The role of the protein-binding on the mode of drug action as well the interactions with other drugs

Comprehensive insight into the binding of sunitinib, a multi-targeted anticancer drug to human serum albumin

Binding studies between a multi-targeted anticancer drug, sunitinib (SU) and human serum albumin (HSA) were made using fluorescence, UV-vis absorption, circular dichroism (CD) and molecular docking analysis. Both fluorescence quenching data and UV-vis absorption results suggested formation of SU-HSA complex. Moderate binding affinity between SU and HSA was evident from the value of the binding constant (3.04×10⁴M^-1), obtained at 298K. Involvement of hydrophobic interactions and hydrogen bonds as the leading intermolecular forces in the formation of SU-HSA complex was predicted from the thermodynamic data of the binding reaction. These results were in good agreement with the molecular docking analysis. Microenvironmental perturbations around Tyr and Trp residues as well as secondary and tertiary structural changes in HSA upon SU binding were evident from the three-dimensional fluorescence and circular dichroism results. SU binding to HSA also improved the thermal stability of the protein. Competitive displacement results and molecular docking analysis revealed the binding locus of SU to HSA in subdomain IIA (Sudlow's site I). The influence of a few common ions on the binding constant of SU-HSA complex was also noticed.

Life-threatening haematological complication occurring in a cat after chronic carbimazole administration

Case summary

An 11-year-old spayed female domestic shorthair cat with a history of hyperthyroidism treated with carbimazole for 7 months was presented for a check-up after a few episodes of vomiting. The cat had been receiving prednisolone at 0.5 mg/kg PO q12h for recent pancreatitis and concurrent inflammation of liver and small intestines confirmed by biopsies. Clinical examination revealed pale mucous membranes with a capillary refill time of <2 s. Haematology showed severely decreased packed cell volume (16%), and increased prothrombin time (42 s), partial thromboplastin time (>120 s) and fibrinogen serum concentration (3.5 g/l). Morphological changes of thrombocytes in the absence of thrombocytopenia were also noted. In-saline agglutination test was positive. Abdominal radiographic and ultrasonographic examinations excluded the presence of organ abnormalities and peritoneal effusion. Blood biochemistry was unremarkable. Feline leukaemia virus and feline immunodeficiency virus tests were negative. On the basis of these findings, immune-mediated anaemia secondary to chronic carbimazole administration was suspected. Prednisolone was increased to 2 mg/kg PO q24h and carbimazole tablets were stopped. Despite close monitoring and intensive care, the cat died the same evening of admission to the hospital.

Relevance and novel information

This report suggests that severe haemotoxicity may occur as a sequel of chronic carbimazole administration in cats. Routine bloodwork and accurate follow-up of cats under treatment with thyrotoxic therapy may be advisable, in order to detect haematological changes before lethal complications occur.

Pregnancy in a patient with congenital analbuminaemia

Congenital analbuminaemia is a rare autosomal recessive disorder that is characterised by a severe reduction or total absence of serum albumin. This condition has implications for therapeutics as a large proportion of commonly used drugs are plasma protein bound where albumin is the primary component of plasma protein. This is the first case report of pregnancy in a patient with congenital analbuminaemia in the medical literature. In the absence of drug dosage guidelines for patients with congenital analbuminaemia, a list of drugs which may be required for this patient during pregnancy, delivery and/or emergency situations were compiled by a multidisciplinary team. Our patient suffered from polyhydramnios during her pregnancy which was successfully managed with albumin transfusions and had a normal vaginal delivery with no complications in the intrapartum or postpartum period. The management and unique challenges of pregnancy in a patient with congenital analbuminaemia are discussed.

When Is It Important to Measure Unbound Drug in Evaluating Nanomedicine Pharmacokinetics?

Nanoformulations have become important tools for modifying drug disposition, be it from the perspective of enabling prolonged drug release, protecting the drug molecule from metabolism, or achieving targeted delivery. When examining the in vivo pharmacokinetic properties of these formulations, most investigations either focus on systemic concentrations of total (encapsulated plus unencapsulated) drug, or concentrations of encapsulated and unencapsulated drug. However, it is rare to find studies that differentiate between protein-bound and unbound (free) forms of the unencapsulated drug. In light of the unique attributes of these formulations, we cannot simply assume it appropriate to rely upon the protein-binding properties of the traditionally formulated or legacy drug when trying to define the pharmacokinetic or pharmacokinetic/pharmacodynamic characteristics of these nanoformulations. Therefore, this commentary explores reasons why it is important to consider not only unencapsulated drug, but also the portion of unencapsulated drug that is not bound to plasma proteins. Specifically, we highlight those situations when it may be necessary to include measurement of unencapsulated, unbound drug concentrations as part of the nanoformulation pharmacokinetic evaluation.

Interaction between the photosensitizer lumichrome and human serum albumin: effect of excipients

Lumichrome (Lc) is a photodegradation product of riboflavin that can be used as a photosensitizer (PS) in antibacterial photodynamic therapy (aPDT). The binding of Lc with plasma proteins such as human serum albumin (HSA) could affect its efficiency as PS. Excipients are necessary to prepare stable formulations to be used in aPDT and they may affect the PS-HSA binding. Hydroxypropyl (HP)-α, β, γ-cyclodextrin (CD), polyethylene glycol 400 (PEG400) and Pluronic^® F-127 (PF127) were selected as model excipients in this study. The intrinsic HSA fluorescence quenching and absorption and fluorescence spectroscopy were used to evaluate the Lc-HSA interaction in the absence and presence of excipients. Nano-differential scanning calorimetry (DSC) was used to determine the effect of excipients on HSA. The photostability of the samples was also evaluated. The combined results showed a modest interaction between Lc and HSA which was reduced mainly by HPβCD. No major alterations of the HSA nano-DSC thermogram were observed after addition of excipients. HSA did enhance Lc photodegradation. The presence of PF127 did also induce photochemical destabilization of Lc independent of HSA. In conclusion, HPαCD, HPγCD and PEG400 seemed to be the excipients more suitable for use in topical preparations containing Lc.

Predictors of Thiopurine Treatment Failure in Biologic-Naïve Ulcerative Colitis Patients

INTRODUCTION

Thiopurines (azathioprine and 6-mercaptopurine) have been used in the management of UC patients for over three decades. Nearly half of patients with UC treated with thiopurines fail to achieve remission or lose remission during treatment. Factors associated with thiopurine failure are poorly understood. The primary aim of our study was to investigate patient-related factors which are associated with thiopurine failure.

METHODS

TNF-alpha antagonist-naïve patients with histological diagnosis of UC, receiving thiopurine therapy, with follow-up data from 1 to 3 years were included in the study. Data regarding demographics, laboratory results, and disease characteristics were collected. The primary endpoint was failure of thiopurine therapy, defined as treatment with steroids, therapeutic escalation to TNF-alpha antagonist therapy, or need for surgery.

RESULTS

Of the 563 patients identified using ICD-9 codes, 78 TNF-alpha antagonist-naïve patients with a histological diagnosis of UC, receiving thiopurine treatment, were identified. Over the three-year follow-up period, 38 patients failed thiopurine treatment. On adjusted Cox regression, BMI < 25 kg/m(2) (HR 3, 95 % CI 1.55-5.83; p value = 0.001) was significantly associated with thiopurine failure. Furthermore, although not statistically significant, there was a strong trend toward thiopurine failure among patients with serum albumin level < 4 g/dL (HR 1.98, 95 % CI 0.97-4; p value = 0.06), non-smoking status (HR 2.2, 95 % CI 0.96-5.06; p value = 0.06), and higher degree of colon inflammation (HR 1.49, 95 % CI 0.96-2.32; p value = 0.08).

DISCUSSION

Our results show that low body mass index is associated with increased risk of failure of thiopurine treatment. Furthermore, there was a strong trend toward thiopurine failure among patients with low serum albumin level (<4gm/dL). These factors should be considered as markers of non-response to thiopurine monotherapy for patients with moderately severe ulcerative colitis.

Specific cellular incorporation of a pyrene-labelled cholesterol: lipoprotein-mediated delivery toward ordered intracellular membranes

In the aim of testing tools for tracing cell trafficking of exogenous cholesterol, two fluorescent derivatives of cholesterol, 22-nitrobenzoxadiazole-cholesterol (NBD-Chol) and 21-methylpyrenyl-cholesterol (Pyr-met-Chol), with distinctive chemico-physical characteristics, have been compared for their cell incorporation properties, using two cell models differently handling cholesterol, with two incorporation routes. In the Caco-2 cell model, the cholesterol probes were delivered in bile salt micelles, as a model of intestinal absorption. The two probes displayed contrasting behaviors for cell uptake characteristics, cell staining, and efflux kinetics. In particular, Pyr-met-Chol cell incorporation involved SR-BI, while that of NBD-Chol appeared purely passive. In the PC-3 cell model, which overexpresses lipoprotein receptors, the cholesterol probes were delivered via the serum components, as a model of systemic delivery. We showed that Pyr-met-Chol-labelled purified LDL or HDL were able to specifically deliver Pyr-met-Chol to the PC-3 cells, while NBD-Chol incorporation was independent of lipoproteins. Observations by fluorescence microscopy evidenced that, while NBD-Chol readily stained the cytosolic lipid droplets, Pyr-met-Chol labelling led to the intense staining of intracellular structures of membranous nature, in agreement with the absence of detectable esterification of Pyr-met-Chol. A 48 h incubation of PC-3 cells with either Pyr-met-Chol-labelled LDL or HDL gave same staining patterns, mainly colocalizing with Lamp1, caveolin-1 and CD63. These data indicated convergent trafficking downwards their respective receptors, LDL-R and SR-BI, toward the cholesterol-rich internal membrane compartments, late endosomes and multivesicular bodies. Interestingly, Pyr-met-Chol staining of these structures exhibited a high excimer fluorescence emission, revealing their ordered membrane environment, and indicating that Pyr-met-Chol behaves as a fair cholesterol tracer regarding its preferential incorporation into cholesterol-rich domains. We conclude that, while NBD-Chol is a valuable marker of cholesterol esterification, Pyr-met-Chol is a reliable new lipoprotein fluorescent marker which allows to probe specific intracellular trafficking of cholesterol-rich membranes.

Artocarpin Induces Apoptosis in Human Cutaneous Squamous Cell Carcinoma HSC-1 Cells and Its Cytotoxic Activity Is Dependent on Protein-Nutrient Concentration

Artocarpin, a natural prenylated flavonoid, has been shown to have various biological properties. However, its effects on human cutaneous squamous cell carcinoma (SCC) have not been previously investigated. We set out to determine whether artocarpin has cytotoxic effects on SCC cells and whether its pharmacological activity is dependent on protein-nutrient concentration. Our results showed that treatment of HSC-1 cells (a human cutaneous SCC cell line) with artocarpin decreased cell viability and induced cell apoptosis by increasing caspase 3/7 activity. These effects were more pronounced at low fetal bovine serum (FBS) concentrations. Artocarpin induced an increase in the level of phospho-p38 and a decrease in the levels of phospho-ERK, phospho-JNK, phospho-Akt, phospho-mTOR, and phospho-S6K. High FBS concentrations in the culture media inhibited and delayed the uptake of artocarpin from the extracellular compartment (culture media) into the intracellular compartment, as determined by high performance liquid chromatography (HPLC) analysis. In conclusion, artocarpin induces apoptosis in HSC-1 cells through modulation of MAPK and Akt/mTOR pathways. Binding of artocarpin to proteins in the FBS may inhibit cellular uptake and reduce the cytotoxic activity of artocarpin on HSC-1 cells. Therefore, artocarpin may have potential use in the future as a form of treatment for cutaneous SCC.

Effect of a Pluronic(®) P123 formulation on the nitric oxide-generating drug JS-K

PURPOSE

O(2)-(2,4-dinitrophenyl)1-[(4-ethoxycarbonyl)piperazin-1-yl]diazen-1-ium-1,2-diolate] or JS-K is a nitric oxide-producing prodrug of the arylated diazeniumdiolate class with promising anti-tumor activity. JS-K has challenging solubility and stability properties. We aimed to characterize and compare Pluronic(®) P123-formulated JS-K (P123/JS-K) with free JS-K.

METHODS

We determined micelle size, shape, and critical micelle concentration of Pluronic(®) P123. Efficacy was evaluated in vitro using HL-60 and U937 cells and in vivo in a xenograft in NOD/SCID IL2Rγ (null) mice using HL-60 cells. We compared JS-K and P123/JS-K stability in different media. We also compared plasma protein binding of JS-K and P123/JS-K. We determined the binding and Stern Volmer constants, and thermodynamic parameters.

RESULTS

Spherical P123/JS-K micelles were smaller than blank P123. P123/JS-K formulation was more stable in buffered saline, whole blood, plasma and RPMI media as compared to free JS-K. P123 affected the protein binding properties of JS-K. In vitro it was as efficacious as JS-K alone when tested in HL-60 and U937 cells and in vivo greater tumor regression was observed for P123/JS-K treated NOD/SCID IL2Rγ (null) mice when compared to free JS-K-treated NOD/SCID IL2Rγ (null) mice.

CONCLUSIONS

Pluronic(®) P123 solubilizes, stabilizes and affects the protein binding characteristics of JS-K. P123/JS-K showed more in vivo anti-tumor activity than free JS-K.

Serum albumin and α-1 acid glycoprotein impede the killing of Schistosoma mansoni by the tyrosine kinase inhibitor Imatinib

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The Abl tyrosine-kinase inhibitor Imatinib is toxic to S. mansoni in vitro but not in vivo in rodents.

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Blood components like serum albumin and alpha-1 acid glycoprotein (AGP) negated Imatinib’s toxicity in vitro.

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Erythromycin partially restored the toxicity of Imatinib in vitro.

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High levels of AGP upon infection make rodents poor models for examining some small molecule inhibitors.

In the search for new drugs and drug targets to treat the flatworm disease schistosomiasis, protein kinases (PKs) have come under particular scrutiny because of their essential roles in developmental and physiological processes in schistosome parasites. In this context the application of the anti-cancer Abl tyrosine kinase (TK) inhibitor Imatinib (Gleevec/Glivec; STI-571) to adult Schistosoma mansoni in vitro has indicated negative effects on diverse physiological processes including survival.

Motivated by these in vitro findings, we performed in vivo experiments in rodent models of S. mansoni infection. Unexpectedly, Imatinib had no effect on worm burden or egg-production. We found that the blood components serum albumin (SA) and alpha-1 acid glycoprotein (AGP or orosomucoid) negated Imatinib’s deleterious effects on adult S. mansoni and schistosomula (post-infective larvae) in vitro. This negative effect was partially reversed by erythromycin. AGP synthesis can increase as a consequence of inflammatory processes or infection; in addition upon infection AGP levels are 6–8 times higher in mice compared to humans. Therefore, mice and probably other rodents are poor infection models for measuring the effects of Imatinib in vivo. Accordingly, we suggest the routine evaluation of the ability of AGP and SA to block in vitro anti-schistosomal effects of small molecules like Imatinib prior to laborious and expensive animal experiments.

Interactive association of drugs binding to human serum albumin

Human serum albumin (HSA) is an abundant plasma protein, which attracts great interest in the pharmaceutical industry since it can bind a remarkable variety of drugs impacting their delivery and efficacy and ultimately altering the drug’s pharmacokinetic and pharmacodynamic properties. Additionally, HSA is widely used in clinical settings as a drug delivery system due to its potential for improving targeting while decreasing the side effects of drugs. It is thus of great importance from the viewpoint of pharmaceutical sciences to clarify the structure, function, and properties of HSA–drug complexes. This review will succinctly outline the properties of binding site of drugs in IIA subdomain within the structure of HSA. We will also give an overview on the binding characterization of interactive association of drugs to human serum albumin that may potentially lead to significant clinical applications.

Interaction of rifampicin embedded in phospholipid nanoparticles with blood plasma lipoproteins

The drug formulations of antituberculous remedy rifampicin in nanoparticles less than 30 nm based on soy phosphatidylcholine and sodium oleate was elaborated in Institute of Biomedical Chemistry. The distribution of rifampicin in blood plasma fractions after incubation with this formulation and with free rifampicin was studied. This goal was stimulated by the literature data about activation of macrophages LDL receptors in cases of M. tuberculosis infection. Plasma was incubated 30 min with free rifampicin or rifampicin encapsulated into the nanoformulation followed by ultracentrifugation and subsequent rifampicin determination by HPLC in lipoprotein fractions. In the case of free rifampicin it appeared mainly in the plasma protein fraction and in HDL (41% and 38%, correspondentely). But after incubation of rifampicin in nanoparticles the drug redistribution was observed. Its proportion in these factions decreased 2-3-fold, and it was found mainly in LDL (60% as compared with 21% for free rifampicin). The increased association of rifampicin encapsulated into phospholipid nanoparticles with LDL is considered as facilitating factor for macrophages delivery and thus for antituberculosis efficiency as well

Exogenous albumin inhibits sorafenib-induced cytotoxicity in human cancer cell lines

Sorafenib is an orally administered multikinase inhibitor that exhibits anti-angiogenic and anti-tumor activity. Sorafenib is also known to bind to protein (>99.5%), suggesting protein binding may be involved in sorafenib pharmacokinetic variability. Albumin is a major drug-binding protein. In this study, we examined the effect of albumin on sorafenib-induced cytotoxicity using two in vitro culture cell lines, human hepatoma Huh-7 cells and androgen-independent prostate cancer PC-3 cells. The cells were cultured and incubated, and cytotoxicity was assessed. Results were confirmed by western blotting. The presence of exogenous albumin markedly blocked the sorafenib-induced cytotoxicity in the two cell lines. Albumin concentration, the change of pharmacological signal transduction as Raf-B, vascular endothelial growth factor (VEGF), and phosphorylation of MEK1/2 or ERK1/2 were found to be decreased by sorafenib. Co-incubation of warfarin, a representative coumarin anticoagulant and potent binding activity, with albumin enhanced the cytotoxic effects by sorafenib. These mechanisms depend on the high binding proper ties of sorafenib and exogenous albumin. Furthermore, we investigated the effects of endo genous albumin expression on sorafenib-induced cytotoxicity using the siRNA knockdown system or transfected expression vector assay. However, the cytotoxic effects by sorafenib showed little change either with the knockdown or overexpression of albumin. Our results suggest that particular care should be taken with albuminemia or the combined use of drugs with a high affinity for albumin, such as warfarin, and sorafenib in the treatment of cancer patients. Our findings may be useful to the cancer therapeutic strategy by sorafenib.

The interactions of amphiphilic antisense oligonucleotides with serum proteins and their effects on in vitro silencing activity

Antisense oligonucleotides (AONs) are a class of compounds with high therapeutic potential. One of the challenges facing this platform is the development of effective techniques to achieve cellular delivery. AON conjugates, in which traditional AONs are attached to certain biomolecules, can exhibit improved intracellular bioavailability in the absence of delivery systems. In this study, the lipophilic moieties docosahexaenoic acid, cholesterol, and docosanoic acid (DSA) were conjugated to various phosphorothioated DNA and chemically-modified 2'-fluoro-arabinonucleic acid AONs via an amino-hexanol-linker added to the 5'-end of the molecule. The gene silencing potential of these compounds was evaluated in vitro in the absence or presence of a transfecting agent (polyion complex micelle). Incubation with sub-micromolar concentration of DSA-conjugates could, in the absence of serum proteins, downregulate more than 60% of the targeted mRNA under carrier-free and carrier-loaded delivery methods. Gene silencing activity of carrier-free DSA-conjugates was, however, decreased in a dose-dependent fashion by adding albumin in the transfection medium. Supplementing the medium with free fatty acid prevented the interaction of the DSA-conjugate with albumin, and restored its silencing activity. These findings suggest that strategies aiming at preventing the association of hydrophobized AONs to serum proteins at the site of action may improve their activity.

Targeted non-covalent self-assembled nanoparticles based on human serum albumin

Human serum albumin (HSA) is a biological nanocarrier that forms non-covalent complexes with a number of synthetic and biomolecules. Previously we demonstrated radiolabeled HSA-based nanoparticles can form non-covalent complexes with fluorescent cyanine dyes yielding imaging agents for surgical guidance towards tumor draining lymph nodes. Here the self-assembly approach enabled rapid clinical translation. Based on this experience we reasoned it would be interesting to expand this non-covalent technology to a targeted approach. Therefore, the ability of HSA to form non-covalent self-assembled complexes with peptides via near-infrared (NIR) cyanine dyes was explored. Föster resonance energy transfer (FRET) quenching interactions between HSA-Cy5 and the non-covalently bound fluorescent molecules indocyanine green (ICG), IR783-CO(2)H and three IR783-labeled targeting peptides were used to monitor complex assembly and disassembly. The host-guest interactions between HSA and IR783-labeled peptides enabled the formation of (bio)nanoparticles that are coated with peptides, which may target α(v)β(3)-integrins, the chemokine receptor 4 (CXCR4), or somatostatin receptors. The potential of CXCR4-targeted (bio)nanoparticles in sentinel lymph node procedures is demonstrated in vivo. By non-covalently binding NIR-dye labeled peptides to an already clinically approved HSA-scaffold, we have readily formed targeted bionanoparticles.

Albumin-binding capacity (ABiC) is reduced in patients with chronic kidney disease along with an accumulation of protein-bound uraemic toxins

BACKGROUND

Albumin is an important transport protein for non-water-soluble protein-bound drugs and uraemic toxins. Its transport capacity is reduced in patients with advanced chronic kidney disease (CKD) and unbound fractions of uraemic toxins are related to complications of CKD. We investigated whether this reduction could be quantified and how it correlated with the stages of CKD. Albumin-binding capacity (ABiC) is a dye-based method that quantifies the remaining binding capacity of one major binding site (site II) of the albumin molecule.

METHODS

Blood samples from 104 CKD patients were incubated with a binding site-specific fluorescent marker and the amount of unbound marker was determined by means of fluorescence detection after filtration. Measurements in a pooled human plasma were used for reference. Glomerular filtration rate and serum indoxyl sulphate (IS) levels were also determined.

RESULTS

Impairment of renal function was associated with a reduction in ABiC (mean ± SD: 118 ± 12; 111 ± 11; 99 ± 8 and 79 ± 9% for Stages 1/2, 3, 4 and 5, respectively; P < 0.001) and an increase in IS (3.9 ± 1.1; 6.2 ± 3.2; 16.3 ± 14.9 and 56.1 ± 28.6 μmol/L for Stages 1/2, 3, 4 and 5, respectively; P < 0.001). In dialysis patients, ABiC was lower in those with urine outputs <500 mL/day than in those with preserved urine output (73.7 ± 6.0 versus 83.8 ± 8.5%; P < 0.001).

CONCLUSION

Impaired albumin function in CKD patients can be quantified, is related to severity of kidney disease and is associated with an accumulation of uraemic albumin-bound retention solutes.

Evaluation of drug-human serum albumin binding interactions with support vector machine aided online automated docking

MOTIVATION

Human serum albumin (HSA), the most abundant plasma protein is well known for its extraordinary binding capacity for both endogenous and exogenous substances, including a wide range of drugs. Interaction with the two principal binding sites of HSA in subdomain IIA (site 1) and in subdomain IIIA (site 2) controls the free, active concentration of a drug, provides a reservoir for a long duration of action and ultimately affects the ADME (absorption, distribution, metabolism, and excretion) profile. Due to the continuous demand to investigate HSA binding properties of novel drugs, drug candidates and drug-like compounds, a support vector machine (SVM) model was developed that efficiently predicts albumin binding. Our SVM model was integrated to a free, web-based prediction platform (http://albumin.althotas.com). Automated molecular docking calculations for prediction of complex geometry are also integrated into the web service. The platform enables the users (i) to predict if albumin binds the query ligand, (ii) to determine the probable ligand binding site (site 1 or site 2), (iii) to select the albumin X-ray structure which is complexed with the most similar ligand and (iv) to calculate complex geometry using molecular docking calculations. Our SVM model and the potential offered by the combined use of in silico calculation methods and experimental binding data is illustrated.

In vitro influence of fatty acids and bilirubin on binding of mycophenolic acid to human serum albumin

Mycophenolic acid (MPA) is 98-99% bound to albumin. Because MPA is restrictively cleared and has a low extraction coefficient, increase in its free fraction related to decreased albumin binding results in lower total concentrations but unchanged unbound concentrations. Multiple factors, including hypoalbuminemia, impaired renal function, and accumulated mycophenolic acid glucuronide are known to reduce MPA protein binding. Little is known about the influence of fatty acids and bilirubin on this issue. By using quenching fluorescence method, the aims of this study were to investigate in vitro the binding properties of MPA, then the influence of myristic acid and bilirubin on MPA binding to albumin. The estimate of dissociation constant (Kd) of MPA was 13.2 [CI 95 12.7-13.8] μM. In the presence of myristic acid (concentration range 4-100 μM), apparent Kd (Kd(app)) of MPA was approximately 1.5-10-fold greater. For myristic acid/albumin molar ratio reachable in clinical settings (2:1 and 5:1), Kd(app) of MPA rose about a factor 1.5 and 2.2, respectively. In the presence of bilirubin (concentration range 0.5-5 μM), Kd(app) of MPA was approximately 1.5-5-fold greater than MPA Kd. For bilirubin/albumin molar ratio reachable in clinical settings (1:4 and 1:2), Kd(app) of MPA rose about a factor 1.5 and 1.9, respectively. These data suggest that hypertriglyceridemia or cholestasis may significantly increase MPA free fraction in clinical settings, thereby lowering MPA total concentration in plasma while the free concentration remains unchanged. These results may help to optimize the therapeutic drug monitoring of MPA.