The effect of entrapment in liposomes on the in vivo distribution of [3H]methotrexate in a primate.

Entrapment of methotrexate (MTX) plus [3',5', 9(n)-3H]methotrexate into positively charged liposomes greatly alters the subsequent distribution of [3H]MTX in a cynomologous monkey (Macaca irus) after a single i.v. injection ([3H]MTX; refers to total radioactivity derived from purified [3H]MTX). When [3H]MTX is incorporated into small, sonically disrupted liposomes, the level of the entrapped [3H]MTX in the total plasma volume was still 50% of the total injected dose after 4 hr, which is 100 times greater than the level found when the same amount of free MTX (1 to 4 mg) plus [3H]MTX was injected. When entrapped in larger mechanicanically disperesed liposomes, however, the plasma levels of liposome-entrapped [3H]MTX at 4 hr was only 6-fold greater than free [3H]MT. The liposome-entrapped MTX (refers to MTX measured by dihydrofolate reductase assay) did not show detectable breakdown in the plasma whereas free MTX showed up to 97% breakdown. Increased clearance of [3H]MTX entrapped in mechanically dispersed liposomes was complemented by its much greater uptake into tissues, especially spleen, compared with sonically disrupted liposomes. There was over a 160-fold increased uptake by the spleen of liposome-entrapped [3H] MTX relative to free [3H]MTX, whereas for sonically disrupted liposomes the comparable ratio was 20. Although this liposome-entrapped MTX showed significant breakdown, it was less than that found after injection of free MTX. In certain tissues, especially the small intestine, a reduced uptake of liposome-entrapped [3H]MTX WAS FOUND. Uptake of liposome-entrapped [3H]MTX into liposomes led to a much lower renal clearance of [3H]MTX, especially in the case of sonically disrupted liposomes. Possible reasons for these effects and the relationship of our findings to those of others are discussed.