Voltage-gated calcium (Ca2+) channels (VGCC) play an integral role in the regulation of membrane ion conductance, cellular excitability and neurotransmitter release. VGCC are composed of the pore-forming α1 subunit and auxiliary α2δ ppm and β subunits that modulate channel expression and function. Among the low voltage activated channels is the Cav3 channel subtype, which mediates T-type calcium currents that may be targeted for treatment of epilepsy, especially children absence epilepsy and chronic pain (Huguenard, 1998, Cribbs et al., 2000, Perez-Reyes et al., 2009, Perez-Reyes, 2010).
The T-type or “low voltage-activated” calcium channels are so named because they open for shorter duration (T=transient) than the L-type (L=long lasting) calcium channels. T-type channels are activated at relatively negative membrane potentials (˜−60 mV). In many types of neurons, Ca2+ influx through T channels triggers low-threshold Ca2+ spikes, which in turn elicit a burst of action potentials mediated by voltage-gated sodium (Na+) channels. Brief burst firing is thought to play an important role in the synchronized activity of the thalamus and neuronal pacemaker under physiological conditions, but it also underlies a wide range of thalamocortical dysrhythmias under pathological conditions such as neuropathic pain or seizures, T channels can be activated by mild depolarization of the cell membrane (Talley et al., 1999, Perez-Reyes, 2003, Perez-Reyes, 2010, Pexton et al., 2011, Todorovic and Jevtovic-Todorovic, 2011).
Molecular cloning has revealed three distinct T channel proteins, designated Cav3.1, Cav3.2 and Cav3.3. The Cav3.1 and Cav3.3 channels are expressed predominantly, though not exclusively, in the CNS. In contrast, the Cav3.2 channel is not only present in the CNS, but also expressed in peripheral nerve cell bodies and nerve endings of afferent fibers (Huguenard, 1998, Cribbs et al., 2000, Perez-Reyes et al., 2009, Perez-Reyes, 2010). The Cav3.2 channel is highly expressed in dorsal root ganglion (DRG) neurons, whereas little Cav3.1 and virtually no Cav3.3 are expressed in the small diameter DRG neurons (Nelson et al., 1992). The Cav3.2 channels are also expressed at a lower level in several non-neuronal tissues, including heart, liver, kidney, and pituitary. Both diabetic neuropathy and chronic constriction injury models in rats lead to DRG neuron-specific upregulation of the Cav3.2 channel and the T current density. This pathological adaptation results in enhanced excitability of sensory neurons and causes hyperalgesia and allodynia (Jagodic et al., 2007, Jagodic et al., 2008, Latham et al., 2009, Messinger et al., 2009, Yue et al., 2013). Conversely, knockout or antisense knockdown of the Cav3.2 isoform produces analgesic effects (Messinger et al., 2009).
T-type channel inhibitors have two known uses in the clinic. The anti-absence seizure effects of ethosuximide and lamotrigine are thought to be mediated by the inhibition of T channel activity in the thalamus (Gomora et al., 2001, Huguenard, 2002). However, both drugs are weak and not specific against the T channel (Xie et al., 1995, Zhang et al., 1996). The antihypertensive effect of mibefradil is conventionally attributed to its inhibition of the T channel. However, mibefradil has poor selectivity with about 3-10 times more potent inhibition of the T-type than of the L-type Ca2+ current or the voltage-gated Na+ current (Avdonin et al., 2000). Because there are no selective T channel blockers, it is unclear whether and to what extend the inhibition of T channel activity at therapeutically relevant concentrations contributes to the therapeutic usefulness of a wide range of drugs, such as analgesics, antiepileptics, neuroprotectants, antipsychotics, antidepressants, antiarrhythmics and antihypertensives.
Targeting a T-channel, particularly the Cav3.2 isoform, would be highly useful in reduction of thermal hyperalgesia and mechanical allodynia under pathological conditions, for example diabetic neuropathy. Several efforts to discover potent and selective T-type Ca2+ channels have been described in the literature, as exemplified below.
1,4-Substituted piperidines, for example, “compound 30” (3,5-dichloro-N-{[1-(3,3-dimethylbutyl)-3-fluoropiperidin-4-yl]methyl}benzamide) and “TTA-P2” (3,5-dichloro-N-((1-((2,2-dimethyltetrahydro-2H-pyran-4-yl)methyl)-4-fluoropiperidin-4-yl)methyl)benzamide) were synthesized by Merck and found to potently block the T-Type Cav3.2 channel [J. Med. Chem. 51, 3692, (2008); J. Med. Chem. 51, 6471, (2008); US 2010/0222387; US 2013/8501773]. TTA-A2 suppresses active wake, promotes slow-wave sleep (Kraus et al., 2010), and prevents weight gain in mice on a high-fat diet (Uebele et al., 2009).
A scaffold hopping approach afforded ML218 (3,5-Dichloro-N-[[(1α,5α,6-exo,6α)-3-(3,3-dimethylbutyl)-3-azabicyclo[3.1.0]hex-6-yl]methyl]benzamide, CID 45115620) a selective T-Type Ca2+ inhibitor. ML218 possess acceptable in vivo rat PK and was efficacious in a preclinical Parkinson model. Thus, ML218 is a useful new biologic probe to study T-Type Ca2+ function in vitro and in vivo (Xie et al., 2010, Xiang et al., 2011).
Certain lactam acetamides have been described by Abbott and others as Cav2.2 and Cav3.2 calcium channel blockers, and ABT-639 has been reported as a Cav3.2 calcium channel blocker for treatment of diabetic neuropathic pain through peripheral action, because ABT-639 is presumed to not penetrate the blood brain barrier (Jarvis et al., 2014).
N-Piperidinyl acetamide derivatives as calcium channel blockers have been described by Zalicus Pharmaceuticals, Ltd. [U.S. Pat. No. 8,569,344 (2013); U.S. Pat. No. 8,377,968 (2013)]. A piperidine-based compound, Z944, inhibits Cav3 channels in a voltage-dependent manner and is able to attenuate thalamic burst firing and suppress absence seizures in rats (Tringham et al., 2012). Z944 has shown promising results in clinical Phase I studies of pain in humans (Lee, 2014).
Despite the fact that many T-type Ca2+ channel inhibitors have been discovered and have advanced to different stages of development, no FDA-approved selective T-type channel inhibitory compounds are available for clinical applications.