Encapsulation of 8-azaguanine in single multiple compartment liposomes

Preparation of PEGylated liposomes incorporating lipophilic lomeguatrib derivatives for the sensitization of chemo-resistant gliomas

Liposomal delivery is a well-established approach to increase the therapeutic index of drugs, mainly in the field of cancer chemotherapy. Here, we report the preparation and characterization of a new liposomal formulation of a derivative of lomeguatrib, a potent O⁶-methylguanine-DNA methyltransferase (MGMT) inactivator. The drug had been tested in clinical trials to revert chemoresistance, but was associated with a low therapeutic index. A series of lomeguatrib conjugates with distinct alkyl chain lengths - i.e. C12, C14, C16, and C18 - was synthesized, and the MGMT depleting activity as well as cytotoxicity were determined on relevant mouse and human glioma cell lines. Drug-containing liposomes were prepared and characterized in terms of loading and in vitro release kinetics. The lipophilic lomeguatrib conjugates did not exert cytotoxic effects at 5 μM in the mouse glioma cell line and exhibited a similar MGMT depleting activity pattern as lomeguatrib. Overall, drug loading could be improved by up to 50-fold with the lipophilic conjugates, and the slowest leakage was achieved with the C18 derivative. The present data show the applicability of lipophilic lomeguatrib derivatization for incorporation into liposomes, and identify the C18 derivative as the lead compound for in vivo studies.

Study of azathioprine encapsulation into liposomes

The factors influencing the encapsulation of azathioprine (AZA) into liposomes were investigated to find out the conditions for its optimal entrapment. Similar studies for comparison were also carried out on 6-mercaptopurine (6-MP), of which AZA is a prodrug. AZA and also 6-MP show higher encapsulation efficiencies in MLVs as compared to LUVs. Variation in phospholipid composition does not seem to affect the loading capacity of either of the two drugs. The encapsulation efficiency of both the drugs improves upon addition of cholesterol in the bilayer, but the effect is seen only up to 30% cholesterol. Thereafter the effect becomes constant. AZA shows better incorporation in the positively charged liposomes as compared to those with neutral or negative charge. The entrapment of 6-MP is, however, found to be independent of the charge on the liposomes. Entrapment efficiency for both the drugs markedly depends on the pH of the hydration medium, yielding better entrapment efficiencies at high pH values. The rise in solute concentration initially causes increase in the entrapment of the two drugs which is followed by a decreasing phase.

Liposomal membrane. I. Chemical damage of liposomal membranes with functional detergent

The interaction and reaction between liposomal membrane and a functional detergent, N-hexadecyl-N-(imidazol-4-yl)methyl-N,N-dimethylammonium chloride hydroperchlorate (Im-I), have been investigated in conjunction with the leakage of bromothymol blue encapsulated as a marker in the bilayers of liposomes. Im-I carries an imidazole moiety and was expected to behave as a simple lipase model. The reaction with Im-I significantly enhanced the leakage of bromothymol blue encapsulated in the egg lecithin and dipalmitoyl phosphatidylcholine liposomes. During the course of reaction with Im-I, the formation of acyl-imidazole intermediate was clearly identified, which was certainly connected with the bromothymol blue release. From various kinetic results on bromothymol blue release and acyl-imidazole formation, it has been suggested that the bromothymol blue release from liposomal bilayer may be caused by the local and instantaneous decomposition of lipids when Im-I penetrates into the bilayer. However, it has also been demonstrated that the immediate reconstruction of liposomes retains the barrier function to protect against the further release of bromothymol blue.