The purpose of this study is to measure the safety and efficacy of LY4175408 in participants with selected advanced cancer. In addition, this study will evaluate how much LY4175408 gets into the bloodstream, how it is broken down, and how long it takes the body to get rid of it. Participation could last up to 4 years.

Inclusion Criteria:

• Have one of the following advanced or metastatic solid tumor cancers:

  • Non-small cell lung cancer (NSCLC)
  • Small cell lung cancer (SCLC)
  • Endometrial cancer
  • Triple negative breast cancer (TNBC) (characterized by HR-negative disease and HER2-negative expression according to American Society of Clinical Oncology (ASCO) - College of American Pathologists guidelines).

• Received all standard therapies for which the individual was deemed to be an appropriate candidate by the treating investigator (except in Cohort B1 and B2 expansion, which require participants who are treatment naive in the advanced metastatic setting); OR the individual is refusing the remaining most appropriate standard of care treatment; OR there is no standard therapy available for the disease.

• Eastern Cooperative Oncology Group (ECOG) performance status of greater than or equal to (≤) 1.

• For dose optimization/dose and expansion cohorts (Cohort A2, Cohort B/C): Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1.

• Cohorts B1/B2: NSCLC with no known actionable genomic alterations. ≤2 prior lines of systemic therapy for advanced or metastatic disease in safety lead-in; no prior systemic therapy in expansion

Exclusion Criteria:

• Prior treatment with a protein tyrosine kinase 7 (PTK7) antibody-drug conjugate (ADC) with a topoisomerase I inhibitor as payload (prior therapy with a PTK7 ADC with a non-topoisomerase I inhibitor payload OR non-PTK7 ADC with any payload is permitted). Prior topoisomerase I-based ADCs are not allowed in cohorts A2, B1 or B2.
• Any serious unresolved toxicities from prior therapy.
• Individual with known or suspected history of uncontrolled central nervous system (CNS) metastases.
• Individual with active uncontrolled systemic bacterial, viral, fungal, or parasitic infection.
• Significant cardiovascular disease.
• Prolongation of QT interval corrected for heart rate using Fridericia's formula (QTcF) > 470 milliseconds (ms)
• History of pneumonitis/interstitial lung disease.
• Individuals who are pregnant, breastfeeding, or plan to breastfeed during the trial or within 30 days of the last dose of trial intervention.