Upon resistance, plasma concentrations of Vemurafenib are lower than at the begin of the therapy

A finding that could suggest that low levels of Vemurafenib in the blood are a reason that the tumours no longer respond…. Drug concentrations can become lower when the body e.g. makes more of the enzymes that degrade those drugs.

http://abstracts.asco.org/144/AbstView_144_129906.html