Pradeep K. Dhal

Site-Specific Antibody–Drug Conjugation through Glycoengineering

Antibody-drug conjugates (ADCs) have been proven clinically to be more effective anti-cancer agents than native antibodies. However, the classical conjugation chemistries to prepare ADCs by targeting primary amines or hinge disulfides have a number of shortcomings including heterogeneous product profiles and linkage instability. We have developed a novel site-specific conjugation method by targeting the native glycosylation site on antibodies as an approach to address these limitations. The native glycans on Asn-297 of antibodies were enzymatically remodeled in vitro using galactosyl and sialyltransferases to introduce terminal sialic acids. Periodate oxidation of these sialic acids yielded aldehyde groups which were subsequently used to conjugate aminooxy functionalized cytotoxic agents via oxime ligation. The process has been successfully demonstrated with three antibodies including trastuzumab and two cytotoxic agents. Hydrophobic interaction chromatography and LC-MS analyses revealed the incorporation of ~1.6 cytotoxic agents per antibody molecule, approximating the number of sialic acid residues. These glyco-conjugated ADCs exhibited target-dependent antiproliferative activity toward antigen-positive tumor cells and significantly greater antitumor efficacy than naked antibody in a Her2-positive tumor xenograft model. These findings suggest that enzymatic remodeling combined with oxime ligation of the native glycans of antibodies offers an attractive approach to generate ADCs with well-defined product profiles. The site-specific conjugation approach presented here provides a viable alternative to other methods, which involve a need to either re-engineer the antibody sequence or develop a highly controlled chemical process to ensure reproducible drug loading.

Functional polymers as therapeutic agents: Concept to market place ☆

Biologically active synthetic polymers have received considerable scientific interest and attention in recent years for their potential as promising novel therapeutic agents to treat human diseases. Although a significant amount of research has been carried out involving polymer-linked drugs as targeted and sustained release drug delivery systems and prodrugs, examples on bioactive polymers that exhibit intrinsic therapeutic properties are relatively less. Several appealing characteristics of synthetic polymers including high molecular weight, molecular architecture, and controlled polydispersity can all be utilized to discover a new generation of therapies. For example, high molecular weight bioactive polymers can be restricted to gastrointestinal tract, where they can selectively recognize, bind, and remove target disease causing substances from the body. The appealing features of GI tract restriction and stability in biological environment render these polymeric drugs to be devoid of systemic toxicity that are generally associated with small molecule systemic drugs. The present article highlights recent developments in the rational design and synthesis of appropriate functional polymers that have resulted in a number of promising polymer based therapies and biomaterials, including some marketed products.

Biologically active polymeric sequestrants: Design, synthesis, and therapeutic applications

In recent years, functional polymers exhibiting inherently biological activities have been receiving increasing attention as polymer-based human therapeutic agents. These polymeric drugs exhibit unique pharmaceutical properties that are fundamentally different from their traditional small-molecule counterparts. However, unlike polymeric drug delivery systems, examples of polymers possessing intrinsically therapeutic properties are relatively scarce. By virtue of their high-molecular-weight characteristics, these polymeric drugs can be confined to the gastrointestinal (GI) tract, where they can selectively recognize, bind, and remove target disease-causing substances from the body. Being confined to the GI tract and non-biodegradable, these polymeric drugs are free from toxic effects that are associated with traditional systemic drugs. This report highlights recent developments in the rational design and synthesis of appropriate functional polymers that have resulted in a number of promising polymer-based therapeutic agents, including some marketed products.

Ammonium and Guanidinium Functionalized Hydrogels as Bile Acid Sequestrants: Synthesis, Characterization, and Biological Properties

Abstract Novel cationic polymers as bile acid sequestrants (BAS) have been considered to be an attractive long‐term therapy for the treatment of hypercholesterolemia. Due to the poor in vivo efficacy of the first generation of BAS like cholestyramine and colestipol, there is a need for discovering new generations of potent BAS. As part of our polymeric drug discovery efforts, we have developed a facile route to prepare functional hydrogels bearing pendant amine and guanidinium groups. The polymeric amines were prepared either by direct polymerization of amine containing monomers or by chemical modification of suitable polymeric precursors. Polymers bearing guanidinium groups were obtained by a polymer analog reaction on crosslinked polymeric amines (primary or secondary) using a readily available guanylating agent in aqueous medium. Incorporation of guanidinium groups to these polymers occurs under mild reaction condition. A numbers of polymer structures with pendant amine and guanidinium groups located at varying distances from the polymer backbone were obtained. These polymeric ammonium and guanidinium salts were evaluated in vivo as BAS and hence cholesterol lowering agents.

Three Generations of Bile Acid Sequestrants

Cholestyramine, the first bile acid sequestrant to be marketed, has been in use for over 20 years. Despite its low potency, requiring 16-24 g of polymer to achieve 20% LDL cholesterol reduction in hypercholesterolemic individuals, only one other sequestrant, colestipol, has come to market in the ensuing period. GelTex Pharmaceuticals has been involved for over six years in the discovery and development of new, more potent polymeric sequestrants. Two binding mechanisms are presented — one that operates via an aggregate binding structure and one that is effective via a defined site binding structure. These two binding mechanisms are compared and contrasted through bile acid binding isotherms. The best of these new sequestrants bind bile acids through a combination of hydrophobicity and ion exchange. Optimization and balancing of each of these interactions led us to more potent materials. The first of these, colesevelam hydrochloride is expected to be three to four times more potent than cholestyramine. A third generation product is still in research at GelTex. With another twofold increase in potency possible, single tablet therapy may become a reality.

Hydrophobically Modified Poly(Allylamine) Hydrogels Containing Internal Quaternary Ammonium Groups as Cholesterol Lowering Agents: Synthesis, Characterization, and Biological Studies

Syntheses and physico-chemical characterization of a series of amphiphilic cationic hydrogels bearing pendant quaternary ammonium groups based on poly(allylamine) backbone are reported. These amphiphilic hydrogels were prepared by crosslinking soluble poly(allylamine) with epichlorohydrin followed by alkylation of the resulting gel with different functional alkyl chlorides bearing quaternary ammonium groups. Equilibrium swelling characteristics of these amphiphilic hydrogels were determined under a variety of solvent conditions. The roles of different parameters such as degree of crosslinking, degree of alkylation, and nature of alkylating agents on the swelling properties of these hydrogels were investigated. As part of a continuing structure-activity relationship study for the discovery of systemically non-absorbed cholesterol lowering agents, these amphiphilic hydrogels were also evaluated for their bile acid sequestration properties. Preliminary in vivo studies suggest these cationic hydrogels are potent bile acid sequestrants.

Bio)polymeric Hydrogels as Therapeutic Agents

Hydrogels derived from both natural and synthetic polymers have gained significant scientific attention in recent years for their potential use as biomedical materials to treat human diseases. While a great deal of research efforts have been directed towards investigating polymeric hydrogels as matrices for drug delivery systems, examples of such hydrogels exhibiting intrinsic therapeutic properties are relatively less common. Characteristics of synthetic and natural polymers such as high molecular weight, diverse molecular architecture, chemical compositions, and modulated molecular weight distribution are unique to polymers. These characteristics of polymers can be utilized to discover a new generation of drugs and medical devices. For example, polymeric hydrogels can be restricted to the gastrointestinal tract, where they can selectively recognize, bind, and remove the targeted disease-causing substances from the body without causing any systemic toxicity that are associated with traditional small molecule drugs. Similarly hydrogels can be implanted at specific locations (such as knee and abdomen) to impart localized therapeutic benefits. The present article provides an overview of certain recent developments in the design and synthesis of functional hydrogels that have led to several polymer derived drugs and biomedical devices. Some of these examples include FDA-approved marketed products.