COVID-19 in patients with classic and variant hairy cell leukemia.

Hairy cell leukemia (HCL), similar to its variant HCLv, is a B-cell malignancy associated with decreased humoral immunity. We prospectively monitored the largest cohort of patients with HCL/HCLv to date (n = 503) for COVID-19 by symptoms, antibody, and polymerase chain reaction (PCR) and/or antigen positivity. Fifty percent (253 of 503) of the patients with HCL/HCLv (238 HCL and 15 HCLv) had evidence of COVID-19, with 210 (83%) testing positive by PCR or rapid-antigen test. Of the 43 patients without positive tests, all had nucleocapsid antibodies indicating COVID-19 exposure, 7 recalled no symptoms, and 36 had mild symptoms. Of the 210 who tested positive, 23, 46, 129, and 12 cases occurred in 2020, 2021, 2022, and 2023, respectively. Among them, 175 began treatment for HCL/HCLv 0.4 to 429 (median, 66) months before, and 132 had their last dose of anti-CD20 monoclonal antibody 0.2 to 229 (median, 63) months before. Two patients died, including a young woman who began rituximab 2 months after first-line cladribine before vaccine availability. Nearly all patients with HCL/HCLv recovered uneventfully from COVID-19 including those without vaccination or those with significant immunosuppression and recent treatment. However, decreased normal B cells from HCL or treatment was associated with lower spike antibody levels as a response to COVID-19 (P = .0094) and longer recovery time (P = .0036). Thus, in a large cohort of patients with HCL/HCLv and in the first to determine relationships between COVID-19 outcome and immune markers, mortality was relatively low (∼1%), sequelae were uncommon, and recovery from COVID-19 was longer if normal B cells were low after recent treatment. The trials are registered at www.clinicaltrials.gov as #NCT01087333 and #NCT04362865.

Dabrafenib plus trametinib in BRAFV600E-mutated rare cancers: the phase 2 ROAR trial.

BRAFV600E alterations are prevalent across multiple tumors. Here we present final efficacy and safety results of a phase 2 basket trial of dabrafenib (BRAF kinase inhibitor) plus trametinib (MEK inhibitor) in eight cohorts of patients with BRAFV600E-mutated advanced rare cancers: anaplastic thyroid carcinoma (n = 36), biliary tract cancer (n = 43), gastrointestinal stromal tumor (n = 1), adenocarcinoma of the small intestine (n = 3), low-grade glioma (n = 13), high-grade glioma (n = 45), hairy cell leukemia (n = 55) and multiple myeloma (n = 19). The primary endpoint of investigator-assessed overall response rate in these cohorts was 56%, 53%, 0%, 67%, 54%, 33%, 89% and 50%, respectively. Secondary endpoints were median duration of response (DoR), progression-free survival (PFS), overall survival (OS) and safety. Median DoR was 14.4 months, 8.9 months, not reached, 7.7 months, not reached, 31.2 months, not reached and 11.1 months, respectively. Median PFS was 6.7 months, 9.0 months, not reached, not evaluable, 9.5 months, 5.5 months, not evaluable and 6.3 months, respectively. Median OS was 14.5 months, 13.5 months, not reached, 21.8 months, not evaluable, 17.6 months, not evaluable and 33.9 months, respectively. The most frequent (≥20% of patients) treatment-related adverse events were pyrexia (40.8%), fatigue (25.7%), chills (25.7%), nausea (23.8%) and rash (20.4%). The encouraging tumor-agnostic activity of dabrafenib plus trametinib suggests that this could be a promising treatment approach for some patients with BRAFV600E-mutated advanced rare cancers. ClinicalTrials.gov registration: NCT02034110 .

Dabrafenib plus trametinib in patients with relapsed/refractory BRAF V600E mutation-positive hairy cell leukemia.

BRAF V600E is the key oncogenic driver mutation in hairy cell leukemia (HCL). We report the efficacy and safety of dabrafenib plus trametinib in patients with relapsed/refractory BRAF V600E mutation-positive HCL. This open-label, phase 2 study enrolled patients with BRAF V600E mutation-positive HCL refractory to first-line treatment with a purine analog or relapsed after ≥2 prior lines of treatment. Patients received dabrafenib 150 mg twice daily plus trametinib 2 mg once daily until disease progression, unacceptable toxicity, or death. The primary endpoint was investigator-assessed objective response rate (ORR) per criteria adapted from National Comprehensive Cancer Network-Consensus Resolution guidelines. Secondary endpoints included duration of response (DOR), progression-free survival (PFS), overall survival (OS), and safety. Fifty-five patients with BRAF V600E mutation-positive HCL were enrolled. The investigator-assessed ORR was 89.0% (95% confidence interval, 77.8%-95.9%); 65.5% of patients had a complete response (without minimal residual disease [MRD]: 9.1% [negative immunohistochemistry of bone marrow {BM} biopsy], 12.7% [negative BM aspirate flow cytometry {FC}], 16.4% [negative immunohistochemistry and/or FC results]; with MRD, 49.1%), and 23.6% had a partial response. The 24-month DOR was 97.7% with 24-month PFS and OS rates of 94.4% and 94.5%, respectively. The most common treatment-related adverse events were pyrexia (58.2%), chills (47.3%), and hyperglycemia (40.0%). Dabrafenib plus trametinib demonstrated durable responses with a manageable safety profile consistent with previous observations in other indications and should be considered as a rituximab-free therapeutic option for patients with relapsed/refractory BRAF V600E mutation-positive HCL. This trial is registered at www.clinicaltrials.gov as #NCT02034110.

Consensus opinion from an international group of experts on measurable residual disease in hairy cell leukemia.

A significant body of literature has been generated related to the detection of measurable residual disease (MRD) at the time of achieving complete remission (CR) in patients with hairy cell leukemia (HCL). However, due to the indolent nature of the disease as well as reports suggesting long-term survival in patients treated with a single course of a nucleoside analog albeit without evidence of cure, the merits of detection of MRD and attempts to eradicate it have been debated. Studies utilizing novel strategies in the relapse setting have demonstrated the utility of achieving CR with undetectable MRD (uMRD) in prolonging the duration of remission. Several assays including immunohistochemical analysis of bone marrow specimens, multi-parameter flow cytometry and molecular assays to detect the mutant BRAF V600E gene or the consensus primer for the immunoglobulin heavy chain gene (IGH) rearrangement have been utilized with few comparative studies. Here we provide a consensus report on the available data, the potential merits of MRD assessment in the front-line and relapse settings and recommendations on future role of MRD assessment in HCL.

Development of a distributed international patient data registry for hairy cell leukemia.

Hairy cell leukemia (HCL) is a rare lymphoproliferative disorder, comprising only 2% of all leukemias. The Hairy Cell Leukemia Foundation (HCLF) has developed a patient data registry to enable investigators to better study the clinical features, treatment outcomes, and complications of patients with HCL. This system utilizes a centralized registry architecture. Patients are enrolled at HCL Centers of Excellence (COE) or via a web-based portal. All data are de-identified, which reduces regulatory burden and increases opportunities for data access and re-use. To date, 579 patients have been enrolled in the registry. Efforts are underway to engage additional COE's to expand access to patients across the globe. This international PDR will enable researchers to study outcomes in HCL in ways not previously possible due to the rarity of the disease and will serve as a platform for future prospective research.

T cell responses against SARS-CoV-2 and its Omicron variant in a patient with B cell lymphoma after multiple doses of a COVID-19 mRNA vaccine.

Anti-SARS-CoV-2 antibodies are crucial for protection from future COVID-19 infections, limiting disease severity, and control of viral transmission. While patients with the most common type of hematologic malignancy, B cell lymphoma, often develop insufficient antibody responses to messenger RNA (mRNA) vaccines, vaccine-induced T cells would have the potential to 'rescue' protective immunity in patients with B cell lymphoma. Here we report the case of a patient with B cell lymphoma with profound B cell depletion after initial chemoimmunotherapy who received a total of six doses of a COVID-19 mRNA vaccine. The patient developed vaccine-induced anti-SARS-CoV-2 antibodies only after the fifth and sixth doses of the vaccine once his B cells had started to recover. Remarkably, even in the context of severe treatment-induced suppression of the humoral immune system, the patient was able to mount virus-specific CD4+ and CD8+ responses that were much stronger than what would be expected in healthy subjects after two to three doses of a COVID-19 mRNA vaccine and which were even able to target the Omicron 'immune escape' variant of the SARS-CoV-2 virus. These findings not only have important implications for anti-COVID-19 vaccination strategies but also for future antitumor vaccines in patients with cancer with profound treatment-induced immunosuppression.

Diagnosis and treatment of hairy cell leukemia as the COVID-19 pandemic continues.

Hairy cell leukemia (HCL) is an indolent B-cell malignancy, usually driven by the BRAF V600E mutation. For 30 years, untreated and relapsed HCL was successfully treated with purine analogs, but minimal residual disease (MRD) remained in most patients, eventually causing relapse. Repeated purine analogs achieve decreasing efficacy and increasing toxicity, particularly to normal T-cells. MRD-free complete remissions (CRs) are more common using rituximab with purine analogs in both 1st-line and relapsed settings. BRAF inhibitors and Ibrutinib can achieve remission, but due to persistence of MRD, must be used chronically to prevent relapse. BRAF inhibition combined with Rituximab can achieve high MRD-free CR rates. Anti-CD22 recombinant immunotoxin moxetumomab pasudotox is FDA-approved in the relapsed setting and is unique in achieving high MRD-free CR rates as a single-agent. Avoiding chemotherapy and rituximab may be important in ensuring both recovery from COVID-19 and successful COVID-19 vaccination, an area of continued investigation.

Long term follow-up of a phase II study of cladribine with concurrent rituximab with hairy cell leukemia variant.

Hairy cell leukemia variant (HCLv) responds poorly to purine analogue monotherapy. Rituximab concurrent with cladribine (CDAR) improves response rates, but long-term outcomes are unknown. We report final results of a phase 2 study of CDAR for patients with HCLv. Twenty patients with 0 to 1 prior courses of cladribine and/or rituximab, including 8 who were previously untreated, received cladribine 0.15 mg/kg on days 1 to 5 with 8 weekly rituximab doses of 375 mg/m2 beginning day 1. Patients received a second rituximab course ≥6 months after cladribine, if and when minimal residual disease (MRD) was detected in blood. The complete remission (CR) rate from CDAR was 95% (95% confidence interval, 75-100). Sixteen (80%) of 20 patients (95% confidence interval, 56-94) became MRD negative according to bone marrow at 6 months. The median duration of MRD-negative CR was 70.1 months, and 7 of 16 are still MRD negative up to 120 months. With a median follow-up of 69.7 months, 11 patients received delayed rituximab, and the 5-year progression-free survival (PFS) and overall survival (OS) were 63.3% and 73.9%, respectively. Five patients with TP53 mutations had shorter PFS (median, 36.4 months vs unreached; P = .0024) and OS (median, 52.4 months vs unreached; P = .032). MRD-negative CR at 6 months was significantly associated with longer PFS (unreached vs 17.4 months; P < .0001) and OS (unreached vs 38.2 months; P < .0001). Lack of MRD in blood at 6 months was also predictive of longer PFS and OS (P < .0001). After progression following CDAR, median OS was 29.7 months. CDAR is effective in HCLv, with better outcomes in patients who achieve MRD-negative CR. This trial is registered at www.clinicaltrials.gov as #NCT00923013.

Vemurafenib and Rituximab in Patients with Hairy Cell Leukemia Previously Treated with Moxetumomab Pasudotox.

The purine nucleoside analogues cladribine and pentostatin are highly-active first-line therapeutic treatments for hairy cell leukemia (HCL), resulting in complete response rates of 80% to 90%. However, HCL patients continue to relapse, and sooner or later, most require subsequent lines of treatment. This report presents the cases of four relapsed patients with classic HCL who were treated with vemurafenib (mostly at the low dose of 240 mg twice daily for 16 weeks) combined with rituximab after the failure of several lines of therapy including cladribine with or without rituximab and moxetumomab pasudotox. Two patients achieved minimal residual disease negative complete response after combined treatment with vemurafenib and rituximab, with a hematologic response ongoing after 38 months from the end of treatment in one patient and a relapse of cytopenias occurring after 13 months in the other patient. A third patient normalized her blood counts and this hematologic response, which was not evaluated in the bone marrow at the end of treatment, was lost after 18 months. The last patient died due to infection and multi-organ failure, too early to verify response to vemurafenib. Two patients who had relapsed after vemurafenib and rituximab derived meaningful clinical benefit from retreatment with the same agents, but eventually relapsed again and started indefinite therapy with dabrafenib and trametinib leading to normalization of the blood counts (despite heavy bone marrow infiltration in the only patient so far evaluable in that regard). The outcomes of these cases indicate that novel targeted agents and, in particular, vemurafenib, combined with rituximab, improve the prognosis of HCL patients, even those heavily pretreated with PNAs and moxetumomab pasudotox.

Hairy cell leukemia and COVID-19 adaptation of treatment guidelines.

Standard treatment options in classic HCL (cHCL) result in high response rates and near normal life expectancy. However, the disease itself and the recommended standard treatment are associated with profound and prolonged immunosuppression, increasing susceptibility to infections and the risk for a severe course of COVID-19. The Hairy Cell Leukemia Foundation (HCLF) has recently convened experts and discussed different clinical strategies for the management of these patients. The new recommendations adapt the 2017 consensus for the diagnosis and management with cHCL to the current COVID-19 pandemic. They underline the option of active surveillance in patients with low but stable blood counts, consider the use of targeted and non-immunosuppressive agents as first-line treatment for cHCL, and give recommendations on preventive measures against COVID-19.

Phase 2 study of ibrutinib in classic and variant hairy cell leukemia.

Hairy cell leukemia (HCL) is a rare B-cell malignancy, and there is a need for novel treatments for patients who do not benefit from purine analogs. Ibrutinib, an oral agent targeting Bruton tyrosine kinase in the B-cell receptor signaling pathway, is highly effective in several malignancies. Its activity in HCL was unknown, so we conducted a multisite phase 2 study of oral ibrutinib in patients with either relapsed classic or variant hairy cell leukemia. The primary outcome measure was the overall response rate (ORR) at 32 weeks, and we also assessed response at 48 weeks and best response during treatment. Key secondary objectives were characterization of toxicity and determination of progression-free survival (PFS) and overall survival (OS). Thirty-seven patients were enrolled at 2 different doses (24 at 420 mg, 13 at 840 mg). The median duration of follow-up was 3.5 years (range, 0-5.9 years). The ORR at 32 weeks was 24%, which increased to 36% at 48 weeks. The best ORR was 54%. The estimated 36-month PFS was 73% and OS was 85%. The most frequent adverse events were diarrhea (59%), fatigue (54%), myalgia (54%), and nausea (51%). Hematologic adverse events were common: anemia (43%), thrombocytopenia (41%), and neutropenia (35%). Ibrutinib can be safely administered to patients with HCL with objective responses and results in prolonged disease control. Although the initial primary outcome objective of the study was not met, the observation of objective responses in heavily pretreated patients coupled with a favorable PFS suggests that ibrutinib may be beneficial in these patients. This trial was registered at www.clinicaltrials.gov as #NCT01841723.

Moxetumomab pasudotox in heavily pre-treated patients with relapsed/refractory hairy cell leukemia (HCL): long-term follow-up from the pivotal trial.

BACKGROUND: Moxetumomab pasudotox is a recombinant CD22-targeting immunotoxin. Here, we present the long-term follow-up analysis of the pivotal, multicenter, open-label trial (NCT01829711) of moxetumomab pasudotox in patients with relapsed/refractory (R/R) hairy cell leukemia (HCL). METHODS: Eligible patients had received ≥ 2 prior systemic therapies, including ≥ 2 purine nucleoside analogs (PNAs), or ≥ 1 PNA followed by rituximab or a BRAF inhibitor. Patients received 40 µg/kg moxetumomab pasudotox intravenously on Days 1, 3, and 5 of each 28-day cycle for up to six cycles. Disease response and minimal residual disease (MRD) status were determined by blinded independent central review. The primary endpoint was durable complete response (CR), defined as achieving CR with hematologic remission (HR, blood counts for CR) lasting > 180 days. RESULTS: Eighty adult patients were treated with moxetumomab pasudotox and 63% completed six cycles. Patients had received a median of three lines of prior systemic therapy; 49% were PNA-refractory, and 38% were unfit for PNA retreatment. At a median follow-up of 24.6 months, the durable CR rate (CR with HR > 180 days) was 36% (29 patients; 95% confidence interval: 26-48%); CR with HR ≥ 360 days was 33%, and overall CR was 41%. Twenty-seven complete responders (82%) were MRD-negative (34% of all patients). CR lasting ≥ 60 months was 61%, and the median progression-free survival without the loss of HR was 71.7 months. Hemolytic uremic and capillary leak syndromes were each reported in ≤ 10% of patients, and ≤ 5% had grade 3-4 events; these events were generally reversible. No treatment-related deaths were reported. CONCLUSIONS: Moxetumomab pasudotox resulted in a high rate of durable responses and MRD negativity in heavily pre-treated patients with HCL, with a manageable safety profile. Thus, it represents a new and viable treatment option for patients with R/R HCL, who currently lack adequate therapy. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT01829711; first submitted: April 9, 2013. https://clinicaltrials.gov/ct2/show/NCT01829711.

Treatment of hairy cell leukemia.

INTRODUCTION: Purine analogs made dramatic improvements for patients with hairy cell leukemia (HCL), but patients often relapse, require multiple treatments, and may become refractory. Major developments in treatment of relapsed/refractory HCL occurred with discovery of disease biology. New agents increase the complexity of clinical decision-making. AREAS COVERED: Anti-CD22 recombinant immunotoxin Moxetumomab Pasudotox (Moxe), CD20 Mabs rituximab and obinutuzumab, BRAF/MEK inhibitors vemurafenib and dabrafenib-trametinib, and Bruton's tyrosine kinase (BTK) inhibitor ibrutinib have been tested in HCL. All show efficacy but with different treatment durations and response rates, including for eradicating minimal residual disease (MRD). Side effects differ and must be considered when selecting treatment. Studies from PubMed indexed papers and abstracts presented at major international conferences are included. EXPERT OPINION: Rituximab with either purine analog or BRAF-inhibitor achieves high rates of MRD-free complete remission (CR). Moxe achieves MRD-free CR without chemotherapy toxicities. Moxe should be considered prior to splenectomy or development of adenopathy. BRAF/MEK inhibition and ibrutinib are effective options but most patients remain MRD+, requiring indefinite treatment or rituximab to prevent relapse. Under investigation is MRD elimination with CD20 antibody combined with Moxe or BRAF inhibitor. High-risk diseases including HCL variant and IGHV4-34+ unmutated HCL require further investigation.

Concurrent chronic lymphocytic leukemia/small lymphocytic lymphoma and hairy cell leukemia: clinical, pathologic and molecular features.

Simultaneous occurrence of hairy cell leukemia (HCL) and chronic lymphocytic leukemia/small lymphocytic lymphoma (termed CLL) is very rare. Clinical characteristics, pathology and management of these cases have not been well described. We present six patients with CLL and HCL or HCL variant (HCL-v). Of six patients, three were initially diagnosed with CLL and later developed concurrent HCL. Two patients had concurrent HCL or HCL-v and CLL at initial diagnosis. One had HCL first, followed by concurrent CLL. Polymerase chain reaction analysis demonstrated B-cell clonality in all cases, with two distinct clonal populations in four cases, and three clonal populations in one case. Five patients were treated with a combination of a purine analog such as fludarabine, cladribine, and pentostastin with either rituximab or ibrutinib, while one received dabrefenib and trametinib. All patients achieved a durable response to either CLL or HCL-directed therapy with reduction or ablation of coexisting B-cell clones.

Development of Recombinant Immunotoxins for Hairy Cell Leukemia.

Hairy cell leukemia (HCL) is an indolent B-cell malignancy with excellent initial response to purine analogs pentostatin or cladribine, but patients are rarely, if ever, cured. Younger patients will usually need repeat chemotherapy which has declining benefits and increasing toxicities with each course. Targeted therapies directed to the BRAF V600E mutation and Bruton's tyrosine kinase may be helpful, but rarely eradicate the minimal residual disease (MRD) which will eventually lead to relapse. Moxetumomab pasudotox (Moxe) is an anti-CD22 recombinant immunotoxin, which binds to CD22 on HCL cells and leads to apoptotic cell death after internalization and trafficking of the toxin to the cytosol. Phase I testing achieved a complete remission (CR) rate of 57% in relapsed/refractory HCL. Most CRs were without MRD and eradication of MRD correlated with prolonged CR duration. Patients were often MRD-free after five years. Important mild-moderate toxicities included capillary leak and hemolytic uremic syndromes which could be prevented and managed conservatively. A phase 3 trial met its endpoint of durable CR with acceptable toxicity, leading to FDA approval of Moxe for relapsed/refractory HCL, under the name Lumoxiti. Moxe combined with rituximab is currently being evaluated in relapsed/refractory HCL to improve the rate of MRD-free CR.

Population pharmacokinetics, efficacy, and safety of moxetumomab pasudotox in patients with relapsed or refractory hairy cell leukaemia.

AIMS: To characterize the pharmacokinetics (PK) of moxetumomab pasudotox, an anti-CD22 recombinant immunotoxin, in adults with relapsed or refractory hairy cell leukaemia, we examined data from a phase 1 study (Study 1001; n = 49) and from the pivotal clinical study (Study 1053; n = 74). METHODS: Data from both studies were pooled (n = 123) to develop a population PK model. Covariates included demographics, disease state, liver and kidney function, prior treatment, and antidrug antibodies (ADAs). Exposure-response and exposure-safety were analysed separately by study. A 1-compartment model with linear elimination from the central compartment and 2 clearance (CL) rates was developed. RESULTS: Moxetumomab pasudotox was cleared more rapidly after cycle 1, day 1 (CL1 = 24.7 L/h) than subsequently (CL2 = 3.76 L/h), with high interindividual variability (116 and 109%, respectively). In Study 1053, patients with ADA titres >10 240 showed ~4-fold increase in CL. Higher exposures (≥median) were related to higher response rates, capillary leak syndrome and increased creatinine (Study 1053 only), or grade ≥3 adverse events (Study 1001 only). Clinical benefits were still observed in patients with lower exposure or high ADA titres. CONCLUSION: Despite a high incidence of immunogenicity with increased clearance, moxetumomab pasudotox demonstrated efficacy in hairy cell leukaemia.

Randomized Phase II Study of First-Line Cladribine With Concurrent or Delayed Rituximab in Patients With Hairy Cell Leukemia.

PURPOSE: Single-agent purine analog, usually cladribine, has been the standard first-line therapy of hairy cell leukemia (HCL) for 30 years. High complete remission (CR) rates often include minimal residual disease (MRD), leading to relapse and repeated treatments. Rituximab can clear MRD, but long-term results are unknown and optimal timing of rituximab undefined. PATIENTS AND METHODS: Patients were randomly assigned to first-line cladribine 0.15 mg/kg intravenously days 1-5 with 8 weekly doses of rituximab 375 mg/m2 begun either day 1 (concurrent, CDAR) or ≥ 6 months later (delayed) after detection of MRD in blood. MRD tests included blood and bone marrow (BM) flow cytometry, and BM immunohistochemistry. RESULTS: Sixty-eight patients with purine analog-naïve classic HCL were randomly assigned 1:1 to concurrent versus delayed arms. At 6 months after CDAR versus cladribine monotherapy, CR rates were 100% versus 88% (P = .11), MRD-free CR rates 97% versus 24% (P < .0001, primary end point), and blood MRD-free rates 100% versus 50% (P < .0001), respectively. At 96 months median follow-up, 94% versus 12% remained MRD free. Compared with CDAR, delayed rituximab after cladribine achieved lower rate (67% of 21 evaluable patients; P = .0034) and durability (P = .0081, hazard radio favoring CDAR, 0.094) of MRD-free CR. Nevertheless, 12 patients in the delayed arm remained MRD free when restaged 6-104 (median, 78) months after last delayed rituximab treatment. Compared with cladribine monotherapy, CDAR led to brief grade 3/4 thrombocytopenia (59% v 9%; P < .0001) and platelet transfusions without bleeding (35% v 0%; P = .0002), but higher neutrophil (P = .017) and platelet (P = .0015) counts at 4 weeks. CONCLUSION: Achieving MRD-free CR of HCL after first-line cladribine is greatly enhanced by concurrent rituximab and less so by delayed rituximab. Longer follow-up will determine if MRD-free survival leads to less need for additional therapy or cure of HCL.

Expression of the muscle-associated gene MYF6 in hairy cell leukemia.

Hairy cell leukemia (HCL) is a purine analog-responsive B-cell malignancy containing the BRAF V600E mutation, expressing CD22, CD11c, CD103, tartrate resistant acid phosphatase (TRAP) CD25, CD123, and annexin 1A. BRAF V600E and the latter 4 markers are usually absent in the more aggressive and chemoresistant variant HCLv. To evaluate differences between HCL and HCLv, expression microarrays comparing HCL with HCLv were performed for 24694 genes using 47323 probes. Microarray data from 35 HCL and 27 HCLv purified samples showed the greatest HCL-HCLv difference in the muscle-associated gene MYF6, expressed by its 2 probes 18.5- and 10.8-fold higher in HCL than HCLv (p<0.0001). By real-time quantitative PCR (RQ-PCR), 100% of 152 classic HCL samples were MYF6-positive, vs 5 (6%) of 90 blood donors. MYF6-expression was also detected in 18 (35%) of 51 with HCLv, 11 (92%) of 12 with HCL expressing unmutated IGHV4-34, 35 (73%) of 48 with chronic lymphocytic leukemia (CLL), and 1 (8%) of 12 with mantle cell lymphoma. Hypomethylation status of MYF6 supported expression in HCL more than HCLv. Posttreatment blood samples becoming negative by flow cytometry remained MYF6+ by RQ-PCR in 42 (48%) of 87 HCL patients, and MYF6 RQ-PCR could detect 1 HCL in 105 normal cells. MYF6, universally expressed in HCL and in most CLL samples, may be a useful biomarker for these leukemias. Further studies are underway to determine the role of MYF6 in HCL.