Grafting Amino Drugs to Poly ( styrene-alt-maleic Anhydride ) as a Potential Method for Drug Release

Sistemas de liberação de fármacos baseados em conjugados polímero-fármaco fornecem um tratamento melhorado com menor toxidade ou efeitos colaterais e são usados no tratamento de diferentes doenças. Conjugados de poli(estireno-alt-anidrido maleico) (PSMA) biodegradável, com agentes terapêuticos tais como cloridrato de amantadina, amlopidina, gabapentina, zonisamida e mesalamina, foram gerados pela formação de ligações de amidas dos fármacos amino que reagiram com os grupos anidridos do PSMA. As quantidades de fármacos covalentemente conjugados foram determinadas por ressonância magnética nuclear (NMR) de H, e a taxa de liberação in vitro em solução tampão (pH 1.3) foi estudada à temperatura do corpo (37 C). Em estudos de cinética, modelos de dissolução diferentes foram examinados a fim de se obter dados de liberação do fármaco e os dados coletados se ajustaram bem a equação de Korsmeyer-Peppas, revelando um mecanismo de difusão Fickaniano dominante na liberação do fármaco em condições in vitro.


Introduction
Nowadays, in order to improve the curative effect of drugs, drug delivery systems are studied and developed.Two general procedures are used to supply the polymers containing pendant bioactive substituents.One method consists of the polymerization of drug containing monomers. 1 In this method, bioactive compounds, which have reactive functional groups, such as carboxyl, [2][3][4][5][6] hydroxyl, [7][8][9][10][11] sulfhydryl [12][13][14] and amino, [15][16][17][18][19] can be converted to their derivatives with the capability of polymerization, and then the drug containing monomers becomes polymerized by currently polymerization methods.Another one uses drugs attached to the prefabricated polymers. 20In these systems, drugs or their derivatives are chemically linked to the synthesized 21,22 or natural 23,24 polymeric chains by chemical reaction.
Poly(styrene-alt-maleic anhydride) (PSMA) is a commercially available copolymer, including reactive anhydride groups.][27] PSMA copolymers were applied to clinically deliver neocarzinostatine, [28][29][30][31] which is an antitumor protein.Henry et al. 31 showed that showed that alkyl amine derivatives of PSMA are capable of destabilizing biological membranes at acidic pH values and showed how this activity can be modulated for use in intracellular drug delivery applications.
In this work, our group reported novel types of polymer-drug conjugates based on PSMA and amino medicinal compounds, such as amantadine (antiviral and anti-parkinson), [32][33][34] amlodipine (anti-hypertensive), 35,36 gabapentin (treatment of epilepsy), [37][38][39] zonisamide (anti-convulsant) 40,41 and mesalamine (anti-inflammatory). 42 Amino drugs containing -NH 2 were linked up with PSMA by amide bond formation, and then, hydrolysis reactions were carried out in buffer solution (pH 1.3) at 37 °C, as the human gastric simulated conditions.Drug release kinetics and release mechanisms are afforded by acquiring experimental data in different ranges of times.

Materials and apparatus
Styrene, maleic anhydride, methanol, hydrogen chloride, benzoyl peroxide, potassium chloride and organic solvents were purchased from Merck Chemicals branch in Iran, and used without further purification.Dimethylformamide (DMF) was dried over MgSO 4 for 72 h, and then distilled once, and tetrahydrofuran (THF) was refluxed on Na/benzophenone for 8 h and distilled twice before use.Amantadine, gabapentin, amlodipine, zonisamide and mesalamine were received from Alborz-Daro Antibiotic Manufacturer, Iran.Buffer solution (pH 1.3) was prepared by addition of KCl aqueous solution (25 mL, 0.2 mol L -1 ) to HCl (33.6 mL, 0.2 mol L -1 ) in a volumetric flask.
All the UV-Vis spectrophotometric data were recorded by using a Shimadzu UV-265 spectrophotometer.A Shimadzu 435-U-04 FTIR spectrophotometer was used to obtain the FTIR (Fourier infrared spectroscopy) spectra of powder pressed KBr pellet samples. 1 H nuclear magnetic resonance (NMR) spectra were performed in DMSO-d 6 sample solutions by a Bruker 300 MHz spectrometer.Molecular weight of PSMA was determined by a Waters-150 GPC analysis instrument (mobile phase: THF, flow rate: 1.0 mL min -1 , and column temperature: 30 °C).

Preparation of poly(styrene-alt-maleic anhydride)
Poly(styrene-alt-maleic anhydride) (PSMA) was prepared through a thermally initiated free-radical polymerization of styrene and maleic anhydride 31 (Scheme 1a): a mixture of equimolar amounts (0.0432 mol) of styrene (4.99 mL) and maleic anhydride (4.236 g) was added to THF (25 mL) solution with benzoyl peroxide (0.0043 mol, 1.042 g) as polymerization initiator agent, and then, poured into a flask equipped with a Scheme 1.General synthetic scheme: (a) copolymerization of PSMA and (b) grafting of amino drugs to PSMA.Vol. 24, No. 7, 2013   reflux condenser and a magnetic stirrer under nitrogen atmosphere.Polymerization was carried out at 80 °C for 7 h.The viscous liquid, which resulted from the polymerization reaction, was washed in cold methanol to precipitate pure PSMA polymer.The purified PSMA was filtered off and dried under vacuum at room temperature.

General procedure for preparation of drug-loaded polymer
The grafting of selected drugs onto PSMA was carried out by reacting the drug amino group with the anhydride groups of PSMA to form amide bonds (Scheme 1b).Each drug was added with an equimolar ratio of monomers.For this purpose, PSMA (0.005 mol, 1.00 g) was dissolved in dry DMF (25 mL) by magnetic stirrer, and then, 0.005 mol of a selected drug, amantadine (0.939 g), gabapentin (0.856 g), zonisamide (1.061 g), mesalamine (0.766 g) and except for amlodipine (0.0025 mol, 1.202 g), due to its larger molecules and lower reactivity, were added to the solutions.The reaction was carried out at room temperature under nitrogen atmosphere for 12 h.Drug loaded polymer was purified by washing with distilled water and dried under vacuum at room temperature.The crude product of PSMA-MSE was gummy and sticky, so it was washed with cold diethyl ether to reach its powder form.

In vitro drug release study
In vitro drug release studies were performed by placing drug loaded polymer samples in a predefined volume of buffer (pH 1.3) at 37 °C.The quantity of released drugs was measured by UV-Vis spectrophotometer; the absorbance was recorded in the maximum wavelength value (λ max ) related to each drug (Table 1).Standard calibration curves (drug concentration vs. absorbance at appropriate λ max ) for suitable dilutions of drugs in buffer solution (pH 1.3) were first obtained.Sampling to determine the amount of released drugs (in ppm) was performed at different times (ranges as shown in Figure 1).

PSMA copolymerization
The free radical copolymerization of PSMA is facile and the copolymer readily affordable.As long as the polymerization was going on, the vinylic peaks disappeared in the 1 H NMR spectrum of PSMA, and aliphatic peaks such as methylene (CH 2 ) and methyne (CH) replaced them.Overlapped broad peaks between d 1.1-2.8 and 6-7.9 ppm are due to methylene/methane and aromatic ring protons of styrene, respectively.Methyne protons of maleic anhydride appear between d 3.3-3.5 ppm.Average molecular weight of PSMA was determined by gel permeation chromatography (GPC) (Table 2).In the FTIR spectrum for PSMA, anhydride group peaks appear at 1856, 1799 (cyclic anhydride C=O) and 1224 cm -1 (cyclic C-O-C).
Grafted drugs to PSMA Spectroscopic characterizations of the functional groups proved that drugs were grafted with covalent bonds to PSMA.The FTIR spectra show that the carboxylic acid and amide groups are formed after the drug amine group attacked the anhydride groups of PSMA.It is noticeable that the corresponding peaks of anhydride groups of PSMA are not entirely removed due to the unreacted remaining parts of the polymeric chains.Table 3 summarizes the FTIR spectra of drug loaded PSMA samples.
Using 1 H NMR data, the molar ratio of attached drug per monomer unit was calculated separately for each PSMA-drug copolymer sample.The percentages of attached drugs per monomer unit were about 17.8, 23.0, 39.3, 33.6 and 25.7 for amantadine, amlodipine, gabapentin, zonisamide and mesalamine, respectively.
In vitro drug release investigations of drug-loaded PSMA All the drugs used in this work are oral, hence, in vitro hydrolysis investigations of drug-loaded PSMAs were done according to the simulated gastric juice condition of human body 43 at acidic pH values (range of 1.1-1.5).In such acidic medium, drug-loaded PSMA hydrolyses were completed gradually as shown in Figure 1, in this way, the drug release is more controlled and permanent.In another study, which was done in simulated intestine media (at higher pHs such as 5, 6 and 7.4), the drug release was carried out by cleavage of the polymeric backbone and were more rapidly accomplished.
The drug release percentage and the cumulative release profiles of amantadine, amlodipine, gabapentin, zonisamide and mesalamine from their polymeric backbones, PSMA-AAE, PSMA-ALE, PSMA-GBN, PSMA-ZNE and PSMA-MSE are presented in Figure 1.Drugs are released rather fast at the beginning of the process.However, a mild variation appeared as the process proceeds.
In order to analyze the kinetics of drug release from PSMA matrix, it was clear that, in vitro release results were fitted by first order equations, according to Higuchi 45 and Korsmeyer-Peppas.A first order model (equation 1) describes the systems in which the rate of drug release is linearly proportional to the drug concentration on the polymer matrix.[51] (1)

derived a
(2) where The obtained kinetic data of drugs release from drug-loaded PSMA, together with the coefficient of determination (r 2 ), are listed in Table 4. Fitting data to the three kinetic models and investigation of the r 2 values show that the Kormeyer-Peppas model agrees with the release curves most accurately.

Water uptake
The water diffusion into PSMA-drug copolymers is based on Fickian diffusion that supposes proportionality between the flow and concentration gradient.The water uptake (S w ) of the conjugated polymer was determined gravimetrically in distilled water at room temperature overnight (W s ).Before each measurement, the dry weight of polymer (W d ) which was dried under vacuum overnight was also determined.The excess of water was gradually removed with filter paper.The water uptake was determined with the following equation: 52 (4) Swelling percentages were obtained as 30, 75, 66, 55 and 70% for PSMA, PSMA-AAE, PSMA-ALE, PSMA-GBN and PSMA-ZNE, respectively.It is worth mentioning that PSMA-MSE is entirely soluble in water.
These results indicate that, drug-loaded PSMA systems have more water uptake potential in comparison with the original polymer.This is expected due to the possible hydrogen bonding of the carboxylic acid groups, in the backbone of the drug loaded polymeric chains, increasing its swelling.

Mechanism of drug release
To explain the nature of the drug release mechanism, Korsmeyer et al. 49 developed a simple empirical model, which described drug release from a polymeric system.Some processes may be classified as either purely diffusion or purely erosion controlled; many others can only be interpreted as being governed by both.4][55] The equation can be written as: (5)   where (C t /C ∞ ) in equation 5, is the mass fraction of drug released at time t and n is a characteristic of the drug-polymer system.By determining the release exponent, it is possible to obtain information about the physical mechanism controlling the drug release from a particular device. 50,56ased on the value of this exponent n, the drug transport was classified as given in Table 5. 57 The exponent n in this work varies from 0.256 to 0.441 for all the formulations (Table 4), i.e., less than 0.45; therefore, this suggests that Fickian diffusion is the main release mechanism under the conditions used.Drug mass transfer flow in this mechanism is related to the product of a molecular diffusivity and a concentration gradient.In order to further characterize the drug release process, the mean dissolution time (t m ) was calculated for each drug according to the following equation. 54(6)   This parameter reflects the level of drug release retarding by the polymer matrix and the in vitro media.The t m obtained values are presented in Figure 2.
Mean dissolution times are within 40.4 to 99.9 h.Among the used drugs, zonisamide and gabapentin exhibit the highest and lowest dissolution times, respectively.This can be attributed to the resonance between S=O and NH bonds (between PSMA and zonisamide) which in turn causes the bond cleavage to occur in longer times.

Conclusions
The covalent grafting of five well-known drugs to PSMA was successfully done by the formation of amides from the reaction of amino group drugs with the anhydride groups of PSMA.In vitro release of amantadine, amlodipine, gabapentin, zonisamide and mesalamine from their corresponding drug-loaded PSMA was also studied, and indicated that the drug release can occur smoothly during long times, and this can be considered as an useful method for drug delivery.The kinetic analysis and the mean dissolution time indicate that Fickian diffusion controls the release of the drugs under study, and also zonisamide and gabapentin exhibit the slowest and the fastest release rates, respectively.

Figure 2 .
Figure 2. Mean dissolution time for drugs under study.

Table 2 .
Average molecular weight (M w ) and polydispersity (PDI = M w M n

Table 3 .
FTIR results of drug grafted PSMA samples

Table 4 .
Release kinetic parameters of the drugs