(T1130-12-83) Exploration of Synergistic Polymer Combinations for Ocular Drug Delivery

Purpose: Ocular drug delivery vehicles (ODDVs) have been explored for the prolonged release of ophthalmic formulations for the prevention of rapid clearance from the eye via the tear ducts or loss as a result of mechanical shear such as blinking. Contact lenses, implants, and in situ gelling systems have been researched to improve ocular drug delivery. The use of hydrogel systems as ODDVs is attractive because of the ability to relatively easily incorporate ophthalmic therapeutics, to retain on the eye after the formation of an in situ gel, and to reduce frequency of administration lowering drop burden for patients.¹ In addition, an aqueous starting solution allows for easy bottle fill and administration while the in situ gel transition allows the thickened substance longer residence time on the eye.² Among the numerous physical and chemical gelation mechanisms, ionic gelation is of interest because of the presence of calcium, sodium, and potassium ions in tear fluid.³ Gellan gum, a well-established gelling polymer^1-5 and primary gelling agent in products such as Timoptic® was selected as a positive control for the studies. We explored polymers with ion sensitive mechanisms as well as viscosity enhancers for the development of a novel combination for an ODDV. We set out to investigate novel polymer combinations whose gelation mechanisms produced a synergistic effect in the form of increases in rheological parameters.

Methods: Solutions (% (w/v)) of low acyl gellan gum (GG) (CPKelco), poly(n-isopropylacrylamide) (PNIPAAm) (Sigma-Aldrich), Methocel™ F4M (HPMC) (Dupont), GENU® pectin (pectin) (CPKelco), xyloglucan (Sigma-Aldrich) , carrageenan CG-129 (CG) (CPKelco), Protanal CR 8133 (SA) (Dupont), hyaluronan (HA) (Santa Cruz Biotechnology), and Cekol® 700 (CMC) were made by adding polymer in 10 mL deionized water and heating between 70 – 90 ˚C for 30 – 60 minutes with magnetic stirring. Polymer was then left to cool and set at room temperature overnight. STF was prepared with 6.78 g/L sodium chloride, 2.18 g/L sodium bicarbonate, 0.0084 g/L calcium chloride, and 1.38 g/L potassium chloride. Sodium hydroxide was used to adjust STF to pH 7.4. For rheological analysis, 1 mL polymer was combined with 1 mL deionized water or STF and rotated on a Rotator Genie (Scientific Industries, Bohemia, NY) at low speed for 5 revolutions. 1.1 mL of sample (polymer with or without STF) were added to a Kinexus rotational rheometer (Malvern Panalytical, Westborough, MA) and a series of rheological sequences were performed. Amplitude oscillation was performed over a shear strain range [0.100 – 100.00 Pa] at 34 ˚C. Frequency sweeps were performed at 1.000 Hz at 34 ˚C using shear strains generated from amplitude tests. Finally, viscometry was performed by applying increasing shear stress for 30 min over the range [0.100 – 100.00 Pa] at 34 ˚C.

Results: Eight candidates from a larger pool were chosen based on their ability to visibly thicken a solution using a qualitative inverted tube test combined with initial rheological screening test results. Demonstration of elastic modulus greater than viscous modulus (G’ > G”) and frequency independence indicative of elastic behavior, and a phase angle (δ) below 30˚ which confirmed elastic deformation common for the behavior of a solid rather than a liquid were the selection criteria. Further screening narrowed down the candidates to two polymer combinations as a result of their interaction with gellan gum in the form of increases in gel strength (G’, Pa). The rheology method was validated using GG + STF as the positive control (Fig 3). The combination of GG + SA +STF showed modest increases in gel strength (p < 0.05) which matched results published in the literature (Fig 4) but surprisingly lead candidates (GG + CG + STF and GG + HA + STF) showed never before seen strong synergistic increases (p < 0.0001) in (G’, Pa) when compared to GG + STF. Frequency independence was observed in both combinations.

Conclusion: Polymer combinations that were selected for further testing exhibited strong synergy when compared to a positive control. Synergy between polymers has not been reported in the literature and so this represents the first reported synergism among binary polymer combinations. Statistically significant increases in gel strength in two combinations (GG + CG +STF) and (GG + HA +STF) were observed for the first time. These combinations also demonstrated increases in viscosity and shear thinning behavior providing further characterization of a gelled substance. Stronger gelling capacity and increased viscosity are properties desirable of an ODDV to remain on the surface of the eye for prolonged periods. This data identified two potentially synergistic combinations worth exploring further for the development of an ODDV for prolonged delivery of ophthalmics based on increases in gel strength, frequency independence, and shear thinning properties.

References: Dubashynskaya et al. (2020), Pharmaceutics 12, 22
Wu et al. (2019), Asian J Pharm Sci. 14, 1-15
Li P et al. (2018), Drug Dev Ind Pharm. 44, 829-836
Cassano et al. (2021) Gels, 7, 130
Liu Y et al. (2010), AAPS PharmSciTech. 11, 610-20

Acknowledgements and

Funding: These studies were partially supported by Nevakar, Inc and Countermeasures Against Chemical Threats, NIH Grant #U54AR055073.

Figure 2: Polymer Synergy Screening. Binary polymer combinations were screened for potential synergy. Amplitude oscillation was performed on singular polymer and polymer combinations in the presence of STF (n = 1).

Figure 5: Synergism in novel combinations. A. Amplitude oscillation performed on samples of gellan gum (GG) and carrageenan (CG) with and without STF as singular polymer samples and binary polymer combinations (n = 3). *, P < 0.0001, Student’s t test, GG + CG + STF vs GG + STF (n = 3). B. Amplitude oscillation performed as in A on samples of gellan gum (GG) and hyaluronan (HA) (n = 3). *, P < 0.0001, Student’s t test, GG + HA + STF vs GG + STF (n = 3).

Figure 6: Frequency independence. Frequency test performed on samples of gellan gum (GG) and hyaluronan (HA) with and without STF as singular polymer samples and binary polymer combinations (n = 3). A straight curve is indicative of frequency independence and characteristic of a solid substance.