Panagiotis
Manesiotis
*a,
Sakina
Kashani
b and
Peter
McLoughlin
b
aSchool of Chemistry and Chemical Engineering, Queen's University Belfast, David Keir Building, Stranmillis Road, BT9 5AG, Belfast, UK. E-mail: p.manesiotis@qub.ac.uk; Fax: +44 (0)28 9097 6524; Tel: +44 (0)28 9097 4515
bPharmaceutical and Molecular Biotechnology Research Centre, Waterford Institute of Technology, Waterford, Ireland
First published on 23rd April 2013
Molecularly Imprinted Polymers (MIPs) against imiquimod, a highly potent immune response modifier used in the treatment of skin cancer, were synthesised using a template analogue strategy and were compared with imprints of the drug itself. An investigation of the complexation between the functional monomer and the template analogue revealed an association constant of 1376 ± 122 M−1, significantly higher than previously reported values for similar systems. The binding characteristics of the synthesised imprinted polymers were evaluated and extremely strong binding for imiquimod was observed while imprinting factors as high as 17 were calculated. When applied as sorbents in solid-phase extraction of imiquimod from aqueous, urine and blood serum samples, clean extracts and recoveries up to 95% were achieved, and it is concluded that while imiquimod imprints exhibited higher capacity for the drug, template analogue imprints are more selective. The results obtained suggest potential applications of imiquimod imprints as sorbents in rapid extraction and monitoring of undesirable systemic release of the drug.
The requirement of a template or a mould around which the molecularly imprinted polymer will be built can in some cases be a limiting factor, as quite often the targeted compound can be prohibitively expensive, incompatible with the polymerisation protocol, e.g. heat or light sensitive, insoluble in polymerisation solvents of choice, dangerous to the researchers' health or simply unavailable. In such cases scientists have employed template substitutes or “dummy” templates, as they are often called, whereby a compound of closely related structure to the substance of interest is imprinted instead of the actual target itself. The resulting polymers have been shown to exhibit excellent recognition properties not only for the substitute template, but also for the actual target, largely depending on the similarity of the chosen template to the targeted structure.7–9 9-Ethyladenine is a typical example of such a template, used by Spivak et al. as an organic solvent soluble analogue of adenine in the study of nucleotide base imprinted polymers.10–12
In the present report, our aim was to develop molecularly imprinted polymers that will enable selective separation and quantitation of imiquimod (IMQ), a highly potent prescribed medication that acts as an immune response modifier. Imiquimod was approved by the FDA for treatment of various types of skin cancer, as well as genital warts, in 1997, and it is formulated as a 5% patient-applied cream under the trade name Aldara™ (3M Healthcare). Although imiquimod is applied topically, a systemic release of the drug is known to occur, with severe side-effects reported in some patient cases.13,14 Thus, a rapid and robust method for isolation and pre-concentration of imiquimod from biological fluids could be an invaluable tool in early detection of systemic release and curtailment of its impact on patient well-being.
Imiquimod is sparingly soluble in organic or aqueous solvents at neutral pH. This poses an immediate limitation to the development of an imprinting protocol, hence a template analogue strategy was originally chosen. However, based on the observation that the drug is more soluble in acidic environments, it was possible to reach the desirable concentration by slight modification of the pre-polymerisation solution composition and thus prepare an imiquimod imprinted polymer, whose performance was subsequently compared to the template analogue based material.
NMR spectra were obtained using a Jeol ECX 400 spectrometer (Tokyo, Japan). HPLC measurements were performed using an Agilent 1200 system equipped with a diode array detector. Surface area analysis was performed using a Micromeritics Gemini VI Nitrogen sorption analyser (Particular Sciences, Dublin, Ireland).
Staircase frontal chromatography was performed on 9IBA-MIP and 9IBA-NIP using solutions of 9IBA in CH3CN–1% CH3COOH as the mobile phase and mixing them in a step-wise fashion of 10% increments with the pure mobile phase to produce a staircase frontal chromatogram with a total of 30 steps in the concentration range 10−6 to 10−3 mol L−1. From each step the corresponding amount of bound analyte was calculated and the collected results were plotted against the corresponding concentration of 9IBA to produce binding isotherms that were fitted using the appropriate binding model.
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Scheme 1 Proposed primary (black) and secondary (grey) mode of interaction between the functional monomer (MAA) and 9IBA, and the chemical structure of imiquimod (IMQ). |
The system studied here is not very dissimilar to the ones reported previously however it was decided to calculate the binding constant between the specific compounds involved. Interestingly, our findings suggest a much stronger association between 9IBA and methacrylic acid, and a Ka = 1376 ± 122 M−1 was calculated when 9IBA's H2 was monitored (Fig. 1), almost an order of magnitude higher than the values previously reported. Job plot experiments verified a 1:
1 complex stoichiometry, warranting the use of a 1
:
1 binding model to fit the obtained experimental results (Fig. 1, inset).
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Fig. 1 1H NMR titration binding isotherm of 9IBA with MAA: experimental data (◇) and the corresponding fitted curve. Inset: Job plot for the 9IBA–MAA complex. |
This significantly higher Ka value can be attributed to the fact that methacrylic acid has different pKa value than the carboxylic acids used in previous studies and more importantly, to the concentration of host (9IBA) used in this study (1 mmol L−1) compared to 20–35 mmol L−1 used in previous studies. The lower concentration used here should eliminate the influence of self-association of both guest and/or host molecules occurring at higher concentrations, but also probe stronger complexation events, occurring at low concentrations. In any case, a Ka of this magnitude is in agreement with the findings of fundamental studies on MIPs, whereby exceptionally strong binding against adenine derivatives using MAA as the functional monomer has been reported.10,11
9IBA-MIP | 9IBA-NIP | IMQ-MIP | IMQ-NIP | |
---|---|---|---|---|
Surface area (m2 g−1) | 241.5 | 210.2 | 229.8 | 210.7 |
Pore volume (cm3 g−1) | 0.44 | 0.56 | 0.51 | 0.49 |
Pore diameter (Å) | 82.0 | 99.5 | 102.7 | 128.9 |
In agreement with the findings of Shea et al.,19 MAA-based MIPs against adenine derivatives exhibit extremely long retention times for their template when tested using the porogen as a mobile phase. Thus, when 9IBA was injected on the 9IBA-MIP column prepared here, no elution was observed after 1.5 hours of analysis using acetonitrile as a mobile phase. Similarly, IMQ, adenine and cytosine were also retained for >1.5 h while thymine, uracil and 5-FU all eluted at or near the solvent front on both columns (data not shown). The latter observation is attributed to the lack of complementarity between the imide functionality of the pyrimidine bases and the donor–acceptor hydrogen bond array of methacrylic acid.
In order to facilitate elution of the injected analytes from the imprinted and non-imprinted columns, 1% acetic acid was added to the mobile phase, aiming to disrupt hydrogen bonding of the analytes to the stationary phase and reduce elution times. Indeed, under the new analysis conditions 9IBA eluted in 28 minutes while IMQ was retained for 63 minutes (Table 2). An imprinting factor (IF = kMIP/kNIP) of 17 was calculated for IMQ, more than 2.5 times higher than the IF calculated for the template itself (IF = 6.4), supporting the design and choice of 9IBA as a template analogue. Adenine and cytosine did not elute from the imprinted column after 1.5 h however their retention times on the non-imprinted column were significantly reduced to 10 and 6.2 minutes respectively (Fig. 2). This observation clearly demonstrates the shape, size and functional group complementarity of the synthesised imprinted polymer: adenine, slightly smaller and lacking the non-functional bulky side chain of 9IBA, is tightly bound by the binding cavities while cytosine, much smaller but with a very similar functional group orientation to 9IBA, is also strongly bound. Thymine, uracil and 5-FU eluted again at or near the solvent front on both columns.
Analyte | t R (min) | k | IF | t R (min) | k | IF | ||||
---|---|---|---|---|---|---|---|---|---|---|
9IBA–MIP | 9IBA–NIP | 9IBA–MIP | 9IBA–NIP | IMQ–MIP | IMQ–NIP | IMQ–MIP | IMQ–NIP | |||
5-Fluorouracil | 1.6 | 1.0 | 1.3 | 0.4 | 3.1 | 1.3 | 1.1 | 0.8 | 0.4 | 1.9 |
Uracil | 3.8 | 1.3 | 4.3 | 0.8 | 5.5 | 3.9 | 1.3 | 4.2 | 0.7 | 5.8 |
Thymine | 4.3 | 1.4 | 5.1 | 0.9 | 5.9 | 2.7 | 1.7 | 2.7 | 1.2 | 2.1 |
9IBA | 28.0 | 5.1 | 38.0 | 5.9 | 6.4 | >90.0 | 10.1 | >125.0 | 12.2 | >9.8 |
Imiquimod | 63.0 | 4.5 | 87.0 | 5.1 | 17.0 | >90.0 | 16.2 | >125.0 | 20.2 | >5.9 |
Adenine | >90.0 | 10.0 | >125.0 | 13.0 | >9.7 | >90.0 | 19.7 | >125.0 | 24.8 | >4.8 |
Cytosine | >90.0 | 6.2 | >125.0 | 7.6 | >16.6 | >90.0 | 11.4 | >125.0 | 14.0 | >8.6 |
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Fig. 2 Retention times (tR, min) of imiquimod and related substances on (a) 9IBA and (b) IMQ imprinted polymer packed HPLC columns (50 mm × 4.6 mm i.d.). Conditions: mobile phase: CH3CN–1% CH3COOH, flow-rate: 1 mL min−1, inj. vol.: 5 μL of 1 mmol L−1 solution, DAD detection at 240 or 260 nm. |
HPLC evaluation of IMQ-MIP and IMQ-NIP revealed similar binding characteristics, albeit retention times were significantly longer. Thus, apart from adenine and cytosine, IMQ and 9IBA were also not eluted from the imprinted column after 1.5 h of analysis. Notably, retention times of IMQ-NIP were longer compared to those of 9IBA-NIP, possibly due to the higher amount of MAA used, contributing to an increase in non-specific binding. This led to an overall decrease in the corresponding calculated imprinting factors; nonetheless, these values have only qualitative interest as the exact retention time on the imprinted column could not be determined.
Overall, the two imprinted polymers exhibit similar behaviour towards the tested analytes with IMQ-MIP being potentially the better sorbent for the drug. This is attributed to the presence of the drug itself in the pre-polymerisation mixture, producing binding sites of superior complementarity to the targeted structure, but also to the higher ratio of MAA:
template, potentially resulting in more points of attachment for the drug, although the majority of them are of a non-specific nature, as demonstrated by the longer retention times exhibited by IMQ-NIP.
Frontal chromatography was performed using 9IBA as the test analyte on its corresponding imprinted and non-imprinted polymers; the quantity of the analyte required for the analysis and the price as well as toxicity of IMQ prohibited its use in such experiments. In spite of this, when a 9IBA rebinding experiment was attempted in the low concentration range using IMQ-MIP as the stationary phase, elution times of each step were excessive and did not permit accurate collection of experimental points.
The obtained results revealed the superior binding performance of 9IBA-MIP versus its non-imprinted counterpart (Fig. 3). In order to calculate the corresponding binding isotherms and binding capacities, each isotherm was fitted using the most common binding models used to model similar materials, namely the single binding site model (Langmuir), two-binding site model (Bi-Langmuir), the continuous distribution model (Freundlich)20 and the Langmuir–Freundlich model.21 The obtained results are reported in Table 3 together with the corresponding sums of least squares (R2) for each model, as a measure of fit quality.
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Fig. 3 Frontal chromatography binding isotherms of 9IBA on 9IBA-MIP (◆) and 9IBA–NIP (◇) packed HPLC columns and the corresponding curves fitted to the Bi-Langmuir model (dashed lines). |
Binding model | 9IBA–MIP | 9IBA–NIP | ||||
---|---|---|---|---|---|---|
K a (L mol−1) | N (μmol g−1) | R 2 | K a (L mol−1) | N (μmol g−1) | R 2 | |
a Average Ka and N values calculated from affinity distribution curves. b Average Ka calculated using Ka = a1/m. | ||||||
Langmuir | 6.0 × 103 | 90.8 | 0.9839 | 1.8 × 103 | 7.7 | 0.9998 |
Bi-Langmuir | 2.3 × 104 | 38.9 | 0.9996 | 8.5 × 105 | 0.04 | 0.9999 |
0.5 × 103 | 142.6 | 1.7 × 103 | 7.8 | |||
Freundlicha | 3.9 × 104 | 66.3 | 0.9919 | 2.0 × 104 | 3.1 | 0.9918 |
Langmuir–Freundlichb | 1.1 × 103 | 162.9 | 0.9969 | 1.7 × 103 | 8.0 | 0.9998 |
The Langmuir model provides a poor fit in the case of 9IBA-MIP, not unexpectedly, given the well documented heterogeneous character of imprinted polymers. The best fit is achieved when the Bi-Langmuir model is applied, assuming a strong and a weak family of binding sites with affinity constants of 2.3 × 104 L mol−1 and 0.5 × 103 L mol−1 and a calculated population of 38.9 μmol g−1 and 142.6 μmol g−1 respectively. The Freundlich and Langmuir–Freundlich models also offer good fittings of the experimental data, with the former yielding average parameters closer to the “stronger” sites predicted by the Bi-Langmuir model and the latter closer to the “weaker” sites. Although it is conceivable that there are more than just two types of binding sites in the matrix of the imprinted polymer, a combination of the above information provides an accurate representation of the affinity characteristics of the polymer.
In the case of 9IBA-NIP, the Langmuir and Langmuir–Freundlich models yield nearly identical values of average affinity constant and number of binding sites, while the same parameters are calculated for the “weak” sites using the Bi-Langmuir model, which constitute the majority of sites in the polymer. The Freundlich model does not offer as good a fit, and the parameters derived are not comparable with the other models. Overall, it appears that 9IBA-MIP possesses high energy and low energy sites, the latter being larger in population and with an affinity constant comparable to the low energy sites of the non-imprinted counterpart. This indicates that the low energy, or non-specific, binding sites are of similar nature in both polymers.
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Fig. 4 MI-SPE recoveries (%) of imiquimod on (a) 9IBA and (b) IMQ imprinted polymers and the corresponding imprinting factors (dashed lines). Conditions: loading: 1 mL of sample; 1st wash: 1 mL H2O, 2nd wash: 1 mL CH3CN–1% CH3COOH; elution: 1 mL CH3OH–5% CH3COOH. Blood and urine samples spiked with 10 μg mL−1 imiquimod. |
Both imprinted polymers achieved high recoveries of the drug from urine and blood serum samples, albeit IMQ-MIP was more consistent across the different sample matrices and average recoveries of up to 95% were reached. However, similar to the observations during chromatographic evaluation, a significant amount of non-specifically bound IMQ, up to 64.7%, was recovered from IMQ-NIP, resulting in average imprinting factors of 1.6. On the other hand, 9IBA-NIP exhibited substantially lower non-specific binding, less than half of what was measured on IMQ-NIP in some cases, thus imprinting factors for 9IBA polymers were as high as 4.3 in H2O and 3.3 in urine. It is noteworthy that both polymers exhibited their lowest recoveries and highest imprinting factors in the aqueous samples while the opposite was observed as sample complexity increased. The nature and composition of biological samples appear to “push” the hydrophobic drug onto the polymer stationary phases, resulting in higher recoveries in blood serum samples, albeit at the expense of selectivity. Nonetheless, both materials produced clean extracts as seen in Fig. 5, and no interference to the quantitation of IMQ was detected.
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Fig. 5 Typical HPLC chromatograms of the different fractions collected by MI-SPE of blood serum samples using (a) 9IBA-MIP and (b) IMQ-MIP. From the bottom, 1st wash (H2O), 2nd wash (CH3CN–1% CH3COOH), eluted sample (CH3OH–5% CH3COOH). |
Thus, it is concluded from this study that 9IBA imprinted polymers exhibit higher selectivity for IMQ, while possessing a smaller number of binding sites, resulting in reduced capacity. The opposite is true for IMQ-MIPs that exhibit marginally higher capacity but appear to be less specific towards the drug.
Based on the above observations on polymer performance, and considering the clear advantages of 9IBA compared to imiquimod in terms of solubility, availability, toxicity and cost, as well as the extensive polymer pre-conditioning required to eliminate template bleeding, 9IBA is proposed as an excellent template analogue and its use is fully justified by the experimental results acquired in this study.
This journal is © The Royal Society of Chemistry 2013 |