Abstract
Drug distribution in the brain is generally associated with an affinity for fatty brain tissues and therefore known to be species- and concentration-independent. We report here the effect of target affinity on brain tissue binding for 10 small molecules designed to inhibit brain heat shock protein 90 (HSP90), a widespread protein whose expression is 1–2% of total cytosolic proteins in eucaryotes. Our results show that increasing the test item concentrations from 0.3 to 100 µM increased the unbound fraction 32-fold for the most potent molecules, with no change for the inactive one (1.1 fold change). Saturation of HSP90 led to normal concentration-independent brain tissue binding. In vivo pharmacokinetics performed in rats showed that the overall volume of distribution of compounds is correlated with their affinity for HSP90. The in vitro binding and in vivo pharmacokinetics (PK) performed in rats showed that small molecule HSP90 inhibitors followed the principle of target-mediated drug disposition. We demonstrate that assessing unbound fractions in brain homogenate was subject to HSP90 target interference; this may challenge the process of linking systemic-free drug concentrations to central nervous system unbound concentrations necessary to establish the proper pharmacokinetics/pharmacodynamics (PK/PD) relation needed for human dose prediction.
Highlights
According to the principles of the free drug hypothesis, only the unbound drug at the site of action exerts a pharmacological effect
The combination of unbound fractions in plasma/blood and brain homogenate can be used to calculate the ratio of free concentrations in brain relative to plasma in vivo; this ratio at steady state is defined as Kp,uu (Cu_Brain/Cu_Plasma)
We present brain tissue binding data for inhibitors of heat shock protein 90 (HSP90), a widespread and highly expressed protein constituting around 1–2% of cytosolic proteins in eukaryotes [9,10]
Summary
According to the principles of the free drug hypothesis, only the unbound (free) drug at the site of action exerts a pharmacological effect. The combination of unbound fractions in plasma/blood and brain homogenate can be used to calculate the ratio of free concentrations in brain relative to plasma (or blood) in vivo; this ratio at steady state is defined as Kp,uu (Cu_Brain/Cu_Plasma) This ratio expresses the extent of brain disposition, the ability of a drug to freely pass the blood–brain barrier (when Kp,uu is close to unity) and allows for predicting the human efficacious dose, taking into account the difference between systemic and central exposures. The level of HSP90 is sufficiently high that it affects the level of non-specific binding of HSP90 inhibitors to brain tissues This phenomenon has previously been reported for HSP90 inhibitors in a peripheral setting and termed “target-mediated drug disposition” [11]. The relation between affinity for HSP90 and binding to brain tissues will be discussed as well as its consequence on the development of small molecule inhibitors of HSP90 in a CNS setting
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