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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.

A diagnosis of leukemia can have a profound effect on patients' health-related quality of life (HRQoL), however this has not been measured prospectively in patients with hairy cell leukemia (HCL). At the request of patients living with HCL who had identified this gap in knowledge about the disease, we conducted a longitudinal study of HRQoL among patients enrolled in the HCL Patient Data Registry (PDR). From September 1, 2018 to September 1, 2020, 165 patients were enrolled in the study and completed the baseline survey. The Functional Assessment of Cancer Therapy - Leukemia (FACT-Leu) was used to measure patients' HRQoL. Results show that newly diagnosed HCL patients reported the lowest HRQoL, followed by patients in relapse and those on "watch and wait." Factors associated with higher (better) FACT-Leu total scores in the multivariable analysis included older age, higher social support, and greater physical activity. These same factors were associated with lower levels of fatigue. In rare diseases where it is difficult to perform large prospective studies, patient/researcher collaborations are critical for the identification of studies that are of importance to patients and their families in order to maximize the benefits of the research and improve the lives of patients living with HCL., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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.

Hairy cell leukemia is an uncommon hematologic malignancy characterized by pancytopenia and marked susceptibility to infection. Tremendous progress in the management of patients with this disease has resulted in high response rates and improved survival, yet relapse and an appropriate approach to re-treatment present continuing areas for research. The disease and its effective treatment are associated with immunosuppression. Because more patients are being treated with alternative programs, comparison of results will require general agreement on definitions of response, relapse, and methods of determining minimal residual disease. The development of internationally accepted, reproducible criteria is of paramount importance in evaluating and comparing clinical trials to provide optimal care. Despite the success achieved in managing these patients, continued participation in available clinical trials in the first-line and particularly in the relapse setting is highly recommended. The Hairy Cell Leukemia Foundation convened an international conference to provide common definitions and structure to guide current management. There is substantial opportunity for continued research in this disease. In addition to the importance of optimizing the prevention and management of the serious risk of infection, organized evaluations of minimal residual disease and treatment at relapse offer ample opportunities for clinical research. Finally, a scholarly evaluation of quality of life in the increasing number of survivors of this now manageable chronic illness merits further study. The development of consensus guidelines for this disease offers a framework for continued enhancement of the outcome for patients.

The natural history of hairy cell leukemia (HCL) includes frequent and potentially life-threatening infections. Prior to the development of effective therapy, the incidence in patients followed for several years was as high as 60%, with infection as a prime cause of death in patients. Studies of the immune system of patients with HCL identified several potential reasons, including profound neutropenia and monocytopenia. In addition, treatment including chemotherapy and splenectomy further compromised the immune system. The success of new therapies has changed the frequency and severity of infections in patients with HCL. During the initial phase of treatment, however, infection risk remains high, with incidence ranging from 30 to 50%. Attempts to ameliorate the risk with growth factors in conjunction with treatment have not been successful, but lower doses of drugs and/or combination therapy have been tried with reported success. In the majority of patients, successful therapy results in normalization of the neutrophil count and marked reduction in the severity and frequency of infections. Interestingly, after purine nucleoside treatment, there is profound depression of CD4+ cells without development of the opportunistic infections seen with patients with human immunodeficiency virus (HIV). Studies to reduce morbidity and mortality should focus on initial induction regimens, as well as confirming the long-term benefit of treatment on risk of infection.

Hairy cell leukemia (HCL) is a chronic lymphoproliferative disease of B-cell origin manifested by pancytopenia and splenomegaly. Before 1980 the only effective treatment for HCL was splenectomy, which resolved the cytopenia but did not eliminate the disease from the bone marrow. In addition, the majority of patients progressed after splenectomy and required further treatment. Pentostatin (Nipent; SuperGen, San Ramon, CA) is a purine antimetabolite that was found in phase I studies to induce profound lymphocytopenia Although in vitro studies suggested that T lymphocytes were most sensitive to pentostatin, patients with B-cell chronic lymphatic leukemia and low-grade non-Hodgkin's lymphoma responded to treatment in the initial phase I trials. Due to evidence that the drug was effective in lymphoproliferative disease, patients with HCL were treated with pentostatin. The promising initial results led to phase II studies in both untreated and previously treated patients. These studies demonstrated that pentostatin was highly effective as a single agent, with complete responses seen in 60% to 90% of patients. These responses were durable without maintenance chemotherapy and were seen in patients previously treated with interferon or chemotherapy. Toxicity was usually mild, with nausea and skin rashes predominating. When seen, infections resulting from neutropenia occurred early in treatment. The high response rates and low toxicity suggest that pentostatin should be considered as one of the standard treatments for HCL.

Twenty-four patients with advanced hairy cell leukemia treated with 2'-deoxycoformycin (dCF) were studied after achieving complete remission to determine the impact of treatment on survival, disease-free survival, long-term complications of treatment, and response to retreatment. At a median follow-up time of 82 months (range, 54 to 104 months), 23 of 24 patients remain alive. One patient has died of recurrent disease refractory to treatment. Of the remaining 23 patients, 11 have relapsed at a median time of 30 months (range, 7 to 80 months) after treatment completion. Of these 11 patients, 7 have been retreated with dCF or 2'-chlorodeoxyadenosine (2-CdA), including one patient that was retreated twice. All seven patients have responded, with five patients achieving second complete remission. Two patients have had normalization of blood cell counts, but repeat bone marrows have not been performed. No serious infections have been seen in dCF-treated patients during follow-up. One case of Hodgkin's disease and three cases of skin malignancies have developed in these 24 patients. From initiation of treatment, survival is 93 months (range, 63 to 116 months). We concluded that dCF significantly prolongs the survival of patients with advanced hairy cell leukemia without resultant long-term complications. It is too early to predict if this therapy will be curative for the patients still in remission.

Hairy cell leukemia is a malignant B-cell disorder characterized by splenomegaly and pancytopenia. The malignant cell is morphologically unique and characterized by fine cytoplasmic projections. Although studies of the cell have revealed important information about its proliferative capacity, cell surface, and membrane composition, less is known about the metabolic characteristics of the cell. We have previously investigated the oxidative metabolism of the hairy cell and have suggested that hairy cells might have a unique glucose metabolism compared to normal lymphocytes. This is indicated by a high rate of [6-14C]glucose oxidation in short-term culture consistent with an active Kreb's cycle and a high ratio of [6-14C]glucose oxidation to [1-14C] glucose oxidation. In this study, we evaluated an additional group of patients with hairy cell leukemia prior to or after treatment with the experimental drug 2'-deoxycoformycin (dCF). We found that in seven of eight patients the leukemic cells had a pattern similar to that previously described and that all of these seven patients had a significant response to therapy. The cells of the eighth patient had minimal Kreb's cycle activity, and at the time of study the patient was resistant to therapy with dCF. The metabolic activity of hairy cells may distinguish them from other lymphoid populations and may be a marker for sensitivity to dCF.

Our experience suggests a potential role of fludarabine phosphate in the management of hairy cell leukemia. On the basis of this experience, the activity of F-AMP needs to be further evaluated in this disease.

The combined effects of interferon (IFN) and 2'-deoxycoformycin (DCF) on 2',5'-oligoadenylate (2-5A) synthetase and adenosine deaminase (ADA) activity were examined in vitro in peripheral blood mononuclear cells from patients with hairy cell leukemia (HCL) and chronic lymphocytic leukemia (CLL). In HCL cells, the effects of IFN and DCF on both OAS and ADA activity were strictly antagonistic. Antagonism in inducing OAS activity was also observed in CLL cells, although, in general, little interaction between DCF and IFN occurred in these cells. If OAS and ADA are important in the antitumor effects of IFN and DCF, our results suggest that combination therapy with IFN and DCF could be counterproductive.

The immune function of patients with hairy cell leukemia (HCL) and solid tumors was evaluated before and after treatment with the investigational drug 2'-deoxycoformycin (pentostatin; dCF). Thirteen HCL patients received doses of dCF of 2 to 4 mg/m2 intravenously at 2- to 6-week intervals for up to 15 courses. After completion of treatment, 12 of 13 patients had resolution of severe monocytopenia and five of nine had normal monocyte antibody dependent cellular cytotoxicity. There was statistically significant depression of total lymphocytes, T cells, and B cells. Evaluation of T subsets showed a decrease in CD4+ cells. Immunoglobulin G (IgG) in sera were decreased from baseline, while IgM and IgA were unaffected. There was no significant effect on skin-test reactivity or large granular lymphocyte numbers. Lymphoblastic transformation was variably affected. Natural-killer (NK) cell function was improved or unchanged after dCF treatment. Reevaluation of seven patients at 21 to 119 weeks after receiving dCF demonstrated that recovery to normal T- and B-cell numbers and subsets does occur. Five solid tumor patients were given dCF at 4 mg/m2 intravenously at 1- to 2-week intervals for up to five courses. There was significant reduction in T cells, B cells, CD4+, and CD8+ cells with no statistically significant effect on the other immune parameters. We conclude that low doses of dCF can cause persistent immunosuppression though recovery may occur after the drug is stopped. In patients followed after completion of dCF, there was no associated increase in second malignancies or unusual infections.

Four patients with the leukemic phase of hairy cell leukemia were treated with leukapheresis. Two patients failed to respond, and the other two had only transient responses; hematologic improvement lasted one month in one case and four months in the second. The patient with a four-month response underwent a second series of leukapheresis resulting in a response lasting at least 8 months. Two of the four patients subsequently had an adequate trial of prolonged chlorambucil therapy and continued to have a clinical response. We conclude that leukapheresis has little to offer to the majority of hairy cell patients for the long-term management of post-splenectomy patients who develop the leukemic phase. However, an occasional patient can have a transient, and even, rarely, a prolonged response.

Ten patients with progressive hairy cell leukemia were treated with 2'deoxycoformycin (dCF) by intravenous bolus (4 mg/m2) given every other week. All ten patients are evaluable for response and nine of the ten patients have achieved a complete remission. In addition to clearing of hairy cells from the bone marrow, eight patients had resolution of their monocytopenia. Seven of the nine patients remain in unmaintained remission with a median duration of 6.2 months. Two patients have had relapse in the bone marrow alone and continue to have normal peripheral blood counts. They are being followed without treatment. Toxicity was minimal at this low dose with one patient having a mild reversible reduction in creatinine clearance. Four other patients had reversible neutropenia. There were no significant infections associated with treatment. Low-dose deoxycoformycin administered intravenously every other week represents an extremely effective treatment for hairy cell leukemia.

2'-Deoxycoformycin (pentostatin [dCF]), a potent inhibitor of adenosine deaminase (ADA), was administered in a biweekly low-dose (2 to 4 mg/m2) intravenous (IV) schedule to patients with advanced hairy cell leukemia. Twenty-three patients were treated, including 12 patients previously treated by splenectomy and five patients treated with interferon. Twenty-one of 23 patients had objective responses, including 20 who achieved a complete remission (CR). Responses occurred rapidly, with an average time to CR of 5.4 months. Treatment was not continued once CR was achieved, and 15 of 20 patients remain in remission with an average duration of 12.6 months. CRs were achieved in both patients previously treated with interferon (three of five) and patients with marked splenomegaly (three of three). Relapses, when seen, have occurred in the bone marrow alone and the one patient who required retreatment was reinduced into CR. Toxicity has been mild and reversible, with nausea and vomiting, conjunctivitis, and skin rash as the main complications of treatment. dCF is the most effective single agent in the treatment of hairy cell leukemia, inducing a high percentage of CRs in all subgroups. Two multiinstitutional trials are now underway to compare its effectiveness v alpha interferon.

Twelve evaluable patients with progressive hairy cell leukemia were treated with deoxycoformycin at a dose of 4 mg/m2 every 2 weeks. Five patients had not been splenectomized, and one had failed to respond to interferon-alpha. Complete remission, as defined by absence of hairy cells in the bone marrow and normalization of the peripheral blood and regression of splenomegaly, was obtained in 11 of 12 patients (92%). These patients have remained in unmaintained remission for 1+ to 13 months with an average of 7.5 months. Two of these patients had a bone marrow relapse at 8 and 12 months, respectively. During treatment the monocytopenia corrected, and, after complete remission was obtained, marrow was aspirable. Toxicity was mild and reversible. There were no significant infections associated with this treatment. It was of interest that we could treat two patients with creatinine clearance of 50 and 60 ml/min using lower doses (and 2-3 mg/m2) than our conventional therapy of 4 mg/m2 every 2 weeks. They obtained a complete remission after 6 and 10 treatments, respectively. Low-dose deoxycoformycin has proven to be an excellent treatment for hairy cell leukemia.