Decreased Protein Binding Research Articles

A study of factors influencing drug disposition in chronic liver disease, using the model drug (+)‐propranolol.

The pharmacokinetics, following i.v. administration of (+)-propranolol (40 mg) have been compared to in vitro measurement of protein binding and biochemical parameters of liver function in six normal subjects and twenty patients with stable chronic liver disease. The clearance of (+)-propranolol decreased with evidence of increasing severity of impairment of liver function correlating significantly with a fall in serum albumin, a rise in bilirubin and a prolongation in prothrombin index. The clearance of (+)-propranolol correlated with and was numerically similar to the clearance of indocyanine green in normal subjects and also in patients with chronic liver disease. Protein binding was decreased in chronic liver disease, but this change was not related to changes in plasma proteins. In normal subjects and patients without ascites the volume of distribution increased with decreases in protein binding. Ascites was associated with a further increase in the volume of distribution. The considerable variation in half-life largely depends on changes in liver blood flow, the degree of protein binding and the plasma protein pool size.

Clinical pharmacokinetics of anticonvulsants.

Anticonvulsant therapy was among the first areas to benefit from clinical pharmacokinetic studies. The most important advantage is that the frequent interindividual variation in the plasma level/dose ratio for these drugs can be circumvented by plasma level monitoring. For several anticonvulsants the brain concentration is shown to parallel the plasma concentration. Phenytoin (diphenylhydantoin) is stil the most important anticonvulsant and the one for which kinetics have been thoroughly investigated in man. These investigations have revealed several reasons for the wellknown difficulties in using this drug clinically. The absorption rate and fraction are very much dependent on the pharmaceutical preparation, and changes of brand may alter the plasma level of phenytoin in spite of unaltered dose. The elimination capacity is saturable causing dose dependent kinetics, which again means disproportional changes in plasma level with changes in dose. Great individual variations exist in the rate of metabolism, and several pharmacokinetic drug interactions are known. As an optimum therapeutic plasma concentration range has been established monitoring plasma levels must be strongly advocated. Interpretation of plasma levels in uraemic patients must take into account decreased protein binding of the drug. Carbamazepine is probably as effective as phenytoin. The elimination is a first order process, but the rate of metabolism increases after a few weeks' treatment. An active metabolite (epoxide) may be the cause of some side-effects. Combined treatment with other anticonvulsant drugs decreases the half-life and more frequent dosing may be necessary. An optimum therapeutic concentration range has been suggested and plasma monitoring is advocated, along with that of the active metabolite, the epoxide. Phenobarbitone is still much used but its kinetics have been investigated to a lesser extent. The main problem is the variability in the rate of elimination. In children the half-life of phenobarbitone is only half of that in adults. An optimum therapeutic plasma range has been established and monitoring is recommended. Primidone may have an anticonvulsant activity in itself, but its main metabolite is phenobarbitone. The relatively rapid elimination of primidone is offset by the long half-life of phenobarbitone. An optimum therapeutic range has been suggested, but plasma level monitoring must include determination of phenobarbitone. Ethosuximide. The clinical pharmacokinetics of this important petit mal anticonvulsant is not well known. It has a relatively long half-life (in adults 2 to 3 days; in children shorter). An optimum therapeutic range has been suggested, and routine monitoring of plasma levels may be recommended. Diazepam exerts a repid anticonvulsant activity when the plasma concentration exceeds approximately 500ng/ml after intravenous injection. The kinetic pattern is complex in man. Clonazepam. The clinical pharmacokinetics are still not fully investigated but a therapeutic range has been suggested...

Vitamin A in liposomes. Inhibition of complement binding and alteration of membrane structure

Incorporation of vitamin A aldehyde (retinal) into liposomes had an inhibitory effect on the amount of human complement protein bound in the presence of specific antiserum. The total membrane-bound protein was directly measured on liposomes which were washed after incubation in antiserum and fresh human serum (complement). At every concentration of complement, decreased protein binding was found with liposomes which contained retinal. Binding of the third component of complement (C3) was also measured directly on washed liposomes and was found to be decreased in the presence of retinal. The diminution in protein binding due to retinal was not caused by differences in the amount of antibody bound and this was shown by two experiments. First, specific antibody protein binding to liposomes was directly measured and was essentially unaffected by retinal. Second, liposomes were prepared from lipid extracts of sheep erythrocytes. These liposomes were used as as immunoadsorbants to remove antisheep erythrocyte antibodies. The immunoadsorbant capacity was the same in both the presence and the absence of retinal. A further conclusion from these experiments was that retinal did not change the number of liposomal glycolipid antigen molecules available for antibody binding and thus presumably did not change the total number of lipid molecules present on the outer surface of the liposomes. Retinal did have an effect on the geometric structure of the liposomes. Size distribution measurements were performed in the diameter range of 1–6.35 μm by using an electronic particle size analyzer (Coulter Counter). Liposomes containing retinal were shifted toward smaller sizes and had less total surface are and volume. It was suggested that retinal-containing liposomes may have had a tighter packing of the molecules in the phospholipid bilayer. This effect of retinal on liposomal structure may have been responsible for the observed decreased binding of C3 and total complement protein.

Transfer of Medium Chain Fatty Acids from Blood to Spinal Fluid in Patients with Cirrhosis

A test meal of medium chain triglycerides (0.5 g per kg of lean body mass) was administered to 12 patients with advanced alcoholic cirrhosis of the liver and to 12 noncirrhotic control subjects. Octanoic acid was measured in serum samples obtained 1 or 2 hr after the test meal. Serum octanoic acid concentrations were 2 times higher in the cirrhotic patients than in the control subjects due to the impaired clearance of octanoic acid from the portal venous circulation (portasystemic anastmosis and parenchymal insufficiency). However, the spinal fluid octanoic acid levels were 4 to 5 times higher in the cirrhotic subjects as compared with the control subjects. This could be explained by studies of ultrafiltrates of serum in vitro, which demonstrated decreased protein binding of octanoic acid in serum at higher levels of serum octanoic acid, as observed in the cirrhotic patients. Application of the in vitro model to the in vivo studies demonstrated that spinal fluid concentrations can be approximated from the serum octanoic acid concentrations. The data are consistent with a passive diffusion of octanoic acid into the cerebrospinal fluid. The possibility of harmful effects of medium chain fatty acids in the cerebrospinal fluid is discussed.

PHYSIOLOGY OF URINARY CORTISOL EXCRETION.

The mechanisms leading to the renal excretion of cortisol have been studied over a wide range of plasma cortisol concentrations. In 3 normal adult males with endogenous plasma cortisol suppressed by dexamethasone, cortisol was infused intravenously in successively increasing amounts and simultaneous renal clearances of cortisol and inulin were determined after brief equilibration at each new concentration of plasma cortisol. In other subjects, clearances of endogenous cortisol and creatinine were measured. Urinary cortisol was quantitated by a glass fiber chroma tographic method, plasma cortisol was estimated as Porter-Silber chromogen, and the percentage of protein binding of cortisol in plasma was determined at 37 C using an ultracentrifugation technique. As plasma cortisol was increased stepwise by infusion, there was a progressive decrease in protein binding, an increase in the filtration of unbound cortisol, and linear increases in both the excretion and reabsorption of cortisol. While the mechanism (s) for reabsorption of cortisol by renal tubules remain (s) unknown, these data are consistent with possible glomerular filtration of unbound cortisol followed by passive backdiffusion of most filtered cortisol. If reabsorption of cortisol is an active process, there is no evidence for a tubular maximum with plasma levels up to 121 μg/100 ml. In contrast to the very low renal clearance of endogenous cortisol in normal subjects, the suppressed group subjected to cortisol infusion showed a high, relatively constant rate of cortisol clearance, averaging 54 ml/min, over a wide range of plasma concentrations. These observations lend basic support to the clinical usefulness of urinary “free” cortisol measurements in the diagnosis of chronic glucocorticoid excess.