Abstract 2134: Use of methionine gamma-lyase-loaded erythrocytes to induce effective methionine depletion in cancer therapy
Authors: Gay Fabien , Aguera Karine , Senechal Karine , Lorenzi Philip , Scheer Alexander , Horand Françoise , Bourgeaux Vanessa 
Published: July 2017

Abstract
Methionine (Met) dependence is a cancer-specific metabolic defect that has emerged as a target during the last two decades. The use of methionine gamma-lyase (MGL; EC number 4.4.1.11), a bacterial Met-catabolizing enzyme, is a promising strategy for treatment of Met-dependent cancers. However, one challenge is that MGL has a very short half-life (~2 hours), resulting in a short-term Met depletion in vivo. Additionally, its cofactor, pyridoxal 5’-phosphate (PLP) is rapidly eliminated from plasma (Yang et al., 2004). PEGylation extends the MGL half-life in mice to up to 38 hours. Nevertheless, frequent injections are still necessary for maintaining an effective Met depletion over time (Sun et al., 2003). In addition, the low bioavailability of PLP remains a major hurdle due to both scavenging by plasma proteins and very short half-life (&lt; 15 minutes) in the blood stream (Zempleni, 1995). Studies showed that pyridoxine (PN), a vitamin B6 vitamer, can easily cross red blood cells (RBCs) membrane to be rapidly converted to PLP via an enzymatic reactions cascade (Anderson et al., 1971). Considering those particular characteristics of RBCs, we propose that RBC-encapsulated MGL (ERY-MET) will protect the enzyme from degradation/immune reactions and therefore overcome the pharmacodynamic limitations for its use in the treatment of Met-dependent cancers. Pharmacokinetics, pharmacodynamics, and safety parameters of MGL-loaded RBCs were evaluated in healthy mice. In parallel, the role of RBC in PLP biosynthesis from exogenous uptake of PN was investigated in vivo. Finally, we investigated the antitumoral effect of repeated injections of MGL-loaded RBCs combined with daily intragastric administration of PN in two subcutaneous xenografted mouse models for human gastric and glioblastoma tumors. The MGL half-life increased from &lt; 24 hours to ~ 10 days when encapsulated in RBC, with no toxicity reported after one injection of ERY-MET. Following intragastric administration (3.2 mg/kg), PN was rapidly (15 minutes) converted into PLP within the RBC, resulting in an increase of MGL activity (holoenzyme). Combining a weekly single intravenous injection of ERY-MET for 5 days, in association with daily PN supplementation by gavage led to a sustained Met depletion in plasma and induced a 85% inhibition of tumor growth 45 days following implantation of glioblastoma cells. In the gastric mouse model, tumor growth was inhibited by 72% at the same time point (45 days).This study clearly demonstrated that encapsulation of MGL in erythrocytes both strongly improved the half-life and contributed to provide active cofactor. In parallel, repeated injections of ERY-MET were effective against tumor growth in mouse models. Thus, due to the RBC intrinsic characteristics, ERY-MET represents a new promising treatment against a broad scope of cancers that rely on Met metabolism.
Citation Format: Fabien Gay, Karine Aguera, Karine Senechal, Philip Lorenzi, Alexander Scheer, Françoise Horand, Vanessa Bourgeaux. Use of methionine gamma-lyase-loaded erythrocytes to induce effective methionine depletion in cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2134. doi:10.1158/1538-7445.AM2017-2134