Lineage-specific detection of residual disease predicts relapse in patients with chronic myeloid leukemia stopping therapy.

ABSTRACT: Patients with chronic myeloid leukemia who are eligible for treatment-free remission (TFR) may still relapse after tyrosine kinase inhibitor (TKI) cessation. There is a need for accurate predictors of outcome to enable patients with a favorable profile to proceed while avoiding futile attempts. Sensitive detection of residual disease in total leukocytes at treatment cessation is associated with relapse but is not highly discriminatory, likely because it is a composite measure of residual leukemia derived from different cell lineages, whereas only some lineages are relevant for relapse. We prospectively measured BCR::ABL1 DNA as a predictive yes/no binary test in 5 cellular fractions from 48 patients meeting conventional criteria for TKI discontinuation. The median BCR::ABL1 DNA level was higher in granulocytes and T cells, but not in other lineages, in patients who relapsed. Among the 40 patients undergoing their first TFR attempt, we defined 3 groups with differing relapse risk: granulocyte-positive group (100%), granulocyte-negative/T-cell-positive group (67%), and granulocyte-negative /T-cell-negative group (25%). These data show the critical importance of lineage-specific assessment of residual disease in the selection of patients who can attempt to achieve TFR with a high expectation of success and, concurrently, defer patients who have a high probability of relapse.

Solute Carrier Family 29A1 Mediates In Vitro Resistance to Azacitidine in Acute Myeloid Leukemia Cell Lines.

Azacitidine (AZA) is commonly used hypomethylating agent for higher risk myelodysplastic syndromes and acute myeloid leukemia (AML). Although some patients achieve remission, eventually most patients fail AZA therapy. Comprehensive analysis of intracellular uptake and retention (IUR) of carbon-labeled AZA (14C-AZA), gene expression, transporter pump activity with or without inhibitors, and cytotoxicity in naïve and resistant cell lines provided insight into the mechanism of AZA resistance. AML cell lines were exposed to increasing concentrations of AZA to create resistant clones. 14C-AZA IUR was significantly lower in MOLM-13- (1.65 ± 0.08 ng vs. 5.79 ± 0.18 ng; p < 0.0001) and SKM-1- (1.10 ± 0.08 vs. 5.08 ± 0.26 ng; p < 0.0001) resistant cells compared to respective parental cells. Importantly, 14C-AZA IUR progressively reduced with downregulation of SLC29A1 expression in MOLM-13- and SKM-1-resistant cells. Furthermore, nitrobenzyl mercaptopurine riboside, an SLC29A inhibitor, reduced 14C-AZA IUR in MOLM-13 (5.79 ± 0.18 vs. 2.07 ± 0.23, p < 0.0001) and SKM-1-naive cells (5.08 ± 2.59 vs. 1.39 ± 0.19, p = 0.0002) and reduced efficacy of AZA. As the expression of cellular efflux pumps such as ABCB1 and ABCG2 did not change in AZA-resistant cells, they are unlikely contribute to AZA resistance. Therefore, the current study provides a causal link between in vitro AZA resistance and downregulation of cellular influx transporter SLC29A1.

BCR-ABL1 genomic DNA PCR response kinetics during first-line imatinib treatment of chronic myeloid leukemia.

Accurate quantification of minimal residual disease (MRD) during treatment of chronic myeloid leukemia (CML) guides clinical decisions. The conventional MRD method, RQ-PCR for BCR-ABL1 mRNA, reflects a composite of the number of circulating leukemic cells and the BCR-ABL1 transcripts per cell. BCR-ABL1 genomic DNA only reflects leukemic cell number. We used both methods in parallel to determine the relative contribution of the leukemic cell number to molecular response. BCR-ABL1 DNA PCR and RQ-PCR were monitored up to 24 months in 516 paired samples from 59 newly-diagnosed patients treated with first-line imatinib in the TIDEL-II study. In the first three months of treatment, BCR-ABL1 mRNA values declined more rapidly than DNA. By six months, the two measures aligned closely. The expression of BCR-ABL1 mRNA was normalized to cell number to generate an expression ratio. The expression of e13a2 BCR-ABL1 was lower than that of e14a2 transcripts at multiple time points during treatment. BCR-ABL1 DNA was quantifiable in 48% of samples with undetectable BCR-ABL1 mRNA, resulting in MRD being quantifiable for an additional 5-18 months (median 12 months). These parallel studies show for the first time that the rapid decline in BCR-ABL1 mRNA over the first three months of treatment is due to a reduction in both cell number and transcript level per cell, whereas beyond three months, falling levels of BCR-ABL1 mRNA are proportional to the depletion of leukemic cells.

Increased peroxisome proliferator-activated receptor γ activity reduces imatinib uptake and efficacy in chronic myeloid leukemia mononuclear cells.

Imatinib is actively transported by organic cation transporter-1 (OCT-1) influx transporter, and low OCT-1 activity in diagnostic chronic myeloid leukemia blood mononuclear cells is significantly associated with poor molecular response to imatinib. Herein we report that, in diagnostic chronic myeloid leukemia mononuclear cells and BCR-ABL1+ cell lines, peroxisome proliferator-activated receptor γ agonists (GW1929, rosiglitazone, pioglitazone) significantly decrease OCT-1 activity; conversely, peroxisome proliferator-activated receptor γ antagonists (GW9662, T0070907) increase OCT-1 activity. Importantly, these effects can lead to corresponding changes in sensitivity to BCR-ABL kinase inhibition. Results were confirmed in peroxisome proliferator-activated receptor γ-transduced K562 cells. Furthermore, we identified a strong negative correlation between OCT-1 activity and peroxisome proliferator-activated receptor γ transcriptional activity in diagnostic chronic myeloid leukemia patients (n=84; P<0.0001), suggesting that peroxisome proliferator-activated receptor γ activation has a negative impact on the intracellular uptake of imatinib and consequent BCR-ABL kinase inhibition. The inter-patient variability of peroxisome proliferator-activated receptor γ activation likely accounts for the heterogeneity observed in patient OCT-1 activity at diagnosis. Recently, the peroxisome proliferator-activated receptor γ agonist pioglitazone was reported to act synergistically with imatinib, targeting the residual chronic myeloid leukemia stem cell pool. Our findings suggest that peroxisome proliferator-activated receptor γ ligands have differential effects on circulating mononuclear cells compared to stem cells. Since the effect of peroxisome proliferator-activated receptor γ activation on imatinib uptake in mononuclear cells may counteract the clinical benefit of this activation in stem cells, caution should be applied when combining these therapies, especially in patients with high peroxisome proliferator-activated receptor γ transcriptional activity.

Dasatinib targets chronic myeloid leukemia-CD34+ progenitors as effectively as it targets mature cells.

Dasatinib is effective in most chronic phase chronic myeloid leukemia patients both in first-line therapy and following imatinib failure. While imatinib uptake into CD34(+) cells is low compared to mononuclear cells, few data evaluate how well dasatinib targets primitive CML cells. This study compares intracellular concentration of dasatinib and Bcr-Abl kinase inhibition in CML-CD34(+) progenitors and mononuclear cells induced by dasatinib. The intracellular concentrations of dasatinib were similar between CML-CD34(+) and mononuclear cells (P=0.8). Similarly, there was no significant difference in the degree of dasatinib-mediated Bcr-Abl kinase inhibition. ABCB1 (MDR1) and ABCG2 inhibitors neither increased dasatinib intracellular concentration nor enhanced dasatinib-mediated Bcr-Abl kinase inhibition. In contrast to nilotinib, we show that dasatinib is not an ABCB1 inhibitor. Thus, dasatinib targets CML-CD34(+) progenitors as effectively as it targets mononuclear cells. ABCB1 and ABCG2 efflux pumps do not appear to influence the intracellular dasatinib concentration in CML-CD34(+) progenitors.

Adverse outcomes for chronic myeloid leukemia patients with splenomegaly and low in vivo kinase inhibition on imatinib.

Variability in the molecular response to frontline tyrosine kinase inhibitor (TKI) therapy in chronic myeloid leukemia may be partially driven by differences in the level of kinase inhibition induced. We measured in vivo BCR::ABL1 kinase inhibition (IVKI) in circulating mononuclear cells after 7 days of therapy. In 173 patients on imatinib 600 mg/day, 23% had low IVKI (<11% reduction in kinase activity from baseline); this was associated with higher rates of early molecular response (EMR) failure; lower rates of major molecular response (MMR), and MR4.5 by 36 months, compared to high IVKI patients. Low IVKI was more common (39%) in patients with large spleens (≥10 cm by palpation). Notably 55% of patients with large spleens and low IVKI experienced EMR failure whereas the EMR failure rate in patients with large spleens and high IVKI was only 12% (p = 0.014). Furthermore, patients with large spleen and low IVKI had a higher incidence of blast crisis, inferior MMR, MR4.5, and event-free survival compared to patients with large spleen and high IVKI and remaining patients. In nilotinib-treated patients (n = 73), only 4% had low IVKI. The combination of low IVKI and large spleen is associated with markedly inferior outcomes and interventions in this setting warrant further studies.

Chronic phase chronic myeloid leukemia patients with low OCT-1 activity randomized to high-dose imatinib achieve better responses and have lower failure rates than those randomized to standard-dose imatinib.

BACKGROUND: The functional activity of the organic cation transporter 1 (OCT-1) protein (OCT-1 activity) is an excellent predictor of molecular response and progression-free survival in patients with newly diagnosed chronic phase chronic myeloid leukemia treated with imatinib as front-line therapy. DESIGN AND METHODS: In this study the predictive value of OCT-1 activity in patients treated with imatinib 400 mg/day or 800 mg/day was evaluated in relation to trough imatinib plasma levels assessed in 100 patients enrolled in the Tyrosine Kinase Inhibitor Optimization and Selectivity (TOPS) trial. RESULTS: The rate of major molecular responses by 24 months in patients on imatinib 400 mg/day was significantly higher in those with high OCT-1 activity than in those with low OCT-1 activity (low OCT-1 activity, 57% of patients; high OCT-1 activity, 100%; P < 0.001); the corresponding difference in patients treated with imatinib 800 mg/day did not reach statistical significance (low OCT-1 activity, 68%; high OCT-1 activity, 95%; P = 0.073). In addition, the combination of low trough imatinib levels (< 1200 ng/mL) and low OCT-1 activity defined a group of patients who had the lowest rates of major molecular response (47%) by 24 months compared to all other patients (81%, P = 0.009). These patients were also at the highest risk of failed imatinib therapy when compared to all other patients (P<0.001). CONCLUSIONS: High-dose imatinib leads to superior molecular responses in patients with low OCT-1 activity. In this group trough imatinib levels may define a group with inferior outcomes. Among patients with high OCT-1 activity, neither higher imatinib dose nor monitoring imatinib trough levels was found to be of significant clinical value. Hence OCT-1 activity determined prior to the start of therapy in newly diagnosed CML patients provides a valuable prognostic tool to determine the optimal up-front dose of imatinib in patients with newly diagnosed chronic phase chronic myeloid leukemia.

Widespread Aberrant Alternative Splicing despite Molecular Remission in Chronic Myeloid Leukaemia Patients.

Vast transcriptomics and epigenomics changes are characteristic of human cancers, including leukaemia. At remission, we assume that these changes normalise so that omics-profiles resemble those of healthy individuals. However, an in-depth transcriptomic and epigenomic analysis of cancer remission has not been undertaken. A striking exemplar of targeted remission induction occurs in chronic myeloid leukaemia (CML) following tyrosine kinase inhibitor (TKI) therapy. Using RNA sequencing and whole-genome bisulfite sequencing, we profiled samples from chronic-phase CML patients at diagnosis and remission and compared these to healthy donors. Remarkably, our analyses revealed that abnormal splicing distinguishes remission samples from normal controls. This phenomenon is independent of the TKI drug used and in striking contrast to the normalisation of gene expression and DNA methylation patterns. Most remarkable are the high intron retention (IR) levels that even exceed those observed in the diagnosis samples. Increased IR affects cell cycle regulators at diagnosis and splicing regulators at remission. We show that aberrant splicing in CML is associated with reduced expression of specific splicing factors, histone modifications and reduced DNA methylation. Our results provide novel insights into the changing transcriptomic and epigenomic landscapes of CML patients during remission. The conceptually unanticipated observation of widespread aberrant alternative splicing after remission induction warrants further exploration. These results have broad implications for studying CML relapse and treating minimal residual disease.

Lineage of measurable residual disease in patients with chronic myeloid leukemia in treatment-free remission.

Approximately half of patients with chronic myeloid leukemia (CML) in sustained deep molecular response who discontinue tyrosine kinase inhibitors (TKIs) remain in treatment-free remission (TFR). Some of these patients have measurable residual disease (MRD) by BCR-ABL1 mRNA testing, and most have detectable BCR-ABL1 DNA by highly sensitive methods. We used fluorescence-activated cell sorting and BCR-ABL1 DNA PCR to investigate the lineage of residual CML cells in TFR. Twenty patients in TFR for >1 year provided blood for sorting into granulocytes, monocytes, B cells, T cells, and NK cells. MRD was identified predominantly in the lymphoid compartment and never in granulocytes. B cells were more often BCR-ABL1 positive than T cells (18 vs 11/20 patients) and at higher levels (median 10-4.9 vs 10-5.7; P = 0.014). In 13 CML patients studied at diagnosis lymphocytes expressing BCR-ABL1 mRNA comprised a small proportion of total leukocytes. These data improve our understanding of TFR biology, since it is now clear that MRD in the blood of TFR patients need not imply the persistence of multipotent CML cells. Lineage-specific assessment of MRD could be explored as a means to improve the prediction of TFR.

Gene expression signature that predicts early molecular response failure in chronic-phase CML patients on frontline imatinib.

In chronic-phase chronic myeloid leukemia (CP-CML) patients treated with frontline imatinib, failure to achieve early molecular response (EMR; EMR failure: BCR-ABL1 >10% on the international scale at 3 months) is predictive of inferior outcomes. Identifying patients at high-risk of EMR failure at diagnosis provides an opportunity to intensify frontline therapy and potentially avoid EMR failure. We studied blood samples from 96 CP-CML patients at diagnosis and identified 365 genes that were aberrantly expressed in 13 patients who subsequently failed to achieve EMR, with a gene signature significantly enriched for stem cell phenotype (eg, Myc, ß-catenin, Hoxa9/Meis1), cell cycle, and reduced immune response pathways. We selected a 17-gene panel to predict EMR failure and validated this signature on an independent patient cohort. Patients classified as high risk with our gene expression signature (HR-GES) exhibited significantly higher rates of EMR failure compared with low-risk (LR-GES) patients (78% vs 5%; P < .0001), with an overall accuracy of 93%. Furthermore, HR-GES patients who received frontline nilotinib had a relatively low rate of EMR failure (10%). However, HR-GES patients still had inferior deep molecular response achievement rate by 24 months compared with LR-GES patients. This novel multigene signature may be useful for selecting patients at high risk of EMR failure on standard therapy who may benefit from trials of more potent kinase inhibitors or other experimental approaches.

The new allosteric inhibitor asciminib is susceptible to resistance mediated by ABCB1 and ABCG2 overexpression in vitro.

Asciminib (previously ABL001), which binds the myristate-binding pocket of the Bcr-Abl kinase domain, is in phase I clinical trials as monotherapy and in combination with imatinib, nilotinib and dasatinib for the treatment of patients with refractory CML or Ph+ ALL. Asciminib sensitivity was evaluated in asciminib naïve BCR-ABL1+ cell lines K562 (negligible ABCB1/ABCG2 expression), K562-Dox (ABCB1-overexpressing through doxorubicin exposure) and K562-ABCG2 (ABCG2 overexpression via transduction) with results demonstrating asciminib efflux by both ABCB1 and ABCG2 transporters. K562-Dox and K562-ABCG2 cells demonstrated increased LD50asciminib vs K562 control cells: 256 and 299 nM respectively vs 24 nM, p < 0.001. Sensitivity was completely restored with specific inhibitors cyclosporine (ABCB1) and Ko143 (ABCG2): K562-Dox LD50asciminib+cyclosporine = 13 nM, K562-ABCG2 LD50asciminib+Ko143 = 15 nM (p < 0.001). When asciminib resistance was modelled in vitro, ABCB1 and ABCG2 overexpression was integral in the development of asciminib resistance. In K562 asciminib-resistant cells, ABCG2 expression increased prior to BCR-ABL1 overexpression and remained high (up to 7.6-fold greater levels in resistant vs control cells, p < 0.001). K562-Dox asciminib-resistant cells had increased ABCB1 expression (2.1-fold vs control cells p = 0.0033). KU812 asciminib-resistant cells overexpressed ABCB1 (5.4-fold increase, p < 0.001) and ABCG2 (6-fold increase, p < 0.001) before emergence of a novel myristate-binding pocket mutation (F497L). In all three cell lines, asciminib resistance was reversible upon chemical inhibition of ABCB1, ABCG2 or both (p < 0.001). When K562 asciminib-resistant cells were treated with asciminib in combination with clinically achievable doses of either imatinib or nilotinib, reversal of the resistance phenotype was also observed (p < 0.01). Overexpression of efflux transporters will likely be an important pathway for asciminib resistance in the clinical setting. Given the lack of evidence for ABCG2-mediated transport of nilotinib or imatinib at clinically relevant concentrations, our data provide an additional rationale for using asciminib in combination with either TKI.

Long-term treatment-free remission of chronic myeloid leukemia with falling levels of residual leukemic cells.

Following the achievement of deep molecular response on tyrosine kinase inhibitors (TKIs), approximately half of patients with chronic myeloid leukemia (CML) can discontinue TKI and remain in treatment-free remission (TFR). The ALLG CML8 study enrolled 40 imatinib-treated patients with undetectable BCR-ABL1 mRNA (approximately MR4.5). Molecular relapse was defined as detectable BCR-ABL1 on two consecutive tests or any single value >0.1%. With a median follow-up of 8.6 years (range 5.7-11.2 years), 18 patients remain in continuous TFR (45.0%; 95% confidence interval 31.9-63.4%). The latest relapse detected was 27 months after stopping imatinib. No patient progressed to advanced phase. Twenty-two patients met criteria for imatinib re-treatment and all regained undetectable molecular response. Nine patients in long-term TFR were monitored by highly sensitive individualized BCR-ABL1 DNA PCR in a sufficient number of samples to enable more precise quantification of residual leukemia. BCR-ABL1 DNA decreased from a median of MR5.0 in the first year of TFR to MR6.1 in the sixth year of TFR. Our results support the long-term safety and remarkable stability of response after imatinib discontinuation in appropriately selected CML patients. Serial high sensitivity testing provides a new and unexpected finding of gradually reducing CML cells in patients in long-term TFR.

A Method for Next-Generation Sequencing of Paired Diagnostic and Remission Samples to Detect Mitochondrial DNA Mutations Associated with Leukemia.

Somatic mitochondrial DNA (mtDNA) mutations have been identified in many human cancers, including leukemia. To identify somatic mutations, it is necessary to have a control tissue from the same individual for comparison. When patients with leukemia achieve remission, the remission peripheral blood may be a suitable and easily accessible control tissue, but this approach has not previously been applied to the study of mtDNA mutations. We have developed and validated a next-generation sequencing approach for the identification of leukemia-associated mtDNA mutations in 26 chronic myeloid leukemia patients at diagnosis using either nonhematopoietic or remission blood samples as the control. The entire mt genome was amplified by long-range PCR and sequenced using Illumina technology. Variant caller software was used to detect mtDNA somatic mutations, and an empirically determined threshold of 2% was applied to minimize false-positive results because of sequencing errors. Mutations were called against both nonhematopoietic and remission controls: the overall concordance between the two approaches was 81% (73/90 mutations). Some discordant results were because of the presence of somatic mutations in remission samples, because of either minimal residual disease or nonleukemic hematopoietic clones. This method could be applied to study somatic mtDNA mutations in leukemia patients who achieve minimal residual disease, and in patients with nonhematopoietic cancers who have a matched uninvolved tissue available.

Degree of kinase inhibition achieved in vitro by imatinib and nilotinib is decreased by high levels of ABCB1 but not ABCG2.

Imatinib and nilotinib interact with ABCB1 and ABCG2. However, whether they are substrates or inhibitors is a source of conjecture. Here, in vitro, Bcr-Abl kinase inhibition was used to elucidate the impact of ABCB1/ABCG2 overexpression on imatinib and nilotinib transport. High levels of ABCB1 protein in K562-Dox cells resulted in a significantly increased 50% inhibitory concentration (IC(50)) compared with parental K562 cells for imatinib (IC(50)(IM); 9 µM to 19 µM, p = 0.002) and nilotinib (IC(50)(NIL); 345 nM to 620 nM, p = 0.013). This difference was abrogated by ABCB1 inhibitors. However, overexpression of ABCG2 did not significantly increase IC(50)(IM) or IC(50)(NIL) or significantly decrease IC(50) upon ABCG2 inhibition. Inhibition of ABCB1 but not ABCG2 resulted in a substantial increase in intracellular nilotinib when used at 150 nM but no increase when used at 2 µM. Imatinib and nilotinib appear to be transported by ABCB1 but do not interact strongly with ABCG2. Furthermore, ABCB1 efflux of nilotinib may be concentration-dependent with transport occurring at clinically relevant concentrations.

Functional activity of the OCT-1 protein is predictive of long-term outcome in patients with chronic-phase chronic myeloid leukemia treated with imatinib.

PURPOSE: Organic cation transporter-1 (OCT-1) activity (OA), a measure of the OCT-1-mediated influx of imatinib into CML mononuclear cells (MNCs), is predictive of major molecular response (MMR) at 12 and 24 months in patients with untreated CML. We now report the impact of OA on loss of response, disease transformation, and survival after 5 years of imatinib. PATIENTS AND METHODS: OA is defined as the difference in intracellular concentration of carbon-14-imatinib with and without OCT-1 inhibition. OA was measured in blood from 56 patients with untreated chronic-phase CML. RESULTS: More patients who had high OA (ie, > median OA value) achieved MMR by 60 months compared with patients who had low OA (89% v 55%; P = .007). A low OA was associated with a significantly lower overall survival (87% v 96%; P = .028) and event-free survival (EFS; 48% v 74%; P = .03) as well as a higher kinase domain mutation rate (21% v 4%; P = .047). These differences were highly significant in patients who averaged less than 600 mg/d of imatinib in the first 12 months but were not significant in patients averaging >/= 600 mg/d. Patients with very low OA (ie, quartile 1) were the only group who developed leukemic transformation (21% in quartile 1 v 0% in all other quartiles; P = .002). CONCLUSION: Measurement of OA pretherapy is a predictor for the long-term risk of resistance and transformation in patients with imatinib-treated CML. Early dose-intensity may reduce the negative prognostic impact of low OA. We propose that OA could be used to individualize dosage strategies for patients with CML to maximize molecular response and optimize long-term outcome.

Most CML patients who have a suboptimal response to imatinib have low OCT-1 activity: higher doses of imatinib may overcome the negative impact of low OCT-1 activity.

Interpatient variability in intracellular uptake and retention (IUR) of imatinib may be due to variable function of the OCT-1 influx pump. OCT-1 activity was measured in pretherapy blood from chronic myeloid leukemia (CML) patients by calculating the difference in IUR of [(14)C]-imatinib with and without OCT-1 inhibition. Of patients with higher than median (high) OCT-1 activity, 85% achieved major molecular response (MMR) by 24 months, versus 45% with no more than a median (low) OCT-1 activity. Assessing patients receiving 600 mg imatinib per day and those averaging fewer than 600 mg over 12 months of therapy revealed patients with high OCT-1 activity achieved excellent molecular response regardless of dose, whereas response of patients with low OCT-1 activity was highly dose dependent. Of patients with low OCT-1 activity who received fewer than 600 mg, 45% failed to achieve a 2-log reduction by 12 months, and 82% failed to achieve a MMR by 18 months, compared with 8% and 17% in the cohort with high OCT-1 activity and dose less than 600 mg/day (P = .017 and P = .022). OCT-1 activity is an important determinant of molecular response to imatinib, with predictive value closely linked to dose. This pretherapy assay identifies patients at greatest risk of suboptimal response where dose intensity is critical, and those likely to respond equally well to standard dose imatinib.

OCT-1-mediated influx is a key determinant of the intracellular uptake of imatinib but not nilotinib (AMN107): reduced OCT-1 activity is the cause of low in vitro sensitivity to imatinib.

Intrinsic sensitivity of newly diagnosed chronic myeloid leukemia (CML) patients to imatinib (IC50(imatinib)) correlates with molecular response. IC50(imatinib) is defined as the in vitro concentration of drug required to reduce phosphorylation of the adaptor protein Crkl by 50%. We now show that interpatient variability in IC50(imatinib) is mainly due to differences in the efficiency of imatinib intracellular uptake and retention (IUR). In 25 untreated CML patients, the IC50(imatinib) strongly correlated (R (2) = -0.484, P = .014 at 2 muM imatinib) with the IUR of [(14)C]imatinib. The addition of prazosin, a potent inhibitor of OCT-1 cellular transporter, reduced the IUR and eliminated interpatient variability. IC50 values for the more potent BCR-ABL inhibitor nilotinib (AMN107) did not correlate with IC50(imatinib) (R(2) =-0.0561, P > .05). There was also no correlation between IC50(nilotinib) and the IUR for [(14)C]nilotinib (R (2) = 0.457, P > .05). Prazosin had no effect on nilotinib IUR, suggesting that influx of nilotinib is not mediated by OCT-1. In conclusion, whereas OCT-1-mediated influx may be a key determinant of molecular response to imatinib, it is unlikely to impact on cellular uptake and patient response to nilotinib. Determining interpatient and interdrug differences in cellular uptake and retention could allow individual optimization of kinase inhibitor therapy.