Interferencia entre dos aeroturbinas

Another major clinical concern is the observation of molecular resistance in IM- treated patients. This is generally observed in CML patients at the advanced stage of disease, with an estimated 4-year resistance rate of 20% in later CP and 70% to 90% in AP/BC phases (220, 222), although, it has also been seen in CP CML patients. IM resistance can be described either as primary (“refractoriness”), defined as a failure to achieve CHR at 3 months, a cytogenetic response at 6 months or a MCR at 12 months; or secondary (“acquired”), defined as loss of established IM response or progression to AP or BC (231). It has also been speculated that there are 2 broad categories of IM resistance: BCR-ABL- independent and BCR-ABL-dependent.

1.2.9.1 BCR-ABL-independent mechanisms of resistance

BCR-ABL-independence suggests that the leukaemia cells are no longer reliant on BCR-ABL to drive proliferation; but have acquired additional oncogenic mutations and mechanisms, which are responsible for their growth and survival. Donato et

al. demonstrated that compensatory SRC activation within a CML cell may play a role in CML progression to BP and BCR-ABL independent IM resistance (232, 233). It was found that patients who have progressed whilst being treated with IM had an elevated SRC kinase activity, while the BCR-ABL transcripts and protein were reduced. Furthermore, it has also been shown that the SRC family kinase (SFK), LYN, is persistently activated in patients who have failed IM therapy and who harbour no BCR-ABL mutations (234). These data have led to the development of new agents that target the SRC kinase and inhibit other non-ATP binding sites (to be discussed further later). The induction of autocrine GF secretion is another potential mechanism for IM resistance. Adaptive autocrine secretion of GM-CSF was shown to mediate BCR-ABL-independent TKI resistance via activation of the anti-apoptotic JAK2-STAT5 pathway. This GM- CSF-induced resistance could then be overcome by the inhibition of JAK2 (235). A further possibility is that IM resistant cells are inherently refractory to TKI treatment and resistance reflects intrinsic properties of CML stem cells, to sustain survival in the presence of drug treatment. Thus, BCR-ABL is no longer a relevant target for IM in these cells and any specific BCR-ABL inhibitor would be ineffective in this situation.

1.2.9.2 BCR-ABL-dependent mechanisms of resistance

Conversely, resistance in Ph+ leukaemias may be dependent on BCR-ABL. Proposed mechanisms of resistance include BCR-ABL amplification- where multiple copies of BCR-ABL gene can be detected in the interphase nuclei in patients who relapse after initially responding to IM, resulting in overexpression of the BCR-ABL oncoprotein (236-238). However, BCR-ABL dependent IM resistance is most often attributed to the development of point mutations within the ABL-kinase domain, which are found in more than 50% patients with acquired

resistance (132, 238-240). In 2001, Gorre and colleagues described the development of BCR-ABL mutations in 11 patients with BC phase CML or Ph+ ALL who relapsed on IM therapy (238). BCR-ABL gene amplification was only detected in three patients. Following sequencing of the ATP-binding pocket and the activation loop of the kinase domain, an identical cytosine to thymidine mutation at ABL nucleotide 944 in 6 of 9 assessable patients was identified. This mutation resulted in a single amino-acid change at position 315, is designated T315I. Threonine 315 (also known as the “gatekeeper” residue) is located at the binding site of ABL and forms a crucial hydrogen bond interaction with IM (241). The T315I mutation disrupts this bond interaction which, along with the bulkier isoleucine side chain, sterically inhibits IM binding. This results in complete insensitivity to IM therapy at clinically achievable doses (238). The T315I mutant remains the most clinically significant as it linked to poor prognosis in CML patients (242). To date, more than 100 kinase domain mutations have been described in vivo and from in vitro screens (240, 241, 243, 244), each conferring a different transforming potential and level of resistance relative to wild-type BCR- ABL. These include mutations within the ATP binding site (P-loop), activation loop (A-loop) or the caboxy terminus. Some mutations only confer a moderate degree of resistance; however, the mutations within the P-loop are often associated with a poorer outcome (239). This is probably due to the restriction of IM-binding due to mutations altering and destabilising the conformation of the ATP-binding site (240). However, it is also worth noting that the presence of BCR-ABL mutations does not always explain resistance in IM-treated patients (245).

It has been noted that the concentration of drug within a target cell is a further important treatment consideration; therefore, the active efflux of chemotherapeutic drugs from target cells could be a mechanism of drug resistance. The ABC

transporter (ABCB1 (or MDR-1)) is a cell-surface transmembrane protein that mediates multi-drug resistance in a variety of malignancies through the regulation of chemotherapeutic agent efflux. Cells from BC CML patients have demonstrated a higher expression of ABCB1 compared with those from CP patients and has been linked to the development of IM resistance (237). Several groups have also shown that IM is a substrate for ABCB1 (246, 247). However, the role of ABCB1 in CML remains unclear as in vitro studies have shown that overexpression of ABCB1 in K562 cells does not confer resistance to IM (248). Furthermore, inhibition of ABCB1 did not enhance the therapeutic effect of IM against BCR-ABL activity in primitive CML cells (249). Another related ABC transporter, ABCG2, is also present on HSCs and has been suggested as another candidate for regulating intracellular IM availability. However, Jordanides and colleagues demonstrated that IM is an inhibitor of, but not a substrate for ABCG2 and therefore does not modulate the intracellular IM concentrations within CML stem cells (250).