Biologic Therapy: Interferons, Interleukin-2, and Adoptive Cellular Immunotherapy

The InterferonsInterleukin-2Adoptive Cellular ImmunotherapyTherapeutic ApplicationsConclusionsReferences

Biologic therapy for cancer may be defined as the use of compounds, or their derivatives, that can be found within the body to treat malignancy. The recent era of biologic therapy began with the identification and isolation of interferon (IFN)[1] and has been expanded with interleukin-2 (IL-2, aldesleukin [Proleukin]), the hematopoietic growth factors, and the retinoids. The technique of gene cloning has brought an increasing number of potentially useful compounds to the clinic. As we increase our understanding of the mechanisms of action of these and conventional cytotoxic drugs, it is likely that chemotherapy and biologic therapy will become less sharply differentiated. In this chapter, the IFNs, IL-2, and adoptive cellular immunotherapy will be reviewed. The hematopoietic growth factors and retinoids will be covered in a separate chapter.

The Interferons

The interferons constitute a family of naturally occurring proteins that were first recognized for their ability to confer on cells resistance to viral infection [1]. Although a multitude of other functions have been subsequently described, the property of viral interference remains necessary for classification as an interferon.

As listed in Table 1, the IFNs are designated alpha interferon (IFN-alfa), beta interferon (IFN-beta [Betaseron]), and gamma interferon (IFN-gamma [Actimmune]). The genes encoding these proteins and their receptors have been sequenced and cloned. IFN-alfa and IFN-beta are class I (virus-induced) IFNs. These molecules share many biologic and structural properties and compete for binding to the same receptor. Both are acid stable, have cysteine-cysteine disulfide bonds necessary for biologic activity, and lack introns in their encoding genes. It is likely that their genes diverged from a common ancestor. Gene transcription is increased in response to the same inducers, and the molecular control of gene expression may be under control of the same promoter/enhancer sequences.

Alpha Interferon: At least 24 distinct species of IFN-alfa have been recognized. They are structurally similar, share 80% to 90% nucleotide sequence homology, and are coded for by genes located on chromosome 9. They are produced in vivo by leukocytes in response to viruses or double-stranded RNA. They differ subtly in biologic activity, but the exact roles of the numerous species are unknown. Most are nonglycosylated.

Two recombinant human IFN-alfa molecules are licensed for clinical use in the United States, IFN alfa-2a (Roferon-A) and IFN alfa-2b (Intron A). Each has a bioavailability of at least 80% when given by intramuscular or subcutaneous injection.

Beta Interferon: There is only one known species of IFN-beta. It shares 25% amino acid homology with IFN-alfa, and the encoding gene is located on chromosome 9. It is normally produced by fibroblasts and epithelial cells in response to the same inducers as IFN-alfa.

Gamma Interferon: The one known species of IFN-gamma is a class II IFN (mitogen induced). Distinct from the other IFNs both structurally and functionally, it is encoded by a gene with three introns located on chromosome 12. The gene product lacks disulfide bonds and is acid labile. T lymphocytes can be induced to produce IFN-gamma in vivo in response to soluble microbial antigen stimulation or by nonspecific mitogens such as IL-2.

Biologic Activity

The broad classes of action currently recognized for interferons are characterized as (1) antiviral, (2) antiproliferative, (3) regulator of differentiation, (4) modulator of lipid metabolism, (5) inhibitor of angiogenesis, (6) antitumoral, and (7) immunoregulator, with effects that include monocyte/macrophage activation, enhanced major histocompatibility complex (MHC) class I expression (IFN-alfa and IFN-beta), enhanced MHC class II expression (IFN-gamma), augmentation of natural-killer (NK) cell activity, stimulation of proliferation and differentiation of B-cells, and increased cytotoxic T-cell activity. In general, IFN-gamma is a much more potent immunomodulator and IFN-alfa more effective antitumor agent in vivo.

Cell-Surface Receptors: All biologic activities of the IFNs require binding to specific high-affinity cell-surface receptors. The two distinct receptor classes, type I and type II, correlate with the classes of IFNs that are bound (reviewed in Rubinstein and Orchansky)[2]. All species of IFN-alfa and IFN-beta bind only to type I receptors, with similar affinity. IFN-gamma has no affinity for type I receptors, binding only to type II receptors.

Interferon Antibodies: Although IFNs are products of human cells, they usually act in a paracrine fashion and are not present in high concentrations in the circulation. Perhaps it is the difference in serum concentration or subtle differences in structure that have made the recombinant proteins immunogenic. In general, antibodies to recombinant IFN-alfa can be detected in up to two thirds of patients receiving the material, although only half of them are neutralizing antibodies [3,4]. Development of neutralizing antibodies is often accompanied by the loss of IFN-related side effects. Administration of nonrecombinant IFN-alfa may restore responsiveness in patients whose disease has relapsed after the development of neutralizing anti-IFN-alfa antibodies [5]. The antibodies cross-react with several natural IFNs, but the specificity is different in each patient. In some patients, the antibodies may disappear despite continuation of therapy.

Toxicity

In general, IFN-related side effects resolve shortly after treatment is discontinued. The common side effects are either acute (arising after the first few doses) or subacute or more chronic (arising beyond 1 week of therapy)[6].

Acute Effects: Patients who receive initial IFN-alfa therapy at doses above 2 million units almost always experience an acute first-dose effect. The symptom complex has been described as an influenza-like syndrome and classically consists of fever, chills, myalgia, tachycardia, and headache.

These symptoms are more common and more intense in older patients, especially at higher doses, and can be alleviated by premedication with acetaminophen. In addition, many patients tolerate IFN better with evening administration. If these simple measures are not successful, a nonsteroidal anti-inflammatory medication can provide relief. With daily dosing these symptoms lessen, usually disappearing within 7 to 10 days.

Chronic Effects: Beyond the first week of therapy, the pattern of toxicity changes. The most troublesome chronic complaint is profound fatigue, sometimes accompanied by anorexia. This is most prominent in patients receiving daily doses or doses above 10 million units and is more frequent in older patients and patients with poor pretreatment performance status. Neurologic effects (encephalopathy with somnolence, confusion, and memory loss) also occur at high doses [7]. These side effects, however, may be more common in patients with preexisting organic brain abnormalities.

The white blood cell and platelet counts usually fall to about 50% of pretreatment values during IFN therapy. These effects are readily reversible on withdrawal of IFN and are not associated with a high risk of infection or bleeding. In patients with compromised marrow function, neutropenia is more prominent. Autoimmune hemolytic anemia and thrombocytopenia have been documented in rare instances [8,9].

Dose-related elevations of hepatic transaminases are seen in 20% to 30% of patients but are usually not dose limiting. Although acute renal failure has been reported, it is very uncommon. Proteinuria (less than 1 g/d) is the most frequently encountered renal side effect, affecting 15% to 20% of patients. The proteinuria is not associated with azotemia. Impotence and loss of libido can also occur in men undergoing IFN treatment.

Autoimmune phenomena can complicate the course of IFN therapy in approximately 5% of patients. Most commonly, patients will develop hypothyroidism, hemolytic anemia, or connective tissue disorders, but cardiomyopathy, porphyria, and glomerulonephritis are seen occasionally.

Interleukin-2

The interleukins are a family of polypeptides originally named for their ability to mediate interactions between leukocytes. Initially called T-cell growth factor, IL-2 was recognized as a product of activated T-cells that stimulates the proliferation and enhances the function of other T-cells and such immunocompetent cells as NK cells and B-cells. NK cells activated by IL-2 develop lymphokine-activated killer (LAK) activity. IL-2–activated B-cells generate secretory rather than membrane-associated IgM, and macrophages gain cytolytic maturation and elaborate transforming growth factor-beta (TGF-beta) upon stimulation with IL-2.

The IL-2 gene, located on chromosome 4, has been cloned. The gene codes for a 153–amino acid protein and is structurally unrelated to the genes that encode the other interleukins and the IFNs. A 20–amino acid signal sequence is cleaved, and two residues are N-glycosylated. This yields the mature single-chain IL-2 molecule, which has one disulfide bond necessary for biologic activity. Small concentrations of IL-2 can be detected in blood and follow a circadian variation.

In vitro, the major stimuli for IL-2 production are mitogen stimulation of T-cells and antigen recognition in combination with MHC. Costimulation of T-cells through CD28 is required for optimal synthesis of IL-2.

Biologic Activity

The interaction between IL-2 and effector cells is mediated through IL-2 binding to cell-surface receptors. The IL-2 receptor exists in three known forms with different affinities. The high-affinity receptor, which mediates much of the biologic activity of IL-2, is a trimeric protein representing 10% of IL-2–binding sites. Although it has been detected on large granular lymphocytes, monocytes, and B-cells, this receptor is located predominantly on activated T-cells. The intermediate affinity IL-2R is a heterodimer that is capable of signal transduction in the presence of large concentrations of IL-2. A circulating soluble form of the low-affinity single-chain IL-2 receptor has been associated with various disease states, including adult T-cell leukemia/lymphoma and hairy-cell leukemia. These soluble receptors are not involved in mediating the cellular effects of IL-2.

The rationale for exploring IL-2 as an anticancer agent relates to its immunomodulatory effects; T-cell stimulation; enhancement of NK-cell cytotoxicity; generation of LAK-cell activity; induction of secretion of other lymphokines such as IFN-gamma, tumor necrosis factor-alpha (TNF-alpha), TGF-beta, IL-6, and IL-1; and stimulation of macrophage cytotoxic activity [10].

Toxicity

The toxicity of IL-2 has hindered its widespread implementation as a therapeutic agent. As with other biologic therapies, it is likely that the maximum tolerated dose is not the most immunologically effective dose, and it is probable that dose and schedule effects will differ among different tumor types. Much of the current investigation into IL-2 is directed toward development of effective schedules with a minimum of toxic side effects.

The common side effects of IL-2 are listed in Table 2. The most significant such effect is the vascular leak syndrome, which consists of weight gain, edema, ascites, pleural effusions, hypotension, and oliguric renal impairment. The mechanism by which the syndrome is created is unknown, but it is associated with high circulating levels of nitrogen oxides [11], the stable metabolite of the endogenous vasodilator nitric oxide, and may be alleviated by inhibitors of nitric oxide synthetase [12]. LAK-mediated damage of endothelial cells may also play a role. Hematologic toxicity is also frequent, with anemia and thrombocytopenia severe enough to require transfusion occurring in approximately 60% and 15% of patients, respectively. Transient oliguric renal dysfunction is also common.

Other (< 20% of patients given high doses)

Adoptive Cellular Immunotherapy

The aim of adoptive cellular immunotherapy is to enhance the immune recognition and destruction of a tumor by the administration of immunocompetent cells to the tumor-bearing host. It is a complex and expanding field of cancer treatment [13]. How these cells mediate antitumor activity is unclear, but the mechanism probably involves indirect cytokine-mediated effects as well as direct-contact tumor-cell killing.

Three broad classes of cells are being investigated: LAK cells, in vitro sensitized (IVS) lymphocytes, and tumor-infiltrating lymphocytes (TILs)(Table 3). Since these cells are often administered in combination with IL-2, the specific role of the adoptive cellular immunotherapy is difficult to determine. Current data are insufficient to conclude that adoptive cellular immunotherapy adds any benefit to IL-2 therapy alone.

Therapeutic Applications

Hairy-cell and chronic myelogenous leukemia (CML), severe thrombocytosis, multiple myeloma, Kaposi's sarcoma, various lymphomas, melanoma, and renal-cell carcinoma have been shown to respond to IFN or IL-2 treatment, with or without adoptive cellular immunotherapy.

Hairy-Cell Leukemia

Hairy-cell leukemia was the first cancer shown to be highly responsive to IFN [14]. This disease represents a malignant proliferation of mature B-lymphocytes. Approximately 500 new cases are diagnosed each year in the United States, accounting for 2% of adult leukemias. Clinical splenomegaly is present at diagnosis in 70% of patients, and the majority have some degree of bone marrow failure. Episodes of infection are common, and the risk is increased when the peripheral blood neutrophil count falls below 1 billion/L. The hairy cells in peripheral blood and bone marrow have distinctive cytoplasmic projections and usually stain positively for tartrate-resistant acid phosphatase.

About 90% of patients with hairy-cell leukemia require treatment at diagnosis or during the course of the disease. The accepted criteria for treatment are hemoglobin values less than 10 g/dL or the need for transfusion, platelet counts less than 100 billion/L, neutrophil counts less than 1 billion/L or repeated infections, a leukemic phase with white blood cell counts more than 30 billion/L and more than 50% hairy cells, symptomatic splenomegaly, bulky or painful lymphadenopathy (uncommon), bone lesions (uncommon), or autoimmune vasculitis (uncommon)[15].

Splenectomy: Before IFN was available, the median survival of patients with hairy-cell leukemia was 4 to 5 years, and most patients died of infection. Conventional chemotherapy was usually not beneficial. The mainstay of therapy was splenectomy.

Patients most likely to benefit from splenectomy are those who have cytopenias in the absence of extensive marrow replacement (less than 85% cellularity). Hematologic response is independent of the size of the spleen. In this setting, 40% of patients have normalization of peripheral blood counts, and a further 50% to 60% will have some improvement in at least one cell line after splenectomy.

However, most patients eventually require treatment for progressive disease. In the unfavorable group of patients with preoperative marrow cellularity of greater than 85%, the median time to progression was reported to be only 5 months [16]. Given the risks of splenectomy, the transience of many of the responses, and the fact that newer systemic therapies, including IFN, are highly effective in patients who have marked splenomegaly, the importance of splenectomy has diminished.

Interferon: Numerous investigators have confirmed the therapeutic efficacy of recombinant human IFN-alfa 2b and IFN-alfa 2a. Although few patients have been treated with IFN-beta, this agent has also shown promising activity against hairy-cell leukemia, whereas IFN-gamma is not active against this disease.

The standard dose of IFN-alfa is 3 million units given subcutaneously three times a week. Daily administration of IFN-alfa for the first 4 to 6 months may accelerate the response. Doses as low as 0.2 million U/m²/d have been investigated [17], and although they maintain significant activity, the remission rates are inferior. However, in patients who suffer significant toxicity or who cannot tolerate the optimum dose, lower doses are useful.

In the first weeks of IFN therapy, many patients experience transient minor falls in blood counts. If these are significant or complicated by infection, the use of granulocyte colony-stimulating factor (G-CSF, filgrastim [Neupogen]) may reverse the neutropenia [18]. When a patient responds to treatment, a return to normal values is seen first in the platelet count (median, 2 months) and then in the neutrophil count and hemoglobin level (median, 4 to 5 months). Improvement in the peripheral blood precedes clearing of the bone marrow, which may take up to 10 months. Side effects usually are not dose limiting. While the optimal duration of IFN treatment is not clearly defined, most investigators recommend a minimum of 12 months.

Overall response rates are 80% to 90%, but most responses are partial remissions. Previously untreated patients have a significantly higher response rate (complete response rate, 70%) than previously treated patients (complete response rate, 15%). Only 5% to 15% of patients do not show improvement in any parameter.

The time to relapse after cessation of therapy varies widely, with a median of 25 months reported [19]. Residual marrow hairy-cell infiltrate greater than 30% and post-treatment platelet count less than 160 billion/L predict a shorter remission duration. However, reappearance of hairy cells in the bone marrow in the absence of abnormal peripheral blood counts is not a sufficient criterion to mandate therapy. High levels of soluble IL-2R at diagnosis may be predictive of response. Virtually all patients who relapse after treatment cessation remain sensitive to IFN and achieve a second remission of a duration comparable to the first (R. Kurzrock, unpublished data).

The postulated mechanisms of IFN activity in hairy-cell leukemia include a direct antiproliferative effect on the malignant clone, modification of oncogene expression, promotion of differentiation, suppression of responsiveness to B-cell growth factor, and modulation of effector cells including NK cells [20], which are known to be functionally deficient in hairy-cell leukemia. Indeed, restoration of NK cell and T-helper activity as well as endogenous IFN-alfa production correlate with clinical response to IFN. TNF is elevated in patients with hairy-cell leukemia and is an autocrine growth factor for the malignant cells but an inhibitor of normal progenitors. IFN can decrease the serum levels of TNF and the autocrine growth stimulation of hairy-cell leukemia cells in vitro.

Purine Analogs: The purine nucleoside analogs also have significant activity in hairy-cell leukemia. Both pentostatin (Nipent) and cladribine (Leustatin) can elicit response rates of 90% or greater. Their main advantage is that the treatment period is shorter than that for IFN-alfa (3 months for pentostatin and 7 days for cladribine, vs 1 year for IFN-alfa)[21-25]. However, in contrast to the immune effector-cell recovery that occurs with IFN-alfa) treatment, both pentostatin and cladribine cause a profound and prolonged lymphopenia with a reversal of the T4/T8 ratio and a reduction in absolute T4 counts to levels usually associated with severe immunosuppression. Patients must be monitored closely to detect any delayed effects of this immune dysregulation, although no increase in opportunistic infections or cancers has been reported [26]. One study of pentostatin used together with IFN-alfa achieved high partial response rates, but it is unclear whether this approach is superior to either drug alone [27].

Given the range of effective therapies available, the goal for patients who have hairy-cell leukemia is a normal life expectancy [23]. To achieve this goal, neutropenic patients must be monitored carefully until their cytopenia responds to therapy. Infection remains the major cause of morbidity and mortality, and G-CSF may therefore be a useful supportive therapy [18]. In addition to gram-negative sepsis, these patients are at risk for unusual infections, including those due to disseminated atypical mycobacteria. Persistent fevers and night sweats without an obvious source should alert the physician to this possibility.

Chronic Myelogenous Leukemia

CML was the first human cancer found to be associated with a consistent chromosomal abnormality, the Philadelphia (Ph¹) chromosome, t(9;22)(q34;q11). Many chemotherapeutic agents can elicit complete hematologic responses in CML, although durable cytogenetic improvement is unusual. In the absence of durable cytogenetic response, progression to blastic phase remains inevitable.

IFN-alfa can induce a complete hematologic remission in about 70% of patients with early chronic-phase disease (within 1 year from diagnosis). It is only in this patient subgroup that significant suppression of the Ph¹-positive clone may be seen. Although 20% to 30% of patients with late chronic-phase CML (beyond 1 year from diagnosis) obtain complete hematologic remission, cytogenetic responses are minor and transient, and a long-term benefit is unlikely. Overall, about 40% to 50% of CML patients in early chronic phase achieve a cytogenetic improvement.

Approximately 15% to 25% of all patients treated achieve a complete cytogenetic response (100% diploid metaphases). Responses are durable in most of these patients, with a complete cytogenetic response persisting at a median follow-up of 3 years (Table 4)[28,29]. Nevertheless, the BCR/ABL gene transcript remains detectable by the polymerase chain reaction [30], which is capable of detecting 1 leukemic cell among more than 100,000 normal cells. Patients achieving a cytogenetic response have a survival advantage over those treated with conventional chemotherapy [29].

Response to IFN-alfa is dose dependent. The recommended dose is 5 million U/m² daily, a considerably higher dose than that used in hairy-cell leukemia. Mild myelosuppression (white blood cell count, 2 billion to 4 billion/L) is usually achieved with this dose. The median time to maximum hematologic response is 6 to 7 months. If patients have not shown a significant hematologic improvement by then, alternative therapy should be instituted. The median time to achieving a cytogenetic response is 12 months. Most patients develop tachyphylaxis to the early IFN side effects of fever, chills, and flu-like symptoms. However, at these higher doses, late neurologic effects (chronic fatigue and memory loss) and loss of libido in men may occur.

Because IFN is most active against small-volume disease, it has been used to treat recurrent CML after allogeneic bone marrow transplantation. Two studies [31,32] reported on a total of 12 patients with hematologic relapse after allogeneic bone marrow transplantation. Complete hematologic remission was obtained using IFN-alfa in eight patients, with a cytogenetic response seen in four. The use of IFN-alfa as maintenance therapy after bone marrow transplantation in Ph¹-positive CML patients at high risk of relapse is, therefore, attractive [33].

IFN-gamma has been less extensively studied, but the rates of response achieved so far are inferior to those elicited by IFN-alfa [34-36]. Overall, hematologic responses (7 complete and 14 partial) occurred in 21 of 55 chronic-phase patients given IFN-gamma, and only 7 attained any cytogenetic response.

Severe Thrombocytosis

Treatment of thrombocytosis in myeloproliferative disorders is one of the most promising new clinical applications of IFN-alfa. It was first observed that IFN-alfa lowers the platelet count in severe thrombocytosis complicating CML [37], and this effect has been repeatedly demonstrated in essential thrombocythemia and other myeloproliferative disorders. Reports on more than 100 treated patients have been published [37-44].

The induction dose of IFN-alfa in patients with severe thrombocytosis is generally 3 to 5 million U/d, and the maintenance dose, 2 to 3 million U/d. This approach has yielded an overall response rate (defined as a more than 50% reduction in platelet count) of greater than 80% (Table 5). Responses are achieved rapidly, with a median time to response of 1 to 3 weeks. Neutrophil counts do not drop below physiologic levels. Ongoing treatment is required, since the platelet count rises once IFN-alfa is withdrawn. Patients have received IFN-alfa therapy for more than 4 years with continuous response [43,44], and virtually all initial thrombotic and hemorrhagic complications of thrombocytosis resolved during treatment and did not recur.

Multiple Myeloma

The first published report of IFN for multiple myeloma described a 100% response rate in four patients [45]. Since then the role of IFN has been explored in myeloma primarily in three situations-as a single agent in untreated or relapsed/refractory patients, as part of combined chemotherapy induction, and in the maintenance of a chemotherapy-induced or bone marrow transplantation-induced remission.

As with other tumor types, the exact mechanism of IFN's action against myeloma is unclear. However, it has been shown to have a direct cytotoxic effect on myeloma cells and growth-inhibitory effects (reviewed in Avvisati and Mandelli)[46].

Previously Untreated Patients: When IFN-alfa is used as a single agent in previously untreated patients, there is a distinct correlation between the dose of this agent and response at doses between 6 and 30 million U/m²/d [47-49]. However, the doses necessary to achieve high response rates are not tolerated, with severe central nervous system side effects occurring at doses over 6 million U/m²/d. Some reports describe a higher response rate in IgA myeloma [48]. Using tolerable doses, response rates in previously untreated patients are 10% to 40%, with a median time to response of 2 months and a median response duration of 14 to 20 months. In a randomized trial, however, IFN-alfa alone was inferior to standard melphalan (Alkeran) and prednisone chemotherapy [48].

Relapsed Patients: In patients who relapse, IFN-alfa produces a response rate of 10% to 25%. This is similar to that achieved by standard chemotherapy. However, in patients who are refractory to chemotherapy, IFN-alfa as a single agent has produced an 18% response rate [50]. Similarly, the addition of IFN-alfa to high-dose dexamethasone improved the response rate in patients for whom high-dose dexamethasone alone had failed [51]. In patients with relapsed disease that responds to conventional chemotherapy, the combination of IFN-alfa and dexamethasone may prolong disease-free survival [52]. The activity of this combination has been confirmed: Among 66 patients who had less than 75% tumor reduction with standard induction chemotherapy, 23 (35%) had further reductions in their monoclonal bands in response to IFN-alfa plus dexamethasone [53].

Combination With Chemotherapy: The rationale for combining standard chemotherapeutic agents and interferon as induction treatment for myeloma is based on laboratory evidence of synergy [54]. Phase II studies have reported 75% response rates with interferon, melphalan, and prednisone [55] and 80% response rates with interferon, vincristine (Oncovin), carmustine (BiCNU), melphalan, cyclophosphamide (Cytoxan, Neosar), and prednisone [56].

Results of large phase III trials conducted to confirm these promising response rates were recently published (Table 6)[57-63]. One study showed a higher response rate with melphalan, prednisone, and IFN-alfa than with melphalan/prednisone alone (68% vs 42%, P < .0001)[57], whereas in a similar study, results were comparable for both groups (33% vs 44%, P = NS)[59]. In no case, however, was there a difference in survival. Interestingly, there was a survival advantage for patients with IgA or Bence Jones myeloma who received IFN [57]. However, IFN has not been proved to add to the long-term outcome of standard chemotherapy. Also, the combination of IFN-alfa and dexamethasone has achieved results similar to those seen with dexamethasone alone in previously untreated patients [64].

Maintenance Therapy: Once a “plateau phase” is reached in myeloma, ongoing chemotherapy is of no benefit. It may be that, at this time, the clonogenic cells are in the G0 phase of the cell cycle and, therefore, are not sensitive to chemotherapeutic agents. Because IFN-alfa has activity against these noncycling cells, it may be beneficial in prolonging the plateau phase [65].

Results are available from four studies of IFN maintenance (Table 7)[53,66-68]. Among the largest series, two showed a significant prolongation of remission duration and one an improved survival. A study from the Southwest Oncology Group using a lower dose of IFN failed to confirm the benefit of IFN [69], but it is possible that the lower dose contributed to the negative finding.

Similarly, the use of maintenance IFN-alfa is being studied in remissions after autologous bone marrow transplantation. Preliminary results of the Medical Research Council trial suggest that IFN-alfa may prolong remission duration (reviewed in Jagannath and Barlogie)[70].

Kaposi's Sarcoma

Kaposi's sarcoma is a common tumor in persons infected with the human immunodeficiency virus (HIV). Clinical manifestations range from a single asymptomatic cutaneous lesion to a disseminated and life-threatening malignancy. In some cases, lesions are cosmetically unacceptable but do not threaten the function of vital organs. Since opportunistic infections remain the most common cause of death in people who have the acquired immunodeficiency syndrome (AIDS), the risks and benefits of treating Kaposi's sarcoma should be carefully considered for each individual.

Intralesional injection of IFN-alfa (3 million to 5 million units three times a week for 4 weeks) is effective for isolated symptomatic lesions and causes minimal side effects [71]. Systemic IFN-alfa has been investigated as a single agent at low doses (1 million to 7.5 million U/m², 3 to 7 days a week), intermediate doses (10 million to 15 million U/m², 3 to 7 days a week), and high doses (20 million to 50 million U/m², 3 to 7 days a week).

The high-dose schedules are poorly tolerated because of severe myelosuppression and neurotoxicity. Overall results from published series (reviewed in Krown) [72] show total (complete plus partial) response rates of 7%, 23%, and 32% with low-, intermediate-, and high-dose protocols, respectively. In patients who respond, regression is usually documented after 4 to 8 weeks of treatment, although maximal response may not occur for up to 6 or 8 months. Ongoing therapy is necessary to sustain responses, and the median response duration ranges between 4 and 18 months [73,74].

Most series have determined that patients with relatively well preserved immunologic function are more likely to respond, suggesting an immunomodulatory action for IFN-alfa [75]. Patients without prior opportunistic infections or night sweats and a CD4 count above 200 to 400/mm³ are most likely to benefit from treatment.

Multiple trials have established that combination treatment with chemotherapeutic agents and increases hematologic toxicity without improving results. When indicated on the basis of the CD4 count, zidovudine (Retrovir) can be combined with IFN in doses of less than 8 to 10 million U/d with an acceptable incidence of neutropenia [76]. The addition of granulocyte macrophage colony-stimulating factor (GM-CSF, sargramostim [Leukine]) to the regimen may allow further IFN dose escalation [77].

Kaposi's sarcoma rarely affects patients without HIV, and so, IFN-alfa has not been adequately investigated in this small population.

Hodgkin's Disease and Non-Hodgkin's Lymphomas

Hodgkin's Disease: Few data are available regarding IFN-alfa therapy for Hodgkin's disease. In the two largest reported series [78,79], 8 of 44 heavily pretreated patients responded. If IFN is to have a role in this disease, it will likely be in conjunction with other treatment modalities.

Low-Grade Follicular Non-Hodgkin's Lymphoma: In non-Hodgkin's lymphoma, IFN-alfa has shown promising activity against low-grade follicular disease. A 40% to 50% response rate can be achieved using 3 to 50 million U/m² administered intramuscularly from three to seven times a week. Most responses are partial, and the median response duration is 8 months. Importantly, there does not appear to be cross-resistance between IFN and cytotoxic drugs. There might be a dose-response relationship, as many patients who responded received higher doses.

Three randomized studies using the combination of chemotherapy and IFN-alfa in follicular non-Hodgkin's lymphoma have been reported. Two large multi-institutional studies demonstrated an improved duration of remission when intermittent IFN-alfa was added to combination chemotherapy [80,81]. The third study, using single-agent chlorambucil for induction, did not demonstrate any improvement in remission rate from the addition of IFN-alfa [82]. Preliminary data show a median remission duration of 2 years in the chlorambucil-only arm, whereas no patients in the IFN-alfa arm have yet achieved a remission this long (P = .013). A survival advantage was documented in the two largest studies [80,81], but not in an Eastern Cooperative Oncology Group study with longer follow-up [83].

Maintenance Therapy: Several studies have explored low-dose (2 million U/m² three times a week) IFN-alfa maintenance therapy following remission induction in low-grade non-Hodgkin's lymphoma. McLaughlin et al [84] investigated IFN-alfa maintenance after remission induced by cyclophosphamide, doxorubicin (Adriamycin), vincristine, prednisone, and bleomycin (Blenoxane) in low-grade non-Hodgkin's lymphoma. The failure-free survival rate was 47% in the IFN-treated patients at 5 years. This result compared favorably with that for a previously treated control group, who had 29% failure-free survival at 5 years (P = .01).

A minor improvement in relapse-free survival was also reported in a smaller Italian study (reviewed in Gaynor and Fisher) [85], in which only 4 of 14 IFN-treated patients relapsed versus 10 of 25 patients given no maintenance therapy. An improved disease-free survival was also reported by the European Organization for Research and Treatment of Cancer [86]. Although promising, these results should be interpreted cautiously, since the patient numbers are small and no survival benefit has been demonstrated.

Intermediate- or High-Grade Disease: Aviles et al [87] reported that IFN maintenance may improve survival in patients with diffuse large-cell lymphoma who achieve a complete response after combination chemotherapy.

Cutaneous T-Cell Lymphoma

Cutaneous T-cell lymphomas are a heterogeneous group of malignancies of mature helper T-cells. They are characterized by primary skin involvement of several forms, from small papules to diffuse erythroderma or large skin tumors. Although the natural histories of these forms vary, patients with any of the forms usually die of advanced lymphoma or secondary infections. Despite predominant skin involvement initially, systemic spread occurs early in the course of the disease. Therefore, although local therapy can effectively control early stages of the disease, systemic therapy has been evaluated in an attempt to decrease the late progression of the disease. Chemotherapeutic agents can induce high response rates, but no survival advantage is obtained [88,89].

Single-Agent Therapy With IFN-alfa: Bunn et al [90] first reported the activity of recombinant IFN-alfa in cutaneous T-cell lymphomas. Twenty patients with advanced stages of the disease refractory to two or more previous therapies were treated with IFN-alfa at a dose of 50 million U/m² intramuscularly, three times per week. Nine patients achieved partial remissions lasting a median of 5 months [90]. Numerous other reports [91-95] have confirmed these observations (Table 8), with response rates of 45% to 65% (reviewed in Bunn et al)[89]. However, with IFN-alfa as a single agent, most of the responses were partial and not long lasting.

Combination Therapy With IFN-alfa: No studies are available for the combination of IFN-alfa with conventional chemotherapy. However, the agent has been used in combination with other therapeutic modalities. The combination of psoralen and ultraviolet A light (PUVA) is effective for the control of skin lesions in cutaneous T-cell lymphoma. Kuzel et al [96] used IFN-alfa at a dose of 12 million U/m² concurrently with PUVA in 39 patients and continued for 2 years. The overall response rate was 90%, with a complete responses in 62% and partial responses in 28%. The median response duration was 28 months [96]. Other smaller studies have confirmed these encouraging results (Table 8)[97,98].

Retinoids are also effective in the management of cutaneous T-cell lymphomas, with overall response rates of 58%, although responses are usually of short duration. IFN-alfa has been combined with retinoids, including isotretinoin (13-cis-retinoic acid, Accutane) and etretinate (Tegison). The response rate with this combination is approximately 60%, with 10% complete responses (Table 8). These results are similar to those achieved with IFN-alfa as a single agent, and so it is unclear whether the combination adds anything to IFN-alfa alone [88].

Nucleoside analogs, including fludarabine (Fludara), 2-chlorodeoxyadenosine (2CdA), and pentostatin (Nipent), are a new group of chemotherapeutic agents with significant activity against lymphoid malignancies. They are effective as single agents, with pentostatin producing response rates of 40% to 60%; 2CdA, 20% to 30%; and fludarabine, 20%. IFN-alfa has been used in combination with fludarabine [99] and pentostatin [100], and the overall response rate is 40% to 45%, with a complete response rate of 5% to 10% (Table 8). Therefore, these combinations do not seem to be superior to those of IFN-alfa alone.

Other Interferons: IFN-beta and IFN-gamma also have activity in cutaneous T-cell lymphomas, although they seem to be less effective than IFN-alfa. The overall response rates with these agents are 25% and 30% [89], respectively.

Melanoma and Renal-Cell Carcinoma

Both melanoma and renal-cell carcinoma have shown some response to IFN-alfa and IL-2. It is important when reviewing the results, however, to remember that the natural histories of these tumors can be highly variable and that accurate prognostic indicators are not always available. Thus, interpretation of the results of small phase II trials in selected patients is very difficult.

Melanoma: In vitro, melanoma cells are among the most sensitive of all tumor cells to the antiproliferative effects of IFN-alfa [101]. This level of sensitivity has not been reproduced in vivo, however.

As a single agent in metastatic disease, IFN-alfa reproducibly yields overall response rates of 5% to 30% (mean, 15%), based on 11 studies reporting on a total of 315 patients (reviewed in Kirkwood and Ernstoff)[102]. The doses used ranged from 10 to 50 million U/m², usually given three times a week. There was no clear evidence of a dose-response relationship. Although the median response duration was only 4 months, some patients remained free of disease for many years. Indicators of likely response to interferon are low tumor burden and disease limited to the skin, lymph nodes, or soft tissue [102]. IFN-alfa has no activity against central nervous system disease and does not prevent metastasis to the central nervous system, even in responding patients.

In an attempt to improve these results, IFN-alfa has been used in combination with standard chemotherapeutic agents, although in vitro data fail to support claims of synergism against melanoma cells [102]. On the basis of an early report showing a 30% response rate for dacarbazine and IFN-alfa [103], larger randomized studies were begun. The results from phase III studies published to date have shown mixed results (Table 9)[104-107].

Falkson et al [105] showed a significantly improved response rate, duration of response, and survival. Other studies, however, failed to detect any improvement in response rate or survival. If combination therapy imparts a survival benefit, it is likely to be smaller than that suggested by Falkson et al [105]. Two of the studies [105,107] reported prolonged remission duration among IFN-alfa-treated patients. IFN-alfa in conjunction with combination chemotherapy has not produced better results than combination chemotherapy alone.

If IFN-alfa can prolong remission duration, it would be reasonable to consider it as adjuvant treatment following complete resection of high-risk melanoma. Several studies of this approach are being carried out by cooperative groups. IFN-gamma has also been investigated in this setting but has been found to offer no benefit [108].

IL-2, alone or combined with adoptive cellular immunotherapy [109], has also been investigated in melanoma. As a single agent, it has achieved a response rate of 10% to 20% [110]. The use of IL-2 in combination with chemotherapy has produced response rates of 13% to 42%, which are not clearly superior to those with chemotherapy alone [111-114]. Despite animal models suggesting synergistic activity of IL-2 with IFN-alfa, the response rates obtained with this combination are not superior to the results with either agent alone [115].

More recently, chemotherapy has been given in combination with IL-2 and IFN-alfa (a schedule called biochemotherapy). With single-agent cisplatin (Platinol), the combination achieved a response rate of 54%. Using combination chemotherapy together with the biologic agents, the response rate has ranged from 33% to 63% (depending on the schedule, Table 10)[116-119]. The impact on survival remains to be determined [120].

Renal-Cell Carcinoma: Renal-cell carcinoma was one of the first tumors in which IFN activity was demonstrated [121]. Average response rates obtained using single-agent IFN-alfa are 10% to 15%, with most responses being partial. Long-term results, however, do not show prolongation of survival [122,123]. IFN-beta and IFN-gamma have no advantage over IFN-alfa.

On review, Muss [122] could find no evidence for a dose-response relationship and recommended IFN-alfa, 5 to 10 million units three times a week, as the range of doses conferring the best therapeutic index. The median duration of response averages 6 to 10 months, and only 20% of responses are maintained beyond 12 months. Patients who have undergone prior nephrectomy and have had a long disease-free interval as well as those with metastatic disease confined to the lung are more likely to respond. The limited data available do not support adjuvant IFN-alfa treatment after complete resection of renal-cell carcinoma [124].

IL-2 has proven activity in metastatic renal-cell carcinoma [125]. Both bolus and continuous-infusion IL-2 generate responses in 10% to 30% of patients, but the continuous-infusion protocol is better tolerated [126]. More recently, low-dose subcutaneous IL-2 has been used and produced similar results [127]. As in melanoma, the addition of adoptive cellular immunotherapy to IL-2 does not predictably improve response rates in metastatic renal-cell carcinoma. High pretreatment levels of C-reactive protein and IL-6 predict a diminished response to IL-2 [128] and may be useful in selecting patients most likely to benefit from this therapy.

The combination of IL-2 and IFN has achieved higher response rates in some studies [124]. One preliminary report on this combination used together with fluorouracil achieved a response rate of 47%, but these results need further confirmation [129].

Conclusions

With or without adoptive cellular immunotherapy, IL-2 remains an innovative treatment, but as currently administered it produces a low response rate in two relatively uncommon tumors (melanoma and renal-cell carcinoma) and significant toxicity. The feasibility of administering multidrug chemotherapy, IFN, and IL-2 has been established [130]; however, toxicity must be effectively managed and response rates improved significantly above those achievable with single-modality treatment before this approach gains a confirmed place in the treatment of patients with these cancers.

Although its mechanism of action is still incompletely understood, IFN-alfa is now firmly established as a powerful therapeutic compound in hairy-cell leukemia. IFN-alfa has also shown efficacy against other neoplasms, although these are mainly hematologic malignancies; it has less pronounced activity in solid tumors. In general, IFN-alfa is most effective against early or low-volume disease. Other strategies that merit exploration include synergistic combinations with retinoids, radiation potentiation, and augmentation of chemotherapy-induced cytotoxicity [131].

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