Pharmacokinetics/Pharmacodynamics and Analysis of the Effect of β-Amyloid Peptide on Acetylcholine Neurocycle and Alzheimer’s Disease Medications
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The brain of Alzheimer’s disease (AD) is characterized by accumulations of β-amyloid peptide aggregates which promote neurodegentartive dysfunction. Comprehensive understanding of the interaction between β-amyloid aggregates and acetylcholine (ACh) neurocycle is required to uncover the physiological processes related to AD and might result in improving therapeutic approaches for AD. Pharmacokinetics (PK) and pharmacodynamics (PD) techniques were applied to allow predicting the extent of the interaction of certain doses of AD drugs and β-amyloid inhibitors and levels of ACh as well. Although many researchers focused on the β-amyloid interactions, the mechanisms by which β-amyloid affects cholinergic neurons and reduction of ACh are still unclear. The prediction of ACh and drug concentrations in the tissues and body needs an understanding of the physiology and mechanisms of β-amyloid aggregates processes and their compilation into a mechanistic model In this work, two hypotheses are proposed to investigate the dynamic behavior of the interaction between β-amyloid peptide aggregates and cholinergic neurocycle and the possible therapeutic approaches through proposing pharmacokinetic/pharmacodynamics (PK/PD) models to represent the impact of β-amyloid aggregates in AD. The effect of β-amyloid peptide aggregates is formulated through incorporating β- amyloid aggregates into non-linear model for the neurocycle of ACh where the presynaptic neuron is considered as compartment 1 and both synaptic cleft and postsynaptic neurons are considered as compartment 2. In the first hypothesis which is choline leakage hypothesis, β-amyloid peptide aggregates are considered to be located in the membrane of the presynaptic neuron and create pathways inside the membrane to allow for the intracellular choline to leak outside the cholinergic system. It is observed that β-amyloid aggregates via the choline leakage hypothesis could cause significant reductions of ACh and choline levels in both compartments. Furthermore, the process rates of ACh synthesis and hydrolysis have been affected negatively by a wide range of β-amyloid aggregate concentrations. It is found that as the input rate of β-amyloid aggregates to compartment 1 increases, the loss of choline from compartment 1 increases leading to an increase in the intracellular concentration of β-amyloid. In the second hypothesis, β-amyloid peptide aggregates are proposed to interact with the enzyme ChAT which is responsible for the synthesis of ACh in compartment 1; three different kinetic mechanisms are suggested to account for the interaction between β-amyloid aggregates and ChAT activity. In the first and second kinetic mechanisms, β-amyloid aggregate is supposed to attack different species in the enzyme. It is found that there is a significant decrease in the rate of ACh synthesis in compartment 1 and ACh concentrations in both compartments. However, it is observed that there is no effect on choline levels in both compartments, the rate of ACh hydrolysis in compartment 2, pH, and ACh levels in compartment 2. In the third kinetic mechanism, all species in ChAT are attacked by β-amyloid aggregates; it is observed that at very high input rates of β-amyloid aggregates, the oscillatory behavior dominates all components of the neurocycle of ACh. The disturbance observed in ACh levels in both compartments explains the harmful effect of the full attack of β-amyloid aggregates to all species of ChAT. It is found that to contribute significantly in ACh neurocycle, choline leakage hypothesis needs concentration of β-amyloid aggregates lower than that needed in ChAT activity hypothesis which is in agreement with experimental observations. The significant decrease in ACh levels observed in both choline leakage and loss of ChAT activity hypotheses leads to cognitive loss and memory impairment which were observed in individuals with AD. A one-compartment drug PK/PD model is proposed to investigate a therapeutic approach for inhibiting β-amyloid aggregation via choline leakage hypothesis where the maximum feed rate of β-amyloid (KL2 = 1) is considered. The drug is assumed to interact with the tissues of the presynaptic neurons where β-amyloid aggregates are located. The PK/PD model is built based on the effect of β-amyloid aggregates via choline leakage hypothesis where the maximum feed rate of β-amyloid aggregates is considered. The dynamic behavior of all concentrations of β-amyloid aggregates, choline, ACh, acetate, and pH in both compartments in addition to the rate of ACh synthesis in compartment 1 and ACh hydrolysis are investigated by monitoring the impacts of the drug on β-amyloid aggregates and cholinergic neurocycle over a wide range of the input drug dosage. The PK/PD model is able to predict the reduction in levels of β-amyloid aggregates and the increase in choline and ACh, in both compartments as well as both rates of ACh synthesis and hydrolysis catalyzed. The parameters of the PK/PD model such as maximum concentration (Cmax), maximum time (Tmax), area under the curve (AUC), and maximum effect (Emax) were investigated. It was found that it takes a longer time (Tmax) (3-5 h) to reach Emax as the drug dose increases. Furthermore, AUC was found to increase with increasing drug dosage. The results of the current work show that drugs / therapeutic agents inhibiting β- amyloid aggregation in the brain represent a likely successful therapeutic approach to give systematic highlights to develop future trials, new diagnostic techniques, and medications for AD. This study is helpful in designing PK and PD and developing experimental animal models to support AD drug development and therapy in the future.