Singlet-oxygen liposomes

Fukuzawa, K. et ah. Rate constants for quenching singlet oxygen and activities for inhibiting lipid peroxidation of carotenoids and alpha-tocopherol in liposomes. Lipids, 33, 751, 1998. 

Figure 14.4 shows that 3-CRYP (like all carotenoids in homogeneous solution and all except ZEA in liposomes) exhibits a linear plot with the quenching of 02 increasing as the concentration of the carotenoid increases. While ZEA shows a bell-shaped plot and zero singlet oxygen quenching at concentrations >70 d,M (see Figure 14.5). Such behavior of ZEA is symptomatic of its unique...

FIGURE 14.5 Rate of decay of 02 against ZEA concentration in air-saturated solutions of DPPC unilamellar liposomes using RB as singlet oxygen sensitizer (a similar, but less marked, effect is observed with PBA as sensitizer). 

Shih MK and Hu ML. 1999. Relative roles of metal ions and singlet oxygen in UVA-induced liposomal lipid... 

A few current examples are shown below and include unsymmetrically substituted derivatives 14227, the cationic derivative 15228, the neutral derivative 16226 and the classic anionic derivative 17229 230, (tetrabenzoporphyrinato)zinc(II) , an intermediate structure between that of porphyrins and phthalocyanines, and have also been investigated in this respect. However, its quantum yield for singlet oxygen generation (0.023) in liposomes is much lower than that of Photofrin (0.19)177. 

The distribution of the singlet oxygen between the external solvent and the liposomes can be further assessed by measuring the quenching rates of substrates whose reaction rate is very little solvent dependent. Alkylfuranes fulfill this requirement. Data obtained for several furanes are included in Table 4. 

The nonsulfonated, nonwater soluble zinc phthalocyanine (Figure 13, M = 2H) encapsulated in unilamellar liposomes has been extensively studied by Reddi et al. [46]. This very hydrophobic Pc requires administration by incorporation into detergents or liposomes. In ethanol, ZnPc produces singlet oxygen with 102 = 0.4-0.5 (, 0.65). However, in the liposomes the singlet oxygen yield is reported to approach unity. However, since the furan used in these experiments may be also consumed by other oxidative processes, the rate of 102 production may be over estimated. While ZnPc appears to have favorable photophysical and pharmacokinetic properties, the difficulty in incorporating sufficient material even in liposomes may present an insurmountable practical problem for clinical use. 

Suwa, K., Kimura, T., and Schaap, A.P. (1978) Reaction of singlet oxygen with cholesterol in liposomal membranes. Effect of membrane fluidity on the photo-oxidation of cholesterol, Photochem. Photobiol., 28 469 473. 

Photosensitive liposomes may contain a photosensitizer that can induce peroxidation of the membrane lipids by type I and/or type II photodynamic sensitization (Srinath and Jain, 1994). In the type I process, the electronically excited sensitizer reacts directly with the lipids. The result is the formation of radicals that can react further with molecular oxygen or other components present at the reaction site. In the type II process, the electronically excited sensitizer reacts directly with molecular oxygen via energy transfer to produce singlet oxygen. 

---