Multimodal functionalization of PLA for biological applications

Aliphatic polyesters, such as polyglycolide, polylactide, poly(ε-caprolactone), are an attractive class of polymers, which can be used for a broad range of practical applications from packaging to more sophisticated biomedical devices. One of the main reasons for the growing interest in this type of degradable polymers is that their physical and chemical properties can be opportunely tuned by proper functionalization. Due to the lack of functionalities along the polymer backbone, many efforts have been focused on the preparation of aliphatic polyesters with pendant functional groups. In general, two approaches have been used, to date: the first uses lactone derivatives bearing functional groups for the ring- opening polymerization; the second strategy consists in the post-polymerization modification of the polymer backbone, whose chain-ends can be tailored for specific applications, by direct grafting of functional groups. Within the family of aliphatic polyesters, polylactide (PLA) is one of the most promising industrial products for plastics with the prospective to replace polymers such as PET, polystyrene and polycarbonate. Currently, the main applications of PLA are in short-term packaging, owing its biodegradability, and in biomedical applications (drug delivery and tissue engineering) owing to its biocompatibility. Besides these characteristics, PLA offers many advantages such as being produced from renewable resources, being FDA-approved for biomedical applications, and commercially available in different molecular weights. Moreover, PLA can be formulated in nanoparticles useful as biocompatible “nanocontainers” for gene and drug delivery. Various hydrophobic and hydrophilic drugs can be efficiently encapsulated into the PLA-based nanoparticles resulting in a sustained and controlled release of the payload over the time. The great potentialities and versatility of smart polymeric nanomaterials render them one of the most exciting interfaces between chemistry and biology. This thesis describes the synthesis and the characterization by complementary techniques (such as NMR spectroscopy, GPC, and MALDI-ToF analyses) of new PLA derivatives obtained by two main approaches. The CuAAC click reaction represents the common synthetic strategy exploited for the PLA functionalization under mild conditions. All the newly synthesized PLA derivatives are eventually formulated in nanoparticles or micelles loaded with selected antitumoral drugs (i.e., salinomycin and doxorubicin) and intended to biological assays on osteosarcoma cells. Osteosarcoma is the most common primary malignant tumor of bone, with an annual incidence worldwide of approximately 5.6 cases per million, typically diagnosed in children and young adults and currently treated with surgery and intensive chemotherapy protocols based on the combination of cytotoxic drugs. The present thesis is structured in three chapters. The first chapter reports an overview of the main characteristics of PLA. It describes the currently available procedures for its production and functionalization, focusing on the characteristics of PLA-based block polymers and summarizing the main biological applications as drug delivery system. The chapter II reports an innovative method for the functionalization of commercial PLA with terminal alkyne units and the subsequent click coupling with azido-derivatives. The application of the solvent-free technique to the PLA derivatization represents an innovation to produce alkyne-grafted PLA without use of solvents, catalysts and in mild conditions. The “clickable” PLA was obtained by an unprecedented solvent-free intra-chain amidation, using propargylamine as alkyne donor. Alkyne-grafted PLA derivatives have been exploited as building blocks for access to a variety of functionalized polymers by Cu(I)-catalyzed cycloaddition reaction (CuAAC) with three different azides. The commercial methoxypolyethylene glycol azide (m-PEG-N3) and the azide-fluor 545 have been selected as models of hydrophilic polymer and fluorescent probe, respectively. Moreover, the CuAAC click coupling with a newly synthesized azide-folate (FA-N3) has been also investigated. All the three PLA derivatives (PLA- PEG, PLA-Flu and PLA-FA) have been formulated in nanoparticles loaded with Salinomycin, by nanoprecipitation technique. The chapter III reports a multistep synthetic procedure for the preparation of a three-arm star PLA-PEG, decorated with the integrin-targeting RGD peptide, designed for antitumoral applications. Arginine-glycine-aspartic acid peptide (Arg-Gly-Asp or RGD peptide) is a cell recognition motif, specifically recognized by the αvβ3 integrin receptor, over-expressed in tumors and involved in the regulation of tumor angiogenesis. The three-arm star PLA was prepared, in a typical core-first approach, by ring-opening polymerization (ROP) of lactide using glycerol as a trifunctional alcohol initiator. In this step, the setting of the correct reaction time was imperative to finely control the molecular weight of the product. The grafting of alkyne moieties on the terminal hydroxyl groups of the star PLA, carried out by esterification with pentynoic anhydride, was followed by CuAAC click reaction with polyethylene glycol monoazide (monoazide-PEG), to obtain an amphiphilic star PLA-PEG copolymer. To decorate the star polymer with the tumor-targeting ligand RGD peptide, the hydroxyl terminal groups of PEG have been esterified with pentynoic anhydride to finally coupled the RGD-azide. As the decoration of nanoparticles with RGD peptide has recently emerged as a useful strategy for targeting tumor cells, the final star PLA-PEG-RGD will be formulated in micelles incorporating a suitable anticancer drug (i.e., Doxorubicin) for biological assays on osteosarcoma cells.