BACKGROUND OF THE INVENTION

Field of Invention

The present invention is related to a pH responsive hydrogel, namely poly(Methacrylic acid-grafted-Ethylene Glycol) P(MAA-g-EG), that is synthesized by photopolymerization process that is initiated by visible light.

Brief Description of the Prior Art

Controlled drug release is an important research area in pharmaceutical research and still has significant issues that had to be addressed since it is challenging to match a patient's needs at the proper time and/or at the proper site. Smart materials that are responsive to pH, temperature and concentration or as such are attracting significant attention for controlled release applications. Especially polymeric hydrogels are widely studied group of polymers since they are biocompatible, able to retain water, and have high permeability properties. Although hydrogels have great capacity to attain water, they are insoluble due to the presence of crosslinked structures or the crystalline regions embodied in their structures. This makes these polymers strong candidates for use in biomedical and pharmaceutical applications especially for drug delivery systems, wound dressings, contact lenses, catheters and as such. Hydrogels are often categorized under "smart materials" due to the functional groups embodied in their structures. Particularly, pH sensitive hydrogels are attracting attention since the environmental pH changes cause modification on the physical state of the hydrogel such as the shape and size which enables the regulation of drug release. Among those hydrogels, P(MAA-g-EG) hydrogels are especially attracting attention for their uses as pH responsive polymeric networks. Although these hydrogels are known to be effective, there is still a need for more efficient responsive hydrogels with uniform cross-linked network structures; and with more efficient hydrogels we refer to enhanced swelling properties and biocompatibility and the preservation of the gel integrity after swelling.

Also there is still a need for more effective way of synthesizing these P(MAA-g-EG) hydrogels. Photo initiated free radical polymerization using UV light is an already known procedure for the synthesis of pH responsive hydrogels. Here in this invention visible light induced photopolymerization of pH responsive P(MAA-g-EG) hydrogels is suggested and by this method lesser amount of initiator use and shorter reaction time is actuated. Also with the process developed in this invention, an enhanced biocompatible polymer with advanced swelling properties is also generated.

SUMMARY OF THE INVENTION

One aspect of the invention is directed to a pH responsive hydrogel, namely poly(Methacrylic acid-grafted-Ethylene Glycol) P(MAA-g-EG), that is synthesized through photopolymerization induced by visible light in the presence of a photo- initiator and a co-initiator. In a selected embodiment of the invention the photo- initiator is selected to be Eosin Y and the co-initiator is selected to be Triethanolamine (TEA). The resulting P(MAA-g-EG) hydrogel turned out to be a hydrogel with enhanced swelling properties with less toxicity.

Another aspect of the invention is the method of synthesizing a pH responsive hydrogel, poly(Methacrylic acid-grafted-Ethylene Glycol) P(MAA-g-EG) from monomers methacrylic acid (MAA) and poly(ethylenglycol) monomethyl ether monomethacrylate and the crosslinking agent tetraetylene glycol dimethacrylate by visible light initiated photopolymerization.

In a preferred embodiment of the invention, Eosin Y is used as the photo-initiator and Triethanolamine (TEA) is used as the co-initiator in the disclosed method of synthesis. And in a more preferred embodiment of the invention the method of synthesizing P(MAA-g-EG) hydrogel is characterized with the following steps i. The monomers methacrylic acid (MAA) and poly(ethylenglycol) monomethyl ether monomethacrylate (PEGMMA), the crosslinking agent tetraetylene glycol dimethacrylate, tetraetylene glycol dimethacrylate, a photo-initiator and a co-initiator are dissolved in water-ethanol mixture and

ii. the resulting solution is exposed to visible light for 20 minutes

iii. and washed with water and finally freeze dried

In the final embodiment of the invention, the use of pH responsive P(MAA-g-EG) hydrogel as a controlled release polymer for the drug delivery applications is disclosed.

The invention is the first example to visible light photopolymerization of p(MAA-g- EG) hydrogel. And it is proved that the method of synthesis enclosed in this invention is advantageous over UV-light initiated photopolymerization since less amount of photo-initiator is used and the reaction time is also much shorter. The resulting p(MAA-g-EG) hydrogel is proved to have enhanced swelling behavior, and since less amount of initiator is employed, the resulting hydrogel is less toxic hence more biocompatible. These properties make the hydrogel a good candidate for pharmaceutical applications especially for controlled release applications.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. pH-responsive p(MAA-g-EG) hydrogel disk with a height of 0.25 cm and a diameter of 0.5 cm that is photopolymerized by laser light with a wavelength of 514 nm.

Figure 2. Dynamic swelling behavior of (a) pH responsive hydrogel P(MAA-g-EG) which had been photopolymerized under UV light (b) pH responsive hydrogel P(MAA-g-EG) that were photopolymerized under visible light. Figure 3. Reversible swelling/deswelling behavior of (a) pH responsive hydrogel P(MAA-g-EG) which had been photopolymerized under UV light, (b) pH responsive hydrogel P(MAA-g-EG) which had been photopolymerized under visible light.

Figure 4. Release behavior of pregabalin at pH=2.2 and pH=7.0 from (a) pH responsive P(MAA-g-EG) hydrogel that was photopolymerized under visible light.

DETAILED DESCRIPTION OF THE INVENTION

In this detailed description of the invention, some exemplary references are used for illustrative purposes only therefore it should be understood that the invention is not limited to the scope of these particular embodiments. Also the terminology used here in the detailed description is for the purpose of description but does not intend to limit the scope of the invention.

The main aspect of the invention is a novel pH responsive hydrogel, namely poly(Methacrylic acid-grafted-Ethylene Glycol) P(MAA-g-EG), that is synthesized via visible light induced photopolymerization method. The p(MAA-g-EG) polymer that is subject to this invention is synthesized from the monomers: methacrylic acid (MAA) and poly(ethylenglycol) monomethyl ether monomethacrylate and tetraetylene glycol dimethacrylate as the crosslinking agent.

A preferred embodiment of the invention is the pH responsive hydrogel as described above wherein Eosin Y is used as the photo-initiator and Triethanolamine (TEA) is used as the co-initiator in the method of synthesis.

A more preferable embodiment of the invention is the pH responsive hydrogel as described above wherein the method of synthesis is characterized by a process comprising the following steps i. The monomers methacrylic acid (MAA) and poly(ethylenglycol) monomethyl ether monomethacrylate (PEGMMA), the crosslinking agent tetraetylene glycol dimethacrylate, tetraetylene glycol dimethacrylate, a photo-initiator and a co-initiator are dissolved in water-ethanol mixture and

ii. the resulting solution is exposed to visible light for 20 minutes

iii. and washed with water and finally freeze dried

Another aspect of the invention is the method of synthesizing a pH responsive hydrogel, namely P(MAA-g-EG) from monomers methacrylic acid (MAA) and poly(ethylenglycol) monomethyl ether monomethacrylate and the crosslinking agent tetraetylene glycol dimethacrylate via visible light induced photopolymerization.

The method of synthesizing the pH responsive hydrogel, namely poly(Methacrylic acid-grafted-Ethylene Glycol) P(MAA-g-EG) as described above wherein Eosin Y is used as the photo-initiator and Triethanolamine (TEA) is used as the co-initiator.

In the preferred embodiment of the invention the method of synthesizing the pH responsive hydrogel as described above, P(MAA-g-EG) is characterized by a process comprising the steps: i. The monomers methacrylic acid (MAA) and poly(ethylenglycol) monomethyl ether monomethacrylate (PEGMMA), the crosslinking agent tetraetylene glycol dimethacrylate, tetraetylene glycol dimethacrylate, a photo-initiator and a co-initiator are dissolved in water-ethanol mixture and

ii. the resulting solution is exposed to visible light for 20 minutes

iii. and washed with water and finally freeze dried

Another aspect of the invention is the use of the pH responsive hydrogel as a drug delivery the active ingredient. One final aspect of the invention is a pharmaceutical composition comprising the P(MAA-g-EG) hydrogel as described in this invention, an active ingredient and pharmaceutically acceptable excipients. The active ingredient is selected from a group of anticonvulsant drugs comprising gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate and zonisamide and most preferably the active ingredient is selected to be pregabalin.

In one embodiment of the invention, the pharmaceutical composition as described above is in the form of a tablet, capsule, intravenous formulation, intranasal formulation, transdermal formulation, formulation for muscular injection, syrup, suppository or aerosol.

Hydrogels as used here in this invention refers to hydrophilic polymer chains those may be either natural or synthetic polymers and are capable of absorbing high amounts of water. Hydrogels find a wide range of application areas such as targeted drug delivery, tissue engineering, biosensors, and diapers as such. pH-responsive hydrogels with the definition as used in the this invention refers to hydrogels that are capable of detecting environmental pH changes that cause a corresponding modification on the physical properties of the polymer such as shape and size and as a result of this change they release the load they have. This type of hydrogels are also known as "smart" or "intelligent" gels since they are capable of sensing the environmental changes, in this case pH changes there are also other types of hydrogels that are responsive to other environmental changes such as temperature, concentration and as such.

The active ingredient or a pharmaceutical formulation containing the active ingredient may be administered whether systemically or at the site including but not limited to oral, topical, pulmonary, rectal, vaginal, and parenteral. It is preferable to formulate an active ingredient as a pharmaceutical composition comprising at least one active ingredient together with one or more pharmaceutically acceptable excipients such as carriers, fillers, diluents, buffers, adjuvants, stabilizers, or other materials. The formulations may be prepared by any method known in the pharmaceutical literature and may be presented in unit dosage form. Formulations may be in the following forms but not limited to tablets, capsules, syrup, lozenges, pills, cachets, sachets, pills, ointments, gels, creams, sprays, pastes, aerosols or suppositories.

EXAMPLES The following examples are provided to illustrate the present invention and are not intended to limit the scope of the invention.

Example 1

Synthesis of poly(Methacrylic acid-grafted-Ethylene Glycol) P(MAA-g-EG)

0.042 moles of methacrylic acid (MAA) is mixed with 0.002 moles of poly(ethylene glycol) monomethyl ether monomethacrylate (PEGMAA) and 2.19X10 ^"4 moles of the crosslinking agent tetraethylen glycol dimethacrylate (TEGDMA) are mixed in an 50:50 (w:w) ethanol-water mixture and then 6.705xl0 ^"6 moles of photo-initiator Eosin Y and 0.0304xl0 ^"6 moles of co-initiator triethanolamine is added to the mixture. This pre-polymer solution is then exposed to laser light at a wavelength of 514 nm for 20 minutes. The resulting hydrogel is then washed with distilled water for 1-2 days in order to get rid of the excess photoinitiator and the cross linking agent. At the final step the hydrogels are dried with the freeze-dry method. (Figure 1.)

Example 2

Dynamic weight swelling experiments for P(MAA-g-EG) on alternating pH conditions Dynamic weight swelling experiments were carried out in different pH solutions in order to further characterize pH sensitivity of the hydrogel. DMGA buffer solutions at 0.1 M and with a range of pH 3.8 to pH 7.0 were used at 37°C. The hydrogel was kept in DMGA buffer solution with a specific pH and weighed at specified time intervals for the calculation of equilibrium weight swelling ratio. The weight swelling ratio, q represents the swelling ratio of the network and was calculated using the following equation:

w _s

q ⁼ W _d

where W _s is the weight of the swollen hydrogel and W _d is the initial weight of the dried hydrogel. Reversible swelling/deswelling behavior of all types of hydrogels was characterized using 0.1 M DMGA buffer solutions with pH values within 2.2-7.0 range at 37°C. For these experiments, ionic strength of DMGA buffers were maintained with sodium chloride and the temperature of the medium was kept at 37°C.

As a result of these experiments it had been proved that hydrogels cured with visible light had higher swelling ratios compared to their UV photopolymerized counterparts. Swelling ratios as high as 37 is obtained for P(MAA-g-EG) hydrogel synthesized with visible light, while this ratio was around 17 for hydrogels formed with UV light. It had also been demonstrated that, smaller pore sizes were obtained for visible light cured hydrogels despite higher swelling ratios compared to UV polymerized hydrogels. This could be explained by the higher number of pores that are present in visible light cured hydrogels compared to UV polymerized hydrogels, which lead to improved swelling ratios. This is also supported by SEM images, where UV polymerized hydrogel had a higher solid content and fewer number of pores compared to the ones synthesized with visible light.

Reversibility of swelling was also tested for hydrogels formed with both UV and visible light Gured hydrogels through exposure of samples at low pH (pH:2.2) and high pH (pH:7.0) buffers (Figure 2). It was observed that, hydrogels synthesized under different conditions can all respond to repeated changes in pH, where highest swelling was obtained for P(MAA-g-EG) hydrogel cured under visible light.

Example 3

Loading and release studies of pregabalin on P(MAA-g-EG) hydrogels Pregabalin loading studies were carried out with visible light cured P(MAA-g-EG) hydrogels. P(MAA-g-EG) hydrogels were incubated for 16 hours in 10 ml pregabalin stock solution, which was prepared in 1 x PBS (pH 7.4) at a concentration of 0.25 mg/ml. Next, hydrogels were collapsed with the addition of 2 μΐ of 6 M HC1 in order to keep the drug within the network. In order to eliminate any drug adsorption on the surface, pregabal in-loaded hydrogels were rinsed with distilled water.

UV-visible spectrophotometer at 210 nm wavelength was used to measure pregabalin absorption. Standard calibration curve for the absorption of pregabalin in aqueous solution was generated for determination of concentration in unknown samples. Loading efficiency of pregabalin into hybrid gels was calculated as follows:

Loading Efficiency =— - * 100 (2) where ₀ represents pregabalin mass in the stock solution initially and Mf is the final pregabalin mass remaining in the solution at the end of 16 hours. As a result of this loading efficiency experiment experiment it was observed that Pregabalin was loaded as 56.67±8.34% into P(MAA-g-EG).

Pregabalin-loaded hydrogels were placed in 1 x PBS (pH 7.4) at 100 rpm and 37°C for release experiments. For every 30 minutes up to 7 hours, 500 μΐ samples were taken and replaced with 1 x PBS (pH:7.4) to sustain sink conditions. The same experiment was repeated at low pH conditions, where the solution buffer was replaced with lxPBS at a pH of 2. Pregabalin concentrations obtained from absorption measurements were used to calculate mass released at time t (M _t ) using the following equation: M _t = C _t * V +∑C _t _i * V _s (3) where C _t is the concentration of pregabalin in the release solution at time t, V is the total volume of release solution (10 ml) and V _s is the sample volume that is taken (500 μΐ). Using the M _t values, the % release of pregabalin was determined as expressed with the following equation:

% Mass Released =— * 100 (4) where M _∞ is the total weight of pregabalin released during the experiment. Once the diffusion behavior of pregabalin from pH responsive and hybrid hydrogels were obtained, the results were modeled using the power-law correlation:

where k represents the characteristic constant of the hydrogel and n is the exponential coefficient, which describes the mode of transport mechanism. The value of n has been found reported as 0.45 for Fickian Diffusion and 0.89 for swelling-controlled diffusion for cylindrical geometry. The fittings were done based on the first 60% of the collected release data to obtain a more accurate fit.

As a result of the release studies it had been observed that in neutral pH environment, 86.37% of pregabalin was released from hydrophilic pH responsive P(MAA-g-EG) hydrogel within 400 minutes, whereas in acidic pH only 15.32% of pregabalin release was observed (Figure 4). Since pH of the medium (pH:7.4) was greater than the pKa value of MA A (pKa:5.4), pH responsive hydrogel network retained an expanded structure, and resulted in higher drug release. As expected, at acidic condition minimum amount of release was observed for the hydrogels.

The transport of pregabalin from the hydrogeP sample was modeled using equation (5). Exponent n in the equation describes the mode of transport, and was determined for each condition at high (pH:7.4) and low (pH:2.2) pH conditions. Values of n closer to 0.45 designate a Fickian mode of diffusion, whereas values closer to 0.89 designate a swelling-controlled diffusion behavior.