Review articleHPMA copolymer–anticancer drug conjugates: design, activity, and mechanism of action
Introduction
The idea of using macromolecules as carriers of (anticancer) drugs developed continuously over the last 90 years. Ehrlich in 1906 coined the phrase ‘magic bullet’, recognizing the importance of biorecognition [1]. DeDuve discovered that many enzymes are localized in the lysosomal compartment and the lysosomotropism of macromolecules [2]. The conjugation of drugs to synthetic and natural macromolecules was initiated nearly 50 years ago. Jatzkewitz used a dipeptide spacer to attach a drug to polyvinylpyrrolidone in the early fifties [3] and Ushakov's group synthesized numerous water-soluble polymer–drug conjugates in the sixties and seventies (see, for example, Ref. [4]). Mathé et al. [5] pioneered conjugation of drugs to immunoglobulins, setting the stage for targeted delivery. Finally, Ringsdorf [6] presented the first clear concept of the use of polymers as targetable drug carriers.
The development of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers as anticancer drug carriers was the result of systematic research on hydrophilic biomedical polymers performed in one of the author's (J.K.) laboratory at the Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences (IMC) in Prague. In the late sixties and seventies, the IMC was a unique place to study hydrophilic biomedical polymers. Hydrogels were designed there by Wichterle and Lı́m [7] as well as soft contact lenses [8]. This was the driving force behind a detailed study of the relationship between the structure of soluble and cross-linked hydrophilic polymers and their biocompatibility [9], [10], [11], [12], [13], [14]. Based on these investigations, hydrogels have been used successfully in human medicine [15] and basic data on the structure–properties relationship were obtained, which permitted the design of new water-soluble polymeric drug carriers [16], [17].
A new hydrophilic, biocompatible polymer, based on N-(2-hydroxypropyl)methacrylamide (HPMA) [18], [19] was chosen as a candidate for a soluble polymeric drug carrier. The α-carbon substitution and the N-substituted amide bond ensured hydrolytic stability of the side-chains [13]. In addition, the crystallinity of the monomer guaranteed the absence of divinyl compounds (a problem with hydrophilic esters of the 2-hydroxyethyl methacrylate type) and the linearity of the macromolecules. The possibility to control the molecular weight distribution of macromolecular therapeutics is a prerequisite for their elimination from the organism. Oligopeptide side-chains were designed as drug attachment sites [20]. The important observation that oligopeptide sequences attached to HPMA copolymers were degradable in vivo and thus had potential as drug attachment/release sites was encouraging and crucial for the further development of HPMA copolymer based macromolecular therapeutics [21]. The chemistry of the synthesis of HPMA copolymer based macromolecular therapeutics was studied in detail [22], [23], [24]. Insulin [25] and ampicillin [26] were attached to HPMA copolymers by aminolysis of reactive polymeric precursors, whereas polymer conjugates containing N-(4-aminobenzensulfonyl)-N′-butylurea were prepared by copolymerization of HPMA and polymerizable derivatives of the drug [27].
The above mentioned initial studies resulted in numerous collaborations and indeed in an independent interest of other laboratories to pursue the potential of HPMA based therapeutics. This review attempts to present the state of the art developments of HPMA copolymer based macromolecular therapeutics and indicate the directions to be taken for the discovery of novel molecular targets and the design, synthesis, and evaluation of second generation conjugates. The review concentrates on HPMA copolymer conjugates only, however, the conclusions may be valid for other water-soluble macromolecular therapeutics.
Section snippets
Biological rationale
The rationale for the use of water-soluble polymers as carriers of anticancer drugs is based on the mechanism of cell entry. Whereas the majority of low molecular weight drugs enter the cell interior by diffusion via the plasma membrane, the entry of macromolecules is restricted to endocytosis [2]. Macromolecules captured by this mechanism are channeled to the lysosomal compartment of the cell. Endocytosis is a common term encompassing phagocytosis and pinocytosis. Phagocytosis describes the
Design of HPMA copolymer based macromolecular therapeutics
The design of macromolecular therapeutics must be based on a sound biological rationale. The HPMA copolymer–drug conjugate should be biorecognizable at two levels: at the plasma membrane to increase the recognition and internalization by a subset of target cells and intracellularly by lysosomal enzymes to release the drug from the carrier. The latter is a prerequisite for transport of the drug into the cytoplasm and nucleus resulting in biological activity [33], [34], [35], [36].
Biocompatibility of macromolecular therapeutics
Binding of anticancer drugs and targeting moieties such as antibodies to polymeric carriers improves their biocompatibility [36]. Using a non-toxic and non-immunogenic polymeric carrier, the decrease of non-specific toxicity of an attached drug (when compared to an unbound drug) may be mainly attributed to the change in body distribution. For example, anthracycline antibiotics have a non-specific cardiotoxicity and bone marrow toxicity, limiting the dose which can be administered. Binding of
Enhanced permeability and retention effect
It is now well accepted that the enhanced permeability and retention (EPR) effect is the predominant mechanism by which soluble macromolecular anticancer drugs exhibit their therapeutic effect on solid tumors. The phenomenon is attributed to high vascular density of the tumor, increased permeability of tumor vessels, defective tumor vasculature, and defective or suppressed lymphatic drainage in the tumor interstitium [74], [75]. Other factors, however, may have an opposite effect. For example,
New targeting strategies
There is a possibility of applying concepts of cell biology in targeted drug delivery. The incorporation of certain chemical features (targeting moieties) into a macromolecule can enormously enhance its rate of uptake by cells, by causing it to adhere to the plasma membrane being internalized [36], [98]. New developments in the design of targetable macromolecular therapeutics are discussed below.
Experimental cancer
HPMA copolymer conjugates have shown activity toward various tumor models. HPMA copolymer–daunomycin conjugate was active in the treatment of experimental Walker sarcoma [78] and L1210 leukemia [116], [117]. HPMA copolymer–doxorubicin conjugates were active against L1210 leukemia [118], B16F10 melanoma [119], M5076 [67], LS174T human colorectal carcinoma xenografts [67] and sensitive and resistant human ovarian carcinoma models [77], [94], [95], [97], [120]. HPMA copolymer platinates have shown
Gene and oligonucleotide delivery
Many laboratories are studying strategies to deliver genes or antisense oligonucleotides into somatic cells of patients [163]. With increasing interest in non-viral delivery systems, ways to overcome membrane barriers for gene delivery into the cytoplasm and/or nucleus are being evaluated [164]. Antisense oligonucleotides have been covalently bound to HPMA copolymers via disulfide [165] and amide [166] bonds.
Novel vectors, block and graft copolymers of HPMA, and comonomers containing quaternary
Mechanism of action of macromolecular therapeutics
Data on the mechanism of action of HPMA copolymer–DOX conjugates on human ovarian carcinomas in vitro and in vivo seem to support our hypothesis that macromolecular therapeutics activate different signaling pathways and possess different anticancer effects than free drugs. As a result of different pathways of internalization and subcellular trafficking, the HPMA copolymer–DOX conjugate might be more protected from cell detoxification mechanisms, resulting in an enhanced activation of apoptosis,
Conclusions and future directions
HPMA copolymers [18] and their conjugates with drugs (reviewed in Ref. [36]) have been one of the most extensively studied systems [176]. HPMA copolymer–anticancer drug conjugates have been found to be active against numerous cancer models and are in clinical trials. The scientific evidence as well as the results of clinical trials seem to indicate the great potential of macromolecular therapeutics in cancer treatment. The fact that the maximum tolerated dose of DOX in patients was several
Acknowledgements
Thanks to our Prague coworkers (Drs K. Ulbrich, B. Řı́hová, J. Strohalm, V. Chytrý, and V. Šubr) – that is the place where everything started; thanks to our coworkers at the University of Utah (Drs C.M. Peterson, J. Spikes, V. Omelyanenko, J.-G. Shiah, N.L. Krinick, D. Putnam, M. Demoy, J. Callahan, K. Fowers, A. Tang, M. Tijerina, C. Wang, and D. Wang) and in the United Kingdom (Drs J.B. Lloyd, R. Duncan, and L.C. Seymour) – those are the places where, in addition to Prague and Milan, the HPMA
References (178)
- et al.
Lysosomotropic agents
Biochem. Pharmacol.
(1974) - et al.
Poly[N-(2-hydroxypropyl)methacrylamide]. I. Radical polymerization and copolymerization
Eur. Polym. J.
(1973) - et al.
Poly[N-(2-hydroxypropyl)methacrylamide]. II. Hydrodynamic properties of diluted polymer solutions
Eur. Polym. J.
(1974) - et al.
Tyrosinamide residues enhance pinocytic capture of N-(2-hydroxypropyl)methacrylamide copolymers
Biochim. Biophys. Acta
(1984) - et al.
Targeting of N-(2-hydroxypropyl)methacrylamide copolymers to liver by incorporation of galactose residues
Biochim. Biophys. Acta
(1983) - et al.
Biological properties of targetable poly[N-(2-hydroxypropyl)methacrylamide]–antibody conjugates
J. Control. Release
(1985) Biodegradation of polymers for biomedical use
Controlled biodegradability of polymers – a key to drug delivery systems
Biomaterials
(1984)- et al.
Cis-aconityl spacer between daunomycin and macromolecular carriers: a model of pH sensitive linkage releasing drug from a lysosomotropic conjugate
Biochem. Biophys. Res. Commun.
(1981) - et al.
Polymers containing enzymatically degradable bonds. 5. Hydrophilic polymers degradable by papain
Biomaterials
(1980)