CHOLINERGIC TRANSMISSION AND NEUROPATHOLOGY

The cholinergic system is integral to the central and peripheral nervous systems and to the neuromuscular junction of skeletal and smooth muscles. Due to its wide distribution, the cholinergic system has an extremely broad physiological relevance both in normal and pathological states. In the brain, the cholinergic system plays a central role in analgesia, cognition, memory and neuroprotection, whereas in the skeletal muscle the system is responsible for contraction. Impairment in cholinergic transmission results in selective loss of cholinergic neurons with subsequent neuropathological conditions such as myasthenia gravis (MG) and Alzheimer's disease (AD), as well as neuropsychiatric disorders such as post-traumatic stress disorder (PTSD), anxiety, attention deficit and depression. In addition, cholinergic impairments further exacerbate neurological diseases of different etiology including Sjögren disease and muscular dystrophy.

TREATMENT OF CHOLINERGIC DISEASES: “THE CONVENTIONAL WISDOM”

Acetylcholinesterase (AChE) is an enzyme responsible for the control of neurotransmission at cholinergic synapses and neuromuscular junctions by hydrolyzing the neurotransmitter acetylcholine (ACh). Rapid hydrolysis of ACh removes excess neurotransmitter from the synapse, preventing overstimulation and tetanic excitation of the postsynaptic cell. For this reason, AChE is the target protein of pesticides and chemical warfare agents. At the same time, controlled use of AChE inhibitors plays a leading role in therapeutic strategies designed to enhance the cholinergic system. The rationale behind the clinical use of anticholinesterases is that AChE blockade extends the half-life of released ACh, thereby enhancing postsynaptic signals in patients with compromised cholinergic function. Indeed, the extensive use of anticholinesterase therapies highlights their clinical value. Nevertheless, accumulated experience attests to the limited extent and duration of clinical effects achieved with pharmacological AChE inhibitors.

ANTISENSE TECHNOLOGY AND ACHE

Proteins like AChE are produced in cells from information encoded by genes, the structural units of DNA. The information in DNA is reproduced as RNA, which is translated inside the cell to protein. Ester (which Amarin acquired in 2007) scientists along with Professor Mona Soreq at the Hebrew University have developed a platform technology allowing for specific arrest of AChE production in living cells and tissues. This technology is based on disrupting AChE biosynthesis with very small amounts of short, chemically synthesized DNA chains called antisense oligonucleotides. Antisense oligonucleotides are uniquely and specifically targeted against the messenger RNA encoding AChE rather than against the biochemical activity of the protein. By preventing the production of AChE protein rather than simply blocking the breakdown of ACh, antisense-based therapies act against both the catalytic and the non-catalytic activities of AChEs.

Antisense Strategy for Blocking AChE Expression

 

A NOVEL STRESS-RESPONSE PROTEIN: THE ACHE-R VARIANT

A new dimension to the question of specificity with regard to AChE inhibitors emerged as molecular cloning revealed that alternative splicing gives rise to three distinct AChE variants. The presumed target of all anticholinesterase therapeutics is the "synaptic" AChE-S variant with its unique 40 amino acid, amphiphilic, C-terminal peptide. A second, erythrocyte-bound form, AChE-E, is presumed to participate in scavenging blood-borne inhibitors, including drugs of abuse. Until recently, AChE-S and AChE-E were considered the primary factors in the development of anticholinesterase drugs. However, research has shown that the third AChE variant, the rare "readthrough" AChE-R, undergoes dramatic upregulation during physiologic stressors, such as disease or exposure to AChE inhibitors. The AChE-R variant lacks an anchoring domain and is soluble. As a result, AChE-R serves to limit the quantity of ACh that reaches the ACh receptor and thereby limit cholinergic overexcitation. However, accumulation of AChE-R will limit cholinergic activation which has been shown to compromise normal synaptic function. Thus, this specific AChE-R variant provides a novel valid target and opens a new therapeutic window for the development of innovative drugs for the treatment of neurological diseases. Amarin’s EN101 for myasthenia gavis is the lead program utilizing this platform technology.