Labile lipids

The selectivity and mildness of the Pd(0)-catalyzed deprotection of allyl (All) esters and the allyloxycarbonyl (Aloe) urethanes117 181 reaction also allowed for the successful and efficient application of this blocking group technology in the synthesis of acid- and base-labile lipidated pepti-... 

Fig. 2. Degradation kinetic of lipoplexes obtained with stable lipid 3 and pH labile lipids 2 and 4 at pH 5, and charge ratios 1.7 and 6. DNA release induced by degradation is measured by the % of fluorescence of intercalated ethidium bromide at 590 nm " 260 nm (100% refers to naked DNA)...

The results of transfection with pH labile lipid 4 and stable lipid 3 in the absence of serum, with or without bafilomycin A, are shown in Fig. 3. Formulations containing lipids 1 and 2 gave results similar to those obtained with 3 and 4 and for the sake of clarity are not represented in the figure. 

Rosenthal, A.F., Vargas, L.A., Isaacson, Y.A., and Bittman, R., A simple synthesis of phosphonate-containing lipids. Introduction of the phosphonic acid moiety into hydrolytically labile compounds, Tetrahedron Lett., 977,1975. 

Molsidomine and pirsidomine, are both stable as solids at room temperature in the absence oflight [137]. However the ring opened metabolites SIN-1A and C87-3786 are both photo labile and sensitive to an oxidising environment resulting ultimately in the release of superoxide and NO in stoichiometric quantities [138]. Generation of these two species is an obvious problem due to the resulting formation of peroxynitrite and the generation of OH, which may initiate lipid peroxidation [139-141] (see Eq. (19)). Such concerns over the formation of peroxynitrite from SIN-1A or C87-3786 are warranted since their cytotoxic effects show close consistency with cellular studies doped with neat peroxynitrite [142, 143]. 

The synthesis of lipidated peptides is implicated by the base-lability of the thioester and the acid sensitivity of the prenyl group double bonds. Thus, new protecting groups are required which can be removed under extremely mild, preferably neutral, conditions. 

Acid- and base-sensitive lipidated peptides can be selectively deprotected by enzymatic hydrolysis of choline esters.[13al Choline esters of simple peptides, but also of sensitive peptide conjugates like phos-phorylated and glycosylated peptides,1141 nucleopep-tides1151 and lipidated peptides,113,1631 can be cleaved with acetyl choline esterase (AChE) and butyryl choline esterase (BChE) under virtually neutral conditions with complete chemoselectivity. Acid-labile farnesyl groups and base-sensitive thioesters are not attacked. 

Acid-labile linkages of carbohydrates are commonly those of furano-sidic sugar residues or of deoxy monosaccharides.50,63 Consequently, neuraminic acid and fucose residues, which occur as terminal monosaccharides of protein- and lipid-linked glycans, are removed by hydrolysis with a dilute acid. The linkages formed by these two sugar residues can conveniently be differentiated by subjecting a portion of... 

It must be emphasized that the problem of unsubstituted hydroxyl groups is usually studied employing free lipid A prepared by treatment of LPS with acid. The demonstration of a free hydroxyl group at C-4 in monophosphated lipid A therefore does not exclude the possibility that, in LPS, a substituent, present in acid-labile linkage, could be bound to 0-4 of lipid A. This possibility has thus far been excluded for E. coli Re LPS which was analyzed by 1H- and 13C-n.m.r. and shown to contain an unsubstituted hydroxyl group at C-4 of GlcN(I) (34,110). 

The dehydration-rehydration procedure is characterized by its mildness and is thus compatible with most labile materials. The amounts of lipids and vaccine materials described below are typical for the preparations made for animal work described here but could be scaled up or down depending on the number of animals and the amount of antigens used. 

Quantitative entrapment of vaccines into small (up to about 200 nm diameter) liposomes in the absence of microfluidization (which can damage DNA and other labile materials when extensive) can be carried out by a novel one-step method (7) as follows SUVs (e.g., cationic) prepared as in section Preparation of Small Unilamellar Vesicles are mixed with sucrose to give a range of sucrose-to-lipid weight/weight ratio of 1.0 to 5.0 and the appropriate amount of plasmid DNA (e.g., 10-500 pg) and/or protein (e.g., up to 1 mg). The mixture is then rapidly frozen and subjected to dehydration by freeze-drying, followed by rehydration as in section Preparation of Vaccine-Containing Dehydration-Rehydration Vesicles. ... 

As expected, acid-labile PEG did not increase the circulation time of the lipoplexes as compared to PEGylated lipoplexes but allowed for the same level to be reached, indicating the stability of the acid-sensitive PEG-lipid moiety in the blood. 

Shin J, Shum P, Thompson D. Acid-triggered release via dePEGylation of DOPE liposomes containing acid-labile vinyl ether PEG-lipids. J Contr Rel 2003 91 187. 

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