Purine Nucleoside Phosphorylase Deficiency Treatment & Management

Guidelines for the diagnosis and management of primary immunodeficiencies have been established. ^[20] The following treatment may be indicated in patients with purine nucleoside phosphorylase (PNP) and adenosine deaminase deficiencies:

Bone marrow transplantation

The treatment options discussed included allogeneic hematopoietic stem cell transplantation (HSCT) from a matched family donor (MFD), a mismatched haploidentical donor, and unrelated bone marrow donor. ^[21]

Hassan et al ^{[22, 23]} reported results of transplantation for the Inborn Errors Working Party of the European Group for Blood and Marrow Transplantation and European Society for Immunodeficiency. In this multicenter study, 106 patients with ADA-deficient SCID received 119 transplants from matched sibling donors (MSDs), matched family donors (MFDs), matched unrelated donors (MUDs), and haploidentical donors. Survival was 86% in MSDs, 81% in MFDs, and 66% in MUDs, compared with 43% in haploidentical donors. Survival was also greater in patients who did not receive pretransplantation conditioning compared with those that received myeloablative therapy, 81% versus 54%. T- and B-cell engraftment was achieved in all groups.

Metabolic abnormalities improved with reduction of deoxy-ATP (dATP) levels following transplant.

In PNP deficiency, human leukocyte antigen (HLA)-matched bone marrow transplantations have been successful in patients who have received pretransplantation conditioning. Haploidentical bone marrow transplantations were difficult to engraft, partly because patients did not receive a conditioning protocol before transplantation. Because residual T-cell function may be present in PNP deficiency, the transplant may have been rejected. ^{[7, 24]}

Successful immune reconstitution has been reported in a patient with PNP deficiency using transplantation of stem cells from umbilical-cord blood. ^[25] The conditioning regimen consisted of busulfan, cyclophosphamide, and antithymocyte globulin (ATG). The patient's neurologic impairments resolved.

A nonmyeloablative conditioning regimen of busulfan and fludarabine has resulted in successful immune reconstitution. ^[26] The conditioning regimen lowered the risk of vaso-occlusive disease when 2 alkylating agents (eg, busulfan and cyclophosphamide) that potentiate hepatotoxicity were used together.

One important issue in the treatment of children with severe T-cell disorders is whether to use a preparative regimen before stem cell infusion for immunosuppression to prevent rejection and myeloablation and, thus, to allow donor T-cell, B-cell, and monocytic-cell engraftment. This is an important issue with all options involving hematopoietic stem cells or umbilical cord blood.

In patients who received T-cell–depleted transplants of bone marrow grafts without prior cytoreduction, graft failures occurred in 30-50%. ^{[27, 28]}

In both murine models and in patients with severe combined immunodeficiency (SCID), normal-to-high natural killer (NK)-cell activity is associated with a higher incidence of graft failure or delayed immunologic reconstitution; this is perhaps the foremost cause for graft rejection.

The nature of the preparative regimen and the relative importance of immunosuppression versus ablation have not yet been fully defined and may depend on the nature of the hematopoietic stem cell graft. The optimal combination of ablative agents (eg, busulfan) with immunosuppressive agents (eg, ATG, cyclophosphamide), and/or newer agents (eg, fludarabine, alemtuzumab) has not been systematically studied and should be the focus of future clinical trials.

The risks of the preparative regimen are known and include sterility, liver, heart and lung toxicity, and malignancy. These risks must be balanced against morbidity and mortality associated with graft rejection and repeated transplantation, poor T-cell engraftment, and/or poor B-cell function. Children with severe T-cell dysfunction may also have serious infections that cannot be eliminated.

Ultimately, the goal of bone marrow transplantation is to provide complete T-cell, B-cell, and NK-cell function. The choice of preparative regimen is complicated by the heterogeneity of NK-cell and B-cell function, which can be expected to develop in various forms of SCID when successful T-cell engraftment occurs. This heterogeneity is not necessarily the case when T cell–depleted haploidentical bone marrow transplantation is performed.

The role of myeloablation in the preparative regimen before transplantation remains controversial. Some groups achieved stable immune reconstitution by transplanting umbilical-cord blood in patients with thymic dysplasia and SCID without ablative therapy. However, patients with some severe T-cell immunodeficiency disorders, such as reticular dysgenesis, combined immunodeficiency (CID), thymic dysplasia, and Wiskott-Aldrich syndrome, require an immunosuppression regimen for preparation.

Patients with PNP deficiency probably also require pretransplantation conditioning. Some groups use a preparative regimen in SCID with high NK-cell function. Further studies are necessary to determine whether this is a true or only theoretical advantage. In addition, patients may need posttransplantation graft versus host disease (GVHD) prophylaxis with cyclosporine and corticosteroids, which affect the function of mature T cells in the umbilical cord preparation.

Preparative regimen and GVHD prophylaxis

Transplantation groups disagree on the need for a preparative regimen for transplantations in both ADA and PNP deficiency. Groups that favor a preparative regimen disagree on what regimen should be used. Preparative regimens have included myeloablative treatments (busulfan, cyclophosphamide, ATG), nonmyeloablative treatments (busulfan, fludarabine), or busulfan alone.

Pretransplantation conditioning for patients typically includes busulfan at 1 mg/kg (1.25 mg/kg if patient < 2 y) given orally every 6 hours on days -9 through -6 (transplantation is on day 0). The dose of busulfan is adjusted on the basis of first-dose kinetics (steady-state level or 400-600 ng/mL). This is followed by cyclophosphamide at 50 mg/kg intravenously (IV) on days -5 through -2 and ATG 30 at mg/kg given IV on days -3 through -1. Recently, alemtuzumab has been introduced as part of a reduced-intensity conditioning regimen in combination with busulfan and/or fludarabine for HSCT in children with malignant and nonmalignant diseases. Alemtuzumab is a humanized monoclonal antibody to CD52 antigen, and this action may help reduce GVHD by depleting T-cells. These reduced-intensity conditioning regimens appear to have improved tolerability and the success rate of HSCT for PNP deficiency.

Prophylaxis for acute GVHD includes a continuous IV infusion of cyclosporin A beginning on day -2 (target whole blood levels of 250-350 ng/mL) and methylprednisolone at 10 mg/kg/d IV on days 5-7, 5 mg/kg/d on days 8-10, and 3 mg/kg/d on days 11-13, followed by a 10% weekly reduction taper. Patients are evaluated daily for acute GVHD during hospitalization and at least weekly after their discharge home for the first 100 days after transplantation. Corticosteroids are generally discontinued by day 60 after transplantation, and cyclosporin A is discontinued between days 100 and 365, depending on clinical evidence of GVHD.

Enzyme replacement therapy

Approximately, 150-160 patients with ADA deficiency worldwide have received polyethylene glycol (PEG)–adenosine deaminase, and approximately 90 patients were receiving treatment in 2006. In the experience of these investigators, in 31 patients who received PEG–adenosine deaminase, 14 patients went on to HSCT, 14 patients continued to do well on PEG–adenosine deaminase therapy, and 3 patients died. PEG–adenosine deaminase therapy was generally well-tolerated. Immune reconstruction varied. In the first 6 months of therapy, absolute lymphocyte count (ALC), CD3+, and CD4+ T cell numbers did increase, with good lymphoproliferation responses to phytohemagglutinin (PHA), and thymopoiesis increased. However, over a prolonged time, T-cell numbers and naive T cells were reduced compared with normal controls. Metabolic abnormalities improved with PEG–adenosine deaminase but not as effective as that seen with HSCT.

The bovine-derived ADA replacement enzyme pegademase (Adagen) was approved by the FDA in 1990. In October 2018, the FDA approved elapegademase (Revcovi) for treatment of adenosine deaminase severe combined immune deficiency (ADA-SCID) in adults and children. The drug had been available as an orphan drug prior to approval. Enzyme replacement helps prevent potentially serious, life-threatening infections in this patient population.

In PNP deficiency, RBC transfusions have offered limited improvement.

Initial attempts to develop a PEG–PNP similar to PEG–ADA were disappointing in that both human and bovine PNP enzymes were too unstable at 37°C. Subsequently, the more stable hexameric Escherichia coli PNP was used and replaced 3 arginine residues in each of the 6 subunits with lysine by means of site-directed mutagenesis. ^[29] In murine models, this PEG–PNP was biologically active. This enzyme also had the ability to phosphorylate Ado, which may permit its use in the treatment of ADA deficiency in addition to PNP deficiency.

Recently, a novel technique to intracellularly transduce PNP protein was reported. ^[30] They used an 11 amino acid human immunodeficiency virus (HIV) transactor (TAT) protein transduction domain (PTD) and created a fusion protein with PNP protein. This PNP–PTD fusion protein rapidly transduced lymphocytes in vitro. Using lymphocytes from patients with PNP deficiency and PNP-/- mice, they demonstrated that the PNP–PTD fusion protein rapidly restored metabolic function and T-cell function in vitro. Furthermore, because the PNP–PTD was cellularly transduced, neutralizing antibodies had little effect.

These studies hold promise of treating patients with PNP deficiency with enzyme replacement therapy.

Live viral immunizations (eg, with oral polio vaccine) should be avoided.

Trimethoprim-sulfamethoxazole (Bactrim, Septra) is used for P carinii prophylaxis.

Fluconazole is used as prophylaxis against Candida species.

Gene therapy

Since the initial trials of ADA gene therapy performed at the National Institute of Health (NIH) and University of Southern California, gene therapy for ADA deficiency has been performed in Milan and London. ^{[31, 32]} ( ^[33] ) ^[34]

In these centers, autologous CD34+ human stem cells were transfected using retroviral vectors encoding the ADA gene.

In contrast to the US trials, these patients received mild conditioning regimens prior to infusion of the gene-modified human stem cells, consisting of either busulfan (Milan) or melphalan (London). PEG–adenosine deaminase was discontinued prior to gene therapy.

In the Milan experience, in 6 of 8 children who were monitored more than 6 months after gene therapy, vector-adenosine deaminase+ cells progressively became most of the T cells, B cells, and NK cells. Stable engraftment of gene corrected cells was seen in 0.1-10% of the myeloid cells. Thymopoiesis, T-cell number and function, and B-cell antibody responses improved. Metabolic abnormalities with decreased deoxyribonucleotides levels were seen. No adverse events or toxicity related to gene therapy were observed.

In London, one patient received ADA gene therapy. This patient experienced similar immune reconstitution and improvement of deoxyribonucleotides toxic metabolites as that observed in the Milan experience.

Despite immunologic reconstruction and decreased deoxyribonucleotides toxic metabolites, significant cognitive and behavioral abnormalities have persisted following HSCT and PEG–adenosine deaminase therapy. Specifically, the London group documented significant reduction in both verbal and performance intelligence quotient (IQ) levels. Metabolic detoxification is not complete with both HSCT and PEG–adenosine deaminase therapies, especially in erythrocytes. However, with gene therapy, erythrocyte ADA enzyme activity does improve, leading to decreased levels of deoxyribonucleotides metabolites in erythrocytes. Clinically, these patients have normal development to date.

PNP deficiency, similar to ADA deficiency, may be amenable to correction with gene therapy. ^[35] In reports of in vitro studies, gene therapy corrected PNP-deficient cells. In a murine model, Liao P et al reported successful gene therapy using a lentiviral vector containing the human PNP gene (lentiPNP). ^[36] Lymphocytes from a PNP-deficient patient and PNP deficient (-/-) were transduced with the lentiPNP gene and then transplanted into PNP (-/-) mice. The lentiPNP transduction corrected the abnormalities associated with PNP deficiency.

A proposed alternative to gene therapy is in situ repair of the defective gene. The principle is to synthesize a short oligodeoxyribonucleotide complementary to the section of the defective gene containing the error (except for the site corresponding to the error). Here, an oligomer contains the nucleotide complementary to that of the normal gene. The oligomer is transfected into the cells by using liposome vectors and binds to its complementary sequence in the defective gene. DNA repair enzymes then delete the defective sequence and insert the correct sequence.

Supportive care

All the patients are hospitalized in single rooms with high-efficiency particulate air-filtration systems.

All blood product transfusions are irradiated with 25 Gy before their administration to prevent GVHD.

Live viral immunizations (eg, with oral polio vaccine) should be avoided.

Trimethoprim-sulfamethoxazole (Bactrim, Septra) is used for P jiroveci prophylaxis.

Antifungal prophylaxis (fluconazole) is used as prophylaxis against infection with Candida species.

Consult a hematologist or an immunologist skilled in bone marrow transplantation.

Because patients with purine nucleoside phosphorylase are susceptible to viral, fungal, and bacterial infections, limit these patients' exposure to other persons.