This is a Phase 1a/1b, open-label, dose escalation and expansion study to evaluate the safety and anti-tumor activity of DAY301, a PTK7-directed antibody-drug conjugate in participants with advanced or metastatic solid tumors. The study comprises of 2 phases: Phase 1a dose escalation where participants will be administered DAY301 at escalating dose levels to assess safety and tolerability, and to determine the maximum tolerated dose (MTD) and/or the recommended dose (RD); In Phase 1b dose expansion, DAY301 will be evaluated in dose expansion cohorts.

Inclusion Criteria:

• Histologically or cytologically confirmed diagnosis of advanced or metastatic solid tumors of the following histologies:
• Ovarian cancer
• Esophageal squamous cell carcinoma
• Triple-negative breast cancer
• Non-small cell lung cancer
• Small cell lung cancer
• Head and neck squamous cell carcinoma
• Gastric/gastroesophageal junction adenocarcinoma
• Cervical squamous cell carcinoma
• Endometrial cancers

(Participants must have been previously treated with standard of care systemic therapy, or for whom no standard therapy is available).

• Availability of tumor tissue sample (either an archival specimen or a fresh biopsy) at screening
• Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1).
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• Adequate organ function.

Exclusion Criteria:

• Prior use of PTK7 targeting treatment (Phase 1a) or prior use of PTK7 targeting treatments and/or topoisomerase 1 (TOP1) inhibitor-based antibody-drug conjugate (ADC) (Phase 1b).

• Phase 1b disease-specific exclusion criteria:

  1. Cohort 1: Neuroendocrine tumors or endometrial sarcoma (eg, stromal sarcoma, leiomyosarcoma, or other types of pure sarcomas)
  2. Cohort 2: Ovarian cancer that progressed >6 months after the last dose of platinum-based chemotherapy (platinum-sensitive disease), or disease that did not respond (partial response [PR] or complete response [CR]) to or progressed ≤91 days after the last dose of first-line platinum-based chemotherapy (primary platinum-refractory disease)
  3. Cohort 3: nasopharyngeal primary tumors.

• History of small bowel obstruction requiring hospitalization within 3 months prior to the first dose of study treatment.

• Ascites requiring frequent paracentesis (more often than approximately every 4 weeks) for symptomatic management, or new onset within 4 weeks prior to the first dose of study treatment. Patients with an indwelling catheter may be considered eligible, after consultation with the medical monitor.

• Active or progressing brain metastases or evidence of leptomeningeal disease.

• Persistent toxicities from previous systemic antineoplastic treatments of Grade >1, excluding alopecia and vitiligo.

• Systemic antineoplastic therapy within five half-lives or 4 weeks, whichever is shorter, prior to first dose of study treatment, including investigational agents.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.