Neuroscience is one of the field of expertise of the Sussex Drug Discovery Centre, and as a medicinal chemist novice to this area of drug discovery, I decided to extend my knowledge of drug delivery to the brain and pharmacokinetics for central nervous system (CNS) pharmacological agents. A significant amount of extremely interesting literature covers neuropharmacokinetics, and I rapidly came across a number of essential principals such as the importance of high blood brain barrier (BBB) permeability or the complexity of predicting pharmacokinetics/pharmacodynamics (PK/PD) relationships for CNS drugs. Among these dozens of publications, one scientific article1 from a group of DMPK scientists at Pfizer particularly attracted my attention, since it was reporting that brain tissue binding is species-independent, something that I would not have expected to be true. Indeed, I was familiar with the well-known phenomenon of interspecies differences in plasma protein binding and would have expected the same for brain tissue binding. As a result, I got extremely curious in finding out why the story was different with brain tissues and how the team at Pfizer conducted their study to draw this conclusion.
Before getting into the details of this study, let’s first set the scene to refresh one’s knowledge of pharmacokinetics and make sure that we are all on the same page. One of the most important, well accepted and widely applied concept in drug discovery and development, used to establish PK/PD models among other things, is the free drug hypothesis. This concept stipulates that in vivo efficacy is solely determined by the free (unbound) drug concentration at the site of action rather than total (bound and unbound) drug concentration, and that only the free drug is able to distribute from the systemic circulation across membranes to tissues. This notion which is key to understand the action of all drugs becomes even more essential in the context of CNS pharmacological agents. Indeed, the CNS is an extravascular compartment separated from the systemic circulation by the BBB and only free drug from the plasma can cross this biological barrier. The capacity of a drug to diffuse through the BBB is referred to as CNS permeability or CNS penetration, and is controlled and influenced by a number of physicochemical properties. Once in the CNS, the drug undergoes binding to the brain tissues and only a portion of the free drug in the plasma is free in the CNS to exert a pharmacological effect.
A number of techniques have been developed and used to determine the unbound drug concentration in the brain, such as in vivo microdialysis, cerebrospinal fluid sampling, and a combined measurement of brain tissue binding and brain distribution from plasma and brain concentration time courses. The latest approach is routinely used in pharmaceutical research since it requires less compound-specific optimisation and provides a direct measurement in the compartment of interest. In addition, it is a less challenging, more reproducible and higher throughput technique than the other methods. The fraction unbound (fu) of drugs in brain tissues can be obtained from two different biological systems: brain homogenates or brain slices. Although brain slices are considered more physiologically relevant, brain homogenates are more widely used, since they are easier to handle and store than slices, and can be readily obtain from commercial suppliers. Both systems afford good correlation with each other, indicating that brain tissue binding is primarily governed by nonspecific binding to lipophilic components rather than binding to intact structural elements.
Similarly to plasma protein binding, brain tissue binding has been routinely determined in multiple species to account for any potential species dependence. However from 2007, a few studies using a limited number of drugs or a limited number of species indicated that brain tissue binding might be species-independent. Pfizer decided to conduct the present study to confirm the species-independent phenomenon of brain tissue binding and determine if it was not limited to certain species, certain classes of pharmaceutical agents or a certain nature of brain binding. With this objective, a large number of structurally diverse compounds, covering a wide range of physicochemical properties (logD from -1.43 to 6.01, MWt from 151 to 823 g/mol, tPSA from 12 to 220 Å2) and brain binding characteristics (fu from 0.0005 to 0.5) in multiple animal species, was selected. The brain tissue binding of these compounds was assessed in seven species and strains (Wistar Han rat, Sprague-Dawley rat, CD-1 mouse, Hartley guinea pig, beagle dog, cynomolgus monkey and human), commonly used in neuroscience drug discovery.
Rigorous statistical orthogonal regression was used to analyse the concordance of the data between species, strain and within strain. The results unambiguously demonstrate that the drugs unbound fractions were strongly correlated (R2 ranging from 0.93 to 0.99) across the various species and strains tested. Importantly, the cross-species/strain correlations were extremely similar to the interassay correlation with the same species. Statistical analysis were performed and indicated that no correction was required for the extrapolation of fraction unbound from one species to the other species/strains. These results suggest that determination of brain tissue binding in a single species can replace multispecies determinations. The authors argue that the difference in composition of the brain relative to the plasma is a likely cause of the species-independent nature of brain tissue binding. Indeed, the brain has a much higher lipid contents (11% lipid and 7.9% protein versus 0.65% lipid and 18% protein in the plasma) which could account for the predominant non-specific binding of drugs to brain tissues. The absence of species-specific brain proteins selectively binding to drugs could be another potential explanation.
As stated earlier, the results from this study allowed the DMPK scientists at Pfizer to conclude that measuring brain tissue binding is essential for drugs intended to be used as CNS pharmacological agents, but that a single species measurement is sufficient. In addition, these results demonstrated that brain tissue binding variations can be ruled out to explain observed interspecies differences in the behaviour of CNS drugs.
I personally found the scientific paper1 describing the above study extremely enlightening and hope that this blog article will be of interest to other medicinal chemists or scientists working in the field of neuroscience.
Blog written by Tristan Reuillon
- Drug Metabolism and Disposition 2011, 39, 1270-1277, Species Independence in Brain Tissue Binding Using Brain Homogenates (doi: 10.1124/dmd.111.038778)