Oral USC-093, a novel homoserinamide analogue of the tyrosinamide (S)-HPMPA prodrug USC-087 has decreased nephrotoxicity while maintaining antiviral efficacy against human adenovirus infection of Syrian hamsters.

Adenovirus infections of immunocompromised humans are a significant source of morbidity and mortality. Presently, there is no drug specifically approved for the treatment of adenovirus infections by the FDA. The state-of-the-art treatment of such infections is the off-label use of cidofovir, an acyclic nucleotide phosphonate. While cidofovir inhibits adenovirus replication, it has dose-limiting kidney toxicity. There is an apparent need for a better compound to treat adenovirus infections. To this end, we have been developing acyclic nucleotide phosphonate prodrugs that utilize an amino acid scaffold equipped with a lipophilic modifier. Here, we compare the antiviral potential of two prodrugs of HPMPA that differ only in the amino acid-based promoiety: USC-087, based on an N-hexadecyl tyrosinamide, and USC-093, based on an N-hexadecyl serinamide. Oral administration of both compounds was very efficacious against disseminated HAdV-C6 infection in immunosuppressed Syrian hamsters, suppressing virus replication and mitigating pathology even when treatment was withheld until 4 days after challenge. We saw only marginal efficacy after respiratory infection of hamsters, which may reflect suboptimal distribution to the lung. Importantly, neither compound induced intestinal toxicity, which was observed as the major adverse effect in clinical trials of brincidofovir, a prodrug of cidofovir which also contains a C-16 modifier. Notably, we found that there was a significant difference in the nephrotoxicity of the two compounds: USC-087 caused significant kidney toxicity while USC-093 did not, at effective doses. These findings will be valuable guidepoints in the future evolution of this new class of potential prodrugs to treat adenovirus infections.

Therapeutic strategies against BK polyomavirus infection in kidney transplant recipients: Systematic review and meta-analysis.

BACKGROUND: The selection of antiviral therapy for BK polyomavirus (BKPyV) infection has been extensively debated. Our study aimed to assess the efficacy and safety of various treatments for BKPyV infection. METHODS: We searched PubMed, EMBASE, and Web of Science databases for relevant studies regarding drug treatments for BKPyV viremia/DNAemia published between January 1, 1970 and September 30, 2022. Two independent authors screened the published studies, extracted pertinent data, and evaluated their methodological quality. A meta-analysis was performed using the RevMan software version 4.2.2. RESULTS: A total of 33 published studies involving 986 patients were included in the meta-analysis. Overall, therapeutic interventions comprised immunosuppression reduction alone or in combination with leflunomide, intravenous immunoglobulin (IVIG), cidofovir, or mTOR inhibitor (mTORi) therapy. The meta-analysis revealed that the efficacy of immunosuppression reduction alone for serum BKPyV clearance was 68% (95% confidence interval [CI]: 0.58-0.77; I2 = 78%). Moreover, the efficacy of immunosuppression reduction in combination with leflunomide, cidofovir, IVIG, or mTORi therapy for serum BKPyV clearance was 61% (95% CI: 0.47-0.74; I2 = 83%), 71% (95% CI: 0.63-0.78; I2 = 0), 87% (95% CI: 0.82-0.93; I2 = 45%), and 80% (95% CI: 0.59-1.00; I2 = 58%), respectively. Compared to immunosuppression reduction alone, immunosuppression reduction combined with IVIG therapy offered a statistically significant benefit in serum BKPyV clearance (P < 0.01) with minimal adverse reactions, whereas other adjunctive drug treatments did not demonstrate considerable effects. CONCLUSIONS: Reducing immunosuppression remains the primary approach for treating BKPyV infection. Although the combination treatment with IVIG proved to be most effective, other agents might offer varied antiviral advantages of high heterogeneity, which should be substantiated in future long-term randomized controlled trials.

Disulfide-incorporated lipid prodrugs of cidofovir: Synthesis, antiviral activity, and release mechanism.

The double-stranded DNA (dsDNA) viruses represented by adenovirus and monkeypox virus, have attracted widespread attention due to their high infectivity. In 2022, the global outbreak of mpox (or monkeypox) has led to the declaration of a Public Health Emergency of International Concern. However, to date therapeutics approved for dsDNA virus infections remain limited and there are still no available treatments for some of these diseases. The development of new therapies for treating dsDNA infection is in urgent need. In this study, we designed and synthesized a series of novel disulfide-incorporated lipid conjugates of cidofovir (CDV) as potential candidates against dsDNA viruses including vaccinia virus (VACV) and adenovirus (AdV) 5. The structure-activity relationship analyses revealed that the optimum linker moiety was C2H4 and the optimum aliphatic chain length was 18 or 20 atoms. Among the synthesized conjugates, 1c exhibited more potency against VACV (IC50 = 0.0960 µM in Vero cells; IC50 = 0.0790 µM in A549 cells) and AdV5 (IC50 = 0.1572 µM in A549 cells) than brincidofovir (BCV). The transmission electron microscopy (TEM) images revealed that the conjugates could form micelles in phosphate buffer. The stability studies in the GSH environment demonstrated that the formation of micelles in phosphate buffer might protect the disulfide bond from glutathione (GSH) reduction. The dominant means of the synthetic conjugates to liberate the parent drug CDV was by enzymatic hydrolysis. Furthermore, the synthetic conjugates remained sufficiently stable in simulated gastric fluid (SGF), simulated intestinal fluid (SIF), and pooled human plasma, which indicated the possibility for oral administration. These results indicated 1c may be a broad-spectrum antiviral candidate against dsDNA viruses with potential oral administration. Moreover, modification of the aliphatic chain attached to the nucleoside phosphonate group was involved as an efficient prodrug strategy for the development of potent antiviral candidates.

In Vitro efficacy of cyclosporine a and various antiseptics and antiviral drugs on adenovirus genotype 8, a common cause of epidemic keratoconjunctivitis.

PURPOSE: To evaluate the in vitro efficacy of cidofovir, ganciclovir, povidone-iodine, chlorhexidine, and cyclosporine A on adenovirus genotype 8. METHODS: Conjunctival samples were collected from patients with adenoviral conjunctivitis and cultured in A549 cells. Adenovirus diagnosis was confirmed by RT-PCR. For each drug, the 50% cytotoxic concentration (CC 50 ) was determined. Subsequently, the antiviral activity was tested at concentrations below CC 50, and the 50% inhibitor concentration (IC 50 ) of drugs was determined RESULTS: While the IC 50 of cidofovir against adenovirus genotype 8 was 3.07 ± 0.8 µM, ganciclovir, povidone-iodine, chlorhexidine, and cyclosporine A were not found to be effective against adenovirus genotype 8 at concentrations below the CC 50 value. CONCLUSIONS: Cidofovir was found effective and the IC 50 value was within the ranges in the literature. Ganciclovir and cyclosporine A were found to be ineffective at doses below the cytotoxic dose, povidone-iodine and chlorhexidine was found to be highly cytotoxic.

Daily low dose intravesical cidofovir for the treatment of BK virus associated hemorrhagic cystitis after allogeneic stem cell transplantation.

INTRODUCTION: BK virus associated hemorrhagic cystitis(BKV-AHC) is a serious complication observed after allogeneic stem cell transplantation and the current therapeutic options are scarce with substantial renal side effects. Although the guidelines recommend intravenous cidofovir application with caution to nephrotoxicity, there are few studies which investigated intravesical administration and reported similar therapeutic results with less renal side effects. METHODS: We administered low dose, daily and consecutive (75 mg/day, for 5 days) intravesical cidofovir to 25 patients with BKV-AHC that developed after (ASCT). RESULTS: The response rate in our cohort was 92% and relapse was not encountered in 84% of the patient population during one year of follow-up. The median BK urine viral load significantly decreased from 260,000,000 IU/mL to 53,000,000 IU/mL after a week of treatment (p = 0.0001). Rise in serum creatinine was observed in 5 patients during treatment and post-treatment nephrotoxicity was seen in only 1 patient. CONCLUSIONS: Daily low dose intravesical cidofovir might be an effective treatment option for BKV-AHC after ASCT with favorable less systemic side effects.

Filociclovir is a potent inhibitor of human adenovirus F41.

Clusters of acute non HepA-E hepatitis cases in previously healthy children have been reported globally. At least, 1010 cases were identified in 35 countries, 5% of those cases required liver transplantation and 2% died. The exact cause is not yet known, but there is circumstantial evidence suggesting that human adenovirus F41 (HAdV-F41) might be playing a role. No antiviral drug has been approved for treating human adenovirus infections. Furthermore, HAdV-F41 is notoriously difficult to grow in cell culture, which hindered studying the efficacy of an antiviral compound against this virus. Here, we show that filociclovir (FCV), a nucleoside analog, is a potent inhibitor of HAdV-F41 in cell culture using 2 approaches, namely immunostaining of infected cells and virus yield reduction assay. The activity of FCV was compared to 3 other known antivirals: cidofovir (CDV), ganciclovir (GCV) and valganciclovir (VGCV). Among the 4 compounds examined in this study, FCV was the most potent, with an EC50 of 3.5 µM. These compounds can be ranked by potency as follows: FCV > CDV > GCV ≥ VGCV. In addition, FCV was 10-fold more potent than CDV in a virus yield reduction assay. This report provides timely and valuable methodologies to the research community for testing antivirals against HAdV-F41. Our findings also support the continued development of FCV for various therapeutic applications, including pediatric hepatitis, if a causal relationship is firmly established in the future.

Targeting ectromelia virus and TNF/NF-κB or STAT3 signaling for effective treatment of viral pneumonia.

Viral causes of pneumonia pose constant threats to global public health, but there are no specific treatments currently available for the condition. Antivirals are ineffective when administered late after the onset of symptoms. Pneumonia is caused by an exaggerated inflammatory cytokine response to infection, but tissue necrosis and damage caused by virus also contribute to lung pathology. We hypothesized that viral pneumonia can be treated effectively if both virus and inflammation are simultaneously targeted. Combined treatment with the antiviral drug cidofovir and etanercept, which targets tumor necrosis factor (TNF), down-regulated nuclear factor kappa B-signaling and effectively reduced morbidity and mortality during respiratory ectromelia virus (ECTV) infection in mice even when treatment was initiated after onset of clinical signs. Treatment with cidofovir alone reduced viral load, but animals died from severe lung pathology. Treatment with etanercept had no effect on viral load but diminished levels of inflammatory cytokines and chemokines including TNF, IL-6, IL-1ß, IL-12p40, TGF-ß, and CCL5 and dampened activation of the STAT3 cytokine-signaling pathway, which transduces signals from multiple cytokines implicated in lung pathology. Consequently, combined treatment with a STAT3 inhibitor and cidofovir was effective in improving clinical disease and lung pathology in ECTV-infected mice. Thus, the simultaneous targeting of virus and a specific inflammatory cytokine or cytokine-signaling pathway is effective in the treatment of pneumonia. This approach might be applicable to pneumonia caused by emerging and re-emerging viruses, like seasonal and pandemic influenza A virus strains and severe acute respiratory syndrome coronavirus 2.

Topical Astodrimer Sodium, a Non-Toxic Polyanionic Dendrimer, Demonstrates Antiviral Activity in an Experimental Ocular Adenovirus Infection Model.

There is no approved antiviral therapy for adenovirus (HAdV) ocular infections. Astodrimer sodium (SPL7013) is a polyanionic dendrimer with antiviral activity. The current study evaluated the ocular tolerability and anti-adenoviral efficacy of topical SPL7013 in rabbit ocular models. In a tolerability study, rabbits were treated with 3% SPL7013, vehicle, or 0.5% cidofovir. Their eyes were graded using the Draize scale. In antiviral efficacy studies, HAdV5 inoculated eyes were treated with 3% SPL7013, vehicle, or 0.5% cidofovir. Eyes were cultured for the virus on days 0, 1, 3, 4, 5, 7, 9, 11, and 14. Viral titers were determined. There were no differences in Draize scores between 3% SPL7013 and vehicle on any day. Cidofovir produced significantly higher Draize scores on day 12 than SPL7013 and vehicle. The 3% SPL7013 and 0.5% cidofovir significantly reduced daily viral titers and positive cultures per total compared with vehicle on several different days. The 3% SPL7013 and 0.5% cidofovir significantly reduced the duration of HAdV5 shedding compared to vehicle. The 3% SPL7013 demonstrated significantly more antiviral activity compared with vehicle in the Ad5/NZW rabbit ocular model. The 3% SPL7013 induced "minimal" to "practically non-irritating" Draize scores in the ocular tolerability study. Further development of astodrimer sodium as a topical antiviral therapy for adenoviral ocular infections is indicated.

Antiviral Drugs Against Herpesviruses.

The discovery of the nucleoside analogue, acyclovir, represented a milestone in the management of infections caused by herpes simplex virus and varicella-zoster virus. Ganciclovir, another nucleoside analogue, was then used for the management of systemic and organ-specific human cytomegalovirus diseases. The pyrophosphate analogue, foscarnet, and the nucleotide analogue, cidofovir, have been approved subsequently and constitute the second-line antiviral drugs. However, the viral DNA polymerase is the ultimate target of all these antiviral agents with a possible emergence of cross-resistance between these drugs. Recently, letermovir that targets the viral terminase complex was approved for the prophylaxis of human cytomegalovirus infections in hematopoietic stem cell transplant recipients. Other viral targets such as the protein kinase and the helicase-primase complex are also evaluated for the development of novel potent inhibitors against herpesviruses.

In vitro co-infection by cytomegalovirus improves the antiviral activity of ganciclovir against human adenovirus.

Human adenovirus (HAdV) infection has an important clinical impact in the immunosuppressed population and is associated with high morbidity and mortality rates. The lack of a specific, safe and effective antiviral treatment against HAdV makes necessary the search for new therapeutic options. The aim of this study was to evaluate the in vitro activity of ganciclovir (GCV) against HAdV in co-infection by human cytomegalovirus (HCMV) and HAdV in cellular cultures. Quantitative real-time polymerase chain reaction (qPCR) was used to measure HAdV and HCMV DNA replication efficiency in monocultures and in co-infection situations in the presence of both cidofovir (CDV) and GCV. The effects of GCV and CDV were also evaluated in a burst assay (used to measure the production of virus particles) for both viruses, alone and in combination. GCV decreased by 1-log the HAdV DNA replication efficiency in co-infection with HCMV compared with its activity in HAdV monoculture. The burst assay showed that the reductions in virus yield in the presence of GCV were higher for HCMV and co-infection than for HAdV in monoculture (145.2±35.5- vs. 116.4±27.3- vs. 23.0±10.0-fold, respectively, P<0.05). The improved anti-HAdV activity of GCV during co-infection may be because of the more efficient phosphorylation of GCV by the HCMV protein kinase, UL97. Patients treated with GCV as pre-emptive therapy for HCMV infection may be considered as low-risk for developing HAdV infections; however, further evaluations are required to confirm these results.

Application of human lymphoid cells for the evaluation of antivirals against human adenovirus type 19: Zalcitabine has superior activity compared to cidofovir.

Human adenovirus type 19 (HAdV-19) is a major cause of the epidemic keratoconjunctivitis. Outbreaks of keratoconjunctivitis are problematic to human health, especially for infants, the elderly, and immunocompromised individuals. However, the development of anti-HAdV drugs has been hampered by inconvenient screening systems; therefore, development of a simple screening method is highly desirable. In this study, we identified that HAdV-19 can infect a human lymphoid cell line transformed with human T-cell leukemia virus (MT-2 cells). MT-2 cells supported HAdV-19 replication and showed apparent cytopathic effects within five days post-infection. Using a thiazolyl blue tetrazolium bromide (MTT)-based colorimetric assay on MT-2 cells, we were able to detect the anti-HAdV-19 activities of previously reported nucleoside/tide compounds, including (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (cidofovir), 2',3'-dideoxycytidine (zalcitabine) and 3'-deoxy-3'-fluorothymidine (trifluridine). Compared with previous methods, this system represents a more simple and rapid method to screen anti-HAdV-19 agents.

Radiolabeling of cidofovir with technetium-99m and biodistribution studies

Radiolabeling cidofovir with technetium-99m (99mTc-CDV) is an innovative procedure that enables real-time monitoring of the drug. Essays were performed in vitro, showing high radiolabel stability within 24 h. Blood clearance, biodistribution studies, and scintigraphic images were performed in healthy mice in order to evaluate the profile of the drug in vivo. 99mTc-CDV showed biphasic blood circulation time and significant kidney uptake, indicating that 99mTc-CDV is preferentially eliminated by the renal route. Bones also showed important uptake throughout the experiment. In summary, cidofovir was successfully labeled with technetium-99m and might be used in further studies to track the drug.

New 2-Oxoimidazolidine Derivatives: Design, Synthesis and Evaluation of Anti-BK Virus Activities in Vitro.

A series of novel 2-oxoimidazolidine derivatives were synthesized and their antiviral activities against BK human polyomavirus type 1 (BKPyV) were evaluated in vitro. Bioassays showed that the synthesized compounds 1-{[(4E)-5-(dichloromethylidene)-2-oxoimidazolidin-4-ylidene]sulfamoyl}piperidine-4-carboxylic acid (5) and N-Cyclobutyl-N'-[(4E)-5-(dichloromethylidene)-2-oxoimidazolidin-4-ylidene]sulfuric diamide (4) exhibited moderate activities against BKPyV (EC50 =5.4 and 5.5 µm, respectively) that are comparable to the standard drug Cidofovir. Compound 5 exhibited the same cytotoxicity in HFF cells and selectivity index (SI50 ) as Cidofovir. The selectivity index of compound 4 is three times less than that of Cidofovir due to the higher toxicity of this compound. Hence, these compounds may be taken as lead compound for further development of novel ant-BKPyV agents.

Cidofovir Diphosphate Inhibits Adenovirus 5 DNA Polymerase via both Nonobligate Chain Termination and Direct Inhibition, and Polymerase Mutations Confer Cidofovir Resistance on Intact Virus.

Human adenovirus (AdV) can cause fatal disease in immune-suppressed individuals, but treatment options are limited, in part because the antiviral cytidine analog cidofovir (CDV) is nephrotoxic. The investigational agent brincidofovir (BCV) is orally bioavailable, nonnephrotoxic, and generates the same active metabolite, cidofovir diphosphate (CDVpp). However, its mechanism of action against AdV is poorly understood. Therefore, we have examined the effect of CDVpp on DNA synthesis by a purified adenovirus 5 (AdV5) DNA polymerase (Pol). CDVpp was incorporated into nascent DNA strands and promoted a nonobligate form of chain termination (i.e., AdV5 Pol can extend, albeit inefficiently, a DNA chain even after the incorporation of a first CDVpp molecule). Moreover, unlike a conventional mismatched base pair, misincorporated CDVpp was not readily excised by the AdV5 Pol. At elevated concentrations, CDVpp inhibited AdV5 Pol in a manner consistent with both chain termination and direct inhibition of Pol activity. Finally, a recombinant AdV5 was constructed, containing Pol mutations (V303I and T87I) that were selected following an extended passage of wild-type AdV5 in the presence of BCV. This virus had a 2.1-fold elevated 50% effective concentration (EC50) for BCV and a 1.9-fold increased EC50 for CDV; thus, these results confirmed that viral resistance to BCV and CDV can be attributed to mutations in the viral Pol. These findings show that the anti-AdV5 activity of CDV and BCV is mediated through the viral DNA Pol and that their antiviral activity may occur via both (nonobligate) chain termination and (at high concentration) direct inhibition of AdV5 Pol activity.

In vitro comparison of acyclovir, ganciclovir and cidofovir against equid alphaherpesvirus 3 and evaluation of their efficacy against six field isolates / Comparación in vitro de aciclovir, ganciclovir y cidofovir contra alfa-herpesvirus equino 3 y evaluación de la eficacia de los mismos frente a 6 aislamientos de campo del virus

Equid alphaherpesvirus 3 (EHV3) is the etiological agent of equine coital exanthema (ECE), which is a venereal, highly contagious disease, characterized by the formation of papules, vesicles, pustules and ulcers on the external genitalia of mares and stallions. EHV3 remains in a latent state after a successful infection and there are latently infected animals in which the virus is reactivated and generally re-excreted subclinically. There are no available vaccines for this condition and prevention is based on the clinical examination of mares prior to mating, which allows to segregate those showing clinical signs. As this approach does not eliminate the risk of contagion in stallions from subclinically infected mares, there is a need for a specific EHV3 treatment. Nowadays, there exist various antiviral compounds of proven effectiveness for other alphaherpesviruses affecting humans and animals. The aim of the present study was to compare the efficacy of three antiviral compounds, acyclovir, ganciclovir and cidofovir against EHV3 in vitro, and to assess their efficacy against six EHV3 Argentinian field isolates. To determine the efficacy of these compounds in vitro, three parameters were analyzed: reduction of plaque number, reduction of plaque size and reduction of viral production. Additionally, the effectiveness of the three compounds at an optimum concentration previously determined in this study was investigated for the EHV3 field isolates. Based on our results, ganciclovir was the most potent antiviral compound to reduce EHV3 replication in vitro and may thus be a valuable candidate for treatment and prevention of ECE in mares and stallions.

El alfa-herpesvirus equino 3 (EHV3) es el agente etiológico del exantema coital equino (ECE), enfermedad venérea, altamente contagiosa y caracterizada por la aparición de pápulas, vesículas, pústulas y úlceras en los genitales externos de yeguas y padrillos. Luego de la primo-infección, el EHV3 se mantiene en el animal en un estado de latencia a partir del cual puede reactivar y excretarse, generalmente de manera subclínica. No existen vacunas, por lo que la prevención se basa en la detección de las lesiones clínicas previo al servicio, y la segregación de estos animales. Sin embargo, este abordaje no previene la infección del padrillo por parte de yeguas que excretan el virus de manera subclínica, y por lo tanto existe la necesidad de un tratamiento específico contra el EHV3. En la actualidad, existen varios compuestos antivirales de probada eficacia contra herpesvirus humanos y veterinarios. El objetivo de este trabajo es comparar la eficacia de 3 compuestos antivirales, aciclovir, ganciclovir y cidofovir, contra EHV3 in vitro, y evaluar la eficacia de los mismos contra 6 cepas de campo argentinas de EHV3. Para determinar la eficacia de los compuestos in vitro se evaluaron 3 parámetros: reducción del número de placas de lisis, reducción del tamaño de placas de lisis y reducción de la producción de virus. Adicionalmente, la efectividad de los compuestos en una concentración óptima, previamente determinada en este estudio, fue determinada para 6 cepas de campo argentinas de EHV3. De acuerdo con los resultados obtenidos, ganciclovir fue el compuesto más potente en reducir la replicación del EHV3 in vitro, y por lo tanto podría considerarse un potencial candidato para el tratamiento y la prevención del ECE en yeguas y padrillos.

Model of population pharmacokinetics of cidofovir in immunocompromised children with cytomegalovirus and adenovirus infection.

Objectives: To describe cidofovir pharmacokinetics and assess the link between concentration and safety/efficacy in children. Patients and methods: An observational study was conducted in 13 immunocompromised children receiving cidofovir for adenovirus and/or cytomegalovirus infection. A population pharmacokinetic model was built and AUC0-24 was derived for each patient. Virological success was defined as a decrease of the viraemia by ≥1 log10 copies/mL within 15 days of cidofovir initiation. The association between AUC0-24 and virological success was assessed using a Wilcoxon test. An AUC0-24 cut-off value was determined using a Fisher's exact test. Results: Overall, 86 blood samples were analysed. A two-compartment model with first-order absorption and elimination best described the cidofovir data. Virological success (VS) was reached in 6/8 children with adenovirus viraemia and in 1/4 children with cytomegalovirus viraemia. Patients with VS displayed a non-significant higher median AUC0-24 compared with patients with virological failure: 48.6 (range 8.9-72.6) versus 19.1 (6.9-22.7) mg·h/L. Adenovirus-viraemic patients with an AUC0-24 value below 19.1 mg·h/L had a higher probability of treatment failure (P = 0.03). Aviraemic children with stool and/or nasopharyngeal adenovirus carriage cleared the viral carriage within a month of cidofovir initiation. During treatment, 1/13 children developed a tubulopathy but none of them had an increase in creatininaemia. Conclusions: Cidofovir appears safe and reasonably well tolerated and seemed to have efficacy in a subset of patients with adenovirus/cytomegalovirus infection. Therapeutic drug monitoring may be useful in children receiving cidofovir and, in the case of adenovirus infection, targeting an AUC0-24 above 19.1 mg·h/L could be associated with higher probability of virological success.

Efficacy of Cidofovir in Treatment of BK Virus-Induced Hemorrhagic Cystitis in Allogeneic Hematopoietic Cell Transplant Recipients.

BK virus-associated hemorrhagic cystitis (BK-HC) is a common complication after allogeneic hematopoietic stem cell transplantation (allo-HCT), with incidences up to 70%. Cidofovir is an antiviral agent with growing evidence as a therapeutic intervention. To assess the safety profile and efficacy of intravenous and intravesical cidofovir in allo-HCT patients with BK-HC, a retrospective study was undertaken of the allo-HCT cohort who received cidofovir for symptomatic BK-HC (hematuria with BK viruria or viremia) from January 2010 until March 2017 in a single transplant center in Ontario, Canada. The primary outcome measure was a reduction in BK-HC severity (graded from 1 to 4); secondary outcomes included overall survival, BK virus titers, and the onset of acute kidney injury. Twelve allo-HCT patients received cidofovir for BK-HC, with pretreatment clinical severity of 3 (50%) or 4 (50%). Cidofovir was administered via intravenous (33%), intravesical (58%), or both modalities (8%). After a median cumulative dose of 10 mg/kg (range, 1 to 37), mean BK-HC grade decreased significantly by 1.8 (3.5 precidofovir, 1.7 postcidofovir, P < .01). Sixty-six percent of patients had at least partial response to cidofovir, with similar response rates between intravenous (66%) and intravesical (62%) administration. Sixty-seven percent of patients died, and 33% of patients experienced renal toxicity, including 2 patients receiving intravesical therapy. In this retrospective series, there was a significant reduction in BK-HC severity after cidofovir administration; most patients achieved at least partial response after cidofovir administration. Even with intravesical instillation, acute kidney injury remains a potential complication of cidofovir. Although cidofovir may be an efficacious therapy for BK-HC, albeit with potential demonstrated toxicities, further prospective trials are needed.