Severe zoledronic acid (ZOL)-associated hypophosphatemia and prognosis in patients with castration-resistant prostate cancer (CRPC).

Abstract

e15180 Background: ZOL therapy is associated with severe (i.e., grade ≥ 3) hypocalcemia and hypophosphatemia in a subset of patients with metastatic CRPC. However, as opposed to the distinct clinical picture of hypocalcemia, the symptoms of hypophosphatemia are less pathognomonic. Furthermore, serum phosphate abnormalities are not regularly reported. Methods: To characterize the rate and clinical impact of severe hypophosphatemia in CRPC patients undergoing ZOL therapy, we identified CRPC patients receiving at least 3 doses of ZOL at our Centre between 01/2004 and 03/2011. Patient demographics, disease characteristics and laboratory parameters were extracted using the Oncology Symptom Control and Information Resource database, and by means of manual chart review. Results: 12 of 90 evaluable patients developed grade ≥ 3 hypophosphatemia (nadir) after 364±299 days (mean±SD) following the first dose of ZOL. While only one patient presented with concomitant severe hypocalcemia, the hypophosphatemia nadir coincided with rising PSA readings in 9 out of 11 informative patients. Severe ZOL-associated hypophosphatemia identified patients with worse outcome (median overall survival from time of CRPC diagnosis to death 685 days) compared to patients without documented hypophosphatemia (907 days, HR 0.52, p=0.049; n=42), or patients with grade 1-2 hypophosphatemia (1035 days, HR 0.44, p=0.016; n=36). Otherwise, the prognostic nomogram developed by Armstrong et al appears not to capture the poor prognosis of patients with severe ZOL-associated hypophosphatemia. Conclusions: Grade 3-4 hypophosphatemia occurs in about 15% of CRPC patients undergoing ZOL therapy and is associated with worse prognosis when compared to patients with absent or mild hypophosphatemia. The latter is usually transient and likely related to increased parathyroid hormone levels due to calcium decreases as a consequence of bisphosphonate therapy. On the other hand, the distinct clinical behavior of CRPC presenting with ZOL-associated severe hypophosphatemia suggests that secreted tumor-associated factors such as fibroblast growth factor 23 may contribute to this phenomenon.