Anthracycline antibiotics with high liposome entrapment: structural features and biological activity.

We evaluated the entrapment of 21 different water-insoluble aglycones or anthracycline antibiotics in multilamellar liposomes composed of dimyristoyl phosphatidyl choline and dimyristoyl phosphatidyl glycerol at a 7:3 molar ratio. The drug:lipid weight ratio was 1:15 to 1:50. The different analogues tested were modified at position 4 in the aglycone portion (4-demethoxy) and/or positions 2' (halo), 3' (hydroxy, acetoxy), or 4' (epi, acetoxy) in the sugar portion. The entrapment efficiency was assessed by measuring the amount of free drug remaining in the supernatant after centrifugation of the liposomes and by direct examination of the pellets by fluorescent microscopy. Optimal entrapment (greater than 98%) was observed with only four compounds: 4-demethoxyadriamycinone; 2'-iododaunorubicin; 4-demethoxydaunorubicin; and 2'-iodo-3'-hydroxy-4'-epi-4-demethoxydoxorubicin (Compound 22). All other compounds showed significant drug precipitation outside the multilamellar vesicles when observed by fluorescent microscopy. Compound 22, entrapped in liposomes, was evaluated in vivo against i.p. L-1210 leukemia by the i.p. route, and liver metastases of M5076 reticulosarcoma by the i.v. route. In both models, liposome-entrapped Compound 22 was more active than doxorubicin at the optimal dose [median survival (given in percentage) of treated to control animals was for L-1210, greater than 600 versus 212; for M5076, 200 versus 133]. 4-Demethoxy and 2'-iodo are structural modifications that markedly enhance the affinity of anthracycline antibiotics for lipid bilayers without compromising biological activity. These findings will serve as a guideline to obtain liposome-anthracycline preparations, with optimal formulation characteristics, enhanced tumor-targeting properties, and non-cross-resistance with doxorubicin.