Influence of changes in protein binding on the central activity of antidepressants.

The central effect (expressed as analgesic response), protein binding and brain uptake of mianserin were measured in mice receiving drug intraperitoneally. A significant decrease of the central effect of mianserin (30 mg kg-1) was seen in mice with experimental inflammation when compared with control animals (reaction time (s) = 12.12 +/- 1.22 vs 25.56 +/- 2.92; P less than 0.001) and the dose-analgesia response curve (10-60 mg kg-1) was significantly shifted to the right in mice with inflammation. In serum of mice with inflammation, unbound concentration of mianserin was decreased from 19.37 +/- 0.73 to 17.83 +/- 0.30% (P less than 0.05) and seromucoid levels were significantly increased (P less than 0.001). Following the intraperitoneal administration of 30 mg kg-1 of mianserin, brain uptake decreased in diseased mice when compared with control animals (P less than 0.02), suggesting that the decrease in analgesia was secondary to a decrease in drug delivery to the brain because of increased protein binding.     

The interaction of human and bovine serum proteins with CYP3A in human liver microsomes: inhibition of testosterone 6beta-hydroxylation by albumin,alpha-globulins,alpha(1)-acid glycoprotein and gamma-globulins

The effects of human and bovine serum proteins on CYP3A activity, using testosterone as the probe substrate, were investigated in human liver microsomes. Serum albumin, alpha-globulins, and alpha(1)-acid glycoprotein (alpha(1)-AGP) of both species significantly inhibited testosterone 6beta-hydroxylation. When the inhibitory effects of serum proteins were compared with serum protein binding data, human alpha-globulins, with a ratio (relative metabolic activity/unbound fraction) of 0.3, showed higher, and bovine alpha(1)-AGP, with the ratio of 1.4, showed lower inhibitory effects than those expected from protein binding of testosterone. The effects of the other serum proteins were close to those expected from protein binding, according to the free drug hypothesis. The K(i) values obtained from the Dixon plots were 0.32% (w/v, 48 microM) for human serum albumin (HSA), 0.48% for human alpha-globulins, and 0.23% (52 microM) for human alpha(1)-AGP. K(i) values of bovine serum albumin, bovine alpha-globulins and bovine alpha(1)-AGP were 3-5 times higher than those of the respective human proteins. The results suggest a direct interaction of some of these serum proteins with the active site of the CYP3A isoform. Since the bovine serum proteins showed weaker inhibitory effects than human serum proteins, the wide use of BSA, which is viewed as interchangeable with HSA, needs to be cautioned.     

Dialyzable serum cofactor(s) required for the protein-mediated transport of DL-propranolol into rat brain

To elucidate the characteristics of promotion factor(s) in rat serum required for the protein-mediated transport of drugs into the brain, we examined the brain uptake of DL-propranolol as a model drug using the in vivo brain uptake index (BUI) method in rats. The protein-mediated transport was not observed in rats injected with the buffer solution containing either various concentrations of purified rat alpha 1-acid glycoprotein (alpha 1-AGP) or rat albumin. When the filtrate from rat serum was used as an injection vehicle to which a physiological concentration of purified rat serum protein(s) was added, the protein-mediated transport of DL-propranolol was observed in the rat brain. Moreover, the ability of protein-mediated transport of DL-propranolol was reduced in rats injected with the dialyzed serum compared with the undialyzed serum. These results suggest that the dialyzable promotion factor in serum is required for the protein-mediated transport of DL-propranolol into the brain.     

Differences in the serum protein binding of prazosin in man and rat

The serum protein binding of prazosin in man and rat has been studied in vitro by equilibrium dialysis. Prazosin was more extensively bound in human serum than in rat serum with binding ratios (B/F) of 14.3 +/- 3.4 and 4.4 +/- 0.2 (corresponding to 93.4 and 81.4% bound), respectively. This difference in binding between the species was partly due to qualitative differences between human and rat serum albumin, but also to the lower concentration of albumin in rat serum. Rat serum albumin (RSA) apparently showed two different classes of binding sites for prazosin, one with high (KD = 5.78 X 10(-6) M) and one with low (KD = 1.1 X 10(-4) M) affinity; the former is suggested as representing alpha 1-acid glycoprotein (alpha 1-AGP) with one binding site for prazosin per molecule, the latter as representing RSA with 0.28 binding sites per molecule. Human serum albumin (HSA) and human alpha 1-AGP both showed one class of binding sites with KD values of 2.7 X 10(-5) and 1.95 X 10(-6) M, respectively. HSA possessed 0.5 and human alpha 1-AGP 1 binding site for prazosin per molecule. The binding parameters obtained for the isolated serum proteins overestimated to some degree the total serum protein binding of prazosin in man. This was explained by a specific deviation from the law of mass action. HSA was the major binding protein in human serum at therapeutic concentrations, with ca. 60% of the total binding, the remaining 40% being bound to alpha 1-AGP. Anticipating that the high affinity binding site on the RSA preparation represents the binding of prazosin to alpha 1-AGP, then this protein accounts for 70% of the binding in rat serum, while rat serum albumin accounts for approximately 23%. The binding of prazosin to lipoproteins was insignificant in both species. The observed differences between man and rat in the serum protein binding of prazosin implicate differences in the two species with respect to prazosin pharmacokinetics and the pharmacological effect.     

The binding characteristics of some adrenergic beta-receptor antagonists to human serum proteins

Binding of pindolol and 8 related compounds was studied in vitro by equilibrium dialysis. The overall binding in serum was compared with the binding to the main, isolated, serum proteins. Most substances show both saturable and non-saturable binding in serum. The saturable and main binding is to alpha 1-AGP, the low non-saturable binding corresponds to albumin and lipoprotein binding. The binding to alpha 1-AGP is characterized by approximately one binding site and association constants K ranging from 10(4) to 10(6) M-1. The binding of pindolol to alpha 1-AGP is strongly inhibited by propranolol, lidocaine, erythromycin, imipramine and TBEP. Significant correlations were found between log NK and log partition coefficient octanol-phosphate buffer suggesting that the protein binding of the 9 adrenergic beta-receptor antagonists to all serum proteins, including alpha 1-AGP, is predominantly hydrophobic in nature.     

Fluorescent investigations of binding of phenprocoumon to alpha 1-acid glycoprotein

The fluorescence of phenprocoumon is greatly enhanced on binding to a single site on alpha 1-acid glycoprotein (alpha 1-AGP). Advantage is taken of this phenomenon to estimate a binding constant for the binding of phenprocoumon to alpha 1-AGP. The fluorescence intensity and binding constant of the phenprocoumon: alpha 1-AGP complex decreased with pH from 6.5 to 8.5, suggesting that phenprocoumon binding to alpha 1-AGP is significantly affected by microenvironmental change in alpha 1-AGP. A variety of drugs, including chlorpromazine and dicumarol, significantly inhibited phenprocoumon binding to alpha 1-AGP. Fatty acids seem to displace phenprocoumon from its binding site on alpha 1-AGP, whereas the addition of neutral salts and sialic acid did not cause the displacement of phenprocoumon. It is concluded that the phenprocoumon binding site is located in the hydrophobic protein structure of alpha 1-AGP.     

Prazosin binding to human alpha 1-acid glycoprotein (orosomucoid), human serum albumin, and human serum. Further characterization of the 'single drug binding site' of orosomucoid

The plasma protein binding of the alpha 1-adrenergic blocking agent prazosin was investigated by means of circular dichroism (CD) and equilibrium dialysis (ED) measurements. The interaction of prazosin with human alpha 1-acid glycoprotein (alpha 1-AGP) results in pronounced negative extrinsic Cotton effects at 255 nm and a smaller negative band at 285 nm which are associated with the binding of prazosin to only one site of the protein. Various basic drugs, and warfarin also, at 50 microM displace prazosin 10 microM from its binding site on alpha 1-AGP and reduce the CD-spectra at 255 nm by 26% (disopyramide), 52% (mepivacaine), about 70% (verapamil, biperiden), and 90-100% (trihexyphenidyl, warfarin). (+/-)-Propranolol reduces the CD-spectra by 76%, its (-)-isomer by 89%, and the (+)-isomer by 65%. ED experiments indicated that the binding of prazosin to alpha 1-AGP is saturable with an association constant of 48 000 M-1 and 0.85 binding sites per protein molecule. Displacement of prazosin from alpha 1-AGP by the same drug as used for the CD experiments at displacer/prazosin ratios of 5 resulted in comparable reductions of the fraction bound as obtained by the CD experiments. Prazosin was also highly bound to human serum albumin (600 microM) with about 80-85% bound at prazosin concentrations from 1-100 microM. Since prazosin binding to human serum is only slightly higher (80-90%) it is concluded that prazosin binding in serum is largely mediated by the albumin fraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of plasma proteins with cytochromes P450 mediated metabolic reactions: inhibition by human serum albumin and alpha-globulins of the debrisoquine 4-hydroxylation (CYP2D) in liver microsomes of human, hamster and rat

The effect of human serum albumin (HSA), alpha1-acid glycoprotein (alpha1-AGP), and alpha- and gamma-globulins on the in vitro metabolism of debrisoquine in human, hamster and rat liver microsomes was studied. Interaction of albumin with cytochrome P450 mediated phenytoin metabolism has been reported. Since plasma protein binding of phenytoin is high, in the present study a weakly protein bound drug, debrisoquine, was studied. Debrisoquine is a substrate of CYP2D6. The debrisoquine 4-hydroxylation was measured using a radio-TLC method. Among the four plasma proteins, alpha-globulins had the strongest inhibitory effect on the debrisoquine 4-hydroxylase activity. The inhibition with 2% alpha-globulins was 42+/-18% for human and higher for hamster and rat liver microsomes (65-71%). HSA had less effect than alpha-globulins. In the presence of HSA, the decrease in activity was between 18 and 35% for all liver microsomes studied. The debrisoquine 4-hydroxylase activity was not significantly changed by alpha1-AGP or gamma-globulins. Using an ultra-filtration method, the protein binding of debrisoquine to 4% HSA, 0.5% alpha1-AGP, 2% alpha-globulins and 2% gamma-globulins was found to be 22, 20, 22 and 5%, respectively. Since the observed inhibition is inconsistent with level of protein binding, it appears, particularly in the case of alpha-globulins, that the plasma proteins interact with CYP2D directly.     

A comparative study of the interaction of warfarin with human alpha 1-acid glycoprotein and human albumin

The interaction of warfarin with human alpha 1-acid glycoprotein (alpha 1-AGP) and human albumin (HA) has been investigated using fluorescence and circular dichroism techniques. The fluorescence of warfarin is greatly enhanced following binding to alpha 1-AGP or HA, the binding constant for a single site being estimated by the Scatchard method. The binding constants for the two serum proteins are similar, but the thermodynamic parameters differ. The binding constants increase as the pH is raised to 9.0. Various basic drugs, such as chlorpromazine, propranolol and imipramine, markedly inhibited the binding of warfarin to alpha 1-AGP. But, some acidic drugs, including phenylbutazone, effectively displaced warfarin bound to HA. The difference in CD spectra observed for alpha 1-AGP and HA indicated that the drug-binding sites of the two proteins might have different asymmetries. It thus appears that the mode of interaction of warfarin with the two proteins differs.     

Inverse stereoselectivity in the binding of acenocoumarol to human serum albumin and to alpha 1-acid glycoprotein

Stereoselective binding of rac-acenocoumarol to human serum albumin (HSA) and alpha 1-acid glycoprotein (alpha 1-AGP) was investigated by affinity chromatography and by combined ultrafiltration (UF) and circular dichroism (CD) methods. For HSA, the ratio of the enantiomeric constants was KR/KS = 2, while for alpha 1-AGP, KS/KR = 3.     

Distribution of moricizine in human blood: binding to plasma proteins and erythrocytes.

Using equilibrium dialysis and incubation experiments, we determined the binding of moricizine to human plasma, isolated plasma proteins, and erythrocytes. The mean (% +/- SD) plasma protein binding at various moricizine concentrations ranged from 81.2 +/- 2.1 to 89.9 +/- 2.1%. There was no apparent relationship between drug concentration and extent of binding in pooled plasma over the concentration range tested. However, protein concentration-dependent binding was observed with albumin and alpha 1-acid glycoprotein (alpha 1-AGP). The unbound fraction of moricizine fell from 61 to 19% and from 70 to 17% with increasing albumin (5 and 50 g/L, respectively) and alpha 1-AGP (0.2 and 1.2 g/L) concentrations. The binding of moricizine to beta-lipoprotein (5 g/L) was 70.6 +/- 3.1% and to gamma-globulin (12 g/L) was 13.6 +/- 3.3%. Moricizine partitioned into erythrocytes, showing an erythrocyte/plasma drug concentration ratio of 1.325 +/- 0.070 and erythrocyte/buffer ratio of 8.561 +/- 0.620. An estimation could be made that 57% of total drug in whole blood was associated with erythrocytes, 39% bound to plasma proteins, and 4% was free. The results of this study demonstrated that erythrocytes, albumin, and alpha 1-AGP were the major binding components in blood.     

Stereoselective binding of propranolol enantiomers to human alpha 1-acid glycoprotein and human plasma.

The binding of propranolol enantiomers to human albumin (ALB), alpha 1-acid glycoprotein (alpha 1-AGP) and plasma was studied. (-) propranolol is more bound than (+)propranolol to alpha 1-AGP (P less than 0.001) and to plasma (P less than 0.05). In solutions containing ALB at a constant concentration (580 mumol/l) and alpha 1-AGP at increasing concentrations, the binding of both isomers increases but the stereo selectivity is evident throughout the alpha 1-AGP concentration range examined (25-100 mumol/l).     

Protein binding studies of furosemide and penbutolol

Serum protein binding of furosemide and penbutolol, the active principles of Betasemid (Hoe 9358), was studied by equilibrium dialysis. Membranes from commercial dialysis tubes were used within commercially available cells. Serum drug and portion unbound in buffer were determined by quantitative thin-layer chromatography. In the range of 1-20 micrograms drug/ml serum, 96 +/- 0.3% of furosemide vs. 88 +/- 4% of penbutolol were bound to proteins. The same results were obtained, when the two substances simultaneously interacted with proteins. Thus, specific protein binding sites for both compounds were demonstrated. For both drugs, Scatchard plots revealed two classes of specific binding sites with statistical mean values/protein of 0.4 and 5 in the case of furosemide and 0.04 and 0.3 in the case of penbutolol. Binding energies were 27.6 and 19.6 kJ/mol furosemide vs. 31.5 and 23.9 kJ/mol penbutolol. Further, serum protein binding of furosemide was studied by ultrafiltration. A micropartition system MPS-1 was used. Results were the same as those from equilibrium dialysis.     

Influence of plasma protein binding on the brain uptake of an antifungal agent, terbinafine, in rats

The intracarotid injection technique has been used to determine the unidirectional brain uptake of an antifungal, lipophilic agent, terbinafine (Lamisil, Sandoz Basle), in the rat. Ultrafiltration showed it to be highly bound to human plasma, human serum albumin (HSA), alpha 1-acid glycoprotein (AAG) and lipoproteins (VLDL, LDL, HDL). The effect of plasma protein binding of the drug on brain uptake was also examined with the technique. The lowest brain uptake was observed in the presence of plasma (6%); it varied from 23 to 30% with physiological concentrations of VLDL, LDL and HSA and was significantly higher (43-45%) in the presence of physiological concentrations of AAG and HDL. The free fraction as determined in-vitro and the brain uptake of the drug varied inversely with the plasma protein concentrations; however, the brain uptake was higher than expected from in-vitro measurements. These data indicate that the amount of circulating Lamisil available for brain penetration exceeds its free fraction; they also show that plasma proteins differently reduce the brain transport of the drug.     

Protein binding of chloroquine enantiomers and desethylchloroquine.

The protein binding of racemic chloroquine, its enantiomers and desethylchloroquine to plasma, purified human albumin, and alpha 1-acid glycoprotein (alpha 1-AGP) was determined by equilibrium dialysis. The binding was not concentration dependent. (+)-Chloroquine bound more to plasma (66.6 +/- 1.9%) and albumin (45.9 +/- 0.8%) than (-)-chloroquine (48.5 +/- 2.4% and 35.3 +/- 0.6%, respectively). These differences were statistically significant. (-)-Chloroquine bound more to alpha 1-AGP (47.5 +/- 0.7%) than (+)-chloroquine (34.5 +/- 0.5%). The binding of desethylchloroquine to alpha 1-AGP is higher than to albumin (38.9 +/- 0.9% and 21.1 +/- 0.4%, respectively.     

Multiple human serum binding of two thienopyridinic derivatives,ticlopidine and PCR 2362, and their distribution between HSA, α1-acid glycoprotein and lipoproteins

The binding of two drugs, ticlopidine and PCR 2362, chemically related to thienopyridin, potent antiaggregant agents, was studied in vitro to serum and to the corresponding isolated proteins, HSA, α₁-AGP, VLDL, LDL and HDL, using equilibrium dialysis at pH 7.4 and 37°. The binding of these drugs to HSA and lipoproteins was non-saturable. The binding capacity of the lipoproteins was much greater than that of HSA and appeared to be dependent on lipid content. The binding capacities of the apoproteins were less than 10% of that observed for the native lipoproteins suggesting that drug-lipoprotein binding involves drug solubilisation in the lipid phase of lipoproteins rather than a classical binding to definite sites. However drug binding to α₁-AGP was saturable with n = 3 for both and K = 89,000 and 33,000 for ticlopidine and PCR 2362, respectively. At physiological concentration, α₁-AGP binding capacity represented 15% of total serum binding capacity which could double in pathological states, in which the level of this protein is increased.     

Plasma protein binding of penbutolol in pregnancy

Penbutolol is a not cardioselective beta-adrenergic blocking drug; it is lipid soluble and differs in its protein binding from the other members of its group because shows linkage to alpha 1-glycoprotein, with no detectable binding to albumin. AAG levels change during pregnancy and so the binding of [3H]-penbutolol was compared in 11 pregnant patients and in 10 healthy women. Binding was obtained by ultrafiltration and measurement of the free fraction by scintillation spectrometry. The free penbutolol fraction was significantly higher in the pregnant women than in the controls (6.06 +/- 0.34 compared with 3.55 +/- 0.29, P less than 0.001). The AAG levels in the pregnant women were significantly lower (0.40 +/- 0.03 g/l) than in the controls (0.77 +/- 0.06 g/l) (P less than 0.001) which showed a significant correlation with the bound/free penbutolol ratio (r = 0.61, P less than 0.005). On the other hand there was no significant correlation with the extent of penbutolol's protein binding even though the albumin levels were lower in the pregnant women (2.83 +/- 0.17 compared with 4.86 +/- 0.17; P less than 0.001). Penbutolol's nK1a for AAG was lower in pregnant women, and this suggests that the fall in AAG levels is not the only factor involved in the reduced binding of penbutolol in pregnancy.     

Binding of oxprenolol and propranolol to serum,albumin and α1-acid glycoprotein in man and other species

Species differences in binding of basic drugs have only occasionally been studied and we have therefore measured the binding of the β-adrenergic blockers oxprenolol and propranolol in (1) serum of healthy humans, dogs, rats and rabbits and of rabbits with experimental arthritis, (2) a solution of albumin of these species and (3) a solution of human α₁-AGP. In humans, dogs, rats and arthritic rabbits, binding of oxprenolol and propranolol was much higher in serum than in albumin solution; in healthy rabbits serum binding was very low and not different from albumin binding. For both drugs, concentration-dependency was seen in serum of dogs, humans and rats and of arthritic rabbits; a similar concentration-dependency was found for human α₁-AGP solution, but not for human albumin and for serum of healthy rabbits.Tris (2-butoxyethyl)-phosphate (TBEP), a known displacer of drugs from α₁-AGP in humans, decreased binding in serum of all species except the rabbit. For both β-blockers, species differences in capacity constants were found; species differences in affinity constants were present only for propranolol. These results suggest that in humans, dog and rat, but much less in rabbits, oxprenolol and propranolol bind mainly to α₁-AGP and that binding to α₁-AGP is more important for oxprenolol than for propranolol.     

Binding of aprindine and moxaprindine to human serum, α1-acid glycoprotein and serum of healthy and diseased humans

Summary A comparison was made between the binding of the anti-arrhythmic agents aprindine and moxaprindine to human serum, to human serum albumin (HSA), to ₁-acid glycoprotein (₁-AGP) and to a mixture of HSA and ₁-AGP. In serum from healthy volunteers (n=4) the binding of aprindine-HCl 5 µg/ml (13.8 µM) was 93.8% (SD±1.0), and that of moxaprindine-HCl 5 µg/ml (12.8 µM) was 94.1% (SD±1.1). Their binding to the mixture of ₁-AGP and albumin approximated their binding to serum. For ₁-AGP, the binding was similar for both compounds, whereas for HSA the binding of aprindine was more pronounced than that of moxaprindine: for both products the affinity coefficient for binding to ₁-AGP was about 100 times greater than that for binding to albumin. In serum from rheumatoid patients and from patients with renal failure a small but significant increase in binding of aprindine and moxaprindine was observed, approximately 1%. Increased and decreased binding was seen in serum from cirrhotic patients; for example, for aprindine the range in cirrhosis was 96.7%–79.8%, and the range in controls was 95.0%–92.4%. Free drug fraction and ₁-AGP concentration were inversely correlated. The results show that ₁-AGP plays an important role in the binding of aprindine and moxaprindine, and that alteration in the binding of the two compounds in disease states to a large extent can be explained by changes in serum ₁-AGP concentration.     

Binding of vinca alkaloid analogues to human serum albumin and to alpha 1-acid glycoprotein

The binding of a series of vinca alkaloid analogues having eburnane or indolo[2,3-a]quinolizidine skeletons was studied with human serum albumin (HSA) by affinity chromatography and with alpha 1-acid glycoprotein by means of competition experiments. On HSA the binding occurs at the benzodiazepine-indole binding site via hydrophobic interaction and shows slight stereoselectivity preferring the trans isomers. The binding to alpha 1-AGP proved to be highly stereoselective in favour of the trans isomers having 3(S),16(R)eburnane or 1(R),12b(S)indolo[2,3-a]quinolizidine absolute configurations.