Preparation of polyethylene glycol-tissue plasminogen activator adducts
that retain functional activity: characteristics and behavior in three
animal species
H Berger and SV Pizzo
Department of Pharmacology, Wellcome Research Laboratories, Reserch
Triangle Park, NC 27709.
Conditions were defined for the derivatization of recombinant tissue
plasminogen activator (rt-PA) with polyethylene glycol (PEG) so as to
retain functional activity as a possible means of producing a t-PA species
with a prolonged circulating lifetime. Derivatives with a wide range of
retention of activities were prepared by varying the concentration and
species of activated PEG. The specific activities of the PEG-rt-PA
derivatives were dependent on the method of assay. Assays using preformed
fibrin gave higher estimates of retention of activity than assays using
soluble components. Plasma elimination studies in mice and rats indicated
prolonged circulating lifetimes for the radiolabeled PEG-rt-PA derivatives
after a rapid clearance and distribution phase; however, the disappearance
of functional activity was much more rapid than the disappearance of
radiolabeled material. The PEG-rt-PA derivatives appeared to accumulate in
tissues above their interstitial fluid concentrations and were rapidly
inactivated, apparently by reaction with the plasma protease inhibitors.
These results were consistent with the inactivation of the PEG-rt-PA
derivatives in rat plasma in vitro. A somewhat longer half-life (t1/2) of
the one derivative studied was observed in dogs (t1/2, 16 minutes) as
compared with the rat (t1/2, five minutes). This was sufficient to confer
thrombolytic activity upon the derivative (administered by bolus injection)
in contrast to native rt-PA. The potential of PEG-modified rt-PA as a
long-lived thrombolytic agent in humans will depend, however, on whether
there will be a further extension of the t1/2 because of a reduction in
clearance and/or a reduction in the rate of inactivation.
Volume 71,
Issue 6,
pp. 1641-1647,
06/01/1988
Copyright © 1988 by The American Society of Hematology
