Background: Downstream inhibition of the mammalian Target of Rapamycin (mTOR) pathway by sirolimus affects a variety of cellular functions including cap-dependent protein translation and cell cycle progression from the G1-to-S phase. Inhibition of mTOR also leads to dysregulation of the upstream signaling pathway that couples growth factor-receptor binding to mTOR activation through the PI-3 kinase/Akt pathway and may render PTEN-induced resistant lymphoblasts sensitive to agents that target the mTOR pathway. This hypothesis is supported by preclinical data which shows that sirolimus inhibits growth of ALL lines in vitro and has activity in a murine model of leukemia including ALL xenografts. Based upon these preclinical data we have piloted a Phase I trial of sirolimus in pediatric patients with relapsed acute leukemia.

Methods: Pediatric patients with ≥ 2nd relapse of acute leukemia were enrolled in a Phase I dose escalation trial of oral daily sirolimus. We used a starting dose that is known to be well tolerated in pediatric renal transplant recipients and results in levels that inhibit ALL growth in vitro. At dose level 1, a loading dose of 9 mg/m2 was given on day 0, and 3 mg/m2 was given daily on days 1 to 21 or 28. Dose level 2 has a loading dose of 12 mg/m2 and daily dose of 4mg/m2. Sirolimus trough levels were obtained on days 3, 7, and end of cycle. Bone marrow aspirates (BM) were performed prior to therapy, and on day 7 (if no peripheral blasts) and day 21 or 28. Peripheral blood (PB) mononuclear cells and PB and/or BM lymphoblasts were obtained on days 0, 3, 7, 14, and 21/28 to evaluate the effect of sirolimus on intracellular targets, including ribosomal protein S6 (a pharmocodynamic marker of mTOR inhibition), 4e-BP1 and STAT5.

Results: To date, 5 males and 3 females, ages 1–21, have been enrolled on study at the first 2 dose levels. Six patients had ALL, 1 infant had MLL(r) ALL, and 1 had AML. They had received 2–4 prior therapies. Three patients with ALL had stable disease at the end of the first cycle. One had a decrease in BM lymphoblasts from 39% to 12% by day 28 and a drop in absolute blast count (ABC) in the PB from 1134 to 290 but remained thrombocytopenic. The patient with MLL(r) infant ALL had a decrease in PB ABC from 62,415 to 0 by day 14 but had no change in BM lymphoblast % on day 21. Two of the 3 patients with stable disease progressed during their second cycle of therapy. The remaining 5 patients had progressive disease or were removed from study prior to the end of cycle 1 and were non-evaluable. At dose level 1 average trough level on day 7 was 10.9 (range 1–20.7) and at the end of the first cycle for non-progressors was 8.5 (range 5.8–12.2). There have been no DLTs attributable to sirolimus in any cycle to date. Preliminary immunoblots show hypophosphorylation of S6 in patients’ PB and BM after initiating sirolimus therapy.

Conclusions: Sirolimus was well tolerated in pediatric patients due to its ease of administration and lack of toxicity. At the first and second dose levels there have been 2 patients and 1 patient with stable disease, respectively. Preliminary biologic data shows evidence of inhibition of mTOR, manifested by a decrease in phosphorylated S6, suggesting that S6 may be used as a biomarker for response to mTOR inhibition. Although sirolimus at these doses had a modest impact on the leukemia burden, it may be more effective when used in concert with other cytotoxic agents that inhibit cell growth and survival. Combined therapy is being investigated.

Author notes

Disclosure:Off Label Use: Rapamycin is not indicated for acute leukemia.