Chemistry. Imatinib was identified through high-throughput screening against the BCR-ABL kinase. The lead compound of this series, a 2-phenylaminopyrimidine, had low potency and poor specificity, inhibiting both serine/threonine and tyrosine kinases (Buchdunger et al., 2001). The addition of a 3′-pyridyl group at the 3′ position of the pyrimidine enhanced its potency. Further modifications resulted in improved activity against PDGFR and c-KIT and loss of serine/threonine kinase inhibition. Introduction of N-methylpiperazine as a polar side chain greatly improved water solubility and oral bioavailability, yielding imatinib, an inhibitor of the closed, or inactive, configuration of the kinase:
Dasatinib (BMS-354825, sprycel), a second-generation BCR-ABL inhibitor, was developed using a series of substituted 2-(aminopyridyl) and 2-(aminopyrimidinyl) thiazol-5-carboxyamides. It inhibits the Src kinase, and unlike imatinib, it binds both the open and closed configurations of the BCR-ABL kinase (Shah et al., 2004).
Nilotinib (AMN107, tasigna) was designed to have increased potency and specificity compared to imatinib. Its structure, based on crystallographic studies of the BCR-ABL kinase, promotes hydrogen bonding to Glu286 and Asp381 (Weisberg et al., 2005) and overcomes mutations that cause imatinib resistance.
Mechanism of Action. Crystallographic and mutagenesis studies indicate that imatinib and nilotinib bind to a segment of the kinase domain that fixes the enzyme in a closed or nonfunctional state, in which the protein is unable to bind its substrate/phosphate donor, ATP (Weisberg et al., 2005). The three BCR-ABL kinase inhibitors differ in their potency of inhibition, their binding specificities, and their susceptibility to resistance mutations in the target enzyme. Dasatinib [(IC50) = <1 nM] and nilotinib [(IC50) = <20 nM] (Weisberg, 2005) inhibit BCR-ABL kinase more potently than does imatinib [(IC50) = 100 nM].
Mechanisms of Resistance. Resistance to the tyrosine kinase inhibitors arises from point mutations in three separate segments of the kinase domain (Figure 62–1). The contact points between imatinib and the enzyme become sites of mutations in drug-resistant leukemic cells; these mutations prevent tight binding of the drug and lock the enzyme in its open configuration, in which it has access to substrate. Most such mutations hold the enzyme in its open and enzymatically active confirmation. The most common resistance mutations affect amino acids 255 and 315, both of which serve as contact points for imatinib; these mutations confer high-level resistance to imatinib and nilotinib. Dasatinib is unaffected by mutation at 255 but is ineffective in the presence of mutation at 315. Nilotinib retains inhibitory activity in the presence of most point mutations (except at 315) that confer resistance to imatinib (Weisberg et al., 2005; O'Hare et al., 2005).
Other mutations affect the phosphate-binding region and the "activation loop" of the domain with varying degrees of associated resistance. Some mutations, such as at amino acids 351 and 355, do not affect response to dasatinib or nilotinib but confer low levels of resistance to imatinib (O'Hare et al., 2005; Corbin et al., 2003). This finding may explain the clinical response of some resistant patients to dose escalation of imatinib.
Molecular studies of circulating tumor cells have detected resistance-mediating kinase mutations prior to initiation of therapy, particularly in patients with Ph+ acute lymphoblastic leukemia (ALL) (Roche-Lestienne et al., 2003) or CML in blastic crisis. This finding strongly supports the hypothesis that drug-resistant cells arise through spontaneous mutation and expand under the selective pressure of drug exposure. Mutations may become detectable in the peripheral blood of patients receiving imatinib in the accelerated phase and in the late (>4 years from diagnosis) chronic phase of CML (Branford et al., 2003), heralding the onset of drug resistance.
Mechanisms other than BCR-ABL kinase mutation play a minor role in resistance to imatinib. Amplification of the wild-type kinase gene, leading to overexpression of the enzyme, has been identified in tumor samples from patients resistant to treatment (Morel et al., 2003). The multidrug resistant (MDR) gene, which codes for a drug efflux protein, confers resistance experimentally but has not been implicated in clinical resistance.
Finally, Philadelphia chromosome-negative clones lacking the BCR-ABL translocation and displaying the karyotype of myelodysplastic cells may emerge in patients receiving imatinib for CML and may progress to myelodysplasia (MDS) and to acute myelocytic leukemia (AML). Their origin is unclear.
Imatinib. Imatinib is well absorbed after oral administration and reaches maximal plasma concentrations within 2-4 hours. The elimination t1/2 of imatinib and its major active metabolite, the N-desmethyl derivative, are ~18 and 40 hours, respectively. Mean imatinib area under the curve (AUC) increases proportionally with increasing dose in the range 25-1000 mg (Peng et al., 2004). Food does not change the pharmacokinetic profile of imatinib. Doses >300 mg/day achieve trough levels of 1 μM, which correspond to in vitro levels required to kill BCR-ABL–expressing cells. Inhibition of the BCR-ABL tyrosine kinase in white blood cells from patients with CML reaches a maximum in the dose range of 250-750 mg/day. Nonrandomized studies suggest that response may be restored in a minority of resistant patients with doses of 600 or 800 mg/day, as opposed to the standard 400 mg/day (Kantarjian et al., 2004). In the treatment of GI stromal cell tumors (GIST), higher doses (600 mg/day) may improve response rates.
CYP3A4 is the major enzyme responsible for metabolism of imatinib. CYPs 1A2, 2D6, 2C9, and 2C19 play minor roles in its metabolism. Clinicians must be cautious in introducing drugs that might interact with imatinib and CYP3A4. A single dose of ketoconazole, an inhibitor of CYP3A4, increases the maximal imatinib concentration in plasma and its plasma AUC by 26% and 40%, respectively. Co-administration of imatinib and rifampin, an inducer of CYP3A4, lowers the plasma imatinib AUC by 70%. Likewise, imatinib, as a competitive CYP3A4 substrate, inhibits the metabolism of simvastatin and increases its plasma AUC by 3.5-fold (O'Brien et al., 2003).
Dasatinib. Dasatinib also is well absorbed after oral administration. As for other BCR-ABL kinase inhibitors, the bioavailability of dasatinib is significantly reduced at neutral gastric pH, resulting from antacids and H2 blockers, but is unaffected by food. The plasma t1/2 of dasatinib is 3-5 hours, and clearance depends on CYP3A4 metabolism. Its metabolites are inactive. Dasatinib exhibits dose proportional increases in AUC, and its clearance is constant over the dose range of 15-240 mg/day. No significant changes in dasatinib pharmacokinetics were observed on repeated dosing or administration of food. Dasatinib was originally approved at a dose of 70 mg twice a day. A randomized phase III trial comparing 50 mg twice daily, 70 mg twice daily, 100 mg daily, and 140 mg daily concluded that all four schedules were equally effective in patients with CML, although the 100-mg daily dose improved progression-free survival (Shah et al., 2008).
Dasatinib is metabolized primarily by CYP3A4, although FMO3 and UGT also contribute to a minor degree. Dasatinib plasma concentrations are affected by inducers and inhibitors of CYP3A4 in a similar fashion to imatinib.
Nilotinib. Approximately 30% of an oral dose of nilotinib (400 mg twice daily) is absorbed after administration, and the drug achieves peak concentrations in plasma 3 hours after dosing. Unlike the other BCR-ABL inhibitors, nilotinib's bioavailability increases significantly in the presence of food (Kantarjian et al., 2006). The drug has a long plasma t1/2 (~17 hours), and plasma concentrations reach a steady state only after 8 days of daily dosing. Doses >400 mg/day are associated with nonlinear increases in median serum concentrations, indicating saturation of GI absorption.
Like dasatinib and imatinib, nilotinib undergoes elimination through metabolism by CYP3A4; metabolism is affected by inducers, inhibitors, and competitors of the CYP3A4 pathway. Nilotinib is a substrate and inhibitor of P-glycoprotein.
Pharmacokinetics and Clinical Uses. These protein tyrosine kinase inhibitors have efficacy in diseases in which the ABL, kit, or PDGFR have dominant roles in driving the proliferation of the tumor, reflecting the presence of a mutation that results in constitutive activation of the kinase, either by fusion with another protein or via point mutations. Thus, imatinib shows remarkable therapeutic benefits in patients with chronic-phase CML (BCR-ABL), GIST (kit mutation positive), chronic myelomonocytic leukemia (EVT6-PDGFR translocation), hypereosinophilia syndrome (FIP1L1-PDGFR), and dermatofibrosarcoma protuberans (constitutive production of the ligand for PDGFR) (Druker, 2004). It is the agent of choice for GIST patients with metastatic disease and as adjuvant therapy of c-kit– positive GIST (DeMatteo et al., 2009). GIST biology is particularly instructive, as patients with an exon 11 mutation of kit have a significantly higher partial response rate (72%) than those with no detectable kit mutations (9%) (Heinrich et al., 2003). The currently recommended dose of imatinib is 400-600 mg/day. Dasatinib is approved for patients with CML resistant or intolerant to imatinib in both chronic (100 mg/day) and advanced phases of disease (70 mg twice daily), and for use combined with cytotoxic chemotherapy in patients with Ph+ ALL who are resistant or intolerant to prior therapies. Nilotinib is approved for patients with CML resistant to or intolerant of prior imatinib therapy.
Toxicity. Imatinib, dasatinib, and nilotinib cause GI distress (diarrhea, nausea, and vomiting), but these symptoms usually are easily controlled. All three drugs promote fluid retention, which may lead to dependent edema, and peri-orbital swelling. Dasatinib may cause pleural effusions. Nilotinib may prolong the QT interval, and should be used with caution in patients with underlying heart disease or arrhythmias, although ventricular arrhythmias have not been reported. Significant myelosuppression occurs infrequently but may require transfusion support, dose reduction, or discontinuation of the drug. All three drugs in this class can be associated with hepatotoxicity. Most nonhematological adverse reactions are self limited and respond to dose adjustments. After the adverse reactions, such as edema, myelosuppression, or GI symptoms have resolved, the drug may be reinitiated and titrated back to effective doses.