Design and Measurement of Drug Tissue Concentration Asymmetry and Tissue Exposure-Effect (Tissue PK-PD) Evaluation

The "free drug hypothesis" assumes that, in the absence of transporters, the steady state free plasma concentrations equal to that at the site of action that elicit pharmacologic effects. While it is important to utilize the free drug hypothesis, exceptions exist that the free plasma exposures, either at C_max, C_trough, and C_average, or at other time points, cannot represent the corresponding free tissue concentrations. This "drug concentration asymmetry" in both total and free form can influence drug disposition and pharmacological effects. In this review, we first discuss options to assess total and free drug concentrations in tissues. Then various drug design strategies to achieve concentration asymmetry are presented. Last, the utilities of tissue concentrations in understanding exposure-effect relationships and translational projections to humans are discussed for several therapeutic areas and modalities. A thorough understanding in plasma and tissue exposures correlation with pharmacologic effects can provide insightful guidance to aid drug discovery.

Systemic exposure following intravitreal administration of therapeutic agents: an integrated pharmacokinetic approach. 1. THR-149

Intravitreal (IVT) injection of pharmacological agents is an established and widely used procedure for the treatment of many posterior segment of the eye diseases. IVT injections permit drugs to reach high concentrations in the retina whilst limiting systemic exposure. Beyond the risk of secondary complications such as intraocular infection, the potential of systemic adverse events cannot be neglected. Therefore, a detailed understanding of the rules governing systemic exposure following IVT drug administration remains a prerequisite for the evaluation and development of new pharmacological agents intended for eye delivery. We present here a novel mathematical model to describe and predict circulating drug levels following IVT in the rabbit eye, a species which is widely used for drug delivery, pharmacokinetic, and pharmacodynamic studies. The mathematical expression was derived from a pharmacokinetic model that assumes the existence of a compartment between the vitreous humor compartment itself and the systemic compartment. We show that the model accurately describes circulating levels of THR-149, a plasma kallikrein inhibitor in development for the treatment of diabetic macular edema. We hypothesize that the model based on the rabbit eye has broader relevance to the human eye and can be used to analyze systemic exposure of a variety of drugs delivered in the eye.

Emerging biological therapies for the treatment of age-related macular degeneration

INTRODUCTION

Age-related macular degeneration (AMD) is the leading cause of blindness in individuals over age 50 in developed countries. Current therapy for nonexudative AMD (neAMD) is aimed at modifying risk factors and vitamin supplementation to slow progression, while intravitreal anti-vascular endothelial factor (VEGF) injections are the mainstay for treatment of choroidal neovascularization in exudative AMD (eAMD).

AREAS COVERED

Over the past decade, promising therapies have emerged that aim to improve the current standard of care for both diseases. Clinical trials for neAMD are investigating targets in the complement cascade, vitamin A metabolism, metformin, and tetracycline, whereas clinical trials for eAMD are aiming to decrease treatment burden through novel port delivery systems, increasing drug half-life, and targeting new sites of the VEGF cascade. Stem cell and gene therapy are also being evaluated for treatment of neAMD and eAMD.

EXPERT OPINION

With an aging population, the need for effective, long term, low burden treatment options for AMD will be in increasingly high demand. Current investigations aim to address the shortcomings of current treatment options with breakthrough treatment approaches. Therapeutics in the pipeline hold promise for improving the treatment of AMD, and are on track for widespread use within the next decade.

Beyond Antibodies: The DARPin® Drug Platform

The DARPin^® drug platform was established with a vision to expand the medical use of biologics beyond what was possible with monoclonal antibodies. It is based on naturally occurring ankyrin repeat domains that are typically building blocks of multifunctional human proteins. The platform allows for the generation of diverse, well-behaved, multifunctional drug candidates. Recent clinical data illustrate the favorable safety profile of the first DARPin^® molecules tested in patients. With the positive phase III results of the most advanced DARPin^® drug candidate, abicipar, the DARPin^® drug platform is potentially about to achieve its first marketing approval. This review highlights some of the key milestones and decisions encountered when transforming the DARPin^® platform from an academic concept to a biotech drug pipeline engine.

A DARPin targeting activated Mac-1 is a novel diagnostic tool and potential anti-inflammatory agent in myocarditis, sepsis and myocardial infarction

The monocyte β₂-integrin Mac-1 is crucial for leukocyte–endothelium interaction, rendering it an attractive therapeutic target for acute and chronic inflammation. Using phage display, a Designed-Ankyrin-Repeat-Protein (DARPin) was selected as a novel binding protein targeting and blocking the α_M I-domain, an activation-specific epitope of Mac-1. This DARPin, named F7, specifically binds to activated Mac-1 on mouse and human monocytes as determined by flow cytometry. Homology modelling and docking studies defined distinct interaction sites which were verified by mutagenesis. Intravital microscopy showed reduced leukocyte–endothelium adhesion in mice treated with this DARPin. Using mouse models of sepsis, myocarditis and ischaemia/reperfusion injury, we demonstrate therapeutic anti-inflammatory effects. Finally, the activated Mac-1-specific DARPin is established as a tool to detect monocyte activation in patients receiving extra-corporeal membrane oxygenation, as well as suffering from sepsis and ST-elevation myocardial infarction. The activated Mac-1-specific DARPin F7 binds preferentially to activated monocytes, detects inflammation in critically ill patients, and inhibits monocyte and neutrophil function as an efficient new anti-inflammatory agent.

Beyond antibodies: ankyrins and DARPins. From basic research to drug approval

This Pharmacological Perspective describes the pathway that, starting from the deep understanding of ankyrins - a family of proteins with high variability-binding and high specificity-binding characteristics - led to the development of a new class of recombinant-binding proteins, the DARPins (designed ankyrin repeat proteins). These are envisaged as alternatives to mAbs and related biologics, with the potential to overcome certain shortcomings of mAbs. DARPins have relatively low molecular weights (14-21kDas) and more favorable PK profiles than mAbs, are stable proteins that can be easily produced in Escherichia coli and can be used in their monovalent form or conjugated to other moieties, for example, polyethylene glycol (PEG) to enhance their half-life. DARPins can also be engineered to produce bi-specific or tri-specific compounds that bind different epitopes of the same target or two different targets. Abicipar, a first-in-class anti-VEGF-A DARPin had similar efficacy compared to anti-VEGF biologics (bevacizumab, ranibizumab) in preclinical studies and was not inferior to ranibizumab in the treatment of age-related macular degeneration (AMD) with a reduced number of intravitreal injections. Abicipar has recently been submitted for regulatory approval for use in AMD.