58 Background: Zoledronic acid (ZOL) therapy is associated with severe (i.e., grade 3/4 according to CTCAEv4.0) hypocalcemia and hypophosphatemia in a subset of patients with metastatic castration-resistant prostate cancer (mCRPC) to bone. However, as opposed to the distinct clinical picture of hypocalcemia, the symptoms of hypophosphatemia are less pathognomonic. Furthermore, the rate of hypophosphatemia is not regularly reported. Methods: To characterize the rate and circumstances of severe hypophosphatemia in mCRPC patients undergoing ZOL therapy, we identified mCRPC patients receiving at least 3 doses of ZOL therapy at our Centre between 2004 and 03/2011. Patient demographics, disease and laboratory parameters were extracted by using the Oncology Symptom Control and Information Resource database, and by means of manual chart review. Results: 11 of 112 patients (10%) developed grade 3/4 hypophosphatemia (nadir) after 241±223 days (mean±SD) following the first dose of ZOL. Only one patient presented with concomitant severe hypocalcemia. Interestingly, the hypophosphatemia nadir coincided with rising PSA readings in 9 out of 10 informative patients. Furthermore, ZOL-associated severe hypophosphatemia identified mCRPC patients with worse outcome (time from CRPC diagnosis to death 628±317 days, versus 901±526 days in CRPC patients without severe ZOL-associated hypophosphatemia, p < 0.028). Conclusions: 10% of mCRPC patients undergoing ZOL therapy presented with grade 3/4 hypophosphatemia. While hypocalcemia usually occurs within the first 6 months of ZOL therapy, ZOL-associated severe hypophosphatemia is a relatively late event. It appears to occur at the time of disease progression and is associated with a poor outcome.
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.
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