CYP19A1 and breast carcinoma: In contrast, direct comparison of the third-generation AIs anastrozole and letrozole has shown that although letrozole suppresses aromatase activity, oestrone and oestrone sulphate levels to a greater degree than anastrozole, these differences in potency do not translate to clinically significant differences in the efficacy of these agents for the second-line treatment of hormone-sensitive advanced breast cancer (Rose et al, 2002).