Extracorporeal enzyme reactors for depletion of phenylalanine in phenylketonuria

Amino Acid Degrading Enzymes and their Application in Cancer Therapy

BACKGROUND

Amino acids are essential components in various biochemical pathways. The deprivation of certain amino acids is an antimetabolite strategy for the treatment of amino acid-dependent cancers which exploits the compromised metabolism of malignant cells. Several studies have focused on the development and preclinical and clinical evaluation of amino acid degrading enzymes, namely L-asparaginase, L-methionine γ-lyase, L-arginine deiminase, L-lysine α-oxidase. Further research into cancer cell metabolism may therefore define possible targets for controlling tumor growth.

OBJECTIVE

The purpose of this review was to summarize recent progress in the relationship between amino acids metabolism and cancer therapy, with a particular focus on Lasparagine, L-methionine, L-arginine and L-lysine degrading enzymes and their formulations, which have been successfully used in the treatment of several types of cancer.

METHODS

We carried out a structured search among literature regarding to amino acid degrading enzymes. The main aspects of search were in vitro and in vivo studies, clinical trials concerning application of these enzymes in oncology.

RESULTS

Most published research are on the subject of L-asparaginase properties and it's use for cancer treatment. L-arginine deiminase has shown promising results in a phase II trial in advanced melanoma and hepatocellular carcinoma. Other enzymes, in particular Lmethionine γ-lyase and L-lysine α-oxidase, were effective in vitro and in vivo.

CONCLUSION

The findings of this review revealed that therapy based on amino acid depletion may have the potential application for cancer treatment but further clinical investigations are required to provide the efficacy and safety of these agents.

Phenylalanine ammonia lyase (PAL): From discovery to enzyme substitution therapy for phenylketonuria

Phenylketonuria (PKU) is a genetic inborn error in metabolism that impacts many people globally, with profound individual and societal consequences when left untreated. The journey of phenylalanine ammonia lyase (PAL) from plant enzyme to enzyme substitution therapy for PKU is a fascinating story that illustrates the importance of collaboration between basic scientists and industry in the drug development process. The story begins with the curiosity of plant physiologists about the origin of lignin, a polymer involved in maintaining the rigidity of plants. They learned that the critical element in this synthesis was an intermediary enzyme that deaminates phenylalanine to cinnamic acid and ammonia (later called phenylalanine ammonia lyase or PAL). Recognition of this ability to metabolize phenylalanine led to subsequent consideration of PAL as a treatment for PKU. This was initially attempted as enteral therapy with extracted enzyme, but that showed only minimal efficacy. Crucially, further development of PAL as a therapy for PKU required quantities of enzyme that could only be obtained after successfully cloning the gene, expressing the enzyme in vitro and modifying the protein via PEGylation to enable parenteral administration of this non-mammalian enzyme. Ultimately, PEGylated PAL was developed as an enzyme substitution therapy for PKU now approved under the name "Palynziq." The multidisciplinary academic-industrial partnership engaged throughout this process has been key to the successful pursuit of this therapeutic possibility and serves as a model for the development of future innovative therapies.

Biochemical Evaluation of Phenylalanine Ammonia Lyase from Endemic Plant Cyathobasis fruticulosa (Bunge) Aellen. for the Dietary Treatment of Phenylketonuria

Enzyme substitution therapy with the phenylalanine ammonia lyase (PAL) is a new approach to the treatment of patients with phenylketonuria (PKU). This enzyme is responsible for the conversion of phenylalanine to trans-cinnamic acid. We assessed the PAL enzyme of the endemic plant Cyathobasis fruticulosa (Bunge) Aellen. for its possible role in the dietary treatment of PKU. The enzyme was found to have a high activity of (64.9±0.1) U/mg, with the optimum pH, temperature and buffer (Tris-HCl and l-phenylalanine) concentration levels of pH=8.8, 37 °C and 100 mM, respectively. Optimum enzyme activity was achieved at pH=4.0 and 7.5, corresponding to pH levels of gastric and intestinal juice, and NaCl concentration of 200 mM. The purification of the enzyme by 1.87-fold yielded an activity of 98.6 U/mg. PAL activities determined by HPLC analyses before and after purification were similar. Two protein bands, one at 70 and the other at 23 kDa, were determined by Western blot analysis of the enzyme. This enzyme is a potential candidate for serial production of dietary food and biotechnological products.

Reduction of l-phenylalanine in protein hydrolysates using l-phenylalanine ammonia-lyase from Rhodosporidium toruloides

l-Phenylalanine ammonia-lyase (PAL, EC 4.3.1.25) from Rhodosporidium toruloides was utilized to remove l-phenylalanine (l-Phe) from different commercial protein hydrolysates. A casein acid hydrolysate (CAH, l-Phe ~2.28 %) was employed as a model substrate. t-Cinnamic acid resulting from deamination of l-Phe was extracted, analyzed at λ = 290 nm, and used for PAL activity determination. Optimum reaction conditions, optimized using successive Doehlert design, were 35 mg mL−1 of CAH and 800 mU mL−1 of PAL, while temperature and pH were 42 °C and 8.7, respectively. Reaction kinetics of PAL with CAH was determined under optimized conditions. Then, removal of l-Phe from CAH was tested. Results showed that more than 92 % of initial l-Phe was eliminated. Similar results were obtained with other protein hydrolysates. These findings demonstrate that PAL is a useful biocatalyst for l-Phe removal from protein hydrolysates, which can be evaluated as potential ingredients in foodstuffs for PKU patients.

Recombinant l-phenylalanine ammonia lyase from Rhodosporidium toruloides as a potential anticancer agent

The recombinant producer strain expressing Rhodosporidium toruloides l-phenylalanine ammonia lyase (PAL) has been obtained, and a purification procedure of PAL has been developed. The purified enzyme, PAL, has the following biochemical and catalytic characteristics: Km for l-Phe of 0.49 mM, pH optimum at 8.5, and temperature optimum at 50°C. PAL exhibited a significant cytotoxic effect toward the following cell lines: MCF7 (IC50 = 1.97 U/mL), DU145 (IC50 = 7.3 U/mL), which are comparable with E. coli l-asparaginase type-II cytotoxicity in vitro. Administration of PAL (200-400 U/kg) to L5178y-bearing mice for five times (a total dose of 1000-2000 U/kg) was well tolerated and showed the increase of life span (ILS) = 12-16%, P < 0.05. Data obtained suggest that PAL from R. toruloides has a potential for cancer treatment.

Structural and biochemical characterization of the therapeutic Anabaena variabilis phenylalanine ammonia lyase

We have recently observed promising success in a mouse model for treating the metabolic disorder phenylketonuria with phenylalanine ammonia lyase (PAL) from Rhodosporidium toruloides and Anabaena variabilis. Both molecules, however, required further optimization in order to overcome problems with protease susceptibility, thermal stability, and aggregation. Previously, we optimized PAL from R. toruloides, and in this case we reduced aggregation of the A. variabilis PAL by mutating two surface cysteine residues (C503 and C565) to serines. Additionally, we report the structural and biochemical characterization of the A. variabilis PAL C503S/C565S double mutant and carefully compare this molecule with the R. toruloides engineered PAL molecule. Unlike previously published PAL structures, significant electron density is observed for the two active-site loops in the A. variabilis C503S/C565S double mutant, yielding a complete view of the active site. Docking studies and N-hydroxysuccinimide-biotin binding studies support a proposed mechanism in which the amino group of the phenylalanine substrate is attacked directly by the 4-methylidene-imidazole-5-one prosthetic group. We propose a helix-to-loop conformational switch in the helices flanking the inner active-site loop that regulates accessibility of the active site. Differences in loop stability among PAL homologs may explain the observed variation in enzyme efficiency, despite the highly conserved structure of the active site. A. variabilis C503S/C565S PAL is shown to be both more thermally stable and more resistant to proteolytic cleavage than R. toruloides PAL. Additional increases in thermal stability and protease resistance upon ligand binding may be due to enhanced interactions among the residues of the active site, possibly locking the active-site structure in place and stabilizing the tetramer. Examination of the A. variabilis C503S/C565S PAL structure, combined with analysis of its physical properties, provides a structural basis for further engineering of residues that could result in a better therapeutic molecule.

Phenylalanine ammonia lyase, enzyme substitution therapy for phenylketonuria, where are we now?

Phenylketonuria (PKU) is an autosomal recessive genetic disorder in which mutations in the phenylalanine-4-hydroxylase (PAH) gene result in an inactive enzyme (PAH, EC 1.14.16.1). The effect is an inability to metabolize phenylalanine (Phe), translating into elevated levels of Phe in the bloodstream (hyperphenylalaninemia). If therapy is not implemented at birth, mental retardation can occur. PKU patients respond to treatment with a low-phenylalanine diet, but compliance with the diet is difficult, therefore the development of alternative treatments is desirable. Enzyme substitution therapy with a recombinant phenylalanine ammonia lyase (PAL) is currently being explored. This enzyme converts Phe to the harmless metabolites, trans-cinnamic acid and trace ammonia. Taken orally and when non-absorbable and protected, PAL lowers plasma Phe in mutant hyperphenylalaninemic mouse models. Subcutaneous administration of PAL results in more substantial lowering of plasma and significant reduction in brain Phe levels, however the metabolic effect is not sustained following repeated injections due to an immune response. We have chemically modified PAL by pegylation to produce a protected form of PAL that possesses better specific activity, prolonged half-life, and reduced immunogenicity in vivo. Subcutaneous administration of pegylated molecules to PKU mice has the desired metabolic response (prolonged reduction in blood Phe levels) with greatly attenuated immunogenicity.

Trends in enzyme therapy for phenylketonuria

Phenylketonuria (PKU) is an inborn error of amino acid metabolism caused by phenylalanine hydroxylase (PAH) deficiency. Dietary treatment has been the cornerstone for controlling systemic phenylalanine (Phe) levels in PKU for the past 4 decades. Over the years, it has become clear that blood Phe concentration needs to be controlled for the life of the patient, a difficult task taking into consideration that the diet becomes very difficult to maintain. Therefore alternative models of therapy are being pursued. This review describes the progress made in enzyme replacement therapy for PKU. Two modalities are discussed, PAH and phenylalanine ammonia-lyase PAH. Developing stable and functional forms of both enzymes has proven difficult, but recent success in producing polyethylene glycol-modified forms of active and stable PAH shows promise.

Biopterin responsive phenylalanine hydroxylase deficiency

PURPOSE

Phenylketonuria (PKU) is an autosomal recessive disorder caused by mutations in the phenylalanine hydroxylase (PAH) gene. There have been more than 400 mutations identified in the PAH gene leading to variable degrees of deficiency in PAH activity, and consequently a wide spectrum of clinical severity. A pilot study was undertaken to examine the response to 6-R-l-erythro-5,6,7,8-tetrahydrobiopterin (BH4) in patients with atypical and classical PKU.

METHODS

PAH gene mutation analysis was performed using denaturing gradient gel electrophoresis and gene sequencing. Patients with classical, atypical, or mild PKU were orally given BH4 10 mg/kg. Blood phenylalanine and tyrosine levels were determined using tandem MS/MS at 0 hours, 4 hours, 8 hours, and 24 hours intervals.

RESULTS

Thirty-six patients were given a single oral dose of 10 mg/kg of BH4. Twenty one patients (58.33%) responded with a decrease in blood phenylalanine level. Of the patients that responded, 12 were classical, 7 atypical, and 2 mild. The mean decline in blood phenylalanine at 24 hours was > 30% of baseline. There were 15 patients who did not respond to the BH4 challenge, 14 of those had classical and one had atypical PKU. Mapping the mutations that responded to BH4 on the PAH enzyme showed that mutations were in the catalytic, regulatory, oligomerization, and BH4 binding domains. Five patients responding to BH4 had mutations not previously identified.

CONCLUSION

The data presented suggest higher than anticipated number of PKU mutations respond to BH4, and such mutations are on all the domains of PAH.

Clinical protocol. Gene therapy of Canavan disease: AAV-2 vector for neurosurgical delivery of aspartoacylase gene (ASPA) to the human brain

This clinical protocol describes virus-based gene transfer for Canavan disease, a childhood leukodystrophy. Canavan disease, also known as Van Bogaert-Bertrand disease, is a monogeneic, autosomal recessive disease in which the gene coding for the enzyme aspartoacylase (ASPA) is defective. The lack of functional enzyme leads to an increase in the central nervous system of the substrate molecule, N-acetyl-aspartate (NAA), which impairs normal myelination and results in spongiform degeneration of the brain. No effective treatment currently exists; however, virus-based gene transfer has the potential to arrest or reverse the course of this otherwise fatal condition. This procedure involves neurosurgical administration of approximately 900 billion genomic particles (approximately 10 billion infectious particles) of recombinant adeno-associated virus (AAV) containing the aspartoacylase gene (ASPA) directly to affected regions of the brain in each of 21 patients with Canavan disease. Pre- and post-delivery assessments include a battery of noninvasive biochemical, radiological, and neurological tests. This gene transfer study represents the first clinical use of AAV in the human brain and the first instance of viral gene transfer for a neurodegenerative disease.

Study on a novel strategy to treatment of phenylketonuria

To replace the low phenylalanine (phe) diet treatment and to improve the quality of life of the phenylketonuria (PKU) patients, A phenylalanine ammonia-lyase (PAL) cDNA from petroselinum crispum was subcloned into expression vectors, pMG36e and pNZ8048, and transformed L. lactis by electroporation. The pMG36e PAL/L. lactis and pNZ8048 PAL/L.L. were screened and characterized by using PCR and HPLC, and prepared as a liquid type preparation that were given orally to treat hyperphenylalaninemia-rats. The phe levels of the rat plasma sample were determined by HPLC. The data showed that the plasma phe levels in hyperphenylalanemia (HPA) rats receiving preparations made from the engineered L. lactis were significantly reduced compared with non-treated HPA rats. The effect of the pNZ8048PAL/L.L. showed a higher expression of PAL and better cure results than pMG36ePAL/L.L. These results point a potential way for PKU treatment.

Affinity hemodialysis for antiviral therapy. I. Removal of HIV-1 from cell culture supernatants, plasma, and blood

We tested an affinity hemodialysis technique designed to efficiently remove HIV and toxic viral proteins from blood. Miniature polyethersulfone hollow-fiber dialysis cartridges (200-500 nm pore) were packed with anti-HIV antibodies covalently coupled to agarose beads and sealed inside the cartridge. Cell culture fluids, plasma, or infected blood (7-15 ml) containing HIV-1 were circulated over the cartridge at 0.7-10 ml/min and the rate of removal of HIV measured by PCR and p24 ELISA. The technique removed up to 98% of HIV-1 particles from cell culture supernatants. Affinity hemodialysis also efficiently captured cultured HIV from human blood plasma (90%) and native HIV from infected blood (83% to 100%). Viral capture followed first-order kinetics (t(1/2) = 2.8 h). Variations in antibody type, matrix linkage (protein G versus direct coupling), bead pore size, and temperature of operation (25-37 degrees C) had only small effects. Although some binding was nonspecific, direct binding to the immobilized antibodies appeared to be the predominant mechanism.

National Institutes of Health Consensus Development Conference Statement: phenylketonuria: screening and management, October 16-18, 2000

OBJECTIVE

To provide health care providers, patients, and the general public with a responsible assessment of currently available data regarding screening for and management of phenylketonuria (PKU).

PARTICIPANTS

A nonfederal, nonadvocate, 14-member panel representing the fields of pediatrics, genetics, human development, public policy, nursing, and molecular physiology and including patient representatives. In addition, 19 experts in pediatrics, medical genetics, psychology, pediatric neurology, biochemical and molecular genetics, and gene therapy presented data to the panel and to a conference audience of 312.

EVIDENCE

The literature was searched using Medline for January 1980 through July 2000, and an extensive bibliography of 3394 references was provided to the panel. Experts prepared abstracts for their conference presentations with relevant citations from the literature. Scientific evidence was given precedence over clinical anecdotal experience.

CONSENSUS PROCESS

The panel, answering predefined questions, developed its conclusions based on the scientific evidence presented in open forum and the scientific literature. The panel composed a draft statement, which was read in its entirety and circulated to the experts and the audience for comment. Thereafter, the panel resolved conflicting recommendations and released a revised statement at the end of the conference. The panel finalized the revisions within a few weeks after the conference. The draft statement was made available on the World Wide Web immediately after its release at the conference and was updated with the panel's final revisions. The statement is available at http://consensus.nih.gov.

CONCLUSIONS

Genetic testing for PKU has been in place for almost 40 years and has been very successful in preventing severe mental retardation in thousands of children and adults. Metabolic control is necessary across the lifespan of individuals with PKU. A comprehensive, multidisciplinary, integrated system is needed to delivery of care to individuals with PKU. Greatly needed are consistency and coordination between screening, treatment, data collection, and patient support programs. There should be equal access to culturally sensitive, age-appropriate treatment programs. Ethically sound, specific policies for storage, ownership, and use in future studies of archived samples remaining from PKU testing should be established. Research into the pathophysiology of PKU and relationship to genetic, neural, and behavioral variation is strongly encouraged. Uniform policies must be established to remove financial barriers to the acquisition of medical foods and modified low-protein foods and to provide access to support services needed to maintain metabolic control in individuals with PKU. Research on nondietary alternative treatments for PKU is strongly encouraged. To achieve optimal statistical power and cross-cultural applicability, it will be beneficial to use data acquired via national and international collaboration.phenylketonuria, hyperphenylalanimea, phenylketonuria screening, phenylalanine-restricted diet, maternal phenylketonuria, newborn screening, phenylalanine monitoring, phenylketonuria outcomes.

A different approach to treatment of phenylketonuria: phenylalanine degradation with recombinant phenylalanine ammonia lyase

Phenylketonuria (PKU), with its associated hyperphenylalaninemia (HPA) and mental retardation, is a classic genetic disease and the first to have an identified chemical cause of impaired cognitive development. Treatment from birth with a low phenylalanine diet largely prevents the deviant cognitive phenotype by ameliorating HPA and is recognized as one of the first effective treatments of a genetic disease. However, compliance with dietary treatment is difficult and when it is for life, as now recommended by an internationally used set of guidelines, is probably unrealistic. Herein we describe experiments on a mouse model using another modality for treatment of PKU compatible with better compliance using ancillary phenylalanine ammonia lyase (PAL, EC 4.3.1.5) to degrade phenylalanine, the harmful nutrient in PKU; in this treatment, PAL acts as a substitute for the enzyme phenylalanine monooxygenase (EC 1.14.16.1), which is deficient in PKU. PAL, a robust enzyme without need for a cofactor, converts phenylalanine to trans-cinnamic acid, a harmless metabolite. We describe (i) an efficient recombinant approach to produce PAL enzyme, (ii) testing of PAL in orthologous N-ethyl-N'-nitrosourea (ENU) mutant mouse strains with HPA, and (iii) proofs of principle (PAL reduces HPA)-both pharmacologic (with a clear dose-response effect vs. HPA after PAL injection) and physiologic (protected enteral PAL is significantly effective vs. HPA). These findings open another way to facilitate treatment of this classic genetic disease.

New approaches to the treatment of phenylketonuria

Phenylketonuria (PKU) is the most common of all aminoacidopathies and is caused by autosomal recessive deficiency of the hepatic phenylalanine hydroxylase system. The diagnosis of PKU should be multifactorial and based on a protein overload test that reveals increased plasma phenylalanine levels during the ingestion of a normal diet, a phenylalanine tolerance test, and in vitro and in vivo activity of the liver enzyme. An individualized diagnosis that characterizes the severity of the disease in each patient provides objective and effective criteria for the dietary treatment of each particular case.

Keratinocyte gene therapy for adenosine deaminase deficiency: a model approach for inherited metabolic disorders

Disorders in which there is toxic buildup of circulating substrate may be treated by furnishing an enzyme reservoir capable of metabolically processing the excess substrate. The epidermal keratinocyte is a potential site for such a reservoir. In this study, we explore the capacity of genetically modified keratinocytes to metabolize extracellular substrate in a culture model that resembles in vivo epidermal architecture. Keratinocytes from adenosine deaminase (ADA)-deficient patients were transduced with a retroviral vector encoding the human ADA gene and the capacity of this tissue to deaminate deoxyadenosine (dAdo) in vitro was measured. The results show that at a substrate concentration of 10 microM, ADA-corrected keratinocytes deaminate dAdo at a rate of 0.38 nmol/min.10(6) cells. These results indicate that keratinocytes process extracellular substrate at rates that suggest complete substrate conversion in a single pass. This study provides a strong indication that the epidermis, the largest and most accessible tissue of the body, is a valuable site for designing clinically relevant gene therapies.

Effect of high-dose tyrosine supplementation on brain function in adults with phenylketonuria

OBJECTIVES

To characterize abnormalities of brain function in patients with phenylketonuria (PKU) who had relaxed or stopped the dietary regimen and to test whether oral high-dose tyrosine (Tyr) supplementation has a beneficial effect.

DESIGN

Comparison with a control group; double-blind, placebo-controlled study comprising six test times; crossover treatment groups; oral high-dose Tyr therapy (100 mg/kg body weight per day) or placebo administration for 4 weeks.

SUBJECTS

Twenty-four early-treated patients with PKU aged 20.8 (16 to 25) years; 24 control subjects.

METHODS

Plasma concentrations of phenylalanine and Tyr were monitored. Neuropsychologic tasks, visual evoked potentials, and spectral analysis of electroencephalographic activity were used to evaluate brain function.

RESULTS

When patients with PKU were compared with control subjects, deficits in certain aspects of brain function were confirmed (i.e., a decreased ability to sustain attention, prolonged latencies of visual evoked potential peaks N1 and P2, and a reduced amount of fast-wave activity on the electroencephalogram). Baseline plasma phenylalanine and Tyr concentrations were in the typical range of adult patients with PKU. The plasma Tyr concentration increased approximately 200% during Tyr supplementation, but no beneficial effects were observed.

CONCLUSIONS

High-dose Tyr supplementation cannot be recommended as an "alternative" treatment for patients with PKU after relaxation or termination of strict dietary adherence.

Cell transplantation in liver-directed gene therapy

Somatic cell gene therapy is a new field of biomedical research that encompasses a variety of traditional basic research and clinical disciplines. This new approach to therapeutics has the potential to prevent, treat, or cure a variety of inherited and acquired diseases. Two divergent strategies of hepatocyte transplantation are being employed in animal models and clinical trials in an attempt to correct genetic deficiencies. Allogeneic hepatocyte transplantation has two main advantages over autologous cell transplantation. First, invasive surgical procedures are not required in the recipient. Second, allogeneic cells can be administered repetitively, so that multiple harvests are not necessary. The major drawbacks to allogeneic hepatocyte transplants are rejection and the risks of immunosuppression. Although there is no clinical experience with the treatment of genetic disease by allogeneic hepatocyte transplantation, a variety of animal models have been characterized, including the Gunn rat (UDP-glucuronosyl transferase deficient), the Nagase analbuminemic rat, and the Watanabe heritable hyperlipidemic rabbit (LDL receptor deficient). The use of genetically corrected autologous cells represents a different and more elegant approach to the correction of inherited disease. A segment of liver is harvested from the affected individual. Recombinant retroviruses are used to transduce normal genes--with a variety of promoter/enhancer constructs--into the patients own hepatocytes. The genetically corrected hepatocytes are then transplanted back into the patient. This approach, known as ex vivo gene therapy, eliminates the risk of rejection and the need for immunosuppression. The safety and efficacy of this approach has been proven in a variety of preclinical animals models, including Watanabe rabbits, dogs, and Papio spp. A clinical trial for the treatment of familial hypercholesterolemia is currently in progress. A number of approaches for the reintroduction of hepatocytes into the recipient have been proposed, including catheter-mediated delivery into the inferior mesenteric vein, the umbilical vein, or into the spleen. Candidate diseases, which are likely to result in the first clinical trials include familial hypercholesterolemia, ornithine transcarbamylase deficiency, Crigler-Najjar syndrome, alpha 1-antitrypsin deficiency, and phenylketonuria.

Enzymes as agents for the treatment of disease

The replacement of genetically deficient enzymes in patients with inherited metabolic disorders by infusion of purified enzymes or by organ transplantation has had very limited success, although good results with bone marrow transplantation have been obtained in some patients with mucopolysaccharidosis, Gaucher disease and inherited immunodeficiency diseases. Genetic engineering of the patient's lymphocytes may ultimately render these approaches redundant, at least for some of these diseases. Treatment of chronic pancreatic insufficiency and of disaccharidase deficiency with oral enzymes can be very effective; therapy can be monitored in the latter by measuring the breath hydrogen excretion and in the former by a range of tests of which stool chymotrypsin assay is the most convenient. Treatment of acute myocardial infarction by intracoronary perfusion of thrombolytic enzymes can improve both cardiac function and long-term survival if given early enough. Successful reperfusion can be identified by changes in the kinetics of serum enzyme release and clearance, especially for the isoenzymes and isoforms of creatine kinase. In cancer chemotherapy, L-asparaginase has long been a useful adjunct in the treatment of acute lymphoblastic leukemia, but recent experience suggests a role in acute nonlymphoblastic leukemia as well.

Clinical application of somatic gene therapy in inborn errors of metabolism

Rapid advances in recombinant DNA and gene transfer technologies provide the potential for somatic gene therapy of inborn errors of metabolism in which the genetically defective function will be restored by transfer of a normal gene into somatic cells. The therapeutic potential and safety of gene therapy has been explored in cultured cells and experimental animals, but therapeutic clinical trials have not yet been proposed or performed. The technologies which may make somatic gene replacement therapy feasible need to be considered and criticised from a clinical perspective. Clinical trials will be necessary to determine the efficacy of somatic gene therapy and address concerns about safety.