Figure 3.

HDAC inhibitor pretreatment blocks [¹¹C]martinostat uptake in brain. (A) Pretreatment with hydroxamic acids (iv) resulted in a 10–14% blockade of tracer uptake in whole brain for the known hydroxamic acids, SAHA and givinostat (n = 1/group), with greater, 18–22%, blockade for the two novel compounds, PK5 and PK6 (n = 2/group; mean ± SEM). Imaging experiments revealed robust 40% blockade of whole brain [¹¹C] levels by the hydroxamic acid, CN54 (n = 1/group). (B) Pretreatment with the protypical orthoaminoanilide benzamide, CI-994, via ip injection revealed modest blockade (9–14%) with limited impact of dose (15–60 mg/kg) acute pretreatment blocking time (2.5–4 h) or duration of treatment (7 days treatment with final treatment 4 h prior to tracer administration). (C) Pretreatment (ip) with the HDAC subtype-selective benzamides Cpd60 and RGFP966 also resulted in limited, 7–10%, blockade of [¹¹C]martinostat binding in whole brain. A 500 mg/kg ip dose of sodium valproate, administered 30 min prior to tracer administration blocked uptake by 16%. (D) Brain penetrance of novel benzamides was evaluated via blocked tracer binding for the tool compound CN147. CN147 demonstrated robust 23% tracer blockade at 5 mg/kg (ip, 2.5 h pretreatment). At 1 mg/kg (ip), maximal blocking effects were observed 4 h after pretreatment and were maintained at a lower dose of 0.1 mg/kg. Consistent with the slow-binding kinetics of benzamide HDAC inhibitors, no appreciable blockade was observed following a short, 5 min, pretreatment time with CN147, 5 mg/kg, ip (data not shown).