PROSTATE-SPECIFIC ANTIGEN DECREASES AFTER WITHDRAWAL OF ANTIANDROGEN THERAPY WITH BICALUTAMIDE OR FLUTAMIDE IN PATIENTS RECEIVING COMBINED ANDROGEN BLOCKADE

Purpose: We determined whether decreases in prostate specific antigen (PSA) would occur after withdrawal of double-blinded antiandrogen ther apy with flutamide or bicalutamide for clinical progression or increas ing PSA concentration in patients receiving combined androgen blockade for advanced prostate cancer. Materials and Methods: PSA concentratio ns were determined weekly for at least 6 weeks and then every other we ek for 6 weeks in 22 patients with stage D2 prostate cancer. All patie nts were withdrawn from antiandrogen therapy (8 flutamide and 14 bical utamide) due to progression or an increasing PSA concentration. Object ive response was evaluated before antiandrogen withdrawal and at week 12. Results: In 4 of 8 patients (50%) withdrawn from flutamide and 4 o f 14 (29%) withdrawn from bicalutamide serum PSA concentrations decrea sed by 50% or more. PSA responses after withdrawal of flutamide therap y occurred within the first few days, whereas those after withdrawal o f bicalutamide therapy occurred within 4 to 8 weeks. Of 4 patients ass essed for objective response 2 had stable disease and 2 had progressio n. A PSA response was observed in the 2 patients with stable disease b ut not the 2 with progression. Conclusions: For patients with stage D2 prostate cancer and disease progression or an increasing PSA concentr ation, withdrawal of antiandrogen therapy with bicalutamide or flutami de may result in a PSA response. The time to PSA response is longer wi th bicalutamide than with flutamide. The clinical significance of the antiandrogen withdrawal phenomenon is unknown.