The therapeutic efficacy of an
immunotoxin, SWAII-
SPDB-dg.
ricin A chain, recognizing the leukocyte-
differentiation antigen CD24, was evaluated against SCLC cell lines in tissue culture and in 2 nude-mouse models. The first model used conventional s.c. solid-
tumor xenografts. The second used small
tumor-cell deposits established in s.c. implanted sponge matrices and allowed us to directly estimate the killing efficiency of the
immunotoxin under experimentally defined conditions in vivo. It also mimics the clinical setting of disseminated
tumor cells which form the basis of residual disease in SCLC. The cytotoxic potency of SWAII-
SPDB-dg.
ricin A chain was demonstrated in tissue culture by the inhibition of 3H-leucine incorporation and by the selective elimination of CD24-positive
tumor cells in clonogenic assays. In nude mice, SWAII-
SPDB-dg.
ricin A chain was cleared from the blood circulation with biphasic kinetics: an initial alpha phase of 1 hr and a second beta phase of 20.5 hr. Following i.v. injection of a dose equivalent to 30% of the LD50, the
immunotoxin delayed the growth of SW2 solid-
tumor xenografts by 16 days. The therapeutic efficacy of SWAII-
SPDB-dg.
ricin A chain was further demonstrated by the selective elimination of clonogenic SW2 cells from small
tumor-cell deposits established in sponge matrices. Regrowth of the solid
tumors after the initial response and the clonogenic activity in the sponge-derived cell population were mediated by CD24-positive cells, excluding the selection of CD24-negative mutants during
immunotoxin therapy.