Sirolimus Plus MEC Chemotherapy Has Significant Activity in High Risk AML Patients Especially Those Who Exhibit in Vivo Inhibition of the mTOR Pathway

Mammalian target of rapamycin (mTOR) inhibitors enhance cytotoxic chemotherapy effects in primary acute leukemia cells in preclinical assays. This prompted a multi-center evaluation of a combination of mTOR inhibitor plus induction chemotherapy in AML. As mTOR is frequently but not uniformly activated in primary AML samples, it is unclear which patients benefit from this targeted approach. Thus, we sought to monitor mTOR kinase activity during therapy to determine whether target activation and/or inhibition predicted clinical response. We previously reported our preliminary experience monitoring pS6 in AML blasts by flow during clinical trials combining sirolimus and AML induction chemotherapy (Kasner et al, ASH 2011, #230). Here we provide the final clinical and pharmacodynamic results from this cohort of subjects.

Subjects had relapsed/refractory AML or untreated AML with unfavorable risk factors (e.g. therapy-related, prior MDS or MPN, or age >60 without favorable karyotype) with a median age of 60.5 years (range 32–77). Subjects received oral sirolimus (12 mg on day 1, then 4 mg daily on days 2–9) plus MEC (mitoxantrone 8 mg/m²/day, etoposide 100 mg/m²/day, cytarabine 1 gm/m²/d on days 4–8) on one of two successive clinical trials. Clinical response was assessed at hematologic recovery or day 42 using IWG criteria (CR, CRp, PR vs. non-response). Pharmacodynamic samples were collected from blood or marrow at baseline, 2 hours post-sirolimus dose on days 1 and 4, and at trough on day 4 (prior to chemotherapy administration). Concurrent blood rapamycin concentration was measured by immunoassay or HPLC. Whole blood/marrow fixation was performed using published methods (Perl, et al. Clin. Cancer Res. 2012). Positive gates for pS6 were created by comparing blasts in ex vivo stimulated (phorbol ester/PMA) and inhibited (rapamycin) conditions and/or autofluorescence (FMO) controls.

We enrolled 52 subjects in 2 consecutive trials; 51 were evaluable for clinical response. Toxicity was similar to published MEC data. 3 infectious deaths occurred (6%). Prolonged aplasia was not observed. 24/51 (47%) subjects responded, with 18 CR (35%), 1 CRp, and 5 PR's observed. Mean peak and trough rapamycin concentrations on day 4 were 22.0 and 8.9 ng/ml, respectively, and did not differ among clinically responding or non-responding subjects. Median survival time for the whole group was 243 days (longest follow up 1584 days). Among the 24 subjects achieving CR or PR, median duration of time to the first event (relapse or death) was 261 days. 20 subjects were able to proceed to a stem cell transplant following therapy.

Serial flow cytometric analysis was performed in 46 subjects, of which 37 provided paired day 1 and day 4 flow samples and were evaluable for clinical response at count recovery. The overall response rate (ORR) among subjects with baseline constitutive pS6 was 14/27 (52%, 9 CR, 1 CRp, 4 PR). The ORR for subjects without constitutive pS6 was 4/10 (40%, 3 CR, 1 PR). Subjects with >50% reduction in pS6 positive blasts on day 4 were considered to be biochemically sensitive to rapamycin, while subjects with <50% reduction or increased pS6 were considered rapamycin-resistant. Categorizing subjects based upon the achievement of CR/CRp/PR vs. NR, the reduction in blasts' pS6 percent on day 4 was 72% among clinically responding subjects and 43% among those without clinical response. The ORR in rapamycin sensitive patients was 10/15 (67%, 6 CR, 4 PR, 5 NR), while in resistant subjects was 4/12 (33%, 3 CR, 1 CRp, 8 NR). Combining rapamycin resistant subjects and those with no basal pS6, the ORR was 8/22 (36%, 6 CR, 1 CRp, 1 PR, 14 NR).

Sirolimus plus MEC is a tolerable and active regimen for patients with high risk AML. The addition of an mTOR inhibitor augmented chemotherapy response particularly among those with demonstrable baseline mTOR activation and target inhibition during therapy. These results demonstrate the diversity of AML with reference to the activation of ribosomal S6 and suggest that phospho-flow monitoring may be an effective tool for patient selection for use of signaling inhibitors in AML. Future trials of this regimen may benefit from enrichment for subjects with mTOR activation and/or rapamycin sensitivity assessment.

Off Label Use: Rapamycin. FDA approved for solid organ transplant. Investigational use for treatment of leukemia. Weiss:Celgene: Consultancy.