A Study of HER3-DXd in Subjects With Locally Advanced or Metastatic Solid Tumors

Inclusion Criteria

Participants must meet all of the following criteria to be eligible for enrollment into the study:

1. Sign and date the informed consent form prior to the start of any study-specific qualification procedures. A separate tissue screening consent will be obtained from all subjects to meet the baseline tumor tissue requirement.

2. Participants aged ≥18 years (follow local regulatory requirements if the legal age of consent for study participation is >18 years old).

3. Has locally advanced unresectable or metastatic disease (not curable by surgery or radiation) as follows:

Cutaneous (acral and non-acral) melanoma

1. Histologically or cytologically confirmed cutaneous (acral or non-acral) melanoma

2. Disease progression while on or after having received treatment with ≥1 prior line of anti-programmed cell death protein (PD-1) or anti-programmed death-ligand 1 (PD-L1) based therapy (previous use of other immune checkpoint inhibitors [ICIs] [ie, anti-CTLA4, anti- LAG-3] is acceptable). Prior anti-PD-(L)1 therapy in the adjuvant setting is allowed if there is recurrence within 12 weeks of the last dose. If the participant had BRAFm melanoma, they must have had disease progression on BRAF/MEK inhibitor therapy as well.

Squamous cell carcinomas of the head and neck

3. Squamous cell carcinoma of the head and neck (with a primary location of oral cavity,oropharynx, larynx, hypopharynx) that is human papillomavirus (HPV) positive or negative (as determined by local standard). Excludes tumor location in the nasopharynx, nasal cavity, paranasal sinuses, and unknown primary locations.

4. Disease progression after having received treatment with ≥1 and <3 prior lines of systemic therapy in the unresectable recurrent or metastatic setting.

Must have had disease progression on anti-PD-(L)1 (either as monotherapy or in combination with chemotherapy or other therapies). Must also have had disease progression on a platinum-based chemotherapy (PBC) regimen either in the recurrent or metastatic setting or in the locally advanced setting with curative intent.

Gastric or GEJ adenocarcinoma

5. Tumor tissue must be confirmed as negative for HER2 expression (immunohistochemistry [IHC] 0/1+ or IHC 2+/in situ hybridization negative) as classified by American Society of Clinical Oncology/College of American Pathologists (ASCO-CAP) guidelines and determined prior to enrollment by assessment in a local laboratory that is Clinical Laboratory Improvement Amendments certified (US sites) or accredited based on specific country regulations.

6. Disease progression after having received treatment with ≥2 prior lines of therapy that include PBC with or without anti-PD-1 therapy.

Ovarian Carcinoma

7. Pathologically documented high-grade serous epithelial ovarian cancer, primary peritoneal cancer, or fallopian tube cancer.

8. Documented disease progression ≥4 weeks after the last dose of PBC and <6 months of last dose of PBC in the advanced or metastatic setting. Prior use of folate reductase alpha targeting antibody-drug conjugate (ADC) (ie, mirvetuximab soravtansine) is allowed.

Cervical Cancer

9. Pathologically or cytologically documented recurrent or persistent squamous, adenosquamous, or adenocarcinoma of the uterine cervix.

10. Disease progression after having received ≥1 line of systemic therapy in the recurrent or metastatic setting. This may include prior anti-PD-(L)1 treatment and/or tissue factor directed ADC (tisotumab vedotin [TV]) per regional standard of care.

Endometrial Cancer

11. Pathologically or cytologically documented endometrial cancer (carcinoma of any histological sub-type or endometrial carcinosarcoma), irrespective of microsatellite instability (MSI) or mismatch repair (MMR) status.

12. Documented disease progression after having received ≥1 prior line of therapy (maximum of 3) PBC containing systemic treatment and an anti-PD(L)-1 therapy-containing regimen (combined or sequential) in the advanced/metastatic setting.

Bladder Cancer

13. Pathologically or cytologically documented locally advanced/unresectable or metastatic urothelial carcinoma of the bladder, renal pelvis, ureter, or urethra. Histological variants are allowed if urothelial histology is predominant. Small cell/neuroendocrine tumors are not allowed even if mixed histology.

14. Relapsed or progressed after treatment with ≥1 prior line of therapy (maximum of 3) that contains anti-PD-(L)1 therapy in the perioperative or metastatic setting. At least 1 line of therapy must also contain one of the following treatment modalities: chemotherapy or enfortumab vedotin. Prior fibroblast growth factor receptor (FGFR)-inhibitor treatment for those who are eligible are allowed.

• Required treatments can be given in combination or sequentially

• Prior cisplatin-based therapy or PD-(L)1 inhibitor therapy given for the treatment of muscle invasive urothelial carcinoma is counted as 1 line of therapy

• The same regimen administered twice in different disease settings will be counted as 1 line of prior therapy

• A minimum of 20 subjects in the second-line setting who have previously received enfortumab vedotin and pembrolizumab in combination will be enrolled.

Esophageal Carcinoma

15. Pathologically or cytologically documented esophageal squamous cell carcinoma.

16. Must have documented disease progression after having received 2 prior lines of therapy including previous PBC with or without an anti-PD-1 therapy-containing regimen (combined or sequential) in the advanced/metastatic setting.

Pancreatic Carcinoma

17. Pathologically or cytologically documented unresectable or metastatic pancreatic adenocarcinoma.

18. Relapsed or disease progression after having received 1 prior line of systemic therapy in the locally advanced/metastatic setting.

Prostate Cancer

19. Pathologically or cytologically documented unresectable locally advanced or metastatic castration-resistant prostate cancer (CRPC).

20. Adenocarcinoma of the prostate without neuroendocrine differentiation or small cell histology.

21. Surgically or medically castrated, with testosterone levels of <50 ng/dL.

22. Documented objective progression as determined by radiographic progression for subjects with measurable disease after androgen deprivation.

23. Relapsed or disease progression after having received treatment with ≥1 of the following novel hormonal agents: abiraterone, enzalutamide, apalutamide, or darolutamide.

24. Relapsed or disease progression after having received ≥1 cytotoxic chemotherapy regimen that included a taxane.

25. Has ≥1 measurable lesion on CT or MRI as per RECIST v1.1 by investigator assessment. Prostate cancer participants with bone only disease may be eligible.

26. Provides a pretreatment tumor tissue sample of sufficient quantity, as defined in the Study Laboratory Manual. The following tissue samples can be provided as the pretreatment tumor tissue sample:

27. Tissue collected from a biopsy (from ≥1 lesion not previously irradiated) performed since progression while on or after treatment with the most recent systemic cancer therapy regimen and prior to signing of the tissue ICF

OR

2. Pretreatment tumor biopsy from ≥1 lesion not previously irradiated and amenable to sampling after signing of the tissue ICF. The pretreatment tissue requirement may be waived after discussion and agreement with the Sponsor.

3. Has Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1 at screening.

Exclusion Criteria

Participants who meet any of the following criteria will be disqualified from entering the study:

1. Has HER2-positive gastric cancer as classified by ASCO-CAP guidelines and determined prior to enrollment by assessment in a local laboratory that is Clinical Laboratory Improvement Amendments certified (US sites) or accredited based on specific country regulations.

2. Has nasopharyngeal cancer.

3. Has mucosal or uveal melanoma.

4. Has a history of (non-infectious) interstitial lung disease (ILD), that required corticosteroids, has current ILD/pneumonitis, or suspected ILD/pneumonitis cannot be ruled out by imaging at screening.

5. Has clinically severe respiratory compromise (based on the investigator's assessment) resulting from intercurrent pulmonary illnesses

6. Is receiving chronic systemic corticosteroids dosed at >10 mg prednisone or equivalent anti-inflammatory activity or any form of immunosuppressive therapy prior to Cycle 1 Day 1.

Participants who require use of bronchodilators, inhaled or topical steroids, or local steroid injections may be included in the study.

7. Had prior treatment with an anti-HER3 antibody and/or antibody-drug conjugate (ADC) that consists of an exatecan derivative that is a topoisomerase I inhibitor (eg, trastuzumab deruxtecan).

8. Has history of other active malignancy within 3 years prior to Cycle 1 Day 1, except the following:

9. Adequately treated nonmelanoma skin cancer

10. Adequately treated intraepithelial carcinoma of the cervix

11. Any other curatively treated in situ disease

12. Has any evidence of severe or uncontrolled diseases (eg, active bleeding diatheses, active serious infection) psychiatric illness/social situations, geographical factors, substance abuse, or other factors that, in the investigator's opinion, make it high risk for the subject to participate in the study or that would jeopardize compliance with the protocol

13. Has previously received irinotecan treatment in the advanced or metastatic disease setting.