The authors genotyped 10 ABCB1 single-nucleotide polymorphisms ( SNPs ) in 683 patients with major depressive disorder treated for at least 2 weeks , of whom 576 completed 8 weeks of treatment with escitalopram , sertraline , or extended-release venlafaxine ( all substrates for P-glycoprotein ) in a large randomized , prospective , pragmatic trial .