Characteristics and Risk Factors for BK Virus-Associated Cystitis after Post-Transplant Cyclophosphamide (PTCy) -Based Allogeneic Hematopoietic Cell Transplants (HCT)

Introduction
BK virus reactivation has been associated with cystitis and urethritis (BK-C), including hemorrhagic cystitis, in HCT recipients, significantly impacting post-HCT morbidity. The risk for BK-C appears increased in patients receiving post-transplant cyclophosphamide. Here, we studied BK virus reactivation and BK-C incidence in all consecutive HCT recipients within the Center for Immuno-Oncology of the National Cancer Institute.

Methods
Between 2015-2023, 189 patients underwent allogeneic HCT on one of several clinical protocol (NCT-03922724, -03983850, -04959175, -05436418, -03663933, -02579967). Engrafted patients with ≥28 days follow up, no prior allogeneic HCT within 1 year and no BK-C at the time of conditioning (n=169) were included in the study. Urine and blood BK quantitative PCRs were performed weekly from conditioning start until D100 post-HCT. In the absence of a standard serologic test, patients with > 1 positive sample before D100 were considered at risk for BK-C.

Results
With median follow-up 20 months (range 1–96), 133 (79%) of 169 patients were considered to be at known risk for BK-C; 87/133 (65%) were BK positive already at day of transplant (D0). 100-day cumulative incidence of BK-C was 72% (95% CI 0.63-0.78), with median onset 30 days post-HCT (IQR 21-41) and median symptom duration of 26 days (IQR 14-47). Most frequent symptoms were dysuria (85/96 [88.5%], median duration 22 days [IQR 12-41]), followed by hematuria (76/96 [79%], median duration 14 days [IQR 7-32]), and urgency (68/96 [71%], median duration 18 days [IQR 11-33]), with 57 (59%) patients developing clots (Bedi grade ≥3). Almost 49% (47/96) of patients required opioid administration, including IV opioids in 20 (21%) patients. Hydronephrosis was detected in 9 (9%) patients; 2 required nephrostomy tube placement. In 19% of cases, symptoms persisted after D100.
BK-C incidence was higher in males (80% vs 60%, p=0.02); marrow graft and sirolimus recipients had higher BK-C incidence compared with PBSC (81% vs 58%, p=0.01) and calcineurin inhibitor recipients (78% vs 41%, p=0.001), respectively. Reducing PTCy dose did not decrease the risk of BK-C, however PTCy 25 mg/kg on day +4 recipients (n=3) developed BK-C later (median 49 days) compared with PTCy 25 mg/kg/day on days +3/+4 (n=46, median 36 days) and PTCy 50 mg/kg/day on days +3/+4 (n=84, median 27 days, p=0.001). Patients receiving myeloablative conditioning had the highest incidence of BK-C (81%) compared with patients receiving reduced intensity (73%) and non-myeloablative (0%) conditioning (p<0.001). Urine BK detection at D0 was associated with earlier BK-C onset (median 26 vs 40 days, p=0.0001). Daily urine output during PTCy administration inversely correlated with time to BK-C onset (-0.23, p=0.03).
In the first 3 weeks post-HCT, BK urine levels were significantly higher in patients developing BK-C compared with those who did not (Mann-Whitney p<0.03); also, BK virus levels in urine showed ability to predict BK-C in a short timeframe, for instance, urine BK cut off 7.14 log10 UI/ml at D0 had an AUC of 0.94 (sens=100%, spec=85%) for predicting BK-C onset by D7. In the PTCy 50 mg/kg/day on days+3/+4 group, ROC analysis using BK urine value on D14 showed an AUC of 0.72, identifying a cutoff of 8.19 log10 IU/mL (sens=73%, spec=72%) as a predictor for future BK-C development. Early immune reconstitution defined by lymphocyte subset numerical counts at D14 did not predict subsequent development of BK-C.
After BK-C onset, kinetics of weekly urine BK levels did not predict symptom duration: at D100, 90% of patients still had a detectable BK in urine, even if asymptomatic. BK-C symptoms lasted longer in patients receiving busulfan-based conditioning (median 28 [95% CI 23-40] vs 16 [95% CI 8-29] days, p=0.02) and higher cyclophosphamide-equivalent dose (75°quartile, median 27 days [95% CI 22-39] vs 25°quartile, median 19 days [95% CI 10-29], p=0.04).

Conclusion
We observed a high incidence of BK-C in PTCy-based HCT recipients. The use of marrow graft, sirolimus, and myeloablative busulfan-based conditioning are associated with increased risk of BK-C; decreasing PTCy dose does not decrease BK-C incidence but postpones its onset and reduced its duration. The use of busulfan and cyclophosphamide appeared to be relevant factors for prolonged BK-C symptoms. Monitoring BK virus in urine can predict BK-C occurrence within a short timeframe, allowing early intervention.