Erythropoietin
Erythropoietin (EPO) is a 30-kD glycoprotein that regulates red cell production by binding to an erythroid progenitor cell surface receptor [58]. EPO is used widely for treating anemia of critical illness or anemia induced by chemotherapy [58]. In the human, EPO is produced by peritubular cells in the kidneys of the adult and in hepatocytes in the fetus. EPO is also a multifunctional tissue-protecting agent that exerts antiapoptotic, antiinflammatory, antioxidative, angiogenic, and neurotrophic effects [59]. EPO showed promise as a neuroprotective agent in animal models of TBI [60]. However, in a recent randomized clinical trial of 200 patients (EPO, n = 102; placebo, n = 98) with severe closed head injury enrolled within 6 hr of injury, the administration of EPO failed to improve favorable outcomes by 20% at 6 months [61]. Of note, most preclinical studies employed the moderate controlled cortical impact injury (an open head injury model) TBI to investigate the effect of EPO on lesion reduction, cell death and functional recovery [60]. These limited preclinical results might be inadequate for predicting the treatment response in patients with severe closed head injury. Currently, there are several clinical trials of EPO in TBI to determine the effect of EPO on secondary brain injury and functional recovery (www.clinicaltrials.gov, NCT00987454), and on numbers of circulating endothelial progenitor cells in patients with persistent symptoms during the subacute period after TBI (NCT02148367, NCT02226848).
Carbamylated EPO, an EPO derivative without hematopoietic activity (i.e., without effects on hematocrit and lack of risk for thrombosis), has been shown to improve functional recovery in animal models of TBI [62-64]. The tissue-protective activities of EPO can be mimicked by a small, nonerythropoietic peptide pyroglutamate helix B surface peptide (pHBSP, also known as ARA 290), which inhibits inflammation and improves cognitive function in rats following mild TBI complicated by hemorrhagic shock [65]. pHBSP is safe and effective in the treatment of the neuropathic symptoms of sarcoidosis [66] and type 2 diabetes [67]. pHBSP may hold a therapeutic potential for treatment of TBI because there is lower risk of thrombotic adverse effects
EPO stimulates the maturation, differentiation and survival of hematopoietic progenitor cells to maintain erythropoiesis and has been widely used for treatment of anemia. Although low levels of EPO and EPO receptors exist in normal adult brains, increased expression of EPO and the EPO receptors is found in neurons, neural progenitor cells, glial cells, and endothelial cells in response to injury (Grasso et al., 2004). EPO (5000 U/kg ip) was demonstrated to cross the blood-brain barrier and to protect against brain injury in rats (Brines et al., 2000; Wang et al., 2004). Acute EPO administration (within 6-h post-TBI) provides neuroprotection (that is, decreased lesion volume and cell loss) as well as enhances neurogenesis, and subsequently improves sensorimotor and spatial learning functions in rat and mouse models (Brines et al., 2000; Cherian et al., 2007; Xiong et al., 2008; Zhang et al., 2009b). Delayed administration of EPO (5000 U/kg, ip for 14 days) from day 1 following TBI in rats significantly increased dentate gyrus neurogenesis and improved spatial memory (Lu et al., 2005). Post-TBI treatment (6-h or 24-h post-injury) with EPO (5000 U/kg) significantly increased the expression of BDNF and improved spatial learning following injury in rats (Mahmood et al., 2007b). Our recent studies demonstrate that a multiple-dose treatment with EPO (5000 U/kg/day for 3 days initiated at day 1 post-injury) is more effective than a single-dose EPO therapy in improving functional recovery in rats after TBI (Xiong et al., 2010a). Treatment with EPO also contributes to neurovascular remodeling, leading to improved neurobehavioral outcomes following TBI (Xiong et al., 2010a; Zhang et al., 2009b). EPO enhances VEGF secretion from neural progenitor cells; the treatment of such cells with EPO leads to the upregulation of VEGF receptor 2 expression in cerebral endothelial cells, promoting angiogenesis (Wang et al., 2008).
Our previous study showed that delayed (24 h post injury) EPO treatment improves neurological functional recovery without reducing lesion volume after TBI (Xiong et al., 2010a). In addition to its effects on neurogenesis and angiogenesis, EPO may improve neurological recovery partially through enhancement of axonal plasticity. Axonal sprouting from the intact corticospinal tract was increased in the denervated side of the gray matter of both cervical and lumbar levels of the spinal cord at day 35 after TBI. However, the corticospinal tract axonal sprouting was significantly enhanced at both cervical and lumbar spinal cord in the EPO-treated TBI animals (Zhang et al., 2010). The contralesional corticospinal tract axonal sprouting was highly and positively correlated with sensorimotor recovery after TBI, suggesting axonal sprouting induced by EPO treatment may contribute to functional recovery after TBI.
In a small clinical trial for treating stroke with EPO, intravenous administration of EPO is well tolerated in acute ischemic stroke and associated with an improvement in clinical outcome at one month (Ehrenreich et al., 2002). However, in a recent large clinical trial of stroke, EPO treatment has not resulted in benefits (increasing higher mortality compared to placebo controls) (Ehrenreich et al., 2009). Combination of tissue plasminogen activator with EPO may be one of the important factors for this failed clinical trial, because a very large number of EPO-treated stroke patients also received tissue plasminogen activator treatment and combination of EPO with tissue plasminogen activator has been demonstrated to cause detrimental effects in animal models of stroke (Jia et al., 2010).
A phase III trial of EPO in patients with TBI has been planned (ClinicalTrials.gov, NCT00987454). A phase II trial investigating the safety of treatment with Darbepoetin Alfa (a long-acting form of EPO) in patients with severe TBI is ongoing (ClinicalTrials.gov, NCT00375869). A phase II/III trial to investigate the early administration of EPO to TBI patients is ongoing (ClinicalTrials.gov, NCT00260052). The high doses of EPO used for the treatment of stroke and TBI significantly increased hematocrit (Mahmood et al., 2007b; Xiong et al., 2008), which may cause adverse vascular effects such as deep venous thrombosis (Lapchak, 2008). However, non-hematopoietic EPO analogs, such as the carbamylated form of EPO (CEPO), are as effective as hematopoietic EPO in neuroprotection and are not associated with the hematopoietic side effects (Lapchak, 2008; Mahmood et al., 2007b), indicating their potential application to TBI therapy. The optimal EPO dose, dosing interval, and number of doses for reducing brain injury that promote neurorestoration and improve functional recovery have not been fully investigated after TBI. The EPO doses for the TBI clinical trials are based on stroke trials. Lack of preclinical data on these important aspects highlights the importance of fully evaluating EPO and its analogs for both acute protection and chronic restoration of function after TBI