Use of Bisphosphonates to Treat Severe Idiopathic Hypercalcaemia in a Young Ragdoll Cat
J.L. Whitney, V.R.D. Barrs, M.R. Wilkinson, K.A. Briscoe, and J.A. Beatty
Because the pathogenesis for idiopathic hypercalcemia remains unknown, treatment for this condition can be difficult. Response to dietary changes have produced mixed results, and most cats eventually require medical management, such as prednisolone, to control the hypercalcemia (4,7,8). When neither dietary modification nor treatment with prednisolone are successful in lowering the high circulating calcium concentrations, treatment with an oral bisphosphonate (i.e., alendronate; Fosamax) has been suggested as another option (4,7,8).
Despite the fact that protocols for alendronate have been published in numerous proceedings and book chapters (5,7,8), no case studies of cats with idiopathic hypercalcemia had been reported in a refereed scientific journal until this present report.
In this case report by Whitney et al. (9), the authors describe a cat with well-documented idiopathic hypercalcemia. This cat failed to respond completely to prednisolone but responded well to long-term treatment using alendronate, with resolution of all clinical and biochemical signs of hypercalcemia.
A 3-year-old Ragdoll cat was examined for investigation of polyuria, polydipsia, vomiting, weight loss, and hypercalcemia. Physical examination was unremarkable (body weight, 3.19 kg). Serum biochemical abnormalities included hypercalcemia (total calcium, 3.8 mmol/L [15.2 mg/dl]; reference range, 1.75-2.6 mmol/L) and hypophosphatemia (1.8 mmol/L; reference range, 2.1-2.8 mmol/L).
Repeat analysis confirmed total (3.74 mmol/L [15 mg/dl]) and ionized (1.8 mmol/L; reference range, 1.2-1.32 mmol/L) hypercalcemia. Urine specific gravity was 1.040 with normal concentrations of urea nitrogen and creatinine. On microscopic examination of the urine, occasional calcium oxalate crystals were identified.
Thoracic radiographs and abdominal ultrasound examination were unremarkable. Parathyroid glands could not be identified on cervical ultrasound examination. Serum intact parathyroid hormone (iPTH) was low, consistent with a parathyroid-independent process (iPTH < 12 pg/ml; reference range, 22-122 pg/ml). Idiopathic hypercalcemia was diagnosed and the cat was discharged on prednisolone (5 mg once daily, PO).
On day 14, the cat was reported to be lethargic and had lost weight (down to 3.06 kg). Persistent total hypercalcemia (3.7 mmol/L [14.8 mg/dl]) and ionized hypercalcemia (1.73 mmol/L) were detected. The frequency of prednisolone therapy was increased to 5 mg, BID. At one month, the owners reported ongoing lethargy and the cat had become anorectic. Serum total hypercalcemia (3.94 mmol/L [15.8 mg/dl]) and ionized hypercalcemia (1.87 mmol/L) persisted so the cat was switched to oral alendronate (Fosamax) 5 mg once weekly, PO.
At recheck, 1 month later, the cat was bright and eating well. No abnormalities were found on physical examination and she had gained weight. Total hypercalcemia was identified (3.85 mmol/L [15.4 mg/dl]). The dose of alendronate was increased to 10 mg once weekly, PO. One month later, the frequency of administration was subsequently increased to 10 mg every 3 days because of persistent hypercalcemia.
At assessment 5 months after initial presentation, the cat was bright with a good appetite and had gained 1.1 kg. No abnormalities were detected on physical examination, serum biochemistry or urinalysis. The serum total calcium (2.6 mmol/L [10.4 mg/dl]) and ionized calcium (1.6 mmol/L) were normal. The dose frequency of alendronate has been gradually reduced. At the time of writing, 18 months after initiating bisphosphonates, the cat is clinically and biochemically normal. The current dose of alendronate is 10 mg administered once weekly.
This is the first reported case of the use of oral bisphosphonate, alendronate, in the successful long-term management of idiopathic hypercalcemia in a cat (9).
Pharmacology of the bisphosphonates
There is a single reported case of the treatment of a cat with concurrent idiopathic hypercalcemia and chronic kidney disease with IV pamidronate (16). However, intravenous treatment with bisphosphonates is almost never needed in cats with idiopathic hypercalcemia, since the hypercalcemia is chronic and the cats are usually not in an acute crisis. Use of an oral bisphosphonates such as alendronate is preferred.
Dosing of alendronate in cats
The dosing regime in this cat (10 mg of alendronate once weekly without food) is similar to what others have reported for cats. If no decrease in serum calcium occurs after 1 month of therapy, the weekly dose can be gradually increased as high as 30 mg per week (5,7,8).
Food substantially reduces the bioavailability and absorption of oral alendronate, so it is recommended that it be administered on an empty stomach (generally after at least a 12-hour fast) (5,7,8). The oral bioavailability of alendronate in the fasted state is about 0.7% in humans, and less than 2% in all species studied (11,12).
Potential side effects of alendronate
Care must be taken when dosing cats with oral alendronate. In human patients oral alendronate administration has been associated with esophagitis and esophageal stricture in up to 15% of patients (18-22). It is important to ensure that the medication does not stick in the esophagus, which could potentially lead to esophagitis. To minimize this risk, owners should immediately administer 5-6 ml of water orally to their cat after dosing to enhance the passage of the alendronate tablet into the stomach (5,7,8).
Effectiveness of alendronate
Overall, it appears that alendronate treatment is relative safe and effective for cats with idiopathic hypercalcemia, but further studies are needed. Oral bisphosphonates will likely replace prednisolone as the second choice for treatment of this disorder in cats.
Dietary therapy for idiopathic hypercalcemia?
However, I also believe that we need to reexamine the use of dietary therapy in these cats, since it is very likely that diet may play a role in the pathogenesis of idiopathic hypercalcemia. In my next post, I’ll discuss the role of diet, why high-fiber diets generally fail to lower calcium in these cats, and what diets may actually help some cats with mild hypercalcemia restore normocalcemia without the use of potentially toxic drugs.
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