In presence of ER hypersensitivity even a small amount of ER may be sufficient for sustained growth signalling. On the other hand, ER disruption operated by fulvestrant is not complete, particularly in the initial phase of treatment. From phase III trials, indeed, The invertigators know that with the standard 250mg monthly dose the steady state of circulating drug is reached only after 5-6 injections. This may play a role since, as long as ER downregulation is concerned, a clear dose-response relationship has been reported. In such a situation, fulvestrant efficacy may be partial, particularly because the concomitant AI discharge yields a restoration of physiologic postmenopausal levels of circulating oestrogens. New dosing schedule are currently under investigation both to accelerate the achievement of the steady state (loading dose) and to achieve higher circulating drug levels (high dose) (86).
In this trial the investigators will be using the so-called 'loading dose'.
Further potential strategies to improve fulvestrant efficacy in this setting are:
A) avoid the restoration of circulating oestrogens; B) interfere with molecular mechanisms that produce ER hypersensitivity by targeting the EGFR/ERBB2/ERB3 system.
A) avoid the restoration of circulating oestrogens: this should be achieved by holding the AI treatment. Because some cases of progression upon AIs may be related to an inefficient inhibition of the aromatase it is a logical step to test whether changing AI class (from type I, steroidal, to type II, non steroidal, and vice-versa) (87), may improve fulvestrant efficay. In this view, pts in this trial will be randomized to receive fulvestrant (loading dose) with or without the alternate class AI treatment. Circulating oestrogens levels will be tracked to verify inhibition of aromatase for pts assigned to concurrent AI treatment.
B) Interfere with growth factors-mediated ER hypersensitivity: although fulvestrant is able to overcome the ER hypersensitivity of LTED (88) and produce a growth arrest, this activity may not be complete because of incomplete ER disruption, but also because of a direct stimulation of growth by the hyperactivated EGFR/ERBB2/ERB3 system. Laboratory evidence support this hypothesis. Indeed, breast cancer cell lines exposed to long-term treatment with fulvestrant became insensitive to the drug and restore growth (89). This growth does not appear, however, related to the development of direct resistance to the drug, since ER mediated signalling continue to be efficiently suppressed in these cells; rather it may be driven by the use of alternative growth-stimulating pathway, including the EGFR system. Indeed, it can be abrogated by the EGFR-tyrosine Kinase inhibitor Gefitinib (IRESSA™) and by an MAPK-inhibitor (90). Lapatinib (GW572016) is an orally active small molecule that reversibly inhibits ErbB1 and ErbB2 tyrosine kinases, which in turn blocks phosphorylation and activation of Erk1/2 (p-Erk1/2) and Akt (p-Akt) in ErbB1- and/ or ErbB2-expressing tumor cell lines and xenografts (91-94). Lapatinib elicits cytostatic or cytotoxic antitumor effects depending on the cell type (95;96). Because ErbB2-containing heterodimers exert potent mitogenic signals, simultaneously interrupting both ErbB1 and ErbB2 signaling is an appealing therapeutic approach. Moreover, ErbB3 signaling is also involved in lapatinib action. Indeed ErbB3 is kinase-dead and relies on ErbB2 for transactivation: ErbB2-ErbB3 heterodimers are potent activators of the PI3K-Akt survival pathway (97;98), which can, in turn, inhibited by lapatinib.
Based on its molecular mechanism of action, on its fair toxicity profile and on its promising, although preliminary, activity data, Lapatinib appears an ideal candidate to combine with Fulvestrant in the attempt to improve its efficacy in patients progressing on AIs.
