Distribution of free and liposome-entrapped [3H]methotrexate in the central nervous system after intracerebroventricular injection in a primate.

[3H]Methotrexate ([3H]MTX) was entrapped inside sonically disrupted, positively charged, [14C]cholesterol-labeled liposomes. Its distribution and metabolic integrity was then studied after a single injection into the cerebrospinal fluid (CSF) in the lateral cerebroventricles of cynomolgus monkeys ( Macaca fascicularis ). Between 24 and 96 hr after injection, a maximal 4-fold increase in average tissue levels, compared with the levels found after injection of free [3H]MTX, was found for total central nervous system (brain plus spinal cord). Greater increased levels of [3H]MTX after injection of liposome-entrapped [3H]MTX compared with free [3H]MTX were found in CSF, but considerable variation was noted according to the site of sampling. Metabolic degradation of free MTX, as measured by the difference between MTX determined by 3H-specific activity and MTX determined by the dihydrofolate reductase assay, was detected at 24 hr, and 96 hr after injection only 11% of the free MTX in CSF represented intact MTX. In contrast, after injection of liposome-entrapped MTX 72% was found to be intact at the same time. Consistent with this protection against metabolic degradation, a 14C:3H ratio of 0.9 to 1.1 in CSF for up to 24 hr after injection was found, suggesting that most of the liposome-entrapped [3H]MTX injected had remained in intact liposomes during this time period. Distribution within brain tissue was also studied at 24 hr postinjection, and it was found that [3H]MTX levels after injection of liposome-entrapped [3H]MTX were higher than after injection of the free form near tissue-CSF interfaces, but less than the free form in deep areas of the brain. Considerable leakage from the central nervous system to plasma was found to occur soon after injection for both the free and the liposome-entrapped [3H]MTX, but the liposome-entrapped [3H]MTX showed 10- to 20-fold higher steady-state plasma levels. This was attributed to a much lower rate of removal from plasma due to the much lower renal excretion of liposome-entrapped MTX. These findings are compared with the behavior found after systemic injection of liposome-entrapped MTX and other compounds. The implications of these results for a possible chemotherapeutic use of liposome-entrapped drugs directly injected into the central nervous system are briefly discussed.