Promyelinating drugs promote functional recovery in an autism spectrum disorder mouse model of Pitt-Hopkins syndrome.

Pitt-Hopkins syndrome is an autism spectrum disorder caused by autosomal dominant mutations in the human transcription factor 4 gene (TCF4). One pathobiological process caused by murine Tcf4 mutation is a cell autonomous reduction in oligodendrocytes and myelination. In this study, we show that the promyelinating compounds, clemastine, sobetirome and Sob-AM2 are effective at restoring myelination defects in a Pitt-Hopkins syndrome mouse model. In vitro, clemastine treatment reduced excess oligodendrocyte precursor cells and normalized oligodendrocyte density. In vivo, 2-week intraperitoneal administration of clemastine also normalized oligodendrocyte precursor cell and oligodendrocyte density in the cortex of Tcf4 mutant mice and appeared to increase the number of axons undergoing myelination, as EM imaging of the corpus callosum showed a significant increase in the proportion of uncompacted myelin and an overall reduction in the g-ratio. Importantly, this treatment paradigm resulted in functional rescue by improving electrophysiology and behaviour. To confirm behavioural rescue was achieved via enhancing myelination, we show that treatment with the thyroid hormone receptor agonist sobetirome or its brain penetrating prodrug Sob-AM2, was also effective at normalizing oligodendrocyte precursor cell and oligodendrocyte densities and behaviour in the Pitt-Hopkins syndrome mouse model. Together, these results provide preclinical evidence that promyelinating therapies may be beneficial in Pitt-Hopkins syndrome and potentially other neurodevelopmental disorders characterized by dysmyelination.

Ester-to-amide rearrangement of ethanolamine-derived prodrugs of sobetirome with increased blood-brain barrier penetration.

Current therapeutic options for treating demyelinating disorders such as multiple sclerosis (MS) do not stimulate myelin repair, thus creating a clinical need for therapeutic agents that address axonal remyelination. Thyroid hormone is known to play an important role in promoting developmental myelination and repair, and CNS permeable thyromimetic agents could offer an increased therapeutic index compared to endogenous thyroid hormone. Sobetirome is a clinical stage thyromimetic that has been shown to have promising activity in preclinical models related to MS and X-linked adrenoleukodystrophy (X-ALD), a genetic disease that involves demyelination. Here we report a new series of sobetirome prodrugs containing ethanolamine-based promoieties that were found to undergo an intramolecular O,N acyl migration to form the pharmacologically relevant amide species. Several of these systemically administered prodrugs deliver more sobetirome to the brain compared to unmodified sobetirome. Pharmacokinetic properties of the parent drug sobetirome and amidoalcohol prodrug 3 are described and prodrug 3 was found to be more potent than sobetirome in target engagement in the brain from systemic dosing.

Structure-Activity Relationships of Central Nervous System Penetration by Fatty Acid Amide Hydrolase (FAAH)-Targeted Thyromimetic Prodrugs.

Thyroid hormone (TH) action is of clinical interest in treating demyelinating diseases of the central nervous system (CNS). Two amide prodrugs of sobetirome, a potent thyroid hormone agonist, were previously shown to significantly improve CNS selective distribution of the parent drug through hydrolysis in the CNS by fatty acid amide hydrolase (FAAH). This concept is elaborated upon here with a series of 29 amide prodrugs targeting FAAH. We identify that conservative aliphatic modifications such as the N-methyl (4), N-ethyl (5), N-fluoroethyl (15), and N-cyclopropyl (18) substantially favor selective CNS distribution of the parent drug in mice. Additionally, lead compounds exhibit moderate to good rates of hydrolysis at FAAH in vitro suggesting both enzymatic and physicochemical properties are important parameters for optimization. Both 4 and 15 were orally bioavailable while retaining appreciable CNS parent drug delivery following an oral dose. The pharmacokinetic parameters of 4 over 24 h postdose (i.v. and p.o.) were determined.

Myelin repair stimulated by CNS-selective thyroid hormone action.

Oligodendrocyte processes wrap axons to form neuroprotective myelin sheaths, and damage to myelin in disorders, such as multiple sclerosis (MS), leads to neurodegeneration and disability. There are currently no approved treatments for MS that stimulate myelin repair. During development, thyroid hormone (TH) promotes myelination through enhancing oligodendrocyte differentiation; however, TH itself is unsuitable as a remyelination therapy due to adverse systemic effects. This problem is overcome with selective TH agonists, sobetirome and a CNS-selective prodrug of sobetirome called Sob-AM2. We show here that TH and sobetirome stimulated remyelination in standard gliotoxin models of demyelination. We then utilized a genetic mouse model of demyelination and remyelination, in which we employed motor function tests, histology, and MRI to demonstrate that chronic treatment with sobetirome or Sob-AM2 leads to significant improvement in both clinical signs and remyelination. In contrast, chronic treatment with TH in this model inhibited the endogenous myelin repair and exacerbated disease. These results support the clinical investigation of selective CNS-penetrating TH agonists, but not TH, for myelin repair.

Block of cyclic nucleotide-gated channels by tetracaine derivatives: role of apolar interactions at two distinct locations.

A series of new tetracaine derivatives was synthesized to explore the effects of hydrophobic character on blockade of cyclic nucleotide-gated (CNG) channels. Increasing the hydrophobicity at either of two positions on the tetracaine scaffold, the tertiary amine or the butyl tail, yields blockers with increased potency. However, shape also plays an important role. While gradual increases in length of the butyl tail lead to increased potency, substitution of the butyl tail with branched alkyl or cyclic groups is deleterious.

A Thyroid Hormone-Based Strategy for Correcting the Biochemical Abnormality in X-Linked Adrenoleukodystrophy.

X-linked adrenoleukodystrophy (X-ALD) is a rare, genetic disorder characterized by adrenal insufficiency and central nervous system (CNS) demyelination. All patients with X-ALD have the biochemical abnormality of elevated blood and tissue levels of very long chain fatty acids (VLCFAs), saturated fatty acids with 24 to 26 carbons. X-ALD results from loss of function mutations in the gene encoding the peroxisomal transporter ABCD1, which is responsible for uptake of VLCFAs into peroxisomes for degradation by oxidation. One proposed therapeutic strategy for genetic complementation of ABCD1 is pharmacologic upregulation of ABCD2, a gene encoding a homologous peroxisomal transporter. Here, we show that thyroid hormone or sobetirome, a clinical-stage selective thyroid hormone receptor agonist, increases cerebral Abcd2 and lowers VLCFAs in blood, peripheral organs, and brains of mice with defective Abcd1. These results support an approach to treating X-ALD that involves a thyromimetic agent that reactivates VLCFA disposal both in the periphery and the CNS.

Targeting Fatty-Acid Amide Hydrolase with Prodrugs for CNS-Selective Therapy.

The blood-brain barrier (BBB) can be a substantial impediment to achieving therapeutic levels of drugs in the CNS. Certain chemical functionality such as the carboxylic acid is a general liability for BBB permeability preventing significant CNS distribution of a drug from a systemic dose. Here, we report a strategy for CNS-selective distribution of the carboxylic acid containing thyromimetic sobetirome using prodrugs targeted to fatty-acid amide hydrolase (FAAH), which is expressed in the brain. Two amide prodrugs of sobetirome were shown to be efficient substrates of FAAH with Vmax/KM values comparable to the natural endocannabinoid FAAH substrate anandamide. In mice, a systemic dose of sobetirome prodrug leads to a substantial ∼60-fold increase in brain distribution (Kp) of sobetirome compared to an equimolar systemic dose of the parent drug. The increased delivery of sobetirome to the brain from the prodrug was diminished by both pharmacological inhibition and genetic deletion of FAAH in vivo. The increased brain exposure of sobetirome arising from the prodrug corresponds to ∼30-fold increased potency in brain target engagement compared to the parent drug. These results suggest that FAAH-targeted prodrugs can considerably increase drug exposure to the CNS with a concomitant decrease in systemic drug levels generating a desirable distribution profile for CNS acting drugs.