Recombinant Methioninase Lowers the Effective Dose of Regorafenib Against Colon-Cancer Cells: A Strategy for Widespread Clinical Use of a Toxic Drug

The Combination of Methionine Restriction and Docetaxel Synergistically Arrests Androgen-independent Prostate Cancer But Not Normal Cells

Background/Aim

Androgen-independent prostate cancer (AIPC) is resistant to androgen-depletion therapy and is a recalcitrant disease. Docetaxel is the first-line treatment for AIPC, but has limited efficacy and severe side-effects. All cancers are methionine-addicted, which is termed the Hoffman effect. Recombinant methioninase (rMETase) targets methionine addiction. The purpose of the present study was to determine if the combination of docetaxel and rMETase is effective for AIPC.

Materials and Methods

The half-maximal inhibitory concentrations (IC50) of docetaxel and rMETase alone were determined for the human AIPC cell line PC-3 and Hs27 normal human fibroblasts in vitro. The synergistic efficacy for PC-3 and Hs27 using the combination of docetaxel and rMETase at their IC50s for PC-3 was determined.

Results

The IC50 of docetaxel for PC-3 and for Hs27 was 0.72 nM and 0.94 nM, respectively. The IC50 of rMETase for PC-3 and for Hs27 was 0.67 U/ml and 0.76 U/ml, respectively. The combination of docetaxel and rMETase was synergistic for PC-3 but not Hs27 cells.

Conclusion

The combination of a relatively low concentration of docetaxel and rMETase was synergistic and effective for AIPC. The present results also suggest that the effective concentration of docetaxel can be reduced by using rMETase, which may reduce toxicity. The present results also suggest the future clinical potential of the combination of docetaxel and rMETase for AIPC.

The potential of methioninase for cancer treatment

Cancer cells are addicted to L-methionine (L-Met) and have a much greater requirement for L-Met than normal cells due to excess transmethylation, termed the Hoffman effect. By targeting this vulnerability through dietary restriction of L-Met, researchers have been able to achieve promising results in inhibiting tumor growth and eradicating cancer cells. Methioninase (EC 4.4.1.11; METase) catalyzes the transformation of L-Met into α-ketobutyrate, ammonia, and methanethiol. The use of METase was initially limited due to its poor stability in vivo, high immunogenicity, and enzyme-induced inactivating antibodies. These issues could be partially resolved by PEGylation, encapsulation in erythrocytes, and various site-directed mutagenesis. The big breakthrough came when it was discovered that METase is effectively administered orally. The enzyme L-asparaginase is approved by the FDA for treatment of acute lymphoblastic leukemia. METase has more potential as a therapeutic since addiction to L-Met is a general and fundamental hallmark of cancer.

Methionine gamma lyase: Structure-activity relationships and therapeutic applications

Methionine gamma lyase (MGL) is a bacterial and plant enzyme that catalyzes the conversion of methionine in methanthiol, 2-oxobutanoate and ammonia. The enzyme belongs to fold type I of the pyridoxal 5'-dependent family. The catalytic mechanism and the structure of wild type MGL and variants were determined in the presence of the natural substrate as well as of many sulfur-containing derivatives. Structure-function relationship studies were pivotal for MGL exploitation in the treatment of cancer, bacterial infections, and other diseases. MGL administration to cancer cells leads to methionine starvation, thus decreasing cells viability and increasing their vulnerability towards other drugs. In antibiotic therapy, MGL acts by transforming prodrugs in powerful drugs. Numerous strategies have been pursued for the delivering of MGL in vivo to prolong its bioavailability and decrease its immunogenicity. These include conjugation with polyethylene glycol and encapsulation in synthetic or natural vesicles, eventually decorated with tumor targeting molecules, such as the natural phytoestrogens daidzein and genistein. The scientific achievements in studying MGL structure, function and perspective therapeutic applications came from the efforts of many talented scientists, among which late Tatyana Demidkina to whom we dedicate this review.