AIT-082 as a potential neuroprotective and regenerative agent in stroke and central nervous system injury

Cubanes in Medicinal Chemistry

Cubane is a highly strained saturated hydrocarbon system that has historically been of interest in theoretical organic chemistry. More recently it has become a molecule of interest for biological applications due to its inherent stability and limited toxicity. Of greater significance is the ability to potentially functionalize cubane at each of its carbon atoms, providing complex biologically active molecules with unique spatial arrangements for probing active sites. These characteristics have led to an increased use of cubane in pharmaceutically relevant molecules. In this Perspective we describe synthetic methodology for accessing a range of functionalized cubanes and their applications in pharmaceuticals. We also provide some perspectives on challenges and future directions in the advancement of this field.

Development of Precision Small-Molecule Proneurotrophic Therapies for Neurodegenerative Diseases

Age-related neurodegenerative diseases, such as Alzheimer's disease, will represent one of the largest future burdens on worldwide healthcare systems due to the increasing proportion of elderly in our society. As deficiencies in neurotrophins are implicated in the pathogenesis of many age-related neurodegenerative disorders, it is reasonable to consider that global neurotrophin resistance may also become a major healthcare threat. Central nervous system networks are effectively maintained through aging by neuroprotective and neuroplasticity signaling mechanisms which are predominantly controlled by neurotrophin receptor signaling. Neurotrophin receptors are single pass receptor tyrosine kinases that form dimeric structures upon ligand binding to initiate cellular signaling events that control many protective and plasticity-related pathways. Declining functionality of the neurotrophin ligand-receptor system is considered one of the hallmarks of neuropathological aging. Therefore, it is imperative to develop effective therapeutic strategies to contend with this significant issue. While the therapeutic applications of cognate ligands for neurotrophin receptors are limited, the development of nonpeptidergic, small-molecule ligands can overcome these limitations, and productively regulate this important receptor system with beneficial effects. Using our advanced knowledge of the high-dimensionality complexity of receptor systems, the future generation of precision medicines targeting these systems will be an attainable goal.

Effects of Neurotrophic Support and Amyloid-Targeted Combined Therapy on Adult Hippocampal Neurogenesis in a Transgenic Model of Alzheimer's Disease

Although it is recognized that multi-drug therapies may be necessary to combat AD, there is a paucity of preclinical proof of concept studies. We present a combination treatment paradigm, which temporally affects different aspects of Alzheimer's disease (AD)-like pathology, specifically Aβ-toxicity and neurogenesis. At early stages of AD-like pathology, in TgCRND8 mice, we found that combating Aβ pathology with scyllo-inositol ameliorated deficits in neurogenesis. Older TgCRND8 mice with established amyloid load had decreased progenitor cell proliferation and survival compared to non-transgenic mice, regardless of scyllo-inositol treatment. The prolonged exposure to Aβ-pathology leads to deficits in the neurogenic niche, thus targeting Aβ alone is insufficient to rescue neurogenesis. To support the neurogenic niche, we combined scyllo-inositol treatment with leteprinim potassium (neotrofin), the latter of which stimulates neurotrophin expression. We show that the combination treatment of scyllo-inositol and neotrofin enhances neuronal survival and differentiation. We propose this proof of concept combination therapy of targeting Aβ-pathology and neurotrophin deficits as a potential treatment for AD.

Validating Eaton's Hypothesis: Cubane as a Benzene Bioisostere

Pharmaceutical and agrochemical discovery programs are under considerable pressure to meet increasing global demand and thus require constant innovation. Classical hydrocarbon scaffolds have long assisted in bringing new molecules to the market place, but an obvious omission is that of the Platonic solid cubane. Eaton, however, suggested that this molecule has the potential to act as a benzene bioisostere. Herein, we report the validation of Eaton's hypothesis with cubane derivatives of five molecules that are used clinically or as agrochemicals. Two cubane analogues showed increased bioactivity compared to their benzene counterparts whereas two further analogues displayed equal bioactivity, and the fifth one demonstrated only partial efficacy. Ramifications from this study are best realized by reflecting on the number of bioactive molecules that contain a benzene ring. Substitution with the cubane scaffold where possible could revitalize these systems, and thus expedite much needed lead candidate identification.

Role of purinergic receptors in CNS function and neuroprotection

The purinergic receptor family contains some of the most abundant receptors in living organisms. A growing body of evidence indicates that extracellular nucleotides play important roles in the regulation of neuronal and glial functions in the nervous system through purinergic receptors. Nucleotides are released from or leaked through nonexcitable cells and neurons during normal physiological and pathophysiological conditions. Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system (CNS), participate in the synaptic processes, and mediate intercellular communications between neuron and gila and between glia and other glia. Glial cells in the CNS are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of purinergic receptors, which are integral to their activation. Astrocytes release adenosine triphosphate (ATP) as a "gliotransmitter" that allows communication with neurons, the vascular walls of capillaries, oligodendrocytes, and microglia. Oligodendrocytes are myelin-forming cells that construct insulating layers of myelin sheets around axons, and using purinergic receptor signaling for their development and for myelination. Microglia also express many types of purinergic receptors and are known to function as immunocompetent cells in the CNS. ATP and other nucleotides work as "warning molecules" especially by activating microglia in pathophysiological conditions. Studies on purinergic signaling could facilitate the development of novel therapeutic strategies for disorder of the CNS.

Biosensor measurement of purine release from cerebellar cultures and slices

We have previously described an action-potential and Ca²⁺-dependent form of adenosine release in the molecular layer of cerebellar slices. The most likely source of the adenosine is the parallel fibres, the axons of granule cells. Using microelectrode biosensors, we have therefore investigated whether cultured granule cells (from postnatal day 7–8 rats) can release adenosine. Although no purine release could be detected in response to focal electrical stimulation, purine (adenosine, inosine or hypoxanthine) release occurred in response to an increase in extracellular K⁺ concentration from 3 to 25 mM coupled with addition of 1 mM glutamate. The mechanism of purine release was transport from the cytoplasm via an ENT transporter. This process did not require action-potential firing but was Ca²⁺dependent. The major purine released was not adenosine, but was either inosine or hypoxanthine. In order for inosine/hypoxanthine release to occur, cultures had to contain both granule cells and glial cells; neither cellular component was sufficient alone. Using the same stimulus in cerebellar slices (postnatal day 7–25), it was possible to release purines. The release however was not blocked by ENT blockers and there was a shift in the Ca²⁺ dependence during development. This data from cultures and slices further illustrates the complexities of purine release, which is dependent on cellular composition and developmental stage.

Proposal of a guanine-based purinergic system in the mammalian central nervous system

Guanine-based purines have been traditionally studied as modulators of intracellular processes, mainly G-protein activity. However, they also exert several extracellular effects not related to G proteins, including modulation of glutamatergic activity, trophic effects on neural cells, and behavioral effects. In this article, the putative roles of guanine-based purines on the nervous system are reviewed, and we propose a specific guanine-based purinergic system in addition to the well-characterized adenine-based purinergic system. Current evidence suggest that guanine-based purines modulate glutamatergic parameters, such as glutamate uptake by astrocytes and synaptic vesicles, seizures induced by glutamatergic agents, response to ischemia and excitotoxicity, and are able to affect learning, memory and anxiety. Additionally, guanine-based purines have important trophic functions affecting the development, structure, or maintenance of neural cells. Although studies addressing the mechanism of action (receptors and second messenger systems) of guanine-based purines are still insufficient, these findings point to the guanine-based purines (nucleotides and guanosine) as potential new targets for neuroprotection and neuromodulation.

Effects of AIT-082, a purine derivative, on tremor induced by arecoline or oxotremorine in mice

The effects of AIT-082, a hypoxanthine derivative, on tremor in mice were investigated. The mice received intragastric administration of AIT-082 for consecutive 60 days at doses of 150, 300 and 600 mg.kg(-1). The results showed that AIT-082 not only effectively inhibited the tremor induced by arecoline or oxotremorine, but also alleviated the tremor intensity and significantly shortened the tremor durations. The inhibition of tremor was perhaps associated with the central cholinergic nerve depressant effects as well as the stimulation of proliferation and differentiation of nerve cells.

A study of the purine derivative AIT-082 in G93A SOD1 transgenic mice

AIT-082 is a purine derivative with neuroprotective and neurotrophic activity that is desirable in a candidate therapy for Amyotrophic Lateral Sclerosis. Consequently, we investigated the effect of AIT-082 in a transgenic mouse model of ALS. AIT-082 (0, 1, 3, 10, 30, 60, 100 mg/kg) was given to TgN(SOD1-G93A)1Gur transgenic mice from age 30 days until death. The age at onset of clinical signs of disease and the age at death were recorded for each animal. Disease progression was measured by the weekly average distance run in a running wheel. Analysis was made by the Kaplan Meier method with log rank statistics, log rank for trend and Cox regression. Neuropathological study of the brain, spinal cord, muscles and other organs was undertaken at death. In a second experiment we studied the effect of AIT-082 (30 mg/kg) at the onset of disease and during survival of transgenic G93A SOD1 mice, beginning dosing at different ages (20, 30, 40, 60, 80 days). Disease onset was mildly earlier (i.e. worse) at 1 and 10 mg/kg AIT-082 and mildly delayed at 30 mg/kg. This improvement did not reach the usual statistical significance. There was no difference in the age at death for any treatment dose. There was no difference in the neuropathology of treated and untreated G93A mice. However, there was an early improvement in the running wheel function at all tested doses. Using Cox regression, after adjustment for sex, the mice in the running wheels had slightly delayed onset of disease without change in survival and, after adjustment for exercise, the female mice had slightly improved survival. Consequently, AIT-082 would not be an attractive candidate for ALS clinical trials as monotherapy and justification for its use in combination therapy would require additional laboratory support. There was dissociation between the endpoints of disease progression (as judged by running wheel performance) and disease onset and survival. AIT-082 improved early running wheel performance yet led to accelerated late decline and had no impact on survival. It is possible that the drug facilitates early sprouting that leads to accelerated late decline.

Protection of sensory function in diabetic rats by Neotrofin

We investigated the ability of Neotrofin, an agent that enhances endogenous nerve growth factor (NGF) levels, to prevent phenotypic, functional and structural changes that occur in the peripheral nerve of streptozotocin-diabetic rats. Eight weeks of Neotrofin treatment prevented depletion of NGF protein in plantar foot skin and sciatic nerve of diabetic rats and increased NGF protein in associated skeletal muscles. These effects were accompanied by maintenance of normal nerve levels of the neuropeptides substance P and calcitonin gene related peptide. Thermal hypoalgesia and conduction slowing of large sensory fibres in diabetic rats were ameliorated by Neotrofin treatment, whereas there was no effect on conduction slowing in large motor fibres or on reduced myelinated fibre axonal calibre. Enhancing endogenous production of neurotrophic factors using small molecules may be an alternative to either exogenous treatment with neurotrophic factors or gene therapy as a therapeutic approach to treating diabetic neuropathy.

Neotrofin increases heme oxygenase-1 selectively in neurons

There is evidence that heme oxygenase plays a role in cellular defense against reactive oxygen species and thereby has neuroprotective effects. We examined the interaction of Neotrofin, a cognitive-enhancing and neuroprotective drug, with the heme oxygenase system. In adult rats, both a single administration or seven daily injections of Neotrofin at 10, 30 or 100 mg/kg intraperitoneally increased HO-1 immunoreactivity in neurons of the hippocampal formation and its connections including CA1-4, fornix, septal nuclei, hippocampal commissure, septohippocampal nucleus, fimbria, anteroventral thalamic nucleus, frontal and parietal cortex. Prominent HO-1 staining of neuronal cells in the proximity of blood vessels and circumventricular organs was also observed. Increasing doses of Neotrofin resulted in an increase in the number of neuronal populations expressing HO-1 with 100 mg/kg evoking a widespread neuronal cell response in brain. Quantification by ELISA confirmed that intraperitoneal administration of 100 mg/kg Neotrofin caused a significant increase in HO-1 protein expression in the hippocampus. The increase was evident by 6 h post-injection, peaked at 24 h with a 4-fold increase, and persisted for at least 48 h. Similarly, oral administration of 100 mg/kg Neotrofin produced a 5-fold increase in HO-1 protein 24 h post-administration. The effect of Neotrofin on HO-1 appears to be at the transcriptional level, as suggested by an increase in HO-1 mRNA levels. Neotrofin treatment was also associated with a significant increase in HO-2 mRNA levels in whole brain homogenate. These data may explain the neuroprotective effects of Neotrofin in models of excitotoxic neuronal injury.

Neotrofin is transported out of brain by a saturable mechanism: possible involvement of multidrug resistance and monocarboxylic acid transporters

Neotrofin (AIT-082; leteprinim potassium) is transported out of brain by a saturable mechanism and in this study the mechanisms mediating this efflux were evaluated. Intracerebroventricular coadministration of [(14)C]Neotrofin with verapamil, a P-glycoprotein inhibitor, probenecid, an organic anion transporter inhibitor, 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK571), a multidrug resistance-associated protein inhibitor, and salicylate or benzoate, both monocarboxylic acid transporter substrates, inhibited the efflux of [(14)C]Neotrofin. Additionally, Neotrofin inhibited the efflux of [(3)H]quinidine from brain. Compounds can diffuse from cerebrospinal fluid (CSF) into extracellular fluid of brain parenchyma and thus, efflux of [(14)C]Neotrofin after intracerebroventricular administration may indicate active transport across choroid plexus epithelium, brain capillary endothelium, or both. To determine whether [(14)C]Neotrofin efflux occurs at the brain capillary endothelium, experiments were performed in which [(14)C]Neotrofin was administered intraparenchymally. The t(1/2) for [(14)C]Neotrofin disappearance from brain after intraparenchymal administration was significantly lower than that for [(3)H]sucrose and the efflux of Neotrofin was inhibited by 600-fold excess of unlabeled Neotrofin, verapamil, MK571, and salicylate. Together, these data suggest that a saturable mechanism for the efflux of Neotrofin is located at the blood-brain barrier and possibly the blood-CSF barrier. It is likely that multiple transporters are involved in the efflux of Neotrofin and these may include multidrug resistance and monocarboxylic acid transporters. These data are discussed in detail with respect to the site of transporter expression, the recent identification of numerous multidrug resistance-associated protein and monocarboxylic acid transporter homologs, the existence of other potential brain efflux transporters, and the availability of specific pharmacological agents with which to distinguish these transporters.

AIT-082 is neuroprotective against kainate-induced neuronal injury in rats

4-[[3-(1,6-dihydro-6-oxo-9-purin-9-yl)-1-oxopropyl]amino]benzoic acid (AIT-082) is an hypoxanthine derivative that stimulates in vitro neurite outgrowth and the production of adenosine and neurotrophins from astrocytes. These effects may predict an in vivo neuroprotective activity of the drug. Thus, we evaluated whether AIT-082 protected against a long-term excitotoxicity of hippocampal neurons following status epilepticus induced in rats by i.p. injection of kainate (12 mg/kg). The epileptogenic effect of kainate was evaluated by monitoring behavioral signs and by electroencephalographic (EEG) recording (80% of the animals showed status epilepticus with a latency of 96.8 +/- 7.4 min starting from the injection). In surviving rats (40% of the injected animals) the neurotoxic effect was evaluated by measuring glutamic acid decarboxylase (GAD) activity, as an index of loss of hippocampal GABAergic neurons, by evaluating the body weight after 7 days and by histological examination of hippocampi. The GAD activity was reduced by 44 +/- 8%, and neuronal loss (about 70%) was found in the CA3c, the CA1 area, and in the dentate gyrus. A single dose of diazepam (20 mg/kg; i.p., 20 min before the kainate injection) almost completely inhibited both seizures and neurotoxicity, ensuring survival of animals. AIT-082 (60 mg/kg/day; i.p., for 7 days, starting from 20 min before the kainate injection) did not modify the seizures caused by kainate but, like diazepam, it decreased kainate-induced mortality, the reduction of GAD activity, and the loss of hippocampal neurons. These data confirm that AIT-082 is of potential interest for the experimental therapy of neurodegenerative disorders.

Effect of a memory-enhancing drug, AIT-082, on the level of synaptophysin

Our objective is to study the effect of AIT-082 on the level of synaptophysin in cultured pheochromocytoma (PC12) cells. The drug AIT-082, a unique purine hypoxanthine derivative, is under development for the treatment of Alzheimer's disease (AD). We analyzed synaptophysin protein as an index of synaptic numbers and density and indirectly neuronal transmission. PC12 cells were treated with nerve growth factor (NGF) (50 ng/ml) and/or different doses of AIT-082 (5-50 ng/ml) obtained from NeoTherapeutics, CA. In the western immunoblots of conditioned media and cell lysates, we detected synaptophysin as 36-40 kDa protein bands. When PC12 cells were treated with NGF and samples were analyzed at 24 or 48 hours after treatment, the secretion of synaptophysin was drastically reduced in the conditioned medium. A significant reduction in the intracellular levels of synaptophysin in NGF-treated samples was also noted. By contrast, when PC12 cells were treated with AIT-082, the secretion of synaptophysin was increased in the conditioned medium as compared to the control. There was also a significant increase in the intracellular levels of synaptophysin in AIT-082-treated cultures. NGF treatment resulted in sympathetic neuronal phenotypes in PC12 cells. As it is known that the immunoreactivity of the synaptophysin protein correlates with the density of the synaptic terminal, our results suggest that treatment by AIT-082 could enhance neurotransmitter release at the presynaptic terminal, which may play a role in the improvement of cognition seen in AD subjects.