Efficient Synthesis Methods Of The Tetrasaccharide Building Blocks Of Leishmania Lipophosphoglycan(LPG)as A Vaccine Candidate For Leishmaniasis

Leishmania is a genus of obligate intracellular protozoan parasites that cause a group of diseases called leishmaniasis.The disease is considered as one of the most serious health problems with fatal complications if left without medical intervention.The geographic distribution of leishmaniasis is wide;millions of people are already infected with thousands of new cases annually,particularly in poor communities and developing countries.The disease is transmitted by phlebotomine sand flies and characterized by a variety of clinical manifestations,ranging from mild self-healing cutaneous form,to moderate disfiguring mucocutaneous lesion,and to the most severe visceral form of leishmaniasis.The level of the parasite that existed in the human host during the infection process is low,which makes the early diagnosis of the disease very difficult.The disease is a persisting health problem;yet,no preventive agents have gained positive eligibility for human use against leishmaniasis.In addition,the available chemotherapeutic options are inadequate due to their toxic effect and the emergence of resistance among Leishmania species.Therefore,the development of new and efficient prophylactic measures,such as immunotherapeutic agents and vaccines,should be applied to prevent and control the disease.Even though leishmaniasis is an old disease,unfortunately,there is no available vaccine to protect humans against its fatal complications.The ability of previously infected patients with leishmaniasis to produce resistance against the subsequent clinical infections made the vaccination approach possible.Leishmania lipophosphoglycan(LPG)has been confirmed to be an attractive vaccine target due to its unique structure,dense distribution,and accessibility.Therefore,this component of the Leishmania cell wall has been investigated extensively.Leishmania cell wall components are sophisticated and play a significant role in both host survival and human infectivity.It can withstand the hydrolytic effects of many enzymes that meant to degrade it.Moreover,it can facilitate the immune escape of the parasite against macrophages and other immune defense mechanisms.Therefore,leishmaniasis is considered dangerous with fatal complications if left untreated.The most crucial portion of the Leishmania parasite cell wall is the dense glycolipid complex on the cell surface known as LPG.The parasite cell surface LPG consists of four domains:glycophosphatidylinositol anchor(GPI),the phosphosaccharide glycan core,phosphoglycan repeating units,and the tetrasaccharide cap.All these domains proved to play a crucial role in parasite-immune escape and infectivity,which render the chemical synthesis of these domains of Leishmania cell wall of great interest.In this project,our concentration will be mainly on the unique domains of LPG that existed on the cell wall of the Leishmania parasite during its dimorphic life cycle within the carrier and host.We aim to develop new and efficient chemical synthesis procedures of the cryptic tetrasaccharide cap[Galp(1?4)-?-[Manp-(1?2)-?-Manp-(1?2)-?]-Manp]located on the Leishmania LPG as a potent vaccine candidate for leishmaniasis.Chemical synthesis in the area of membrane-bound glycoconjugates is increasingly developed and gaining more attention.Many researchers have devoted their lives to this challenging task,and various studies and different methods have been used in this regard;however,specific work in the area of Leishmania LPG cell wall or parts of its fragments is minimal because the chemical synthesis process is very complicated.Herein,we report our work involving new and efficient strategies for the chemical synthesis of the tetrasaccharide moiety building blocks of Leishmania LPG,which is considered as the most potent part and crucial in the parasite survival and infectivity.This part is unique among all other eukaryotic cell wall components,rendering it as a potential target for further studies and investigation.Although the tetrasaccharide cap of the Leishmania cell wall has been synthesized in many different ways,still,there are some obstacles to be overcome like yield,purity,and stereoselectivity.Given the above problems,this paper takes the tetrasaccharide cap of the Leishmania LPG cell wall as a research hotspot of chemical glycobiology.This research aims to prove the ability to efficiently synthesize the building blocks found in the tetrasaccharide cap of Leishmania LPG by applying new and efficient approaches.The chemical synthetic aggregation strategy was started by the preparation of the desired intermediates.The glycan chain extension was carried out by protecting group operation and chemical glycosidation,which allow the gram-level synthesis of the acceptors and donors' building blocks.This method lays a foundation for the further synthesis of high-purity,structurally uniform,and structurally defined glycoconjugates.The structural confirmation of such glycosylation,structure-activity relationships,and functional studies provide essential prerequisites for sugar-based vaccines and drugs.The major achievement and innovation of this project are as follows:1.Cheap,green,and reusable reagents were designed to be used during the total chemical synthesis,which would facilitate the mass preparation and industrial production of the products.2.The high regioselectivity of the "cyanide effect" was flexibly used to protect the 2,3-hydroxyl groups of the building blocks.This new approach resulted in efficient and large-scale preparation of the essential intermediates required for the preparation and synthesis of the acceptors and donors building blocks of the tetrasaccharide cap through the reasonable protection strategy.3.The gram-level synthesis of the acceptors and donors building blocks was achieved by different protecting group operations and glycosidation strategies based on the designed intermediates.The synthetic acceptors and donors building blocks of the tetrasaccharide cap will be furtherly proceeded to complete the entire cap structure,which in turn will be extensively subjected to further biological experiments after linking to a suitable protein carrier to work as a vaccine or semi-vaccine candidate against leishmaniasis and study the immune response toward these potential vaccines.