A series of 436 Munro database chemicals were studied with respect to their corresponding experimental LD50 values to investigate the possibility of establishing a global QSAR model for acute toxicity. Dragon molecular descriptors were used for the QSAR model development and genetic algorithms were used to select descriptors better correlated with toxicity data. Toxic values were discretized in a qualitative class on the basis of the Globally Harmonized Scheme: the 436 chemicals were divided into 3 classes based on their experimental LD50 values: highly toxic, intermediate toxic and low to non-toxic. The k-nearest neighbor (k-NN) classification method was calibrated on 25 molecular descriptors and gave a non-error rate (NER) equal to 0.66 and 0.57 for internal and external prediction sets, respectively. Even if the classification performances are not optimal, the subsequent analysis of the selected descriptors and their relationship with toxicity levels constitute a step towards the development of a global QSAR model for acute toxicity.

A combination of theoretical and experimental studies has provided correlations between molecularly imprinted polymer composition, morphology, and recognition behavior obtained using a series of bupivacaine-imprinted methacrylic acid (MAA)–ethylene glycol dimethacrylate copolymers differing in molar ratios of the respective monomers. Results extracted from analysis of molecular dynamics (MD) trajectory data demonstrated that stability and frequency of interactions between bupivacaine and the monomers in the prepolymerization phase were strongly affected by minor changes in polymer composition, which in turn affected binding site affinity and heterogeneity of the imprinted polymers. Moreover, through the characterization of polymer morphology, we show that higher molar fractions of MAA resulted in polymeric materials with increased pore size, a feature that enhanced the binding capacity of the polymers. Furthermore, the results presented point at the strength of MD for predicting MIP-template binding capacity and affinity.

In this report, principal component analysis (PCA) has been used to explore the influence of template complexation in the pre-polymerization phase on template molecularly imprinted polymer (MIP) recognition and polymer morphology. A series of 16 bupivacaine MIPs were studied. The ethylene glycol dimethacrylate (EGDMA)-crosslinked polymers had either methacrylic acid (MAA) or methyl methacrylate (MMA) as the functional monomer, and the stoichiometry between template, functional monomer and crosslinker was varied. The polymers were characterized using radioligand equilibrium binding experiments, gas sorption measurements, swelling studies and data extracted from molecular dynamics (MD) simulations of all-component pre-polymerization mixtures. The molar fraction of the functional monomer in the MAA-polymers contributed to describing both the binding, surface area and pore volume. Interestingly, weak positive correlations between the swelling behavior and the rebinding characteristics of the MAA-MIPs were exposed. Polymers prepared with MMA as a functional monomer and a polymer prepared with only EGDMA were found to share the same characteristics, such as poor rebinding capacities, as well as similar surface area and pore volume, independent of the molar fraction MMA used in synthesis. The use of PCA for interpreting relationships between MD-derived descriptions of events in the pre-polymerization mixture, recognition properties and morphologies of the corresponding polymers illustrates the potential of PCA as a tool for better understanding these complex materials and for their rational design.

This report demonstrates that the diversity of hydrogen bond interactions present in molecularly imprinted polymer pre-polymerization mixtures, typically associated with binding-site heterogeneity, can also contribute to morphological characteristics that may influence polymer–template recognition. Comparisons have been made between a series of bupivacaine molecularly imprinted methacrylic acid (MAA)–ethylene glycol dimethacrylate (EGDMA) copolymers and a series of analogous methyl methacrylate (MMA)–EGDMA copolymers using comprehensive molecular dynamics studies of the respective pre-polymerization mixtures, template–polymer binding studies and detailed BET surface area and BJH porosity analyses. The role of the carboxylic acid functionality of MAA, and in particular the acidic proton, in generating morphological features conducive to analyte access (slit-like rather than ink bottle-like structures) and recognition is discussed.

Warfarin is one of the most commonly used drugs in anticoagulent therapy. Notwithstanding its wide use, achieving correct dosage is often a major challenge due to its narrow therapeutic window.[1] The bioavailability of warfarin is believed to be greatly influenced by the environment-dependent composition of the ensemble of isomers present. While the different structures of warfarin have been discussed in earlier publications,[2] details of the mechanism underlying the formation of the cyclic hemiacetal (Figure 1) had not yet been investigated.

Figure 1. Cyclization reaction of warfarin.

Figure 2. Transition state in presence of one water molecule.

We have now studied the reaction by means of density functional calculations. Comparison of results from calculations performed on the isolated warfarin molecule and in presence of water molecules (compare Figure 2) highlight the importance of intermolecular interactions in the key proton transfer step for the reaction to proceed. A viable model for the mechanism underlying the isomerisation shall be presented.

References

[1]             J. Ansell, J. Hirsh, L. Poller, H. Bussey, A. Jacobsen and E. Hylek, Chest, 126, 204S (2004).

[2]            B. C. G. Karlsson, A. M. Rosengren, P. O. Andersson and I. A. Nicholls, J. Phys. Chem. B, 111,10520 (2007).

The anticoagulent drug warfarin exhibits chameleon-like isomerism, where the environment-dependent composition of the ensemble of structures greatly influences its bioavailability. Here, the mechanism of conversion between the major isomeric forms is studied. The dramatic differences in transition state energies, as determined by density functional calculations, highlight the necessity for the involvement of intermolecular interactions in the key proton transfer step. A viable model for the mechanism underlying the isomerization reactions is presented.

The role of the structural diversity of the widely used anticoagulant drug warfarin on its distribution in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayer membranes was investigated using a series of both restrained (umbrella sampling) and unrestrained molecular dynamics simulations. Data collected from unrestrained simulations revealed favorable positions for neutral isomers of warfarin, the open side chain form (OCO), and the cyclic hemiketal (CCO), along the bilayer normal close to the polar headgroup region and even in the relatively distant nonpolar lipid tails. The deprotonated open side chain form (DCO) was found to have lower affinity for the DOPC bilayer membrane relative to the neutral forms, with only a small fraction interacting with the membrane, typically within the polar headgroup region. The conformation of OCO inside the lipid bilayer was found to be stabilized by intramolecular hydrogen bonding thereby mimicking the structure of CCO. Differences in free energies, for positions of OCO and CCO inside the bilayer membrane, as compared to positions in the aqueous phase, were −97 and −146 kJ·mol^–1. Kinetic analysis based on the computed free energy barriers reveal that warfarin will diffuse through the membranes within hours, in agreement with experimental results on warfarin’s accumulation in the plasma, thus suggesting a passive diffusion mechanism. We propose that this membrane transport may be an isomerization-driven process where warfarin adapts to the various local molecular environments encountered under its journey through the membrane. Collectively, these results improve our understanding of the influence of warfarin’s structural diversity on the drug’s distribution and bioavailability, which in turn may provide insights for developing new formulations of this important pharmaceutical to better address its narrow therapeutic window.

Warfarin is an oral anticoagulant drug used to prevent thrombolic disorders such as myocardial infarction and stroke by inhibiting the active site of vitamin-K dependent epoxide reductase (VKOR) [1]. Despite being in widespread use and having a narrow therapeutic window, its mechanisms of action are not yet fully understood and incorrect warfarin dosage often leads to severe side effects. A factor limiting our understanding of warfarin’s bioavailability is warfarin’s structural diversity, which has been shown to be strongly affected by the nature of molecular environment e.g. solvent polarity and pH [2-7]. One of the major factors contributing to a drug’s biological effect is membrane transport, a process involving exposure of warfarin to environments of quite different character. Since a drug’s transport across membrane may include both active transport by carriers as well as diffusion-controlled processes, it may be envisaged that in order to fully predict warfarin’s anticoagulant effect these mechanisms must be carefully elucidated.

Molecular dynamics (MD) simulations have previously been performed in order to obtain detailed information on static equilibrium as well as dynamic properties of small organic drugs in biomembranes. One of the most studied lipids in cell membrane simulations has been dipalmitoylphosphatidylcholine (DPPC) which is the most abundant phospholipid in cell membranes. Here we present lipid bilayer membrane transport properties for a series of warfarin structures previously reported in the literature using a fully solvated DPPC membrane model. Data extracted from simulations shed light on differences in membrane partioning as well as mobilities of warfarin isomers studied and a mechanism by which warfarin permeates through membranes in vivo is presented.

References

1. Landefeld, C.; Beyth, R. Am. J. Med. 1993, 95, 315-328.
2. Karlsson, B. C. G.; Rosengren, A. M.; Andersson, P. O.; Nicholls, I. A. J. Phys. Chem. B 2007, 111, 10520-10528.
3. Karlsson, B. C. G.; Rosengren, A. M.; Andersson, P. O.; Nicholls, I. A. J. Phys. Chem. B 2009, 113, 7945-7949.
4. Karlsson, B. C. G.; Rosengren, A. M.; Näslund, I.; Andersson, P. O.; Nicholls, I. A. Submitted 2010.
5. Rosengren, A. M.; Karlsson, B. C. G.; Näslund, I.; Andersson, P. O.; Nicholls, I. A. Submitted 2010.
6. Nicholls, I. A.; Karlsson, B. C. G.; Rosengren, A. M.; Henschel, H. J. Mol. Recognit. 2010, In press.
7. Henschel, H.; Karlsson, B. C. G.; Rosengren, A. M.; Nicholls, I. A. Submitted 2010.

A series of steady-state fluorescence anisotropy experiments has been performed to demonstrate the presence of a deprotonated open side chain form of warfarin in organic environments. We explain the observed emission-wavelength-dependent anisotropy of warfarin in ethanol, 2-propanol, and acetonitrile due to the coexistence of neutral isomers and deprotonated open side chain forms displaying different fluorescence decay kinetics. To investigate solvent-solute interactions in more detail, a series of molecular dynamics simulations was performed to study warfarin solvation and to predict the time scale of rotational diffusion displayed by this compound. Predictions obtained provide an explanation for the nonzero values in anisotropy observed for neutral isomers of warfarin associated with the short fluorescence lifetime (tau < 0.1 ns) and for an approximately zero anisotropy observed for the deprotonated open side chain form, which is associated with the longer fluorescence lifetime (tau = 0.5-1.6 ns). Finally, we address the potential use of fluorescence anisotropy for an increased understanding of the structural diversity of warfarin in protein binding pockets.

Warfarin is a clinically important drug widely used in the treatment of thrombolic disorders e.g. myocardial infarction and stroke.¹ When administered, 99% of the drug present in blood is bound to the transport protein human serum albumin (HSA).² On account of the fact that HSA demonstrates polymorphism and warfarin has a narrow therapeutic index, careful monitoring of the effect of drug-dosage must be performed.

Currently, warfarin’s anticoagulant effect is measured by an indirect method in which the clotting time is measured and correlated to the amount of warfarin present. As current methods for self-monitoring are limited, the development of alternative robust and more sensitive methods is desirable.

In this study, we have developed a non-covalent molecularly imprinted polymer³ (MIP) system with selectivity for warfarin.⁴ The HSA-like binding properties of this MIP were established in previous efforts to develop polymers capable of HSA-like binding of warfarin.⁵

In principle, the fluorophoric nature of warfarin should allow for the fluorescence spectroscopy-based detection of the drug. Recent efforts by us,^6-8 using a series of theoretical and spectroscopic studies have highlighted the complex nature of warfarin. In particular, the medium dependent isomerization of this drug illustrates why spectroscopy based methods for the direct detection of the drug has not been forthcoming. Results from these studies have been used to develop a method for the in situ detection of warfarin using time resolved fluorescence spectroscopy.

(1)      Landefeld, C.; Beyth, R. Anticoagulant-related bleeding - epidemiology, prediction and prevention. Am. J. Med. 1993, 95, 315-328.

(2)      Yacobi, A.; Udall, J. A.; Levy, G. Comparative pharmacokinetics of coumarin anticoagulants.18 Serum-protein binding as a determinant of warfarin body clearance and anticoagulant effect. Clin. Pharmacol Ther. 1976, 19, 552-558.

(3)      Alexander, C.; Andersson, H. S.; Andersson, L. I.; Ansell, R. J.; Kirsch, N.; Nicholls, I. A.; O'Mahony, J.; Whitcombe, M. J. Molecular imprinting science and technology: A survey of the literature for the years up to and including 2003. Journal of Molecular Recognition 2006, 19, 106-180.

(4)      Rosengren, A. M.; Karlsson, B. C. G.; Näslund, I.; Andersson, P. O.; Nicholls, I. A. Time resolved fluorescence spectroscopic detection of the anticoagulant warfarin: A sensor-based method for direct detection in blood plasma. 2010, Submitted.

(5)      Karlsson, B. C. G.; Rosengren, A. M.; Näslund, I.; Andersson, P. O.; Nicholls, I. A. Synthetic Human Serum Albumin Sudlow I binding site mimics. 2010, Submitted.

(6)      Karlsson, B. C. G.; Rosengren, A. M.; Andersson, P. O.; Nicholls, I. A. The Spectrophysics of Warfarin: Implications for Protein Binding J. Phys. Chem. B 2007, 111, 10520-10528.

(7)      Karlsson, B. C. G.; Rosengren, A. M.; Andersson, P. O.; Nicholls, I. A. Molecular Insights on the Two Fluorescence Lifetimes Displayed by Warfarin from Fluorescence Anisotropy and Molecular Dynamics Studies. J. Phys. Chem. B 2009, 113, 7945-7949.

(8)      Nicholls, I. A.; Karlsson, B. C. G., Rosengren, A. M.. Henschel, H. Warfarin: an Environment-Dependent Switchable Molecular Probe. J. Mol. Recognit. 2010, in press.

Here, we report the design, synthesis, and characterization of molecularly imprinted polymer (MIP) derived mimics of the human serum albumin (HSA) Sudlow I site-the binding site for the anticoagulant warfarin. MIP design was based upon a combination of experimental (H-1 NMR) and computational (molecular dynamics) methods, Two MIPs and corresponding nonimprinted reference polymers were synthesized and characterized (scanning electron microscopy; nitrogen sorption; and Fourier transform infrared spectroscopy). MIP-ligand recognition was examined using radioligand binding studies, where the largest number of selective sites was found in a warfarin-imprinted methacrylic acid ethylene dimethacrylate copolymer (MAA-MIP). The warfarin selectivity of this MIP was confirmed using radioligand displacement and zonal chromatographic studies. A direct comparison of MIP-warfarin binding characteristics with those of the HSA Sudlow I binding site was made, and similarities in site population (per gram polymer or protein) and affinities were observed. The warfarin selectivity of the MIP suggests its potential for use as a recognition element in a MIP-based warfarin sensor and even as a model to aid in understanding and steering blood-plasma protein-regulated transport processes or even for the development of warfarin sensors.

The further evolution of molecularly imprinted polymer science and technology necessitates the development of robust predictive tools capable of handling the complexity of molecular imprinting systems. A combination of the rapid growth in computer power over the past decade and significant software developments have opened new possibilities for simulating aspects of the complex molecular imprinting process. We present here a survey of the current status of the use of in silico-based approaches to aspects of molecular imprinting. Finally, we highlight areas where ongoing and future efforts should yield information critical to our understanding of the underlying mechanisms sufficient to permit the rational design of molecularly imprinted polymers. 

In principle, molecularly imprinted polymer science and technology provides a means for ready access to nano-structured polymeric materials of predetermined selectivity. The versatility of the technique has brought it to the attention of many working with the development of nanomaterials with biological or biomimetic properties for use as therapeutics or in medical devices. Nonetheless, the further evolution of the field necessitates the development of robust predictive tools capable of handling the complexity of molecular imprinting systems. The rapid growth in computer power and software over the past decade has opened new possibilities for simulating aspects of the complex molecular imprinting process. We present here a survey of the current status of the use of in silico-based approaches to aspects of molecular imprinting. Finally, we highlight areas where ongoing and future efforts should yield information critical to our understanding of the underlying mechanisms sufficient to permit the rational design of molecularly imprinted polymers.

The development of in silico strategies for the study of the molecular imprinting process and the properties of molecularly imprinted materials has been driven by a growing awareness of the inherent complexity of these systems and even by an increased awareness of the potential of these materials for use in a range of application areas. Here we highlight the development of theoretical and computational strategies that are contributing to an improved understanding of the mechanisms underlying molecularly imprinted material synthesis and performance, and even their rational design.

The need for materials with predetermined ligand-selectivities for use in sensing and separation technologies, e.g. membranes and chromatography, has driven the development of molecularly imprinted polymer science and technology. Over recent years, the need to develop robust predictive tools capable of handling the complexity of molecular imprinting systems has become apparent The current status of the use of in silica techniques in molecular imprinting is here presented, and we highlight areas where new developments are contributing to improvements in the rational design of molecularly imprinted polymers.

The complex nature of the structure of the anticoagulant warfarin is reflected in the diversity of binding modes observed in warfarin–protein recognition systems. A series of theoretical, ¹H-NMR and steady state and time resolved fluorescence spectroscopic studies, have been used to establish correlations between the molecular environment provided by various solvent systems and the relative concentrations of the various members of warfarin's ensemble of isomers. A consequence of these observations is that the judicious choice of solvent system or molecular environment of warfarin allows for manipulation of the position of the equilibrium between isomeric structures such as the hemiacetal and open phenol-keto forms, the latter even possible in a deprotonated form, where in each case unique spectroscopic properties are exhibited by the respective structures. Collectively, warfarin's capacity to adapt its structure as a function of environment in conjunction with the fluorescence behaviours of the various isomers together provide an environment-dependent molecular switch with reporter properties, which allows for the simultaneous detection of warfarin in different states with lifetimes spanning the range < 0.10–5.5 ns. These characteristics are here used to examine warfarin binding domains in a series of materials (solvents, protein, inorganic matrix and synthetic polymer). Moreover, these studies demonstrate the potential for using warfarin, or other switchable analogues thereof, as a tool for studying molecular level characteristics, for example local dielectricity. Copyright © 2010 John Wiley & Sons, Ltd.

The influence of the physical properties of incubation medium on the rebinding of template to bupivacaine molecularly imprinted and non-imprinted methacrylic acid–ethylene dimethacrylate co-polymers has been studied. Principal component analysis (PCA) was employed to identify the factors with the greatest influence on binding. While the dielectric constant (D) made a significant contribution to describing the observed binding, the influence of polarity as reflected in the Snyder polarity index (SPI) was also demonstrated to make a significant contribution. The use of solvents containing hydroxyl functionality in particular was observed to exert unique effects on recognition. The variation in solvent influence on binding at constant D motivates more complex analyses when studying MIP–ligand recognition.

In order to examine the physical mechanisms underlying molecularly imprinted polymer¹ (MIP)–ligand recognition, polymers with selectivity for the local anaesthetic bupivacaine have been synthesised and their ligand-recognition characteristics examined. As several previous studies have pointed at the complexity of the rebinding characteristics and the dependence on rebinding media,^2-4 we used chemometric strategies for the analysis of ligand-MIP binding in various media.⁵

In a previous study we presented results from a chemometric analysis showing that rebinding of bupivacaine to the MIP in different solvent mixtures and at different temperatures follow a complicated non-linear relationship.⁶ The results from that analysis, motivated an investigation into the significance of the solvent physical characteristics (molecular and bulk) on rebinding properties. In this work,⁷ principal component analysis was employed to identify the factors with the greatest influence on binding. While the dielectric constant made a significant contribution to describing the observed binding, the influence of polarity as reflected in the Snyder polarity index was also demonstrated to also make a significant contribution. The use of solvents containing hydroxyl functionality was observed to exert unique effects on recognition. The variation in solvent influence on binding at constant dielectricity motivates more complex analyses when studying MIP-ligand recognition.

(1)      Alexander, C.; Andersson, H.S.; Andersson, L.I.; Ansell, R.J.; Kirsch, N.; Nicholls, I.A.; O'Mahony, J.; Whitcombe, M.J. Molecular imprinting science and technology: A survey of the literature for the years up to and including 2003. Journal of Molecular Recognition 2006, 19, 106-180.

(2)      Andersson, L.I. Efficient sample pre-concentration of bupivacaine from human plasma by solid-phase extraction on molecularly imprinted polymers. Analyst 2000, 125, 1515-1517.

(3)      Karlsson, J.G.; Andersson, L.I.; Nicholls, I.A. Probing the molecular basis for ligand-selective recognition in molecularly imprinted polymers selective for the local anaesthetic bupivacaine. Analytica Chimica Acta 2001, 435, 57-64.

(4)      Karlsson, J.G.; Karlsson, B.; Andersson, L.I.; Nicholls, I.A. The roles of template complexation and ligand binding conditions on recognition in bupivacaine molecularly imprinted polymers. Analyst 2004, 129, 456-462.

(5)      Nicholls, I.A.; Andersson, H.S.; Charlton, C.; Henschel, H.; Karlsson, B.C.G.; Karlsson, J.G.; O’Mahony, J.; Rosengren, A.M.; Rosengren, K.J.; Wikman, S. Theoretical and computational strategies for rational molecularly imprinted polymer design. Biosensors and Bioelectronics 2009, 25, 543-552.

(6)      Rosengren, A.M.; Karlsson, J.G.; Andersson, P.O.; Nicholls, I.A. Chemometric models of template-molecularly imprinted polymer binding. Analytical Chemistry 2005, 77, 5700-5705.

(7)      Rosengren, A.M; Golker, K.; Wiklander, J.G.; Nicholls, I.A. Dielectric constants are not enough: Principal component analysis of the influence of solvent properties on molecularly imprinted polymer–ligand rebinding. Biosensors and Bioelectronics 2009, 25, 553-557.

Molecular imprinting is a technique for creating polymeric recognition materials with predetermined ligand selectivities.¹ A molecularly imprinted polymer (MIP) with selectivity for the local anaesthetic bupivacaine has been synthesised in order to examine the physical mechanisms underlying MIP–ligand recognition characteristics. As rebinding characteristics has shown to be complex, we use chemometric strategies for the analysis of ligand-MIP binding in various media.^2-4 The use of chemometrics simplify the selection of optimal experimental parameters as well as the extraction of significant information generated from multivariate data analysis.⁵

Previously we have presented results from a chemometric analysis pointing at a complex non-linear relationship when studying binding of bupivacaine to the MIP in different solvent mixtures and at different temperatures.⁶ The results motivated an investigation into the significance of the solvent physical characteristics (molecular and bulk) on rebinding properties. In this work, principal component analysis was employed to identify the factors with the greatest influence on binding. While the dielectric constant made a significant contribution to describing the observed binding, the influence of polarity as reflected in the Snyder polarity index was also demonstrated to make a significant contribution. The use of solvents containing hydroxyl functionality was observed to exert unique effects on recognition. The variation in solvent influence on binding at constant dielectricity motivates more complex analyses when studying MIP-ligand recognition. Collectively, the results provided general insights concerning the complex interplay between the mechanisms controlling ligand recognition in MIPs.

References

1. Alexander, C.; Andersson, H.S.; Andersson, L.I.; Ansell, R.J.; Kirsch, N.; Nicholls, I.A.; O’Mahony, J.; Whitcombe, M.J. Journal of Molecular Recognition 2006, 19, 106-180.
2. Andersson, L.I. Analyst 2000, 125, 1515-1517.
3. Karlsson, J.G.; Andersson, L.I.; Nicholls, I.A. Analytica Chimica Acta 2001, 435, 57-64.
4. Karlsson, J.G.; Karlsson, B.; Andersson, L.I.; Nicholls, I.A. Analyst 2004, 129, 456-462.
5. Nicholls, I.A.; Andersson, H.S.; Charlton, C.; Henschel, H.; Karlsson, B.C.G.; Karlsson, J.G.; O’Mahony, J.; Rosengren, A.M.; Rosengren, K.J.; Wikman, S. Biosensors and Bioelectronics 2009, 25, 543-552.
6. Rosengren, A.M.; Karlsson, J.G.; Andersson, P.O.; Nicholls, I.A. Analytical Chemistry 2005, 77, 5700-5705.

A molecularly imprinted polymer (MIP) is a polymeric material with selective recognition for an analyte.¹ In order to examine the physical mechanisms underlying MIP–ligand recognition, polymers with selectivity for the local anaesthetic bupivacaine have been synthesised and their ligand-recognition characteristics examined. As several previous studies have pointed at the complexity of the rebinding characteristics and the dependence on rebinding media,^2-4 we used chemometric strategies for the analysis of ligand-MIP binding in various media.⁵

In a previous study we presented results from a chemometric analysis showing that rebinding of bupivacaine to the MIP in different solvent mixtures and at different temperatures follow a complicated non-linear relationship.⁶ The results from that analysis, motivated an investigation into the significance of the solvent physical characteristics (molecular and bulk) on rebinding properties. In this work, principal component analysis was employed to identify the factors with the greatest influence on binding. While the dielectric constant made a significant contribution to describing the observed binding, the influence of polarity as reflected in the Snyder polarity index was also demonstrated to also make a significant contribution. The use of solvents containing hydroxyl functionality was observed to exert unique effects on recognition. The variation in solvent influence on binding at constant dielectricity motivates more complex analyses when studying MIP-ligand recognition.

References

1. Alexander, C.; Andersson, H.S.; Andersson, L.I.; Ansell, R.J.; Kirsch, N.; Nicholls, I.A.; O’Mahony, J.; Whitcombe, M.J. Journal of Molecular Recognition 2006, 19, 106-180.
2. Andersson, L.I. Analyst 2000, 125, 1515-1517.
3. Karlsson, J.G.; Andersson, L.I.; Nicholls, I.A. Analytica Chimica Acta 2001, 435, 57-64.
4. Karlsson, J.G.; Karlsson, B.; Andersson, L.I.; Nicholls, I.A. Analyst 2004, 129, 456-462.
5. Nicholls, I.A.; Andersson, H.S.; Charlton, C.; Henschel, H.; Karlsson, B.C.G.; Karlsson, J.G.; O’Mahony, J.; Rosengren, A.M.; Rosengren, K.J.; Wikman, S. Biosensors and Bioelectronics 2009, 25, 543-552.
6. Rosengren, A.M.; Karlsson, J.G.; Andersson, P.O.; Nicholls, I.A. Analytical Chemistry 2005, 77, 5700-5705.

Warfarin is an important oral anticoagulant drug that demonstrates a molecular-environment dependent structural diversity. Previous investigations of warfarin’s ensemble of isomers in organic solvent-based environments have pointed to the importance of the closed-ring cyclic hemiketal form of the drug in non-polar environments, e.g. the interior of proteins, enzymes and biomembranes. Detection of the presence of these isomers in polar environments has not yet been reported. Here, we demonstrate the presence of the cyclic hemiketal structural form of warfarin under aqueous conditions. This finding underscores the importance considering all structural isomers of this drug when making predictions on warfarin’s bioavailability.

The relationship between molecularly imprinted polymer (MIP) morphology and template-rebinding over a series of warfarin-imprinted methacrylic acid co(ethylene dimethacrylate) polymers has been explored. Detailed investigations of the nature of template recognition revealed that an optimal template binding was obtained with polymers possessing a narrow population of pores (~3–4 nm) in the mesopore size range. Importantly, the warfarin-polymer rebinding analyses suggest strategies for regulating ligand binding capacity and specificity through variation of the degree of cross-linking, where polymers prepared with a lower degree of cross-linking afford higher capacity though non-specific in character. In contrast, the co-existence of specific and non-specific binding was found in conjunction with higher degrees of cross-linking and resultant meso- and macropore size distributions.

Warfarin is an anticoagulant drug extensively used in the treatment and prevention of thrombotic disorders. Previous studies have shown that warfarin binds extensively to blood plasma proteins and that only a small fraction of the drug is unbound and thus available for therapeutic function. Both warfarin's narrow therapeutic window and the susceptibility of anticoagulant function to patient-dependent factors necessitate regular monitoring. In this study, we have shown that the lifetimes for each of the various bound and free forms of the drug in blood plasma can be quantified in situ by time-correlated single-photon counting fluorescence spectroscopy over the clinically significant concentration range. A relationship between the blood coagulation and the distribution of fluorescence lifetimes was observed. The in situ detection of clinically relevant concentrations of warfarin in its respective bound and unbound forms could provide a prognostic tool for use in patient treatment.

Warfarin is an oral anticoagulant drug, used in the treatment of thrombolic disorders.¹ Since the therapeutic window of warfarin is narrow, careful monitoring of the effect of drug dosage is important. To date, the effect of warfarin treatment is monitored indirectly in which the clotting time (prothrombin time) is measured. The development of an alternative method, ideally both more robust and more sensitive, for the determination of warfarin effect on blood coagulation is desirable.

It was envisaged that a warfarin-selective synthetic antibody prepared by molecular imprinting² could provide the basis for the development of a novel method to facilitate the direct determination of warfarin in blood, and ideally even allow correlation the extent of patient blood coagulation. A large number of factors have been shown to be important and influence molecularly imprinted polymer (MIP) performance, such as type and amount of monomer incorporated in the final MIP matrix as well as rebinding media.^3,4 Recent studies on polymer composition have pointed at the influence of the choice of monomer and template on the textural properties of a MIP.^5-7

We have prepared a series of warfarin imprinted methacrylic acid ethylene dimethacrylate co-polymers, with varying degrees of crosslinking, using a non-covalent molecular imprinting strategy. Characterization of polymer morphology and MIP-ligand radioligand binding studies were performed in order to understand the molecular basis for recognition in warfarin MIPs and to optimize polymer composition. Furthermore, we conclude that polymers prepared with a high degree of cross-linking demonstrated the highest binding capacity, however this rebinding is proposed to be predominately non-specific in character. In contrast, the co-existence of specific and non-specific binding was found within polymers with meso- and macroporous size distributions. Finally, detailed investigations of warfarin-MIP rebinding revealed that the optimal polymer composition was represented by a material with a narrow population of pores (~3-4 nm), a size range similar to the diameter of warfarin (~1 nm).

(1)      Landefeld, C.; Beyth, R. Anticoagulant-related bleeding - epidemiology, prediction and prevention. American Journal of Medicine 1993, 95, 315-328

(2)      Alexander, C.; Andersson, H.S.; Andersson, L.I.; Ansell, R.J.; Kirsch, N.; Nicholls, I.A.; O'Mahony, J.; Whitcombe, M.J. Molecular imprinting science and technology: A survey of the literature for the years up to and including 2003. Journal of Molecular Recognition 2006, 19, 106-180.

(3)      Kempe, H.; Kempe, M. Novel method for the synthesis of molecularly imprinted polymer bead libraries. Macromolecular Rapid Communications 2004, 25, 315-320.

(4)      Rosengren, A.M.; Karlsson, J.G.; Andersson, P.O.; Nicholls, I.A. Chemometric models of template-molecularly imprinted polymer binding. Analytical Chemistry 2005, 77, 5700-5705.

(5)      O'Mahony, J.; Molinelli, A.; Nolan, K.; Smyth, M.; Mizaikoff, B. Anatomy of a successful imprint: Analysing the recognition mechanisms of a molecularly imprinted polymer for quercetin. Biosensors and Bioelectronics 2006, 21, 1383-1392.

(6)      Al Kobaisi, M.; Tate, M.; Rix, C.; Jakubov, T.; Mainwaring, D. The effect of molecular imprinting on the pore size distribution of polymers. Adsorption 2007, 13, 315-321.

(7)      Urraca, J. L.; Carbajo, M. C.; Torralvo, M. J.; González-Vázquez, J.; Orellana, G.; Moreno-Bondi, M. C. Effect of the template and functional monomer on the textural properties of molecularly imprinted polymers. Biosensors and Bioelectronics 2008, 24, 155-161.

Here we report on a novel method for the direct in situ measurement of specific isomeric forms of the anticoagulant warfarin using time correlated single-photon counting (TCSPC) spectroscopy in conjunction with synthetic Sudlow I binding site receptors. The method is highly robust over the clinically significant concentration range, and demonstrates the potential of the binding site mimics in conjunction with the spectroscopic strategy employed here for the determination of this important pharmaceutical in clinical or even environmental samples.

Warfarin is a clinically important drug widely used in the treatment of thrombolic disorders such as myocardial infarction and stroke [1]. When administered, 99% of the drug present in blood is bound to the transport protein human serum albumin (HSA) [2]. On account of the fact that HSA demonstrates polymorphism and warfarin has a narrow therapeutic window, careful monitoring of the effect of drug-dosage must be performed. Currently, warfarin’s anticoagulant effect is measured by an indirect method in which the clotting time is measured and correlated to the amount of warfarin present. As current methods for self-monitoring are limited, the development of alternative robust and more sensitive methods is desirable.

Molecular imprinting is a technique for the preparation of synthetic polymeric receptors (MIPs) with selective recognition for predetermined analyte [3]. In this study [4], we have used a non-covalent MIP system with selectivity for warfarin. The HSA-like binding properties of this MIP was developed from previous attempts to develop polymers with HSA-like binding of warfarin [5].

In principle the fluorophoric nature of warfarin should allow for the fluorescence spectroscopy-based detection of the drug. Recent efforts by us [6-8], using a series of theoretical and spectroscopic studies, have highlighted the complex nature of warfarin. In particular, the medium dependent isomerization of this drug illustrates why spectroscopy-based methods for the direct detection of the drug has not been forthcoming. Results from these studies have been used to develop a method for the in situ detection of warfarin using time correlated single photon counting (TCSPC). The application of this method for the detection of warfarin in blood is highlighted.

References

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4. Rosengren, A. M.; Karlsson, B. C. G.; Näslund, I.; Andersson, P. O.; Nicholls, I. A. 2010, Submitted.
5. Karlsson, B. C. G.; Rosengren, A. M.; Näslund, I.; Andersson, P. O.; Nicholls, I. A. 2010, Submitted.
6. Karlsson, B. C. G.; Rosengren, A. M.; Andersson, P. O.; Nicholls, I. A. J. Phys. Chem. B 2007, 111, 10520-10528.
7. Karlsson, B. C. G.; Rosengren, A. M.; Andersson, P. O.; Nicholls, I. A. J. Phys. Chem. B 2009, 113, 7945-7949.
8. Nicholls, I. A.; Karlsson, B. C. G., Rosengren, A. M. J. Mol. Recognit. 2010, in press.