Pyridinium Metabolite Of Haloperidol Research Articles

Potent inhibition of CYP2D6 by haloperidol metabolites: stereoselective inhibition by reduced haloperidol.

We evaluated the inhibitory effect of haloperidol and its metabolites on CYP2D6 activity in order to better understand the potential role of these metabolites in drug interactions involving haloperidol. The inhibitory effects of haloperidol and five of its metabolites on dextrorphan formation from dextromethorphan, a marker probe of CYP2D6 activity, were measured in human liver microsomal preparations. Apparent kinetic parameters for enzyme inhibition were determined by nonlinear regression analysis of the data. Racemic reduced haloperidol and its metabolite, RHPTP competitively inhibited dextromethorphan O-demethylation with estimated Ki values (0.24 microM and 0.09 microM, respectively) that were substantially lower than that of haloperidol (0.89 microM). The inhibitory effect of S(-)-reduced haloperidol was more potent than the R(+)-enantiomer, with estimated Ki values of 0.11 microM and 1.1 microM, respectively. The pyridinium metabolite of haloperidol, HPP+ inhibited the enzyme activity noncompetitively with a Ki value of 0.79 microM. The N-dealkylated metabolites of haloperidol (FBPA and CPHP) had a diminished inhibitory potency. While FBPA showed no notable inhibitory effect on dextrorphan formation, CPHP showed moderate competitive inhibition with a Ki value of 20.9 microM. The principal metabolites of haloperidol inhibit CYP2D6, suggesting that they might contribute to the inhibitory effects of the drug. Reduced haloperidol seems to inhibit CYP2D6 activity in an enantioselective manner with the physiologically occurring S(-) enantiomer being more potent.

Empirical and molecular modeling study of the pyridinium species RHPP+, an abundant and potentially neurotoxic metabolite of haloperidol

The “reduced” haloperidol pyridinium metabolite (RHPP+) is found in the brain, plasma and urine of patients treated with the neuroleptic drug haloperidol (HP). RHPP+ is suspected to be neurotoxic through a mechanism that entails interference with the mitochondrial electron transport chain. We have studied the conformation of this flexible molecule in solution (using NMR) and in the solid state by single crystal X-ray analysis. Using the solid state structure as initial input, molecular dynamics runs indicated that the molecule preferably exists in an unfolded, rather than a folded conformation. We propose that the interaction of RHPP+ with complexes in the mitochondrial respiratory chain is stabilized primarily by an ionic bond involving the cationic nitrogen and secondarily by hydrogen-bond anchoring originating from the hydroxy group. A comparison with HPP+, the “unreduced” pyridinium metabolite of HP, suggests that this latter interaction may—among other considerations such as lipophilicity—account for differences in the in vitro toxicological profiles of RHPP+ and HPP+, which carries a ketone group in lieu of the hydroxy.

Effects of a quaternary pyridinium metabolite of haloperidol (HP+) on the viability and catecholamine levels of cultured PC12 cells

Haloperidol has been found to be metabolized to a pyridinium ion (HP+; 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-pyridinium). HP+ is structurally similar to the toxic metabolite of the dopaminergic neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), N-methyl-4-phenylpyridinium (MPP+). HP+ is toxic towards dopaminergic neurons and was proposed to be associated with some of the extrapyramidal side effects of haloperidol. We therefore investigated the neurotoxicity of HP+ towards cultured PC12 cells. At high concentrations, HP+ reduced the viability of PC12 cells as measured by trypan blue exclusion and the MTT method. However, HP+ decreased intracellular dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and dihydroxyphenylalanine (DOPA) levels at lower concentrations than those required to compromise cell viability. The immunoreactivity of tyrosine hydroxylase was not affected by the treatment with HP+. It was subsequently demonstrated that HP+ can release [3H]DA preloaded in rat striatum slices. Thus, it is proposed that HP+ decreases dopamine content in PC12 cells through actively releasing amines from the cells and (or) blocking the reuptake of the released amines.

Two pyridinium metabolites of haloperidol are present in the brain of patients at post-mortem.

We have shown in patients taking the antipsychotic drug haloperidol (HP) that two pyridinium metabolites (HPP+ and RHPP+) are present in blood and urine in nM concentrations. These metabolites are structurally analogous to MPP+, the neurotoxic metabolite of the well-known parkinsonian-producing protoxin, MPTP. In this study we measured the concentrations of HPP+ and RHPP+ in seven regions of the brain (putamen, substantia nigra, globus pallidus, caudate, hippocampus, cerebellum and occipital cortex) obtained at post-mortem from three patients who were taking HP before death. Blood, urine, and bile from one patient were analysed as well. HPP+ was present in all regions (except for substantia nigra in one patient and globus pallidus in another); the amount/g ranged from 1.6-8.3 pMol but there was no preferential sequestration of the metabolite in dopaminergic regions. Similarly, RHPP+ was present relatively uniformly in all regions; the amount/g ranged from 1.1-7.6 pMol. The concentrations of HPP+ and RHPP+ in one patient were 24 and 13 nM in blood, 660 and 230 nM in urine, and 13.0 and 1.4 microM in bile, respectively. The presence of these pyridinums in brain adds another important piece of information to the case that, at least for HP, metabolite-induced neurotoxicity could contribute to the extrapyramidal side-effects in patients receiving long-term therapy.

Hyperactive behavioural effects induced by intranigral infusion of a pyridinium metabolite of haloperidol in rats

Amphetamine-like behaviours such as sniffing and head movements were observed in rats intranigrally injected with a pyridinium metabolite of haloperidol (HP+). Photobeam interruptions by rats injected with HP+ were over 200 times more frequent than those of control rats 24 h after the operation. This apparent dopamine agonist effect of HP+ may be relevant to some of the clinical side effects of haloperidol such as acute dystonia and chronic tardive dyskinesia, which have been proposed to be the result of dopaminergic hyperactivity.

Haloperidol and its tetrahydropyridine derivative (HPTP) are metabolized to potentially neurotoxic pyridinium species in the baboon.

The in vivo metabolic fate of haloperidol (HP) and its tetrahydropyridine analog HPTP have been examined in the baboon to investigate the formation of potentially neurotoxic pyridinium metabolites that have been observed previously in humans. Urine samples collected from baboons treated with HPTP were shown to contain, in addition to the parent drug, the corresponding reduced HPTP (RHPTP), generated by reduction of the butyrophenone carbonyl group. RHPTP was characterized by comparison with a synthetic standard using HPLC with electrochemical detection and HPLC/MS/MS. Another compound identified by LC/MS/MS was a glucuronide metabolite of RHPTP. The HP pyridinium (HPP+) and reduced pyridinium (RHPP+) metabolites were shown to be present in urine from both HP and HPTP treated baboons by HPLC using fluorescence detection. The urinary excretion profile of HPP+ and RHPP+ in both groups was essentially identical and, in contrast to that observed in rodents, closely paralleled the profile found in humans treated with HP. These data in the baboon suggest that the metabolic processes involved in the production of the pyridinium metabolites of HP are similar to those in humans. Furthermore, the HPTP-treated baboon may be an appropriate model in which to study the role of pyridinium metabolites in the induction of tardive dyskinesia.

Hyperactive behavioural effects induced by intranigral infusion of a pyridinium metabolite of haloperidol in rats

Hyperactive behavioural effects induced by intranigral infusion of a pyridinium metabolite of haloperidol in rats

Quantitative analysis of two pyridinium metabolites of haloperidol in patients with schizophrenia

A pyridinium metabolite (HPP+) of the neuroleptic drug haloperidol has been identified in rats and in the urine of patients. The purpose of this study was to measure the steady-state blood and plasma concentrations and daily urinary excretion of HPP+ in patients treated with haloperidol. HPP+ was measured by HPLC with fluorescence detection. The chromatograms also revealed the presence of a previously unknown pyridinium species, which was identified in urine by liquid chromatography/mass spectrometry/mass spectrometry as 4-(4-chlorophenyl)-1-4-(4-fluorophenyl)-4-hydroxybutylpyridinium (RHPP+). Concentrations of RHPP+ were then measured by HPLC. The steady-state concentrations of HPP+ or RHPP+ in blood and plasma from 34 patients were virtually identical. The plasma concentrations of each metabolite were related to the daily dose of haloperidol and to its plasma concentrations. Nonlinearity in the elimination of RHPP+ was suggested by the increase in the ratio between RHPP+ and HPP+ plasma concentrations with dose or steady-state concentrations of haloperidol. The concentrations of RHPP+ in plasma and urine generally exceeded those of HPP+; the ratio between them in plasma ranged from 0.9 to 14.1. The daily urinary excretion of HPP+ and RHPP+ accounted for 0.40% +/- 0.18% and 2.3% +/- 1.4% of the haloperidol dose, respectively. The renal clearance of each species was 4.5 +/- 2.5 and 11.3 +/- 5.3 L/hr, respectively. The presence of these pyridinium species in humans raises the concern that they may be neurotoxic in a manner similar to the dopaminergic pro-neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

On the metabolism of haloperidol.

1. p-Fluorobenzoyl-propionic acid, 4-(4-chlorophenyl)-4-hydroxy-piperidine, and reduced haloperidol were confirmed as metabolites of haloperidol. Their formation was catalysed by hepatic microsomes and was NADPH dependent. 2. The pyridinium metabolite of haloperidol (HP+) was identified. It is proposed that haloperidol first undergoes dehydration to form its 1,2,3,6-tetrahydropyridine analogue (HTP). HTP is then further metabolized to HP+, HTP N-oxide and its N-dealkylated product, 4-chlorophenyl-1,2,3,6-tetrahydropyridine (CPTP). HTPN-oxide was metabolized to CPTP and HTP. All these metabolites were confirmed by comparison with synthesized compounds using h.p.l.c. and h.p.l.c.-mass spectrometry. 3. Three unknown metabolites were present in microsomal metabolic incubations of haloperidol. One of them was tentatively characterized by h.p.l.c.-mass spectrometry as an oxygenated product of haloperidol, another appears to be the 2-pyridine analogue of haloperidol. The third metabolite was shown to be a neutral compound of unknown structure, which was not haloperidol N-oxide nor 4-hydroxy-4'-fluorobutyrophenone. In addition, HTP was metabolized to a further unknown product with a similar u.v. spectrum to that of HTP. 4. The identification of these metabolites led to the hypothesis that the metabolism of haloperidol is similar to that of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and may therefore produce motor neurone toxicity by a similar mechanism.