In Vivo and In Vitro Evidence for Brain Uptake of 4-Phenylbutyrate by the Monocarboxylate Transporter 1 (MCT1)

Purpose

4-Phenylbutyrate (4-PBA) is expected to be a potential therapeutic for several neurodegenerative diseases. These activities require 4-PBA transport into the brain across the blood-brain barrier (BBB). The objective of the present study was to characterize the brain transport mechanism of 4-PBA through the BBB.

Methods

The brain transport of 4-PBA across the BBB was investigated following intravenous (IV) injection and internal carotid artery perfusion (ICAP) in vivo. The mechanism of transport was examined using TR-BBB cells, an in vitro model of the BBB.

Results

The volume of distribution (V_D) of 4-PBA by rat brain was about 7-fold greater than that of sucrose, a BBB impermeable vascular space marker, suggesting the blood-to-brain transport of 4-PBA through the BBB in the physiological state. [¹⁴C]4-PBA uptake by TR-BBB cells showed time-, pH- and concentration-dependence with a K _m of 13.4 mM at pH 7.4 and 3.22 mM at pH 6.0. The uptake was Na⁺ independent, and was significantly inhibited by alpha-cyano-4-hydroxycinnamate (a typical inhibitor for monocarboxylate transport), endogenous monocarboxylate compounds and monocarboxylic drugs. Lactate and valproate competitively inhibited [¹⁴C]4-PBA uptake with K _i value of 13.5 mM and 7.47 mM, respectively. These results indicate the role of monocarboxylate transporters (MCTs) in 4-PBA transport into the brain at the BBB. TR-BBB cells expressed mRNA of rMCT1, 2, and 4, especially, rMCT1 showed high mRNA expression level. In addition, [¹⁴C]4-PBA uptake was inhibited by rMCT1 specific small interfering RNA.

Conclusion

The transport mechanism of 4-PBA from blood to brain across the BBB likely involves MCT1.