Traumatic brain injury is a heterogenous disorder that is the result of both primary and secondary injury mechanisms. It can vary in severity and is frequently associated with high morbidity in all forms that negatively impacts the health and readiness of individual soldiers. As a result, considerable attention has been given to understanding the complex array of secondary injury mechanisms in order to develop neuroprotective therapies. Although many interventions have been evaluated for this purpose, over 30 phase III prospective clinical trials evaluating various therapies have failed to reach their primary endpoints.21-26 Furthermore, many of these therapies have been evaluated as post-injury interventions. While this may serve civilian TBI patients, where approximately 60-75% of patients present for medical evaluation shortly after injury27,28 evaluation and documentation rates following acute TBI, particularly mild TBI, has historically been low.29 A number of factors unique to military service have been identified as barriers to acute evaluation and care, including various in-theater assessment barriers, exposure to other, concurrent injuries that are often more severe, symptoms that are common to various other diagnoses or attributed to the stresses of military service, and co-existing mental health comorbidities. This is further complicated by a history of underreporting amongst military personal for a disease process that continues to rely on accurate reporting of history and clinical symptoms to make a diagnosis. Suggested hypotheses to explain this underreporting have included concerns amongst affected soldiers that reporting symptoms may result in removal from their units, colleagues and responsibilities, fear of delays return to home and family following deployment, beliefs that existing symptoms are minor and will resolve on their own, fear of stigmatization and a reduced ability to recognize TBI symptoms until return to less structured garrison and civilian lifestyles.30,31,32 Prophylactic neuroprotection for high-risk training and operations would theoretically address some of these issues. In the current study, we demonstrate that CN-105 improves functional outcomes and reduces microglial activation in rat hippocampi when administered prior to TBI. These findings, along with its ease of administration and safety profile, make it an attractive prospect for providing a degree of neuroprotection to soldiers who are deemed by military leadership and military healthcare providers to be at high risk for TBI.
The development of ApoE mimetic neuroprotective therapies was based on initial observations demonstrating that endogenous ApoE played an adaptive role following brain injury by reducing neuroinflammation and secondary neuronal injury.33,34,35 Although the intact ApoE lipoprotein is too large to cross the blood brain barrier and thus cannot serve as a viable therapeutic36, ApoE peptides can be created from the apoE receptor binding region, which is believed to mediate its anti-inflammatory and neuroprotective effects via interactions with the glial LRP-1 receptor.37,38 Importantly, there is convergent evidence that ApoE-based neuroprotectants improve outcomes in a variety of preclinical animal models that recreate the different features of TBI pathology, including closed head injury, parenchymal and subarachnoid hemorrhage, and ischemia (Table 2).
CN-105 was developed to optimize potency and CNS penetration by linearizing the polar surface of the helical receptor binding region of ApoE. CN-105 has been demonstrated to reduce glial activation in vitro, and in vivo and to improve histological and functional outcomes in a number of preclinical models of brain injury. Moreover, CN-105 is stable and can be stored in lyophilized form or in solution. Importantly, phase 1 single and multiple dose escalation studies have demonstrated linear and predictable pharmacokinetic profile, and a favorable toxicity profile both in the Phase 1 and ongoing Phase 2 trials.20 Our current observations demonstrating prophylactic efficacy of CN-105 reducing vestibulomotor deficit was likely a function of adequate blood levels at the time of injury. Of note, the measured half-life in humans ~3.5 hours, is considerably longer than in rodent models (<1.5 hours) (Table 1 and Ref. 20) and increases the feasibility of prophylactic administration in the military setting. This data indicates that prophylactic dosing of CN-105 may be effective in improving functional outcomes and microglial activation in the hippocampus when administered prior to traumatic brain injury. Considering that balance skills have been associated with increased hippocampal volumes59, the reduction in microglial activation may have contributed to the improved vestibulomotor function in CN-105 treated mice. However, the short half-life associated with intravenous dosing in a murine model may limit the prophylactic window required to achieve therapeutic tissue concentrations at the time of injury.
Although CN-105 represents an excellent candidate for clinical translation in the setting of traumatic brain injury, there are several potential limitations, which should be addressed. As a peptide, CN-105 has minimal oral bioavailability, and current clinical trials utilize intravenous administration.16,17,18,20 Although this does not represent a challenge for administration following injury, it would not be optimal for repeated prophylactic administration. To this end, we are exploring minimally or noninvasive routes of delivery, such as intranasal, subcutaneous, or transdermal administration. The mechanism(s) by which CN-105 exerts its neuroprotective effects remains incompletely defined, although convergent data suggests that both apoE and the apoE mimetic peptides exert direct neuroprotective and anti-inflammatory effects via interaction with the LRP-1 receptor, which is present on neurons and glia.37,38 A better understanding of the physicochemical nature of this interaction may allow the rational development of small molecule therapies. Finally, although we demonstrate as proof of principle that prophylactic administration of CN-105 improves recovery and functional outcome after TBI, as long as adequate blood/tissue concentrations are achieved, intraperitoneal administration is not feasible in the clinical setting. Moreover, rodent models are not always ideal for studies of human clinical pharmacokinetics or disease intervention; for example, the half-life of CN-105 in a clinical trial is approximately 3.5 hours, which compares favorably to the much shorter half-life in rodents (Guptill paper) Nevertheless, the positive effects on vestibulomotor function in the study group of this trial are encouraging. With safety and tolerability in humans previously established20, future studies should aim to determine if the neuroprotective effects of CN-105 seen in murine TBI trials can translate into outcome benefits in clinical trials.