# Chronic Lymphocytic Leukemia

## Introduction

Chronic lymphocytic leukemia (CLL) is the most common leukemia in adults in the Western world, characterized by the clonal proliferation and accumulation of mature CD5-positive B lymphocytes in the blood, bone marrow, lymph nodes, and spleen. The incidence is approximately 4.5 per 100,000 per year, with a median age at diagnosis of 72 years and a male-to-female ratio of approximately 2:1. CLL is a remarkably heterogeneous disease: some patients have an indolent course and never require treatment, living decades with stable disease, while others progress rapidly and require early intervention. The treatment paradigm has undergone a fundamental transformation, shifting from chemoimmunotherapy to targeted therapy with BTK inhibitors and BCL2 inhibitors as the cornerstone of management.

## Diagnosis

The diagnosis of CLL requires an absolute B-lymphocyte count of 5,000 per microliter or greater persisting for 3 or more months, as defined by the International Workshop on CLL (iwCLL) criteria. Flow cytometry immunophenotyping reveals a characteristic profile: CD5-positive, CD19-positive, CD20-positive (dim), CD23-positive, CD200-positive, with dim surface immunoglobulin expression. CD10 and FMC7 are characteristically negative. The CLL score (Matutes score) assigns points for CD5 positivity, CD23 positivity, FMC7 negativity, weak surface immunoglobulin, and weak CD79b expression. A score of 4 to 5 is consistent with classic CLL, while a score of 3 or less should prompt consideration of alternative diagnoses such as mantle cell lymphoma or marginal zone lymphoma.

The peripheral blood smear in CLL demonstrates a monotonous population of small, mature-appearing lymphocytes accompanied by characteristic "smudge cells," which are fragile CLL lymphocytes that rupture during smear preparation. Monoclonal B-cell lymphocytosis (MBL), the precursor state to CLL, is defined by the presence of a CLL immunophenotype with fewer than 5,000 B-lymphocytes per microliter and no associated cytopenias or lymphadenopathy. MBL progresses to CLL requiring treatment at a rate of approximately 1 to 2% per year.

## Prognostic Factors

### FISH Cytogenetics (Hierarchical Dohner Model)

The hierarchical FISH model described by Dohner and colleagues stratifies CLL patients into prognostic categories. Deletion of 13q14, when present as the sole abnormality, is the most common finding (approximately 55% of patients) and carries a favorable prognosis with a median overall survival exceeding 10 years. Trisomy 12 is found in 10 to 20% of patients and confers intermediate risk. Deletion of 11q22-23, which involves the ATM gene, is present in 10 to 20% of patients and carries an adverse prognosis. Deletion of 17p13, which removes one copy of the TP53 tumor suppressor gene, is found in 5 to 10% of patients and carries a very adverse prognosis, with resistance to chemoimmunotherapy. Normal FISH is found in 15 to 20% of patients and confers intermediate risk.

### Molecular Markers

IGHV mutation status is one of the most important prognostic markers in CLL. Mutated IGHV (defined as less than 98% homology to the germline sequence) indicates a post-germinal-center origin and carries a favorable prognosis, while unmutated IGHV (98% or greater homology) indicates a pre-germinal-center origin and carries an adverse prognosis. TP53 mutations, present in approximately 10% of patients at diagnosis (in addition to those with del(17p)), confer an adverse prognosis and mandate the use of targeted therapy rather than chemoimmunotherapy. Additional mutations in NOTCH1, SF3B1, BIRC3, and ATM carry intermediate-to-adverse prognostic significance. Complex karyotype (3 or more chromosomal abnormalities) is adverse, particularly in the context of TP53 aberrations.

### Clinical Staging

Two complementary staging systems are used in CLL. The Rai staging system classifies patients from stage 0 (lymphocytosis alone) through stage I (lymphadenopathy), stage II (hepatomegaly or splenomegaly), stage III (anemia with hemoglobin below 11 g/dL), to stage IV (thrombocytopenia with platelet count below 100,000). The Binet staging system classifies patients as stage A (fewer than 3 involved nodal areas), stage B (3 or more involved nodal areas), or stage C (hemoglobin below 10 g/dL or platelets below 100,000).

| Rai Stage | Features | Risk | Binet Stage | Features |
|---|---|---|---|---|
| 0 | Lymphocytosis alone | Low | A | <3 involved nodal areas |
| I | Lymphadenopathy | Intermediate | B | ≥3 involved nodal areas |
| II | Hepato/splenomegaly | Intermediate | C | Hb <10 g/dL or platelets <100,000 |
| III | Anemia (Hb <11 g/dL) | High | -- | -- |
| IV | Thrombocytopenia (plt <100,000) | High | -- | -- |

### CLL-IPI (International Prognostic Index)

The CLL International Prognostic Index integrates multiple prognostic variables into a single scoring system: TP53 status (4 points), IGHV mutation status (2 points), beta-2-microglobulin above 3.5 mg/L (2 points), Rai stage III-IV or Binet stage C (1 point), and age above 65 years (1 point). The resulting risk groups are low (0-1 points), intermediate (2-3), high (4-6), and very high (7-10).

## Indications for Treatment (iwCLL Criteria)

Treatment is indicated for Rai stage III-IV or Binet stage C disease with cytopenias resulting from bone marrow failure, progressive lymphocytosis (lymphocyte doubling time less than 6 months or greater than 50% increase in 2 months), massive, progressive, or symptomatic lymphadenopathy or splenomegaly, autoimmune cytopenias that respond poorly to corticosteroids, or constitutional symptoms including B symptoms and fatigue of sufficient severity to affect functional capacity. A cardinal principle of CLL management is that treatment should never be initiated based on the absolute lymphocyte count alone.

<image>A comprehensive CLL prognostic factor diagram. At the top, show the hierarchical FISH cytogenetics model (Dohner) as a horizontal bar with categories from favorable to adverse: del(13q) sole → normal → trisomy 12 → del(11q) → del(17p), with median overall survival for each category. Below, show a molecular prognostic panel including IGHV mutation status (mutated vs. unmutated), TP53 mutation, and NOTCH1/SF3B1 status with arrows indicating favorable or adverse prognosis. At the bottom, integrate these into the CLL-IPI scoring system showing the point allocation for each variable and the four risk groups with their 5-year overall survival rates. Use color coding from green (favorable) through yellow (intermediate) to red (adverse). Medical education poster format.</image>

## First-Line Treatment

### BTK Inhibitors

Ibrutinib (Imbruvica) was the first-in-class covalent Bruton tyrosine kinase inhibitor approved for CLL. The RESONATE-2 trial demonstrated markedly superior progression-free and overall survival compared to chlorambucil in treatment-naive patients aged 65 and older. The ECOG-E1912 trial showed that ibrutinib plus rituximab was superior to FCR in younger patients in both PFS and OS. Ibrutinib is administered at 420 mg daily on a continuous basis until disease progression or intolerance. The toxicity profile includes atrial fibrillation (10 to 15%), bleeding (due to platelet dysfunction rather than thrombocytopenia), hypertension, arthralgias, diarrhea, and increased infection risk. Ibrutinib should be held for 3 to 7 days before and after invasive surgical procedures.

Acalabrutinib (Calquence) is a second-generation covalent BTK inhibitor with greater kinase selectivity and fewer off-target effects compared to ibrutinib. The ELEVATE-TN trial demonstrated superior PFS for acalabrutinib with or without obinutuzumab compared to chlorambucil-obinutuzumab in treatment-naive patients. The ELEVATE-RR trial compared acalabrutinib directly against ibrutinib in relapsed CLL and demonstrated non-inferior PFS with a significantly lower rate of cardiovascular toxicity, including atrial fibrillation (9.4% versus 16%). Acalabrutinib is dosed at 100 mg twice daily and requires an acidic gastric environment for absorption, necessitating avoidance of proton pump inhibitors.

Zanubrutinib (Brukinsa) is a second-generation covalent BTK inhibitor with the highest selectivity among the approved agents. The ALPINE trial compared zanubrutinib to ibrutinib in relapsed CLL and demonstrated a superior overall response rate and PFS with significantly fewer cardiac adverse events, including atrial fibrillation (2.5% versus 10.1%). Zanubrutinib is dosed at 160 mg twice daily or 320 mg once daily.

### Venetoclax + Obinutuzumab (Fixed-Duration)

The CLL14 trial evaluated a fixed-duration regimen of venetoclax plus obinutuzumab administered for 12 months compared to chlorambucil plus obinutuzumab. The venetoclax-based combination demonstrated markedly superior PFS (76% versus 35% at 5 years) and high rates of undetectable measurable residual disease. The venetoclax ramp-up schedule begins at 20 mg daily and increases weekly over 5 weeks to the target dose of 400 mg daily. Obinutuzumab is administered during cycles 1 through 6, followed by venetoclax monotherapy to complete 12 months of total treatment. Tumor lysis syndrome risk stratification, based on tumor burden (absolute lymphocyte count and lymph node size) and renal function, is critical during the ramp-up period. High-risk patients, defined as those with any lymph node 10 cm or larger, or an absolute lymphocyte count of 25,000 or greater combined with a lymph node 5 cm or larger, should undergo inpatient initiation with continuous monitoring. Venetoclax is the treatment of choice for del(17p) or TP53-mutated CLL regardless of the treatment setting.

### Chemoimmunotherapy (Declining Role)

FCR (fludarabine, cyclophosphamide, and rituximab) was the historical gold standard for young, fit patients based on the GCLLSG CLL8 trial, which demonstrated an overall survival benefit. FCR may still be considered for the specific subset of young, fit patients with IGHV-mutated CLL who lack del(17p) or TP53 mutations, as it is the only CLL regimen with the potential for long-term treatment-free remission, with approximately 50% of IGHV-mutated patients remaining in remission at 10 years. Bendamustine plus rituximab (BR) serves as a less toxic chemoimmunotherapy alternative for older patients. Chlorambucil plus obinutuzumab, while still available as a low-intensity option, has been largely superseded by targeted therapy.

## Relapsed/Refractory CLL

### Treatment Selection

The approach to relapsed CLL depends on the prior therapy received. Patients who received a prior BTK inhibitor should be treated with a venetoclax-based regimen. Those who received prior venetoclax should be treated with a BTK inhibitor. Patients who are refractory to both BTK inhibition and venetoclax (double-refractory CLL) should be considered for pirtobrutinib, clinical trial enrollment, or cellular therapy.

### Pirtobrutinib (Jaypirca)

Pirtobrutinib is a non-covalent (reversible) BTK inhibitor that, unlike covalent BTK inhibitors, retains activity against the C481S BTK resistance mutation that commonly mediates resistance to ibrutinib, acalabrutinib, and zanubrutinib. The BRUIN trial demonstrated an overall response rate of 73% and a median PFS of approximately 18 months in relapsed CLL patients who had previously received a covalent BTK inhibitor. Pirtobrutinib is administered at 200 mg daily and is generally well tolerated. It is FDA-approved for CLL after 2 or more prior therapies including a BTK inhibitor.

### CAR T-Cell Therapy

Lisocabtagene maraleucel (liso-cel, Breyanzi) has received FDA approval for CLL after 2 or more prior lines of therapy based on the TRANSCEND CLL 004 trial, which demonstrated an overall response rate of 45% and a complete remission rate of 18%. CAR T-cell therapy is indicated for double-refractory CLL that has failed both BTK inhibitor and venetoclax-based therapy.

### Allogeneic HSCT

The role of allogeneic HSCT has diminished considerably with the availability of targeted therapies. It may still be considered for young patients who are refractory to all available targeted agents. Reduced-intensity conditioning is preferred, though significant graft-versus-host disease and non-relapse mortality risks remain.

## Richter Transformation

Richter transformation refers to the development of an aggressive lymphoma in the setting of pre-existing CLL, occurring in 2 to 10% of CLL patients. Approximately 80% of cases transform to diffuse large B-cell lymphoma and 15% to a Hodgkin-like lymphoma. Clinical features that should raise suspicion include rapidly enlarging lymphadenopathy, new B symptoms, markedly elevated LDH, and discordant PET avidity with one or more sites demonstrating high SUV uptake disproportionate to the remainder of the disease. Excisional biopsy is essential for diagnosis, as core needle biopsy may miss the transformed component. Prognosis is poor, with a median overall survival of 6 to 12 months with R-CHOP. Clonally related Richter transformation (arising from the CLL clone) carries a worse prognosis than clonally unrelated transformation. Treatment options include clinical trials, checkpoint inhibitors, CAR T-cell therapy, and allogeneic HSCT.

<image>A treatment algorithm for CLL showing first-line and relapsed settings. First-line: Branch by TP53 status. If del(17p)/TP53 mutated: venetoclax + obinutuzumab (fixed-duration 12 months) OR BTKi (continuous). If TP53 wild-type: three options shown as parallel boxes - (1) BTKi continuous therapy (ibrutinib, acalabrutinib, or zanubrutinib), (2) venetoclax + obinutuzumab fixed-duration 12 months, (3) FCR (only for young, fit, IGHV-mutated patients without TP53 aberration). Show treatment selection factors (age, comorbidities, IGHV status, patient preference for fixed vs. continuous therapy). At relapse: sequence based on prior therapy - prior BTKi → venetoclax; prior venetoclax → BTKi; double-refractory → pirtobrutinib, CAR T (liso-cel), or clinical trial. Include a box for Richter transformation with separate management pathway. Clean clinical algorithm with decision diamonds and treatment boxes.</image>

## Key Clinical Pearls
- Never treat CLL based on lymphocyte count alone; iwCLL treatment indications must be met
- Second-generation BTK inhibitors (acalabrutinib, zanubrutinib) have lower atrial fibrillation risk than ibrutinib; prefer in patients with cardiac risk factors
- Venetoclax + obinutuzumab offers fixed-duration treatment (12 months) with deep, durable responses; appealing for patients who prefer time-limited therapy
- TP53-aberrant CLL should NEVER be treated with chemoimmunotherapy; always use targeted therapy (BTKi or venetoclax-based)
- FCR remains the only CLL regimen with potential for cure-like long-term remission, but ONLY in the subset of young, fit, IGHV-mutated patients without TP53 aberrations
- Tumor lysis syndrome with venetoclax requires careful risk stratification and ramp-up; high-risk patients (any lymph node >=10 cm or ALC >=25,000 + lymph node >=5 cm) should be admitted for initial dosing

## References
1. Hallek M, et al. iwCLL guidelines for diagnosis, indications for treatment, response assessment, and supportive management. Blood. 2018;131(25):2745-2760.
2. Fischer K, et al. Venetoclax and obinutuzumab in patients with CLL (CLL14). N Engl J Med. 2019;380(23):2225-2236.
3. Shanafelt TD, et al. Ibrutinib-rituximab or chemoimmunotherapy for chronic lymphocytic leukemia (ECOG-E1912). N Engl J Med. 2019;381(5):432-443.
4. Brown JR, et al. Zanubrutinib or ibrutinib in relapsed or refractory chronic lymphocytic leukemia (ALPINE). N Engl J Med. 2023;388(4):319-332.
5. Mato AR, et al. Pirtobrutinib in relapsed or refractory B-cell malignancies (BRUIN). Lancet. 2021;397(10277):892-901.