We identified 2 patients with
ALK translocated cancers that developed clinical acquired resistance to crizotinib. Patient A, with IMT, achieved a partial response with crizotinib therapy lasting 8 months but subsequently developed re-growth of several tumor lesions and had these surgically removed (
Table S1) (
10). Both pre- and post-treatment tumor specimens had evidence of viable tumor, expressed ALK by IHC and contained an
ALK translocation (). There was no evidence of
ALK amplification in the post-treatment tumor (data not shown). This patient’s tumor was known to harbour the
RANBP2-ALK translocation (
10). Sequencing of the entire ALK kinase domain demonstrated that one of the clinically progressing tumor lesions contained a F1174L mutation (). This was not detected by direct sequencing, or by cloning and sequencing of individual clones, in the 2 other progressing lesions, or by direct sequencing in 2 other tumours that were clinically stable at the time of surgery (
Table S1). Furthermore it was not detected in the pre-treatment tumor specimen even by cloning and sequencing of individual clones (
Table S1). Patient B, with
EML4-ALK NSCLC, achieved a partial response with crizotinib treatment but developed acquired resistance following 5 months of therapy. At the time of progression a liver biopsy of a growing lesion was performed (
Table S1).
EML4-ALK variant 1 was identified by RT-PCR but no secondary mutations in
ALK were detected in the acquired resistance tumor specimen.
We next evaluated the biologic impact of the F1174L mutation. Both
RANPB2-ALK and
RANBP-ALK F1174L led to IL-3 independent growth of Ba/F3 cells () but the growth was faster in the presence of the F1174L mutation. This increased growth rate was mirrored by a greater baseline ALK phosphorylation of RANBP2-ALK F1174L compared to RANBP2-ALK () and by increased downstream AKT and ERK 1/2 phosphorylation (). The
RANB2-ALK F1174L cells were significantly more resistant to crizotinib () and the F1174L mutation diminished crizotinib mediated inhibition of ALK signalling and blocked apoptosis (
Figures S1A and B). We also introduced the F1174L mutation into the background of
EML4-ALK found in NSCLC(
3). Similar to
RANPB2-ALK, the
EML4-ALK F1174L Ba/F3 cells grew faster (), had a greater baseline ALK phosphorylation () and were more resistant to crizotinib growth inhibition than
EML4-ALK Ba/F3 cells (). Consistent with these findings on growth, greater concentrations of crizotinib were required to inhibit ALK phosphorylation in the
EML4-ALK F1174L cells compared to those with
EML4-ALK alone (). Collectively, our studies demonstrate that the F1174L mutation imparts both biologic and drug resistance properties to cancers harbouring
ALK translocations. Furthermore, patients with neuroblastoma harbouring the F1174L
ALK mutation may have a transient or no clinical benefit from crizotinib treatment using the current dosing schedules (
13).
Crizotinib is administered continuously daily (250 mg BID), reaching a median through plasma concentration of 57 nM of free drug, and is clinically effective at this dosing in
ALK rearranged IMT and NSCLC (
9,
10,
13). Our preclinical studies suggest that higher doses of crizotinib could be used to overcome the F1174L mediated resistance mechanism (). This could potentially be achieved using intermittent administration of higher doses of crizotinib, to achieve a higher Cmax, sufficient to inhibit ALK phosphorylation in the presence of F1174L. Similarly, some imatinib resistance mutations (including F359V, M244V, Q252H and E355G), many of which effect the conformational change in ABL, cause a relative drug resistance which can be overcome by higher drug doses
in vitro and in some cases clinically by using higher doses of imatinib (
14,
15). In order to develop additional therapeutic strategies we evaluated both a structurally unrelated ALK kinase inhibitor, TAE684, and the heat shock protein (HSP) 90 inhibitor 17-AAG in the crizotinib resistant models (
7,
16). Although the F1174L mutation slightly increased the IC
50 for TAE684 against RANBP-ALK Ba/F3 cells (59 nM with F1174L; 22 nM without), the IC
50 was still substantially below the concentrations of crizotinib (IC
50 200 nM) required to inhibit growth and ALK phosphorylation in the parental RANBP-ALK Ba/F3 cells (Figures and ). Similar findings were observed for EML4-ALK Ba/F3 cells ( and data not shown). The effects of TAE684 on growth were also mirrored at the level of ALK phosphorylation (). Recent clinical studies have identified anti-tumor activity of the HSP90 inhibitor IPI-504 in
ALK translocated NSCLC (
17). We thus evaluated the effects of the HSP90 inhibitor 17-AAG in models harbouring the F1174L mutation (). Ba/F3 cells with and without F1174L were equally sensitive to 17-AAG
in vitro (). As many ALK inhibitors are in pre-clinical development and several HSP90 inhibitors currently undergoing clinical development, our findings provide direct therapeutic strategies for patients that develop crizotinib resistance.
The structural basis for the crizotinib resistance of the F1174L mutation is not entirely clear. Examination of the recently published crystal structure of ALK in an inactive conformation reveals that the F1174L mutation is not in direct contact with the ATP-binding pocket, where both crizotinib and TAE684 are expected to bind () (
18,
19). Thus the F1174L mutation is unlikely to confer resistance via direct steric interactions. Crizotinib is known to bind an
inactive conformation of MET (
20) and a recently released crystal structure in complex with ALK indicates that it binds a distinct inactive conformation in ALK (PDB ID 2XP2). The activating F1174L mutation must promote the active conformation of the kinase, and therefore may disfavor binding of crizotinib analogous to some imatinib resistance mutations in ABL (
14). A more definitive understanding of the mechanism of resistance, and the differential effect of the mutation on crizotinib versus TAE684, will require detailed binding and structural studies of these inhibitors with the F1174L mutant.
The current study highlights the need to study drug resistance mechanisms from cancer patients treated with kinase inhibitors. The molecular, cellular and structural understanding of drug resistance mechanisms will continue to reveal therapeutic insights for the development of future anti-cancer therapies.