Molecular Predictors for the Response to Nilotinib Treatment After Acquired Imatinib Failure In Ph+ Chronic Myeloid Leukemia

A switch to the 2nd generation tyrosine kinase inhibitor nilotinib has been proven to be effective in case of resistance or intolerance to imatinib for the majority of patients with Ph+ chronic myeloid leukemia (CML) in chronic phase (CP). Besides mutations in the BCR-ABL kinase domain and various BCR-ABL-independent mechanisms, e.g. clonal evolution and activation of pathways bypassing BCR-ABL, efficacy of imatinib and nilotinib are dependent on intracellular drug levels, which are influenced by the activity of the efflux transporter protein multidrug resistance 1 (MDR1). Cell culture data suggest overexpression of MDR1 as a cause of resistance to nilotinib (Mahon et al., Cancer Res 2008). Moreover, Dulucq et al. (Blood 2008) report the association of MDR1 single nucleotide polymorphisms (SNPs) with the chance to achieve major molecular response (MMR) on first-line imatinib therapy.

In order to allow risk stratification, we sought to elucidate molecular markers, e.g. MDR1 gene expression, BCR-ABL transcript burden, BCR-ABL mutation status and common SNPs in the MDR1 gene regarding their potency to predict therapy-related endpoints, such as MMR, complete cytogenetic response (CCyR), and progression-free survival (PFS) during second line therapy with nilotinib in imatinib-resistant CML-CP patients.

A cohort of 83 imatinib-resistant patients in chronic phase CML treated with nilotinib was investigated within the AMN107A2101 phase I/II trial. Baseline BCR-ABL mutations were detected by D-HPLC and direct sequencing. MDR1 and BCR-ABL mRNA expression levels were determined by qRT-PCR using LightCycler™ technology, normalized against beta-glucuronidase (GUS) expression. BCR-ABL levels were standardized according to the international scale (IS). MDR1 SNPs (1236C>T, 2677G>T) were investigated using conventional sequencing. Log-rank tests were performed to compare the time to MMR (BCR-ABL IS ≤0.1%), CCyR, and PFS.

By 12 or 24 months, patients with MDR1/GUS ratios ≥2 achieved MMR in an estimated rate of 34%, CCyR was attained in rates of 53% and 58%, and PFS rates were 88% and 75%, whereas those with initial MDR1/GUS ratios <2 showed MMR in 13% (p=0.026), CCyR in 25% and 35% (p=0.028), and PFS in 71% and 50% (p=0.027), respectively (Table). Further, combining BCR-ABL load and MDR1 expression prior to nilotinib produced a significant association with subsequent molecular and cytogenetic response and PFS. By 12 or 24 months, patients presenting with BCR-ABL IS <13% and MDR1/GUS ≥2 showed MMR in 56%, CCyR in 75% and 81% and longer PFS in 93% and 85% compared to those with BCR-ABL IS ≥13% and MDR1/GUS <2 achieving MMR rates of 5% (p<0.001), CCyR rates of 26% and 38% (p=0.002) and PFS rates of 72% and 41%, respectively (p=0.013). Also, BCR-ABL kinase domain mutation status prior to the onset of nilotinib was significantly associated with PFS under nilotinib therapy: Patients without any mutation showed PFS rates of 90% and 71% at 12 or 24 months, respectively; whereas those harboring less sensitive or resistant mutations (Hughes et al., JCO 2009) attained only 26% PFS (p=0.028). Except for patients harboring less sensitive or resistant mutations there was no significant difference between unmutated and mutated patients. 31 of 39 patients revealed SNPs at the 1236 position and 33 of the 39 showed SNPs at the 2677 position. Nilotinib efficacy was not affected by MDR1 SNPs. Neither an association with the molecular outcome nor MDR1 gene expression under nilotinib therapy was found.

The rates of MMR, CCyR, and PFS on nilotinib 2nd line therapy are significantly associated with baseline levels of MDR1 gene expression and tumor burden of imatinib-resistant CML-CP patients. Further, baseline BCR-ABL mutations predict the probability of PFS rates but do not seem to do so for MMR and CCyR. These findings might deliver new molecular tools for the clinical risk stratification for treatment with nilotinib in case of resistance under imatinib front line therapy and suggest the need for prospective validation.

Reynolds: Novartis Oncology: Employment, Equity Ownership. Woodman: Novartis Oncology: Employment. Robin: Novartis Oncology: Employment. Hofmann: Novartis Oncology: Honoraria, Research Funding. Hochhaus: Novartis Oncology: Honoraria, Research Funding. Müller: Novartis Oncology: Honoraria, Research Funding.