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Phenserine (also known as eseroline phenylcarbamate) is a mildly toxic, chirally pure, lipophilic, orally bioavailable tartaric acid, next-generation acetylcholinesterase inhibitor clinically used for the treatment of Alzheimer’s Disease. It is a derivative of physostigmine.

Unlike those currently marketed acetylcholinesterase inhibitors, phenserine has a dual mechanism of action. It acts as an acetylcholinesterase inhibitor to keep the level of acetylcholine high in the synapse. This agent also reduces the amount of β-amyloid precursor protein (β-APP) produced as well as the formation of amyloid-β peptide (Aβ) aggregated in the brain. The anti-amyloid activity can confer the disease-modifying effects in Alzheimer’s Disease patients.

Phenserine is generally well-tolerated in both healthy individuals and Alzheimer’s disease patients. Rapid clearance of the drug from the blood may be a reason for its high tolerance. Common side effects include vomiting, nausea, dizziness, and peripheral edema.

Phenserine was patented by the United States Government for the use of Alzheimer’s Disease between 1995 and 1998. In 1995, eseroline phenylcarbamate was chosen to be a lead compound for Alzheimer's Disease-specific drugs by the National Institute on Aging. This drug was then reported to be ineffective on 7 February 2005. Phenserine was under development in treating dementia and Alzheimer’s Disease between 1995-2005. The drug is now used to synthesize posiphen, a follow-on compound to treat Alzheimer's Disease.

Alzheimer’s Disease
Alzheimer’s Disease is a chronic neurodegenerative disorder characterized by the loss of memory and cognitive functions. The most common cause of the disease is the presence of senile plaques, which are some aggregated amyloid-β peptide (Aβ). Phenserine mainly aims to repress the formation of Aβ, thereby prevents the formation of senile plaques and reduces the risk, onset, and progression of the disease.

The drug represses the binding of Aβ oligomers onto the surface proteins of neurons and prevents the oligomers from altering the synaptic structures of neurons. Neurons are protected from structure shifting which can then transmit action potentials and form synapses with one another normally. Synapses are essential in transferring information from nerve to nerve, which these clefts comprise consciousness and, thoughts and memory. Proper formation of synapses contributes to healthy neuronal communication, cognition, and memory formation in patients so they will be less likely to have Alzheimer’s Disease.

Phenserine did not pass the clinical trial phase III in 2003. This indicated that the medicine did not show a significant effect in treating Alzheimer’s disease over the long term. This meant its therapeutic effect was similar to placebos when the drug was applied to hundreds of patients at the same time. The therapeutic efficacy in a large group of patients was low and the agent was not suitable to be marketed.

Traumatic Brain Injury
Phenserine was originally developed to be an Alzheimer’s Disease-specific drug. It was shown in 2019 that phenserine could also be used in reducing neurodegeneration after Traumatic Brain Injury (TBI). Symptoms of TBI including concussion and diffuse axonal injury can then be reduced. Clinically translatable doses of this agent had significantly reduced the neurodegeneration and collective neuropathological cascades in a mouse model of TBI.

There have not been any clinical trials in humans yet. The pharmacodynamics, pharmacokinetics and side effects are still unknown.

Mechanism of action
Phenserine, a phenylcarbamate analog of physostigmine, is categorized as a long duration reversible acetylcholinesterase inhibitor(AChEI). Acetylcholinesterase is an enzyme that catalyzes the hydrolysis of acetylcholine (ACh), a neurotransmitter, into choline and acetate. Phenserine acts as an AChE inhibitor to block the AChE action and increase both levels and duration of ACh action at all cholinergic postganglionic synapses. Positive results of improved memory and cognition in experimental paradigms in rodents and dogs, as well as in Alzheimer’s subjects was reported in treatment with phenserine.

In addition to AChE inhibition, phenserine has a dual mechanism of action that reduces the amount of β-amyloid precursor protein (β-APP) in neuronal cell culture. The β-APP gene is expressed and transcribed into β-APP mRNA, which is translated into β-APP. Cleavage of β-APP to β-amyloid peptide(Aβ) is done by beta-secretase and gamma-secretase.

Phenserine interacts with a regulatory element in the 5’-untranslated region of the β-APP gene. It decreases the translational efficiency of β-APP mRNA into β-APP. Moreover, phenserine lowers the oligomeric form of Aβ by changing the activities of β and γ secretases. Altering the activities of β and γ secretases, the drug interrupts the cleavage of β-amyloid precursor protein (β-APP) and blocks the formation of Aβ. Aβ is found to be the main component of amyloid plaque in the brains of Alzheimer’s patients.

Drug Synergism
Phenserine refers to (-)-phenserine which is an active enantiomer for acetylcholinesterase (AChE) inhibition. (+)-phenserine is shown to possess weaker inhibition activity and can be applied at a higher dosage.

(-)-phenserine can attenuate the progression of Alzheimer’s disease in combination with (+)-Posiphen, an experimental drug that potentially treats Alzheimer’s disease.

Absorption
Phenserine is approximately 7-fold more lipophilic than (-)-physostigmine, a commonly used drug for Alzheimer’s disease. High lipophilicity indicates improved oral absorption and preferential distribution within the brain.

Phenserine has a high and predictable oral bioavailability, a complete bioavailability of 1 is estimated. It is rapidly absorbed by and eliminated from the body as revealed in a single-dose bioavailability study on a healthy subject. Plasma blood concentration of phenserine has a direct association with erythrocyte AChE inhibition.

Distribution
Phenserine can be distributed in the body for many hours after absorption. It is characterized as a long-lasting non-competitive AChEI. Long-lasting action is illustrated by a relatively long half-life of 8 to 10 hours. After the enduring effect of phenserine has passed, it can be removed from the body quickly. Rapid plasma clearance of phenserine ensures complete drug clearance before administration of the next dose.

Metabolism
After distributing to the targeted location in the brain, phenserine is quickly metabolized. The metabolism mechanism for the drug is identified as phase I metabolism, also called functionalization reaction. Phase I metabolism is mainly carried out by the cytochrome P450 monooxygenase system (CYP P450). Two actions N-methylation and hydroxylation in the system occur predominantly through the CYP3A4 pathway, an isoform of the CYP P450 family.

Toxicity
There can be possible toxicity during the metabolism of phenserine. As a result of preferential brain selectivity, the drug is shown to be significantly less toxic than (-)-physostigmine. It is generally safe and well-tolerated in both healthy individuals and Alzheimer’s disease patients when administered as a single oral dose of 5mg or 10mg. The most common adverse events (AEs) are dizziness, nausea, and peripheral edema. Intravenous administration of 20mg/kg phenserine into rats have not been associated with toxic effects or death.

Side effects
In the early clinical trials of phenserine, adverse events present include:
 * Nausea
 * Vomiting
 * Dizziness
 * Headache
 * Nasopharyngitis
 * Hypertension

The causes of the above side effects are still unknown. The occurrences of those side effects increased with the dosage of phenserine applied to the patients. For instance, only 9% of patients had nausea with 10-mg BID of the drug but 19% of patients had nausea with 15-mg BID of it. 5% of patients had vomiting with 10-mg BID of the medicine but 11% of patients had vomiting with 15-mg BID of it. 3% of patients had headaches with 10-mg BID of the agent but 8% of patients had nausea with 15-mg BID of it. The adverse effects were reported to be more significant when phenserine was used with acetylcholine.

There were no severe adverse events and deaths occurred in the clinical trials of phenserine.

Chemistry
Phenserine is obtained as a crystalline compound that is prepared and chosen from extensive medicinal chemistry studies focused on optimizing the structure and activity relation of hexahydropyrrolo [2,3b] indole carbamates and their ring B and C hetero analog.

==== Synthetic methodology ====
 * 1) Efficient cascade reactions for the introduction of 3a-substitute, an alkyl or alkenyl group.
 * 2) Assembly of quaternary carbon center
 * 3) Reductive cyclisation to key pyrroloindoline structure

Phenserine is synthesized amongst various novel carbamates for preclinical evaluation.

Taxonomy
Phenserine belongs to pyrroloindoles, a class of organic compounds. Compounds in this class contain a pyrroloindole moiety, a tricyclic heterocycle that consists of a pyrrole ring fused to an indole. Pyrrole is a five-membered aromatic compound and indole is bicyclic, consists of a six-membered benzene ring fused with a five-membered pyrrole ring.

Physostigmine also belongs to the class of pyrroloindoles. Phenserine is an unsubstituted phenylcarbamate of (-)-physostigmine. Biologically, phenserine is used as an L(+)-tartrate salt to increase aqueous solubility. (-)-phenserine L(+)-tartrate salt has a chiral center in the 3a position, presented as chirally pure (3aS)-, -(-), enantiomer. There are two enantiomers of phenserine: (+)-phenserine and (-)-phenserine. Only (-)-phenserine exerts antiacetylcholinesterase activity.

Drug Discovery
Phenserine was first chosen as a lead clinical candidate for Alzheimer’s Disease treatment by National Institute on Aging in 1995. It was because phenserine was a potent, pseudo-irreversible non-competitive inhibitor acetylcholinesterase.

Phase I Trial
Phase I trial is the first step of the clinical testing of the drug. It aimed to test the safety of phenserine. Axonyx, a US biopharmaceutical company that focused on treating Alzheimer’s Disease, carried out this trial in late 1999. Healthy elderly volunteers were chosen and received single and multiple oral doses of a safe therapeutic range of phenserine. Phenserine passed the test in early 2000.

Phase II Trial
After the success in the phase I trial, a phase II trial proceeded in late 2000. It aimed to test the usable doses in Alzheimer's Disease and look for optimum balance between therapeutic efficacy and adverse effects. The phase II trial was a 12-week, double-blind, placebo-controlled, and randomized study. 72 mild to moderate Alzheimer’s Disease patients were divided into two groups, with 24 individuals receiving placebo and 48 individuals receiving phenserine. In the first 2 weeks, 5 mg BID of the drug was given to the individuals in the phenserine group. The dosage applied increased to 10 mg BID in the next 10 weeks. In late 2000, Axonyx announced that the agent passed the phase II trial.

Phase IIb Trial
After the applied dosage of phenserine was determined in the previous stage, a phase IIb trial was carried out in mid-2003. Phase IIb trial aimed to assess the effect of phenserine on AβPP and Aβ in plasma and cerebrospinal fluid in Alzheimer's Disease patients. 75 mild to moderate patients received 10 and 15 mg BID of the medicine over 6 months. The study failed to reach a statistical significance to prove that it was useful for the disease. A phase III trial was then proceeded to retest its therapeutic efficacy.

Phase III Trial
In order to confirm the results of the phase IIb trial, phenserine underwent a phase III trial in 2003. This stage was also a double-blind, placebo-controlled, and randomized study. 375 mild to moderate Alzheimer’s Disease patients were administered with the agent orally for 6 months. 150 of them received 10 mg BID of phenserine, 149 of them received 15 mg BID of it and 76 of them received placebo.

On Feb 7 2005, Axonyx Inc announced that phenserine did not show a statistically significant result in treating Alzheimer’s Disease compared to placebo after the phase III trial. Axonyx terminated the development of the drug after the announcement.

Further Development
The research on phenserine did not stop after the announcement of its failure in treating Alzheimer's Disease. In 2008, this medicine was further reviewed through meta-analysis and literature review. It was regarded as a drug that had failed to treat Alzheimer’s Disease because of failing to reach a high enough statistical significance. In 2009, QR Pharma decided to synthesize posiphen from phenserine. It is an enantiomer of phenserine, which claimed to be a new hope to treat Alzheimer's Disease. Since phenserine was unable to pass the phase III trials, it is now used as a precursor compound to make posiphen.