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

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.