Residency · Residency · Hematology Thrombosis

Acute Lymphoblastic Leukemia

Introduction

Acute lymphoblastic leukemia (ALL) is a malignant neoplasm arising from the clonal proliferation of lymphoid precursors (lymphoblasts) that accumulate in the bone marrow, blood, and extramedullary sites. The disease demonstrates a bimodal age distribution, with the first and most prominent peak occurring in children aged 2 to 5 years and a second peak in adults over 60 years. The treatment outcomes differ dramatically between these age groups: pediatric ALL is one of the great success stories of modern oncology, with cure rates exceeding 90%, while adult ALL achieves cure rates of only 40 to 50% with current therapy. B-cell ALL accounts for 75 to 80% of cases and T-cell ALL for 20 to 25%, and these lineages have distinct biological features, genetic landscapes, and treatment considerations.

Classification

WHO 5th Edition / ICC 2022

The modern classification of ALL recognizes an expanding number of genetically defined subtypes. B-ALL with t(9;22)/BCR-ABL1, designated Philadelphia-positive (Ph+) ALL, is found in approximately 25% of adult ALL but only 3% of pediatric cases. Historically carrying an adverse prognosis, outcomes have improved dramatically with the incorporation of tyrosine kinase inhibitors. KMT2A (MLL)-rearranged ALL is associated with an adverse prognosis and is particularly common in infant ALL. The t(12;21)/ETV6-RUNX1 translocation is the most common genetic abnormality in pediatric ALL, occurring in approximately 25% of childhood cases, and carries an excellent prognosis. Hyperdiploidy (51 to 65 chromosomes) is another favorable-risk pediatric subtype, also present in approximately 25% of cases. Hypodiploidy (fewer than 44 chromosomes) carries an adverse prognosis. The t(1;19)/TCF3-PBX1 translocation is associated with intermediate risk.

Intrachromosomal amplification of chromosome 21 (iAMP21) is an adverse-risk entity that requires intensive therapy. BCR-ABL1-like (Ph-like) ALL is a recently recognized subtype that shares a gene expression profile with Ph+ ALL but lacks the BCR-ABL1 fusion. It encompasses ABL-class fusions, JAK-STAT pathway alterations, and CRLF2 rearrangements, carries an adverse prognosis, and accounts for approximately 15% of all ALL cases. DUX4-rearranged ALL carries a favorable prognosis even when accompanied by ERG deletion. Additional emerging entities include MEF2D-rearranged, ZNF384-rearranged, and ETV6-RUNX1-like ALL.

T-ALL is a distinct lineage with its own biology. Early T-cell precursor ALL (ETP-ALL) is a subset with myeloid-like immunophenotypic features and was historically associated with a poor prognosis, though outcomes have improved significantly with intensive chemotherapy regimens.

Diagnostic Workup

The diagnostic evaluation of ALL begins with bone marrow aspirate and biopsy, which reveals 20% or more lymphoblasts (or 25% or more by some pediatric protocols). Morphologically, blasts are classified as L1/L2 (small to medium-sized cells with scant cytoplasm) or L3 (Burkitt-like, with prominent cytoplasmic vacuolation), though this morphologic classification has limited prognostic significance in the modern era.

Flow cytometry immunophenotyping is essential for lineage assignment and subclassification. B-ALL blasts are characteristically positive for CD19, CD10 (common ALL antigen, cALLA), CD22, CD79a, TdT (terminal deoxynucleotidyl transferase), and CD34. T-ALL blasts express cytoplasmic CD3, CD7, CD2, and TdT, with CD1a positivity in cortical T-ALL and variable CD4/CD8 expression (double positive or double negative).

Cytogenetic analysis includes conventional karyotyping and a FISH panel targeting BCR-ABL1, KMT2A rearrangements, ETV6-RUNX1, and iAMP21. Molecular studies include RT-PCR for BCR-ABL1 fusion transcript and next-generation sequencing panels to identify Ph-like gene expression profiles and IKZF1 deletions (which carry adverse prognostic significance). Cerebrospinal fluid analysis by cytology and flow cytometry for CNS staging is performed at diagnosis in conjunction with the first dose of intrathecal chemotherapy. CNS status is classified as CNS-1 (no blasts in CSF), CNS-2 (fewer than 5 white blood cells per microliter with identifiable blasts), or CNS-3 (5 or more white blood cells per microliter with blasts, or cranial nerve palsy).

<image>A diagnostic and immunophenotypic classification diagram for ALL. Show two main branches: B-ALL and T-ALL. For B-ALL, display the maturation stages (pro-B, common/pre-B, mature B) with their characteristic immunophenotypic markers shown as a horizontal bar chart (CD19, CD10, CD20, CD22, cytoplasmic μ, surface Ig, TdT, CD34). For T-ALL, show maturation stages (pro-T, pre-T, cortical T, medullary T) with markers (CD7, cCD3, CD2, CD1a, CD4, CD8, sCD3, TdT). On the right side, list the key genetic abnormalities for each lineage with their prognostic impact (green arrow for favorable: hyperdiploidy, ETV6-RUNX1; red arrow for adverse: BCR-ABL1, KMT2A, hypodiploidy, Ph-like). Include the frequency of each genetic subtype in adult vs. pediatric ALL. Medical education diagram style.</image>

Treatment - Adult B-ALL

Philadelphia-Negative ALL

Adolescent and Young Adult (AYA) - Pediatric-Inspired Regimens

A paradigm shift in the treatment of adolescent and young adult ALL has been the recognition that patients aged 15 to 39 years achieve superior outcomes when treated with pediatric-inspired regimens rather than traditional adult protocols. Protocols such as CALGB 10403 and those modeled on AALL0434 incorporate the key elements of pediatric therapy: induction, consolidation, interim maintenance, delayed intensification, and prolonged maintenance. The critical chemotherapeutic components include high-dose methotrexate, asparaginase (PEG-asparaginase at 2,500 IU/m2 or the longer-acting calaspargase pegol), vincristine, dexamethasone, and CNS-directed therapy. Asparaginase is a cornerstone of ALL therapy that depletes circulating asparagine, an amino acid upon which lymphoblasts are uniquely dependent. Its toxicity profile includes pancreatitis, hepatotoxicity with coagulopathy, venous thrombosis, and hypersensitivity reactions, all of which require vigilant monitoring.

Older Adults (>40 years)

Patients over 40 years of age generally cannot tolerate the full intensity of pediatric-inspired regimens. The Hyper-CVAD protocol (cyclophosphamide, vincristine, doxorubicin, and dexamethasone alternating with high-dose methotrexate and cytarabine) has been widely used in the United States for this population. More recently, Mini-Hyper-CVD combined with inotuzumab ozogamicin with or without blinatumomab has shown promising results in older patients in studies from MD Anderson. Venetoclax-based combinations are also being explored as emerging treatment options.

Philadelphia-Positive ALL

The treatment of Ph+ ALL has been transformed by the incorporation of tyrosine kinase inhibitors. First-generation TKI therapy with imatinib at 600 to 800 mg daily combined with chemotherapy (as in the GRAAPH-2005 and UKALL14 trials) improved outcomes substantially compared to the pre-TKI era. Second-generation TKIs, particularly dasatinib at 140 mg daily, offer the additional advantage of superior CNS penetration. Third-generation TKI therapy with ponatinib has been evaluated in the PhALLCON trial, which demonstrated that ponatinib at 30 to 45 mg daily combined with reduced-intensity chemotherapy was superior to imatinib-based regimens in achieving MRD-negative complete remission and event-free survival.

Perhaps the most paradigm-changing development in Ph+ ALL is the emergence of chemotherapy-free approaches. The GIMEMA D-ALBA protocol combines dasatinib with dexamethasone and blinatumomab without conventional cytotoxic chemotherapy, and has achieved remarkable results: 98% MRD-negative complete remission and 2-year overall survival of 87%. This approach may ultimately eliminate the need for intensive chemotherapy and transplantation in many Ph+ ALL patients. Allogeneic HSCT in first complete remission, which was historically standard for all Ph+ ALL patients, now has a diminishing role as TKI-immunotherapy combinations achieve deeper and more durable remissions.

Ph-Like ALL

Ph-like ALL is a biologically heterogeneous subtype that shares the gene expression profile of Ph+ ALL but lacks the BCR-ABL1 fusion. The underlying genetic alterations include ABL-class fusions (which may respond to TKI therapy with dasatinib or ponatinib), JAK-STAT pathway alterations (which may respond to ruxolitinib), and CRLF2 rearrangements. Accurate identification requires gene expression profiling, and the treatment approach involves adding the appropriate targeted agent to backbone chemotherapy.

Immunotherapy in ALL

Blinatumomab (Bispecific T-Cell Engager, BiTE)

Blinatumomab is a bispecific T-cell engager antibody that simultaneously binds CD19 on leukemic blasts and CD3 on T cells, redirecting cytotoxic T lymphocytes to destroy CD19-positive target cells. In the relapsed/refractory setting, the TOWER trial demonstrated a median overall survival of 7.7 months with blinatumomab compared to 4.0 months with standard chemotherapy. In the MRD setting, the BLAST trial showed a 78% MRD conversion rate with improved relapse-free survival. The practice-changing E1910 trial evaluated blinatumomab added to consolidation therapy in newly diagnosed Ph-negative B-ALL and demonstrated 3-year overall survival of 89.4% compared to 72.8% with standard consolidation alone. This result has established blinatumomab as a standard component of first-line consolidation therapy.

Blinatumomab is administered as a continuous intravenous infusion at 9 micrograms per day during week 1, escalating to 28 micrograms per day for the remainder of the 28-day treatment cycle. The step-up dosing strategy mitigates the risk of the principal toxicities, which include cytokine release syndrome and neurotoxicity (manifesting as tremor, seizures, or encephalopathy).

Inotuzumab Ozogamicin (CD22 ADC)

Inotuzumab ozogamicin is an antibody-drug conjugate consisting of an anti-CD22 antibody linked to calicheamicin, a potent cytotoxic agent. The INO-VATE trial in relapsed/refractory B-ALL demonstrated a composite CR/CRi rate of 80.7% compared to 29.4% with standard chemotherapy, and a median overall survival of 7.7 months compared to 6.7 months. Dosing consists of 0.8 mg/m2 on day 1 and 0.5 mg/m2 on days 8 and 15 of the first cycle, followed by 0.5 mg/m2 on days 1, 8, and 15 of subsequent cycles. The most clinically significant toxicity is sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD), particularly in patients who proceed to HSCT after inotuzumab exposure. To mitigate this risk, inotuzumab should be limited to 2 cycles when transplant is planned.

CAR T-Cell Therapy

Chimeric antigen receptor (CAR) T-cell therapy has established a transformative role in relapsed/refractory ALL. Tisagenlecleucel (Kymriah), an anti-CD19 CAR T-cell product, is FDA-approved for pediatric and young adult relapsed/refractory B-ALL based on the ELIANA trial, which demonstrated a complete remission rate of 82% and 12-month event-free survival of 50%. Brexucabtagene autoleucel (Tecartus), another anti-CD19 CAR T-cell product, is FDA-approved for adult relapsed/refractory B-ALL based on the ZUMA-3 trial, which achieved a CR/CRi rate of 71% and median overall survival of 25.4 months in responding patients.

CRS management follows established protocols, with tocilizumab (an anti-IL-6 receptor antibody) used for grade 2 or higher CRS and dexamethasone for immune effector cell-associated neurotoxicity syndrome (ICANS). CD19 antigen loss is a significant mechanism of relapse, occurring in approximately 30% of patients who relapse after anti-CD19 CAR T therapy. Bispecific and dual-targeting CAR constructs are under development to address this limitation.

Measurable Residual Disease (MRD)

MRD is the single strongest independent prognostic factor in ALL, surpassing all other clinical and genetic variables in its predictive power. Assessment methods include multiparameter flow cytometry (sensitivity of 10^-4), RT-qPCR for fusion gene transcripts (sensitivity of 10^-4 to 10^-5), and next-generation sequencing of immunoglobulin heavy chain or T-cell receptor gene rearrangements using the clonoSEQ platform (sensitivity of 10^-6). MRD is assessed at multiple time points including end of induction, end of consolidation, and before and after HSCT. MRD positivity after induction should prompt consideration of therapy intensification, addition of blinatumomab, or early transplant referral. MRD negativity in favorable-risk patients may allow avoidance of transplant in first complete remission.

<image>A treatment pathway diagram for adult B-ALL showing the integration of immunotherapy into the treatment algorithm. Start with "Newly Diagnosed Adult B-ALL" and branch into Ph-negative and Ph-positive. For Ph-negative: Induction (pediatric-inspired for AYA, Hyper-CVAD for older) → MRD assessment → consolidation with blinatumomab (E1910 trial result highlighted) → MRD-guided decision for HSCT vs. maintenance. For Ph-positive: TKI-based induction (dasatinib or ponatinib + steroids ± blinatumomab as in D-ALBA trial) → MRD assessment → HSCT if MRD+ or continue TKI + immunotherapy if MRD-. Show a separate relapsed/refractory pathway: blinatumomab, inotuzumab ozogamicin, or CAR T-cell therapy (tisagenlecleucel/brexucabtagene), with response rates for each. Include MRD assessment points as checkpoints throughout the pathway. Modern clinical pathway style with trial names annotated.</image>

CNS Prophylaxis and Treatment

Without adequate CNS-directed therapy, the risk of CNS relapse in ALL ranges from 30 to 50%. Intrathecal chemotherapy is the backbone of CNS prophylaxis and consists of methotrexate at 12 mg (or triple intrathecal therapy with methotrexate, cytarabine, and hydrocortisone) administered with each chemotherapy cycle. The total number of intrathecal doses varies by protocol but typically ranges from 12 to 16 over the course of treatment. High-dose systemic methotrexate at doses exceeding 1 g/m2 achieves therapeutic concentrations in the cerebrospinal fluid and provides additional CNS coverage. For patients with CNS-3 disease, more intensive intrathecal therapy is administered, and cranial radiation (18 to 24 Gy) may be incorporated in some protocols, though the role of radiation is diminishing with the availability of effective systemic and intrathecal agents.

Maintenance Therapy

Maintenance therapy is a distinctive and essential component of ALL treatment that has no parallel in AML management. The standard regimen consists of daily oral mercaptopurine at 75 mg/m2 and weekly oral methotrexate at 20 mg/m2, continued for 2 to 3 years. Pharmacogenomic testing for TPMT and NUDT15 polymorphisms is mandatory before initiating maintenance therapy. Patients who are homozygous for loss-of-function variants in either gene require dramatic dose reductions (up to 10-fold for TPMT) to prevent fatal myelosuppression. NUDT15 polymorphisms are of particular importance in East Asian populations, where they are more prevalent. For Ph+ ALL, the tyrosine kinase inhibitor is continued throughout the maintenance phase, often indefinitely.

Key Clinical Pearls

• Blinatumomab added to consolidation is now standard in newly diagnosed Ph-negative B-ALL based on E1910 (OS benefit confirmed)
• The D-ALBA protocol (dasatinib + blinatumomab without conventional chemotherapy) is a paradigm shift for Ph+ ALL and may eliminate the need for transplant in many patients
• TPMT and NUDT15 genotyping MUST be performed before starting maintenance therapy to prevent fatal myelosuppression
• Inotuzumab ozogamicin is highly effective in relapsed B-ALL but limit to 2 cycles if transplant is planned (SOS/VOD risk)
• MRD negativity at end of induction is the strongest predictor of long-term survival in ALL
• Pediatric-inspired regimens for AYA patients (up to age 39-40) are superior to adult protocols; asparaginase is a critical but toxic component

References

1. Faderl S, et al. Adult acute lymphoblastic leukemia: concepts and strategies. Cancer. 2010;116(5):1165-1176.
2. Kantarjian H, et al. Blinatumomab versus chemotherapy for advanced acute lymphoblastic leukemia (TOWER). N Engl J Med. 2017;376(9):836-847.
3. Litzow MR, et al. Blinatumomab for MRD-negative B-ALL in first remission (E1910). N Engl J Med. 2024;391(8):e18.
4. Foa R, et al. Dasatinib-blinatumomab for Ph-positive acute lymphoblastic leukemia in adults (GIMEMA D-ALBA). N Engl J Med. 2024;390(6):497-508.
5. Kantarjian HM, et al. Inotuzumab ozogamicin versus standard therapy for acute lymphoblastic leukemia (INO-VATE). N Engl J Med. 2016;375(8):740-753.

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