Abstract

Recently, we have shown that BMS-214662, a cytotoxic farnesyltransferase inhibitor (FTI), targets primitive progenitor cells (PPC) in chronic myeloid leukaemia (CML) [

Copland et al,
Blood
2005
;
106
:
204a
]. These PPC are believed to be responsible for the molecular persistence which occurs following treatment with imatinib mesylate (IM) in CML. We have also shown that neither dasatinib (BMS-354825) nor nilotinib (AMN107) targets these PPC in vitro [
Copland et al,
Blood
2006
;
107
:
4532
;
Jorgensen et al,
Blood
2005
;
106
:
314a
]. To further investigate the efficacy of BMS-214662 against CML stem cells we performed long term culture-initiating cell (LTC-IC) assays with both chronic phase CML and normal CD34+ progenitors to assess drug selectivity. The CD34+ cells were treated for 72 hours under the following conditions

  1. no drug control,

  2. IM 5μM,

  3. dasatinib 150nM,

  4. BMS-214662 250nM,

  5. IM + BMS-214662,

  6. dasatinib + BMS-214662 before LTC-IC assay.

Compared to the no drug control, CD34+ CML cells showed increased colony formation in the IM and dasatinib arms (181 and 178% respectively) indicating that, by inhibiting proliferation, these drugs exert a protective effect on CML PPC. The addition of BMS-214662 to either IM or dasatinib significantly reduced the number of colonies compared to either agent alone (P=0.045 and P=0.029 respectively). The BMS-214662 containing arms showed a dramatic reduction in total colony numbers to < 2.5% of the no drug control (P=0.038). In 2/3 experiments, FISH showed that, after treatment with BMS-214662, the majority of cells from residual colonies were BCR-ABL negative. In addition, BMS-214662 was less toxic to normal LTC-IC from healthy donors. Using CFSE-based flow cytometry to track cell division, we assessed BMS-214662 alone and in combination with IM or dasatinib in blast crisis (BC) CML. BMS-214662 reduced the number of PPC compared to the no drug control and acted synergistically with IM or dasatinib to reduce the total number of viable cells and PPC in BC CML. To determine if the effects of BMS-214662 in CML were due to inhibition of Ras, we used the CFSE method, caspase-3 activity to measure apoptosis and Ki-67/7AAD cell cycle analysis to assess quiescence to compare BMS-214662 with another FTI with equivalent Ras inhibition, BMS-225975. As previously, after 6 days culture, not only had BMS-214662 significantly reduced the number of CML PPC compared to the no drug control (P=0.018), but also compared to BMS-225975 containing arms (P=0.024). There was no significant difference between the no drug control and BMS-225975 arms. Caspase-3 activity was highest in the BMS-214662-containing arms. After 72 hours, Ki-67/7AAD analysis showed reduced G0 cells in the BMS-214662 arms. The disparate effects of BMS-214662 and BMS-225975 suggest that the efficacy of BMS-214662 in CML is not via inhibition of Ras. We then determined the efficacy of BMS-214662 in Ba/F3 cells expressing different BCR-ABL kinase mutations (WT BCR-ABL, T315I, M351T and H396P) using viable cell counts and 3H thymidine proliferation assays. BMS-214662 was equipotent in both WT BCR-ABL and mutant BCR-ABL kinase expressing cells. These results provide further evidence that BMS-214662 selectively targets CML stem cells, acts in synergy with IM or dasatinib and may prove useful in the management of IM-resistant and BC CML.

Disclosures: Novartis have provided the TLH lab with research funding.; TLH is on the advisory committees for Novartis (imatinib, nilotinib) and Bristol Myers Squibb (dasatinib).; MC and TLH have both received travel funding travel from Bristol Myers Squibb to present at meetings. TLH has also received funding from Novartis for travel.

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