“First-line therapy with ATRA-ATO with two initial doses of idarubicin results in superior event-free survival, compared to conventional ATRA-chemotherapy in patients with high-risk APL,” said first author Uwe Platzbecker, MD, of the University Hospital Leipzig, department for hematology, cellular therapy, hemostaseology, and infectious diseases, in Leipzig, Germany, at the annual meeting of the European Hematology Association (EHA) in Madrid, Spain.
“We believe that the trial may support the implementation of this regimen as a new standard of care in all patients with high-risk APL,” he said.
In the treatment of low and intermediate risk APL, a subtype of acute myeloid leukemia (AML), the combination of ATRA and ATO has become standard since being shown in a pivotal 2013 study to be superior versus ATRA and chemotherapy. The approach is approved by the Food and Drug Administration in the treatment of adults with newly diagnosed low-risk APL.
Importantly, the improved survival with ATRA/ATO approach may result from “reduced severe hematologic toxicity together with similar antileukemic efficacy,” compared with the regimen that include chemotherapy, the authors of the 2013 study speculated.
However, the treatment regimen has not been evaluated in randomized trials in patients with high-risk APL, defined as having a white blood cell count of more than 10,000 cells per μL.
For those patients, the conventional treatment remains ATRA with a chemotherapy backbone, Dr. Platzbecker explained.
To evaluate if the improvements extend to high-risk APL patients without compromising safety, Dr. Platzbecker and colleagues conducted the open-label, prospective APOLLO trial, involving newly diagnosed high-risk APL who were enrolled between 2016 and 2022 at 143 sites in six European countries.
The patients were randomized into one of two groups: ATRA/ATO, involving treatment consisting of two doses of idarubicin (12 mg/m2) on days 1 and 3 at the time of induction therapy, in addition to ATO 0.15 mg/kg and ATRA 45 mg/m2, daily until complete remission, or the ATRA-chemotherapy arm, involving standard ATRA also with idarubicin induction, followed by three cycles of chemotherapy-based consolidation as well as 2 years of maintenance treatment.
While the study was prematurely discontinued in August 2022 because of COVID-19–related recruitment delays and expiration of the study drug, the maintenance and observational periods are ongoing.
Of 131 patients with high-risk APL who were evaluable for the outcome analysis, 68 were in the ATRA/ATO group and 63 in the ATRA-chemotherapy arm.
Overall, participants had a mean age of 46, 50% were female, their median Eastern Cooperative Oncology Group score was 1. Their median white blood cell count was 36 × 109/L, with 39% having a white blood cell count greater than 50 × 109/L.
Molecular resistance occurred in 1.7% in the ATRA/ATO arm vs 5.5% in the ATRA chemotherapy arm, which was not statistically significant (P = .268); however, the incidence of molecular relapse was much lower without chemotherapy, at 1.6% with ATRA/ATO vs 14% with ATRA and chemotherapy.
For the primary endpoint, with a median follow-up of 31 months, the 2-year rate of event-free survival those in the ATRA/ATO arm was 88% vs 70% in the ATRA plus chemotherapy regimen (P = .02). The 5-year event-free survival continued to favor ATRA-ATO (87% vs 55%; P = .0034).
The estimated 5-year overall survival was 93% vs 82% for ATRA/ATO vs ATRA-chemotherapy, respectively, which was not significantly different (P = .17).
There were no significant differences between the arms in complete response (93% with ATRA/ATO vs 91% with ATRA-chemotherapy; P = .65), and rates of early death (within the first 30 days) were also similar across arms, at 7% vs 10%, respectively.
Death while in complete remission occurred in zero patients in the ATRA/ATO arm and three in the ATRA chemotherapy arm.
In terms of toxicities, the ATRA/ATO group had significantly lower rates of hematologic toxicity versus ATRA-chemotherapy, including rates of thrombocytopenia grade 1-4 and neutropenia grade 3-4 (P < .001), while there were no significant differences between the groups in hepatic toxicities (11.8% and 14.3%, respectively; P = .08) or differentiation syndrome (1.5% vs 4.8%; P = .27).
QTc prolongation grade 3-4 occurred in 4.4 patients receiving ATRA/ATO, compared with 0 in the ATRA-chemotherapy group; however, Dr. Platzbecker said the cases had no clinical implications.
Asked to elaborate on the regimens’ toxicities in the press briefing, Dr. Platzbecker noted that “what is very important especially for patients, is [lower rates] of issues such as hair loss and constipation that are much less common with the ATRO/ATO regimen.”
“In addition, we know from the early experiences with this that younger patients are being cured by this regimen,” hence improving pregnancy prospects for women.
A take-home message from the overall results is that the ATRO/ATO regimen for high-risk APL patients should represent “a new treatment paradigm” that will “hopefully soon” be reflected in guideline recommendations, Dr. Platzbecker said in an interview.