PH sensitive liposomes

Liposome pH-sensitivity is assessed by comparing the release kinetics of encapsulated HPTS/DPX between liposomes dispersed in neutral HEPES and acidic MES buffers (pH between 5.0 and 6.5). The method relies on monitoring the dequenching of HPTS fluorescence as it is released (X. . 413 nm and A., 512 nm) (see Note 9). 

An alternative approach is the use of pH-sensitive fluorophores (Lichtenberg and Barenholz, lOSS). These probes are located at the lipid-water interface and their fluorescence behavior reflects the local surface pH, which is a function of the surface potential at the interface. This indirect approach allows the use of vesicles independent of their particle size. Recently, techniques to measure the C potential of Liposome dispersions on the basis of dynamic light scattering became commercially available (Muller et al., 1986). 

Figure 2. Liposomes with a specific character a), temperature-sensitive liposome b). target-sensitive liposome c). pH-sensitive liposome. Closed triangles and rectangles in lipid bilayer indicate amphiphiles which change the liposome s hydration with pH changes.

The pH-sensitive liposomes consist of mixtures of several saturated egg phosphatidylcholines and several A -acylamino acids. The release of drug is suggested to be a function of acid-base equilibrium effected by the interaction between ionizable amino acids and N-acy-lamino acid headgroups of the liposomes. There appears to be a close relation between Tc and pH effect [72],... 

Figure 9 pH-sensitive paramagnetic liposomes at physiological pH and above, Gd(DTPA-BMA) is entrapped in the liposome and the relaxivity is low due to the low water exchange rate (/cex) through the liposome membrane. At lower pH, the liposome is disrupted and the relaxivity increases.172... 

Sakaguchi N, Kojima C, Harada A, Kono K (2008) Preparation of pH-sensitive poly (glycidol) derivatives with varying hydrophobicities their ability to sensitize stable liposomes to pH. Bioconjug Chem 19 1040-1048... 

Yuba E, Kojima C, Sakaguchi N, Harada A, Koiwai K, Kono K (2008) Gene delivery to dendritic cells mediated by complexes of lipoplexes and pH-sensitive fusogenic polymer-modified liposomes. J Control Release 130 77-83... 

Budker V, Gurevich V, Hagstrom JE, Bortzov F, Wolff JA (1996) pH-sensitive, cationic liposomes a new synthetic virus-like vector. Nat Biotechnol 14 760-764... 

Use of the pH-sensitive membrane-impermeable flurophore pyranine based on the ratiometric method, which determines directly level of dissociation of pyranine from the ratio between the charged (nnprotonated) pyranine and total pyranine in the intraliposome aqneons phase Addition of impermeable DPX, which acts as a quencher to pyranine fluorescence, into the liposome external medium ensures lack of contribution of extraliposome medium pyranine fluorescence (18,22). This method is considered invasive as the pyranine has to be added in the hydration medium prior to liposome preparation and cannot be used for pH determination of intraliposome aqueous phase... 

Connor J, Yatvin MB, Huang L. pH-sensitive liposomes acid-induced liposome fusion. Proc Natl Acad Sci USA 1984 81(6) 1715-1718. 

Meyer O, et al. Cationic liposomes coated with polyethylene glycol as carriers for oligonucleotides. J Biol Chem 1998 273 15621 Shi G, et al. Efficient intracellular drug and gene delivery using folate receptor-targeted pH-sensitive liposomes composed of cationic/anionic lipid combinations. J Control Release 2002 80 309. 

Guo X, Szoka E. Steric stabilization of Fusogenic Liposomes by a low-pH sensitive PEG-diortho ester-lipid conjugate. Bioconj Chem 2001 12 291. 

Simoes S, Slepushkin V, Duzgunes N, et al. On the mechanisms of internalization and intracellular delivery mediated by pH-sensitive liposomes. Biochim Biophys Acta 2001 1515 23-37. 

Kakudo T, Chaki S, Futaki S, et al. Transferrin-modified liposomes equipped with a pH-sensitive fusogenic peptide an artificial viral-like delivery system. Biochemistry 2004 43(19) 5618-5628. 

Cholera Toxin Subunit B. a marker for caveolae-dependent endocytosis. partly co-localizes with liposomal FITC>dextran indicating that pH-sensitive liposomes are taken up-at least to a certain extend— via caveotae pathway... 

Figure 2 Principle of spectral bio-imaging HUVEC incubated with pH-sensitive DOPE CHEMS liposomes (loaded with fluorescein isothiocyanate-dextran), cholera toxin subunit B (Alexa Fluor 594 labeled), and diamidino-phenylindole-dihydrochloride.

Huth US, Schubert R, Peschka-Siiss R. Investigating the uptake and intracellular fate of pH-sensitive liposomes by flow cytometry and spectral bio-imaging. J Control Release 2006 110(3) 490-504. 

Polyelectrolytes have recently found application in the development of pH sensitive liposomal controlled release systems. This application arises from the fact that polyelectrolytes may be used both to stabilize liposomes, and to disrupt liposomes in a pH dependent manner. Although the use of liposomes in oral pharmaceutical compositions has been discussed [424], liposomes generally suffer from poor stability and are therefore prone to leakage of the entrapped active agents. To overcome this problem, several authors have stabilized the liposomes using polyelectrolytes. For example, Tirrell and coworkers have employed ionene [425], and polyethylene imine) [426] to stabilize liposomes. Similarly, Sato and coworkers have studied maleic acid copolymers [427], and Sumamoto and coworkers have studied liposomes [428] coated with polysaccharides. In related work, Kondo and coworkers have emphasized the use of carboxymethyl chitin to produce artificial red blood cells [429-435]. 

Legendre, J.Y. and Szoka, F.C. (1992) Delivery of plasmid DNA into mammalian cell lines using pH-sensitive liposomes-comparison with cationic liposomes. Pharm. Res., 9, 1235-1242. 

Venugopalan P, Jain S, Sankar S, Singh P, Rawat A, Vyas SP (2002) pH-sensitive liposomes mechanism of triggered release to drug and gene delivery prospects. Pharmazie 57( 10) 659-671... 

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