I have been blogging about companion diagnostics for about nine years. Here are links to some of my notes over the years on this topic (see: More Details About Roche's Companion Diagnostics Strategy; Consideration of a Broader Definition for "Companion Diagnostics"; Some Interesting Insights into Companion Diagnostics). In the earlier years, companion diagnostics was defined as the use of biomarkers to screen for the most suitable patients for a particular chemotherapy given that patient's neoplasm. Let's refer to this approach as searching for the most appropriate therapy. A classic example is the use of the HER-2/neu assay prior to the initiation of treatment with Herceptin (see: A Closer Look at Companion Diagnostics Strategies).
In a note posted in June 4, 2015, I suggested that the definition for companion diagnostics was broadening in the sense that biomarkers were being used in connection with cancer immunotherapy to monitor the patient's response to treatment (see: Cancer Immunotherapy Meets Companion Diagnostics). In a list of the various uses for companion diagnostics, I suggested the following: [to] monitor response to treatment with a particular therapeutic product for the purpose of adjusting treatment to achieve improved safety or effectiveness. We thus see the emergence of the new category of assessing the effectiveness of therapy with biomarkers.
There are now new ways to test for appropriateness and effectiveness of therapy for cancer patients. For example, we are testing patients' serum both before and after therapy for circulating tumor DNA (ctDNA) (see: AstraZeneca Partners with Roche on Development of a Companion Diagnostic; Illumina Launches Company to Develop a Pre-symptomatic Cancer Test) and circulating tumor cells (see: "Liquid Biopsy" Used to Refer to Detection of Any Serum Cancer DNA; Rapid Adoption of the Term "Liquid Biopsy" on the Web; "Liquid Biopsy" Used to Refer to Detection of Any Serum Cancer DNA).
There has been some resistance in the pharmaceutical industry to companion diagnostics. Companies understand that testing for the appropriateness of a particular drug has the potential to reduce sales of the product. The FDA understands this and has pushed aggressively for the use of companion diagnostics when new drugs are approved by the agency (see: Companion Diagnostics Gains Wider Acceptance in FDA Despite Challenges; Personalized Medicine and Companion Diagnostics Go Hand-in-Hand). I suspect that the opposition to lab tests for the effectiveness of treatment may continue although this type of testing may result in more drug being administered rather than less if the patient's current dose is deemed insufficient. In other words, companion diagnostics could serve to increase drug sales.