Amphotericin B blunts erythropoietin response to anemia

Tolerability profile of the current antifungal armoury

The tolerability of available antifungal agents is essential to the final outcome of the management of invasive mycoses. There are limited classes of antifungal agents for use, and they can have serious direct toxicities and/or drug-drug interactions. In this review, we examine the common toxicities noted for antifungal agents and attempt to both identify the issues around the adverse events and provide clinical context for their occurrence in these fragile patients.

Evaluation of Amphotericin B Lipid Formulations for Treatment of Severe Coccidioidomycosis

Patients with severe coccidioidomycosis infections are often treated with either amphotericin B lipid complex (ABLC) or liposomal amphotericin B (L-AmB). Outcome data with these agents in severe coccidioidomycosis cases are currently lacking. The purpose of this study is to evaluate the efficacy and toxicity of ABLC and L-AmB in treating severe coccidioidomycosis. A retrospective pre-post study design was employed. Chart reviews were completed from 1 January 2005 to 31 December 2014 for all patients who received lipid-based amphotericin B. Inclusion criteria included having a follow-up complement fixation (CF) titer or a treatment emergent adverse event (TEAE) prior to follow-up. Patients with meningeal involvement and pregnant patients were excluded. Treatment outcomes were assessed based on documented completion of therapy as well on symptoms, complement fixation titer, and changes to laboratory monitoring parameters. A total of 108 patients were identified, 69 of whom met the inclusion criteria. There were no statistical differences in demographics or disease burden in those that received ABLC and those that received L-AmB, except that those who received L-AmB were more likely to have previously diagnosed chronic kidney disease (n_L-AmB = 4, 12.5% vs n_ABLC = 0, 0.0%; P = 0.042) and to have a lower creatinine clearance at the start of therapy (L-AmB = 79.6 mg/dl versus ABLC = 100.4 mg/dl; P = 0.008). Successful treatment was achieved in 27 (73.0%) of ABLC patients and 22 (68.8%) of L-AmB patients (P = 0.700). Amphotericin B was discontinued due to documented completion of therapy for 17 (45.9%) ABLC patients and 18 (56.3%) L-AmB patients (P = 0.553). Acute kidney injury (AKI) was the documented reason of treatment cessation for 10 (27.0%) ABLC and 1 (3.1%) L-AmB patient (P = 0.007). ABLC and L-AmB both appear to be equally efficacious in the treatment of severe coccidioidomycosis. L-AmB may have less renal toxicity than ABLC and may be the preferred agent in baseline renal impairment.

Prognostic implications of baseline anaemia and changes in haemoglobin concentrations with amphotericin B therapy for cryptococcal meningitis

OBJECTIVES

Anaemia represents a common toxicity with amphotericin B-based induction therapy in HIV-infected persons with cryptococcal meningitis. We sought to examine the impact of amphotericin-related anaemia on survival.

METHODS

We used data from Ugandan and South African trial participants to characterize the variation of haemoglobin concentrations from diagnosis to 12 weeks post-diagnosis. Anaemia severity was classified based on the haemoglobin concentration at cryptococcal meningitis diagnosis, and nadir haemoglobin values during amphotericin induction. Cox proportional hazard models were used to estimate 2- and 10-week mortality risk. We also estimated 10-week mortality risk among participants with nadir haemoglobin < 8.5 g/dL during amphotericin induction and who survived ≥ 2 weeks post-enrolment.

RESULTS

The median haemoglobin concentration at meningitis diagnosis was 11.5 g/dL [interquartile range (IQR) 9.7-13 g/dL; n = 311] with a mean decline of 4.2 g/dL [95% confidence interval (CI) -4.6 to -3.8; P < 0.001; n = 148] from diagnosis to nadir value among participants with baseline haemoglobin ≥ 8.5 g/dL. The median haemoglobin concentration was 8.1 g/dL (IQR 6.5-9.5 g/dL) at 2 weeks, increasing to 9.4 g/dL (IQR 8.2-10.9 g/dL) by 4 weeks and continuing to increase to 12 weeks. Among participants with haemoglobin < 8.5 g/dL at diagnosis, mortality risk was elevated at 2 weeks [hazard ratio (HR) 2.7; 95% CI 1.5-4.9; P < 0.01] and 10 weeks (HR 1.8; 95% CI 1.1-2.2; P = 0.03), relative to those with haemoglobin ≥ 8.5 g/dL. New-onset anaemia occurring with amphotericin therapy did not have a statistically significant association with 10-week mortality (HR 2.0; 95% CI 0.5-9.1; P = 0.4).

CONCLUSIONS

Amphotericin induced significant haemoglobin declines, which were mostly transient and did not impact 10-week mortality. Individuals with moderate to life-threatening anaemia at baseline had a higher mortality risk at 2 and 10 weeks post-enrolment.

David and Goliath: chemical perturbation of eukaryotes by bacteria

Environmental microbes produce biologically active small molecules that have been mined extensively as antibiotics and a smaller number of drugs that act on eukaryotic cells. It is known that there are additional bioactives to be discovered from this source. While the discovery of new antibiotics is challenged by the frequent discovery of known compounds, we contend that the eukaryote-active compounds may be less saturated. Indeed, despite there being far fewer eukaryotic-active natural products these molecules interact with a far richer diversity of molecular and cellular targets.

Recent approaches to antifungal therapy for invasive mycoses

Invasive fungal infections with primary and opportunistic mycoses have become increasingly common in recent years and pose a major diagnostic and therapeutic challenge. They represent a major area of concern in today's medical fraternity. The occurrence of invasive fungal diseases, particularly in AIDS and other immunocompromised patients, is life-threatening and increases the economic burden. Apart from the previously known polyenes and imidazole-based azoles, newly discovered triazoles and echinocandins are more effective in terms of specificity, yet some immunosuppressed hosts are difficult to treat. The main reasons for this include antifungal resistance, toxicity, lack of rapid and microbe-specific diagnoses, poor penetration of drugs into sanctuary sites, and lack of oral or intravenous preparations. In addition to combination antifungal therapy, other novel antimycotic treatments such as calcineurin signaling pathway blockers and vaccines have recently emerged. This review briefly summarizes recent developments in the pharmacotherapeutic treatment of invasive fungal infections.

Drug safety profile of darbepoetin alfa for anemia of chronic kidney disease

Anemia of chronic kidney disease due to deficiency of erythropoietin is common and has clinical consequences. Erythropoiesis stimulating agents including darbepoetin alfa (DA) are effective in correcting anemia. DA is generally well tolerated and has side effect profile similar to recombinant human erythropoietin. It has a long half-life permitting infrequent dosing. DA has been tested extensively in preclinical and clinical studies and significant experience has accumulated in clinical practice. Global safety profile of DA must consider recent data indicating worse survival, poor cardiovascular outcomes and thrombotic risks of targeting near normal hemoglobin levels and administering high doses of erythropoiesis stimulating agents. Strategies to achieve and maintain a reasonable, individualized target hemoglobin level with minimal variations in hemoglobin level are needed.

Acute amphotericin B overdose

OBJECTIVE

To report the clinical course of a woman with cryptococcal meningitis and no previous cardiac disease who developed a fatal cardiac arrhythmia after an acute overdose of amphotericin B and to review its toxicity.

CASE SUMMARY

A 41-year-old woman with a history of proliferative glomerulonephritis from systemic lupus erythematosus was admitted with a diagnosis of cryptococcal meningitis. Liposomal amphotericin B was prescribed at the standard dose of 5 mg/kg/day; however, amphotericin B deoxycholate 5 mg/kg was inadvertently administered (usual dose of the deoxycholate formulation is 0.5-0.8 mg/kg/day). The patient developed cardiac arrhythmias, acute renal failure, and anemia. The medication error was noticed after she had received 2 doses of amphotericin B deoxycholate, and it was then discontinued. Despite treatment in the intensive care unit, the woman died on the sixth day after admission.

DISCUSSION

Amphotericin B deoxycholate has been reported to produce significant cardiac toxicity, with ventricular arrhythmias and bradycardia reported in overdoses in children and in adults with preexisting cardiac disease, even when administered in conventional dosages and infusion rates. Use of the Naranjo probability scale indicated a highly probable relationship between the observed cardiac toxicity and amphotericin B deoxycholate therapy in this patient.

CONCLUSIONS

Given the fulminant course of amphotericin B deoxycholate overdosage and lack of effective therapy, stringent safeguards against its improper administration should be in place.

Amphotericin B blunts erythropoietin response to hypoxia by reinforcing FIH-mediated repression of HIF-1

Amphotericin B (AmB) is widely used for treating severe systemic fungal infections. However, long-term AmB treatment is invariably associated with adverse effects such as anemia. The erythropoietin (EPO) suppression by AmB has been proposed to contribute to the development of anemia. However, the mechanism whereby EPO is suppressed remains obscure. In this study, we investigated the possibility that AmB inhibits the transcription of the EPO gene by inactivating HIF-1, which is a known key transcription factor and regulator of EPO expression. EPO mRNA levels were markedly attenuated by AmB treatment both in rat kidneys and in Hep3B cells. AmB inactivated the transcriptional activity of HIF-1α, but did not affect the expression or localization of HIF-1 subunits. Moreover, AmB was found to specifically repress the C-terminal transactivation domain (CAD) of HIF-1α, and this repression by AmB required Asn803, a target site of the factor-inhibiting HIF-1 (FIH); moreover, this repressive effect was reversed by FIH inhibitors. Furthermore, AmB stimulated CAD-FIH interaction and inhibited the p300 recruitment by CAD. We propose that this mechanism underlies the unexplained anemia associated with AmB therapy.

Human Immunodeficiency Virus Hematology

The advent of potent antiretroviral therapy has altered the expected natural history of human immunodeficiency virus (HIV) infection and of many previously associated opportunistic complications, including malignancies. At the same time, HIV suppression hasn’t affected all of these complications equally and the longer expected survival of infected patients may allow the development of newer complications. Additionally, the use of potent antiretroviral combination therapy may itself lead to hematological toxicities. Together these changes affect the consultation role of the hematology-oncology specialist in comprehensive HIV care and demand ongoing education.

In Section I, Dr. Paul Volberding reviews the biology of antiretroviral drug development and the progression in discovering new agents as the viral life cycle is further elucidated. He briefly summarizes the process of combining agents to achieve the degree of viral suppression required for long-term clinical benefit.

In Section II, Dr. Kelty Baker reviews the effects of HIV and its therapy on hematologic dyscrasia and clotting disorders. She summarizes how therapy may decrease certain previously common manifestations of HIV disease while adding new problems likely to result in referral to the hematologist. In addition, she addresses the role of secondary infections, such as parvovirus, in this spectrum of disorders.

In Section III, Dr. Alexandra Levine discusses the still challenging aspects of HIV associated non-Hodgkin’s lymphoma and the association between HIV infection and Hodgkin’s disease. She addresses current controversies in the pathogenesis of HIV related lymphomas and summarizes a number of recent trials of combination chemotherapy, with or without monoclonal antibodies, in their management. Additionally, she reviews the complex relationship of HIV disease with multicentric Castleman’s disease and recent attempts to manage this disorder.

Darbepoetin alfa: a novel erythropoiesis-stimulating protein

OBJECTIVE

To review the pharmacology, pharmacokinetics, clinical efficacy, and safety of darbepoetin alfa.

DATA SOURCES

Pertinent references were identified by a MEDLINE search (1995-January 2001) of the medical literature, review of English-language literature and references of these articles, product information, and abstracts from professional meetings.

STUDY SELECTION

Clinical efficacy data were gathered from all available trial data citing darbepoetin alfa. Additional information concerning pharmacology, pharmacokinetics, and safety was also reviewed.

DATA SYNTHESIS

Darbepoetin alfa is a new erythropoiesis-stimulating protein with a threefold longer half-life than recombinant human erythropoietin (r-HuEPO). Darbepoetin alfa is approved for intravenous and subcutaneous administration in patients requiring and not requiring dialysis. Clinical studies in patients with chronic kidney disease (CKD) have shown darbepoetin alfa to be equivalent to r-HuEPO in terms of increases in hemoglobin concentration, percentage of patients obtaining target hemoglobin, and average time to reach target hemoglobin concentration. Trials are currently ongoing in patients receiving cancer chemotherapy. The adverse event profile appears to be similar between the 2 agents.

CONCLUSIONS

The equivalent efficacy and safety profile, as well as the longer half-life, may make darbepoetin alfa an attractive alternative to r-HuEPO in patients with CKD. Since these patients need to receive r-HuEPO 1-3 times weekly at the expense of increased healthcare utilization to improve their hemoglobin, agents such as darbepoetin alfa, with longer durations of action, may reduce healthcare expenses. In addition, enhanced patient compliance may be realized with once-weekly or once every-other-week administration.

Erythropoietin in stem cell transplantation

Anemia is universal after allogeneic and autologous stem cell transplantation, with both increased red cell utilization and decreased production playing a role. This anemia sometimes is associated with a relative erythropoietin deficiency. In allogeneic stem cell transplantation, randomized trials have demonstrated improved erythropoiesis and a decrease in red cell transfusions in recombinant human erythropoietin (rHuEPO)-treated patients. Studies of rHuEPO in patients undergoing autologous stem cell transplants, however, have not shown a benefit. The role of rHuEPO in stem cell mobilization and treatment of delayed erythropoiesis has yet to be defined and further studies are needed.

Agents for treatment of invasive fungal infections

The incidence of fungal infections continues to rise as the population of immunocompromised individuals increases. Despite the enlarging numbers of infections, there are only a few antifungal agents for treatment of deep-seated, invasive infections. These agents include amphotericin B, flucytosine, terbinafine, and several azoles. Progress has been made in understanding the role of these agents in a variety of infections and this article examines in detail these agents and their prophylactic, empiric, and therapeutic uses in invasive mycoses. This article focuses on general concepts of antifungal therapies and provides a detailed review of each antifungal agent available for treatment of deep-seated mycoses.

Amphotericin B colloidal dispersion

Amphotericin B colloidal dispersion (ABCD) is a colloidal dispersion of a stable complex of amphotericin B with cholesteryl sulphate in a 1:1 proportion, forming uniform disk-shaped particles. ABCD is associated with less nephrotoxicity than conventional amphotericin B deoxycholate. Infusion-related adverse events are more frequent in patients receiving ABCD than in patients receiving liposomal amphotericin B or amphotericin B deoxycholate. ABCD has been shown in a randomised, double-blind study, to be an effective alternative to amphotericin B deoxycholate for empirical treatment of patients with fever and neutropenia. ABCD is active in the treatment of invasive Candida spp. and Aspergillus spp. infections in immunocompromised hosts, however most of the data supporting its use for these types of infections is derived from non-comparative open-label clinical trials of patient refractory to or intolerant of conventional antifungal therapy. ABCD is approved by the US FDA for the treatment of invasive aspergillosis in patients where renal impairment of unacceptable toxicity precludes the use of amphotericin B deoxycholate in effective doses, and in patients with invasive aspergillosis where prior amphotericin B deoxycholate therapy has failed. Two other lipid formulations of amphotericin B, amphotericin B lipid complex and liposomal amphotericin B, are available and, like ABCD, are associated with reduced nephrotoxicity as compared to amphotericin B deoxycholate. The role of ABCD in comparison with these other lipid formulations of amphotericin B is discussed herein. High cost remains an issue with all lipid formulations of amphotericin B.

Administration of fluconazole in children below 1 year of age

For this review, 78 studies regarding the use of fluconazole in a total of 726 children below 1 year of age were evaluated. The range of fluconazole dosage was 2-50 mg kg-1 day-1, with 162 days being the maximum duration of treatment. According to current experience, fluconazole seems to be well tolerated and efficacious against systemic candidosis and candidaemia in children below 1 year of age, including neonates and very low-birthweight infants (VLBWIs). The recommended daily dosage is 6 mg kg-1. (In Germany, fluconazole is approved for children between 1 and 16 years in cases in which there is no therapeutic alternative for treatment of systemic infections caused by Candida spp. and Cryptococcus neoformans in a dosage of 3-6 mg kg-1 day-1 and for superficial Candida infections in a dosage of 1-2 mg kg-1 day-1.) In patients with impaired renal function, the daily dose should be reduced in accordance with the guidelines given for adults. In neonates during the first 2 weeks of life, this dosage should be administered only every 72 h. In weeks 2-4 of life, the same dose should be given every 48 h, following which daily dosing is appropriate. This posology is derived from the age-related pharmacokinetics of fluconazole, with a higher volume of distribution and a prolonged plasma elimination half-life, especially during the first month of life. Drug monitoring during treatment should be performed to ensure therapeutic plasma concentrations of fluconazole within a range between 4 and 20 micrograms ml-1. The benefit of fluconazole should be investigated in prospective studies for treatment of systemic candidosis with administration of higher dosages as well as for early empiric therapy in VLBWIs.

Antifungal agents. Part I. Amphotericin B preparations and flucytosine

Traditionally, amphotericin B has been the cornerstone of antifungal treatment. Toxicity, however, is a major dose-limiting factor of amphotericin B deoxycholate. Nevertheless, it continues to have a major role in the treatment of deep-seated mycotic infections. Recently, less nephrotic lipid formulations, including amphotericin B lipid complex, amphotericin B cholesteryl sulfate, and liposomal amphotericin B, have been introduced. The pharmacologic properties, main indications, recommended dosages, related costs, and adverse effects of these various preparations are summarized in this review. Orally administered flucytosine is useful in certain infections, particularly cryptococcal meningitis, but it should be used with caution in patients with renal insufficiency.

[Use of fluconazole in children less than 1 year old: review]

For this review, 78 publications for use of fluconazole in children below 1 year of age were evaluated with a total of 726 patients. The range of fluconazole dosage was 2-50 mg/kg/day with 162 days as maximum duration of treatment. According to the present experience, fluconazole seems to be an efficacious and well tolerated therapy against systemic candidosis and candidemia in children below 1 year of age, including neonates and very low birth-weight infants (VLBWI). The recommended daily dosage is 6 mg/kg. In patients with impaired renal function, the daily dose should be reduced in accordance with the guidelines given for adults. In neonates during the first two weeks of life, this dosage should be administered only every 72 hours. In weeks two to four of life, the same dose should be given every 48 hours. After that daily dosing is appropriate. This posology is derived from the age-related pharmacokinetics of fluconazole with a higher volume of distribution and a prolonged plasma elimination half life especially during the first month of life. Drug monitoring during treatment should be performed to ensure therapeutic plasma concentrations of fluconazole within a range between 4 and 20 micrograms/ml. The benefit of fluconazole should be investigated in prospective studies for treatment of systemic candidosis with administration of higher dosages as well as for early empiric therapy in VLBWI.

Influence of selected clinical events on the need for red blood cell transfusion in patients with severe cytopenia following myeloablative chemotherapy

The transfusional need in patients treated with myeloablative chemotherapy appears to be greater than that resulting from bone marrow suppression alone. The erythrocyte transfusional need may be expressed as delta hb = the daily loss of haemoglobin (mmol/l/day), and multivariate analyses of factors influencing delta hb have been carried out. We have studied 124 patients, mainly with acute myeloid leukaemia, treated with 537 courses of chemotherapy, inducing cytopenia (a white cell count < 1.0 x 10(9)/l) in 476 cases. The transfusional need was 2.5-3 times greater than expected, suggesting the presence of haemolysis and/or occult bleedings. Using multiple regression analysis and repeated measurement analysis the main factor influencing the transfusional need was type/dose of chemotherapy. The higher the dose of cytarabine the greater then transfusional need. Other factors significantly influencing the transfusional need were tumour burden, fever, treatment-related events such as positive blood cultures, systemic fungal infection, mucosal bleeding, melaena/haematemesis and diarrhoea. Finally, it was found that the transfusional need decreased gradually during the cytopenic period.

Rapid infusion of amphotericin B in dextrose

OBJECTIVE

To review the data examining the use of rapid infusion of amphotericin B in dextrose infusions.

DATA SOURCES

A MEDLINE search of the English-language literature and review of pertinent references' bibliographies was used to identify articles evaluating the effect of amphotericin B infusion rates on the incidence of adverse reactions.

STUDY SELECTION AND DATA EXTRACTION

Controlled and uncontrolled studies involving humans are reviewed; emphasis is placed on recent comparative trials. Pertinent information, as judged by the authors, was selected for discussion.

DATA SYNTHESIS

Amphotericin B, a polyene antifungal agent with significant toxicity, remains the agent of choice for many serious fungal infections. The potential benefits of rapid administration of amphotericin B in reducing the incidence and/or severity of adverse reactions were noted soon after its introduction. Recent studies have examined the tolerability of rapid (0.75-1 h) amphotericin B infusions. Results of studies assessing the tolerability of rapid amphotericin B infusions suggest that tolerance to infusion-related reactions develops during therapy. Comparative trials have obtained variable results. The comparative trials supporting rapid amphotericin B infusion have generally used crossover designs, enrolled small numbers of patients, and excluded patients with significant renal or cardiovascular dysfunction.

CONCLUSIONS

Rapid amphotericin B infusions should be avoided during initiation of therapy when infusion-related reactions tend to be most problematic, and in patients with cardiovascular disease, renal dysfunction, and potassium disorders because of the potential risk for cardiac arrhythmias. The literature currently available is conflicting and insufficient to support the routine use of rapid amphotericin B infusion.

Cisplatin-associated anemia: an erythropoietin deficiency syndrome

Cisplatin-based therapy results in a cumulative anemia that is disproportionate to the effects on other blood cells. The severity of this treatment-induced anemia and the resultant transfusion requirement in cancer patients correlate with cisplatin-induced renal tubular dysfunction. Observed/expected serum erythropoietin (EPO) ratios decline with progressive cisplatin therapy and are proportionate to the degree of renal dysfunction. Recovery from anemia and of observed/expected serum EPO ratios in patients occurs after cessation of cisplatin therapy, along with restoration of renal tubular function. Creatinine clearance, however, remains permanently depressed. Cisplatin-treated rats develop progressive renal dysfunction and anemia that persists for many weeks, without effects on white blood cell counts. The anemia is also associated with a lack of expected EPO and reticulocyte response. With EPO administration, cisplatin-treated rats exhibit a greater reticulocyte response and hematocrit increment then non-cisplatin-treated rats given EPO, indicating minimal erythroid precursor cell damage from cisplatin. These results indicate the primary etiology of cisplatin-associated anemia is a transient, but persisting EPO deficiency state resulting from cisplatin-induced renal tubular damage, which can be prevented or treated by hormone (EPO) replacement.

Effect of recombinant human erythropoietin after allogenic bone marrow transplantation

The hematologic effects of recombinant human erythropoietin after allogeneic bone marrow transplantation (BMT) were studied. Nineteen patients received 150 U/kg/day of C127 mouse-cell-derived recombinant human erythropoietin (rHu EPO) as a daily continuous intravenous infusion until hematocrit exceeded 35%. These data were compared with a treatment-matched historical control group of 43 patients. RHu EPO-treated patients recovered erythropoiesis more rapidly and became independent from erythrocyte transfusions after a median of 17 days, which was 7 days earlier than the control patients.

The Use of Erythropoietin in the Treatment of Post-bone Marrow Transplatation Anemia

The issue of the role of erythropoietin (Epo) in the erythroid reconstitution after bone marrow transplantation (BMT) has been addressed in several recent studies. A defective Epo production in response to anemia has been shown to occur in patients undergoing allogeneic BMT unlike in most of those subjected to an autologous rescue. The factors involved in the inadeguate Epo production in BMT are discussed, with particular attention to the role of the immunosuppressive drug cyclosporin-A, which has been shown to inhibit Epo production in both in vivo and in vitro models. The observation of defective Epo production eventually led to the development of clinical trials of recombinant human Epo (rhEpo) administration in BMT patients; the aims of these studies were to stimulate erythroid engraftment, hence reducing blood transfusion exposure. Although the number of patients studied up to now is relatively small, a benefit from rhEpo administration in terms of accelerated erythroid engraftment seems very likely, and it may also be associated with decreased transfusional needs in most treated patients. However, further studies are needed to better define indications, dosages and schedules of rhEpo in BMT patients.

Antifungal agents used for deep-seated mycotic infections

The increased use of immunosuppressive regimens in organ transplantation and in the treatment of malignant lesions and the epidemic of acquired immunodeficiency syndrome (AIDS) are major reasons for the greater prevalence of fungal infections seen in clinical practice during the past decade. The traditional cornerstone of antifungal treatment, amphotericin B, continues to play a major role in deep-seated mycotic infections. The indications for intravenously administered miconazole have become limited. Orally administered flucytosine remains useful in certain infections, particularly cryptococcal meningitis. The new orally administered antifungal agents ketoconazole and fluconazole have been approved for clinical use and have supplanted amphotericin B in certain situations. Investigational antifungal agents, including liposomal amphotericin B, itraconazole, and saperconazole, hold promise for the future. Active investigation in the development of new antifungal agents is expected to continue.

Control of erythropoietin production

The cloning and expression of the human erythropoietin gene has not only resulted in an important therapeutic advance but has led to the dissemination of reliable immunoassays and molecular probes for the study of normal and disordered erythropoietin physiology. We are now beginning to understand the cellular and molecular basis for the control of erythropoietin secretion, and have begun to identify abnormal erythropoietin physiology in a wide variety of disease states. This avenue of investigation will continue to expand and should lead to important new therapies for disorders of red blood cell production.