https://doi.org/10.15255/KUI.2012.014
Published: Kem. Ind. 62 (1-2) (2013) 19–31
Paper reference number: KUI-14/2012
Paper type: Review
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Preparation and Immunomodulatory Properties of Modified Peptidoglycan Fragments

R. Ribić and S. Tomić Pisarović

Abstract

Immunostimulators, known also as adjuvants, are added to vaccines to accelerate, extend or amplify the specific immune reaction to a specific antigen. One well known class of immunomodulating compounds is based on muramylpeptides which are fragments of peptidoglycans, natural polymers that build up the cell wall of bacteria. Muramyldipeptide, N-acetylmuramyl- L-alanyl-D-isoglutamine (MDP, Fig. 1) is the smallest structural unit of the peptidoglycan monomer (PGM, Fig. 2) which shows immunostimulating activity. PGM isolated from Brevibacterium divaricatum, acts in itself as an effective adjuvant, and several derivatives were prepared to study the possible influence of different substituents on the immunomodulatory activity. Thus, lipophilic derivatives tert-butyloxycarbonyl-L-tyrosyl-PGM and (adamant- 1-yl)acetyl-PGM (Fig. 3) were prepared and their activities studied. They were also shown to be good substrates for N-acetylmuramyl-L-alanine amidase from human serum (Scheme 1) which specifically hydrolyzes the lactylamide bond. MDP which is an integral part of PGM and proven to be an effective adjuvant was further synthetically modified and obtained derivatives tested as possible immunomodulators. Romutide (MDP-Lys(L18)), approved by Food and Drug Administration (FDA), and mifamurtide (L-MTP-PE), approved by European Medicines Agency (EMA), highlight among many other MDP derivatives (Fig. 4). Since N-acetylglucosamine in the structure of MDP is not essential for the immunostimulating effect, desmuramyldipeptides (Fig. 5) with different acyl groups at N-terminus of L-Ala-D-isoGln dipeptide were prepared. In adamantyl desmuramyldipeptides such as adamantylamide dipeptide (Fig 6), adamantyl tripeptides (Fig. 7) and desmuramylpeptides with (adamant-1-yl)carboxyamido group (Fig. 8), lipophilic adamantane moiety is bound to the dipeptide part. Binding of some specific sugars to immune active substances may help their targeted delivery. An example is mannose which enables mannosylated compounds to interact with lectins specific for mannose, such as mannose receptors (MR) expressed at macrophages and dendritic cells. Therefore, it is possible to increase the activity of the parent immunologically active compound by mannosylation. One of the ways is the preparation of mannosylated liposomes by using mannosylated lipids (Fig. 9). Mannosylation can also influence the direction of the immune reaction. This is shown by the examples of mannosylated PGM and adamantyl tripeptides. Mannosylated PGM derivatives (Scheme 2) are the first PGM derivatives comprising carbohydrate substituents. Mannosylation changed the immunostimulating activity of PGM, but did not affect the susceptibility of the lactylamide bond to hydrolysis with N-acetylmuramyl-L-alanine amidase (Scheme 3). Adamantyl tripeptides, structurally related to PGM, can be mannosylated using the same method (Scheme 4). The greatest potential showed the mannosylated (adamant-1-yl)tripeptide (Fig. 10) whose immunostimulating activity is comparable to that of PGM. Numerous MDP derivatives have been synthesized with the intention of improving the pharmacological properties and reducing the side effects of the parent molecule. Furthermore, the study of their structure-activity relationship contributes to the clarification of the mechanism of action of MDP. All presented examples indicate that relatively small changes in the primary structure of peptidoglycan fragments affect their immune reaction. Mannosylation is particularly important modification of muramylpeptide adjuvants since it may allow the targeted delivery of these active substances.

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Keywords

immunomodulation, muramyldipeptide, peptidoglycan monomer, adamantyl desmuramyldipeptides, mannose