Favorable therapeutic index of a p210BCR-ABL-specific tyrosine kinase inhibitor;: activity on lineage-committed and primitive chronic myelogenous leukemia progenitors

Purpose: In order to assess the effect of the tyrosine kinase inhibitor CGP57148B on lineage-committed and primitive chronic myelogenous leukemia (CML) progenitor cells, peripheral blood progenitor cells (PBPC) mobilized in chronic phase CML were exposed to this compound in vitro. Methods: Both shortterm (II week) and long-term exposure (greater than or equal to 2 weeks) to CGP57148B were investigated using suspension culture, semisolid (methylcellulose) assay or stroma-dependent long-term culture (LTC). The proportion of bcr/abl-positive progenitors was determined after direct plating [2 weeks in colony-forming cell (CFC) assay] as well as after 2 or 6 weeks LTC (LTC always followed by CFC replates). Results: Incubation of CML PBPC over 48 h in suspension culture with 100 mu M CGP57148B reduced the proportion of bcr/abl-positive colonies to 4.4 +/- 4.3% (n=5) after direct plating, 6.6 +/- 4.2% (n = 5) after 2 weeks LTC and 5 =/- 5.6% (n = 2) after 6 weeks LTC. At this dose, survival of drug-exposed normal PBPC was 53 +/- 4.2%, 51 +/- 2.8% and 54.5 +/- 4.9% (n = 2), respectively. Incubation with CGP57148B at a concentration of 10 mu M over 1 week under LTC conditions reduced the number of bcr/abl-positive colonies to 11.8 +/- 6.1% (n= 5) after direct plating, 12 +/- 6.4% (n=4) after 2 weeks LTC and 14.3 +/- 11.4% (n=3) after 6 weeks LTC; survival of normal PBPC was 84.5 +/- 2.1%, 93 +/- 4.2% and 86 +/- 1.4% (n=2), respectively. Following long-term exposure to CGP57148B at a concentration of 1 mu M, the proportion of remaining bcr/abl-positive colonies was 35%, 9% and 25% of untreated CML samples after direct plating as well as after 2 and 6 weeks LTC, respectively. The respective values for 10 mu M CGP57148B were 10%, 11% and 19%. Long-term exposure of normal progenitors to CGP57148B yielded a survival of 98%, 100% and 93% (1 I-IM) or 77%, 86% and 80% (10 mu M), respectively. Conclusion: The results support the use of CGP57148B either for short-term exposure in vitro (e.g. purging) or for continuous treatment of CML in vivo.