Repositioning drugs for traumatic brain injury - N-acetyl cysteine and Phenserine

Abstract

Traumatic brain injury (TBI) is one of the most common causes of morbidity and mortality of both young adults of less than 45 years of age and the elderly, and contributes to about 30% of all injury deaths in the United States of America. Whereas there has been a significant improvement in our understanding of the mechanism that underpin the primary and secondary stages of damage associated with a TBI incident, to date however, this knowledge has not translated into the development of effective new pharmacological TBI treatment strategies. Prior experimental and clinical studies of drugs working via a single mechanism only may have failed to address the full range of pathologies that lead to the neuronal loss and cognitive impairment evident in TBI and other disorders. The present review focuses on two drugs with the potential to benefit multiple pathways considered important in TBI. Notably, both agents have already been developed into human studies for other conditions, and thus have the potential to be rapidly repositioned as TBI therapies. The first is N-acetyl cysteine (NAC) that is currently used in over the counter medications for its anti-inflammatory properties. The second is (-)-phenserine ((-)-Phen) that was originally developed as an experimental Alzheimer's disease (AD) drug. We briefly review background information about TBI and subsequently review literature suggesting that NAC and (-)-Phen may be useful therapeutic approaches for TBI, for which there are no currently approved drugs.

Keywords: N-acetyl cysteine; Phenserine; Traumatic brain injury.

Fig. 1

a Structure N-acetyl cysteine. b Post injury administration of NAC (50 mg/kg daily for 4 days) significantly improves MWM performance. MWM performance as measured by latency to reach the goal platform was compared between groups: TBI, TBI-NAC, and Sham. Both Sham and TBI-NAC groups have significantly shorter latencies to reach the goal platform as compared to the TBI group. Additionally, treatment with NAC after TBI improved performance in the MWM that reached sham levels. Data are presented as the mean ± SEM. *p < .05, ***p ≤ .001, sham relative to TBI. † p < .05 TBI-NAC relative to TBI. c Number of times animals crossed the within a 7.5 cm radius of the platform border during the probe trial. A one-way ANOVA showed significant differences between groups. Fisher’s LSD post hoc showed that sham and TBI-NAC (50 mg/kg daily for 4 days) had significantly better retention of the platform location as compared to TBI alone. Data are presented as the mean ± SEM. Brackets indicate comparisons between groups. *p < 0.05, **p < 0.01

Fig. 2

Number of clinical symptoms at seven days

Fig. 3

(−)-Phen is primarily metabolized by N-dethylation to yield (−)-N1- and (−)-N8-norphenserine that then are further N-dethylated to (−)-N1,N8-bisnorphenserine. Their brain/plasma ratios under steady-state conditions are shown

Fig. 4

Human peripheral blood mononuclear cells (PBMCs) were isolated, from human blood, then cultured in the presence and absence of (−)-Phen for 24 h, and were then challenged with phytohemaggultanin (PHA: 3 μg/ml; Sigma-Aldrich) to induce inflammation and cytokine production. The detection limit for these assays is <1 pg/ml for IL-1β and <3 pg/ml for IL-10. The intra-and interassay CV was <10%. All results are expressed in pg/ml [68]

Fig. 5

Cultured hippocampal neurons in triplicate were prepared from 18 to 20 day rat (Sprague Dawley) embryos and were cultured for 7 days. They were treated with (−)-Phen (5 uM) followed by addition of an excitotoxic concentration of glutamate (50 uM). Neuronal viability was assessed 24 h after addition of glutamate (MTS assay). The results are plotted as percent neuronal survival ± SEM [58]

Fig. 6

Human SH-SY5Y were treated with and without (−)-Phen and challenged with oxidative stress (H2O2: 100 uM). Cell viability was quantified by MTS assay at 24 h. * designates comparisons with cells challenged with H2O2 (*p < 0.05, N ≥ 4 per group). Viability with (−)-Phen (30 uM) treatment was no different from control unchallenged cells

Fig. 7

a and b (−)-Phen (2.5 mg/kg, i.p.) was administered for 21 consecutive days to rats and mice. Animals were killed within 2 h of their final (−)-Phen dose or saline; a brain (cortex) sample was taken and immediately frozen to –70oC and thereafter analysed for Aβ by ELISA. (−)-Phen significantly (p < 0.05) lowered APP, Aβ (1-40 and 1-42) levels vs. controls. This decline, particularly in Aβ42 levels, was likewise found in wild type mice dosed with (−)-Phen (2.5 mg/kg and 7.5 mg/kg, i.p.) for 21 days [80]

Fig. 8

Administration of (−)-Phen to humans, by gradually escalating the dose to achieve 15 mg BID resulted in a decline (~20%) in levels of Aβ42 evaluated in the plasma time-dependently following the final (−)-Phen dose. This reduction coincided with the achieval of peak plasma (−)-Phen concentrations and suggests that the maintenance of long-term steady-state levels of (−)-Phen by slow-release formulations could provide a sustained lowering of Aβ42 in humans [83]. Phenserine tartrate was administered orally to healthy volunteers. Days 1-28: 10 mg BID; Days 29-34: 15 BID; Day 35: 15 mg. Blood samples were drawn on Days 1, 28, 29 and 35. Plasma samples were analyzed for Aβ1-42 using a sandwich ELISA. Plasma phenserine concentrations were determined by LC/MS/MS

Fig. 9

Primary SVZ progenitors cells were isolated from the lateral and medial ganglionic eminence of mouse embryos at embryonic day E13.5, and following trituration to a single cell suspension were grown as neurospheres for day 6 or 7 in vitro in the presence and absence of (−)-Phen analogs (0.01 μM) – which increased cell survival [78, 104]

Fig. 10

a and b. A time-dependent plasma AChE inhibition achieved by (−)-Phen in anesthetized rats following a single dose, in which cholinesterase inhibition was achieved by the combined action of (−)-Phen and its primary metabolites. (−)-Phen and active metabolites readily enter the brain (see Fig. 3), and thereby induce brain AChE inhibition and elevate acetylcholine levels [110]. By contrast, (−)-Physostigmine at a higher dose achieves lower plasma AChE inhibition, has less brain uptake than (−)-Phen, is short-lived in vivo, and is associated with greater adverses actions [72]. b: Time-dependent plasma AChE inhibition and predicted brain pharmacokinetics of (−)-Phen and primary metabolites in humans after single acute dosing

Fig. 11

mild TBI mice demonstrate a deficit in visual memory compared with control uninjured (Sham) animals (**p < 0.01), in which (−)-Phen administration significantly ameliorated (at both doses **p < 0.01 vs. mTBI alone) [58]. A washout period of 2 days before cognitive evaluation ensured no confound in relation to any direct action to improve cognition. These data are thus interpreted as evidence for an effect of Phen against post injury pathology allowing reduced cognitive deficits in (−)-Phen treated animals (mTBI: mild TBI, Phen: (−)-Phen)

Fig. 12

Y-Maze was assessed two days after (−)-Phen washout, evaluating two clinically translatable doses (2.5 and 5.0 mg/kg BID × 5 days) initiated post mTBI. Whereas mTBI challenged mice demonstrate a significant deficit in spatial memory vs. control uninjured (Sham) animals (**p < 0.01). (−)-Phen administration significantly ameliorated this deficit (##p < 0.01 for 2.5 mg/kg and #p < 0.05 for 5 mg/kg vs. mTBI alone) [58]. Likewise, these data are interpreted as evidence for a positive effect against post injury pathology allowing reduced cognitive deficits in (−)-Phen treated animals (mTBI: mild TBI, Phen: (−)-Phen)