Drug Development

Facilitate clinical trials of the most effective drugs by partnering with biotech firms or pharmaceutical companies to hasten drug development and approval.

Investigations of the Mechanism of Action of TagretinR/Bexarotene on Amyloid Clearance in Transgenic Mouse Models

Funding year(s): 
2012
Funding to date: 
$50,343

Recent studies from the laboratory of Dr. Gary Landreth (Cramer P. et. al (2012) Science 335) have demonstrated that Bexarotene (Targretin), a highly selective, blood-brain barrier-permeant, FDA-approved, RXR agonist for the treatment of cutaneous T-cell lymphoma, can rapidly reduce amyloid plaque burden and rescue behavioral deficits in transgenic mouse models of AD. The proposed mechanism of action is via transcriptional activation of PPARγ:RXR- and LXR:RXR-regulated genes, including ApoE, ABCA1 and ABCG1 expression, that facilitates Aβ clearance and promotes microglial phagocytosis. In support of this proposal, the authors reported that treatment of primary microglia or astrocytes with Bexarotene stimulated the expression of apoE, ABCA1 and ABCG1 and secretion of highly lipidated HDL particles, leading to degradation of soluble Aβ42 in a PPARγ-, LXR-, and apoE-dependent manner.

Most notably, Landreth and colleagues observed the rapid removal of both diffuse and compact Aβ plaques in the cortex and hippocampus of APPswe/PS1DE9 transgenic mice (APP/PS1 mice) after acute treatment with Bexarotene. Targretin or vehicle (H2O) was orally administered in 6-month-old APP/PS1 mice daily for three, seven or 14 days. The authors observed the progressively enhanced expression of apoE, ABCA1 and ABCG1 and elevated HDL levels in both the hippocampus and cortex of Targretin-treated mice, accompanied by a sustained 30 percent reduction in soluble Aβ levels throughout the 14-day treatment period. Insoluble Aβ levels were reduced by 40 percent after 72 hours and progressively decreased over the subsequent 14 days. Total and thioflavin-S+ Aβ plaques were reduced by ~75 percent after seven and 14 days of Bexarotene treatment.

Furthermore, abundant Aβ-laden microglia were observed after three days of Bexarotene treatment, suggesting their involvement in the phagocytic removal of Aβ deposits. To assess whether Targretin could decrease Aβ burden in older animals with greater plaque deposition, 11-month-old APP/PS1 mice were treated with Targretin for seven days and again, there was found significantly reduced levels of soluble and insoluble Aβ40 and Aβ42, a 50 percent reduction in plaque number, and a concurrent increase in expression of apoE, the cholesterol transporters, and HDL levels. Thus, the authors concluded that acute Targretin treatment is efficacious at both early and later stages of pathogenesis in this mouse model.

In view of the significant implications of these landmark findings for the development of novel AD therapeutics, we performed a small pilot study (N=3 each for vehicle and drug) using 8-month-old male APP/PS1 mice that were treated orally with a commercial source of Bexarotene for seven days. We failed to observe any differences in hippocampal or cortical amyloid plaque area or plaque counts between vehicle and bexarotene-treated animals. We confirmed the levels of ABCA1 were elevated in the brains of Bexarotene-treated animals, indicating access of the compound into the CNS. We now propose to extend our preliminary findings by assessing the impact of Targretin on Abeta levels and amyloid pathology in larger cohorts of APP/PS1 and "5X FAD" transgenic mouse models.
 

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Optimization of Novel ACAT Inhibitors for Alzheimer's Disease

Funding year(s): 
2011
Funding to date: 
$150,000

The goal of this project is to test three novel ACAT inhibitors to determine whether they will prevent development of amyloid pathology and alter APP processing in AD mice in order to prevent and treat AD.

High cholesterol is associated with cardiovascular disease and also regulates the production of the toxic Abeta peptide in Alzheimer’s disease (AD). Professor Kovacs previously found that drugs specifically targeting an enzyme of the cholesterol pathway, acyl-coenzyme A: cholesterol acyltransferase (ACAT) inhibitors reduce generation of the toxic Abeta peptides in cells and animal models of AD. Most importantly, she and her team recently used a potentially clinically relevant ACAT inhibitor, CI-1011, in aged mice after abundant amyloid deposition. CI-1011 has previously reached phase III clinical trials for prevention of atherosclerosis (but was later discontinued). In aged mice, CI-1011 dramatically decreased diffuse amyloid, which is most toxic for neuronal function (Huttunen, 2010).

Their data show that ACAT inhibition can reverse existing amyloid pathology, not just prevent novel pathology from forming. ACAT inhibitors are not yet marketed against cardiovascular disease. Dr. Kovacs and associates have recently designed and screened a library of 45 novel ACAT inhibitors, based on the structure of CI-1011. From this screen, they identified three novel ACAT inhibitors that reduce Abeta generation. They will optimize these three inhibitors by screening a novel library of compounds based on their structure and determine whether the novel inhibitors prevent development of amyloid pathology and alter APP processing in AD mice in absence of toxicity. These studies should result in one or more novel ACAT inhibitor with the potential to prevent and treat AD.

Researchers: 

Mechanisms of Retinoid X Receptor-Mediated Abeta Clearance in Alzheimers' Disease

Funding year(s): 
2011
Funding to date: 
$100,000

The goal of this project is to investigate the mechanisms through which RXRs (retinoid x nuclear receptor) promote amyloid clearance from the brain.

There is now persuasive evidence that sporadic, late-onset forms of AD arise from impaired clearance of Abeta from the brain. Moreover, the elevated risk associated with possession of an ApoE4 allele is correlated with a reduced capacity to remove Abeta from the brain. These findings are consistent with the ability of ApoE to facilitate the proteolytic degradation of Abeta, and ApoE4 is less efficient in doing so. These data argue that elevation of ApoE levels in the brain would enhance the physiological mechanisms that aid Abeta clearance. ApoE expression and subsequent lipidation is transcriptionally regulated through the coordinated actions of the nuclear receptors PPARg and LXR. These ligand-activated receptors form obligate heterodimers with RXRs to create a functional transcription factor. Preliminary data demonstrating the oral administration of the RXR-specific agonist, bexarotene, results in the rapid, ApoE-dependent clearance of soluble Abeta in the interstitial fluid within hours. Moreover, bexarotene treatment provokes the loss of more than 60 percent of amyloid plaques from aged APP/PS1 mice within 72 hours, coincident with the appearance of amyloid-laden, phagocytic microglia. This latter effect is postulated to arise from the ability of the nuclear receptors to stimulate the conversion of microglia into M2 or “alternative activation” states associated with suppression of inflammatory gene expression and induction of genes promoting phagocytosis.

Thus, the effects of the activation of RXRs on AD pathology arise from two distinct transcriptional programs that operate in different cell types. The genetic targets of RXR action in the brain are largely unknown. This application represents a collaboration between the laboratories of Drs. David Holtzman, Rudolph Tanzi and Gary Landreth that is focused on investigating the mechanisms through which RXRs promote amyloid clearance from the brain.

Effect of Bexarotene on Abeta in APP Tg Mice Expressing ApoE3 and ApoE4

Funding year(s): 
2011
Funding to date: 
$100,000

The goal of this project is to determine the effects of bexarotene on both Abeta and ApoE metabolism in the presence of human Abeta and human ApoE isoforms (any of two or more functionally similar proteins that have a similar but different amino acid sequence) because it is relevant to potential effects of similar drugs in humans.

A large amount of data strongly suggests the aggregation and deposition of the Abeta peptide in the brain is the initiating event in the pathological cascade known as Alzheimer’s disease (AD). This event appears to initiate a series of events, including exacerbation of Tau-related pathology, direct damage to neurons and synapses and damage to blood vessels from cerebral amyloid angiopathy (CAA). ApoE is the strongest genetic risk factor for AD, and is an important component of amyloid plaques that plays a direct role in determining whether, when and how much Abeta deposits accumulate in the brain (for review, see (Kim et al., 2009a)). The laboratory of Gary Landreth has found that activation of the retinoid X receptor (RXR) pathway, by a drug called bexarotene, strongly alters the levels of several proteins linked to lipid metabolism in the brain, including ApoE, ABCA1 and others. His lab also has found that bexarotene has very strong effects in decreasing soluble Abeta levels over hours as well as clearing amyloid plaques over days.

This finding has important treatment implications for AD. The Landreth lab data shows the effects of bexarotene on Abeta require ApoE; however, the experiments were done in mice that express murine ApoE. While murine ApoE is homologous to human ApoE, it is not identical at the amino acid level and some properties are different. For example, in APP transgenic mice that develop Abeta deposition, murine ApoE is much more amyloidogenic than any of the human ApoE isoforms (Fagan et al., 2002; Holtzman et al., 2000; Holtzman et al., 1999). Thus, it is important to determine the effects on bexarotene on both Abeta and ApoE metabolism in the presence of human Abeta and human ApoE isoforms, as this is more relevant to potential effects of this or similar drugs in humans. In these experiments, they will utilize 5XFAD mice, which express human mutant APP that leads to Abeta deposition beginning at around 4 months of age. The 5X FAD mice express human ApoE3 and human ApoE4. They will assess the effects of bexarotene on interstitial fluid Abeta levels over hours as well as its effects on behavior and Abeta deposition and associated pathology over weeks and months.

Design, Synthesis and Characterization of Novel and Potent Gamma Secretase Modulators: Physiochemical and Pharmacokinetic Properties

Funding year(s): 
2009
Funding to date: 
$200,000

During the past year, our laboratory at UCSD (Wagner laboratory), in close collaboration with the Tanzi laboratory at MGH, discovered, synthesized and characterized (in vitro) a novel series of molecules able to potently prevent the formation of what is currently thought to be the pathogenic culprit of Alzheimer’s disease (AD). Through the use of novel, selective and proprietary chemical substitutions, in conjunction with an arsenal of biochemical screening assays (carried out at both UCSD and MGH), we were able to demonstrate structure activity relationships (SAR) that support these SGSMs as very promising, potent and potentially disease modifying therapeutic agents for AD. The next step in the evaluation of the most potent and with the best aqueous solubility or the most water soluble SGSMs, which would be the focus of year two of the CAF-funded project, will entail a thorough pharmacological or in vivo examination of these molecules in order to identify the best or the “lead” drug candidate.