Long-term follow-up of patients with aplastic anemia and refractory anemia responding to combination therapy with recombinant human granulocyte colony-stimulating factor and erythropoietin

In our previous study, approximately 60% of aplastic anemia (AA) and refractory anemia (RA) patients treated with recombinant human granulocyte colony-stimulating factor (rhG-CSF) and recombinant human erythropoietin (rhEpo) showed a multilineage response. In this study, we analyzed the long-term follow-up of the multilineage responders (multi-R). In the follow-up analysis of 11 multi-R (6 AA and 5 RA), 10 patients (5 AA and 5 RA) were evaluable. The range of time from the start of treatment to the final contact was 50 to 125 months. Analysis of survival times revealed a significant difference between multi-R and non-multi-R among AA patients given this treatment (P = .016). One AA and 1 RA patient among the multi-R developed acute leukemia. Of 7 living multi-R, 3 AA and 2 RA patients did not need transfusion at final contact. Four of them maintained the target hemoglobin concentration of more than 11 g/dL for quality-of-life benefit. The findings suggested that this result is an important advantage of this treatment.     

Bilineage response in refractory aplastic anemia patients following long-term administration of recombinant human granulocyte colony-stimulating factor.

5 patients with refractory aplastic anemia (AA) received long-term administration (2-11 + months) of recombinant human G-CSF (rhG-CSF) in doses from 250-500 micrograms/body/day by intravenous infusion or 75-300 micrograms/body/d by subcutaneous injection. All 5 evaluable patients showed a substantial increase in absolute neutrophil count (ANC) with a recovery of myeloid components in the bone marrow after 1 to 2 months of treatment. Interestingly, 2 out of the 5 patients showed a dramatic improvement in severe anemia after 2 to 4 months of treatment accompanying a recovery of erythroid components in the bone marrow. In addition, there was no serious infection before or during therapy. Long-term administration of rhG-CSF was well tolerated because of its minimal toxicity. Clonal assay revealed a recovery of myeloid progenitors in all patients and a recovery of erythroid progenitors in 3 out of the 5 patients. These results suggest that long-term administration of rhG-CSF at least mobilizes residual myeloid as well as erythroid progenitor cells and induces a bilineage response in severe refractory AA.     

Treatment of severe aplastic anemia with an immunosuppressive agent plus recombinant human granulocyte-macrophage colony-stimulating factor and erythropoietin

To evaluate the therapeutic potential of hematopoietic growth factors (HGFs) during immunosuppressive treatment (IST) of severe aplastic anemia (SAA), 38 patients with newly diagnosed SAA received IST alone (group I), or IST plus recombinant human erythropoietin and granulocyte-macrophage colony-stimulating factor (rhEPO + rhGM-CSF) (group II). Eleven patients in each group received antilymphocyte globulin (ALG) for IST, and eight patients in each group received cyclosporine (CSA). Complete remission rates at one year were 26% and 74% for group I and group II patients, respectively. The ALG-treated subgroup showed the greatest differences between treatments. Compared with patients receiving ALG alone, patients treated with ALG plus HGFs had significantly better one-year survival (100% vs. 54.5%, P < 0.05), complete remission rates (91% vs. 36%, P < 0.05), more rapid and complete hematologic recovery, greater reductions in transfusion requirements, and lower infection rates. The data suggest a potential role for rhEPO + rhGM-CSF therapy in SAA patients receiving IST. Am. J. Hematol. 59:185–191, 1998. © 1998 Wiley-Liss, Inc.     

Treatment of aplastic anemia in children with recombinant human granulocyte colony-stimulating factor

Twenty children (aged 1 to 17 years) with severe or moderate aplastic anemia were treated with recombinant human granulocyte colony- stimulating factor (rhG-CSF) at a dose of 400 micrograms/m2 per day administered as a 30-minute intravenous (IV) infusion daily for 2 weeks. This treatment increased the neutrophil counts (2.7- to 28.0- fold) in 12 of the 20 patients. Increasing doses (800 or 1,200 micrograms/m2 per day) were administered to five patients who had not responded to the initial dose, and three showed an increase in neutrophil count. Differential counts of bone marrow (BM) aspirates showed an increase in the myeloid/erythroid ratio. The response was transient, however, and the neutrophil count returned to baseline within 2 to 10 days of discontinuing treatment. No severe toxicity attributable to rhG-CSF was observed. The results suggest that this agent is effective in stimulating granulopoiesis in children with aplastic anemia. Our study also indicates that rhG-CSF will be particularly useful in managing patients with aplastic anemia complicated by bacterial or fungal infection.     

Responsiveness of bone marrow erythropoietic stem cells (CFU-E and BFU-E) to recombinant human erythropoietin (rh-Ep) in vitro in aplastic anemia and myelodysplastic syndrome

Responsiveness of bone marrow erythropoietic stem cells (CFU-E and BFU-E) to recombinant human erythropoietin (rh-Ep) was examined in vitro in 23 patients with aplastic anemia and 14 with myelodysplastic syndrome (MDS) to investigate the clinical use of rh-Ep for these diseases. Bone marrow mononuclear cells were cultured by methylcellulose methods for CFU-E and BFU-E assays. In normals, the CFU-E numbers reached a plateau of increase at Ep doses of almost 2-5 units, and no further increase was observed with the addition of larger Ep doses. In aplastic anemia, the responses of CFU-E to Ep were relatively good in nonsevere type and generally poor in severe type. However, the CFU-E numbers increased with increasing doses of Ep in some of the patients with aplastic anemia. Among the patients with MDS, the responses of CFU-E to Ep were relatively good in primary acquired refractory anemia (PARA) and primary acquired sideroblastic anemia. On the other hand, the responses of CFU-E to Ep were poor in refractory anemia with an excess of blasts (RAEB) and RAEB in transformation among the MDS patients. BFU-E responses to Ep were poor in severe aplastic anemia, RAEB, and RAEB-T. However, there are Ep responsive patients in some of aplastic anemia and PARA. High titers of rh-Ep were suggested to be effective clinically in some patients with aplastic anemia and those with PARA.     

Failure of combination therapy with recombinant granulocyte colony-stimulating factor and erythropoietin in myelodyspplastic syndromes

Summary Recombinant human granulocyte colonystimulating factor (rhG-CSF) and erythropoietin (rhE-PO) were used to treat ten patients with myelodysplastic syndromes (MDS). None of the patients showed a favorable response in erythrocyte and platelet counts following 10 weeks' treatment, although favorable responses in neutrophil counts were observed in eight of ten patients (80.0%) and in seven of eight patients (87.5%) following 2 weeks' and 10 weeks' treatment, respectively. However, one patient with refractory anemia had a delayed favorable response in erythrocyte and neutrophil counts at week 14 in spite of the cessation of combination therapy at week 10. These results indicate that combination therapy with rhG-CSF and rhEPO is not beneficial to patients with MDS, based on the presently used protocol.     

Effect of androgen therapy and anemia on serum erythropoietin levels in patients with aplastic anemia and myelodysplastic syndromes

Immunoreactive serum erythropoietin (EPO) was measured in anemic and non-anemic patients with acquired non-severe aplastic anemia (AA; n = 22) and myelodysplastic syndromes (MDS; n = 31) receiving or not androgens to examine the effect of androgen therapy and anemia on EPO levels in these disorders. Soluble transferrin receptor (TfR) and absolute reticulocyte count (ARC) were also assayed in order to evaluate erythropoietic activity. AA and MDS patients were stratified for anemia and androgen treatment as follows: 12 untreated anemic patients; 17 anemic patients during androgen therapy; 14 non-anemic patients without any treatment (>1 year); and 10 non-anemic patients on androgen therapy. Although EPO levels in non-anemic patients were significantly higher than in healthy controls (n = 29) no statistically significant differences in Hb and EPO values were found between non-anemic patients receiving or not androgen therapy. In the linear regression analysis between Hb and log EPO concentration, no statistically significant differences in the slopes between untreated and androgen-treated anemic groups nor between both groups and patients with iron deficiency anemia (n = 23) were observed. However, the y intercept (log EPO) of regression line was significantly higher in androgen-treated anemic patients than in the androgen therapy-free anemic group. Serum TfR levels were higher in treated than in untreated anemic patients, whereas ARC was not different between both groups. These data seemingly indicate that (1) androgens at pharmacological doses do not increase serum EPO levels in non-anemic AA and MDS patients, and (2) in patients with AA and MDS, androgen-driven EPO stimulation is appreciably enhanced by anemia. Am. J. Hematol. 57:113–118, 1998. © 1998 Wiley-Liss, Inc.     

Effect of recombinant human granulocyte macrophage-colony stimulating factor in long-term marrow cultures from patients with aplastic anemia.

The hematopoietic system in patients with aplastic anemia (AA) shows both quantitative and qualitative deficiencies, i.e., reduced numbers of hematopoietic progenitor cells (HPC) and impaired HPC proliferation in long-term marrow cultures (LTMC). Since recombinant human granulocyte macrophage-colony stimulating factor (rhGM-CSF) has been shown to be a potent stimulator of normal hematopoiesis, both in vivo and in vitro, in the present study we wanted to assess the possibility of stimulating hematopoiesis in LTMC from 17 patients with AA, by weekly addition of rhGM-CSF (10 ng/ml). In LTMC from 11 patients (group of responders), rhGM-CSF induced a significant increase (4.8-fold, compared with untreated cultures) in the levels of myeloid progenitor cells; in contrast, in six patients (group of nonresponders), myeloid progenitors were refractory to this cytokine. In the group of responders, rhGM-CSF also induced a pronounced increment in the levels of nonadherent and adherent cells (5.99- and 5.18-fold, respectively, compared with untreated cultures). Among the different myelopoietic lineages, rhGM-CSF preferentially stimulated the macrophagic lineage; this was evident both at the progenitor and mature cell levels. Interestingly, the effect of rhGM-CSF in LTMC from AA patients was only transient. Indeed, the effects mentioned above were observed only during the first three weeks of culture; afterwards, myeloid progenitor and nonadherent cell levels in treated cultures declined, practically reaching the levels observed in untreated cultures. At the moment, we do not know whether this transient stimulatory effect is due to the production of inhibitory cytokines, by macrophages generated in response to rhGM-CSF, or to the exhaustion of the HPC pool in AA cultures. In all 17 patients, rhGM-CSF had no effect on the kinetics of erythroid or multipotent progenitor cells. These results are in keeping with clinical studies in which it has been observed that most AA patients treated with rhGM-CSF show increments in circulating monocytes and granulocytes, as well as in bone marrow cellularity. However, little or no effect is observed on erythropoiesis. The actual mechanisms involved in the in vitro effects of rhGM-CSF on myeloid progenitor cells from AA bone marrow are still not completely understood. Future studies on this issue should be encouraged, since they may help to understand the in vivo (clinical) effects of this cytokine.     

A survey of the effectiveness of cyclophosphamide in patients with severe aplastic anemia

The members of the American Society of Hematology were surveyed to determine their experience in the treatment of patients with severe aplastic anemia (AA) with cyclophosphamide (CYT). Among the 401 members responding, 50 had treated AA patients, and three remissions were reported. These remissions can be explained either by response to CYT in a subgroup of patients with AA due to an immune mechanism or to spontaneous remission. Clinical trials of the efficacy of cytotoxic drugs in the treatment of AA patients will not likely permit a distinction between drug effect and spontaneous remission until the subgroup of patients with immune-mediated AA is identified through the development of more sensitive and specific in vitro tests.     

Treatment of platelet alloimmunization with intravenous immunoglobulin in a child with aplastic anemia

A 9-year-old girl with severe refractory aplastic anemia had become refractory to platelet transfusion due to platelet alloimmunization. As a result, a huge right parieto-temporo-occipital cerebral hematoma and a subarachnoid hemorrhage subsequently occurred. Intravenous immunoglobulin (IVIG) (Green Cross, Taiwan) at a dose of 0.4 g/kg/day for 5 days, followed by another 3 doses during the following 5 days, and combined with large-dose platelet transfusions effectively controlled the bleeding by elevating the platelet count to above 100 × 10³/mm³. The patient soon recovered with only a mild sequela. Two months later, because of multiple caries and a periodontal abscess, dental extraction needed to be performed. IVIG (0.4 g/kg/day × 6 days) enabled surgery to proceed by elevating the platelet count to above 55 × 10³/mm³, and no bleeding complications occurred during or after the procedure. The potential benefit of high-dose IVIG in modulating platelet alloimmunization is a result of increasing the survival of transfused platelets. Thus, this therapy is recommended when patients with platelet alloimmunization have critical bleeding episodes or undergo surgical procedures. © 1995 Wiley-Liss, Inc.     

Results and follow-up of a phase III randomized study of recombinant human-granulocyte stimulating factor as support for immunosuppressive therapy in patients with severe aplastic anaemia

In patients with idiopathic severe aplastic anaemia who are treated with immunosuppressive agents to combat T lymphocyte-mediated destruction of haematopoietic progenitor cells, neutropenia is a major cause of infections and toxicity. Evidence from preliminary studies suggests that recombinant human glycosylated granulocyte colony-stimulating factor (lenograstim) increases the number and functionality of neutrophils in patients with severe aplastic anaemia. This randomized, parallel-group, multicentre study was conducted to evaluate the efficacy and safety of subcutaneous lenograstim during the first 12 weeks of standard immunosuppressive therapy in 102 patients with de novo severe aplastic anaemia. The addition of lenograstim to standard therapy resulted in an increase in the proportion of patients showing complete neutrophil response (83.0%vs 44.9%; P < 0.0001). This was seen even among patients with very severe aplastic anaemia (69.2%vs 31.6%; P = 0.012). In patients receiving lenograstim, median time to complete neutrophil response was shorter (6.3 vs 16.1 weeks; P = 0.0001) and mean duration of first neutrophil response was longer (P = 0.0248) than in the control group. At a median follow-up of 5 years, no difference was observed between the groups in term of survival, haematological response and occurrence of secondary leukaemia (one patient in each group). We conclude that lenograstim support of immunosuppressive therapy might be used for patients with severe aplastic anaemia as it significantly enhances neutrophil recovery but does not modify the overall response and survival.     

Treatment of the anemia of myelodysplastic syndromes using recombinant human granulocyte colony-stimulating factor in combination with erythropoietin [see comments]

We treated myelodysplastic syndrome patients (MDS) with both recombinant human granulocyte colony-stimulating factor (G-CSF) and recombinant human erythropoietin (EPO) to determine whether such combination therapy resulted in improvement of their anemias. Twenty- four of 28 patients begun on study completed the protocol and were evaluable for erythroid responses. Therapy was initiated with G-CSF at 1 micrograms/kg administered by daily subcutaneous injection and adjusted to either normalize or double the neutrophil count. EPO was then administered by daily subcutaneous injection at a dose of 100 U/kg and dose-escalated to 150 and 300 U/kg every 4 weeks while continuing the G-CSF. Changes in absolute reticulocyte count, hematocrit level, and need for RBC transfusions were compared with pretreatment values as well as other blood cell counts. Ten of 24 patients (42%) had erythroid responses, whereas all patients had neutrophil responses. Six previously transfused patients no longer required RBC transfusions during the treatment period. Erythroid responses were found to be independent of patient age, French-American-British subtype, duration of disease, prior RBC transfusion requirements, or cytogenetic abnormalities at presentation. Pretreatment serum EPO levels were lower in erythroid-responding as compared with nonresponding patients (median 157 v 600 U/L; P = .05). The combined treatment modality was generally well tolerated. We conclude that a substantial percentage of MDS patients had both erythroid and myeloid responses when treated with the combination of G-CSF and EPO.     

Treatment of the anemia of predialysis patients with recombinant human erythropoietin: a randomized, placebo-controlled trial

Recombinant human erythropoietin (r-HuEPO) was administered in two phases to 12 patients with chronic renal insufficiency (creatinine clearances of 0.17-0.51 ml/second [10-30 ml/minute]) and uremic anemia. In addition to the routine tests done as part of a multicenter clinical trial, our patients had serial red cell mass measurements, quantitation of bone marrow stem cells, and marrow cytogenetic analysis. During the first eight weeks (acute phase), an equal number of patients was randomized to placebo or one of three doses of r-HuEPO (50, 100 or 150 unit/kg intravenously three times weekly). All three patients receiving 150 unit/kg responded by increasing their packed cell volume (PCV) to the normal range within eight weeks. There were lesser responses in PCV at the two lower doses of r-HuEPO and no response in the placebo group. The 51Cr red cell mass also increased significantly in a dose-related manner in patients receiving r-HuEPO but did not change in the placebo group. Marrow studies revealed increases in erythroid, megakaryocyte, and granulocyte-monocyte progenitor cells in those patients on r-HuEPO, but no mutagenic effects were seen. Subsequently, ten patients received open label r-HuEPO. During this maintenance phase, all ten achieved or maintained a normal PCV. Several adverse events occurred, but none were definitely linked to r-HuEPO. Recombinant human erythropoietin is an effective and potent treatment of anemia caused by renal failure.     

Increased production of tumor necrosis factor-alpha by peripheral blood mononuclear cells in the patients with aplastic anemia

The activity of tumor necrosis factor-alpha (TNF-alpha) in the supernatant of cultured peripheral blood mononuclear cells (PBMC) was measured in patients with aplastic anemia. It was significantly higher in patients with aplastic anemia than in normal controls, both when PBMC were unstimulated or when they were stimulated with PHA. Results from aplastic anemia patients were also significantly higher than patients who had received allogeneic bone marrow transplants. In aplastic anemia patients, the TNF-alpha value produced by PBMC upon stimulation and the platelet count were inversely correlated, as well those patients who had high TNF-alpha values tended to have lower hemoglobin and leukocyte values although this was not significant statistically. These results suggest that the increased production of TNF-alpha by PBMC plays a role in the severe suppression of hematopoiesis in aplastic anemia.     

Recombinant human granulocyte-macrophage colony-stimulating factor plus recombinant human erythropoietin may improve anemia in selected patients with myelodysplastic syndromes

The purpose of this study was to improve erythropoiesis in patients with anemia due to myelodysplastic syndromes (MDS). We treated 13 patients first with recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) for 6 weeks, then with recombinant human erythropoietin (rhEpo) and rhGM-CSF for the next 12 weeks. Five patients had refractory anemia (RA), 3 refractory anemia with ringed sideroblasts (RAS), and 5 refractory anemia with excess of blasts (RAEB). Ten patients were transfusion-dependent at the time of inclusion. Eleven patients completed this phase II study. Five responded with an increase in hemoglobin level (3 patients) or a reduction in transfusion requirement (2 patients). We registered no response in the remaining 6 patients during treatment. Patients responding to combined treatment had relatively low concentrations of plasma Epo and plasma ferritin before treatment with rhEpo and a normal karyotype throughout the study. Long-term bone marrow cultures did not predict the response. Still, responders seemed to have a higher number of colony-forming progenitors than non-responders. In conclusion, combined therapy with rhGM-CSF and rhEpo may stimulate hematopoiesis and correct or improve anemia in some patients with MDS. © 1993 Wiley-Liss, Inc.     

Aplastic anemia successfully treated with rituximab: the possible role of aplastic anemia-associated autoantibodies as a marker for response

A 1-yr-old Japanese male infant developed hepatitis-associated aplastic anemia (AA), and anti-thymocyte globulin (ATG) plus cyclosporine A (CsA) was administered without any appreciable effects. Laboratory examination of the patient's serum obtained before therapy revealed various autoantibodies, such as PA-IgG, anti-platelets, anti-single-stranded DNA (ssDNA), and anti-double-stranded DNA (dsDNA) antibodies (Abs) in addition to anti-DRS-1 Abs and anti-moesin Abs, both of which are known to be detectable in approximately 40% of all patients presenting with AA. He was therefore treated with 17.5 mg/kg/d rituximab 5.5 months after ATG/CsA therapy. The same rituximab therapy was repeated three times once a month thereafter. His neutrophil counts started to increase 50 d after the first rituximab therapy and he achieved a complete remission at 16 months after the last rituximab administration. All of the autoantibodies including anti-ssDNA, dsDNA, DRS-1, and moesin became undetectable when he attained the remission. Anti-CD20 monoclonal antibody therapy may be effective in a subset of patients with AA characterized by the presence of autoantibodies.     

再生障碍性贫血和骨髓增生异常综合征患者骨髓CD34+细胞及其粒细胞集落刺激因子受体的研究

目的:检测再生障碍性贫血(AA)和骨髓增生异常综合征(MDS)患者CD34 细胞占骨髓单个核细胞(BMMNC)的比率及其表面粒细胞集落刺激因子受体(G-CSFR)的表达率。方法:用流式细胞术(FCM)检测13例AA、22例MDS及12例非血液病患者CD34 细胞占BMMNC的比率及其表面G-CSFR的表达率。结果:AA组与对照组、AA组与MDS组、MDS-难治性贫血(RA)组与难治性贫血伴原始细胞增多(RAEB)组BMMNC中CD34 细胞的比率比较差异有统计学意义(P<0.05),但G-CSFR的表达率差异无统计学意义(P>0.05)。多数重型AA(SAA)患者(3/4)及少数慢性AA(CAA)患者(1/9)BMMNC中的CD34 细胞少于0.1%。多数G-CSFR表达率低(<14%)的患者(7/9)外周血中性粒细胞减少;而表达率正常(14.0%~28.9%)的患者(1/6)很少见;表达率高(>28.9%)的患者(3/7)也可存在中性粒细胞减少。结论:造血干细胞减少是AA的主要发病机制之一,其表面G-CSFR的表达率不是影响AA的主要因素;MDS患者CD34 细胞比率升高是一个预后不良的指标,G-CSFR的检测可部分解释MDS患者外周血中性粒细胞减少的原因。     

A cohort study of the nature of paroxysmal nocturnal hemoglobinuria clones and PIG-A mutations in patients with aplastic anemia

Abstract:  Background:  Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by the clonal expansion of blood cells, which are deficient in glycosylphosphatidylinositol anchored proteins (GPI-APs). As PNH frequently occurs during the clinical course of acquired aplastic anemia (AA), it is likely that a process inducing bone marrow failure in AA is responsible for the selection of GPI-AP deficient blood cells or PNH clone. Objective:  To explore the nature and mutation of a PNH clone in AA. Methods:  We performed regular repeated flow cytometric analyses of CD59 expression on peripheral blood cells from a cohort of 32 patients with AA. Mutation of phosphatidylinositol glycan class A (PIG-A) was also studied. Results:  Fifty-one episodes of occurrences of CD59 negative granulocytes out of a total cohort 167 flow cytometric analyses (31%) were observed in 22 patients (69%). CD59 negative erythrocytes were less apparent than the granulocytes. Repeated occurrences of PNH clones were observed in 16 patients. Most of the emerging PNH clones were transient in nature. They were more frequently detected during episodes of lower white blood cell and platelet counts. Persistence and expansion of the GPI-AP deficient blood cell populations to the level of clinical PNH were seen in only four patients (12.5%). Analysis of PIG-A gene demonstrated eight mutations among the four patients, with two and four independent mutations in two patients. Conclusions:  Our study indicates that PIG-A mutations of hematopoietic stem cells with resultant PNH clones, are relatively common among AA patients. It also supports the hypothesis of selection of the PNH clone by a process or condition associated with or responsible for bone marrow failure in AA. However, there must be an additional factor favoring expansion or growth of the clone to the level of clinical or florid PNH.     

Incidence and risk factors of aplastic anemia in Latin American countries: the LATIN case-control study

Background

Associations between aplastic anemia and numerous drugs, pesticides and chemicals have been reported. However, at least 50% of the etiology of aplastic anemia remains unexplained.

Design and Methods

This was a case-control, multicenter, multinational study, designed to identify risk factors for agranulocytosis and aplastic anemia. The cases were patients with diagnosis of aplastic anemia confirmed through biopsy or bone marrow aspiration, selected through an active search of clinical laboratories, hematology clinics and medical records. The controls did not have either aplastic anemia or chronic diseases. A total of 224 patients with aplastic anemia were included in the study, each case was paired with four controls, according to sex, age group, and hospital where the case was first seen. Information was collected on demographic data, medical history, laboratory tests, medications, and other potential risk factors prior to diagnosis.

Results

The incidence of aplastic anemia was 1.6 cases per million per year. Higher rates of benzene exposure (≥30 exposures per year) were associated with a greater risk of aplastic anemia (odds ratio, OR: 4.2; 95% confidence interval, CI: 1.82–9.82). Individuals exposed to chloramphenicol in the previous year had an adjusted OR for aplastic anemia of 8.7 (CI: 0.87–87.93) and those exposed to azithromycin had an adjusted OR of 11.02 (CI 1.14–108.02).

Conclusions

The incidence of aplastic anemia in Latin America countries is low. Although the research study centers had a high coverage of health services, the underreporting of cases of aplastic anemia in selected regions can be discussed. Frequent exposure to benzene-based products increases the risk for aplastic anemia. Few associations with specific drugs were found, and it is likely that some of these were due to chance alone.