Methods of determining plasma and tissue binding of drugs. Pharmacokinetic consequences

Rapid microsample analysis of imipramine and desipramine by reversed-phase high-performance liquid chromatography with ultraviolet detection

A rapid and highly sensitive HPLC assay method was developed to measure small amounts of imipramine and its major metabolite, desipramine. The assay involved simple extraction procedures using clomipramine as the internal standard. The mobile phase consisted of acetonitrile (60%) and 0.01 M triethylamine in distilled water (40%) with the pH adjusted to 3.0. Separations were achieved on a C18 column and the effluent measured for UV absorption at 260 nm. The chromatographic separation was excellent, with no interference from endogenous serum constituents. This assay was suitable for measuring drug concentrations in the range of 10-1000 ng/ml using a 0.1-ml serum sample. The method was applied to a drug disposition study in transgenic mice with increased plasma alpha 1-acid glycoprotein.

The antimalarial drug halofantrine is bound mainly to low and high density lipoproteins in human serum

1. The major serum proteins which bind halofantrine were identified by size exclusion chromatography. In addition, the binding affinity of halofantrine to human erythrocytes and serum proteins was measured by an erythrocyte partitioning technique. The influence of serum-drug binding on the distribution of halofantrine in whole blood was estimated by simulating several disease-related changes in the levels of the most important binding proteins. 2. The chromatographic resolution of serum preincubated with halofantrine allowed a quantitative analysis of binding to low density lipoproteins, high density lipoproteins, alpha 1-acid glycoprotein and albumin using the erythrocyte partitioning technique. Very low density lipoproteins did not bind halofantrine to a significant extent. 3. In whole blood halofantrine is bound to serum proteins (83%) and to erythrocytes (17%). Low density lipoproteins (affinity constant nKP = 44.4 l g-1) and high density lipoproteins (nKP = 14.4 l g-1) were the most important binding proteins in serum. alpha 1-acid glycoprotein (nKP = 4.39 l g-1) and albumin (nKP = 0.27 l g-1) had relatively low binding affinities. 4. The concentration of serum proteins influences both the fraction of unbound drug and the fraction of drug associated with the erythrocytes. Changes in serum protein concentrations often encountered in malaria are likely to increase both the unbound fraction and the fraction bound to the erythrocytes.

Placental transfer of drugs administered to the mother

Drugs administered to mothers have the potential to cross the placenta and reach the fetus. Under particular circumstances, the comparison of the drug concentration in the maternal and fetal plasma may give an idea of the exposure of the fetus to the maternally administered drugs. In this review drugs are classified according to their type of transfer across the placenta. Several drugs rapidly cross the placenta and pharmacologically significant concentrations equilibrate in maternal and fetal plasma. Their transfer is termed 'complete'. Other drugs cross the placenta incompletely, and their concentrations are lower in the fetal than in maternal plasma. The majority of drugs fit into 1 of these 2 groups. A limited number of drugs reach greater concentrations in fetal than maternal plasma. It is said that these drugs have an 'exceeding' transfer. The impression prevails that suxamethonium chloride (succinylcholine chloride) and doxorubicin do not cross the placenta. However, a careful analysis of the literature suggests that this impression is wrong and that all drugs cross the placenta, although the extent transfer varies considerably. The following parameters were considered as possible factors determining the extent of placental transfer: (i) the molecular weight of the drug; (ii) the pKa (pH at which the drug is 50% ionised); and (iii) the extent of drug binding to the plasma protein. Drugs with molecular weights greater than 500D have an incomplete transfer across the human placenta. Strongly dissociated acid drug molecules should have an incomplete transfer, but this does not seem to be an absolute rule. For example, ampicillin and methicillin transfer completely and they are strongly dissociated at physiological pH. The extent of drug binding to plasma protein does not influence the type of drug transfer across the human placenta.

Studies on paracetamol binding to serum proteins

There is a limited amount of data on the binding of paracetamol to plasma proteins. It has been suggested that binding might influence the ability of some analytical methods to quantify the total amount of drug present in the plasma fraction--the basis of clinical experience in risk assessment and antidote usage. We have investigated the binding of paracetamol to plasma proteins using an ultrafiltration technique. In overdose and spiked uraemic plasma samples the mean percentage of paracetamol bound was 24.1 (1SD = 7.0) with no significant correlation with drug levels or degree of uraemia. There is a small but significant increase in binding with increasing serum albumin concentration both in plasma (rs = 0.549, P = 0.014) and in pure serum albumin solutions (rs = 0.848, P < 0.001).

General treatment of competitive binding as applied to the potentiometric ion probe technique: application to the interaction of nonsteroidal anti-inflammatory drugs with bovine serum albumin

The binding of naproxen, ketoprofen, phenylbutazone, salicylic acid, azapropazone, and indobufen to bovine serum albumin was studied by applying the potentiometric ion probe technique. An ion-selective electrode for the ion probe 1-anilino-8-naphthalene-sulfonate was utilized for the purposes of this study. A modified site-oriented competitive binding model was used for the estimation of the drugs' binding parameters, considering different number of binding sites on the competing binding class(es) for the probe and the drug. Calculations were based exclusively on the concentration data of the free probe. The model's ability for accurate estimations of binding parameters was evaluated by simulation studies. The following values of binding parameters were found at 25 degrees C for the drugs under study; naproxen, n1 = 9.1, k1 = 9.4 x 10(5) M-1; ketoprofen, n1 = 8.8, k1 = 10.8 x 10(5) M-1; phenylbutazone, n1 = 3.2, k1 = 1.4 x 10(5) M-1; salicylic acid, n1 = 2.6, k1 = 1.8 x 10(5) M-1, n2 = 21.5, k2 = 1.0 x 10(4) M-1; azapropazone, n1 = 0.5, k1 = 7.8 x 10(5) M-1, n2 = 26.3, k2 = 1.9 x 10(4) M-1; indobufen, n1 = 5.8, k1 = 5.8 x 10(5) M-1, n2 = 19.9, k2 = 3.8 x 10(5) M-1, where ni the number of binding sites of the i class and ki the corresponding association constant.

Plasma protein binding of risperidone and its distribution in blood

The plasma protein binding of the new antipsychotic risperidone and of its active metabolite 9-hydroxy-risperidone was studied in vitro by equilibrium dialysis. Risperidone was 90.0% bound in human plasma, 88.2% in rat plasma and 91.7% in dog plasma. The protein binding of 9-hydroxy-risperidone was lower and averaged 77.4% in human plasma, 74.7% in rat plasma and 79.7% in dog plasma. In human plasma, the protein binding of risperidone was independent of the drug concentration up to 200 ng/ml. The binding of risperidone increased at higher pH values. Risperidone was bound to both albumin and alpha 1-acid glycoprotein. The plasma protein binding of risperidone and 9-hydroxy-risperidone in the elderly was not significantly different from that in young subjects. Plasma protein binding differences between patients with hepatic or renal impairment and healthy subjects were either not significant or rather small. The blood to plasma concentration ratio of risperidone averaged 0.67 in man, 0.51 in dogs and 0.78 in rats. Displacement interactions of risperidone and 9-hydroxy-risperidone with other drugs were minimal.