A single-center experience with BK virus nephropathy

BACKGROUND: BK virus nephropathy has an increasing role in renal transplant dysfunction, since new, highly potent immunosuppressive drugs have been introduced into therapy following renal transplantation. Diagnosis of acute impairment of renal transplant function is complicated by difficulty in differentiating BK virus nephropathy from acute rejection. PATIENTS AND METHODS: We retrospectively described the findings and therapeutic approaches of 6 consecutive patients with BK virus nephropathy in our transplantation center (75 - 80 transplantations/ year). BK virus nephropathy was classified according to Drachenberg et al. [2004]. RESULTS: We observed an incidence rate of < 1% for BK nephropathy in our center. Four patients had a pattern B whereas 2 patients revealed a pattern C of BK virus nephropathy. Focal C4d-positive staining of peritubular capillaries were found in 2 of the 6 cases. For earlier detection of BK nephropathy, a diagnostic algorithm for each patient after renal transplantation was established. Urine was continuously monitored by cytology for decoy cells and PCR for BK virus DNA. If PCR was also positive for the BK virus in plasma, biopsy of the renal allograft was performed. Thereby diagnosis could be confirmed sooner. For treatment of BK nephropathy in our center, we reduced immunosuppressive agents and initiated a virustatic treatment with cidofovir in the first 3 cases. However, results were not satisfactory and two allografts were lost. We then reconsidered our therapeutic approach and switched the immunosuppressive treatment to leflunomide with consistent low dose steroids. We use therapeutic drug monitoring for leflunomide and aim at a target level of 40 - 100 microg/ml. We lost no allograft with BK nephropathy since using this therapeutic approach. CONCLUSION: In our center, leflunomide therapy, but not cidofovir, was effective in patients with BK virus nephropathy of the renal allograft.     

Native kidney BK virus nephropathy associated with acute lymphocytic leukemia

Background

Polyoma BK virus nephropathy is a common complication after renal transplantation and is rarely seen in non-renal transplant recipients. There are only a couple of case reports of BK virus nephropathy in native kidneys in non-transplant patients, including a recent report of a 73-year-old patient with chronic lymphatic leukemia. A variety of treatment options, including leflunomide and cidofovir, were reported in these patients.

Case diagnosis/treatment

Here we report the case of a 10-year-old boy with acute lymphatic leukemia who presented with non-oliguric hypertensive acute kidney injury at the 12th maintenance cycle of his chemotherapy. The workup supported the clear diagnosis of BK virus nephropathy with tubulointerstitial changes, and the patient responded favorably to intravenous immunoglobulin therapy.

Conclusions

Pediatric nephrologists need to consider BK virus nephropathy as a differential diagnosis of acute kidney injury in immunocompromised non-transplant patients.     

Experiences with leflunomide in solid organ transplantation

BACKGROUND: Leflunomide (Arava), a drug widely used for treatment of rheumatoid arthritis, has a very promising background in experimental transplantation. Its activity in experimental models of chronic rejection, its synergy with calcineurin phosphatase inhibitors, and its inhibitory effects on herpes virus replication are compelling reasons to pursue its clinical evaluation in transplantation. We report the use of this drug over the past 3 years in various clinical situations. METHODS: A retrospective review was performed in 53 liver and kidney transplant recipients receiving Arava. A single-dose pharmacokinetic (PK) study was first performed in stable, renal transplant recipients, and an initially targeted serum level of 100 microg/mL (300 microM) was calculated to require a loading dose of 1200-1400 mg over a 7-day period. We correlate the appearance of toxicity with serum levels of active drug and review the outcomes in patients whose clinical condition required dose reductions of conventional immune suppressive drugs. RESULTS: Fifty-three patients received leflunomide from 5 days to more than 430 days, and 37 patients received the drug for more than 60 days. The primary toxicity was anemia in the renal transplant patients and elevation of liver enzymes in the liver transplant patients. At comparable oral doses, serum levels were substantially lower and anemia more common in patients with serum creatinine >3 mg/dL. In liver and renal recipients with serum creatinine <3 mg/dL, the drug was well tolerated and dose-limiting side effects occurred in less than 15% when drug serum levels were less than 80 microg/ml. Patients with serum creatinine >3 mg/dL often required serum levels of active drug reduced to <60 microg/mL. In 12 of 18 renal patients treated for 200 days or more, the dose of cyclosporine or Prograf was reduced by a mean of 38.5% and stopped in one patient. The prednisone dose was reduced by a mean of 25% in these same 13 patients. Cyclosporine or FK506 was stopped completely in four liver recipients and reduced by 65% in another patient. No evidence of acute rejection developed in any of these liver or kidney transplant patients. CONCLUSION: Leflunomide seems to possess substantial immune suppressive potency in renal and liver transplant recipients and may be safely dosed for more than 300 days. The data suggest that calcineurin phosphatase inhibitors and prednisone can be safely reduced in patients with serum levels of active drug above 50 microg/mL. Because of a wide inter-patient range of active metabolite terminal half-life (>300%), monitoring of serum levels would seem to be an important part of its evaluation.     

Treatment of Refractory BK Virus-Associated Nephropathy With Cidofovir

BK virus-associated nephropathy (BKVN) has become recognized as an important cause of allograft dysfunction in renal transplant recipients and despite reduction in immunosuppression, 30-40% of recipients ultimately progress to allograft loss. Cidofovir is an antiviral agent that demonstrates in vitro activity against murine polyomavirus and has been proposed for treatment of BKVN in renal allograft recipients. We describe the clinical course, renal function, serial renal histology and urine and blood viral load measurements in two consecutive patients with refractory BKVN who were treated with low-dose cidofovir (0.25 mg/kg IV). In each case, renal dysfunction and BK viral load progressed despite reduced immunosuppression, and persistent BK virus infection was documented in serial renal allograft biopsy specimens. Administration of low-dose cidofovir was associated with clearance of BK virus DNA from blood and allograft, and stabilization of renal function in both patients, without significant toxicity. These preliminary data suggest that low-dose cidofovir may be tolerated, even among renal transplant recipients with significant renal dysfunction due to BKVN. Prospective, controlled trials are warranted to further define the optimal dose, toxicity and potential role of cidofovir in renal transplant recipients with BK virus nephropathy.     

Polyoma Virus Nephropathy in Native Kidneys After Lung Transplantation

Polyoma virus nephropathy is recognized as an emerging clinical problem in renal transplantation; however, polyoma in native kidneys is unusual. We report a patient who developed polyoma nephropathy in his native kidneys 15 months after successful lung transplantation. His immunosuppression consisted of tacrolimus, mycophenolate mofetil, and large doses of steroids because of three rejection episodes. When the condition was recognized, cidofovir was an effective treatment (3 doses of 2-3mg/kg); however, his renal function deteriorated nonetheless. Tubulitis and interstitial cell infiltration in his native kidneys were evidence that the changes were in response to viral injury. Polyoma nephropathy of native kidneys is unusual. An earlier course of cisplatin treatment because of metastatic seminoma prior to lung transplantation may have been contributory to pre-existing renal injury. After cidofovir was begun, the polyoma viral load in serum and urine decreased substantially; however, after high-dose steroid treatment of two rejection episodes, each time a significant increase in viral load was seen. We stained biopsies of native kidneys from 30 recipients of other organs. The biopsies were done for various reasons but not because polymoma virus was suspected. We found no additional cases.     

Effect of leflunomide and cidofovir on replication of BK virus in an in vitro culture system

BACKGROUND: Cidofovir and Leflunomide are used empirically in the treatment of BK virus nephropathy. The aim of this study is to quantify the antiviral activity of these drugs. METHODS: BK virus was grown in a cell-culture system. The rate of viral replication in the presence or absence of the drug being tested was assessed using a quantitative polymerase chain reaction assay. RESULTS: The inhibitory concentration, effective concentration, and selectivity index for Leflunomide are 39.7+/-6.9, 11.3+/-2.8, and 3.8+/-0.8 microg/mL, respectively. For Cidofovir, these indices were, respectively, 63.9+/-17.2, 36.3+/-11.7, and 2.3+/-0.8 microg/mL. CONCLUSIONS: The in vitro activity of Cidofovir and Leflunomide is modest, and the selectivity index is low. There is a need to develop more effective and less toxic anti-BK virus drugs for clinical use.     

BK virus infection,replication, and diseases in pediatric kidney transplantation

Polyomavirus-associated nephropathy is diagnosed in 2–8% of pediatric renal transplants and often precedes renal allograft dysfunction. Without intervention, however, significant graft dysfunction is observed in more than 50% of cases, although progressive early graft loss is reported in only three of 32 (9%) of cases. No specific treatment is available, but early decrease in immunosuppression is followed by declining human polyomavirus type 1 (BK virus) replication and improved outcome. The data suggest differences between pediatric and adult kidney transplantation. Possibly, pediatric patients might be able to mount a more vigorous BK virus-specific immune response than adult patients under similar modulation of immunosuppression. Also the role of cidofovir and leflunomide is still unresolved in pediatric patients. Larger prospective trials are needed to better define the impact of BK virus immunity for replication and disease as well as the role of reducing immunosuppression with or without cidofovir or leflunomide in pediatric transplant patients.     

Mitochondrial Changes in Cidofovir Therapy for BK Virus Nephropathy

Polyoma (BK) virus nephropathy (BKVN) is often treated with the nucleotide analog cidofovir. An adverse effect of this drug class is proximal tubular toxicity, and ultrastructural abnormalities in proximal tubular mitochondria have been observed in patients treated with similar drugs for other viral infections. We report similar changes in biopsies from BKVN treated with cidofovir. Renal allograft biopsies showing BKVN, on which electron microscopy was performed, were categorized into 3 groups: initial diagnosis (BD), postcidofovir treatment (CT), and posttreatment with immunosuppression reduction (IR). Nineteen cases from each group were randomly selected. Mitochondrial changes were present in 6 biopsies from patients receiving CT therapy (31.5%), ranging from diffuse mitochondrial swelling to profound morphologic changes. No similar abnormalities were seen in other groups. In those with atypical mitochondria, the mean number of cidofovir doses was 2.67, with an average interval between last dose and biopsy of 2.17 weeks. CT patients without mitochondrial changes had a mean of 4.6 doses and an average interval between last dose and biopsy of 27.2 weeks. Some renal transplant patients treated with cidofovir display alterations in proximal tubular mitochondria akin to those seen with similar drugs. The findings support the mitochondrial toxicity of nucleotide analogs.     

Quantitative viral load monitoring and cidofovir therapy for the management of BK virus-associated nephropathy in children and adults

BACKGROUND: BK virus (BKV)-associated nephropathy (BKVAN) has been increasingly recognized as an important cause of renal transplant dysfunction. We report the role of quantitative viral load monitoring in the management of BKVAN. METHODS: We developed a real-time quantitative polymerase chain reaction (PCR) assay for BKV detection in urine and plasma. Four renal allograft recipients, including two children, with BKVAN were treated with low-dose cidofovir and followed prospectively. RESULTS: The PCR assay showed a detection limit of 10 viral copies with an intra-assay coefficient of variation of 19%. All four patients with BKVAN demonstrated intranuclear inclusions on allograft biopsy and a progressive rise in serum creatinine; three patients underwent multiple biopsies before the diagnosis of BKVAN was made. Three of the patients experienced a "viral syndrome" before the onset of renal dysfunction. One child also demonstrated an echogenic renal mass. All of the patients demonstrated strongly positive urinary PCR values (>100,000 copies/microL). BKV DNA was also detected in the plasma of three patients. All the patients were treated with intravenous low-dose cidofovir (0.25-1 mg/kg per dose, every 2-3 weeks, without probenecid). BK viruria resolved within 4 to 12 weeks (after 1-4 doses) of the cidofovir therapy, and all patients remain with stable renal function 6 to 26 months posttherapy. CONCLUSIONS: Quantitative PCR for BKV is a sensitive and reliable method for following the course of the infection in renal transplant patients. In addition, cidofovir therapy may be useful in the treatment of some of these patients, and its role needs to be investigated further.     

HLA Type-Independent Method to Monitor Polyoma BK Virus-Specific CD4+ and CD8+ T-Cell Immunity

(Re)activation of quiescent viral diseases is a major problem in immunosuppressed transplant patients. Polyoma BK virus-associated nephropathy (PVAN) caused by active polyoma BK virus (BKV) infection became a main reason for graft loss in kidney transplantation. After diagnosis, most transplant centers react by reducing immunosuppression (IS) to allow the immune system to control the infection. However, the impact of reduced IS on BKV immunity is not well researched. Here we present an HLA type-independent method to monitor BKV-specific T-cell immunity. Applying our method, viral protein 1-specific CD4+ and CD8+ T-cell responses were detected in patients with serum BKV-DNA levels >250 000 copies/mL. In addition, specific T-cell responses were also found in allograft-infiltrating cells. The method can be used to assess the impact of decreased immunosuppression on BKV immunity and to clarify the role of specific T cells in the pathogenesis of PVAN. We strongly recommend its implementation in future clinical studies.     

Clinical Courses of Renal Transplant Recipients with High BK Viremia

Objective

We sought to investigate the clinical courses of renal transplant recipients with plasma BK viral loads >10⁴ copies/mL.

Methods

A single-center retrospective review was performed of 88 kidney transplant patients in whom high BK viremia (defined as plasma BKV load >10⁴ copies/mL) was detected more than once from January 1, 2004, to December 31, 2011.

Results

At the time of transplantation, the mean recipient and donor ages were 44.5 ± 11.1 and 43.9 ± 11.3 years, respectively, and 59 subjects (67.0%) were male. The median times to first BK positivity and high BK viremia after transplantation were 44 and 136 days, respectively. Within 3 months after transplantation, we detected, 56 cases of high BK viremia (63.6%). The mean duration of high BK viremia was 8.2 ± 7.7 months. When plasma BKV load was first >4 logs, the mean log BKV load was 5.50 ± 1.11 log copies/mL, which rose to a maximum of 5.82 ± 1.11. At these times, mean serum creatinine concentrations were 1.67 ± 0.79 and 2.64 ± 2.78 mg/dL, respectively. There were 31 cases (35%) of biopsy-proven BK nephropathy patients among 51 (58%) biopsies. Treatment modalities included discontinuation or dose reduction of mycophenolic acid drugs (n = 68) and switch from tacrolimus to cyclosporine (n = 9), cidofovir (n = 9), and leflunomide (n = 3). Based on the serum creatinine elevation after high BK viremia, patients were divided into group 1 (n = 27; 30.1%), whose maximal creatinine change was <0.5 mg/dL, and group 2, with a greater alteration. On multivariate logistic regression analysis, the maximal plasma BK viral load was significantly associated with a greater serum creatinine elevation (P < .001).

Conclusions

High BK viremia mostly occurred within 3 months after kidney transplantation. About 30% of renal allograft recipients with high BK viremia maintained stable renal function. Maximal plasma BK viral load was the only independent risk factor for high serum creatinine elevation.     

BK virus infection in kidney transplantation: a case for early intervention

The prognosis for renal allograft survival in a patient with BK virus-associated nephropathy is distressing because nearly 50% of affected grafts are lost. With these statistics in mind, attempting clinical intervention before BK virus disease develops seems appropriate. Noninvasive screening tools such as tests to detect decoy cells in urine can enable diagnosis of BK viral infection in its early stages. Although it is not entirely clear which patients with BK infection will have BK disease develop, it may be possible to intervene in this early warning period to prevent the further development of BK virus-associated nephropathy. We present a case of a 50-year-old man in whom asymptomatic BK viruria developed 6 weeks after he received a non-HLA matched kidney from a deceased donor. This BK viruria was accompanied, subsequently, by an unexplained increase in serum creatinine level 2 months after transplantation. Following a change in his immunosuppressive regimen as therapy for presumed renal allograft dysfunction associated with BK viral infection, the decoy cells in his urine disappeared and his creatinine levels stabilized.     

来氟米特对糖尿病肾病大鼠MCP-1表达的影响

目的：通过建立糖尿病肾病（DN）大鼠模型来研究来氟米特对大鼠肾脏MCP-1表达的影响,以进一步探讨来氟米特对DN大鼠的治疗作用及其机制。方法：健康雄性Wistar大鼠30只随机分为3组：对照组、DN组及来氟米特组,应用链脲佐菌素（STZ）诱导左肾切除大鼠DN模型,来氟米特组在成模后每日灌胃来氟米特5mg/kg。于实验第8周、12周末各组分别取5只动物测24h尿量及24h尿蛋白定量;心内采血测肌酐（Scr）、尿素氮（BUN）;电镜标本观察肾组织病理改变,免疫组化法检测肾组织MCP-1蛋白的表达。结果：来氟米特组大鼠尿蛋白、Scr、BUN与同期DN组相比明显减少。病理组织学观察说明来氟米特可以减轻肾脏损害。经来氟米特治疗后,MCP-1的表达减少。结论：来氟米特可减少DN大鼠肾组织MCP-1的表达,减轻肾脏损害。     

Quantification of BK Viral Load in Asymptomatic Renal Allograft Recipients

Introduction: Polyoma BK virus (BKV) has recently been identified to cause renal allograft dysfunction, which manifests as polyomavirus-associated nephropathy (PVAN). However, the presence and level of BKV DNA in renal allograft patients with good and stable renal function have remained undetermined. Methods: In this prospective study, serum samples were collected from a total of 45 renal allograft recipients with serum creatinine <155 μmol/L. In 17 patients, whose duration of transplantation was under 2 years, samples were collected at 3–4-month intervals for up to 2 years after transplantation. BK viral load was quantified using quantitative polymerase chain reaction (Q-PCR). Results: The BK viral load in asymptomatic renal allograft recipients was independent of the duration of transplantation and did not correlate with allograft function. The mean (± SD) level of viremia was 552.80 ± 1931.00 genome copies/mL, with 92.9% of patients having low levels of viremia corresponding to <1 × 10³ copies/mL. In contrast, patients with proven PVAN had levels in the range of 10⁶ copies/mL. Conclusions: The prevailing BK viral load in asymptomatic renal allograft patients is quantifiably low. Our findings may guide optimal immunosuppressive modulation in PVAN cases, where judicious manipulation of immunosuppression is required without inciting allograft rejection.     

Prospective Monitoring of Polyomavirus BK Replication and Impact of Pre-Emptive Intervention in Pediatric Kidney Recipients

Polyoma BK virus (BKV)-associated nephropathy (PVAN) is a relevant cause of poor renal allograft survival. In a prospective analysis, we monitored BKV DNA in blood and urine samples from 62 consecutive pediatric kidney recipients. In patients with BKV replication, we analyzed the impact of reduction of maintenance immunosuppression on viral load kinetics and PVAN in patients with BKV replication. BKV-specific immunity was concomitantly evaluated on blood samples of viremic patients, by measuring the frequency of BKV-specific interferon-gamma-producing and cytotoxic T cells, and BKV IgG antibody levels. At a median follow-up of 24 months, BK viruria was observed in 39 of 62 patients, while BK viremia developed in 13 patients (21%). In all viremic patients, immunosuppression reduction resulted in the clearance of viremia, and prevented development of PVAN, without increasing the rate of acute rejection or causing graft dysfunction. As a consequence of immunosuppression adjustment, an expansion of BKV-specific cellular immunity was observed that coincided with viral clearance. We conclude that treating pediatric kidney transplant patients pre-emptively with immunosuppression reduction guided by BKV DNA in blood is safe and effective to prevent onset of PVAN. BKV-specific cellular immunity may be useful to guide this intervention.     

Severe crescentic BK virus nephropathy with favourable outcome in a transplanted patient treated with Leflunomide

Herein reported is a severe case of BK virus nephropathy, probably caused by an intense/overimmunosuppression and identified 17 months after transplantation. The diagnostic biopsy showed extracapillary proliferation and typical cytopathic lesions, both in tubular epithelial cells and in those of the glomerular crescents. Severe inflammatory infiltrates, tubulitis, tubular atrophy and fibrosis were also observed. Immunohistochemistry and molecular biology disclosed the presence of an AS variant of the BK virus with a high viral load, both in renal tissue and urine. Immunosuppression was reduced and Leflunomide therapy administered for a month. Although this led to an improvement in the renal function, the therapy had to be suspended due to the onset of a skin rash. A second biopsy was performed 7 months later. The cellular crescents were no longer present and there was no evidence of either histologic or immunohistochemical findings consistent with an active BK virus infection. Tubular atrophy and interstitial fibrosis were still present. In addition, fibrotic crescents, which may be interpreted as late scarring changes of previous epithelial proliferation, were found. Although molecular investigation still showed the presence of the BK virus, the viral load in renal tissue, urine and serum was greatly reduced. Indeed, serum and urine viral load was still low at the last control, five months after the second biopsy. The morphologic and clinical evolution are reported and the possible role of the therapy is discussed.     

Successful Treatment for BK Virus Nephropathy by Leflunomide in a Kidney Transplant Patient: A Case Report

IntroductionThe immunosuppressant agents in kidney transplantation (KT) may lead to various complications such as opportunistic infections and malignancies. BK virus associated nephropathy is a significant complication following KT, and it can result in graft failure. BK virus causes tubulointerstitial nephritis, ureter stenosis, and even graft failure in KT recipients with impaired immune system. We described a 63-year-old woman, who was a hepatitis C carrier and on dialysis for 22 years before KT, who received cadaveric-donor KT 2 years previously. She reported decreasing urine output and general weakness. The serum creatinine level was slightly increased from 2.94 to 4.38 mg/dL.MethodsImmunosuppressant medications including prednisolone, everolimus, cyclosporin, and mycophenolate sodium were continued as maintenance therapy post KT. Kidney biopsy was performed due to deterioration of graft function.ResultsThe kidney biopsy showed consistent results with early-stage polyomavirus nephropathy, characterized by focal viral cytopathic changes with positive immunohistochemical signals and mesangial proliferative glomerulonephritis, immune-complex-mediated (Fig 1 and Fig 2). Negative C4d staining at peritubular capillary was reported. The dosage of mycophenolate sodium was tapered from 720 to 360 mg daily and that of everolimus increased from 0.5 to 1.0 mg daily due to BK viral infection with BK nephropathy. The serum creatinine level was 2.75 mg/dL after treatment.ConclusionEarly detection of BK nephropathy and decreasing immunosuppressant agents are the mainstay of treatment. Substituting leflunomide for mycophenolate sodium and increasing dosage of everolimus has been proposed to solve BK nephropathy. We presented that the use of leflunomide in such situation is in a timely manner.     

Use of cidofovir in polyomavirus BK viral nephropathy in two renal allograft recipients

Polyomavirus BK viral allograft nephropathy is a potentially reversible cause of deteriorating function of kidney allografts. Initial treatment involves reducing immunosuppressive medications, with low-dose cidofovir an effective alternative in refractory cases. We describe two cases of BK viral allograft nephropathy responding to low-dose cidofovir after a reduction in immunosuppressive medications failed to clear the virus or stabilize the deterioration in renal function. There were no significant side-effects from this treatment in either patient.     

Treatment of BK Virus-Associated Nephropathy With Cidofovir in Renal Transplantation

BK virus-associated nephropathy (BKVN) has become recognized as an important cause of allograft dysfunction among transplant recipients. Despite reduction in immunosuppression, 30%-40% of recipients progress to allograft loss. Cidofovir is an antiviral agent that has been proposed for treatment of BKVN. We describe the clinical course, renal function, and blood viral measurement in 6 renal transplant recipients with BKVN who were treated with low doses of cidofovir. Administration of cidofovir was associated with clearance of BK virus DNA from blood and stabilization of renal function in 5 cases. These data suggest that cidofovir may be useful as adjuvant therapy for BKVN.     

In vivo activity of leflunomide: pharmacokinetic analyses and mechanism of immunosuppression.

BACKGROUND: Leflunomide is an experimental drug with demonstrated ability to prevent and reverse acute allograft and xenograft rejection. The two biochemical activities reported for the active metabolite of leflunomide, A77 1726, are inhibition of tyrosine phosphorylation and inhibition of dihydroorotate dehydrogenase, an enzyme necessary for de novo pyrimidine synthesis. These activities can be distinctly separated in vitro by the use of uridine, which reverses the anti-proliferative effects of A77 1726 caused by inhibition of de novo pyrimidine synthesis. We report the effect of uridine on the in vivo immunosuppressive activities of leflunomide. METHODS: We first quantified the serum levels of A77 1726, the active metabolite of leflunomide, after a single treatment of leflunomide (5, 15, and 35 mg/kg). Additionally, we quantified the levels of serum uridine and of nucleotide triphosphates in the liver, spleen, and lymph nodes of Lewis rats after the administration of a single dose of uridine (500 mg/kg; i.p.). Lewis rats heterotopically transplanted with brown Norway or Golden Syrian hamster hearts were treated for 50 or 75 days with leflunomide (5, 15, and 35 mg/kg/day; gavage) alone or in combination with uridine (500 mg/ kg/day; i.p.). Hematocrits were determined and the levels of alloreactive or xenoreactive immunoglobulin (Ig)M and IgG were determined by flow cytometric analysis. The allograft and xenografts, small bowel, liver, kidney, and spleen were subjected to pathological examination. RESULTS: A linear relationship was observed between the serum A77 1726 concentrations in Lewis rats and the dose of leflunomide administered. Peak A77 1726 concentrations were 20.9, 71.8 and 129.3 mg/l (77.5, 266.1 and 478.8 microM) for the 5, 15, and 35 mg/kg doses of leflunomide, respectively. The concentration of uridine in the serum of normal Lewis rats is 6.5 microM; after i.p. administration of 500 mg/kg uridine, the serum uridine concentrations peaked at 384.1 microM in 15-30 min. The rapid elimination of uridine was not reflected in the lymphoid compartments, and the pharmacokinetics of pyrimidine nucleotides in the spleen resembled that of A77 1726. This dose of uridine, when administered daily (500 mg/kg/day, i.p.), weakly antagonized the immunosuppressive activities of leflunomide (5, 15, and 35 mg/kg/day) in the allotransplantation model. In contrast, in the xenotransplantation model, the same concentration of uridine completely antagonized the immunosuppressive activities of low-dose leflunomide (15 mg/kg/day) and partially antagonized the immunosuppressive activities of high-dose leflunomide (35 mg/kg/day). Toxicities associated with high-dose leflunomide (35 mg/kg/day) were anemia, diarrhea, and pathological changes in the small bowel and liver. These toxicities were significantly reduced by uridine co-administration. CONCLUSION: These studies reveal that the blood levels of A77 1726 in Lewis rats satisfy in vitro requirements for both inhibition of de novo pyrimidine synthesis and protein tyrosine kinase activity. Our data also illustrate that the in vivo mechanism of immunosuppression by leflunomide is complex and is affected by at least the following four factors: type and vigor of the immune response, availability of uridine for salvage by proliferating lymphocytes, species being investigated, and concentration of serum A77 1726.