Binding of antiarrhythmic drugs to purified human alpha 1-acid glycoprotein

Effect of Rythmol (propafenone HCl) administration during pregnancy in Wistar rats

Propafenone is a well-known Class 1C antiarrhythmic agent that has sodium channel blocking properties as well as the ability to block 13 other channels and a modest calcium antagonistic effect. Propafenone has a profound electrophysiologic effect on auxiliary atrioventricular circuits and in patients with atrioventricular nodal reentry tachycardia can obstruct conduction in the fast conducting pathway. Furthermore, propafenone is less likely than other Class 1C drugs to cause proarrhythmia. However, although this medicine can pass through the placenta, the effects during pregnancy remain unknown. Here, we investigated the potential teratogenic and genotoxic effects of Rythmol during rat development. Pregnant Wistar rats received 46.25 mg/kg body weight of propafenone daily by gavage from Gestation Day (GD) 5 to GD 19. At GD 20, the dams were dissected, and their fetuses were assessed via morphologic, skeletal, and histologic investigation. In addition, a comet assay was used to measure DNA impairment of fetal skull osteocytes and hepatic cells. The study showed that propafenone treatment of pregnant rats led to a marked decrease in gravid uterine weight, number of implants/litter, number of viable fetuses, and bodyweight of fetuses but a clear increase in placental weight and placental index in the treated group. Frequent morphologic abnormalities and severe ossification deficiency in the cranium bones were observed in the treatment group. Various histopathological changes were observed in the liver, kidney, and brain tissues of maternally treated fetuses. Similarly, propafenone induced DNA damage to examined samples. Thus, our study indicates that propafenone may be embryotoxic in humans.

Pharmacokinetic and Pharmacodynamic Considerations for Drugs Binding to Alpha-1-Acid Glycoprotein

According to the free drug hypothesis only the unbound drug is available to act at physiological sites of action, and as such the importance of plasma protein binding primarily resides in its impact on pharmacokinetics and pharmacodynamics. Of the major plasma proteins, alpha-1-acid glycoprotein (AAG) represents an intriguing one primarily due to the high affinity, low capacity properties of this protein. In addition, there are marked species and age differences in protein expression, homology and drug binding affinity. As such, a thorough understanding of drug binding to AAG can help aid and improve the translation of pharmacokinetic/pharmacodynamic (PK/PD) relationships from preclinical species to human as well as adults to neonates. This review provides a comprehensive overview of our current understanding of the biochemistry of AAG; endogenous function, impact of disease, utility as a biomarker, and impact on PK/PD. Experimental considerations are discussed as well as recommendations for understanding the potential impact of AAG on PK through drug discovery and early development.

High-throughput analysis of drug dissociation from serum proteins using affinity silica monoliths

A noncompetitive peak decay method was used with 1 mm×4.6 mm id silica monoliths to measure the dissociation rate constants (kd) for various drugs with human serum albumin (HSA) and α1-acid glycoprotein (AGP). Flow rates up to 9 mL/min were used in these experiments, resulting in analysis times of only 20-30 s. Using a silica monolith containing immobilized HSA, dissociation rate constants were measured for amitriptyline, carboplatin, cisplatin, chloramphenicol, nortriptyline, quinidine, and verapamil, giving values that ranged from 0.37 to 0.78 s(-1). Similar work with an immobilized AGP silica monolith gave kd values for amitriptyline, nortriptyline, and lidocaine of 0.39-0.73 s(-1). These kd values showed good agreement with values determined for drugs with similar structures and/or affinities for HSA or AGP. It was found that a kd of up to roughly 0.80 s(-1) could be measured by this approach. This information made it possible to obtain a better understanding of the advantages and possible limitations of the noncompetitive peak decay method and in the use of affinity silica monoliths for the high-throughput analysis of drug-protein dissociation.

Accessory cholera enterotoxin, Ace, from Vibrio cholerae: structure, unfolding, and virstatin binding

Vibrio cholerae accessory cholera enterotoxin (Ace) is the third toxin, along with cholera toxin (CT) and zonula occludens toxin (Zot), that causes the endemic disease cholera. Structural characterization of Ace has been restricted because of the limited production of this toxic protein by V. cholerae. We have cloned, overexpressed, and purified Ace from V. cholerae strain O395 in Escherichia coli to homogeneity and determined its biological activity. The unfolding of the purified protein was investigated using circular dichroism and intrinsic tryptophan fluorescence. Because Ace is predominantly a hydrophobic protein, the degree of exposure of hydrophobic regions was identified from the spectral changes of the environment-sensitive fluorescent probe 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (bis-ANS) that quenches the fluorescence of tryptophan residues of Ace in a concentration-dependent manner. Results showed that bis-ANS binds one monomeric unit of Ace with a 1:1 stoichiometry and a K' of 0.72 μM. Ace exists as a dimer, with higher oligomeric forms appearing upon glutaraldehyde cross-linking. This study also reports the binding of virstatin, a small molecule that inhibits virulence regulation in V. cholerae, to Ace. The binding constant (K=9×10(4) M(-1)) and the standard free energy change (ΔG°=-12 kcal mol(-1)) of Ace-virstatin interaction have been evaluated by the fluorescence quenching method. The binding does not affect the oligomeric status of Ace. A cell viability assay of the antibacterial activity of Ace has been performed using various microbial strains. A homology model of Ace, consistent with the experimental results, has been constructed.

A site-directed mutagenesis study of drug-binding selectivity in genetic variants of human alpha(1)-acid glycoprotein

Human alpha(1)-acid glycoprotein (AGP), a major carrier of many basic drugs in circulation, consists of at least two genetic variants, namely A and F1*S variant. Interestingly, the variants of AGP have different drug-binding properties. The purpose of this study was to identify the amino acid residues that are responsible for the selectivity of drug binding to genetic variants of AGP using site-directed mutagenesis. First, we screened amino acid residues in the region proximal to position 100 that are involved in binding of warfarin and dipyridamole, which are F1*S-specific ligands, and of propafenone, which is an A-specific ligand, using ultrafiltration. In the F1*S variant, His97, His100, and Trp122 were involved in either warfarin- or dipyridamole-binding, while Glu92, His100, and Trp122 participated in the binding of propafenone in the A variant. Exchange of the residue at position 92 between AGP variants reversed the relative strength of propafenone binding to the two variants, but had a markedly different effect on binding of warfarin and dipyridamole. These findings indicate that the amino acid residue at position 92 plays a significant role in drug-binding selectivity in AGP variants, especially for drugs that preferentially bind to the A variant.

Enantioselective plasma protein binding of propafenone: mechanism, drug interaction, and species difference

The interaction of propafenone (PPF) enantiomers with human plasma, human serum albumin (HSA), alpha(1)-acid glycoprotein (AGP), as well as with plasma from rat, rabbit, and cow was investigated using indirect chiral high performance liquid chromatography (HPLC) and ultrafiltration techniques. The stronger binding of the S-PPF found in human plasma was due to AGP. Two classes of binding sites in AGP were identified: one with high-affinity and small binding capacity (K(1(S)) = 7.65 x 10(6) M(-1), n(1(S)) = 0.50; K(1(R)) = 2.81 x 10(6) M(-1), n(1(R)) = 0.46), which revealed stereoselectivity; the other with low-affinity and high-binding capacity (n(2(S)) K(2(S)) = 9.95 x 10(3) M(-1); n(2(R)) K(2(R)) = 9.74 x 10(3) M(-1)). The binding to HSA was found to be weak and not enantioselective (nK(S) = 2.08 x 10(3) M(-1), nK(R) = 2.05 x 10(3) M(-1)). The interaction between enantiomers observed in human plasma was confirmed as a competitive type interacting at the high-affinity site in AGP. The binding mode of both enantiomers with AGP was mainly hydrophobic bond. PPF enantiomers had higher-binding affinity for the F-S variant of human AGP. Drug-drug binding interaction studies showed that verapamil, diazepam, nifedipine, furosemide, nitrendipine, and nimodipine did not affect the binding of PPF enantiomers except quinidine and aprindine at the therapeutic concentration. Comparative studies indicated considerable species-dependent binding stereoselectivity between plasma of the four species investigated.

Use of plasma proteins as solubilizing agents in in vitro permeability experiments: Correction for unbound drug concentration using the reciprocal permeability approach

The purpose of the present study was to explore the applicability of the reciprocal permeability approach to correct for changes in thermodynamic activity when in vitro permeability data are generated in the presence of plasma proteins. Diazepam (DIA), digoxin (DIG), and propranolol (PRO) permeability was assessed in the presence of bovine serum albumin (BSA) and bovine alpha-1-acid glycoprotein (AAG). The reciprocal permeability approach was subsequently employed to calculate the true permeability coefficient (Papp(corr)) and the operational protein association constant (nK(a)). For BSA binding, good agreement was observed between the Papp(corr) values and Papp values obtained in the absence of protein. For PRO and AAG, where binding affinity was high, deviation in the reciprocal permeability plots was evident suggesting ligand depletion at low drug/high protein concentrations. Bidirectional DIG permeability data in the presence of either BSA or AAG indicated that neither protein had an effect on the efflux transporters involved in DIG permeability. The data suggest that plasma proteins can be utilized in permeability experiments with no adverse effects on transporter function and that the reciprocal permeability approach can be used to correct permeability data for changes in unbound drug concentration.

Therapeutic drug monitoring of antiarrhythmic drugs

Most antiarrhythmic drugs fulfil the formal requirements for rational use of therapeutic drug monitoring, as they show highly variable plasma concentration profiles at a given dose and a direct concentration-effect relationship. Therapeutic ranges for antiarrhythmic drugs are, however, often very poorly defined. Effective drug concentrations are based on small studies or studies not designed to establish a therapeutic range, with varying dosage regimens and unstandardised sampling procedures. There are large numbers of nonresponders and considerable overlap between therapeutic and toxic concentrations. Furthermore, no study has ever shown that therapeutic drug monitoring makes a significant difference in clinical outcome. Therapeutic concentration ranges for antiarrhythmic drugs as they exist today can give an overall impression about the drug concentrations required in the majority of patients. They may also be helpful for dosage adjustment in patients with renal or hepatic failure or in patients with possible toxicological or compliance problems. Their use in optimising individual antiarrhythmic therapy, however, is very limited.

Propafenone binding interaction with human alpha 1-acid glycoprotein: assessing experimental design and data evaluation

Binding data on racemic RS-propafenone as well as individual R- and S-drug enantiomers interacting reversibly with human alpha 1-acid glycoprotein, as obtained by a high-performance liquid chromatographic method, are evaluated according to three different approaches introduced, respectively, by Scatchard, Bjerrum, and by Tobler and Engel. A non-linear curve-fitting procedure was applied to compute the binding parameters exclusively for the binary system comprising the examined protein and R- and S-propafenone, individually. The exactness of the study design rather than the numerical values were the focus of attention in the evaluation of the data found.

Stereoselective binding of beta-blockers to purified rat alpha 1-acid glycoprotein

The stereoselectivity of binding of four beta-blockers, pindolol, propranolol, oxprenolol and acebutolol, to purified rat alpha 1-acid glycoprotein (AAG) was examined using equilibrium dialysis. Pindolol and propranolol were bound stereoselectively to AAG, whereas binding of oxprenolol was non-stereospecific. Neither of the enantiomers of acebutolol bound to either AAG or any other plasma protein. The affinity of (+)-pindolol was 25 times that of (-)-pindolol, as determined in a single enantiomer experiment. Both enantiomers of propranolol demonstrated two classes of binding sites in AAG, the total binding for the high affinity site for (+)-propranolol being double that of (-)-propranolol, which could explain the higher binding of the (+)-enantiomer in racemate experiments. These results further showed that stereoselective binding to a rat AAG is not a property common to all beta-blockers.

Fluorescence studies of beta-adrenergic ligand binding to alpha 1-acid glycoprotein with 1-anilino-8-naphthalene sulfonate, isoprenaline, adrenaline and propranolol

The present study shows that ANS (1-anilino-8-naphthalene sulfonate), propranolol, isoprenaline, adrenaline and dopamine have common binding sites on AAG (alpha 1-acid glycoprotein). A fluorescence technique was employed to characterize the interaction between the ligands and AAG at 20-22 degrees. The binding of ANS to AAG caused increased fluorescence intensity at emission and excitation wavelengths of 400 and 470 nm. In this situation, propranolol displaced ANS in a concentration-dependent mode with an apparent dissociation constant of 6.2 +/- 0.01 microM, whereas isoprenaline did not reduce the ANS-AAG fluorescence. However, in the presence of AAG, catecholamines caused a marked increase of fluorescence at excitation and emission wavelengths of 250 and 325 nm, respectively. These wavelengths were employed to characterize the binding of isoprenaline, adrenaline and propranolol to AAG. Two subsets of binding sites were demonstrated. The Kd values were 0.87 +/- 0.03 and 25.1 +/- 10.7 microM for ANS, 0.76 +/- 0.09 and 133 +/- 30.4 microM for propranolol, 140 +/- 14 and 2.18 +/- 0.58 mM for isoprenaline, 137 +/- 24 and 14.8 +/- 0.1 mM for adrenaline, respectively. AAG had identical high affinity binding capacity for these ligands (n approximately 1). However, the second class of binding sites showed ligand-dependent binding capacity: n = 1 for ANS, n approximately 10 for propranolol, n approximately 15 for adrenaline, n approximately 20 for isoprenaline, respectively. ANS, propranolol, dopamine and adrenaline caused concentration-dependent inhibition of isoprenaline binding to AAG with apparent dissociation constants of 5.1 +/- 1.8 microM, 6.4 +/- 1.1 microM, 0.57 +/- 0.13 mM and 1.5 +/- 0.46 mM, respectively.

Tissue distribution of propafenone in the rat after intravenous administration

Tissue distribution of propafenone has been studied in the rat. Measurement of propafenone was made in several tissues: plasma, heart, kidney, lung, liver, muscle, fat and brain, after i.v. administration of 2 mg/kg of the drug. The plasma propafenone kinetics profile can be described by a two-compartmental model. The pharmacokinetic parameters, derived from plasma levels, showed a t1/2 beta of 55.4 min, the central Vd/kg of 2.4 ml/kg, the Cl of 62.8 ml/min.kg and the AUC0-oo of 31.6 micrograms.min/ml. The analysis of the propafenone tissue distribution showed the highest concentration of drug in the lung, followed by the heart and kidneys. A significant concentration was found in brain, muscle and adipose tissue, with concentration ratios (tissue/plasma) above 1. The half-life values obtained for individual organs and tissues are similar to those obtained in plasma, around 1 h. In the post-distributive phase, plasma and tissue concentrations decline in parallel.

Interaction of propafenone enantiomers with human alpha 1-acid glycoprotein

The interaction of propafenone enantiomers with human alpha 1-acid glycoprotein was studied using high-performance liquid chromatography. Each of the two optical antipodes interacted with one class of high-affinity binding sites characterized by Ka(R) = (6.18 +/- 0.93) x 10(5) M-1, n(R) = 1.34 +/- 0.09 for the (R)-isomer and Ka(S) = (8.93 +/- 1.82) x 10(5) M-1, n(S) = 0.99 +/- 0.08 for the (S)-isomer. Nonspecific binding to secondary low-affinity high-capacity binding site(s) was only slightly greater in the case of the (S)-enantiomer (n'k'(S) = (1.06 +/- 0.09) x 10(4) M-1) compared to the (R)-enantiomer (n'k'(R) = (6.87 +/- 0.72) x 10(3) M-1). It was concluded that both enantiomers interact with common single class of high-affinity binding sites on AAG (along with nonspecific binding) exhibiting only slight stereoselectivity for propafenone.

Reversal of acquired resistance to adriamycin in CHO cells by tamoxifen and 4-hydroxy tamoxifen: role of drug interaction with alpha 1 acid glycoprotein

Tamoxifen and 4-OH tamoxifen were used to reverse multidrug resistance (MDR) in CHO cells with acquired resistance to adriamycin (CHO-Adrr). Because alpha 1 acid glycoprotein (AAG) can bind a range of calcium channel blockers that also reverse MDR and rises in malignancy, its interactions with tamoxifen and 4-OH tamoxifen were also studied. Tamoxifen decreased the IC50 of 10 microM adriamycin 4.8-fold in the parent CHO-K1 cell line and 16-fold in CHO-Adrr. Similarly 4-OH tamoxifen decreased the IC50 3-fold in the parent cells, but 13-fold in the resistant cells. Tamoxifen and 4-OH tamoxifen were similarly potent in reversing MDR, although their anti-oestrogen potency differs 100-fold. AAG was added in increasing concentrations to the combination of adriamycin and tamoxifen. As AAG concentrations increased from 0.5 to 2 mg ml-1 (the range found in vivo) the effect of tamoxifen on reversing MDR was gradually decreased. At the highest AAG concentrations, there was complete reversal of the effects of both tamoxifen and 4-OH tamoxifen. AAG was found to bind 3H-tamoxifen in a non-saturable non-specific manner, in contrast to the binding of tamoxifen to albumin. Thus the use of tamoxifen as a reversal agent for MDR in vivo may be impaired by high binding to AAG. However, at the lower range of normal values of AAG, there was still an effect of 10 microM tamoxifen. It may be desirable to select patients for modifier studies based on AAG plasma levels.

Pharmacokinetics of diphenhydramine after dose ranging in nonpregnant ewes

Studies were conducted to characterize the pharmacokinetics of diphenhydramine in nonpregnant ewes after iv administration of 25-, 50-, 100-, and 200-mg doses of diphenhydramine hydrochloride on a crossover basis. Plasma drug concentration versus time data exhibited multiexponential characteristics. The initial distribution half-life increased from 5 to 9 min and the elimination half-life from 34 to 68 min as the dose was increased. There was also an increase in the volume of distribution (from 3 to 6 L/kg) with increasing dose. The elimination half-life and the volume of distribution after a 200-mg dose were significantly greater than after a 25-mg dose. There was, however, a linear increase in AUC0 infinity as dose was increased. The average total body clearance (approximately 5 L/h/kg) remained unchanged regardless of dose. The free fraction of diphenhydramine determined by equilibrium dialysis averaged 0.229 +/- 0.080, and the extent of drug binding to plasma protein was independent of the drug concentrations encountered (30-780 ng/mL) in the nonpregnant sheep in vivo. Concentration-independent binding of the drug was also confirmed by in vitro binding studies over the drug concentration range 10-2000 ng/mL. Therefore, it appears that changes in the volume of distribution are likely to be a result of changes in tissue uptake or binding of the drug as a function of dose.

Influence of lignocaine on plasma protein binding and pharmacokinetics of verapamil in dogs

The effect of lignocaine (lidocaine) on the plasma protein binding of verapamil has been studied in-vitro and in-vivo in dogs. The binding of verapamil was ca 85%. In-vitro addition of lignocaine at therapeutic concentrations displaced verapamil from its plasma binding sites. Lignocaine in this regard was equipotent with tris(2-butoxyethyl)phosphate, suggesting an interaction at the level of alpha 1-acid glycoprotein binding sites. On in-vivo administration of 4 mg kg-1 in a bolus to dogs in which steady state concentrations of verapamil were present, the free fraction of verapamil increased transiently. During the lignocaine maintenance infusion, it then decreased to a level higher than that before administration of the local anaesthetic. The free verapamil concentrations increased suddenly upon the administration of the lignocaine loading dose, and then returned to values slightly higher than those before lignocaine. After a bolus injection of verapamil during a lignocaine infusion, the verapamil total plasma concentrations were lower than during a saline infusion, but the free concentrations were not different. The volume of distribution of verapamil was increased, whereas the blood clearance had not changed; the lignocaine infusion did not change the hepatic blood flow, as measured by indocyanine green clearance. These results show that lignocaine displaces verapamil in-vitro and in-vivo from its plasma protein binding sites, but the ensuing pharmacokinetic changes do not lead to significant changes in free verapamil concentrations.

Use of 1-anilino-8-naphthalene sulfonate as a fluorescent probe in the investigation of drug interactions with human alpha-1-acid glycoprotein and serum albumin

We report a rapid method for the characterization of the human serum albumin (HSA) and alpha-1-acid glycoprotein (AAG) interactions with drugs. The binding of 1-anilino-8-naphthalene sulfonate (ANS) to AAG and HSA was measured by fluorescence spectroscopy. Fluorescence data indicated that ANS was bound tightly to at least one site on AAG, with an affinity constant of 1.35 x 10(6) M-1. The fluorescence of an ANS:AAG complex was quenched by the binding of various drugs. Fluorescence quenching of the HSA:ANS complex showed a single site with an affinity constant of 0.72 x 10(6) M-1. The interaction of AAG and HSA with ANS or other drugs was also studied by comparative equilibrium dialysis. [14C]Pipequaline was used as an AAG and HSA site marker. [14C]Pipequaline seems to share sites I (azapropazone) and II (diazepam and ibuprofen) of HSA. However, high concentrations of warfarin were unable to displace [14C]pipequaline. On the other hand, it was shown that palmitic acid decreased, whereas bilirubin increased the pipequaline binding.

Modulation of dipyridamole action by alpha 1 acid glycoprotein. Reduced potentiation of quinazoline antifolate (CB3717) cytotoxicity by dipyridamole

Dipyridamole potentiates the cytotoxicity of N10-propargyl-5,8-dideazafolic acid (CB3717), an antifolate inhibitor of thymidylate synthase, by inhibiting both thymidine (TdR) salvage and deoxyuridine (UdR) efflux. Dipyridamole binds to the serum component alpha 1acid glycoprotein (alpha 1AGP) and hence the effects of alpha 1AGP on dipyridamole-induced changes in nucleoside transport and CB3717 cytotoxicity have been investigated. Using A549 lung cancer cells in vitro, alpha 1AGP reduced the inhibition of nucleoside transport by dipyridamole in a concentration-dependent manner. Between 10 and 200 times the concentration of dipyridamole was needed to inhibit TdR uptake to the same degree in medium containing 1 mg/ml alpha 1AGP (a physiological concentration) when compared to the uptake in alpha 1AGP-free medium. Although dipyridamole inhibited UdR efflux more than TdR efflux, inhibition of UdR efflux was reduced less than the inhibition of TdR efflux in the presence of 1 mg/ml alpha 1AGP. Thus, clinically achievable levels of dipyridamole (2.5-7.5 microM), even in the presence of physiological alpha 1AGP concentrations, caused significant inhibition of nucleotide uptake and efflux. The cytotoxicity of CB3717 was increased 2-3-fold by 3 and 10 microM dipyridamole in alpha 1AGP-free medium, whereas dipyridamole did not significantly (P greater than or equal to 0.05) potentiate CB3717 cytotoxicity in the presence of 1 mg/ml alpha 1AGP. Measured free dipyridamole levels indicated that the impaired inhibition of nucleoside transport and the lack of potentiation of CB3717 cytotoxicity in the presence of alpha 1AGP was due solely to the binding of dipyridamole to alpha 1AGP. It is concluded that alpha 1AGP levels will be a major determinant of the ability of dipyridamole to modulate the activity of antimetabolites in vivo.

Different binding of propranolol enantiomers to human alpha 1-acid glycoprotein

The binding of propranolol enantiomers to human alpha 1-acid glycoprotein was studied using high performance liquid chromatography in order to provide insight into binding models and to describe individual binding parameters of both enantiomers. The binding of (-)-propranolol was shown to be saturable with one major binding site (n = 0.81, k = 2.73 x 10(5)/M). The saturation process achieved its upper asymptotic value at drug/protein molar ratio of approximately 1. In the case of the opposite (+)-enantiomer the binding isotherm did not show evidence of saturation even at higher drug/protein molar ratios (up to 50). The individual binding parameters for (+)-enantiomer were n = 0.38, k = 3.4 x 10(6)/M and n'k' = 1.39 x 10(4)/M for the saturable and nonsaturable binding component, respectively. At drug/protein molar ratio 2 the circular dichroism measurements confirmed the existence of different binding models for individual propranolol enantiomers.

Interaction between propranolol and propafenone in healthy volunteers

The effects of propafenone on the pharmacokinetics and pharmacodynamics of propranolol were evaluated in 12 healthy male subjects. Both propafenone and propranolol were each administered alone for one week followed by concomitant administration for an additional week. Blood samples, obtained at steady-state, were analyzed for propafenone and its two metabolites as well as for propranolol and 4-hydroxypropranolol. Left ventricular function, exercise performance and electrocardiographic intervals were assessed. Coadministration of propranolol did not produce any significant change in propafenone kinetics including peak plasma concentration (Cmax), time to peak plasma concentration (Tmax), elimination rate constant (t1/2), mean steady-state plasma concentration (Css), or area under the concentration vs time curves. However, concomitant propafenone administration significantly increased Cmax (83%), Tmax (55%), t1/2 (30%), and Css (213%) which were accompanied by significant decreases in plasma levels of 4-hydroxy-propranolol. Propafenone and propranolol significantly reduced supine systolic and diastolic blood pressure by 2.5 to 15.4%. The combination did not reduce diastolic blood pressure further (64.0 +/- 2.8 to 59.7 +/- 1.7 mmHg) nor did it produce a supplemental reduction in heart rate (12% reduction with propranolol, 10% reduction with concomitant administration). Propranolol, but not propafenone, significantly decreased end-diastolic volume index (13%), stroke volume index (15%), and velocity of circumferential fiber shortening (19%). The combination did not cause any further changes in echocardiographic measurements. Electrocardiographic intervals were not altered by either drug use alone or in combination.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of alpha-1-acid glycoprotein administration on propranolol binding and beta blockade in rats

Alpha-1-acid glycoprotein (AAG), 750 mg/kg, was administered to rats to determine its effect on propranolol binding and beta blockade. Anesthetized rats received [3H]propranolol i.v., followed in 15 min by human AAG or bovine serum albumin, 750 mg/kg. AAG treatment produced a human AAG concentration in serum of 7.76 +/- 1.17 mg/ml, several times higher than the endogenous serum AAG concentration in stressed rats. AAG treatment significantly increased the heart rate response to isoproterenol, compared to albumin (95.4 +/- 19.6 vs 28.3 +/- 16.7% of baseline value, measured 45 min after propranolol, P less than 0.001). AAG-treated rats had greater [3H]propranolol binding in serum (93.0 +/- 3.2 vs 76.7 +/- 3.0%, P less than 0.01) and a lower calculated unbound [3H]propranolol concentration in serum (2.7 +/- 1.3 vs 7.4 +/- 3.1 X 10(6) dpm/ml, P less than 0.001) than albumin-treated rats. These data demonstrate that AAG can alter propranolol pharmacokinetics and pharmacodynamics even when administered after the propranolol effect is evident. Because the reported affinity of propranolol for cardiac beta receptors is 10,000 times greater than its affinity for AAG, these data suggest that AAG acted by altering propranolol disposition rather than by directly competing with beta receptors for drug.

Binding of prazosin and propranolol at variable alpha 1-acid glycoprotein and albumin concentrations

1. The effect of variable alpha 1-acid glycoprotein (AAG) and albumin (HSA) concentrations on the binding of prazosin and propranolol was assessed in plasma after surgery and in mixtures of AAG/HSA with concentrations mimicking those found in vivo. 2. On the pre-operative day the binding of prazosin and propranolol was 94.8% and 89.0%, respectively and 97.3% and 93.2%, respectively, 5 days after surgery. 3. In solutions containing mixtures of highly purified AAG and HSA representing the pre-operative state, 88.6% and 83.9% binding of prazosin and propranolol was observed, whereas for solutions mimicking post-operative plasma, the equivalent values were 94.6% and 91.4%, respectively. 4. The ratios between bound and unbound concentrations of both drugs were closely correlated to the concentrations of AAG, but not to the concentrations of HSA. 5. The present study demonstrates that AAG is responsible for the binding variability of prazosin and propranolol in plasma from post-operative patients.

Effects of coadministration of propafenone on the pharmacokinetics of digoxin in healthy volunteer subjects

Previous reports have suggested an interaction between propafenone and digoxin. We investigated the pharmacokinetics of IV digoxin when given alone (Phase I), after pretreatment with propafenone 150 mg every 8 hours for seven days (Phase II), and after propafenone 300 mg every 8 hours for 7 days (Phase III). The total body clearance of digoxin during Phase I was 2.45 ml/min/kg and was 2.17 ml/min/kg during Phase II (NS) and decreased to 1.92 ml/min/kg during Phase III (P less than 0.05). The renal clearance and half-life of digoxin were not significantly altered by propafenone. There was a trend towards a decrease in the volume of distribution of digoxin from 9.43 L/kg in Phase I, to 9.33 L/kg in Phase II, and 8.02 L/kg in Phase III. Similarly there was a trend towards a decreased nonrenal clearance of digoxin from 1.21 ml/min/kg during Phase I to 1.01 ml/min/kg during Phase II and to 0.75 ml/min/kg during Phase III. The changes in volume of distribution and nonrenal clearance parallel each other resulting in no change in the elimination half-life of digoxin. It is postulated that the mechanism of this interaction is due to decreases in the volume of distribution and nonrenal elimination of digoxin by propafenone. The degree of this interaction was related to the dose of propafenone. The magnitude of this interaction may be greater in patients and, thus, may require a reduction in the digoxin dose.

In vitro protein binding of propafenone in normal and uraemic human sera

The protein binding of propafenone, a Class I antiarrhythmic agent, was studied in vitro using a selective and sensitive electron-capture detection gas-liquid capillary chromatographic assay method developed in our laboratory. The concentration-dependency of the serum protein binding of propafenone was confirmed in vitro by equilibrium dialysis, using serum obtained from healthy human subjects and patients with chronic renal failure. In normal serum the unbound fraction of propafenone was 0.027 at a propafenone concentration of 0.25 microgram.ml-1, 0.041 within the therapeutic concentration range (0.5-2 micrograms.ml-1), 0.138 at a propafenone concentration of 25 micrograms.ml-1, and 0.187 when the propafenone concentration was increased to 100 micrograms.ml-1. There was no evidence of significant concentration-dependent changes in unbound fraction within the propafenone concentration range of 0.5-1.5 micrograms.ml-1. However, concentration-dependent binding was demonstrated at concentrations greater than 1.5 micrograms.ml-1. A high-affinity, low-capacity binding site (K1 = 6.53 x 10(5) l.mol-1; n1P1 = 1.73 x 10(-4) mol.l-1) and a low-affinity, high-capacity binding site (K2 = 8.77 x 10(3) l.mol-1; n2P2 = 8.57 x 10(-3) mol. x l-1) were identified. In pooled uraemic serum the unbound fraction of propafenone was approximately 50% of that of normal serum throughout the concentration range studied (1-5 micrograms.ml-1). In sera from patients with chronic renal failure the increase in propafenone binding ratio or the decrease in unbound fraction was associated with the increase in alpha 1-acid glycoprotein concentrations, and there was a correlation (r = 0.8302) between alpha 1-acid glycoprotein concentration and the propafenone binding ratio.

Delipidation of alpha 1-acid glycoprotein. Propranolol binding to this glycoprotein and its modification by extracted material and exogenous lipids

Propranolol binding to human alpha 1-acid glycoprotein (AAG) delipidated by two methods is described. Commercial AAG (99% pure) was either precipitated by ethanol-acetone and then washed by ether, or it was precipitated by ethanol. Binding capacity was quantified by the product n x Ka where n denotes the number of binding sites and Ka the association constant (M-1). Propranolol binding to nondelipidated AAG (n x Ka = 0.113 +/- 0.013 microM-1) was clearly increased after precipitation by ethanol-acetone (n x Ka = 0.386 +/- 0.109 microM-1) or precipitation by ethanol (n x Ka = 0.312 +/- 0.096 microM-1). Binding capacity potentiation cannot be due to modification of AAG microheterogeneity forms, as two-dimensional gel electrophoresis pattern of AAG in presence of concanavalin A was not altered after both methods. Recombination of precipitated AAGs with supernatant dry residue resulted in the abrogation of observed potentiation. Moreover, addition of a polar lipid, linoleic acid, (from 30 to 300 microM) strongly inhibited propranolol binding. These results indicated that glycoprotein precipitation by ethanol provided a simple method to further study binding inhibitors associated with isolated AAG.

Buffer and pH effects on propranolol binding by human albumin and alpha 1-acid glycoprotein

Propranolol binding to isolated human alpha-1-acid glycoprotein (AGP) and human albumin (HSA) was studied by equilibrium dialysis at 37 degrees C. With AGP (0.067%) and HSA (4%), total propranolol concentration was varied from 0.7 to 93,000 ng mL-1. Over this concentration range the percentage drug bound to HSA declined from 49 to 39% while that to AGP declined from 68 to 4%. Two classes of sites were identified on AGP with n1k1 = 8.50 X 10(4) M-1 and n2k2 = 3.12 X 10(4) M-1. With a pH 7.4 phosphate buffer, propranolol binding to AGP was greatest when the protein was initially dissolved in pH 7.4 water compared with pH 7.2 water or the phosphate buffer. Thus, the method of AGP solution preparation affected propranolol binding by this protein. For both AGP and HSA, greater drug binding was noted with phosphate buffers in comparison with a physiological buffer. With phosphate buffers, decreasing pH from 7.4 to 7.0 decreased propranolol binding by AGP, while decreasing pH from 7.7 to 7.4 had little effect. With HSA, the percent propranolol bound consistently decreased on lowering pH from 7.7 to 7.0.

Propafenone. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in the treatment of arrhythmias

Propafenone is a Class I antiarrhythmic agent with weak beta-adrenoceptor antagonist activity which can be given both intravenously and orally. Dosage must be individualised because of dose-dependent pharmacokinetics, a wide range of clinically effective plasma concentrations (64 to 3271 micrograms/L) after comparable doses, the presence of an active metabolite (5-hydroxy-propafenone) and genetically determined metabolic oxidation. In non-comparative studies propafenone 450 and 900 mg/day orally significantly suppressed premature ventricular complexes and couplets in 96% and 75% of patients, respectively, and abolished ventricular tachycardia in 75% of patients. Efficacy was confirmed in placebo-controlled studies in which propafenone 300 to 900mg daily suppressed premature ventricular complexes (greater than 80%) in 77% of patients; 87% of patients had significant reductions in couplets and abolition of ventricular tachycardia. In patients with ventricular arrhythmias refractory to other antiarrhythmic agents, propafenone 450 to 1200 mg/day suppressed arrhythmias in 63% of patients (in long term therapy 66%). Electrically induced arrhythmias were prevented by intravenously administered propafenone in 12 to 23% of patients. However, long term oral therapy was effective in 77% of patients selected using programmed electrical stimulation. Propafenone was also effective in suppressing atrial and AV nodal/junctional re-entrant tachycardias and Wolff-Parkinson-White tachycardias involving accessory pathways. A limited number of comparisons with other antiarrhythmic drugs indicate that the antiarrhythmic efficacy of propafenone is superior or similar to that of quinidine, disopyramide and tocainide, and comparable to that of lignocaine (lidocaine), flecainide and metoprolol against ventricular arrhythmias and a smaller number of atrial arrhythmias. Cardiovascular side effects indicate a proarrhythmic effect similar to that with other Class I drugs, occasional precipitation of congestive heart failure and conduction abnormalities; the latter two occur more often in patients with underlying ventricular dysfunction. Non-cardiovascular side effects (neurological, gastrointestinal) are well tolerated and generally resolve with continued therapy or dosage reduction. Thus, propafenone is an effective antiarrhythmic agent, and is a useful addition to currently available drugs, although further studies will be required to determine clearly its place in therapy compared with more established antiarrhythmic drugs.

Binding of catecholamines to alpha-1 acid glycoprotein, albumin and lipoproteins in human serum

The binding of catecholamines in human serum was determined by equilibrium dialysis at 37 degrees. For serum concentrations of 10-15 nM the bound fractions were 28.8 +/- 2.2%, 25.7 +/- 1.7% and 22.2 +/- 2.2% for (+/-)-isoproterenol (IPR), (+/-)-norepinephrine (NE) and (+/-)-epinephrine (EPI), respectively. At higher serum concentrations saturation occurred. Alpha-1 acid glycoprotein (AAG) possessed one high affinity binding site and approximately 10 low affinity sites. The catecholamines were bound to AAG with the same order of potency for both classes of binding sites: IPR (Kd1: 100 microM Kd2: 2.2 mM) greater than NE (Kd1: 120 microM, Kd2: 6.5 mM) greater than EPI (Kd1: 140 microM, Kd2: 14 mM). Human serum albumin (HSA) and lipoproteins (SLP) interacted with the catecholamines in a non-saturable manner. IPR showed the strongest and EPI the weakest association to both of these serum protein fractions. (-)-Propranolol was able to inhibit the binding of IPR in serum and to isolated AAG, but not to HSA or to SLP. The present results show that AAG is an important catecholamine-binding protein in human serum. AAG, but not HSA or SLP, possesses binding sites shared by adrenergic receptor stimulators and blockers.