Pharmacological Dilutional Therapy Using the Vasopressin Antagonist Tolvaptan for Young Patients With Cystinuria: A Pilot Investigation.

Objective: To perform a pilot study of short-term safety, tolerability, and impact on urinary stone risk parameters of the vasopressin V2-receptor antagonist tolvaptan (which increases urinary excretion of free water) among adolescents and young adults with cystinuria.
Materials and Methods: We enrolled cystinuria patients age 12-25 years. Subjects were treated for 4 days at low-dose tolvaptan (0.3 mg/kg/day, maximum 30 mg) and 4 days at high dose (0.6 mg/kg/day, maximum 60 mg). Twenty-four-hour urine collections were done at baseline, day 3-4 of the dosing period, day 7-8 of the dosing period, and 3-6 days after washout. Primary outcome was cystine capacity (mg/L, target capacity > 0). Secondary outcomes included other urinary/serum parameters, tolerability, and thirst response.
Results: Two females (17, 23 years) and 2 males (13, 24 years) were enrolled. Cystine capacity respectively went from baseline of -312, -82, -353, and -628 mg/L to 97, 111, 75, and -3 mg/L on high dose (Figure 1). Twenty-four-hour volume went from 1.96, 3.0, 2.1, and 0.91 L to 11.74, 6.5, 9.9, and 2.8 L on high dose (Figure 2). There were no abnormalities in serum electrolytes or liver enzymes. Subjects did experience extreme thirst (9/10 on visual scale), but none discontinued treatment or reduced dose.
Conclusion: Dilutional therapy with tolvaptan increased both cystine capacity and urinary volumes. This treatment approach has the potential to reduce recurrence of stones in this population. Further investigation should study longer term effects and safety, and determine optimal dosing to improve tolerability.
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