Current Topics in Medicinal Chemistry - Volume 17, Issue 32, 2017

Antibody Drug Conjugates (ADCs) use targeting ability of monoclonal antibodies to deliver potent cytototoxic payloads to their intended target. The linker encompasses a conjugating functionality suitable for attachment to the antibody, a spacer unit that typically incorporates a hydrophilic element and a trigger which releases the potent cytototoxic warhead. Understanding the conflicting requirements of ADC design, providing stability in systemic circulation but efficient payload release once the ADC reaches its intended target, is crucial to effective linker development. ADC linker design has been approached in a variety of different ways, with increasingly elegant solutions continuing to be reported as understanding of the intricate design complexities increases. This review focuses on the synthetic approaches used in ADC linkers, and the impact of linker design on antibody conjugation, ADC pharmacokinetics and payload release. Linker approaches utilized in commercial ADCs as well as ADCs currently in clinical, pre-clinical and early stage development are discussed.

Overexpressed cell-surface receptors are hallmarks of many disease states and are often used as markers for targeting diseased cells over healthy counterparts. Cell adhesion peptides, which are often derived from interacting regions of these receptor-ligand proteins, mimic surfaces of intact proteins and, thus, have been studied as targeting agents for various payloads to certain cell targets for cancers and autoimmune diseases. Because many cytotoxic agents in the free form are often harmful to healthy cells, the use of cell adhesion peptides in targeting their delivery to diseased cells has been studied to potentially reduce required effective doses and associated harmful side-effects. In this review, multiple cell adhesion peptides from extracellular matrix and ICAM proteins were used to selectively direct drug payloads, signal-inhibitor peptides, and diagnostic molecules, to diseased cells over normal counterparts. RGD constructs have been used to improve the selectivity and efficacy of diagnostic and drug-peptide conjugates against cancer cells. From this precedent, novel conjugates of antigenic and cell adhesion peptides, called Bifunctional Peptide Inhibitors (BPIs), have been designed to selectively regulate immune cells and suppress harmful inflammatory responses in autoimmune diseases. Similar peptide conjugations with imaging agents have delivered promising diagnostic methods in animal models of rheumatoid arthritis. BPIs have also been shown to generate immune tolerance and suppress autoimmune diseases in animal models of type-1 diabetes, rheumatoid arthritis, and multiple sclerosis. Collectively, these studies show the potential of cell adhesion peptides in improving the delivery of drugs and diagnostic agents to diseased cells in clinical settings.

The recent approval of trastuzumab emtansine (Kadcyla®) and brentuximab vedotin (Adcetris ®) has spurred tremendous investment in new approaches for the targeted delivery of pharmaceutical agents. Targeted delivery approaches, such as Antibody Drug Conjugates (ADCs), typically rely on an endogenous or exogenous “trigger” that results in the release of the pharmacologically active agent at the intended site of action. Lysosomal and intracellular triggers include proteolytic cleavage, glycolytic cleavage, phosphatase cleavage, hydrolytic cleavage, and reductive cleavage. Recent work has also illustrated that exogenous triggers and extracellular enzymes can be harnessed to result in linker cleavage at the site of action. As these linker technologies have grown, so also has our understanding of the biophysical parameters that drive exposure and stability. The growth in targeted delivery approaches has also driven advancement in bioanalytical strategies for assessing the distribution, processing, and metabolism of these agents. This review provides a systematic overview of each of these areas, particularly focusing on recent advancements in the field that has the potential to expand the scope of therapeutic areas that ADCs and other targeted delivery approaches can be designed to address.

Non-cleavable linkers are used in a number of different modalities for various reasons, such as linking an active drug moiety to half-life extending molecules, to groups that enable a specific tissue or cell targeting or to facilitate active uptake into target cells. Non-cleavable linkers do not have a designated weak point in their structure that can lead to cleavage by proteases, hydrolases or chemically by pH changes. Consequently, when designing a conjugate, the choice of a non-cleavable over a cleavable linker is usually a consequence of pursuing a certain mode of action where the stability of the complex is more important than a fast liberation of the active moiety. Linkers of various length, polarity, stability and flexibility are used for different types of conjugates and the linker design is mostly driven by the particular purpose and desired mode of action. This article reviews non-cleavable linkers applied predominantly in Antibody Drug Conjugates (ADCs), and how they influence these conjugates in terms of ADME properties (absorption, distribution, metabolism and elimination) and safety.