Rapid hydroxylation of methtryptoline (1-methyltetrahydro-beta-carboline) in rat: identification of metabolites by chiral gas chromatography-mass spectrometry

Discovery of Highly Potent Serotonin 5-HT2 Receptor Agonists Inspired by Heteroyohimbine Natural Products

The serotonin 5-HT₂ receptors are important pharmaceutical targets involved in signaling pathways underlying various neurological, psychiatric, and cardiac functions and dysfunctions. As such, numerous ligands for the investigation of these receptors' activity and downstream effects have been developed synthetically or discovered in nature. For example, the heteroyohimbine natural product alstonine exhibits antispychotic activity mediated by 5-HT_2A/2C agonism. In this work, we identified a heteroyohimbine metabolite containing a serotonin pharmacophore and truncated the scaffold, leading to the discovery of potent agonist activity of substituted tetrahydro-β-carbolines across the 5-HT₂ receptor family. Extensive SAR development resulted in compound 106 with EC₅₀ values of 1.7, 0.58, and 0.50 nM at 5-HT_2A, 5-HT_2B, and 5-HT_2C, respectively. Docking studies suggest a π-stacking interaction between the tetrahydro-β-carboline core and conserved residue Trp^6.48 as the structural basis for this activity. This work lays a foundation for future investigation of these compounds in neurological and psychiatric disorders.

Metabolic hydroxylation of 1-methyl-1,2,3,4-tetrahydro-beta-carboline in humans

We characterized the metabolites of 1-methyl-1,2,3,4-tetrahydro-beta-carboline (MTBC) in human urine by gas chromatography-negative ion chemical ionization mass spectrometry (GC-NICIMS) and developed an analytical method using GC-NICIMS for their quantitative determination. When tetradeuterated MTBC was orally administered to a human subject, two peaks of the deuterated metabolites appeared on mass fragmentograms of the urine samples after administration. They were identified as tetradeuterated 6-hydroxy-MTBC (6-OH-MTBC) and 7-hydroxy-MTBC (7-OH-MTBC), indicating that MTBC was metabolically hydroxylated in humans. The proposed GC-NICIMS method could sensitively and selectively determine urinary 6-OH-MTBC and 7-OH-MTBC without interference from their artifactual formation during analysis. Its application to urine analysis has revealed that MTBC is excreted in human urine predominantly as the two hydroxylated metabolites, in which 6-OH-MTBC is present in both free and conjugated forms, whereas the 7-OH-MTBC of a conjugated form is much more than the 7-OH-MTBC of a free form.

Mitochondrial respiratory inhibition by N-methylated beta-carboline derivatives structurally resembling N-methyl-4-phenylpyridine

Mitochondrial accumulation and respiratory inhibition are critical steps in the actions of N-methyl-4-phenylpyridinium ion (MPP+), the toxic metabolite of the parkinsonism-inducing agent, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. We examined the respiratory characteristics of 2-methylated beta-carbolines (2-Me beta Cs) and 2-methylated 3,4-dihydro-beta-carbolines (2-MeDH beta Cs), which encompass the MPP+ structure. As indoleamine derivatives, they could have endogenous roles in idiopathic parkinsonism. With rat liver mitochondria, the order for inhibition of NAD(+)-linked O2 consumption (6-min preincubations) was as follows: MPP+ = 2-methylharmine greater than 2-methylharmol = 2-methylharmaline much greater than 2-methylharmalol greater than 2-methylnorharman greater than 6-OH-2-methylharmalan much greater than 2-methylharman. Similar to MPP+, 2-MeDH beta C/2-Me beta C inhibition was potentiated by tetraphenylboron and reversed by dinitrophenol, consistent with the involvement of cationic forms. However, the participation of neutral forms was indicated by the 2-MeDH beta C/2-Me beta C inhibitory time courses, which were unlike MPP+. The neutral forms probably arise via indolic nitrogen deprotonation because the characteristics of a cationic beta-carboline that cannot N-deprotonate, 2,9-dimethylnorharman, mirrored MPP+ rather than 2-Me beta Cs. Succinate-supported respiration was also significantly blocked by 2-MeDH beta Cs/2-Me beta Cs, but results with tetraphenylboron and 2,9-dimethylnorharman indicated that cationic forms were less important than in the inhibition of NAD(+)-linked respiration. We suggest that the relatively potent inhibition by certain 2-MeDH beta Cs/2-Me beta Cs involves neutral forms for passive mitochondrial entry and cationic as well as neutral forms that act at several respiratory sites. Respiratory inhibition could reasonably underlie the reported neurotoxicity of 2-Me beta Cs.

Dopamine uptake inhibitory capacities of beta-carboline and 3,4-dihydro-beta-carboline analogs of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) oxidation products

Potentially endogenous beta-carboline and 3,4-dihydro-beta-carboline alkaloidal compounds were compared, generally as 2-methylated (quaternary) and normethylated pairs, to the neurotoxin, 1-methyl-4-phenyl-dihydropyridinium ion (MPP+), with respect to inhibition of [3H]dopamine uptake into rat striatal synaptosomal preparations. Although less potent than MPP+, several compounds displayed IC50 values for inhibition in the moderate range (12-24 microM). Notably, quaternization generally did not improve inhibitory potency, and the 3,4-dihydro-compounds often were more effective inhibitors than their heteroaromatic analogs. The partially competitive nature of inhibition by one of the more effective pairs, 2-methyl-harmine and harmine, was consistent with uptake of the beta-carbolines by the synaptosomal dopamine uptake system, as was the fact that the accumulation of 2-[14C]methyl-harmine was significantly reduced by low Na+ media and by nomifensine, a potent inhibitor of the dopamine transporter. When viewed with reports that certain 2-methyl-beta-carbolines show MPP+-like toxicity in vitro and in vivo, these studies support the proposal that a mammalian beta-carbolinium compound may be taken up by nigrostriatal neurons and provoke the neuronal degeneration underlying Parkinson's disease.

In vitro study of the aromatic hydroxylation of 1-methyltetrahydro-beta-carboline (methtryptoline) in rat

The incubation of 1-methyltetrahydro-beta-carboline (1-MeTHBC) with hepatocytes isolated from 3-methylcholanthrene-treated rats led to formation of the 5-, 6- and 7-hydroxylated products. The hydroxylating activity was associated with the microsomal fraction as indicated by testing different subcellular fractions. The highest activity for hydroxylating 1-MeTHBC was found in liver which was about ten times as active as lung. Only a trace amount of hydroxylating activity was present in brain and kidney tissue. Analysis using chiral gas chromatography revealed an unequal abundance of enantiomers in all three products. The formation of the 5-, 6- and 7-hydroxylated products was confirmed in vivo by analysis of 24 h urine samples after intraperitoneal administration of 1-MeTHBC to 3-methylcholanthrene-treated rats.

Acetaldehyde and its condensation products as markers in alcoholism

Several studies show that recently abstaining alcoholics generate higher circulating levels of acetaldehyde than nonalcoholics following ethanol administration. It is conceivable that levels of stable adducts (tetrahydroisoquinolines and tetrahydro-beta-carbolines) derived from acetaldehyde condensations with biogenic amines also might be increased in alcoholics consuming ethanol, thus serving in body fluids as chemical markers that are more persistent than acetaldehyde itself. Limited human and rat studies indicate that urinary excretion of an oxidized tryptamine condensation product (harmane) and of an acetaldehyde/serotonin condensation product is elevated by chronic ethanol. Salsolinol, the derivative of acetaldehyde and dopamine, does not appear to be a meaningful urinary marker, but levels of the related pyruvic acid/dopamine product may be increased by ethanol. Blood assays of condensation products have been limited in number and equivocal. Condensation product measurements are complicated not only by artifacts (formation during analyses), but by other inherent problems. Products of interest often are constituents of diets and alcoholic beverages. For this and perhaps endogenous metabolic reasons, traces of condensation products are normally excreted by nondrinking individuals. Furthermore, the assays require high sensitivity and specificity and are not easily adapted to routine use. Thus, although several condensation products have initial appeal as clinical or pathological indicators in chronic alcoholism, thorough and statistically sound studies are needed before conclusions can be reached concerning any particular biogenic amine-derived product.