Effects of CSFS and their combinations with chemotherapeutic agents (CH) on leukemic blasts (LB) in children (MTT-assay).

Thirty-five samples of bone marrow (BM) from 17 patients (pts) with ALL and 18 pts with AML (aged 9 m-20 yrs, median 7.7 yrs) were obtained. Using MTT-assay the sensitivity of LB to Ara-C, VP-16, DOX, G(GM)-CSF and their combinations was measured. LC50 was higher in pts with AML than with ALL: to Ara-C 1.94-fold (p < 0.05), to VP-16 1.62-fold (p = 0.2), to DOX 3.9-fold (p < 0.05). Incubation with G-CSF increased the viability of ALL and AML LB--104.3% and 104.1% respectively (the viability of leukemic cells without CSF accepted as 100%). Incubation with GM-CSF decreased the viability of ALL LB (96.5%) and increased the viability of AML LB (139.1%) (p = 0.08). Combining Ara-C with G- or GM-CSF resulted in equal or increased LC50 (compared with LC50 of Ara-C alone) in 100% cases of AML. For ALL: LC50 of "Ara-C+G-CSF" was equal or increased in 63.6% cases; LC50 of "Ara-C+GM-CSF"-in 62.5%. For VP-16 and DOX all pts (ALL, AML) except two had equal or increased LC50 of "CH+CSF" (compared with LC50 of CH alone). These data show: 1) AML LB were less sensitive to the investigated CH than ALL LB. 2) The LC50 of "CH+CSF" was equal or increased compared to the LC50 of CH for the absolute majority of cases with VP-16 and DOX. The same results were obtained with AML and in about 60% cases of ALL. The effect of the increasing of cytototoxity of CH in presence of CSF probably exists mostly at higher concentrations of CH than those that can be achieved in clinical practice.

Hematopoietic growth factors in cancer treatment.

The introduction of hematopoietic growth factors during the past five years has changed the scenery of antitumor treatment. Growth factors following high-dose chemotherapy or bone marrow transplantation have become established as part of many treatment protocols. The main benefits are earlier recovery of neutrophils resulting in fewer days with fever, antibiotics and hospitalization. Growth factors were found to reduce the treatment-related morbidity and to improve the practicability of therapeutic regimens. First studies on a prospective chemotherapy dose intensification supported by growth factors are underway. As a further effect of growth factors, an attempted enhancement of antitumor cytotoxicity by recruitment of tumor cells to chemosensitivity or by modulation of antitumor drug metabolism appears realistic based on first data on growth factor priming in AML. New ways to support high-intensity and myeloablative antitumor strategies are opened by the autologous transplantation of peripheral blood progenitor cells mobilized by growth factors and also by the use of new factors like SCF and the synergistic combination of growth factors as part of future strategies against malignant disorders.

Hematopoietic recovery following high-dose combined alkylating-agent chemotherapy and autologous bone marrow support in patients in phase-I clinical trials of colony-stimulating factors: G-CSF, GM-CSF, IL-1, IL-2, M-CSF.

Hematopoietic recovery in 115 patients with metastatic breast cancer or metastatic melanoma, enrolled in phase-I studies of recombinant growth factors while undergoing treatment with high-dose chemotherapy with autologous bone marrow support, was examined with assays of bone marrow progenitor cells and peripheral blood progenitor cells, and by evaluation of peripheral blood counts. Groups of patients receiving hematopoietic cytokine support [with interleukin-1 (IL-1), interleukin-2 (IL-2), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), or monocyte CSF (M-CSF)] post marrow infusion were compared with contemporaneous control patients not receiving growth factor support. Patients receiving GM-CSF demonstrated statistically significant increases in the growth of granulocyte/macrophage colony-forming units (CFU-GM) in the bone marrow and peripheral blood compared with control patients. The effect of GM-CSF was dose dependent in the early period post marrow infusion (day +6) with bone marrow CFU-GM colonies at doses 8-16 micrograms/kg/day 34 times those measured in controls. Significant increases in bone marrow multipotential progenitor cells (CFU-GEMM) were seen in patients receiving GM-CSF day +21 post marrow infusion. Patients receiving IL-1 demonstrated significant increases in bone marrow CFU-GM at day +21, maximal at dosages of 24-32 ng/kg/day. There were no significant increases in burst forming unit-erythroid (BFU-E) among any study group. Patients receiving G-CSF had significantly increased absolute neutrophil counts (ANC) and total white blood cell counts (WBC) by day +11 post transplant compared with control patients. Patients receiving GM-CSF demonstrated significantly increased WBC (greater than 2000/mm3) at day +11 and ANC greater than 500/mm3 at day +16. Optimal dose of G-CSF and GM-CSF to stimulate neutrophil recovery post transplant was 4-8 micrograms/kg/day and 8-16 micrograms/kg/day, respectively. Platelet recovery did not differ among the six study groups. These data demonstrate accelerated myeloid recovery after high-dose chemotherapy and autologous bone marrow support in patients receiving either G-CSF or GM-CSF. Moreover, GM-CSF and IL-1 stimulate myelopoiesis at the level of bone marrow CFU-GM, while G-CSF causes earlier neutrophil recovery peripherally.

A phase I/II trial of recombinant granulocyte-macrophage colony-stimulating factor for children with aplastic anemia.

Nine pediatric patients (median age, 8 years; range, 0.7 to 19 years), eight with refractory aplastic anemia and one with newly diagnosed aplasia, were enrolled in a phase I/II trial of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) administered via continuous intravenous infusion. Doses ranged from 8 to 32 micrograms/kg/d. Six of eight evaluable patients responded with a significant rise in neutrophil count (median fourfold increase; range, 2.5- to 31-fold) during the 28-day induction period. Five patients completed 2 further months of therapy (maintenance) with persistent or improved neutrophil responses. Three patients had bone marrow aspirates suggestive of increased erythropoiesis, although only one patient had improvement in peripheral hematocrit and platelet count. In the five patients completing maintenance, three experienced a rapid return to baseline counts after rhGM-CSF was discontinued, one maintained a neutrophil response for 2 months after drug discontinuation, and one has maintained a trilineage response for greater than 1 year off study. Drug therapy was well tolerated. Toxicity was minimal at doses from 8 to 16 micrograms/kg/d. Fever and rash were more commonly seen at 32 micrograms/kg/d. No patient developed an infection during the course of rhGM-CSF administration. These results demonstrate that rhGM-CSF increases peripheral neutrophil counts in children with refractory and newly diagnosed aplastic anemia and may be able to stimulate a multilineage response in a more limited number. Randomized, prospective trials are necessary to determine if rhGM-CSF administration will impact favorably on the morbidity and mortality of severe aplastic anemia.

Subcutaneous recombinant granulocyte-macrophage colony-stimulating factor used as a single agent and in an alternating regimen with azidothymidine in leukopenic patients with severe human immunodeficiency virus infection.

We investigated the effects of recombinant human granulocyte-macrophage colony-stimulating factor (rGM-CSF) administered by the subcutaneous route, first alone and then alternating with azidothymidine (AZT), in leukopenic patients with severe human immunodeficiency virus (HIV) infection. Ten patients with acquired immunodeficiency syndrome (AIDS) or related disorders, five of whom could not tolerate conventional doses of AZT, were administered rGM-CSF subcutaneously for 12 days. They then were administered an alternating regimen using AZT for 1 week, followed by 5 days of subcutaneous rGM-CSF and 2 days without any medication. During the initial 12 days of GM-CSF administration, there was an increase in the mean white blood cell (WBC) value. In addition, rGM-CSF stimulated circulating monocytes as evidenced by an increase in superoxide anion production and expression of surface HLA-DR antigen. However, at the same time rGM-CSF increased the serum HIV p24 antigen in each of the six evaluable patients from 189 x/divided by 2.02 pg/mL (geometric mean x/divided by SEM) at entry to 375 x/divided by 2.11 pg/mL (P less than .05). During the subsequent period of alternating AZT and rGM-CSF treatment, serum HIV p24 antigen fell below the day 14 value in most patients, particularly after the weeks of AZT administration. The mean T4 cell value increased in patients who had not previously received AZT, but generally did not change in those who had prior AZT exposure. Hematologic toxicity appeared to be somewhat reduced compared with continuous full-dose AZT therapy, and two patients with previous AZT hematologic toxicity tolerated this alternating regimen for 25 weeks. Additional regimens simultaneously combining these two agents are worth exploring.

Use of recombinant human granulocyte-macrophage colony-stimulating factor in graft failure after bone marrow transplantation.

The effect of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) was evaluated in 37 patients with marrow graft failure after allogeneic (n = 15), autologous (n = 21), or syngeneic (n = 1) bone marrow transplantation. rhGM-CSF was administered by 2-hour infusion at doses between 60 and 1,000 micrograms/m2/d for 14 or 21 days. At doses of less than 500 micrograms/m2, rhGM-CSF was well-tolerated and did not exacerbate graft-versus-host disease in allogeneic transplant recipients. No patient with myelogenous leukemia relapsed while receiving rhGM-CSF. Twenty-one patients reached an absolute neutrophil count (ANC) greater than or equal to 0.5 x 10(9)/L within 2 weeks of starting therapy while 16 did not. None of seven patients who received chemically purged autologous marrow grafts responded to rhGM-CSF. The survival rates of GM-CSF-treated patients were significantly better than those of a historical control group.

Maintenance treatment of patients with myelodysplastic syndromes using recombinant human granulocyte colony-stimulating factor.

Myelodysplastic syndromes (MDS) are characterized by chronic refractory cytopenias resulting in increased risk of infection, bleeding, and conversion to acute leukemia. In an effort to improve these cytopenias we have treated 18 patients over a 6- to 8-week period with increasing daily subcutaneous doses of recombinant human granulocyte colony-stimulating factor (G-CSF). Sixteen patients responded with improvement in neutrophil counts. On cessation of treatment these counts returned to baseline values over a 2- to 4-week period. To maintain these improved blood counts 11 patients were treated with G-CSF for more prolonged periods. Ten patients again responded with an increase in total leukocyte counts (1.6- to 6.4-fold) and absolute neutrophil counts (ANC) (3.6- to 16.3-fold), with responses persisting for 3 to 16 months. A significantly decreased risk of developing bacterial infections was noted during periods with ANC greater than 1,500/mm3 as compared with periods of time with ANC less than 1,500/mm3. Two anemic patients had a greater than 20% rise in hematocrit over the study period, and 2 additional patients had a decrease in red blood cell transfusion requirements during G-CSF treatment. Bone marrow myeloid maturation improved in 7 of 9 maintenance phase patients. Three patients progressed to acute myeloid leukemia during treatment. The drug was generally well-tolerated and no severe toxicities were noted. These data demonstrated that G-CSF administered to MDS patients by daily subcutaneous administration was well-tolerated and effective in causing persistent improvement of the neutrophil levels and marrow myeloid maturation. These effects were associated with a decreased risk of infection and, in some patients, with decreased red blood cell transfusion requirements.

A phase I study of therapy with recombinant granulocyte-macrophage colony-stimulating factor administered by IV bolus or continuous infusion.

Sixteen patients with primary or secondary bone marrow failure were treated with recombinant human granulocyte-macrophage colony-stimulating factor (rGM-CSF) given as either an intravenous bolus or by continuous infusion. The dose range studied was from 15 micrograms/m2/d to 960 micrograms/m2/d. Administration of rGM-CSF on a bolus schedule failed to elicit a hematologic response, but resulted in side effects of epigastric distress and eructation in over 30% of administered courses. Administration of rGM-CSF by continuous infusion resulted in a dose-dependent increase in the total leukocyte, granulocyte, and eosinophil counts. The mean maximal rise in granulocyte count was 8.5-fold. After cessation of therapy, blood counts returned to near baseline in most patients by 7 days. A 36 percent decrease from baseline in mean serum cholesterol level was observed in the continuous infusion group, but not in patients receiving rGM-CSF as an IV bolus. Fever, fatigue, and bone pain were dose-limiting in the continuous infusion group at a dose of 240 micrograms/m2/d. The maximally tolerated dose was 480 micrograms/m2/d. No life-threatening toxicities were observed in either group. Our data demonstrate that continuous infusion rGM-CSF is biologically active and non-toxic at a dose of 120 micrograms/m2/d in patients with bone marrow failure.

Phase I/II study with GM-CSF in patients with myelodysplastic syndromes.

The availability of rh GM-CSF has allowed the in vivo treatment of patients with cytopenia. Therefore a phase I/II trial was initiated to study the effect of rh GM-CSF in patients with myelodysplastic syndromes who were not eligible for other kinds of therapy. rh GM-CSF has been tested in 10 patients in doses from 15 micrograms/m2 to 150 micrograms/m2 given intravenously over 8 hours for a cycle of 7 days followed by an interval of 14 days and a second 7-day treatment course. A dose dependent increase in leukocyte count was observed in 9 out of 10 patients. No change in reticulocyte numbers was seen and only one patient experienced an increase in platelet count. Toxicity mainly consisted of mild local phlebitis at the site of infusion and sternal pain after bolus injection. An increase in blast cell counts in some patients necessitated the start of low dose Ara-C therapy.

Potential role of granulocyte-macrophage colony stimulating factor in patients with HIV infection.

Abnormalities in leukocyte number and function contribute to the high incidence of infection in patients with HIV infection. Leukopenia is a frequent occurrence in patients with AIDS and AIDS-related diseases and is a major dose limiting factor in the treatment of HIV infected individuals with antiviral compounds and chemotherapy. Granulocyte-macrophage colony stimulating factor (GM-CSF) is a hematopoietic hormone that stimulates the growth and differentiation of myeloid progenitor cells in vitro and enhances the function of mature monocytes and neutrophils. Studies on the effects of this agent in patients with AIDS, indicate that GM-CSF causes increased production of neutrophils, monocytes, and eosinophils in a dose dependent fashion. Leukocytes produced in response to GM-CSF function normally as judged by in vitro tests. The effects of GM-CSF on HIV replication and expression in vivo are uncertain. Studies of the use of GM-CSF alone and in combination with antiretroviral, antimicrobial, antineoplastic agents or other hematopoietins and cytokines will help define its ultimate clinical utility in patients with HIV infection.

Phase I/II study of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) in bone marrow failure.

We performed a Phase I/II study of the administration of recombinant human GM-CSF to patients suffering from severe bone marrow failure, either due to aplastic anemia or myelodysplastic syndrome. Doses ranging from 15 micrograms/m2 to 480 micrograms/m2 were administered as an intravenous infusion daily for 7 days. Temporary improvements were seen in granulocyte counts, monocyte counts and reticulocyte counts. There was no reduction in erythrocyte transfusion requirements and no effect was observed on platelet counts. There was only minimal toxicity consisting of transient low back discomfort, anorexia, myalgias/arthralgias, and low grade fever. Our data suggest that prolonged use of GM-CSF might benefit some patients with severe marrow failure.

Recombinant granulocyte/macrophage-colony stimulating factor in human and murine bone marrow transplantation.

Recombinant granulocyte/macrophage-colony stimulating factor (rGM-CSF) is a protein that mediates a broad range of biological activities including inducing proliferation of bone marrow progenitor cells and up-regulating the functional capabilities of neutrophils, monocytes, and macrophages. We have administered rGM-CSF in vivo in the context of allogeneic (murine) and autologous (human) bone marrow transplantation (BMT). In a murine allogeneic model of T-cell depletion constructed to favor graft failure/rejection of donor marrow, ex vivo treatment of marrow with rGM-CSF significantly promoted engraftment. However, when rGM-CSF was infused in vivo, engraftment was significantly impaired. We have initiated a phase I clinical trial of rGM-CSF in human autologous BMT for patients with acute lymphoblastic leukemia (ALL). rGM-CSF has been administered for 14-21 days in a dose escalation study (16-128 micrograms/M2/day). At high doses of rGM-CSF, neutrophil recovery has returned earlier and to higher levels than previously observed in historical controls. rGM-CSF related toxicity in this study has been mild.

The effects of recombinant human granulocyte-macrophage colony-stimulating factor on phagocyte kinetics in man.

Four patients with advanced resistant malignant disease received recombinant human granulocyte-macrophage colony stimulating factor (rhGM-CSF) for 10 days. All developed a moderate neutrophilia and monocytosis over this period. A transient phagocytopenia was observed during the first hour of administration. Radionuclide labelling studies showed that this cytopenia was due to sequestration predominantly within the lungs and that the recovery was due to re-entry of the same cells into the circulation. Studies of neutrophil lobularity during this time showed no reduction in lobe count suggesting that there had been little if any release of immature cells from bone marrow reserves. Skin window responses were present in 2 out of 3 patients during the period of neutropenia showing that cells were also present in the marginated pool of the skin at this time.

Phase I study of recombinant DNA granulocyte macrophage colony stimulating factor.

In this Phase I study of rh GM-CSF three patients have been entered at each of the following dose levels--0.3, 1, 3, 10 and 30 micrograms/kg/day. The mean total white cell count (x 10(9)/l) over the first ten days of rh GM-CSF rose from 11 to 14 at 3 micrograms/kg, 8 to 23 at 10 micrograms/kg and 7 to 27 at 30 micrograms/kg. Side effects included transient pyrexias after the first two infusions of rh GM-CSF and bone pains which were severe and required analgesia in two patients receiving 30 micrograms/kg and one receiving 10 micrograms/kg. No neutralizing antibodies to rh GM-CSF have been detected in the six patients tested to date. Patients are now being studied at 60 micrograms/kg/day.

Yeast-expressed granulocyte-macrophage colony-stimulating factor in cancer patients: a phase ib clinical study.

The in vivo effect of yeast-derived recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) was investigated in 29 patients with advanced malignancy in phase Ib trial. Patients were treated at six different dose levels (30-1000 micrograms/m2/day) with either daily intravenous bolus injection or 24 hours continuous infusion for 5 days or 2 weeks. Administration of rh GM-CSF resulted in a broad spectrum of dose-, route-, and schedule-dependent hematopoietic effects. Sustained infusion of rh GM-CSF elicited a maximum 17-fold average peak increase of the total white blood cell (WBC) count with mainly neutrophils, eosinophils, and monocytes accounting for this rise, and increases in bone marrow cellularity with a shift to immature myeloid elements. Elevation of lymphocytes, platelets and reticulocytes was not induced. Within one week after discontinuation of treatment the leukocytosis had disappeared. Adverse reactions encountered with rh GM-CSF seen in 65% of the patients studied were never life-threatening and always reversible. They included mild myalgias, facial flushing, low-grade fever, headache, bone discomfort, nausea, dyspnoea and transient decline of platelet counts. These results suggest that rh GM-CSF can be safely administered at the doses and schedules employed and that it can induce in vivo some of the biological effects reported in in vitro studies. Although no objective antitumour responses have been seen, the ability of rh GM-CSF to increase turnover and function of leukocytes in vivo may prevent neutropenia and infections, when GM-CSF is adjunctively added to cytotoxic cancer therapy.

Phase I study of recombinant human granulocyte-macrophage colony-stimulating factor in patients with bone marrow failure and malignancy.

Twenty-five patients with malignancy and/or bone marrow failure were treated with recombinant human granulocyte macrophage colony-stimulating factor (rh GM-CSF) (specific activity 5 x 10(7) units/mg, purity greater than 95%) given by continuous intravenous infusion at various fixed dose levels (15-500 micrograms/m2/day), as part of a phase I study. Treatment was associated with marked increases in white blood cell counts (2-70 fold), consisting mainly of neutrophilic granulocytes. Significant increases were also observed in eosinophils, monocytes and lymphocytes. In addition six patients had multilineage responses characterized by increases in platelet counts (greater than 2 fold and greater than 100,000/mm3) and reticulocyte counts (greater than 2 fold). Three of these 6 patients did not require red cell or platelet transfusion for 17 to 37 weeks. Treatment also resulted in an increase in bone marrow cellularity and myeloid: erythroid cell ratio. The common side effects were constitutional symptoms and bone pain. These findings demonstrate that rh GM-CSF is well tolerated when given by continuous IV infusion and is a potent stimulator of hematopoiesis in vivo.

Phase I clinical study for recombinant human granulocyte colony-stimulating factor.

The pharmacokinetics, the safety and the efficacy of purified recombinant human granulocyte colony-stimulating factor (rh G-CSF) expressed in Chinese hamster ovary (CHO) cells were studied in normal healthy adults. Following the single subcutaneous dose, G-CSF levels in sera elevated in a dose-dependent way, with C-max of approximately 140 pg per ml, T-max of 4 hours and T-half of 5 hours at a dose of 10 micrograms per ml. In accord with the elevation, blood neutrophil counts rose promptly and transiently without any changes in the other blood cell counts. Degree and duration of the neutrophilia were apparently proportional to the rh G-CSF dose and, in turn, the G-CSF levels in sera. Single daily injections of rh G-CSF at a dose of 10 micrograms per body per day for 7 consecutive days did not bring on any accumulation of rh G-CSF in sera, and repeatedly induced neutrophilias following every injection. During the course of the injections, the chemiluminescence generating ability of blood neutrophils also rose. Peak value of the transient neutrophilias and level of the abilities tended to go down after 3 or 4 consecutive injections, presumably due to the limited number of mature neutrophils which can be released from the bone marrow in response to rh G-CSF. However, it was found that the M/E ratio becomes higher after the repeated dosing with an increased proportion of myeloid immature cells as well as increase in the levels of both erythroid and myeloid precursor cells in the bone marrow.(ABSTRACT TRUNCATED AT 250 WORDS)

Phase I/II study of recombinant human granulocyte colony-stimulating factor in patients receiving intensive chemotherapy for small cell lung cancer.

Twelve patients with advanced small cell carcinoma of the bronchus were treated by continuous infusion of recombinant human granulocyte colony-stimulating factor (rh G-CSF) at the following dose levels: 1 microgram, 5 micrograms, 10 micrograms, 20 micrograms and 40 micrograms/kg/day for 5 days. No toxicities resulted from the treatment and in all 12 patients the number of peripheral neutrophils increased rapidly to a maximum of 100 x 10(9)/l in one patient at 10 micrograms/kg/day. The neutrophils were shown to be functionally normal in tests of their mobility and bactericidal activity. During the Phase II part of the patients were treated using a combination of i.v. Adriamycin, Ifosfamide and Etoposide. The chemotherapy was repeated every 3 weeks. rh G-CSF was given to each patient for 14 days on alternate cycles of chemotherapy and reduced the period of absolute neutropenia considerably (median of 80%), with a return to normal, or above normal, neutrophil counts within 2 weeks after day 1 of chemotherapy. Ten severe infective episodes were observed during the 20 cycles of chemotherapy which did not include rh G-CSF, while only one infective episode occurred in 20 courses when treated with rh G-CSF. These results demonstrate the utility of rh G-CSF in restoring functional neutrophils to patients undergoing intensive chemotherapy.

Granulocyte-macrophage colony-stimulating factor induces the expression of the CD11b surface adhesion molecule on human granulocytes in vivo.

The CD11b (Mol) molecule is a member of a family of surface glycoproteins that are essential for adhesion-dependent granulocyte functions. Brief exposure of granulocytes to human granulocyte-macrophage colony-stimulating factor (GM-CSF) in vitro increases the surface expression of CD11b and increases granulocyte adhesiveness. To assess the possible in vivo significance of these observations we studied the effect of GM-CSF on CD11b, CD11a (LFA-1), and CD11c (gp 150, 95) expression on granulocytes from nine adult patients with sarcoma who were receiving GM-CSF as part of a phase I trial. GM-CSF was administered as a continuous infusion at a dose of 32 or 64 micrograms/kg/d. Granulocyte CD11b, CD11a, and CD11c expression was determined by indirect immunofluorescence staining of whole blood, thereby minimizing in vitro manipulation. A transient leukopenia developed within 15 minutes of initiation of GM-CSF treatment that was associated with a marked increase in the surface antigen density of CD11b. A mean 1.7-fold increase (P = .001) in the percentage of CD11b-positive granulocytes and a mean 2.1-fold increase (P = .002) in CD11b surface antigen density was noted after 12 hours of treatment. No change in CD11a or CD11c expression was observed over the first 12 hours. The level of CD11b expression was followed in six patients for up to 5 days of treatment with GM-CSF. Compared with the 12-hour value, three of six patients showed a subsequent decrease in CD11b expression, two remained constant, and one showed a continued increase in CD11b surface density. Fluorescence-activated cell sorting of granulocytes into high- and low-density CD11b-positive groups revealed a preponderance of immature myeloid forms in the low-density CD11b fraction, which suggests that the late decrease in CD11b expression in some patients may be related to a greater proportion of circulating immature myeloid forms in the peripheral blood. This study suggests that GM-CSF administered as a continuous infusion rapidly upregulates the expression of granulocyte CD11b in vivo. The influence of this phenomenon on in vivo granulocyte aggregation may be clinically relevant with regard to the toxicity of GM-CSF and deserves further investigation.