Enantioselective plasma protein binding of propafenone: mechanism, drug interaction, and species difference

Enantioselectivity in Drug Pharmacokinetics and Toxicity: Pharmacological Relevance and Analytical Methods

Enzymes, receptors, and other binding molecules in biological processes can recognize enantiomers as different molecular entities, due to their different dissociation constants, leading to diverse responses in biological processes. Enantioselectivity can be observed in drugs pharmacodynamics and in pharmacokinetic (absorption, distribution, metabolism, and excretion), especially in metabolic profile and in toxicity mechanisms. The stereoisomers of a drug can undergo to different metabolic pathways due to different enzyme systems, resulting in different types and/or number of metabolites. The configuration of enantiomers can cause unexpected effects, related to changes as unidirectional or bidirectional inversion that can occur during pharmacokinetic processes. The choice of models for pharmacokinetic studies as well as the subsequent data interpretation must also be aware of genetic factors (such as polymorphic metabolic enzymes), sex, patient age, hepatic diseases, and drug interactions. Therefore, the pharmacokinetics and toxicity of a racemate or an enantiomerically pure drug are not equal and need to be studied. Enantioselective analytical methods are crucial to monitor pharmacokinetic events and for acquisition of accurate data to better understand the role of the stereochemistry in pharmacokinetics and toxicity. The complexity of merging the best enantioseparation conditions with the selected sample matrix and the intended goal of the analysis is a challenge task. The data gathered in this review intend to reinforce the importance of the enantioselectivity in pharmacokinetic processes and reunite innovative enantioselective analytical methods applied in pharmacokinetic studies. An assorted variety of methods are herein briefly discussed.

Analysis of free drug fractions in serum by ultrafast affinity extraction and two-dimensional affinity chromatography using α1-acid glycoprotein microcolumns

In the circulatory system, many drugs are reversibly bound to serum proteins such as human serum albumin (HSA) and alpha1-acid glycoprotein (AGP), resulting in both free and protein-bound fractions for these drugs. This report examined the use of microcolumns containing immobilized AGP for the measurement of free drug fractions by ultrafast affinity extraction and a two-dimensional affinity system. Several drugs known to bind AGP were used as models to develop and evaluate this approach. Factors considered during the creation of this method included the retention of the drugs on the microcolumns, the injection flow rate, the microcolumn size, and the times at which a second AGP column was placed on-line with the microcolumn. The final system had residence times of only 110-830ms during sample passage through the AGP microcolumns and allowed free drug fractions to be determined within 10-20min when using only 3-10μL of sample per injection. This method was used to measure the free fractions of the model drugs at typical therapeutic levels in serum, giving good agreement with the results obtained by ultrafiltration. This approach was also used to estimate the binding constants for each drug with AGP in serum, even for drugs that had significant interactions with both AGP and HSA in such samples. These results indicated that AGP microcolumns could be used with ultrafast affinity extraction to measure free drug fractions in a label-free manner and to study the binding of drugs with AGP in complex samples such as serum.

Rational use of plasma protein and tissue binding data in drug design

It is a commonly accepted assumption that only unbound drug molecules are available to interact with their targets. Therefore, one of the objectives in drug design is to optimize the compound structure to increase in vivo unbound drug concentration. In this review, theoretical analyses and experimental observations are presented to illustrate that low plasma protein binding does not necessarily lead to high in vivo unbound plasma concentration. Similarly, low brain tissue binding does not lead to high in vivo unbound brain tissue concentration. Instead, low intrinsic clearance leads to high in vivo unbound plasma concentration, and low efflux transport activity at the blood-brain barrier leads to high unbound brain concentration. Plasma protein and brain tissue binding are very important parameters in understanding pharmacokinetics, pharmacodynamics, and toxicities of drugs, but these parameters should not be targeted for optimization in drug design.

Stereoselective binding of doxazosin enantiomers to plasma proteins from rats, dogs and humans in vitro

AIM

(±)Doxazosin is a long-lasting inhibitor of α1-adrenoceptors that is widely used to treat benign prostatic hyperplasia and lower urinary tract symptoms. In this study we investigated the stereoselective binding of doxazosin enantiomers to the plasma proteins of rats, dogs and humans in vitro.

METHODS

Human, dog and rat plasma were prepared. Equilibrium dialysis was used to determine the plasma protein binding of each enantiomer in vitro. Chiral HPLC with fluorescence detection was used to measure the drug concentrations on each side of the dialysis membrane bag.

RESULTS

Both the enantiomers were highly bound to the plasma proteins of rats, dogs and humans [(-)doxazosin: 89.4%-94.3%; (+)doxazosin: 90.9%-95.4%]. (+)Doxazosin exhibited significantly higher protein binding capacities than (-)doxazosin in all the three species, and the difference in the bound concentration (Cb) between the two enantiomers was enhanced as their concentrations were increased. Although the percentage of the plasma protein binding in the dog plasma was significantly lower than that in the human plasma at 400 and 800 ng/mL, the corrected percentage of plasma protein binding was dog>human>rat.

CONCLUSION

(-)Doxazosin and (+)doxazosin show stereoselective plasma protein binding with a significant species difference among rats, dogs and humans.

Stereoselective binding of chiral drugs to plasma proteins

Chiral drugs show distinct biochemical and pharmacological behaviors in the human body. The binding of chiral drugs to plasma proteins usually exhibits stereoselectivity, which has a far-reaching influence on their pharmacological activities and pharmacokinetic profiles. In this review, the stereoselective binding of chiral drugs to human serum albumin (HSA), α1-acid glycoprotein (AGP) and lipoprotein, three most important proteins in human plasma, are detailed. Furthermore, the application of AGP variants and recombinant fragments of HSA for studying enantiomer binding properties is also discussed. Apart from the stereoselectivity of enantiomer-protein binding, enantiomer-enantiomer interactions that may induce allosteric effects are also described. Additionally, the techniques and methods used to determine drug-protein binding parameters are briefly reviewed.

Plasma protein binding: from discovery to development

The importance of plasma protein binding (PPB) in modulating the effective drug concentration at pharmacological target sites has been the topic of significant discussion and debate amongst drug development groups over the past few decades. Free drug theory, which states that in absence of energy-dependent processes, after steady state equilibrium has been attained, free drug concentration in plasma is equal to free drug concentration at the pharmacologic target receptor(s) in tissues, has been used to explain pharmacokinetics/pharmacodynamics relationships in a large number of cases. Any sudden increase in free concentration of a drug could potentially cause toxicity and may need dose adjustment. Free drug concentration is also helpful to estimate the effective concentration of drugs that potentially can precipitate metabolism (or transporter)-related drug-drug interactions. Disease models are extensively validated in animals to progress a compound into development. Unbound drug concentration, and therefore PPB information across species is very informative in establishing safety margins and guiding selection of First in Human (FIH) dose and human efficacious dose. The scope of this review is to give an overview of reported role of PPB in several therapeutic areas, highlight cases where PPB changes are clinically relevant, and provide drug metabolism and pharmacokinetics recommendations in discovery and development settings.

Stereoselective binding of mexiletine and ketoprofen enantiomers with human serum albumin domains

AIM

To investigate the stereoselective binding of mexiletine or ketoprofen enantiomers with different recombinant domains of human serum albumin (HSA).

METHODS

Three domains (HSA DOM I, II and III) were expressed in Pichia pastoris GS115 cells. Blue Sepharose 6 Fast Flow was employed to purify the recombinant HSA domains. The binding properties of the standard ligands, digitoxin, phenylbutazone and diazepam, and the chiral drugs to HSA domains were investigated using ultrafiltration. The concentrations of the standard ligands, ketoprofen and mexiletine were analyzed with HPLC.

RESULTS

The recombinant HSA domains were highly purified as shown by SDS-PAGE and Western blotting analyses. The standard HSA ligands digitoxin, phenylbutazone and diazepam selectively binds to DOM I, DOM II and DOM III, respectively. For the chiral drugs, R-ketoprofen showed a higher binding affinity toward DOM III than S-ketoprofen, whereas S-mexiletine bound to DOM II with a greater affinity than R-mexiletine.

CONCLUSION

The results demonstrate that HSA DOM III possesses the chiral recognition ability for the ketoprofen enantiomers, whereas HSA DOM II possesses that for the mexiletine enantiomers.

Enantioselective drug–protein interaction between mexiletine and plasma protein

Objectives

This study examined the interaction of mexiletine enantiomers with human plasma, human serum albumin (HSA), and human α1-acid glycoprotein (hAGP), and characterized the binding modes of mexiletine enantiomers with hAGP in the molecular level.

Methods

Enantiomer separation of mexiletine was performed using precolumn derivatization chiral HPLC. The ultrafiltration technique was used to separate the free mexiletine in plasma matrix. Molecular dynamics simulations and free energy calculations were assessed using molecular mechanics and the generalized Born surface area method.

Key findings

Significant differences in enantioselective binding to human plasma were observed (R > S). The hAGP–mexiletine binding profile exhibited similar enantioselectivity (R > S) to that in human plasma, whereas HSA–mexiletine interaction was S > R at pH 7.4. Moreover, the results of comparative studies indicated that mexiletine had the highest binding affinity for F1-S, a variant of hAGP. Based on the computational studies, residues such as Arg90, Leu79, Ser89 and Phe89 showed an energy difference of more than −0.35 kcal/mol between the enantiomers.

Conclusions

hAGP may be one of the key proteins leading to the enantioselective protein bindings of mexiletine in human plasma (R > S). The residues Arg90, Leu79, Ser89 and Phe89 of hAGP may have important roles in the observed enantioselectivity.

Ryanodine receptor inhibition potentiates the activity of Na channel blockers against spontaneous calcium elevations and delayed afterdepolarizations in Langendorff-perfused rabbit ventricles

BACKGROUND

Na channel blockers are effective in suppressing delayed afterdepolarizations (DADs) in isolated Purkinje fibers. However, in isolated mouse ventricular myocytes lacking calsequestrin, only those Na channel blockers that also inhibit type 2 ryanodine receptor channels were effective against spontaneous Ca elevation (SCaE) and DADs.

OBJECTIVE

To test the hypothesis that combined Na channel and type 2 ryanodine receptor channel blocker ((R)-propafenone) is more effective than a Na channel blocker (lidocaine) in suppressing SCaE and DADs in the intact rabbit ventricles.

METHODS

We compared (R)-propafenone (3 μmol/L) with lidocaine (50 μmol/L) on SCaE and DADs by using epicardial optical mapping of intracellular calcium (Ca(i)) and membrane voltage in Langendorff-perfused rabbit hearts. SCaE and DADs were induced by rapid pacing trains and isoproterenol (0.3 μmol/L) infusion. One arbitrary unit is equivalent to the Ca transient amplitude of paced beats.

RESULTS

SCaEs were observed at the cessation of rapid pacing in all hearts at baseline. (R)-Propafenone nearly completely inhibited DADs and SCaE (0.04 arbitrary units [95% confidence interval 0.02-0.06] vs 0.23 arbitrary units [95% confidence interval 0.18-0.28] at baseline; n = 6 hearts; P <.001). Lidocaine also significantly reduced the SCaE but was significantly (P <.05) less effective than (R)-propafenone. Both drugs increased the rise time of action potential upstroke and reduced conduction velocity to a similar extent, suggesting a significant inhibition of I(Na).

CONCLUSIONS

Both Na channel blockers significantly reduced tachycardia-induced SCaEs in the rabbit ventricles, but (R)-propafenone was significantly more effective than lidocaine. These data suggest that type 2 ryanodine receptor inhibition potentiates the activity of Na channel blockers against SCaE and DADs in the intact hearts.

In vitro-in vivo extrapolation of clearance: modeling hepatic metabolic clearance of highly bound drugs and comparative assessment with existing calculation methods

In vitro-in vivo extrapolation (IVIVE) is an important method for estimating the hepatic metabolic clearance (CL) of drugs. This study highlights a problematic area observed when using microsomal data to predict in vivo CL of drugs that are highly bound to plasma proteins, and further explores mechanisms for human CL predictions by associating additional processes to IVIVE disconnect. Therefore, this study attempts to develop a novel IVIVE calculation method, which consists of adjusting the binding terms in a well-stirred liver model. A comparative assessment between the IVIVE method proposed here and previously published methods of Obach (1999. Drug Metab Dispos 27:1350-1359) and Berezhkovskiy (2010. J Pharm Sci 100:1167-1783) was also performed. The assessment was confined by the availability of measured in vitro and in vivo data in humans for 25 drugs highly bound to plasma proteins, for which it can be assumed that metabolism is the major route of elimination. Here, we argue that a difference in drug ionization and binding proteins such as albumin (AL) and alpha-1-acid glycoprotein (AAG) in plasma and liver also needs to be considered in IVIVE based on mechanistic studies. Therefore, converting unbound fraction in plasma to liver essentially increased the predicted CL values, which resulted in much more accurate estimates of in vivo CL as compared with the other IVIVE methods tested. The impact on CL estimate was more apparent for drugs binding to AL than to AAG. This is a mechanistic rational for explaining a considerable proportion of the divergence between previously estimated and observed CL values. Human CL was predicted within 1.5-fold, twofold, and threefold of the observed CL for 84%, 96%, and 100% of the compounds, respectively. Overall, this study demonstrates a significant improvement in the mechanism-based prediction of metabolic CL for these 25 highly bound drugs from in vitro data determined with microsomes, which should facilitate the application of physiologically based pharmacokinetic (PBPK) models in drug discovery and development.

High-throughput analysis of drug dissociation from serum proteins using affinity silica monoliths

A noncompetitive peak decay method was used with 1 mm×4.6 mm id silica monoliths to measure the dissociation rate constants (kd) for various drugs with human serum albumin (HSA) and α1-acid glycoprotein (AGP). Flow rates up to 9 mL/min were used in these experiments, resulting in analysis times of only 20-30 s. Using a silica monolith containing immobilized HSA, dissociation rate constants were measured for amitriptyline, carboplatin, cisplatin, chloramphenicol, nortriptyline, quinidine, and verapamil, giving values that ranged from 0.37 to 0.78 s(-1). Similar work with an immobilized AGP silica monolith gave kd values for amitriptyline, nortriptyline, and lidocaine of 0.39-0.73 s(-1). These kd values showed good agreement with values determined for drugs with similar structures and/or affinities for HSA or AGP. It was found that a kd of up to roughly 0.80 s(-1) could be measured by this approach. This information made it possible to obtain a better understanding of the advantages and possible limitations of the noncompetitive peak decay method and in the use of affinity silica monoliths for the high-throughput analysis of drug-protein dissociation.

Binding of licarbazepine enantiomers to mouse and human plasma proteins

Racemic licarbazepine (Lic) is the active metabolite of oxcarbazepine (OXC) and eslicarbazepine acetate (ESL), appearing in human plasma as S-licarbazepine (S-Lic) and R-licarbazepine (R-Lic). However, human metabolism of OXC and ESL to Lic differs in the S-Lic/R-Lic enantiomeric ratio observed in plasma. S-Lic appears in higher proportion after ESL administration than after OXC (95% versus 80%). Enantioselective pharmacokinetics of Lic enantiomers have been found in mice after their separate administration and in humans following OXC treatment. Since protein binding of drugs may be enantioselective and a determining factor of pharmacokinetics, the binding of S-Lic and R-Lic to mouse and human total plasma proteins and, specifically, to human serum albumin (HSA) and alpha(1)-acid glycoprotein (AGP) were herein investigated for the first time. Free and bound fractions of S-Lic and R-Lic were separated by ultrafiltration after previous in vitro incubation of spiked plasma samples and protein solutions with each enantiomer at 10, 25 and 50 microg/ml. The results revealed that the extent of binding of Lic enantiomers to total plasma proteins was 30% and independent of the drug concentration and species considered. The data also suggest that the binding of Lic enantiomers to HSA is greater than that to AGP. Moreover, absence of enantioselectivity in the binding of Lic enantiomers to mouse and human plasma proteins and to HSA and AGP is evident. In conclusion, these findings suggest that the enantioselectivity observed in vivo in the biodisposition of S-Lic and R-Lic is not dependent on their affinity to plasma proteins.

Stereoselective protein binding of tetrahydropalmatine enantiomers in human plasma, HSA, and AGP, but not in rat plasma

Tetrahydropalmatine (THP) is one of the active alkaloid ingredients of Rhizoma Corydalis. THP has a chiral center, and the stereoselective pharmacokinetics and tissue distribution have been reported. The aim of the present article is to study the stereoselective protein binding of THP using equilibrium dialysis followed by HPLC-UV analysis. The results showed that THP stereoselectively binds to human serum albumin (HSA), alpha(1)-acid glycoprotein (AGP), and proteins in human plasma. The fraction binding of (+)-THP was significantly higher than that of (-)-THP, whereas such stereoselectivity was not found in rat plasma. The affinity of HSA and AGP to (+)-THP, expressed as nK(A), were 9.0 x 10(3) M(-1) and 2.34 x 10(5) M(-1), respectively, which were notablely higher than to (-)-THP, with the nK(A) of 3.4 x 10(3) M(-1) and 1.44 x 10(5) M(-1), respectively. The binding site of HSA for (-)-THP was Site I, whereas for (+)-THP was both Site I and Site II. The F1/S variants of AGP were proved to be the key variants (-)- and (+)-THP binding to both. Finally, the AGP binding drugs, such as mifepristone, were demonstrated to reduce the fraction binding of (-)- and (+)-THP with pure AGP (1 mg/ml) but did not affect the fraction binding of both (-)- and (+)-THP with proteins in human plasma. It can be concluded that protein binding of THP is species dependent and stereoselective, both HSA and AGP contribute to the stereoselective binding to THP enatiomers, and AGP binding drugs may not cause the drug-drug interaction on THP in healthy human plasma.