Limiting dilution analysis of a novel tripeptide anticancer agent Ambamustine (PTT-119): effect on K-562, CCRF-SB and multidrug resistant LoVo-Dk cell lines.

Cell suspensions of normal human bone marrow were mixed with human acute lymphoblastic or myelogenous leukemic cells of the CCRF-SB or K-562 lines. After incubating the cell mixtures in vitro with different dose levels of Ambamustine (PTT-119), a quantity of 10(4) treated cells were dispensed into microculture plates, and graded cell numbers of the lines used to contaminate the normal marrow were added. Limiting dilution analysis (LDA) was used to estimate the frequency of leukemic cells persisting after treatment. Incubation with 50 micrograms/mL of PTT-119 produced a total elimination of K-562 acute myelogenous blasts, whereas nearly 0.17 and 0.27 leukemic cells were still present in the cell mixtures after treatment with 5 and 25 micrograms/mL, respectively. When normal bone marrow was contaminated with CCRF-SB lymphoblastic cells, incubation with either 50 or 25 micrograms/mL of PTT-119 produced a complete clearing of leukemic cells, whereas with 5 micrograms/mL the leukemic cells in each well were 0.18. When PTT-119 was incubated with LoVo-DX, a colon cancer cell line which expresses the pleiotropic drug resistance MDR phenotype, virtually complete inhibition of clonogenic colonies was observed with as little as 5 micrograms/mL. This suggests that PTT-119 could be used in clinical trials as a non-cross-resistant agent in multidrug protocol.