All types of cancer cells are addicted to methionine, which is known as the Hoffman effect. Restricting methionine inhibits the growth and proliferation of all tested types of cancer cells, leaving normal cells unaffected. Targeting methionine addiction with methioninase (METase), either alone or in combination with common cancer chemotherapy drugs, has been shown as an effective and safe therapy in various types of cancer cells and animal cancer models. About six years ago, recombinant METase (rMETase) was found to be able to be taken orally as a supplement, resulting in anecdotal positive results in patients with advanced cancer. Currently, there are 8 published clinical studies on METase, including two from the 1990s and six more recent ones. This review focuses on the results of clinical studies on METase-mediated methionine restriction, in particular, on the dosage of oral rMETase taken alone as a supplement or in combination with common chemotherapeutic agents in patients with advanced cancer.

Background. Enzyme prodrug therapy is a promising strategy to treat solid malignancies. The utilization of two-component systems, including an enzyme and a non-toxic prodrug, to generate cytotoxic compounds directly at the surface of the tumor cell can be successful strategy in reducing the overall toxic load on the body.Aim. To determine antitumor activity of the pharmacological pair C115H methionine γ-lyase (C115H MGL) conjugated with daidzein (C115H MGL-Dz) and of S-alk(en)yl-L-cysteine sulfoxides against various types of solid tumors in vitro and in vivo.Materials. MTT-test was used to determine the cytotoxicity of C115H MGL-Dz in the presence of S-alk(en)yl-L-cysteine sulfoxides in vitro against Sw620 (colon cancer), Panc1 (pancreatic cancer), and 22Rv1 (prostate cancer). Apopto- sis induction and cell cycle alteration in 22Rv1, Sw620, and SKBR3 cell lines were studied using the Muse® Caspase-3/7 and Muse® Cell Cycle Assay kits. In vivo anticancer activity was studied on Sw620, Panc1, and 22Rv1 subcutaneous xenografts in Balb/c nude mice.Results. The C115H MGL-Dz had the maximum cytotoxic activity in the presence of S-propyl-L-cysteine sulfoxide (propiin) with IC50 values: 3.88 and 5.4 for Panc1 and 22Rv1, respectively. Dipropyl thiosulfinate formed by the β-eli-mination of propiin catalyzed by C115H MGL-Dz, induces apoptosis through both the activation of caspases and alternative pathways, and also it inhibits cell division, contributing to a decrease in the concentration of cells in the G2/M phase. The anticancer efficacy of the pharmacological pair C115H-Dz/propiin in vivo indicated a significant decrease in Panc1 tumor volume (tumor growth inhibition (TGI) 67.5 %, p = 0.004), Sw620 (TGI 22.07 %, p = 0.12) and 22Rv1 (TGI 70 %, p = 0.043).Conclusion. Pharmacological pair C115H MGL-Dz/propiin was capable of suppressing tumor development in malignant solid tumors and might be considered as a potential anticancer approach in cancer prodrug therapy.