In vivo modulation of macrophage tumoricidal activity by oral administration of the liposome-encapsulated macrophage activator CGP 19835A.

The present study evaluated the in vivo biological activity of synthetic muramyl tripeptide, CGP 19835A, when encapsulated into phosphatidylcholine liposomes (POPC-19835A) and administered as an p.o. immunomodulator to BALB/c mice. Liposomes were rapidly absorbed in the intestine and reached the systemic circulation within 4 h. Alveolar macrophages harvested from the lungs of mice 24 h after a single p.o. feeding of POPC-19835A were tumoricidal toward syngeneic murine renal cell carcinoma target cells. Repeated daily feedings with POPC-19835A generated sustained activation of the alveolar macrophages. Activation of peritoneal macrophages to the tumoricidal state required at least three daily feedings of POPC-19835A. In vitro studies demonstrated the release of tumor necrosis factor alpha and interleukin 6 by macrophages activated by POPC-19835A in the presence of gamma-interferon. Interleukin 1 and nitric oxide were not induced in macrophages by this liposomal preparation. Daily administration of POPC-19835A after i.v. injection of renal cell carcinoma tumor in BALB/c mice inhibited the development of experimental lung metastasis and confirmed the potential role of long-term therapy with this new p.o. immunomodulator.