Abstracts for the AACR meeting next month.
Number: 1244
Title: Phosphatidylserine-targeting antibody reactivates tumor immunity and destroys tumor vasculature in mice
Presentation
Time: Monday, Apr 08, 2013, 1:00 PM - 5:00 PM
Location: Hall A-C, Poster Section 5
Author
Block: Yi Yin, Xianming Huang, Dan Ye, Philip Thorpe. UT Southwestern Medical Ctr., Dallas, TX
Abstract
Body:
The immunosuppressive lipid, phosphatidylserine (PS), becomes exposed on tumor blood vessels and tumor cells responding to therapy. The
exposed PS contributes to the immunosuppressed tumor microenvironment. Bavituximab is a phosphatidylserine (PS)-targeting antibody that
is being combined with chemotherapy in clinical trials in cancer patients. Here, we tested the hypothesis that innate immunity against cancer
can be elicited by combining chemotherapy with a monoclonal antibody that indirectly binds exposed phosphatidylserine (PS). We used a
murine version of bavituximab, 2aG4, plus docetaxel to treat prostate tumors in mice. Combination treatment markedly retarded tumor
growth, prolonged survival times and delayed progression to androgen-independent disease. Antibody treatment reduced the presence of
myeloid-derived suppressor cells (MDSCs) in the tumors and caused macrophages to repolarize from the immunosuppressive,
proangiogenic M2-like state into a tumoricidal M1-like state. The reactivated macrophages destroyed antibody-coated tumor vasculature
and tumor cells. Addition of 2aG4 to MDSCs in vitro caused them to differentiate into M1-like macrophages and DCs. Thus, PS-targeting
antibodies appear to reactivate innate immunity in tumors resulting in tumor growth inhibition.
Number: 2850
Presentation
Title: Predicting anti-tumor responses to phosphatidylserine targeting antibodies using tumor imaging
Presentation
Time: Tuesday, Apr 09, 2013, 8:00 AM -12:00 PM
Location: Hall A-C, Poster Section 21
Author
Block:
Jian Gong1, Richard Archer1, Van Nguyen1, Christopher C.W. Hughes2, Jeff Hutchins1, Bruce Freimark1. 1Peregrine Pharmaceuticals, Inc.,
Tustin, CA; 2University of California, Irvine, Irvine, CA
Abstract
Body:
Phosphatidylserine (PS) is a phospholipid normally residing in the inner leaflet of the plasma membrane and becomes exposed on tumor
vascular endothelial cells and tumor cells in response to chemotherapy, irradiation and oxidative stresses in the tumor microenvironment.
Binding of antibodies targeting PS on the tumor endothelial cells and tumors induces the recruitment of immune cells and engages the immune
system to destroy tumor vasculature. The antibodies also enhance anti-tumor immunity by blocking the immunosuppressive action of PS. A
chimeric PS-targeting antibody, bavituximab, is being used in combination with chemotherapy to treat patients with solid tumors in Phase II
trials. Fully human antibody PGN635 binds PS through the interaction of beta-2-glycoprotein 1 (ß2GP1) in the same manner as bavituximab.
Using human PC-3 prostate tumor xenografts in SCID mice, we demonstrate that targeting of PS in tumors by PGN635 is enhanced by
chemotherapy. Combination of PGN635 with docetaxel inhibited tumor growth compared to the control IgG plus docetaxel group (p<0.05).
Near-infrared optical imaging of PS in tumors with PGN650, an F(ab')2 antibody fragment of PGN635, showed tumor growth inhibition in
mice treated with docetaxel correlates with PS expression levels in the tumors. Maximal uptake of the PS imaging was observed when
chemotherapy was given 24 hours before the imaging probe.
Number: 4326
Presentation
Title: Phosphatidylserine-targeting ‘betabodies’ for the treatment of cancer
Presentation
Time: Tuesday, Apr 09, 2013, 1:00 PM - 5:00 PM
Location: Hall A-C, Poster Section 36
Author
Block:
Xianming Huang1, Dan Ye1, Troy Luster2, E. Sally Ward1, Philip Thorpe1. 1UT Southwestern Medical Ctr., Dallas, TX; 2Human Genome
Science, Maryland, MD
Abstract
Body:
Bavituximab is a chimeric monoclonal antibody that is being combined with chemotherapy to treat patients with lung or pancreatic cancer in
randomized Phase II clinical trials. Bavituximab targets the immunosuppressive lipid, phosphatidylserine (PS), which becomes exposed on
the outer membrane surface of tumor blood vessels and tumor cells in tumors responding to therapy. The antibody acts by destroying tumor
vasculature and by reactivating tumor immunity. Here, we generated new PS-targeting therapeutics by fusing domains of the PS-binding
plasma protein, mouse ß2-glycoprotein I (ß2GP1), to the Fc region of mouse IgG2a. Such ‘betabodies’ potentially have the following
advantages: they bind directly to PS, and do not require a cofactor protein (ß2GP1) for binding; they can be made fully human; they are
smaller in size (100 KDa); and they have slower blood clearance rates. A construct was generated that bound strongly to PS-expressing
cells and plates, localized to tumor vascular endothelium in vivo, and had a ß-phase blood half-life of approximately five days after
intravenous injection into mice as compared with one day for a murine version of bavituximab, 2aG4. Betabodies could potentially be the
next generation of PS-targeting cancer therapeutics.
Number: 5166
Presentation
Title: Membrane phospholipid asymmetry mediates the selective packaging of protein factors in cancer microparticles
Presentation
Time: Wednesday, Apr 10, 2013, 8:00 AM -12:00 PM
Location: Hall A-C, Poster Section 24
Author
Block: Songwang Hou1, Carol Williams2, Ming Zhao1. 1Northwestern University, Chicago, IL; 2Medical College of Wisconsin, WI
Abstract
Body:
Objective: Growing evidence indicates that cancer cell-derived microparticles (MPs) play important regulatory roles on cellular and system
levels. These activities are attributed to protein factors inside MPs. It has been known that contents in MPs are not a passive representation
of the host cell, but a collection of factors packaged in a selective fashion. In the current study, we hypothesize that membrane phospholipid
dynamics promotes the selective packaging of protein factors by modulating the electrostatic density inside MPs.
Methods: The dynamics of phospholipid species across the membrane bilayer, including phosphatidylethanolamine (PE), phosphatidylserine
(PS) and phosphatidylinositides (PIs) are characterized using specific molecular probes. We used exogenous probes, Duramycin and bovine
lactadherin, to stain the presence of PE and PS, respectively, on the outer surface of MP membrane. Endogenously expressed probes
include GFP-tagged C2 domain of lactadherin (LacC2-GFP), and GFP-tagged 2PH-PLCdelta, are used to assess the relative density of PS
and PI, respectively, in the inner leaflet of MP membranes. We examined the distribution of small Rho GTPases, including RhoA, Cdc42 and
Rac1, using GFP-labeled reporters. To further examine the electrostatic interactions, we used RhoA mutants with the polybasic amino acids
mutated to Gln. Additionally, we constructed a GFP-PBR to validate the electrostatic-dependent packaging mechanism.
Results: PE is externalized to the outer surface of MPs. This is correlated with a greater density of negatively charged phospholipids,
particularly PS, in the inner leaflet. The process is independent of apoptosis or the generation of apoptotic bodies. The redistribution of PE to
the MP surface is correlated with an elevated presence of small GTPases in a PBR-dependent manner. Mutation of PBR abolished RhoA
uptake in MPs. The incorporation of PBR to GFP significantly enhanced GFP localization in MPs. Conclusion: PE externalization to MP
surface likely results in a greater density of negatively charged phospholipids, including PS, and to a lesser extent, PIs. An elevated density of
anionic phospholipids provides an electrostatic attraction for membrane-associated proteins to MPs. This may provide a general mechanism
for the selective sequestration of signal transduction factors in cancer cell-derived MPs.
