Filgrastim is approved to help prevent chemotherapy-induced neutropenia in patients who have received certain chemotherapy agents. Currently, the dosing is 5 mcg/kg. However, patients can receive only 300 mcg or 480 mcg due to vial-size availability. As a result, it is common practice in oncology pharmacy that patients who weigh less than 70 kg typically receive 300 mcg since 70 kg x 5 mcg/kg equals 350 mcg, which would round up to 480 mcg. There are no data to support this; however, over the years it has become common practice to dose filgrastim in this manner.

Elsayed and colleagues performed a chart review of 91 patients who had chemotherapy-induced neutropenia and had received two consecutive doses of filgrastim 300 mcg. Patients were divided into three groups: low weight (<60 kg); medium weight (>60 kg and <85 kg); and high weight (>85 kg). The scientists looked at complications that result from chemotherapy-induced neutropenia to determine how effective the dose was. For patients weighing <60 kg, 300 mcg would be considered the correct dose, so the researchers examined adverse events in these patients versus patients who were >60 kg and still received 300 mcg, which would technically be a suboptimal dose since the dosing is 5 mcg/kg.

The researchers found that infections, delays in chemotherapy, and hospitalizations due to chemotherapy-related febrile neutropenia were very low (5%). Patients in the medium-weight category (>60 kg and <85 kg), did not have higher rates of infection. Patients in the higher weight category (>85 kg) did experience a higher rate of infections (5% vs. 33%, P = .001). About 26% of patients in the low-weight group missed the next cycle of chemotherapy, followed by 24% in the medium group and 25% in the high-weight group. Only 6% of patients in the low-weight category and 5% of patients in the medium-weight category required a dose reduction, whereas 8% required a reduction in chemotherapy doses in the high-weight category. In the low- and medium-weight groups, no patient had an infection within 2 weeks or febrile neutropenia or any hospitalization, whereas 33% in the higher weight category had an infection within 2 weeks (none had febrile neutropenia or a hospitalization within 2 weeks).

The results demonstrated that in patients who weighed up to 85 kg, it was acceptable to give the suboptimal dosing at 300 mcg. The cost of the treatment was decreased by 43% by giving the medium-weight (61-84 kg) 300-mcg dose versus the 480-mcg dose that the patients normally would have received. This resulted in a cost savings of $147,274 for the institution. This study shows potential for changing practices in filgrastim dosing in patients receiving chemotherapy, and the change could potentially result in cost savings for many institutions. Studies on a larger scale with a greater subject population would be required to validate these results. If more studies showed the same findings, this could potentially change the oncology guidelines and provide data to validate optimal dosing in specific weight groups.

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