Differentiation induction in myelodysplasia and acute myeloid leukaemia: use of synergistic drug combinations.

DNA synthesis inhibitors and vincristine greatly enhance the response of leukaemic and dysplastic cells to differentiation inducing agents such as retinoic acid (RET). Differentiation induction therapy is an attractive therapeutic approach in myelodysplasia (MDS) and in acute myeloid leukaemia (AML) in the elderly, since it should be possible to increase the production of mature cells, at the expense of precursor cells, without incurring the complications of intensive cytotoxic therapy. Single agent differentiation therapy has, however, not been highly successful. We have therefore investigated the use of synergistic combinations of agents. We treated nine patients (6 with MDS, 3 with AML) with 13-cis-retinoic acid (up to 100 mg/m2/day) in combination with either 6-thioguanine (20-40 mg/day in 14-57 day courses) or with vincristine (1-2 mg as a single injection during a four-day course of RET). Seven patients responded with an increase in the mature cells of at least one haemopoietic lineage. A concomitant decrease in marrow blasts was observed in 3/4 responding patients. The retention of dysplastic and karyotypic abnormalities and lack of a hypoplastic phase all suggested that differentiation induction was occurring in vivo. Prior failure to respond to therapy with single agents (RET in two and cytosine arabinoside in five patients) suggests that the synergy observed in vitro operates in vivo. In-vitro studies on marrow cells from seven patients demonstrated synergistic differentiation induction in 6/7 samples. The seventh patient was one of the two who did not respond clinically. The second of these clinically unresponsive patients had cells which were relatively refractory to RET in vitro, suggesting that in-vivo and in-vitro responses may be related.