These results extend previously reported findings from this study that showed a correlation between sunitinib activity and GIST kinase genotype in patients who have metastatic/unresectable GIST and have experienced imatinib failure.33
Data on the relative responsiveness of different molecular subgroups of imatinib-resistant GIST may help to optimize treatment of patients with GIST and may help to better understand the basis of sunitinib activity in these patients. Such studies may also advance understanding of the mechanisms of resistance and may facilitate development of strategies to circumvent it.
The analyses reported here assessed the effect of tumor kinase genotype on sunitinib activity by using clinical study data complemented by in vitro cellular assays. Although sunitinib demonstrated clinical activity against GISTs of the three most common primary genotypes, both datasets indicated that primary and secondary mutations in the pathogenic kinase strongly influence sunitinib activity. Both the clinical benefit and the objective response rates with sunitinib were higher in patients with primary KIT
exon 9 mutations than with exon 11 mutations (clinical benefit rates: 58% v
34%; objective response rates: 37% v
= .002). Similarly, PFS and OS were significantly longer in patients with primary KIT
exon 9 mutations or a wild-type genotype than in those with KIT
exon 11 mutations. These results are the converse of those reported for imatinib, in which objective response rates were higher and PFS and OS were longer in patients with GIST who harbored exon 11 mutations than in those who had exon 9 mutations or a wild-type genotype.3,14,15
Notably, the potency of sunitinib against wild-type and exon 9-mutant KIT was superior to that of imatinib in vitro, whereas both drugs exhibited similar potency against KIT exon 11 mutant kinases. A possible explanation is that these mutational sites have different structural effects on KIT, with different consequences for interaction with the two TKIs. Indeed, exon 9 mutations were recently reported to have structural consequences similar to ligand-mediated receptor dimerization.34
This mechanism of kinase activation appears distinct from that caused by mutation of the intracellular juxtamembrane domain encoded by exon 11.35
Others have also observed the impact of mutational site on TKI potency in vitro: by using an isogenic BaF3 model, the imatinib IC50
in cells that expressed exon 9 mutations was found to be approximately eight-fold higher than that obtained in cells that expressed the exon 11 V559D mutation.36
These results suggest that the greater clinical benefit seen for sunitinib-treated patients with exon 9-mutant or wild-type imatinib-resistant GISTs may be related to the greater potency of sunitinib against these kinases. They also suggest that genotypically defined subsets of patients may experience different clinical outcomes when treated with first-line imatinib than with sunitinib. Sunitinib is currently approved only as second-line therapy for GIST, but studies are being planned to evaluate its efficacy and safety as first-line treatment. On the other hand, sunitinib has yet to be tested in imatinib-naïve patients, and the majority of patients in this study with primary KIT
exon 11 mutations had acquired secondary KIT
mutations that confer imatinib resistance. Studies in imatinib-naïve patients will be required to definitely assess the effect of a primary exon 11 mutation alone on sunitinib activity in vivo.
This study also showed that secondary kinase mutations were significantly more common in GISTs with primary KIT
exon 11 than exon 9 mutations and that they did not occur in GISTs with a wild-type genotype, which is consistent with previous reports that secondary kinase mutations are common in GISTs that exhibit secondary imatinib resistance but not in those that exhibit primary resistance.16,17
Moreover, the frequency of secondary mutations is likely to have been underestimated in this analysis, because only one patient in our analysis was found to have different secondary mutations in different lesions, and intra- and interlesion heterogeneity of secondary mutations in GISTs has been documented by others.20,25
Only a limited number of small-needle biopsy specimens were available per patient in our study (mean, 1.4 biopsy specimens per patient; range, 0-3). In particular, it is probable that further sampling would have revealed secondary mutations in those tumors with primary KIT
exon 11 mutations that appeared to lack them. Because exon 11 mutants are strongly inhibited by imatinib, secondary resistance is more likely to require the selection and subsequent expansion of clones expressing a second, resistance-conferring mutation than GISTs with exon 9 mutations or a wild-type genotype, which are more likely to be intrinsically resistant to imatinib. Consistent with this, the median duration of prior imatinib treatment for patients who had primary exon 11 mutations was 22.8 months, compared with 12.5 and 10.5 months for patients who had exon 9 mutations or a wild-type genotype, respectively (). However, it is worth noting that, although the duration of imatinib treatment was a significant prognostic factor for PFS and OS in a univariate analysis, it was not a significant factor in a multivariate analysis (data not shown). Although multivariate analyses performed on such a small sample must be interpreted with caution, they confirmed that primary and secondary KIT
genotype were significant prognostic factors for PFS and were marginally significant prognostic factors for OS.
Consistent with previous studies,16,18-28
mutations in patients with imatinib-resistant GIST enrolled on the current study tended to cluster in exons 13 and 14, which encode the drug/ATP binding pocket of the receptor, or in exon 17, which encodes the kinase activation loop. Of note, our in vitro studies showed that sunitinib potently inhibited the kinase activity of KIT receptors that contained secondary mutations in the drug/ATP binding pocket and that are resistant to imatinib, such as V654A (exon 13) and T670I (exon 14). These secondary mutations were coexpressed with a common primary mutation (V560D), which recreated the situation often observed in GISTs that exhibit secondary imatinib resistance. Previous ex vivo studies have also shown that sunitinib inhibits imatinib-resistant KIT receptors that contain mutations in the drug/ATP binding pocket.29,30
However, the in vitro studies performed here also showed that sunitinib was relatively ineffective at inhibiting KIT receptors that contained secondary mutations localized to the activation loop. Consistent with these in vitro findings, PFS and OS were longer and the clinical benefit rate was higher for patients in the clinical trial who had secondary KIT
exon 13 or 14 (ie, ATP-binding-pocket) mutations than those with secondary KIT
exon 17 or 18 (ie, activation-loop) mutations.
The results of this study provide one explanation for the activity of sunitinib in patients with imatinib-refractory GIST that has been seen in this and other trials.13
However, antiangiogenic effects of sunitinib treatment also may contribute to its effectiveness. In addition to KIT and PDGFRA activity, sunitinib also selectively inhibits PDGFRB and all three isotypes of VEGFR, whereas imatinib inhibits PDGFRB but not VEGFRs. Studies in animal models indicate that dual inhibition of PDGFR and VEGFR produces greater antiangiogenic effects than inhibition of only one or the other,37-39
which suggests that sunitinib may produce greater antiangiogenic effects than imatinib and that these effects may contribute to its activity against imatinib-refractory GISTs.
Of note is our observation that secondary KIT mutants that involve the activation loop are insensitive to both sunitinib and imatinib. Given that different tumor clones in one individual may acquire imatinib resistance because of different secondary mutations, including those involving the KIT activation loop,20,25
not all imatinib-resistant tumors may respond well to sunitinib therapy. Conversely, some GISTs with secondary KIT
activation-loop mutations may still be susceptible to sunitinib because of its potent antiangiogenic effects. Additional research of this issue is warranted.