In cohort @ ( n = @ ) , median progression-free survival was @ months ( @-@ @ ) for the letrozole group and @ months ( @-not estimable ) for the palbociclib plus letrozole group ( HR @ , @-@ @ ; one-sided p < @ ) ; in cohort @ ( n = @ ) , median progression-free survival was @ months ( @-@ @ ) for the letrozole group and @ months ( @-@ @ ) for the palbociclib plus letrozole group ( HR @ , @-@ @ ; one-sided p = @ ) .