Evidence of a cyclosporine-binding protein in human erythrocytes

Whole blood or plasma: what is the ideal matrix for pharmacokinetic-driven drug candidate selection?

In the present era of drug development, quantification of drug concentrations following pharmacokinetic studies has preferentially been performed using plasma as a matrix rather than whole blood. However, it is critical to realize the difference between measuring drug concentrations in blood versus plasma and the consequences thereof. Pharmacokinetics using plasma data may be misleading if concentrations differ between plasma and red blood cells (RBCs) because of differential binding in blood. In this review, factors modulating the partitioning of drugs into RBCs are discussed and the importance of determining RBC uptake of drugs for drug candidate selection is explored. In summary, the choice of matrix (plasma vs whole blood) is an important consideration to be factored in during drug discovery.

The development of high-content screening (HCS) technology and its importance to drug discovery

INTRODUCTION

High-content screening (HCS) was introduced about twenty years ago as a promising analytical approach to facilitate some critical aspects of drug discovery. Its application has spread progressively within the pharmaceutical industry and academia to the point that it today represents a fundamental tool in supporting drug discovery and development.

AREAS COVERED

Here, the authors review some of significant progress in the HCS field in terms of biological models and assay readouts. They highlight the importance of high-content screening in drug discovery, as testified by its numerous applications in a variety of therapeutic areas: oncology, infective diseases, cardiovascular and neurodegenerative diseases. They also dissect the role of HCS technology in different phases of the drug discovery pipeline: target identification, primary compound screening, secondary assays, mechanism of action studies and in vitro toxicology.

EXPERT OPINION

Recent advances in cellular assay technologies, such as the introduction of three-dimensional (3D) cultures, induced pluripotent stem cells (iPSCs) and genome editing technologies (e.g., CRISPR/Cas9), have tremendously expanded the potential of high-content assays to contribute to the drug discovery process. Increasingly predictive cellular models and readouts, together with the development of more sophisticated and affordable HCS readers, will further consolidate the role of HCS technology in drug discovery.

Distribution of cyclosporin in organ transplant recipients

Cyclosporin is an immunosuppressive agent with a narrow therapeutic index. The total concentration of cyclosporin in blood is usually monitored to guide dosage adjustment and to compensate for substantial interindividual and intraindividual variability in cyclosporin pharmacokinetics. Cyclosporin is a highly lipophilic molecule and widely distributes into blood, plasma and tissue components. It mainly accumulates in fat-rich organs, including adipose tissue and liver. In blood, it binds to erythrocytes in a saturable fashion that is dependent on haematocrit, temperature and the concentration of plasma proteins. In plasma, it binds primarily to lipoproteins, including high-density, low-density and very-low-density lipoprotein, and, to a lesser extent, albumin. The unbound fraction of cyclosporin in plasma (CsA(fu)) expressed as a percentage is approximately 2%. It has been shown that both the pharmacokinetic and pharmacodynamic properties of cyclosporin are related to its binding characteristics in plasma. Furthermore, there is some evidence to indicate that the unbound concentration of cyclosporin (CsA(U)) has a closer association with both kidney and heart allograft rejection than the total (bound + unbound) concentration. However, the measurement of CsA(fu) is inherently complex and cannot easily be performed in a clinical setting. Mathematical models that calculate CsA(fu), and hence CsA(U), from the concentration of plasma lipoproteins may be a more practical option, and should provide a more accurate correlate of effectiveness and toxicity of this drug in transplant recipients than do conventional monitoring procedures. In conclusion, the distribution characteristics of cyclosporin in blood, plasma and various tissues are clinically important. Further investigations are needed to verify whether determination of CsA(U) improves the clinical management of transplant recipients.

Properties of the Ca2+ influx reveal the duality of events underlying the activation by vanadate and fluoride of the Gárdos effect in human red blood cells

The properties of the 45Ca2+ influx by human red blood cells (RBC) induced by NaVO3 or NaF were compared. The NaVO3-induced 45Ca2+ influx was slower and less extensive than that induced by NaF. Both processes were saturable with Ca2+. Substitution of Na+ by K+ inhibited the 45Ca2+ influx induced by NaVO3 but stimulated that by NaF. The NaVO3-induced Ca2+ influx was sensitive to nifedipine (IC50 = 50 mol/l), Cu2+ (IC50=9 mol/l), DTNB (5,5'-dithiobis-(dinitrobenzoic acid)) (IC50 = 12 mol/l) (maximal inhibition 16%, 18%, and 28%, respectively, if NaF was used as inducer). On the other hand, tetrodotoxin (TTX) and cyclosporin A inhibited only the NaF-induced 45Ca2+ influx (IC50 = 21 mol/l and 28 mol/l, respectively). Pig RBC, known not to display the NaVO3-induced Ca2+ influx, exhibited Ca2+ influx induced by NaF. The results show that NaVO3 activates the Ca2+ influx via a pathway homologous to the L-type Ca2+ channel while the NaF-induced Ca2+ influx is mediated via the TTX-sensitive Na+ channel in the presence of NaF with possible participation of calcineurin or cyclophilin. Thus, the Gardos effect induced by NaVO3 and NaF represents two phenomena activated by different mechanisms present in the cryptic state in the RBC membrane.

In-vivo distribution and erythrocyte binding characteristics of cyclosporin in renal transplant patients

The pharmacokinetic parameters of cyclosporin, a potent immunosuppressive agent, show large intra- and inter-individual variability, possibly because of the different analytical methods used. A recently developed cyclosporin-specific radioimmunoassay has been used to study the in-vivo distribution and binding characteristics of cyclosporin in whole blood, plasma and erythrocytes of fifteen renal transplant patients. The profiles of cyclosporin concentration-time curves after an oral dose of cyclosporin had either one peak (ten patients, group A) or two (five patients, group B). Essentially no difference was observed between the two groups in the relationship between equilibrium cyclosporin concentrations in erythrocyte and plasma as a function of whole-blood concentration. The equilibrium in-vivo cyclosporin concentrations in erythrocyte and plasma were, however, markedly lower than those previously observed under in-vitro conditions. The ratio of cyclosporin concentration in erythrocytes (CE) to that in plasma (CP) changed with time, in inverse proportion to the change in cyclosporin concentration in blood, over the range 0.63-2.80 in individual patients with an average of 1.36 +/- 0.07 (mean +/- s.e.m.) for group A and 1.42 +/- 0.23 for group B. The apparent cyclosporin binding affinity (Kd) to erythrocytes under in-vivo conditions averaged 452.2 +/- 47.6 nM (543.5 +/- 57.2 ng mL-1) for group A and 419.4 +/- 41.2 nM (504.1 +/- 49.5 ng mL-1) for group B, whereas apparent cyclosporin binding capacity (Bmax) of the blood cell averaged 0.83 +/- 0.07 nmol mL-1 for group A and 0.78 +/- 0.07 nmol mL-1 for group B. Significantly reduced average Kd (262.7 +/- 40.2 nM or 315.8 +/- 48.9 ng mL-1, P < 0.01) and Bmax (0.56 +/- 0.08 nmol mL-1, P < 0.05) values were observed during the period after Tmax (4-12 h after the drug ingestion) in group A patients. Apparent Kd and Bmax, determined by a nonlinear regression technique, were 131.6 +/- 29.4 and 1088.0 +/- 114.7 nM (158.2 +/- 35.4 and 1307.8 +/- 137.9 ng mL-1) and 0.178 +/- 0.024 and 0.814 +/- 0.078 nmol mL-1, respectively, during the 4-12 h period in group A patients. These findings reveal distinct differences in in-vivo distribution of cyclosporin and the binding characteristics of the compound to erythrocytes from those previously observed under in-vitro conditions. The significantly lower Kd of cyclosporin binding to erythrocytes during the elimination phase suggests a potential effect of cyclosporin-containing erythrocytes or of cyclosporin contained in erythrocytes during cyclosporin treatment.

Alternative cyclosporine metabolic pathways and toxicity

There are some indications from clinical studies (41,43) for aberrant cyclosporine metabolism resulting in formation of potentially toxic metabolites. When the activity of cytochrome P450 3A enzymes is low, more substrate is available for hypothetical alternative pathways of cyclosporine. There are several reasons for low P450 3A activity in a liver graft such as inter-individual genetic variability (43,49,84), cold ischemia and reperfusion damage, changes of the P450 activity during cholestasis (85) or other liver diseases (86), the influence of cytokines (87) and drug interactions such as inhibition or enzyme induction (88). Furthermore, low concentrations of cytochrome P450 3A influence the cyclosporine blood trough concentrations. The P450 3A concentration as estimated by the erythromycin breath test can be used to calculate the initial cyclosporine dose required to obtain cyclosporine blood trough concentrations in the therapeutic window (89). In vitro such alternative pathways comprising 3-methylcholanthrene-inducible (44,46,47) and/or ethinyl estradiol-inducible cytochrome P450 enzymes (48) could be identified and resulted in production of cyclized cyclosporine metabolites. The exact identification of the P450 enzymes involved requires metabolism of cyclosporine using reconstituted purified enzymes or single P450 enzymes expressed in cell lines. In addition, it remains to be clarified whether cyclosporine itself or its metabolite AM1 is the substrate for cyclization. Because cyclized metabolites have a low affinity to cyclophilin (58,59) they are mainly found in plasma. When more cyclized metabolites are formed primarily the concentration of cyclosporine metabolites in plasma increases. The free fraction of cyclosporine at 37 degrees C was found to be 1%-1.5% (90,91) of the cyclosporine concentration in blood. To date, nothing is known about the free fraction of cyclosporine metabolites. Because distribution characteristics of the cyclized metabolites in blood and urine are different from those of cyclosporine, it can be speculated that the free fraction of the cyclized metabolites is higher than that of cyclosporine. This might be reflected by a higher renal clearance resulting in relatively higher concentrations in urine compared with blood (61; Figure 3). If this is the case, a shift in the metabolite pattern with increased concentrations of cyclized metabolites will lead to an overproportional increase of the free fraction of cyclosporine metabolites. Although it is tempting to assume that cyclization is the alternative pathway explaining cyclosporine toxicity in patients with low concentrations of P450 3A enzymes in the liver (Figure 6), this has not yet been proven and will require not only quantification of P450 3A but of the complete P450 enzyme pattern in the liver in combination with characterization of the cyclosporine metabolite pattern by HPLC with special respect to the cyclized metabolites AM1c and AM1c9. Also, it is still unclear whether or not the cyclized metabolites contribute to cyclosporine toxicity. At least, it is unlikely that they are involved in covalent binding to macromolecules in the liver and kidney (44,71). In a clinical study using an HPLC method which allowed the specific quantification of 16 cyclosporine metabolites it was shown that the blood trough concentrations of the cyclized metabolite AM1c9 is elevated during early nephrotoxicity in liver graft recipients (82) and it was shown in an in vitro model that AM1c9 increases endothelin production and therefore might have a negative effect on renal hemodynamics.(ABSTRACT TRUNCATED)

Characterization of cyclosporine A uptake in human erythrocytes

More than 70% of cyclosporine A (CsA) is bound to erythrocytes at whole blood concentrations of 50-1000 ng.ml-1. Cytosolic CsA is bound to the erythrocyte peptidyl-prolyl cis-trans isomerase cyclophilin. Measurements of serum CsA levels under clinical conditions are hampered by a temperature-dependent translocation of CsA into erythrocytes during cooling of the probes to room temperature. In order to characterize the kinetics of CsA uptake and to find a specific uptake inhibitor, we developed a method to measure the velocity of uptake based on rapid cooling of the erythrocyte suspension. The total erythrocyte-binding capacity for CsA amounted to 43 x 10(-5) nmol per 10(6) erythrocytes or 2.6 x 10(5) molecules per erythrocyte. Whereas the erythrocyte-binding capacity of CsA was temperature-independent between 10 degrees C and 42 degrees C, uptake kinetics of CsA were temperature-dependent. The Arrhenius plot for CsA uptake in human erythrocytes was linear and no transition temperature between 0 degree C and 42 degrees C could be detected. Therefore the CsA uptake process in human erythrocytes did not fulfil the criteria of carrier-mediated transport. This indicates that CsA diffuses passively into human erythrocytes. Hence, erythrocyte CsA uptake cannot be specifically inhibited.

Distribution and protein binding of FK506, a potent immunosuppressive macrolide lactone, in human blood and its uptake by erythrocytes

The distribution of FK506 in the blood was estimated in-vitro. At a drug level of 5 ng mL-1, FK506 mainly distributed in erythrocytes (95-98%) in dog, monkey and human blood, and its distribution was affected by drug concentration, temperature, and haematocrit values. In erythrocytes most of FK506 was distributed in cytoplasmic components and was bound strongly to a protein having a molecular weight of 10-11 kDa. The molecular weight of this protein agrees with FK506-binding protein found in various cells. Greater than 98.8% of FK506 was bound to the plasma proteins in all species studied. FK506 bound to various plasma proteins such as lipoproteins, globulins, alpha 1-acid glycoprotein and albumin.

Cyclosporin metabolism in transplant patients

The immunosuppressant cyclosporin, a cyclic undecapeptide, is metabolized to more than 30 metabolites. Cytochrome P450IIIA enzymes located in liver and small intestine are responsible for the biotransformation of cyclosporin and its metabolites and are the site of several drug interactions. It is still under discussion, whether the cyclosporin metabolites are involved in the immunosuppressive and/or toxic activities of cyclosporin. While isolated metabolites show not more than 10-20% of the activity of the mother compound in vitro, metabolite combinations have additive and synergistic effects. Isolated metabolites show no toxic effects in rat models while there is an association between metabolite blood concentrations and cyclosporin toxicity in several clinical studies. Possible mechanisms for the toxic effect of cyclosporin metabolites are covalent binding to macromolecules in liver and kidney, alteration of the cytochrome P450 pattern in liver and kidney, increased endothelin production in the kidney and synergistic effects of cyclosporin combinations on mesangial cells. Liver dysfunction leads to an alteration of the metabolite patterns and to increased concentrations of cyclosporin metabolites in blood. In conclusion there is evidence that cyclosporin metabolites may contribute to cyclosporin toxicity and high metabolite blood concentrations in patients should not be tolerated.

Do we know the site of action of cyclosporin?

The site and mode of action of cyclosporin A (CsA) have been subjects of study ever since CsA was discovered and demonstrated to be a selective suppressor of allograft rejection. In this article, Bernard Erlanger traces progress to date and presents evidence that the site of action is not cytoplasmic cyclophilin but a lymphocyte cell-surface receptor that might be related in structure to cyclophilin.

Cyclosporin A and its metabolites, distribution in blood and tissues

Cyclosporin A (CsA), a non-myelotoxic immunosuppressant, and its metabolites are widely distributed in the body. Highest concentrations of CsA have been detected in the pancreas, adipose tissue and liver, lowest concentrations in brain, muscle, blood and other body fluids. Metabolites are distributed differently to CsA. In addition to lipid partition, intracellular binding to cyclophilin, a peptidyl-prolyl cis-trans isomerase, appears to play a role in its tissue distribution. The temperature dependence of such binding in erythrocytes poses difficulty in serum or plasma measurements. Tissue specific processes may also influence action and toxicity of CsA and its metabolites; thus, a better understanding of the complex distribution pattern of CsA and its metabolites would be important for establishing improved strategies and selection of appropriate specific methodologies for drug monitoring.

Therapeutic drug monitoring of cyclosporin. Practical applications and limitations

Cyclosporin, a potent immunosuppressive agent used to prevent rejection of transplanted organs, has a narrow therapeutic range and various toxic effects, mostly concentration-dependent. The kinetics of this drug present a large intra- and interindividual variability due to many factors resulting in marked variations of blood cyclosporin concentrations, and in a poor correlation between administered dose and concentrations. The knowledge of cyclosporin peculiarities and of factors affecting blood concentrations can provide a rational basis for establishing an adequate therapy for the individual patient, in conjunction with other laboratory and clinical data. Cyclosporin monitoring is a method of evaluating whether the therapeutic choice is correct. Cyclosporin concentrations can be measured in blood, plasma and serum using radioimmunoassay or high performance liquid chromatography. Different results are obtained, depending on the technique and on biological fluids used. Cyclosporin measurement presents many problems and difficulties. There is a need for standardisation and for quality assessment programmes. The recent development of monoclonal antibodies may represent a significant advance for cyclosporin monitoring. The most important factors affecting blood concentrations are: type of transplant, bile deficit, gastrointestinal dysfunction, food, variations of lipoprotein concentrations, impairment of liver function, age, drug coadministration. Therapeutic drug monitoring should be undertaken on a regular basis after the initiation of therapy with cyclosporin. After discharge from the hospital the patient and the attending physician should be aware of the factors which may require changes in cyclosporin therapy.

Identification of cyclophilin as the erythrocyte ciclosporin-binding protein

Previous studies on the distribution of circulating ciclosporin have shown that the majority of the drug is associated with erythrocytes. In order to investigate the nature of ciclosporin-erythrocyte binding, binding studies were performed on isolated erythrocytes. At therapeutic concentrations (approx. 0.5 microgram/ml in whole blood) greater than 90% of the erythrocyte associated ciclosporin was found in the cytosol. The cytosolic binding capacity was approximately (2-2.5).10(5) molecules of ciclosporin per cell. A lower affinity binding of the drug to the plasma membrane occurred only at higher ciclosporin concentrations. The ciclosporin-binding species was purified from erythrocyte cytosol using ciclosporin-Affigel affinity chromatography. This revealed a 16 kDa protein, similar in size to the ciclosporin-binding protein, cyclophilin, previously identified in lymphocyte cytosol. Immunochemical analysis using rabbit anti-bovine spleen cyclophilin antisera revealed that the erythrocyte ciclosporin-binding protein was either cyclophilin or a closely related protein. It is concluded that intracellular ciclosporin-binding within erythrocytes is mostly attributable to the presence of a single protein or protein family represented by cyclophilin. The presence of (2-2.5).10(5) copies of this binding protein within each erythrocyte is responsible for the ciclosporin found associated with erythrocytes.

Saturable binding of cyclosporin A to erythrocytes: estimation of binding parameters in renal transplant patients and implications for bioavailability assessment

Cyclosporin (CyA) exhibits saturable binding to erythrocytes. A one-site binding model was fitted to data from renal transplant patients receiving CyA therapy. The average maximum binding capacity is 2560 micrograms CyA/liter of packed erythrocytes; the unbound CyA concentration associated with 50% saturation of the binding site is 67 micrograms/liter. Analysis indicates that whole-blood CyA measurement to monitor drug therapy should be viewed cautiously, particularly when the hematocrit varies considerably. The error in estimating absolute bioavailability at steady state from whole-blood measurements, resulting as a consequence of the saturable binding, has been explored. Although extrapolation to the therapeutic situation, which involves transient drug administration, is difficult, errors of up to 25% are anticipated. When an accurate estimate of bioavailability is required, analysis based on plasma data is proposed. For bioequivalence testing, both blood and plasma CyA data are equally acceptable.

Cyclophilin binds to the region of cyclosporine involved in its immunosuppressive activity

Although several cytosolic proteins including calmodulin and cyclophilin have been shown to bind cyclosporine, the direct involvement of these proteins in the immunosuppressive activity of cyclosporine remains to be established. In the present study, a quantitative immunoassay for cyclophilin was developed which made it possible to compare its relative affinity for cyclosporine and any of its analogues. The binding of cyclophilin to cyclosporine coated on a solid phase was revealed by anti-cyclophilin rabbit antiserum followed by antiglobulin-enzyme conjugate. This reaction could be inhibited by addition of free cyclosporine or certain cyclosporine analogues. By studying the binding of cyclophilin to more than fifty cyclosporine derivatives modified singly on each of the eleven amino acid residues, it could be shown that cyclophilin binds to the residues of cyclosporine known to be critical for its immunosuppressive activity. These data identify cyclophilin as a highly discriminating stereospecific binding protein for cyclosporine.