Phase Ib Trial of the mTOR Inhibitor Everolimus Given in Combination with Multiagent Chemotherapy in Relapsed Acute Lymphoblastic Leukemia

Introduction: Treatment of relapsed childhood acute lymphoblastic leukemia (ALL) remains a significant clinical challenge with a need for novel, more effective therapies. Pre-clinical studies have demonstrated that mTOR inhibitors have single agent activity in ALL and are synergistic with other chemotherapy agents typically used to treat ALL, including vincristine, corticosteroids, doxorubicin and L-asparaginase.

Methods: Dana-Farber Cancer Institute Protocol 11-237 tested the combination of everolimus (given orally on days 1-32) with a standard reinduction regimen of prednisone (40 mg/m²/daily orally on days 4-32), weekly vincristine (1.5 mg/m²/dose, max 2 mg, on days 4,11,18 and 25), PEG-asparaginase (2,500 U/m²/dose IV on days 5 and 18) and doxorubicin (30 mg/m²/dose on days 4 and 5) with dexrazoxane pretreatment (300 mg/m²/dose on days 4 and 5). Intrathecal cytarabine was administered on day 1 and intrathecal methotrexate, cytarabine and hydrocortisone (IT-MAH) was administered on days 18 and 32; patients (pts) with cerebrospinal fluid blasts at study entry received additional IT-MAH on days 11 and 25. Pts with first bone marrow relapse ALL occurring greater than 18 months after continuous remission were eligible for participation. The primary aim was to identify a maximum tolerated dose of everolimus in combination with this backbone utilizing a classic "3 + 3" dose escalation design. Three dose levels (DL) of everolimus were tested (2-, 3-, and 5-mg/m²/day). 5 mg/m²/day (DL3) was the highest dose tested because it was the recommended phase 2 dose (RP2D) of single-agent everolimus in children with solid tumors. Secondary aims included assessment of second complete remission (CR2) rate and levels of end-reinduction minimal residual disease (MRD) assessed by allele-specific oligonucleotide PCR. On-target activity of everolimus was assessed 6 hours after the first dose of everolimus by examining phosphorylated S6 (pS6) levels in peripheral blood blasts by flow cytometry.

Results: Thirteen pts with first relapse B-ALL were enrolled. (Table 1) Median age at enrollment was 10 yrs (range: 2-22 yrs). Median duration of first CR was 3.7 yrs (range 1.5-7.7 yrs). One pt treated at DL 3 stopped study drug early due to Grade 4 infection (meningitis due to Rothia mucilaginosa) and was inevaluable for dose limiting toxicity (DLT) assessment. Table 2 displays the most common Grade 2 or higher adverse events (AE's) attributable to everolimus. There were no Grade 5 events and no episodes of interstitial pneumonitis. No DLTs were observed at DL1 or DL2. One of 6 evaluable pts treated at DL3 experienced a DLT (Grade 4 hyperbilirubinemia). 12 of the 13 pts achieved CR (92%). Of the 12 pts who achieved CR, 9 (75%) had low end-reinduction MRD (²0.001). (Table 1) Of the 7 pts with evaluable samples (2 at DL1, 1 at DL2 and 4 at DL3), 6 (86%) had decrease in pS6 in peripheral blasts (>20% decrease in mean fluorescence intensity), consistent with on-target mTOR inhibition. Only 1 pt (treated at DL1) did not have evidence of pS6 inhibition; this pt achieved CR2 with low MRD.

Discussion: Everolimus given in combination with a standard 4-drug reinduction was well tolerated at all dose levels tested in pediatric pts with first relapse ALL. CR2 rates were comparable to published re-induction regimens, with a promising proportion of pts with low end-reinduction MRD. An expansion cohort is currently enrolling to further assess toxicity of this combination, particularly infectious risks, in order to define the RP2D.