Regarding the relative merits of Femara(=Letrozole) & Arimidex in terms of efficacy & side effects,studies done in the late 90's & early 2000's appeared to give a slight edge to Femara over Arimidex at least over the short to medium term.
http://clincancerres.aacrjournals.or...t/7/9/2620.pdf (2001)
Table 4 Tolerability data: indirect comparison of newer aromatase inhibitors versus megestrol acetate in second-line trials
Adverse event (%) Anastrozolea Letrozole Exemestane71 Megestrol
1 mg o.d.b 2.5 mg 25 mg 160 mg q.i.d
Headache 14 13 NA 9–13
Hot flushes 13 6 13 4–11
Nausea 18 11 9 5–23
Vomiting 8 3 NA 5–7
Diarrhea 9 6 NA 3–11
Weight gain 3 6 8e 9–17
Rash 6 6 2 3–12
Dyspnea 11 9 1 3–28
Asthenia 18 NA NA 7–20
Fatigue NA 11 8 10–22
However the long term thorough comparison of Femera & Arimidex is still awaiting since Femara among other things is affecting cortisol & as a very potent suppressor of estrogen is more likely to cause bone problems (osteoporosis, fractures...) & perhaps affect other organs such as eyes & brain more intensely than Arimidex. In addition it has been found that while Letrozole is a superior overall suppressor of estrogens compared to Arimidex (95 to 99% vs 75-85%) , it is less effective in suppressing one of the 2 main estrogens which happens to be the one which is the more cancer cell stimulating. So Arimidex may one day be found to be better than Femara for long term treatments.
These may be some of the reasons for a new large scale international clinical trial comparing the 2 drugs recently started:
http://meeting.jco.org/cgi/content/abstract/24/18_suppl/10672
Journal of Clinical Oncology, 2006 ASCO Annual Meeting Proceedings (Post-Meeting Edition).
Vol 24, No 18S (June 20 Supplement), 2006: 10672
© 2006 American Society of Clinical Oncology
The Head to Head trial: Letrozole vs anastrozole as adjuvant treatment of postmenopausal patients with node positive breast cancer
An other possible reason is competition between the manufacturers of the 2 drugs if it is funded by one of them. This already happened in one of the early head-to-head trials.