© ASCO/Scott Morgan
CHICAGO—A dual inhibitor of FLT3 and Axl may produce more durable responses than other FLT3 inhibitors and improve survival in patients with FLT3-positive, relapsed or refractory acute myeloid leukemia (AML), according to a speaker at the 2015 ASCO Annual Meeting.
The FLT3/Axl inhibitor, ASP2215, has not been compared against other FLT3 inhibitors directly, and the data presented were from a phase 1/2 study.
However, the speaker said ASP2215 provided “potent and sustained” inhibition of FLT3 and produced an overall response rate (ORR) of 52% among FLT3-positive patients.
The median duration of response for these patients was 18 weeks, and their median overall survival was about 27 weeks.
Mark J. Levis, MD, PhD, of the Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins in Baltimore, Maryland, presented these results at ASCO as abstract 7003.*
“We’ve been studying FLT3 inhibitors for a number of years now,” Dr Levis began, “and we think they show significant clinical promise, [but] they also have problems.”
He noted that some of these drugs haven’t been particularly safe or well-tolerated. They can cause gastrointestinal toxicity, hand-foot syndrome, QT prolongation, and myelosuppression.
However, the most intriguing problem with FLT3 inhibitors, according to Dr Levis, is the emergence of resistance-conferring point mutations observed in studies of some of the newer drugs, such as sorafenib and quizartinib.
“So in that context, here we have ASP2215,” Dr Levis said. “This is a type 1 FLT3 tyrosine kinase inhibitor, and, as such, it has activity against not only wild-type and ITD-mutated FLT3 but also against those resistance-conferring point mutations typically found in the activation loop at the so-called gatekeeper residue (F691L).”
With this in mind, he and his colleagues conducted a phase 1/2 trial of ASP2215. The study was sponsored by Astellas Pharma Global Development, Inc., the company developing ASP2215.
The trial was open to patients with relapsed or refractory AML, irrespective of their FLT3 mutation status. The researchers’ goal was to identify a safe, tolerable dose of ASP2215 that fully inhibited FLT3.
The team used a standard 3+3 design, with dose levels ranging from 20 mg to 450 mg once daily. They expanded every cohort until they reached a dose-limiting toxicity.
In all, the trial enrolled 198 patients, 24 in the dose-escalation cohorts and 174 in the dose-expansion cohorts. The patients’ median age was 62 (range, 21-90), and 53.1% were male.
Nearly 66% of patients had FLT3 mutations, 29.4% were FLT3-negative, and 5.2% had unknown FLT3 status. About 35% of patients had received 1 prior line of therapy, 26.3% had 2, 33.5% had 3 or more, and 5.7% had an unknown number of prior therapies.
In the 194 patients who were evaluable for safety, treatment-emergent adverse events included diarrhea (13.4%), fatigue (12.4%), AST increase (11.3%), ALT increase (9.3%), thrombocytopenia (7.7%), anemia (7.2%), peripheral edema (7.2%), constipation (6.7%), nausea (6.7%), dizziness (6.2%), vomiting (5.7%), and dysgeusia (5.2%).
Serious adverse events included febrile neutropenia (27.3%), sepsis (11.9%), disease progression (10.3%), pneumonia (8.8%), hypotension (5.7%), and respiratory failure (5.7%).
“The kinds of side effects we saw were typical for a relapsed/refractory AML population,” Dr Levis said. “Nothing really stood out. Any trial of relapsed/refractory AML is going to have febrile neutropenia and sepsis, and those were our dominant, serious adverse events. There was no real safety signal here that was unique to the drug, we felt.”
The researchers said the maximum-tolerated dose of ASP2215 was 300 mg, as 2 patients who received the 450 mg dose experienced dose-limiting toxicities. One was grade 3 diarrhea, and the other was grade 3 AST elevation.
Response and survival
Among the 127 patients who were FLT3-positive, the ORR was 52% (n=66). The complete response (CR) rate was 6.3% (n=8). The composite CR rate, which includes CRs with incomplete hematologic recovery (CRi) and incomplete platelet recovery (CRp), was 40.9% (n=52). And the partial response (PR) rate was 11% (n=14).
“As we scale up the dose, the PRs shift on over to CRis, and the dominant response is, in fact, a complete response with incomplete count recovery,” Dr Levis said. “The categories where we had the largest number of responses were the 120 mg and 200 mg categories. We didn’t really have enough patients in the 300 mg category to comment on it. ”
For the FLT3-positive patients, the median duration of response was 126 days.
“The duration of response really stood out here,” Dr Levis said. “It’s over 4 months. That is something we really didn’t see with the other drugs, and I suspect that is a reflection of the suppression of the outgrowth of these resistance mutations.”
Unfortunately, FLT3-wild-type patients did not fare as well. The ORR among these patients was 8.8%. None of the patients achieved a CR, 3 had a composite CR (5.3%), and 2 had a PR (3.5%).
Among the FLT3-positive patients, the median overall survival was about 27 weeks. It was 128 days in the 20 mg dose cohort (n=13), 105.5 days in the 40 mg cohort (n=8), 201 days in the 80 mg cohort (n=12), 199 days in the 120 mg cohort (n=40), and 161 days in the 200 mg cohort (n=45). Dr Levis did not present survival data for the 300 mg or 450 mg cohorts, which included 7 and 2 patients, respectively.
“[Relapsed/refractory AML] is a population that has a median survival of about 3 months with conventional therapy, at least by historical publications,” Dr Levis noted. “If you look at survival in this trial, patients treated at the FLT3-inhibitory doses [had a] greater than 6-month median survival.”
He added that studies of ASP2215 in combination with other agents are ongoing in patients with newly diagnosed AML. And phase 3 trials of ASP2215 at the 120 mg dose, with the option of scaling up to 200 mg, are planned.
*Information in the abstract differs from that presented at the meeting.