Non-MAO A binding of clorgyline in white matter in human brain

Monoamine Oxidase B (MAO-B): A Target for Rational Drug Development in Schizophrenia Using PET Imaging as an Example

Monoamine oxidase B (MAO-B) is an important high-density enzyme involved in the generation of oxidative stress and central in the catabolism of dopamine, particularly in brain subcortical regions with putative implications in the pathophysiology of schizophrenia. In this chapter, we review postmortem studies, preclinical models, and peripheral and genetic studies implicating MAO-B in psychosis. A literature search in PubMed was conducted and 64 studies were found to be eligible for systematic review. We found that MAO-B could be identified as a potential target in schizophrenia. Evidence comes mostly from studies of peripheral markers, showing reduced platelet MAO-B activity in schizophrenia, together with preclinical results from MAO-B knock-out mice resulting in a hyperdopaminergic state and behavioral disinhibition. However, whether brain MAO-B is altered in vivo in patients with schizophrenia remains unknown. We therefore review methodological studies involving MAO-B positron emission tomography (PET) radioligands used to quantify MAO-B in vivo in the human brain. Given the limitations of currently available treatments for schizophrenia, elucidating whether MAO-B could be used as a target for risk stratification or clinical staging in schizophrenia could allow for a rational search for newer antipsychotics and the development of new treatments.

Monoamine Oxidase Inhibition by Plant-Derived β-Carbolines; Implications for the Psychopharmacology of Tobacco and Ayahuasca

The monoamine oxidases (MAOs) are flavin-containing amine oxidoreductases responsible for metabolism of many biogenic amine molecules in the brain and peripheral tissues. Whereas serotonin is the preferred substrate of MAO-A, phenylethylamine is metabolized by MAO-B, and dopamine and tyramine are nearly ambivalent with respect to the two isozymes. β-Carboline alkaloids such as harmine, harman(e), and norharman(e) are MAO inhibitors present in many plant materials, including foodstuffs, medicinal plants, and intoxicants, notably in tobacco (Nicotiana spp.) and in Banisteriopsis caapi, a vine used in the Amazonian ayahuasca brew. The β-carbolines present in B. caapi may have effects on neurogenesis and intrinsic antidepressant properties, in addition to potentiating the bioavailability of the hallucinogen N,N-dimethyltryptamine (DMT), which is often present in admixture plants of ayahuasca such as Psychotria viridis. Tobacco also contains physiologically relevant concentrations of β-carbolines, which potentially contribute to its psychopharmacology. However, in both cases, the threshold of MAO inhibition sufficient to interact with biogenic amine neurotransmission remains to be established. An important class of antidepressant medications provoke a complete and irreversible inhibition of MAO-A/B, and such complete inhibition is almost unattainable with reversible and competitive inhibitors such as β-carbolines. However, the preclinical and clinical observations with synthetic MAO inhibitors present a background for obtaining a better understanding of the polypharmacologies of tobacco and ayahuasca. Furthermore, MAO inhibitors of diverse structures are present in a wide variety of medicinal plants, but their pharmacological relevance in many instances remains to be established.

Specific binding of [(18)F]fluoroethyl-harmol to monoamine oxidase A in rat brain cryostat sections, and compartmental analysis of binding in living brain

We investigated [(18)F]fluoroethyl-harmol ([(18)F]FEH) as a reversible and selective ligand for positron emission tomography (PET) studies of monoamine oxidase A (MAO-A). Binding of [(18)F]FEH in rat brain cryostat sections indicated high affinity (KD = 3 nM), and density (Bmax; 600 pmol/g). The plasma free fraction was 45%, and untransformed parent constituted only 13% of plasma radioactivity at 10 min after injection. Compartmental analysis of PET recordings in pargyline-treated rats showed high permeability to brain (K1; 0.32 mL/g/min) and slow washout (k2; 0.024/min), resulting in a uniformly high equilibrium distribution volume (VD; 20 mL/g). Using this VD to estimate unbound ligand in brain of untreated rats, the binding potential ranged from 4.2 in cerebellum to 7.2 in thalamus. We also calculated maps of rats receiving [(18)F]FEH at a range of specific activities, and then estimated saturation binding parameters in the living brain. In thalamus, striatum and frontal cortex KD was globally close to 300 nM and Bmax was close to 1600 pmol/g; the 100-fold discrepancy in affinity suggests a very low free fraction for [(18)F]FEH in the living brain. Based on a synthesis of findings, we calculate the endogenous dopamine concentration to be 0.4 μM in the striatal compartment containing MAO-A, thus unlikely to exert competition against [(18)F]FEH binding in vivo. In summary, [(18)F]FEH has good properties for the detection of MAO-A in the rat brain by PET, and may present logistic advantages for clinical research at centers lacking a medical cyclotron. We made a compartmental analysis of [(18)F]fluoroethylharmol ([(18)F]FEH) binding to monoamine oxidase A (MAO-A) in living rat brain and estimated the saturation binding parameters from the binding potential (BPND). The Bmax was of comparable magnitude to that in vitro, but with apparent affinity (300 nM), it was 100-fold lower in vivo. PET imaging with [(18) F]FEH is well suited for quantitation of MAO-A in living brain.

Monoamine oxidase: radiotracer chemistry and human studies

Monoamine oxidase (MAO) oxidizes amines from both endogenous and exogenous sources thereby regulating the concentration of neurotransmitter amines such as serotonin, norepinephrine, and dopamine as well as many xenobiotics. MAO inhibitor drugs are used in the treatment of Parkinson's disease and in depression stimulating the development of radiotracer tools to probe the role of MAO in normal human biology and in disease. Over the past 30 years since the first radiotracers were developed and the first positron emission tomography (PET) images of MAO in humans were carried out, PET studies of brain MAO in healthy volunteers and in patients have identified different variables that have contributed to different MAO levels in brain and in peripheral organs. MAO radiotracers and PET have also been used to study the current and developing MAO inhibitor drugs including the selection of doses for clinical trials. In this article, we describe the following: (1) the development of MAO radiotracers; (2) human studies including the relationship of brain MAO levels to genotype, personality, neurological, and psychiatric disorders; and (3) examples of the use of MAO radiotracers in drug research and development. We will conclude with outstanding needs to improve the radiotracers that are currently used and possible new applications.

Modulation of monoamine oxidase (MAO) expression in neuropsychiatric disorders: genetic and environmental factors involved in type A MAO expression

Monoamine oxidase types A and B (MAO-A, MAO-B) regulate the levels of monoamine neurotransmitters in the brain, and their dysfunction may be involved in the pathogenesis and influence the clinical phenotypes of neuropsychiatric disorders. Reversible MAO-A inhibitors, such as moclobemide and befloxatone, are currently employed in the treatment of emotional disorders by inhibiting the enzymatic degradation of dopamine, serotonin and norepinephrine in the central nervous system (CNS). It has been suggested that the irreversible MAO-B inhibitors selegiline and rasagiline exert a neuroprotective effect in Parkinson's and Alzheimer's diseases. This effect, however, is not related to their inhibition of MAO activity; in animal and cellular models, selegiline and rasagiline protect neuronal cells through their anti-apoptotic activity and induction of pro-survival genes. There is increasing evidence that MAO-A activity, but not that of MAO-B, is implicated in the pathophysiology of neurodegenerative disorders, but also in gene induction by MAO-B inhibitors; on the other hand, selegiline and rasagiline increase MAO-A mRNA, protein, and enzyme activity levels. Taken together, these results suggest that each MAO subtype exerts effects that modulate the expression and activity of the other isoenzyme. The roles of MAO-A and -B in the CNS should therefore be re-evaluated with respect to the "type-specificity" of their inhibitors, which may not be unconditional during chronic treatment. Mao-a expression, in particular, may be implicated in pathogenesis and phenotypes in neuropsychiatric disorders. MAO-A expression is modified by mao polymorphisms affecting its transcriptional efficiency, as well as by mutations and polymorphism of parkin, Sirt1, FOXO, microRNA, presenilin-1, and other regulatory proteins. In addition, childhood maltreatment has been shown to have an impact upon adolescent social behavior in children with mao-a polymorphisms of low transcriptional activity. Low MAO-A activity may increase the levels of serotonin and norepinephrine, resulting in disturbed neurotransmitter system development and behavior. This review discusses genetic and environmental factors involved in the regulation of MAO-A expression, in the contexts of neuropsychiatric function and of the regulation of neuronal survival and death.

Distribution of monoamine oxidase proteins in human brain: implications for brain imaging studies

Positron emission tomography (PET) imaging of monoamine oxidases (MAO-A: [(11)C]harmine, [(11)C]clorgyline, and [(11)C]befloxatone; MAO-B: [(11)C]deprenyl-D2) has been actively pursued given clinical importance of MAOs in human neuropsychiatric disorders. However, it is unknown how well PET outcome measures for the different radiotracers are quantitatively related to actual MAO protein levels. We measured regional distribution (n=38) and developmental/aging changes (21 hours to 99 years) of both MAOs by quantitative immunoblotting in autopsied normal human brain. MAO-A was more abundant than MAO-B in infants, which was reversed as MAO-B levels increased faster before 1 year and, unlike MAO-A, kept increasing steadily to senescence. In adults, regional protein levels of both MAOs were positively and proportionally correlated with literature postmortem data of MAO activities and binding densities. With the exception of [(11)C]befloxatone (binding potential (BP), r=0.61, P=0.15), correlations between regional PET outcome measures of binding in the literature and MAO protein levels were good (P<0.01) for [(11)C]harmine (distribution volume, r=0.86), [(11)C]clorgyline (λk3, r=0.82), and [(11)C]deprenyl-D2 (λk3 or modified Patlak slope, r=0.78 to 0.87), supporting validity of the latter imaging measures. However, compared with in vitro data, the latter PET measures underestimated regional contrast by ∼2-fold. Further studies are needed to address cause of the in vivo vs. in vitro nonproportionality.

Reversible inhibitors of monoamine oxidase-A (RIMAs): robust, reversible inhibition of human brain MAO-A by CX157

Reversible inhibitors of monoamine oxidase-A (RIMA) inhibit the breakdown of three major neurotransmitters, serotonin, norepinephrine and dopamine, offering a multi-neurotransmitter strategy for the treatment of depression. CX157 (3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathiin-10,10-dioxide) is a RIMA, which is currently in development for the treatment of major depressive disorder. We examined the degree and reversibility of the inhibition of brain monoamine oxidase-A (MAO-A) and plasma CX157 levels at different times after oral dosing to establish a dosing paradigm for future clinical efficacy studies, and to determine whether plasma CX157 levels reflect the degree of brain MAO-A inhibition. Brain MAO-A levels were measured with positron emission tomography (PET) imaging and [(11)C]clorgyline in 15 normal men after oral dosing of CX157 (20-80 mg). PET imaging was conducted after single and repeated doses of CX157 over a 24-h time course. We found that 60 and 80 mg doses of CX157 produced a robust dose-related inhibition (47-72%) of [(11)C]clorgyline binding to brain MAO-A at 2 h after administration and that brain MAO-A recovered completely by 24 h post drug. Plasma CX157 concentration was highly correlated with the inhibition of brain MAO-A (EC(50): 19.3 ng/ml). Thus, CX157 is the first agent in the RIMA class with documented reversible inhibition of human brain MAO-A, supporting its classification as a RIMA, and the first RIMA with observed plasma levels that can serve as a biomarker for the degree of brain MAO-A inhibition. These data were used to establish the dosing regimen for a current clinical efficacy trial with CX157.

Applying neuroimaging ligands to study major depressive disorder

The recent increase in radioligands available for neuroimaging major depressive disorder has led to advancements in our understanding of the pathophysiology of this illness and improved antidepressant development. Major depressive disorder can be defined as an illness of recurrent major depressive episodes of persistently low mood, dysregulated sleep, appetite and weight, anhedonia, cognitive impairment, and suicidality. The main target sites investigated with radioligand neuroimaging include receptor sites that regulate in response to lowered monoamine levels, targets related to removal of monoamines, uptake of ligands related to regional brain function, and target sites of antidepressants.

Brain monoamine oxidase A activity predicts trait aggression

The genetic deletion of monoamine oxidase A (MAO A), an enzyme that breaks down the monoamine neurotransmitters norepinephrine, serotonin, and dopamine, produces aggressive phenotypes across species. Therefore, a common polymorphism in the MAO A gene (MAOA, Mendelian Inheritance in Men database number 309850, referred to as high or low based on transcription in non-neuronal cells) has been investigated in a number of externalizing behavioral and clinical phenotypes. These studies provide evidence linking the low MAOA genotype and violent behavior but only through interaction with severe environmental stressors during childhood. Here, we hypothesized that in healthy adult males the gene product of MAO A in the brain, rather than the gene per se, would be associated with regulating the concentration of brain amines involved in trait aggression. Brain MAO A activity was measured in vivo in healthy nonsmoking men with positron emission tomography using a radioligand specific for MAO A (clorgyline labeled with carbon 11). Trait aggression was measured with the multidimensional personality questionnaire (MPQ). Here we report for the first time that brain MAO A correlates inversely with the MPQ trait measure of aggression (but not with other personality traits) such that the lower the MAO A activity in cortical and subcortical brain regions, the higher the self-reported aggression (in both MAOA genotype groups) contributing to more than one-third of the variability. Because trait aggression is a measure used to predict antisocial behavior, these results underscore the relevance of MAO A as a neurochemical substrate of aberrant aggression.

Evidence that brain MAO A activity does not correspond to MAO A genotype in healthy male subjects

BACKGROUND

A functional polymorphism in the promoter region of the monoamine oxidase A (MAO A) gene has two common alleles that are referred to as the high and low MAO A genotypes. We report the first in vivo human study to determine whether there is an association between MAO A genotype and brain MAO A activity in healthy male subjects.

METHODS

Brain MAO A activity was measured with positron emission tomography and [(11)C]clorgyline in 38 healthy adult male nonsmokers genotyped for MAO A polymorphism.

RESULTS

There was no significant difference in brain MAO A activity between the high (n = 26) and low (n = 12) MAO A genotypes.

CONCLUSIONS

The lack of an association between the high and low MAO A genotype and brain MAO A activity suggests that this polymorphism by itself does not contribute to differences in brain MAO A activity in healthy adult male subjects.

Monoamine oxidase A rather than monoamine oxidase B inhibition increases nicotine reinforcement in rats

Although nicotine is considered to be responsible for the addictive properties of tobacco, growing evidence underlines the importance of non-nicotine components in smoking reinforcement. It has been shown that tobacco smoke contains monoamine oxidase (MAO) A and B inhibitors and decreases MAO-A and MAO-B activity in smokers. Here, we investigated the effects of clorgyline hydrochloride (irreversible MAO-A inhibitor; 2 mg/kg/day), selegiline (irreversible MAO-B inhibitor; 4 mg/kg) and the beta-carboline norharmane hydrochloride (reversible MAO-B inhibitor; 5 mg/kg/day) treatments on nicotine self-administration (30 microg/kg/infusion, free base) in rats. Independent of the responsiveness to novelty and locomotor activity stimulation, only clorgyline hydrochloride treatment increased the intake of nicotine in a fixed-ratio schedule (FR5) of reinforcement. When a progressive-ratio schedule was implemented, both clorgyline hydrochloride and norharmane hydrochloride treatments potentiated the reinforcing effects of nicotine, whereas selegiline had no effect. Taken together, these results indicate that MAO-A inhibition interacts with nicotine to enhance its rewarding effects in rats and suggest that other compounds present in tobacco, such as beta-carboline, may also play an important role in sustaining smoking behavior in humans.

Translational neuroimaging: positron emission tomography studies of monoamine oxidase

Positron emission tomography (PET) using radiotracers with high molecular specificity is an important scientific tool in studies of monoamine oxidase (MAO), an important enzyme in the regulation of the neurotransmitters dopamine, norepinephrine, and serotonin as well as the dietary amine, tyramine. MAO occurs in two different subtypes, MAO A and MAO B, which have different substrate and inhibitor specificity and which are different gene products. The highly variable subtype distribution with different species makes human studies of special value. MAO A and B can be imaged in the human brain and certain peripheral organs using PET and carbon-11 (half-life 20.4 minutes) labeled mechanism-based irreversible inhibitors, clorgyline and L -deprenyl, respectively. In this article we introduce MAO and describe the development of these radiotracers and their translation from preclinical studies to the investigation of variables affecting MAO in the human brain and peripheral organs.

Comparison of the binding of the irreversible monoamine oxidase tracers, [(11)C]clorgyline and [(11)C]l-deprenyl in brain and peripheral organs in humans

The monoamine oxidase A and B (MAO A and B) radiotracers [(11)C]clorgyline (CLG) and [(11)C]L-deprenyl (DEP) and their deuterium labeled counterparts (CLG-D and DEP-D) were compared to determine whether their distribution and kinetics in humans are consistent with their physical, chemical and pharmacological properties and the reported ratios of MAO A:MAO B in post-mortem human tissues. Irreversible binding was consistently higher for DEP in brain, heart, kidneys and spleen but not lung where CLG >DEP and not in thyroid where there is no DEP binding. The generally higher DEP binding is consistent with its higher enzyme affinity and larger free fraction in plasma while differences in regional distribution for CLG and DEP in brain, heart, thyroid and lungs are consistent with different relative ratios of MAO A and B in humans.

A review of graphical methods for tracer studies and strategies to reduce bias

Graphical techniques provide simple methods for the analysis of data from tracer studies. They provide considerable ease of computation compared to the optimization of individual model parameters in the solution of the differential equations generally used to describe the binding of tracers. The theoretical work of Patlak which was applied to irreversible tracers formed the basis for extensions of graphical techniques to reversibly binding tracers. The advantage of graphical methods is that they are not dependent upon a particular model structure but provide a measure of tracer binding that can be interpreted in terms of a model structure if desired. They provide a visual way to distinguish the type of binding whether reversible or irreversible in the initial studies of new ligands. Conditions under which the graphical techniques can be applied are considered as well as problems encountered with slow binding components. One problem in the use of these methods particularly the method for reversible tracers is the bias generated due to the presence of statistical noise. Some recently proposed techniques for reducing the noise are considered.

Monoamine oxidase A imaging in peripheral organs in healthy human subjects

Monoamine oxidase (MAO) catalyzes the oxidative deamination of many biogenic and dietary amines. Though studies of MAO have focused mainly on its regulatory role in the brain, MAO in peripheral organs also represents a vast mechanism for detoxifying vasoactive compounds as well as for terminating the action of physiologically active amines, which can cross the blood brain barrier. Indeed, robust central and peripheral MAO activity is a major requirement in the safe use of many CNS drugs, particularly antidepressants, and thus an awareness of the MAO inhibitory potential of drugs is essential in therapeutics. In this study, we examined the feasibility of quantifying MAO A in peripheral organs in healthy human subjects using comparative positron emission tomography (PET) imaging with carbon-11 (t(1/2): 20.4 min) labeled clorgyline ([(11)C]clorgyline) a suicide inactivator of MAO A and its deuterium labeled counterpart ([(11)C]clorgyline-D2). Heart, lungs, kidneys, thyroid, and spleen showed a robust deuterium isotope effect characteristic of MAO and the magnitude of the effect was similar to that of trancylcypromine, an irreversible MAO inhibitor used in the treatment of depression. Liver time-activity curves were not affected by deuterium substitution precluding the estimation of liver MAO in vivo. In organs showing an isotope effect, MAO A is greatest in the lungs and kidneys followed by the thyroid and heart. This method, which has been previously applied in the human brain, opens the possibility to also directly assess the effects of different variables including smoking, dietary substances, drugs, disease, and genetics on peripheral MAO A in humans.

Strategy for the formation of parametric images under conditions of low injected radioactivity applied to PET studies with the irreversible monoamine oxidase A tracers [11C]clorgyline and deuterium-substituted [11C]clorgyline

The construction of parametric positron emission tomography images of enzyme or receptor concentration obtained using irreversibly binding radiotracers presents problems not usually encountered with reversibly binding radiotracers. Difficulties are most apparent in brain regions having low blood flow and/or high enzyme or receptor concentration and are exacerbated with noisy data. This is especially true when minimal doses of radiotracer are administered. A comparison was recently reported of the irreversible monoamine oxidase A (MAO A) radiotracers [11C]clorgyline (CLG) and deuterium-substituted [11C]clorgyline (CLG-D) in the human brain using region of interest (ROI) analysis in which the authors observed an unexpected loss of image contrast with CLG-D compared with CLG. In order to more fully investigate patterns of binding of these irreversibly binding radiotracers, a strategy was devised to reduce noise in the generation of parametric images of the model term related to enzyme or receptor concentration. The generalized linear least squares (GLLS) method of Feng et al. (1995), a rapid linear method that is unbiased, was used for image-wide parameter estimation. Since GLLS can fail in the presence of large amounts of noise, local voxels were grouped within the image to increase the signal, and the GLLS method was combined with the standard nonlinear estimation methods when necessary. Voxels were grouped together depending on their proximity and whether they fell within a specified range of the time-integrated image. It was assumed that voxels meeting both criteria are functionally related. Simulations reflecting varying enzyme concentrations were performed to assess precision and accuracy of parameter estimates in the presence of varying amounts of noise. Using this approach, images were generated of the combination parameter lambdak3 (lambda = K1/k2, where K1 and k2 are plasma-to-tissue and tissue-to-plasma transport constants, respectively) that is related to enzyme concentration as well as images of the transport constant K1 for individual subjects. Reasonably high-quality images of both K1 and lambdak3 were obtained for CLG and CLG-D for individual subjects even with low injected doses averaging 6 mCi. While there were no differences in the K1 images, the lambdak3 images revealed the loss of contrast previously reported for CLG-D using the ROI analysis. This method should be generalizable to other tracers and should facilitate the analysis of group differences.