The factors underlying the large interindividual variability in response to glucocorticoids in Giant Cell Arteritis are poorly understood. The investigators hypothesize that a part of this variability is related to pharmacokinetic factors determined by genetic polymorphism: hepatic clearance involving cytochromes P450 of the subfamily 3A (CYP3A) and drug efflux leukocyte conditioned by P-glycoprotein involved in multidrug resistance drugs (ABCB1). The investigators have designed a multicentric prospective pharmacokinetical and pharmacogenetic cohort study to assess the link between prednisolone clearance and the relapse risk in giant cell arteritis.