Metoclopramide metabolism and determination by high-pressure liquid chromatography

Photodegradation, toxicity and density functional theory study of pharmaceutical metoclopramide and its photoproducts

Pharmaceuticals as ubiquitous organic pollutants in the aquatic environment represent substances whose knowledge of environmental fate is still limited. One such compound is metoclopramide, whose direct and indirect photolysis and toxicological assessment have been studied for the first time in this study. Experiments were performed under solar radiation, showing metoclopramide as a compound that can easily degrade in different water matrices. The effect of pH-values showed the faster degradation at pH = 7, while the highly alkaline conditions at pH = 11 slowed photolysis. The highest value of quantum yield of metoclopramide photodegradation (ϕ = 43.55·10^-4) was obtained at pH = 7. Various organic and inorganic substances (NO₃^-, Fe(III), HA, Cl^-, Br^-, HCO₃^-, SO₄^2-), commonly present in natural water, inhibited the degradation by absorbing light. In all experiments, kinetics followed pseudo-first-order reaction with r² greater than 0.98. The structures of the photolytic degradation products were tentatively identified, and degradation photoproducts were proposed. The hydroxylation of the aromatic ring and the amino group's dealkylation were two major photoproduct formation mechanisms. Calculated thermochemical quantities are in agreement with the experimentally observed stability of different photoproducts. Reactive sites in metoclopramide were studied with conceptual density functional theory and regions most susceptible to ^•OH attack were characterized. Metoclopramide and its degradation products were neither genotoxic for bacteria Salmonella typhimurium in the SOS/umuC assay nor acutely toxic for bacteria Vibrio fischeri.

Influence of Cation Transporters (OCTs and MATEs) on the Renal and Hepatobiliary Disposition of [11C]Metoclopramide in Mice

PURPOSE

To investigate the role of cation transporters (OCTs, MATEs) in the renal and hepatic disposition of the radiolabeled antiemetic drug [11C]metoclopramide in mice with PET.

METHODS

PET was performed in wild-type mice after administration of an intravenous microdose (<1 μg) of [11C]metoclopramide without and with co-administration of either unlabeled metoclopramide (5 or 10 mg/kg) or the prototypical cation transporter inhibitors cimetidine (150 mg/kg) or sulpiride (25 mg/kg). [11C]Metoclopramide PET was also performed in wild-type and Slc22a1/2(-/-) mice. Radiolabeled metabolites were measured at 15 min after radiotracer injection and PET data were corrected for radiolabeled metabolites.

RESULTS

[11C]Metoclopramide was highly metabolized and [11C]metoclopramide-derived radioactivity was excreted into the urine. The different investigated treatments decreased (~2.5-fold) the uptake of [11C]metoclopramide from plasma into the kidney and liver, inhibited metabolism and decreased (up to 3.8-fold) urinary excretion, which resulted in increased plasma concentrations of [11C]metoclopramide. Kidney and liver uptake were moderately (~1.3-fold) reduced in Slc22a1/2(-/-) mice.

CONCLUSIONS

Our results suggest a contribution of OCT1/2 to the kidney and liver uptake and of MATEs to the urinary excretion of [11C]metoclopramide in mice. Cation transporters may contribute, next to variability in the activity of metabolizing enzymes, to variability in metoclopramide pharmacokinetics and side effects.

An accelerated background subtraction algorithm for processing high-resolution MS data and its application to metabolite identification

Background: Metabolite identification without radiolabeled compound is often challenging because of interference of matrix-related components. Results: A novel and an effective background subtraction algorithm (A-BgS) has been developed to process high-resolution mass spectral data that can selectively remove matrix-related components. The use of a graphics processing unit with a multicore central processing unit enhanced processing speed several 1000-fold compared with a single central processing unit. A-BgS algorithm effectively removes background peaks from the mass spectra of biological matrices as demonstrated by the identification of metabolites of delavirdine and metoclopramide. Conclusion: The A-BgS algorithm is fast, user friendly and provides reliable removal of matrix-related ions from biological samples, and thus can be very helpful in detection and identification of in vivo and in vitro metabolites.

Drug-drug interactions in pharmacologic management of gastroparesis

BACKGROUND

Gastroparesis is a disorder characterized by delayed gastric emptying due to chronic abnormal gastric motility. The treatment of the disease often entails the co-administration of several classes of pharmacological agents. These agents may be metabolized via the same pathway. Inhibition or induction of a shared metabolic pathway leads to change in the systemic levels of prescribed drugs, possibly leading to undesired clinical outcomes.

PURPOSE

This review discusses different pharmacological treatment for gastroparesis patients and describes the potential for drug-drug interactions (DDIs) in some of the combinations that are currently used. Prokinetic agents such as metoclopramide and domperidone are the cornerstone in treatment of gastroparesis. Antiemetic agents such as promethazine and ondansetron are frequently administered to gastroparesis patients to reduce nausea and vomiting. Gastroparesis is prevalent in diabetic patients and therefore antidiabetic agents are also prescribed. Many of these co-administered drugs are metabolized via common drug metabolizing enzymes and this can trigger potential DDIs. The scientific literature was reviewed from the years 1975-2014 for original research articles and reviews that evaluated DDIs in gastroparesis. Many commonly prescribed combinations were predicted to cause potential DDIs in gastroparesis patients. This review will help inform about potential hazardous combinations. This information will hopefully lead to less adverse effects and more successful gastroparesis management.

Metoclopramide is metabolized by CYP2D6 and is a reversible inhibitor, but not inactivator, of CYP2D6

1. Metoclopramide is a widely used clinical drug in a variety of medical settings with rare acute dystonic events reported. The aim of this study was to assess a previous report of inactivation of CYP2D6 by metoclopramide, to determine the contribution of various CYPs to metoclopramide metabolism, and to identify the mono-oxygenated products of metoclopramide metabolism. 2. Metoclopramide interacted with CYP2D6 with Type I binding and a Ks value of 9.56 ± 1.09 µM. CYP2D6 was the major metabolizer of metoclopramide and the two major products were N-deethylation of the diethyl amine and N-hydroxylation on the phenyl ring amine. CYPs 1A2, 2C9, 2C19, and 3A4 also metabolized metoclopramide. 3. While reversible inhibition of CYP2D6 was noted, CYP2D6 inactivation by metoclopramide was not observed under conditions of varying concentration or varying time using Supersomes(TM) or pooled human liver microsomes. 4. The major metabolites of metoclopramide were N-hydroxylation and N-deethylation formed most efficiently by CYP2D6 but also formed by all CYPs examined. Also, while metoclopramide is metabolized primarily by CYP2D6, it is not a mechanism-based inactivator of CYP2D6 in vitro.

IN SILICO PREDICTION OF DRUG BINDING TO CYP2D6: IDENTIFICATION OF A NEW METABOLITE OF METOCLOPRAMIDE

Patients with cancer often take many different classes of drugs to treat the effects of their malignancy and the side effects of treatment, as well as their comorbidities. The potential for drug-drug interactions that may affect the efficacy of anticancer treatment is high, and a major source of such interactions is competition for the drug-metabolizing enzymes, cytochromes P450 (P450s). We have examined a series of 20 drugs commonly prescribed to cancer patients to look for potential interactions via CYP2D6. We used a homology model of CYP2D6, together with molecular docking techniques, to perform an in silico screen for binding to CYP2D6. Experimental IC50 values were determined for these compounds and compared with the model predictions to reveal a correlation with a regression coefficient of r2= 0.61. Importantly, the docked conformation of the commonly prescribed antiemetic metoclopramide predicted a new site of metabolism that was further investigated through in vitro analysis with recombinant CYP2D6. An aromatic N-hydroxy metabolite of metoclopramide, consistent with predictions from our modeling studies, was identified by high-performance liquid chromatography/mass spectrometry. This metabolite was found to represent a major product of metabolism in human liver microsomes, and CYP2D6 was identified as the main P450 isoform responsible for catalyzing its formation. In view of the prevalence of interindividual variation in the CYP2D6 genotype and phenotype, we suggest that those experiencing adverse reactions with metoclopramide, e.g., extrapyramidal syndrome, are likely to have a particular CYP2D6 genotype/phenotype. This warrants further investigation.

Effects of acute moderate hypoxemia on kinetics of metoclopramide and its metabolites in chronically instrumented sheep

Hypoxemia is known to induce various physiological changes which can result in alteration in drug pharmacokinetics. To examine the effect of acute moderate hypoxemia on metoclopramide (MCP) pharmacokinetics, a continuous 14-hour infusion of MCP during a normoxemic, hypoxemic and subsequent normoxemic period was conducted in eight adult sheep. Arterial blood and urine samples were collected to examine the effects on the pharmacokinetics of MCP and its deethylated metabolites. MCP and its mono- and di-deethylated metabolites were quantitated using a GC/MS method. Steady-state concentrations of MCP were achieved in each of the three periods. During hypoxemia, MCP plasma steady-state concentration increased significantly from 50.72 +/- 1.06 to 63.62 +/- 1.79 ng/mL, and later decreased to 55.83 +/- 1.15 ng/mL during the post-hypoxemic recovery period. Total body clearance (CL(TB)) of MCP was significantly decreased from 274.2 +/- 48.0 L/h to 205.40 +/- 28.2 L/h during hypoxemia, and later restored to 245.8 +/- 44.2 L/h during the post-hypoxemic period. Plasma mono-deethylated MCP concentration (32.78 +/- 1.73 ng/mL) also increased, compared to the control group (21.20 +/- 1.39 ng/mL), during hypoxemia and subsequent normoxemic period. Renal excretion of MCP and its metabolites was also decreased during hypoxemia, while urine flow was increased with a concomitant decrease in urine osmolality. Thus, the results indicate that acute moderate hypoxemia affects MCP pharmacokinetics.

The gastroprokinetic and antiemetic drug metoclopramide is a substrate and inhibitor of cytochrome P450 2D6

Metoclopramide is increasingly prescribed for conditions previously treated with cisapride, but its metabolic enzymology and drug interactions are poorly understood. Using human liver microsomes (HLMs) and recombinant human cytochromes P450 (P450), we identified the major route of metoclopramide oxidation and the P450 isoforms involved. We also documented the ability of metoclopramide to inhibit the P450 system, using isoform-specific substrate reaction probes of CYP1A2, 2C19, 2C9, 2D6, 2E1, and 3A4. Metoclopramide was predominantly N-dealkylated to monodeethylmetoclopramide, a metabolite that has not so far been described in humans. Formation rate of this metabolite followed Michaelis-Menten kinetics (K(m), 68 +/- 16 microM; V(max), 183 +/- 57 pmol/min/mg of protein; n = 3 HLMs). Of the isoform-specific inhibitors tested, 1 microM quinidine was a potent inhibitor of metoclopramide (25 microM) monodeethylation [by an average of 58.2%; range, approximately 38% (HL09-14-99) to 78.7% (HL161)] with K(i) values highly variable among the HLMs tested (K(i), mean +/- S.D., 2.7 +/- 2.8 microM; range, 0.15 microM in HL66, 2.4 microM in HL09-14-99, and 5.7 microM in HLD). Except troleandomycin, which inhibited metoclopramide metabolism in only one HLM (by approximately 23% in HL09-14-99), the effect of other inhibitors was minimal. Among the recombinant human P450 isoforms examined, monodeethylmetoclopramide was formed at the highest rate by CYP2D6 (V = 4.5 +/- 0.3 pmol/min/pmol of P450) and to a lesser extent by CYP1A2 (0.97 +/- 0.15 pmol/min/pmol of P450). The K(m) value derived (approximately 53 microM) was close to that from HLMs (68 microM). Metoclopramide is a potent inhibitor of CYP2D6 at therapeutically relevant concentrations (K(i) = 4.7 +/- 1.3 microM), with negligible effect on other isoforms tested. Further inhibition of CYP2D6 was observed when metoclopramide was preincubated with HLMs and NADPH-generating system before the substrate probe was added (maximum rate of inactivation, K(inact) = 0.02 min(-1), and the concentration required to achieve the half-maximal rate of inactivation, K'(i) = 0.96 microM), suggesting mechanism-based inhibition. Metoclopramide elimination is likely to be slowed in poor metabolizers of CYP2D6 or in patients taking inhibitors of this isoform, whereas metoclopramide itself could reduce the clearance of CYP2D6 substrate drugs.

Preparation and in vivo evaluation of parenteral metoclopramide-loaded poly(alkylcyanoacrylate) nanospheres in rats

Metoclopramide-loaded poly(alkylcyanoacrylate) (PACA) nanospheres were prepared by emulsifier-free polymerization in aqueous media at ambient conditions. The optimum polymerization conditions for metoclopramide sorption on PACA nanospheres in the presence of dextran (DEX) or hydroxypropyl-beta-cyclodextran (HPCD) in the polymerization medium were studied and the feasibility of either poly(isobutylcyanoacrylate) (PIBCA) or poly(ethylcyanoacrylate) (PECA) nanospheres as parenteral prolonged release drug delivery system of metaclopramide in rats was also investigated. The optimum time for the addition of metaclopramide after initiating the polymerization was 1 h, which results in 14.8 +/- 0.4 and 9.2 +/- 0.5% of drug loading for PIBCA and PECA, respectively. The HPCD in the polymerization medium of PECA nanospheres improved the drug adsorption compared to DEX at all times, but the difference was only significant (p < 0.05) when metoclopramide was added at 0 and 30 min. Wistar rats were given subcutaneous (s.c.) injections of metoclopramide solution (5 mg/kg) and three different metoclopramide nanospheres suspensions (10 mg/kg) on two phases. The drug solution is rapidly absorbed, distributed, and eliminated. The maximum drug concentration was observed after 30 min of s.c. administration of all the tested nanosphere formulations. PECA-HPCD showed the highest concentration (3.16 +/- 0.66 mg/L) followed by PIBCA-DEX (1.95 +/- 0.37 mg/L) and PECA-DEX (1.68 +/- 0.28 mg/L). The AUCs of PECA-DEX, PECA-HPCD and PIBCA-DEX were 4.8, 1.88 and 2.43 times higher than that of the solution form, respectively. Following PECA-DEX the maximum drug concentration, 1.68 +/- 0.28 mg/L, rapidly decreased to 0.54 +/- 0.05 mg/L. The drug was successfully maintained around this serum drug concentration up to 12 h in rats and the mean drug concentration was reduced to 0.2 +/- 0.02 mg/L, 63% reduction, after 24 h of nanosphere administration. The developed aqueous parenteral prolonged release preparation (PECA-DEX) could be used as a promising intermittent formula for metoclopramide or other drugs when the oral route is not accessible, especially during managing chronic nausea in patients with advanced cancer.

Formulation optimization of controlled-release pellets of metoclopramide hydrochloride using dissolution fit factor approach

The purpose of this study was to optimize the formulation variables for the preparation of ethyl cellulose-coated nonpareils loaded with metoclopramide hydrochloride (MCL). The approach to evaluate the effectiveness of formulation parameters was monitored by release rate testing using dissolution fit factors as a tool. The content of ethyl cellulose used in the formulation was based on the drug-loaded weight. The interrelationship of each developed formulation and the reference formulation Gastro-Timelets and their respective dissolution curves were evaluated using Moore's equation: [equation: see text]. The relationship between the ethyl cellulose content in the formulation and the dissolution fit factor f2 can be described as the following regression equation: Y = -0.054X2 + 3.347X - 1.915 (r2 = 0.99). The optimum ethyl cellulose content obtained from the equation was 30.8%. The type and content of plasticizer used in the formulation to achieve the greatest f2 were determined to be Myvacet 9-40 at the concentration of 25%. Results indicated that using the release rate testing approach with the dissolution fit factor as a tool could provide valuable information for formulation optimization.

Identification of metoclopramide metabolites in the urine of cattle by gas chromatography-mass spectrometry and high-performance liquid chromatography-photodiode array detection

The urinary metabolites of metoclopramide (4-amino-5-chloro-N-[2-diethylaminoethyl]-2-methoxybenzamide) were identified in cows. The drug was administered intravenously, voided urine was collected, and individual urine extracts were analysed by gas chromatography-mass spectrometry and high-performance liquid chromatography-photodiode array detection. The parent compound and one major metabolite (4-amino-5-chloro-N-[2-(ethylamino)ethyl]-2-methoxybenzamide) were common to all individuals. In addition to the parent and major metabolite, a second, minor metabolite was identified in two cows as 4-amino-5-chloro-N-[2-(diethylamino)ethyl]-2-hydroxybenzamide. The identity of the minor metabolite was confirmed by comparison with a standard synthesized by a new method. Metabolite identification and characterization in food animal species allows the design of safety and environmental impact studies and relative metabolite ratios between dose treatment groups.

Effects of cross-gender steroid hormone treatment on prolactin concentrations in humans

It is well known that peripheral prolactin levels are significantly higher in menarcheal women than in men. Higher levels of prolactin in menarcheal women are related to exposure to higher levels of estrogen in women than in men. Increased exposure to androgens in men has also been proposed as a possible reason to account for lower prolactin levels in men; however, this suggestion has not been conclusively proven. The current study sought to evaluate the cross-gender effects of male and female hormones on basal levels and the pituitary store of prolactin in humans. Four groups of individuals were involved: normal men and women, male and female transsexuals primed with female hormones and testosterone, respectively for at least 6 months. A metoclopramide challenge test was carried out on each subject of each group. Subjects were rested for 1 h, with an indwelling catheter in the antecubital vein, before a blood sample was collected for estimation of basal hormone levels. Following an oral ingestion of 10 mg of metoclopramide, blood samples were collected at 15, 30, 60, 90, 120, 150 and 180 min. Prolactin, estradiol and testosterone concentrations were measured by radioimmunoassay. Basal levels as well as metoclopramide-induced releases of prolactin (as measured by area under the curve) in normal women were significantly higher (p less than 0.05) than corresponding levels in normal men. Following long-term priming with female hormones, the pattern of response to metoclopramide in male transsexuals was dramatically changed.(ABSTRACT TRUNCATED AT 250 WORDS)

An automated high-performance liquid chromatographic trace enrichment method for the determination of metoclopramide in serum and its application to a bioequivalence human volunteer study

An automated trace enrichment method for metoclopramide is described. Serum was injected on to a short column packed with PLRP-S 100A (a polymeric reversed-phase material). The unwanted components were washed off with borate buffer (pH 9.8) before switching to a Spherisorb ODS column for the separation. The limit of detection in serum samples was 2 ng/ml. The method was used in a pharmacokinetic study to compare the biovailability of two formulations of the drug.

Metoclopramide--a review

Metoclopramide has wide applications in both clinical and experimental medicine. It is useful in the management of gastro-oesophageal reflux and gastric stasis. It is being used increasingly in the management of nausea and vomiting, and at high doses will significantly relieve the emesis that is induced by cytotoxic agents. Metoclopramide also has an important place in the investigation of the role of dopamine in physiological and pathological processes.

Antiemetic effect and pharmacokinetics of high dose metoclopramide in cancer patients treated with cisplatin-containing chemotherapy regimens

Fifteen cancer patients receiving cisplatin-containing chemotherapy participated in two antiemetic studies. In Study 1 they received standard antiemetics in low doses on demand, and in Study 2 the same patients participated in an open randomized cross-over study between metoclopramide 1 and 2 mg/kg i.v. X 5. Serum metoclopramide was determined by HPLC. Self-reporting of nausea using a visual analogue scale (VAS) was compared with observer rated scores. Tolerability and volume vomited were assessed by nurse observers. The biological half-life of metoclopramide was 9.9 h, the volume of distribution was 9.9 l/kg and the clearance was 0.68 l/h/kg. The pharmacokinetics of high dose metoclopramide was linear in the range 0.15-2 mg/kg X 5, with very little accumulation. Compared to standard antiemetics, both high dose regimens of metoclopramide had a significant effect on nausea, but no effect on the volume vomited. Self reports of nausea were significantly correlated with observer rated values. Tolerance of high dose metoclopramide was good except in 3 patients who left the study because of restlessness and trismus. It is concluded that high dose metoclopramide probably can be administered for several consecutive days without appreciable accumulation of the drug. Self-reporting of nausea by patients on VAS is a simple and feasible method of evaluation. The finding that metoclopramide affects nausea but not vomiting supports the hypothesis that nausea and vomiting should be evaluated separately in assessing antiemetic efficacy.

Pharmacokinetic studies of high-dose metoclopramide with and without forced diuresis

The pharmacokinetics of high-dose metoclopramide (10 mg/kg body wt. in five infusions of 2 mg/kg body wt. each) was studied in 11 patients (5 females, 6 males) in two groups: group A with and group B (consisting of five patients) without forced diuresis. When the drug was infused, forced diuresis had no influence on the pharmacokinetics of metoclopramide (serum level after the 1st infusion was 851 +/- 361 ng/ml in group A versus 840 +/- 348 ng/ml in group B; after the 5th infusion it was 2,005 +/- 588 ng/ml in group A versus 2,463 +/- 1,350 ng/ml in group B). There were significant differences in the 24-h serum levels (582 +/- 308 ng/ml in group A versus 379 +/- 170 ng/ml in group B; P less than 0.05) and in the elimination half life (8.5 +/- 2.6 h in group A versus 6.1 +/- 1.1 h in group B; P less than 0.05). The results demonstrate that the dosage regimen originally suggested by Gralla for cytostatic drugs, with forced diuresis for high-dose metoclopramide therapy, may also be applied, with no dosage reduction, with to other cytostatic drugs which do not require forced diuresis.

Drug level monitoring: chromatography of some minor groups of drugs

Some important facts about the chromatographic separation of a number of selected categories of drugs are summarized. The data refer to the chromatographic method of choice, stationary phase, mobile phase (carrier gas), detection procedure and (where available) method sensitivity. Also, fundamental instrumental parameters, namely injector, column and detector temperature, carrier gas and mobile phase flow-rate and gradient set-up are reported here. In all cases also the source material used for analysis is specified. The data are presented in table form, each table dealing with a particular category of drugs. The following categories of drugs are being dealt with: anthelmintics, antiarteriosclerotics, antibacterials, anticholinergics and cholinergics, anticoagulants, antidiabetics, antiemetics, antimycotics, antihistamines, antimalarials, antiparasitics, antiparkinsonics, antitussives, antiulcer drugs, antiviral compounds, appetite depressants and immunosuppressives.

Successful administration of metoclopramide for the treatment of nausea in patients with advanced liver disease. A double-blind controlled trial

A double-blind comparison of metoclopramide versus placebo was performed on 8 cirrhotic patients with nausea (8 cases) and heartburn (3 of the 8 cases) plus mild portal-systemic encephalopathy. As metoclopramide is a dopamine antagonist and dopamine-inadequate neurotransmission has been implicated in the pathogenesis of hepatic coma, this study was also designed to evaluate the effects of metoclopramide on mental state. The study included basal, placebo, metoclopramide, and final periods; each period lasted for 2 wk. Throughout the study patients received 3 g/day of neomycin and an 1800-cal diet containing 40 g/day of mixed protein. During the placebo and metoclopramide phases patients received either two 10-mg metoclopramide capsules t.i.d. or identical placebo capsules. During the study, biweekly liver function tests and portal-systemic encephalopathy parameters were evaluated. A self-evaluation for the presence of nausea and heartburn was also obtained. To monitor the dopamine-blockade effect of metoclopramide, serum prolactin levels were measured. Metoclopramide significantly suppressed the subjective signs of nausea (7 of 8 cases) and heartburn (all cases). Serum prolactin levels were 22 +/- 21 ng/ml, 30 +/- 31 ng/ml, 110 +/- 57 ng/ml (p less than 0.01), and 18.6 +/- 2 ng/ml during basal, placebo, metoclopramide, and final periods, respectively. In spite of these signs of dopamine blockade, no deterioration in mental state, asterixis, electroencephalograms, blood ammonia levels, or psychometric testings were observed. In addition, no extrapyramidal signs were noticeable during any period of the study. One patient presented transient somnolence at the end of the metoclopramide period. We conclude that dopamine blockade is not associated with the appearance of portal-systemic encephalopathy. Metoclopramide is a safe and effective treatment for nausea and heartburn in patients with advanced liver disease.

Determination of bromopride in human plasma and urine by high-performance liquid chromatography

Bromopride was measured in plasma and urine using reversed-phase high-performance liquid chromatography employing ultraviolet absorption detection. The limit of detection in plasma was 2 ng/ml, sufficient for pharmacokinetic studies of the drug. Plasma concentrations of bromopride reached mean peak levels (55 ng/ml) at 1 h after single oral doses of 20 mg and declined with a half-life of 4.9 h. Less than a mean of 10% of an oral dose was excreted unchanged in the urine. The assay could also be used to measure metoclopramide in these bio-fluids.

Evaluation of a high pressure liquid chromatographic technique for metoclopramide analysis

High-dose metoclopramide has found an established place in the prophylaxis of nausea and vomiting which may be induced by cancer chemotherapy. A simple reliable high pressure liquid chromatographic technique for the measurement of this drug in serum has been developed and evaluated. The methodology will find application in the study of metoclopramide pharmacokinetics and in the monitoring of high-dose metoclopramide therapy in individual patients with neoplastic disease.

Nonlinear bioavailability of metoclopramide in the rat: evidence for saturable first-pass metabolism

In vivo and in vitro experiments were conducted in rats to examine the possibility of either extrahepatic metabolism or saturable first-pass effect as an explanation for the unusual presystemic clearance of metoclopramide (I) previously reported. In vivo studies involved two-thirds hepatectomized rats and animals pretreated with carbon tetrachloride to induce hepatic necrosis, whereas in vitro studies involved incubation of equal amounts of I (5.0 mumol/mL) with various tissue homogenates (viz., liver, kidney, and lung) or their 9000 X g supernatant fractions. Results suggest that the metabolism of I principally occurs in the rat liver, and there was no evidence suggesting the involvement of kidney or lung tissue in the metabolism of I. Forty-eight-hour cumulative urinary excretion studies following oral and intravenous administration of less than or equal to 5.0 mg/kg of metoclopramide hydrochloride were conducted. The bioavailability (F) values of I at dosage levels 0.1, 0.5, 1.0, and 5.0 mg/kg were 0.49, 0.75, 0.77, and 0.83, respectively. It is concluded that the liver is the primary organ for the metabolism of I in the rat and that the drug exhibits dose-dependent hepatic first-pass metabolism.

The determination of metoclopramide in plasma by reversed-phase ion-pair high-performance liquid chromatography

Methodology based on reversed-phase ion-pair high-performance liquid chromatography is described for the determination of metoclopramide in plasma. The chromatography was optimized in terms of the peak shape for the drug and its resolution from endogenous plasma components by investigating the effects of quaternary ammonium (competing) ions and alkylsulphate (pairing) ions in an acidic mobile phase containing acetonitrile (20%) and 20 mM acetic acid. Optimum chromatographic conditions were obtained with an ODS-Hypersil column and a mobile phase containing 20% acetonitrile, 20 mM acetic acid, 0.6 mM sodium octylsulphate and 0.5 mM tetrabutylammonium chloride. A simplified method of sample preparation is described in which only 1 ml of plasma is required. The limit of detection (at 310 nm) was 7 ng/ml and no interference from endogenous plasma components or from any drugs commonly used in the treatment of cancer was observed. Consequently the methodology should be applicable to pharmacokinetic studies on metoclopramide, when used clinically to control the gastro-intestinal side-effects of chemotherapy.

Metoclopramide. An updated review of its pharmacological properties and clinical use

Since previously reviewed in the Journal (Vol. 12, No. 2), metoclopramide has been confirmed as an effective drug in treating and preventing various types of vomiting and as a useful agent in oesophageal reflux disease, gastroparesis, dyspepsia, and in a variety of functional gastrointestinal disorders. Of considerable importance is the recent evidence of its efficacy when administered intravenously in high dosages in preventing severe vomiting associated with cisplatin. Good results have been achieved in patients not previously treated with cisplatin, but further studies are needed to determine its level of efficacy in patients who have experienced severe vomiting during earlier courses of cytotoxic therapy. Side effects consisting of mild sedation, diarrhoea and reversible extrapyramidal reactions have occurred, but are tolerated by many patients.

Comparison of inhibitory effects of substituted benzamides and classical neuroleptics on operant behavior in rats and squirrel monkeys

The operant behavioral effects of the substituted benzamides metoclopramide, sultopride and sulpiride were compared with those of chlorpromazine and haloperidol using lateral hypothalamic self-stimulation of the brain in rats and Sidman avoidance responding in rats and squirrel monkeys. All the drugs inhibited both phenomena dose-dependently in rats. The order of potency was: haloperidol greater than chlorpromazine greater than metoclopramide greater than sultopride greater than sulpiride. Sulpiride only produced moderate inhibition even at 128 mg/kg s.c. All drugs also suppressed Sidman avoidance in squirrel monkeys, which were far more sensitive than rats to the inhibitory effects of all three benzamides. In monkeys, sulpiride 8 mg/kg i.m. apparently suppressed Sidman avoidance with a delayed onset and a prolonged duration, which contrasted with the rapid onset and short duration of metoclopramide action. Thus, long-term observation of the behavioral effects of benzamides in squirrel monkeys may provide more precise information on the clinical antipsychotic efficacy of substituted benzamides.

The pharmacokinetics of a new benzamide drug clebopride, in the rat and the dog

After intravenous, intramuscular and oral administration of clebopride in the rat and the dog its apparent volume of distribution is high (1.6-3.2 1 kg-1) and it has a longer biological half-life than metoclopramide in both species. High clearance values and concentrations of metabolites in plasma after oral administration indicate that the drug is subjected to an extensive first pass metabolism in the rat. Thus, clebopride administered orally gives relatively low concentrations in the systemic circulation in rats even though it is rapidly absorbed. The metabolic processes appear to become saturated at high doses which is reflected in dose-dependent kinetics. Linear kinetics were observed in the dog, although enterohepatic recycling could occur.

Single-dose pharmacokinetics of metoclopramide

The time courses of plasma metoclopramide concentrations were followed in six subjects after oral and intravenous single dose administration. Plasma concentration-time data following i. v. administration in each subject were found to fit a two compartment model with a mean terminal half-life of 4.55 h +/- 0.80 h and a mean distribution half-time of 0.35 h +/- 0.09 h. Volumes of distribution were high (3.43 +/- 1.181 . kg-1), and clearances (0.53 +/- 0.191 . kg-1 h-1) approached liver plasma flow. This suggests that metoclopramide occurs at higher concentrations in tissues than in plasma, and that its clearance is probably limited by liver blood flow rather than liver metabolic capacity. The postabsorption decline in metoclopramide plasma levels after oral administration was also biexponential in each subject. The terminal half-life was 5.17 h +/- 0.98 h. Mean volume of distribution and mean clearance were similar to intravenous values (after adjustment for bioavailability). Oral absorption was rapid with peak plasma concentrations being reached at a mean time of 0.93 h. A mean bioavailability of 0.77 was calculated for the six subjects, and it was postulated that this incomplete availability is due to a first-pass effect. The inter-individual variation in the degree of "first-pass' was considerable (0.47--1.14).

The pharmacokinetics of single doses of metoclopramide in renal failure

The pharmacokinetics of metoclopramide have been studied after intravenous and oral dosing (10 mg) to 6 patients with chronic renal failure. The mean terminal half-life was 13.9 h after intravenous and 14.8 h after oral administration. Total body clearance after i.v. dosing was 16.7 l/h. Oral bioavailability was 71.8%. In comparison to previous studies on normal subjects these results indicate that clearance of metoclopramide in renal failure is approximately 30% of normals. This difference is not accounted for by the change in renal clearance and suggests impaired metabolism or an alteration in enterohepatic circulation of metoclopramide in renal failure.

Electron-capture gas chromatographic assay for metoclopramide in plasma

An original electron-capture gas chromatographic assay has been developed for the quantiation of metoclopramide in human plasma. The method involves derivatization with heptafluorobutyryl imidazole after alkaline extraction, acid backwash, and a further alkaline extraction. Plasma levels of metoclopramide as low as 5 micrograms/l can be measured using 1 ml of plasma, and no interference from related substances or commonly prescribed drugs has been found. The percentage recovery of drug from plasma ranges from 88% to virtually 100%, and the between-run variation in the assay is 4.3%. The assay has been used for the study of metoclopramide pharmacokinetics in man following intravenous single-dose administration. The resultant plasma concentration vs. time curve was biexponential, with a terminal half-life of 5.0 h, and a distribution half-time of 0.3 h.

The pharmacokinetics of metoclopramide in man with observations in the dog

1 The pharmacokinetics of metoclopramide have been studied in eight normal male volunteers. 2 The mean plasma beta half-life was 156.7 min after i.v. administration of 10 mg metoclopramide. 3 After oral dosing of 10 mg the mean half-life was 196.6 min and after 20 mg 317.5 min (P less than 0.05). 4 Bioavailability of a 10 mg oral dose of metoclopramide varied between 32 and 97%. 5 A major urinary metabolite was metoclopramide-N-4-sulphate and the amounts of conjugates appearing in urine to 24 h correlated significantly with the bioavailability. 6 In the dog the metabolic fate of metoclopramide is different to man with conjugation being a minor metabolic pathway. The half-life in the dog does not appear to be dose dependent. 7 The wide differences in bioavailability of metoclopramide in man may contribute to the unpredictable occurrence of side effects.

Pharmacokinetics of metoclopramide intravenously and orally determined by liquid chromatography

1 A rapid and sensitive method, based on liquid chromatography, has been developed for determination of metoclopramide concentrations in plasma and urine samples. Concentrations down to 15 nmol/1 (5 ng/ml) of plasma and 100 nmol/1 (30 ng/ml) of urine could be determined with a relative standard deviation of less than or equal to 10%. The method was used to study disposition of metoclopramide in healthy volunteers following single doses intravenously and orally as aqueous solution and a slow release tablet. 2 The initial distribution after intravenous administration was very rapid. The elimination half-life postdistribution was 4.9 h. The apparent volume of distribution, Vd, was 3.0 1/kg body weight. On average 19% was excreted unchanged after intravenous administration of 5 and 10 mg (15 and 30 mumol) of drug. The rate of absorption of metoclopramide was delayed after administration of a slow release tablet and the maximum plasma concentration was about 50% lower than after a solution. The extent of bioavailability was the same following the two different formulations suggesting a first-pass elimination of 25-40%.

Modification of metoclopramide GLC assay: application to human biological specimens

A modified electron-capture GLC assay for metoclopramide in human biological specimens is reported. This assay involves the incorporation of a back-extraction method to remove endogenous contaminants. Its applicability was demonstrated by studying the time course of metoclopramide in plasma and urine from a human subject. The lowest quantifiable metoclopramide concentration in plasma was 7 ng/ml, provided 0.5 ml of plasma was used.

Rapid thin-layer chromatographic photodensitometric method for the determination of metoclopramide and clebopride in the presence of some of their metabolic products

Metoclopramide and its newly developed analogue clebopride, together with some of their metabolic products are quantitated, following extraction from biological tissues and fluids, and subsequent separation on silica gel thin-layer chromatographic plates. Diazotisation, followed by coupling with N-(1-naphthyl)ethylenediammonium dichloride, carried out on the thin-layer plate, is utilised for visualisation. The intensity of the spots is measured by photodensitometric analysis. The effect of variation of various experimental conditions is studied. The method has proven to be satisfactory for the measurement of 20 ng/ml of these compounds in biological material; the results are well within the accepted limits of deviation.

L-Dopa inhibits metoclopramide stimulation of the lower esophageal sphincter in man

Opossum lower esophageal sphincter smooth muscle contains inhibitory dopaminergic receptors. Since metoclopramide is a dopaminergic antagonist in many experimental situations, the present study was designed to investigate whether this mechanism could explain the lower esophageal sphincter (LES) stimulating action of metoclopramide in man. The interactions of (1) oral L-dopa, a dopamine precursor, and metoclopramide; and (2) L-dopa and the cholinergic agent, bethanechol, on lower esophageal sphincter pressure (LESP) in normal subjects were examined. Oral L-dopa significantly inhibited LESP response to either oral metoclopramide 20 mg (P less than 0.05), or intravenous metoclopramide 20 mg (P less than 0.05). In contrast, L-dopa did not inhibit the LESP response to subcutaneous bethanechol (0.07 mg/kg). Mean basal LESP measured 50 min after ingestion of 1000 mg L-dopa, 19.3 +/- 3.1 mm Hg, was significantly less than basal LESP after L-dopa placebo, 29.3 +/- 4 mm Hg (P less than 0.01). It is concluded that (1) L-dopa inhibited the metoclopramide-induced rise in LESP but not peak stimulation of LESP by bethanechol; (2) there is evidence for the possibility of LES dopaminergic inhibitory receptors in man; and (3) these data are consistent with the hypothesis that metoclopramide acts on the LES by blocking a dopaminergic pressure-lowering mechanism.

Sensitive electron-capture gas-liquid chromatographic assay for the de-ethylated metabolite of metoclopramide

A procedure is described for determining 4-amino-5-chloro-2-methoxy-N-(2-ethylaminoethyl)benzamide, a metabolite formed by de-ethylation of metochlopramide, in urine from rats. The sampe is extracted (at pH approximately 13) with chloroform and, after treatment with heptafluorobutyric anhydride, derivatives of the extracted compounds are analyzed by gas-liquid chromatography, with electron-capture detection and diazepam as internal standard. The drug and its metabolite are separated, and the latter can be determined in the range 0.4 to 1.85 microgram/ml in the sample. The behaviour of the metabolic during chemical-ionization and electron-impact mass spectrometry is discussed.