Congenital athymia is an ultra rare condition in which infants are born without a functioning thymus. The thymus is crucial for the development, selection, and maturation of T cells, which are essential in effectively fighting infection and regulating the immune system. Without a functioning thymus, children develop severe immunodeficiency, susceptible to life-threatening infections. Without adequate medical treatment and management, children with congenital athymia usually do not live past the first few years of life. Multiple genetic and syndrome disorders, mutations and deficiencies are associated with congenital athymia. Supportive care, such as strict prevention measures, prophylactic antimicrobials, immunoglobulin replacement and isolation have been the mainstay management of children with congenital athymia. Rethymic is a regenerative tissue-based therapy that is indicated for immune reconstitution in pediatric patients with congenital athymia. The reengineered tissue is implanted in the thigh muscle to help a child with congenital athymia build a functioning immune system to reduce the number of potentially life-threatening infections.
For individuals with congenital athymia who received allogeneic processed thymus tissue-agdc, the evidence included 10 prospective, single-center, open-label studies evaluating a total of 95 participants. Relevant outcomes are disease-specific survival, change in disease status, quality of life, treatment-related mortality and treatment-related morbidity. The primary evidence of efficacy was based on a comparison of survival between the study participants (n=95) and natural history populations (n=49). Natural history data were obtained from a contemporaneous population with congenital athymia who were observed from 1991-2017 and received only supportive care. The Kaplan-Meier estimated survival rates for the study participants who received allogeneic processed thymus tissue was 77% (95% confidence interval [CI], 0.670 to 0.841) at 1 year and 76% (95% CI, 0.658 to 0.832) at 2 years. For patients who were alive at 1 year after treatment, the survival rate was 94% at a median follow-up of 10.7 years. In comparison, the 2-year survival rate was 6%, with all patients dying by 3 years of age in the external historical control. In addition to the observed survival benefit, the efficacy is also supported by evidence of immune reconstitution based on decreased frequency and severity of infections. Overall, there were 29 deaths following treatment in the study population. The most common cause of death was infection and most of these deaths occurred during the first year after treatment, which is not unexpected as immune reconstitution following treatment takes about 6-12 months after transplantation. No major limitations in the study relevance were identified. Limitations in study design and conduct relates to the lack of randomized controlled trials. Congenital athymia is an ultra rare disorder and therefore conducting randomized trials would be challenging. The evidence 1) included data from a large number of study participants, especially given the rarity of the disease, with a long duration of follow-up (over 25 years), 2) the natural history is uniform and well-characterized, and 3) most importantly, there was a large survival effect that was consistent and persistent in this otherwise rapidly fatal disease, despite the heterogeneous underlying genetic anomalies and diverse comorbid conditions. The evidence is sufficient to determine that the technology results in an improvement in the net health outcome.
This policy is designed to address medical guidelines that are appropriate for the majority of individuals with a particular disease, illness, or condition. Each person's unique clinical circumstances may warrant individual consideration, based on review of applicable medical records.
Allogeneic processed thymus tissue-agdc is considered medically necessary for pediatric individuals with congenital athymia if they meet criteria 1 through 8:
Diagnosis of severe combined immunodeficiency has been ruled out.
Documentation that following infection control measures can reasonably be maintained until the development of thymic function is established. (See policy guidelines for additional details).
Antimicrobial prophylaxis to prevent bacterial, fungal, and viral infections
Immunoglobulin replacement therapy
Strict infection control, sanitation, and isolation protocols to limit exposure to infectious pathogens
Absence of comorbidities, in the opinion of the treating clinician, that are reasonably likely to result in severe complications, including death from administration of allogeneic processed thymus tissue (for example, pre-existing renal impairment, or cytomegalovirus or Epstein-Barr virus infection).
Screened for anti-human leukocyte antigen (HLA) antibodies prior to receiving allogeneic processed thymus tissue-agdc. Individuals testing positive for anti-HLA antibodies should receive allogeneic processed thymus tissue-agdc from a donor who does not express those HLA alleles.
If the individual has received a prior hematopoietic cell transplantation or a solid organ transplant, HLA matching of allogeneic processed thymus tissue-agdc to recipient alleles that were not expressed in the donor is required.
Allogeneic processed thymus-agdc is considered investigational when the above criteria are not met.
Allogeneic processed thymus-agdc is considered investigational for all other indications.
Repeat treatment with allogeneic processed thymus-agdc is considered investigational.
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CPT |
N/A |
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ICD10 PCS |
XW020D8 |
Introduction of Engineered Allogeneic Thymus Tissue into Muscle, Open Approach, New Technology Group 8 (eff 10/01/2023) |
| HCPCS | C9399 | Unclassified drugs or biologicals |
| J3490 | Unclassified drugs | |
| J3590 | Unclassified biologics |
| CD10 CM | Q89.9 | Other specified congenital malformations |
| D82.1 | Di George's Syndrome |
The recommended dose range is 5,000 to 22,000 mm2 of allogeneic processed thymus tissue-agdc/m2 recipient body surface area.
1 surgical implantation per lifetime.
Allogeneic processed thymus tissue-agdc is administered by a single surgical procedure. The dosage is determined by the total surface area of the allogeneic process thymus tissue-agdc slices and recipient body surface area. A slice is defined as the contents on a single fiber membrane; the allogeneic processed thymus tissue-agdc slices are variable in size and shape. The manufacturer calculates the dose in advance of the specific individual; the amount of product provided is adjusted at the manufacturing facility to ensure the maximum dose for the individual cannot be exceeded. Up to 42 cultured allogeneic processed thymus tissue-agdc slices will be provided to each individual. Individuals with evidence of maternal engraftment or an elevated response to phytohemagglutinin should receive allogeneic processed thymus tissue-agdc with immunosuppressive medications.
To decrease the risk of graft-versus-host disease, immunosuppressive therapy should be administered prior to and/or after treatment according to the disease phenotype and pre-treatment phytohemagglutinin response in accordance with the recommendations in Table 2 and 3 of the product label.
Immune reconstitution sufficient to protect against infection is unlikely to develop prior to 6 to 12 months after treatment with allogeneic processed thymus tissue-agdc. For some patients, it may take up to 2 years.
Immunizations should not be administered in patients who receive allogeneic processed thymus tissue-agdc until the individual’s immune-function has been restored. It is recommended prescribers reference the following criteria before deciding to administer an inactivated vaccine: immunosuppressive therapies have been discontinued, immunoglobulin replacement therapy has been discontinued, the total CD4+ T cell count is > 200 cells/mm3, and there are more CD4+ T cells than CD8+ T cells. It is recommended that no more than 2 inactivated vaccines be given per month. Live vaccines should not be administered until individuals have met the recommended criteria for inactivated vaccines and received vaccinations with inactivated agents.
Congenital athymia (CA) is an ultra-rare condition in which individuals are born without a functioning thymus. Estimated incidence in the United States is approximately 17 to 24 infants for every 4 million.1,The thymus is crucial for the development, selection and maturation of T cells, which are essential in effectively fighting infection and regulating the immune system.2, Without a functioning thymus, individuals are profoundly immunodeficient, have significant increased susceptibility for infection, and have greater tendency to develop autologous graft-versus-host disease (GVHD). These infections and autoimmune conditions can be fatal, and with only supportive care, children with congenital athymia typically do not survive beyond 2 to 3 years of age.3,s. Congenital athymia may be associated with other conditions, such as: DiGeorge syndrome (with or without 22q11.2 deletion syndrome); mutations in the genes TBX1, CHD7, (CHARGE syndrome), and FOXN1 (FOXN1 deficiency); diabetic embryopathy.4,5,
Currently, all 50 states in the USA offer newborn screening testing that identifies T cell receptor excision circles. This test may identify infants who have congenital athymia in addition to severe combined immunodeficiency (SCID)6,Confirmatory diagnosis requires confirmation of low naïve T cells by flow cytometry.6,
Diagnosis of congenital athymia requires confirmation of profoundly low naïve T cells by flow cytometry and genetic evaluation after initial positive results via SCID newborn screening. Individuals with congenital athymia will have less than 50 naïve T cells/mm3 or naïve T cells comprising less than 5% of the total T cells. However, congenital athymia individuals and a subset of individuals with SCID also present with a lack of T cells but normal levels of B cells and Natural Killer cells (T-B+ NK+). To differentiate these individuals, a genetic panel for known T-B+ NK+ SCID gene mutations , genetic testing to identify gene associated with either condition, or testing of hematopoietic stem cells using an artificial thymic organoid system can be used. Accurate identification of the causes for immunodeficiency in these individuals is important as to ensure appropriate treatment decisions (e.g. receipt of hematopoietic stem cell transplant vs. cultured thymus tissue implantation).6,
Medical treatment of congenital athymia typically focuses on supportive care. Strict infectious disease prevention measures (e.g., masks, sterile gowns, gloves, frequent handwashing), isolation, and antimicrobial prophylaxis to prevent bacterial, viral and fungal infections are critical. Because B cell function is usually reduced , immunoglobulin replacement should be considered. Immunosuppressive agents including steroids or calcineurin inhibitors may be required to manage inflammatory reactions and reduce the risk of autologous GVHD. Hematopoietic stem call transplantation has been performed in patients with congenital athymia , but the efficacy is low.
On October 8, 2021, allogeneic processed thymus tissue-agdc (Rethymic®; Enzyvant) was approved by the U.S. Food and Drug Administration (FDA) for the treatment of congenital athymia in pediatric patients.
