Background: PI3K/mTOR signaling, a critical pathway in cell proliferation, metabolism, and apoptosis, is often dysregulated in acute lymphoblastic leukemia (ALL). A phase 1 trial of the mTOR inhibitor temsirolimus combined with etoposide and cyclophosphamide was performed in children with relapsed/refractory (r/r) ALL. Methods: Temsirolimus was administered intravenously (IV) on days 1 and 8 with cyclophosphamide 440 mg/m2 and etoposide 100 mg/m2 IV daily days 1-5. The starting dose level (DL) of temsirolimus was 7.5 mg/m² (DL1) with escalation to 10 mg/m² (DL2), 15 mg/m² (DL3), and 25 mg/m² (DL4). MRD was performed centrally. PI3K pathway inhibition was measured by phosphoflow cytometry (PFC) analysis of peripheral blood (PB) from treated patients (pts). Results: Sixteen heavily pretreated r/r ALL pts ages 2-19 years with marrow blasts > 25% were enrolled; 15 were evaluable [10 B-ALL/5 T-ALL]. One dose-limiting toxicity (DLT) of grade (Gr) 4 pleural and pericardial effusions with pneumonitis/lung infection leading to Gr 5 cardiorespiratory arrest occurred in a pt treated at DL3. No further DLTs were seen in the DL3 expansion and DL4 cohorts. Gr 3/4 non-hematologic toxicities occurring in ≥ 3 pts included febrile neutropenia, elevated ALT, hypokalemia, mucositis, and tumor lysis syndrome and were independent of dose. Of 15 evaluable pts, 4 (27%; 2 B-ALL/2 T-ALL) had a complete response (CR) after cycle 1, comprised of 1 pt at each DL. Three had MRD < 0.01%. Three pts (20%; 2 B-ALL/1 T-ALL) had partial response (PR). Overall response rate (CR+PR = ORR) was 47%. Pharmacodynamic PFC studies compared phosphoprotein levels pre (day 0) and post treatment (days 3-5) in 9 consenting pts with available PB. All tested pts showed basal activation of PI3K pathway signaling. Dose-dependent inhibition of mTOR targets phosphorylated (p) S6 and/or p4EBP1 was observed in 9/9 and 6/9 pts, respectively, following temsirolimus and chemotherapy treatment. Various patterns of compensatory upregulation of pPI3K, pmTOR, pAkt, and/or pERK was observed. Conclusions: Temsirolimus at 25 mg/m² combined with salvage etoposide and cyclophosphamide has an acceptable safety profile in high-risk pediatric patients with r/r ALL. Responses were observed at all DLs. mTOR target inhibition was achieved and appeared to correlate with dose level. Future testing of other PI3K/mTOR pathway inhibitors in combination with chemotherapy may be warranted with a goal of further increasing response in r/r ALL. Clinical trial information: NCT01614197

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