BCR-ABL1 is a fusion gene formed as a result of the t(9;22)(q34;q11) chromosomal translocation, the translocation that results in the formation of the Philadelphia chromosome. The Abelson murine leukaemia viral oncogene homolog 1 (ABL1) gene from 9q34 is translocated downstream to a region at 22q11 known as breakpoint cluster (BCR). The fusion gene encodes for a constitutionally active tyrosine kinase that has been shown to drive the expression chronic myeloid leukaemia phenotype. BCR-ABL1 gene has also been on implicated in the pathogens is of acute lymphoblastic leukaemia and in rare cases of acute myeloid leukaemia. The gene has been targeted with unparalleled success by the first tyrosine kinase inhibitor approved in clinical practice, imatinib.
Molecular Biology of BCR-ABL
The ABL1 proto-oncogene is located on chromosome 9 at q34. Chromosome 22 has the BCR gene at 22q11. The ABL1 gene translocated downstream to the BCR gene as a result of the t(9;22)(q34;q11) translocation. ABL1-BCR translocation also occurs and may express but is of no clinical significance.
The BCR-ABL1 fusion gene and it’s variants
The breakpoint of the ABL1 gene may be upstream exon 1a, between exon 1a and 1b or downstream exon 1b but it is almost always upstream exon 2. With rare exceptions all transcripts of BCR1-ABL1 gene have exon 2-11 of the ABL1 gene. The BCR breakpoints are variable and determine the size as well as the pathogenic properties of the BCR-ABL1 gene.. The breakpoint on the BCR gene are clustered in three regions known major cluster, minor cluster and micro cluster (Table 1). Depending on the location of breakpoint on the BCR gene three types of protein are synthesized. The p210 transcript is associated with CML and some patients with Philadelphia positive acute lymphoblastic leukaemia. The shorter p190 transcript is associated with philedelphia positive acute lymphoblastic leukaemia and some patients of chronic myeloid leukaemia. The CML that carry this mutation show monocytosis and have a more aggressive course. The p230 is the largest and the rarest of the BCR-ABL1 transcripts. It is associated with a more indolent course and is found in patients with the rare chronic neutrophilic leukaemia. Atypical transcripts e1a3, e13a3 and e6a2 have been described.
Table 1: The BCR-ABL1 fusion genes
|| Major Cluster
|| Minor Cluster
||Between alternative exon 2, e2’ and e2
||between exons e19 and e20
|| CML, e14a2 shown to have thrombocytosis in some studies., Ph+ ALL
|| Ph + ALL; CML that tends to have monocytosis and an agressive course
|| Chronic Neutrophilic Leukaemia, Small reports describing patients with a course resembling classical CML
Mutagenicity of BCR-ABL1
ABL1 is a nuclear kinase whose activity is tightly regulated by the cell. BCR-ABL1 translocation results loss of regulation and the kinase is cosntitutively active. Sustitution of ABL1 at the N termnal by segments of the BCR gene result in the synthesis of a protein that has the capacity to dimerise. Dimerisation transphosphorylates and then aurtophsophorylates the the kinase fully activating it. The precise mechanism how BCR-ABL1 leads to chronic myeloid leukaemia is not known but activation of phosphatidylinositol kinase, RAS/Mitogen activated protein kinase and JAK/STAT pathway has been demonstrated in BCR-ABL1 positive cells. These pathways are involved in cellular growth and differentiation. The BCR-ABL1 kinase also phosphorylates proteins involved in adhesion and migration and this may have a role in premature release of myeloid cells in circulation. CML cells have a two to sixfold increase in reactive oxygen species and have impaired DNA repair. Reactive oxygen species can induce DNA double strand breaks. The results is additional mutations and these are believed to be responsible for blast crisis and acclerates phase.
Tyrosine kinase inhibitors targeting the BCR-ABL1 protein induce a remission in most patients of CML. About half the patients who have achieved sustained complete molecular response relapse on discontinuation of the tyrosine kinase inhibitors. This suggests that the stemat least some CML stem cells are not BCR-ABL1 dependent for growth. Experimental observations support this hypothesis.
Targeting the BCR-ABL1 Gene
The BCR-ABL1 gene was the first gene to be targeted by a tyrosine kinase inhibitor, imatinib. Imatinib was followed by dasatinib, nilotinib, Busotinib and Panotinob. Imatanib, Dasatinib and Nilotinib are approved to first line use. Imatinib has resulted in a 85% 8 year survival. Dasatinib and nolitinib are active in imatinib resistant CML and are now approved for first line use. Drug resistance results from mutations in BCR-ABL1 kinase. The T315I mutation or the gatekeeper mutations impaires access of TKIs to the BCR-ABL1 kinase making most drugs inactive. Panotinib can inhibit the T315I mutation.
Barnes DJ Melo JV. Molecular Basis of Chronic Myleoid Leukaemia. In Chronic Myeloproliferative Disorders: Cytogenetic and Molecular Anomalies. Bain Barbra J (Ed) 2003.
Dasatinib is a thiazole carboxamide oral multikinase inhibitor that inhibits BCR-ABL and SRC kinases that is used for the treatment of chronic myeloid leukaemia and BCR-ABL positive acute lymphoblastic leukaemia.
Mechanism of Action
The kinases inhibited by dasatinib include
- SRC family (SRC, LCK, YES, FYN)
- Epirin A receptor
- Platelet derived growth factor β (PDGF-β) receptors
BCR-ABL kinase inhibition
As an inhibitor of BCR-ABL kinase dasatinib is 325 times more potent than imatinib and 16 times more potent than nilotinib. Unlike imatininb and nilotinib dasatinib binds the active and the inactive configurations of the enzyme. It is effective against mutations that have destabilized the inactive configuration. Unlike imatinib chronic inhibition of the kinase in not achieved and does not appear to be necessary for action. Activity of dasatinib appears to be related to the peak levels and the side effects to the nadir. In keeping with this observation, once a day therapy with dasatinib is as effective and less toxic than the same dose administered in two daily doses. Though dasatinib targets cells that are more premature than imatinib, it has no action against the CML stem cell. Dasatinib therapy, like imatinib and nilotinib, is needed lifelong. Dasatinib is ineffective against cells carrying the T315I mutation.
Inhibition of Other Kinases
Dasatinib has immunomodulatory effects. Inhibition of PDGF-β receptors may be responsible for the chylous effusion seen with dasatinib.
Dasatinib is rapidly absorbed on oral administration and peak levels are achieved in 0.5-3 hours. Absorption is not affected by food. The solubility of dasatinib decreases with increasing pH. It is highly protein bound (96%). The volume of distribution is large (2505L) indicating extensive extravascular distribution. The concentrations achieved depend on the total dose and schedule. Once a day therapy results in doubling of Cmax and halving of Cmin. Efficacy of dasatinib is related to the average concentration. The toxicity of dasatinib increases with age and with Cmin. These observations suggest that once a day therapy would be efficacious and less toxic and this is borne out in clinical trials (Haematologica. 2010; 95: 232–240). Animal studies show that dasatinib crosses the blood brain barrier and achieves therapeutic concentrations in the CSF. In patients of Ph+ve ALL the CSF levels have ranged from 5-28% of plasma levels and this has correlated with sustained responses in patients with Ph +ve CML in BC or Ph +ve ALL (Blood 2008; 112:1005-12). Dasatinib is metabolized by the liver and the metabolites are eliminated in the feces. There are no active metabolites. Only about 4% of the drug is eliminated in the urine mostly in the form of metabolites. No dose reduction is needed for liver of kidney dysfunction.
Four potential processes at that drugs can interact with dasatinib include absorption, protein binding, drug metabolism and effect of the QT interval.
- Dasatinib and Inhibitors of acid secretion: Famotidine decreases dasatinib exposure by 61%. H-2 antagonists and proton pump inhibitors should not be used with dasatinib. Antacids may be used but an interval of at least 2 hours must separate dasatinib and antacids.
- Protein Binding: Dasatinib is highly protein bound. Displacement from protein binding has the potential tom increase the dasatininb levels. No such interaction has been identified.
- Dasatinib Metabolism: Dasatinib is a substrate for and an inhibits CYP3A4 and dasatinib levels may be affected by inhibitors and inducers of CYP3A4.
- Inhibitors of CYP3A4: Ketoconazole may increase the exposure to dasatinib. Other drugs that may show a similar effects include iraconazole, voraconazole, verapamil, erythromycin, clarythromycin and cyclosporine.
- Inducers of CYP3A4: Rifampicin induces CYP3A4 and reduces exposure to dasatinib by 80%. Phenobarbital, phenytoin, carbamazepine are expected to have the same effect.
- Drugs prolonging QT interval: Dasatinib prolongs the QT interval and should be used with caution with other agents that prolong QT.
- Others: Dasatinib increases the simvastatin by about 20%
- Myelosupression: Myelosupression manifests as neutropenia and /or thrombocytopenia and is more frequent in patients with advanced CML or acute lymphoblastic leukaemia. It is less common when the drug is administered at a dose of 100mg once a day dose than other dosing schedules. It is reversible of withholding the drug.
- Fluid Retention: Fluid retention is seen in about one-third of the patients. About 4-10% have severe fluid retention. The incidence of pleural effusions is lower when the drug is administered at a dose of 100mg once a day.
- Cardiopulmonary toxicity: Cardiopulmonary toxicity manifests as congestive cardiac failure, QT prolongation and pulmonary hypertenssion. Pericardial effusions may be seen.
- Hepatotoxicity: Hepatic toxicity manifests as elevated transaminases.
- Musculoskeletal system: Musculoskeletal toxicity manifests as arthralgia and myalgia and cramps.
- Gastrointestinal toxicity: Gastrointestinal toxicity includes nausea, vomiting, dyspepsia, diarrhoea and gastrointestinal bleed.
- Others: Fatigue, asthenia and anorexia may be seen. Dasatinib use has been associated with hypocalcaemia and hypophosphataemia
- Chronic Myeloid Leukaemia: Dasatinib is indicated for chronic phase, accelerated phase and blast crisis of chronic myeloid leukaemia. It may be used in first line setting or in patients who progress or are intolerant to imatinib. It is not effective in the T515I mutation.
- Ph +ve Acute Lymphoblastic Leukaemia resistant or intolerant to previous TKI therapy.
- Chronic myeloid Leukaemia:
- Chronic Phase: 100mg once a day. The dose may be increased to a maximum of 140mg once a day.
- Acclerated Phase and Blast Crisis: 140mg once a day. The dose may be increased to a maximum of 180mg once a day.
- Ph +ve Acute Lymphoblastic Leukaemia: 140mg once a day. The dose may be increased to a maximum of 180mg once a day.
For dose modification for neutropenia and thrombocytopenia refer to sprycel.com.