before randomisation 6. Eastern Cooperative Oncology Group performance status (ECOG PS) 0-1 7. Adequate bone marrow and organ function within 14 days before randomisation as defined by the following laboratory values:

• absolute neutrophil count ≥ 1.5 × 109/L,

• platelets ≥ 100 × 109/L,

• haemoglobin ≥ 9.0 g/dL:

• estimated glomerular filtration rate (eGFR) ≥ 30 mL/min by a Cockcroft-Gault formula,

• INR ≤ 1.5,

• serum creatinine < 1.5 mg/dL,

• total bilirubin < ULN, except for patients with Gilbert's Syndrome who may only be included if the total bilirubin is ≤ 3.0 × ULN or direct bilirubin ≤ 1.5 × ULN,

• aspartate transaminase (AST) < 2.5 × ULN,

• alanine transaminase (ALT) < 2.5 × ULN. 8. Adequate treatment washout period before randomisation (refer to protocol for detailed information) 9. Evidence of post-menopausal status or negative serum pregnancy test for females of childbearing potential (refer to protocol for detailed information) Post-menopausal status is accepted for women, who at the time of initiation of study medication, either

• had underwent bilateral oophorectomy, or

• are ≥ 60 years of age, or

• are < 60 years of age and amenorrhoeic for 12 or more months (in the absence of chemotherapy, tamoxifen, toremifen, or ovarian suppression)

• and/or whose FSH- and oestradiol-blood values are within the postmenopausal range per local laboratory normal range.

  10. Female subjects must not donate, or retrieve for their own use, ova from the time of randomisation and throughout the study treatment period, and for at least 7 months after the final study drug administration.

Exclusion Criteria:

Patients eligible for inclusion in this study must not meet any of the following criteria:

1. 1. Non-operable breast cancer including inflammatory breast cancer

2. cT1a/b, cN0 breast cancer

3. Any previous history of invasive breast cancer

4. Primary malignancies within 5 years, with the exception of

   • adequately resected non-melanoma skin cancer
   • curatively treated in-situ disease

5. Any evidence for existing metastatic disease (confirmed by CT Thorax/Abdomen, bone scan, or other methods according to clinical practice

6. Previous or concurrent treatment with cytotoxic agents for any reason (except non-oncological reasons)

7. Concurrent treatment with other experimental drugs and participation in another clinical trial with any investigational drug within 30 days prior to study entry

8. Severe and relevant co-morbidity that would interact with the application of cytotoxic agents or the participation in the study/inadequate organ function

9. Reasons indicating risk of poor compliance

10. Woman of child-bearing potential defined as a woman physiologically capable of becoming pregnant, and not using highly effective methods of contraception during the study treatment and for 7 months after stopping the treatment.

11. Use of oral (oestrogen and progesterone), transdermal, injected, or implanted hormonal methods of contraception as well as hormonal replacement therapy.

12. Has substance abuse or any other medical conditions such as clinically significant cardiac or psychological conditions, that may, in the opinion of the investigator, interfere with the subject's participation in the clinical study or evaluation of the clinical study results.

13. Patients with a medical history of myocardial infarction (MI) within 6 months before randomisation, symptomatic congestive heart failure (CHF) (New York Heart Association Class II to IV), Subjects with troponin levels above ULN at screening (as defined by the manufacturer), and without any myocardial related symptoms, should have a cardiologic consultation before enrolment to rule out MI.

14. Corrected QT interval (QTcF) prolongation to > 470 msec (females) based on average of the screening 12-lead ECG.

15. History of (non-infectious) ILD / pneumonitis that required steroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at screening.

16. Lung criteria:

    • Lung-specific intercurrent clinically significant illnesses including, but not limited to, any underlying pulmonary disorder
    • Any autoimmune, connective tissue or inflammatory disorders (e.g., Rheumatoid arthritis, Sjogren's, sarcoidosis etc.) where there is documented, or a suspicion of pulmonary involvement at the time of randomisation.
    • Prior pneumonectomy (complete)
    • Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals

17. Active primary immunodeficiency, known human immunodeficiency virus (HIV) infection, or active hepatitis B or C infection. Patients positive for hepatitis C (HCV) antibody are eligible only if polymerase chain reaction is negative for HCV RNA. Patients should be tested for HIV prior to randomisation if required by local regulations or ethics committee (EC).

18. Receipt of live, attenuated vaccine (mRNA and replication deficient adenoviral vaccines are not considered attenuated live vaccines) within 30 days prior to the first dose of trastuzumab deruxtecan or carboplatin.

    Note: Patients, if enrolled, should not receive live vaccine during the study and up to 30 days after the last dose of IMP.

19. Known allergy or hypersensitivity to study treatment (T-DXd), to comparator (SoC-) treatment, or any of the study drug / comparator (SoC-) excipients.

20. History of severe hypersensitivity reactions to other monoclonal antibodies.

21. Pregnant or breastfeeding female patients, or patients who are planning to become pregnant