Discovery of Benzylpiperazine Derivatives as CNS-Penetrant and Selective Histone Deacetylase 6 Inhibitors

Abstract

Inhibition of histone deacetylase 6 (HDAC6) in the brain is a highly attractive therapeutic target for the treatment of neurodegenerative diseases. The low blood-brain barrier permeability of most known HDAC6 inhibitors, however, prevents their application as central nervous system (CNS) drugs. To overcome this problem, we designed and synthesized benzylpiperazine derivatives using a hybrid strategy of combining HDAC6 inhibitors and brain-penetrant histamine H₁ receptor antagonists. Introducing the benzylpiperazine units to the cap region of hydroxamate-type HDAC6 inhibitors led us to identify isozyme-selective and CNS-penetrant HDAC6 inhibitor KH-259 (1) with the appropriate pharmacokinetic and safety properties. Intraperitoneal administration of KH-259 (10 mg/kg) had antidepressant activity and increased acetylated α-tubulin in the brain without promoting acetylated histone H3K9. These findings indicate that our hybrid strategy of combining HDAC6 inhibitors and histamine H₁ receptor antagonists is an effective methodology for designing CNS-penetrant HDAC6 inhibitors.

Figure 1

Chemical structures of HDAC6 inhibitors with a “Y-shaped” CAP group.^,−

Figure 2

Drug design of lead compound 2 hybridizing cetirizine and tubastatin A.

Figure 3

(A) Antidepressant effects of the identified compounds. Immobility time during the tail-suspension test after administration of the compounds (Vehicle: n = 10, FLX, n = 6, compound 2: n = 10, compound 8: n = 10, compound 1, n = 10) (ANOVA; F_4,41 = 4.73, p = 0.031). Evaluation of acetylated α-tubulin (B) and acetylated histone H3K9 (C) levels in the brain. Data are expressed as mean ± SE *p < 0.05, **p < 0.01, compared with vehicle-injected stressed (control) mice (Dunnett’s multiple-comparison post-hoc test).

Scheme 1. Synthetic Route of Compound 1 (KH-259)

Reagents and conditions: (a) K₂CO₃, MeCN, rt, 99%; (b) 2N NaOH aq., MeOH/THF, rt, 93%; (c) O-(tetrahydropyran-2-yl)-hydroxylamine, EDCI·HCl, HOBt, DIPEA, DMF, rt, 82%; (d) 4N HCl in EtOAc, EtOAc, rt, 97%.