[RESULTS] In cohort 1 ( n = 66 ) , median progression-free survival was 5.7 months ( 2.6-10 .5 ) for the letrozole group and 26.1 months ( 11.2-not estimable ) for the palbociclib plus letrozole group ( HR 0.299 , 0.156-0 .572 ; one-sided p < 0.0001 ) ; in cohort 2 ( n = 99 ) , median progression-free survival was 11.1 months ( 7.1-16 .4 ) for the letrozole group and 18.1 months ( 13.1-27 .5 ) for the palbociclib plus letrozole group ( HR 0.508 , 0.303-0 .853 ; one-sided p = 0.0046 ) .