Clinical outcomes of second-generation tyrosine kinase inhibitors versus imatinib in older patients with CML.

Age and comorbidities are important factors to be considered in the selection of tyrosine kinase inhibitors (TKIs) for first-line treatment in patients with chronic myeloid leukemia in chronic phase (CML-CP). However, it is yet unclear whether TKI selection, particularly, imatinib versus second-generation TKIs (2GTKIs), impacts treatment outcomes in the clinical practice. To address this, we compared the clinical outcomes of prospectively registered 452 patients with CML-CP treated with imatinib and 2GTKIs, taking into consideration their age and/or comorbidities. A total of 136 patients (30.1%) were classified into an older cohort (≥65 years) and 316 (69.9%) into a younger cohort (18-64 years). The TKI selection did not vary based on age (70.6% received 2GTKIs in the younger cohort and 66.2% in the older cohort). The median follow-up period was 5.4 years. Treatment responses including the cumulative incidence of deep molecular response (BCR-ABL1 international scale ≤0.0032%) at any time were similar between the two age cohorts regardless of the type of TKI. The 5-year overall survival (OS) in the older cohort was lower than that in the younger cohort (95.9% vs 83.8%; p < 0.0001), whereas the 5-year OS in patients treated with 2GTKIs was not influenced by age factors and comorbidities. Therefore, our results suggest that the selection of 2GTKIs as first-line treatment is an effective option for both younger and older CML-CP patients with or without comorbidities. This trial was registered at UMIN-CTR as 00003581.

A longitudinal analysis of paroxysmal nocturnal haemoglobinuria-type cells in patients with bone marrow failure: Results of a prospective multi-centre study in Japan.

To determine the prevalence and clinical relevance of glycosylphosphatidylinositol-anchored protein-deficient (GPI[-]) cell populations (paroxysmal nocturnal haemoglobinuria [PNH]-type cells) in patients with acquired aplastic anaemia (AA) or myelodysplastic syndrome (MDS), we prospectively studied peripheral blood samples of 2402 patients (1075 with AA, 900 with MDS, 144 with PNH, and 283 with other anaemia) using a high-sensitivity flow cytometry assay in a nationwide multi-centre observational study. PNH-type cells were detected in 52.6% of AA and 13.7% of MDS patients. None of the 35 patients with refractory anaemia (RA) with ringed sideroblasts or the 86 patients with RA with excess of blasts carried PNH-type cells. Among the 317 patients possessing PNH-type granulocytes, the percentage of PNH-type granulocytes increased by ≥10% in 47 patients (14.8%), remained unchanged in 240 patients (75.7%), and decreased by ≥10% in 30 patients (9.5%) during 3 years of follow-up. PNH-type granulocyte expansion occurred more frequently (27.1%) in the 144 patients who originally carried PNH-type granulocytes ≥1% than in the 173 patients with PNH-type granulocytes <1% (4.6%). This study confirmed that PNH-type cells are undetectable in authentic clonal MDS patients, and the presence of ≥1% PNH-type granulocytes predicts a higher likelihood of PNH-type cell expansion than with <1% PNH-type granulocytes.

Artificial cerebrospinal fluid restores aspirin-inhibited physiological hemostasis through recovery of platelet aggregation function.

BACKGROUND: Optimal hemostasis provides safety and reliability during neurosurgery which improves surgical outcomes. Previously, artificial cerebrospinal fluid (aCSF) and its component sodium bicarbonate were found to facilitate physiological hemostasis by amplifying platelet aggregation. This study aimed to verify whether aCSF amplifies platelet-dependent hemostasis in the presence of antiplatelet agents. METHODS: We prepared platelet-rich plasma (PRP) or washed platelets using aspirin (acetylsalicylic acid, (ASA)) or normal saline (NS). We evaluated samples treated with a commercially available aCSF solution or NS for amplification of aggregation, activation of integrin αIIbß3, phosphatidylserine (PS) exposure, P-selectin (CD62P) expression, and formation of microparticles (MPs). We assessed the effect of aCSF on in vivo hemostasis in the presence of ASA by measuring the tail bleeding time in ASA-or NS-injected C57BL/6 N mice. RESULTS: Compared with NS, aCSF amplified ASA-inhibited platelet aggregation by recovering platelet activation including PS exposure, MP release, CD62P expression, and integrin αIIbß3 activation. When using washed platelets, aCSF almost completely counteracted the inhibition of platelet aggregation by ASA. Prolonged bleeding time from the amputated tail of ASA-injected mice was significantly shortened by the treatment with aCSF compared to NS. Sodium bicarbonate also directly amplified ASA-inhibited platelet aggregation. CONCLUSIONS: aCSF and sodium bicarbonate facilitate physiological hemostasis through the recovery of inhibited platelet aggregation even in the presence of ASA. The utilization of aCSF in the operative field may be advantageous for facilitating hemostasis in patients with impaired platelet function and contribute to improving outcomes of neurosurgery.

A SARS-CoV-2 Delta variant containing mutation in the probe binding region used for RT-qPCR test in Japan exhibited atypical PCR amplification and might induce false negative result.

INTRODUCTION: A recent pandemic of SARS-CoV-2 infection has caused severe health problems and substantially restricted social and economic activities. RT-qPCR plays a vital role in the diagnosis of SARS-CoV-2 infection. The N protein-coding region is widely analyzed in RT-qPCR to diagnose SARS-CoV-2 infection in Japan. We recently encountered two cases of SARS-CoV-2-positive specimens showing atypical amplification curves in the RT-qPCR. METHODS: We performed whole-genome sequencing of 63 samples (2 showing aberrant RT-qPCR curve and 61 samples infected with SARS-CoV-2 simultaneously in the same area) followed by Phylogenetic tree analysis. RESULTS: We found that the viruses showing abnormal RT-qPCR curves were Delta-type variants of SARS-CoV-2 with a single nucleotide mutation in the probe-binding site. There were no other cases with the same mutation, indicating that the variant had not spread in the area. After searching the database, hundreds of variants were reported globally, and one in Japan contained the same mutation. Phylogenetic analysis showed that the variant was very close to other Delta variants endemic in Japan but quite far from the variants containing the same mutation reported from outside Japan, suggesting sporadic generation of mutant in some domestic areas. CONCLUSIONS: These findings propose two key points: i) mutations in the region used for SARS-CoV-2 RT-qPCR can cause abnormal amplification curves, and ii) various mutations can be generated sporadically and unpredictably; therefore, efficient and robust screening systems are needed to promptly monitor the emergence of de novo variants.

The clinical significance of PNH-phenotype cells accounting for < 0.01% of total granulocytes detected by the Clinical and Laboratory Standards Institute methods in patients with bone marrow failure.

Small populations of glycosylphosphatidylinositol-anchored protein-deficient (GPI[-]) cells accounting for up to 0.01% of total granulocytes can be accurately detected by a high-sensitivity flow cytometry (FCM) assay established by the Clinical and Laboratory Standards Institute (CLSI method) and have a prognostic value in bone marrow failure (BMF); however, the significance of GPI(-) granulocytes accounting for 0.001-0.009% of granulocytes remains unclear. To clarify this issue, we examined the peripheral blood of 21 BMF patients in whom minor (around 0.01%) populations of GPI(-) granulocytes had been previously detected by a different high-resolution FCM method (OPTIMA method, which defines ≥ 0.003% GPI(-) granulocytes as an abnormal increase) using both the CLSI and OPTIMA methods simultaneously. These two methods detected an "abnormal increase" in GPI(-) granulocytes in 10 patients (48%) and 17 patients (81%), respectively. CLSI detected 0.002-0.005% (median, 0.004%) GPI(-) granulocytes in 7 patients who were deemed positive for PNH-type cells according to the OPTIMA method, which detected 0.003-0.012% (median 0.006%) GPI(-) granulocytes. The clone sizes of GPI(-) cells detected by each assay were positively correlated (r = 0.994, p < 0.001). Of the seven patients who were judged positive for PNH-type cells by OPTIMA alone, five received immunosuppressive therapy, and all of them achieved a partial or complete response. GPI(-) granulocytes detected in BMF patients by the CLSI method should thus be considered significant, even at percentages of < 0.01%.

Prognostic effect of comorbidities in patients with chronic myeloid leukemia treated with a tyrosine kinase inhibitor.

Comorbidities at diagnosis among patients with chronic myeloid leukemia in chronic phase (CML-CP) may affect their overall survival (OS) rate even in the tyrosine kinase inhibitor (TKI) era. However, the prognostic impact of comorbidities in patients with CML-CP treated with a second-generation TKI (2GTKI) has not been elucidated. We evaluated the effect of comorbidities on survival using the Charlson Comorbidity Index (CCI) in patients with CML-CP treated with imatinib or a 2GTKI (nilotinib and dasatinib). From April 2010 to March 2013, 506 patients with CML-CP were registered for the population-based cohort study, and 452 with a median age of 56 y were assessable. Treatment groups included 139 patients receiving imatinib, 169 receiving nilotinib, and 144 receiving dasatinib. Comorbidities were diagnosed in 99 patients. CCI scores were stratified as follows: 2, 353 patients; 3, 72 patients; and ≥4, 27 patients. Treatment response did not vary relative to CCI scores. However, across the entire cohort, the OS rate was significantly lower among patients with higher CCI scores than in those with a CCI score of 2 (94.4% in score 2, 89.0% in score 3, and 72.8% in score ≥4; P < .001). Multivariate analysis identified a CCI score of ≥4 as a strong adverse prognostic factor for OS rather than the disease-specific risk factor, older age, performance status, or selection of TKI (Wald test, P < .01). Our results demonstrated that comorbidities at diagnosis were the most important predictive factor for successful treatment, regardless of the TKI type used in CML-CP. This trial was registered at UMIN-CTR as 00003581.

Intronic Alternative Polyadenylation in the Middle of the DMD Gene Produces Half-Size N-Terminal Dystrophin with a Potential Implication of ECG Abnormalities of DMD Patients.

The DMD gene is one of the largest human genes, being composed of 79 exons, and encodes dystrophin Dp427m which is deficient in Duchenne muscular dystrophy (DMD). In some DMD patient, however, small size dystrophin reacting with antibody to N-terminal but not to C-terminal has been identified. The mechanism to produce N-terminal small size dystrophin remains unknown. Intronic polyadenylation is a mechanism that produces a transcript with a new 3' terminal exon and a C-terminal truncated protein. In this study, intronic alternative polyadenylation was disclosed to occur in the middle of the DMD gene and produce the half-size N-terminal dystrophin Dp427m, Dpm234. The 3'-rapid amplification of cDNA ends revealed 421 bp sequence in the downstream of DMD exon 41 in U-251 glioblastoma cells. The cloned sequence composing of the 5' end sequence of intron 41 was decided as the terminal exon, since it encoded poly (A) signal followed by poly (A) stretch. Subsequently, a fragment from DMD exon M1 to intron 41 was obtained by PCR amplification. This product was named Dpm234 after its molecular weight. However, Dpm234 was not PCR amplified in human skeletal and cardiac muscles. Remarkably, Dpm234 was PCR amplified in iPS-derived cardiomyocytes. Accordingly, Western blotting of cardiomyocyte proteins showed a band of 234 kDa reacting with dystrophin antibody to N-terminal, but not C-terminal. Clinically, DMD patients with mutations in the Dpm234 coding region were found to have a significantly higher likelihood of two ECG abnormal findings. Intronic alternative splicing was first revealed in Dp427m to produce small size dystrophin.

Identification of the shortest splice variant of Dp71, together with five known variants, in glioblastoma cells.

BACKGROUND: Dystrophin Dp71 mRNA is produced from the most distal alternative promoter of the DMD gene, mutations in which cause Duchenne muscular dystrophy (DMD). Dp71 is characterized by a wide variety of splice variants. In addition to being associated with cognitive disturbance in patients with DMD, Dp71 may also play a role in tumorigenesis. This study analyzed Dp71 transcripts in glioblastoma, the most common and most lethal type of cerebral malignancy. METHODS: Dp71 mRNA in the U-251 glioblastoma cell line was analyzed by reverse-transcription polymerase chain reaction (RT-PCR). The amplified products were subcloned and sequenced. RESULTS: RT-PCR amplification of the 5' end of the Dp71 transcript yielded a product of expected size, indicating transcription from the Dp71 promoter in glioblastoma. Amplification of full-length Dp71, from exon G1 to DMD exon 79, yielded a product of expected size, as well as a faint, smaller sized band. Sequencing of 17 clones revealed six different alternatively spliced variants, with only one clone being of full-length Dp71 containing all 18 exons. Ten clones lacked exon 78 (Dp71b), indicating that Dp71b was a major type of Dp71 in glioblastoma. In addition, three clones lacked both exons 71 and 78 (Dp71ab), one clone lacked exons 71, 73 and 78 (Dp71ab △73), one clone lacked exons 71-74 and 78 (Dp71bc), and one clone lacked exons 68-76 and 78 (Dp71b△68-76). This novel transcript was the shortest Dp71 variant, with a predicted stop codon in exon 77 and was predicted to produce a 24.8 kDa protein, consisting of 216 amino acids including 15 amino acids from exon 77. This novel product was classified as Dp71g because of its unique C-terminal amino acid sequence. CONCLUSIONS: Six splice variants of Dp71 were identified in glioblastoma cells, with Dp71b being the most abundant. Deletion of exon 78 was an apparent default splicing pathway in glioblastoma, being observed in 16 of 17 clones. Glioblastoma cells contained the shortest Dp71 transcript (Dp71b△68-76) identified to date, with a unique C-terminal amino acid sequence. These findings suggest the need to assess the function of Dp71 variants in glioblastoma.

Essential human resources for antimicrobial stewardship teams in Japan: Estimates from a nationwide survey conducted by the Japanese Society of Chemotherapy.

Implementation of antimicrobial stewardship programs (ASPs) with multidisciplinary antimicrobial stewardship teams (ASTs) is critical for appropriate antimicrobial use at healthcare facilities. Although the Japanese medical reimbursement system was revised to allow fees for ASP implementation, several concerns remain, including understaffing and enforcement of the recommendations on ASTs and ASPs in practice. Furthermore, there are no recommendations on full-time equivalents (FTEs) of the core members in ASTs in Japan. This committee report presents our recommendations on ASTs based on an analysis of the nationwide survey on implemented ASPs and staff FTEs at 1358 healthcare facilities conducted by the Japanese Society of Chemotherapy. Our report provides a directive for structural and financial support of ASTs and should aid in planning for the enhancement of AST practices and the organization of new ASTs.

Establishment of a flow cytometry assay for detecting paroxysmal nocturnal hemoglobinuria-type cells specific to patients with bone marrow failure.

Minor populations of glycosylphosphatidylinositol-anchored protein-deficient (GPI[-]) cells in the peripheral blood may have a prognostic value in bone marrow failure (BMF). Our objective is to establish the optimal flow cytometry (FCM) assay that can discriminate GPI(-) populations specific to BMF from those of healthy individuals. To identify a cut-off that discriminates GPI(-) rare cells from GPI(+) cells, we determined a position of the borderline that separates the GPI(-) from GPI(+) cells on a scattergram by testing more than 30 healthy individuals, such that no GPI(-) dot fell into the upper left quadrant where fluorescein-labeled aerolysin (FLAER)-CD11b+ granulocytes and CD55-CD59- glycophorin A+ erythrocytes were positioned. This method allowed us to define ≥ 0.003% CD11b+FLAER- granulocytes and ≥ 0.005% glycophorin A+CD55-CD59- erythrocytes to be specific to BMF patients. Longitudinal cross-validation studies showed minimal (< 0.02%) inter-laboratory differences in the GPI(-) cell percentage. An analysis of 1210 patients with BMF revealed a GPI(-) cell population in 56.3% of patients with aplastic anemia and 18.5% of patients with myelodysplastic syndrome. The GPI(-) granulocyte percentages was 0.003-0.01% in 3.7% of patients. This FCM assay effectively identified an increase in the percentage of GPI(-) rare cells that are specific to BMF patients and allowed different laboratories to accurately detect 0.003-0.01% of pathological GPI(-) cells.

Features of vascular adverse events in Japanese patients with chronic myeloid leukemia treated with tyrosine kinase inhibitors: a retrospective study of the CML Cooperative Study Group database.

This study investigated the incidence rate and features of vascular adverse events (VAEs) in Japanese patients with chronic myeloid leukemia (CML) who were treated with tyrosine kinase inhibitors (TKIs). The analysis included 369 CML patients in the chronic or accelerated phases, selected from the CML Cooperative Study Group database; 25 events in 23 (6.2%) of these patients were VAEs. At the time of VAE incidence, nine patients were on treatment with imatinib, 12 with nilotinib, three with dasatinib, and one with bosutinib. VAE incidence comprised 13 cases of ischemic heart disease (IHD), eight of cerebral infarction (CI), and four of peripheral arterial occlusive disease (PAOD). IHD incidence rate in the study population was higher than that in the age-matched general population, particularly in nilotinib-treated patients, while CI incidence rate was almost equivalent. Compared with the Suita score, the SCORE chart and the Framingham score risk assessment tools detected more patients with high or very high risk of VAEs. In conclusion, incidence of IHD requires closer monitoring in nilotinib-treated patients. More detailed investigations for determining the most useful tool to predict VAE incidence and long-term analysis of therapy-related VAE cases are needed for improving safety during TKI therapy.

UNC569-induced Morphological Changes in Pigment Epithelia and Photoreceptor Cells in the Retina through MerTK Inhibition in Mice.

Mer proto-oncogene tyrosine kinase (MerTK), which is expressed in the retinal pigment epithelium (RPE), regulates phagocytosis of shed photoreceptor outer segments (POS). To investigate the effects of drug-induced MerTK inhibition on the retina, UNC569, a specific MerTK inhibitor, was orally administered to male mice at a concentration of 60, 100, or 150 mg/kg for up to 14 days. Furthermore, MerTK inhibition in the retinal tissue sample was examined using a phosphorylation assay following a single dose of UNC569 at 100 mg/kg. In electron microscopic examination, UNC569 at 100 mg/kg or more increased phagosomes and phagolysosomes in the RPE. In addition, UNC569 at 150 mg/kg increased chromatin-condensed nuclei in the outer nuclear layer, indicating the early phase of apoptosis of photoreceptor cells. MiR-183, miR-96, and miR-124, which are enriched in photoreceptor cells, were elevated in the plasma of mice following treatment of 150-mg/kg UNC569, in conjunction with the photoreceptor lesion. Additionally, 100-mg/kg UNC569 inhibited MerTK phosphorylation in the retina. These results suggest that MerTK inhibition impaired phagocytic function of the retina, leading to accumulation of shed POS within the POS layer and increasing phagosomes and phagolysosomes in the RPE to delay POS renewal, resulting in apoptosis of photoreceptor cells.

Detection of Dystrophin Dp71 in Human Skeletal Muscle Using an Automated Capillary Western Assay System.

BACKGROUND: Dystrophin Dp71 is one of the isoforms produced by the DMD gene which is mutated in patients with Duchenne muscular dystrophy (DMD). Although Dp71 is expressed ubiquitously, it has not been detected in normal skeletal muscle. This study was performed to assess the expression of Dp71 in human skeletal muscle. METHODS: Human skeletal muscle RNA and tissues were obtained commercially. Mouse skeletal muscle was obtained from normal and DMDmdx mice. Dp71 mRNA and protein were determined by reverse-transcription PCR and an automated capillary Western assay system, the Simple Western, respectively. Dp71 was over-expressed or suppressed using a plasmid expressing Dp71 or antisense oligonucleotide, respectively. RESULTS: Full-length Dp71 cDNA was PCR amplified as a single product from human skeletal muscle RNA. A ca. 70 kDa protein peak detected by the Simple Western was determined as Dp71 by over-expressing Dp71 in HEK293 cells, or suppressing Dp71 expression with antisense oligonucleotide in rhabdomyosarcoma cells. The Simple Western assay detected Dp71 in the skeletal muscles of both normal and DMD mice. In human skeletal muscle, Dp71 was also detected. The ratio of Dp71 to vinculin of human skeletal muscle samples varied widely, indicating various levels of Dp71 expression. CONCLUSIONS: Dp71 protein was detected in human skeletal muscle using a highly sensitive capillary Western blotting system.

[Management of paroxysmal nocturnal hemoglobinuria: an update].

The accepted therapeutic strategy for treating paroxysmal nocturnal hemoglobinuria (PNH) has drastically changed since eculizumab was introduced in 2010 in Japan. Its clinical impact has paved way for the new area of anti-complement therapy, and it has sparked further research and development of anti-complement drugs. The long-term safety and efficacy of eculizumab was established in clinical settings by analyzing post-marketing surveillance that is mandatory in Japan and in clinical trial settings. However, new clinical complications such as resistance to eculizumab, breakthrough hemolysis, extravascular hemolysis, and invasive meningococcal disease (IMD) have emerged after the introduction of eculizumab. Therefore, it is important to strategize effective ways to tackle these issues occurring during eculizumab treatment. In Japan, it is an urgent priority to reach a consensus for managing IMD in patients treated with eculizumab and for establishing protocols for prophylaxis to prevent meningococcal infection.

Polyuria-associated hydronephrosis induced by xenobiotic chemical exposure in mice.

Hydronephrosis is a commonly found disease state characterized by the dilation of renal calices and pelvis, resulting in the loss of kidney function in the severest cases. A generally accepted etiology of hydronephrosis involves the obstruction of urine flow along the urinary tract. In the recent years, we have developed a mouse model of hydronephrosis induced by lactational exposure to dioxin and demonstrated a lack of anatomical obstruction in this model. We also showed that prostaglandin E2 synthesis system plays a critical role in the onset of hydronephrosis. In the present study, we found that neonatal hydronephrosis was not likely to be associated with functional obstruction (impaired peristalsis) but was found to be associated with polyuria and low urine osmolality with the downregulation of proteins involved in the urine concentrating process. The administration of an antidiuretic hormone analog to the dioxin-exposed pups not only suppressed the increased urine output but also decreased the incidence and severity of hydronephrosis. In contrast to the case in pups, administration of dioxin to adult mice failed to induce polyuria and upregulation of prostaglandin E2 synthesis system, and the adult mice were resistant to develop hydronephrosis. These findings suggest the possibility that polyuria could induce hydronephrosis in the absence of anatomical or functional obstruction of the ureter. It is concluded that the present animal model provides a unique example of polyuria-associated type of hydronephrosis, suggesting a need to redefine this disease state.

Posttranslational modifications in type I collagen from different tissues extracted from wild type and prolyl 3-hydroxylase 1 null mice.

Type I collagen extracted from tendon, skin, and bone of wild type and prolyl 3-hydroxylase 1 (P3H1) null mice shows distinct patterns of 3-hydroxylation and glycosylation of hydroxylysine residues. The A1 site (Pro-986) in the α1-chain of type I collagen is almost completely 3-hydroxylated in every tissue of the wild type mice. In contrast, no 3-hydroxylation of this proline residue was found in P3H1 null mice. Partial 3-hydroxylation of the A3 site (Pro-707) was present in tendon and bone, but absent in skin in both α-chains of the wild type animals. Type I collagen extracted from bone of P3H1 null mice shows a large reduction in 3-hydroxylation of the A3 site in both α-chains, whereas type I collagen extracted from tendon of P3H1 null mice shows little difference as compared with wild type. These results demonstrate that the A1 site in type I collagen is exclusively 3-hydroxylated by P3H1, and presumably, this enzyme is required for the 3-hydroxylation of the A3 site of both α-chains in bone but not in tendon. The increase in glycosylation of hydroxylysine in P3H1 null mice in bone was found to be due to an increased occupancy of normally glycosylated sites. Despite the severe disorganization of collagen fibrils in adult tissues, the D-period of the fibrils is unchanged. Tendon fibrils of newborn P3H1 null mice are well organized with only a slight increase in diameter. The absence of 3-hydroxyproline and/or the increased glycosylation of hydroxylysine in type I collagen disturbs the lateral growth of the fibrils.

Distinct interaction of nilotinib and imatinib with P-Glycoprotein in intracellular accumulation and cytotoxicity in CML Cell Line K562 cells.

Nilotinib, a second-generation tyrosine kinase inhibitor (TKI), has been approved for first-line chronic myeloid leukemia (CML) treatment. The improved clinical response of nilotinib over that of the first generation TKI, imatinib, has been thought to be a result of its high potency of inhibition of BCR-ABL kinase. This study aimed to characterize differences between nilotinib and imatinib in the intracellular accumulation and cytotoxic effect on the CML cell line K562. Accumulation of nilotinib in K562 cells was from 4.7- to 9.0-fold higher than that of imatinib. The cytotoxic effect of nilotinib on K562 cells was 14.2-fold higher than that of imatinib. Inhibition experiments in K562 cells, and examination of the cellular uptake using influx transporter-transfected human embryonic kidney (HEK) 293 cells, suggested that the influx transporters OCT1 and OATP1A2, which have been reported to mediate accumulation of imatinib in CML cells, contributed little to the uptake of nilotinib. Nilotinib was found to accumulate in imatinib-resistant K562 (K562/IM) cells overexpressing the efflux transporter P-glycoprotein (P-gp), although cytotoxic assays showed that K562/IM cells displayed 20000-fold greater resistance to nilotinib over the parent K562 cells. In conclusion, the present findings suggest that intracellular accumulation of nilotinib in CML cells contributes to its clinical response and efficacy in CML patients. Although nilotinib has been reported to be effective against imatinib-resistant ABL kinase mutants, the drug could not overcome imatinib resistance acquired by P-gp-overexpression. These results imply that classification of mechanisms of drug resistance is important for suitable strategies to treat imatinib-resistant CML patients.