Tirasemtiv

Tirasemtiv (CK-2017357) is the first direct fast skeletal muscle troponin activator to enter clinical development, representing a novel pharmacological class that targets the sarcomere rather than neuromuscular signaling or motor neuron survival. Discovered at Cytokinetics, Inc. through a high-throughput screen of ATP hydrolysis rates in rabbit fast skeletal myofibrils, tirasemtiv selectively binds the fast skeletal muscle troponin complex with a dissociation constant of approximately 40 nanomolar, slows the rate of calcium release from troponin C, and shifts the calcium-force relationship of fast skeletal muscle fibers leftward, thereby amplifying force production at submaximal stimulation frequencies without altering maximal tetanic force. Selectivity for fast skeletal muscle troponin over slow skeletal troponin is approximately 95-fold (Kd 3800 nM for slow skeletal), and the compound does not activate cardiac muscle troponin, a critical safety distinction from calcium sensitizers that act on cardiac isoforms.

The compound emerged from a medicinal chemistry optimization of the imidazo[4,5-b]pyrazin-2-one scaffold. The initial high-throughput screening hit (CK-1303249, compound 1) displayed an AC40 of 7.0 micromolar in the myofibrillar ATPase assay; iterative optimization of the N-1 and C-6 substituents, culminating in a 3-pentyl group at N-1 and an ethynyl group at C-6, yielded tirasemtiv with an AC40 of 0.1 micromolar and a skinned fiber DF30 of 0.3 micromolar. The 3-pentyl substitution eliminated stereochemistry from the molecule, simplifying manufacture and regulatory characterization. Oral bioavailability is high in preclinical species (98 percent in rat, 93 percent in dog) and the compound exhibits near-dose-proportional pharmacokinetics in humans with a terminal half-life of approximately 10 to 15 hours.

In preclinical models, tirasemtiv produced dose-dependent increases in submaximal isometric force in rat extensor digitorum longus muscle, improved forelimb grip strength by 38 percent, grid hang time by 125 percent, and rotarod performance by 150 percent in the B6SJL-SOD1(G93A) transgenic mouse model of amyotrophic lateral sclerosis (ALS) at a 25 percent baseline deficit milestone, and modestly prolonged survival (median 118 versus 115 days, hazard ratio 0.66). In preclinical models of spinal muscular atrophy, tirasemtiv improved submaximal muscle force, grip strength, and fatigue resistance.

Clinical development proceeded through an extensive program: Phase 1 dose-escalation in healthy volunteers established a maximum tolerated dose of 2000 mg and characterized the pharmacokinetic-pharmacodynamic relationship between plasma concentration and submaximal muscle force; Phase 2 trials demonstrated proof of concept in myasthenia gravis (dose-dependent improvement in Quantitative Myasthenia Gravis score, p = 0.02) and generated signals in peripheral artery disease; the 711-patient BENEFIT-ALS Phase 2b trial failed its primary ALSFRS-R endpoint but showed a 50 percent reduction in the rate of slow vital capacity decline; and the pivotal VITALITY-ALS Phase 3 trial in ALS failed to demonstrate a statistically significant effect on the primary slow vital capacity endpoint (p = 0.56), with high discontinuation rates (34 percent on tirasemtiv versus 12 percent on placebo) driven by dizziness, fatigue, and nausea confounding interpretation. Development was halted in November 2017.

The principal tolerability limitations of tirasemtiv, specifically dizziness (51 percent versus 20 percent on placebo), fatigue (33 percent versus 14 percent), and nausea (22 percent versus 8 percent), led Cytokinetics to develop the second-generation fast skeletal troponin activator reldesemtiv (CK-2127107), which was designed for improved tolerability but similarly failed its Phase 3 ALS trial (COURAGE-ALS, stopped for futility in 2023). This monograph reviews the chemistry, synthesis, and structural pharmacology of tirasemtiv; the troponin activation mechanism in molecular detail; the comprehensive pharmacokinetic record in preclinical species and humans; the preclinical pharmacology across ALS, SMA, and exercise physiology models; the clinical evidence base across ALS, myasthenia gravis, and peripheral artery disease; sourcing and quality verification; reconstitution and handling; stack-interaction considerations; adverse-event profile; and a comparative assessment of five sarcomere-targeting and neuromuscular function compounds against tirasemtiv on five competency standards.