Displacement of thiopental from human serum albumin by associated drugs

Interactions of fentanyl with blood platelets and plasma proteins: platelet sensitivity to prasugrel metabolite is not affected by fentanyl under in vitro conditions

BACKGROUND

Clinical trials indicate that fentanyl, like morphine, may impair intestinal absorption and thus decrease the efficacy of oral P2Y₁₂ inhibitors, such as clopidogrel, ticagrelor, and prasugrel. However, the ability of fentanyl to directly negate or reduce the inhibitory effect of P2Y₁₂ receptor antagonists on platelet function has not been established. A series of in vitro experiments was performed to investigate the ability of fentanyl to activate platelets, potentiate platelet response to ADP, and/or diminish platelet sensitivity to prasugrel metabolite (R-138727) in agonist-stimulated platelets. The selectivity and specificity of fentanyl toward major carrier proteins has been also studied.

METHODS

Blood was obtained from healthy volunteers (19 women and 12 men; mean age 40 ± 13 years). Platelet function was measured in whole blood, platelet-rich plasma and in suspensions of isolated platelets by flow cytometry, impedance and optical aggregometry. Surface plasmon resonance and molecular docking were employed to determine the binding kinetics of fentanyl to human albumin, α₁-acid glycoprotein, apolipoprotein A-1 and apolipoprotein B-100.

RESULTS

When applied at therapeutic and supratherapeutic concentrations under various experimental conditions, fentanyl had no potential to stimulate platelet activation and aggregation, or potentiate platelet response to ADP, nor did it affect platelet susceptibility to prasugrel metabolite in ADP-stimulated platelets. In addition, fentanyl was found to interact with all the examined carrier proteins with dissociation constants in the order of 10^-4 to 10^-9 M.

CONCLUSIONS

It does not seem that the delayed platelet responsiveness to oral P2Y₁₂ inhibitors, such as prasugrel, in patients undergoing percutaneous coronary intervention, results from direct interactions between fentanyl and blood platelets. Apolipoproteins, similarly to albumin and α₁-acid glycoprotein, appear to be important carriers of fentanyl in blood.

Serum albumin saturation test based on non-esterified fatty acids imbalance for clinical employment

Fatty acids are fundamental as energy and structural source to the human cells. They are not usually found free in human circulation. Alteration in fatty acids metabolism is linked to diseases such as diabetes, preeclampsia, heart disease, and some infectious diseases. Increased levels of non-esterified fatty acids (NEFA) may cause cell dysfunction and lipotoxicity. Since physiologically fatty acids are transported bound to albumin, we propose here a simple and cheap test that consists of albumin isoelectric focusing determination to measure the potential systemic NEFA cytotoxicity. For validation of this method, albumin isoelectric focusing in 51 serum samples from 40 critically ill patients and 11 controls was compared with NEFA/albumin ratios measured by HPLC. We called this approach an albumin saturation test. This test may indicate to physicians the potential NEFA lipotoxicity guiding them throughout better patient management. The albumin saturation test can point out serum albumin-NEFA saturation through a cheap assay that could be performed by any care facility.

Opioid binding sites in human serum albumin

BACKGROUND

Human serum albumin (HSA) is an important carrier for opioids. However, the locations of the binding sites remain unclear. In the present study, we have characterized opioid-HSA interactions using multiple biochemical and biophysical techniques to reveal: (a) the location of the binding site(s); (b) whether naloxone shares the binding site with morphine; and (c) whether opioid agonists share their binding site(s) with general anesthetics.

METHODS

Elution chromatography to determine the global interactions and tryptophan intrinsic fluorescence to determine the localized interactions of opioids with HSA were used. Competition studies using isothermal titration calorimetry were used to determine the overlap of binding site(s) among opioid agonists, antagonists, and general anesthetics. An automatic docking calculation was used to predict the possible binding sites and to assess findings of the solution studies.

RESULTS

For elution chromatography with immobilized HSA, the retention times of naloxone, morphine, and fentanyl were prolonged but shorter than that of propofol. The inhibition of tryptophan fluorescence by naloxone was not affected by morphine or fentanyl. The calorimetric heat profiles of propofol and halothane interaction with HSA were changed significantly, but not equally by morphine, naloxone, or fentanyl. Consistent with direct binding studies, docking results demonstrated that opioids share sites with general anesthetics; a distinct binding site for naloxone was revealed near the sole tryptophan in HSA that is not shared with morphine.

CONCLUSIONS

The interaction of opioids with HSA is weak in comparison with propofol. Naloxone has a distinct binding site in HSA not shared with opioid agonists. Opioids share binding sites with general anesthetics in HSA.

A critical review: does thiopental continuous infusion warrant therapeutic drug monitoring in the critical care population?

Thiopental is a barbiturate used in traumatic brain injuries (TBIs) to reduce intracranial pressure (ICP) and to manage cerebral ischemia. As thiopental follows Michaelis-Menten kinetics, therapeutic drug monitoring (TDM) has been used in practice to improve efficacy and reduce adverse effects. However, its role is still debatable, and TDM is not widely practiced. Current evidence suggests that thiopental therapy may improve mortality and functional outcome in a subpopulation of patients with severe TBI with elevated ICP refractory to conventional medical therapy. Several analytical methods are available to quantify thiopental concentrations. This review uses a previously published 9-step decision-making algorithm to determine whether TDM of thiopental in TBI is warranted. There seems to be poor correlation between thiopental concentration and pharmacological response in terms of neurological response, ICP, electroencephalography, and drug toxicity. There is no established therapeutic range for thiopental continuous infusion due to a wide range of plasma concentrations corresponding to efficacy (25-50 mg/L) and toxicity (30-70 mg/L) and the resulting overlap between the 2. Thiopental exhibits intrapatient and interpatient variability due to age, obesity, renal and hepatic dysfunction, Michaelis-Menten kinetics, and hepatic enzyme autoinduction. Available evidence suggests that TDM of thiopental continuous infusion is not beneficial in improving efficacy or avoiding toxicity. There are however 2 possible scenarios in which TDM may provide additional information to sound clinical judgment. The first is providing patient-specific plasma target concentration to guide titration of therapy. The second scenario is differentiating between brain death and barbiturate-induced coma.

Study on glutathionesulfonic acid sodium salt as biodistribution promoter for thiopental sodium

The effects of glutathione (GSH) and glutathionesulfonic acid sodium salt [N-(N-gamma-L-glutamyl-L-beta-sulfoalanyl)glycine sodium salt, GSO3Na], which is a minor metabolite of GSH, on the pharmacokinetics of thiopental sodium were investigated in rats. The concomitant use of GSO3Na with thiopental sodium significantly increased the tissue-to-plasma concentration ratio (Kp) of thiopental sodium 60 min after its administration in the heart, lung, brain, liver, kidney, and spleen, while GSH did not affect them. On the other hand, the Kp value of thiopental sodium 5 min after its administration with concomitant GSO3Na decreased significantly only in the spleen. Neither GSO3Na nor GSH changes the pharmacokinetic parameters of thiopental sodium. Significant change of the binding ratio of thiopental sodium to bovine serum albumin (BSA) was not observed by the addition of less than 5-fold GSO3Na. About 50% of thiopental sodium was bound to the brain, lung or liver, however, no significant change of this binding ratio was observed by the concomitant use of GSO3Na. The partition coefficient of thiopental sodium apparently increased by the concomitant use of GSO3Na but not by GSH. This phenomenon seemed to be concerned with a mechanism to increase the Kp values of thiopental sodium in the tissues. The increment in the drug distribution to tissues with concomitant GSO3Na observed in this study is useful information for the application of drug combinations as a biodistribution promoter.

Drug interactions with patient-controlled analgesia

Patient-controlled analgesia (PCA) has become standard procedure in the clinical treatment of pain. Its widespread use in patients with all kinds of diseases opens a variety of possible interactions between analgesics used for PCA and other drugs that might be administered concomitantly to the patient. Many of these drug interactions are of little clinical importance. However, some drug interactions have been reported to result in serious clinical problems. Drug interactions can either predominantly affect the pharmacokinetics or pharmacodynamics of the drug. Most important pharmacokinetic drug interactions occur at the level of drug metabolism or protein binding. Acceleration of methadone metabolism caused by cytochrome P450 (CYP) 3A4 induction by antiretroviral drugs or rifampicin (rifampin) has caused methadone withdrawal symptoms. Lack of morphine formation from codeine as a result of CYP2D6 inhibition by quinidine results in an almost complete loss of the analgesic effects of codeine. Alterations of methadone protein binding caused by an inhibition of alpha1-acid glycoprotein synthesis by alkylating substances are another possibility for predominantly pharmacokinetically based drug interactions during PCA. Furthermore, inhibition of P-glycoprotein by anticancer drugs could result in altered transmembrane transport of morphine, methadone or fentanyl, although this has not been shown to be of clinical relevance. Synergistic effects of systemically administered opioids with spinally or topically delivered opioids or anaesthetics have been reported frequently. The same is true for the opioid-sparing effects of coadministered non-opioid analgesics. Antidepressants, anticonvulsants or alpha2-adrenoreceptor agonists have also been shown to exert additive analgesic effects when administered together with an opioid. Inconsistent findings, however, are reported regarding the treatment of patients with opioid-induced nausea and sedation, since coadministration of antiemetics either increased or decreased the respective adverse effects or revealed additional unwanted drug effects.

Serum protein binding of propofol in patients with renal failure or hepatic cirrhosis

BACKGROUND

Serum protein binding is a limiting factor in the access of drugs to the central nervous system. Disease-induced modifications of the degree of binding may influence the effect of anesthetic drugs.

METHODS

The protein binding of propofol, an intravenous anaesthetic agent which is highly bound to serum albumin, has been investigated in serum samples from healthy volunteers, from patients with chronic renal failure not undergoing hemodialysis, from patients with chronic renal failure included in a regular hemodialysis program, and from patients with hepatic cirrhosis. Protein binding was determined by the ultrafiltration technique using an Amicon Micropartition System, MPS-1.

RESULTS

The percentage of unbound propofol (mean(SD)) in healthy volunteers (n = 16) was 0.98 (0.48) % showing a high interindividual variability. Chronic renal failure did not significantly modify serum protein binding of propofol. In the chronic renal failure group not undergoing regular hemodialysis (n = 9), unbound propofol was 0.92 (0.34) %. In addition, patients in periodic dialysis did not show changes in propofol binding either compared before (1.11 (0.33) %; n = 13) or after hemodialysis (0.87 (0.38) %; n = 12). A slight decrease in albumin concentration was found in all renal patients (P < 0.05) in comparison to healthy volunteers. Creatinine and urea concentrations were higher in these patients (P < 0.01) but in the postdialysis group, the differences in urea levels were not significant when compared with those of volunteers. No changes in the degree of propofol binding were observed in patients with hepatic cirrhosis (0.97 (0.30) %; n = 14) when compared with the group of healthy volunteers. Significant differences were observed in albumin (P < 0.01) and bilirubin (P < 0.05) concentrations. Considering all subjects, the degree of binding did not correlate with biomedical data.

CONCLUSION

Due to the absence of significant changes in the protein binding it is unlikely that there will be an exaggerated pharmacological response in patients with renal and hepatic disease following the administration of a standard propofol dose, although due to interpatient variability careful titration can be recommended.

Protein binding of methohexital. Study of parameters and modulating factors using the equilibrium dialysis technique

This paper describes the parameters that characterize methohexital-albumin binding and the influence of physiological or analytical factors on this binding. Two useful and reproducible methods for measuring the free concentration-equilibrium dialysis (ED) and ultrafiltration (UF)-are described and their performances are compared. Methohexital binds exclusively to albumin according to a two-class binding model. The first is a saturable class site of high affinity constant (KA = 11 200 M-1) and a number of sites per albumin molecule of 1. The second is a non-saturable site of poorer affinity (nKA = 810 M-1). The bound fraction of methohexital in the therapeutic range and at physiological albumin concentration is 86.7 +/- 0.9% in isolated albumin solution. In serum, it ranges from 80 to 84.5%, according to subjects (n = 6). Binding is inhibited by the presence of endogenous compounds of serum (for a given albumin concentration the bound fraction decreases from 90.3% in isolated albumin solution to 82.6% in serum), probably by free fatty acids. An increase in the bound fraction is observed when the pH is increased from 7 to 9. This phenomenon may be explained by a higher affinity of the drug towards the basic (B-form) conformation of the albumin molecule, in analogy with the close barbiturate thiopental. A decrease in the bound fraction against temperature is shown, as though binding forces diminished with increase in temperature. Indeed, the binding modification is less pronounced in the presence of serum endogenous compounds. As expected, there is no evidence of any effect of heparin anticoagulant on the bound fraction. Methohexital binding is strongly modified by the albumin concentration; the bound fractions change from 67 to 91% in the albumin range 150-900 microM.

Pharmacokinetics of thiopental after single and multiple intravenous doses in critical care patients

Thiopental was administered to neurosurgical patients for cerebral protection and its pharmacokinetic parameters were determined after a single bolus of 540, 1000 or 1500 mg (3 subjects) or after multiple doses of 250 mg (5 subjects) and 500 mg (2 subjects) every two hours for up to 7 days. The data were analysed by a two- or three-compartment model and linear kinetics. After a single iv bolus, the mean initial volume of distribution (V1) was 0.481 l.kg-1, and the steady-state volume of distribution (Vss) was 2.16 l.kg-1. The distribution (t1/2 alpha) and elimination (t1/2 beta) half-lives were 0.590 and 5.89 h, respectively, and the mean residence time (MRT) was 7.44 h. The clearance was 5.41 ml.min-1.kg-1. With repeated injections, the pharmacokinetic parameters for each patient were estimated taking into account all administered doses and blood samples, which were taken whenever possible daily at steady state and after the last dose. The variability observed in the pharmacokinetic parameters of thiopental reflected by the coefficient of variation (CV%) was wide but was of similar magnitude within patients (CVintra) as it was between patients (CVinter). The steady-state trough plasma concentration (Cmin obs) ranged from 4.8 to 30 mg.l-1 (mean 16.0 mg.l-1 and median 14.3 mg.l-1). Peak concentrations (Cmax obs) ranged from 8.35 to 45 mg.l-1 (25.4 mg.l-1, and median 23.3 mg.l-1). The values of V1 and Vss were similar to those obtained after a single dose. For V1, the mean was 0.333 l.kg-1. The mean Vss was 2.68 l.kg-1, with a CVintra of 12.6 to 56% and a CVinter of 13.2%. A shorter distribution half-life t1/2 alpha was noted on multiple dosing; the mean value was 0.122 h. The elimination half-life t1/2 beta and the mean residence time became longer due to a decrease in clearance. For t1/2 beta the mean value was 16.3 h. The mean MRT was 21.9 h, CVintra 9.19 to 48.5%, and the CVinter 35.3%. The mean clearance was 2.16 ml.min-1.kg-1, CVintra 7.28 to 25.5%, and the CVinter 20.4%. This value is 50% lower than after a single dose. Identification of the kinetic parameters of thiopental allows simulation of the effects of doses on subsequent plasma levels and will permit a priori prediction of day to day adjustment of drug dosage.