A companion diagnostic device can be an in vitro diagnostic device or an imaging tool that provides information that is essential for the safe and effective use of a corresponding therapeutic product. The use of an IVD companion diagnostic device with a particular therapeutic product is stipulated in the instructions for use in the labeling of both the diagnostic device and the corresponding therapeutic product, as well as in the labeling of any generic equivalents and biosimilar equivalents of the therapeutic product.

Because the companion diagnostic test is designed to be paired with a specific drug, the development of both products requires close collaboration between experts in both the Food and Drug Administration (FDA)’s device center, which evaluates the test to determine whether it may be cleared or approved, and FDA’s drug center, which evaluates the drug to determine whether it may be approved.

The process works best when development of the test begins before the drug enters clinical trials, increasing the likelihood that the participants in the trials are the patients most likely to benefit from the treatment.
Companion diagnostics can provide information such as the following:

• Test results that identify a population in which the therapeutic product will achieve greater (or little) effectiveness
• Test results that identify a patient population that should not receive a particular therapeutic product due to the possibility for therapy-related serious adverse events
• Test results that identify the characteristics of a disease, condition, or disorder to specifically determine what type of treatment is appropriate
• Test results that are the basis for selecting a safe and efficacious therapeutic dose.

Development of companion diagnostics has been supported by regulatory agencies, such as the FDA. The biopharmaceutical industry has also embraced the co-development of companion diagnostics, with companies either partnering on their development or pursuing in-house programs.
Genetic Engineering & Biotechnology News notes that a frequently cited example of a successful personalized therapeutic and companion diagnostic pair is Genentech’s antibody-based drug, Herceptin® (trastuzumab), which treats patients with cancers that overexpress the HER2 receptor (such as breast cancer), and the companion diagnostic test, HercepTest. The diagnostic test was developed to select patients that will benefit from treatment with Herceptin, a blockbuster drug.