Turmeric and Its Major Compound Curcumin on Health: Bioactive Effects and Safety Profiles for Food, Pharmaceutical, Biotechnological and Medicinal Applications

The data reviewed indicate that extracts of Curcuma longa exhibit anti-inflammatory activity after parenteral application in standard animal models used for testing anti-inflammatory activity. It turned out that curcumin and the volatile oil are at least in part responsible for this action. It appears that when given orally, curcumin is far less active than after i.p. administration. This may be due to poor absorption, as discussed. Data on histamine-induced ulcers are controversial, and studies on the secretory activity (HCl, pepsinogen) are still lacking. In vitro, curcumin exhibited antispasmodic activity. Since there was a protective effect of extracts of Curcuma longa on the liver and a stimulation of bile secretion in animals, Curcuma longa has been advocated for use in liver disorders. Evidence for an effect on liver disease in humans is not yet available. From the facts that after oral application only traces of curcumin were found in the blood and that, on the other hand, most of the curcumin is excreted via the faeces it may be concluded that curcumin is absorbed poorly by the gastrointestinal tract and/or underlies presystemic transformation. Systemic effects therefore seem to be questionable after oral application except that they occur at very low concentrations of curcumin. This does not exclude a local action in the gastrointestinal tract.

Substances cross the blood-brain barrier (BBB) by a variety of mechanisms. These include transmembrane diffusion, saturable transporters, adsorptive endocytosis, and the extracellular pathways. Here, we focus on the chief characteristics of two mechanisms especially important in drug delivery: transmembrane diffusion and transporters. Transmembrane diffusion is non-saturable and depends, on first analysis, on the physicochemical characteristics of the substance. However, brain-to-blood efflux systems, enzymatic activity, plasma protein binding, and cerebral blood flow can greatly alter the amount of the substance crossing the BBB. Transport systems increase uptake of ligands by roughly 10-fold and are modified by physiological events and disease states. Most drugs in clinical use to date are small, lipid soluble molecules that cross the BBB by transmembrane diffusion. However, many drug delivery strategies in development target peptides, regulatory proteins, oligonucleotides, glycoproteins, and enzymes for which transporters have been described in recent years. We discuss two examples of drug delivery for newly discovered transporters: that for phosphorothioate oligonucleotides and for enzymes.

Data from in vitro plasma protein binding experiments that determine the fraction of protein-bound drug are frequently used in drug discovery to guide structure design and to prioritize compounds for in vivo studies. However, we consider that these practices are usually misleading, because in vivo efficacy is determined by the free (unbound) drug concentration surrounding the therapeutic target, not by the free drug fraction. These practices yield no enhancement of the in vivo free drug concentration. So, decisions based on free drug fraction could result in the wrong compounds being advanced through drug discovery programmes. This Perspective provides guidance on the application of plasma protein binding information in drug discovery.