To Evaluate the Safety, Tolerability, Pharmacokinetics and Antitumor Activity of DAY301 in Participants With Locally Advanced or Metastatic Solid Tumors

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 (ADC) 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 * Cervical squamous cell carcinoma * Endometrial cancers (Participants must have been previously treated with standard of care systemic therapy, have refused standard therapy, or have no standard therapy 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) inhibitors (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: Nasopharyngeal primary tumors. 3. Cohort 3: Ovarian cancer that progressed \>6 months after the last dose of platinum-based chemotherapy (platinum-sensitive disease), or disease that did not respond (PR or complete response \[CR\]) to or progressed ≤91 days after the last dose of first-line platinum-based chemotherapy (primary platinum-refractory disease) .- 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.