Hydrophilic biomaterials: From crosslinked and self-assembled hydrogels to polymer-drug conjugates and drug-free macromolecular therapeutics

This "Magnum Opus" accentuates my lifelong belief that the future of science is in the interdisciplinary approach to hypotheses formulation and problem solving. Inspired by the invention of hydrogels and soft contact lenses by my mentors, my six decades of research have continuously proceeded from the synthesis of biocompatible hydrogels to the development of polymer-drug conjugates, then generation of drug-free macromolecular therapeutics (DFMT) and finally to multi-antigen T cell hybridizers (MATCH). This interdisciplinary journey was inspiring; the lifetime feeling that one is a beginner in some aspects of the research is a driving force that keeps the enthusiasm high. Also, I wanted to illustrate that systematic research in one wide area can be a life-time effort without the need to jump to areas that are temporarily en-vogue. In addition to generating general scientific knowledge, hydrogels from my laboratory have been transferred to the clinic, polymer-drug conjugates to clinical trials, and drug-free macromolecular systems have an excellent potential for personalizing patient therapies. There is a limit to life but no limit to imagination. I anticipate that systematic basic research will contribute to the expansion of our knowledge and create a foundation for the design of new paradigms based on the comprehension of mechanisms of physiological processes. The emerging novel platform technologies in biomaterial-based devices and implants as well as in personalized nanomedicines will ultimately impact clinical practice.

HPMA Copolymer-Drug Conjugates with Controlled Tumor-Specific Drug Release

Over the past few decades, numerous polymer drug carrier systems are designed and synthesized, and their properties are evaluated. Many of these systems are based on water-soluble polymer carriers of low-molecular-weight drugs and compounds, e.g., cytostatic agents, anti-inflammatory drugs, or multidrug resistance inhibitors, all covalently bound to a carrier by a biodegradable spacer that enables controlled release of the active molecule to achieve the desired pharmacological effect. Among others, the synthetic polymer carriers based on N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers are some of the most promising carriers for this purpose. This review focuses on advances in the development of HPMA copolymer carriers and their conjugates with anticancer drugs, with triggered drug activation in tumor tissue and especially in tumor cells. Specifically, this review highlights the improvements in polymer drug carrier design with respect to the structure of a spacer to influence controlled drug release and activation, and its impact on the drug pharmacokinetics, enhanced tumor uptake, cellular trafficking, and in vivo antitumor activity.

Succinoylated Poly[N-(2- Hydroxyethyl)-L-Glutamine] Derivatives for Drug Delivery

A series of succinoylated poly[N-(2-)- L-glutamine] (PHEG) derivatives was synthesized by reacting PHEG with succinic anhydride in the presence of N,N-dimethylaminopyridine as a catalyst. The size of the derivatives were measured by dynamic light scattering in buffers (pH 5.5 and 7.4, respectively) the lysosomal and physiological pH. The degradability of the succinoylated polymers toward cathepsin B was followed by gel permeation chromatography. It was demonstrated that an increase of modification results in decreased biodegradability. Conjugation of mitomycin C (MMC) with a succinoylated PHEG derivative through a collagenase-sensitive Pro-Leu-Gly-Pro- Leu spacer resulted in a water-soluble MMC conjugate. This conjugate was shown to be hydrolytically stable in buffers of lysosomal and physiological pH and able to release MMC in the presence of the bacterial collagenase clostridium histolyticum.

Studies on PEGylated and Drug-Loaded PAMAM Dendrimers

Three methods were investigated for their suitability for the activation of poly(ethylene glycol) 2000 mono-methyl ether en route to conjugation with dendrimers, using 4-nitrophenylchloroformate as the activator. The use of acetonitrile as a solvent gave the best results. Poly(ethylene glycol) (PEG) grafted polyamidoamine (PAMAM) dendrimers were synthesized and characterized; the use of acetonitrile as a solvent gave the best result. A series of PEG conjugated PAMAM dendrimers with varying degrees of substitution of the dendrimer surface functional group by PEG were prepared. The encapsulation efficiency and the in vitro release characteristics of these PEG conjugated PAMAM dendrimers were studied. The percentage coverage of PAMAM dendrimer surface with PEG had little effect on the encapsulation efficiency but affected the release of methotrexate. IR spectra showed that many of the encapsulated methotrexate molecules were located within the cavity of the dendrimer.

Perspectives On: Polymeric Drugs and Drug Delivery Systems

Therapeutic uses of a variety of drug carrier systems have significant impact on the treatment and potential cure of many chronic diseases, including cancer, diabetes, mellitus, rheumatoid arthritis, HIV infection, and drug addiction. Drug delivery technology is a multidisciplinary science involving the physical, biological, medicinal, pharmaceutical, biomedical engineering and biomaterial fields. Polymeric systems can deliver drugs directly to the intended site of action and can also improve efficacy while minimizing unwanted side effects elsewhere in the body, which often limit the long-term use of many drugs. In this article, some recent publications on several polymeric drug conjugates, gene delivery systems and polymer implants are addressed.

Antitumor efficacy of colon-specific HPMA copolymer/9-aminocamptothecin conjugates in mice bearing human-colon carcinoma xenografts

The antitumor activity of a colon-specific N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer - 9-aminocamptothecin (9-AC) conjugate (P-9-AC) was assessed in orthotopic and subcutaneous animal (HT29 xenograft) tumor models. P-9-AC treatment of mice bearing orthotopic colon tumors, with a dose of 3 mg/kg of 9-AC equivalent every other day for 6 weeks, resulted in regression of tumors in 9 of 10 mice. A lower dose of P-9-AC (1.25 mg/kg of 9-AC equivalent) every other day for 8 weeks inhibited subcutaneous tumor growth in all mice. No liver metastases were observed. Colon-specific release of 9-AC from polymer conjugates enhanced antitumor activity and minimized the systemic toxicity.

Azo compounds in colon-specific drug delivery

Azo compounds have the potential to act as drug carriers that facilitate the selective release of therapeutic agents to the colon, and also to effect the oral administration of those macromolecular drugs that require colon-specific drug delivery. With some further research-driven refinements, these materials may lead to more efficient treatments for local conditions, such as colonic cancer or inflammatory bowel disease. This article provides an overview of the azo-based systems developed to date, identifies the requirements for an ideal carrier, and highlights the directions for further developments in the field of azo group-facilitated colonic delivery.

Biodistribution and pharmacokinetics of colon-specific HPMA copolymer--9-aminocamptothecin conjugate in mice

A water soluble N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-9-aminocamptothecin (9-AC) conjugate was designed for oral colon-specific drug delivery in the treatment of colon cancer. Comparative studies between the polymer conjugate and free drug have been performed to assess their biodistribution and pharmacokinetics in mice. After oral administration of equal doses of the polymer conjugate or free 9-AC, the drug concentrations in major organs at fixed time points were determined using an HPLC-fluorescence assay. Only 2+/-1% of 9-AC released from the polymer conjugate was detected in the small intestine (SI), and the mean peak concentration of free 9-AC was 45-fold higher than that from released drug. Colon-specific release of 9-AC produced high local concentrations. The mean peak concentration of released 9-AC in cecal contents, feces, cecal tissue, and colon tissue were, respectively, 3.2-fold, 3.5-fold, 2.2-fold and 1.6-fold higher than that using free 9-AC. In plasma, the high and sharp drug concentration profile from free drug was in contrast to the relatively low and flat pharmacokinetic profile obtained from drug released from the HPMA copolymer. There was no significant difference between released and free drug for the area under the concentration-time curve (AUC) and bioavailability values. As a consequence of the colon-specific release of unmodified 9-AC from the polymer conjugate, antitumor efficacy can be anticipated to be enhanced due to prolonged colon tumor exposure to higher and more localized drug concentrations.

Colon-specific 9-aminocamptothecin-HPMA copolymer conjugates containing a 1,6-elimination spacer

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymer-9-aminocamptothecin (9-AC) conjugate for oral colon-specific drug delivery was designed, synthesized, and characterized. The drug, 9-AC, was attached to the polymer carrier via a spacer containing a combination of an aromatic azo bond and a 4-aminobenzylcarbamate group. The design of the spacer ensured a fast and highly efficient release of unmodified 9-AC from the polymer in the colon by azo bond cleavage followed by a 1,6-elimination mechanism. An in vitro degradation study indicated that this conjugate was stable in simulated upper GI tract conditions, including small intestine (SI) contents, SI mucosa suspension, and in PBS (pH 1.5 and 7.4). A fast release of the unmodified drug (85+/-10% of 9-AC in 12 h) was detected in rat cecal contents. This drug delivery system has potential in the treatment of colon cancer.

The effect of the linker on the hydrolysis rate of drug-linked ester bonds

Tailoring the length of a sulfide containing linker adjusts the hydrolysis of a drug-linked ester bond to values appropriate for once-a-week administrations. A model drug of paclitaxel was coupled using a hydrolyzable linker to a poly(ethylene glycol) macromonomer, via a conjugate addition reaction between a thiol and an acrylamide. The macromonomers were synthesized in three steps with an average overall yield of 70%. By changing the length of the linker from 3-sulfanylpropionyl to 4-sulfanylbutyryl, the half-life time of the release of the drug could be increased from 4.2+/-0.1 to 14.0+/-0.2 days. Drug-containing hydrogels were prepared by radical photopolymerization of these macromonomers with either the 3-sulfanylpropionyl or the 4-sulfanylbutyryl linker. The release of the drug from these hydrogels followed similar trends as the release of the drug from the soluble polymer-drug conjugates. The synthetic methodology employed does not involve the use of coupling reagents in the final conjugation between the drug and the polymer, excluding the presence of potential toxic residuals. The conjugation method is relatively simple and is applicable to nearly any hydroxyl-containing drugs.