Intravesical Allogeneic Platelet Concentrate for Refractory BK Virus-Associated Haemorrhagic Cystitis After Allogeneic Haematopoietic Stem Cell Transplantation.

Background/Objectives: BK virus-associated haemorrhagic cystitis (BKV-HC) is a severe complication of allogeneic haematopoietic stem cell transplantation (allo-HSCT) for which no standard treatment exists. Intravesical platelet-rich plasma has been proposed, but recipients are profoundly thrombocytopenic, and an autologous product cannot be obtained. We report the outcome of an intravesical allogeneic platelet concentrate (allo-PC) prepared from irradiated single-donor apheresis platelets in patients refractory to all available treatment. Methods: Single-centre retrospective series of 14 consecutive patients with Grade 3-4 BKV-HC treated between January 2018 and June 2021, selected from 84 patients who developed haemorrhagic cystitis during this period. All had failed hyperhydration, forced diuresis, continuous bladder irrigation and cidofovir; 10 had undergone previous cystoscopic cauterisation, 10 had immunosuppression reduction, and 4 had hyperbaric oxygen, all without resolution. The median duration of haematuria before allo-PC was 45.5 days. The primary outcome was resolution of macroscopic haematuria; transfusion requirement, catheter removal and readmission were reported separately. Results: Macroscopic haematuria resolved in 12 of 14 patients (85.7%, 95% CI 60.1-96.0) after a median of 5.5 days (IQR 4.0-12.5). Seven patients (50.0%, 95% CI 26.8-73.2) met all secondary criteria. Product platelet concentration correlated inversely with time to resolution (rs = -0.917, 95% CI -0.977 to -0.725). Because a fixed 50 mL volume was given, the delivered dose varied 18-fold (0.30-5.62 × 109 platelets/kg). Body weight also correlated with time to resolution (rs = +0.791), but in partial correlation the association with platelet concentration persisted after adjustment for weight (rs = -0.783, p = 0.004) and for age (rs = -0.827, p = 0.002), whereas the association with weight did not persist after adjustment for platelet concentration (rs = +0.292, p = 0.38). No procedure-related complication exceeded Clavien-Dindo Grade I. Conclusions: In patients who had exhausted all available treatment, intravesical allo-PC was followed by resolution of haematuria in most cases. The association between product platelet content and time to resolution was not explained by body weight or age. These uncontrolled findings are hypothesis-generating.