Dasatinib induces complete cytogenetic response and loss of F359C in an imatinib resistant chronic myelocytic leukemia patient

To the editor: We would like to report our experience on a chronic myelocytic leukemia (CML) patient with F359C mutation. Mutations in BCR/ABL kinase domain are well-known cause of resistance to tyrosine kinase inhibitors in CML. Although F359C mutation confers resistant to imatinib in vitro, there has been limited report on its effect in vivo. A 33-year-old male with CML in chronic phase responded initially imatinib but lost his response 20 months later. The patient had F359C at the time of progress. The patient did not respond to either increased imatinib dose (800 mg/day) or nilotinib. Dasatinib induced complete hematologic, cytogenetic, and molecular remission in the patient. The F359C mutation became undetectable. The F359C mutation is resistant to imatinib and (most likely) nilotinib, but responsive to dasatinib treatment. The precise frequency of the F359C mutation is unknown but certainly very low [1–6]. In 2006, Hughes et al. reported the mutation was less than 2% in CML and Philadelphia chromosome positive (Ph1) acute lymphoblastic lymphoma patients (ALL) [1]. Only one patient with F359C was detected in the main nilotinib trial imatinib resistant/intolerant CML patients in chronic phase (CP) were enrolled [2]. In the START-C trial, 8 patients with F359C/I/V were reported in 2 years follow up (the exact number of patients with the F359C mutation is not reported) [3,4]. Guilhot et al. reported 2 patients with the F359C mutation CML in accelerated phase (AP) on imatinib [5]. Schnittger et al. found 3 patients with the mutation out of 88 imatinib resistant CML patients [6]. Functional role of this mutation was evaluated in vitro [2,7]. In cellular proliferation-based analysis showed cells with the F359C mutation have IC50 > 1,000 nM for imatinib and 250 nM for nilotinib [2,7]. There was no recovery when these cells were treated with 5 nM dasatinib [7]. In parallel to these laboratory data, the results from the patients reported in different trials showed that the F359C mutation confers resistance to imatinib treatment [2–6]. In the nilotinib trial, 9 of 11 patients with F359C/V mutation progressed during treatment [2]. However, the resistance to nilotinib might be overcome by higher doses considering its IC50 is around 250 nM, and the plasma trough concentrations at the maximum tolerated dose of nilotinib (400 mg twice daily) are in the range of 2000 nM [8]. On the other hand, dasatinib is effective in patients with imatinib-resistant mutations including F359C. Two patients with CML in AP and F359C detected at base line achieved major hematologic and cytogenetic response with dasatinib at 8-month follow up [5]. In addition, 4 of 8 patients achieved complete cytogenetic response (CCR), 3 had partial CR, and 1 complete hematologic response (CHR) in the START-C trial [4]. No report available on the fate of the F359C mutation on extended follow up. A 33-year-old male was diagnosed with CML in CP and put on imatinib 400 mg daily in July 2005 (94% of cells analyzed were Ph1 chromosome). Despite initial good response (CHR with reduction of Ph1 chromosomes to 25%, leukocytosis with basophilia and thrombocytosis occurred after 20 months. Imatinib dose was increased to 800 mg daily with no improvement in CBC and cytogenetics. Direct sequencing of BCR-ABL kinase domains including the nucleotide binding loop (P-loop), the catalytic domain and the activation loop, displayed a F359C point mutation—TTC to TGC nucleotide change leading to Phe-to Cys amino acid substitution (Fig. 1). Anagrelide was added to treatment and nilotinib replaced imatinib. Platelet counts were normalized initially but increased again when anagrelide was discontinued. Dasatinib replaced nilotinib, which resulted in a CHR and decrease in Ph1 positive cells <1% in 2 weeks. The patient has been on dasatinib to date (19 months) with no available donor for bone marrow transplantation. The patient achieved a CCR, and BCR-ABL transcripts decreased to the level below the quantifiable limits of the assay (the limit of detection for the assay is at 1 in 100,000). Direct sequencing confirmed the absence of the F359C mutation. BCR-ABL1 quantification was performed by sensitive Real-Time RT-PCR using the LightCycler technology (limit of detection is at 1 in 100,000 copies) [9]. BCR-ABL1 kinase domain mutation analysis was performed using seminested reverse transcriptase-polymerase chain reaction followed by direct sequencing as previously described [10]. Most of the mutations in the ABL kinase domain impair imatinib as TKI by disruption of the critical interactions between imatinib and Abl, or by preventing the kinase from adopting the specific inactive conformation that is required for imatinib mesylate binding [11]. It is postulated that residues such as F359, which neighbor the activation loop, are critical for maintaining the particular inactive imatinib-bound conformation. The F359 residue is also involved in van der Waals contacts with both imatinib and the activation loop in the imatinib-bound form [12]. F359 mutations that are associated with amino acid substitutions in the catalytic region include (Phe359Ala) F359A, Phe359Val (F359V), Phe359Ile (F359I), or Phe359Cys (F359C) as detected in this patient. Like imatinib mesylate, nilotinib binds to the inactive Abl kinase conformation, with the P-loop folding over the ATP-binding site, and the activation-loop blocking the substrate binding site, to disrupt the ATPphosphate-binding site and inhibit the catalytic activity of the enzyme [13]. Dasatinib was originally developed as a Src kinase inhibitor and is structurally distinct from imatinib. Compared with imatinib, dasatinib exhibits increased potency but reduced selectivity and binds both active and inactive conformations of Abl [12]. Overriding imatinib resistance with dasatinib in other mutations involving F359 residue, such as F359I, has been reported and its efficacy against BCR-ABL with mutations at this site is explained by the fact that it does not reside near this residue [12,14]. This case report supports that dasatinib should be considered as first treatment of choice for patients with the F359 mutation. Considering the active process of acquiring new mutations or other mechanisms that can cause resistant to dasatinib in the long term, allogeneic transplantation should be considered in these patients.