Dissertation - Open Access
Doctor of Philosophy (PhD)
Treatment of central nervous system (CNS) diseases, such as Alzheimer’s disease, Parkinson’s diseases, and brain tumors, is still a challenge due to the existence of the blood-brain barrier (BBB). The BBB as a membrane barrier that separates systemic blood circulation from the CNS is structurally formed by endothelial cells, astrocyte, pericyte and neurons and is one of the crucial protection mechanisms for the CNS. The BBB protects the brain by preventing foreign compounds (non-endogenous compounds) from entering the brain and helps maintain brain homeostasis. In addition, the presence of drug efflux pumps (i.e., P-glycoproteins, multidrug-resistance proteins) in the BBB helps pump some of foreign compounds out of the brain. Because of the BBB, a drug molecule usually exists in the CNS in low concentration. Difficulty in achieving its therapeutic concentration caused by the BBB is often the major cause for a drug to fail to treat a CNS disease. To cross the BBB and deliver a drug to the brain, various strategies have been attempted. These strategies can be mainly divided into two categories: invasive approaches (i.e., neurosurgical or disruption of BBB) and noninvasive approaches. A nanocarrier drug-delivery system with a brain targeting agent is an effective noninvasive approach to cross the BBB and deliver a drug to the brain. Although the BBB prevents foreign compounds from entering the CNS, endogenous compounds such as glucose, amino acids, neurotransmitters, and glutathione (GSH) are able to enter the CNS through their corresponding transporters or receptors present at the BBB. The ligand or ligand analogue of these transporters or receptors have been employed to develop brain targeting agents to help deliver drug to the brain. GSH is a tripeptide serving as a major molecule in the body to protect cells from oxidative stress and reactive toxic species. GSH is a hydrophilic and cell membrane impermeable molecule. As an endogenous molecule, GSH crosses the BBB and reaches the brain through a Na+-dependent GSH transporter. Thanks to the overexpression of GSH transporters in the BBB, successes have been made by using GSH to develop brain targeting agents or bran targeting drug delivery systems to facilitate drug entry to the brain. In our previous work, we have successfully designed and synthesized 2-(2- cholesteroxyethoxyl) ethyl 3’-S-glutathionylpropionate (COXP) by connecting the hydrophilic GSH molecule with a hydrophobic cholesterol through a two-ethylene glycol unit linker. COXP is an amphiphilic molecule and can self-assemble to form micelles with a CMC value of 3.9 μM. The low μM of CMC suggests that COXP micelle is stable enough when diluted in the blood circulation after administration and has a potential for clinical applications as a drug delivery system. Micelle is one of the major nanocarrier drug delivery systems. The COXP micelle is featured with GSH molecules on the micelle surface to serve as a ligand for the GSH transporter recognition for brain targeting. Using 1,1’-dioctadecyl-3,3,3’,3’-tetramethylindo-tricarbocyanine iodide (DiR), a near IR fluorescent and hydrophobic dye, as a fluorescence tracker and the whole-body fluorescent imaging technique with mice, our previous data showed that COXP micelles increased the delivery of DiR to the brain by ~20 folds when compared with free DiR. The work not only provides a proof of concept that COXP micelle is an effective brain drug delivery system but also validates our strategy of using GSH as a brain targeting ligand for designing a novel brain targeting molecule. The aim of this dissertation was to identify more effective brain targeting agents and brain-targeting drug delivery systems by conducting structural modification of COXP. The structural modification mainly involved two parts of the COXP structure: i). replacement of the linker between GSH and cholesterol; ii). replacement of the hydrophobic cholesterol structure with a hydrophobic fatty alcohol. All designed compounds are amphiphilic molecules and expected to form micelles. Chapter 2 describes the structural modification through a replacement of the linker between GSH and cholesterol. Three compounds were designed with three different linkers: i) N5-(1-((carboxymethyl)amino)-3-((3-((10,13-dimethyl -17-(6- methylheptan-2-yl)-2,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H-cyclopenta[a]phenanthren-3-yl)oxy)-3-oxopropyl) thiol)-1-oxopropan-2-yl)glutamine (CLG); ii) 19-amino-14-((carboxymethyl)carbamoyl)-1-((10,13-dimethyl-17- (6-methylheptan-2-yl)-2,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H cyclopenta[a]phenanthren-3-yl) oxy)-1,9,16-trioxo-2,5,8-trioxa-12-thia-15-azaicosan-20-oic acid (COLG); iii) 16-amino-11- ((carboxymethyl)carbamoyl)-1-((10,13-dimethyl-17- (6-methylheptan-2-yl)- 2,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H-cyclopenta[a]phenanthren-3-yl) oxy)- 1,6,13-trioxo-9-thia-2,5,12-triazaheptadecan-17-oic acid (CNLG). All three were able to form a micelle with a CMC value of 1.34 μM, 5.70 μM, and 6.01 μM for CLG, COLG, and CNLG, respectively. A preliminary ex-vivo work with mice using DiR as a fluorescence tracking agent showed that the micelles from these three compounds were all effective braintargeting micelles with CNLG micelles being the most effective brain targeting micelles. Results from the whole-body imaging showed that CNLG micelles delivered DiR to the brain 40 folds higher when compared with free DiR, significantly higher than COXP micelles. Chapter 3 describes the structural modification through a replacement of the cholesterol structure with a fatty alcohol. Two compounds were designed: i) N5-(1- ((carboxymethyl)amino)-3-((3-(octadecyloxy)-3-oxopropyl) thio)-1-oxopropan-2-yl)glutamine (OG); ii) 2-amino-7-((carboxymethyl)carbamoyl)-5,12-dioxo-13, 16,19- trioxa-9-thia-6-azaheptatriacontanoic acid (OLG). Both OG and OLG were able to form micelles with a CMC value of 12.48 μM for OG and 2.0 μM for OLG, respectively. Both micelles were brain targeting micelles. The OG micelles increased the delivery of DiR to the brain by 63 folds at 2 h compared with free DiR. The OLG micelles were even better and increased the delivery of DiR to the brain by 98.3 folds at 30 min compared with free DiR. Both OG micelles and OLG micelles delivered DiR to the brain substantially higher than COXP micelles. In conclusion, we have successfully developed five GSH derivatives as a brain targeting agents based on the structural modification of COXP. The results from this dissertation further validate the use of GSH as an effective ligand for the development of novel brain targeting agents. The results also confirm that a change of the linker and replacement of the cholesterol with a hydrophobic alcohol are valid structural modifications and can lead to more effective brain targeting agents and brain targeting micelles.
Number of Pages
South Dakota State University
Huang, Yue, "Design, Synthesis, and Evaluation of Glutathione Derivatives as Brain Targeting Agents" (2021). Electronic Theses and Dissertations. 5642.
Available for download on Friday, July 26, 2024