Ensitrelvir eradicates persistent SARS-CoV-2 infection in a follicular lymphoma patient treated with anti-CD20 antibodies.

Patients with hematological malignancies, especially B-cell malignancies, who received anti-CD20 antibodies exhibit a poor immune response to the mRNA coronavirus disease 2019 (COVID-19) vaccine within 6-12 months after the last administration. These patients occasionally present with severe COVID-19 symptoms. Additionally, patients with hematologic diseases who have persistent COVID-19 after receiving anti-CD20 antibodies, postpone chemotherapy for the primary disease. Despite the efficacy of ensitrelvir in shortening the duration of symptoms, evidence of improved prognosis is lacking. However, prognosis might be improved if ensitrelvir treatment could reduce the viral load and shorten the time to postpone chemotherapy. It is unclear whether viral reduction directly improves prognosis. However, faster viral reduction may lead to faster resumption of chemotherapy for the underlying disease, resulting in better prognosis. Here, we present a case wherein we administered ensitrelvir fumaric acid to a 75-year-old woman with persistent COVID-19 after anti-CD20 antibody treatment. Her symptoms resolved quickly, with a reduction of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral load, and she could continue receiving chemotherapy for lymphoma. Our findings suggest that ensitrelvir administration should be considered in patients with SARS-CoV-2 infection after anti-CD20 antibody treatment.

Prognostic value of metabolic tumor volume of extranodal involvement in diffuse large B cell lymphoma.

Extranodal involvement predicts poor outcomes of diffuse large B cell lymphoma (DLBCL), but the impact of the metabolic tumor burden (MTV) of extranodal sites using positron emission tomography has not been clarified. This study aimed to assess the impact of extranodal MTV on overall survival (OS). We retrospectively analyzed 145 newly diagnosed DLBCL patients and verified the prognostic impact of each extranodal and nodal MTV. Multivariate Cox hazards modelling using both extranodal and nodal MTV as covariables identified extranodal MTV as a significant factor for OS (hazard ratio [HR] 1.072, 95% confidence interval [CI] 1.019-1.129, P = 0.008), but not nodal MTV. Multivariate Cox modelling using restricted cubic splines demonstrated that the impact of total MTV depends on the MTV of extranodal sites, not of nodal sites. When both the number and MTV of extranodal involvements were used as covariables, extranodal MTV remained a significant predictor of OS (HR 1.070, 95%CI 1.017-1.127, P = 0.009), but the number of extranodal sites did not. Extranodal MTV potentially had a more significant role on prognosis than nodal MTV. When considering prognostic impacts, the MTV of extranodal involvement is significantly more important than the number.

Prognostic utility of a geriatric nutritional risk index in combination with a comorbidity index in elderly patients with diffuse large B cell lymphoma.

Reflecting the increasing risk in elderly patients with diffuse large B cell lymphoma (DLBCL), prognostic predictors other than the International Prognostic Index have attracted more attention. This study presents the first analysis of the prognostic utility of the Geriatric Nutritional Risk Index (GNRI) in combination with the Charlson Comorbidity Index (CCI) for overall survival (OS) in elderly DLBCL patients. A multicentre retrospective was conducted on a cohort of 451 patients (≥65 years). The GNRI and CCI were independent predictors in a multivariate Cox proportional hazard model. There was a nonlinear correlation between the GNRI and OS in a Cox model with restricted cubic spline. Multivariate receiver operating characteristic curves showed a significant improvement in prediction accuracy when the GNRI was added to CCI. Adding the GNRI to CCI yielded a significant category-free net reclassification improvement (0·556; 95% CI: 0·378-0·736, P < 0·001) and integrated discrimination improvement (0·094; 95% CI: 0·067-0·122, P < 0·001). The decision curve analysis demonstrated the clinical net benefit associated with the adoption of the GNRI. The GNRI was not only a predictor of OS but also remarkably improved the prognosis prediction accuracy when incorporated with the CCI, having the ability to stratify the prognosis of elderly DLBCL patients.

Association of the Geriatric 8 with treatment intensity and prognosis in older patients with diffuse large B-cell lymphoma.

Because of the heterogeneity among older patients with diffuse large B-cell lymphoma (DLBCL), the establishment of an easy-to-use geriatric assessment tool is an unmet need. We verified the impact of the Geriatric 8 (G8) on treatment stratification and overall survival (OS). We conducted a retrospective, multicentre analysis of older patients (≥65 years) with DLBCL. The primary endpoint was OS. The total average relative dose intensity (tARDI) was defined as the average delivered dose intensity divided by the planned dose intensity through all cycles. A total of 451 patients were diagnosed with DLBCL from 2007 to 2017, and 388 patients received standard regimens. A multivariate Cox model confirmed that the G8 was a significant predictor of OS (hazard ratio 0·88, 95% confidence interval 0·828-0·935). A Cox model with restricted cubic spline showed a linear association between the G8 and the mortality risk. The G8 had a significant impact on OS in elderly patients with DLBCL. The upper limit of tARDI for standard regimens to improve OS might be appropriate at ≥80% for patients with high G8 scores and 60% for patients with low G8 scores. However, the standard regimens should be given to all patients regardless of the G8 score to improve OS.

Utility of the Geriatric 8 for the Prediction of Therapy-Related Toxicity in Older Adults with Diffuse Large B-Cell Lymphoma.

BACKGROUND: The management of severe adverse events (AEs) is important in safely and effectively providing chemotherapy to older adults with diffuse large B-cell lymphoma (DLBCL). However, reports on simple and DLBCL-specific predictive models for treatment-related toxicity in elderly individuals are scarce. The aim of this study was to examine the usefulness of Geriatric 8 (G8) in predicting treatment-related severe AEs, nonhematological toxicity, and febrile neutropenia in older adults with DLBCL in real-world practice. MATERIALS AND METHODS: We conducted a multicenter, retrospective study on 398 consecutive patients with DLBCL (aged ≥65 years) who received standard therapy at three centers in Japan (University of Fukui Hospital, the Fukui Prefectural Hospital, and the Japanese Red Cross Fukui Hospital), between 2007 and 2017. RESULT: Multivariate logistic analysis demonstrated that the G8 score was an independent predictive factor for severe AEs. Moreover, a logistic regression model with restricted cubic spline showed a nonlinear association between the incidence of severe AEs and the G8 score. According to receiver operating characteristic analysis, the most discriminative cutoff value of the G8 for the incidence of severe AEs was 11, with an area under the curve value of 0.670. AEs occurred most often in the first course of chemotherapy and decreased as the course progressed. CONCLUSION: The G8 score, an easy-to-use geriatric assessment tool, can be a useful prediction model of treatment-related severe AEs during standard therapy in older adults with DLBCL. IMPLICATIONS FOR PRACTICE: In older patients with diffuse large B-cell lymphoma (DLBCL), to accurately predict the risk of severe adverse events (AEs) in advance is essential for safe and effective treatment. This study demonstrated that the Geriatric 8 score, a simple and established geriatric assessment tool, indicated a high predictive ability for occurrence of therapy-related severe AEs in elderly patients with DLBCL who were treated with standard treatment.

Successful Treatment of Breakthrough Trichosporon asahii Fungemia by the Combination Therapy of Fluconazole and Liposomal Amphotericin B in a Patient with Follicular Lymphoma.

Invasive trichosporonosis is a rare and lethal fungal infection that occurs in immunocompromised patients. Breakthrough trichosporonosis can occur in patients treated with echinocandins since Trichosporon spp. are resistant to these antifungal agents. We report a case of breakthrough Trichosporon asahii fungemia. A 62-year-old Japanese woman with relapsed follicular lymphoma was treated empirically with broad-spectrum antibiotics and micafungin due to an intermittent fever during reinduction chemotherapy. After four cycles of anti-cancer chemotherapy, she experienced a high neutropenic fever and T. asahii was subsequently detected from a blood culture. The patient was not given voriconazole due to the contraindication for use with carbamazepine, and she was successfully treated with fluconazole plus liposomal amphotericin B without any serious complications. The combined therapy of fluconazole and liposomal amphotericin B may therefore be useful in treating T. asahii fungemia, especially in patients receiving antiepileptic agents.

[Lineage switch from T-lymphoblastic leukemia to myeloid leukemia at relapse].

A 38-year-old woman was referred to our hospital because of fever, general malaise, and abnormal blood count. The white blood cell count was 19,500/µl, with 72% lymphoblast. Bone marrow examination showed increased cellularity with 94% lymphoblast. Flow cytometry revealed the following T-cell lineages: cyCD3 (+), CD5 (+), CD7 (+), and CD34 (+). Chromosome analysis revealed hypodiploidy. The patient was diagnosed with early T-cell precursor lymphoblastic leukemia (ETP-ALL). After two cycles of induction chemotherapy, she achieved complete remission, but the disease relapsed after one cycle of consolidation therapy. At the time of relapse, leukemic cells were myeloperoxidase positive and showed a loss of T-cell surface antigen, suggesting that a lineage switch occurred. The patient did not respond to the second induction therapy. She subsequently received 3+7 (idarubicin+cytarabine) for acute myeloid leukemia (AML), but she deceased due to refractory leukemia. At the time of relapse, genome sequencing was performed and mutations of NRAS, TP53, and MLLT-PICALM fusion genes were revealed. Here, we report a case of ETP-ALL who relapsed with a lineage switch to AML in concordance with refractory disease.

Venetoclax and alvocidib are both cytotoxic to acute myeloid leukemia cells resistant to cytarabine and clofarabine.

BACKGROUND: Cytarabine (ara-C) is the major drug for the treatment of acute myeloid leukemia (AML), but cellular resistance to ara-C is a major obstacle to therapeutic success. The present study examined enhanced anti-apoptosis identified in 3 newly established nucleoside analogue-resistant leukemic cell line variants and approaches to overcoming this resistance. METHODS: HL-60 human AML cells were used to develop the ara-C- or clofarabine (CAFdA)-resistant variants. The Bcl-2 inhibitor venetoclax and the Mcl-1 inhibitor alvocidib were tested to determine whether they could reverse these cells' resistance. RESULTS: A 10-fold ara-C-resistant HL-60 variant, a 4-fold CAFdA-resistant HL-60 variant, and a 30-fold CAFdA-resistant HL-60 variant were newly established. The variants demonstrated reduced deoxycytidine kinase and deoxyguanosine kinase expression, but intact expression of surface transporters (hENT1, hENT2, hCNT3). The variants exhibited lower expression of intracellular nucleoside analogue triphosphates compared with non-variant HL-60 cells. The variants also overexpressed Bcl-2 and Mcl-1. Venetoclax as a single agent was not cytotoxic to the resistant variants. Nevertheless, venetoclax with nucleoside analogs demonstrated synergistic cytotoxicity against the variants. Alvocidib as a single agent was cytotoxic to the cells. However, alvocidib induced G1 arrest and suppressed the cytotoxicity of the co-administered nucleoside analogs. CONCLUSIONS: Three new nucleoside analogue-resistant HL-60 cell variants exhibited reduced production of intracellular analogue triphosphates and enhanced Bcl-2 and Mcl-1 expressions. Venetoclax combined with nucleoside analogs showed synergistic anti-leukemic effects and overcame the drug resistance.

The impact of diagnostic wait time on the survival of patients with diffuse large B-cell lymphoma: effect modification by the International Prognostic Index.

Despite the importance of a prompt diagnosis to improve cancer patients' survival, little has been reported on diagnostic delay in diffuse large B-cell lymphoma (DLBCL). A single-centre, retrospective study was conducted to examine the association between diagnostic wait time (DWT), the interval from the initial hospital visit to diagnosis, and survival in patients with DLBCL. A total of 193 patients were enrolled from 2007 to 2017 in our institution. A covariate-adjusted Cox proportional hazards model with restricted cubic spline was used to evaluate the impact of DWT on survival, with a subgroup analysis according to the International Prognostic Index (IPI). DWT was not associated with survival in the entire DLBCL population, but the impact of DWT on survival differed between IPI < 3 and ≥ 3; prolongation of DWT steadily exacerbated the prognosis in patients with IPI ≥ 3, whereas there was a patient population with IPI < 3 who had a high mortality rate despite rather early diagnosis. The opposite trend in the effect of DWT on survival between patients with IPI < 3 and ≥ 3 offset survival in all DLBCL patients. DWT had no observable impact on outcomes in the entire DLBCL population, but longer DWT worsened the prognosis, particularly in patients with IPI ≥ 3.

Combination of panobinostat with ponatinib synergistically overcomes imatinib-resistant CML cells.

The major mechanism of imatinib (IM) resistance of CML is the reactivation of ABL kinase either through BCR-ABL gene amplification or mutation. We investigated the cytotoxicity of a pan-ABL tyrosine kinase inhibitor, ponatinib, and a pan-histone deacetylase inhibitor, panobinostat, against IM-resistant CML cells in vitro. Two different IM-resistant cell lines, K562/IM-R1 and Ba/F3/T315I were evaluated in comparison with their respective, parental cell lines, K562 and Ba/F3. K562/IM-R1 overexpressed BCR-ABL due to gene amplification. Ba/F3/T315I was transfected with a BCR-ABL gene encoding T315I-mutated BCR-ABL. Ponatinib inhibited the growth of both K562/IM-R1 and Ba/F3/T315I as potently as it inhibited their parental cells with an IC50 of 2-30 nM. Panobinostat also similarly inhibited the growth of all of the cell lines with an IC50 of 40-51 nM. This was accompanied by reduced histone deacetylase activity, induced histone H3 acetylation, and an increased protein level of heat shock protein 70, which suggested disruption of heat shock protein 90 chaperone function for BCR-ABL and its degradation. Importantly, the combination of ponatinib with panobinostat showed synergistic growth inhibition and induced a higher level of apoptosis than the sum of the apoptosis induced by each agent alone in all of the cell lines. Ponatinib inhibited phosphorylation not only of BCR-ABL but also of downstream signal transducer and activator of transcription 5, protein kinase B, and ERK1/2 in both K562/IM-R1 and Ba/F3/T315I, and the addition of panobinostat to ponatinib further inhibited these phosphorylations. In conclusion, panobinostat enhanced the cytotoxicity of ponatinib towards IM-resistant CML cells including those with T315I-mutated BCR-ABL.

[Sudden blast crisis of chronic myeloid leukemia after a 13-year durable remission following allogeneic bone marrow transplantation and donor lymphocyte infusion].

We describe a rare case of sudden blast crisis of chronic myeloid leukemia that occurred after a 13-year durable remission, following allogeneic bone marrow transplantation and donor lymphocyte infusion. A 55-year-old Japanese man was diagnosed with chronic-phase chronic myeloid leukemia 24 years previously. He underwent allogeneic bone marrow transplantation 2 years after diagnosis. Although the disease recurred 6 years after transplantation, the patient achieved remission again by a donor lymphocyte infusion. Despite a 13-year durable remission, the disease later relapsed into a sudden blast crisis. Prednisolone and vincristine combined with imatinib mesylate effectively achieved a major molecular response. However, the disease relapsed repeatedly with central nervous system infiltration. Dasatinib and intrathecal methotrexate, cytarabine, and dexamethasone administration via the Ommaya reservoir controlled disease progression. Nevertheless, the disease became refractory to treatment with the emergence of a T315I Bcr-Abl gene mutation. The patient eventually died 43 months post crisis.

Combination of guanine arabinoside and Bcl-2 inhibitor YC137 overcomes the cytarabine resistance in HL-60 leukemia cell line.

Cytarabine (ara-C) is the key agent for treating acute myeloid leukemia. After being transported into leukemic cells, ara-C is phosphorylated, by several enzymes including deoxycytidine kinase (dCK), to ara-C triphosphate (ara-CTP), an active metabolite, and then incorporated into DNA, thereby inhibiting DNA synthesis. Therefore, the cytotoxicity of ara-C depends on the production of ara-CTP and the induction of apoptosis. Here, we established a new ara-C-resistant acute myeloid leukemia cell line (HL-60/ara-C60) with dual resistance characteristics of the anti-antimetabolic character of decreased ara-CTP production and an increase in the antiapoptotic factors Bcl-2 and Bcl-XL. We further attempted to overcome resistance by augmenting ara-CTP production and stimulating apoptosis. A relatively new nucleoside analog, 9-ß-d-arabinofuranosylguanine (ara-G), and the small molecule Bcl-2 antagonist YC137 were used for this purpose. HL-60/ara-C60 was 60-fold more ara-C-resistant than the parental HL-60 cells. HL-60/ara-C60 cells exhibited low dCK protein expression, which resulted in decreased ara-CTP production. HL-60/ara-C60 cells were also refractory to ara-C-induced apoptosis due to overexpression of Bcl-2 and Bcl-XL. Combination treatment of ara-C with ara-G augmented the dCK protein level, thereby increasing ara-CTP production and subsequent cytotoxicity. Moreover, the combination of ara-C with YC137 produced a greater amount of apoptosis than ara-C alone. Importantly, the three-drug combination of ara-C, ara-G and YC137 provided greater cytotoxicity than ara-C+ara-G or ara-C+YC137. These findings suggest possible combination strategies for overcoming ara-C resistance by augmenting ara-CTP production and reversing refractoriness against the induction of apoptosis in ara-C resistant leukemic cells.

Aurora B inhibitor barasertib and cytarabine exert a greater-than-additive cytotoxicity in acute myeloid leukemia cells.

Barasertib, an aurora B inhibitor, terminates cell division, introduces polyploidy, and consequently causes apoptosis. In the present study, we evaluated the effect of the combination of barasertib and cytarabine (ara-C), a key agent for leukemia chemotherapy, on leukemic cells in vitro. Human leukemia HL-60 cells and HL-60/ara-C20 cells, a 20-fold ara-C-resistant variant, were used. The 50% growth inhibitory concentrations of an active metabolite of barasertib, barasertib-hydroxyquinazoline-pyrazol-aniline (Barasertib-HQPA), and ara-C were 51 nM and 300 nM for HL-60 cells and 70 nM and 5300 nM for HL-60/ara-C20 cells, respectively. Barasertib-HQPA induced polyploidy with a subsequent induction of sub-G1 phase apoptosis, indicating the M-phase specific cytotoxicity. Cells treated with the S-phase specific ara-C accumulated in S phase and subsequently died through apoptosis. When HL-60 cells were treated with barasertib-HQPA and ara-C in combination, a greater-than-additive apoptosis was induced. This enhancement was obtained when the cells were treated with barasertib-HQPA prior to ara-C (37.9% sub-G1) or with both concurrently (31.2% sub-G1), but not with ara-C prior to barasertib-HQPA (17.8% sub-G1). The combination effects were similarly obtained in HL-60/ara-C20 cells with 19.7% sub-G1 for barasertib-HQPA→ara-C, 18.4% sub-G1 for both concurrently, and 13.8% sub-G1 for ara-C→barasertib-HQPA, and another leukemic U937 cells with 25.4% sub-G1 for barasertib-HQPA→ara-C, 28.2% sub-G1 for both concurrently, and 16.0% sub-G1 for ara-C→barasertib-HQPA. Barasertib-HQPA inhibited aurora B autophosphorylation and histone H3 phosphorylation in all the cell lines. Barasertib-HQPA did not inhibit DNA synthesis, allowing ara-C incorporation into DNA for its cytotoxicity. Thus, barasertib-HQPA and ara-C provided a greater-than-additive cytotoxicity in leukemic cells in vitro.

Prognostic significance of the Geriatric 8 score alone and included with genetic risk group in older adults with acute myeloid leukemia.

INTRODUCTION: As the numbers of older adult patients with acute myeloid leukemia (AML) continue to increase, the establishment of a simple geriatric assessment specifically for AML represents an unmet need. This study aimed to assess the impact of the Geriatric 8 (G8) score on overall survival (OS). MATERIALS AND METHODS: We retrospectively analyzed 100 patients ≥60 years old with newly diagnosed AML. RESULTS: Multivariate Cox modeling identified G8 score as a significant prognostic factor for OS (hazard ratio 0.891, 95% confidence interval [CI] 0.808-0.983). A linear association between G8 score and mortality risk was confirmed in a Cox model with restricted cubic spline. Multivariate receiver operating characteristic curves demonstrated a significant improvement in prediction ability when G8 score was added to cytogenetic risk group. The combination of G8 score and cytogenetic risk group yielded a significant continuous net reclassification improvement (0.718; 95%CI 0.353-1.082; P < 0.001). Decision curve analysis showed a clinical net benefit associated with adding G8 score to cytogenetic risk group. DISCUSSION: G8 score not only offered a strong prognostic factor for OS, but also markedly improved prediction accuracy for mortality when incorporated with cytogenetic risk group.

The Relative Dose Intensity Changes during Cycles of Standard Regimens in Patients with Diffuse Large B-Cell Lymphoma.

No studies have focused on the trajectory of the average relative dose intensity (ARDI) during cycles of first-line chemotherapy for patients with diffuse large B-cell lymphoma. To evaluate the impact of attenuating ARDI during cycles on overall survival, we conducted a multi-centre, longitudinal, observational retrospective study. A total of 307 analysable patients were enrolled. Multivariate Cox hazards modelling with restricted cubic spline models revealed prognostic benefits of higher ARDI up to, but not after, cycle 6. According to group-based trajectory modelling, patients were classified into five groups depending on the pattern of ARDI changes. Among these, two groups in which ARDI had fallen significantly to less than 50% by cycles 4-6 displayed significantly poorer prognosis, despite increased ARDI in the second half of the treatment period (log-rank p = 0.02). The Geriatric Nutritional Risk Index offered significant prediction of unfavourable ARDI changes (odds ratio 2.540, 95% confidence interval 1.020-6.310; p = 0.044). Up to cycle 6, maintenance of ARDI in all cycles (but particularly in the early cycles) is important for prognosis. Malnutrition is a significant factor that lets patients trace patterns of ARDI changes during cycles of chemotherapy associated with untoward prognosis.

Darinaparsin in patients with relapsed or refractory peripheral T-cell lymphoma: results of an Asian phase 2 study.

Darinaparsin is a novel organic arsenical compound of dimethylated arsenic conjugated to glutathione, with antitumor activity and a mechanism of action markedly different from other available agents. This phase 2, nonrandomized, single-arm, open-label study evaluated the efficacy and safety of intravenous darinaparsin (300 mg/m2 over 1 hour, once daily for 5 consecutive days, per 21-day cycle) and its pharmacokinetics at multiple doses in 65 Asian patients with relapsed or refractory peripheral T-cell lymphoma (PTCL). The primary end point was the overall response rate (ORR). The ORR based on central assessment was 19.3% (90% confidence interval, 11.2-29.9), which was significantly higher than the predefined threshold of 10% (P = .024). The ORR was 16.2% in patients with PTCL-not otherwise specified and 29.4% in patients with angioimmunoblastic T-cell lymphoma. Tumor size decreased in 62.3% of patients. Treatment-emergent adverse events (TEAEs) were observed in 98.5% of patients. Grade ≥3 TEAEs with an incidence rate of ≥5% included anemia (15.4%), thrombocytopenia (13.8%), neutropenia (12.3%), leukopenia (9.2%), lymphopenia (9.2%), and hypertension (6.2%). Darinaparsin is effective and well tolerated, with TEAEs that were clinically acceptable and manageable with symptomatic treatment and dose reductions. This trial was registered at www.clinicaltrials.gov as #NCT02653976.

Japanese subgroup analysis in the Asian phase II study of darinaparsin in patients with relapsed or refractory peripheral T-cell lymphoma.

A Japanese subgroup analysis from the Asian phase II study of darinaparsin in patients with relapsed or refractory peripheral T-cell lymphoma (PTCL) was performed to evaluate the efficacy and safety outcomes of the Japanese population. In this Asian phase II study, darinaparsin was administered to 65 patients, including 37 Japanese patients. In the Japanese population, the histopathological type of PTCL was PTCL, not otherwise specified in 26 patients (70.3%), angioimmunoblastic T-cell lymphoma in 9 patients (24.3%) and anaplastic large cell lymphoma, anaplastic lymphoma kinase (ALK) -negative in 2 patients (5.4%), and the median patient age was 70.0 (range: 43-85). 94.6% and 35.1% of the Japanese population had previously received multi-agent and single-agent regimen, respectively. The efficacy and safety were summarized and compared between the overall and Japanese populations. Based on central assessment, the overall response rate was 22.2% (8/36; 90% confidence interval [CI]: 11.6-36.5) in the Japanese population and 19.3% (11/57; 90% CI: 11.2-29.9) in the overall population. There were no essential differences in the safety profile of darinaparsin between the Japanese population and the overall population. The results of this subgroup analysis indicate that the efficacy and safety profiles of the Japanese subpopulation were broadly consistent with that of the overall population, and that darinaparsin is potentially an effective treatment with a manageable safety profile in Japanese patients with relapse or refractory PTCL.

Induction of DNA strand breaks is critical to predict the cytotoxicity of gemtuzumab ozogamicin against leukemic cells.

Gemtuzumab ozogamicin (GO) consists of the CD33 antibody linked to calicheamicin. The binding of GO to the CD33 antigen on leukemic cells results in internalization followed by the release of calicheamicin, thereby inducing DNA strand breaks. We hypothesized that the induction of DNA strand breaks would be a surrogate marker of GO cytotoxcity. Here, two GO-resistant variants (HL/GO-CSA [225-fold], HL/GO [200-fold]) were established by serially incubating human leukemia HL-60 cells with GO with or without a P-glycoprotein (P-gp) inhibitor, cyclosporine A, respectively. The CD33 positivity was reduced in both variants. The HL/GO-CSA cells showed an increased multidrug resistance protein-1 (MRP1) transcript, and an MRP1 inhibitor partially reversed GO resistance. The HL/GO cells had neither P-gp nor MRP1 overexpression. Microarray analysis and Western blotting indicated elevated levels of DNA repair-associated proteins in both variants. Two other leukemic subclones, showing either P-gp or MRP1 overexpression, were also GO-resistant. Using single cell gel electrophoresis analysis, it was determined that GO-induced DNA strand breaks increased dose-dependently in HL-60 cells, whereas the number of breaks was reduced in the GO-resistant cell lines. The induction of DNA strand breaks was correlated with GO sensitivity among these cell lines. The CD33 positivity and the expression levels of transporters were not proportional to drug sensitivity. Using primary leukemic cells, the induction of DNA strand breaks appeared to be associated with GO sensitivity. Thus, GO-induced DNA strand breaks as the final output of the mechanism of action would be critical to predict GO cytotoxicity.