Liposome partitioning

Liposomes, which are lipid bilayer vesicles prepared from mixtures of lipids, also provide a useful tool for studying passive permeability of molecules through lipid. For example, this system has been used to demonstrate the passive nature of the absorption mechanism of monocarboxylic acids [101]. 

Liposome partitioning of ionizable drugs can be determined by titration, and has been correlated with human absorption [102-104]. A new absorption potential parameter has been suggested, as calculated from liposome distribution data and the solubility-dose ratio, which shows an excellent sigmoidal relationship with human passive intestinal absorption (Eq. 2) [102, 103]. 

APsuv is the absorption potential measured from the distribution in small unilamellar vesicles (SUV) at pH 6.8, the solubility was measured at pH 6.8 in simulated intestinal fluid, V is the volume of intestinal fluid, and dose is a mean single oral dose. Liposome partitioning is only partly correlated with octanol/water distribution. 

A further partition system based on the use of liposomes, and marketed as Transil , has also been investigated [105, 106]. 

A further partihon system based on the use of liposomes, and commercialized under the name Transil [110, 111], has shown its utiUty as a UpophiUcity measure in PBPK modeling [112]. Fluorescent-labeled liposomes, called fluorosomes, are another means of measuring the rate of penetration of small molecules into membrane bilayers [113, 120]. Similarly, a colorimetric assay amenable to HTS for evaluating membrane interactions and penetrahon has been presented [116]. The platform comprises vesicles of phospholipids and the chromahc Upid-mimehc polydiacetylene. The polymer undergoes visible concentrahon-dependent red-blue transformahons induced through interactions of the vesicles with the studied molecules. 

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B. W. Drug liposome partitioning as a tool for the prediction of human passive intestinal absorption. Pharm. Res. 1999, 16, 882-888. 

The pH-metric method, which also requires no phase separation, has been used to determine dmg-liposome partitioning [149,162,385-387], The method is the same as that described in Section 4.14, except that FAT-LUV-ET liposomes are used in place of octanol. SUV liposomes have also been used [385,386]. To allow for pH gradients to dissipate (Section 5.6) in the course of the titration, at least 5-10 min equilibration times are required between successive pH readings. 

With liposome partitioning, however, the rule slips to diff 1-2. This means SIP partitioning starts at about pH 8.5 for weak bases whose pKa values are near 9.5 (e.g., Figs. 5.7b, 5.11). So, all who published anomalous values of log P em may need to get out their slide rules 1429—4-321 (What we know now was not known then.)... 

PREDICTION OF ABSORPTION FROM LIPOSOME PARTITION STUDIES ... 

Ottiger, C. Wunderli-Allenspach, H., Immobilized artificial membrane (IAM)-HPLC for partition studies of neutral and ionized acids and bases in comparison with the liposomal partition system, Pharm. Res. 16, 643-650 (1999). 

Rogers, J. A. Choi, Y. W., The liposome partitioning system for correlating biological activities for imidazolidine derivatives, Pharm. Res. 10, 913-917 (1993). 

K Balon, BU Riebesehl, BW Muller. Determination of liposome partitioning of ionizable drugs by titration. J Pharm Sci 88 802-806, 1999. 

Liposome partition Immobilized artificial membrane column Liposome electrokinetic chromatograph Solid-supported lipid membrane etc... 

Formelova, J, Breier, A, Gemeiner, P, and Kurillova, L. (1991). Trypsin entrapped within liposomes. Partition of a low-molecular-mass substrate as the main factor in kinetic control of hydrotyelfe. Czech. 

The results again show that liposome partitioning gives superior information and describes biological effects related to drug-membrane interactions more correctly than log Poet especially in the case of charged drug molecules. 

Balon et al. (1999a) reported improved correlation between liposome partition and human intestinal absorption in comparison to the correlation of intestinal absorption to logDow- Tammela et al. (2004) showed a good agreement between CaCo2 permeation results and liposome data. 

For liposome partition experiments freshly prepared liposomes of defined size are incubated with an aqueous solution of the analyte. The partition coefficient is calculated from the ratio of compound present in aqueous environment and lipid environment in equilibrium. 

However, the throughput of liposome partition assays is limited as preparation and validation of liposomes are very time consuming. That seems to be the main reason why liposome partitioning is not widely used in the pharmaceutical industry to date. On the other hand, the results obtained using liposome partitioning show that liposomes are unique tools to study the bilayer membrane affinity (MA) of drugs and their correlation to intestinal membrane absorption. 

Balon K, Riebesehl BU, Muller BW (1999) Drug liposome partitioning as a tool for the prediction of human passive intestinal absorption. Pharm Res 16 882-888 Balon K, Riebesehl BU, Muller BW (1999) Determination of liposome partitioning of ionizable drugs by titration. J Pharm Sci 88 802-806... 

Several experiments have demonstrated that a correlation exists between log P and the degree of transcellular absorption of a homologous series of compounds. However, for structurally different compounds, such correlation could not be shown. The liposome partitioning system is employed increasingly as an alternative to the octanol approach (13, 14). 

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