Table 1.

Natural lipid As and various analogs and derivatives of lipid As achieved by chemical synthesis

Compound	Bacterial origin	Group that realized the synthesis	Year	Reference	Key structural and synthetic features	Biological outcomes
Natural lipid As and lipid A derivatives bearing saturated lipids and both phosphate groups
1	E. coli	Boons	2007	43	Natural lipid A with six acyl chains; synthesis based on an orthogonally protected disaccharide	Lower potency to induce cytokine production than E. coli LPS, suggesting the role of Kdo residues in LPS
10	S. typhimurium	Boons	2007	43	Natural lipid A with seven acyl chains.	Significantly lower potency to induce cytokine production than 1
11	E. coli	Boons	2007	43	A lipid A derivative with shortened lipids	Higher potency to induce cytokine production than 1
12	S. typhimurium	Boons	2007	43	A lipid A derivative with shortened lipids	Higher potency to induce cytokine production than 10
13	S. typhimurium	Boons	2007	43	A lipid A derivative devoid of the anomeric phosphate group	Lower potency to induce cytokine production than 12
		1 and 10–13 were synthesized by a similar strategy; their difference in biological activities indicated the importance of the number and length of the fatty acid lipid chains in lipid A
26	Ru. gelatinosus	Fukase	2008	45	Natural lipid A with six acyl chains; synthesis by using lapidated monosaccharide blocks	Potent ability to antagonize LPS endotoxic activity.
27, 28	C. trachomatis	Kosma	2004	54	C. trachomatis lipid A with tetra- or pentaacyl lipid chains.
Natural lipid As and lipid A derivatives bearing unsaturated lipids
34a,b	R. sphaeroides	Christ	1994	37, 55, 56	A derivative of natural lipid A; use of Alloc and All groups to protect the hydroxyl and phosphate groups	Devoid of LPS agonistic properties; strong in vitro antagonistic activity; suppress TNF-α production induced by LPS
40 (E5531)	R. sphaeroides	Yamatsu	1995	57–59	A derivative of natural lipid A with 3,3′-O-acyl groups substituted for hydrolytically stable ether linkages and C-6′-hydroxyl group blocked as methyl ether	Strong antagonist of cytokine release of induced by LPSs; completely devoid of LPS agonistic activity; protecting BCG-primed mice from LPS-induced lethality and E. coli infection-caused death.
41 (E5564)	R. sphaeroides	Christ	2002	60–62	Second generation of the above lipid derivatives	Excellent antagonist activity; clinical use in the trade name of Eritoran for the treatment of severe sepsis
Unnatural lipid A derivatives bearing two phosphate groups
42–44	artificial	Savage	2003	40	Containing four lipid chains with incrementally shortened length	Different solubility in water; used to study the interaction of lipid A with polymyxin B
54–56	E. coli	Kusama	1990	69	Lipid A derivatives containing anomeric α/β-phosphonooxyethyl group instead of labile α-glycosylphosphate.	α-glycosides had comparable antitumor activity as natural lipid A; β-glycoside was less active
64, 65	E. coli	Kusama	1991	74	Derivatives of lipid A biosynthetic precursor; containing anomeric α-phosphonooxyethyl	The first generation of potent antitumor lipid A derivatives with enhanced stability and reduced toxicity.
66, 67	E. coli	Fukase	2001	75, 76	Lipid A derivatives with tritium-labeled α-phosphonooxyethyl	Useful for the study of the interactions between lipid A and their acceptors
Lipid A analogs carrying an anomeric carboxylic acid
74a–c, 75a–c	E. coli	Shiozaki	2000	79, 80	Lipid A derivatives with a carboxylic acid moiety in place of the glycosyl phosphate; 74a–c and 75a–c had six and four lipid chains; different 6′-O-substituents	74a–c was a LPS agonist but 75a–c was a LPS antagonist, indicating the importance of the number of lipid chains; the carboxyl group and the 6′-O-substituent did not have a significant impact
92a–c, 93a–c, 94a–c	artificial	Shiozaki	2001	81	Anomeric carboxylic acid; unnatural 3,3′-O-ether linkage; different 6′-C-substituents; 92 and 94 had four lipid chains; 93 had six	Improved stability; 92 and 94 were strong LPS antagonists; 93b was inactive; 93a,c were moderate LPS agonists; ether linkage and anomeric carboxylic acid did not have a major impact
104a–c	artificial	Shiozaki	2003	51	Anomeric carboxymethyl group and 3,3′-O-ether linkages.	Improved stability; LPS antagonists; ether linkage and anomeric carboxylic acid did not have a major impact
111a,b–116a,b	artificial	Fukase	2006	84	Monosaccharide analogs of lipid A with aspartic acid or phosphoserine in place of non-reducing end phosphorylated glucosamine unit	Most compounds except 115a,b showed antagonistic activity
Monophosphoryl lipid A and related derivatives
119–121	S. minnesota	Jiang	2007	50	S. minnesota R595 MPLA derivatives; different substituents at the 3-O-position	All were strong adjuvants; 121 was not more toxic than natural R595 MPLA 118 that devoid of 3-O-acyl chain
127a-f	S. minnesota	Johnson	1999	52	S. minnesota R595 MPLA derivatives with variant fatty acids	Fatty acid chain length was important for inducing proinflammatory cytokines
133–140	E. coli K4	Corsaro	2016	94	Semi-synthetic lipid A analogs derived from selective modification of the C-6′-OH, lipid pattern and phosphate	Some of them, especially 136, showed promising immunoadjuvant activity
146	N. meningitidis	Guo	2009	95	MPLA derivative with an amino group at the reducing end, readied for coupling with other carbohydrate antigens	Useful as a vaccine carrier for the development of self-adjuvant conjugate vaccines
147–150	N. meningitidis	Guo	2014	96	MPLA derivative with varied acyl chains with an amino group at the reducing end.	Used to discover new and optimized vaccine carriers and adjuvants.
156	E. coli	Guo	2010	46	MPLA derivative with an alkyne group at the reducing end for click reaction	Useful as a vaccine carrier for the development of self-adjuvant conjugate vaccines
157–160	P. gingivalis	Ogawa, Boons	2007, 2008	99, 100	Tetra-acylated MPLAs with branched lipid chains and devoid of 4′-O-phosphate	159 and 160 showed potent antagonist activity, which was determined by their acylation pattern.
Non-phosphorylated lipid As and derivatives
161–164	R. sin-1	Carlson, Vandenplas	2002	101, 102	Lack of both phosphate groups; aminogluconate in place of the reducing end glucosamine unit	Inhibited TNF-α release
165, 166	R. sin-1	Boons	2003	41	Reducing end anomeric carbon had different oxidation states	The gluconolactone unit in 166 was important for its antagonistic activity.
173, 174	R. sin-1	Boons	2004	42	Devoid of 3-O-acylation	Antagonists of LPS but less potent than 166, indicating the biological importance of 3-O-acyl chain
188, 189	R. sin-1	Boons	2007	44	189 contained the 27-hydroxyoctacosanoic group at its 2′-N-position	Lipid chain length, not the OH group on it, was critical for its antagonistic activity
LPS analogs and Kdo-lipid A conjugates
199 (ReLPS)	E. coli mutant	Kusumoto	2001	53, 110	First chemical synthesis of a short LPS with two Kdo units linked to lipid A; Kdo fluorides with bulky 4,5-O-protecting groups were used for α-glycosylation	199 showed slightly less potent cytokine-inducing activity than that of the natural ReLPS
212	H. pylori	Fukase.	2007	107	Lipid A-Kdo conjugate with fewer but longer acyl chains; devoid of 4-O′-phosphate; Kdo fluorides was used for α-glycosylation	TLR-4 antagonists; Kdo residue could enhance the inhibitory activity
221	H. pylori	Fukase, Fujimoto	2011	114	Lipid A-Kdo conjugate with an ethanolamine unit in its structure; Kdo N-phenyltrifluoroacetimidate and a microfluidic glycosylation protocol was developed and used for α-glycosylation	Inhibition of cytokine production; the presence of a Kdo unit in the structure reversed the bioactivity of its parent lipid A 220