CDK4/6 Inhibitor Palbociclib for Treatment of KMT2A-Rearranged Acute Myeloid Leukemia: Interim Analysis of the AMLSG 23-14 Trial
Authors: Fröhling Stefan , Agrawal Mridul , Jahn Nikolaus , Fransecky Lars R. , Baldus Claudia D. , Wäsch Ralph , Lübbert Michael , Walter Gina , Jensen Patrizia , Scholl Claudia , Habdank Marianne , Döhner Konstanze , Döhner Hartmut , Schlenk Richard F. 
Published: October 2019

Abstract
Background: Approximately 5% of adult acute myeloid leukemia (AML) cases are associated with balanced translocations of chromosome 11q23, and AML with t(9;11)(p22;q23) is recognized as a distinct entity by the WHO Classification. Similarly, the presence of t(4;11)(q21;q23), which accounts for 8-10% of B-cell precursor acute lymphoblastic leukemia (ALL) in patients over the age of 20 years, defines a distinct entity termed "B-lymphoblastic leukemia with t(v;11q23)" according to the WHO Classification. On the molecular level, t(11q23) result in fusion of the KMT2A (also called MLL) gene, which encodes a histone 3 lysine 4 methyltransferase, to a broad spectrum of more than 70 partner genes. The prognosis of patients with relapsed/refractory KMT2A-rearranged leukemia is very poor, and new treatment approaches are needed. Using in vitro and in vivo experimental models, we previously identified cyclin dependent kinase 6 (CDK6) as a potential therapeutic target in KMT2A-rearranged leukemias (Placke et al. Blood. 2014;124:13-23).
Aims: To evaluate the tolerability and efficacy of the small-molecule CDK4/6 inhibitor palbociclib in KMT2A-rearranged AML and ALL within a genotype-guided clinical trial (AMLSG 23-14; ClinicalTrials gov. Identifier NCT02310243).
Methods: Patients with KMT2A-rearranged leukemia, either relapsed/refractory or newly diagnosed but ineligible for intensive chemotherapy, are enrolled. The study is a phase Ib/IIa trial with a safety/tolerability part in the phase Ib using the standard palbociclib dose of 125 mg once daily for 21 days in a 28-day cycle. Based on a 3+3 modified Fibonacci design, a dose deescalation to 100 mg and 75 mg in case of toxicity is possible in sequential cohorts. If no or only one limiting toxicity is observed among 6 patients at one dose level, this dose level will be taken forward to the phase IIa expansion part of the study. Limiting toxicities are defined as toxicities attributable to palbociclib, expected or unexpected. The expansion part of the study is based on Simon's optimal 2-stage design with 18 patients and 43 patients in the 2 stages.
Results: The phase Ib of the study has been completed with recruitment of 6 patients with relapsed/refractory leukemia (AML, n=3; treatment-related AML, n=2; ALL, n=1; refractory to intensive chemotherapy, n=2; relapse, n=4 [following allogeneic stem cell transplantation, n=3; following chemotherapy, n=1]). Cytogenetic results were as follows: t(9;11), n=3; t(6;11), n=1; t(11;19), n=1; t(4;11), n=1. The median white blood cell count (WBC) at study inclusion was 7.05 G/l (range, 0.9-61.0). To control hyperleukocytosis, 3 patients were treated with hydroxyurea during the first week of palbociclib and one patient with corticosteroids. No limiting toxicity occurred during the first 28-day cycle, the limiting-toxicity assessment period. White blood cell counts rapidly decreased after one week of palbociclib at a dose of 125 mg/day and remained low until week 3 (median, 1.6 G/l; range, 0.6-1.9). The median WBC after one week of drug holiday was 1.9 G/l (range, 1.3-7.3). Response assessment revealed one partial remission, 3 disease stabilizations, and 2 cases of progressive disease. Four patients completed further treatment cycles (median, 2; range 2-6), with one patient achieving a complete remission with incomplete hematologic recovery after cycle 2. This patient, a 76-year-old man with t(11;19)-positive de novo AML refractory to chemotherapy with daunorubicin and cytarabine, relapsed after cycle 6, and correlative laboratory studies are underway to determine potential resistance mechanisms.
Conclusions: Palbociclib is well tolerated in patients with refractory/relapsed KMT2A-rearranged leukemia with no occurrence of limiting toxicities and has clinical activity in this prognostically unfavorable subset of AML/ALL. Therefore, the study will be taken forward to the efficacy part with accrual of further patients. In addition, the protocol is currently amended as a basket trial with inclusion of patients with locally advanced/metastatic chordoma based on preclinical evidence that CDK4/6 dependence represents a specific liability of chordoma cells that could be exploited for therapeutic benefit.

Disclosures
Lübbert: Celgene: Other: Travel Funding; Ratiopharm: Other: Study drug valproic acid; Janssen-Cilag: Other: Travel Funding, Research Funding. Schlenk:Amgen: Research Funding; Pfizer: Honoraria, Research Funding.