Anthracyclines entrapped in small-sized, sterically stabilized
liposomes have the advantage of long circulation time, reduced systemic toxicity, increased uptake into systemic
tumors, and gradual release of their payload. To date, there is no information on the behavior of these
liposomes in
brain tumors. The objective of this study was to compare the biodistribution and clinical efficacy of free
doxorubicin (F-DOX) and stealth
liposome-encapsulated
DOX (SL-DOX) in a secondary
brain tumor model. Nine days after
tumor inoculation Fischer rats with a right parietal malignant
sarcoma received an intravenous dose of 6 mg/kg of either F-DOX or SL-DOX for evaluation of
drug biodistribution. For therapeutic trials a single dose of 8 mg/kg was given 6 or 11 days after
tumor induction, or alternatively, weekly doses (5 mg/kg) were given on Days 6, 13, and 20.
Liposome-encapsulated DOX was slowly cleared from plasma with a t1/2 of 35 hours. Free-DOX maximum
tumor drug levels reached a mean value of 0.8 microgram/g and were identical in the adjacent brain and contralateral hemisphere. In contrast, SL-DOX
tumor levels were 14-fold higher at their peak levels at 48 hours, declining to ninefold increased levels at 120 hours. A gradual increase in
drug levels in the brain adjacent to
tumor was noted between 72 and 120 hours (up to 4 micrograms/g). High-performance liquid chromatography analysis identified a small amount of aglycone metabolites within the
tumor mass from 96 hours and beyond, after SL-DOX injection. Cerebrospinal fluid levels were barely detectable in
tumor-bearing rats treated with F-DOX up to 120 hours after
drug injection (< or = 0.05 microgram/ml), whereas the levels found after SL-DOX were 10- to 30-fold higher. An F-DOX single-dose treatment given 6 days after
tumor inoculation increased the rats' life span (ILS) by 135% over controls (p < 0.05) but was not effective if given on Day 11. In contrast, SL-DOX treatment resulted in an ILS of 168% (p < 0.0003) with no difference when given after 6 or 11 days. Treatment with three weekly doses of SL-DOX produced an ILS of 189% compared to 126% by F-DOX (p < 0.0002). The authors conclude that the use of long-circulating
liposomes as cytotoxic
drug carriers in
brain tumor results in enhanced
drug exposure and improved therapeutic activity, with equal effectiveness against early small- and large-sized
brain tumors.