Colony-stimulating Factors

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 which is produced by monocytes‚ fibroblasts, and endothelial cells. G-CSF regulates the production of neutrophils within the bone marrow and affects neutrophil progenitor proliferation‚^2‚3 differentiation,^2‚4 and selected end-cell functional activation (including enhanced phagocytic ability‚⁵ priming of the cellular metabolism associated with respiratory burst‚⁶ antibody dependent killing,⁷ and the increased expression of some functions associated with cell surface antigens⁸). G-CSF is not species specific and has been shown to have minimal direct in vivo or in vitro effects on the production of hematopoietic cell types other than the neutrophil lineage.

Preclinical Experience

Filgrastim was administered to monkeys‚ dogs‚ hamsters‚ rats‚ and mice as part of a preclinical toxicology program which included single-dose acute‚ repeated-dose subacute‚ subchronic‚ and chronic studies. Single-dose administration of Filgrastim by the oral‚ intravenous (IV)‚ subcutaneous (SC)‚ or intraperitoneal (IP) routes resulted in no significant toxicity in mice‚ rats‚ hamsters‚ or monkeys. Although no deaths were observed in mice‚ rats‚ or monkeys at dose levels up to 3450 mcg/kg or in hamsters using single doses up to approximately 860 mcg/kg‚ deaths were observed in a subchronic (13- week) study in monkeys. In this study‚ evidence of neurological symptoms was seen in monkeys treated with doses of Filgrastim greater than 1150 mcg/kg/day for up to 18 days. Deaths were seen in 5 of the 8 treated animals and were associated with 15- to 28-fold increases in peripheral leukocyte counts‚ and neutrophil-infiltrated hemorrhagic foci were seen in both the cerebrum and cerebellum. In contrast‚ no monkeys died following 13 weeks of daily IV administration of Filgrastim at a dose level of 115 mcg/kg. In an ensuing 52-week study‚ one 115 mcg/kg dosed female monkey died after 18 weeks of daily IV administration of Filgrastim. Death was attributed to cardiopulmonary insufficiency.

In subacute‚ repeated-dose studies‚ changes observed were attributable to the expected pharmacological actions of Filgrastim (ie‚ dose-dependent increases in white cell counts‚ increased circulating segmented neutrophils‚ and increased myeloid:erythroid ratio in bone marrow). In all species‚ histopathologic examination of the liver and spleen revealed evidence of ongoing extramedullary granulopoiesis; increased spleen weights were seen in all species and appeared to be dose-related. A dose-dependent increase in serum alkaline phosphatase was observed in rats‚ and may reflect increased activity of osteoblasts and osteoclasts. Changes in serum chemistry values were reversible following discontinuation of treatment.

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