Supramolecular structure organization and rheological properties modulate the performance of hyaluronic acid-loaded thermosensitive hydrogels as drug-delivery systems

Abstract

The challenges for developing new pharmaceutical formulations based on natural and synthetic polymers has led to innovation into the design of systems responsive environmental stimuli such as temperature. However, the presence of hydrophilic or hydrophobic molecules, charged groups, or metallic elements can affect their structural behavior and their biopharmaceutical performance This work aims to study and characterize the morphology and structure of polymeric formulations based on Poloxamer (PL) 407 (15 % and 30 % m/v) and its binary with PL 338 (15 % PL 407 + 15 % PL 338) and hyaluronic acid (0.5 % m/v), as drug delivery systems of local anesthetic bupivacaine (0.5 % m/v) and ropivacaine (0.5 % m/v) hydrochloride. For this, it was performed SANS analysis for determination of supramolecular organization and lattice parameters; calorimetry was done to characterize their thermodynamic parameters; rheological analysis flow curve, consistency and adhesion calculation, Maxwell model study. Also, it was performed drug release profiles and calculation of diffusion coefficients. It was identified lamellar structures in PL 407 15 % formulations, and coexistence of cubic and hexagonal phases in PL 407 30 % and binary formulations, however hyaluronic acid, bupivacaine or ropivacaine seem do not affect the type of supramolecular structure. In addition, these additives can modulate viscosity among poloxamers chains, increasing micelle-micelle interactions as it happens in presence of bupivacaine. On the other hand, addition of hyaluronic acid can promote increased structural stabilization by hydrophilic interactions between hyaluronic and micellar corona. It reflects the ability how to control the drug release, as in case of binary system that retained bupivacaine for longer time than other systems, as well it happens when hyaluronic acid is added in PL 407 15 % and PL 407 30 %.

Introduction

The possibility of designing new pharmaceutical formulations based on natural and synthetic polymers has led to the development of systems that can respond to distinct stimuli such as temperature, pH gradient, chemical and light, and electrical and magnetic fields. However, the presence of hydrophilic or hydrophobic molecules, charged groups, or metallic elements can affect their structural behavior and impact the drug release rate and biopharmaceutical performance [1], [2].

Poloxamers (PL) are copolymers composed of hydrophilic polyethylene oxide (PEO) and hydrophobic polypropylene oxide (PPO) blocks, represented as PEO_x-PPO_y-PEO_x (Fig. 1 A), where x and y confer different physicochemical properties and enable their interaction with hydrophilic and hydrophobic interfaces. In aqueous solutions, these copolymers self-organize into micelles at appropriate temperatures and concentrations [3], [4], [5]. With increasing temperature, micelles tend to enhance their structural order, with dehydrating PPO units organizing themselves (“packing”) into lamellar, cubic, and hexagonal phases, producing partially rigid structures with high elastic moduli [6]. This reversible phenomenon, known as gelation, is characterized by the temperature (or temperature range) of sol–gel transition (Tsol-gel), as they remain fluid or viscoelastic semi-solid gels depending on the environmental temperature [7], [8].

Over the last two decades, studies on PL-based binary systems (for example, PL407 + PL338) as drug-delivery matrixes have accumulated; however, controversies regarding the supramolecular structure and interference of drug incorporation and other additives (as different polymers) persist, along with how they impact biopharmaceutical properties of pre-formulations. Indeed, these critical questions need to be resolved, as interferences between the copolymer concentration and temperature are necessary for micellization processes and gelation in binary systems, an alternative strategy to modulate the micellar structure, hydrogel rheological properties, formulation stability over physical condition changes, solubility, and drug release rate [2], [6], [9]. Having said that, we investigated the binary systems phase behavior composed of two hydrophilic PL, PL407 and PL338. Despite their structural similarities, both polymers show some differences regarding to molecular weight, ethylene oxide (EO): propylene oxide (PO) relationships and CMC (Fig. 1), which determines their differential phase organization behavior and supramolecular organization according to their final concentration and ratio into the formulation. Other important point is their potential low cytotoxicity, justifying their selection as hydrogels matrices [9], [10].

Previous studies have discussed interactions between PL and natural polymers [10], [11], [12]. However, establishing how additive incorporation can impact hydrogel phase organization could, in turn, determine the structural requirements for modulating their release rates and mechanisms [7], [9]. Conversely, negative charges and hydroxyl groups in hyaluronic acid (HA) chains at physiological pH afford the HA molecule hydrophilic characteristics (Fig. 1 B). In addition, its high molar mass confers unique viscoelastic and rheological properties [10], [13], [14]. We have previously reported the influence of low HA concentrations on the PL407-PL338 hydrogel matrix for intra-articular applications; however, the drug model used, lidocaine base, also had a differential function in structural changes [9].

Herein, we investigated two chemically related amino-amide local anesthetics, bupivacaine hydrochloride (BVC) and ropivacaine hydrochloride (RVC), widely used for intraoperative and postoperative pain relief (Fig. 1 C). BVC is more hydrophobic than RVC, mainly due to the butyl group in its chemical structure, whereas RVC presents a propyl radical. This primary chemical difference determines their mechanism of action mediated by blocking sodium channels, the duration of anesthetic effects, and cardiac toxicity [1]. Although both drugs have been investigated in several nanocarrier systems, in the present report, we aimed to present a physico-chemical look at how the molecular study of drug and hydrogel component interactions should also be explored from an active drug role perspective, considering their chemical structure and availability as salt forms. We synthesized HA-loaded PL hydrogels containing BVC or RVC and performed small-angle neutron scattering (SANS), differential scanning calorimetry (DSC), and rheological property examinations to determine viscoelasticity, flow resistance, adhesion, and cohesion, as well as their release profiles.