Controlled release kinetics of p-aminosalicylic acid from biodegradable crosslinked polyesters for enhanced anti-mycobacterial activity

doi:10.1016/j.actbio.2015.11.032

Acta Biomaterialia

Volume 30, 15 January 2016, Pages 168-176

https://doi.org/10.1016/j.actbio.2015.11.032 Get rights and content

Abstract

Unlike conventional polymeric drug delivery systems, where drugs are entrapped in polymers, this study focuses on the incorporation of the drug into the polymer backbone to achieve higher loading and sustained release. Crosslinked, biodegradable, xylitol based polyesters have been synthesized in this study. The bioactive drug moiety, p-aminosalicylic acid (PAS), was incorporated in xylitol based polyesters to impart its anti-mycobacterial activity. To understand the influence of the monomer chemistry on the incorporation of PAS and its subsequent release from the polymer, different diacids have been used. Controlled release profiles of the drug from these polyesters were studied under normal physiological conditions. The degradation of the polyesters varied from 48% to 76% and the release of PAS ranged from 54% to 65% of its initial loading in 7 days. A new model was developed to explain the release kinetics of PAS from the polymer that accounted for the polymer degradation and drug concentration. The thermal, mechanical, drug release and cytocompatibility properties of the polymers indicate their suitability in biomedical applications. The released products from these polymers were observed to be pharmacologically active against Mycobacteria. The high drug loading and sustained release also ensured enhanced efficacy. These polymers form biocompatible, biodegradable polyesters where the sustained release of PAS may be tailored for potential treatment of mycobacterial infections.

Statement of significance

In the present work, we report on novel polyesters with p-aminosalicylic acid (PAS) incorporated in the polymer backbone. The current work aims to achieve controlled release of PAS and ensures the delivered PAS is stable and pharmacologically active. The novelty of this work primarily involves the synthetic chemistry of polymerization and detailed analysis and efficacy of active PAS delivery. A new kinetic model has been developed to explain the PAS release profiles. These polymers are biodegradable, cytocompatible and anti-mycobacterial in nature.

Graphical abstract

Introduction

The burden of mycobacterial infections worldwide and particularly in the developing countries continues to be a major healthcare challenge. Mycobacterial infections like tuberculosis (TB) [1] and leprosy [2] claim million lives worldwide. Other common mycobacterial diseases include buruli ulcer caused by Mycobacterium ulcerans [3] and respiratory diseases caused by rapidly growing Mycobacteria (RGM) [4], [5]. Some diseases have also been reported to be caused by Mycobacterium avium and Mycobacterium xenopi [6]. These diseases can be effectively treated by multi-drug therapy.

p-Aminosalicylic acid (PAS) is a well-known drug used in conjunction with first line drugs like isoniazid, rifampicin, etc. [7] that is active against mycobacterium and also finds use in treating TB. In recent years, this drug has found various other applications including the treatment of cancer and inflammatory bowel disease (IBD) [8], etc. PAS exhibits modest activity against Influenza virus A [9], [10]. PAS has also been derivatized to more potent forms to treat diseases such as influenza and manganism, etc.

In spite of its widespread potential as a cure for various diseases, its short serum half-life (15 min to 1 h), makes it difficult to use PAS for treatment. As an alternative, PAS loaded into different polymers to be delivered at the site of interest for enhanced efficacy has been proposed. In addition, owing to its anti-inflammatory properties, PAS loaded polymers may be well suited for regenerative medicine such that the release of PAS can suppress implantation-induced inflammation and potential bacterial infections.

As discussed in a recent review [11], loading of the drug in the polymer backbone ensures controlled release, higher loading and better processability. PAS has been incorporated in methacrylic polymers with maximum loading of ∼14% [12]. The low aminosalicylate uptake of these polyesters is a major limitation and has resulted in their limited success as therapeutics for mycobacterial diseases. Previous studies [13], [14] have reported the incorporation of aminosalicylates (PAS and 5-aminosalicylic acid) in poly(anhydride esters) (PAEs) backbone for drug release applications. PAEs are a preferred class of polymers for drug delivery applications. However, the stability of the drug in these polymers is questionable [13]. Moreover, the anhydride bonds, being extremely labile to hydrolysis, degrade very fast in the body [15]. There has, therefore, been an increasing demand for delivery systems that offer sustained release of such drugs while minimizing the usual side-effects associated with high dosage. Crosslinked polymers have gained increasing importance in this regard owing to their easily tunable properties with regards to drug/biomolecule delivery [16], [17]. Since, it is imperative to incorporate PAS in a polymer backbone with efficient loading to mediate its sustained release over time; polyesters may serve as suitable alternatives. Crosslinked polyesters offer sustained drug delivery [18] and are also capable of supporting tissue regeneration [19].

In the present study, we have synthesized crosslinked polyesters from xylitol and a diacid. Two different diacids, namely, adipic acid or sebacic acid were selected to study the effect of chain length on the polymer properties and release of PAS. It has been studied earlier how these slight modifications in the polymer chemistry offer a plethora of release rates and properties [20], [21]. As the stability of PAS is dependent on thermal conditions and the three functional groups on PAS are susceptible to conjugation, the reaction scheme was specially designed to successfully incorporate PAS in the polymer backbone and it remained pharmacologically active after release. The activity of these PAS-based polyesters against Mycobacterium smegmatis was also studied.

Section snippets

Materials

Adipic acid (AA), sebacic acid (SA) (SRL laboratories, India), p-aminosalicylic acid (PAS, I), benzyl chloroformate (Cbz-Cl) and xylitol (all from Sigma Aldrich) were used for the synthesis of crosslinked polyesters. The di-carboxylic acids were recrystallized in ethanol (Merck, India) and kept at 4 °C prior to use in order to remove organic impurities. Solvents used at various stages of the work include ethyl acetate (EtoAc), chloroform, dimethylsulfoxide (DMSO) and N,N-dimethylformamide (DMF),

Synthesis

The protection of –NH₂ group in PAS by Cbz-Cl for preventing its reaction with the diacyl chlorides is an important step in the synthesis of these polyesters. Synthesis without amine protection will result in amide linkages instead of ester bonds. Amide bonds are not susceptible to hydrolysis and are only cleaved under the influence of enzymes. The presence of three functional groups on PAS makes it susceptible to a number of chemical modifications. The carboxyl group is prone to form an

Conclusions

In the present work, we have synthesized two polyesters incorporating PAS in the backbone of the polymer. These polyesters show controlled release profiles and can hence be used as reservoirs for the controlled delivery of PAS over required periods of time. These polyesters also have controlled degradation rates and may be used as therapeutic implants for MDR-TB, which will degrade over time under physiological conditions. A detailed study of the drug release kinetics reveals that the release

Acknowledgments

The authors would like to thank Department of Biotechnology (DBT), India (BT/PR5977/MED/32/242/2012) for funding this work. K.C. acknowledges Ramanujan fellowship from the Department of Science and Technology (DST), India. G.M. thanks Department of Science and Technology (DST), India for J.C. Bose fellowship.

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Full length articleControlled release kinetics of p-aminosalicylic acid from biodegradable crosslinked polyesters for enhanced anti-mycobacterial activity

Abstract

Statement of significance

Graphical abstract

Introduction

Section snippets

Materials

Synthesis

Conclusions

Acknowledgments

Lancet Infect. Dis.

Tuberculosis

Polymer

Adv. Drug Deliv. Rev.

Acta Biomater.

Acta Biomater.

Vib. Spectrosc.

J. Chem. Thermodyn.

Lancet

Gastroenterology