PH pH-sensitive polyethylene glycol-conjugated urokinase nanogels (PEG-UK) is a new form of urokinase (UK) nanogels that could release UK at certain pH values. In our former study, we demonstrated that the pH value in the infarcted brain significantly declined to the level that could trigger the delivery of UK from PEG-UK. Thrombolysis is recommended as the first choice for ischemic stroke within the time window. However, it is common for the patients to miss the thrombolysis time window, which is one of the major causes of bad prognosis from ischemic stroke. It remains promising for seeking therapeutic approaches for ischemic stroke by investigating potential protective reagents delivered out of the usually thrombolysis time window. In this study, the protective effect of administration of PEG-UK outside the usual time window and the underlying mechanisms were investigated. PEG-UK was administrated 2 h and a half after ischemic stroke Delayed administration of PEG-UK significantly ameliorated the severity of neurological deficits of permanent middle cerebral occlusion (pMCAO) rats and reduced the infiltration of inflammatory cells and the concentration of interleukin 1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) in the brain tissues. The content of water and the leakage of Evans Blue (EB) in the PEG-UK group were also decreased. Maintenance of the expression of platelet-derived growth factor-C (PDGF-C) and inhibition of the upregulation of metalloproteinase proteins, low-density lipoprotein receptor-related protein (LRP), nuclear factor kappa B (NF-kappa B) p65 and cyclooxygenase-2 (Cox-2) were observed through western blotting and realtime PCR in the PEG-UK group. Besides, delayed administration of PEG-UK attenuated the up regulation of Caspase8 and Caspase9 and the cleavage of Caspase3 and poly (ADP-ribose) polymerase 1 (PARP1) in ischemic lesion sites. Moreover, PEG-UK treatment also inhibited the upregulation and phosphorylation of N-methyl-D-aspartic acid receptors (NMDARs), which has been revealed to play a vital role in mediating excito-neurotoxicity in ischemic stroke. In conclusion, through the inhibition of LRP/NF-kappa B/Cox-2 pathway, the Caspase cascade and activation of NMDARs, administration of PEG-UK outside the usual time window could still exert protective effects in pMCAO rats through the maintenance of the integrity of BBB and the inhibition of apoptosis and excito-neurotoxicity. (C) 2020 Elsevier Inc. All rights reserved.