1. INTRODUCTION:

In the recent past, the synthesis of new biodegradable polymers have attained significant interest to fulfil the unmet needs in healthcare. Several families of polymers like polyesters, polyurethanes, polyanhydrides, polyamides, poly(amino acids) and polyorthoesters have found interesting applications in catering medical needs^1-3. Now a days, the focus has shifted towards designing and synthesising biodegradable polymers with tailored properties for specific applications by several approaches which include developing novel synthetic polymers with unique chemistry to increase the diversity of polymer structure, establishing biosynthetic processes to form stimuli-sensitive polymer structures and adopting combinatorial and computational approaches in biomaterial design^4-6. Polymers have found many applications in drug delivery because of their ability to control and to sustain drug release, thereby providing an opportunity for improving the potential and reliability of drugs. Many investigations have attempted to functionalise polyesters to improve their ability to entrap different molecules and to modify drug release. Polyesters with different pendant reactive moieties such as hydroxyl^7,8 carboxyl^9-11 and amino¹² groups have been synthesised.

However, their preparation often involved many synthetic steps in order to modify the physical-chemical properties of the polymer chain^9,13. Carboxylated polymers provide several advantages in designing delivery systems due to versatile nature of carboxyl groups. The carboxyl groups have been explored for various applications from enhancing protein entrapment in a nanoparticulate system¹⁴ to utilise their reactivity for polymer drug conjugation¹⁵. The negatively ionizable carboxyl functional group also increased the calcification properties of biomaterials¹⁶. More recently, the ability of particles exposing carboxyl groups to adhere to live malignant cells was reported¹⁷. In particular, polyesters such as polylactide (PLA), polyglycolide (PGA) and their copolymer poly(lactideco- glycolide) (PLGA) have been extensively used for drug delivery applications because of their superior biocompatibility and biodegradability. Lactic acid based polyesters (PLA) are well-known biodegradable thermoplastic polymers and would appear to meet these strict requirements. The raw material, lactic acid, is nontoxic, naturally occurring and derived from natural renewable sources (e.g., corn) by microbial fermentation of biomass. PLA has excellent physical properties, which mimic conventional thermoplastics and it is truly a biodegradable polymer, which decomposes rapidly and completely in a typical compost environment to yield carbon dioxide, water and biomass^18-21. The hydrolysis of PLA polymers has been widely studied both in vivo and in vitro. Hydrolytic degradation of poly(DL- and L-lactic acids) proceeds by random hydrolytic chain scission of the ester linkages, eventually producing the monomeric lactic acid²². Polymers such as cellulose acetate phthalate(CAP) and cellulose acetate trimellitate(CAT) are entering coating materials for bio-medical purposes. However, human body cannot assimilate these cellulose derivatives. In the study, aliphatic/aromatic copolyesters are systematically synthesised from lactic acid, terephthalic acid and diols. The effects of diols on structure and properties of the resulting copolymers are investigated. Depending on their chain microstructure and properties, their drug release pattern is studied.

2. EXPERIMENTAL:

2.1 Materials

Lactic acid (Merck AR grade) and Terephthalic acid (Lancaster AR grade) were recrystallised from deionised water and used. Ethylene glycol (Lancaster, AR grade) and 1,4 Butane diol (Lancaster AR grade) were dried with CaO overnight and then distilled under reduced pressure. Titanium tetra isopropoxide, used as catalyst, purchased from Lancaster was used as such. All the other materials and solvents used were of analytical grade.

2.2. Synthesis

The copolyesters were synthesised by a two step melt polycondensation method. The polycondensation flask was a three neck flask equipped with a nitrogen inlet, a condenser and a thermometer. A magnetic stirrer was used to stir the reaction mixture. As an example, the synthesis of poly (butylene lactate – co – butylene terephthalate), PBLT has been described. The reaction mixture is 0.1mol Lactic acid, 0.1 mol terephthalic acid and 0.2mol 1,4 BD. The reaction mixture is purged with nitrogen and heated in an oil bath. The temperature of the reaction mixture is raised to 150°C in 20min. Then the temperature is gradually raised in 10°C steps every minute to the fixed reaction temperature of 210°C to remove water being the esterification by product. When water ceased to be generated, a predetermined amount of titanium tetra isopropoxide, (TTiPO) (0.1mmol) catalyst is added to the reaction mixture. Subsequently, the pressure of the reaction system was gradually decreased and polycondensation is continued at 210°C under a final reduced pressure lower than 0.5mmHg. The reaction was terminated when the rotation of the mechanical stirrer is stopped. The resulting crude copolymers were dissolved in chloroform and then poured into excess of dry cold methanol to purify the polyester. The precipitated copolyesters were dried in a dessicator for further characterisation (Fig. 1.).

2.3 Characterisation:

2.3.1. Intrinsic Viscosity

The intrinsic viscosity, of polymer solutions in chloroform was measured at 30°C in a constant temperature bath using Ubbelhode Viscometer.

PBLT

PEGLT

Figure 1. Summary of the synthesis reaction of aliphatic/aromatic copolyesters PBLT and PEGLT

2.3.2. Solubility test

Solubility of random copolyesters PBLT and PEGLT were determined various solvents qualitatively. It may be noted that the two polyester samples exhibit the similar solubility pattern despite their different compositions. Polyesters maintain a good solubility in acetone, CHCl_3,THF, DMF and DMSO. The copolyesters are insoluble in water, methanol and ethanol.

2.3.3. Fourier- Transform Infrared (FTIR) Spectroscopy

IR Spectra of the copolyesters were recorded using a perkin Elmer IR spectrometer in the range of 3800cm-¹ to 480cm-¹. The samples were embedded in KBr pellets.

2.3.4. Nuclear Magnetic Resonance(NMR)

¹H NMR spectra were obtained with a Joel Model GS X 300 MHz NMR Spectrometer by using CDCl₃ as solvent and internal standard, respectively. The measurements were carried out at room temperature.

2.3.5. Hydrolytic tests

The hydrolytic test was investigated in acid and alkaline regions of various pH values.

2.3.6. Coating of the core(uncoated) Tablets

PBLT and PBGLT were separately used as coating materials. Their 40% solution in chloroform were used as coating solution. The coating solution was sprayed over core tablets in a coating pan with continuous hot air flow. The coating pan was allowed to rotate until the solvent evaporated and the tablets were dried. Drug release from coated tablets was tried as a function of weight gain by coating material on tablets and 10-12% weight gain by coating material on tablets gave the best result.

2.3.7. Preparation of Dichlofenac Sodium Standard Calibration Curve for Drug Release Measurement

For the preparation of standard curve of diclofenac sodium for its quantitative determination in the subsequent experiments, phosphate buffer solution of pH 7.4 was used as the medium. Absorbance of some known solutions of the drug were measured at its λ_max ( 274nm ) on a UV-VIS spectrophotometer. The standard curve was constructed by plotting the absorbance of the drug against its concentration in the suitable region.

2.3.8. Dissolution Studies

The dissolution for both the core and coated tablets were performed in order to evaluate the efficiency of the copolyesters as coating material on the release of the drug. Simulated gastric fluid was prepared by dissolving 2g of NaCl and 7 ml of HCL in 1 litre distilled water and pH was adjusted to 1.2 and simulated intestinal fluid was prepared by mixing 0.1M KH₂PO₄ and 0.1M Na₂HPO₄ solution and then pH was adjusted to 7.4. One of the simulated gastric fluid (pH 1.2) heated at 37°C was initially used for the dissolution studies and the solution was replaced after 2 hours by one litre of simulated intestinal fluid (pH 7.4) heated previously at 37°C.

Samples (5ml) were withdrawn from the simulated gastric fluid at 30 min intervals for 2 hours and from simulated intestinal fluid at 15 min intervals for 1 hour.

3. RESULTS AND DISCUSSION:

3.1. Intrinsic Viscosity and solubility studies

The inherent viscosity of the copolyesters was measured in chloroform using ubbelhode viscometer.

The intrinsic viscosity and solubility of the copolyesters are presented in table 1 and 2 respectively.

TABLE – 1 Intrinsic Viscosity of Copolyesters PBLT and PEGLT

Polymer	Inherent viscosity ηinh (dL/g)
PBLT	0.9
PEGLT	0.82

The synthesised copolyesters are soluble in CHCl₃, acetone, DMSO and insoluble in alcohols and water.

3.2. Fourier- Transform Infrared (FTIR) Spectroscopy

FTIR spectra of copolymers for BD and EG polyesters are shown in the Figure 2. Band characteristics of aromatic esters are observed between 1725cm^–1 and 1722cm^-1(C=O stretching) and 728 cm^–1 and 723cm^-1 (ring C–H out of plane bending).

The C-H symmetric stretching of aliphatic-CH₂- group observed at 2963-2960cm^-1. Strong vibrational modes observed at 1274-1271cm^-1 and 1116-1112cm^–1 are associated with C–O–C asymmetric stretching modes of both aromatic and aliphatic esters.

Fig. 2. IR spectra of copolymers PBLT and PEGLT

3.3. ¹H NMR spectroscopy

The chemical shift values obtained from ¹H NMR spectra of the copolyesters are as follows. Aromatic protons of terephthalic group was observed at 8.1-8.08ppm(singlet) ,methylene protons of diols (-CO-O-CH2-) at 4.7-3.3ppm(multiplet), methyl protons of LA at 1.9-1.2ppm and methine protons of LA at 5.3-5.1ppm.

TABLE 2 Solubility of Copolyesters PBLT and PEGLT

S. No	Polymer	Acetone	CHCl₃	DMSO	Methanol	Ethanol	THF	DMF	Water
1	PBLT	+ + +	+ + +	+ +	- -	- -	+ +	+ +	- -
2	PEGLT	+ + +	+ + +	+ +	- -	- -	+ +	+ +	- -

+++ - Freely Soluble, ++ - Soluble, -- Insoluble

Fig. 3. ¹H NMR spectra of copolymers PBLT and PEGLT

3.4. ¹³C NMR Spectroscopy

The chemical shift values obtained from ¹³C NMR Spectra of the copolyesters are as follows. Aromatic group of terephthalic was observed at 134-129ppm(multiplet), methylene carbons of diols (-CO-O-CH₂) at 20. 39-25.4,69.32 – 69.55ppm(multiplet), methyl group of LA at 20-16ppm(multiplet) and methine group of LA at 69-62ppm(multiplet) (Fig. 4.).

Fig. 4. ¹³C NMR spectra of copolymers PBLT and PEGLT

3.5. Hydrolytic degradation test

From the hydrolytic test, it was found that copolyester samples remained unchanged in simulates gastric fluid (PH-1.2) and gradually degraded in simulated intestinal fluid (PH-7.4). In acid region, the copolymer samples did not swell. Whereas in alkaline region they swell and the ester linkage was hydrolysed.

3.6. Drug release pattern of the copolyesters

In vitro release study was carried out for both PBLT and PEGLT coated diclofenac sodium in the simulated gastric fluid (pH 1.2) and also in the simulated intestinal fluid (pH 7.4). Dissolution of drug from its dosage form is dependent on many factors which include not only the physico-chemical properties of the drug but also the formation of the dosage form and the process of manufacture ²³. Such statement is also true for enteric – coated preparations. In this study it was found that the copolyesters did not degrade or swell in the gastric fluid for two hours when coated on a core tablet, whereas in the intestinal fluid the coating gradually degraded and drug release was observed from PBLT and PEGLT coated tablets. Fig-5 reveals that 80.5% of diclofenac sodium released from the PEGLT and 74.5% of diclofenac sodium released from the PBLT coated tablets in the simulated intestinal fluid within 180 min. In acid region especially in pH 1.2 the ester linkage of the compound is resistant and in alkaline region especially of pH 7.4 the compound is susceptible. As a result, ester linkage is gradually hydrolyzed. In pH 1.2, the coating remains intact but in alkaline region release of the drug. Therefore, the swelling and hydrolysis of the ester as well as diffusion of drug particles simultaneously play an important role on the release behaviour of the drug.

The drug release pattern of both PBLT and PEGLT coated diclofenac sodium corresponds to the BP drug release profile of enteric – coated tablets²⁴, but considering the amount of drug release in the intestinal environment PEGLT seems to be better than PBLT. The toxicological investigation of PBLT and PEGLT are in progress, if they proved to be nontoxic, would be usable as enteric coating material.

Fig. 5. Percentage of drug release Vs Time (min) from core, PBLT coated and PEGLT coated tablets in simulated intestinal fluid (pH 7.4).

4. CONCLUSION

The aliphatic-aromatic random copolyesters PBLT and PEGLT were synthesised by two step melt polycondensation method using lactic acid, terephthalic acid and 1,4 butane diol/ ethylene glycol, in the presence of a highly effective catalyst titanium tetra isopropoxide. The synthesised random copolyesters were characterised by means of solubility, viscosity measurements, IR, ¹H NMR, ¹³C NMR spectral analysis and hydrolytic degradation tests. The polyesters exhibit good solubility in many common solvents which is an important quality in view of its applications. The hydrolytic degradation test shows that the copolyester samples remains intact in the acidic medium (pH<6) but gradually degraded in alkaline medium (pH>7). In pH 1.2 the coating material remains intact that is, negligible release of the drug happens whereas in alkaline region that is, in pH 7.4 the polyesters swells and was degraded resulting the release of the drug. It was observed that PEGLT releases more diclofinac sodium than PBLT. The investigation can be considered as a potentially interesting tool for the design of new drug delivery systems.

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Received on 30.03.2012 Modified on 16.04.2012

Asian J. Research Chem. 5(5): May 2012; Page 611-615