Elsevier

Progress in Neurobiology

Volume 113, February 2014, Pages 40-55
Progress in Neurobiology

Allopregnanolone as regenerative therapeutic for Alzheimer's disease: Translational development and clinical promise

https://doi.org/10.1016/j.pneurobio.2013.08.004Get rights and content

Highlights

  • Allopregnanolone is a regenerative therapeutic candidate that promotes neurogenesis and restoration of cognitive function.

  • Endogenous allopregnanolone is important for normal brain development and is reduced with aging and Alzheimer's.

  • Intermittent allopregnanolone reduces Alzheimer's pathology; increases cholesterol homeostasis, white matter generation.

  • Formulation and route of administration allow for fine-tuning of pharmacokinetics and safety profile.

  • Treatment regimen must synergize with systems biology of regeneration and prevention/reduction of Alzheimer's.

Abstract

Herein, we review a translational development plan to advance allopregnanolone to the clinic as a regenerative therapeutic for neurodegenerative diseases, in particular Alzheimer's. Allopregnanolone, an endogenous neurosteroid that declines with age and neurodegenerative disease, was exogenously administered and assessed for safety and efficacy to promote neuro-regeneration, cognitive function and reduction of Alzheimer's pathology. Allopregnanolone-induced neurogenesis correlated with restoration of learning and memory function in a mouse model of Alzheimer's disease and was comparably efficacious in aged normal mice. Critical to success was a dosing and treatment regimen that was consistent with the temporal requirements of systems biology of regeneration in brain. A treatment regimen that adhered to regenerative requirements of brain was also efficacious in reducing Alzheimer's pathology. With an optimized dosing and treatment regimen, chronic allopregnanolone administration promoted neurogenesis, oligodendrogenesis, reduced neuroinflammation and beta-amyloid burden while increasing markers of white matter generation and cholesterol homeostasis. Allopregnanolone meets three of the four drug-like physicochemical properties described by Lipinski's rule that predict the success rate of drugs in development for clinical trials. Pharmacokinetic and pharmacodynamic outcomes, securing GMP material, development of clinically translatable formulations and acquiring regulatory approval are discussed. Investigation of allopregnanolone as a regenerative therapeutic has provided key insights into mechanistic targets for neurogenesis and disease modification, dosing requirements, optimal treatment regimen, route of administration and the appropriate formulation necessary to advance to proof of concept clinical studies to determine efficacy of allopregnanolone as a regenerative and disease modifying therapeutic for Alzheimer's disease.

Introduction

Neurogenic mechanisms in brain are novel therapeutic targets to sustain neurological function and to prevent, delay or treat neurodegenerative diseases such as Alzheimer's and Parkinson's (Brinton, 2013). More than a decade of research has amassed since adult mammalian neurogenesis was detected in human brain (Eriksson et al., 1998) and has been confirmed and extensively studied in preclinical animal models. The seminal study by Eriksson et al. demonstrated unambiguously that the cell-birth marker BrdU indeed labeled adult human neurogenesis. It was recently demonstrated that a significant amount of neurogenesis occurs in the healthy aging human brain measured by a radiometric carbon dating method to birthdate neuronal and nonneuronal genomic DNA during the human lifespan (Spalding et al., 2013). The two most prominent adult neurogenic niches are the hippocampal dentate gyrus subgranular zone (SGZ) (Altman and Das, 1965, Cameron et al., 1993) and subventricular zone (SVZ) of the lateral ventricles (Altman, 1969, Luskin, 1993) reviewed by Liu and Brinton (2010). Although the evidence is less robust, data exist to support additional neurogenic niches in mammals including the cortical layers (evidence for interneurons generated after neocortical development), hypothalamus (Kokoeva et al., 2007), and cerebellum (Lee et al., 2005, Ponti et al., 2008).

Newly born dentate gyrus granule cells of the hippocampal neurogenic niche localize to the granule cell layer in the dentate gyrus and integrate into the adjacent molecular cell layer. As the new neural cells mature, dendrites sprout into the molecular cell layer to receive glutamatergic afferents from the perforant pathway of the entorhinal cortex and their axonal projections form mossy fiber synapses in the CA3 subfield to strengthen neural circuitry and function of the hippocampus. In less-described cortical neurogenesis, newly born interneurons, most destined to be GABAergic, are dispersed throughout the relatively voluminous cortical space. In the neocortex and dentate gyrus, adult neurogenic rates of replacement are similar despite differences in cell type, function, and relative numeric ratios to surrounding mature cells (Cameron and Dayer, 2008). Regenerative potential of these brain regions is possible throughout the lifespan with a neurogenic decline in aged brain (Cameron and McKay, 1999, Kuhn et al., 1996). In the SGZ, immature granule cells reach their highest level within weeks following birth in rodents; early postnatal rodents are roughly equivalent to birth maturity in humans.

Newborn granule cells are present in advanced aged mammalian brains and animal models demonstrate that the new granule cells possess intrinsic neuronal properties required for integration into the neural network. While the significance of functional neurogenesis in human AD remains unknown, new evidence demonstrates that a significant amount of neurogenesis occurs in the healthy aging human brain. Knoth et al. examined a large number of post-mortem brains that covered the lifespan range of 0–100 years. It was reported that human neurogenesis declines with age and doublecortin-positive cells decreased about tenfold from childhood to old age (Knoth et al., 2010) and the numbers roughly parallel the rate of neurogenesis in aging rodent models (Fig. 1). Recently, Spalding et al. used a 14C labeling method that relied on a serendipitous spike in atmospheric 14C levels resulting from nuclear bomb fallout (Spalding et al., 2013). The results of these studies revealed that the age of the 14C-labeled DNA within adult hippocampal neurons indicate that the neurons were born during adulthood. Spalding et al. determined that each year approximately 1.75 percent of the neurons turnover within the self-renewing fraction with only a modest decline during aging. A best-fit scenario model predicted that approximately 35 percent of the hippocampal cells were cycling corresponding to slightly less than the proportion that constitute the entire dentate gyrus region. The authors estimated that the hippocampal dentate gyrus of human brain produces around 700 new neurons per day. At this rate, enough neurons could be replaced in the hippocampus to regenerate the entire hippocampal neurogenic region over the lifespan suggesting that the potential amount of neurogenesis is very significant. Interestingly, the rate of neurogenesis in adult humans was similar by comparison to the rate determined in adult rodents but the rate did not decline as steeply as is known in rodents suggesting that humans may rely on neurogenesis more during the aging process. Compared to developmental periods there are relatively few new granule neurons generated in the advanced aged rodent hippocampus yet the cells are functionally capable of survival and retain normal granule cell morphology and spine density (Morgenstern et al., 2008). Adult born neurons may also be distinctly different and possess higher plasticity and functionality than aged mature neurons. Compared to the perinatal period of rapid brain development, adult neurogenic granule cells mature at a slower rate which could be attributed to less than optimal levels of neurosteroids and other neurotrophic factors. Newly born adult rodent model neurons that survive possess appropriate synaptic density and can form afferent glutamatergic connections (Morgenstern et al., 2008). Newborn dentate granule cells have been shown to functionally integrate into the neural circuitry (van Praag et al., 2002) and to influence hippocampal-dependent processes including spatial pattern recognition (Clelland et al., 2009).

Glutamatergic and cholinergic synaptic inputs penetrate the SGZ of the adult dentate gyrus, yet the adult dentate gyrus SGZ retains an embryonic-like state with nearly exclusive GABAAR chloride channel depolarizing inputs influencing the local microenvironment of progenitors in the neurogenic niche (Tozuka et al., 2005). Mature neurons typically respond to GABA as an inhibitory neurotransmitter. In neural progenitor cells, the chloride ion gradient is reversed and GABA is excitatory to efflux of chloride. Depolarization of the progenitor cell activates molecular events that regulate cell proliferation. Adult-generated granule cells initially receive GABAergic input from local interneurons, isolated from extrinsic excitatory input (Overstreet Wadiche et al., 2005). GABAAR excitation initiates a cascade of events leading to calcium influx in adult neuroprogenitor cells subsequently inducing the accumulation of a neurogenic transcription factor, NeuroD (Tozuka et al., 2005). Upon GABA exposure, Type-2a cells display inward currents in SGZ (Tozuka et al., 2005). Depolarization events promote subsequent transcription factor-activated processes, mitosis, and granule cell maturation in the weeks following cell division (Overstreet-Wadiche et al., 2006).

Section snippets

Allopregnanolone in brain development, aging and Alzheimer's disease (AD)

Humans are naturally exposed to Allo and other neurosteroids throughout life. During the third trimester of human pregnancy endogenous levels of Allo in humans can naturally reach 150–200 nmol/L for both mother and fetus (Fig. 1) (Luisi et al., 2000). The placenta is both a barrier and a major source of Allo and other neurosteroids during gestation. During embryonic development, neuroepithelial cells proliferate in response to neurogenic cues, often from contact with the vascular niche, to form

Allo efficacy in promoting neurogenesis

Our studies have demonstrated a correlation between Allo-induced neural progenitor cell survival and improved memory function in the triple transgenic mouse model of AD (3xTgAD) (Brinton, 2013, Chen et al., 2011, Irwin et al., 2011, Singh et al., 2011, Wang et al., 2010). The 3xTgAD mouse was developed by overexpression of Swedish mutant APP, mutant P301L Tau in the homozygous mutant of presenilin 1 (M146V) knock-in mouse and age-dependently develop cortical and hippocampal tangle-like

Allo in vivo once per week promotes neurogenesis and reduces AD pathology

To further advance efforts to assess the preclinical efficacy of Allo for AD, our group designed long-term treatment studies. The studies were designed to test Allo using the same age of enrollment (3xTgAD male 3-months of age; prior to overt intraneuronal Aβ), efficacious dose of 10 mg/kg via subcutaneous route of administration, matching our previous studies. In addition to neurogenic efficacy, the long-term studies were extended to determine the disease modifying effects afforded by the

Allopregnanolone and Lipinski's rule of 5

Lipinski's rule of five is a mnemonic developed by Christopher A. Lipinski and colleagues for predicting success of a drug by oral route of administration (Lipinski et al., 2001). They found that four criteria with values close to multiples of five were predictive of oral availability. While the rules are based on requirements for oral administration of a drug, they are widely applied as an initial indicator for successful drug development. The rule of five predicts that poor absorption or

Concluding remarks

Preclinical and translational research evidence support therapeutic development of allopregnanolone for neurodegenerative diseases including Alzheimer's. The dose and treatment regimen of Allo must be tailored to a route of administration and pharmacokinetic profile required for neurogenesis and AD pathology reduction. Allo induces both rapid neural progenitor cell cycle gene expression (Wang et al., 2005) and induces key regulators of cholesterol homeostasis (Chen et al., 2011) while reducing

Disclosure statement

Patents pending on allopregnanolone as a therapeutic for mild cognitive impairment and Alzheimer's disease.

Acknowledgments

The work that led to this Review was supported by grants from the NIH National Institute on Aging (U01 AG031115), the Alzheimer Drug Discovery Foundation, the Kenneth T. and Eileen L. Norris Foundation, and The California Institute for Regenerative Medicine (DR2-05410) to R. D. Brinton. The invaluable contributions of M. Rogawski and G. Bauer of the University of California at Davis; J. Wang of the University of Mississippi, and S. Chen of the University of Southern California to our discovery

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