Unique Hydrolysis of an Ester-Type Prodrug of Levodopa in Human Plasma: Relay-Type Role Sharing between Alpha-1 Acid Glycoprotein and Human Serum Albumin

Hydrolytic Metabolism of Withangulatin A Mediated by Serum Albumin Instead of Common Esterases in Plasma

BACKGROUND AND OBJECTIVES

The oral bioavailability of withangulatin A (WA) is low and may undergo first-pass metabolism because of the presence of two esters bonds. This study aimed to identify the hydrolysis behavior and mechanism of WA, thus enriching its structure-pharmacokinetic relationship.

METHODS

The in vivo pharmacokinetic studies of WA in rats were first investigated, followed by in vitro assays including metabolic stability, phenotyping identification and metabolic kinetics assays. After screening out the responsible enzymes with higher catalytic capacity, molecular docking study was performed to demonstrate the interaction mode between WA and metabolic enzymes. Then, metabolites in human serum albumin (HSA) were identified by LC-TOF-MS/MS.

RESULTS

In rats, the oral bioavailability of WA was only 2.83%. In vitro, WA was hydrolyzed in both rat and human plasma and could not be inhibited by selective esterase inhibitors. Physiologic concentration of HSA not recombinant human carboxylesterases (rhCES) could significantly hydrolyze WA, and it had a similar hydrolytic capacity with human plasma to WA. Furthermore, WA could stably bind to HSA by forming hydrogen bonds with Lys199 and Arg410, accompanied by the metabolic reaction of the lactone ring opening.

CONCLUSION

The study showed that WA underwent obvious hydrolysis in rat and human plasma, which implied a strong first-pass effect. Serum albumin rather than common esterases primarily contributed to the hydrolytic metabolism of WA in plasma.

Hydrolase Activity of the Genetic Variants of Human Alpha-1-Acid Glycoprotein

In human plasma, the main agent of hydrolysis of the ester-type prodrug of levodopa, designated ONO-2160, is alpha-1-acid glycoprotein (AGP), which is a mixture of the F1*S and A variants at molar ratios of 3:1 to 2:1. In this study, the mechanism of AGP esterase-like activity was investigated by evaluating the contribution of the F1*S and A variants to ONO-2160 hydrolysis and identifying the AGP hydrolase active site. We found that although both variants hydrolyzed ONO-2160, their hydrolase activities were different. The intrinsic plasma clearance of the F1*S variant (0.441 mL/h/mg protein) was approximately 30 times higher than that of the A variant (0.0148 mL/h/mg protein), indicating that the F1*S variant contributed the most to AGP esterase-like activity. To identify the hydrolase active site of AGP, we performed inhibition studies of ONO-2160 hydrolysis using 12 AGP-binding drugs with various ligand-binding constants and binding selectivities to the two AGP variants. Inhibition of activity was positively correlated with the constant of ligand binding to the F1*S variant. In addition, compounds with high affinity to the F1*S variant inhibited ONO-2160 hydrolysis the most. Together, our data indicate that ONO-2160 is predominantly hydrolyzed by the F1*S variant at its ligand-binding site.

Development of Prodrugs for Treatment of Parkinson's Disease: New Inorganic Scaffolds for Blood-Brain Barrier Permeation

The treatment of Parkinson's disease (PD) has not been consistently modified for more than 60 years. L-DOPA, the blood-brain barrier permeable precursor prodrug of dopamine, is to date the only effective therapy on the market. However, it is well known that prolonged treatment with L-DOPA leads to several side effects, which may affect the patient's life expectancy (i.e., the wearing-off phenomenon, on-off fluctuations, and dyskinesia). For this reason, modifications, and supplements to L-DOPA treatment have been and are being studied, which, however, have not yet resulted in a valid alternative to the cornerstone drug. This review aims to summarize the main formulations currently in use for PD treatment, explaining advantages and disadvantages for each class. The attention will be focused on the promising prodrug concept, aimed at finding a suitable L-DOPA substitute with improved pharmacokinetic behavior. In this respect, new potential candidates which show interesting properties for the intended scope, the so-called dicarba-closo-dodecaboranes(12) (carboranes), will be discussed. Carboranes are inorganic molecular icosahedral boron-carbon clusters with 12 vertices and 20 deltahedral faces. They have been extensively studied for applications in medicine as potential pharmacophores, reagents in boron neutron capture therapy (BNCT) and radiotherapy. Here, we discuss them as inorganic scaffolds for dopamine delivery at the central nervous system (CNS) level.

Serum Albumin: A Multifaced Enzyme

Human serum albumin (HSA) is the most abundant protein in plasma, contributing actively to oncotic pressure maintenance and fluid distribution between body compartments. HSA acts as the main carrier of fatty acids, recognizes metal ions, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays esterase, enolase, glucuronidase, and peroxidase (pseudo)-enzymatic activities. HSA-based catalysis is physiologically relevant, affecting the metabolism of endogenous and exogenous compounds including proteins, lipids, cholesterol, reactive oxygen species (ROS), and drugs. Catalytic properties of HSA are modulated by allosteric effectors, competitive inhibitors, chemical modifications, pathological conditions, and aging. HSA displays anti-oxidant properties and is critical for plasma detoxification from toxic agents and for pro-drugs activation. The enzymatic properties of HSA can be also exploited by chemical industries as a scaffold to produce libraries of catalysts with improved proficiency and stereoselectivity for water decontamination from poisonous agents and environmental contaminants, in the so called “green chemistry” field. Here, an overview of the intrinsic and metal dependent (pseudo-)enzymatic properties of HSA is reported to highlight the roles played by this multifaced protein.

Into the Labyrinth of the Lipocalin α1-Acid Glycoprotein

α₁-acid glycoprotein (AGP), also known as Orosomucoid (ORM), belongs to the Lipocalin protein family and it is well-known for being a positive acute-phase protein. AGP is mostly found in plasma, with the liver as main contributor, but it is also expressed in other tissues such as the brain or the adipose tissue. Despite the vast literature on AGP, the physiological functions of the protein remain to be elucidated. A large number of activities mostly related to protection and immune system modulation have been described. Recently created AGP-knockout models have suggested novel physiological roles of AGP, including regulation of metabolism. AGP has an outstanding ability to efficiently bind endogenous and exogenous small molecules that together with the complex and variable glycosylation patterns, determine AGP functions. This review summarizes and discusses the recent findings on AGP structure (including glycans), ligand-binding ability, regulation, and physiological functions of AGP. Moreover, this review explores possible molecular and functional connections between AGP and other members of the Lipocalin protein family.

Species Difference in Hydrolysis of an Ester-type Prodrug of Levodopa in Human and Animal Plasma: Different Contributions of Alpha-1 Acid Glycoprotein

Previously, we found that ONO-2160, an ester-type prodrug of levodopa (3-hydroxy-l-tyrosine), was mainly hydrolyzed in human plasma by α₁-acid glycoprotein (AGP) with a partial contribution of albumin. In this study, we investigated whether ONO-2160 was hydrolyzed in the plasma of preclinical species (dog, rabbit, rat, and mouse) and humans and whether AGP and albumin are involved in its hydrolysis. ONO-2160 was hydrolyzed to some extent in the plasma of all tested species with the order of magnitude mouse > human > rabbit > rat > dog. Except for dogs, ONO-2160 was partially hydrolyzed by animal AGP and albumin. This indicated that, similar to albumin, AGP possesses esterase-like activity in mice, rats, and rabbits, as well as humans. A comparison of the values of intrinsic clearance per milliliter of plasma demonstrated that AGP was the major contributor to the hydrolysis of ONO-2160 in rabbit plasma, whereas albumin was primarily responsible for the hydrolysis of ONO-2160 in mouse plasma. This was confirmed by experiments using AGP-knockout mouse plasma. This study reports the first evidence for the existence of species differences in the hydrolysis of ONO-2160 in plasma related to the different contributions of AGP and albumin.