Outcomes of Relapsed/Refractory Patients with IDH1/2 Mutated AML Treated with Non-Targeted Therapy: Results from the NCRI AML Trials

Introduction: The increasing delineation of acute myeloid leukemia (AML) has identified a number of genetic mutations which may be amenable to targeted therapies. However, such mutations typically only occur in a minority of patients, and this relative paucity presents challenges in drug development. Even for more common mutations such as FLT3 ITD, randomised trials can take many years to complete, and there is the issue of how to deal with patients who are tested but not eligible. Earlier phase trials therefore tend to be single arm studies, and often recruit in the relapsed/refractory population, where eligibility is known up front, and it is possible to obtain an early read out for efficacy. Such is the case for the recent evaluations of enasidenib and ivosidenib in IDH1/2 mutated patients. However, with single-arm studies there a need to contextualise results. We therefore looked at outcomes from the United Kingdom NCRI trials of AML for patients with IDH1/IDH2 mutations who were relapsed or refractory to therapy.

Methods: A database search identified patients within the UK NCRI AML trials with an IDH1/IDH2 mutation, who had received intensive induction and who were either: in second relapse, relapsed post-transplant, refractory to two courses of induction, or who relapsed within 1 year of remission. Outcomes were measured from the point of eligibility: patients who were multiply eligible were included only once, at their first point of eligibility. The primary outcome was overall survival, with achievement of complete remission, with or without peripheral count recovery, as secondary outcome. Cox regression analysis was used to identify prognostic factors within the cohort of patients. Cytogenetics are evaluated using the MRC classification.

Results: A total of 757 patients were identified with IDH1/2 mutation (IDH1 alone n=247; IDH2 alone n=504, both n=6). Of these 211 patients satisfied the relapsed/refractory criteria (IDH1 alone n=81; IDH2 alone n=128; both IDH1/2 n=2; refractory n=28; relapsed post SCT n=34; relapsed within 1 year with no SCT n=138; second relapse n=11 - Table). Median age was 54 years (range 22-77); 51% were male; and 95% of patients had intermediate risk cytogenetics. Remissions were achieved in 43/211 patients (20%; refractory 50%; relapsed post SCT 15%; relapsed within 1 year 17%; second relapse 9% - Table). Patients with IDH1 mutations had a remission rate of 23%; for IDH2 mutated patients, the rate was 18%. Median survival was 4.4 months for IDH1 mutated patients, and 6.6 months for IDH2 mutations; 2 year survival was 17%, 21% respectively. Split by age, median survival was 4.0 and 9.4 months respectively (2-year survival 19%; 27%) for patients aged <60; in patients aged 60 or over, median survival was 5.2, 2.9 months (2 year survival 13%; 8%). In multivariable analyses no presenting factor was significantly associated with survival among IDH1 patients. In particular, there was no significant difference in survival by age or between the four different eligibility groups. By contrast, among IDH2 patients, patients in second relapse had worst survival, followed by those relapsing post transplant, those relapsing within 1 year, and those with disease refractory to two courses of therapy (p=0.001); older patients had significantly worse survival (p=0.004 for age older or younger than 60).

Conclusions: These results give context to the recent findings in single arm studies of ivosidenib for relapsed/refractory IDH1 mutated patients, and enasidenib for patients harbouring an IDH2 mutation. In the two studies reported, median survival was respectively 8.8 and 9.3 months, compared to 4.4 and 6.6 months in a younger group of patients identified from the UK NCRI AML trials treated with a variety of therapies. In both monotherapy trials the median survival was extended: however, reported one-year survival was not greatly improved (enasidenib 1 year survival 39% vs 34% for the NCRI cohort; ivosidenib, approximately 35% vs 32%). The difference in survival for IDH2 mutated patients in the NCRI cohort, by age and route to eligibility indicates that the interpretation of the results of single arm studies, in a heterogeneous condition such as AML, is fraught with difficulties. Ideally the magnitude of benefit should be assessed using randomised data from large scale collaborations and platform trials.

Table: Outcomes for IDH1/IDH2 mutated relapsed/refractory patients in the UK NCRI AML trials.

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