Drug-diagnostic co-development: challenges and issues.

Diagnostic tests have become increasingly important for optimizing drug use. Ideally, a companion diagnostic test is developed concurrently with a corresponding therapeutic product (co-development). However, the diagnostic test may also be developed to optimize treatment with previously approved therapeutic agents (follow-up-development). In co-development, the effectiveness of an agent in marker-defined patients is confirmed by an enrichment trial design. In follow-up-development, a biomarker is validated by prospective and/or retrospective analyses of unselected design trials. A prospectively designed trial is the gold-standard approach to biomarker validation, but retrospective validation can be used to efficiently determine effective treatments for marker-defined patients. Accumulation and systematization of examples of drug-diagnostic development will aid in the effective development of companion diagnostics.