Anti-CD52

The Campath series of mAbs was originally produced at the Cambridge University Pathology Department in the 1980s. Alemtuzumab (Campath) is a humanized IgG1 mAb directed against the CD52 antigen. CD52 is a nonmodulating glycoprotein expressed on lymphocytes, monocytes, and macrophages but not on stem cells or bone marrow progenitor cells. As CD52 is expressed by both B and T lymphocytes, alemtuzumab is immunosuppressive. This mAb causes lymphocyte lysis via ADCC and complement fixation and may also induce apoptosis. Alemtuzumab is approved by the FDA for patients with chronic lymphocytic leukemia (CLL) who have been treated with alkylating agents and have failed fludarabine therapy and for patients with relapsing multiple sclerosis. This mAb is used for CTCL off-label. Per package insert, alemtuzumab is administered as an infusion over 2 hours thrice a week in a dose-escalating manner starting at 3 mg, then increasing to 10 mg, and then up to 30 mg for a total of 12 weeks depending on tolerability. Because of marked immunosuppression due to the drug, careful monitoring for cytomegalovirus (CMV) reactivation and appropriate prophylaxis for PCP/herpes simplex virus/varicella zoster virus are recommended. Responses are generally evaluated at the end of the 12 weeks. However, alemtuzumab administration in CTCL differs from that recommended for CLL (see below).

In 2003, a phase 2 study conducted by Lundin and colleagues reported on 22 patients with refractory, advanced, CD52-positive CTCL (7 patients with Sézary syndrome [SS] and 15 with advanced mycosis fungoides [MF]) successfully treated with alemtuzumab with an overall RR of 55% (32% complete response [CR], 23% partial response [PR]). The investigators reported better responses in erythrodermic patients with SS than in those with plaques or skin tumors and clearing of Sézary cells in 6 of 7 patients. After 10 years, this early observation was explained by Clark and colleagues when they determined that in leukemic SS, alemtuzumab depleted recirculating benign and malignant central memory T cells in blood and skin of patients with SS, but did not affect a diverse population of sessile skin-resident effector memory T cells found in MF. Low-dose alemtuzumab (10 mg) was also associated with lack of infections in alemtuzumab-treated patients with SS despite the complete absence of T cells in the blood, suggesting that sessile skin-resident effector memory T cells can protect the skin from pathogens even in the absence of T-cell recruitment from the circulation.

Because high-dose alemtuzumab is associated with profound immunosuppression and infectious complications including bacterial sepsis and CMV reactivation in two-thirds of patients treated with alemtuzumab, alternative administration routes and dosages were explored. The efficacy of intermittent low-dose subcutaneous (SQ) alemtuzumab was tested in 14 patients with SS (11 with relapse and 3 untreated). Most patients in this study received a reduced dose of 3 mg on day 1 and then 10 mg on alternating days SQ. Overall, 12 (85.7%) of the 14 patients achieved a clinical response, with 3 CRs (21.4%) at a median follow-up of 16 months with time to treatment failure of 12 months. Importantly, there was no hematologic toxicity or infection observed at the 10-mg dose level, whereas almost a third of the patients receiving 15 mg or more experienced infectious complications. The study showed that low-dose intermittent SQ alemtuzumab therapy resulted in durable clinical responses and reduced risk of infections.

To summarize, alemtuzumab is successfully used for symptomatic relief and palliation in patients with SS and MF. Alemtuzumab is safe and effective even when used in low doses SQ in short courses; it is considered to be safe even in patients with a very poor performance status and in the very elderly. All patients treated with alemtuzumab inevitably show relapse of the condition. However, re-treatment with the same regimen is acceptable and may result in clinical responses.

Anti-CD30

CD30 (Ki-1 antigen) is a cell surface leukocyte activation transmembrane protein of 120 kDa belonging to the tumor necrosis factor receptor superfamily that is expressed on activated B and T lymphocytes and on malignant hematopoietic cells including Hodgkin lymphoma (HL), anaplastic large cell lymphoma (ALCL), primary cutaneous ALCL (PCALCL), lymphomatoid papulosis (LyP), and MF with large cell transformation (LCT) and can also be expressed at low levels in nontransformed MF. In fact, it may carry a prognostic significance in nontransformed disease even when observed in low levels. Consequently, CD30 represents an attractive therapeutic target in these pathologic entities. Numerous unsuccessful clinical trials targeting CD30 were attempted until the recently developed ADC brentuximab vedotin (CD30-monomethyl auristatin E [MMAE], SGN-35) was shown to be safe and effective in lymphoid malignancies expressing CD30.

Historically, CD30 antigen was evaluated as a therapeutic target in various CD30-expressing malignancies, including HL, systemic ALCL and PCALCL, as well as MF. Initial clinical trials using the chimeric unconjugated anti-CD30 antibody SGN-30 and the fully human anti-CD30 antibody MDX-060 conducted in HL did not reveal satisfactory clinical responses. The second-generation anti-CD30 antibodies, the defucosylated fully human MDX-1401 and XmAb2513, were evaluated in clinical trials, but responses were also limited. In the 1990s, attempts to increase the cytotoxic potential of the CD30-targeting antibody led to development of the bispecific mouse antibodies targeting CD30 and CD16 with the goal of recruiting natural killer (NK) cells. There was 1 CR in a patient with heavily pretreated HL in this clinical trial, but significant immunogenicity was observed, and although the molecule was abandoned, the approach was considered to be promising. At present, AFM13, a tetravalent bispecific (antihuman CD30 and antihuman CD16A) recombinant antibody construct is being investigated for the treatment of HL; the results of this clinical trial are pending.

In CTCL, the efficacy of naked chimeric mAb brentuximab (anti-CD30) was tested in patients with PCALCL, LyP, MF with LCT, as well as multiple simultaneous subtypes of CTCL. The overall RR was 70% (16 of 23 patients) with 10 patients achieving a CR and another 6 patients achieving a PR; 9 of the 10 patients who achieved a CR and 5 of the 6 patients who achieved a PR were in remission at their follow-up evaluation (median duration, 84 days). The adverse events during the study were mild or moderate.

To increase antitumor activity, the chimeric mAb brentuximab (anti-CD30) was conjugated with the antitubulin agent MMAE (or vedotin). After binding CD30, the conjugate is rapidly internalized inside the cell by endocytosis. Once inside the lysosome, cathepsin B and other proteolytic enzymes cleave MMAE off the conjugate. Once released, it binds to tubulin and causes cell cycle arrest. In 2003, the successful use of CD30-MMAE ADC named brentuximab vedotin was first reported in cell lines and mouse models leading to the launch of phase 1 clinical trials in HL and ALCL. After unprecedented results in phase 1 clinical trials with overall responses of nearly 50% and reduction in tumor size in most patients, several phase 2 clinical trials were launched. The overall RR in patients with heavily pretreated HL and ALCL ranged from 75% to 86% with CRs of 34% to 53%, leading to accelerated FDA approval of this drug for the treatment of relapsed and refractory HL and ALCL. The recommended dose was 1.8 mg/kg every 3 weeks for up to 16 cycles. In CTCL, brentuximab vedotin also demonstrated significant activity. However, the expression of CD30 in MF is highly variable. Phase 2 clinical trials of patients with MF and SS with various degrees of CD30 ⁺ expression on skin biopsies is ongoing.

Adverse events of brentuximab vedotin are usually manageable, but include potentially serious neutropenia and hyperkalemia. Fatigue, nausea, anemia, upper respiratory tract infection, diarrhea, fever, rash, thrombocytopenia, cough, and vomiting have also been reported. Progressive, and often irreversible peripheral neuropathy with intense pain and hypersensitivity to cold, beginning in the hands and feet and sometimes involving the arms and legs, is cumulative and constitutes an important clinical consideration because it may limit prolonged administration of the drug. The mechanism of this toxicity is not entirely clear, but diffusion of MMAE in the tumor microenvironment and cytotoxicity of bystander cells may in part explain its activity. Progressive multifocal leukoencephalopathy associated with brentuximab vedotin therapy, which led to death, has been reported and resulted in the FDA issuing a black box warning. Owing to potentially irreversible and serious side effects when the drug is used for long term, its use probably should be limited to the current approved duration, which can be broken down to several shorter courses with periods of drug holiday. The loss of CD30 expression after brentuximab vedotin has not been reported.

Anti-CD25

CD25 is the α-subunit of interleukin-2 receptor (IL-2R). CD25 is a transmembrane protein present on activated B and T cells and NK cells; it is also a marker for CD4 ⁺ FoxP3 ⁺ regulatory T cells (Tregs) and is constitutively expressed on a small proportion of resting memory T cells and a significant proportion of malignant CD4 ⁺ cells in CTCL, most B-cell neoplasms, and some leukemias. Thus, CD25 was historically considered to be an attractive target for targeting CTCL cells. The levels of the soluble form of CD25, sIL-2R, may be elevated in these diseases and is occasionally used to track disease progression and prognosis.

IL-2R is composed of 3 subunits: α (CD25, p55), β (CD122, p75), and γ (CD132, p64). The β and γ (CD122, CD132) subunits are essential for the molecule targeting CD25 to be effective because only interleukin (IL)-2 binding to the intermediate- and high-affinity receptors (IL-2R-β/γ, CD122, CD132 or IL-2R-α/β/γ, CD25, CD122, and CD133) results in signal transduction. Denileukin diftitox (DD, DAB389IL-2, Ontak) is a fusion protein in which the receptor-binding domain of DT has been replaced by the IL-2 molecule, making it capable of binding the IL-2R. After DD binds to the high-affinity IL-2 receptor, it is internalized by receptor-mediated endocytosis, the molecule is proteolytically cleaved within the lysosomal compartment, and the DT portion translocates into the cytoplasm where it inhibits messenger RNA prolongation and ultimately, protein synthesis.

Initial phase 1 studies using original DD molecules indicated minimal activity in CTCL. In these early studies, the heterogeneity of CD25 expression was extremely variable and ranged from 40% to less than 5% in the analyzed samples; they also demonstrated generation of neutralizing antibodies. The presence of antitoxin antibodies did not preclude response to treatment, and the presence of CD25 did not differentiate between responders and nonresponders. Of note, all clinical studies using DD did not use rigorous end point criteria, which are now standard for the assessment of patients with MF and SS.

The original DAB486-IL-2 molecule was later modified (in-frame deletion of 97 amino acids from the receptor-binding domain) to produce a fusion protein with a 5-fold greater affinity for target cells, resulting in an overall increase in its half-life and a 10-fold increase in potency. This new DD showed improved RRs in the mid-30s percentages. There was no correlation between the expression of the CD25 or CD122 subunits and a clinical response, but expression of CD132, which is necessary for a response, was not assayed.

The pivotal phase 3 trial, a randomized, blinded, multicenter study for stage IB to IVA MF/SS, required greater than 20% of lymphocytes within the skin biopsy to stain positive for CD25. A total of 71 patients were randomly assigned to receive either a 9 or 18 μg/kg/d dose intravenously (IV) for 5 consecutive days; treatment was repeated every 21 days for up to 8 cycles. There was further randomization of the population into those with disease of stage IIA or less and IIB or more (68% with stages > IIB). The overall RR was about 30% with 10% of patients exhibiting a CR. The overall RR between the 2 dosage groups did not reach statistical difference; however, there was a trend to superiority in efficacy of the higher dose in patients with greater than stage IIB disease. The median duration of response for those with an objective result was almost 7 months (from 2.7 to >46 months). Approximately 68% of patients had a significant clinical improvement of their pruritus. Based on these results, the FDA approved the use of DD in patients with CTCL relapse and positive test result for CD25.

Adverse effects (AEs) include hypersensitivity reactions, mild to moderate flulike symptoms, fever, chills, asthenia, arthralgia, headache, myalgia, gastrointestinal symptoms, rash, transient lymphopenia, and infections. Because premedication with steroids and antihistamines were not permitted with this clinical trial, the observed toxicities were more severe than those experienced by patients receiving effective premedication in clinical practice. A capillary leak syndrome (defined as edema, hypoalbuminemia, and hypotension) occurred in up to 25% of the patients and was usually seen in the first 14 days. Premedication with dexamethasone, diphenhydramine, and acetaminophen is recommended, and adequate saline hydration should continue because this has also shown to diminish the incidence of vascular leak syndrome. Since FDA approval, there have been case reports of other AEs, including thyrotoxicosis, retinopathy, and vision loss. To improve DD efficacy, combination therapies were investigated in small clinical trials, including with HDAC inhibitors, bexarotene (Targretin), and systemic steroids. The combination studies all demonstrated improved RRs, but it has not been confirmed in larger clinical trials.

Recently, several meta-analyses have been performed of all combined DD phase 3 clinical trials analyzing the efficacy, response duration, and safety of DD in CTCL. The data indicate that DD showed a significant overall RR, progression-free survival (PFS), and failure of progression of disease when compared with placebo. The significant toxicity observed in clinical trials can be reduced using a premedication regimen and resolved to placebo levels after the second or third course of treatment. Recently, as a part of a postmarketing commitment, the pharmaceutical company Eisai has conducted a clinical trial to assess the efficacy and safety of E7777 (improved purity DD) in patients with persistent and recurrent CTCL: the results of this clinical trial are pending.