Antitumor activity of enkephalin analogues in inhibiting PYB6 tumor growth in mice and immunological effects of methionine enkephalinamide

Abstract

Recent evidence has implicated enkephalins as immunomodulators. Several studies have reported the regulation of tumor growth by methionine enkephalin (ME). However, there has been little effort to relate the immunological significance of enkephalins to the development of anticancer drugs. The present study had three aims: first, to compare the antitumor activity of the synthetic peptide, D-[Ala 2]methionine enkephalinamide (MEA), with endogenous enkephalins on PYB6 fibrosarcoma tumor growth; second, to determine whether tumor growth inhibition was mediated by an opiate receptor; and third, to investigate the effects of MEA on selected immune responses. Female B6C3F1 mice were injected i.p. daily for 7 days with 50–4000 μg/kg of ME, MEA, leucine enkephalin (LE) or D-[Ala 2]leucine enkephalinamide (LEA), beginning 1 day after PYB6 inoculation. ME and MEA, but not LE or LEA, decreased the PYB6 growth rate. The dose of 50 μg/kg MEA exerted the maximum inhibition of tumor growth (nearly 72% on day 15 post tumor transplantation). MEA was not directly toxic to PYB6 tumor cells, as evaluated by the measurement of DNA synthesis and cellular ATP levels of PYB6 cells exposed to MEA in vitro. No 3H-etorphine specific bindings were detected on the cell membrane or sonicates of splenic lymphocytes or PYB6 cells. Therefore, the antitumor activity by MEA is likely mediated by an indirect mechanism. Immunological studies indicated that MEA selectively enhanced the lymphoproliferative response to the T-cell mitogen, concanavalin A, but not to the B-cell mitogen, lipopolysaccharide. Natural killer cell activity was unaffected both after in vivo and in vitro exposure to MEA and the number of IgM antibody-forming cells in response to the T-dependent antigen, sheep red blood cells, was unaffected after in vitro exposure to MEA.