New pathways in drug discovery for alzheimer’s disease

Molecular drug targets and therapies for Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder that is characterized by normal memory loss and cognitive impairment in humans. Many drug targets and disease-modulating therapies are available for treatment of AD, but none of these are effective enough in reducing problems associated with recognition and memory. Potential drug targets so far reported for AD are β-secretase, Γ-secretase, amyloid beta (Aβ) and Aβ fibrils, glycogen synthase kinase-3 (GSK-3), acyl-coenzyme A: cholesterol acyl-transferase (ACAT) and acetylcholinesterase (AChE). Herbal remedies (antioxidants) and natural metal-chelators have shown a very significant role in reducing the risk of AD, as well as lowering the effect of Aβ in AD patients. Researchers are working in the direction of antisense and stem cell-based therapies for a cure for AD, which mainly depends on the clearance of misfolded protein deposits — including Aβ, tau, and alpha-synuclein. Computational approaches for inhibitor designing, interaction analysis, principal descriptors and an absorption, distribution, metabolism, excretion and toxicity (ADMET) study could speed up the process of drug development with higher efficacy and less chance of failure. This paper reviews the known drugs, drug targets, and existing and future therapies for the treatment of AD.

Experimental inhibition of fibrillogenesis and neurotoxicity by amyloid-beta (Aβ) and other disease-related peptides/proteins by plant extracts and herbal compounds

Amyloid-β (Aβ) fibrillogenesis and associated cyto/neurotoxicity are major pathological events and hallmarks in diseases such as Alzheimer's disease (AD). The understanding of Aβ molecular pathogenesis is currently a pharmacological target for rational drug design and discovery based on reduction of Aβ generation, inhibition of Aβ fibrillogenesis and aggregation, enhancement of Aβ clearance and amelioration of associated cytotoxicity. Molecular mechanisms for other amyloidoses, such as transthyretin amyloidosis, AL-amyloidosis, as well as α-synuclein and prion protein are also pharmacological targets for current drug therapy, design and discovery. We report on natural herbal compounds and extracts that are capable binding to and inhibiting different targets associated with AD and other amyloid-associated diseases, providing a basis for future therapeutic strategies. Many herbal compounds, including curcumin, galantamine, quercetin and other polyphenols, are under active investigation and hold considerable potential for future prophylactic and therapeutic treatment against AD and other neurodegenerative diseases, as well as systemic amyloid diseases. A common emerging theme throughout many studies is the anti-oxidant and anti-inflammatory properties of the compounds or herbal extracts under investigation, within the context of the inhibition of cyto/neurotoxicity and anti-amyloid activity.

Role of biochemical Alzheimer's disease biomarkers as end points in clinical trials

Amyloid-beta (Abeta) peptides, and total and phosphorylated tau are potential biomarkers for use in the development of treatments for Alzheimer's disease. Abeta(1-41) forms extracellular amyloid plaques, while tau and phospho-tau form intracellular neurofibrillary tangles in the brains of Alzheimer's disease patients. Plasma and cerebrospinal fluid concentrations of Abeta decreased following the clinical administration of gamma-secretase inhibitors and increased following the clinical administration of an anti-Abeta antibody. Therapies targeting Abeta decreased tau and phospho-tau concentrations in the cerebrospinal fluid. These biochemical biomarkers appear to be useful to establish therapeutic dosing for Phase III trials. Pivotal registration trials that rely on clinical measures as primary end points can utilize biochemical biomarkers as secondary outcomes indirectly measuring Alzheimer's disease pathology.

Development of semagacestat (LY450139), a functional gamma-secretase inhibitor, for the treatment of Alzheimer's disease

BACKGROUND

Alzheimer's disease is thought to be caused by increased formations of neurotoxic amyloid beta (A beta) peptides, which give rise to the hallmark amyloid plaques. Therefore, pharmacological agents that reduce A beta formation may be of therapeutic benefit.

OBJECTIVE

This paper reviews the pharmacology and chemical efficacy of an A beta-lowering agent, semagacestat (LY450139).

METHODS

A review of the published literature pertaining to semagacestat was obtained using several electronic search engines; unpublished data on file at Eli Lilly and Co. were used as supplementary material.

RESULTS/CONCLUSIONS

Semagacestat treatment lowers plasma, cerebrospinal fluid and brain A beta in a dose-dependent manner in animals and plasma and cerebrospinal fluid A beta in humans, compared with placebo-treated patients. On the basis of extant data, semagacestat seems to be well tolerated, with most adverse events related to its actions on inhibition of peripheral Notch cleavage. Thus far, clinical efficacy has not been detectable because of the short duration of the current trials. Phase III trials with 21 months of active treatment are currently underway.

Monitoring the amyloid beta-peptide in vivo--caveat emptor

As a wave of 'disease modifying' (DM) therapies for Alzheimer's disease (AD) progresses towards the later stages of clinical development, an evaluation of our ability to measure relevant pharmacodynamic effects of such therapies is warranted. Reducing accumulation of amyloid beta (Abeta)-peptide in the brain parenchyma is the primary objective of most current DM approaches. Although a number of methods are available to measure Abeta in blood, cerebrospinal fluid (CSF) and the cerebrum, putative DM-induced changes in the levels of the peptides may not be fully captured, and the reasons for any such changes are not fully understood. Additional candidate biofluid (tau and isoprostanes) and imaging (MRI, FDG-PET) measures may provide alternative supporting evidence of drug activity and subsequent clinical efficacy in patient populations.

Antioxidants have a rapid and long-lasting effect on neuritic abnormalities in APP:PS1 mice

Senile plaques are a major pathological hallmark of Alzheimer's disease (AD). Compelling evidence suggests that senile plaques lead to structural alterations of neuronal processes and that local toxicity may be mediated by increased oxidative stress. Anti-oxidant therapy can alleviate the neuronal abnormalities in APP mice, but the time-course of this beneficial effect is unknown. We used multiphoton microscopy to assess in vivo the characteristics of antioxidant treatment on senile plaques and neurites in AD model mice (APPswe/PS1dE9). We observed that α-phenyl-N-tert-butyl nitrone (PBN), Ginkgo biloba extract (EGb 761) and Trolox had no effect on the size of existing senile plaques. However, all anti-oxidants had a straightening effect on curved neurites. This effect was detected as soon as 4 days after commencing the treatment, and was maintained after 1 month of daily treatment, with no further increase in the effect. The straightening of neurites persisted 15 days after stopping the treatment. These data indicate that neuronal plasticity is fast and still active in adult animals, and suggest that amelioration of the neuritic distortions associated with senile plaques with antioxidants is both rapid and long lasting.

Safety, tolerability, and effects on plasma and cerebrospinal fluid amyloid-beta after inhibition of gamma-secretase

OBJECTIVES

gamma-Secretase inhibitors may be useful as disease-modifying drugs for the treatment of Alzheimer disease. LY450139 is a gamma-secretase inhibitor currently in clinical development, with doses being optimized through the use of biomarkers.

METHODS

To further characterize biomarker responses to LY450139, single oral doses of 60, 100, or 140 mg were administered to volunteers without neuropsychiatric disease. Extensive safety assessments were obtained along with measures of changes in amyloid-beta (Abeta) in plasma and cerebrospinal fluid (CSF). A measure of the change in plasma Abeta1-40 was derived (area above the curve), which was determined by both the magnitude and duration of Abeta1-40 reduction.

RESULTS

A total of 31 subjects (ages 49-53 years, 19 men) were enrolled. With the possible exception of headache, no clinically significant adverse events or laboratory changes were observed. A dose-proportional increase in drug exposure was present in plasma and in CSF. A dose-dependent change in plasma Abeta1-40 area above the curve was also demonstrated. Using the 140-mg dose, a maximum 72.6% reduction in plasma Abeta1-40 was demonstrated that did not return to baseline for more than 12 hours. Cerebrospinal fluid concentrations of Abeta were unchanged 4 hours after drug administration.

CONCLUSIONS

These data show that single doses of LY450139 up to 140 mg are accompanied by a dose-dependent plasma Abeta response. No response in CSF Abeta was apparent 4 hours after dosing.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.