Robert J. Kreitman

Efficacy of the Anti-CD22 Recombinant Immunotoxin BL22 in Chemotherapy-Resistant Hairy-Cell Leukemia

BACKGROUND Hairy-cell leukemia that is resistant to treatment with purine analogues, including cladribine, has a poor prognosis. We tested the safety and efficacy of an immunotoxin directed against a surface antigen that is strongly expressed by leukemic hairy cells. METHODS RFB4(dsFv)-PE38 (BL22), a recombinant immunotoxin containing an anti-CD22 variable domain (Fv) fused to truncated pseudomonas exotoxin, was administered in a dose-escalation trial by intravenous infusion every other day for a total of three doses. RESULTS Of 16 patients who were resistant to cladribine, 11 had a complete remission and 2 had a partial remission with BL22. The three patients who did not have a response received low doses of BL22 or had preexisting toxin-neutralizing antibodies. Of the 11 patients in complete remission, 2 had minimal residual disease in the bone marrow or blood. During a median follow-up of 16 months (range, 10 to 23), 3 of the 11 patients who had a complete response relapsed and were retreated; all of these patients had a second complete remission. In 2 of the 16 patients, a serious but completely reversible hemolytic-uremic syndrome developed during the second cycle of treatment with BL22. Common toxic effects included transient hypoalbuminemia and elevated aminotransferase levels. CONCLUSIONS BL22 can induce complete remissions in patients with hairy-cell leukemia that is resistant to treatment with purine analogues.

Phase I trial of recombinant immunotoxin anti-Tac(Fv)-PE38 (LMB-2) in patients with hematologic malignancies

PURPOSE To evaluate the toxicity, pharmacokinetics, immunogenicity, and antitumor activity of anti-Tac(Fv)-PE38 (LMB-2), an anti-CD25 recombinant immunotoxin that contains an antibody Fv fragment fused to truncated Pseudomonas exotoxin. PATIENTS AND METHODS Patients with CD25(+) hematologic malignancies for whom standard and salvage therapies failed were treated with LMB-2 at dose levels that ranged from 2 to 63 microg/kg administered intravenously over 30 minutes on alternate days for three doses (QOD x 3). RESULTS LMB-2 was administered to 35 patients for a total of 59 cycles. Dose-limiting toxicity at the 63 microg/kg level was reversible and included transaminase elevations in one patient and diarrhea and cardiomyopathy in another. LMB-2 was well tolerated in nine patients at the maximum-tolerated dose (40 microg/kg QOD x 3); toxicity was transient and most commonly included transaminase elevations (eight patients) and fever (seven patients). Only six of 35 patients developed significant neutralizing antibodies after the first cycle. The median half-life was 4 hours. One hairy cell leukemia (HCL) patient achieved a complete remission, which is ongoing at 20 months. Seven partial responses were observed in cutaneous T-cell lymphoma (one patient), HCL (three patients), chronic lymphocytic leukemia (one patient), Hodgkins disease (one patient), and adult T-cell leukemia (one patient). Responding patients had 2 to 5 log reductions of circulating malignant cells, improvement in skin lesions, and regression of lymphomatous masses and splenomegaly. All four patients with HCL responded to treatment. CONCLUSION LMB-2 has clinical activity in CD25(+) hematologic malignancies and is relatively nonimmunogenic. It is the first recombinant immunotoxin to induce major responses in cancer. LMB-2 and similar agents that target other cancer antigens merit further clinical development.

Immunotoxin therapy of cancer

Rationally designed anticancer agents that target cell-surface antigens or receptors represent a promising approach for treating cancer patients. However, antibodies that bind these targets are often, by themselves, non-cytotoxic. By attaching potent toxins we can dramatically improve the clinical utility of some anti-tumour antibodies. Here we describe the construction and clinical utility of several recombinant immunotoxins; each of which is composed of antibody Fv fragments fused to powerful bacterial toxins. Results from clinical trials indicate that recombinant immunotoxins and similar agents that are designed to combine antibody selectivity with toxin cell-killing potency will be useful additions to cancer therapy.

Phase I Study of SS1P, a Recombinant Anti-Mesothelin Immunotoxin Given as a Bolus I.V. Infusion to Patients with Mesothelin-Expressing Mesothelioma, Ovarian, and Pancreatic Cancers

Purpose: To determine the toxicities, maximum tolerated dose (MTD) and pharmacokinetics of the recombinant immunotoxin SS1P (anti-mesothelin dsFv-PE38) in patients with mesothelin-expressing cancers. Experimental Design: SS1P given as a 30-min i.v. infusion every other day (QOD) for six or three doses was administered to 34 patients with advanced mesothelioma (n = 20), ovarian (n = 12), and pancreatic (n = 2) cancer. Results: The initial cohort of 17 patients received SS1P QOD × 6 doses and the MTD was 18 μg/kg/dose. Dose-limiting toxicities (DLT) included grade 3 uticaria (one patient) and grade 3 vascular leak syndrome (two patients). To allow further SS1P dose escalation, 17 patients were treated on the QOD × 3 schedule and the MTD was 45 μg/kg/dose. The DLT was grade 3 pleuritis and was seen in two of two patients treated at a dose of 60 μg/kg and in one of nine patients treated at a dose of 45 μg/kg. At the MTD of 45 μg/kg, the mean Cmax of SS1P was 483 ng/mL and half-life was 466 min. Of the 33 evaluable patients treated, 4 had minor responses, 19 had stable disease (including 2 with resolution of ascites), and 10 had progressive disease. Conclusions: SS1P is well tolerated with pleuritis as the DLT at the highest dose level. Evidence of clinical activity was noted in a group of heavily pretreated patients. Phase II clinical trials of SS1P are being planned for malignant mesothelioma and other mesothelin-expressing malignancies.

Immunotoxins for targeted cancer therapy

Immunotoxins are proteins that contain a toxin along with an antibody or growth factor that binds specifically to target cells. Nearly all protein toxins work by enzymatically inhibiting protein synthesis. For the immunotoxin to work, it must bind to and be internalized by the target cells, and the enzymatic fragment of the toxin must translocate to the cytosol. Once in the cytosol, 1 molecule is capable of killing a cell, making immunotoxins some of the most potent killing agents. Various plant and bacterial toxins have been genetically fused or chemically conjugated to ligands that bind to cancer cells. Among the most active clinically are those that bind to hematologic tumors. At present, only 1 agent, which contains human interleukin-2 and truncated diphtheria toxin, is approved for use in cutaneous T-cell lymphoma. Another, containing an anti-CD22 Fv and truncatedPseudomonas exotoxin, has induced complete remissions in a high proportion of cases of hairy-cell leukemia. Refinement of existing immunotoxins and development of new immunotoxins are underway to improve the treatment of cancer.

Phase I Trial of Recombinant Immunotoxin RFB4(dsFv)-PE38 (BL22) in Patients With B-Cell Malignancies

PURPOSE To conduct a phase I trial of recombinant immunotoxin BL22, an anti-CD22 Fv fragment fused to truncated Pseudomonas exotoxin. PATIENTS AND METHODS Forty-six pretreated patients with CD22+ non-Hodgkins lymphoma (NHL; n = 4), chronic lymphocytic leukemia (CLL; n = 11), and hairy cell leukemia (HCL; n = 31) received 265 cycles at 3 to 50 microg/Kg every other day x 3 doses. RESULTS BL22 was active in HCL, with 19 complete remissions (CRs; 61%) and six partial responses (PRs; 19%) in 31 patients. Of 19 CRs, 11 were achieved after one cycle and eight after two to 14 cycles. All 25 responders benefited clinically with one cycle. The CR rate was 86% in patients enrolled at > or = 40 microg/Kg every other day x 3, and 41% at lower doses (P = .011). The median duration for CR was 36 months (range, 5 to 66 months), and eight patients remain in CR at 45 months (range, 29 to 66 months). Lower but significant activity occurred in CLL. Neutralizing antibodies occurred in 11 (24%) of 46 patients (all HCL). A reversible hemolytic uremic syndrome requiring plasmapheresis was observed in one patient with NHL during cycle 1 and in four patients with HCL during cycle 2 or 3. The maximum-tolerated dose (MTD) evaluated at cycle 1 was 40 microg/Kg IV. QOD x 3. The most common toxicities at 30 to 50 microg/Kg every other day x 3 included hypoalbuminemia, transaminase elevations, fatigue, and edema. CONCLUSION BL22 was well tolerated and highly effective in HCL, even after one cycle. Phase II testing is underway to define the efficacy with one cycle and to study safety when additional cycles are needed for optimal response.

Phase I Trial of Anti-CD22 Recombinant Immunotoxin Moxetumomab Pasudotox (CAT-8015 or HA22) in Patients With Hairy Cell Leukemia

PURPOSE To conduct a phase I dose-escalation trial assessing safety and response of recombinant immunotoxin moxetumomab pasudotox (CAT-8015, HA22) in chemotherapy-resistant hairy cell leukemia (HCL). PATIENTS AND METHODS Eligible patients had relapsed/refractory HCL after ≥ two prior therapies and required treatment because of abnormal blood counts. Patients received moxetumomab pasudotox 5 to 50 μg/kg every other day for three doses (QOD ×3), with up to 16 cycles repeating at ≥ 4-week intervals if patients did not experience disease progression or develop neutralizing antibodies. RESULTS Twenty-eight patients were enrolled, including three patients each at 5, 10, 20, and 30 μg/kg, four patients at 40 μg/kg, and 12 patients at 50 μg/kg QOD ×3 for one to 16 cycles each (median, four cycles). Dose-limiting toxicity was not observed. Two patients had transient laboratory abnormalities consistent with grade 2 hemolytic uremic syndrome with peak creatinine of 1.53 to 1.66 mg/dL and platelet nadir of 106,000 to 120,000/μL. Drug-related toxicities in 25% to 64% of the 28 patients included (in decreasing frequency) grade 1 to 2 hypoalbuminemia, aminotransferase elevations, edema, headache, hypotension, nausea, and fatigue. Of 26 patients evaluable for immunogenicity, 10 patients (38%) made antibodies neutralizing more than 75% of the cytotoxicity of 1,000 ng/mL of immunotoxin, but this immunogenicity was rare (5%) after cycle 1. The overall response rate was 86%, with responses observed at all dose levels, and 13 patients (46%) achieved complete remission (CR). Only 1 CR lasted less than 1 year, with the median disease-free survival time not yet reached at 26 months. CONCLUSION Moxetumomab pasudotox at doses up to 50 μg/kg QOD ×3 has activity in relapsed/refractory HCL and has a safety profile that supports further clinical development for treatment of this disease.

Immunotoxins in cancer therapy.

Several antibodies have now been approved for the therapy of cancer. Two of these, rituximab (Rituxan) and alemtuzumab (CAMPATH; ILEX Oncology Inc), are capable of inducing cell death and tumor regressions when given alone. The third, trastuzumab (Herceptin), is not very effective by itself, but does potentiate the activity of paclitaxel (Taxol) and other chemotherapeutic agents. Unfortunately, most antibodies do not kill cancer cells unless they are armed with a cytotoxic agent, such as a radioisotope, a cytotoxic drug, or a protein toxin. Each of these have advantages and disadvantages.

Ablation of cerebellar Golgi cells disrupts synaptic integration involving GABA inhibition and NMDA receptor activation in motor coordination.

The role of inhibitory Golgi cells in cerebellar function was investigated by selectively ablating Golgi cells expressing human interleukin-2 receptor alpha subunit in transgenic mice, using the immunotoxin-mediated cell targeting technique. Golgi cell disruption caused severe acute motor disorders. These mice showed gradual recovery but retained a continuing inability to perform compound movements. Optical and electrical recordings combined with immunocytological analysis indicated that elimination of Golgi cells not only reduces GABA-mediated inhibition but also attenuates functional NMDA receptors in granule cells. These results demonstrate that synaptic integration involving both GABA inhibition and NMDA receptor activation is essential for compound motor coordination. Furthermore, this integration can adapt after Golgi cell elimination so as not to evoke overexcitation by the reduction of NMDA receptors.

Phase II Trial of Recombinant Immunotoxin RFB4(dsFv)-PE38 (BL22) in Patients With Hairy Cell Leukemia

PURPOSE To conduct a phase II trial in chemoresistant hairy cell leukemia (HCL) with BL22, a recombinant anti-CD22 immunotoxin which showed phase I activity in HCL. PATIENTS AND METHODS Eligible patients had relapsed/refractory HCL and needed treatment based on blood counts. Patients were stratified into three groups: response to cladribine less than 1 year, those with a response lasting 1 to 4 years, or no response and uncontrolled infection. Patients received BL22 40 microg/kg every other day for three doses on cycle 1. Those achieving hematologic remission (HR), defined as neutrophils > or = 1,500/mm(3), hemoglobin > or = 11 g/dL, and platelets > or = 100,000/mm(3), were observed. Patients without HR were re-treated at 30 microg/kg every other day for three doses every 4 weeks beginning at least 8 weeks after cycle 1. RESULTS Thirty-six patients were enrolled including 26, nine, and one in groups 1 to 3. The response after one cycle (CR, 25%; PR, 25%) improved when 56% were re-treated (CR, 47%; PR, 25%). CR rate was similar in groups 1 and 2 (P = .7). Twenty-two with baseline spleen height lower than 200 mm had higher CR (64% v 21%; P = .019) and OR rates (95% v 36%; P = .0002) compared to 14 with spleens either absent or higher than 200 mm. The only serious toxicity was reversible grade 3 hemolytic uremic syndrome, not requiring plasmapheresis, in two patients (6%). High neutralizing antibodies were observed in four patients (11%) and prevented re-treatment. CONCLUSION BL22 activity in HCL is confirmed. Best responses to BL22 after cladribine failure are achieved before the patients develop massive splenomegaly or undergo splenectomy.