Methionine depletion by recombinant
methioninase (rMETase) has been demonstrated previously to be highly effective in
tumor-bearing mouse models. However, the therapeutic potential of rMETase has been limited by its short plasma half-life and immunologic effects, including high antibody production in mice and monkeys and
anaphylactic reactions in monkeys. To overcome these limits of rMETase, the
enzyme has been coupled to
methoxypolyethylene glycol succinimidyl
glutarate (MEGC-PEG-5000). In this study, we evaluated the pharmacokinetics, antigenicity and toxicity of
MEGC-PEG-rMETase in Macaca fascicularis monkeys using an escalating-dose strategy. Dose ranging studies at 1,000, 4,000, and 8,000 units/kg i.v. determined that a single dose of 4,000 units/kg was sufficient to reduce plasma
methionine to <5 micromol/L for 12 hours. Pharmacokinetic analysis with the single 4,000 units/kg dose showed that
MEGC-PEG-rMETase holoenzyme activity was eliminated with a biological half-life of 1.3 hours, and the
MEGC-PEG-rMETase apoenzyme was eliminated with a biological half-life of 90 hours, an approximately 36-fold increase compared with non-PEGylated rMETase. A single dose at 2,000 units/kg of
MEGC-PEG-rMETase resulted in an
apoenzyme half-life of 143 hours. A seven-day i.v. administration of 4,000 units/kg every 12 hours resulted in a steady-state depletion of plasma
methionine to <5 micromol/L. The only manifest toxicity was decreased food intake and slight
weight loss. Red cell values and
hemoglobin declined transiently during treatment but recovered after
cessation of treatment. Subsequent challenges on days 29, 50 and, 71 did not result in any immunologic reactions. This result is in contrast to non-PEGylated rMETase, which elicited
anaphylactic reactions in monkeys. Anti-
MEGC-PEG-rMETase antibodies (at 10(-2)) were found on day 29, and these increased to 10(-3) to 10(4) on day 71, 100 to 1,000-fold less than
antibodies elicited by naked rMETase. Although anti-
MEGC-PEG-rMETase antibodies were produced, no
neutralizing antibody was identified, and each challenge dose was effective in depleting plasma
methionine levels. The results of the present study demonstrate that PEGylation greatly prolongs serum half-life of the rMETase
apoenzyme and eliminated
anaphylactic reactions. The results indicate a profile with respect to serum half-life, toxicity, and antigenicity that suggest clinical potential of
MEGC-PEG-rMETase.