Exploring the lack of cross-resistance between aromatase inhibitors: evidence for a difference?

A lack of cross-resistance between the aromatase inhibitors (AIs) provides evidence to suggest that there are clinical differences between these agents. Available data from clinical trials indicate that patients exposed to nonsteroidal AIs may benefit from a steroidal compound of similar biochemical potency, and durable stable disease can be achieved in a significant proportion of patients. To date, there is little evidence suggesting specific pharmacokinetic/pharmacodynamic resistance for individual tumours to particular compounds. To clarify fully this issue, a head-to-head comparative trial in the adjuvant setting is needed and the results of the MA.27 trial randomizing patients to the steroidal AI exemestane vs. the nonsteroidal AI anastrozole will be invaluable in this regard.