Rational targeting of Wzb phosphatase and Wzc kinase interaction inhibits extracellular polysaccharides synthesis and biofilm formation in Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic nosocomial pathogen, and responsible for high mortality and morbidity. Biofilm formation is one of the resistance determinants, where extracellular polysaccharide (EPS) is an essential component. EPS synthesis and its export is regulated by the bacterial Wza-Wzb-Wzc system. Wzc exhibits auto-phosphorylation protein tyrosine kinase activity, while Wzb is a protein tyrosine phosphatase. Wzb mediates dephosphorylation of Wzc. Dephosphorylated Wzc is required for the export of the EPS through porin Wza-Wzc complex. It shows that the interaction of Wzb with Wzc is critical for the export of EPS. Therefore, if the Wzb-Wzc interaction is inhibited, then it might hinder the EPS transport and diminish the biofilm formation. In this study, we have modelled the Wzb, and Wzc proteins and further validated using PSVS, ProSA, RAMPAGE, and PDBsum. The modelled proteins were used for protein-protein docking. The docked protein-protein complex was minimized by Schrodinger software using OPLS_2005 force field. The binding site of the minimized Wzb-Wzc complex was identified by Sitemap. The high throughput virtual screening identified Labetalol hydrochloride and 4-{1-hydroxy-2-[(1-methyl-3-phenylpropyl) amino] propyl} phenol from FDA-approved drug library based on their interaction at the interface of Wzb-Wzc complex. The inhibitor-protein complex was further undergone molecular mechanics analysis using Generalized Born model and Solvent Accessibility (MMGBSA) to estimate the binding free energies. The lead was also used to generate the pharmacophore model and screening the molecule with antimicrobial scaffold. The identified lead was experimentally validated for its effect on EPS quantity and biofilm formation by A. baumannii. Wzb-Wzc interaction is essential for biofilm and EPS export; hence, the identified lead might be useful to regulate the biofilm formation by A. baumannii.

Derivatization of labetalol hydrochloride for its spectrofluorimetric and spectrophotometric determination inhuman plasma: Application to stability study

Two simple, selective and accurate methods were developed for the determination of Labetalol hydrochloride in pure form and pharmaceutical tablets. Both methods are based on derivatization of the studied drug with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBDCl) in alkaline medium (pH7.5).The reaction product was measured spectrofluorimetrically at 540nm after excitation at 476nm (method I) or spectrophotometrically at 480nm (method II). The calibration graphs were rectilinear over the concentration ranges of 0.10-2.0 and 1.0-11.0μgmL^-1 for methods I and II, respectively. The proposed methods were successfully applied to the analysis of commercial tablets without interference from common excipients. Furthermore, the spectrofluorimetric method was utilized for the in vitro determination of labetalol in spiked human plasma, with a percent mean recovery (n=3) of 97.80±1.29%. Moreover, the spectrofluorimetric method was extended to examine the stability study of LBT under different stress conditions such as alkaline, acidic, oxidative, photolytic and a thermal degradation.

Micellar enhanced spectrofluorometric determination of labetalol through complexation with aluminium(III): application to dosage forms and biological fluids

Two simple, sensitive, and specific spectrofluorometric procedures have been developed for the determination of labetalol (LBT) in pharmaceuticals and biological fluids. LBT was found to react with Al3+, both in acetate buffer of pH 4.5 (Procedure I) and borate buffer of pH 8.0 (Procedure II), to produce highly fluorescent stable complexes. The fluorescence intensity could be enhanced by the addition of sodium dodecyl sulfate, resulting in 3.5- and 2.7-fold increases in the fluorescence intensity for Procedures I and II, respectively. In both procedures, the fluorescence intensity was measured at 408 nm after excitation at 320 nm. The different experimental parameters affecting the development and stability of the fluorescent products were carefully studied and optimized. The fluorescence intensity-concentration plots were rectilinear over the range of 0.02-0.1 and 0.01-0.05 microg/mL with a detection limit of 0.003 and 0.001 microg/mL for Procedures I and II, respectively. The proposed method was successfully applied to commercial tablets containing LBT. The results were in good agreement with those obtained using a reference spectrofluorometric method. Furthermore, the method was applied for the determination of LBT in spiked human plasma, and the recovery (n = 4) was 93.30 +/- 2.62%. A proposal of the reaction pathway was postulated for Procedures I and II, respectively.

Characterization of basic drug–human serum protein interactions by capillary electrophoresis

Drug-protein interactions are determining factors in the therapeutic, pharmacodynamic and toxicological drug properties. The affinity of drugs towards plasmatic proteins is apparently well established in bibliography. Albumin (HSA) especially binds neutral and negatively charged compounds; alpha(1)-acid glycoprotein (AGP) binds many cationic drugs, lipoproteins bind to nonionic and lipophilic drugs and some anionic drugs while globulins interact inappreciably with the majority of drugs. In this paper, the characterization of the interaction between cationic drugs, beta-blockers and phenotiazines towards HSA, AGP, and both HSA + AGP mixtures of proteins under physiological conditions by CE-frontal analysis is presented. Furthermore, the binding of these drugs to all plasmatic proteins is evaluated by using ultrafiltration and CE. The results indicate that the hydrophobic character of compounds seems to be the key factor on the interaction between cationic drugs towards proteins. In fact, hydrophobic basic drugs bind in great extension to HSA, while hydrophilic basic drugs present low interactions with proteins and bind especially to AGP.

Transdermal Drug Delivery of Labetolol Hydrochloride: System Development, In Vitro; Ex Vivo and In Vivo Characterization

OBJECTIVE

The aim of the present work was to develop and evaluate matrix type transdermal drug delivery systems (TDDS) of labetolol hydrochloride (L-HCL) effective for 48 hours.

EXPERIMENTAL

The TDDS were prepared by solvent evaporation technique. Six formulations (carrying Eudragit RL100:Eudragit RS 100 in 7.5:4.5, 5.0:5.0, 3.5:8.5 in formulations X-1, X-2, X-3 and Eudragit RL100:PVP K-30 in 9.0:2.0, 5.0:5.0, 4.0:7.0 in formulations Y-1, Y-2, Y-3, respectively) were prepared. All formulations carried 36% w/w of L-HCL, 10-12% w/w of enhancer dimethyl sulfoxide and 2.5-7.5% w/w of plasticizer PEG 400 in methanol-acetone solvent system. The TDDS were evaluated by in vitro drug release, ex vivo skin permeation, stability and in vivo pharmacodynamic studies.

RESULTS

The maximum drug release for X-series was 90.26% in 48 hours (X-1) and for Y-series, it was 83.24% (Y-1). Again formulations X-1 (Kp = 0.221x10(-2) cm hr(-1)) and Y-1 (Kp = 0.210x10(-2) cm hr(-1)) exhibited the best skin permeation potential in the respective series. This might be due to higher permeability characteristics of Eudragit RL100. A shelf life of 2.38 years was predicted for the TDDS. Mean systolic BP of the experimental hypertensive rats was significantly reduced (p<0.01) on TDDS treatment.

CONCLUSION

The TDDS holds promise for clinical trials.

Polymeric alkenoxy amino acid surfactants: II. Chiral separations ofβ-blockers with multiple stereogenic centers

Two amino acid-based (leucine and isoleucine) alkenoxy micelle polymers were employed in this study for the separation of multichiral center-bearing beta-blockers, nadolol and labetalol. These polymers include polysodium N-undecenoxy carbonyl-L-leucinate (poly-L-SUCL) and polysodium N-undecenoxy carbonyl-L-isoleucinate (poly-L-SUCIL). Detailed synthesis and characterization were reported in our previous paper [26]. It was found that poly-L-SUCIL gives better chiral separation than poly-L-SUCL for both nadolol and labetalol isomers. The use of 50-100 mM poly-L-SUCIL as a single chiral selector provided separation of four and three isomers of labetalol and nadolol, respectively. Further optimization in separation of both enantiomeric pairs of nadolol and labetalol was achieved by evaluation of type and concentration of organic solvents, capillary temperature as well type and concentration of cyclodextrins. A synergistic approach, using a combination of poly-L-SUCIL and sulfated beta-CD (S-beta-CD) was evaluated and it showed dramatic separation for enantiomeric pairs of nadolol. On the other hand for labetalol enantiomers, separation was slightly decreased or remain unaffected using the dual chiral selector system. Finally, simultaneous separation of both nadolol and labetalol enantiomers was achieved in a single run using 25 mM poly-L-SUCIL and 5% w/v of S-beta-CD in less then 35 min highlighting the importance of high-throughput chiral analysis.

Chiral separation of labetalol stereoisomers in human plasma by capillary electrophoresis

A newly derivatized cyclodextrin [octakis-(2,3-diacetyl-6-sulfato)-gamma-cyclodextrin] was investigated as a chiral selector in capillary zone electrophoresis in a study of the chiral separation of labetalol stereoisomers. Heptakis(2,3-diacetyl-6-sulfato)-beta-cyclodextrin (HDAS-beta-CD) and octakis(2,3-diacetyl-6-sulfato)-gamma-cyclodextrin (ODAS-gamma-CD) were shown to be effective in separating labetalol stereoisomers. Optimal separating conditions of the four stereoisomers of labetalol were achieved with 10 mM HDAS-beta-CD and 10 mM ODAS-gamma-CD in an acidic pH buffer of low molarity. Data illustrating the effects of capillary length and cyclodextrin concentration on the separation are presented. The longer capillary length and high voltage enabled the baseline separation of all isomers in less than 15 min. The optimized method was applied to the analysis of human control plasma containing labetalol utilizing solid-phase extraction (SPE) in the 96-well format.

Spectrophotometric determination of labetalol in pharmaceutical preparations and spiked human urine

Two simple and sensitive spectrophotometric methods were developed for the spectrophotometric determination of labetaolol (LBT). Both methods are based on the phenolic nature of the drug. The first method (Method I) is based on coupling LBT with diazotized benzocaine in presence of trimethylamine. A yellow colour peaking at 410 nm was produced and its absorbance is linear with the concentration over the range 1-10 microg ml(-1) with correlation coefficient (n=5) of 0.9993. The molar absorptivity was 2.633 x 10(4) l mol(-1) cm(-1). The second method (Method II) involves coupling LBT with diazotized p-nitroaniline in presence of sodium carbonate. An orange colour peaking at 456 nm was obtained and its absorbance is linear with concentration over the range 1-10 microg ml(-1) with correlation coefficient (n=5) of 0.99935. The stoichiometry of the reaction in both cases was accomplished adopting the limiting logarithmic method and was found to be 1:1. The developed method could be successfully applied to commercial tablets. The results obtained were in good agreement with those obtained using the official methods. No interference was encountered from co-formulated drugs, such as hydrochlorothiazide. The method was further extended to the in-vitro determination of LBT in spiked human urine. The % recovery (n=4) were 97.7+/-5.75 and 103.27+/-5.42 using the Methods I and II, respectively.

Spectrofluorometric determination of labetolol in pharmaceutical preparations and spiked human urine through the formation of coumarin derivative

A simple, sensitive and specific spectrofluorimetric method has been developed for the determination of labetalol (LBT). The method is based on the reaction between LBT and ethylacetoacetate in the presence of sulphuric acid to give yellow fluorescent product with excitation wavelength of 312 nm and emission wavelength of 432 nm. The reaction conditions were studied and optimized. The fluorescence intensity-concentration plot is rectilinear over the range 1-15 microgram/ml with minimum detectability limit of 0.8 microgram/ml (2.16 x 10(-6) M). The proposed method was successfully applied to commercial tablets containing LBT, the percentage recoveries agreed well with those obtained using the official methods. Hydrochlorothiazide, which is frequently co-formulated with LBT did not interfere with the assay. The method was further extended to the in-vitro determination of LBT in spiked human urine samples. The percentage recovery was 101.50+/-6.18 (n=6). A proposal of the reaction pathway was postulated.

Partial agonistic activity of labetalol, the arylethanolamine, on beta 3-adrenoceptors in the guinea-pig gastric fundus

1. The agonistic and antagonistic effects of labetalol, the alpha1- and beta-adrenoceptor antagonist, were studied on beta3-adrenoceptors in the guinea-pig gastric fundus. 2. Labetalol caused a concentration-dependent relaxation with a pD2 value of 5.58 +/- 0.09 and an intrinsic activity of 0.64 +/- 0.06, which was not affected by pretreatment with both the selective beta1-adrenoceptor antagonist, (+/-)-atenolol (100 microM), and the selective beta2-adrenoceptor antagonist, (+/-)-butoxamine (100 microM). 3. However, the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist, (+/-)-bupranolol (3-30 microM), shifted the concentration-response curve of labetalol to the right (pA2 value=5.97 +/- 0.08). 4. In the presence of (+/-)-atenolol (100 microM) and (+/-)-butoxamine (100 microM), relaxations to catecholamines [(-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline], to the selective beta3-adrenoceptor agonist, BRL37344, and to the non-conventional partial beta3-adrenoceptor agonist, (+/-)-CGP12177A, were weakly antagonized by labetalol (10 microM). 5. These results indicate that labetalol, the arylethanolamine, acts as a partial agonist on beta3-adrenoceptors in the guinea-pig gastric fundus.

Studies on the photostability and in vitro phototoxicity of Labetalol

The purpose of this study was to obtain information on the photochemical and phototoxic properties of Labetalol, a beta-blocker drug. Preliminary information on the drug photoreactivity was achieved using a flow system with a photochemical reactor on-line with a diode array detection system. Photophysical and photochemical investigations on the drug were performed in aqueous solutions at different pH values using spectrophotometric and fluorimetric methods; the photodegradation quantum yield was found to be 2.7 x 10(-3) at pH 5.8 and 1.5 x 10(-2) at pH 11.5. Forced photodegradation of labetalol solutions under exposure to UVA--UVB radiations (xenon arc lamp) was monitored by reversed-phase liquid chromatography. The main photodegradation products were isolated and characterized by NMR and mass spectrometry; labetalol was found to give 3-amino-1-phenylbutane and salicylamide-4-carboxaldehyde as the main photoproducts. Preliminary phototoxic testings on human keratinocyte cultures were performed evaluating the viability of the cells by the neutral-red uptake assay; mutagenic and photomutagenicity tests were also carried out based on Salmonella typhimurium strains. As a result, labetalol was found to be photolabile,mainly in alkaline medium, but evidences of significant phototoxic and photomutagenic effects by the drug were not observed.

[High performance liquid chromatographic enantiomer separation of beta-blocking agents with a new isothiocyanate type chiral derivatizing agent]

The applicability of (1R,2R)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propyl isothiocyanate [(R,R)-DANI] as a recently developed chiral derivatizing agent for the enantioseparation of a series of beta-blockers is described. The thiourea diastereomers formed were analysed by reversed-phase high-performance liquid chromatography, mixtures of water and methanol or acetonitrile being used for elution. Conditions of derivatizations (temperature, reagent excess and reaction time) were optimized. The effects of organic modifiers on the retention and separation were investigated; the diastereomers could readily be baseline-separated with methanol-containing mobile phases. The reagent allowed the detection of all four stereoisomers of labetalol, which has two chiral centres.

[Separation of enantiomeric labetalol by reversed-phase high performance liquid chromatography]

A reversed-phase high performance liquid chromatographic method for the separation of labetalol enantiomers was developed. In the method, 2, 3, 4, 6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate (GITC) was chosen as the reagent for pre-column chiral derivatization of labetalol to give four diastereomeric thiourea derivatives. These derivatives were efficiently separated on a Nova-Pak C18 column using V(MeOH):V(0.01 mol.L-1(pH 7.00) H2PO4(-)-HPO(4)2- buffer) = 51:49 as the mobile phase and detected by UV detector at a wavelength of 250 nm or fluorescence detector at lambda ex = 340 nm and lambda em = 440 nm. The effects of the pH of mobile phase on the retention and fluorescence absorbance are also discussed.

[The enantiomeric separation of aromatic alcohol amino drugs by thin-layer chromatography]

Two chiral aromatic alcohol amino drugs, Labarol and Bataroc, were resolved by thinlayer chromatography (TLC) on the silica gel GF254 plates (2.5 cm x 10 cm), by using the ammonium-D-10-camphorsulfonate (CSA) as chiral ion-pair interaction agent which was added to the mobile phase in the ammonium form. All developments were carried out at lower temperature (2-4 degrees C, in a refrigerator) in small glass jars of 250 mL volume. These two drugs were not resolved at room temperature (15-30 degrees C). Analytical reagent grade methanol and dichloromethane can be directly used as mobile phase without further drying. The chiral separation occured over a range from 40% to 70% (optimum 67%) dichloromethane in the mobile phase volume ratio and 55% to 80% (optimum 60%) dichloromethane in the mobile phase volume ratio. These separation conditions were easily obtained. This method is relatively inexpensive and attractive.

Pharmacokinetics of beta-adrenoceptor blockers in obese and normal volunteers

AIMS

Obesity can modify the pharmacokinetics of lipophilic drugs. As beta-adrenoceptor blockers (BB) are often prescribed for obese patients suffering from hypertension or coronary heart disease, this study compares the pharmacokinetics of lipophilic beta-adrenoceptor blockers in obese and control subjects.

METHODS

Nine obese (157 +/- 24% of ideal body weight (IBW) mean +/- s.d.) and nine non-obese healthy volunteers (98 +/- 10% IBW), aged 32 +/- 9 years, were included in the study. Subjects were randomly given a single i.v. infusion of one of the following racemic beta-adrenoceptor blockers, whose doses (expressed as base per kg of IBW) were: propranolol (0.108 mg), labetalol (0.99 mg) and nebivolol (0.073 mg). The plasma concentrations of unchanged drugs were measured by h.p.l.c. The ionisation constants and lipophilicity parameters of beta-adrenoceptor blockers were assessed.

RESULTS

The pharmacokinetic data for the three drugs were qualitatively similar. There was a trend towards a greater total distribution volume (Vss) in obese patients than in controls. However, Vss expressed per kg body weight was slightly smaller in obese patients. The relationship between Vss and lipophilicity of five beta-adrenoceptor was studied by combining the current results with those previously obtained with a moderately lipophilic drug (bisoprolol) and a hydrophilic one (sotalol). The Vss of the five drugs was positively and well-correlated (r2 = 0.90; P < 0.01) with their distribution coefficient at pH 7.4 (log D7.4), but not with their partition coefficients. The linear regression coefficients for lean and obese subjects were very similar.

CONCLUSIONS

Lipophilic beta-adrenoceptor blockers seem to diffuse less into adipose than into lean tissues. All electrical forms of the drugs (i.e. cations, neutral forms, or zwitterions) present at physiological pH contribute to their tissue distribution, in both obese and lean subjects. Their tissue distribution in obese patients could be restricted by the sum of hydrophobic forces and hydrogen bonds they elicit with macromolecules in lean tissues.

Stereoselective interaction of drug enantiomers with human serum transferrin in capillary zone electrophoresis

Stereoselective interaction of drugs with human serum transferrin in capillary zone electrophoresis is described. The substances passed a pseudo-stationary protein zone applied in a coated capillary and the possible chiral separation of the optical isomers was followed. Drugs with different structures were screened and the enantiomers of bupivacaine, propranolol and promethazine as well as the diastereomers of labetalol were resolved. Racemic mixtures of atenolol and pindolol enantiomers could not be resolved under these conditions.

Stability of labetalol hydrochloride, metoprolol tartrate, verapamil hydrochloride, and spironolactone with hydrochlorothiazide in extemporaneously compounded oral liquids

The stability of drugs commonly prescribed for use in oral liquid dosage forms but not commercially available as such was studied. Labetalol hydrochloride 40 mg/mL, metoprolol tartrate 10 mg/mL, verapamil hydrochloride 50 mg/mL, and spironolactone 5 mg/mL plus hydrochlorothiazide 5 mg/ mL were prepared in a 1:1 mixture of Ora-Sweet and Ora-Plus (Paddock Laboratories), a 1:1 mixture of Ora-Sweet SF and Ora-Plus (Paddock Laboratories), and cherry syrup and placed in polyethylene terephthalate bottles. The sources of the drugs were tablets. Six bottles were prepared per liquid; three were stored at 5 degrees C and three at 25 degrees C, all in the dark. A sample was removed from each bottle initially and at intervals up to 60 days and analyzed for drug concentration by stability-indicating high-performance liquid chromatography. At least 91% of the initial drug concentration was retained in all the oral liquids for up to 60 days. There were no substantial changes in the appearance or odor of the liquids, or in the pH. Labetalol hydrochloride 40 mg/mL, metoprolol tartrate 10 mg/mL, verapamil hydrochloride 50 mg/mL, plus hydrochlorothiazide 5 mg/ mL in three oral liquids compounded extemporaneously from sweetened vehicles and tablets were stable for up to 60 days when stored without light at 5 and 25 degrees C.

Stability of labetalol hydrochloride with selected critical care drugs during simulated Y-site injection

The compatibility and stability of labetalol hydrochloride in combination with various critical care drugs was evaluated. Labetalol hydrochloride 1.0 mg/mL was combined in 5% dextrose injection with equal volumes of each of the following drugs: dobutamine 2.5 mg/mL (as the hydrochloride salt), dopamine hydrochloride 1.6 mg/mL, morphine sulfate 0.5 mg/mL, nitroglycerin 0.2 mg/mL, and ranitidine 0.6 mg/mL (as the hydrochloride salt). The mixtures were placed in duplicate Y-site administration sets. Visual inspection, pH determination, and high-performance liquid chromatography were performed in duplicate on samples removed at zero, two, and four hours. No change in pH or appearance occurred throughout the study. All drug concentrations remained above 90% of the initial concentration in each combination. Labetalol hydrochloride 1.0 mg/mL and dobutamine 2.5 mg/mL (as the hydrochloride salt), dopamine hydrochloride 1.6 mg/mL, morphine sulfate 0.5 mg/mL, nitroglycerin 0.2 mg/mL, or ranitidine 0.6 mg/mL (as the hydrochloride salt) in 5% dextrose injection were stable and compatible for up to four hours at 20-25 degrees C during simulated Y-site injection.

Reversed-phase high-performance liquid chromatographic separation of the stereoisomers of labetalol via derivatization with chiral and non-chiral isothiocyanate reagents

The antihypertensive agent labetalol is a mixture of two racemates. We report reversed-phase high-performance liquid chromatographic (HPLC) methodology for the separation of the four stereoisomers of labetalol via derivatization with the chiral reagent (4S-cis)-2,2-dimethyl-5-isothiocyanato-4-phenyl-1,3-dioxane. The derivatives were separated on octadecylsilane columns with a methanol-ammonium phosphate buffer mixture as mobile phase. Separations of the diastereomeric forms of labetalol were achieved with the non-chiral derivatizing reagents benzyl isothiocyanate and 1-naphthalenemethyl isothiocyanate. In all cases the derivatives of the R,S/S,R forms eluted before those of the R,R/S,S forms. Isothiocyanates may have general utility in stereoisomer separations of amines by HPLC.

Stability of labetalol hydrochloride in distilled water, simple syrup, and three fruit juices

Labetalol hydrochloride (HCl) is an effective antihypertensive drug. It is available in a tablet dosage form, which may be difficult to administer to geriatric and pediatric patients. The tablets can be suspended in various liquid vehicles, but its stability under these conditions is unknown. The objective of our study was to determine the stability of labetalol HCl in five liquid vehicles, in both plastic and glass prescription bottles, over a four-week period at room temperature and under refrigeration. Labetalol HCl tablets (Trandate, Glaxo) were triturated, and suspended in distilled water, simple syrup, apple juice, grape juice, and orange juice to approximate concentrations of 7-10 mg/mL. After filtration, the liquids were stored in five amber, plastic prescription bottles, and five amber, glass prescription bottles at 4 degrees C and 23 degrees C. The samples were collected at 0, 24, and 72 hours, and 1, 2, and 4 weeks after initiation of the study. Labetalol HCl was measured by an HPLC method. Because there was no substantial change in labetalol HCl concentration during the study period, the tablet dosage from can be reformulated, stored, and administered to elderly or pediatric patients as a liquid dosage form.

Oxidation of beta-blocking agents--VII. Periodate oxidation of labetalol

The oxidation of labetalol with sodium metaperiodate is described. In spite of the bulky substituent on the amino group of labetalol, glycol cleavage of the molecule occurred. Spectrometric methods verified that the aromatic aldehyde formed was 2-hydroxy-5-formylbenzamide and that the amine was 1-methyl-3-phenylpropylamine. The polarographic behaviour of the aldehyde was examined as well. The half-wave potential in Britton-Robinson buffer pH 5.68 was -1.24 V. Direct linearity (r = 0.9999) was observed between the diffusion current and the concentration of the aldehyde. Quantitation of labetalol was carried out polarographically by measuring the concentration of 2-hydroxy-5-formylbenzamide formed in the oxidation.