Immunologic reconstitution during PEG-ADA therapy in an unusual mosaic ADA deficient patient

We report detailed genetic and immunologic studies in a patient diagnosed with adenosine deaminase (ADA) deficiency and combined immune deficiency at age 5 years. At the time of diagnosis, although all other lymphocyte subsets were depleted, circulating CD8(+) T cells with a terminally differentiated phenotype were abundant and expressed normal ADA activity due to a reversion mutation in a CD8(+) T cell or precursor. Over the first 9 months of replacement therapy with PEG-ADA, the patient steadily accumulated mature naïve CD4(+) and CD8(+) T cells, as well as CD4(+)/FOXP3(+) regulatory T cells, consistent with restoration of a functional cellular immune system. While CD19(+) naïve B cells also accumulated in response to PEG-ADA therapy, a high proportion of these B cells exhibited an immature surface marker phenotype even after 9 months, and immunization with neoantigen bacteriophage varphiX174 demonstrated a markedly subnormal humoral immune response. Our observations in this single patient have important implications for gene therapy of human ADA deficiency, as they indicate that ADA expression within even a large circulating lymphocyte population may not be sufficient to support adequate immune reconstitution. They also suggest that an immature surface marker phenotype of the peripheral B cell compartment may be a useful surrogate marker for incomplete humoral immune reconstitution during enzyme replacement, and possibly other forms of hematopoietic cell therapies.

An unconditioned bone marrow transplantation in a child with purine nucleoside phosphorylase deficiency and its unique complication

Purine nucleoside phosphorylase deficiency is a rare immunodeficiency syndrome characterized by recurrent infections, neurological dysfunction, and autoimmunity. Early diagnosis and hematopoietic stem cell transplantation may reverse the dismal prognosis in PNP deficiency. This report presents a new PNP deficiency case successfully transplanted without a conditioning regimen from an HLA-identical family donor, who developed a complication of disseminated BCG infection.

A 9-yr evaluation of carrier erythrocyte encapsulated adenosine deaminase (ADA) therapy in a patient with adult-type ADA deficiency

Adenosine deaminase (ADA) deficiency is an inherited disorder which leads to elevated cellular levels of deoxyadenosine triphosphate (dATP) and systemic accumulation of its precursor, 2-deoxyadenosine. These metabolites impair lymphocyte function, and inactivate S-adenosylhomocysteine hydrolase (SAHH) respectively, leading to severe immunodeficiency. Enzyme replacement therapy with polyethylene glycol-conjugated ADA is available, but its efficacy is reduced by anti-ADA neutralising antibody formation. We report here carrier erythrocyte encapsulated native ADA therapy in an adult-type ADA deficient patient. Encapsulated enzyme is protected from antigenic responses and therapeutic activities are sustained. ADA-loaded autologous carrier erythrocytes were prepared using a hypo-osmotic dialysis procedure. Over a 9-yr period 225 treatment cycles were administered at 2-3 weekly intervals. Therapeutic efficacy was determined by monitoring immunological and metabolic parameters. After 9 yr of therapy, erythrocyte dATP concentration ranged between 24 and 44 micromol/L (diagnosis, 234) and SAHH activity between 1.69 and 2.29 nmol/h/mg haemoglobin (diagnosis, 0.34). Erythrocyte ADA activities were above the reference range of 40-100 nmol/h/mg haemoglobin (0 at diagnosis). Initial increases in absolute lymphocyte counts were not sustained; however, despite subnormal circulating CD20(+) cell numbers, serum immunoglobulin levels were normal. The patient tolerated the treatment well. The frequency of respiratory problems was reduced and the decline in the forced expiratory volume in 1 s and vital capacity reduced compared with the 4 yr preceding carrier erythrocyte therapy. Carrier erythrocyte-ADA therapy in an adult patient with ADA deficiency was shown to be metabolically and clinically effective.

Uricase and other novel agents for the management of patients with treatment-failure gout

Successful management of chronic gout depends on reducing body pools of urate. The benchmark of success is to maintain serum urate levels at less than 6 mg/dL using therapies such as probenecid or allopurinol. In a subset of patients with gout, these medications fail to achieve this benchmark, resulting in ongoing signs and symptoms characteristic of treatment-failure gout. Potential therapies now in clinical development show promise for treating this refractory patient population. In this article, we review the clinical characteristics associated with treatment-failure gout and discuss recent data from clinical trials of febuxostat and uricase.

Control of hyperuricemia in subjects with refractory gout, and induction of antibody against poly(ethylene glycol) (PEG), in a phase I trial of subcutaneous PEGylated urate oxidase

PEG-modified recombinant mammalian urate oxidase (PEG-uricase) is being developed as a treatment for patients with chronic gout who are intolerant of, or refractory to, available therapy for controlling hyperuricemia. In an open-label phase I trial, single subcutaneous injections of PEG-uricase (4 to 24 mg) were administered to 13 such subjects (11 had tophaceous gout), whose plasma uric acid concentration (pUAc) was 11.3 +/- 2.1 mg/dl (mean +/- SD). By day seven after injection of PEG-uricase, pUAc had declined by an average of 7.9 mg/dl and had normalized in 11 subjects, whose mean pUAc decreased to 2.8 +/- 2.2 mg/dl. At doses of 8, 12, and 24 mg, the mean pUAc at 21 days after injection remained no more than 6 mg/dl. In eight subjects, plasma uricase activity was still measurable at 21 days after injection (half-life 10.5 to 19.9 days). In the other five subjects, plasma uricase activity could not be detected beyond ten days after injection; this was associated with the appearance of relatively low-titer IgM and IgG antibodies against PEG-uricase. Unexpectedly, these antibodies were directed against PEG itself rather than the uricase protein. Three PEG antibody-positive subjects had injection-site reactions at 8 to 9 days after injection. Gout flares in six subjects were the only other significant adverse reactions, and PEG-uricase was otherwise well tolerated. A prolonged circulating life and the ability to normalize plasma uric acid in markedly hyperuricemic subjects suggest that PEG-uricase could be effective in depleting expanded tissue stores of uric acid in subjects with chronic or tophaceous gout. The development of anti-PEG antibodies, which may limit efficacy in some patients, is contrary to the general assumption that PEG is non-immunogenic. PEG immunogenicity deserves further investigation, because it has potential implications for other PEGylated therapeutic agents in clinical use.

Pharmacokinetics and pharmacodynamics of intravenous PEGylated recombinant mammalian urate oxidase in patients with refractory gout

OBJECTIVE

To evaluate the efficacy, immunogenicity, and tolerability of intravenous (IV) PEGylated recombinant mammalian urate oxidase (PEG-uricase) for the treatment of severe gout.

METHODS

Single infusions of PEG-uricase (at doses ranging from 0.5 mg to 12 mg) were administered to 24 patients (6 cohorts of 4 patients each) in a phase I clinical trial. Plasma uricase activity (pUox), the plasma urate concentration (pUAc), and the uric acid-to-creatinine ratio (UAc:Cr) in urine were monitored for 21 days after dosing. Adverse events and the IgG antibody response to PEG-uricase were followed up for 35 days.

RESULTS

All patients completed the trial. Maximum pUox was linearly related to the IV dose of PEG-uricase, the area under the curve (AUC) value increased linearly (up to a dose of 8 mg), and the pUox half-life was 6.4-13.8 days. After doses of 4-12 mg, the pUAc fell within 24-72 hours, from a mean +/- SD value of 11.1 +/- 0.6 mg/dl to 1.0 +/- 0.5 mg/dl; the AUC value for the pUAc was equivalent to maintaining the pUAc at 1.2-4.7 mg/dl for 21 days postinfusion. The UAc:Cr ratio in urine fell in parallel with the pUAc. IgG antibodies to PEG-uricase, mostly IgG2 and specific for PEG, developed in 9 patients, who had more rapid enzyme clearance but no allergic reactions. All adverse events were mild to moderate, with gout flares being most common.

CONCLUSION

The bioavailability, efficacy, and tolerability of IV PEG-uricase were greater than the bioavailability, efficacy, and tolerability observed in a previous phase I trial of subcutaneous PEG-uricase. Infusing 4-12 mg of PEG-uricase every 2-4 weeks should maintain the pUAc well below the therapeutic target of 6 mg/dl and greatly reduce renal uric acid excretion. This treatment could be effective in depleting expanded tissue urate stores in patients with chronic or tophaceous gout.

Patients with adenosine deaminase deficiency surviving after hematopoietic stem cell transplantation are at high risk of CNS complications

Adenosine deaminase (ADA) deficiency is a systemic metabolic disease that causes an autosomal recessive variant of severe combined immunodeficiency (SCID) and less consistently other complications including neurologic abnormalities. Hematopoietic stem cell transplantation (HSCT) is able to correct the immunodeficiency, whereas control of nonimmunologic complications has not been extensively explored. We applied HSCT in 15 ADA-deficient patients consecutively treated at our institutions since 1982 and analyzed long-term outcome. Seven patients received transplants without conditioning from HLA-matched family donors (MFDs); the other 8 patients received conditioning and were given transplants either from HLA-mismatched family donors (MMFDs; n = 6) or from matched unrelated donors (MUDs; n = 2). At a mean follow-up period of 12 years (range, 4-22 years), 12 patients are alive with stable and complete immune reconstitution (7 of 7 after MFD, 4 of 6 after MMFD, and 1 of 2 after MUD transplantation). Six of 12 surviving patients show marked neurologic abnormalities, which include mental retardation, motor dysfunction, and sensorineural hearing deficit. We were unable to identify disease or transplantation-related factors correlating with this divergent neurologic outcome. The high rate of neurologic abnormalities observed in long-term surviving patients with ADA deficiency indicates that HSCT commonly fails to control CNS complications in this metabolic disease.

Prolonged pancytopenia in a gene therapy patient with ADA-deficient SCID and trisomy 8 mosaicism: a case report

A patient with adenosine deaminase-deficient severe combined immune deficiency (ADA-SCID) was enrolled in a study of retroviral-mediated ADA gene transfer to bone marrow hematopoietic stem cells. After the discontinuation of ADA enzyme replacement, busulfan (75 mg/m2) was administered for bone marrow cytoreduction, followed by infusion of autologous, gene-modified CD34+ cells. The expected myelosuppression developed after busulfan but then persisted, necessitating the administration of untransduced autologous bone marrow back-up at day 40. Because of sustained pancytopenia and negligible gene marking, diagnostic bone marrow biopsy and aspirate were performed at day 88. Analyses revealed hypocellular marrow and, unexpectedly, evidence of trisomy 8 in 21.6% of cells. Trisomy 8 mosaicism (T8M) was subsequently diagnosed by retrospective analysis of a pretreatment marrow sample that might have caused the lack of hematopoietic reconstitution. The confounding effects of this preexisting marrow cytogenetic abnormality on the response to gene transfer highlights another challenge of gene therapy with the use of autologous hematopoietic stem cells.

Cerebral lymphoma in an adenosine deaminase-deficient patient with severe combined immunodeficiency receiving polyethylene glycol-conjugated adenosine deaminase

Polyethylene glycol-conjugated adenosine deaminase (PEG-ADA) provides an alternate therapy to mismatched stem cell transplantation for patients with ADA-deficient severe combined immunodeficiency. Although replacement therapy with PEG-ADA is effective in preventing infections, immune function does not return to normal, and most patients remain lymphopenic. Information is limited regarding the prognosis of patients on long-term ADA-replacement therapy. Here we present a case of a 10-year-old child who was diagnosed with ADA-severe combined immunodeficiency at 4 weeks of age after contracting pneumonia. Treatment with PEG-ADA was begun, the biochemical markers of ADA deficiency normalized, and his clinical progress was very good without significant infections. At 10 years of age, after presenting with headaches and cranial nerve deficits, he was diagnosed with Epstein-Barr virus-positive malignant brain lymphoma. It did not respond to various regimens of aggressive chemotherapy, and the patient expired 5 months later. We speculate that in this patient the immunologic surveillance by T cells may have been defective with respect to elimination of Epstein-Barr virus-infected cells, hence the formation of neoplasm. The possible mechanisms underlying such pathology are reviewed.

Long-term efficacy of enzyme replacement therapy for Adenosine deaminase (ADA)-deficient Severe Combined Immunodeficiency (SCID)

Adenosine deaminase (ADA)-deficient Severe Combined Immunodeficiency (ADA-deficient SCID) is characterized by impaired lymphocyte development and function resulting from the adenosine metabolism defect. Enzyme replacement therapy with polyethylene glycol-conjugated adenosine deaminase (PEG-ADA) minimizes infectious complications of ADA-deficient patients who have not received bone marrow transplantation. In PEG-ADA therapy, enzymatically active ADA continuously circulates to act as a metabolic sink, detoxifying the adenosine and deoxyadenosine metabolites that accumulate to high levels in the absence of ADA. Studies have shown that upon the initiation of PEG-ADA therapy, the absolute numbers of circulating T and B lymphocytes and NK cells increase and protective immune function develops. However, the long-term efficacy is unknown. This retrospective study was designed to assess the long-term effectiveness of PEG-ADA treatment, based on evaluation of the immune function of nine ADA-deficient SCID patients (age 5-15) treated over the past decade. The results showed that the lymphocyte counts of all of the PEG-ADA treated patients were below the normal range at all times, despite initial improvements. A gradual decline of mitogenic proliferative responses occurred after a few years of treatment and normal antigenic response occurred less than expected. To this date, these low numbers and functions of lymphocytes had been adequate to provide protective immunity. These patients should be followed closely to detect a premature decline in immune function with aging in future decades of PEG-ADA therapy.

CD28 costimulatory signal induces protein arginine methylation in T cells

Protein phosphorylation initiates signal transduction that triggers lymphocyte activation. However, other posttranslational modifications may contribute to this process. Here, we show that CD28 engagement induced protein arginine methyltransferase activity and methylation on arginine of several proteins, including Vav1. Methylation of Vav1 and IL-2 production were reduced by inhibiting S-adenosyl-L-homocysteine hydrolase, an enzyme that regulates cellular transmethylation. Methylated Vav1 was induced in human and mouse T cells and selectively localized in the nucleus, which suggested that this form marks a nuclear function of Vav1. Our findings uncover a signaling pathway that is controlled by CD28 that is likely to be important for T cell activation.

polyethylene glycol-conjugated adenosine deaminase (ADA) therapy provides temporary immune reconstitution to a child with delayed-onset ADA deficiency

We describe the effects of polyethylene glycol-conjugated adenosine deaminase (ADA) replacement therapy on lymphocyte counts, activation, apoptosis, proliferation, and cytokine secretion in a 14-month-old girl with "delayed-onset" ADA deficiency and marked immunodysregulation. Pretreatment lymphopenia affected T cells (CD4, 150/microl; CD8, 459/microl), B cells (16/microl), and NK cells (55/microl). T cells were uniformly activated and largely apoptotic (CD4, 59%; CD8, 82%); and T-cell-dependent cytokine levels in plasma were elevated, including the levels of interleukin 2 (IL-2; 26 pg/ml), IL-4 (81 pg/ml), IL-5 (46 pg/ml), gamma interferon (1,430 pg/ml), tumor necrosis factor alpha (210 pg/ml), and IL-10 (168 pg/ml). Mitogen-stimulated peripheral blood mononuclear cells show reduced IL-2 secretion and proliferation. During the first 5 months of therapy there was clinical improvement and partial immune reconstitution, with nearly normal lymphocyte subset numbers, reduced T-cell activation and CD4-cell apoptosis, and decreased plasma cytokine levels. In parallel, IL-2 secretion and the lymphocyte mitogenic response improved. Between 4 and 7 months, immunoglobulin G antibodies to bovine ADA developed and resulted in the complete reversal of immune recovery.

New insights into adenosine-receptor-mediated immunosuppression and the role of adenosine in causing the immunodeficiency associated with adenosine deaminase deficiency

There is growing interest in manipulating adenosine (Ado) signal transduction to control inflammation and autoimmunity. This concept probably originated with the discovery of severe combined immunodeficiency disease (SCID) in infants with inherited deficiency of adenosine deaminase (ADA). However, the basis for immunosuppression by Ado has not been well defined, and effects of 2'-deoxyadenosine (dAdo), which does not activate Ado receptors, have also been implicated in causing SCID. Here I discuss recent evidence that Ado, acting through its A2A receptor, interferes with NF-kappa B activation in antigen-receptor-stimulated B and T lymphocytes. I also assess the relative contributions of Ado and dAdo to the pathogenesis of ADA-deficient SCID.

Crystallization and preliminary X-ray analysis of human placental S-adenosylhomocysteine hydrolase

A recombinant form of human placental S-adenosylhomocysteine (AdoHcy) hydrolase expressed in E. coli, which was inactivated by a type-I mechanism-based inhibitor, has been crystallized using the hanging-drop vapour-diffusion technique. The crystals grow as flat plates, with unit-cell dimensions a = 96.2, b = 173.6, c = 142.9 A, alpha = beta = gamma = 90 degrees. The crystals exhibit the symmetry of space group C222 and diffract to a minimum spacing of approximately 2.0 A resolution at the Cornell High Energy Synchrotron Source. On the basis of density calculations two monomers of the tetrameric protein are estimated to be present in the asymmetric unit (V(m) = 2.99 A(3) Da(-l)). The self-rotation function clearly indicates the location of the non-crystallographic twofold axis.

Purine nucleoside phosphorylase deficiency (PNP-def) presenting with lymphopenia and developmental delay: successful correction with umbilical cord blood transplantation

Purine nucleoside phosphorylase deficiency is a primary immunodeficiency syndrome characterized by the triad of recurrent infection, neurologic dysfunction, and autoimmunity. This patient presented atypically with few infections and normal T-cell function. Progressive lymphopenia, ataxia, and developmental delay led to diagnosis. Umbilical cord blood transplantation corrected the immunodeficiency.

Cell surface adenosine deaminase binds and stimulates plasminogen activation on 1-LN human prostate cancer cells

Adenosine deaminase (ADA) is expressed intracellularly by all cells, but in some tissues, it is also associated with the cell surface multifunctional glycoprotein CD26/dipeptidyl peptidase IV. By modulating extracellular adenosine, this "ecto-ADA" may regulate adenosine receptor signaling implicated in various cellular functions. CD26 is expressed on the surface of human prostate cancer 1-LN cells acting as a receptor for plasminogen (Pg). Since ADA and Pg bind to CD26 at distinct but nearby sites, we investigated a possible interaction between these two proteins on the surface of 1-LN cells. Human ADA binds to CD26 on the surface 1-LN cells and immobilized CD26 isolated from the same cells with similar affinity. In both cases, ADA binding is diminished by mutation of ADA residues known to interact with CD26. ADA was also found to bind Pg 2 in the absence of CD26 via the Pg kringle 4 (K4) domain. In the presence of 1-LN cells or immobilized CD26, exogenous ADA enhances conversion of Pg 2 to plasmin by 1-LN endogenous urinary plasminogen activator (u-PA), as well as by added tissue Pg Activator (t-PA), suggesting that ADA and Pg bind simultaneously to CD26 in a ternary complex that stimulates the Pg activation by its physiologic activators. Consistent with this, in melanoma A375 cells that bind Pg, but do not express CD26, the rate of Pg activation was not affected by ADA. Thus, ADA may be a factor regulating events in prostate cancer cells that occur when Pg binds to the cell surface and is activated.

Genotype is an important determinant of phenotype in adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency is associated with a broad clinical and mutational spectrum. Defining the relationship of genotype to phenotype among patients with different degrees of immunodeficiency has been complicated because the disease is rare, most mutations are 'private' and patients are often heteroallelic. In recent years, knowledge of ADA structure and systematic expression of mutant alleles have revealed that phenotype is strongly associated with the sum of ADA activity provided by both alleles. A scale for ranking novel ADA alleles based on expression may have utility if newborn screening for primary immunodeficiency disorders is initiated.

Clustered charged amino acids of human adenosine deaminase comprise a functional epitope for binding the adenosine deaminase complexing protein CD26/dipeptidyl peptidase IV

Human adenosine deaminase (ADA) occurs as a 41-kDa soluble monomer in all cells. On epithelia and lymphoid cells of humans, but not mice, ADA also occurs bound to the membrane glycoprotein CD26/dipeptidyl peptidase IV. This "ecto-ADA" has been postulated to regulate extracellular Ado levels, and also the function of CD26 as a co-stimulator of activated T cells. The CD26-binding site of human ADA has been localized by homolog scanning to the peripheral alpha2-helix (amino acids 126-143). Among the 5 non-conserved residues within this segment, Arg-142 in human and Gln-142 in mouse ADA largely determined the capacity for stable binding to CD26 (Richard, E., Arredondo-Vega, F. X., Santisteban, I., Kelly, S. J., Patel, D. D., and Hershfield, M. S. (2000) J. Exp. Med. 192, 1223-1235). We have now mutagenized conserved alpha2-helix residues in human and mouse ADA and used surface plasmon resonance to evaluate binding kinetics to immobilized rabbit CD26. In addition to Arg-142, we found that Glu-139 and Asp-143 of human ADA are also important for CD26 binding. Mutating these residues to alanine increased dissociation rates 6-11-fold and the apparent dissociation constant K(D) for wild type human ADA from 17 to 112-160 nm, changing binding free energy by 1.1-1.3 kcal/mol. This cluster of 3 charged residues appears to be a "functional epitope" that accounts for about half of the difference between human and mouse ADA in free energy of binding to CD26.

Flow cytometry analysis of adenosine deaminase (ADA) expression: a simple and reliable tool for the assessment of ADA-deficient patients before and after gene therapy

Clinical gene therapy trials for adenosine deaminase (ADA) deficiency have shown limited success of corrective gene transfer into autologous T lymphocytes and CD34(+) cells. In these trials, the levels of gene transduction and expression in hematopoietic cells have been assessed by DNA- or RNA-based assays and measurement of ADA enzyme activity. Although informative, these methods are rarely applied to clonal analysis. The results of these assays therefore provide best estimates of transduction efficiency and gene expression in bulk populations based on the assumption that gene transfer and expression are uniformly distributed among transduced cells. As a useful additional tool for evaluation of ADA gene expression, we have developed a flow cytometry (fluorescence-activated cell sorting, FACS) assay capable of estimating the levels of intracellular ADA on a single-cell basis. We validated this technique with T cell lines and peripheral blood mononuclear cells (PBMCs) from ADA-deficient patients that showed severely reduced levels of ADA expression (ADA-dull) by FACS and Western blot analyses. After retrovirus-mediated ADA gene transfer, these cells showed clearly distinguishable populations exhibiting ADA expression (ADA-bright), thus allowing estimation of transduction efficiency. By mixing ADA-deficient and normal cells and using enzymatic amplification, we determined that our staining procedure could detect as little as 5% ADA-bright cells. This technique, therefore, will be useful to quickly assess the expression of ADA in hematopoietic cells of severe combined immunodeficient patients and represents an important tool for the follow-up of patients treated in clinical gene transfer protocols.

Adenosine deaminase deficiency with mosaicism for a "second-site suppressor" of a splicing mutation: decline in revertant T lymphocytes during enzyme replacement therapy

Four patients from 3 Saudi Arabian families had delayed onset of immune deficiency due to homozygosity for a novel intronic mutation, g.31701T>A, in the last splice acceptor site of the adenosine deaminase (ADA) gene. Aberrant splicing mutated the last 4 ADA amino acids and added a 43-residue "tail" that rendered the protein unstable. Mutant complementary DNA (cDNA) expressed in Escherichia coli yielded 1% of the ADA activity obtained with wild-type cDNA. The oldest patient, 16 years old at diagnosis, had greater residual immune function and less elevated erythrocyte deoxyadenosine nucleotides than his 4-year-old affected sister. His T cells and Epstein-Barr virus (EBV) B cell line had 75% of normal ADA activity and ADA protein of normal size. DNA from these cells and his whole blood possessed 2 mutant ADA alleles. Both carried g.31701T>A, but one had acquired a deletion of the 11 adjacent base pair, g.31702-12, which suppressed aberrant splicing and excised an unusual purine-rich tract from the wild-type intron 11/exon 12 junction. During ADA replacement therapy, ADA activity in T cells and abundance of the "second-site" revertant allele decreased markedly. This finding raises an important issue relevant to stem cell gene therapy.

Diabetes insipidus in uricase-deficient mice: a model for evaluating therapy with poly(ethylene glycol)-modified uricase

Uricase-deficient mice develop uric acid nephropathy, with high mortality rates before weaning. Urate excretion was quantitated and renal function was better defined in this study, to facilitate the use of these mice as a model for evaluating poly(ethylene glycol)-modified recombinant mammalian uricases (PEG-uricase) as a potential therapy for gout and uric acid nephropathy. The uric acid/creatinine ratio in the urine of uricase-deficient mice ranges from 10 to >30; on a weight basis, these mice excrete 20- to 40-fold more urate than do human subjects. These mice consistently develop a severe defect in renal concentrating ability, resulting in an approximately sixfold greater urine volume and a fivefold greater fluid requirement, compared with normal mice. This nephrogenic diabetes insipidus leads to dehydration and death of nursing mice but, with adequate water replacement, high urine flow protects adults from progressive renal damage. Treatment of uricase-deficient mice with PEG-uricase markedly reduced urate levels and, when initiated before weaning, preserved the renal architecture (as evaluated by magnetic resonance micros-copy) and prevented the loss of renal concentrating function. PEG-uricase was far more effective and less immunogenic than unmodified uricase. Retention of uricase in most mammals and its loss in humans and some other primates may reflect the evolution of renal function under different environmental conditions. PEG-uricase could provide an effective therapy for uric acid nephropathy and refractory gout in human patients.