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WHAT ARE THE MOST COMMON CAUSES OF THROMBOSIS IN CHILDREN?
A: The incidence of hospital-associated pediatric thrombosis has risen over the last 2 decades. This is due to improved survival of children with chronic conditions and a concomitant increase in the use of central venous catheters—the most common cause of thrombosis in children—and other lifesaving technology.
The presence of inherited thrombophilia in children is more of a risk factor for than a cause of thromboembolism and is usually of greater importance in adolescent children who develop venous thromboembolism without any triggers or who develop an exaggerated response compared to the trigger. Hereditary thrombophilia are classified mainly into the high or low risk for thrombosis groups. The high-risk group includes: deficiencies of the coagulation inhibitors anti-thrombin, protein C, and protein S, while the low-risk group includes factor V Leiden and prothrombin gene mutation.
WHAT DOES TESTING FOR HEREDITARY THROMBOPHILIA INVOLVE?
The most common tests performed in cases of hereditary thrombophilia include those for anti-thrombin, protein C, and protein S activity, as well as factor V Leiden and prothrombin mutation analysis done via polymerase chain reaction. All of these tests together constitute the hypercoagulable panel at my institution. We might also test for plasma homocysteine concentrations, especially if a patient has arterial thrombosis.
Hereditary thrombophilia testing does not influence the immediate care of patients with thrombosis and should be deferred for approximately 3–6 months after an acute episode and after anticoagulant therapy has ceased. This is because both the thrombotic consumptive process and anticoagulants affect tests for coagulation inhibitor activity. However, labs can run molecular testing for factor V Leiden and prothrombin gene mutation at any time.
While labs should perform this testing on a case-by-case basis, it is generally reserved for children with unprovoked thrombotic episodes and a family history of thrombosis. Hereditary thrombophilia testing is usually not recommended if a thrombotic episode is provoked by strong risk factors like major surgery, catheter use, immobility, major trauma, or malignancy. Additionally, comprehensive testing based on a positive family history alone is controversial but might be necessary when prescribing oral contraceptives. In all scenarios, communication between the laboratory and clinicians is essential for deciding when, whom, and what to test.
Labs should always remember that the purpose of these tests is primarily for risk assessment, not for identifying a cause of thrombosis. This means that a patient with a positive result might never actually have a thrombotic episode.
WHAT IS THE BIGGEST CHALLENGE WITH HEMOSTASIS TESTING IN CHILDREN?
The coagulation system of neonates and children evolves with age, which means that pediatric concentrations for a majority of coagulation factors and inhibitors differ markedly from adult concentrations. For example, protein C levels at birth could be anywhere from 17% to 53% of adult levels. These levels usually rise to >50% of adult levels by 6 months, with some reports indicating that full adult levels may not be reached until around 16 years of age.
Differences like this between children and adults have significant biological and clinical implications. In an ideal world, diagnostic laboratories processing pediatric samples would therefore use age, analyzer, and reagent-appropriate reference ranges—but currently this is not always possible. Many hemostatic reference values for preterm infants are lacking, and the ones that researchers have already reported rely on small study groups. Because of this knowledge gap, adult-based reference ranges are often used for the diagnosis of pediatric patients.
ARE DIRECT ORAL ANTICOAGULANTS (DOACS) APPROVED FOR USE IN CHILDREN?
None of the newer DOACs have been approved for use in children. Several clinical trials are still ongoing that will hopefully soon result in guidelines for pediatric DOAC use. These drugs would particularly benefit children on long-term therapy since new DOACs do not need to be monitored and also have fewer food and drug interactions.
HOW CAR-T CELL THERAPY WORKS
Chimeric antigen receptor (CAR)-T cells are autologous T cells that undergo genetic modification to express a receptor that contains four basic components: 1) extracellular single chain variable fragment (scvf) specific to a target antigen, 2) a transmembrane region, 3) intracellular T cell receptor (CD3 zeta chain), and 4) T cell co-receptor domain. The T cell receptor and co-receptor activates the T cell to exert its cytotoxic T cell functions upon the target cell.
These CAR-T cell components are customizable. For example, different scvfs can be used to recognize different targets, or different T cell coregulatory molecules can be added. CAR-T cells, via the scvf, recognize surface targets that are reproducibly expressed on malignant cells and not expressed on tissues that are known to cause irreparable damage to nonmalignant tissues that cannot be readily managed clinically.
CAR-T cells are manufactured from peripheral blood T cells. After collection of starting material by apheresis, the cells are transported to a processing facility where a vector, typically retroviral in nature, containing the genetic material for the CAR is introduced into the T cells. The T cells are then cultured and stimulated to proliferate. Once the desired number of cells for infusion has been obtained, the cells are transported back to the site of infusion. The patient then receives the cells, and is monitored for response in both acute and chronic settings.
The two Food and Drug Administration (FDA)-approved CAR-T cell products in clinical use—tisagenlecleucel (Kymriah) and axicabtagene ciloleucel (Yescarta)—both contain scfvs directed against CD19, a cell surface protein expressed on many B cell malignancies. The CAR constructs for the two commercial products differ mainly in the intracellular costimulatory component that renders the cytotoxic T cell function: CD28 in axicabtagene ciloleucel and CD137/4-1bb in tisagenlecleucel. They also differ in the vector used to deliver the genetic material into T lymphocytes: Tisagenlecleucel is manufactured using a lentiviral vector, and axicabtagene ciloleucel uses a gammaretroviral vector.
It is interesting to speculate that these noted differences between the commercial products might be due to the almost simultaneous and parallel progression of each through the FDA approval process. As data accrue over time, the different profiles of the two CAR-T cell products might become clearer. Tisagenlecleucel and axicabtagene ciloleucel have shown impressive results treating malignancies that, up until this point, have had extremely poor prognoses. Treatment with axicabtagene ciloleucel demonstrated a 58% complete response rate after 2 years in patients with relapsed refractory diffuse large B cell lymphoma. Treatment with tisagenlecleucel yielded an overall survival rate of 73% at 1 year. Both products received FDA approval for large B cell lymphomas; tisagenlecleucel also has approval for relapsed and refractory B acute lymphoblastic leukemia.
Notably, while CAR-T cell products have shown very promising results, they have only gained approval for certain hematologic malignancies and have
less-than-100% response rates. In addition, efforts to use CAR-T cells in solid malignancies have not yet proven successful. So while at this point CAR-T cell therapy has not yet been proven to be a magic bullet, it does have the high potential to become a mainstay in oncologic therapy.
Most clinical and laboratory characterization in patients has taken place in the context of the two FDA-approved CAR-T cell products, and some laboratory profile components in patients are specific to the particular CAR T cell therapy. For instance, B cell aplasia is an expected side effect of CAR-T cells directed against CD19, since normal B cells also express CD19 and are therefore eliminated in the same manner as the malignant cells expressing CD19. However, this side effect would not be expected in a patient who received CAR-T cells directed against a different target antigen not expressed on B cells.
Familiarity with the different types of CAR-T cell products currently in clinical use as well as those in clinical development will be useful for anticipating potential scenarios that might arise during evaluation of these patients. Laboratory involvement is essential throughout the process of CAR-T cell treatment so that care can be delivered in a timely manner that optimizes patient outcomes.