In this episode, I’ll discuss the effects of morbid obesity on filgrastim pharmacokinetics.
The prescribing information for filgrastim recommends dosing based on actual body weight, but this results in large doses for morbidly obese patients. The original data for filgrastim only used patients who were at an ideal body weight, and the effects of giving larger doses to patients are not well studied. A group of authors recently published in Pharmacotherapy a prospective study looking at the effects of morbid obesity on the pharmacokinetics of filgrastim.
The study was a prospective, matched-control study in 30 patients admitted to an inpatient oncology unit. All patients received filgrastim at a dose of 5 mcg/kg using actual body weight, although a +/- 10% range was allowed to have a patient still included in the study. Patients in the morbidly obese group were required to have a weight of >190% of ideal body weight and the matched control patients were required to be 80-124% of IBW.
Systemic clearance (Cl) was 0.111 ± 0.041 mL/min in the morbidly obese group versus 0.124 ± 0.045 mL/min in the non-obese group, a difference that was not statistically significant. There were also no significant differences in the mean area under the curve or the maximal concentration. The time to maximum concentration reached was numerically higher in the morbidly obese group but this did not reach statistical significance with a p value of 0.06.
The authors concluded that:
Calculating subcutaneous filgrastim doses using actual body weight appears to produce similar systemic exposure in morbidly obese and non-obese patients with severe neutropenia.
The study was adequately powered to detect a difference of one standard deviation for the apparent systemic clearance, but not to determine whether a difference in adverse effect exists between morbidly obese and normal-weight patients receiving filgrastim based on actual body weight. In addition, no clinical efficacy factors could be assessed due to the small sample size of the study. Until larger studies are completed there is no reason to think that filgrastim dosing for morbidly obese patients based on actual body weight won’t produce similar effects due to the similar kinetic parameters when compared to patients at their ideal body weight.
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