A study investigating the blood-brain barrier penetration, distribution and elimination of retro-inverso peptides as candidate treatments for Alzheimer’s disease

Alzheimer’s disease (AD) is an age-related neurodegenerative disease that mainly affects elderly individuals. The disease is characterised clinically by cognitive decline and neuropsychiatric symptoms, while its pathological features are the accumulation of senile plaques and neurofibrillary tangles in the brain. Aβ pathology is a central component of the disease. Early aggregated forms of Aβ known as “soluble oligomers” are believed to be a potent neurotoxic agent responsible for nerve cell damage and loss of synapses, so contributing to memory and learning deficits. Therefore, inhibiting the formation of these early aggregates could provide a novel approach to the treatment of AD. A retro-inverso peptide, RI-OR2-TAT, has been developed previously in Lancaster, and this demonstrated good inhibition of early-stage Aβ aggregation, but only at high concentrations of inhibitor relative to Aβ. An improved peptide system was subsequently developed involving attachment of RI-OR2-TAT onto the surface of a lipid nanoparticle scaffold (nanoliposome). The resulting “Peptide inhibitor nanoparticles” (PINPs) demonstrated much more potent inhibition of aggregation, and could penetrate through an artificial BBB. In this project, RI-OR2-TAT, PINPs and a novel retro-inverso peptide, RI-OR2-NAG, were examined for their ability to cross the intact and functional blood-brain barrier (BBB) of WT mice. Their accumulation in the brain, distribution in peripheral tissues, and possible modes of excretion, were examined in WT mice, 1 h following peripheral administration. In fluorescent imaging studies, fluorescein-labelled versions of RI-OR2-TAT, RI-OR2-NAG and PINPs all demonstrated effective penetration across an integral BBB and accumulation in the cortex, hippocampus, and dentate gyrus of the brain, where they were mainly co-localised with glial cells. Luminex technology was used to study the distribution of the first two peptides in the brain, heart, lungs, stomach, liver, spleen, kidneys and small intestine. The results of this study confirmed that these peptides accumulate in small amounts in the brain, but they are mainly found in the small intestine, suggesting a possible elimination route through bile secretions into the duodenum, followed by excretion through the faeces or reabsorption back into the general circulation, for possible excretion through the urine. Finally, as part of the project, the anti-aggregatory properties of the RI-OR2-NAG peptide were investigated, and this proved to be a more effective inhibitor of Aβ aggregation than RI-OR2-TAT. In conclusion, the use of liposomes as an effective carrier nanosystem for targeted delivery could be extended to the RI-OR2-NAG peptide. The results of this PhD thesis project are encouraging and it is hoped that a version of these peptides can progress into clinical trials