3.1.1. Liposomes as Delivery Platforms

Liposomes are the first generation of nanocarriers for drug delivery used in academia and pharmaceutical industries [71]. Liposomes are made of biocompatible and biodegradable molecules that can protect drugs from enzymatic degradation and prevent drug clearance in blood.

Ishii and co-workers developed asialo-erythropoietin (AEPO)-modified PEGylated liposomes (AEPO-liposomes) to treat cerebral I/R injury [48]. Asialo-EPO (AEPO) is a neuroprotective agent that binds to EPO receptor (EPOR) on neuronal cells and activates MAPK and PI3K/Akt pathways for improved outcome of cerebral stroke. The results showed that AEPO-liposomes accumulated in the ischemic lesion for more than 24 h after injection, decreased infarct lesions, reduced the neuronal apoptosis, and ameliorated cerebral I/R injury in rats.

ZL006 is another neuroprotectant that can selectively block the ischemia-induced binding between nNOS (neuronal nitric oxide synthase) and PSD-95 (a scaffolding protein) to prevent ischemic damage [72]. However, this drug is limited to cross the BBB. To solve this problem, HAIYPRH (T7), a peptide for targeting transferrin receptor (TfR), was conjugated to PEGylated liposomes (T7-P-LPs) loaded with ZL006 (T7-P-LPs/ZL006), and the liposomes mediated the transport of ZL006 across BBB in the ischemic stroke model [49]. The results in ex vivo fluorescence imaging and particle biodistribution studies suggested that T7 enhanced the transport of liposomes across the BBB. T7-PLPs/ZL006 also significantly reduced infarct volume and prevented neurological deficit in the rat ischemic stroke model compared to free ZL006. The same group developed another dual targeting delivery system using T7 peptide and stroke homing peptide (SHp, CLEVSRKNC)-conjugated liposome (T7&SHp-P-LPs/ZL006) and demonstrated that this system can penetrate BBB to deliver ZL006 to the ischemia brain tissue. Administration of T7&SHp-P-LPs/ZL006 decreased the infarction sizes in the brain and ameliorated neurological damage [50].

Basic fibroblast growth factor (bFGF) has demonstrated the neuroprotective effect in acute stroke; however, bFGF encounters the same issue that it cannot cross the BBB. To overcome this problem, liposomes were developed to deliver bFGF (bFGF-NL) to the brain in a rat model of cerebral I/R. After treatment with bFGF-NL, in three consecutive days, mice showed improved neurological function, reduced infarct volume, and decreased nuclear fragmentation of neurons. The therapeutic effect is associated with PI3K/Akt activation by bFGF-NL [51].

To address the problem of narrow therapeutic time window of tPA (4.5 h), Fukuta et al. combined tPA and liposomal fasudil (fasudil-Lip) and examined whether they can increase the benefit of tPA to treat cerebral I/R. The results showed that the combined therapy increased neuroprotective effects compared to the administration of tPA or fasudil alone. In addition, the study indicated that administration of fasudil-Lip decreased the cerebral hemorrhage and extended the therapeutic time window of tPA compared to treatment with tPA [52].

To determine when drugs should be administered after reperfusion surgery, Al-Ahmady et al. selected a liposomal formulation made from HSPC:CHOL:DSPE-PEG2000 because PEGylated-liposomes have a long blood circulation. In the study, liposomes were intravenously administered at 0.5 or 48 h after reperfusion. Transmission electronic microscopy, histological, and immunostaining were performed, and the results indicated a higher accumulation of liposomes in cerebral ischemic mice than that in healthy mice. The possible mechanism was that the increased accumulation of liposomes in brain in the early phase (0.5 h) was due to transcellular transport. However, at 48 h after stroke, the liposome deposition was associated with both transcellular and paracellular pathways [53]. This is an interesting study, but it is needed to investigate the time course of liposome accumulation in ischemic stroke tissues to determine the therapeutic delivery time to improve the treatment of stroke.
