Immunotherapy is a milestone in the treatment of poor-prognosis pediatric acute lymphoblastic leukemia (ALL) and is expected to improve treatment outcomes and reduce doses of conventional chemotherapy without compromising the effectiveness of the therapy. However, both chemotherapy and immunotherapy cause side effects, including neurological ones.
1. Introduction
Five-year overall survival of ALL has increased over the past decades and now exceeds over 96%
[1]. Chemotherapy is a crucial part of treating ALL and involves many cytotoxic drugs, which inhibit cancer cells from growing rapidly, but they also damage healthy cells, resulting in a wide range of adverse effects. However, with the current high rate of survival, it would be difficult to improve results with only conventional chemotherapy which has reached its maximum of tolerance and could no longer be pushed to improved results. The target should be to search for the use of intensive multimodal treatment regimens, including high-dose chemotherapy and next-generation drugs
[2]. Precision medicine with immunotherapy and other molecularly targeted treatments offers unique opportunities to customize treatment intensity
[3]. Their advantages also include reducing the need for hematopoietic stem cell transplantation (HSCT), decreasing the burden of toxicities, and fighting persistent residual disease. Recently approved agents for ALL include blinatumomab, inotuzumab ozogamicin (InO), and CAR T-cell therapy, which are expected to improve treatment outcomes and reduce doses of conventional chemotherapy without compromising the effectiveness of the therapy. Nevertheless, the benefits of aggressive chemotherapy versus target therapy for different patient groups remain unclear and all the strategies cause adverse events, such as neurotoxicity, hepatotoxicity, gastrointestinal complication, and secondary malignancies, making neither of these therapies an ideal treatment
[4].
2. Neurotoxicity of Conventional Therapy
The optimal treatment doses are determined based on tolerability, response assessment, and drug pharmacodynamics and pharmacogenomics. The clinical characteristics of the patient and the biological features of the leukemia are the main factors that determine the choice of specific chemotherapeutics. Treatment protocol consists of phases such as induction, intensification, consolidation, and maintenance
[7][5]. Currently, first-line treatment protocols include a variety of medication combinations which involve the use of VCR, L-ASP, corticosteroids, antimetabolites (cytarabine and MTX), and anthracyclines
[3]. However, the dose intensity of conventional chemotherapy has been pushed to its limits, and because children absorb and metabolize drugs differently than adults, toxicity is a key issue in pediatric chemotherapy. To determine further treatment development, attention must be given to some of the unique neurotoxicities associated with MTX, VCR, L-ASP, and their molecular background (
Table 1).
Table 1. Treatments used in ALL and associated neurotoxicity.
Phase of Treatment |
Drugs |
Toxicity-Related Gene |
Mechanism of Neurotoxicity |
Neurotoxicity |
References |
Induction |
Vincristine |
ABCC11 | | 1 | , | ABCC2 | | 2 | , | ABCC4 | | 3 | , | ABCC5 | | 4 | , | ABCB1 | | 5 | , | ABCC10 | | 6 | , | CEP72 | | 7 | , | SLC5A7 | | 8 | , | TUBB1 | | 9 | , | TUBB2A | | 10 | , | TUBB2B | | 11 | , | TUBB3 | | 12 | , | TUBB4A | | 13 | , | MAP4 | | 14 | , | CYP3A4 | | 15 | , | CYP2C8 | | 16 | , | CYP3A5 | | 17 | , | CEP72 | | 18 |
Interferes with the assembly of microtubule structures leading to cell apoptosis. It affects the peripheral nerves but can also contribute to dysfunction of the cranial nerves and autonomic nervous system. |
Peripheral neuropathy, sensory neuropathy: symmetry sensory/tactile impairment, numbness, and tingling in the hands and feet, paresthesia, decreased balance, tendon weakening, visual and hearing problems. |
[8,9] | [6][7] |
L-asparaginase |
ZBTB1 | | 19 | , | GRIA1 | | 20 | , | HLA-DRB1 | | 21 |
L-asparaginase produces three neurotoxic agents: ammonia, L-aspartic acid, and glutamic acid. These two amino acids can induce cell death in CNS neurons by excessive stimulation through NMDA (N-methyl-D-aspartate) receptor, leading to a major intracellular calcium influx and apoptosis. |
Myelosuppression, encephalopathy, hepatic toxicity. |
[10, | [ | 11, | 8 | 12] | ][9][10] |
Consolidation |
Methotrexate (intravenous infusion and intrathecally) |
DHFR19bp | | 22 | , | MTHFR 677C > T | | 23 | , | MTHFR 677TT | | 24 | , | SLC19A1 | | 25 | , | TYMS | | 26 | , | ADORA2A | | 27 |
Methotrexate is an antimetabolite that inhibits Dihydrofolate Reductase and thus tetrahydrofolate formation. This affects the synthesis of macromolecules such as myelin, and reversible leukoencephalopathy has been suggested to be secondary to impaired myelin turnover. Dihydrofolate Reductase inhibition leads to lack of folate and cobalamin, and increase in homocysteine, which is toxic to vascular endothelium may cause seizures and vascular disease.
Dihydrofolate Reductase inhibition results in decreased levels of S-adenosylmethionine, which in turn plays a role in maintaining the myelin sheath, and this deficiency may lead to demyelination after intrathecal methotrexate administration. |
Transverse myelopathy-symptoms include back pain with subsequent weakness, sensory loss and bladder or bowel incontinence, blurred vision, aphasia, anarthria, seizures, aphasia, mental status disorder, stroke-like episodes, delirium, leukoencephalopathy septic meningitis characterized by headache, neck stiffness, nausea, vomiting and potential fever and encephalopathy. |
[13,14,15] | [11][12][13] |
Cytarabine |
DCK | | 28 | , | NT5C2 | | 29 | , | CDA | | 30 | , | RRM1 | | 31 | , | GIT1 | | 32 | , | NT5C 3 | | 33 | , | ENT1 | | 34 | , | SCL29A1 | | 25 |
Cytarabine exhibits preferential toxicity for CNS | 35 | progenitor cells and oligodendrocytes, compromises cell division in vitro, and causes cell death and reduced cell division in vivo. |
Myelosuppression, neurotoxicity. |
[16,17,18,19] | [14][15][16][17] |
Maintenance |
Methotrexate (orally) |
Genes have been described above. |
Mechanism has been described above. |
Seizures, aphasia, mental status disorder, stroke-like episodes, delirium, leukoencephalopathy, cognitive dysfunction, personality changes. |
[13,14,15,20] | [11][12][13][18] |