Design and early clinical evaluation of selective inhibitors of monoamine oxidase

Evolution of ideas about the risk of tyramine syndrome developing during therapy with irreversible non-selective monoamine oxidase inhibitors (to the 70th anniversary of the first use of this group of antidepressants)

We describe the history of studying of the problem of tyramine syndrome during the treatment with antidepressants, irreversible non-selective monoamine oxidase inhibitors. Data on clinical observations are presented. Early hypotheses of pathogenesis are considered. The period of discovery of the relationship between increased blood pressure and the use of foods containing tyramine is outlined. The transformation of terminology is demonstrated. The events associated with the restriction of the use of therapy are characterized. The formation of the opinion about the exaggeration of the risk of developing tyramine syndrome is analyzed. Data on the involvement in the discussion of the problem of tyramine syndrome of society as a whole are given. 

Structural and mechanistic studies of mofegiline inhibition of recombinant human monoamine oxidase B

Mechanistic and structural studies have been carried out to investigate the molecular basis for the irreversible inhibition of human MAO-B by mofegiline. Competitive inhibition with substrate shows an apparent K(i) of 28 nM. Irreversible inhibition of MAO-B occurs with a 1:1 molar stoichiometry with no observable catalytic turnover. The absorption spectral properties of mofegiline inhibited MAO-B show features (lambda(max) approximately 450 nm) unlike those of traditional flavin N(5) or C(4a) adducts. Visible and near-UV circular dichroism spectra of the mofegiline-MAO-B adduct shows a negative peak at 340 nm with an intensity similar to that of N(5) flavocyanine adducts. The X-ray crystal structure of the mofegiline-MAO-B adduct shows a covalent bond between the flavin cofactor N(5) with the distal allylamine carbon atom as well as the absence of the fluorine atom. A mechanism to explain these structural and spectral data is proposed.

Local striatal infusion of MPP+ does not result in increased hydroxylation after systemic administration of 4-hydroxybenzoate

In vivo bilateral microdialysis in the rat striatum was used to investigate hydroxyl radical formation under basal conditions and after intrastriatal administration of the neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). After a short equilibration period, 4-hydroxybenzoate (4HBZ), which scavenges hydroxyl radicals to produce 3,4-dihydroxybenzoate (34DHB), was injected intraperitoneally 15 min before infusion of MPP+. To evaluate the enzymatic contribution to hydroxyl radical formation, two other series of microdialyses were performed following administration of monoamine oxidase B inhibitors, either 1-deprenyl (selegiline) or MDL 72,974A [(E)-2-(4-fluorophenethyl)-3-fluoroallylamine hydrochloride]. Microdialysate samples were analyzed by high-performance liquid chromatography for catecholamines, 3,4-dihydroxyphenylacetate (DOPAC), homovanillate (HVA), along with the hydroxyl radical adduct, 34DHB and its precursor, 4HBZ. MPP+ administration resulted in a massive release of dopamine along with a decrease in DOPAC and HVA in all three groups. A striking effect in all three groups was noted in which MPP+ resulted in a decrease in interstitial 4HBZ to < 50% of the non-MPP+ -treated side. In absolute terms, the amount of 34DHB produced was low but similar in all three groups, even after unilateral MPP+ infusion. When 34DHB was normalized to 4HBZ release to account for differences in precursor availability, there were no significant differences in the 34DHB/4HBZ ratios either with or without MAO inhibitor treatment or after local MPP+ infusion. Systemic 4HBZ administration appears to result predominantly in intra-cellular sampling of hydroxyl radicals which produces different results from local infusion of trapping agents such as salicylate.

Synthesis of stereo (R and S) and geometric (E and Z) isomers of [18F]fluoro-beta-fluoromethylene-m-tyrosine derivatives: in vivo probes of central dopaminergic function

Fluorination of pure R and S enantiomers of (E)-beta-fluoromethylene-m-tyrosine [(E)-FMMT] and its racemic geometric isomer, (Z)-beta-fluoromethylene-m-tyrosine [(Z)-FMMT] with [18F]acetyl hypofluorite ([18F]AcOF) gave a mixture of aromatic ring fluorinated products and a pair of diastereomeric products of addition across the exocyclic double bond. Semipreparative high performance liquid chromatography (HPLC) enabled a complete separation and isolation of these products, namely, 6-[18F]fluoro, 2-[18F]fluoro, and 2,6-[18F]difluoro (E)-FMMT and (Z)-FMMT derivatives. No attempt was made to isolate the individual components of the addition product. Pure racemic 4-[18F]fluoro-(E)-beta-fluoromethylene-m-tyrosine was also synthesized from a substituted (E)-FMMT precursor involving a fluorodestannylation reaction with [18F]F2. The availability of stereo (R and S) isomers of 6-[18F]fluoro and 2-[18F]fluoro (E)-FMMT and those of the racemic (Z)-FMMT along with 4-[18F]fluoro-(E)-beta-fluoromethylene-m-tyrosine would now enable a systematic investigation of the central monoamine oxidase/aromatic amino acid decarboxylase enzyme system with positron emission tomography.

[18F]Fluoro-beta-fluoromethylene-m-tyrosine derivatives show stereo, geometrical, and regio specificities as in vivo central dopaminergic probes in monkeys

Stereo (D and L), geometrical (E and Z), and regiospecific (2-, 4-, and 6-[18F]fluoro) analogs of beta-fluoromethylene-m-tyrosine (FMMT) have been investigated in adult vervet monkeys (Cercopithecus aethiops sabaeus, n = 12) in vivo with positron emission tomography (PET). Brain transport through the blood-brain barrier and central aromatic amino acid decarboxylase (AAAD)-mediated decarboxylation rates were established. Results show strict structural dependency of the kinetic behavior of radiofluorinated FMMT analogs, with the E-isomer exhibiting a higher specificity over the (Z) geometrical counterpart for central dopaminergic structures. The 6-[18F]fluoro substituted L-(E)-FMMT was also favored over the 2- and 4-[18F]fluorosubstituted isomers in terms of their ability to localize in the same brain areas. The role of PET in drug development is also exemplified in this work.

Molecular associations of flavins with betacarbolines and related indoles

The interactions of a set of structurally selected betacarbolines (BC), (9H-pyrido[3,4-b]indoles) and indoles (IND) with two representative flavins (FN): riboflavin (RFN) and flavin mononucleotide (FMN) have been investigated by absorption and fluorescence spectroscopies. Spectral results provided evidence on the formation of 1:1 non-fluorescent molecular complexes, whose stability constants and other related thermodynamic parameters have been estimated from Stern-Volmer quenching analysis. The FMN complexes are somewhat more stable than the RFN complexes. The stabilities of the IND and BC complexes for a given FN follow approximately the order IND approximately tetrahydro BC < dehydro BC < fully aromatic BC. Protonation of the pyridinic nitrogen atom of BCs has a destabilizing effect, which is more pronounced for fully aromatic than for tetrahydro derivatives. The influence of structural factors on the stability of the complexes has been discussed and, aided by theoretical AM1 calculations, a qualitative model for the structure (stacking) and binding forces (cooperative localized charge transfer and dispersion forces) of the complexes has been proposed.

Harman (1-methyl-beta-carboline) is a natural inhibitor of monoamine oxidase type A in rats

Harman (1-methyl-beta-carboline) displaces [3H]pargyline in vitro from high affinity binding sites on membranes from cerebral cortex, provided that experimental conditions are chosen under which [3H]pargyline labels selectively monoamine oxidase type A. Norharman (beta-carboline) is a much weaker displacing compound. It is well known that the type A enzyme can be blocked irreversibly in vivo by treatment of rats with clorgyline. Under these conditions no specific binding of [3H]harman and [3H]pargyline to monoamine oxidase type A was detected in brain, whereas the specific binding was reduced to 5% in liver tissue. The in vitro and ex vivo experiments suggest that there is a specific binding site for harman on monoamine oxidase type A, thereby extending earlier in vitro findings. It has been postulated that harman operates as a natural inhibitor of monoamine oxidase type A in mammals. The present study demonstrates that harman and norharman occur in rat brain, blood plasma, heart, kidney and liver. It further shows that pretreatment with clorgyline induces a time-dependent increase in the blood plasma levels of harman, suggesting the displacement of harman from the enzyme in tissue with its subsequent delivery into the blood. These findings strongly support the hypothesis based on in vitro experiments, that harman binds reversibly to the active site of monoamine oxidase type A in vivo. Dietary sources for mammalian harman play probably only a minor role, because the concentrations in beer and wine as well as other foodstuffs are too low to contribute substantially to endogenous levels of harman.

4-(Aminomethyl)-1-aryl-2-pyrrolidinones, a new class of monoamine oxidase B inactivators

Both 4-(aminomethyl)-1-phenyl-2-pyrrolidinone (4a) and 4-(aminomethyl)-1- (methoxyphenyl)-2-pyrrolidinone (4b) hydrochlorides were synthesized via a six-step sequence, which represents a general approach to 1,4-disubstituted 2-pyrrolidinones. Both of these compounds inactivated monoamine oxidase B and represent the first in a new class of monomamine oxidase inactivators.

[3H]harman binding experiments. I: A reversible and selective radioligand for monoamine oxidase subtype A in the CNS of the rat

Harman (1-methyl-beta-carboline) is an endogenous compound with neurotropic properties in rats and humans. In a novel in vitro binding assay, the binding site of [3H]harman has been characterized in the rat crude mitochondrial (P2) fraction. The binding was saturable and reversible. Only a single high-affinity binding site was detected by kinetic, saturation, and displacement analyses in the cerebral cortex of the rat. The linear Scatchard plots revealed equilibrium dissociation constant (KD) values of approximately 2.5 nM at 0 degrees C, approximately 9 nM at 23 degrees C, and approximately 30 nM at 37 degrees C. Among six CNS regions (hypothalamus, hippocampus, cerebral cortex, striatum, cerebellum, and spinal cord), the highest density of binding sites (Bmax) was determined in the hypothalamus (approximately 5.5 pmol/mg of protein) and the lowest in the spinal cord (approximately 2.0 pmol/mg of protein). Several drugs known to affect serotonergic, adrenergic, dopaminergic, cholinergic, or GABAergic neurotransmission inhibited specific binding at best in the micromolar range. In contrast, potent and selective inhibitors of monoamine oxidase subtype A were active in the lower and middle nanomolar range. The displacing potency (apparent Ki) of substrates and inhibitors of monoamine oxidase correlated positively and highly significantly with the corresponding values of the inhibition of monoamine oxidase activity of subtype A (r = 0.92, p less than 0.001, n = 17) but not of subtype B (r = -0.47, p greater than 0.05, n = 15). In conclusion, [3H]harman was identified as a specific ligand of the active site of the A subtype of monoamine oxidase in rat brain.

Selective inactivation of MAO-B by benzyl-dimethyl-silyl-methanamines in vitro

The alpha-silyl amines benzyl-dimethyl-silyl-methanamine and the p-fluoro and p-chloro derivatives are potent time-dependent inhibitors of rat brain MAO-B. The inhibition exhibits saturation kinetics, takes place in the enzyme active-site and is irreversible. The most potent inhibitor in the series is 4-fluorobenzyl-dimethyl-silyl-methanamine (KI = 11 microM, tau 1/2 = 2.3 min). Its selectivity for the B-form relative to the A-form of rat brain MAO is higher than 10(4). Benzyl-dimethyl-silyl-methanamines may represent a new family of anti-Parkinsonian agents.

The depletion of rat cortical norepinephrine and the inhibition of [3H]norepinephrine uptake by xylamine does not require monoamine oxidase activity

Inhibition of monoamine oxidase A through pretreatment of rats with clorgyline (10 mg/kg ip) or the pro-drug MDL 72,394 (0.5 mg/kg ip) did not block the amine-depleting action of xylamine (25 mg/kg ip). Xylamine treatment resulted in a loss of approximately 60% of the control level of norepinephrine in the cerebral cortex. A 1-hr pretreatment, but not a 24-hr pretreatment, with the monoamine oxidase B inhibitor, L-deprenyl (10 mg/kg ip), prevented the depletion of norepinephrine by xylamine. In addition, pretreatment with MDL 72,974 (1.25 mg/kg ip), a monoamine oxidase B inhibitor without amine-releasing or uptake - inhibiting effects, did not protect cortical norepinephrine levels. Inhibition of monoamine oxidase by either MDL 72,974 or MDL 72,394 did not prevent the inhibition of [3H]norepinephrine uptake into rat cortical synaptosomes by xylamine. These data indicate that monoamine oxidase does not mediate the amine-releasing or uptake inhibiting properties of xylamine. The protection afforded by L-deprenyl following a 1-hr pretreatment most probably was due to accumulation of its metabolite, L-amphetamine, which would inhibit the uptake carrier. A functional carrier is required for depletion since desipramine (20 mg/kg ip) administered 1 hr prior to xylamine, was also able to prevent depletion of norepinephrine.