As we enter the twenty-first century, research at the interface of polymer chemistry and the biomedical sciences has given rise to the first nano-sized (5-100 nm) polymer-based pharmaceuticals, the 'polymer therapeutics'. Polymer therapeutics include rationally designed macromolecular drugs, polymer-drug and polymer-protein conjugates, polymeric micelles containing covalently bound drug, and polyplexes for DNA delivery. The successful clinical application of polymer-protein conjugates, and promising clinical results arising from trials with polymer-anticancer-drug conjugates, bode well for the future design and development of the ever more sophisticated bio-nanotechnologies that are needed to realize the full potential of the post-genomic age.

The dawning era of polymer therapeutics

Poly(ethylene glycol) (PEG) is the most used polymer and also the gold standard for stealth polymers in the emerging field of polymer-based drug delivery. The properties that account for the overwhelming use of PEG in biomedical applications are outlined in this Review. The first approved PEGylated products have already been on the market for 20 years. A vast amount of clinical experience has since been gained with this polymer--not only benefits, but possible side effects and complications have also been found. The areas that might need consideration and more intensive and careful examination can be divided into the following categories: hypersensitivity, unexpected changes in pharmacokinetic behavior, toxic side products, and an antagonism arising from the easy degradation of the polymer under mechanical stress as a result of its ether structure and its non-biodegradability, as well as the resulting possible accumulation in the body. These possible side effects will be discussed in this Review and alternative polymers will be evaluated.

Poly(ethylene glycol) in Drug Delivery: Pros and Cons as Well as Potential Alternatives

Polymers can be conjugated to anticancer drugs and proteins to improve their therapeutic index. Some such conjugates are in routine clinical use and there are exciting advances in development, such as polymer-based combination therapies.

Polymer conjugates as anticancer nanomedicines

In medicine, nanotechnology has sparked a rapidly growing interest as it promises to solve a number of issues associated with conventional therapeutic agents, including their poor water solubility (at least, for most anticancer drugs), lack of targeting capability, nonspecific distribution, systemic toxicity, and low therapeutic index. Over the past several decades, remarkable progress has been made in the development and application of engineered nanoparticles to treat cancer more effectively. For example, therapeutic agents have been integrated with nanoparticles engineered with optimal sizes, shapes, and surface properties to increase their solubility, prolong their circulation half-life, improve their biodistribution, and reduce their immunogenicity. Nanoparticles and their payloads have also been favorably delivered into tumors by taking advantage of the pathophysiological conditions, such as the enhanced permeability and retention effect, and the spatial variations in the pH value. Additionally, targeting ligands (e.g., small organic molecules, peptides, antibodies, and nucleic acids) have been added to the surface of nanoparticles to specifically target cancerous cells through selective binding to the receptors overexpressed on their surface. Furthermore, it has been demonstrated that multiple types of therapeutic drugs and/or diagnostic agents (e.g., contrast agents) could be delivered through the same carrier to enable combination therapy with a potential to overcome multidrug resistance, and real-time readout on the treatment efficacy. It is anticipated that precisely engineered nanoparticles will emerge as the next-generation platform for cancer therapy and many other biomedical applications.

Engineered Nanoparticles for Drug Delivery in Cancer Therapy

Poly(amidoamine) (PAMAM) dendrimers: from biomimicry to drug delivery and biomedical applications

A copolymer of N-(2-hydroxypropyl) methacrylamide (HPMA) and N-methacryloyltyrosinamide was prepared and fractionated using Sepharose 4B/6B (1:1) chromatography to produce eight HPMA copolymer fractions of narrow polydispersity and mean molecular weight (Mw) ranging from 12 to 778 kD. These fractions were radioiodinated and injected intravenously, subcutaneously, and intraperitoneally into rats. Their bloodstream-concentration profiles were monitored and rates of excretion assessed. Following intravenous administration the circulating blood volume available to the copolymers was not molecular-weight-dependent. A molecular-weight threshold limiting glomerular filtration was identified at approximately 45 kD, and preparations greater than this threshold were lost from the bloodstream only slowly by extravasation. Molecular weight did not influence the movement of copolymers from the peritoneal compartment to the bloodstream after intraperitoneal injection. The transfer rates observed could be accounted for by bulk phase lymphatic drainage alone, no transcapillary routes being implicated. Following subcutaneous administration the largest HPMA copolymer fraction (Mw = 778 kD, diameter approximately 30 nm) showed increased retention at the site of the injection, approximately 20% of the dose remaining there after 21 days. This could result from physical restriction of movement or from internalization into local phagocytic cells. The smaller copolymer fractions moved readily into the bloodstream whence they were either lost in the urine or they gradually penetrated into other tissues and organs. Long-term (21 days) body distribution of copolymers following both intraperitoneal and subcutaneous administration showed size-dependent accumulation in organs of the reticuloendothelial system.

Effect of molecular weight (Mw) of N-(2-hydroxypropyl)methacrylamide copolymers on body distribution and rate of excretion after subcutaneous, intraperitoneal, and intravenous administration to rats.

Activity of N-(2-hydroxypropyl)methacrylamide copolymers containing daunomycin against a rat tumour model.

Macromolecular prodrugs for use in targeted cancer chemotherapy: melphalan covalently coupled to N- (2-hydroxypropyl) methacrylamide copolymers

Two N-(2-hydroxypropyl)methacrylamide (HPMA, 1) copolymer-chlorin e6-anti-Thy 1.2 antibody conjugates were synthesized. They differ in the method of antibody binding. In conjugate 11, the polyclonal anti-Thy 1.2 antibody was bound via Nϵ-amino groups of lysine residues. Conjugate 14 contained anti-Thy-1.2 antibodies bound via oxidized carbohydrate groups present near the hinge region in the Fc part of the antibody molecule. The photodynamic activity of these conjugates towards mouse splenocytes in vitro was compared with the activity of a nontargetable HPMA copolymer-chlorin e6 conjugate (15) and free chlorin e6. Whereas free chlorin e6 was cytotoxic both when kept in the dark and when splenocytes were irradiated, the binding of the drug to HPMA copolymers considerably decreased its cytotoxicity in the dark. Conjugates 11 and 14 containing targeting polyclonal anti-Thy 1.2 antibodies were more biologically active compared to the nontargetable conjugate 15. Conjugate 14, which contained anti-Thy 1.2 antibodies bound at a specific location was the most active both in its photodynamic effect on viability of splenocytes and/or suppression of the primary antibody response of mouse splenocytes towards sheep red blood cells in vitro. The results demonstrate the importance of the chemistry of antibody binding on the biological activity of targetable polymeric drugs.

Targetable photoactivatable drugs, 2. Synthesis of N‐(2‐hydroxypropyl)methacrylamide copolymeranti‐thy 1.2 antibody‐chlorin e6 conjugates and a preliminary study of their photodynamic effect on mouse splenocytes in vitro

Targeted polymeric prodrugs based on N-(2-hydroxypropyl) methacrylamide were tested on human peripheral blood lymphocytes or mouse splenocytes triggered in vitro to proliferation by T cell specific (Con A) or B cell specific (S. aureus Cowan 1) mitogens. Only a selective inhibition of 3H- thymidine incorporation by T lymphocytes was observed after in vitro incuba tion with prodrugs prepared by covalent attachment of daunomycin or adriamycin and antibody (anti-CD3; anti Thy 1.2) to biodegradable oligopeptide side chains of soluble synthetic copolymer HPMA. The in vitro results were confirmed in vivo by inhibition of antibody response to thymus dependent (ARS-BGG) or thymus independent (TNP-E. coli) antigens in normal Balb/c mice and in athymic nu/nu Balb/c mice. After administration of biocompatible, antibody targeted prodrugs only the antibody response against ARS-BGG was considerably reduced.

Jiri Strohalm

Papers

Effect of molecular weight (Mw) of N-(2-hydroxypropyl)methacrylamide copolymers on body distribution and rate of excretion after subcutaneous, intraperitoneal, and intravenous administration to rats.

Activity of N-(2-hydroxypropyl)methacrylamide copolymers containing daunomycin against a rat tumour model.

Macromolecular prodrugs for use in targeted cancer chemotherapy: melphalan covalently coupled to N- (2-hydroxypropyl) methacrylamide copolymers

Targetable photoactivatable drugs, 2. Synthesis of N‐(2‐hydroxypropyl)methacrylamide copolymeranti‐thy 1.2 antibody‐chlorin e6 conjugates and a preliminary study of their photodynamic effect on mouse splenocytes in vitro

Selectivity of Antibody-Targeted Anthracycline Antibiotics on T Lymphocytes