Combination of experimental and in silico methods for the assessment of the phototransformation products of the antipsychotic drug/metabolite Mesoridazine

The lack of studies on the fate and effects of drug metabolites in the environment is of concern. As their parent compounds, metabolites enter the aquatic environment and are subject to biotic and abiotic process. In this regard, photolysis plays an important role. This study combined experimental and in silico quantitative structure-activity relationship (QSAR) methods to assess the fate and effects of Mesoridazine (MESO), a pharmacologically active human drug and metabolite of the antipsychotic agent Thioridazine, and its transformation products (TPs) formed through a Xenon lamp irradiation. After 256min, the photodegradation of MESO⋅besylate (50mgL^-1) achieved 90.4% and 6.9% of primary elimination and mineralization, respectively. The photon flux emitted by the lamp (200-600nm) was 169.55Jcm^-2. Sixteen TPs were detected by means of liquid chromatography-high resolution mass spectrometry (LC-HRMS), and the structures were proposed based on MSⁿ fragmentation patterns. The main transformation reactions were sulfoxidation, hydroxylation, dehydrogenation, and sulfoxide elimination. A back-transformation of MESO to Thioridazine was evidenced. Aerobic biodegradation tests (OECD 301 D and 301F) were applied to MESO and the mixture of TPs present after 256min of photolysis. Most of TPs were not biodegraded, demonstrating their tendency to persist in aquatic environments. The ecotoxicity towards Vibrio fischeri showed a decrease in toxicity during the photolysis process. The in silico QSAR tools QSARINS and US-EPA PBT profiler were applied for the screening of TPs with character of persistence, bioaccumulation, and toxicity (PBT). They have revealed the carbazole derivatives TP 355 and TP 337 as PBT/vPvB (very persistent and very bioaccumulative) compounds. In silico QSAR predictions for mutagenicity and genotoxicity provided by CASE Ultra and Leadscope® indicated positive alerts for mutagenicity on TP 355 and TP 337. Further studies regarding the carbazole derivative TPs should be considered to confirm their hazardous character.

Synthesis, receptor binding and functional studies of mesoridazine stereoisomers

The four stereoisomers of mesoridazine were synthesized and evaluated in D2, 5-HT1A, 5-HT2A, 5-HT2C, D1, and D3 receptor binding and functional assays. Two isomers demonstrated potent D2 receptor binding (Ki < 3 nM) and functional antagonism (IC50 < or = 10 nM) activities. These two isomers also showed moderate affinity for the 5-HT2A and D3 receptors. A third isomer was devoid of significant D2 receptor binding, but did have moderate affinity for the 5-HT2A and D3 receptors. The fourth isomer demonstrated poor affinity for all the receptors tested. Most significantly, the stereochemistry of the sulfoxide moiety played a dominant role in the observed structure-activity relationship (SAR).

The metabolism of piperidine-type phenothiazine antipsychotic agents. III. Mesoridazine in dog, human and rat

1. The metabolism of mesoridazine was studied in female rats (20 mg/kg, oral), female dogs (50 mg over 30 h, oral) and adult male volunteers (25 mg, oral). 2. Solvent extracts of urines from each species were directly analysed by h.p.l.c.-mass spectrometry with a plasmaspray interface. In the case of phenolic metabolites the urinary extracts were derivatized with a silylating reagent (with and without prior enzymic hydrolysis) prior to analysis. The structures of metabolites, with the exceptions of mesoridazine N-oxide and phenols, were confirmed by comparison of their chromatographic behaviours and mass spectra with those of authentic standards. 3. Compounds identified in the urine of all three species were mesoridazine, sulforidazine, mesoridazine ring sulphoxide, sulforidazine ring sulphoxide, N-desmethylmesoridazine ring sulphoxide, the lactam of sulforidazine ring sulphoxide and phenolic derivatives of mesoridazine and sulforidazine. Whereas the unconjugated phenolic metabolite of sulforidazine was present in urine of all three species, the conjugated form was identified only in dog and rat urines. Also, the unconjugated phenolic metabolite of mesoridazine was identified only in the urine of dog and human, but rat urine contained only the conjugated form. 4. Other metabolites found were: the lactam of mesoridazine (rat), the lactam of mesoridazine ring sulphoxide (rat and human), mesoridazine N-oxide (human) and sulforidazine N-oxide (dog and human). 5. Mesoridazine and six of its metabolites present in urines of human, rat and dog were quantified by a h.p.l.c.-u.v. method. The mean total excretion of measured analytes in human, rat and dog were 6.3, 2.6 and 29.1%, respectively. The excretion of the lactam of sulforidazine ring sulphoxide was greater in human (0.4%) and rat (0.2%) than dog (0.02%). Moreover, the urinary excretion of the lactam of mesoridazine ring sulphoxide in human and rat constituted 0.4% and 0.2%, respectively. Of the three lactams found in rat the lactam of mesoridazine was present in the least amount (0.05%). 6. Interspecies comparison of the lactam metabolites indicated that both qualitatively and quantitatively human more closely resembled rat than dog. On the other hand, N-oxide metabolites were detected in human and dog but not in rat.

UVA-induced genetic effects of thioridazine, mesoridazine and sulforidazine: an in vitro study

This in vitro study focuses on the UVA-induced reactions with DNA of thioridazine (TRZ), and two of its major metabolites (TRZ-2-sulfoxide or mesoridazine, MRZ; and TRZ-2-sulfone or sulforidazine, SRZ). TRZ binds covalently to DNA upon UVA-irradiation. Under comparable irradiation conditions, MRZ binds to a lesser extent and almost no binding was observed with SRZ. Besides, photo-induced genetic effects were investigated by means of a differential DNA repair test in E. coli. The photo-induced genetic effects in E. coli decreased from TRZ, MRZ to SRZ, which corresponds with their capacity for UVA-induced binding to DNA. TRZ, MRZ and SRZ differed in their rate of photodecomposition rather than in the intrinsic reactivity towards DNA of the instable intermediates formed. Irreversible binding to DNA was also observed upon treatment with peroxidase, which is known to oxidize phenothiazines via the formation of reactive radical cation species. As both the colour of the intermediate and its reactivity towards DNA were comparable for peroxidase treatment and exposure to UVA, we assume that the radical cation is the reactive species in the latter case as well.

Mesoridazine and thioridazine: clinical effects and blood levels in refractory schizophrenics

Seven schizophrenic (according to DSM-III criteria) inpatients completed a two-phase study; each phase had a 1-week drug-free period followed by 6 weeks of a drug trial. The first phase uniformly involved treatment with chlorpromazine, and in the second phase patients received either mesoridazine (N = 3) or thioridazine (N = 4). Clinical ratings (Brief Psychiatric Rating Scale and Clinical Global Impressions) and neuroleptic blood levels were obtained weekly throughout the study. Whereas patients failed to respond to chlorpromazine 1800 mg/day, response to mesoridazine 400 mg/day and to thioridazine 800 mg/day was established on all Brief Psychiatric Rating Scale factors except for anxiety-depression. A higher neuroleptic blood level was achieved with mesoridazine or thioridazine at less than half the reference chlorpromazine dosage. Correlations between neuroleptic blood level and clinical response were positive for mesoridazine, negative for chlorpromazine, and nonsignificant for thioridazine. These findings are consistent with earlier research. We conclude that drug-resistant schizophrenics seem to improve clinically with mesoridazine or thioridazine, unlike with chlorpromazine, and that for mesoridazine this difference may be a function of selective dopamine receptor blockade.

Phenolic metabolites of thioridazine in man

After oral administration of thioridazine or mesoridazine to man, faecal extracts contained mainly unconjugated 7-hydroxy-thioridazine and its demethylated analogue, together with some 3-hydroxy-thioridazine, and 7-hydroxy derivatives of sulphoridazine and desmethyl sulphoridazine. In contrast, urine contained free 7-hydroxy-mesoridazine, and no free 7-hydroxy-thioridazine was detectable. Conjugates of all these phenols were also found. After oral sulphoridazine administration (thioridazine side-chain sulphone), urine contained mainly 7-hydroxy-sulphoridazine and faeces both 7-hydroxy-sulphoridazine and its desmethyl analogue. Sulphoridazine metabolism is therefore much simpler than that of thioridazine. This different elimination pattern in faeces and urine may be due to reduction of sulphoxides but not sulphones by intestinal bacteria.

Mesoridazine -- a pharmacodynamic and pharmacokinetic profile

Available data on the clinical effectiveness and side effects of mesoridazine are reviewed, and an attempt is made to relate clinical efficacy to phenothiazine pharmacokinetics. Both controlled and open-label clinical studies have attested to the efficacy of mesoridazine in schizophrenia, as well as certain other psychiatric disorders. Clinical observations of the effectiveness of mesoridazine in patients refractory to treatment with thioridazine and other neuroleptics may be related to its slow rate of inactivation and to the relatively large proportion of free mesoridazine that is available for penetration to the target sites in the brain.

GLC analysis of thioridazine, mesoridazine, and their metabolites

A GLC method for measuring thioridazine, mesoridazine, their metabolites, and possibly other phenothiazines was developed. By using this method, seven different phenothiazine derivatives, thioridazine, and six known thioridazine metabolites were extracted and separated. This method was tested by assaying plasma samples from 30 hospitalized psychiatric patients receiving thioridazine or mesoridazine.

Albumin binding of thioridazine and some of its metabolites

The binding of thioridazine, northioridazine, mesoridazine and sulforidazine to bovine serum albumin is studied by means of equilibrium dialysis. For these four substances the percent binding in function of the albumin concentration is determined, as well as the apparent binding constant in the concentration range studied. The importance of the hydrophobic character and the chemical structure for the albumin binding is discussed.

Absorption and excretion of thioridazine and mesoridazine in man

Tritium labeled thioridazine and mesoridazine were given to four schizophrenic subjects to determine if differences in reported clinical potency of these two drugs could be explained by different rates of absorption and excretion. Mesoridazine was found to have earlier peak blood levels and lower fecal excretion. However, the blood and fecal differences were too small to be an adequate explanation for the differences in clinical potency suggesting that the rate of metabolic degradation is a more likely explanation for the potency difference. Thioridazine differs from other phenothiazines by containing two sulfur atoms. Thioridazine is metabolized by oxidative demethylation, oxidation at both sulfur atoms to sulfoxides and sulfones and by hydroxylation in the ring followed by glucuronide formation. Monosufoxides, disulfoxide and disufone have been found in the urine and bile of rats after thioridazine administration by inverse isotope dilution analysis. Neither the ring sulfoxide nor the disulfone show significant pharmacological activity, but activity is shown by the side chain monosulfoxide, mesoridazine. In fact it has been postulated that mesoridazine is the active form of thioridazine. Mesoridazine when compared on an equal dose basis to thioridazine is more potent in anti-emotional and hypotensive effects and produces more extrapyramidal symptoms. Since oxidation of the ring sulfur would be expected to decrease potency, it has been theorized that a portion of thioridazine is oxidized within the ring prior to the oxidation of the side chain sulfur atom thus effectively decreasing the potential activity of thioridazine. Thioridazine studies in rats have shown greater excretion in the urine and bile of the side chain sulfoxide than of the ring sulfoxide or of unchanged thioridazine. The difference in potency of these two compounds could alternatively be a result of differences in absorption, reabsorption after biliary excretion or the rate of urinary excretion. The metabolic pathways of mesoridazine in the human are essentially unknown. Because of this, we thought it worthwhile to determine if the difference in potency between thioridazine and mesoridazine is also related to differences in the rate of excretion and absorption. Because phenothiazines are found in extremely low plasma concentrations, we used radioactive compounds to perform this study.