Shedding light on endomitosis.

Zhang et al have taken this transla-tional medicine approach full circle by assessing the capacity of A11 to clear arterial throm-bus in 2 murine stroke models. Although A11 showed considerable promise in these studies, restoring blood flow, for example, when administered 2 hours after carotid artery occlu-sion, the most dramatic and encouraging findings were obtained with a second-generation derivative of A11 (see figure). This variant, termed SLK (single-chain linked first kringle), was produced by coupling the A11 scFv fragment through a linker pep-tide to the first kringle domain of plasminogen. The rationale, experimentally confirmed in this study, was that the first kringle domain of plasminogen would allow SLK to home to, and covalently bond with, terminal lysine residues within partially degraded fibrin associated with the platelet thrombus, similar to the approach recently used to create Delta-plasmin 8 and serving to concentrate the A11 scFv component directly at the platelet thrombus where platelet lysis would then occur. Critically, SLK behaves in mice with many of the advantages that would be hoped for in an optimal therapeutic thrombolytic. First, SLK could reproducibly lyse thrombus and restore blood flow when administered 4 hours after cessation of blood flow or induction of stroke, whereas A11 was ineffective at this time period after arterial occlusion. SLK showed partial protection against infarct (34% reduction) and significantly preserved neurologic function (as measured by postural reflex), without evidence of cerebral bleeding. In contrast, in the same mu-rine model, one-third of mice treated with tissue plasminogen activator died of intracranial hemorrhage. 9 Second, SLK caused minimal throm-bocytopenia (ϳ11% reduction in platelet count at the 4-hour postadministration nadir), with restoration of normal count at 24 hours. Third, SLK bound preferentially to activated platelets and lysed platelets by the same functional mechanism as A11. Fourth, SLK had no affect on platelet aggregation in vitro. Importantly, ex vivo analysis of blood platelets taken from mice 4 hours after SLK administration showed no evidence of platelet activation and did show normal collagen and thrombin agonist responses in terms of P-selectin expression and formation of platelet-monocyte aggregates. Fifth, SLK had no effect on bleeding time. Finally, as a human scFv an-tibody construct, SLK would be less likely to act as a foreign antigen and would be unlikely to activate complement or invoke an inflammatory phagocytic cytokine response because it lacks an Fc domain. The current study did not address SLK's use as a thrombolytic …