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 40 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.  


A partial list of approved diagnostics paired with medicines:

DrugTrade NameCompanion Diagnostic approved
dostarlimab-gxlyJemperliVENTANA MMR RxDx Panel
alpelisibPiqrayFoundationOne® Liquid CDx
rucaparibRubracaFoundationOne® Liquid CDx
olaparibLynparzaFoundationOne® Liquid CDx
alectinibAlecensaFoundationOne® Liquid CDx
erlotinibTarcevaFoundationOne® Liquid CDx
osimertinibTagrissoFoundationOne® Liquid CDx, Guardant360® CDx
gefitinibIressaFoundationOne® Liquid CDx
trastuzumabHerceptinVENTANA HER2 Dual ISH DNA Probe Cocktail
tazemetostatTazverikcobas EZH2 Mutation Test
nivolumab/ipilimumab PD-L1 IHC 28-8 pharmDx
encorafenib/cetuximab therascreen BRAF V600E RGQ PCR Kit
niraparibZejulaMyriad myChoice® CDx
alpelisibPIQRAYtherascreen PIK3CA RGQ PCR Kit
erdafitinibBALVERSAtherascreen FGFR RGQ RT-PCR Kit
atezolizumabTECENTRIQVENTANA PD-L1(SP142) Assay