Poly(ethylene glycol)-Grafted Polymers as Drug Carriers - American

Chapter 20

Poly(ethylene glycol)-Grafted Polymers as Drug Carriers

Downloaded by UNIV OF MELBOURNE on September 13, 2014 | http://pubs.acs.org Publication Date: August 5, 1997 | doi: 10.1021/bk-1997-0680.ch020

Ε. H. Schacht and K. Hoste Polymers Materials Research Group, Institute for Biomedical Technologies (IBITECH), University of Ghent, Krijgslaan 281, S4 bis, Ghent, Belgium

This paper describes the synthesis and evaluation of polyethylene glycol modified dextran and poly[N-(2-hydroxyethyl)-L-glutamine] (PHEG). The graft copolymers show aggregate formation in the liquid and solid state. In vitro and in vivo biological evaluation revealed that the PEG-modified polymers are interesting as potential drug carriers.

Chemotherapy often has limited success because of a lack in cell selectivity for the conventional dosage forms. Undesirable interaction with non-target cells results in unwanted side effects. This phenomenon is severely hampering cancer chemotherapy. Over the past two decades extensive research has been devoted to the design of advanced drug delivery systems that can deliver the active agent to the preferred site of action. Among the various concepts that have been proposed to achieve a more efficient drug delivery is the macromolecular prodrug approach. In this concept, a drug is linked onto a polymeric carrier. This carrier can be inert or biodegradable. The polymer can be designed to contain structural elements that can modify the water/lipid solubility. In addition, interaction with target cells can be promoted by introducing so called targeting groups onto the polymer backbone^/-2). An additional variable is the linkage inbetween the carrier and the drug moiety. Spacer groups can be introduced aiming to provide site selective drug release. Oligopeptides which are a good substrate for target associated enzymes are attractive spacer candidates ft). This polymeric prodrug concept was first presented in a comprehensive model by Ringsdorf in \9Ί5(4). In designing proper macromolecular prodrugs the solubilizing component is an important contributor. Since a lot of drugs and peptidic spacer groups are hydrophobic, attachment of such groups onto hydrophilic polymers seriously reduces the water solubility and limits their parental applicability. Hydrophobic side groups tend to aggregate and eventually cause precipitation. Water solubility of the conjugates can be enhanced by introducing hydrophilic © 1997 American Chemical Society

In Poly(ethylene glycol); Harris, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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solubilizers which can prevent aggregation or prevent aggregates from precipitation. A suitable candidate for promoting polymer-drug conjugate solubility is polyethylene glycol (PEG). PEG is known to be a non-toxic and non-immunogenic water-soluble polymer. Abuchowski and co-workers demonstrated that substitution of proteins with PEG makes the proteins less immunogenic and more stablefJ-7). At present, PEG-enzyme conjugates are accepted for clinical application^. Considering the substantial amount of biological data, PEG is an attractive polymer for modification of biologically active polymers. In the macromolecular prodrug approach PEG-ylated polymers, block copolymers or graft copolymers, are attractive carriers. PEG-containing Block Copolymers A first example of a PEG-containing block copolymer used as drug carrier was reported by Ringsdorf et al (9). Amino terminated PEG was used as initiator for the ring opening polymerization of the N-carboxyanhydride (NCA, Leuch's anhydride) of L-lysine. In the block copolymer the ε-amino groups were partially acylated with palmitoyl groups in order to promote aggregation. Sudan Red 7B solubilisation studies revealed that the block copolymers formed micelles with PEG as a hydrophilic shell and palmitoyl substituted poly(L-lysine) (PLL) as the inner core. Differential Scanning Calorimetry (DSC) with liposomes indicated that the block copolymer interacts with and probably penetrates lipid membranes. Furthermore, the PEG-PLL block copolymers were substituted with sulfidoderivatives of cyclophosphamide (CP), an alkylating antitumor agent(70-71). Normally, such cyclophosphamide derivatives are hydrolyzed rapidly to give the active metabolite 4-hydroxycyclophosphamide. With the PEG-PLL derivatives however, in vitro studies indicated that the cyclophosphamide-block copolymer acts as an intracellular depot for the active metabolite of cyclophosphamide. Cellular uptake of the block copolymer occurs prior to sustained release of the active dmg(12). Another block copolymer, containing PEG, which has been used as a drug carrier is described elsewhere in this book by Kataoka. Kataoka et al. synthesized block copolymers of PEG and poly(aspartic acid) and used them as carriers for the hydro phobic anti-cancer drug adriamycine^ij. In vitro and in vivo micelle formation of PEG-P(Asp(ADR)) was confirmed by different techniques(14). Again, PEG formed the hydrophilic shell of the micellar structure. Furthermore, it was shown that free adriamycine could be physically trapped into the PEG-P(Asp(ADR)) micelle^5). It was demonstrated that micelles formed by block copolymers can act as an efficient reservoir for free drugs. The above described examples illustrate very well that PEG-containing copo lymers can form micellar structures which are capable of carrying hydrophobic drugs. The drug might be covalently linked to the carrier or can be entrapped physically into the micelles. Both cases show the same benefit : introducing PEG results in improved solubilisation of hydrophobic drugs. In addition to PEG containing block copolymers, PEG-grafted copolymers are an interesting alternative as drug carrier. Examples are described in the next section.


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PEG-Grafied Polymers as Drug Carriers


PEG-containing Graft Copolymers A large number of synthetic polymers have been proposed as carriers for preparing macromolecular prodrugs(7 6-23). Among them are the polysaccharide dextran (Figure 1) and the poly(ot-amino acid)derivative, poly-[N-(2-hydroxyethyl)-L-glutamine] (PHEG) (Figure 2). We have selected both polymers for the preparation of macromolecular derivatives of cytotoxic and anti-bacterial agents(24-26). It was observed that the maximal acceptable drug content in the conjugate was limited by a lack of water solubility. In order to improve the solubilizing ability of the carriers, polyethylene glycol was grafted onto the polymer backbone. Synthesis. Dellacherie reported before the preparation of PEG-grafted dextran by reaction of a-methyl-ω-amino-polyethylene glycol with epichlorohydrin activated dextran (Figure 3)(27). Gel permeation chromatography (GPC) indicated the presence of a high molecular weight polydisperse polymer. A drawback of this preparation method is the formation of less defined intermediates during epichlorohydrin activation . Moreover, the alkaline conditions (pH 10-11) required for coupling can induce polysaccharide depolymerization. A more elegant method for synthesizing PEG-substituted dextran was developed in our research group(28). Dextran-PEG was easily obtained by reaction of 4nitrophenyl chloroformate activated dextran with an equivalent amount of a-methylω-amino-polyethylene glycol (PEG-NH ) (Figure 4). 2

PEG-amine was prepared by quantitative conversion of the PEG-hydroxyl group into the tosylate and subsequent amination with aqueous ammonia (Figure 5)(29). In order to remove non-reacted PEG-NH , the reaction mixture was passed over a strong acid ion exchange resin. The reaction product was finally isolated by preparative (GPC). Direct isolation via preparative GPC was not feasible since the PEG-modified polymers tend to solvate free PEG-NH . This was clearly demonstrated by analytical GPC analysis of a mixture PEG-dextran and FITC-labeled PEG(Figure 6). In analytical GPC, PEG-FITC appears as a single peak that can be detected by UV-detection at 492 nm. At this detection wavelength no signal is observed for PEG-dextran. However, analysis of a mixture of dextran-PEGand PEGFITC gives one broad UV-sensitive signal corresponding with the elution peak observed by refractive index detection of dextran-PEG alone. Phase separation of dextran and PEG on a molecular level in a dextran-PEG conjugate can result in the formation of a PEG-core in which free P E G - N H can be 2

2

2

trapped. This implies that conjugates isolated by precipitation or by preparative GPC may be contaminated. In such case N M R analysis can give erratic data concerning conjugate composition. The method described above allows the preparation of PEG-grafted dextrans with well controlled degree of substitution and different PEG lengths. In a similar way, a series of PEG-grafted PHEG-derivatives were prepared(30).



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POLY(ETHYLENE GLYCOL)

OH

-

Figure 1. Dextran

ο II

NH-CH - C (CH

2

)

3-.

2

I 0=C —NHCH CH OH Figure 2. Poly-[N-(2-hydroxyethyl)-L-glutamine] 2

2

Dex—O—0%-CH—CH^-NH—PEG (*)H Figure 3. Dextran-PEG prepared by epichlorohydrin activated dextran


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PEG-NE^

PEG-OH Downloaded by UNIV OF MELBOURNE on September 13, 2014 | http://pubs.acs.org Publication Date: August 5, 1997 | doi: 10.1021/bk-1997-0680.ch020

PEG-Grafted Polymers as Drug Carriers

Ο P-O-C-NH-PEG Ο P-OH

/ V

P_0-C-0—

Poly(ethylene glycol)-Grafted Polymers as Drug Carriers - American

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