Pilot Phase I Clinical Trial of Methioninase on High-Stage Cancer Patients: Rapid Depletion of Circulating Methionine

Methionine (MET) has been shown to be a tumor-selective therapeutic target for cancer, since cancer cells require higher amounts of MET to divide and survive than normal cells. This phenomena is known as MET dependence and is probably due to MET overuse by cancer cells. A pilot clinical trial was initially carried out with non-recombinant METase (METase) produced from Pseudomonas putida and subsequently highly purified. No acute clinical toxicity was observed for any criteria measured in the three patients. The depletion of serum MET started within 30 min of the infusion and was maintained for 4 h after the infusion was completed in patient 1 and patient 2. The lowest serum MET levels were 35% and 19% of the pretreatment level, respectively, in patient 1 and patient 2. Patient 3 received a 10 h i.v. infusion of METase without any sign of side effects. MET was depleted over 200-fold from 23.1 to 0.1 μM by the 10-h infusion of patient 3. No clinical toxicity was observed in any criteria measured in patient 3. Subsequently, another pilot Phase I clinical trial was carried out of serum MET depletion in cancer patients by recombinant METase (rMETase) cloned from Pseudomonas putida and produced in E. coli. Patients with advanced breast cancer, lung cancer, renal cancer, and lymphoma were given a single rMETase treatment at doses ranging from 5000 to 20,000 units by i.v. infusion over 6-24 h. No clinical toxicity was observed in any patient after rMETase treatment. rMETase levels were measured at 0.1 to 0.4 units per ml of serum in the patients which correspond to therapeutic levels in vitro. The lowest serum MET levels in rMETase-treated patients were 0.1% of the pretreatment levels corresponding to approximately 0.1 μM, which also correlates to therapeutic levels in vitro as well as in vivo. The results of the METase and rMETase pilot Phase I clinical trials therefore indicate that i.v. infusion of rMETase is safe and effectively depletes its biochemical target of serum MET, suggesting potential efficacy in future clinical trials.