POLYMERIC BIOMATERIALS AND NANOMEDICINES

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.

Novel methacrylate copolymers functionalized with fluoroarylamide; copolymerization kinetics, thermal stability and antimicrobial properties

In the first step of this study, 2-oxo-2-[(2,4,6-trifluorophenyl)amino]ethyl-2-methylprop-2-enoate(OTFAMA) monomer was synthesized and characterized. Then, a series copolymers were obtained by free-radical copolymerization method of OTFAMA and glycidyl methacrylate (GMA), which is a commercial monomer at 65 °C in 1,4-dioxane solvent. Structural characterizations of synthesized monomer and copolymers were carried out using FTIR, ¹H-¹³C-NMR instruments. The composition of the copolymers was estimated by elemental analysis. The reactivity ratios (r₁ and r₂) were obtained from the various linear graphical methods. The values of r₁ (OTFAMA) = 0.33 and r₂ (GMA) = 0.45 were found from the same graphical methods. The thermal behaviors of all the polymers have been investigated using the differential scanning calorimetry (DSC) and the thermogravimetric analysis (TGA). A kinetic study of the thermal decompostion of copolymers was investigated using thermogravimetric analyzer with non-isothermal methods selected for analyzing solid-state kinetics data. The average activation energy values were calculated via Kissinger and Ozawa models in a period of α = 0.10-0.80. Photo stability of the copolymers was investigated. Also, the biological activity of the copolymers against different bacterial and fungal species has been investigated.

Polymer nanomedicines

Polymer nanomedicines (macromolecular therapeutics, polymer-drug conjugates, drug-free macromolecular therapeutics) are a group of biologically active compounds that are characterized by their large molecular weight. This review focuses on bioconjugates of water-soluble macromolecules with low molecular weight drugs and selected proteins. After analyzing the design principles, different structures of polymer carriers are discussed followed by the examination of the efficacy of the conjugates in animal models and challenges for their translation into the clinic. Two innovative directions in macromolecular therapeutics that depend on receptor crosslinking are highlighted: a) Combination chemotherapy of backbone degradable polymer-drug conjugates with immune checkpoint blockade by multivalent polymer peptide antagonists; and b) Drug-free macromolecular therapeutics, a new paradigm in drug delivery.

Biorecognition: A key to drug-free macromolecular therapeutics

This review highlights a new paradigm in macromolecular nanomedicine - drug-free macromolecular therapeutics (DFMT). The effectiveness of the new system is based on biorecognition events without the participation of low molecular weight drugs. Apoptosis of cells can be initiated by the biorecognition of complementary peptide/oligonucleotide motifs at the cell surface resulting in the crosslinking of slowly internalizing receptors. B-cell CD20 receptors and Non-Hodgkin lymphoma (NHL) were chosen as the first target. Exposing cells to a conjugate of one motif with a targeting ligand decorates the cells with this motif. Further exposure of decorated cells to a macromolecule (synthetic polymer or human serum albumin) containing multiple copies of the complementary motif as grafts results in receptor crosslinking and apoptosis induction in vitro and in vivo. The review focuses on recent developments and explores the mechanism of action of DFMT. The altered molecular signaling pathways demonstrated the great potential of DFMT to overcome rituximab resistance resulting from either down-regulation of CD20 or endocytosis and trogocytosis of rituximab/CD20 complexes. The suitability of this approach for the treatment of blood borne cancers is confirmed. In addition, the widespread applicability of DFMT as a new concept in macromolecular therapeutics for numerous diseases is exposed.

A developed antibody-drug conjugate rituximab-vcMMAE shows a potent cytotoxic activity against CD20-positive cell line

Rituximab is a chimeric monoclonal antibody directed against B-lymphocyte specific antigen CD20, which is used for the treatment of B-cell malignancies. However, the effectiveness of rituximab is limited partly due to treatment resistance. The aim of this study was to develop rituximab-based antibody drug conjugate (ADC) to enhance rituximab activity. In this study, monomethyl auristatin E (MMAE) was covalently conjugated to dithiothreitol -reduced rituximab via a valine-citrulline peptide linker (rituximab-vcMMAE). The conjugates were then characterized by using nonreducing sodium dodecyl sulfate-polyacrylamide electrophoresis (SDS-PAGE) and cell-based enzyme-linked immunosorbent assay (ELISA). The cytotoxic activity of the ADC was evaluated against Raji (human B-cell lymphoma; CD20-positive) and MOLT-4 (T lymphoblast; acute lymphoblastic leukemia; CD20-negative) cell lines. In addition, the colony formation assay was used to identify the propagation ability of ADC-treated cells in vitro. Results from nonreducing SDS-PAGE revealed various species of rituximab-MC-Val-Cit-PABC-MMAE (rituximab-vcMMAE), as compared with unconjugated rituximab. The binding capacity of rituximab-vcMMAE to the CD20-positive cell was similar to that of the parental rituximab. Most importantly, our results revealed that rituximab-vcMMAE was highly potent against the CD20-positive cell line, but not against the CD20-negative cell. At the same time, rituximab-vcMMAE was able to inhibit colony formation in CD20-positive cells. These data indicate that rituximab-vcMMAE may be a highly effective and selective therapy for the treatment of B-cell lymphoma.

The Light at the End of the Tunnel-Second Generation HPMA Conjugates for Cancer Treatment

It is almost four decades since N-(2-hydroxypropyl)methacrylamide (HPMA) - based copolymers arose as drug carriers. Although fundamentals have been established and significant advantages have been proved, the commercialization of this platform technology was hampered due to modest outcome of clinical trial initiated with PK1, the symbol of first generation polymer-drug conjugates. In this review, we illustrate the exciting progress and approaches offered by more effective 2^nd generation HPMA-based polymer-drug conjugates in cancer treatment. For example, a new synthetic strategy endorses inert HPMA polymer with biodegradability, which permitted to prepare high molecular weight HPMA-drug conjugates with simple linear architecture while maintaining good biocompatibility. As expected, extended long-circulating pharmacokinetics and enhanced antitumor activities were achieved in several preclinical investigations. In addition, greater inhibition of tumor growth in combination regimes exhibits the remarkable capability and flexibility of HPMA-based macromolecular therapeutics. The review also discusses the main challenges and strategies for further translation development of 2^nd generation HPMA-based polymer-drug conjugates.

Drug-free macromolecular therapeutics: Impact of structure on induction of apoptosis in Raji B cells

Recently, we developed a new paradigm in macromolecular therapeutics that avoids the use of low molecular weight drugs. The activity of the "drug-free macromolecular therapeutics" is based on the biorecognition of complementary motifs at cell surface resulting in receptor crosslinking and apoptosis induction. The system is composed of two nanoconjugates: (1) a single-stranded morpholino oligonucleotide (MORF1) attached to an anti-CD20 Fab' fragment (Fab'-MORF1); (2) multiple copies of complementary oligonucleotide MORF2 grafted to a linear polymer of N-(2-hydroxypropyl)methacrylamide (HPMA) - P-(MORF2)_x. The two conjugates crosslink CD20 antigens via MORF1-MORF2 hybridization at the surface of CD20⁺ malignant B-cells and induce apoptosis. Preclinical studies in a murine model of human non-Hodgkin's lymphoma showed cancer cells eradication and long-term survivors. The aim of this study was to determine the relationship between the detailed structure of the nanoconjugates and apoptosis induction in Raji cells to allow system optimization. The factors studied include the length of the MORF sequence, the valence of P-(MORF2)_x (varying x), molecular weight of P-(MORF2)_x, incorporation of a miniPEG spacer between Fab' and MORF1 and between polymer backbone and pendant MORF2, and comparison of two Fab' fragments, one from 1F5 antibody (Fab'_1F5), the other from Rituximab (Fab'_RTX). The results of apoptosis induction in human Burkitt's B-cell non-Hodgkin's lymphoma (NHL) Raji cells as determined using three apoptotic assays (Annexin V, Caspase 3, and TUNEL) indicated that: a) An improvement of apoptotic activity was observed for a 28 base pair MORF sequence when compared to MORFs composed of 20 and 25 base pairs. The differences depended on type of assay, concentration and exposure schedule (consecutive vs. premixed). b) The higher the valence of P-(MORF2)_x the higher the levels of apoptosis. c) Higher molecular weight of P-(MORF2)_x induced higher levels of apoptosis. d) A miniPEG₈ spacer was effective in enhancing apoptotic levels in contrast to a miniPEG₂ spacer. e) There was not a statistically significant difference when comparing Fab'_1F5-MORF1 with Fab'_RTX-MORF1.

Indium-based and iodine-based labeling of HPMA copolymer-epirubicin conjugates: Impact of structure on the in vivo fate

Recently, we developed 2nd generation backbone degradable N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-drug conjugates which contain enzymatically cleavable sequences (GFLG) in both polymeric backbone and side-chains. This design allows using polymeric carriers with molecular weights above renal threshold without impairing their biocompatibility, thereby leading to significant improvement in therapeutic efficacy. For example, 2nd generation HPMA copolymer-epirubicin (EPI) conjugates (2P-EPI) demonstrated complete tumor regression in the treatment of mice bearing ovarian carcinoma. To obtain a better understanding of the in vivo fate of this system, we developed a dual-labeling strategy to simultaneously investigate the pharmacokinetics and biodistribution of the polymer carrier and drug EPI. First, we synthesized two different types of dual-radiolabeled conjugates, including 1) (111)In-2P-EPI-(125)I (polymeric carrier 2P was radiolabeled with (111)In and drug EPI with (125)I), and 2) (125)I-2P-EPI-(111)In (polymeric carrier 2P was radiolabeled with (125)I and drug EPI with (111)In). Then, we compared the pharmacokinetics and biodistribution of these two dual-labeled conjugates in female nude mice bearing A2780 human ovarian carcinoma. There was no significant difference in the blood circulation between polymeric carrier and payload; the carriers ((111)In-2P and (125)I-2P) showed similar retention of radioactivity in both tumor and major organs except kidney. However, compared to (111)In-labeled payload EPI, (125)I-labeled EPI showed lower radioactivity in normal organs and tumor at 48h and 144h after intravenous administration of conjugates. This may be due to different drug release rates resulting from steric hindrance to the formation of enzyme-substrate complex as indicated by cleavage experiments with lysosomal enzymes (Tritosomes). A slower release rate of EPI(DTPA)(111)In than EPI(Tyr)(125)I was observed. It may be also due to in vivo catabolism and subsequent iodine loss as literature reported. Nevertheless, tumor-to-tissue uptake ratios of both radionuclides were comparable, indicating that drug-labeling strategy does not affect the tumor targeting ability of HPMA copolymer conjugates.

Polymeric Biodegradable Stent Insertion in the Esophagus

Esophageal stent insertion has been used as a well-accepted and effective alternative to manage and improve the quality of life for patients diagnosed with esophageal diseases and disorders. Current stents are either permanent or temporary and are fabricated from either metal or plastic. The partially covered self-expanding metal stent (SEMS) has a firm anchoring effect and prevent stent migration, however, the hyperplastic tissue reaction cause stent restenosis and make it difficult to remove. A fully covered SEMS and self-expanding plastic stent (SEPS) reduced reactive hyperplasia but has a high migration rate. The main advantage that polymeric biodegradable stents (BDSs) have over metal or plastic stents is that removal is not require and reduce the need for repeated stent insertion. But the slightly lower radial force of BDS may be its main shortcoming and a post-implant problem. Thus, strengthening support of BDS is a content of the research in the future. BDSs are often temporarily effective in esophageal stricture to relieve dysphagia. In the future, it can be expect that biodegradable drug-eluting stents (DES) will be available to treat benign esophageal stricture, perforations or leaks with additional use as palliative modalities for treating malignant esophageal stricture, as the bridge to surgery or to maintain luminal patency during neoadjuvant chemoradiation.