Staphylokinase, a bacterial
plasminogen activator, is a potent, highly
fibrin-specific but antigenic
thrombolytic agent in humans. In an effort to attenuate the antigenicity of wild-type
staphylokinase (
SakSTAR variant), 2 of its 3
immunodominant epitopes were altered by substituting clusters of 2 or 3 charged
amino acids with
alanine, yielding the mutant
SakSTAR.M38 (K35A, E38A, K74A, E75A, R77A), which was less antigenic in inbred New Zealand White rabbits. In the present study, groups of 6 baboons (Papio hamadryas) were randomized to
SakSTAR (group 1) or
SakSTAR.M38 (group 2). The thrombolytic potencies of 50 micrograms/kg compound at baseline, assessed in an extracorporeal
thrombosis model, were similar: 77 +/- 2.9% (mean +/- SEM) clot lysis in group 1 and 83 +/- 3.6% in group 2. Groups 1 and 2 were immunized subcutaneously at 2, 3, and 5 weeks with 500 micrograms
SakSTAR or
SakSTAR.M38, respectively. From 6 weeks, group 1 developed significantly more antibody-related neutralizing activity than group 2 (maximal titer at 8 weeks of 100 +/- 23 micrograms
SakSTAR and of 22 +/- 7.1 micrograms
SakSTAR.M38 neutralized per milliliter of plasma, respectively). Neutralizing activities subsequently decreased gradually to 10-20% of peak values at 18 weeks. At 6 weeks, both groups were resistant to thrombolysis with 50 micrograms/kg of either compound. Rechallenge at 18 weeks with 250 micrograms/kg of the immunizing compound showed a significantly better recovery of the thrombolytic potency of
SakSTAR.M38 (68 +/- 4.5% clot lysis) than of
SakSTAR (39 +/- 5.3% clot lysis). Neither agent degraded
fibrinogen or depleted alpha 2-antiplasmin. Therefore,
SakSTAR.M38 is comparably active and
fibrin-specific but less antigenic than wild-type
SakSTAR. These findings in outbred primates confirm and extend earlier observations in inbred rabbits and provide a basis for the further development of
staphylokinase variants with reduced antigenicity in humans.