INTRODUCTION
This Product Profiler introduces health care professionals to immune globulin subcutaneous (human), Hizentra™, a treatment for patients with primary immunodeficiency disease (PIDD). The term PIDD refers to a large, heterogeneous group of disorders that affect the cells, tissues, and proteins of the immune system. Hizentra™ is the first and only U.S. Food and Drug Administration (FDA)-approved 20% subcutaneous immunoglobulin (SCIg) indicated for PIDD. It can be self-administered by patients under a physician’s care and after training from a physician or other health care provider.
Clinical data have shown Hizentra™ to be a safe and effective treatment of patients with PIDD. However, the safety and efficacy of this product have not been studied in neonates or infants.
When administered on a weekly basis, SCIg provides stable steady-state serum immunoglobulin G (IgG) levels, with lower IgG peak levels and higher IgG trough levels relative to monthly intravenous (IV) treatment (Berger 2008). The following text presents a brief overview of PIDD and current treatment options, followed by a review of the evidence-based literature supporting the FDA-approved indications for the SC administration of human Ig.
DISEASE OVERVIEW
Incidence and Prevalence
PIDDs represent a class of disorders in which the immune system of a person is compromised (NIH 2008). These diseases differ from secondary immune diseases in that they are usually the result of an intrinsic or genetic defect in the immune system (NIH 2008). Secondary immune diseases often can result from another disorder or an external agent such as chemotherapy, viruses, radiation therapy, or other drug-related treatments (Boyle 2007). The lack of a competent and normally functioning immune system poses significant threats to an individual. Antibody production impairments or ineffective/malfunctioning cellular defects lead to recurrent and unusual infections (Boyle 2007, Buckley 2009). Infections may be persistent, severe, become life-threatening, and may be caused by unusual microorganisms typically not harmful to those with normally functioning immune systems (Buckley 2009, NIH 2008).
There are over 150 different forms of PIDD syndromes that are associated with varying degrees of severity (NIAID 2009). Approximately 500,000 Americans suffer from various forms of PIDD syndromes, 5,000 to 10,000 of whom have severe manifestations (NIAID 2009). Although there are several types and forms of PIDDs, they are still considered rare diseases and many of the types are designated with an orphan status, meaning a single type of PIDD may affect fewer than 200,000 people (FDA 2009, NIAID 2009). Naturally, infections are the primary hallmark of patients with PIDDs (NIH 2008). It is common for patients with PIDDs to suffer from chronic ear, sinus, and other infections (NIH 2008). Furthermore, serious infections of the respiratory tract, such as recurrent pneumonia, can result in permanent lung damage (NIH 2008). However, these patients often suffer from additional comorbidities that may or may not be related to the immune system (NIH 2008). Examples of these conditions include anemia, arthritis, autoimmune diseases, and other comorbidities involving the heart, digestive tract, or nervous system (NIH 2008).
Etiology
Significant progress has been made in the past 20 years in cellular and molecular biology (Bacchelli 2007). These advancements have allowed researchers to begin to understand the underlying causes of some PIDDs (Bacchelli 2007). For example, genetic mutations have been identified in the immune system of affected individuals (Fischer 2004). These mutations provide researchers with valuable insight into the normal role and function of genes that affect immune cell development and function, and immune-related homeostatic mechanisms (Fischer 2004). These findings can be studied and applied to diagnosis, genetic counseling, prognosis, and potential therapeutic strategies (Fischer 2004).
Two examples of common PIDDs are Common Variable Immunodeficiency (CVID) and X-Linked Agamma-globulinemia (XLA). These deficiencies share the common etiology of antibody production defects (Buckley 2009). Physical findings can suggest one deficiency over another. For example, enlarged lymph nodes and splenomegaly is commonly present in patients suffering from CVID, while absent or reduced tonsil size and lymph nodes are characteristically found in patients with XLA (Buckley 2009).
Pathophysiology
The immune system is responsible for defending the body against infection and disease. In addition to the innate immune system, it does this using two components of the adaptive immune system, humoral and cell-mediated immunity (Shier 2010). The humoral immune system primarily uses antibodies produced from activated B cells to fight against infection (Shier 2010). The second system, the cell-mediated immune system, uses a compilation of different types of T cells to produce cytokines, secrete toxins, kill virus-infected and cancerous cells, enhance B cell antibody production, provide for future immunity against a specific pathogen, and regulate T cell response (Blaese 2007, Shier 2010). Table 1 describes the various components of the immune system and their functions.
TABLE 1.
Components of the Immune System
| Component | Function |
|---|---|
| B lymphocytes | Production of antibodies: IgG: defends against bacteria, viruses, and toxins; activates complement; main antibody in the plasma IgA: called the secretory Ig; defends against bacteria and viruses on mucosal surfaces IgM: first antibody produced by immature B lymphocytes; reacts with antigens on some red blood cells following incompatible blood transfusions; activates complement IgD: B cell activation; exact function unknown IgE: promotes inflammation and allergic reactions |
| T lymphocytes | Direct attack of viruses, fungi, or transplanted tissue Regulation of immune system Types of T lymphocytes:
|
| Natural killer cells | Killing of virus-infected cells Secretion of chemicals to enhance inflammation |
| Phagocytes | Ingestion and elimination of microorganisms Types of phagocytes:
|
| Complement system | Component of innate immunity Types of complement proteins:
|
Sources: Adapted from Blaese 2007 and Shier 2010.
Defects or disruptions in the presence of any of these components can drastically jeopardize the safety and immune capability of the host. The basis for the classification of PIDD disorders is categorized by the main component of the immune system that is deficient, absent, or defective (Table 2) (Merck Manual 2008).
TABLE 2.
Classifications of Component Defects
| Type of component defect | Description of defect(s) | % of PIDDs | Common associated disorder(s) |
|---|---|---|---|
| B cell defects | Antibody production deficiencies Decreased antibody titers B cell dysfunction |
50–60% | Selective IgA deficiency XLA CVID IgG subclass deficiency Specific antibody deficiency Autosomal recessive agammaglobulinemia Selective IgM deficiency Good’s syndrome |
| T cell defects | Immunoglobulin deficiencies | 5–10% | DiGeorge syndrome ZAP-70 deficiency X-linked lymphoproliferative syndrome Chronic mucocutaneous candidiasis |
| Combined B and T cell defects | Impaired antibody formation | 20% | Severe combined immunodeficiency Ataxia Telangiectasia Wiskott-Aldrich syndrome |
| Natural killer defects | Probable predisposition to viral infection and tumors (very rare) | NA | NA |
| Phagocytic cell defects | Impaired pathogen destruction | 10–15% | Chronic granulomatous disease Leukocyte adhesion deficiency syndrome Chédiak-Higashi syndrome |
| Complement defects | Deficiencies of complement components or inhibitors | ≤2% | Systemic lupus erythematosus Pyogenic infections Autoimmune disorders Angioedema Paroxysmal nocturnal hemoglobinuria Leukocyte adhesion deficiency syndrome |
Sources: Adapted from Bonilla 2005, Merck Manual 2008.
Clinical Presentation and Evaluation
Often, patients with PIDD present with recurrent, persistent, or unusual infections (Buckley 2009). The clinical presentation of PIDD, however, can depend on the extent of an underlying defect. For example, frequent upper respiratory infections may be the result of an antibody or complement deficiency (Buckley 2009). Frequent lower respiratory infections may suggest deficiencies in B or T cells, the complement pathway, or phagocyte defects (Buckley 2009). Infections common to skin and internal organs may suggest a phagocyte deficiency while infections in the blood or central nervous system may suggest an antibody or complement deficiency (Buckley 2009).
Some patients also may present with a variety of other clinical manifestations, including autoimmune or rheumatologic disease, anemia, gastrointestinal, heart or nervous system complications, and skin or liver abscesses (NIH 2008). In fact, one or more of these noninfectious manifestations may be the first or predominant clinical symptom of an underlying immunodeficiency (NIAID 2009).
Certain immunodeficiency diseases may be diagnosed because of the combination of characteristic signs and symptoms (NIH 2008). For example, children with DiGeorge syndrome often have an underdeveloped thymus gland, congenital heart disease, defective or underdeveloped parathyroid glands, and characteristic facial features (NIH 2008). Patients with Wiskott-Aldrich syndrome often develop bleeding problems and skin rashes in addition to increased likelihood of infections (NIH 2008). Unfortunately, it is not common practice to screen for PIDDs during infancy, childhood, or adulthood (Buckley 2009). Consequently, PIDDs usually are detected only after patients experience a multitude of infections and when physicians, parents, and patients recognize that infections are more than just ordinary (NIH 2008).
When a patient is suspected of having a PIDD, a complete patient and family history, a detailed physical examination with pertinent laboratory tests, and diagnostic procedures should be performed and evaluated (NIH 2008). The initial screening tests should include a complete blood count (CBC) with differential, quantitative immunoglobulin levels (IgG, IgM, and IgA), specific antibody response to vaccines, complement screening, and skin tests (NIH 2008). If any of these tests demonstrate abnormalities or if clinical symptoms persist, further immunologic evaluation is needed to identify specific deficiencies (Bonilla 2005, Buckley 2009, NIH 2008).



