All cancer cell types are methionine-addicted, which is termed the Hoffman effect. Cancer cells, unlike normal cells, cannot survive without large amount of methionine. In general, when methionine is depleted, both normal cells and cancer cells synthesize methionine from homocysteine, but cancer cells consume large amounts of methionine and they cannot survive without exogenous methionine. For this reason, methionine restriction has been shown to be effective against many cancers in vitro and in vivo. Methionine restriction arrests cancer cells in the S/G2-phase of the cell cycle. Cytotoxic agents that act in the S/G2-phase are highly effective when used in combination with methionine restriction due to the cancer cells being trapped in S/G2-phase, unlike normal cells which arrest in G1/G0-phase. Combining methionine restriction and chemotherapeutic drugs for cancer treatment is termed the Hoffman protocol. The efficacy of many cytotoxic agents and molecular-targeted drugs in combination with methionine restriction has been demonstrated. The most effective method of methionine restriction is the administration of recombinant methioninase (rMETase), which degrades methionine. The efficacy of rMETase has been reported in mice and human patients by oral administration. The present review describes studies on anticancer drugs that showed synergistic efficacy in combination with methionine restriction, including rMETase administration. It is proposed that the next disruptive generation of cancer chemotherapy should employ current therapy in combination with methionine restriction for all cancer types.
Keywords: Hoffman effect; Hoffman protocol; Methionine restriction; cancer; chemotherapy; combination therapy; recombinant methioninase (rMETase); review; synergy.
Copyright 2023, International Institute of Anticancer Research.