Elsevier

Biomaterials

Volume 235, March 2020, 119728
Biomaterials

HPMA-based star polymer biomaterials with tuneable structure and biodegradability tailored for advanced drug delivery to solid tumours

https://doi.org/10.1016/j.biomaterials.2019.119728Get rights and content

Abstract

Design, controlled synthesis, physico-chemical and biological characteristics of novel well-defined biodegradable star-shaped copolymers intended for advanced drug delivery is described. These new biocompatible star copolymers were synthesised by grafting monodispersed semitelechelic linear (sL) N-(2-hydroxypropyl)methacrylamide copolymers onto a 2,2-bis(hydroxymethyl)propionic acid (bisMPA)-based polyester dendritic core of various structures. The hydrodynamic diameter of the star copolymer biomaterials can be tuned from 13 to 31 nm and could be adjusted to a given purpose by proper selection of the bisMPA dendritic core type and generation and by considering the sL copolymer molecular weight and polymer-to-core molar ratio. The hydrolytic degradation was proved for both the star copolymers containing either dendron or dendrimer core, showing the spontaneous hydrolysis in duration of few weeks. Finally, it was shown that the therapy with the biodegradable star conjugate with attached doxorubicin strongly suppresses the tumour growth in mice and is fully curative in most of the treated animals at dose corresponding approximately to one fourth of maximum tolerated dose (MTD) value. Both new biodegradable systems show superior efficacy and tumour accumulation over the first generation of star copolymers containing non-degradable PAMAM core.

Introduction

Polymer materials have been increasingly studied in recent decades as potential biomaterials for various applications in medicine. Moreover, a considerable number of water-soluble polymer materials with bound anti-cancer therapeutics suitable for drug delivery to tumour tissue have been developed using macromolecular chemistry, given that macromolecules, including polymer drug conjugates, polymer micelles and polymer particles, can passively accumulate in solid tumours due to enhanced permeability and retention (EPR) effect [[1], [2], [3]]. The conjugation of low-molecular-weight drugs with water-soluble polymers confers several advantages, such as prolonged circulation in blood and higher accumulation in tumour tissue, enabling highly effective anti-tumour therapy with minimal drug side effects [4,5]. The bioactive molecules are typically covalently attached to the polymer carrier via biodegradable spacers, thus enabling the safe transport of inactive drug into the blood stream and controlled drug release in the target tissue [[6], [7], [8]].

Based on their excellent water-solubility, biocompatibility and non-immunogenic properties, N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer biomaterials [5,6,9] are among the most studied water-soluble synthetic polymer carriers. In recent decades, conjugates based on HPMA copolymers containing the cytotoxic drugs doxorubicin (DOX), pirarubicin (THP), or docetaxel [10,11], bound via a pH- sensitive hydrazone bond, have been developed as highly potent drug-delivery systems for cancer treatment in vivo [12]. The accumulation of these polymer carriers in tumour tissue is molecular-weight dependent [4]. In addition, the size and shape of polymer carriers is critical to their biological behaviour, and various structures of HPMA copolymers with different molecular weights have thus been designed [13,14].

Such biomaterials as macromolecules or particles with a higher hydrodynamic diameter (in the range of 10–150 nm) accumulate in tumour tissue to a higher extent than small molecules [15]. HPMA polymer carriers with various structures, such as grafted, block, branched and star polymer conjugates have been designed to fulfil the requirements for an effective drug delivery system (DDS), as their higher hydrodynamic diameter predetermines a higher accumulation in tumour tissue than that of linear polymers with a molecular weight below the limit of the renal threshold [5,7]. Despite similarities in the “grafting to” procedure used in the synthesis of grafted and star polymers, biodegradable star-like polymer precursors represent the most effective way to increase molar mass and hydrodynamic size in a defined manner with low molecular-weight dispersity above the limit of renal threshold [16].

Dendrimers are frequently used as cores for star-like polymeric materials due to their well-defined structure and availability in terms of diversity of molecular mass and functional groups. We previously showed that high-molecular-weight star-like polymer DOX conjugates with a polyamidoamine (PAMAM) dendrimer core and HPMA copolymer arms can accumulate in solid tumours in mice to a considerably greater extent than corresponding linear polymer conjugates with DOX [17]. However, the safe elimination of these polymers via the kidney has not yet been verified. The renal threshold for linear polymer carriers based on HPMA is approximately 50–70 kg mol−1 [17]. The PAMAM dendrimer core is non-biodegradable, as well as corresponding star-like polymers, meaning that star-like polymer precursor with either an enzymatically or a reductively cleavable spacer between the core and the polymers arms have been designed in order to ensure their renal excretion [4,18]. A disadvantage of this approach is the multistep synthetic procedure required for star polymers and the slow biodegradation rate of the enzymatically cleavable spacer. Moreover, both enzymatically and reductively degradable spacers are designed to be degradable only within the target cell, meaning that the star-like polymer is transported in the bloodstream and subsequently into tumour tissue as a high-molecular-weight structure of bulky hydrodynamic size. Indeed, the considerable hydrodynamic size can restrict extravasation and penetration within the tumour tissue, decrease eliminability from the organism and decrease the maximum tolerated dose (MTD) [19]. These limitations may exclude these in principle non-degradable materials from translation into clinical studies. The use of biodegradable dendrimer cores may facilitate the design of high-molecular star-like polymer carriers with enhanced tumour accumulation without the need to incorporate a degradable spacer. Polyester cores have been shown to hydrolytically degrade within the organism, and thus represent as a degradable core of high-molecular-weight polymer materials that can be degraded even extracellularly in tumour tissue, thereby facilitating pervasion into tumours [12,20].

In the present study, we focused on the design and novel advanced and controllable synthesis of high-molecular-weight star-like polymeric materials based on HPMA copolymers using 2,2-bis(hydroxymethyl)propionic acid (bisMPA) dendrons and dendrimers as the biodegradable cores [21]. The bisMPA cores are biocompatible dendritic cores, rendering them valuable candidates as a multifunctional core for a new generation of star-like carriers. To obtain high-molecular-weight polymer materials with a well-defined structure, the use of monodispersed semitelechelic linear copolymers with reactive end group functionality close to one is necessary. Therefore, sL copolymers were synthesised using a controlled polymerisation technique termed RAFT (Reversible Addition–Fragmentation chain Transfer) polymerisation using chain transfer agent with suitable reactive groups. The physico-chemical characteristics, degradation kinetics, in vitro cytotoxicity and in vivo anti-tumour activity as well as the biodistribution in tumour-bearing mice are described in detail here.

Section snippets

Chemicals

Tert-butyl alcohol, methanol, ethyl acetate, dimethyl sulfoxide (DMSO), 2,4,6-trinitrobenzene-1-sulfonic acid (TNBSA), N,N-dimethylacetamide (DMA), CuBr, 4-quinolinol, and N,N-diisopropylmethylamine (DIPEA) were purchased from Merck-Sigma Aldrich (Germany). Doxorubicin hydrochloride (DOX) was purchased from Meiji Seika, Japan. Dendritic cores; PFD-G4-TMP-azide (48 end groups), PFD-G3-TMP-azide (24 end groups), and PFD-G2-TMP-azide (12 end groups) dendrimers; PFD-G5-acetylene-ammonium (32 end

Results and discussion

Polymer conjugates based on HPMA with bound DOX, particular with linear, graft, or star-like polymer structures, have been studied extensively in a number of studies and shown to have enhanced efficacy in comparison with free drugs and other systems [4,5,[27], [28], [29], [30], [31], [32], [33], [34]]. Nevertheless, polymer biomaterials that combine the advantages of high-weight star-like conjugates during transport in the bloodstream, EPR-driven tumour accumulation and linear polymers upon

Conclusion

Here, we describe the design, synthesis, and biological evaluation of novel biodegradable star polymer biomaterials based on a biodegradable bisMPA dendrimer or dendron core grafted with biocompatible sL HPMA copolymers. The novel synthetic strategy allows for the adjustment of the molecular weight Mw and Dh of the biomaterials from 87 to 720 kg mol−1 and from 13 to 31 nm, respectively, based on the selection of dendritic core, sL HPMA copolymer, and ratio of polymer to dendritic core. The

Acknowledgments

This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic within the Inter-excellence program (project MSMT LTAUSA18083) and within the National Sustainability Program II (Project BIOCEV-FAR LQ1604) and by the project “BIOCEV” (CZ.1.05/1.1.00/02.0109); Czech Science Foundation (project 17-13283S and 19-05649S); Ministry of Health of the Czech Republic (project 16–28600A); and the National Institutes of Health (P30CA014520).

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