Prostate Specific Antigen Decreases After Withdrawal of Antiandrogen Therapy with Bicalutamide or Flutamide in Patients Receiving Combined Androgen Blockade

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Abstract
Purpose: We determined whether decreases in prostate specific antigen (PSA) would occur after withdrawal of double-blinded antiandrogen therapy with flutamide or bicalutamide for clinical progression or increasing PSA concentration in patients receiving combined androgen blockade for advanced prostate cancer. Materials and Methods: PSA concentrations were determined weekly for at least 6 weeks and then every other week for 6 weeks in 22 patients with stage D2 prostate cancer. All patients were withdrawn from antiandrogen therapy (8 flutamide and 14 bicalutamide) due to progression or an increasing PSA concentration. Objective response was evaluated before antiandrogen withdrawal and at week 12. Results: In 4 of 8 patients (50%) withdrawn from flutamide and 4 of 14 (29%) withdrawn from bicalutamide serum PSA concentrations decreased by 50% or more. PSA responses after withdrawal of flutamide therapy occurred within the first few days, whereas those after withdrawal of bicalutamide therapy occurred within 4 to 8 weeks. Of 4 patients assessed for objective response 2 had stable disease and 2 had progression. A PSA response was observed in the 2 patients with stable disease but not the 2 with progression. Conclusions: For patients with stage D2 prostate cancer and disease progression or an increasing PSA concentration, withdrawal of antiandrogen therapy with bicalutamide or flutamide may result in a PSA response. The time to PSA response is longer with bicalutamide than with flutamide. The clinical significance of the antiandrogen withdrawal phenomenon is unknown.

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