Blood-brain barrier transport and protein binding of flumazenil and iomazenil in the rat: implications for neuroreceptor studies

Natural Blockers of PD-1/PD-L1 Interaction for the Immunotherapy of Triple-Negative Breast Cancer-Brain Metastasis

The limited treatment options for triple-negative breast cancer with brain metastasis (TNBC-BM) have left the door of further drug development for these patients wide open. Although immunotherapy via monoclonal antibodies has shown some promising results in several cancers including TNBC, it cannot be considered the most effective treatment for brain metastasis. This is due to the protective role of the blood-brain barrier (BBB) which limits the entrance of most drugs, especially the bulky ones such as antibodies, to the brain. For a drug to traverse the BBB via passive diffusion, various physicochemical properties should be considered. Since natural medicine has been a key inspiration for the development of the majority of current medicines, in this paper, we review several naturally-derived molecules which have the potential for immunotherapy via blocking the interaction of programmed cell death protein-1 (PD-1) and its ligand, PD-L1. The mechanism of action, physicochemical properties and pharmacokinetics of these molecules and their theoretical potential to be used for the treatment of TNBC-BM are discussed.

Biological barriers, and the influence of protein binding on the passage of drugs across them

Drug-protein binding plays a key role in determining the pharmacokinetics of a drug. The distribution and protein binding ability of a drug changes over a lifetime, and are important considerations during pregnancy and lactation. Although proteins are a significant fraction in plasma composition, they also exist beyond the bloodstream and bind with drugs in the skin, tissues or organs. Protein binding influences the bioavailability and distribution of active compounds, and is a limiting factor in the passage of drugs across biological membranes and barriers: drugs are often unable to cross membranes mainly due to the high molecular mass of the drug-protein complex, thus resulting in the accumulation of the active compounds and a significant reduction of their pharmacological activity. This review describes the consequences of drug-protein binding on drug transport across physiological barriers, whose role is to allow the passage of essential substances-such as nutrients or oxygen, but not of xenobiotics. The placental barrier regulates passage of xenobiotics into a fetus and protects the unborn organism. The blood-brain barrier is the most important barrier in the entire organism and the skin separates the human body from the environment.

Measurement of drug-protein dissociation rates by high-performance affinity chromatography and peak profiling

The rate at which a drug or other small solute interacts with a protein is important in understanding the biological and pharmacokinetic behavior of these agents. One approach that has been developed for examining these rates involves the use of high-performance affinity chromatography (HPAC) and estimates of band-broadening through peak profiling. Previous work with this method has been based on a comparison of the statistical moments for a retained analyte versus nonretained species at a single, high flow rate to obtain information on stationary phase mass transfer. In this study an alternative approach was created that allows a broad range of flow rates to be used for examining solute-protein dissociation rates. Chromatographic theory was employed to derive equations that could be used with this approach on a single column, as well as with multiple columns to evaluate and correct for the impact of stagnant mobile phase mass transfer. The interaction of L-tryptophan with human serum albumin was used as a model system to test this method. A dissociation rate constant of 2.7 (+/-0.2) s(-1) was obtained by this approach at pH 7.4 and 37 degrees C, which was in good agreement with previous values determined by other methods. The techniques described in this report can be applied to other biomolecular systems and should be valuable for the determination of drug-protein dissociation rates.

Role of Site-Specific Binding to Plasma Albumin in Drug Availability to Brain

Many studies have reported greater drug uptake into brain than that predicted based upon existing models using the free fraction (f(u)) of drug in arterial serum. To explain this difference, circulating plasma proteins have been suggested to interact with capillary membrane in vivo to produce a conformational change that favors net drug dissociation and elevation of f(u). Albumin, the principal binding protein in plasma, has two main drug binding sites, Sudlow I and II. We tested this hypothesis using drugs that bind selectively to either site I (warfarin) or site II (ibuprofen), as well as mixed ligands that have affinity for both sites (tolbutamide and valproate). Brain uptake was determined in the presence and absence of albumin using the in situ rat brain perfusion technique. Unidirectional brain uptake transfer constants (K(in)) were measured and compared with those predicted using the modified Kety-Crone-Renkin model: K(in) = F(1-e(-f(u) x PS(u)/F)), where F is perfusion flow and PS(u) is the permeability-surface area product to free drug of brain capillaries. The results demonstrated good agreement between measured and predicted K(in) over a 100-fold range in perfusion fluid albumin concentration using albumin from three different species (i.e., human, bovine, and rat), as well as whole-rat serum. K(in) decreased in the presence of albumin in direct proportion to perfusion fluid f(u) with constant PS(u). The results show that brain uptake of selected Sudlow site I and II ligands matches that predicted by the modified Kety-Crone-Renkin model with no evidence for enhanced dissociation.

Blood-brain barrier permeability to ammonia in liver failure: a critical reappraisal

In patients with acute liver failure (ALF), hyperammonemia is related to development of cerebral edema and herniation. The present review discusses the mechanisms for the cerebral uptake of ammonia. A mathematical framework is provided to allow a quantitative examination of whether published studies can be explained by the conventional view that cerebral uptake of ammonia is restricted to diffusion of the unprotonated form (NH(3)) (the diffusion hypothesis). An increase in cerebral blood flow (CBF) enhanced ammonia uptake more than expected, possibly due to recruitment or heterogeneity of brain capillaries. Reported effects of pH on ammonia uptake were in the direction predicted by the diffusion hypothesis, but often less pronounced than expected. The published effects of mannitol, cooling, and indomethacin in experimental animals and patients were difficult to explain by the diffusion hypothesis alone, unless dramatic changes of capillary surface area or permeability for ammonia were induced. Therefore we considered the possible role of membrane protein mediated transport of NH(4)(+) across the blood-brain barrier (BBB). Early tracer studies in Rhesus monkeys suggested that NH(4)(+) is responsible for 20% or even more of the transport of ammonia from plasma to brain. In other locations, such as in the thick ascending limb of Hendle's loop and in isolated astrocytes, transport protein mediated translocation of NH(4)(+) is predominant. Many of the ion-transporters involved in renal NH(4)(+) reabsorbtion are also present in brain capillary membranes and could mediate uptake of NH(4)(+). Astrocytic uptake of NH(4)(+) is associated with increased extracellular K(+), which is a potent cerebral vasodilator. Such interference between transport of NH(4)(+) and other cations could be clinically important because increased cerebral blood flow often precedes cerebral herniation in acute liver failure. We suggest that protein mediated transport of NH(4)(+) through the brain capillary wall is a realistic possibility that should be more intensely studied.

Different sensitivities to competitive inhibition of benzodiazepine receptor binding of 11C-iomazenil and 11C-flumazenil in rhesus monkey brain

The in vivo binding kinetics of 11C-iomazenil were compared with those of 11C-flumazenil binding in rhesus monkey brain. The monkey was anesthetized with ketamine and intravenously injected with either 11C-iomazenil or 11C-flumazenil in combination with the coadministration of different doses of non-radioactive flumazenil (0, 5 and 20 microg/kg). The regional distribution of 11C-iomazenil in the brain was similar to that of 11C-flumazenil, but the sensitivity of 11C-iomazenil binding to competitive inhibition by non-radioactive flumazenil was much less than that of 11C-flumazenil binding. A significant reduction in 11C-flumazenil binding in the cerebral cortex was observed with 20 microg/kg of flumazenil, whereas a relatively smaller inhibition of 11C-iomazenil binding in the same region was observed with the same dose of flumazenil. These results suggest that 11C-flumazenil may be a superior radiotracer for estimating benzodiazepine receptor occupancy in the intact brain.