Abstract LB-291: Translational studies of the dual mTOR-PI3K inhibitor SF1126
Authors: Su Jingdong , Peng Xiaodong , David Ebenezer , Flowers Christopher , Lonial Sagar , Garlich Joseph 
Published: February 2011

SF1126 is in development in phase I clinical trials as a single agent and interim results have been presented recently in patients with solid tumors (ASCO, 2009, abstract 2558) and multiple myeloma (ASH, 2009, abstract 3879). SF1126 inhibits all four class I phosphatidylinositol 3-kinase (PI3K) isoforms along with other cancer targets such as mTOR, DNA-PK, PIM1, and PLK1. The current ongoing work reported herein describes translational studies providing support for expanding single agent clinical trials (B-cell malignancies such as CLL and mantle cell lymphoma) and combination trials such as with bortezomib in multiple myeloma.

Gene expression profiles from myeloma cell lines exposed to SF1126 have identified that HSP-90AA1 is down regulated in common between 3 different myeloma cell lines, while genes for 14-3-3h, cyclin D1, and EIF4BEP1 are commonly downregulated among 2 of the 3 cell lines tested. Genes that are upregulated in common include Bcl-2, Gab2, and CDK2 further supporting the role of cell cycle regulation as a major effect of PI3K inhibition in myeloma cell lines. Additional data on response to SF1126 exposure will be presented in B-cell, T-cell, and mantle cell lymphoma cell lines and primary patient CLL samples. The in vitro combination of SF1126 with dexamethasone (dex) was examined since dex is so commonly used in myeloma patients. Three day proliferation results on RPMI8266 cells (non-responsive to dexamethasone and KRAS mutated) exposed to 5 uM dex plus 5, 10, or 20 uM SF1126 showed a 12, 46, and 78% decrease, respectively, in cell proliferation as measured by the WST staining method versus a no-SF1126-treatment control. The ability of SF1126 to inhibit hematological cancer stem cells was also evaluated. CD138neg cells were isolated from RPMI8226 cell lines as described in the literature (anti-CD138 antibody-conjugated magnetic beads to serially remove CD138+ cells) to give a highly enriched fraction purported to be cancer stem cell ‘like’. SF1126 at 5, 10, or 20 uM demonstrated an 18, 32, and 50% decrease in CD138neg cell proliferation relative to no-treatment controls. These results compared favorably to lenalidomide in the same experiment which showed no decrease in proliferation. These results show for the first time that SF1126 may have activity towards hematological cells characterized as cancer stem cells. In vitro combinations of SF1126 with bortezomib on several myeloma cell lines have previously been shown to exhibit synergistic effects. SF1126 has also previously been shown to have single agent activity in vivo using MM1R (dexamethasone resistant myeloma) xenograft mouse models. The MM1R xenograft mouse model was thus chosen to study SF1126/bortezomib combinations in vivo. Thirty nude mice with similar MM1R tumor sizes were randomly divided into 5 groups of 6 on day 21 post s.c. tumor cell inoculation and treated as follows (all injections were in 100 uL volume administered iv in the tail vein on Tues/Friday 2X/week schedule): 1) saline control; 2) SF1126 at 20 mg/kg; 3) Bortezomib 0.5 mg/kg (1/2 MTD); 4) Bortezomib followed by SF1126 one hour later; 5) SF1126 followed by bortezomib one hour later. Since bortezomib is given iv in the clinic and SF1126 is given as an iv infusion (90 minutes) in the clinic groups 4 and 5 were designed to determine if there is any difference in order of the agents when given in combination. Tumor volume and body weights were assessed through day 42 (3 weeks of treatment). Groups 2, 3, 4, and 5 showed 49, 47, 74, and 85% inhibition of tumor growth relative to the control (group 1). Both of the combination groups 4 and 5 (regardless of order) showed statistical significance compared to control tumor volume (p&lt;0.005). Excised tumor weights showed the same relationship with groups 2, 3, 4, and 5 showing 23, 22, 56, and 66% less tumor weight versus the group 1 control tumor weight with groups 4 and 5 being significant (p&lt;0.01). No treatment related body weight changes were evident and all groups showed increasing body weight gain over the three week treatment period.

A second combination study in MM1R xenograft nude mice was performed but dosing was changed to 2X/week using a two weeks on/one week off schedule followed by two weeks more treatment to mimic the clinical schedule using bortezomib (days 1,4,8,11 of a 21 day cycle) and taking into account the clinical administration of SF1126 (twice weekly with no holidays). Twenty nude mice with similar MM1R tumor size were randomly divided into 4 groups of 5 on day 20 post s.c. tumor cell inoculation and treated as follows (all injections were in 100 uL volume administered iv in the tail vein on a 2X/week schedule): 1) saline control; 2) SF1126 at 20 mg/kg; 3) Bortezomib 0.5 mg/kg (1/2 MTD); 4) SF1126 followed by bortezomib one hour later.

Tumor volume and body weights were assessed through day 52. This treatment course represented 2 weeks on, one week off, then finally 2 weeks on. Groups 2 and 3 (single agents) showed 70 and 52% inhibition of tumor growth respectively with the combination (group 3) showing the most inhibition at 82% inhibition of tumor growth relative to the control (group 1). Excised tumor weights showed the same relationship with groups 2, 3, and 4 showing 59, 35 and 71% less tumor weight versus the group 1 control tumor weight. No treatment related body weight changes were evident and all groups showed increasing body weight gain over the three week treatment period although the control group showed the largest weight gain.

These results indicate in vivo combination effects and help guide the planning of clinical combination studies using SF1126 and bortezomib in multiple myeloma.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-291.