“…Data supporting the existence of an SLTER for At are less robust. Several studies were able to demonstrate either a linear [4,17,18,26,28,34,55,56] or curvilinear relationship [5,7,22,35,36,53], but many studies did not find any relationship between serum levels and therapeutic effect [2,10,11,12,24,25,27,33,39,46]. These results are based on the sum of the serum levels (C At+Nt ) of At (C At ) and its demethylated metabolite Nt (C Nt ).…”