Plasma protein binding of drugs in pregnancy

The degree of binding to plasma proteins is an important determinant of drug disposition and response. Normal human pregnancy is associated with concentration of plasma proteins, free fatty acids and possibly other endogenous substances interfering with drug binding. The possibility of an associated change in plasma binding capacity therefore needs to be taken into consideration. Experimental studies conducted mostly in vitro have shown that the plasma protein binding of many (but not all) drugs is decreased during pregnancy, particularly during the last trimester. This phenomenon should be taken into account when interpreting serum concentrations of total (free + protein-bound) drug in clinical practice. Notable examples of drugs whose unbound fraction increases during pregnancy include diazepam, valproic acid, phenytoin, phenobarbitone, salicylic acid, pethidine, lignocaine, dexamethasone, sulphafurazole and propranolol. For many drugs, important differences have been demonstrated in the degree of protein binding between maternal and cord plasma. In some cases, this may provide an explanation for the finding of marked differences in total drug concentration between maternal and fetal plasma at the time of delivery.     

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.     

Plasma protein binding of drugs in the elderly

Binding to plasma proteins can affect the pharmacokinetics and pharmacodynamics of drugs. Age is one of many factors which can affect plasma protein binding of drugs. Unfortunately, very few generalities can be drawn from the studies of the effect of age on protein binding. Whether age has an effect on protein binding is dependent not only on the drug, but also on the manner in which the study is conducted. Several studies involve patients with various disease states making assessment of the effect of age alone on protein binding difficult. Results of different studies on the same drug do not always agree--in one case finding no change in protein binding with age and in another, a significant increase or decrease in protein binding. Most drugs which exhibit increased binding (decreased free fraction) in elderly subjects are basic and tend to have a greater affinity for alpha 1-acid glycoprotein than for albumin. The list of drugs exhibiting decreased binding (increased free fraction) in the elderly is longer and includes both acidic and basic drugs. The impact of changes in protein binding with age is dependent on the magnitude of the change, on the pharmacokinetic characteristics of the drug and on its therapeutic index. Some changes, although statistically significant, are not likely to be of importance clinically. From the studies reviewed, the free fraction is changed by greater than 50% in the elderly for only a few drugs, e.g. acetazolamide, diflunisal, etomidate, naproxen, salicylate, valproate and zimeldine.     

Plasma protein binding of etidocaine during pregnancy and labour

Summary Preliminary studies of the ultrafiltration method for measuring the extent of plasma protein binding of etidocaine showed that etidocaine binding was both pH and concentration dependent. Etidocaine (1 µg/ml) was found to bind avidly to a physiological concentration (74 mg/dl) of ₁-acid glycoprotein (₁-AGP) (7.23±0.64%, mean ± SD, unbound). In vitro investigation of etidocaine binding in plasma obtained from blood bank donors and from 19 pregnant women prior to induction of labour, during early labour, mid-labour and delivery showed no difference in etidocaine binding (10.3±3.3%, 7.06±2.66%, 8.15±2.57%, 7.84±3.74% and 9.28±6.06% unbound respectively). There was a significant increase in the mean plasma total free fatty acid (FFA) concentration from pre-labour (0.535±0.240 mM) to delivery (0.948±0.28 mM), while plasma albumin and -lipoprotein concentrations remained constant. ₁-Acid glycoprotein concentration tended to increase slightly from pre-labour to early labour (p<0.1) but was still within the normal physiological range. There was no correlation between etidocaine binding ratio and the concentrations of FFA or plasma proteins except for a poor correlation with the ₁-AGP concentration (r=0.361, p<0.05). Storage of plasma and inadequate control of plasma pH during ultrafiltration appeared to give spurious binding values. These studies with the extensively bound basic drug etidocaine suggest that unlike many acidic drugs which are bound predominantly to serum albumin, the binding of ₁-AGP — bound basic drugs may be unaffected by pregnancy and labour.     

Plasma protein binding of digitoxin and some other drugs in renal disease

Plasma protein binding of most acidic drugs is decreased in uraemia, whereas the binding of basic drugs is usually unchanged or decreased. Decreased protein binding in patients with renal disease mainly relates to drugs binding to albumin. Digitoxin binds to a specific site on the albumin molecule. Conflicting reports exist on digitoxin-protein binding in patients with renal disease. In ten patients with end-stage renal disease treated with haemodialysis we found only a slightly increased free fraction of digitoxin. A heparin-induced increase of the free fraction of digitoxin during haemodialysis has been reported. However, this increase was caused by the generation of non-esterified fatty acidsin vitro. If thisin vitro lipolysis was blocked, no increase of free digitoxin could be detected. Alterations of digitoxin-protein binding in uraemic patients during haemodialysis and during the intervals between haemodialysis treatments are small.     

Plasma protein binding of alpidem in healthy volunteers,in neonates and in liver or renal insufficiency

Summary The binding of alpidem, a new anxiolytic drug, has been studied in plasma from 6 healthy subjects, 12 patients with renal failure, 12 patients with liver cirrhosis and 12 chronic uraemics maintained on haemodialysis, as well as in 12 serum samples from the placental cord, to represent the situation in the newborn.The unbound fraction was 0.61% (healthy volunteers), 1.31% (newborns), 0.86% (cirrhotic patients), 0.72 (patients with renal failure), 0.70% (before haemodialysis) and 0.79% (after haemodialysis). Binding in the volunteers was significantly different from that in neonates and cirrhotics only.Alpidem became bound to isolated albumin (45 g·l^–1) and alpha₁-acid glycoprotein (0.75 g·l^–1) to 97.2% and 97.1%, respectively. The bound fraction of the drug in a mixture of two proteins was 99.1%.For alpidem, it appears that alpha₁-acid glycoprotein may balance the effect of any decrease in the albumin concentration.     

Plasma protein binding of disopyramide in pregnant and postpartum women, and in neonates and their mothers.

1. The protein binding of disopyramide was measured in plasma obtained from nonpregnant women, pregnant women in the first, second, and third trimesters, matched pairs of mothers and neonates (cord plasma), and 1 month postpartum women (n = 6 or 8 of each). 2. Plasma samples spiked with 0.2-12.0 micrograms ml-1 of the drug were ultrafiltered and the free fractions were measured with a fluorescent polarization immunoassay. 3. The mean (+/- s.d.) percentages of free drug at a total concentration of 3.0 micrograms ml-1 observed in the third trimester (46 +/- 9%) and neonate (79 +/- 5%) groups were greater (P less than 0.05 or 0.01) than that in the non-pregnant group (34 +/- 7%). In contrast, the corresponding value observed in the postpartum group (23 +/- 8%) was less (P less than 0.05) than that in the non-pregnant group. In addition, there was a significant (P less than 0.01) difference in the mean percentage of free drug at 3.0 micrograms ml-1 in plasma from mothers (43 +/- 9%) and neonates (79 +/- 5%). 4. A multiple regression analysis indicated that alpha 1-acid glycoprotein (r = -0.88, P less than 0.01), rather than albumin (r = -0.008), dominated the binding of disopyramide within the therapeutic range of drug concentration. An analysis of the binding parameters of disopyramide suggested that alterations in binding were attributable to changes in the capacity rather than the affinity of binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma protein binding of chlorpromazine

More than 90% of the plasma content of chlorpromazine over a concentration range from 0·008 to 15·1 μg/ml was bound to human plasma protein. Binding was affected by the pH of the aqueous medium; with few exceptions the higher values were obtained at the higher pH values. Binding was highest in some of the plasma samples from humans, and successively lower in plasma from dogs, rabbits and rats. Binding of chlorpromazine after administration of the drug to psychiatric patients, and after in vitro addition of the drug to plasma, was reversible. Variation in binding in plasma from different humans was marked; the amount bound varied from 91·0 to 99·0%. Thus the variation in the amount free was from 1·0–9·0%.     

Plasma protein binding of bepridil

The binding of the calcium-channel blocking agent, bepridil HCl (Vascor), to plasma proteins was investigated using radiolabeled bepridil and equilibrium dialysis. Greater than 99.7% of added bepridil-14C was found to freshly collected human plasma. The binding was characterized by a saturable high-affinity site (KD = 32 ng/mL = 87 nM) on alpha1-acid glycoprotein (AAG) or on an AAG-human serum albumin complex and lower affinity binding sites on albumin and other plasma macromolecules. Bepridil that is not bound to plasma proteins is extensively distributed into erythrocytes as evidenced by a red blood cell to free drug distribution coefficient of 71 +/- 7. Despite this high value, the blood to plasma ratio of bepridil averaged only 0.67 in humans, indicating that most of the circulating drug is bound to plasma proteins. Bepridil protein binding was not affected by additions of nonesterified fatty acids. Free fractions of bepridil were enhanced by addition of verapamil, nifedipine, diltiazem, disopyramide, and warfarin but only at concentrations above those achieved clinically. Bepridil was also displaced by the plasticizer, tris-(2-butoxyethyl)phosphate. Plasma obtained from a small number of angina patients prior to bepridil administration showed no differences in ability to bind bepridil compared with plasma obtained from healthy subjects.     

Plasma protein binding of fentanyl

Whole plasma of 15 normal volunteers bound 79·16% ± 0·16 s.e. (n = 45) of fentanyl, at a concentration of 0·6 ng ml^?1. Measurement was by equilibrium dialysis at 37°C, pH 7·4. The largest contribution to binding appears to be due to serum albumin, since 45·52% ± 0·40 s.e. (n = 3) of fentanyl was bound to a solution of purified albumin at a concentration of 46 g litre^?1 in buffer. Physiological concentrations of isolated very low density, low density and high density lipoprotein fractions in buffer bound 18·39% ± 0·65 s.e.; 39·14% ± 0·42 and 21·18% ± 0·51 (n = 15) respectively. A significant correlation was found between percent binding and serum albumin concentration (r = 0·745, P = 0·0022) and oestrogen and progestagen therapy (r = 0·766, P = 0·0014). There was no significant correlation with fasting serum cholesterol, triglyceride, age, sex or the concentration of total protein minus albumin. Binding to fibrinogen and α₁-acid glycoprotein did not occur. Binding increased with increasing pH, temperature and ionic strength of the buffer. The results were compatible with hydrophobic bond formation between fentanyl and proteins. Fentanyl concentration did not affect the percent bound to whole plasma or the protein fractions over a range of 0·6 ng-10 mg ml^?1. Dilution of plasma with buffer gave a linear relation of percent bound or free to log plasma dilution. The binding of fentanyl to pooled plasma of the normal subjects was not affected by a wide variety of anionic, cationic and uncharged drugs when these were tested at a concentration of 20 μg ml^?1. At higher concentrations, aspirin, phenylbutazone and quinidine caused inhibition of fentanyl binding. A linear relation was found between percent bound and concentration of the inhibitory ligand. For aspirin, r = 0·873, P <0·01; for phenylbutazone, r = 0·81, P <0·05; and for quinidine, r = 0·982, P <0·01. Aspirin and phenylbutazone inhibited binding of fentanyl to albumin, while quinidine caused inhibition of binding to lipoproteins of all three densities, but not to albumin. Changes in the concentrations of the common ions of plasma (except H⁺), of free fatty acids and of creatinine did not affect fentanyl binding to whole plasma. 8 M urea reduced binding by 25% of the normal value.     

Plasma protein binding of ceftriaxone

1. The plasma protein binding characteristics of ceftriaxone, a new cephalosporin antibiotic, were determined in human, baboon, rabbit, dog and rat plasma.

2. The protein binding of ceftriaxone was similar and concentration-dependent in human, baboon, rabbit and rat plasma, being highly bound (90-95%) at low concentrations (<100μg/ml) but considerably less bound (approx. 60%) at high concentrations (> 400 μg/ml). Binding in dog plasma was also concentration-dependent but much lower (approx. 25%) at lower concentrations (30 μg/ml) and virtually unbound (2%) at high concentrations (1 mg/ml) over a similar concentration range.

3. Binding of ceftriaxone to human plasma involved two sites: a high affinity-low capacity (saturable) site and a low affinity-high capacity site. Binding to dog plasma apparently was at a single, high affinity-low capacity site.

4. The pharmacokinetics of ceftriaxone in an animal species with binding characteristics similar to man (baboon), appear to be non-linear when based on total drug concentration and linear when based on the free drug concentration. In the dog, pharmacokinetic parameters did not change appreciably if calculated from total or free drug concentrations, due to the low protein binding.     

Plasma protein binding of drugs as a function of age in adult human subjects.

The intent of this study was to determine what influence, if any, increasing age has on the binding of drugs by plasma proteins. Plasma from healthy subjects ranging in age from 21 to 94 years was used. The binding of phenytoin (diphenylhydantoin) (acid), penicillin G potassium (benzylpenicillin potassium), and phenobarbituric acid was determined by equilibrium dialysis of 14C-labeled compounds. No differences were found in total protein concentration; however, albumin was reduced in subjects over 50 years of age. Plasma binding of each drug studied was not related to age; this finding suggests that age per se is not a factor in the binding of drugs by plasma proteins.     

Carbamazepine protein binding and disposition in pregnancy

The pregnancies of five women with epilepsy treated with carbamazepine monotherapy were studied prospectively. Free and total serum concentrations of carbamazepine and its epoxide and dihydrodiol metabolites were analyzed at monthly intervals from the first trimester through 8 weeks postpartum. Assays were by high performance liquid chromatography, and free compounds were separated by ultrafiltration. The mean intrinsic clearance of carbamazepine (clearance of free drug corrected for changes in maternal body weight) did not change appreciably during pregnancy and the postpartum period. The mean free fractions of carbamazepine and the epoxide were elevated during pregnancy (0.25 and 0.50) compared with postpartum (0.22 and 0.43). Mean total maternal carbamazepine and epoxide concentrations were 40 and 48% higher than neonatal levels at birth, but maternal and neonatal free concentrations agreed closely. The ratio of epoxide to parent drug increased during pregnancy, as reported by other authors. Evidence is presented that this may be a result of inhibition of further biotransformation of the epoxide rather than of increased production. Two patients missed at least one dose of carbamazepine during labor, resulting in markedly reduced serum concentrations at delivery.     

Valproic acid disposition and protein binding in pregnancy

Nine epileptic women were followed prospectively through pregnancy on a monthly basis. At each visit, total and free serum valproic acid levels and albumin levels were obtained. Assays were done by gas-liquid chromatography, and free drug was separated by ultrafiltration. Despite an upward dose adjustment in four patients, total valproic acid levels declined as pregnancy proceeded, but free levels did not. Plasma free fractions and clearances increased, but intrinsic clearances, which were adjusted for changes in body weight, remained unchanged.     

Plasma protein binding of furosemide in the elderly

Summary The protein binding of furosemide was investigated in plasma from 22 old and 11 young subjects by equilibrium dialysis. The unbound fraction of furosemide was 3.16% in plasma from the elderly and 1.71% in plasma from the young. A significant correlation was found between the unbound fraction of furosemide and the plasma concentration of albumin. The average number of binding sites was 3.8 (elderly) and 2.7 (young) 10^–6 mol/g albumin. The average association constant (K) was 4.3 (elderly) and 4.2 (young) 10⁵ M^–1. By increasing the concentration of furosemide up to 200 µg/ml buffer the unbound fraction of the drug rose to 5.2% (elderly) and 3.5% (young).