BIOLOGIC RESPONSE MODIFIERS IN PEDRIATIC CANCER

PEDIATRIC ONCOLOGY IN THE 21st CENTURY, PART I

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BIOLOGIC RESPONSE MODIFIERS IN PEDIATRIC CANCER Laura L. Worth, MD, PhD, Sima S. Jeha, MD, and Eugenie S. Kleinerman, MD

Many advances have been made in the development of chemotherapeutic agents, surgical approaches, and radiotherapy for cancer. Despite these advances, several tumors rapidly become resistant to chemotherapy, other tumors grow too close to critical structures to allow their complete surgical removal, and the administration of radiotherapy can be limited by the number of tumor sites involved or the tolerance of the normal tissues in the radiation field. Consequently, new therapeutic approaches are being explored, including the use of biologic response modifiers. The goal of biologic response modifiers is to stimulate the body’s own immune system to help eradicate tumor cells. This stimulation is important because many tumor cells have low or absent major histocompatibility complex (MHC) class I or class I1 molecules, which allows them to escape from immune s~rveillance.~, 60, 88 The use of immunotherapy in the management of cancer began in 1884,12 with the observation of a ”spontaneous regression” of a tumor at the site of an infection. A round cell sarcoma was noted to recur repeatedly after every ”complete” surgical resection. After one surgical excision, an erysipelas infection at the surgical resection site occurred, and the tumor did not recur. This infection was thought to have stimulated the body’s immune system to

Supported in part by CA-42992 (ESK), CA82606 (ESK), and CA16672 Cancer Center Support Core Grant (to The University of Texas M. D. Anderson Cancer Center).

From the Departments of Pediatrics (LLW, SSJ, ESK) and Cancer Biology (ESK), The University of Texas M. D. Anderson Cancer Center, Houston, Texas

HEMATOLOGY/ONCOLOGY CLINICS OF NORTH AMERICA

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help eradicate the residual disease. Based on this observation, Cooley12 directly inoculated an infectious agent to a tumor site in hopes of stimulating an immune response and eradicating the tumor. Immunotherapy has evolved considerably since these early days. The use of socalled biologic response modifiers has become an attractive alternative. Some biologic response modifiers, such as cytokines, are considered natural because the human body produces them. Many biologic response modifiers are less toxic than standard chemotherapeutic agents and result in less damage to the kidneys and liver. Because of their growing importance in the treatment of cancer, biologic response modifiers are now considered the fourth treatment modality, after chemotherapy, radiotherapy, and surgery. This article is divided into three main sections. The first section addresses some of the unique aspects of developing and assessing the effects of biologic response modifiers for clinical trials. The second section looks at biologic response modifiers used in pediatric solid tumors. The final section describes how biologic response modifiers are being used in the treatment of pediatric leukemias and lymphomas. DEVELOPMENT OF BIOLOGIC RESPONSE MODIFIERS

The development of biologic response modifiers differs from that of standard cytotoxic agents. The standard phase I trials used to assess the value of novel chemotherapeutic agents are intended to establish the dose-limiting toxicities of the new agent. Dose-limiting toxicities include bone marrow suppression; breakdown of mucosal membranes; and renal, pulmonary, cardiac, and hepatic toxicities. Because the optimal therapeutic dose for biologic response modifiers is well below the maximally tolerated dose (these agents have a broad dose-response curve), the use of classic phase I trials is not suitable for these compounds. Rather, phase I trials must be designed to measure the specific drug-induced biologic responses created by biologic response modifiers so that the optimal dose for further clinical trials can be determined. Phase I1 or efficacy trials of biologic response modifiers pose special considerations. Many patients who are enrolled on phase I or I1 studies have recurrent or progressive disease that is no longer responsive to chemotherapy. Frequently, these patients have large, bulky disease. Although interleukin (1L)-2 has shown efficacy in advanced renal cell carcinoma and melanoma with bulky disease,I5 many biologic response modifiers are most effective against minimal disease. A biologic response modifier that may be effective against a specific tumor type in the setting of minimal residual disease could end up being discarded in a classic phase I1 trial because it appears to be without activity in the setting of large bulky disease. Biologic response modifiers have mechanisms of action that differ from that of standard chemotherapeutic agents. Frequently, these compounds may take longer to induce an apparent effect. This scenario is illustrated best by the treatment of hemangiomas with

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interferon (IFN)-~L.~~* loo IFN-a acts as an antiangiogenesis factor preventing new vasculature formation. It has little effect on established vasculature. As a result, months of therapy are required before a response is seen. This agent might have been discarded in the classic phase I1 setting if no significant measurable response was noted after only a few cycles of therapy. The definition of a response can differ depending on the intended effect of the biologic response modifier. Some biologic response modifiers are cytostatic, and stable disease, not tumor regression, is the expected outcome. Other biologic response modifiers stimulate an immune response by recruiting lymphocytes and macrophages to the disease site. Such an effect might result in an increased lesion on imaging studies, rather than a decreased one. Consequently, a pulmonary metastasis on standard chest radiography may appear larger rather than smaller after the administration of a biologic response modifier because of an inflammatory response associated with its administration. This change may be mistaken for progressive disease. Additional studies or histologic examination may be needed to show that the enlargement of the mass is not disease progression but rather a fluid-filled cyst or mass devoid of active tumor with an active inflammatory component.10' Many adult tumors respond to the administration of biologic response modifiers. IFN-a alone or in combination with chemotherapeutic agents has been found to be effective in hairy cell leukemia,s3 melan0ma,4~,50, 68 renal cell carcinoma," chronic myeloid leukemia (CML)?2 and multiple myeloma.70,72 IL-2 is an effective agent against renal cell carcinoma and melanoma. Despite documented stimulation of the immune system, however, IL-2 produces no antitumor effect in pediatric sarcomas or neur~blastoma.~ The identification and investigation of agents active specifically in pediatric tumors is a rapidly growing field. BIOLOGIC RESPONSE MODIFIERS IN PEDIATRIC SOLID TUMORS

Osteosarcoma

Osteosarcoma is a primary bone tumor that usually presents in adolescents and young adults when the long bones are undergoing rapid growth. Osteosarcoma is an active area of research for biologic response modifiers. Most patients with osteosarcoma have micrometastases at the time of diagnosis.19 Despite surgical resection and aggressive adjuvant chemotherapy, 30% to 40% of patients develop pulmonary metastases. Many of these patients relapse during the first year, usually while still receiving chemotherapy. This finding implies that the metastases are inherently drug-resistant or that the concentration of drug in the lungs is insufficient. Salvage chemotherapeutic regimens have had limited efficacy.41Some of the biologic response modifiers currently being investigated in osteosarcoma are described subsequently (Table 1).

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Table 1. BIOLOGIC RESPONSE MODIFIERS IN PEDIATRIC SOLID TUMORS Agent

L-MTP-PE ImmTher IL-la IL-2 IL-11 IL-12 IFN-?/ 13-cis-retinoic acid Epogen G-CSF GM-CSF

Proposed Mechanism

Macrophage/ monocyte activator Activation of cytotoxic T lymphocytes Macrophage/monocyte activator Activation of CTL Antiangiogenesis (?) Induction of IL-2, IL-6, colonystimulating factor, tumor necrosis factor, activation of CTLs Stimulate CTL, stimulate the activation and proliferation of NK cells Stimulate platelet production Stimulate IFN-y production, activate NK cell cytotoxicity, activation CTLs Activation of macrophages, NK cells, and CTLs Increase MHC I antigens Differentiation agent Stimulate red blood cell production Stimulate granulocyte progenitor cells Stimulate granulocyte and myeloid progenitor cells

Pediatric Tumor TvDe

Osteosarcoma" Ewing's sarcoma* Osteosarcomat Melanomat Osteosarcoma*

NA Osteosarcomat

Neuroblastoma" NA NA NA

*Clinical trial. tl'reclinical trial. CTL = Cytotoxic T lymphocytes; MHC = major histocompatibility complex; NK = natural killer; G-CSF = granulocyte colony-stimulating factor; GM-CSF = granulocyte-macrophage colonystimulating factor; NA = not appliciable.

Liposome-Encapsulated Muramyl Tripeptide Phosphatidylethanolamine

Muramyl tripeptide phosphatidylethanolamine (MTP-PE) is a derivative of muramyl dipeptide (MDP), the minimal subunit of mycobacteria that is able to activate the immune MTP-PE is lipophilic and can be encapsulated into liposomes (L-MTP-PE).The antitumor effect of MTP-PE is linked to its ability to activate macrophage or monocyte tumoricidal function.28The maximal tolerated dose (MTD) in a standard phase I study was determined to be 6.0 mg/m2, with toxicities limited to flulike symptoms, including chills, fever, myalgias, and fatigue.74 Monocyte tumoricidal activity, the desired biologic response, was maximal at doses of 0.55 to 2.0 mg/m2, however. The monocyte tumoricidal activity as measured by the ability of monocytes isolated from peripheral blood of patients being treated with L-MTP-PE to kill osteosarcoma cells in vitro decreased at the MTD of 6.0 mg/m2, suggesting a suboptimal activation of monocytes at the MTD. Analysis of the serum showed that a dose of 2.0 mg/m2 produced the largest increases in acute-phase reactants, cytokines, white blood cell counts, absolute neutrophil counts, and other indices of stimulated immune function. The dose chosen for the phase I1 and phase I11 study was 2.0 mg/m2, well below the MTD of 6.0 mg/m2.

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Phase I1 trials were crucial in the development of L-MTP-PE. Patients with pulmonary metastases were rendered disease-free by surgery, then treated with L-MTP-PE, the study endpoint being time to relapse. Five patients in this study had a single pulmonary nodule that recurred within 6 weeks of the completion of therapy with L-MTP-PE. Histopathologic examination of these nodules revealed unique features.53Although metastases isolated from patients treated with standard chemotherapeutic agents show central necrosis with a rim of viable tumor at the periphery of a metastasis, lung metastases removed from patients who had received L-MTP-PE showed necrosis and fibrosis at the periphery of the tumor, with viable tumor cells at the center of the lesion. Such tumors are infiltrated with chronic inflammatory cells. This pattern suggested the appearance of pulmonary tuberculosis, in which the infectious lesions are walled off, and necrosis proceeds slowly from the outside so that the lesion is replaced by fibrous t i s s ~ e .This ' ~ finding in lung metastases suggested that the duration of treatment may be crucial for optimal activity of the drug. A significant increase in metastasis-free survival rate was noted for patients treated for 6 months but not for patients who received only 3 months of therapy.52 L-MTP-PE was developed for use with adjuvant chemotherapy and not as a single agent. The antitumor activity of L-MTP-PE is linked directly to its ability to activate the immune function,28,30 whereas chemotherapy in some instances suppresses the immune function. A phase IIb trial designed to assess the interaction between L-MTP-PE and the chemotherapeutic agent ifosfamide showed that ifosfamide did not interfere with the immune system-activating function of L-MTP-PE.51Activation of the immune system by L-MTP-PE did not affect the tumor cytotoxicity of ifosfamide. A randomized phase I11 trial in newly diagnosed osteosarcoma was conducted by the Children's Cancer Group and the Pediatric Oncology Group. The goal of the study was to determine if the addition of ifosfamide, L-MTP-PE, or both to the standard three-drug regimen of doxorubicin, methotrexate, and cisplatin improved the metastasis-free survival rates. Patients were randomized at study entry to one of four different regimens: doxorubicin, methotrexate, and cisplatin with or without L-MTP-PE and doxorubicin, methotrexate, cisplatin, and ifosfamide with or without L-MTP-PE. Although the data are maturing, the four-drug chemotherapy combination with L-MTP-PE (doxorubicin, methotrexate, cisplatin, ifosfamide, and L-MTP-PE) appears to be superior, yielding a disease-free survival rate of 79% at 3 years, compared with 59% seen in patients receiving the four-drug chemotherapy without L-MTP-PE. This improvement in disease-free survival rate is the first to be shown for osteosarcoma in a multicenter study in more than 20 years. Interleukin-la

Other immune modulators that have been used to treat relapsed osteosarcoma include IL-la. This cytokine has many effector functions,

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including the induction of IL-2, IL-6, colony-stimulating factors, tumor necrosis factor (TNF), and intracellular adhesion molecules.6,58, 76 IL-la has been shown to activate T and B cells16,26 and to enhance the cytotoxic activity of lymphocyte^.^^ Used alone, etoposide is not effective against In vitro, however, IL-la increased the cytotoxicity of o~teosarcoma.~~ etoposide against osteosarcoma cells from 9% to 709'0.~~ In a phase I1 study of recombinant IL-la and etoposide in patients with relapsed osteosarcoma, patients were treated daily for 5 days with IL-la, 0.1 mg/ kg, immediately followed by etoposide, 100 mg/m2.101This cycle was repeated every 3 weeks. The study had to be closed because of the unavailability of IL-la. Of the 8 evaluable patients, 3 had stable disease, and 1 had a partial response. The authors of the study concluded that this drug combination warrants further evaluation in a cancer for which few therapeutic options are available for patients with recurrent disease. Future Directions in Osteosarcoma

Because of the success of biologic response modifiers in the treatment of osteosarcoma, the cytokine IL-12 is being investigated. IL-12 exerts a variety of biologic effects on natural killer (NK) and T cells. It activates the cytotoxic activity of NK cells54and facilitates the induction of cytotoxic T lymphocyte^.^^ IL-12 also induces the induction of the Thl response from naive T ~ells,4~, 64 stimulates the production of interferony by NK and T cells, and has been shown to have antiangiogenic activity.94Many of these activities are implicated in the antimetastatic and antitumor effect of IL-12. IL-12 has been shown to be effective when given subcutaneously to patients with renal cell carcinoma73and m e l a n ~ m aIL-12 . ~ administered intravenously in a phase I study produced severe toxic side Because the amount of IL-12 at the tumor site determines the degree of tumor regression, and the main site of relapse in osteosarcoma is the lung, delivery of IL-12 directly to the lung would not only target the site of relapse, but also decrease the side effects associated with systemic therapy. This approach has been investigated in a mouse osteosarcoma model. Mice with microscopic pulmonary metastases were treated with inhalation therapy using an adenoviral vector containing the gene for IL-12. This therapy resulted in the local production of IL-12 in the lung and a dramatic decrease in the number of pulmonary metastases.lo3This approach may prove promising in the treatment of patients with osteosarcoma that has metastasized to the lung. Neuroblastoma

Neuroblastoma, a tumor that arises from neural crest cells, is the most common extracranial solid tumor in children, with approximately 550 new cases each year in the United States. The median age at diagnosis is approximately 22 months. One third of patients with neuro-

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blastoma are less than 1 year old at diagnosis. These patients in general have less advanced disease and a better prognosis. Most patients diagnosed at an age greater than 1 year have tumors that are unresectable or disease that has metastasized to the bone or bone marrow. Despite aggressive therapy, including myeloablative chemotherapy and autologous marrow transplantation, many patients relapse.66 Neuroblastoma is an unusual tumor in that occasional spontaneous regressions have been seen.38,89 If a biopsy specimen is taken from the site where the tumor regressed, differentiated forms of the neuroblastoma, ganglioneuroma, or ganglioneuroblastoma cells are found. This finding suggests that neuroblastoma arises from neuronal crest cells that for some reason do not terminally differentiate. Consequently, drugs that could cause terminal differentiation of these cells could be effective in the treatment of this disease.

134s -Retinoic Acid In vitro, 13-cis-retinoic acid is a vitamin A derivative shown to decrease the proliferation and to cause the differentiation of neuroblastoma cell lines even when the cells are resistant to cytotoxic therapy.69,85 A study was developed to determine if 13-cis-retinoic acid could have the same effect on neuroblastoma cells in vivo as it did in vitro. In a prospective randomized trial, patients received standard chemotherapy with or without 13-cis-retinoic acid or autologous bone marrow transplant with or without 13-cis-retinoic acid. Although the highest diseasefree survival rate was seen in the group that received 13-cis-retinoic acid after autologous bone marrow transplant, the survival rate of patients who also received 13-cis-retinoic acid and standard chemotherapy was higher than that of patients who received chemotherapy alone.66These results suggest that 13-cis-retinoic acid may be inducing the differentiation of neuroblastoma in the presence of minimal residual disease. More information on the use of retinoids in the management of childhood cancer is given elsewhere in this issue. Monoclonal Antibodies Monoclonal antibodies (mAbs) can be used for diagnosis, identification of metastases, and therapy. In pediatric cancers, a-fetoprotein (hepatic tumors), human chorionic gonadotropin (germ cell tumors), and GD2 (neuroblastoma) are tumor-associated antigens that aid in the diagnosis and tracking of tumor progression. GD2 is a disialoganglioside expressed on tumors of neuroectoderma1 origin. GD2 also is expressed in the cerebellum and on peripheral nerves. Tumor imaging with I3l3F8, a murine mAb directed toward GD2, has aided in the detection of metastatic disease and in planning for radiotherapy for neuroblastoma. This radiolabeled antibody can be used to deliver radiation. The use of this antibody to deliver doses of 4000 cGy to the tumor has resulted in a survival advantage for patients with

ne~roblastoma.~~ The anti-GD2 antibody, 14.G2a, facilitates antibodydependent cytotoxicity. In a phase I trial of 14.G2a in combination with IL-2, responses were noted in patients with neuroblastoma, most of whom had bulky disease. Twenty-six patients entered in this trial completed a full course of therapy and were evaluable. Of the 24 evaluable patients with neuroblastoma, 7 had stable disease, and 1 had a partial response. Of the 10 evaluable neuroblastoma patients with marrow disease, 3 had a significant decrease in the percentage of neuroblastoma cells in the bone marrow.34Further investigation of the combination of 14.G2a and IL-2 will involve its use in the setting of minimal residual disease or in an adjuvant setting. Ewing’s Sarcoma

Ewing’s sarcoma is part of a family of tumors that range from sarcoma of the bone and soft tissue to more mature cell types that express neuronal differentiation as found in the peripheral primitive neuroectodermal tumors. Using multimodality treatment plans, advances have been made in the management of Ewing’s sarcoma. Patients with primary tumors of the humerus, femur, or trunk; patients with metastases at diagnosis; and patients with bulky disease have a high risk of relapse, however.44,99 Although high-dose, short-term therapy with vincristine, doxorubicin, and cyclophosphamide (VAC therapy) increased the initial complete response rate in patients with high-risk Ewing’s sarcoma, the relapse rate, 40% to 50% at 2 years, remained high.56Relapsed Ewing’s sarcoma is particularly difficult to treat because of drug resistance and because the incidence of second remissions is extremely low. Two immune-modulating approaches are being investigated: one using a macrophage activator and one using cytokines in the setting of isolated limb perfusion. Liposome-Encapsulated Disaccharide Tripeptide (ImmTher)

Regardless of the sensitivity to chemotherapy, most Ewing‘s sarcoma tumor cells can be killed by activated macro phage^.^^, lo2 The efficacy of dose-intensive VAC therapy in combination with the macrophage-activating agent ImmTher was in~estigated.~~ ImmTher is a new liposome-encapsulated lipophilic disaccharide tripeptide derivative of MDP. The liposomal compound is similar to MTP-PE in that it activates monocyte tumoricidal activity and stimulates the production of IL-1, IL6, IL-8, and TNF. In vitro, however, the disaccharide tripeptide moiety ImmTher was more tumoricidal than MDP. In contrast to MTP-PE, ImmTher increases the expression and production of IL-12 by monocytes,lo3which may enhance the antitumor activity of the drug. ImmTher stimulates human monocytes in vitro to kill several different human Ewing’s sarcoma cell lines.lo3ImmTher ’s role against pulmonary metas-

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tases in Ewing’s sarcoma is being investigated in a weekly schedule given for 52 weeks after the completion of dose-intensive VAC chemotherapy, surgery, radiotherapy, or a combination of these modalities. Tumor Necrosis Factor and Interferony in Limb Perfusion

Another innovative approach to treating extremity soft tissue sarcomas is isolated limb perfusion for the delivery of chemotherapeutic agents and cytokines. This approach was developed by Eggermont et a1,2O who used isolated limb perfusion under hyperthermic conditions with high-dose TNF-a, IFN-y, and melphalan. Because of the systemic side effects of IFN-y, this trial was repeated using only TNF-a and melphalan. The response rate of 81°/P was similar to that obtained using all three agents.20Because of the success in delivering biologic response modifiers by isolated limb perfusion, this approach is being investigated as a gene delivery Malignant Brain Tumors

Tumors of the central nervous system are the most common malignant solid tumor in children and are challenging to cure. Changes in chemotherapy have resulted in only limited progress in central nervous system tumors, and the prognosis after frontline therapy has failed is dismal. The role of biologic response modifiers is being investigated in this setting. Interferon-p

One agent being used in brain tumors is IFN-P. Results have been mixed. A study in Japan reported the effectiveness of IFN-P in combination with carmustine and radiotherapy in 10 of 11patients.96In 2 patients with recurrent primitive neuroectodermal tumors, IFN-P given daily as a single agent resulted in stable disease, with minimal side effects. At the time of the study, the patients had been treated for 14 and 24 months.93The Children’s Cancer Group conducted a phase 1/11 study of IFN-P and hyperfractionated radiotherapy in children with newly diagnosed brainstem gliomas. The investigators concluded that this combination could be tolerated by this group of patients, with occasional dose-limiting toxicities of the liver, blood, and central nervous system. This therapy did not improve the rate of disease control, however, and 30 of 32 patients developed progressive disease at a median of 5 months and died at a median of 9 months after study entry.*l Interleukin-13

IL-13 is an immune regulatory cytokine that is structurally homologous to IL-4. It is unclear why malignant glioma cells have an abundance

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of IL-13 receptors on their surface, but they make this receptor an attractive target for antiglioma therapies.14 An antagonist to IL-4, hIL4.Y124D, inhibits the physiologic effect of IL-4 and IL-13 on normal cells. This same antagonist to IL-4 does not inhibit the IL-13-based cytotoxicity on glioma cells, however. This finding suggests that the IL13 receptors on glioma cells are tumor specific in the presence of an IL4 antagonist and that this antagonist can serve as a receptor-directed cytotoxin without being cytotoxic to normal cells. Hematopoietic Growth Factors in Solid Tumors

Hematopoietic growth factors do not play an important role in the modulation of the immune system in pediatric solid tumors. Instead, .these growth factors frequently are used to stimulate the recovery of granulocytes (e.g., granulocyte-colony stimulating factor [G-CSF]) after chemotherapy so that the dose of chemotherapy can be increased or the time between courses of chemotherapy can be shortened.47G-CSF is used to stimulate the production and mobilization of stem cells for use in transplantation. Epogen, which is created using recombinant DNA technology, has the same amino sequence as erythropoietin. Epogen stimulates red blood cell productionz and has been used to decrease the need for red blood cell transfusions in patients receiving chemotherapy. The use of IL-11, a growth factor produced by stromal cells, increases platelet counts.'* Its use in breast cancer patients treated with multiple cycles of cyclophosphamide and doxorubicin reduced the incidence of severe thromb~cytopenia.~~ BIOLOGIC RESPONSE MODIFIERS IN PEDIATRIC HEMATOLOGIC MALIGNANCIES

Anecdotal reports of spontaneous remissions of leukemia after blood transfusions suggest that the immune system plays an important role in regulating leukemia cell proliferation. Based on Cooley's observation of the regression of a sarcoma in the face of infection, early attempts using immune modulation involved nonspecific stimulation of the immune system in hopes of stimulating a nonspecific activation of the immune system. These early approaches used bacille Calmette-Gukrin, a modified form of the tubercle bacillus, because it had shown increased immune reactions in murine models. When used in patients with advanced disease, however, these approaches were almost uniformly unsuccessful, and these efforts have largely been abandoned? Data from allogeneic bone marrow transplant recipients showed that the immune system plays a crucial role in eradicating minimal residual leukemia: Tcell depletion decreases the incidence and severity of graft-versus-host disease but is associated with a higher relapse rate.I7 Donor leukocyte transfusion can induce remission in CML patients who relapse after

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allogeneic BMT.82Various studies suggest that a cure does not always entail total eradication of leukemia but might rather be a state in which the immune system keeps minimal residual disease under check.86Advances in molecular biology have allowed a better characterization of the various lineages of hematopoietic cells, making these easily accessible cells ideal targets for biologic therapies. Although the use of cytokines continues to be explored and improved, mAbs are being used in various treatment regimens for leukemia.

Monoclonal Antibodies

The development of techniques for generating mAbs has contributed to the armamentarium of targeted leukemia therapy. After attaching to the cell surface, unmodified mAbs activate cell-mediated and complement-mediated cytotoxicity mechanisms, causing cell death.'O The development of recombinant chimeric mAbs that use the constant region of human antibodies and the variable combining region of the mouse antibody had decreased the incidence of the development of human antimurine antibodies (HAMA), improving the potency of mAbs and allowing repeat infusion^.^, 37 Radioactive isotopes and toxins can be conjugated to mAbs to increase their effectiveness against mAbs can be used to target any leukemia cell surface marker. The efficacy of anti-CD20, anti-CD52, and anti-CD33 mAbs is well established. Researchers are studying the use of these agents to intensify chemotherapy, to consolidate maintenance, and to purge stem cells before transplant. IDEC-C2B8 (IDEC- 102, Rituximab)

The surface antigen CD20 is expressed in more than 90% of cells of B-cell lymphomas and in about 50% of cells of B-cell precursor acute lymphocytic leukemias (ALL). CD20 is not shed by the cells; does not circulate in the plasma as a free protein; and is not internalized, modulated, or down-regulated on antibody binding. IDEC-C2B8 (IDEC-102, rituximab) is a chimeric humanized mAb that binds to CD20.84Used in relapsed and refractory low-grade and follicular non-Hodgkin's lymphoma, rituximab has shown response rates of 48% in several trials.6143 In 83% of the patients, circulating B cells were depleted within the first 3 doses, with sustained depletion for 6 to 9 months. Although rituximab depletes normal B cells, it has not been associated with profound immunosuppression. Of initial responders, 40% benefit from repeat therapy after disease progression. Less than 1%of patients develop quantifiable immunoreactivity (HAMA or human antichimeric antibodies [HACA]). Treatment-related toxicity is limited primarily to infusion-related events, including fever, chills, skin rash, nausea, headache, rigor, hypotension, and bronchospasm. These events usually are associated with the first

infusion, are believed to be due to cytokine release, and can be reversed by adjusting the infusion rate. Although rituximab is licensed for the treatment of indolent lymphoma, the role of this mAb in combination with chemotherapy in CD20-positive lymphomas and leukemias is being investigated. The use of rituximab is being investigated for the treatment of Hodgkin’s disease. Approximately 25% of Hodgkin’s lymphomas are CD-20 positive. Some patients with CD20-negative Hodgkin’s disease have responded to rituximab therapy. Although rituximab did not target specifically the tumor cells in the latter cases, it suppressed the normal B cells that sustain Reed-Sternberg cell proliferation. A trial with rituximab in combination with gemcitabine is currently under way for patients with relapsed Hodgkin’s disease. CAMPATH- 1H

CAMPATH-1H is a humanized mAb specific for the surface antigen CD52, a cell surface glycoprotein that is expressed abundantly on most normal and malignant lymphocytes and monocytes. CAMPATH-1H was used initially to control graft-versus-host disease and graft rejection post transplantation.4°,97 The demonstrable immunosuppressive effects of this agent led to its use in autoimmune diseases.8,59 The drug now is under investigation as part of a nonmyeloablative conditioning regimen before allogeneic stem cell transplantation. Although CAMPATH-1H induced only a modest overall response rate in non-Hodgkin’s lymphoma patients with bulky lymph nodes, significant disease regression was seen in blood and bone marrow.39CAMPATH-1H was shown later to be highly effective and well tolerated in patients with chronic lymphocytic leukemia (CLL).*OThe role of CAMPATH-1H as a therapy for minimal versus active disease or single use versus its use in combination with chemotherapy remains to be established. CMA-676 (Gemtuzumab, Zogamicin, Mylotarg)

The surface antigen CD33 is highly expressed on the myeloid blast cells of most patients with acute myeloid leukemia (AML). Mylotarg is a conjugate of humanized anti-CD33 mouse mAb and calicheamicin, a highly potent antitumor antibiotic that is inactive when bound to the mAb.43After binding to the surface of the AML cell, the immunotoxin conjugate is internalized. Within the cell, the antibody linker is hydrolyzed, liberating free calicheamicin, which diffuses to the nucleus and cleaves double-stranded DNA. In preliminary studies, 30% of patients with relapsed AML responded to treatment with m y l ~ t a r g .91~ ~ , Chills, fever, hypotension, and dyspnea were reported shortly after the completion of infusion in some patients. These symptoms usually are mild and reversible. Myelosuppression, especially neutropenia and thrombocytopenia, is common. Mylotarg would seem to be an effective therapy for AML in first relapse. This agent has a favorable safety profile

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and is the only antibody-targeted chemotherapeutic agent approved for use in the United States for the treatment of AML. Ongoing studies are evaluating different doses and schedules of mylotarg alone and in combination with other therapies. Cytokines

The use of G-CSF and granulocyte-macrophage (GM)-CSF in hematologic malignancies allows for safe and timely administration of intensive therapy by shortening the duration of neutropenia. These agents also are used to mobilize stem cells. The use of G-CSF and GM-CSF as recruiting and synchronizing agents to enhance the cytotoxicity of cytosine arabinoside has failed to show a priming advantage.23,77 The use of either factor is not associated with higher relapse rates, however, and they probably are safe when used in the setting of low tumor burden. IL-2 stimulates the proliferation of T cells and induces the elaboration of other cytokines, including IFN-y and TNF.42IL-2 activates NK cells to become lymphokine-activated killer cells.79IL-2 can block leukemia cell growth in vitro and in mouse models. Initial work focused on the use of IL-2 in the setting of advanced disease.87Clinical trials have shown that IL-2 is most effective against low leukemic burden.31,32, 65, 71 Current studies are focusing on the use of IL-2 in patients in remission and post transplantation.', 98 Preliminary evidence indicates that IL-2 may be active in AML but not in ALL. IFN-a revolutionalized the treatment of CML in the 1 9 8 0 ~IFN-a .~~ has shown sporadic success in inducing remissions in childhood ALL but has never achieved the consistent results observed in hairy cell leukemia or CML. Hematologic response is observed in most CML patients treated with IFN-a, with 26% achieving complete cytogenetic response. A subset of these patients have been followed up without maintenance treatment and have remained in cytogenetic remission for 10 years. Researchers developed a pegylated (PEG)-IFN-a, which is a form of rHu-IFN alfa-2a that has been modified chemically by the covalent attachment of a branched methoxypolyethylene glycol moiety. Data from animals and humans indicate that PEG-IFN-a injected once a week is more effective and tolerated better than IFN-a injected 3 times a week. Combined with the tyrosine kinase inhibitor STI-571, discussed elsewhere in this issue, PEG-IFN-a might provide an effective targeted therapy for CML and avoid the morbidity of bone marrow transplant, which is the current standard of care. CONCLUSIONS

Biologic response modifiers are becoming an important addition to surgery, chemotherapy, and radiotherapy in the management of cancer. As this field of research grows and expands, more biologic response modifiers will be incorporated into therapeutic regimens. By stimulating

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the immune system to eradicate minimal residual disease, these agents may improve the disease-free and long-term survival rates of patients with a variety of malignancies. The challenge is to incorporate biologic response modifiers into the treatment armamentarium in ways that will maximize their tumorigenicity.

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Address reprint requests to Laura L. Worth, MD, PhD Department of Pediatrics, Box 87 The University of Texas The M. D. Anderson Cancer Center 1515 Holcombe Boulevard Houston, TX 77030