Induction of acute lymphocytic leukemia differentiation by maintenance therapy

Abstract

Despite extensive study in many malignancies, maintenance therapy has clinically benefited only two diseases: acute lymphocytic leukemia (ALL) and acute promyelocytic leukemia (APL). ALL maintenance therapy utilizes low-dose 6-mercaptopurine (6MP) and methotrexate (MTX), while maintenance in APL primarily consists of all-trans-retinoic acid (ATRA). 6MP and MTX as used in ALL are also now usually added to maintenance ATRA for APL, based on data suggesting an improved disease-free survival. Although the mechanism of action of MTX and 6MP as maintenance is unknown, low-dose cytotoxic agents are potent inducers of differentiation in vitro. Thus, we studied whether maintenance therapy in ALL, like ATRA in APL, may be inducing terminal differentiation of ALL progenitors. The APL cell line NB4, the ALL cell lines REH and RS4;11, and patients' ALL blasts were incubated with ATRA, 6MP, and MTX in vitro. All three drugs inhibited the clonogenic growth of the APL and ALL cell lines without inducing immediate apoptosis, but associated with induction of phenotypic differentiation. The three drugs similarly upregulated lymphoid antigen expression, while decreasing CD34 expression, on patients' ALL blasts. These data suggest that induction of leukemia progenitor differentiation plays an important role in the mechanism of action of maintenance therapy in ALL.

Figure 1

Clonogenic growth of acute lymphocytic leukemia (ALL) cell lines REH and RS4;11 following treatment with all-trans-retinoic acid (ATRA), 6-mercaptopurine (6MP) and methotrexate (MTX). Results represent mean±s.e.m. of five separate experiments with each cell line. P-values are for individual treatment groups compared to control.

Figure 2

Effects of all-trans-retinoic acid (ATRA), 6-mercaptopurine (6MP) and methotrexate (MTX) on the (a) clonogenic growth of and (b) CD15 expression by NB4 cells. Results represent the mean±s.e.m. of four separate experiments. P-values are for individual treatment groups compared to control.

Figure 3

Effects of all-trans-retinoic acid (ATRA), 6-mercaptopurine (6MP) and methotrexate (MTX) on (a) CD19 and (b) CD38 expression by acute lymphocytic leukemia (ALL) cell lines. Representative histograms of (c) CD19 expression by REH cells after 6MP treatment and (d) RS4;11 cells after MTX treatment (control — thin line, drug — thick line). Results represent the mean±s.e.m. of five separate experiments. P-values are for individual treatment groups compared to control.

Figure 4

Effects of all-trans-retinoic acid (ATRA), 6-mercaptopurine (6MP) and methotrexate (MTX) on (a) CD19, (b) CD38 and (c) CD34 expression by acute lymphocytic leukemia (ALL) blasts from five patients. (d) Representative histogram of CD38 expression by ALL blasts following treatment with MTX (control — thin line, MTX — thick line). Results represent the mean±s.e.m. P-values are for individual treatment groups compared to control.