Translational research: the changing landscape of drug discovery.

Drug discovery represents the first step in the creation of new drugs, and takes place in academic institutions, biotech companies, and large pharmaceutical corporations. Until recently, these sectors have each operated independently with little collaboration between those at the forefront of discovery research and those with experience in developing drugs. With the rise of translational research these relationships are shifting and new hubs are emerging, as key players seek to pool the expertise necessary to generate new therapies by linking laboratory discoveries directly to unmet clinical needs. In this article I discuss how the increasing adoption of translational research is leading to novel integrated discovery nexuses that may change the landscape of drug discovery.

Absorption of polyethylene glycol (PEG) polymers: the effect of PEG size on permeability.

Polyethylene glycol (PEG) polymers are large amphiphilic molecules that are highly hydrated in solution. To explore the permeability properties of different sized PEG polymers across epithelial membranes in vivo, we examined the absorption of fluorescently labeled PEG conjugates sized 0.55-20 kDa from the lung, since this system provides a reservoir that limits rapid diffusion of molecules away from the site of delivery and enables permeability over longer times to be examined. Following intratracheal delivery in rats, the PEG polymers underwent absorption with first-order kinetics described by single exponential decay curves. PEG size produced a marked influence on the rate of uptake from the lung, with half-lives ranging from 2.4 to 13 h, although above a size of 5 kDa, no further change in rate was observed. PEG size likewise affected retention in alveolar macrophages and in lung tissue; whereas smaller PEG sizes (<2 kDa) were effectively cleared within 48 h, larger PEG sizes (>5 kDa) remained in lung cells and tissue for up to 7 days. These data demonstrate that PEG polymers can be absorbed across epithelial membranes and that PEG size plays a dominant role in controlling the rate and mechanism of absorption.

The pharmacology of PEGylation: balancing PD with PK to generate novel therapeutics.

Conjugation of macromolecules to polyethylene glycol (PEG) has emerged recently as an effective strategy to alter the pharmacokinetic (PK) profiles of a variety of drugs, and thereby to improve their therapeutic potential. PEG conjugation increases retention of drugs in the circulation by protecting against enzymatic digestion, slowing filtration by the kidneys and reducing the generation of neutralizing antibodies. Often, PEGylation leads to a loss in binding affinity due to steric interference with the drug-target binding interaction. This loss in potency is offset by the longer circulating half-life of the drugs, and the resulting change in PK-PD profile has led in some cases to enabling of drugs that otherwise could not be developed, and in others to improvements in existing drugs. Thus, whereas most approaches to drug development seek to increase the activity of drugs directly, the creation of PEGylated drugs seeks to balance the pharmacodynamic (PD) and pharmacokinetic properties to produce novel therapies that will meet with both increased efficacy and greater compliance in the clinical setting. This review examines some of the PEGylated drugs developed in recent years, and highlights some of the different strategies taken to employ PEG to maximize the overall PK-PD profiles of these compounds.

The lungs as a portal of entry for systemic drug delivery.

The lung is naturally permeable to all small-molecule drugs studied and to many therapeutic peptides and proteins. Absorption can be estimated using a simple animal test, intratracheal instillation. Inhalation offers a noninvasive route for the delivery of peptides and proteins that otherwise must be injected. Peptides that have been chemically altered to inhibit peptidase enzymes exhibit very high bioavailabilities by the pulmonary route. Natural mammalian peptides, less than about 30 amino acids, are broken down in the lung by ubiquitous peptidases and have very poor bioavailabilities. In general, proteins with molecular weights between 6,000 and 50,000 D are relatively resistant to most peptidases and have good bioavailabilities following inhalation. For larger proteins the bioavailability picture is not clear. Although the lung is rich in antiproteases, aggregation of inhaled proteins will stimulate opsonization (coating) by special proteins in the lung lining fluids, which will then mark the aggregated proteins for phagocytosis and intracellular enzymatic destruction. Small peptides and proteins are absorbed more rapidly after inhalation than after subcutaneous injection. For other small molecules, inhalation is also a fast way to get into the body because drug efflux transporters and metabolizing enzymes are present in the lung at much lower levels than the gastrointestinal tract. Lipophilic small molecules are absorbed extremely fast, t(1/2) (abs) approximately 1 to 2 minutes. Water-soluble small molecules are absorbed rapidly t(1/2) (abs) approximately 65 minutes. Small molecules can exhibit prolonged absorption if they are highly insoluble or highly cationic. Encapsulation in slow release particles such as liposomes can also be used to control absorption.

Coupling of dopamine receptors to G proteins: studies with chimeric D2/D3 dopamine receptors.

D2 and D3 dopamine receptors belong to the superfamily of G protein-coupled receptors; they share a high degree of homology and are structurally similar. However, they differ from each other in their second messenger coupling properties. Previously, we have studied the differential coupling of these receptors to G proteins and found that while D2 receptor couples only to inhibitory G proteins, D3 receptor couples also to a stimulatory G protein, Gs. We aimed to investigate the molecular basis of these differences and to determine which domains in the receptor control its coupling to G proteins. For this purpose four chimeras were constructed, each composed of different segments of the original D2 and D3 receptors. We have demonstrated that chimeras with a third cytoplasmic loop of D2 receptor couple to Gi protein in a pattern characteristic of D2 receptor. On the other hand chimeras containing a third cytoplasmic loop of D3 receptor have coupling characteristics like those of D3 receptor, and they couple also to Gs protein. These findings demonstrate that the third cytoplasmic loop determines and accounts for the coupling of dopamine receptors D2 and D3 to G proteins.