Cancer vaccine trial evaluations: immunobridging and potential immunological endpoints.

Therapeutic cancer vaccines are an emerging class of immunotherapy, but challenges remain in effectively adapting approved vaccines to a growing number of adjuvants, combination therapies, and antigen-selection methods. Phase III clinical trials remain the gold standard in determining clinical benefit, but are slow and resource intensive, whilst radiological surrogates often fail to reliably predict clinical benefit. Using immunological surrogates of efficacy, deployed in 'immunobridging trials', could present a viable alternative, safely speeding up cancer vaccine development in a cost-effective manner. Whilst this approach has proven successful in infectious disease vaccines, identifying reliable immunological correlates of protection has proven difficult for cancer vaccines. Most clinical trials, which present the richest source of data to establish a correlate, rely on peripheral blood samples and standard immunoassays that are ill-equipped to capture the complexity of the vaccine-induced anti-tumour response. This review is the first to outline the importance and challenges of establishing immunological surrogates for cancer vaccines in the context of immunobridging trials, evaluating current immunoassay methods, and highlighting the need for techniques that can characterize tumour-infiltrating lymphocytes and the suppressive tumour microenvironment across a range of patients. The authors propose adapting trial designs for surrogate discovery, including combining phase I/II trials and the use of multi-omics approaches. Successful immunological surrogate development could enable future immunobridging trials to accelerate the optimization of approved cancer vaccines without requiring new phase III trials, promoting faster clinical implementation of scientific advances and patient benefits.