Real-world global data strengthen the case for adjuvant imatinib in KIT exon 9–mutant GIST after surgery, while raising fresh questions on optimal dosing and duration.
Adjuvant imatinib significantly delays recurrence and improves survival in patients with resected gastrointestinal stromal tumours (GIST) harbouring KIT exon 9 mutations, according to the largest analysis to date.
The findings, published in JAMA Oncology, help to resolve a long-standing clinical uncertainty in this biologically distinct subgroup.
The international retrospective cohort study of 367 patients found that postoperative imatinib was associated with a markedly reduced early risk of recurrence or death and a significant overall survival benefit compared with observation, particularly in those with high-risk disease.
KIT exon 9 mutations account for fewer than 10% of GIST cases and have historically posed a therapeutic dilemma.
While adjuvant imatinib at 400mg daily for three years is standard for high-risk disease, most prospective trial data derive from the more common exon 11 subtype, leaving clinicians uncertain whether exon 9 tumours, known to be relatively less sensitive to standard-dose imatinib in advanced disease, require different strategies.
Professor John R Zalcberg AO, professor of cancer medicine at Monash University and Melbourne University, consultant medical oncologist at Alfred Health, Epworth Hospital and Cabrini Health and the editor of Oncology Republic, welcomed the findings.
In advanced disease, clinicians often escalate to 800mg, and some have applied that approach postoperatively despite limited evidence.
“In the advanced disease, there appears to be a benefit to 800mg but in this real world data set, there didn’t appear to be a benefit to the people having a higher dose,” Professor Zalcberg said.
“So the standard is 400 milligrams. I would have thought that in people with advanced disease with an exon 9 mutation, we try and treat them at a higher dose if it’s tolerable.
“This study in the high-risk patients, they looked at 800 versus 400 and they didn’t find any benefit, and that’s one of the real strengths of the paper.”
Across the cohort, three-quarters of patients received adjuvant imatinib, with a median treatment duration of just over two years. In multivariable analyses, treatment was associated with a substantially lower early hazard of recurrence or death, although the benefit diminished over time, consistent with the drug’s cytostatic mechanism.
Overall survival was also significantly improved.
Dr Zalcberg added that, while confounding was an inherent limitation of retrospective analyses, the dose comparison was likely more robust than the overall treatment comparison.
The findings arrive amid growing interest in extending the duration of adjuvant therapy beyond three years. While the current study could not definitively address duration, emerging evidence from other trials in unselected GIST populations suggests longer treatment may further improve recurrence-free survival.
“There’s at least some argument for going for longer anyway,” Professor Zalcberg said, pointing to ongoing and recently reported studies comparing three years with five or six years of therapy.
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For clinicians, the results underscore the importance of molecular stratification in GIST, not only for prognosis but also for treatment planning.
“I think it’s essential,” Professor Zalcberg said, cautioning against a one-size-fits-all approach.
“I don’t think people can be adequately managed without getting mutation testing done.”
Despite its limitations, including potential selection bias and heterogeneity in treatment duration, the study provided a benchmark for future prospective research in this rare subgroup.
“To our knowledge, this cohort study provides the strongest evidence available to date supporting the use of adjuvant imatinib in mNIH high-risk GISTs with KIT exon 9 mutations and establishes a benchmark for future prospective trials in this population,” the researchers conclude in JAMA Oncology.
“Although no dose-dependent differences were observed, optimal dosing and treatment duration remain unclear. Recurrence remains frequent (approximately 75%) after resection of high-risk GISTs with KIT exon 9 mutations, underscoring the urgent need for novel therapeutic strategies tailored to this subgroup.”
