Preparation of A Magnetic Nanosensor Based on Cobalt Ferrite Nanoparticles for The Electrochemical Determination of Methyldopa in The Presence of Uric Acid.

AIM AND OBJECTIVE: Methyldopa is one of the medications that is used for the treatment of hypertension. Therefore, the determination of methyldopa in the presence of other biological components is essential. In this work, a promising electrochemical sensor based on CoFe2O4 magnetic nanoparticles modified glassy carbon electrode (CoFe2O4/GCE) was developed for electrochemical determination of methyldopa in the presence of uric acid. Cobalt ferrite nanoparticles were synthesized via chemical method. MATERIALS AND METHODS: Characterizing the CoFe2O4 was investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and cyclic voltammetry techniques. RESULTS: Under the optimal experimental conditions, the current response of the electrochemical sensor obtained with differential pulse voltammetry was increased linearly in the concentration range from 1.45 to 15.1 µmol L-1 with the detection limit of 1.07 µmol L-1 for methyldopa. Also, by using the proposed method, methyldopa and uric acid could be analyzed in a mixture independently. The difference in peak potential for analytes is about 150 mV. CONCLUSION: The present sensor was successfully applied for the determination of methyldopa in the presence of uric acid in biological samples and the pharmaceutical samples with satisfactory results.

Dopamine: mediator of brain polysome disaggregation after L-dopa.

The disaggregation of brain polysomes which is produced by giving large doses of (L)-dopa to rats is not reproduced by administering its metabolite, 3-O-methyldopa, by giving D-dopa, which also depletes the brain of S-adenosylmethionine but is not converted to catecholamines, or by giving the L-dopa after a decarboxylase inhibitor. Polysome disaggregation is potentiated by the prior administration of a monoamine oxidase inhibitor, indicating that formation of a catecholamine is an obligatory requirement. These observations suggest that the mechanism by which L-dopa disaggregates brain polysomes involves its conversion to dopamine within the majority of brain cells.

Enantioseparation of dopa and related compounds by cyclodextrin-modified microemulsion electrokinetic chromatography.

A chiral microemulsion electrokinetic chromatography method has been developed for the enantiomeric separation of 3,4-dihydroxyphenylalanine (dopa), its precursors phenylalanine and tyrosine, and the structurally related substance methyldopa. The separations were achieved using an oil-in-water microemulsion, which consisted of the oil-compound ethyl acetate, the surfactant sodium dodecylsulfate (SDS), the co-surfactant 1-butanol, the organic modifier propan-2-ol and 20mM phosphate buffer pH 2.5 or 2.0 as aqueous phase. For enantioseparation sulfated beta-cyclodextrin was added. The resolution of each racemate was optimized by varying the concentration of the buffer and all components of the microemulsion. Enantioseparation could be achieved for dl-dopa, dl-phenylalanine and dl-tyrosine within 13 min with a resolution of 4.3, 3.1 and 3.3, respectively, and for methyldopa in 17 min (Rs: 1.4). The established methods allowed the detection of dopa, phenylalanine, tyrosine and methyldopa with a limit at 0.5, 1.0, 0.2 and 2.0 microg/ml.

Cyclic voltammetric study of alpha-methyldopa at carbon paste electrode.

The cyclic voltammetric behaviour of alpha-methyldopa at a silicon oil carbon paste electrode has been reported. This allowed the development of a quantitative method to determine alpha-methyldopa in LiCl, KCl, NaCl, HCl, H2SO4 and CH3COOH as supporting electrolytes. For qualitative characteristics, alpha-methyldopa showed an ECC mechanism in terms of electron transfer reaction (placing it in DPI zone) at carbon paste electrode. The values of transfer coefficients alpha and beta were determined. The larger DeltaEp values were obtained due to the use of unmodified carbon paste electrode (CPE) has decreased the rate of electron transfer at the surface of the test electrode. The first order rate constant values (ko) were within 0.10-7.78 +/- 0.1x10(-3) s(-1). Adsorption of analyte was also determined at CPE using repeated scan method.

Clinical pharmacokinetics of methyldopa.

Absorption of methyldopa from the gastrointestinal tract is incomplete and variable; bioavailability after oral administration is about 25% (range 8 to 62%). The average time to reach maximum plasma concentration (tmax) [chemically determined] is 2 hours, when the maximum plasma concentration of active drug accounts for 50% of the radioactivity, the remainder representing various metabolites. Physicochemical determination of methyldopa shows that bi-phasic elimination occurs after both intravenous and oral administration, the half-life of the alpha-phase being 0.21 hours (range 0.16 to 0.26 hours) and of the beta-phase 1.28 hours (range 1.02 to 1.69 hours) in normal subjects. Methyldopa is less than 15% protein bound, whereas the primary metabolite, which most probably is the O-sulphate, is about 50% protein bound. The apparent volume of distribution in the central compartment is about 0.23L/kg (range 0.19 to 0.32L/kg), and the total volume of distribution (calculated as Vdarea) is about 0.60L/kg (range 0.41 to 0.72L/kg) in healthy volunteers. Acid-labile conjugates are formed after oral administration. These acid-labile conjugates, in particular the O-sulphate, are probably formed in the intestinal cells, since they are detected in very small amounts after intravenous administration. Additionally, there is a rapid formation of partly unidentified metabolites after both intravenous and oral administration. After intravenous administration the quantitatively most prominent metabolites are methyldopamine and the glucuronide of dihydroxyphenylacetone, but traces of 5 or 6 other metabolites have also been found and identified. These metabolites are probably formed in the liver, but the complete metabolic pattern is still unknown. The renal clearance of methyldopa (95 ml/min/m2) is more than 50% higher than the endogenous creatinine clearance. Renal excretion of some metabolites is slower. Extrarenal elimination accounts for about 50% of the total body clearance of the drug. Renal excretion is very low in patients with renal failure, resulting in accumulation of both active drug and, in particular, its metabolites. There is a marked accumulation of unidentified metabolites in renal failure patients, which possibly explains the strong and prolonged hypotensive action of methyldopa in these patients.

Uptake of monoamines into central neurones and the blood-brain barrier in the infant rat.

1. A fluorescence histochemical study was undertaken to investigate the ability of systemically administered monoamines to penetrate into the central nervous system of infant rats.2. It was found that after subcutaneous injections of large doses, L-noradrenaline, L-alpha-methyl-noradrenaline, DL-alpha-methyl-dopamine, L-alpha-methyl-dopa and 5-hydroxytryptamine could cross the blood-brain barrier of newborn to 2 week old rats and be taken up by neurones normally containing monoamine as well as by several neurones which normally contained no monoamine.3. 5-Hydroxytryptamine containing neurones took up 5-hydroxytryptamine as well as catecholamines and alpha-methyl-dopa. The pinealocytes, which contain large amounts of 5-hydroxytryptamine, were depleted of their endogenous fluorescence by alpha-methyl-dopa and showed no selective uptake of catecholamines.4. The uptake of monoamines was not antagonized by previous reserpine treatment or by inhibition of catecholamine synthesis, and occurred throughout the whole extent of the monoamine-containing neurones (cell body, axon, terminals).5. The monoamines were also taken up by the endothelial cells and pericytes of capillaries in the central nervous system (CNS) which are thought to constitute the physical barrier to these substances in the adult.6. The peripheral sympathetic nerves to pial blood vessels, the pineal and pituitary glands also took up the monoamines and alpha-methyl-dopa; uptake was also noted in cells of the pituitary gland, in epithelial and connective tissue elements of the meninges and choroid plexuses and in the epithelial, connective and muscular tissue of pial blood vessels.7. It is concluded that the blood-brain barrier to monoamines may not be fully developed in infant rats, at least for high levels of circulating monoamines; the central monoamine-containing neurones possess the ;membrane pump' mechanism for uptake of monoamines from the time of birth, even though they are not fully developed morphologically.

Pharmacokinetics and presystemic gut metabolism of methyldopa in healthy human subjects.

This study examined the pharmacokinetics and metabolism of methyldopa after giving single 250-mg oral and intravenous doses to 16 healthy human volunteers. A 48-hour washout period was allowed between oral and intravenous treatments. Blood and urine samples were collected; methyldopa was assayed in blood and urine, and its metabolites (methyldopa sulfate, alpha-methyldopamine, and alpha-methyldopamine sulfate) were assayed in urine. Pharmacokinetic parameters were recorded as follows: half-life was 2.0 +/- 0.7 hours; total body and renal clearance were 268 +/- 72 and 107 +/- 35 mL/min, respectively; and volume of distribution at steady-state was 33 +/- 11 L. The absolute bioavailability of the drug was 42 +/- 16%. The measurable metabolites in urine after oral and intravenous administration accounted for 27% and 17% of the dose, respectively. Methyldopa sulfate was the most abundant metabolite recorded; its quantity was higher after oral than after intravenous administration, 20.1 +/- 5.7% versus 6.7 +/- 5.3% of the dose (P < .05), suggesting significant presystemic gut metabolism. First-pass gut metabolism for methyldopa was estimated to be 17.6 +/- 6.9% of the dose given.

On the role of alpha-methyldopamine in the antihypertensive effect of alpha-methyldopa.

In renal hypertensive rats the cerebral concentration of alpha-methyldopa, alpha-methyldopamine, alpha-methylnoradrenaline, dopamine and noradrenaline as well as the blood-pressure were determined simultaneously. The antihypertensive effect followed a time course identical to that of the increase in the cerebral concentration of alpha-methyldopamine and of the decrease in the concentration of dopamine, whereas lowering of blood pressure on the one hand, and changes in the levels of alpha-methylnoradrenaline and noradrenaline, on the other, were not related to each other. Dose-response relationships showed the same correlations and lack of correlations, respectively. These results suggest that non-betat-hydroxylated catecholamines play a major role in mediating the antihypertensive effect of alpha-methyldopa or, alternatively, that only the newly biosynthesized alpha-methyl-noradrenaline is effective in lowering blood pressure.

Magnitude of response to levodopa in Parkinson disease as it relates to peripheral and central measurements of levodopa and associated metabolites.

The relationships between magnitude of response to orally administered carbidopa/levodopa (CD/LD) and serum/cerebrospinal fluid (CSF) concentrations of levodopa (LD), 3-O-methyldopa (3-O-MD), and homovanillic acid (HVA) were studied in 15 patients with chronic LD-treated Parkinson disease. The degree of clinical benefit derived from a 25/250 tablet of CD/LD could not be correlated with absolute serum levels of LD, 3-O-MD or LD/3-O-MD ratios. CSF levels of LD and 3-O-MD were also not associated with improvement. CSF levels of HVA, however, did significantly correlate with magnitude of response to LD. Furthermore, CSF HVA levels were not dependent on previous LD dosage. Our data suggest that in chronic LD-treated patients, central factors related to the integrity of the nigrostriatal tract may be a more important determinant of magnitude of response to LD than peripheral elements affecting the amount of LD entering the central nervous system.

Fluorometric determination of methyldopa in biological fluids.

A fluorometric method for the analysis of methyldopa, based on the formation of a fluorophore after oxidation and rearrangement, is described. The drug is isolated from biological fluids by adsorption on alumina and elution with an organic solvent. Fluoresence is linear from 0.1 to 1.5 microgram of methyldopa/ml. The assay has a lower limit of sensitivity of 100 ng/ml and is suitable for pharmacokinetic studies following therapeutic doses in animals and humans.

Applications of paired ion high-pressure liquid chromatography to catecholamines and phenylephrine.

Paired ion high-pressure liquid chromatography was useful for separating intact catecholamines (epinephrine, isoproterenol, levodopa, and methyldopa) and phenylephrine from some of their decomposition products. Furthermore, methyldopa was separated from either hydrochlorothiazide of levodopa. Under identical conditions without 1-heptanesulfonic acid (a counterion for paired ion chromatography) in the mobile phase, these separations were not possible. Paired ion chromatography also was tried successfully for the quantitative determinations of isoproterenol, levodopa, methyldopa, and phenylephrine in some dosage forms. The results were compared with the results obtained using some literature methods.

Excretion of antihypertensive medication into human breast milk: a systematic review.

OBJECTIVE: To establish which antihypertensive medications are safe for use while breastfeeding, by reviewing the available evidence. METHODS: Reports of studies examining the transfer of antihypertensive medications to breastmilk were identified from multiple MEDLINE and EMBASE searches, manual review of bibliographies of articles and textbooks on drug use during lactation. The reports were reviewed and the results were compiled. RESULTS: Prospective cohort studies and case reports constituted the only available evidence. Compilation of these results found that the milk to plasma (M/P) ratios varied widely across the beta-blocker family, the beta-blockers with low protein binding having the highest M/P ratios. The angiotensin-converting enzyme (ACE) inhibitors, methyldopa, and some calcium channel blockers had low M/P ratios. CONCLUSION: The available data to date indicate that ACE inhibitors, methyldopa, beta-blockers with high protein binding, and some calcium channel blockers all appear to be safe treatments of hypertension in a nursing mother. The data suggest that drugs to be avoided are beta-blockers with low protein binding. However, the available evidence is limited and further studies are needed to confirm these findings.

The use of a response surface methodology on HPLC analysis of methyldopa, amiloride and hydrochlorothiazide in tablets.

A multifactor optimisation technique is successfully applied to develop a new HPLC method in which methyldopa, hydrochlorothiazide and amiloride were analysed and determined on a C18 column with detection at 286 nm. The optimal conditions of HPLC separation were determined with the aid of the response surface diagram -- 'window diagram'. The effect of simultaneously varying the pH, proportion aqueous acetic acidum and methanol in the mobile phase were studied to optimise the separation. The mobile phase composition that provides an acceptable resolution methyldopa, hydrochlorothiazide and amiloride in a short elution time is water--methanol (75:25) and pH 3.60. The k' values for methyldopa, hydrochlorothiazide and amiloride after optimisation were 1.40, 2.50 and 5.33, respectively. Relative retention (alpha) for ratio hydrochlorothiazide/methyldopa and amiloride/hydrochlorothiazide were 1.767 and 2.159, respectively. Correlation coefficients of the calibration curves for all analytes were greater than 0.995 and the R.S.D. values for the slope and the intercept with respect to the linearity were less than 2%. A method is applied for the quantitative analysis of Alatan tablets (Lek-Ljubljana). The powdered tablets are extracted with methanol, containing caffeine as the internal standard and assayed by comparison of peak areas after liquid chromatography. The high recovery (for all analytes about 100%) and the low R.S.D. (<2%) confirm good precision and reproducibility of the chromatographic method.

Direct high-performance liquid chromatographic determination of the enantiomeric purity of levodopa and methyldopa: comparison with pharmacopoeial polarimetric methods.

Chiral high-performance liquid chromatography was employed for determination of the enantiomeric purity of levodopa and methyldopa. The determination of D-DOPA in levodopa was accomplished using a chiral ligand-exchange chromatograpy with an ordinary C18 column and a chiral mobile phase containing N,N-dimethyl-L-phenylalanine and Cu(II) acetate or by means of LC on a teicoplanin column in conjunction with ethanol-water (65:35, v/v). Both methods gave good performance, however, the latter was faster and more convenient and suitable for routine analyses. For the determination of D-methyldopa a LC method based on the use of a teicoplanin column in polar organic mode with methanol-acetic acid-triethylamine (1,000:0.05:0.05, v/v/v) mobile phase was developed. The precision, accuracy, linearity and selectivity were satisfactory. In comparison with pharmacopoeial polarimetric methods (according to the European Pharmacopoeia and the Pharmacopoea Bohemoslovaca), the LC methods proved to be much more sensitive giving detection limits 0.04% of D-DOPA and 0.3% of D-methyldopa.

Spectrophotometric investigations of the assay of physiologically active catecholamines in pharmaceutical formulations.

Two simple, sensitive, and accurate spectrophotometric methods are proposed for the determination of levodopa (LD), methyldopa (MD), dopamine hydrochloride (DP), and pyrocatechol (PC) in pure and pharmaceutical preparations. The methods are based on measurement of the absorbances of tris(o-phenanthroline)iron(II) (method A) and tris(bipyridyl)iron(II) (method B) obtained by the oxidation of the catecholamines by iron(III) in the presence of 1,10-phenanthroline and 2,2'-bipyridyl at 510 and 522 nm, respectively. The absorbances were found to increase linearly with increases in the concentrations of the catecholamines, results which were corroborated by the calculated correlation coefficients (0.9990-0.9996). Beer's law was valid over the concentration ranges of 0.04-0.6, 0.06-0.75, 0.06-0.65, and 0.05-0.70 microg/mL in method A and 0.02-1.0, 0.04-1.3, 0.05-1.0, and 0.06-1.1 microg/mL in method B for PC, MD, LD, and DP, respectively. The common excipients and additives did not interfere in their determinations. The proposed methods were successfully applied to the assay of LD, MD, and DP in various dosage forms. The results were validated by statistical analysis.

Coulometric determination of levodopa, methyldopa and carbidopa.

A simple and rapid method for the assay of small quantities of levodopa, methyldopa and carbidopa is presented. The method is based on coulometric titration of the investigated substances with electrogenerated chlorine in the presence of methyl orange as indicator. Results are accurate and reproducible. The method does not require any expensive instrumentation and can be applied in any laboratory for routine analysis of these substances.

Colorimetric assay of methyldopa bulk drug and tablets as its Fe(III) complex.

A new colorimetric method for the determination of methyldopa is described. The method is based on the reaction of methyldopa with ferric chloride in the pH range of 3.0-3.5 and producing a water soluble (1:2) brown complex with maximum absorbance at 423 nm. The conditional stability constant of the complex at the optimum pH 3.0 and ionic strength 0.4 M, is found to be 10(7.5). Beer's low is obeyed up to 6 mmol/l methyldopa concentration. The RSD of the method is 0.708-1.226%. The proposed method was found to be suitable for the accurate and reproducible analysis of methyldopa bulk drug and tablets which is pointed by high recovery values 99.20-99.79%.

Detection of alpha-methyldopa on thin-layer plates using pi-acceptors in 1,4-dioxane.

pi-Acceptors (P-chloranil and chloranilic acid) in 1,4-dioxane were used to detect alpha-methyldopa on thin layer plates. Thin layer plates coated to a thickness of 0.25 mm with silica gel G60 were spotted with solution of alpha-methyldopa from drug formulations and pure alpha-methyldopa solution. The plates were developed in a solvent system of methanol: glacial acetic acid: water (14:5:3). The developed plates were dried and sprayed with pi-acceptors, and further were counter-sprayed with dimethylformamide (DMF). P-Chloranil gave an immediate blue green complex with alpha-methyldopa which intensified on counter-sprayed with DMF. alpha-methyldopa (reference sample and those from drug formulations) gave Rf of 0.85 and excipients from the drug formulations did not interfere with the result.

First report for voltammetric determination of methyldopa in the presence of folic acid and glycine.

In this study, a carbon paste electrode modified with TiO2 nanoparticles and ferrocene monocarboxylic acid (FM) was used to prepare a novel electrochemical sensor. The objective of this novel electrode modification was to seek new electrochemical performances for the detection of methyldopa in the presence of folic acid and glycine. The peak potentials recorded in a phosphate buffer solution (PBS) of pH7.0 were 325, 750 and 880 mV vs. Ag/AgCl/KCl (3.0M) for methyldopa, folic acid and glycine, respectively. Under the optimum pH of 7.0, the oxidation of methyldopa occurred at a potential about 160 mV less positive than that of the unmodified carbon paste electrode (CPE). The response of catalytic current with methyldopa concentration showed a linear relation in the range from 2.0×10(-7) to 1.0×10(-4)M with a detection limit of 8.0 (± 0.2)×10(-8)M.

Adsorptive stripping voltammetry determination of methyldopa on the surface of a carboxylated multiwall carbon nanotubes modified glassy carbon electrode in biological and pharmaceutical samples.

In the present work, a simple carboxylated multiwall carbon nanotubes (CMWCNTs) modified glassy carbon electrode was developed for sensitive determination of methyldopa (MTD). The study of modified electrode and MTD electrochemical behavior at its surface was investigated employing SEM, adsorptive stripping voltammetry, electrochemical impedance spectroscopy and chronocoulometry. These studies show that the oxidation of MTD is facilitated at the surface of GCE which is casted with CMWCNTs and remarkably peak current enhanced comparing to the bare electrode due to its adsorption on the electrode surface. Also, because of the catalytic property of modified electrode onset potential decreased for oxidation of MTD. Under optimized conditions, the calibration curve was linear in two concentration ranges of 0.1-30 and 30.0-300.0 µM with a detection limit of 0.08 µM and relative standard deviation (R.S.D.%) lower than 3.0% (n=5). This modified electrode was used as a sensor for determination of MTD in pharmaceutical and human urine samples with satisfactory results.