Abstract 4062: Chemotherapy-induced therapeutic expression in stem cell vehicles
Authors: Molotkov Andrei , Doubrovin Mikhail , Ali Alessandra , Mintz Akiva 
Published: September 2020

Abstract
Stem cells are being studies as therapeutic vehicles due to their tumor homing properties that make them ideal to track down and attack infiltrating tumor cells that are not easily accessed. In order for stem cells to exert their therapeutic effect, they are engineered with therapeutic payloads that are typically secreted in the vicinity of the tumor. While many have shown that constitutive expression of a therapeutic by a tumor-homing stem cell can potentially treat a variety of cancers, we and others have focused on designing inducible systems that allow us to control the therapeutic expression and thus potentially limit systemic effects. In past work, we created an ultrasound-inducible stem cell that expressed reporter and/or therapeutic genes under the heat shock protein 70 (HSP70) promoter. In the current work, we hypothesize that we can potentially exploit the HSP70 promoter activation that has been reported after cellular stresses induced by standard-of care-chemotherapy, such as temozolomide, that is used to treat Glioblastoma (GBM), a fatal disease with a median survival of only 15 months after diagnosis. To test our hypothesis, we infected bone marrow derived mesenchymal stem cells (MSCs) with a plasmid that expresses GFP and firefly luciferase under the control of the HSP70 promoter and treated them with 100 or 400 ug/ml of temozolomide. We observed significant induction of GFP reporter 24 hours after treatment. We subsequently implanted our stem cells subcutaneously in mice and treated them with temozolomide. 24 hours after treatment, we observed gene activation using bioluminescent and fluorescent imaging. To further test this gene activation in a disease model, we stereotactically implanted our engineered stem cells along with patient derived (PDX) GBM cells intracranially. 5 days post implantation, mice were treated with a single therapeutic dose of temozolomide (200 mg/kg) and sacrificed 24 hours after treatment. After sectioning the brains and examining under fluorescent microscope, we observed significant GFP induction in the temozolomide treated mice compared to untreated controls. In order to demonstrate a biologic effect of induced therapeutics we performed a preliminary experiment where we intracranially implanted engineered stem cells that expressed HSP70 promotor inducible TNFα, a cytokine that we demonstrated can be delivered by stem cells to treat GBM and open the blood brain barrier (BBB) to further systemic therapies. We subsequently induced with 200 mg/kg of systemically administered temozolomide and observed BBB permeability via optical/CT imaging that demonstrated brain deposition of systemically administered cy7-albumin, in contrast to the untreated control. In conclusion, we successfully demonstrated the potential of engineering stem cells to express proteins/therapeutics after induction by temozolomide, a standard-of-care chemotherapy. We are therefore exploring using this strategy to express therapeutics that act in synergy with the inducing chemotherapy agent.
Citation Format: Andrei Molotkov, Mikhail Doubrovin, Alessandra Ali, Akiva Mintz. Chemotherapy-induced therapeutic expression in stem cell vehicles [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4062.