Uricase Therapy of Gout




Key Points





  • Uricolytic therapy with uricase (urate oxidase) degrades uric acid, ultimately resulting in the production of allantoin, which is much more soluble and readily excreted in the urine than is uric acid.



  • As a result of loss of expression of uricase during hominid evolution and a delicate balance of uric acid production and elimination in man, humans are unusually predisposed to develop gout because tissue levels of urate are close to its solubility threshold.



  • As all uricases are highly immunogenic in humans, PEGylated enzymes with reduced antigenicity and prolonged duration of action have been developed for therapeutic use in patients with severe, refractory gout who cannot tolerate, or do not respond to, available uricostatic or uricosuric drugs.



  • Pegloticase is a recombinant PEG modified mammalian uricase that is effective in about half of patients with severe gout refractory to other urate-lowering drugs.



  • Gout patients who are persistent responders to pegloticase demonstrate marked serum urate lowering and a remarkable, rapid decrease (over months) in the size of tophi.



  • Treatment-emergent antibodies limit the therapeutic efficacy of pegloticase and are associated with infusion reactions in a sizeable subset of patients.



  • Plasma urate should be measured before each infusion of pegloticase and treatment discontinued if the PUA is repeatedly ≥6 mg/dL (0.36 mmol/L).





Introduction


Uricase or urate oxidase (EC 1.7.3.3) is a copper-binding enzyme that catalyses the oxidation of uric acid to 5-hydroxyisourate and hydrogen peroxide (H 2 O 2 ) ( Fig. 14-1 ). Subsequent hydrolysis and decarboxylation leads to the formation of allantoin as the end product of purine metabolism in most prokaryotic and eukaryotic organisms (see Fig. 14-1 ).




Figure 14-1


Pathway for the degradation by uricase of urate to allantoin in most mammals and other organisms. Uricase (urate oxidase) catalyses the oxidation of urate to 5-hydroxyisourate. Allantoin is rapidly generated after enzymatic hydrolysis and decarboxylation in lower species. In humans, in addition to the loss of uricase, enzymes that specifically degrade 5-hydroxyisourate to allantoin were lost in evolution, and slower alternative hydrolysis (by transthyretin-related protein) and/or oxidative degradation (over hours) occurs to generate allantoin. Not depicted here is oxidative degradation, in humans and other species, of uric acid to 5-hydroxyisourate via myeloperoxidase and peroxide. This can occur in human inflammatory loci and on the endothelial surface, and neutrophils have been observed to generate allantoin from uric acid.


As a consequence of two independent mutations in the urate oxidase gene during hominoid evolution, humans and some primates are unusual, among mammals, in lacking active uricase and in having uric acid as the end product of purine catabolism. Allantoin is 5 to 10 times more water-soluble than uric acid and readily excreted in the urine. Plasma and tissue urate concentrations in humans are 5- to 10-fold higher than those in mammals that express uricase and, even in the absence of genetic, dietary, and other environmental risk factors for hyperuricemia, are close to the solubility threshold of urate. This renders humans intrinsically prone to urate crystal deposition and gout.




Development of Uricolytic Drug Therapy


The possibility of developing uricolytic therapy with uricase for gout, and the hyperuricemia associated with tumour lysis in patients with malignant disease, has been considered for more than 50 years. In an elegant proof of principle study, published in Science in 1957, London and Hudson were able to demonstrate transient reduction of serum urate in a patient with gout, and in a nongouty subject, following intravenous administration of small quantities of a purified porcine liver uricase ( Fig. 14-2 ). This followed earlier studies in chickens and a single patient by Oppenheimer and Kunkel in the 1940s. Over the years, a number of urate oxidase preparations from a variety of sources have been developed and investigated ( Table 14-1 ). One of these, pegloticase (Krystexxa, Puricase), a recombinant, mammalian, uricase conjugated to methoxypolyethylene glycol (mPEG), received marketing authorization in the United States in September 2010 as an orphan drug for the treatment of adults with chronic gout refractory to conventional therapy. Another PEGylated recombinant uricase, pegadricase (formerly pegsiticase), is currently in early clinical trials, as this is written in 2011.




Figure 14-2


Serum uric acid and urine allantoin in a patient with gout before and after intravenous administration of purified porcine uricase in 1957.

(From London M, Hudson PB. Uricolytic activity of purified uricase in two human beings. Science 1957;125:937-8.)


Table 14-1

Uricases Developed for Clinical Studies/Therapy




















































Name Method Production Species of Origin Current Status References
Hog liver uricase Purified from liver Pig Not in clinical development
Uricozyme Nonrecombinant, purified from fungal cultures Aspergillus flavus No longer manufactured



  • Rasburicase



  • Fasturtec



  • Elitek

Non-PEGylated, recombinant protein produced in strain of Saccharomyces cerevisiae Uricase cDNA from A. flavus


  • Approved for prevention/treatment tumor lysis in children in United States (FDA)



  • Adults and children in Europe (EMA)

Uricase-PEG5 Nonrecombinant, purified from fungal cultures and PEGylated (5-kDa PEG strands) Candida utilis No longer in clinical development
Uricase-PEG5 Nonrecombinant, bacterial protein, PEGylated (5-kDa PEG strands) Arthrobacter protophormiae Not in clinical development



  • Uricase-PEG 20



  • Pegadricase (formerly known as pegsiticase)

Recombinant, PEGylated (20-kDa PEG strands linked via succinimidyl succinimide) Uricase cDNA from Candida utilis Phase II studies



  • Pegloticase



  • Krystexxa



  • Puricase

Recombinant, PEGylated (9 strands 10-kDa PEG covalently attached to each subunit) Mammalian cDNA (mainly porcine, with baboon C-terminal sequence) FDA approval Sept 2010 for treatment of adults with chronic gout refractory to conventional therapy.

Adapted from Terkeltaub R. Learning how and when to employ uricase as bridge therapy in refractory gout. J Rheumatol 2007;34:1955-8.




Nonrecombinant Uricase From Aspergillus flavus


Nonrecombinant fungal uricase from A. flavus (Uricozyme) was developed in France in the 1960s. After an early phase I/II study in 61 adults demonstrated reduction in serum urate levels and urinary excretion of allantoin following intravenous administration of 800 units/day, it was used widely for the prevention and treatment of tumor lysis in Europe in the 1970s and 1980s. A review of children in France with advanced Burkitt lymphoma and mature B-cell acute lymphoblastic leukemia treated with nonrecombinant uricase showed that only 1.7% required dialysis for acute renal failure during induction chemotherapy. This compared very favorably with a report of U.S. experience using standard prophylaxis with allopurinol, hydration, and alkalinization of the urine, in which there were six deaths from renal and metabolic complications and 21% of children with comparable malignancies required hemodialysis following chemotherapy.


In a subsequent review of 134 children with acute leukemia and Burkitt lymphoma treated with chemotherapy in the United States, prophylaxis with nonrecombinant uricase was found to be associated with lower mortality and morbidity from tumor lysis, as well as lower levels of serum urate, than historical controls managed with allopurinol. However, serious allergic reactions including urticaria, bronchospasm with hypoxia, and anaphylaxis occurred in 4.5%; and methemoglobinemia developed in one patient.


There were a few case reports of the use of Uricozyme for the management of hyperuricemia in patients with gout following organ transplants in the 1990s, but more widespread use in patients with treatment-resistant gout was limited by the short half-life, low enzyme yields, and difficulties in purification during the manufacturing process, as well as by the significant risk of severe allergic reactions.


Production of Uricozyme was discontinued following the development and availability of recombinant A. flavus enzyme.




Recombinant Aspergillus flavus Uricase (Rasburicase)


The gene encoding A. flavus urate oxidase was first cloned and expressed in Escherichia coli in 1992. Rasburicase (Elitek, Fasturtek) is a purified, recombinant A. flavus uricase expressed in a genetically modified strain of Saccharomyces cerevisiae. In the United States, rasburicase has had U.S. Food and Drug Administration (FDA) approval since 2002 for short-term, initial control of hyperuricemia in children with leukemia, lymphoma, and solid tumor malignancies who are receiving anticancer therapies expected to result in tumor lysis. In Europe, rasburicase has EMA approval for the prevention and treatment of hyperuricemia associated with tumor lysis in adults, as well as in children, but it is not currently licensed for the treatment of hyperuricemia in patients with gout.


Pharmacokinetics and Pharmacodynamics


Rasburicase is tetramer composed of four monomeric subunits, each with a molecular mass of 34 kDa. It has a half-life of 16 to 21 hours following intravenous administration at doses of 0.15 mg/kg and 0.2 mg/kg. There is no evidence of drug accumulation in children after steady-state levels are achieved at 48 to 72 hours, but there are few data on the pharmacokinetics of rasburicase in adults and the elderly. Clearance of enzyme, by peptide hydrolysis, is not, however, influenced by renal or hepatic function and rasburicase has no effect on cytochrome P450 activity. The licensed recommendation for dosing for the prevention or treatment of tumor lysis is 0.15 to 0.2 mg/kg once daily in 50 ml of normal saline as an intravenous infusion over 30 minutes for 5 days, but there is much data to suggest that lower doses, and single doses, can be used to control hyperuricemia in adults as well as children following tumor lysis.


Efficacy of Rasburicase in Preventing and Treating Tumor Lysis Syndrome


In an open-label phase I/II study of the efficacy and safety of rasburicase in 131 patients, 20 years old or younger, receiving chemotherapy for acute lymphoblastic leukemia or advanced non-Hodgkin lymphoma, treatment with rasburicase 0.2 mg/kg every 12 hours for 48 hours was followed by reduction of plasma urate levels from 9.7 mg/dL to 1.0 mg/dL in 65 children who were hyperuricemic and from 4.3 mg/dL to 0.5 mg/dL in 66 children who did not have raised plasma urate levels before therapy ( p = .0001). None of the patients required dialysis and none developed tumor lysis syndrome. In a single nonblinded randomized controlled comparison of treatment with rasburicase or allopurinol in 52 pediatric patients with lymphoma or leukemia receiving chemotherapy, the frequency of normalization of serum urate was significantly greater in hyperuricemic patients receiving rasburicase (risk ratio [RR] 19.09, 95% confidence interval [CI] 1.28 to 285.41) and the area under the curve of serum urate at 96 hours was significantly lower in the rasburicase group (MD-201, 95% CI –258.05 to –143.95). However, a recent Cochrane systematic review of trials of nonrecombinant and recombinant urate oxidase for the treatment and prevention of tumor lysis syndrome in children with cancer concluded that while it was effective in reducing the serum urate, there was currently no conclusive evidence that it was effective in reducing renal failure or mortality from tumor lysis syndrome in children with malignancies.


Efficacy of Rasburicase in Primary Purine Overproducers


There is a single case report recording the safety and efficacy of intravenous rasburicase (0.2 mg/kg/day for 3 days) in preserving renal function in a 26-day-old boy with Lesch-Nyhan syndrome presenting with renal insufficiency, metabolic acidosis, hyperuricemia, and hyperuricosuria associated with overproduction of uric acid and severe deficiency of hypoxanthine-guanine phosphoribosyl transferase.


Use of Rasburicase for the Treatment of Gout


Although no controlled trials of rasburicase for the treatment of gout have been undertaken, and rasburicase is not licensed or approved for treating gout anywhere in the world, it has been used to treat a small number of difficult cases, where other approaches to lowering urate levels had failed or were inappropriate.


In 2000, Phillips et al. reported regression of tophi following treatment with rasburicase in a patient with end-stage renal disease and severe tophaceous gout that was resistant to allopurinol.


In 2005, Vogt documented successful treatment of a 33-year-old woman with severe progressive tophaceous gout, allopurinol hypersensitivity, and chronic renal insufficiency following transplantation for end-stage renal failure. Serum urate decreased from baseline levels of about 850 μmol/L to less than 50 μmol/L a few days after commencing rasburicase 0.15 mg/kg IV but then returned to baseline despite continuing therapy with infusions every second week for 6 months. However, gout flares gradually ceased, tophi diminished in size, and the recombinant uricase was well tolerated with continuing infusions of rasburicase fortnightly, and subsequently monthly, over 3 years. The prolonged efficacy and tolerance of repeated infusions of rasburicase over such a long period in this patient may have been in part attributable to the co-administration of immunosuppressive treatment with cyclosporine A and prednisolone.


Moolenburgh et al. reported on a 57-year-old man with primary gout who had progressive tophaceous disease, recurrent flares, and persistent hyperuricemia despite combination therapy with allopurinol 900 mg and benzbromarone 200 mg/day. Hyperuricemia persisted in the range 0.45 to 0.55 mmol/L even after further escalation of the allopurinol dose to 1800 mg/day. Treatment with rasburicase 0.2 mg/kg IV over 60 minutes daily for 1 week resulted in an immediate fall in serum urate to less than 0.01 mmol/L. Continuing treatment with weekly infusions of rasburicase was initially associated with serum urate levels that sometimes rose again to more than 0.6 mmol/L immediately before the next infusion, as well as with regular postinfusion flares of gout requiring intravenous dexamethasone for symptom control. These ceased, however, after 3 months of therapy when serum urate levels were consistently in the normal range. Decreasing the frequency of the rasburicase infusions from 1 to 2 weeks was followed by a rise in serum urate to greater than 0.6 mmol/L and a recurrence of gout attacks, but subsequent maintenance treatment with weekly infusions of rasburicase (0.2 mg/kg) was followed by gradual cessation of gout flares, stabilization of the serum urate in the range 0.25 to 0.27 mmol/L and measurable decrease in the size of tophi ( Fig. 14-3 ).




Figure 14-3


Reduction in the size of tophi in a patient with gout following 12 months treatment with rasburicase (0.2 mg/kg weekly).

(From Moolenburgh JD, Reinders MK, Jansen TLThA. Rasburicase treatment in severe tophaceous gout: a novel therapeutic option. Clin Rheumatol 2006;25:749-52.)


One small retrospective study in 10 patients with tophaceous gout, that could not be treated with allopurinol, compared outcomes and short-term safety in five patients who had received IV infusions of rasburicase 0.2 mg/kg daily for 5 days with five patients treated with 6 consecutive monthly infusions of raburicase. Patients in both groups were given 60 mg dexamethasone to prevent gout flares. In the first group a rapid and profound fall in the serum urate immediately following infusion was not sustained at 1 month (511.5 ± 128.4 μmol/L) or 2 months (572 ± 96.2 μmol/L) compared with the baseline serum urate of 573.6 ± 48.2 μmol/L. Infusions were followed by gout flares and there was no noticeable change in the size of tophi. In those given monthly treatment, the serum urate fell significantly from 612.6 ± 162.4 μmol/L at baseline to 341.2 ±91.8 μmol/L ( p = .001) after six infusions, and reduction in the size of tophi was observed in two patients. However, one patient developed bronchospasm and one developed urticaria during the sixth infusion. Two others from this group, who continued with monthly injections of rasburicase, had to discontinue therapy after 8 months because of cutaneous allergic reactions, and one patient developed a new tophus, as well as hyperuricemia that was refractory to further rasburicase after 14 monthly infusions; presumably because of the development of neutralizing antibodies.


Wider use of rasburicase as a debulking agent in patients with tophaceous gout or for the long-term management of patients with gout that cannot be treated with other urate-lowering drugs is limited by its short half-life, the development of neutralizing antibodies, significant risk of immune-mediated serious side effects, and with loss of efficacy after 6 to 15 months of therapy.


Immunogenicity and Adverse Effects Associated With Non-PEGylated Uricases


Most of the information about the immunogenicity and adverse effects associated with the administration of non-PEGylated, nonrecombinant, and recombinant A. flavus urate oxidase comes from trials and clinical experience in children and young adults with malignancy-associated hyperuricemia.


Rasburicase was remarkably well tolerated in both children and adults in the North American multicenter, compassionate use trial with a low (6%) incidence of adverse events. Only 10 of 1069 patients (less than 1%) were withdrawn as a result of events that might be drug related. During the first course of treatment, 12 patients (1.1%) had hypersensitivity reactions (pruritis, urticaria, bronchospasm, or edema) and one had an anaphylactic reaction. Antibodies to rasburicase were detected in 13% of patients after a single infusion. However, the incidence of hypersensitivity reactions in patients treated with rasburicase is much lower after both single and repeated infusions than in patients treated with nonrecombinant uricase, presumably because of greater purity of the recombinant protein and possibly because of increased immunogenicity associated with modification of an active cysteine during the extraction and purification of Uricozyme.


Side Effects Associated With the Generation of Hydrogen Peroxide


Among the more serious adverse events that followed treatment with rasburicase that were not attributable to hypersensitivity were four cases of hemolytic anemia and three patients with methemoglobinemia. In one of the patients who developed hemolysis, glucose-6-phosphate dehydrogenase (G6PD) deficiency was subsequently diagnosed. Hydrogen peroxide (H 2 O 2 ) is generated during uricolysis with uricase (see Fig. 14-1 ). Treatment with all urate oxidase preparations, including rasburicase, is contraindicated in patients with G6PD deficiency because of markedly increased risks of inducing hemolysis and/or methemoglobinemia. Guidelines for the management of tumor lysis syndrome in adults and children recommend screening for G6PD deficiency before treatment with rasburicase in patients with a previous history of drug-induced hemolytic anemia and in patients from certain ethnic groups in which G6PD deficiency occurs more frequently (African, Mediterranean, and Southeast Asian ancestry). Patients should be monitored for evidence of cyanosis during uricase infusions and full blood counts should be undertaken during and after treatment.


It is also important to monitor plasma urate levels regularly. Blood should be collected in prechilled heparinized tubes and plasma urate assayed at 4°C within 4 hours of collection, to avoid spuriously low measurements of serum urate, as a result of urate degradation in vitro by uricase activity in serum at room temperature.




PEGylated Recombinant Uricases


To prolong the duration of uricase activity and reduce antigenicity, efforts in recent years have focused on the development of polyethylene glycol (PEG)–modified forms of recombinant uricase. Some that are, or have been, in development are included in Table 14-1 . One form of PEG-uricase, pegloticase (Krystexxa, Puricase) was given FDA approval for the treatment of adults with chronic gout refractory to conventional therapy in the United States in 2010. It is a recombinant, porcine-like uricase produced in Escherichia coli that is covalently conjugated to methoxypolyethylene glycol (mPEG).




Pegloticase (PEG-uricase, Krystexxa, Puricase)


The preclinical development of pegloticase and some other PEG-uricases is well described in a recent review by Sherman et al. The earliest U.S. patent for “water-soluble non-immunogenic polypeptide compositions” filed by Davis et al. in 1979 included a description of a synthesis of a PEGylated uricase. The development of the PEG-uricase that is pegloticase resulted from collaborative experimental work undertaken by scientists at Duke University and Mountain View Pharmaceuticals between 1995 and 1999. A predominantly porcine-like recombinant uricase containing a few residues from the baboon sequence was chosen from a number of recombinant uricases for coupling to a variable number of strands of methoxy-PEG with differing molecular weights, using p -nitrophenolcarbonate to form stable, covalent urethane bonds between the polymers and the solvent-accessible amino groups on the protein. As had previously been demonstrated, it was found that the number of strands of 5-kDa PEG that inactivated the enzyme by greater than 50% was smaller than the number needed to confer a longer half-life in rodents or to suppress the binding of antiuricase antibodies in vitro. Experiments showed that conjugates with six strands of 10-kDA PEG per subunit had a significantly longer half-life in mice than conjugates with smaller numbers of strands of 20- or 30-kDa PEG, while retaining approximately 90% of the uricolytic activity of the unmodified enzyme. Conjugates with 9 ± 1 strands of 10-kDa PEG per subunit were, however, 10-fold less antigenic than conjugates with six strands.


Previous work had demonstrated that the immunogenicity of of PEGylated porcine-like uricase conjugates could be reduced below the level conferred by the PEGylation alone if the conjugates were prepared from uricase from which all traces of aggregated protein, detectable by light scattering measurements, had been removed. However, even extensive PEGylation with 24 strands of 10-kDa PEG per tetramer could not completely block the immunogenicity in mice of a porcine-like uricase that contained traces of large aggregates. Conjugates with nine strands of 10-kDa PEG per subunit were taken forward for clinical development as they came closest to fulfilling the requirements proposed for the clinical usefulness of a PEGylated drug ( Table 14-2 ). Figure 14-4 shows a molecular model of this conjugate in which 36 strands of 10-kDa PEG are coupled to the most accessible lysine residues of the uricase tetramer using a program adapted from the Add ..PEG program described by Lee et al.



Table 14-2

Requirements for a Clinically Useful PEG-Uricase







  • 1.

    Sufficient reduction of immunogenicity of nonhuman enzyme to permit repeated dosing.


  • 2.

    Retention of sufficient enzymatic activity to be effective at a reasonable dose, e.g., retention of at least 75% of the intrinsic activity of the conjugate.


  • 3.

    Sufficient solubility under physiologic conditions to enable reasonable bioavailability.


  • 4.

    Reproducible and cost-effective synthesis of a conjugate with adequate stability during storage, during shipping, and in vivo.


  • 5.

    A sufficiently long half-life in patients to permit a convenient schedule of administration, e.g., once or twice a month.

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Mar 5, 2019 | Posted by in RHEUMATOLOGY | Comments Off on Uricase Therapy of Gout

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