Native Antibody and Antibody-Targeted Chemotherapy for Acute Myeloid Leukemia

Eric L. Sievers Clinical Research Division Fred Hutchinson Cancer Research Center Seattle, Washington 98109; and Department of Pediatrics University of Washington Seattle, Washington 98105

Native Antibody and Antibody-Targeted Chemotherapy for Acute Myeloid Leukemia

I. Chapter Overview

________________________________________________________________________________________________

CD33 is a normal myeloid surface antigen that is expressed by leukemic blast cells from the vast majority of patients with acute myeloid leukemia (AML). Early clinical studies performed in New York and Seattle demonstrated that unconjugated antibodies directed against CD33 specifically target sites of normal and abnormal hematopoiesis. These findings provided a rationale for the development of a second generation of antibodies capable of delivering cytotoxic agents to leukemic blast cells. One such agent, MylotargTM (gemtuzumab ozogamicin) was approved in 2000 by the U.S. Food and Drug Administration (FDA) for the treatment of patients with CD33-positive AML in first relapse who are 60 years of age or older and who are not considered candidates for other types of cytotoxic chemotherapy. Among 277 adult patients with CD33-positive AML in first relapse, 26% experienced an overall response after Mylotarg monotherapy. Despite Advances in Pharmacology, Volume 51 Copyright 2004, Elsevier Inc. All rights reserved. 1054-3589/04 $35.00

169

170

Eric L. Sievers

myelosuppression, hyperbilirubinemia, and elevated hepatic transminases being commonly observed, the agent was reasonably well tolerated by adult patients with advanced AML. Newer treatment regimens combining Mylotarg and conventional chemotherapy have yielded a surprisingly high remission induction rate in de novo AML patients. These preliminary findings have prompted the planning of prospective, randomized studies in the U.S. and the U.K. that should help us refine our use of this novel immunoconjugate.

II. Introduction

____________________________________________________________________________________________________________

Almost 50 years have passed since Pressman and Korngold showed that antibodies could target tumor cells, and 2 decades have elapsed since Kohler and Milstein made large-scale production of monoclonal antibodies feasible. Why is antibody treatment of AML uncommon? Despite numerous attempts to identify them, leukemia cells rarely express novel antigenic targets that are not otherwise expressed by normal tissues. For this reason, normal cell surface antigens with expression restricted to the hematopoietic system have been selected as targets. With the stunning exception of rituximab in CD20positive lymphomas, most antibodies targeting various normal hematopoietic antigens have proven clinically ineffective as therapeutic agents against hematologic malignancies. In the last few years, however, strategies employing antibodies for the treatment of patients with AML have begun to bear fruit.

A. CD33: A Normal Antigen Expressed During Myeloid Differentiation Investigators on opposite coasts at the Memorial Sloan-Kettering Cancer Center (MSKCC) and the Fred Hutchinson Cancer Research Center (FHCRC) have both selected the normal myeloid antigen CD33 as an attractive target for antibody-based therapy. Targeting CD33 makes sense for a variety of reasons. First, leukemic blast cells from more than 80–90% of AML patients express the antigen at high levels (Dinndorf et al., 1986; Griffin et al., 1984). Second, because nonhematopoietic tissues and normal primitive hematopoietic precursors both lack CD33 expression, relatively selective targeting of a malignant population of cells can be achieved. Because primitive precursor cells remain unscathed, hematopoietic recovery readily occurs over a several-week period. Finally, antibody and any conjugated cytotoxic agent are internalized after CD33 cell surface engagement by antibody. This modulation of the antigen–antibody complex enables the targeted delivery of a radionucleotide, protein toxin, or other cytotoxic substance into the cytoplasm of leukemic cells.

Antibody-Targeted Therapy of AML

171

In a tightly regulated manner, CD33 is expressed as pluripotent hematopoietic stem cells mature and give rise to progenitors with diminished self-renewal capacity and a greater degree of differentiation (Andrews et al., 1983; Dinndorf et al., 1986; Griffin et al., 1984). CD33 is expressed by maturing normal hematopoietic cells, but stem cells lack surface expression of CD33 (Andrews et al., 1989). In marrow long-term culture experiments from some patients with AML, selective ablation of CD33-positive cells from leukemic marrow aspirates resulted in the growth of normal nonclonal granulocytes and monocytes (Bernstein et al., 1987, 1992). Although these findings suggested that selectively targeting and eliminating CD33-positive cells might enable patients with AML to achieve clinical remissions, other investigators have also provided compelling data suggesting that the more rare, clonogenic leukemic cell does not express CD33 and other lineage-associated antigens. Bonnet and Dick (1997) demonstrated growth of AML in an immunodeficient mouse model after infusion of isolated primitive (CD34þ CD38) precursors from human marrow specimens obtained from AML patients. In reconciling these apparently conflicting data, it is conceivable that selective ablation of CD33-positive cells using antibody might rid the body of large numbers of mature leukemic cells without fully deleting the rare progenitor cells from which the leukemia arises. This hypothesis is buttressed by the clinical observation that AML remissions induced by antibody-targeted ablation of CD33-positive cells (described later) were relatively brief if further definitive therapy was not subsequently administered.

B. Unconjugated Anti-CD33 Antibody Cytotoxicity from unconjugated monoclonal antibodies occurs by several mechanisms. In antibody-dependent cellular cytotoxicity (ADCC), granulocytes and tissue macrophages eliminate target cells coated with antibody through binding of the antibody Fc receptor. In complementdependent cellular cytotoxicity (CDC), the Fc portion of immunoglobulin bound to tumor cells induces cell death by complement fixation. ADCC is likely the mechanism associated with the impressive non-Hodgkin’s lymphoma tumor regressions seen in association with anti-CD20 antibodies (Buchsbaum et al., 1992). Data also suggest that ligation of CD20 by antibody interferes with normal signal transduction, directly leading to apoptosis without a significant component of ADCC (Shan et al., 1998). Whereas rituximab has been shown to be effective therapy in certain types of nonHodgkin’s lymphoma, unconjugated antibody approaches targeting CD33 expressed by AML cells have shown limited efficacy for patients with large tumor burdens. However, some benefit might exist for patients with acute promyelocytic leukemia (APL) who harbor minimal residual disease (Jurcic et al., 2000).

172

Eric L. Sievers

Investigators at MSKCC and FHCRC first employed trace radioiodinated anti-CD33 antibodies in patients with advanced AML. Intravenous administration of approximately 5 mg/m2 of antibody resulted in selective targeting and rapid saturation of leukemic blast cells in patients’ peripheral blood and marrow (Appelbaum et al., 1992; Scheinberg et al., 1991). Although no significant clinical efficacy was observed by using these early strategies, investigators from Protein Design Labs and MSKCC performed several clinical evaluations of HuM195, a humanized monoclonal antibody created by grafting the CDR regions of the M195 anti-CD33 murine monoclonal antibody onto the Eu human IgG1 antibody (Co et al., 1992). In a pilot monotherapy study conducted by MSKCC, patients received supersaturating doses of HuM195 at doses of 12 or 36 mg/m2/day on Days 1 through 4, with repeat doses provided on Days 15 through 18 (Caron et al., 1998). Among 10 patients with advanced myeloid leukemias (9 AML and 1 CML) treated, 1 achieved a complete remission. In a larger randomized study, 50 patients with advanced leukemia (median age of 62 years) received either 12 or 36 mg/m2 of HuM195 daily for 4 consecutive days weekly for a total of four courses (Feldman et al., 2003). Two complete remissions and one partial remission were observed among 49 evaluable patients. Nine additional patients experienced decreases in blast counts ranging from 30–74%. The antibody treatments were extremely well tolerated. Infusion-related fevers and chills were commonly observed, but serious organ toxicity was uncommon. No immune responses to HuM195 were detected. Overall, only patients with minimal tumor burden experienced clinical benefit from HuM195 monotherapy. Because HuM195 monotherapy was associated with little toxicity, the unconjugated antibody was evaluated in combination with conventional chemotherapy in a prospective randomized study (Feldman et al., 2002). The primary endpoint of the study was response rate. Treatment consisted of mitoxantrone, cytarabine, and etoposide plus or minus HuM195 given in two courses at the completion of the induction chemotherapy regimen. The study enrolled 191 patients with a median age of 57 years who had AML that was initially refractory to therapy or had relapsed with remission duration of less than 1 year. One quarter of enrolled patients had a history of an antecedent hematologic disorder. Although the two randomized cohorts were reasonably well matched for demographic features, the antibody treatment group included a disproportionately high portion with active infections, primary refractory AML, or a prior antecedent hematological disorder. Inclusion of HuM195 was well tolerated by study patients. Although the overall response rate observed with HuM195 was 36% compared with 28% among those not treated with antibody, this difference was not statistically significant at p ¼ 0.28. Unfortunately, this suggestion of improved response rate did not translate into improved clinical outcome as

Antibody-Targeted Therapy of AML

173

no survival difference was observed between the two treated populations with extended follow-up. Although results from the antibody chemotherapy combination trial for advanced AML patients were disappointing, mature data in patients with APL in complete remission using HuM195 monotherapy as maintenance are encouraging, particularly among patients harboring minimal residual disease. Of 27 APL patients induced into first remission with all-trans retinoic acid (ATRA), followed by idarubicin and cytarabine consolidation therapy, 25 had evidence of residual leukemia by reverse transcription-polymerase chain reaction (RT-PCR) before HuM195 treatment (Jurcic et al., 2000). Subsequently, they received an additional 6 months of maintenance therapy with HuM195 given monthly in two doses separated by 3 or 4 days. Bone marrow aspirates were evaluated serially for the PML/RAR-
mRNA by RT-PCR. Among 22 patients evaluable, HuM195 monotherapy appeared to result in the conversion to RT-PCR negativity in 11 patients. Overall, 25 of 27 (93%) patients with de novo APL remained in clinical complete remission for 7þ to 58þ months, with a median follow-up of 60þ months. Taken together, these results suggest that HuM195 might have reasonable efficacy for APL patients who harbor evidence of residual disease after induction therapy. Because the leukemia progenitor cell in APL is more likely to express CD33, APL might be uniquely amenable to antibody strategies targeting CD33.

C. Anti-CD33 Conjugated with Calicheamicin: Gemtuzumab Ozogamicin Clinical studies of p67.6, an anti-CD33 antibody developed in the laboratory of Dr. Irwin Bernstein in Seattle, demonstrated that rapid and specific targeting of CD33-positive cells could be achieved in vivo (Appelbaum et al., 1992). Unfortunately, a short marrow residence time was observed with conventionally radiolabeled anti-CD33, thus limiting potential clinical efficacy with this approach. Hence, a cytotoxic agent was sought for conjugation to the p67.6 anti-CD33 antibody. Because the original p67.6 murine anti-CD33 antibody was immunogenic, a humanized monoclonal antibody containing approximately 98% human amino acid sequences was created. Gemtuzumab was synthesized by grafting the p67 anti-CD33 murine monoclonal CDR sequences onto a human IgG4 isotype antibody. The IgG4 antibody isotype was selected because it was associated with fewer Fc-dependent functions and a relatively long half-life in circulation. The humanized antibody gemtuzumab had CD33-binding affinity similar to that of the precursor murine p67 antibody. Administration of anti-CD33 antibody results in rapid saturation of CD33 sites throughout the body. The antigen–antibody complex is then rapidly internalized into the cell, enabling antibody-targeted delivery of

174

Eric L. Sievers

a cytotoxic agent into the intracellular space. Calicheamicin, a potent antitumor antibiotic that cleaves double-stranded DNA, was conjugated to a humanized anti-CD33 antibody to create gemtuzumab ozogamicin (Mylotarg). As illustrated in Fig. 1, assays of cell surface binding of gemtuzumab suggest that the antigen–antibody complexes are rapidly internalized (van der Velden et al., 2001). From studies that follow the fate of internalized antibodies, it is suggested that the endocytosed anti-CD33 complexes translocate to lysosomes, where hydrolytic release of calicheamicin from the linker occurs. Unlike conventional chemotherapy agents that cause single- or double-strand lesions through radical intermediates or topoisomerases, the extremely reactive calicheamicin behaves like ionizing radiation by cleaving both DNA strands simultaneously. These site-specific double-stranded DNA breaks result in apoptotic cell death (Ellestad et al., 1995; Sissi et al., 1999; Zein et al., 1988). Gemtuzumab was evaluated in three in vitro tests for specific targeting and killing of leukemia cells: cultured HL-60 leukemia cells, HL-60 human xenograft tumors, and marrow specimens from AML patients in colonyforming assays. Uniformly, leukemia cells were ablated with high specificity compared with that of calicheamicin linked to antibodies directed against nonspecific antigens or unconjugated anti-CD33 antibody.

D. Clinical Studies of Gemtuzumab Ozogamicin In collaboration with Wyeth-Ayerst Research, investigators at FHCRC and the City of Hope National Medical Center conducted a Phase I study of gemtuzumab in which patients with relapsed or refractory CD33-positive AML were treated with escalating doses of drug every 2 weeks for three doses (Sievers et al., 1999). Leukemia was ablated from the blood and marrow of 8 of 40 (20%) patients and blood counts normalized in three (8%) patients. Figure 2 shows the relationship between hematologic parameters and time for a patient who received gemtuzumab at 4 mg/m2 per dose. Gemtuzumab doses up to 9 mg/m2 were generally well tolerated, and a postinfusion syndrome of fever and chills was the most common side effect. Modest and reversible hepatic transaminase elevations and hyperbilirubinemia was observed in several patients who received gemtuzumab at higher dose levels. Subsequent prospective international Phase II studies evaluated gemtuzumab in 142 patients with CD33-positive AML in first untreated relapse (Sievers et al., 2001). Three similar concurrent Phase II studies evaluated safety and efficacy of gemtuzumab in patients with CD33-positive AML in first relapse, lacking history of an antecedent hematologic disorder. The initial report described here detailed findings from 142 adults with a median age of 61 years. Among those in whom cytogenetics were documented, 39% had abnormalities known to be associated with unfavorable outcomes.

Antibody-Targeted Therapy of AML

175

FIGURE 1 Maximal Mylotarg binding to leukocyte subsets. Maximal Mylotarg binding to different leukocyte subsets was analyzed prior to start of the first (hatched bars) and second (open bars) Mylotarg treatment cycle by incubating PB with an excess of Mylotarg in vitro, followed by detection with biotin-conjugated antihuman IgG4 and streptavidin FITC. (A) Maximal Mylotarg binding to AML blast cells (cycle 1: n ¼ 86; cycle 2: n ¼ 35), monocytes (n ¼ 33; n ¼ 45), granulocytes (n ¼ 55; n ¼ 32), and lymphocytes (n ¼ 61; n ¼ 43). Patient numbers differ between leukocyte subsets and between cycle 1 and cycle 2 because not all patients received a second treatment cycle, in some patients the PB sample was not available or of too low quality, or too few events were available for a reliable analysis of all leukocyte subsets. (B) Maximal Mylotarg binding to AML blast cells from patients showing less than 5% blast cells in their PB just prior to the start of the second treatment cycle (blast cell reducers; n ¼ 27) or 5% or more blasts in PB (blast cell nonreducers; n ¼ 31 and n ¼ 26 for cycle 1 and cycle 2, respectively). Maximal Mylotarg binding data for cycle 2 could not be obtained in the blast cell reducers because the number of blast cells was too low to perform a reliable analysis. ND indicates no data. Data are expressed as mean  SD. Significant differences (P < .05; indicated by the asterisks [unpaired t test]) were observed in maximal Mylotarg binding to blast cells between cycle 1 and cycle 2 and between cycle 1 data of the blast cell reducers and blast cell nonreducers.

176

Eric L. Sievers

FIGURE 2 Relationship between hematologic parameters and time for a representative patient (FH-012) who received CMA-676 at 4 mg/m2 per dose. Arrows denote infusions of CMA-676. All counts refer to peripheral blood counts.

Patients received gemtuzumab as a 2-h intravenous infusion at a dose of 9 mg/m2 at 2-week intervals for two doses. Before therapy, patients with elevated peripheral white blood cell counts were given hydroxyurea to reduce these counts to <30,000/ml. Among the 142 patients, 46% had fewer than 5% blasts in the bone marrow after one dose of gemtuzumab, based on morphologic analysis of bone marrow aspirates. Thirty percent achieved an overall remission (OR) characterized by <5% blasts in the bone marrow, >1500 neutrophils/ml, and RBC and platelet transfusion independence. Twenty-three patients (16%) achieved CR (complete remission), and 19 (13%) obtained CRp (complete remission with incomplete platelet recovery to 100,000/ml) to produce the OR rate of 30%. Surprisingly, poor prognostic features, including advanced age and short duration of first CR, did not appear to appreciably influence the likelihood of remission induction using gemtuzumab. A 26% OR rate was seen in patients aged 60 or more, compared with 34% in younger patients. Correspondingly, the OR rate was 28% for patients who had a CR1 of less than 1 year compared with an OR rate of 32% for patients whose first remissions were longer. In addition, similar remission induction rates were observed among favorable, intermediate, and unfavorable risk cytogenetic groups as well. Based on these data, gemtuzumab

Antibody-Targeted Therapy of AML

177

was approved by the FDA in May 2000 as monotherapy for the treatment of patients with CD33-positive AML in first relapse who are >60 years of age and not considered candidates for cytotoxic chemotherapy (Bross et al., 2001). Remission durations after gemtuzumab monotherapy were short lived unless patients received consolidation with hematopoietic stem cell transplantation (HSCT) or further chemotherapy. Relapse-free survival (RFS) was measured from the date of first documented OR to relapse, death, or data cutoff. A considerable number of patients who achieved OR were sufficiently healthy to tolerate subsequent HSCT. In the published report of 142 patients with recurrent AML (Fig. 3), the median RFS was at least 8.9 months among OR patients who received allogeneic (n ¼ 10) or autologous (n ¼ 5) HSCT (Sievers et al., 2001). In contrast, the median RFS was only 2.1 months for the 23 OR patients who received no further therapy. These data suggest that postremission therapy, particularly in the form of allogeneic hematopoietic stem cell transplant, enables the majority of patients who achieve gemtuzumab monotherapy responses to remain in extended remissions.

FIGURE 3 Relapse-free survival for OR patients who received HSCT ( ) and for OR patients who received no further therapy (&) as postremission therapy. Fifteen OR patients received HSCT (median > 8.9 months), and 23 OR patients received no further therapy (median 2.1 months).

178

Eric L. Sievers

Most gemtuzumab-treated patients experienced a postinfusion syndrome of fevers and chills. Because hypotension rarely developed several hours after administration of gemtuzumab, close medical monitoring for several hours following infusion is suggested. Hypotension did not occur in any patients after the second dose of gemtuzumab. Severe neutropenia and thrombocytopenia were regularly observed because gemtuzumab ablates normal myeloid and megakaryocytic precursors. Twenty-eight percent of patients developed serious infection of grade 3 or 4. Mucositis was rarely observed in only 4% of patients. No treatment-related cardiotoxicity, cerebellar toxicity, or alopecia was seen. No patients in the Phase II studies developed antiglobulin or anticonjugate immune responses. For reasons that are not entirely clear, gemtuzumab can induce hepatic dysfunction. Moderate but typically reversible hepatic transaminase and bilirubin elevations were commonly seen. Among 142 patients, 1 patient died of liver failure on Day 22 of study and another died on Day 156 of study with persistent ascites and hepatic splenomegaly. A recent report summarized gemtuzumab monotherapy in a population of 101 patients with first untreated relapse of AML, including 80 treated on the previous studies, who were 60 years and older (Larson et al., 2002). The overall remission rate was 28%. CR was observed in 13% of patients and CRp in 15%. The median survival was 5.4 months for all enrolled patients and 14.5 months and 11.8 months for patients achieving CR and CRp, respectively.

III. Lingering Questions Regarding Immunoconjugate Therapies

_____________________________________________________________________________

A. Is Acute Promyelocytic Leukemia Unusually Sensitive to Gemtuzumab? Because primitive hematopoietic stem cells as defined by CD34þ/ CD38 antigens do not appear to be involved in the neoplastic process in APL (Turhan et al., 1995), it has been hypothesized that gemtuzumab might have a high likelihood of ablating APL progenitor cells. Petti et al. (2001) reported results from a patient with APL who was unusually refractory to conventional approaches and achieved prolonged hematological and molecular remission after two doses of gemtuzumab. In a prospective trial of 19 patients with de novo APL, investigators at the M. D. Anderson Cancer Center evaluated ATRA in combination with gemtuzumab as a possible replacement for anthracycline that is typically used for this disease (Estey et al., 2002a). Once patients achieved CR, eight additional courses of gemtuzumab and ATRA were delivered every 4–5 weeks. Of 19 evaluable patients, 16 (84%) achieved CR and most remained PCR-negative 2–4

Antibody-Targeted Therapy of AML

179

months from the date of achieving remission. Patients in remission tolerated maintenance doses of gemtuzumab quite well. A median of five post-CR courses was given; three patients received eight and four patients received seven post-CR courses of treatment. These findings demonstrate that gemtuzumab is safe in repeated doses, and suggest that it has clinical activity in APL.

B. Is Gemtuzumab Effective in the Treatment of Older Patients with De Novo AML? Investigators at the M. D. Anderson Cancer Center also evaluated gemtuzumab monotherapy in 51 patients aged 65 years or more with newly diagnosed AML and advanced myelodysplastic syndrome (Estey et al., 2002b). Gemtuzumab was given in a compressed dose schedule on Days 1 and 8, or as directed by the product label on Days 1 and 15. Interleukin 11 (IL-11) was administered by random treatment assignment to half of enrolled patients. Among patients who received gemtuzumab monotherapy, only 2 of 26 (8%) entered remission. Among those who also received IL-11, 9 of 25 (36%) achieved CR. After comparing these data with historical results obtained at their center with idarubicin plus cytarabine, the authors concluded that survival with gemtuzumab monotherapy (with or without IL-11) appeared to be inferior.

C. Why Do Liver Toxicities Occasionally Occur with Gemtuzumab Ozogamicin? Moderately severe elevations in hepatic transaminases and bilirubin occurred at a median of 8 days after treatment in about a quarter of 142 treated patients treated in the Phase II studies (Sievers et al., 2001). Although laboratory abnormalities were usually transient and reversible, one patient experienced liver failure and died. A second patient died with persistent ascites and hepatosplenomegaly. A venoocclusive-like disease (VOD) characterized by ascites, weight gain, and moderate elevations in bilirubin was observed in 11 of 271 (4%) patients treated in the collective clinical trials dataset, and in 6 of 120 (5%) patients in the compassionate-use program (Sievers et al., 2000). These collective clinical features have recently been termed sinusoidal obstruction syndrome by McDonald et al. (2002). Among 36 patients who received Mylotarg before transplant, 3 patients (8%) died of VOD (Sievers et al., 2000). Conversely, among 23 consecutive patients who were treated with Mylotarg after transplant, 8 patients (35%) developed fatal liver disease (Rajvanshi et al., 2002). In instances in which hepatic tissue was obtained from patients manifesting signs of sinusoidal occlusion syndrome, a consistent pattern of hepatic endothelial cell damage with marked increase in collagen deposition was observed. Although hepatic Kupffer cells express CD33 and are thus targeted by gemtuzumab, the pathophysiology of

180

Eric L. Sievers

hepatic toxicity remains enigmatic. Gemtuzumab should be given with great caution in patients who have preexisting hepatic injury or a history of allogeneic HSCT.

D. Why Are Many Patients Resistant to Gemtuzumab? Although the overall response rate observed with gemtuzumab is comparable with that of conventional agents, most relapsed AML patients treated on the Phase II studies failed to enter remission. Several biological features of leukemic cells could interfere with Mylotarg-induced cytotoxicity and be manifested as drug resistance. For instance, insufficient levels of CD33 might be expressed by a subpopulation of leukemic blast cells, rendering the cells resistant to cytotoxicity. However, among patients in the Phase II trials with 80% CD33-positive leukemic blasts that stained at four times above background, no correlation between CD33 expression and clinical response was observed (Sievers et al., 2001). Gemtuzumab treatment also does not appear to select for antigen-negative subclones. In instances in which a subsequent relapse occurred after an initial response to gemtuzumab, leukemic cells typically express CD33 at high levels. Several clinical observations suggest that MDR transporter proteins extrude free calicheamicin from leukemic cells. Clinical remissions with gemtuzumab were also associated with low blast cell MDR function (Linenberger et al., 2001). Because elevated blast cell drug efflux might be blocked by cyclosporine A (CSA), induction therapy with gemtuzumab and CSA might target the cytotoxic agent to the leukemic cells and toxicity to normal tissues might be limited. Clinical trials to explore this hypothesis are being designed.

IV. Summary

_________________________________________________________________________________________________________________

Some efficacy has been observed in a small number of AML patients harboring a low disease burden at the time of relapse and others with APL using the unconjugated antibody HuM195. In the setting of antibody–drug conjugates, approximately 25% of patients with AML in first relapse achieved remission after gemtuzumab monotherapy. Because blast cell expression of the MDR phenotype is associated with an inferior response with gemtuzumab, trials exploring MDR inhibition are planned. Without subsequent consolidation treatment, patients who respond to gemtuzumab will likely experience remission durations that average only 2 months. In comparison, gemtuzumab responders who are consolidated with HSCT sustain remissions that exceeded 18 months. Despite data suggesting that gemtuzumab is reasonably well tolerated by adults and children, no randomized trials have compared gemtuzumab monotherapy with combination chemotherapy. Neutropenia and thrombocytopenia were universally

Antibody-Targeted Therapy of AML

181

observed, but mucositis was rarely seen. Gemtuzumab is targeted to CD33expressing cells using an antibody, but liver injury appeared with moderate frequency. New clinical trials should shed light on the etiology of hepatic injury and possibly elucidate what clinical scenarios demand greater caution with gemtuzumab. Although emerging data suggest that combinations of conventional chemotherapy and gemtuzumab result in a relatively high remission induction rate for newly diagnosed AML patients, these findings must be regarded as highly preliminary. Nonetheless, a first generation of antibody-targeted chemotherapy has made its way to the clinics, certainly to be followed by second-generation strategies that improve antileukemic efficacy while simultaneously limiting damage to normal tissues.

References

_______________________________________________________________________________________________________________________

Andrews, R. G., Singer, J. W., and Bernstein, I. D. (1989). ‘‘Precursors of colony-forming cells in humans can be distinguished from colony-forming cells by expression of the CD33 and CD34 antigens and light scatter properties.’’ J. Exp. Med. 169, 1721–1731. Andrews, R. G., Torok-Storb, B., and Bernstein, I. D. (1983). ‘‘Myeloid-associated differentiation antigens on stem cells and their progeny identified by monoclonal antibodies.’’ Blood 62(1), 124–132. Appelbaum, F. R., Matthews, D. C., Eary, J. F., Badger, C. C., Kellogg, M., Press, O. W., Martin, P. J., Fisher, D. R., Nelp, W. B., Thomas, E. D., and Bernstein, I. D. (1992). ‘‘The use of radiolabeled anti-CD33 antibody to augment marrow irradiation prior to marrow transplantation for acute myelogenous leukemia.’’ Transplantation 54(5), 829–833. Bernstein, I. D., Singer, J. W., Andrews, R. G., Keating, A., Powell, J. S., Bjornson, B. H., Cuttner, J., Najfeld, V., Reaman, G., Raskind, W., Sutton, D. M. C., and Fialkow, P. J. (1987). ‘‘Treatment of acute myeloid leukemia cells in vitro with a monoclonal antibody recognizing a myeloid differentiation antigen allows normal progenitor cells to be expressed.’’ J. Clin. Invest. 79(4), 1153–1159. Bernstein, I. D., Singer, J. W., Smith, F. O., Andrews, R. G., Flowers, D. A., Petersens, J., Steinmann, L., Najfeld, V., Savage, D., Fruchtman, S., Arlin, Z., and Fialkow, P. J. (1992). ‘‘Differences in the frequency of normal and clonal precursors of colony-forming cells in chronic myelogenous leukemia and acute myelogenous leukemia.’’ Blood 79(7), 1811–1816. Bonnet, D., and Dick, J. E. (1997). ‘‘Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell.’’ Nature Med. 3(7), 730–737. Bross, P. F., Beitz, J., Chen, G., Chen, X. H., Duffy, E., Kieffer, L., Roy, S., Sridhara, R., Rahman, A., Williams, G., and Pazdur, R. (2001). ‘‘Approval summary: gemtuzumab ozogamicin in relapsed acute myeloid leukemia.’’ Clin Cancer Res. 7(6), 1490–1496. Buchsbaum, D. J., Wahl, R. L., Normolle, D. P., and Kaminski, M. S. (1992). ‘‘Therapy with unlabeled and 131I-labeled pan-B-cell monoclonal antibodies in nude mice bearing Raji Burkitt’s lymphoma xenografts.’’ Cancer Res. 52(23), 6476–6481. Caron, P. C., Dumont, L., and Scheinberg, D. A. (1998). ‘‘Supersaturating infusional humanized anti-CD33 monoclonal antibody HuM195 in myelogenous leukemia.’’ Clin Cancer Res. 4(6), 1421–1428. Co, M. S., Avdalovic, N. M., Caron, P. C., Avdalovic, M. V., Scheinberg, D. A., and Queen, C. (1992). ‘‘Chimeric and humanized antibodies with specificity for the CD33 antigen.’’ J. Immunol. 148(4), 1149–1154.

182

Eric L. Sievers

Dinndorf, P. A., Andrews, R. G., Benjamin, D., Ridgway, D., Wolff, L., and Bernstein, I. D. (1986). ‘‘Expression of normal myeloid-associated antigens by acute leukemia cells.’’ Blood 67(4), 1048–1053. Ellestad, G. A., Ding, W.-D., Zein, N., and Townsend, C. A. (1995). DNA-Cleaving Properties of Calicheamicin Gamma1(I). In ‘‘Enediyne Antibiotics as Antitumor Agents’’ (D. B. Borders and T. W. Doyle, Eds.), pp. 137–160. Marcel Dekker, New York. Estey, E. H., Giles, F. J., Beran, M., O’Brien, S, Pierce, S. A., Faderl, S. H., Cortes, J. E., and Kantarjian, H. M. (2002a). ‘‘Experience with gemtuzumab ozogamycin (‘‘mylotarg’’) and all-trans retinoic acid in untreated acute promyelocytic leukemia.’’ Blood 99(11), 4222–4224. Estey, E. H., Thall, P. F., Giles, F. J., Wang, X. M., Cortes, J. E., Beran, M., Pierce, S. A., Thomas, D. A., and Kantarjian, H. M. (2002b). ‘‘Gemtuzumab ozogamicin with or without interleukin 11 in patients 65 years of age or older with untreated acute myeloid leukemia and high-risk myelodysplastic syndrome: comparison with idarubicin plus continuous-infusion, high-dose cytosine arabinoside.’’ Blood 99(12), 4343–4349. Feldman, E., Kalaycio, M., Weiner, G., Frankel, S., Schulman, P., Schwartzberg, L., Jurcic, J., Velez-Garcia, E., Seiter, K., Scheinberg, D., Levitt, D., and Wedel, N. (2003). ‘‘Treatment of relapsed or refractory acute myeloid leukemia with humanized anti-CD33 monoclonal antibody HuM195.’’ Leukemia 17(2), 314–318. Feldman, E., Stone, R., Brandwein, J., Kalaycio, M., Moore, J., Chopra, R., Jurcic, J., Miller, C., Roboz, G., Levitt, D., Young, D., and O’Connor, J. (2002). ‘‘Phase III randomized trial of an anti-CD33 monoclonal antibody (HuM195) in combination with chemotherapy compared to chemotherapy alone in adults with refractory or first-relapse acute myeloid leukemia (AML).’’ Proceedings of ASCO 21, 261a. Griffin, J. D., Linch, D., Sabbath, K., Larcom, P., and Schlossman, S. F. (1984). ‘‘A monoclonal antibody reactive with normal and leukemic human myeloid progenitor cells.’’ Leuk Res. 8(4), 521–534. Jurcic, J. G., DeBlasio, T., Dumont, L., Yao, T. J., and Scheinberg, D. A. (2000). ‘‘Molecular remission induction with retinoic acid and anti-CD33 monoclonal antibody HuM195 in acute promyelocytic leukemia.’’ Clin. Cancer Res. 6(2), 372–380. Larson, R. A., Boogaerts, M., Estey, E., Karanes, C., Stadtmauer, E. A., Sievers, E. L., Mineur, P., Bennett, J. M., Berger, M. S., Eten, C. B., Munteanu, M., Loken, M. R., Van Dongen, J. J., Bernstein, I. D., and Appelbaum, F. R. (2002). ‘‘Antibody-targeted chemotherapy of older patients with acute myeloid leukemia in first relapse using Mylotarg (gemtuzumab ozogamicin).’’ Leukemia 16(9), 1627–1636. Linenberger, M. L., Hong, T., Flowers, D., Sievers, E. L., Gooley, T. A., Bennett, J. M., Berger, M. S., Appelbaum, F. R., and Bernstein, I. D. (2001). ‘‘Multidrug-resistance phenotype and clinical responses to gemtuzumab ozogamicin.’’ Blood 98(4), 988–994. McDonald, G. B. (2002). ‘‘Management of hepatic sinusoidal obstruction syndrome following treatment with gemtuzumab ozogamicin (mylotarg(r)).’’ Clin Lymphoma 2(Suppl. 1), S35–539. Petti, M. C., Pinazzi, M. B., Diverio, D., Romano, A., Petrucci, M. T., De Santis, S., Meloni, G., Tafuri, A., Mandelli, F., and Lo Coco, F. (2001). ‘‘Prolonged molecular remission in advanced acute promyelocytic leukaemia after treatment with gemtuzumab ozogamicin (Mylotarg CMA-676).’’ Br. J. Haematol. 115(1), 63–65. Rajvanshi, P. S. H., Sievers, E. L., and McDonald, G. B. (2002). ‘‘Hepatic sinusoidal obstruction after gemtuzumab ozogamicin (Mylotarg) therapy.’’ Blood 99(7), 2310–2314. Scheinberg, D. A., Lovett, D., Divgi, C. R., Graham, M. C., Berman, E., Pentlow, K., Feirt, N., Finn, R. D., Clarkson, B. D., Gee, T. S., Larson, S., Oettgen, H., and Old, L. (1991). ‘‘A phase I trial of monoclonal antibody M195 in acute myelogenous leukemia: Specific bone marrow targeting and internalization of radionuclide.’’ J. Clin. Oncol. 9(3), 478–490.

Antibody-Targeted Therapy of AML

183

Shan, D., Ledbetter, J. A., and Press, O. W. (1998). ‘‘Apoptosis of malignant human B cells by ligation of CD20 with monoclonal antibodies.’’ Blood 91(5), 1644–1652. Sievers, E. L., Appelbaum, F. A., Spielberger, R. T., Forman, S. J., Flowers, D., Smith, F. O., Shannon-Dorcy, K., Berger, M. S., and Bernstein, I. D. (1999). ‘‘Selective ablation of acute myeloid leukemia using antibody-targeted chemotherapy: A phase I study of an anti-CD33 calicheamicin immunoconjugate.’’ Blood 93(11), 3678–3684. Sievers, E. L., Larson, R. A., Estey, E., Stadmauer, E. A., Castaigne, S., Berger, M. S., Leopold, L. H., and Appelbaum, F. R. (2000). ‘‘Low incidence of hepatic veno-occlusive disease after treatment with gemtuzumab ozogamicin (Mylotarg, CMA-676): Relationship to hematopoietic stem cell transplantation (Abstract).’’ Blood 96, 206b. Sievers, E. L., Larson, R. A., Stadtmauer, E. A., Estey, E., Lowenberg, B. L., Dombret, H., Karanes, C., Theobald, M., Bennett, J. M., Sherman, M. L., Berger, M. S., Eten, C. B., Loken, M. R., van Dongen, J. J. M., Bernstein, I. D., and Appelbaum, F. R. (2001). ‘‘Efficacy and Safety of Gemtuzumab Ozogamicin in Patients with CD33-Positive Acute Myeloid Leukemia in First Relapse.’’ J. Clin. Oncol. 19(13), 3244–3254. Sissi, C., Aiyar, J., Boyer, S., Depew, K., Danishefsky, S., and Crothers, D. M. (1999). ‘‘Interaction of calicheamicin gamma1(I) and its related carbohydrates with DNA-protein complexes.’’ Proc. Natl. Acad. Sci. USA 96(19), 10643–10648. Turhan, A. G., Lemoine, F. M., Debert, C., Bonnet, M. L., Baillou, C., Picard, F., Macintyre, E. A., and Varet, B. (1995). ‘‘Highly purified primitive hematopoietic stem cells are PMLRARA negative and generate nonclonal progenitors in acute promyelocytic leukemia.’’ Blood 85(8), 2154–2161. van der Velden, V. H. J., te Marvelde, J. G., Hoogeveen, P. G., Bernstein, I. D., Houtsmuller, A. B., Berger, M. S., and van Dongen, J. J. M. (2001). ‘‘Targeting of the CD33calicheamicin immunoconjugate Mylotarg (CMA-676) in acute myeloid leukemia: in vivo and in vitro saturation and internalization by leukemic and normal myeloid cells.’’ Blood 97, 3197–3204. Zein, N., Sinha, A. M., McGahren, W. J., and Ellestad, G. A. (1988). ‘‘Calicheamicin gamma 1I: An antitumor antibiotic that cleaves double-stranded DNA site specifically.’’ Science 240(4856), 1198–1201.