DETERMINE Trial Treatment Arm 02: Atezolizumab in Adult, Paediatric and Teenage/Young Adult Patients With Cancers With High Tumour Mutational Burden (TMB) or Microsatellite Instability-high (MSI-high) or Proven Constitutional Mismatch Repair Deficiency (CMMRD) Disposition

This clinical trial is looking at a drug called atezolizumab. Atezolizumab is approved as standard of care treatment for adult patients with urothelial cancer, non-small cell lung cancer, extensive-stage small cell lung cancer, hepatocellular carcinoma and triple negative breast cancer. This means it has gone through clinical trials and been approved by the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK. Atezolizumab works in patients with these types of cancers which have certain changes in the cancer cells called high tumour mutational burden (TMB) or high microsatellite instability (MSI) or proven (previously diagnosed) constitutional mismatch repair deficiency (CMMRD). Investigators now wish to find out if it will be useful in treating patients with other cancer types which are also TMB/MSH-high or show CMMRD. If the results are positive, the study team will work with the NHS and the Cancer Drugs Fund to see if these drugs can be routinely accessed for patients in the future. This trial is part of a trial programme called DETERMINE. The programme will also look at other anti-cancer drugs in the same way, through matching the drug to rare cancer types or ones with specific mutations.

DETERMINE Treatment Arm 02 (atezolizumab) aims to evaluate the efficacy of atezolizumab in adult, paediatric and teenage/young adult (TYA) patients with rare\* cancers with high TMB or high MSI or proven CMMRD disposition and in common cancers where high TMB/MSI or proven CMMRD is considered to be infrequent. \*Rare is defined generally as incidence less than 6 cases in 100,000 patients (includes paediatric and teenagers/young adult cancers) or common cancers with rare alterations. This treatment arm has a target sample size of 30 evaluable patients. Sub-cohorts may be defined and further expanded to a target of 30 evaluable patients each. The ultimate aim is to translate positive clinical findings to the NHS (Cancer Drugs Fund) to provide new treatment options for rare adult, paediatric and TYA cancers. OUTLINE: Pre-screening: The Molecular Tumour Board (MTB) makes a treatment recommendation for the participant based on molecularly-defined cohorts. Screening: Consenting patients undergo biopsy and collection of blood samples for research purposes. Treatment: Patients will receive atezolizumab until disease progression without clinical benefit, unacceptable adverse events (AEs) or withdrawal of consent. Patients will also undergo collection of blood samples at various intervals while receiving treatment and at EoT. After completion of study treatment, patients are followed up every 3 months for 2 years. THE DETERMINE TRIAL MASTER (SCREENING) PROTOCOL: Please see DETERMINE Trial Master (Screening) Protocol record (NCT05722886) for information on the DETERMINE Trial Master Protocol and applicable documents.