Companion Diagnostics for Cancer Therapy

Companion diagnostics are tests – often assays – that are developed along with a medicine for use in deciding which patients to treat with the medicine.

This paradigm stands apart from most medical practice where drugs and diagnostics are separately ordered by doctors.  The companion diagnostic for a targeted therapy is part of the package with the actual medicine.  Tastuzumab and imatinib were early cancer drugs that required companion diagnostics with administration.   The FDA’s approval of alpelisib in May 2019 came with a requirement that the therascreen PIK3CA RGQ PCR Kit be used on patients to detect the PIK3CA mutation before treatment.

The nature of the companion diagnostic (CDx) can take many forms: nucleic acid–based testing (there are a bunch), as well as less esoteric measures of blood serum and urine (e.g. biomarker proteins and metabolites that show evidence of pharmaceutical action).  Use may help avoid adverse drug reactions.  More than 30 companion diagnostics have been authorized by the FDA.

Complementary Diagnostics

In 2016 FDA employees at a meeting of the American Association for Cancer Research meeting explained the agency’s thinking on companion and complementary diagnostics.  An account stated “ while a complementary oncology in the clinicdiagnostic is able to provide additional information as to how a drug might be used, it is not the same as a companion diagnostic, which is essential for the safe and effective use of a drug.”  Some observers  reserve the word complementary for things like PD-L1 immunohistochemistry (IHC) assays used in conjunction with PD-1/PD-L1 immune checkpoint inhibitors.  

See FDA website on targeted therapies and companion diagnostics.