Background: Ferric carboxymaltose (Injectafer) has a higher rate of hypophosphatemia than other iron formulations. The proposed mechanism for this is a transient increase of intact fibroblast growth factor 23 (FGF23) leading to renal phosphate wasting. Prescribing information for ferric carboxymaltose lists the incidence of hypophosphatemia (defined as <2 mg/dl) occurring 2.1% of patients. Clinical experience at our institution prompted concern the rate of hypophosphatemia might be greater than previously reported. The aim of this evaluation was to assess rates of hypophosphatemia following administration of ferric carboxymaltose in a real-world patient population.
Methods: A retrospective review of the electronic medical record was conducted to identify all patients receiving ferric carboxymaltose at the Geisinger Cancer Institute between July 2019 through October 2019. Patients were included for analysis if they had a serum phosphorus level measured within 30 days following administration as well as a baseline serum phosphorus level preceding the first ferric carboxymaltose administration when available. ED utilization was evaluated via manual chart review to identify chief complaints related to hypophosphatemia.
Results: A total of 4821 patients received ferric carboxymaltose across the system. Of those, 2192 patients received the ferric carboxymaltose infusion within our cancer institute and of those patients 207 had a phosphorus level collected within 30 days of ferric carboxymaltose administration. Of this cohort, the median age at time of ferric carboxymaltose administration was 66 years with most patients being female (63%). 160 patients (77%) had a baseline phosphorus level prior to ferric carboxymaltose administration (range 1-200 days). 118 (57%) patients subsequently developed a phosphorus level < 2.5 mg/dL as detailed in Table 1. The mean reduction in serum phosphorus concentration following ferric carboxymaltose administration was approximately 1mg/dL. A total 91 patients visited the emergency department 133 times within 30 days of injection. The most common chief complaints were generalized weakness (15), shortness of breath (14) cases, and vomiting (8). GFR > 60 ml/min was noted in 46% of patients.
Conclusions: This retrospective evaluation demonstrated a significantly higher rate of hypophosphatemia following ferric carboxymaltose administration than outlined in the package insert. Hypophosphatemia may represent a clinically significant adverse drug reaction which clinicians and patients should be aware. Table 1. Incidence of Hypophosphatemia Following Ferric Carboxymaltose Administration (n=207)>2.5 mg/dL 89 (43%)2 – 2.4 mg/dL 42 (20%)1 – 1.9 mg/dL 68 (33%)<1 mg/dL 8 (4%)Limitations: 1- Exact indications for ordering phosphorus lab following administration are difficult to establish. 2- It is difficult to establish if this hypophosphatemia was of any clinical significance.