We studied bioavailability, treatment schedule dependence, and therapeutic efficacy of orally administered
MST-16, a novel derivative of bis(2,6-dioxopiperazine), against murine
tumors and human
tumor xenografts. The rate of its intestinal absorption was about 50%, and it was immediately metabolized to its parent compound,
ICRF-154. Therapeutic efficacy of
MST-16 was heavily dependent on the treatment schedule: 9 daily
oral administrations and treatment every 4 h on day 1 only were much more effective against s.c.-implanted
L1210 leukemia than a single dose or five daily administrations giving the same total dose. Orally administered
MST-16 showed potent life-prolonging effects (196%, 219% and 148%) in mice inoculated i.p. with P388,
L1210 leukemia, and C-26
colon adenocarcinoma, respectively, but had no effect on
B16 melanoma inoculated in the same way.
MST-16 inhibited more than 80% growth of
Lewis lung carcinoma,
B16 melanoma, and C-38
colon adenocarcinoma implanted s.c., but had only a minor effect on M5076
fibrosarcoma. Lung
metastasis of
Lewis lung carcinoma was also effectively suppressed. Furthermore,
MST-16 significantly inhibited growth of human colon, lung and breast
cancers implanted s.c. in nude mice. We also made a kinetic analysis of the in vitro cell-killing effect by
ICRF-154, the active form of
MST-16 in vivo. It demonstrated a cell cycle phase-specific and time-dependent action, providing a reasonable explanation for the schedule-dependent
therapeutic effect of
MST-16.