The number of people suffering from Alzheimer's disease (AD) is steadily rising and current treatments only provide minor symptom amelioration. Results from recent clinical trials targeting amyloid-? (A?) production or clearance were disappointing, prompting a reexamination of approaches to AD treatment. Brains from AD patients exhibit accumulation of ceramide, a signaling molecule and an integral component of exosomal membranes. One major source of ceramide is through the hydrolysis of sphingomyelin catalyzed by neutral sphingomyelinase 2 (nSmase2). Even though transient increases in ceramide through nSMase2 upregulation are part of normal brain functioning, experimental evidence indicates that chronic nSMase2 upregulation results in negative effects including neuroinflammation and oxidative stress. Recent studies implicate nSMase2 in both A? aggregation and tau protein propagation through exosome secretion from glial cells. Moreover, inhibition of exosome synthesis by genetic or pharmacological inhibition of nSMase2 was shown to block A? aggregation and tau propagation in both in vitro and in vivo AD models, thus opening a new avenue for AD therapeutics. Unfortunately, there are no clinically useful nSMase2 inhibitors. Current inhibitors are weak (M-mM) with poor physicochemical properties and/or limited brain penetration. In collaboration with NCATS we carried out a human nSMase2 high throughput screen (HTS) of >350,000 compounds. Filtering and analysis of HTS hits led to discovery of 2,6-dimethoxy-4-(5-phenyl-4-(thiophen-2-yl)-1H-imidazol -2-yl) phenol (DPTIP) the first nM inhibitor (IC50 = 30 nM). DPTIP was found to be selective and capable of dose-dependently inhibiting exosome release in glial cultures. Unfortunately, in vivo DPTIP exhibited rapid clearance resulting in a short half-life (t1/2< 0.5h) and had poor oral bioavailability (F<5%). Structural modifications (~200 analogs synthesized by our group) have not led to substantial improvements. Given its significant clinical potential, we propose to address the pharmacokinetic limitations by utilizing dendrimer nanoparticles to deliver DPTIP selectively to activated glial cells in the brain. Our team discovered that systemically-administered hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers (~4 nm in size) target activated glia in the injured brain, without the need for targeting ligands, showing minimal uptake in healthy brains. While the dendrimers are endocytosed and retained by activated glial cells in the brain maintaining exposure for >2 weeks, they are rapidly cleared from the periphery (plasma t1/2 ~ 6-24 h). We have validated the brain targeting, safety, and efficacy in multiple small and large animal models, and are in Phase 1 clinical trials with our first dendrimer product (D-NAC in childhood cerebral adrenoleukodystrophy). Herein, we propose to synthesize and evaluate the in vivo pharmacokinetics and target engagement of two differently sized dendrimers conjugated to DPTIP (D-DPTIP) following peroral administration. The optimal conjugate assessed by brain imaging, LC/MS bioanalysis, and functional inhibition of glial nSMase2 activity will be tested for efficacy and safety in two established mouse models of AD. We have assembled a highly experienced team with expertise in dendrimer nanoparticles (Rangaramanujam), pharmacokinetics, biomarkers and target engagement studies (Rais) and pharmacology, drug discovery and clinical translation (Slusher).

Public Health Relevance

Pharmacological and genetic evidence in three Alzheimer's disease (AD) mouse models suggests that the enzyme neutral sphingomyelinase 2 (nSMase2) plays a significant role in the etiology of the disease, however there are no inhibitors to test this hypothesis in clinical trials. Utilizing a high- throughput screening campaign we identified DPTIP, the first nM human nSMase inhibitor; however DPTIP has a short half- life (t1/2< 0.5h) and poor oral availability (F<5%) hampering its clinical use, and chemical modifications (~200 analogs synthesized by our group) have not led to substantial improvements. Given its significant clinical potential, we propose to synthesize and evaluate dendrimer nanoparticle conjugated-DPTIP for their ability to deliver DPTIP to brain and provide efficacy and safety in two established mouse models of AD.

National Institute of Health (NIH)
National Institute on Aging (NIA)
Research Project (R01)
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Drug Discovery for the Nervous System Study Section (DDNS)
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Refolo, Lorenzo
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Johns Hopkins University
Schools of Medicine
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