CYP-2D6 Tamoxifen

Lisey

On the topic of laboratories... When I got tested in 2017, there were only 3 labs in the nation that were certified by the FDA and able to supply to Medicaid / Medicare patients. They have government oversite and require inspections. Kailos Genetics is one of the 3 labs. I know 23andme and other private labs like that are NOT certified because they work directly with the consumer and don't require a doctor to see the results.

The Government requires all certified labs to have a doctor see the results to help explain them if needed (thus why you submit your doctor). If the lab doesn't ask for it, they aren't certified.

sumomo

I've done some reading and would like to share my findings so far. The reason some MOs don't recommend CYP2D6 test is there haven't been enough studies to confirm the correlation between CYP2D6 genotype and Tamoxifen or even between Endoxifen plasma concentration and Tamoxifen efficacy. Most studies are "retrospective", therefore many other factors were involved so you cannot determine how much CYP2D6 genotype contributed to given results. Besides, there have been contradictory findings.

One recent study says it confirmed no efficacy difference between Tamoxifen 20mg vs. 40mg dosage to those who were categorized as poor metabolizer. It also says the Endoxifen concentration was significantly higher in those taking 40mg and it's even higher than the rapid metabolizers who were taking 20mg (https://ascopubs.org/doi/abs/10.1200/JCO.19.01412?journalCode=jco). It seems to me that it's not always good to have a high Edoxifen plasma concentration, which is very confusing to me but I have to interpret that there must be a proper level of Endoxifen concentration and that may be achieved by poor metabolizers taking Tamoxifen 20mg daily.

By the way, in this thread I see some say you pay for CYP2D6 test unless MO orders it. Which country/region is it about? Has anyone taken the test in Canada?