Influence of molecular weight on kinetics release of metronidazole from proline-based polymers prepared by RAFT polymerization

Abstract

In the current study, the influence of polymer molecular weight on the release of metronidazole (MTZ) as a drug model from synthesized proline-based polymers was studied. Using reversible-addition fragmentation chain transfer (RAFT) polymerization, amino acid-based polymers based on N-acryloyl-L-proline (A-L-Pro-OH) with various molecular weights were prepared with relatively high conversions (55–98%, as measured by ¹H NMR spectroscopy). The polymerization process was done at different ratios of [monomer]₀/[CTA]₀/[AIBN]₀. Proline-based polymers with pre-determined molecular weights (M_n = 6000–22 600 in methylated forms) and low polydispersities (M_w/M_n = 1.30–1.37) were utilized for the immobilization of metronidazole. The kinetics release studies of the polymer-MTZ adducts were done at various pH values: 2.0, 7.4, and 8.5 phosphate buffer solutions. The obtained results proved that the hydrolytic behaviors of polymeric prodrugs robustly relied on the molecular weight of the polymer and the pH of the release media. The kinetics of the release process were described using Higuchi and Korsmeyer's models to explain the drug release mechanism.

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Introduction

Nowadays there is an imperative need to develop new technologies for drugs and protein delivery, which represents one of the main challenges of modern biotechnology. Moreover, bioavailability and drug targeting to specific sites can be improved by the development of polymeric prodrugs.^1,2 In frequent polymeric prodrugs, pharmaceuticals are covalently bonded to polymers which act as carriers.³ In addition, the use of polymers as controlled-release systems is one of the most attractive methods for providing active macromolecules with properties of long-term and persistent delivery to living tissue.^4,5 For instance, controlled-release polymers have been used to release macromolecules to the systemic circulation system and to target to certain tissues (e.g. brain or into mucus secretions).^4,6–10

Amino acid-based polymers, in which amino acid moieties may be into the main chain or in the side chain, have been the subject of continuing interest in biomimetic polymer research and various bio-related applications.^11–14 Preparation of polymers having amino acid moieties has been carried out using conventional free radical and controlled radical polymerizations of vinyl monomers containing amino acid residues.¹⁴

Amino acid-based polymers as well as synthetic polypeptides are characterized by their prospect to create highly ordered hierarchical structures via noncovalent forces, like hydrogen bonding, characteristic chain chirality, conformation, and amphiphilicity, as well as their prospect applications as biomedical and biodegradable polymers.^11,13–15

5-Nitroimidazoles, such as metronidazole (MTZ), are usually used as anti-amoebic, antiprotozoal and antibacterial drugs. The discovery of antibacterial and antitrichomonal properties of antibiotic azomycin led to the examination of nitroimidazoles as anti-parasitic agents.^16–18 The parasite reaches preferentially the colon and causes haemorrhage and ulceration. Therefore, metronidazole must be targeted and released in the colon to assure its effective activity versus the parasite but, after oral administration, it is completely and promptly absorbed.

Mathematical modelling is playing a serious role in explaining the mechanism of drug release. Since it facilitate the development of new drug delivery systems in a methodical way instead of trial and inaccuracy methods.¹⁹ The mechanism of drug release from a polymer matrix can be assorted to three systems, relying on physical or chemical properties of the polymer,²⁰ including diffusion, swelling or erosion controlled. In a swelling controlled system, the release of drug is controlled by diffusion of drugs from polymeric matrix as well as by disentanglement of polymeric chains within the matrix resulting in dissolution (chain relaxation) of the polymer. The “anomalous transport” of drug release is often present in swelling controlled systems since both diffusion and chain relaxation can be occurred together.¹⁹ Since only the diffusion contribution was considered, the most widely models for predicting the kinetics of drug release are Higuchi and the power law models.^21,22

Recently, we reported the development of new drug delivery system using alanine and phenylalanine-based polymers. Alanine and phenylalanine-based monomers with different hydrophilicity and chirality as new drug delivery system was prepared by reversible addition-fragmentation chain transfer (RAFT) technique.¹³ The obtained results indicated that the release rate of drug and the behaviour of polymeric prodrugs are highly dependent on the configuration of amino acid-based polymer-MTZ adducts as well as the release media's pH.

Herein, we report the use of RAFT polymerization for the preparation of proline-based polymers with pre-determined molecular weights and well-defined structures as new controlled drug-release system (Scheme 1). Proline was selected as a starting amino acid unit, due to its biological relevance as a major constituent of collagen as a common animal protein and a major fibrous element of bone, skin, cartilage, teeth and tendon. Some research groups have reported the development of L-proline derivatives, polymeric materials, as well as their bio-related phenomena and applications.¹¹ The main goals of the present study are to: (i) prepare well-defined proline-based polymer-MTZ adducts by RAFT polymerization of N-acryloyl-L-proline (A-L-Pro-OH), followed by immobilization of drug; (ii) investigate the influence of polymer molecular weight on the drug release from the proline-based polymeric prodrugs at different pHs; (iii) understand the release mechanism controlling the kinetics of drug release from systems prepared with metronidazole as drug model and proline-based polymers as matrix-forming materials.

Scheme 1 RAFT polymerization of N-acryloyl-L-proline (A-L-Pro-OH) and immobilization of MTZ onto poly(A-L-Pro-OH).

Experimental

Materials

Acryloyl chloride, 4-(dimethylamino) pyridine (DMAP) and N,N′-dicyclohexylcarbodiimide (DCC) (99%) were provided by Aldrich. 2,2′-Azobis(isobutyronitrile) (AIBN, 97%, recrystallized from methanol) and N,N-dimethylformamide (99.5%, dehydrated DMF) were purchased from Kanto Chemical, and methanol (99.8%) was provided by Wako Pure Chemical.

Metronidazole (MTZ) was provided by TCI, Japan. L-Proline (L-Pro-OH) was purchased from ACROS.

Benzyl 1-pyrrolecarbodithioate as a chain transfer agent (CTA) was synthesized and purified as a yellow oil as reported.²³ N-Acryloyl-L-proline (A-L-Pro-OH) (I) was synthesized by reaction of L-Pro-OH with acryloyl chloride according to the reported procedure previously (Scheme S1, ESI†).¹¹

Characterization

¹H NMR (400 MHz) spectra were measured on a JEOL JNM-ECX400. FT-IR spectra were recorded on a JASCO FT/IR-210 spectrometer. Number-average molecular weight (M_n) and molecular weight distribution (M_w/M_n) were determined by Size Exclusion Chromatography (SEC) on a Tosoh HPLC-8220 system equipped with refractive index and ultraviolet detectors at 40 °C using polystyrene as standard. Elemental microanalysis was recorded on Perkin-Elmer 2400-II CHNS/O analyzer. UV spectra were measured on Perkin-Elmer UV/vis spectrophotometer (Lambda-35). The morphology of adducts was examined using Scanning Electron Microscopy (SEM) (JEOL GSM-6610LV).

Synthesis of proline-based polymers

Poly(A-L-Pro-OH) (II_a–c) with different molecular weights were prepared according to the procedure reported previously.¹¹ Typically, in a dry glass ampule equipped with magnet bar, A-L-Pro-OH (I, 8.87 mmol), benzyl 1-pyrrolecarbodithioate (CTA, 0.04–0.18 mmol), AIBN (0.01–0.04 mmol) dissolved in dehydrated methanol (6 mL) and were placed. The solution was degassed by two freeze–evacuate–thaw cycles, flame-sealed off under vacuum, and was stirred at 60 °C for 24 h. The characteristic pale red colour maintained during the polymerization and the reaction was stopped by rapid cooling with liquid nitrogen.

Synthesis of poly(A-L-Pro-OMTZ)s adducts (III_a–c)

Poly(N-acryloyl-L-proline-2-(2-methyl-5-nitro-imidazol-1-yl)-ethylester)s, poly(A-L-Pro-OMTZ)s (III_a–c), were prepared according to the following general procedure.^2,24 At −5 °C, solution of DCC (0.98 g, 4.73 mmol) in DMF (5 mL) was added dropwise to a solution of poly(A-L-Pro-OH) (II_a–c) (0.8 g, 4.73 mmol) and DMAP (57.77 mg, 0.47 mmol) in DMF (10 mL). After keeping the mixture at −5 °C for 0.3 h, metronidazole solution (0.81 g, 4.73 mmol) in DMF (10 mL) was added dropwise within 0.5 h. Stirring of the reaction mixture at −5 °C was continued for 3 h and gradually backed to room temperature and continued stirring for more 20 h. The formed dicyclohexylurea precipitate was taken away by filtration and filtrate was concentrated on rotavapor at 65 °C to 2 mL. The product was obtained by precipitation in water (100 mL, was added dropwisely). The product was washed with water and was dried in oven under vacuum at 55 °C overnight to give (1.51 g, 99%) of poly(A-Pro-OMTZ). The product is water insoluble and soluble in DMF, DMSO and chloroform.

¹H NMR (400 MHz, CDCl₃) (Fig. 1): δ 7.9 (1H, –N–CH=C–, imidazole), 4.7–3.2 (7H, –OCH₂CH₂N–, CH₂NCHCOO), 2.5 (3H, –CH₃), 0.9–2.4 (7H, –CH₂CH<, –NCH₂CH₂CH₂–) ppm.

Fig. 1 ¹H NMR (400 MHz, CDCl₃) spectrum of poly(A-L-Pro-OMTZ).

IR (KBr) (Fig. 2): 3327 (N–H), 2930 (C–H, aliphatic), 2852 (–CH₃), 1629 (–C=C–N–), 1748 (C=O, ester), 1702 (C=O, amide), 1531 (–NO₂), 1450 (C–C), 1364–1189 (C–N), 1089 (C–O) cm⁻¹.

Fig. 2 FT-IR spectrum (KBr) of poly(A-L-Pro-OMTZ).

Determination of the total MTZ content in adducts (III_a–c)

The total metronidazole content immobilized onto polymers (III_a–c) was determined according to the described method in our previous work¹³ as following; by suspension of a known weight of the polymer-MTZ adducts (III_a–c) (2–5 mg) in sodium hydroxide solution (10 mL) of pH 9.0 at 60 °C for 24 h. The amount of metronidazole released was determined by measuring the absorbance at λ_max = 311 nm using UV/vis spectrophotometer.

In Vitro drug release study

Kinetics study of metronidazole release were done at 37 ± 0.2 °C in 0.05 M phosphate buffer solution (pH = 8.5, 7.4, and 2.0). The procedure used as following: a number of bottles containing 3 mL phosphate buffer solution were put in a water shaker thermostat at 37 ± 0.2 °C to attain the desired temperature. Then, 3–5 mg of the polymer-MTZ adduct was added to each bottle and zero time was recorded. At regular time intervals, the extract concentrations of metronidazole were analysed using UV-vis by recording the absorbance at λ_max = 311 nm. The accumulative percentage of drug released was enumerated, using the standard plots of absorbance versus concentration of metronidazole. Release experiments were done in triplicate.

Kinetics of drug release

Data were analysed using Higuchi²⁵ and Korsmeyer et al.²⁶ (eqn (1) and (2), respectively) after completing the in vitro dissolution of all the batches for 48 h.

M_t/M_∞ = kt^1/2 (1)

M_t/M_∞ = k′tⁿ (2)

In these equations, M_t; the accumulative amount of drug released at time (t), M_∞; the amount of drug release after infinite time. Kinetic constants, k and k′, are linked to the characteristics of the polymer-drug system, and the diffusional exponent or release exponent, n, gives an indication about the mechanism of drug release as well as the shape of the tested matrix.²² For a spherical sample, when n is below 0.43, the release assigned by Fickian diffusion mechanism. If n in range of 0.43–0.85, the transport is an anomalous (non-Fickian diffusion), and often termed as first-order release. When n value ≥0.85, the release has been described by case II and super case II transport. This indicates that the drug release rate remains constant all the time and the release is characterized by zero-order. In this case, the drug release is controlled by the erosion and swelling of the polymer.^27,28

Results and discussion

Polymers as controlled-release systems are an appealing method for preparing long term, and continuous delivery of drugs and active macromolecules. The development of new systems for drugs delivery still one of the major challenges. In the current work, we focused on proline-based polymers with characteristic hydrophilicity (Scheme 1). First, proline-based polymers with different molecular weights were prepared by RAFT polymerization to control the synthesis of amino acid-based polymers with pre-determined molar mass, narrow molecular weight distribution and well-defined structure.^{11,13,14,23,29} Then, metronidazole was immobilized as a model drug onto the proline-based polymers via esterification reaction using DCC as condensation agent in addition to DMAP as a catalyst. In the product, the proline moiety is acted as a spacer between the polymer main chain and the drug model, and the chemical structure of the spacer may have a significant effect on the release profile, in addition to the length of the polymer main chain. Finally, in vitro drug release study was carried out to investigate the influence of the polymer molecular weight on the release rate.

Synthesis of proline-based polymers

Acrylamide containing L-proline, N-acryloyl-L-proline (A-L-Pro-OH), was polymerized by RAFT process to afford a well-defined carboxylic acid-containing polymer, which can be also regarded as a weak polyelectrolyte, in which the degree of ionization was controlled by the pH and ionic strength of aqueous solution (Scheme 1). The amino acid-based monomer, A-L-Pro-OH, was synthesized by reaction of acryloyl chloride with L-proline according to the procedure reported previously with a little modification.¹¹ Controlled polymerization of amino acid-based monomer with carboxylic acid moiety is helpful for the preparation of tailored functional polymers for several applications, because of the usage of the carboxylic acid groups as reactive sites for the incorporation of functional molecules by the reaction with various compounds. In this work, carboxylic acid group in proline-based polymer was employed as sites to link the bio-related material, metronidazole.

Well-defined polymer was prepared by RAFT polymerization of carboxylic acid-containing monomer, A-L-Pro-OH without protecting the carboxylic group and by using benzyl 1-pyrrolecarbodithioate as chain transfer agent (CTA) to achieve controlled RAFT polymerization of the acrylamide containing L-proline (A-L-Pro-OH).¹¹ The solubility of poly(A-L-Pro-OH) and its degree of ionization depend on the pH of water, in which it was soluble in alkaline water (pH = 10) however it was insoluble in neutral water (pH ≈ 7) as well as in acidic water (pH = 1). The structures of the amino acid-containing monomer and resulting proline-based polymers (I–III_a–c) were confirmed by ¹H NMR, FT-IR spectra (Fig. S1 and S2, ESI†) as well as elemental microanalysis.

So as to clarify the impact of molecular weight on the drug release from the proline-based polymeric prodrugs at different pH's, poly(A-L-Pro-OH)s with different molar mass were synthesized by RAFT polymerization. For this purpose, the polymerization of A-L-Pro-OH was carried out at different ratios of [M]₀/[CTA]₀ (between 50 and 200), while the molar ratio of [CTA]₀/[AIBN]₀ was held constant at 5. Under these conditions, the determined conversions by ¹H NMR were found to be 55–98%, as shown in (Table 1). For SEC measurements, methyl ester of poly(A-L-Pro-OH) was prepared by treating the carboxylic acid groups using trimethylsilyldiazomethane.¹¹ The methylated polymers, poly(N-acryloyl-L-proline methyl ester)s, showed symmetrical unimodal SEC peaks (RI detector) with relatively narrow molecular weight distributions (M_w/M_n = 1.30–1.37, see Fig. S3–S5, ESI†). Proline-based polymers holding pre-determined molecular weights (M_n = 6000–22 600 in the methylated form, which correspond to M_n = 5500–20 900 in the carboxylic acid form) and low polydispersities were employed for the immobilization of metronidazole.

Table 1 Summary for polymerization conditions of A-L-Pro-OH using CTA at 60 °C for 24 h^a

Polymer code	[M]0/[CTA]0	Conv.^b %	Mn^c (SEC)	[Mw/Mn]^c (SEC)
a [CTA]0/[AIBN]0 = 5.b Calculated by ^1H NMR in CD3OD.c Methylated samples, poly(N-acryloyl-L-proline methyl ester), were determined by SEC (RI detector) and using polystyrene as standards in (DMF, 10 mM LiBr).
IIa	50	55	6000	1.30
IIb	100	98	13 300	1.34
IIc	200	98	22 600	1.37

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Synthesis of polymers (III_a–c) adducts

The proline-based polymers (II_a–c) were reacted with metronidazole as drug model at −5 °C by using DCC as condensation agent in the presence of DMAP as a catalyst. The polymer-MTZ adducts (III_a–c) were formed via the esterification reaction. Racemization and formation of N-acylurea were avoided by carrying out the reaction under cooling conditions^2,30 as shown in Scheme 1. After stirring the reaction overnight at room temperature, the formed dicyclohexylurea was taken away by filtration. Results of immobilization of metronidazole onto proline-based polymers (II_a–c) are summarized in Table 2. The yields of the products, which correspond to the water-insoluble parts, were 92–99% in all cases.

Table 2 Results for immobilized metronidazole onto proline-based polymers (II_a–c)

Polymer code	Yield^a	Conv.^b (%)	C (%)		H (%)		N (%)
			Calc.	Found	Calc.	Found	Calc.	Found
a Water-insoluble part.b Conversions determined by eqn (3).c Calculated values based on the drug loading (92%, 77%, and 64% for IIIa–c, respectively) determined from ^1H NMR measurements.d Calculated values based on 100% of the drug loading.
IIIa	99	92	52.37^c	58.30	5.67^c	7.42	16.99^c	14.17
			52.12^d		5.58^d		17.37^d
IIIb	92	77	52.80^c	58.59	5.75^c	7.43	16.15^c	14.68
			52.12^d		5.58^d		17.37^d
IIIc	95	64	53.22^c	53.99	5.84^c	6.73	15.31^c	13.81
			52.12^d		5.58^d		17.37^d

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The structure of the formed adducts (III_a–c) was confirmed by ¹H NMR, IR spectra as well as elemental microanalysis. The total drug loading (%) was calculated using the ¹H NMR spectrum, in which the integration of the methyl group (3H, –CH₃) resonance around 2.5 ppm was compared with the peak intensity for the sum of (1H, –N–C–H) of polymer and polymer-MTZ adduct around 4.1 ppm (eqn (3)).

where X is the fraction of drug loading.

From ¹H NMR, the drug loading was found to be 92%, 77%, and 64% for (III_a–c), respectively. This decrease in drug loading with increasing the molecular weight can be attributed to the higher polymer chain entanglements which hinder the immobilization of the drug due to steric hindrance effect. Based on the values of the drug loading (92%, 77%, and 64% for III_a–c, respectively) determined from ¹H NMR, the elemental microanalyses data for the adduct (III_a) is found/calculated (%), C: 58.30/52.37; H: 7.42/5.67; N: 16.99/14.17. Relying on nitrogen analysis, the conversion was found to be 83.4%, which indicated that polymer (III_a) had 83.4% in form of the polymer-MTZ adduct (III_a) conjugate and 16.6% in form of poly(A-L-Pro-OH) (II_a). In a similar way, the adducts (III_b and III_c) contained 90.9% and 90.2% in the form of the polymer-MTZ adducts, respectively.

Fig. 3 presents the SEM images of three polymer-MTZ adducts (III_a–c). It was predicted from this figure that the surface of the three adducts are highly porous. The porosity of the materials is significant in natural phenomena as well as practical applications such as adsorption, diffusion and dissolution of drugs.³⁰ During the release, it was written that the drug molecule exist inside a pore will reach outside towards one of the end-points of the pore via naturally diffusion.³¹ However, the drug molecule will diffuse towards the nearest pore to be release, if it is present within the micropores network. The motion of such molecules in the micropore network is very finite due to the available fixed space.³¹ As we can see in Fig. 3, both the diameter of the pores and the distribution of the pore size increase with increasing the polymer molecular weight from III_a to III_c. This leads to heterogeneity of the surface where the surface roughness will be increased upon increasing the molecular weight of the polymer³² which should be restricted the drug diffusion within the pore network.

Fig. 3 SEM micrographs of polymer-MTZ adducts (III_a–c).

Determination of the total MTZ content in adducts (III_a–c)

The total content of metronidazole immobilized onto polymer adducts (III_a–c) were determined by alkaline hydrolysis (pH 9.0) of a known amount of the polymer-MTZ adducts at 60 °C. The concentration of the released drug was established by measuring the absorption at λ_max = 311 nm until reached a constant value. The hydrolysis studies showed that the total content of metronidazole was found to be 15.0, 17.9 and 38.7 mg of MTZ/g of polymer for polymer (III_a–c), respectively.

In vitro drug release investigation

Release kinetics of metronidazole from polymer-MTZ adducts with different molecular weights were carried out. The influence of the media pH on release rate was also studied for each adduct. The molecular weight-dependence of polymer-MTZ adducts on metronidazole release rate in alkaline pH of 8.5 was illustrated in Fig. 4a as cumulative percent release versus time. Release studies were done over 48 h. By increasing the molecular weight of polymer-MTZ adducts from III_a to III_c, the release rate of metronidazole decreased. These results agree with the other studies on the effect of molecular weight on drug release.^31,33,34 The adduct (III_a) having relatively low molecular weight, showed 88% of its drug content after 48 h at 37 °C. The adduct (III_b) having medium molecular weight released 80% of its drug content, whereas the adduct (III_c) with high molecular weight showed release of 71% of its total drug content under the same conditions. Generally, initial burst release was observed for all release studies followed by a slow release. This means that, the control of drug release rate is affected by molecular weights of the used polymer matrix for drug delivery. This decrease in drug release with increasing the molecular weight can be attributed to the fact that the increase in the molecular weight of polymer led to higher chain entanglements which slow down the diffusion of drug molecules through the matrix of the polymer.³⁵ Therefore, slow release at higher molecular weight can be interpreted in terms of the drug getting trapped in the more highly entangled chains of the polymer matrix.

Fig. 4 Influence of polymer molecular weight on the release behaviour of metronidazole from adducts (III_a–c) at 37 °C in phosphate buffer of (a) pH 8.5 and (b) pH 7.4 (over 48 h) and (c) within the first two hours at pH 2.0.

To facilitate comparison between different matrix compositions, the rate of release was analysed using Higuchi,²⁵ Korsmeyer et al.²⁶ equations and the values of main parameter are summarized in Table 3. Generally, the three matrices have good fitness with the two equations. Release rate of drug was calculated from the slope values of the straight line of the Higuchi plot. The values of release rate, k′ showed that the polymer matrices with low molecular weight (III_a) has the highest value of k′ and this value decreases with increasing the polymer molecular weight (Table 3). Release exponent (n) value of polymer (III_a) was 0.86, indicating that the release pattern of the drug characterized by case-II (zero-order). For polymer (III_b), the value of (n) was 0.47, suggesting that the release pattern of the drug was anomalous (non-Fickian). However, the drug release from the highest molecular weight polymer (III_c) is govern by Fickian diffusion (n = 0.43), which is due to the pores in the surface of the polymer-adduct as shown in Fig. 3.

Table 3 Results of kinetics parameters characterized the release behaviour of metronidazole from adducts (III_a–c) at pH 8.5

Polymer code	Higuchi equation		Korsmeyer equation
	K (min^−0.5)	R^2	n	K′ (min^−0.5)	R^2
IIIa	0.010	0.988	0.863	0.030	0.972
IIIb	0.018	0.999	0.474	0.022	0.997
IIIc	0.021	0.934	0.432	0.012	0.970

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In vitro release investigations were done at 37 °C and different pH values, namely at pH of 2.0 (stomach pH), 7.4 (physiological fluids pH), and 8.5 (colon pH) as shown in Fig. 4 and 5. The percentage of the accumulative release of metronidazole from adduct (III_a) with different pH values was shown in Fig. 4c and 5a. At pH 2.0, the released amount of metronidazole from adduct (III_a) is about 25% of its drug content after 48 h, whereas about 61% and 88% of metronidazole were released at pH's 7.4 and 8.5, respectively. The quantity of released metronidazole from adduct (III_b) was 23% of its total drug content after 48 h in a buffer solution of pH 2.0, and significantly higher (37% and 80%) at pH's 7.4 and 8.5, respectively (Fig. 4c and 5b). The release rates of metronidazole from adduct (III_c) after 48 h was higher at higher pH. At pH 8.5, 71% of its drug content was released and by contrast, only 34% and 21% were released at pH's 2.0 and 7.4, respectively (Fig. 4c and 5c). These results demonstrated that the percentages of accumulative release of metronidazole from adducts was increased as the pH of the medium increased independent on the molecular weight of the polymer. As well as, the release rate of metronidazole from adducts (III_a–c) was greatest in alkaline medium (i.e. colon pH) compared to acidic medium (i.e. stomach pH) which showed high stability.

Fig. 5 Influence of phosphate buffer solution of pH's 7.4 and 8.5 on the release behaviour of metronidazole (over 48 h) from adducts (a) III_a, (b) III_b, and (c) III_c at 37 °C.

Conclusions

RAFT polymerization was used to prepare proline-based polymers with various molecular weights. Metronidazole as a drug model was immobilized onto the polymer backbone via proline moiety as a spacer. Release rate is mainly influenced by the molecular weight of prepared polymers in addition to the release media pH. SEM images of three polymer-MTZ adducts (III_a–c) showed that the surface of the three adducts are highly porous.

The release studies showed that high molecular weight product was released slower than the polymer-MTZ adducts having lower molecular weights. This decrease in drug release rate with increasing the molecular weight can be attributed to the higher polymer chain entanglements which slow down the drug molecules diffusion from the polymer matrix due to the trapping of drug into the more highly entangled matrix of polymer chains.

Moreover, the release rate increased by increasing the pH from acidic medium (stomach pH) to alkaline medium (colon pH) which reflects the efficiency of using this system in drug targeting to colon.

Acknowledgements

Dr M. EL-Newehy (ID: 10430, ParOwn Grant # 0907 Cycle, www.mhesr-initiatives.org) thanks the Egyptian Ministry of Higher Education and Scientific Research (MHESR) for funding his stay in Japan.

Notes and references

1. A. Y. A. Alfaifi, M. H. El-Newehy, E. S. Abdel-Halim and S. S. Al-Deyab, Carbohydr. Polym., 2014, 106, 440.
2. E.-R. Kenawy, M. H. El-Newehy, F. I. Abdel-Hay and R. M. Ottenbrite, Biomacromolecules, 2007, 8, 196.
3. C. W. Lee, Macromol. Res., 2004, 12, 63.
4. W. M. Saltzman, N. F. Sheppard Jr, M. A. Mchuch, R. B. Dause, J. A. Pratt and A. M. Dodrill, J. Appl. Polym. Sci., 1993, 48, 1493.
5. R. Langer, Science, 1990, 249, 1527.
6. E. Mathiowitz, W. M. Saltzman, A. Domb, P. Dor and R. Langer, J. Appl. Polym. Sci., 1988, 35, 755.
7. L. Brown, L. Siemer, C. Munoz, E. Edelman and R. Langer, Diabetes, 1986, 35, 692.
8. D. Hoffman, L. Wahlberg and P. Aebischer, Exp. Neurol., 1990, 110, 39.
9. W. Dang and W. M. Saltzman, Biotechnol. Prog., 1992, 8, 527.
10. M. L. Radomsky, K. J. Whaley, R. A. Cone and W. M. Saltzman, Biol. Reprod., 1992, 47, 133.
11. H. Mori, I. Kato, M. Matsuyama and T. Endo, Macromolecules, 2008, 41, 5604.
12. F. Sanda and T. Endo, Macromol. Rapid Commun., 1999, 200, 2651.
13. M. H. El-Newehy, A. S. Elsherbiny and H. Mori, J. Appl. Polym. Sci., 2013, 127, 4918.
14. H. Mori and T. Endo, Macromol. Rapid Commun., 2012, 33, 1090.
15. A. K. A. Al-Lami, A. A. Ibadi and H. H. Radey, Chem. Mater. Res., 2015, 7, 30.
16. Au. Rehman, A. S. Ijaz and A. Raza, J. Iran. Chem. Soc., 2005, 2, 197.
17. M. E. Wolff, Burgers medicinal chemistry, John Wiley & Sons, New York, 4th edn, 1979, p. 428.
18. C. Cosar and L. Julon, Ann. Inst. Pasteur, 1959, 96, 238.
19. D. Y. Arifin, L. Y. Lee and C. H. Wang, Adv. Drug Delivery Rev., 2006, 58, 1274.
20. K. W. Leong and R. Langer, Adv. Drug Delivery Rev., 1987, 1, 199.
21. T. Higuchi, J. Pharm. Sci., 1963, 52, 1145.
22. P. L. Ritger and N. A. Peppas, J. Controlled Release, 1987, 5, 23.
23. H. Mori, K. Sutoh and T. Endo, Macromolecules, 2005, 38, 9055.
24. C. H. Chang, Y. M. Sheu, W. P. Hu, L. F. Wang and J. S. Chen, J. Polym. Sci., Part A: Polym. Chem., 1998, 36, 1481.
25. T. Higuchi, J. Pharm. Sci., 1961, 50, 874.
26. R. W. Korsmeyer, R. Gurny, E. Doelker, P. Buri and N. A. Peppas, Int. J. Pharm., 1983, 15, 25.
27. N. A. Peppas, Pharm. Acta Helv., 1985, 60, 100.
28. H. Mori, M. Matsuyama, K. Sutoh and T. Endo, Macromolecules, 2006, 39, 4351.
29. J. Roman and H. Lucyna, Polym. Bull., 2010, 64, 459.
30. K. A. Mehta, M. S. Kislalioglu, W. Phuapradit, A. W. Malick and N. H. Shah, J. Controlled Release, 2000, 63, 201.
31. V. Lemaire, J. Belair and P. Hildgen, Int. J. Pharm., 2003, 258, 95.
32. K. Kijung, K. Yeoju, K. Na Re and C. Soonja, Polymer, 2011, 52, 439.
33. G. C. Ramkissoon, F. Liu, M. Bandys and S. W. Kim, J. Biomed. Mater. Res., Part A, 1999, 10, 1149.
34. T. T. P. Hsu and R. Langer, J. Biomed. Mater. Res., Part A, 1985, 19, 445.
35. W. M. Saltzman, N. F. Sheppard, M. A. McHug, R. B. Dause, J. A. Partt and A. M. Dodrill, J. Appl. Polym. Sci., 1993, 48, 1493.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14307e

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