Diagnosis and treatment of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL), characterized by a translocation between the promyelocytic leukemia gene (PML) on chromosome 15 and the retinoic acid receptor-alpha (RARalpha) gene on chromosome 17, has become a model for targeted treatment of cancer. Advances in our understanding of the fundamental biology of this disease have led to the development of tools for diagnosis, monitoring of minimal residual disease, and detection of early relapse. Differentiation therapy with all-trans retinoic acid in combination with chemotherapy has significantly improved survival in patients with APL. Moreover, arsenic trioxide, which induces differentiation and apoptosis of APL cells, has become standard treatment for relapsed disease, and its role in the treatment of newly diagnosed APL is under active investigation. The lessons learned from APL have broad applications to other forms of leukemia and to cancer in general, whereby molecularly targeted therapy is directed to specifically defined subgroups.

Monoclonal antibody therapy of APL

Acute promyelocytic leukemia (APL) is a rare subtype of acute myeloid leukemia (AML) for which a number of targeted therapies have been developed. The "targets" have included both genotypic and phenotypic features of the disease. The application of monoclonal antibodies (MAbs) to this disease to date have been limited to a relatively small number of studies where this therapy has been used to supplement effective approaches to the disease. The preliminary results have been promising, and further development of this modality as an effective adjunct to existing treatment regimens will most certainly occur in the near future.

Pentostatin, cyclophosphamide, and rituximab is an active, well-tolerated regimen for patients with previously treated chronic lymphocytic leukemia

PURPOSE

Purine analogs and alkylators are important agents for treating chronic lymphocytic leukemia (CLL). Early studies combining fludarabine and chlorambucil were abandoned owing to increased toxicity from overlapping myelosuppression and immunosuppression. Of the purine analogs active in CLL, pentostatin appears to be the least myelosuppressive. We previously reported that pentostatin and cyclophosphamide (PC) is active and well-tolerated in patients with relapsed or refractory CLL. Subsequently, we added rituximab, and now report on this three-drug combination.

PATIENTS AND METHODS

We treated 46 patients with either previously treated CLL (32 patients) or other low-grade B-cell neoplasms (14 patients). Patients received pentostatin 4 mg/m2, cyclophosphamide 600 mg/m2, and rituximab 375 mg/m2 (PCR). All drugs were administered on the same day (rituximab omitted from cycle 1), and patients received six cycles at 3-week intervals. Filgrastim, sulfamethoxazole/trimethoprim, and acyclovir were administered prophylactically.

RESULTS

The median age was 62 years (range, 30 to 80 years). The median number of prior regimens was two (range, one to seven). For CLL patients, there were 24 responses (75%), including eight complete responses (25%). In fludarabine-refractory patients, 75% responded. Toxicity was acceptable, with grade 3/4 infections (including fever of unknown origin) in 28%. The regimen was well tolerated, with 72% of patients receiving the planned treatment at full dose.

CONCLUSION

PCR is safe and effective in previously treated patients with CLL. In comparison with our prior two-drug regimen, we find that rituximab did not seem to add significantly to the toxicity, but did appear to confer a survival advantage. Based on these results, we are currently studying PCR as initial therapy for patients with CLL.

Immunotherapy for acute myeloid leukemia

Immunotherapeutic strategies have become part of standard cancer treatment. Chimeric and humanized antibodies have demonstrated activity against a variety of tumors. Although the humanized anti-CD33 antibody HuM195 has only modest activity against overt acute myeloid leukemia (AML), it can eliminate minimal residual disease in acute promyelocytic leukemia. High-dose radioimmunotherapy with b-particle-emitting isotopes targeting CD33, CD45, and CD66 can potentially allow intensification of antileukemic therapy before hematopoietic stem cell transplantation. Conversely, a-particle immunotherapy with isotopes such as bismuth-213 or actinium-225 offers the possibility of selective tumor cell kill while sparing surrounding normal tissues. Targeted chemotherapy with the anti-CD33- calicheamicin construct gemtuzumab ozogamicin has produced remissions in relapsed AML and appears promising when used in combination with standard chemotherapy for newly diagnosed AML. T-cell recognition of peptide antigens presented on the cell surface in combination with major histocompatibility complex antigen provides another potentially promising approach for the treatment of AML.

Antibody-based treatment of acute myeloid leukaemia

Monoclonal antibodies have become an important treatment modality in cancer therapy. Genetically engineered chimaeric and humanised antibodies have demonstrated activity against a variety of tumours. Whereas the humanised anti-CD33 monoclonal antibody HuM195 has only modest activity against overt acute myeloid leukaemia (AML), it can eliminate minimal residual disease detectable by reverse transcription-polymerase chain reaction in acute promyelocytic leukaemia. High-dose radioimmunotherapy with beta-particle-emitting isotopes targeting CD33, CD45 and CD66 can potentially allow intensification of antileukaemic therapy before bone marrow transplantation. Conversely, alpha-particle immunotherapy with isotopes such as bismuth-213 or actinium-225 offers the possibility of selective tumour cell kill while sparing surrounding normal cells. Targeted chemotherapy with the anti-CD33-calicheamicin construct gemtuzumab ozogamicin has produced remissions as a single agent in patients with relapsed AML and appears promising when used in combination with standard chemotherapy in the treatment of newly diagnosed AML.

The promise of targeted {alpha}-particle therapy

The use of monoclonal antibodies to deliver radioisotopes directly to tumor cells has become a promising strategy to enhance the antitumor effects of native antibodies. Since the alpha- and beta-particles emitted during the decay of radioisotopes differ in significant ways, proper selection of isotope and antibody combinations is crucial to making radioimmunotherapy a standard therapeutic modality. Because of the short pathlength (50-80 microm) and high linear energy transfer ( approximately 100 keV/microm) of alpha-emitting radioisotopes, targeted alpha-particle therapy offers the potential for more specific tumor cell killing with less damage to surrounding normal tissues than beta-emitters. These properties make targeted alpha-particle therapy ideal for the elimination of minimal residual or micrometastatic disease. Radioimmunotherapy using alpha-emitters such as (213)Bi, (211)At, and (225)Ac has shown activity in several in vitro and in vivo experimental models. Clinical trials have demonstrated the safety, feasibility, and activity of targeted alpha-particle therapy in the treatment of small-volume and cytoreduced disease. Further advances will require investigation of more potent isotopes, new sources and methods of isotope production, improved chelation techniques, better methods for pharmacokinetic and dosimetric modeling, and new methods of isotope delivery such as pretargeting. Treatment of patients with less-advanced disease and, ultimately, randomized trials comparing targeted alpha-particle therapy with standard approaches will be required to determine the clinical utility of this approach.

Cytoreduction with iodine-131-anti-CD33 antibodies before bone marrow transplantation for advanced myeloid leukemias

The monoclonal antibodies M195 and HuM195 target CD33, a glycoprotein found on myeloid leukemia cells. When labeled with iodine-131 ((131)I), these antibodies can eliminate large disease burdens and produce prolonged myelosuppression. We studied whether (131)I-labeled M195 and HuM195 could be combined safely with busulfan and cyclophosphamide (BuCy) as conditioning for allogeneic BMT. A total of 31 patients with relapsed/refractory acute myeloloid leukemia (AML) (n=16), accelerated/myeloblastic chronic myeloid leukemia (CML) (n=14), or advanced myelodysplastic syndrome (n=1) received (131)I-M195 or (131)I-HuM195 (122-437 mCi) plus busulfan (16 mg/kg) and cyclophosphamide (90-120 mg/kg) followed by infusion of related-donor bone marrow (27 first BMT; four second BMT). Hyperbilirubinemia was the most common extramedullary toxicity, occurring in 69% of patients during the first 28 days after BMT. Gamma camera imaging showed targeting of the radioisotope to the bone marrow, liver, and spleen, with absorbed radiation doses to the marrow of 272-1470 cGy. The median survival was 4.9 months (range 0.3-90+ months). Three patients with relapsed AML remain in complete remission 59+, 87+, and 90+ months following bone marrow transplantation (BMT). These studies show the feasibility of adding CD33-targeted radioimmunotherapy to a standard BMT preparative regimen; however, randomized trials will be needed to prove a benefit to intensified conditioning with radioimmunotherapy.

Monitoring PML-RARalpha in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is characterized by a translocation between the promyelocytic leukemia gene (PML) on chromosome 15 and the retinoic acid receptor-alpha (RARalpha) gene on chromosome 17. Reverse-transcription polymerase chain reaction (RT-PCR) amplification of PML-RARalpha messenger RNA can establish the diagnosis of APL, predict response to all-trans retinoic acid and arsenic trioxide, detect minimal residual disease, and predict relapse. Quantitative "real-time" RT-PCR techniques may improve residual disease assessment by facilitating more rapid and standardized results. APL provides a useful model in which therapy is targeted to an underlying genetic aberration and treatment is adapted based on monitoring of residual disease.

Pentostatin and cyclophosphamide: an effective new regimen in previously treated patients with chronic lymphocytic leukemia

PURPOSE

Purine analogs and alkylators are important agents in the treatment of chronic lymphocytic leukemia (CLL). Previously, combinations of fludarabine and chlorambucil were abandoned because of increased toxicity from overlapping myelosuppression and immunosuppression. Of the purine analogs active in CLL, pentostatin may be least myelosuppressive. We hypothesized that combining pentostatin with cyclophosphamide would have less myelotoxicity than combinations using other purine analogs.

PATIENTS AND METHODS

We studied 23 patients with previously treated CLL. All patients received pentostatin 4 mg/m(2). Seventeen patients received cyclophosphamide 600 mg/m(2), and six patients received cyclophosphamide 900 mg/m(2). Both drugs were administered on day 1 of each cycle, and cycles were repeated every 3 weeks for six treatments. Filgrastim, sulfamethoxazole/trimethoprim, and acyclovir were administered prophylactically. The median number of prior treatment regimens was three (range, one to five) with 13 patients (57%) refractory to prior fludarabine therapy.

RESULTS

The cyclophosphamide 900 mg/m(2) dose level was associated with moderate to severe nausea, and we chose cyclophosphamide 600 mg/m(2) as the dose for further study. There were 17 responses (74%; 95% confidence interval, 63% to 85%), including four complete responses. The response rate was 77% in fludarabine-refractory patients. Myelosuppression was acceptable with grade 3/4 neutropenia and thrombocytopenia, seen in 35% and 30% of patients, respectively. The relative sparing of thrombopoiesis can be seen in that only one patient (5%) with an initial platelet count of more than 20,000 required platelet transfusions while receiving therapy.

CONCLUSION

Pentostatin 4 mg/m(2) with cyclophosphamide 600 mg/m(2) is safe and effective in previously treated patients with CLL. On the basis of these results, we are currently studying pentostatin, cyclophosphamide, and rituximab (PCR) therapy in patients with CLL.

Refractory Aspergillus pneumonia in patients with acute leukemia: successful therapy with combination caspofungin and liposomal amphotericin

BACKGROUND

Pulmonary aspergillosis and other invasive fungal infections (IFIs) commonly complicate the management of patients with acute leukemia. Standard amphotericin-based therapies may be ineffective for many patients and the available salvage agents (itraconazole and caspofungin) are reported to possess only moderate activity against resistant infections. Laboratory evidence suggests a synergistic interaction between amphotericin and caspofungin. The authors treated a group of patients with amphotericin-refractory IFIs with the combination of caspofungin and amphotericin (or liposomal amphotericin).

METHODS

A retrospective evaluation of patients with amphotericin-resistant IFIs was conducted. Diagnosis was based on clinical, radiographic, and when available, microbiologic data. Response to combination antifungal therapy was graded as either favorable or unfavorable. Favorable responses included improvement of both clinical and radiographic signs of fungal pneumonia. All other responses were graded as unfavorable.

RESULTS

Thirty patients were included in this analysis. Twenty-six patients had acute leukemia. Based on recently published criteria, the IFIs were classified as proven in 6 patients, probable in 4 patients, and possible in 20 patients. The median duration and dose of amphotericin monotherapy were 12 days (range, 4-65 days) and 7.8 mg/kg (range, 4.2-66.1 mg/kg), respectively. The median duration of combination therapy was 24 days (range, 3-74 days). Eighteen patients (60%) experienced a favorable antifungal response. Twenty patients with acute leukemia received combination therapy for fungal pneumonias arising during intensive chemotherapy treatments. Favorable responses were observed in 15 of these patients (75%), and antifungal response did not depend on the response of the underlying leukemia. Survival to hospital discharge was significantly better (P < 0.001) in patients having a favorable response. Mild to moderate nephrotoxicity was noted in 50% of patients, necessitating the substitution of liposomal amphotericin. Mild elevation of alkaline phosphatase levels occurred in 30% of patients. Caspofungin was temporarily withheld from one patient who developed moderate but reversible biochemical hepatotoxicity.

CONCLUSIONS

The antifungal combination of caspofungin and amphotericin can be administered safely to high-risk patients with hematologic malignancies. Although an absolute assessment of efficacy is limited by the design of this study, encouraging outcomes were noted for many patients. The authors plan to evaluate this regimen further in a randomized clinical trial.

Targeted alpha particle immunotherapy for myeloid leukemia

Unlike beta particle-emitting isotopes, alpha emitters can selectively kill individual cancer cells with a single atomic decay. HuM195, a humanized anti-CD33 monoclonal antibody, specifically targets myeloid leukemia cells and has activity against minimal disease. When labeled with the beta-emitters (131)I and (90)Y, HuM195 can eliminate large leukemic burdens in patients, but it produces prolonged myelosuppression requiring hematopoietic stem cell transplantation at high doses. To enhance the potency of native HuM195 yet avoid the nonspecific cytotoxicity of beta-emitting constructs, the alpha-emitting isotope (213)Bi was conjugated to HuM195. Eighteen patients with relapsed and refractory acute myelogenous leukemia or chronic myelomonocytic leukemia were treated with 10.36 to 37.0 MBq/kg (213)Bi-HuM195. No significant extramedullary toxicity was seen. All 17 evaluable patients developed myelosuppression, with a median time to recovery of 22 days. Nearly all the (213)Bi-HuM195 rapidly localized to and was retained in areas of leukemic involvement, including the bone marrow, liver, and spleen. Absorbed dose ratios between these sites and the whole body were 1000-fold greater than those seen with beta-emitting constructs in this antigen system and patient population. Fourteen (93%) of 15 evaluable patients had reductions in circulating blasts, and 14 (78%) of 18 patients had reductions in the percentage of bone marrow blasts. This study demonstrates the safety, feasibility, and antileukemic effects of (213)Bi-HuM195, and it is the first proof-of-concept for systemic targeted alpha particle immunotherapy in humans.

A single, high dose of idarubicin combined with cytarabine as induction therapy for adult patients with recurrent or refractory acute lymphoblastic leukemia

BACKGROUND

The majority of adult patients who are treated for lymphoblastic disease will either develop recurrent disease or will be refractory to their initial therapy. One option for patients with recurrent/refractory disease is to administer a reinduction regimen that employs a dose-intense combination of anthracycline and cytarabine. These salvage regimens are relatively distinct from the traditional vincristine/prednisone-based programs that are used typically as primary induction therapy. The authors studied a regimen that contained high-dose cytarabine and a single high dose of idarubicin as salvage induction therapy for patients with recurrent or refractory lymphoblastic disease.

METHODS

Twenty-nine previously treated adult patients with recurrent or refractory acute lymphoblastic leukemia were treated with a new intensive regimen. Eight patients had primary refractory disease. Twenty-one patients had recurrent disease, and 16 of these patients developed recurrent disease while they were still receiving their primary therapy. The treatment regimen consisted of cytarabine 3.0 g/m(2) by 3-hour infusion daily for 5 days and idarubicin 40 mg/m(2) given as a single dose on Day 3. Filgrastim (granulocyte-colony stimulating factor) 5 microg/kg administered subcutaneously every 12 hours was started on Day 7 and was continued until the absolute neutrophil count was > 5000/microL. Response was assessed using standard criteria.

RESULTS

There were 11 complete responses (38%; 95% confidence interval, 20-56%). Four patients subsequently underwent allogeneic bone marrow transplantation. Moderate but acceptable toxicity was observed given the severely myelosuppressive nature of the regimen. There was only one treatment-related death (3%). Two patients, both with significant prior exposure to anthracyclines, suffered reductions in left ventricular function to the 20-30% range during episodes of severe systemic infection. After recovery from infection, the ejection fraction in one patient improved to 50%.

CONCLUSIONS

The authors conclude that this regimen has moderate activity and a relatively low incidence of mortality for this high-risk group of patients. This regimen may be most suitable for patients who can undergo potentially curative allogeneic bone marrow transplantation if they achieve a complete response.

Antibody therapy in acute myeloid leukemia: current status and future directions

Monoclonal antibodies have become an important modality for cancer therapy. The genetically engineered, humanized anti-CD33 antibody HuM195 has demonstrated modest activity against overt relapsed acute myeloid leukemia (AML) and more substantial activity against minimal residual disease in acute promyelocytic leukemia. Radioimmunotherpay with beta-particle-emitting isotopes has eliminated large leukemic burdens while minimizing radiation exposure to normal tissues in both nonmyeloablative and myeloablative regimens. Targeted beta-particle immunotherapy with agents such as bismuth 213-labeled HuM195 offers the possibility of a more selective tumor cell kill with less damage to surrounding normal cells. Directed chemotherapy using the anti-CD33-calicheamicin conjugate gemtuzumab ozogamicin (Mylotarg) has produced remissions in patients with relapsed AML.

Radioimmunotherapy for acute leukemia

BACKGROUND

The use of monoclonal antibodies to deliver radioactive isotopes directly to tumor cells has become a promising strategy to enhance the antitumor effects of native monoclonal antibodies. In this article, we summarize the role of radioimmunotherapy in the treatment of leukemia.

METHODS

The authors reviewed the published clinical trials of radioimmunotherapy in acute leukemia.

RESULTS

Radioimmunoconjugates that emit beta-particles, such as 131I-anti-CD33, 90Y-anti-CD33, 131I-anti-CD45, and 188Re-anti-CD66c, deliver significant doses of radiation to the bone marrow and may be particularly effective when used as part of a conditioning regimen for hematopoietic stem cell transplantation. Radioimmunoconjugates that emit short-ranged alpha-particles, such as 213Bi-anti-CD33, are better suited for the treatment of low-volume or residual disease.

CONCLUSIONS

Radiolabeled antibodies can be administered safely to patients with advanced leukemias and have significant antileukemic activity. Radiolabeled antibodies can potentially intensify the antileukemic effects of conditioning regimens when used in conjunction with hematopoietic stem cell transplantation. Whether or not radiolabeled antibodies improve the outcome of patients with leukemia remains to be demonstrated by randomized studies.

Prognostic significance of minimal residual disease detection and PML/RAR-alpha isoform type: long-term follow-up in acute promyelocytic leukemia

The t(15;17) translocation in acute promyelocytic leukemia (APL) yields a PML/RAR-alpha fusion messenger RNA species that can be detected by reverse transcription-polymerase chain reaction (RT-PCR) amplification. Breakpoints within intron 3 of PML produce a short PML/RAR-alpha isoform, whereas breakpoints within intron 6 result in a longer form. Using RT-PCR, serial evaluations were performed on the bone marrow of 82 patients with APL (median follow-up, > 63 months) who received retinoic acid (RA) induction followed by postremission treatment with chemotherapy, RA, and biologic agents. Sixty-four patients attained a clinical complete remission and had at least 2 RT-PCR assays performed after completing therapy. Forty of 47 patients (85%) with newly diagnosed APL who were induced using RA had residual disease detectable by RT-PCR before additional therapy. After 3 cycles of consolidation therapy, residual disease was found in only 4 of 40 evaluable patients (10%). Among newly diagnosed patients who had 2 or more negative RT-PCR assays, only 3 of 41 (7%) had a relapse, whereas all 4 patients (100%) who had 2 or more positive results had a relapse. Among 63 newly diagnosed patients, those who expressed the short isoform appeared to have shorter disease-free and overall survival durations than patients who expressed the long isoform. These data indicate that 2 or more negative RT-PCR assays on bone marrow, performed at least 1 month apart after completing therapy, are strongly associated with long-term remissions. Conversely, a confirmed positive test is highly predictive of relapse.

Novel therapeutics for chemotherapy-resistant acute myeloid leukaemia

Patients with chemotherapy-resistant acute myeloid leukaemia are rarely cured by non-allogeneic transplant therapies. Multiple new investigational agents have become available for treatment of these patients and there are few tools to permit rational drug and clinical trial selection. In this review, we describe the chemical and biological properties of some of these agents and some of their initial clinical activity to date. The selected agents react with either cell surface molecules or signal pathway intermediates and include antibody and antibody conjugates to CD33 and CD45, a fusion protein directed to the granulocyte-macrophage colony-stimulating factor receptor, an anti-sense oligonucleotide to Bcl2, a farnesyl transferase inhibitor, and a protein kinase C agonist/inhibitor. The challenge for the next decade will be how to select patients for particular molecularly targeted therapeutics and how to combine these agents.

Acute myeloid leukemia with t(5;18)(q35;q21)

A case of acute myelocytic leukemia with a translocation (5;18)(q35;q21) is reported. Cytogenetic abnormalities of the long arm of chromosome 5 have long been known to affect hematopoiesis. Although translocations between 5q and other chromosomes have been associated with malignancy, this is the first reported case of a t(5;18) resulting in acute myeloid leukemia. Possible molecular mechanisms underlying the pathogenesis of the disease are discussed.

Antibody therapy for residual disease in acute myelogenous leukemia

The elimination of minimal residual disease remains one of the most promising applications of monoclonal antibody (mAb)-based therapies. An early study showed that treatment with iodine-131 (131I)-labeled anti-CD33 mAb M195 had antileukemic effects when given as postremission therapy to patients with acute promyelocytic leukemia (APL) in second remission. This treatment, however, was limited by significant myelosuppression and by the development of neutralizing human antimouse antibodies. Treatment with native HuM195, a humanized version of murine M195, eliminated minimal residual disease detectable by reverse transcription-polymerase chain reaction (RT-PCR) in 50% of patients. Patients with newly diagnosed APL treated with all-trans retinoic acid followed by HuM195 and consolidation chemotherapy had an 87% 3-year disease-free survival. Radioimmunotherapy with short-ranged, high-energy alpha particle-emitting isotopes may increase the potency of native mAbs while avoiding the nonspecific cytotoxicity of beta-emitting constructs. Targeted alpha particle therapy with bismuth-213-HuM195 showed significant antileukemic activity in patients with relapsed or refractory acute myelogenous leukemia. Antibody-drug conjugates, such as gemtuzumab ozogamicin, composed of a humanized anti-CD33 mAb and calicheamicin, have produced complete remissions in patients with relapsed AML and are likely to be active in the postremission setting.

Parametric images of antibody pharmacokinetics in Bi213-HuM195 therapy of leukemia

Kinetic analysis of gamma camera patient images can provide time-dependent information about antibody behavior. Current region-of-interest-based techniques for the kinetic analysis of these images rely on user selection and drawing of regions to be analyzed. Such analyses do not reveal unexpected kinetic activity outside of the selected regions of interest and do not provide a whole-image assessment regarding the pharmacokinetics of an agent. At Memorial Sloan-Kettering Cancer Center, a method for generating images in which the pixel value represents a kinetic parameter has been developed. This work extends the method into a new application in which whole-body parametric images are used to examine the kinetics of Bi213-HuM195 in patients with leukemia.

METHODS

Bi213-HuM195 is typically administered in multiple injections over 2-4 d, yielding a progressive increase in the amount of antibody administered. Patients are injected with individual doses while positioned in a gamma camera, and imaging is initiated at the start of the injection. The acquisition is performed in dynamic mode with images collected at several time intervals over 1 h. Using software developed in-house, images are corrected for patient movement through iterative alignments, decay corrected, and summed to yield a series of images over regular time intervals. Parametric rate images are obtained by fitting a linear expression to the counts in each pixel. In this study, rate images from a patient's first injection were compared with rate images from the last injection.

RESULTS

The conventional planar images of antibody distribution showed significant uptake in liver, spleen, and marrow, whereas the generated rate images displayed different patterns, sometimes with negative values in liver and spleen and positive values in marrow, reflecting clearance and uptake rates rather than total accumulation. The impact of the progressive increase in antibody administration was observed by comparing the first with the last rate images. Interpatient comparisons were also made and showed that rate image patterns varied depending on patient-specific conditions such as the amount of disease and previous therapies undergone by the patient.

CONCLUSION

Rate images make it possible to succinctly display kinetic information about an agent's behavior over the entire acquired image.

Antibody therapy of acute myelogenous leukemia

Monoclonal antibodies (mAbs) have become an important modality for cancer therapy. A genetically engineered, humanized anti-CD33 antibody HuM195 has demonstrated activity against over relapsed acute myelogenous leukemia (AML) and against minimal residual disease in acute promyelocytic leukemia (APL). Radioimmunotherapy with beta (beta) particle-emitting isotopes has produced significant responses while minimizing radiation exposure to normal tissues in both nonmyeloablative and myeloablative regimens. Targeted alpha (alpha) particle therapy with 213Bi-labeled HuM195 offers the possibility of more selective tumor cell kill. Additionally, directed chemotherapy using an anti-CD33-calicheamicin conjugate (CMA-676) has produced remissions in patients with relapsed AML.

Advances in immunotherapy of hematologic malignancies: cellular and humoral approaches

Monoclonal antibodies have become an important modality for cancer therapy. Genetically engineered chimeric and humanized antibodies have demonstrated activity against overt lymphoma and leukemia, as well as minimal residual disease. Radioimmunotherapy in both nonmyeloablative and myeloablative regimens has produced significant responses and also minimized radiation exposure to normal tissues. Targeted alpha-particle therapy offers the possibility of selective tumor cell kill. Antibody-drug conjugates have produced remissions in acute leukemia. Many proteins potentially act as leukemia or lymphoma-specific antigens for major histocompatibility complex-restricted T cell cytotoxicity. These include the idiotype proteins, breakpoint cluster region (bcr)-abl and other fusion oncoproteins, myeloid-specific differentiation antigens and minor histocompatibility antigens. Clinical trials exploiting the new understanding of the T cell immunology are underway.

Antibody immunotherapy for leukemia

Monoclonal antibodies have become an important modality for cancer therapy. Genetically engineered chimeric and humanized antibodies have demonstrated activity against both overt leukemia and minimal residual disease. High-dose radioimmunotherapy can potentially allow intensification of antileukemic therapy before bone marrow transplantation yet minimize radiation exposure to normal organs. Targeted alpha particle therapy offers the possibility of selective tumor cell kill while sparing surrounding normal cells. Antibody-drug conjugates and immunotoxins have produced complete remissions in leukemia.

Molecular remission induction with retinoic acid and anti-CD33 monoclonal antibody HuM195 in acute promyelocytic leukemia

Despite achieving complete remission with retinoic acid (RA), most patients with acute promyelocytic leukemia (APL) have minimal residual disease detectable by reverse transcription-PCR (RT-PCR) amplification. HuM195, a humanized monoclonal antibody reactive with the cell surface antigen CD33, specifically targets and kills myeloid leukemia cells. We studied whether HuM195 could eliminate minimal residual disease in patients with APL by using RT-PCR. After attaining clinical complete remission with RA and/or chemotherapy, patients received HuM195 twice weekly for 3 weeks. Patients in first remission were given consolidation chemotherapy, generally with three cycles of idarubicin and cytarabine. Patients in second or greater remission did not receive chemotherapy. All patients received six monthly courses of maintenance with two doses of HuM195. Twenty-five of 27 patients treated in first remission had positive RT-PCR determinations before HuM195 treatment. Of the 22 patients evaluable for conversion of positive RT-PCR assays, 11 (50%) became RT-PCR negative after HuM195 treatment without additional therapy. Within the subset of patients who received RA alone as induction, 8 of 18 evaluable patients (44%) had negative RT-PCR determinations after HuM195 treatment but before chemotherapy. Among similar patients treated on earlier studies, 7 of 34 patients (21%) induced into remission with RA and then maintained on the drug for 1 month were RT-PCR negative before chemotherapy (P = 0.07). Twenty-five of 27 patients with newly diagnosed APL (93%) remain in clinical complete remission for 7+ to 58+ months, with median follow-up of 29 months. Seven patients in second or third remission and one patient in molecular relapse were also treated. Only one of these patients became RT-PCR negative after treatment with HuM195. These data suggest that HuM195 has activity against minimal residual disease in APL, particularly in newly diagnosed patients.

Radionuclides as conditioning before stem cell transplantation

Targeted radiotherapy using either radiolabeled monoclonal antibodies or bone-seeking radiopharmaceuticals can potentially result in delivery of high radiation doses to tumor or bone marrow with less toxicity to normal organs. High-dose radioimmunotherapy has produced encouraging results in lymphoma and leukemia. The use of alternative radionuclides and new combinations of radioimmunotherapy with chemotherapy may improve patient outcomes. Comparative trials will be necessary to confirm an improved therapeutic benefit with this approach.

Pharmacokinetics and dosimetry of an alpha-particle emitter labeled antibody: 213Bi-HuM195 (anti-CD33) in patients with leukemia

Data from nine patients with leukemia participating in a phase I activity-escalation study of HuM195, labeled with the alpha-particle emitter 213Bi (half-life = 45.6 min), were used to estimate pharmacokinetics and dosimetry. This is the first trial using an alpha-particle emitter in humans. The linear energy transfer of alpha particles is several hundredfold greater than that of beta emissions. The range in tissue is approximately 60-90 microm.

METHODS

The activity administered to patients ranged from 0.6 to 1.6 GBq. Patient imaging was initiated at the start of each injection. Thirty 1-min images followed by ten 3-min images were collected in dynamic mode; a 20% photopeak window centered at 440 keV was used. Blood samples were collected until 3 h postinjection and counted in a gamma counter. Contours around the liver and spleen were drawn on the anterior and posterior views and around a portion of the spine on the posterior views. No other organs were visualized.

RESULTS

The percentage injected dose in the liver and spleen volumes increased rapidly over the first 10-15 min to a constant value for the remaining hour of imaging, yielding a very rapid uptake followed by a plateau in the antibody uptake curves. The kinetic curves were integrated to yield cumulated activity. The mean energy emitted per nuclear transition for 213Bi and its daughters, adjusted by a relative biologic effectiveness of 5 for alpha emissions, was multiplied by the cumulated activity to yield the absorbed dose equivalent. Photon dose to the total body was determined by calculating a photon-absorbed fraction. The absorbed dose equivalent to liver and spleen volumes ranged from 2.4 to 11.2 and 2.9 to 21.9 Sv, respectively. Marrow (or leukemia) mean dose ranged from 6.6 to 12.2 Sv. The total-body dose (photons only) ranged from 2.2 x 10(-4) to 5.8 x 10(-4) Gy.

CONCLUSION

This study shows that patient imaging of 213Bi, an alpha-particle emitter, labeled to HuM195 is possible and may be used to derive pharmacokinetics and dosimetry. The absorbed dose ratio between marrow, liver and spleen volumes and the whole body for 213Bi-HuM195 is 1000-fold greater than that commonly observed with beta-emitting radionuclides used for radioimmunotherapy.

A phase I trial of humanized monoclonal antibody HuM195 (anti-CD33) with low-dose interleukin 2 in acute myelogenous leukemia

HuM195 is a recombinant humanized IgG1 monoclonal antibody reactive with CD33, a Mr 67,000 glycoprotein expressed on early myeloid progenitor cells and myeloid leukemia cells. HuM195 has been shown to rapidly target and saturate acute myeloid leukemia (AML) cells after i.v. infusion into patients and is capable of mediating antibody-dependent cellular cytotoxicity. This activity is enhanced in vitro when natural killer (NK) effector cells are preincubated with low concentrations of interleukin 2 (IL-2). Previous Phase I trials of HuM195 in patients with relapsed AML demonstrated safety and attainment of complete responses, but significant antileukemic activity appears limited to patients with low leukemia tumor burdens. Therefore, in the present trial, we sought to determine whether low-dose IL-2 could safely enhance the numbers of NK cells and therefore the cytotoxic capability of HuM195 via presumptive NK cell antibody-dependent cellular cytotoxicity in vivo against myeloid leukemia cells. Thirteen patients with relapsed or refractory AML and one patient with advanced myelodysplastic syndrome were treated with 0.6x10(6) IU/m2 of s.c. IL-2 daily for 35 days. Starting on day 15, patients received twice weekly i.v. infusions of HuM195 (3.0 mg/m2) for 3 weeks. Immediately after the HuM195 infusion, the patients received IL-2 i.v. infusions over 2 h at one of three escalating dose levels of 0.5x10(6), 1.0x10(6), and 2.0x10(6) IU/m2. Peripheral blood mononuclear cells were quantitated and immunophenotyped by flow cytometry. Safety, tolerability, bone marrow mononuclear cell morphology, and immunophenotype, as well as responses were assessed. Of the 14 patients who entered the study, 10 were able to complete at least one cycle of therapy. Adverse effects to the s.c. IL-2 were relatively mild and included erythema and induration of the skin at the injection site and low-grade fever. Toxicity from the sequential HuM195 and i.v. IL-2 infusions included nausea, rigors, and fever. Toxicity was IL-2 dose related with dose-limiting toxicity seen at the 2.0x10(6) IU/m2 dose level. Three patients had stable disease at the completion of the first cycle and went on to receive a second cycle of treatment. CD3-positive, CD56-positive, and CD33-positive cells were generally found to significantly decrease immediately after each administration of i.v. IL-2 and HuM195. CD56-expressing cells increased in 6 of 10 patients from the beginning to the end of therapy. Among the 10 evaluable patients, 2 patients had significant decreases in the percentage of blasts in the bone marrow (one of which achieved a complete bone marrow remission), 5 patients had stable levels of bone marrow blasts, and 3 had progression of disease on therapy. The combination of IL-2 and HuM195 shows modest biological activity and clinical antileukemic activity but also produced significant toxicity.

Radioimmunotherapy with alpha-emitting nuclides

This review discusses the application of alpha particle-emitting radionuclides in targeted radioimmunotherapy. It will outline the production and chemistry of astatine-211, bismuth-212, lead-212, actinium-225, bismuth-213, fermium-255, radium-223 and terbium-149, which at present are the most promising alpha-emitting isotopes available for human clinical use. The selective cytotoxicity offered by alpha particle-emitting radioimmunoconstructs is due to the high linear energy transfer and short particle path length of these radionuclides. Based upon the pharmacokinetics of alpha particle-emitting radioimmunoconstructs, both stochastic and conventional dosimetric methodology is discussed, as is the preclinical and initial clinical use of these radionuclides conjugated to monoclonal antibodies for the treatment of human neoplasia.

Radiolabeled anti-CD33 monoclonal antibody M195 for myeloid leukemias

M195, a mouse monoclonal antibody reactive with the early myeloid antigen CD33, has been shown to target leukemia cells in patients and to reduce large leukemic burdens when labeled with 131I. A complementarity-determining region-grafted, humanized version (HuM195) has demonstrated similar targeting of leukemia cells without immunogenicity. We have studied two applications of therapy with 131I-M195. First, to intensify therapy prior to bone marrow transplantation (BMT), we combined 131I-M195 with busulfan and cyclophosphamide. Fifteen patients received first BMT for relapsed or refractory acute myelogenous leukemia or accelerated or blastic chronic myelogenous leukemia; four received second BMT for relapsed chronic or accelerated chronic myelogenous leukemia. Doses of 131I-M195 ranged from 120 to 230 mCi/m2. Few toxicities could be attributed to 131I-M195 therapy, and all patients engrafted. Eighteen patients achieved complete remission. Among those patients receiving first BMT, three have remained in unmaintained remission for 18+ to 29+ months. Six patients relapsed, including one with isolated central nervous system disease 32 months after BMT. Ten patients died in complete remission of transplant-related complications. Second, we studied whether 131I-M195 could reduce minimal residual disease and prolong remission and survival durations safely in patients with relapsed acute promyelocytic leukemia after they attained remission with all-trans-retinoic acid. Seven patients were treated with either 50 or 70 mCi/m2 131I-M195. Toxicity was limited to myelosuppression. As a measure of minimal residual disease, we monitored PML/RAR-alpha mRNA by reverse transcription PCR. Six patients had positive reverse transcription PCR assays prior to receiving 131I-M195; two converted transiently to negative. Median disease-free survival and overall survival of the seven patients were 8 (range, 3-14.5) months and 28 (range, 5.5-43+) months, respectively. This regimen compares favorably with others for relapsed acute promyelocytic leukemia. In an effort to avoid nonspecific cytotoxicity associated with 131I in future trials for minimal residual disease, we have conjugated short-range, alpha particle-emitting radioisotopes to HuM195 using a bifunctional chelate, 2-(p-isothiocyanatobenzyl)-cyclohexyldiethyl-enetriaminep entaacetic acid, with high efficiency and specific activities. 212Bi-HuM195 has demonstrated dose- and specific activity-dependent killing of HL60 cells in vitro. Injection of 213Bi-HuM195 into healthy BALB/c mice produced no effects on weight or viability.

Sequential targeted therapy for relapsed acute promyelocytic leukemia with all-trans retinoic acid and anti-CD33 monoclonal antibody M195

Acute promyelocytic leukemia (APL) provides a model to examine the sequential use of selective oncogene product-targeted and lineage-targeted agents. All-trans retinoic acid (RA) has been shown to produce brief remissions by a novel differentiating mechanism in most patients with APL. M195, a mouse monoclonal antibody (moAb) reactive with the cell-surface antigen CD33, can target myeloid leukemia cells in patients and reduce large leukemic burdens when labeled with 131I. We studied whether 131I-M195 could safely reduce minimal residual disease and prolong remission and survival durations in patients with relapsed APL after they had attained remission with all-trans RA. Seven patients with relapsed APL in second remission were treated with either 70 (n = 2) or 50 (n = 5) mCi/m2 of 131I-M195. As a measure of minimal residual disease, we serially monitored PML/RAR-alpha mRNA by a reverse transcription polymerase chain reaction (RT-PCR) assay. There was no immediate toxicity. Late toxicity was limited to myelosuppression, but no episodes of febrile neutropenia were seen. Six patients had detectable PML/RAR-alpha mRNA after all-trans RA therapy; two had single negative RT-PCR determinations following 131I-M195. Median disease-free survival of the seven patients was 8 months (range 3-14.5). Four patients with median follow-up of 24 months remain alive, and median overall survival exceeds 21+ months (range 5.5-33+). This regimen based on targeted therapy compares favorably to other approaches for the treatment of relapsed APL, including that used in the immediately preceding trial in which patients were induced into remission and maintained with all-trans RA, as well as other chemotherapy-based regimens. These data support further study of moAb-based therapy for minimal residual disease in acute leukemia.

Recent developments in the radioimmunotherapy of cancer

Recent trials using radiolabeled monoclonal antibodies, particularly for hematologic neoplasms, have demonstrated the ability of these agents to destroy tumor cells safely and specifically. Advances in molecular biology, immunology and radiochemistry will allow the continued development of strategies to overcome the remaining obstacles to effective therapy with radioimmunoconjugates.

Excretion of Ascaris lumbricoides during total body irradiation

We describe the excretion of Ascaris lumbicoides, an intestinal roundworm, in the emesis of an asymptomatic patient undergoing total body irradiation. This suggests that Ascaris is sensitive to irradiation.

A phase 1B trial of humanized monoclonal antibody M195 (anti-CD33) in myeloid leukemia: specific targeting without immunogenicity

This trial studied the biodistribution, pharmacology, toxicity, immunogenicity, and biologic characteristics of a trace-labeled, anti-CD33, humanized monoclonal antibody M195 (Hu-M195) in patients with relapsed and refractory myeloid leukemia. Hu-M195 is a computer-modeled, "complementarity-determining region-grafted," IgG1, humanized version of M195. M195 is a murine monoclonal antibody that reacts with CD33, a 67-kD glycoprotein expressed on early myeloid progenitor cells and myeloid leukemia (acute myelogenous leukemia and chronic myelogenous leukemia) cells, but not normal stem cells. 131I-murine-M195 has already shown significant ability to cytoreduce patients with relapsed or refractory myeloid leukemias. Hu-M195 has higher avidity than the original mouse monoclonal antibody and, unlike murine M195, has the capability to mediate antibody-dependent cellular cytotoxicity against leukemia targets. Thirteen patients with relapsed or refractory myelogenous leukemia were treated with Hu-M195 at 4 levels of 0.5, 1.0, 3.0, and 10.0 mg/m2 in a phase I trial. Patients received a total of 6 doses per patient over 18 days. Two patients were retreated for a total of 12 doses. The first dose of Hu-M195 was trace-labeled with 131I to allow detailed pharmacokinetic and biodistribution studies by serial sampling of blood, radioimmunoassays of cells, and whole-body gamma-camera imaging. Cumulative total doses of up to 216 mg of Hu-M195 were administered safely. Reversible fever and rigors were observed after infusion at the highest dose levels. The entire bone marrow was specifically and clearly imaged within hours after infusion, with optimal biodistribution occurring at the 3 mg/m2 level. Adsorption of Hu-M195 onto targets in vivo was demonstrated by flow cytometry; near saturation of available sites occurred at the 3 mg/m2 dose level. Plasma and whole body half lives were 38 and 51 hours, respectively, which may reflect continual replenishment of target sites on new leukemia cells. 131I-Hu-M195 was rapidly internalized into the target cells in vivo within 1 hour. Human antihuman antibody responses were not observed. In conclusion, Hu-M195 can be administered safely in multiple doses, without significant toxicity or any evidence of immunogenicity, and can localize rapidly and efficiently to the bone marrow in patients with myeloid leukemias. Additional phase II trials with this agent alone or in combination with cytokines or isotopes are warranted at the optimal biologic dose.