TP508 is a synthetically derived tissue repair peptide that has previously demonstrated safety and potential efficacy in phase I/II clinical trials for the treatment of diabetic foot ulcers. Recent studies show that a single injection of TP508 administered 24 h after irradiation significantly increases survival and delays mortality in murine models of acute radiation mortality. Thus, TP508 is being developed as a potential nuclear countermeasure. Because of the short plasma half-life of TP508, we hypothesize that increasing the peptide bioavailability would increase TP508 efficacy or reduce the dosage required for therapeutic effects. We, therefore, evaluated the covalent attachment of various sizes of polyethylene glycol to TP508 at either its N-terminus or at an internal cysteine. A size-dependent increase in TP508 plasma half-life due to PEGylation was observed in blood samples from male CD-1 mice using fluorescently labeled TP508 and PEGylated TP508 derivatives. Biological activity of PEGylated TP508 derivatives was evaluated using a combination of biologically relevant assays for wound closure, angiogenesis, and DNA repair. PEG5k-TP508 enhanced wound closure after irradiation and enhanced angiogenic sprouting in murine aortic ring segments relative to equimolar dosages of TP508 without enhancing circulating half-life. PEG30k-TP508 extended the plasma half-life by approximately 19-fold while also showing enhanced biological activity. Intermediate-sized PEGylated TP508 derivatives had enhanced plasma half-life but were not active in vivo. Thus, increased half-life does not necessarily correlate with increased biological activity. Nevertheless, these results identify two candidates, PEG5k-TP508 and PEG30k-TP508, for potential development as second-generation TP508 injectable drugs.

There is increasing evidence that radiation-induced damage to endothelial cells and loss of endothelial function may contribute to both acute radiation syndromes and long-term effects of whole-body nuclear irradiation. Therefore, several drugs are being developed to mitigate the effects of nuclear radiation, most of these drugs will target and protect or regenerate leukocytes and platelets. Our laboratory has demonstrated that TP508, a 23-amino acid thrombin peptide, activates endothelial cells and stem cells to revascularize and regenerate tissues. We now show that TP508 can mitigate radiation-induced damage to endothelial cells in vitro and in vivo. Our in vitro results demonstrate that human endothelial cells irradiation attenuates nitric oxide (NO) signaling, disrupts tube formation and induces DNA double-strand breaks (DSB). TP508 treatment reverses radiation effects on NO signaling, restores tube formation and accelerates the repair of radiation-induced DSB. The radiation-mitigating effects of TP508 on endothelial cells were also seen in CD-1 mice where systemic injection of TP508 stimulated endothelial cell sprouting from aortic explants after 8 Gy irradiation. Systemic doses of TP508 that mitigated radiation-induced endothelial cell damage, also significantly increased survival of CD-1 mice when injected 24 h after 8.5 Gy exposure. These data suggest that increased survival observed with TP508 treatment may be due to its effects on vascular and microvascular endothelial cells. Our study supports the usage of a regenerative drug such as TP508 to activate endothelial cells as a countermeasure for mitigating the effects of nuclear radiation.