Mitigation of off-target adrenergic binding and effects on cardiovascular function in the discovery of novel ribosomal S6 kinase 2 inhibitors

We previously reported the discovery of a novel ribosomal S6 kinase 2 (RSK2) inhibitor, (R)-5-Methyl-1-oxo-2,3,4,5-tetrahydro-1H-[1,4]diazepino[1,2-a] indole-8-carboxylic acid [1-(3-dimethylamino-propyl)-1H-benzoimidazol-2-yl]-amide (BIX 02565), with high potency (IC(50) = 1.1 nM) targeted for the treatment of heart failure. In the present study, we report that despite nanomolar potency at the target, BIX 02565 elicits off-target binding at multiple Adrenergic Receptor subtypes that are important in the control of vascular tone and cardiac function. To elucidate in vivo the functional consequence of receptor binding, we characterized the cardiovascular (CV) profile of the compound in an anesthetized rat CV screen and telemetry-instrumented conscious rats. Infusion of BIX 02565 (1, 3, and 10 mg/kg) in the rat CV screen resulted in a precipitous decrease in both mean arterial pressure (MAP; to -65 ± 6 mm Hg below baseline) and heart rate (-93 ± 13 beats/min). In telemetry-instrumented rats, BIX 02565 (30, 100, and 300 mg/kg p.o. QD for 4 days) elicited concentration-dependent decreases in MAP after each dose (to -39 ± 4 mm Hg on day 4 at T(max)); analysis by Demming regression demonstrated strong correlation independent of route of administration and influence of anesthesia. Because of pronounced off-target effects of BIX 02565 on cardiovascular function, a high-throughput selectivity screen at adrenergic α(1A) and α(2A) was performed for 30 additional RSK2 inhibitors in a novel chemical series; a wide range of adrenergic binding was achieved (0-92% inhibition), allowing for differentiation within the series. Eleven lead compounds with differential binding were advanced to the rat CV screen for in vivo profiling. This led to the identification of potent RSK2 inhibitors (cellular IC(50) <0.14 nM) without relevant α(1A) and α(2A) inhibition and no adverse cardiovascular effects in vivo.