The main goal of this study is to test a new medicine, Risvutatug Rezetecan also called Ris-Rez. We want to see if this medicine can help people with certain types of cancer, whether its safe to use, how well people tolerate it, and how their bodies handle the drug (how its absorbed and broken down). This research is for adolescents and adults who have either: Osteosarcoma, which is a type of bone cancer, or Soft Tissue Sarcoma, which is a type of cancer that starts in soft body tissues (like muscle, fat, or nerves). In both cancer types the cancer must have already been treated, but has come back or spread, and cant be removed by surgery

Inclusion Criteria:

- Participants are eligible to be included in the study only if all of the following criteria apply

• Participants must be ≥ 12 years of age.
• Has histologically confirmed unresectable advanced or metastatic R/R OSA (Cohort 1) or unresectable advanced or metastatic STS (Cohort 2) that has progressed to at least one prior line of systemic therapy.
• Has documented disease progression on the last line of systemic treatment as confirmed by radiological imaging
• Has an ECOG performance status of 0 or 1, or Lansky PS/Karnofsky PS ≥ 70% for adolescent participants, with no deterioration in the 2 weeks prior to first dose/randomization.
• Has adequate organ function.
• All participants, or their legal guardians, must provide signed informed consent and agree to follow the study protocol before starting any study activities

Exclusion Criteria:

- Participants are excluded from the study if any of the following key exclusion criteria apply:

• Has received any prior therapy with an Antibody-drug-conjugates (ADC) with a TOPO1-inhibitor payload.
• Has known sensitivity to study intervention components or excipients or other allergy that, in the opinion of the investigator or medical monitor, contraindicates participation in the study.
• Has severe, uncontrolled or active cardiovascular disorders.
• Known active infectious diseases requiring systemic treatment or known Human immunodeficiency virus (HIV).
• Has symptomatic brain metastases or untreated progression exclusively due to brain metastasis during or after the last treatment prior to screening, evidence of leptomeningeal/meningeal/brainstem metastasis or evidence of spinal cord metastases.
• Has received treatment with an investigational agent within 4 weeks of the first dose of study intervention.
• Is pregnant or breastfeeding.

Study Sponsor will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/gsk-patient-level-data-sharing-july2025.pdf

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or asset(s) with development terminated across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months, but an extension may be granted, when justified, for up to 6 months.