Mechanism Of Action
Colony-stimulating factors are glycoproteins which act on hematopoietic cells by binding to specific cell surface receptors and stimulating proliferation‚ differentiation commitment‚ and some end-cell functional activation.
Endogenous G-CSF is a lineage-specific colony-stimulating factor that is produced by monocytes‚ fibroblasts, and endothelial cells. G-CSF regulates the production of neutrophils within the bone marrow and affects neutrophil progenitor proliferation‚ differentiation, and selected end-cell functions (including enhanced phagocytic ability‚ priming of the cellular metabolism associated with respiratory burst‚ antibody-dependent killing, and the increased expression of some cell surface antigens). G-CSF is not species-specific and has been shown to have minimal direct in vivo or in vitro effects on the production or activity of hematopoietic cell types other than the neutrophil lineage.
In phase 1 studies involving 96 patients with various nonmyeloid malignancies‚ administration of filgrastim resulted in a dose-dependent increase in circulating neutrophil counts over the dose range of 1 to 70 mcg/kg/day. This increase in neutrophil counts was observed whether filgrastim was administered intravenous (1 to 70 mcg/kg twice daily)‚ subcutaneous (1 to 3 mcg/kg once daily)‚ or by continuous subcutaneous infusion (3 to 11 mcg/kg/day). With discontinuation of filgrastim therapy‚ neutrophil counts returned to baseline in most cases within 4 days. Isolated neutrophils displayed normal phagocytic (measured by zymosan-stimulated chemiluminescence) and chemotactic (measured by migration under agarose using N-formyl-methionyl-leucyl-phenylalanine [fMLP] as the chemotaxin) activity in vitro.
The absolute monocyte count was reported to increase in a dose-dependent manner in most patients receiving filgrastim; however‚ the percentage of monocytes in the differential count remained within the normal range. Absolute counts of both eosinophils and basophils did not change and were within the normal range following administration of filgrastim. Increases in lymphocyte counts following filgrastim administration have been reported in some normal subjects and patients with cancer.
White blood cell (WBC) differentials obtained during clinical trials have demonstrated a shift towards earlier granulocyte progenitor cells (left shift)‚ including the appearance of promyelocytes and myeloblasts‚ usually during neutrophil recovery following the chemotherapy-induced nadir. In addition‚ Dohle bodies‚ increased granulocyte granulation‚ and hypersegmented neutrophils have been observed. Such changes were transient and were not associated with clinical sequelae, nor were they necessarily associated with infection.
Filgrastim exhibits nonlinear pharmacokinetics. Clearance is dependent on filgrastim concentration and neutrophil count: G-CSF receptor-mediated clearance is saturated by high concentration of filgrastim and is diminished by neutropenia. In addition, filgrastim is cleared by the kidney.
Subcutaneous administration of 3.45 mcg/kg and 11.5 mcg/kg of filgrastim resulted in maximum serum concentrations of 4 and 49 ng/mL‚ respectively‚ within 2 to 8 hours. After intravenous administration, the volume of distribution averaged 150 mL/kg and the elimination half-life was approximately 3.5 hours in both normal subjects and subjects with cancer. Clearance rates of filgrastim were approximately 0.5 to 0.7 mL/minute/kg. Single parenteral doses or daily intravenous doses‚ over a 14-day period‚ resulted in comparable half-lives. The half-lives were similar for intravenous administration (231 minutes‚ following doses of 34.5 mcg/kg) and for subcutaneous administration (210 minutes‚ following filgrastim dosages of 3.45 mcg/kg). Continuous 24-hour intravenous infusions of 20 mcg/kg over an 11 to 20-day period produced steady-state serum concentrations of filgrastim with no evidence of drug accumulation over the time period investigated. The absolute bioavailability of filgrastim after subcutaneous administration is 60% to 70%.
The pharmacokinetics of filgrastim in pediatric patients after chemotherapy are similar to those in adult patients receiving the same weight-normalized doses, suggesting no age-related differences in the pharmacokinetics of filgrastim products [see Use In Specific Populations].
In a study with healthy volunteers, subjects with moderate renal impairment, and subjects with end stage renal disease (n = 4 per group), higher serum concentrations were observed in subjects with end-stage renal disease. However, dose adjustment in patients with renal impairment is not necessary.
Pharmacokinetics and pharmacodynamics of filgrastim are similar between subjects with hepatic impairment and healthy subjects (n = 12/group). The study included 10 subjects with mild hepatic impairment (Child-Pugh Class A) and 2 subjects with moderate hepatic impairment (Child-Pugh Class B). Therefore, dose adjustment for ZARXIO in patients with hepatic impairment is not necessary.
Animal Toxicology And Pharmacology
Filgrastim was administered to monkeys‚ dogs‚ hamsters‚ rats‚ and mice as part of a nonclinical toxicology program, which included studies up to 1 year duration.
In the repeated-dose studies‚ changes observed were attributable to the expected pharmacological actions of filgrastim (i.e.‚ dose-dependent increases in white blood cell counts‚ increased circulating segmented neutrophils‚ and increased myeloid: erythroid ratio in bone marrow). Histopathologic examination of the liver and spleen revealed evidence of ongoing extramedullary granulopoiesis, and dose-related increases in spleen weight were seen in all species. These changes all reversed after discontinuation of treatment.
Patients With Cancer Receiving Myelosuppressive Chemotherapy
The safety and efficacy of filgrastim to decrease the incidence of infection‚ as manifested by febrile neutropenia‚ in patients with nonmyeloid malignancies receiving myelosuppressive anti-cancer drugs were established in a randomized‚ double-blind‚ placebo-controlled trial conducted in patients with small cell lung cancer (Study 1).
In Study 1, patients received up to 6 cycles of intravenous chemotherapy including intravenous cyclophosphamide and doxorubicin on day 1; and etoposide on days 1, 2, and 3 of 21 day cycles. Patients were randomized to receive filgrastim (n = 99) at a dose of 230 mcg/m2 (4 to 8 mcg/kg/day) or placebo (n = 111). Study drug was administered subcutaneously daily beginning on day 4, for a maximum of 14 days. A total of 210 patients were evaluable for efficacy and 207 were evaluable for safety. The demographic and disease characteristics were balanced between arms with a median age of 62 (range 31 to 80) years; 64% males; 89% Caucasian; 72% extensive disease and 28% limited disease.
The main efficacy endpoint was the incidence of febrile neutropenia. Febrile neutropenia was defined as an ANC < 1000/mm3 and temperature > 38.2°C. Treatment with filgrastim resulted in a clinically and statistically significant reduction in the incidence of infection‚ as manifested by febrile neutropenia, 40% for filgrastim-treated patients and 76% for placebo-treated patients (p < 0.001). There were also statistically significant reductions in the incidence and overall duration of infection manifested by febrile neutropenia; the incidence, severity and duration of severe neutropenia (ANC < 500/mm3); the incidence and overall duration of hospital admissions; and the number of reported days of antibiotic use.
Patients With Acute Myeloid Leukemia Receiving Induction Or Consolidation Chemotherapy
The safety and efficacy of filgrastim to reduce the time to neutrophil recovery and the duration of fever, following induction or consolidation chemotherapy treatment of patients with acute myeloid leukemia (AML) was established in a randomized, double-blind‚ placebo-controlled‚ multi-center trial in patients with newly diagnosed, de novo AML (Study 4).
In Study 4 the initial induction therapy consisted of intravenous daunorubicin days 1, 2, and 3; cytosine arabinoside days 1 to 7; and etoposide days 1 to 5. Patients were randomized to receive subcutaneous filgrastim (n = 259) at a dose of 5 mcg/kg/day or placebo (n = 262) from 24 hours after the last dose of chemotherapy until neutrophil recovery (ANC ≥1000/mm3 for 3 consecutive days or ≥ 10,000/mm3 for 1 day) or for a maximum of 35 days. The demographic and disease characteristics were balanced between arms with a median age of 54 (range 16 to 89) years; 54% males; initial white blood cell count (65% -<25,000 /mm3 and 27% > 100,000/mm3); 29% unfavorable cytogenetics.
The main efficacy endpoint was median duration of severe neutropenia defined as neutrophil count < 500/mm3. Treatment with filgrastim resulted in a clinically and statistically significant reduction in median number of days of severe neutropenia, filgrastim-treated patients 14 days, placebo-treated patients 19 days (p = 0.0001: difference of 5 days (95% CI: -6.0, -4.0)). There was a reduction in the median duration of intravenous antibiotic use, filgrastim-treated patients: 15 days versus placebo-treated patients: 18.5 days; a reduction in the median duration of hospitalization, filgrastim-treated patients: 20 days versus placebo-treated patients: 25 days.
There were no statistically significant differences between the filgrastim and the placebo groups in complete remission rate (69% -filgrastim, 68% -placebo), median time to progression of all randomized patients (165 days -filgrastim, 186 days -placebo), or median overall survival (380 days -filgrastim, 425 days -placebo).
Patients With Cancer Undergoing Bone Marrow Transplantation
The safety and efficacy of filgrastim to reduce the duration of neutropenia in patients with nonmyeloid malignancies undergoing myeloablative chemotherapy followed by autologous bone marrow transplantation was evaluated in 2 randomized controlled trials of patients with lymphoma (Study 6 and Study 9). The safety and efficacy of filgrastim to reduce the duration of neutropenia in patients undergoing myeloablative chemotherapy followed by allogeneic bone marrow transplantation was evaluated in a randomized placebo controlled trial (Study 10).
In Study 6 patients with Hodgkin’s disease received a preparative regimen of intravenous cyclophosphamide, etoposide, and BCNU (“CVP”), and patients with non-Hodgkin’s lymphoma received intravenous BCNU, etoposide, cytosine arabinoside and melphalan (“BEAM”). There were 54 patients randomized 1:1:1 to control, filgrastim 10 mcg/kg/day, and filgrastim 30 mcg/kg/day as a 24 hour continuous infusion starting 24 hours after bone marrow infusion for a maximum of 28 days. The median age was 33 (range 17 to 57) years; 56% males; 69% Hodgkin’s disease and 31% non-Hodgkin’s lymphoma.
The main efficacy endpoint was duration of severe neutropenia ANC < 500/mm3. A statistically significant reduction in the median number of days of severe neutropenia ( ANC < 500/mm3) occurred in the filgrastim-treated groups versus the control group (23 days in the control group‚ 11 days in the 10 mcg/kg/day group‚ and 14 days in the 30 mcg/kg/day group [11 days in the combined treatment groups‚ p = 0.004]).
In Study 9, patients with Hodgkin’s disease and non-Hodgkin’s lymphoma received a preparative regimen of intravenous cyclophosphamide, etoposide, and BCNU (“CVP”). There were 43 evaluable patients randomized to continuous subcutaneous infusion filgrastim 10 mcg/kg/day (n = 19), filgrastim 30 mcg/kg/day (n = 10) and no treatment (n = 14) starting the day after marrow infusion for a maximum of 28 days. The median age was 33 (range 17 to 56) years; 67% males; 28% Hodgkin’s disease and 72% non-Hodgkin’s lymphoma.
The main efficacy endpoint was duration of severe neutropenia. There was statistically significant reduction in the median number of days of severe neutropenia ( ANC < 500/mm3) in the filgrastim-treated groups versus the control group
(21.5 days in the control group versus 10 days in the filgrastim-treated groups, p < 0.001). The number of days of febrile neutropenia was also reduced significantly in this study (13.5 days in the control group versus 5 days in the filgrastimtreated groups‚ p < 0.0001).
In Study 10, 70 patients scheduled to undergo bone marrow transplantation for multiple underlying conditions using multiple preparative regimens were randomized to receive filgrastim 300 mcg/m2/day (n = 33) or placebo (n = 37) days 5 through 28 after marrow infusion. The median age was 18 (range 1 to 45) years, 56% males. The underlying disease was: 67% hematologic malignancy, 24% aplastic anemia, 9% other. A statistically significant reduction in the median number of days of severe neutropenia occurred in the treated group versus the control group (19 days in the control group and 15 days in the treatment group‚ p < 0.001) and time to recovery of ANC to ≥ 500/mm3 (21 days in the control group and 16 days in the treatment group‚ p < 0.001).
Patients Undergoing Autologous Peripheral Blood Progenitor Cell Collection And Therapy
The safety and efficacy of filgrastim to mobilize autologous peripheral blood progenitor cells for collection by leukapheresis was supported by the experience in uncontrolled trials, and a randomized trial comparing hematopoietic stem cell rescue using filgrastim mobilized autologous peripheral blood progenitor cells to autologous bone marrow (Study 11). Patients in all these trials underwent a similar mobilization/collection regimen: filgrastim was administered for 6 to 7 days‚ in most cases the apheresis procedure occurred on days 5‚ 6, and 7. The dose of filgrastim ranged between 10 to 24 mcg/kg/day and was administered subcutaneously by injection or continuous intravenous infusion.
Engraftment was evaluated in 64 patients who underwent transplantation using filgrastim mobilized autologous hematopoietic progenitor cells in uncontrolled trials. Two of the 64 patients (3%) did not achieve the criteria for engraftment as defined by a platelet count ≥ 20‚000/mm3 by day 28. In clinical trials of filgrastim for the mobilization of hematopoietic progenitor cells‚ filgrastim was administered to patients at doses between 5 to 24 mcg/kg/day after reinfusion of the collected cells until a sustainable ANC (≥ 500/mm3) was reached. The rate of engraftment of these cells in the absence of filgrastim post transplantation has not been studied.
Study 11 was a randomized, unblinded study of patients with Hodgkin’s disease or non-Hodgkin’s lymphoma undergoing myeloablative chemotherapy‚ 27 patients received filgrastim-mobilized autologous hematopoietic progenitor cells and 31 patients received autologous bone marrow. The preparative regimen was intravenous BCNU, etoposide, cytosine arabinoside and melphalan (“BEAM”). Patients received daily filgrastim 24 hours after stem cell infusion at a dose of 5 mcg/kg/day. The median age was 33 (range 1 to 59) years; 64% males; 57% Hodgkin’s disease and 43% non-Hodgkin’s lymphoma. The main efficacy endpoint was number of days of platelet transfusions. Patients randomized to filgrastimmobilized autologous peripheral blood progenitor cells compared to autologous bone marrow had significantly fewer days of platelet transfusions (median 6 vs 10 days).
Patients With Severe Chronic Neutropenia
The safety and efficacy of filgrastim to reduce the incidence and duration of sequelae of neutropenia (that is fever‚ infections, oropharyngeal ulcers) in symptomatic adult and pediatric patients with congenital neutropenia‚ cyclic neutropenia‚ or idiopathic neutropenia was established in a randomized controlled trial conducted in patients with severe neutropenia (Study 7).
Patients eligible for Study 7 had a history of severe chronic neutropenia documented with an ANC < 500/mm3 on three occasions during a 6 month period, or in patients with cyclic neutropenia 5 consecutive days of ANC < 500/mm3 per cycle. In addition patients must have experienced a clinically significant infection during the previous 12 months. Patients were randomized to a 4 month observation period followed by filgrastim treatment or immediate filgrastim treatment. The median age was 12 years (range 7 months to 76 years); 46% males; 34% idiopathic, 17% cyclic and 49% congenital neutropenia. Filgrastim was administered subcutaneously. The dose of filgrastim was determined by the category of neutropenia.
Initial dose of filgrastim:
- Idiopathic neutropenia: 3.6 mcg/kg/day
- Cyclic neutropenia: 6 mcg/kg/day
- Congenital neutropenia: 6 mcg/kg/day divided 2 times per day
The dose was increased incrementally to 12 mcg/kg/day divided 2 times per day if there was no response.
The main efficacy endpoint was response to filgrastim treatment. ANC response from baseline (< 500/mm3) was defined as follows:
- Complete response: median ANC > 1500/mm3
- Partial response: median ANC ≥ 500/mm3 and ≤ 1500/mm3 with a minimum increase of 100%
- No response: median ANC < 500/mm3
There were 112 of 123 patients who demonstrated a complete or partial response to filgrastim treatment.
Additional efficacy endpoints included a comparison between patients randomized to 4 months of observation and patients receiving filgrastim of the following parameters:
- incidence of infection
- incidence of fever
- duration of fever
- incidence, duration, and severity of oropharyngeal ulcers
- number of days of antibiotic use
The incidence for each of these 5 clinical parameters was lower in the filgrastim arm compared to the control arm for cohorts in each of the 3 major diagnostic categories. An analysis of variance showed no significant interaction between treatment and diagnosis‚ suggesting that efficacy did not differ substantially in the different diseases. Although filgrastim substantially reduced neutropenia in all patient groups‚ in patients with cyclic neutropenia‚ cycling persisted but the period of neutropenia was shortened to 1 day.