Selective histone deacetylase 6 inhibitors bearing substituted urea linkers inhibit melanoma cell growth

Abstract

The incidence of malignant melanoma has dramatically increased in recent years thus requiring the need for improved therapeutic strategies. In our efforts to design selective histone deactylase inhibitors (HDACI), we discovered that the aryl urea 1 is a modestly potent yet nonselective inhibitor. Structure-activity relationship studies revealed that adding substituents to the nitrogen atom of the urea so as to generate compounds bearing a branched linker group results in increased potency and selectivity for HDAC6. Compound 5 g shows low nanomolar inhibitory potency against HDAC6 and a selectivity of ∼600-fold relative to the inhibition of HDAC1. These HDACIs were evaluated for their ability to inhibit the growth of B16 melanoma cells with the most potent and selective HDAC6I being found to decrease tumor cell growth. To the best of our knowledge, this work constitutes the first report of HDAC6-selective inhibitors that possess antiproliferative effects against melanoma cells.

Figure 1

Structure of 1, a urea containing HDACI. Appendage of substitutions to the nitrogen atoms of the urea generate a branched linker motif that may interact with a side cavity at the enzyme surface.

Figure 2

Comparison of acetylation status of tubulin and histone H3 demonstrates HDAC62substrate selectivity of Nexturastat A.

Scheme 1

Synthesis of proximal nitrogen atom substituted hydroxamic acids Reagents and conditions: (a) R¹-NH₂, NaCNBH₃, rt, 5% AcOH/DCM, 16 h; (b) Aryl-NCO, DCM, rt, 16 h; (c) 50% wt NH₂OH, NaOH, THF/MeOH (1:1), 0 °C to rt, 30 min.

Scheme 2

Synthesis of distal nitrogen atom substituted hydroxamic acids Reagents and conditions: (a) R¹-NH₂, CuI, K₃PO₄, ethylene glycol, iPrOH, 80°C, 18 h; (b) i. triphosgene, sat. aq bicarbonate/DCM (1:1), 0°C, 30 min; ii. methyl 4-(aminomethyl)benzoate-HCl, Et₃N, DCM, rt, 16 h; (c) 50% wt NH₂OH, NaOH, THF/MeOH (1:1), 0°C to rt, 30 min.