1 Introduction
1.8 Poly-ε-caprolactone
Poly-ε-caprolactone (PCL) was made commercially available after efforts to identify biodegradable synthetic polymers. Originally synthesised by the Carothers group in the early 1930s, PCL is most commonly prepared via the ring-opening polymerisation of ε-caprolactone in the presence of a stannous octoate catalyst.[175] Alternative polymerisation methods have
58 since been explored including the use of anionic[176] and cationic[177,178] catalysts as well as free radical ROP.[179] A thorough review of the synthesis of PCL has been written by Labet and Thielemanns.[180] PCL is a hydrophobic, semi-crystalline polymer, though this decreases as a
function of molecular weight[181], with excellent blend-compatibility and low melting point (59-64 °C). While PCL is readily degraded by bacteria and fungi, animals lack suitable enzymes for bulk macromolecular degradation.[182] While enzymes may not be involved, PCL
has been shown to undergo in vivo degradation via a two stage hydrolytic degradation mechanism.[183,184] In stage 1 high molecular weight PCL (> 50,000 g mol-1) undergoes random
hydrolytic scission. If water is unable to penetrate towards the centre of the bulk material only surface erosion occurs, resulting in a lower observed Mn but no change in overall mass. In stage
2 small fragments of PCL (< 3,000 g mol-1) undergo intracellular degradation within the phagosomes of macrophages and giant cells to yield 6-hydroxyl caproic acid.[183] This is subsequently entered into the citric acid cycle and ultimately eliminated as CO2 and H2O.[185]
Sun et al. observed 92% excretion of their tritium-labelled PCL (Mw 3,000) after 135 days.[186]
Devices fabricated from high molecular weight PCL have been shown to exhibit a particularly long degradation and elimination time (ca. 3 years[184,186]), particularly when compared to other biodegradable hydrophobic polymers (e.g. polylactides and polyglycolides). In the 1970’s and 80’s short degradation and total elimination times were considered desirable due to risks perceived with long-proliferating biomaterials. This contributed to a reduced interest in PCL. To date the degradation and elimination of PCL is under-researched and this is likely due to the significant time commitment in producing a comprehensive analysis. Improvements have been made; Lam, Teoh and Hutmacher reported a method of accelerated degradation which followed a similar profile to the in vivo degradation of PCL.[187]
Over the past 30 years there has been a resurging interest in PCL within the field of tissue engineering. Tissue engineering benefits significantly from the properties of PCL for a number of reasons: (i) synthesis is inexpensive and PCL’s low melting point and rheological properties make it easy to manufacture; (ii) it can be made into porous 3D networks which allow for cell growth and the flow of nutrients and metabolic waste;[188] (iii) it already has FDA approval for
use in medical devices and formulations. Commercially PCL has been used for long term, subdermal delivery of female contraceptives under the name “Capronor” and is currently sold as a surgical suture material under the name “MONOCRYL”. While the material exhibits a range of desirable properties its poor tensile strength has limited it to non-load bearing
59 biomedical applications. This has prompted research into PCL blends with carbon nanotubes to improve the overall mechanical properties.[189,190] A more comprehensive review of the tissue engineering applications of PCL are beyond the scope of this work but have been thoroughly covered by a number of other authors in the field.[5,191,192]
The increased focus on PCL for tissue engineering has, in turn, increased interest within adjacent fields such as polymeric drug delivery. Its hydrophobicity provides an ideal environment for the association of similarly hydrophobic drug molecules. However, nanoscale polymer-drug conjugates are rapidly concentrated in the liver and eliminated by the mononuclear phagocyte system (MPS).[193] While this is potentially useful, if targeting these systems is desirable, most systems benefit from increased circulation times for active or passive targeting. A discussion of purely PCL-based nanoscale delivery systems is provided in Chapter 4 of this thesis. To achieve improved circulation times, amphiphilic copolymers of PCL are synthesised by reaction with a suitably hydrophilic material, frequently PEG.
PEG-PCL block copolymer synthesis is typically achieved by ROP of lactones with a PEG macroinitiator.[193] The resulting block copolymers have been used to encapsulate poorly water soluble anti-cancer drugs. Recently, Danafar et al. produced well-defined PEG-PCL diblock copolymer micelles to deliver the natural product sulforaphane with excellent encapsulation efficiency (86%).[194] Eatemadi et al. synthesised PCL-PEG-PCL copolymers and showed a reduction in IC50 of 29% and 46% for cisplatin and doxorubicin respectively, when associated
with the copolymer compared to the free drug.[195] The majority of PCL-PEG nanomaterials are reported as spherical nanoparticles (either micelles or vesicles) however Loverde and co- workers showed that worm-like structures exhibit a 2-fold increase in drug loading compared to their spherical counterparts.[196]
A criticism of PCL-based nanomedicines is that formation of polymer-drug conjugates mainly relies on non-covalent interactions. PCL lacks a repeating pendent functionality through which to facilitate covalent attachment. Some methods for modifying the back-bone structure of PCL have been proposed.
60 R O O N H d O O O O O O O O 5 HN NH HN a c b PCL PCL PCL O H O R O O H N d O O O H O i) LDA ii) -50 °C 15 min, toluene R O O H N d O O O H O x y x y S S R = SH DMPA RT, 2 h, THF Br
Scheme 1.17 – Synthetic strategy for PEG-(PCL)8 star copolymer
Buwalda et al. initially synthesised an 8-arm PEG-PCL star block copolymer before functionalizing with propargyl bromide in an anionic post-polymerization step. Further modification via photoradical thiol-yne addition of benzyl mercaptan yielded PCL with pendent benzylthioether (BTE) groups (Scheme 1.17).[197] The resulting PEG-(PCL-BTE)8
micelles were analysed by TEM and found to have an average diameter of 50 nm and a 50% increase in drug loading and encapsulation efficiency compared to PEG-(PCL)8 (average
diameter ~ 200 nm). The authors attribute these effects to the presence of BTE moieties and some degree of π-π stacking interactions. While post-polymerisation modification of the polymer backbone directly highlighted its effects, the degree of substitution by propargyl bromide was found to be only 8%, highlighting the difficulty in achieving quantitative modification. Zhai et al. described an alternative approach by first synthesising an ε- caprolactone monomer derivative. 1,4-Cyclohexanediol was modified via sequential Michael addition, PCC oxidation and Baeyer-Villiger oxidation before undergoing ROP with mPEG 5000 (Scheme 1.18).[198]
61 OH OH O O + KOtBu THF O OH O O PCC DCM O O O O O O O O O O O OH 113 Sn(Oct)2 O O O 113 O O H O O O x MCPBA, NaHCO3 DCM
Scheme 1.18 – Synthetic route employed by Zhai et al. to yield paclitaxel modified PEG-PCL diblock copolymers [198]
Complete deprotection of pendent t-butyl moieties was reported and esterification of subsequent carboxyl groups introduced pendent vinyl moieties to 28% of the total PCL block. Subsequent reaction with paclitaxel modified 13% of the total PCL block. While the method introduces more synthetic steps than the method proposed by Buwalda et al., significantly more pendent functionalities can be introduced when starting from a pre-modified monomer.
While PEG has been shown to expand the applicability of PCL, these systems ultimately suffer from the same ABC-phenomenon problems as all PEG-based materials. It is therefore desirable to consider alternative hydrophilic polymers and potential conjugation methods to create a new range of amphiphilic materials.