Human Anti-human Antibody Response Research Articles

Phase I trial of anti-GD2 monoclonal antibody hu3F8 plus GM-CSF: Impact of body weight, immunogenicity and anti-GD2 response on pharmacokinetics and survival

ABSTRACTFifty-seven stage 4 patients with refractory/relapsed neuroblastoma were enrolled in a phase I trial (Clinicaltrials.gov NCT01757626) using humanized anti-GD2 monoclonal antibody hu3F8 in combination with granulocyte-macrophage colony-stimulating factor. The influence of body weight and human anti-human antibody (HAHA) on the pharmacokinetics (PK) of hu3F8, and the effect of de novo anti-GD2 response on patient outcome were explored. Serum samples before hu3F8 infusion, and serially up to day 12 during treatment cycle #1, and at 5 min after each hu3F8 infusion for all subsequent cycles were collected. PK was analyzed using non-compartmental modeling. Immunogenicity was assayed by HAHA response, and vaccination effect by induced host anti-GD2 response measured periodically until disease progression or last followup. Progression-free and overall survival was estimated by the Kaplan-Meier method. Despite dosing being based on body weight, smaller patients had consistently lower area-under-the-curve and faster clearance over the 15 dose levels (0.9 to 9.6 mg/kg per treatment cycle) in this trial. Positive HAHA, defined by the upper limit of normal, when measured within 10 days from the last hu3F8 dose received, was associated with significantly lower serum hu3F8. Despite prior sensitization to other anti-GD2 antibody, e.g. mouse 3F8 or ch14.18, 75% of the patients never developed HAHA response even after getting more treatment cycles. Hu3F8 induced a de novo anti-GD2 response in patients, which was prognostic of progression-free survival. We conclude that hu3F8 had low immunogenicity. During treatment, positive HAHA and low body weight affected PK adversely, whereas induced anti-GD2 response was an outcome predictor.

Phase I study of tremelimumab (CP-675 206) plus PF-3512676 (CPG 7909) in patients with melanoma or advanced solid tumours

Background:Tremelimumab, a fully human cytotoxic T-lymphocyte antigen 4 monoclonal antibody, and PF-3512676, a Toll-like receptor-9 agonist, are targeted immune modulators that elicit durable single-agent antitumour activity in advanced cancer.Methods:To determine the maximum tolerated dose (MTD) of these agents combined during this phase I study, patients received intravenous tremelimumab (6.0, 10.0, or 15.0 mg kg−1) every 12 weeks plus subcutaneous PF-3512676 (0.05, 0.10, or 0.15 mg kg−1) weekly. Primary end points were safety and tolerability; secondary end points included pharmacokinetics and antitumour activity.Results:Twenty-one patients with stage IV melanoma (n=17) or advanced solid tumours (n=4) were enrolled. Injection-site reactions (n=21; 100%), influenza-like illness (n=18; 86%), and diarrhoea (n=13; 62%) were the most common treatment-related adverse events (TAEs). Grade ⩾3 TAEs were reported (n=7; 33%). Dose-limiting toxicities (prespecified 6-week observation) occurred in one of the six patients in the 10 mg kg−1 tremelimumab plus 0.05 mg kg−1 PF-3512676 cohort (grade 3 hypothalamopituitary disorder) and two of the six patients in the 15 mg kg−1 tremelimumab plus 0.05 mg kg−1 PF-3512676 cohort (grade 3 diarrhoea). Consequently, 15 mg kg−1 tremelimumab plus 0.05 mg kg−1 PF-3512676 exceeded the MTD. Two melanoma patients achieved durable (⩾170 days) partial response. No human antihuman antibody responses to tremelimumab were observed.Conclusion:Weekly PF-3512676 (⩽0.15 mg kg−1) plus tremelimumab (⩽10 mg kg−1 every 12 weeks) was tolerable.

Induction of Human Anti-Human Antibody Responses (Ab2) after Application of a Humanized Lewis Y Carbohydrate Specific Antibody (Ab1): Connection of Prolonged Disease Stabilization with Ab3 Induction?

Purpose: Detailed analysis of a patient with epithelial Lewis Y (LeY) positive cancer who received twice 50 mg of the humanized Lewis Y carbohydrate specific mAb IGN311 and developed a clinically significant human anti-human antibody (HAHA) response (Ab2). Results: Clinical stabilization of the disease was assigned to in this patient. The HAHA response consisted mainly of IgG1 and was found to be directed against the IGN311 binding site. Consistent with the induction of the HAHA response, CDC activity against Lewis Y positive target cells was completely abolished at day 8 and could not be restored by the second 50 mg infusion indicating complete neutralization of applied IGN311. The ADCC reactivity was also significantly reduced and anti-anti idiotype-specific antibodies (Ab3) were detectable at day 65. Conclusions: Induction of Ab3 antibodies should be considered as an additional factor influencing the efficacy of humanized antibodies. In this context, the potential threat of induced HAHA responses against therapeutic mAbs might have to be reconsidered because they might actually have also beneficial immunological long-term effects leading to an active immunization component induced by therapeutic antibodies.

A first-in-human phase I study of U3-1287 (AMG 888), a HER3 inhibitor, in patients (pts) with advanced solid tumors.

3026 Background: Human epidermal growth factor receptor 3 (HER3) is a key dimerization partner for the HER family and activates oncogenic signaling pathways. Its overexpression in many solid tumors has been linked to poor prognosis. U3-1287 (AMG 888) is a fully human anti-HER3 monoclonal antibody with demonstrated anticancer activity in preclinical models. This study had 2 parts: based on the observed tolerability of ≤20 mg/kg and pharmacokinetics (PK) study from the dose-escalation phase (part 1), 9, 14, and 20 mg/kg Q2W dosages were given in the dose-expansion phase (part 2). Methods: Pts with non–small cell lung cancer (NSCLC) and other refractory solid tumors thought to express HER3 were eligible. Study assessments included adverse event (AE) rates, PK, and tumor response (per modified RECIST and overall metabolic tumor response by EORTC). Human antihuman antibody (HAHA) responses were measured to assess immunogenicity. Results: Part 2 data (as of 9/24/10) are reported. Pts (N=31) with head and neck (1 pt), breast (1), ovary (1), cecum/appendix (1), colorectal cancer (CRC, 10), and NSCLC (17) tumor types were given 9 (n=10), 14 (n=2), and 20 mg/kg (n=19) of U3-1287 (AMG 888). Pts had a median (range) of 5 (2–10) prior chemotherapy regimens. 11 serious AEs were reported in 10 pts; none were attributable to U3-1287 (AMG 888). 16 pts (51.6%) experienced AEs ≥ grade (Gr) 3, with 1 Gr 5 dyspnea and 1 Gr 4 hypoglycemia. Two AEs ≥ Gr 3 were considered drug-related: Gr 3 anemia and Gr 3 rash. Gr 1/2 drug-related AEs in ≥10% of pts were fatigue, diarrhea, and nausea. Of 21 pts assessed, no HAHA responses were observed (by 21 days postdose). PK analysis (n=15) showed that steady state is reached after 3 dosing cycles for pts receiving 9, 14, or 20 mg/kg. Mean steady state Cmin was >10-fold greater than the threshold concentration required for 90% HER3 inhibition in xenograft models. Eight pts (6 NSCLC, 2 CRC) had a best response of stable disease at or beyond day-70 visit. One pt (NSCLC, 20 mg/kg) had significant tumor shrinkage (26.3%) and partial metabolic response by FDG-PET scan. Conclusions: U3-1287 (AMG 888) is well tolerated, with an absence of detectable HAHA responses. Part 2 data support 9–20 mg/kg Q2W dosage for future studies.

HAHA – nothing to laugh about. Measuring the immunogenicity (human anti-human antibody response) induced by humanized monoclonal antibodies applying ELISA and SPR technology

Immunogenicity induced by passively applied proteins is a serious issue because it is directly related to the patient's safety. The out-come of an immune reaction to a therapeutic protein can range from transient appearance of antibodies without any clinical significance to severe life threatening conditions. Within this article, enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR) methodology to measure immunogenicity are compared and the pros and cons are discussed.

FDA Report: Eculizumab (Soliris®) for the Treatment of Patients with Paroxysmal Nocturnal Hemoglobinuria

On March 16, 2007, eculizumab (Soliris; Alexion Pharmaceuticals, Inc. Cheshire, CT), a humanized monoclonal antibody that binds to the human C5 complement protein, received accelerated approval by the U.S. Food and Drug Administration for the treatment of patients with paroxysmal nocturnal hemoglobinuria (PNH) to reduce hemolysis. Eculizumab was studied in a randomized, double-blind, placebo-controlled clinical trial in 87 RBC transfusion-dependent adult PNH patients and in a supportive single-arm study in 96 patients. The eculizumab dose was 600 mg as a 35-minute i.v. infusion administered weekly for the first 4 weeks followed by 900 mg (week 5) then 900 mg every 14 days thereafter. Hemoglobin stabilized in 21 of 43 (48.8%) eculizumab-treated patients, compared with none of 44 placebo-treated patients. Eculizumab-treated patients required significantly fewer RBC transfusions than placebo-treated patients (median, 0 versus 10 units). There was also a significant reduction in the serum lactate dehydrogenase area under the curve with eculizumab compared with placebo treatment. Results of the phase II supportive study were similar to those of the phase III study. The safety database included 196 adult patients with PNH. Significant findings included the development of human anti-human antibody responses in three patients and serious meningococcal infections in three patients. Patients should undergo meningococcal vaccination at least 2 weeks prior to receiving the first eculizumab treatment and have revaccination according to current medical guidelines. Patients must be monitored and evaluated immediately for early signs of meningococcal infections and treated with antibiotics as indicated.

Pharmacokinetic and safety study of subcutaneously administered weekly ING-1, a human engineered™ monoclonal antibody targeting human EpCAM, in patients with advanced solid tumors

ING-1 is a high-affinity, human engineeredtrade mark monoclonal antibody that recognizes a 40 kilodalton epithelial cell adhesion molecule (EpCAM) glycoprotein that is expressed in high levels on most adenocarcinomas and is an attractive target for immunotherapy. ING-1 was administered subcutaneously weekly at doses between 0.1 and 2 mg/kg/week. Pharmacokinetic samples were drawn during weeks 1 and 6. Fourteen patients with advanced refractory cancer received a median of 6 (range 1-9) doses of ING-1. At 1 mg/kg, a 62-year-old man with colon cancer developed reversible grade 3 pancreatitis after the third dose. His plasma ING-1 levels were similar to the other two patients dosed at 1 mg/kg. Two patients dosed at 0.6 mg/kg experienced stable disease at 6 weeks. Peak drug levels increased with dose and time, suggesting drug accumulation with repeated dosing. Low human anti-human antibody response was noted in three of the 13 patients assessed and was directed towards the variable region of ING-1. Weekly ING-1 administered subcutaneously was well tolerated at 0.6 mg/kg/week and further experience at this dose is warranted to demonstrate safety. The risk of pancreatitis and the marginal anti-tumor effect may preclude further monotherapy studies; however, combination studies with chemotherapy are warranted.

Vascular Targeted Therapy With Anti–Prostate-Specific Membrane Antigen Monoclonal Antibody J591 in Advanced Solid Tumors

Based on prostate-specific membrane antigen (PSMA) expression on the vasculature of solid tumors, we performed a phase I trial of antibody J591, targeting the extracellular domain of PSMA, in patients with advanced solid tumor malignancies. This was a proof-of-principle evaluation of PSMA as a potential neovascular target. The primary end points were targeting,toxicity, maximum-tolerated dose, pharmacokinetics (PK), and human antihuman antibody (HAHA) response. Patients had advanced solid tumors previously shown to express PSMA on the neovasculature. They received 111Indium (111ln)-J591 for scintigraphy and PK, followed 2 weeks later by J591 with a reduced amount of 111In for additional PK measurements. J591 dose levels were 5, 10, 20, 40, and 80 mg. The protocol was amended for six weekly administrations of unchelated J591. Patients with a response or stable disease were eligible for re-treatment. Immunohistochemistry assessed PSMA expression in tumor tissues. Twenty-seven patients received monoclonal antibody (mAb) J591. Treatment was well tolerated. Twenty (74%) of 27 patients had at least one area of known metastatic disease targeted by 111In-J591, with positive imaging seen in patients with kidney, bladder, lung, breast, colorectal, and pancreatic cancers, and melanoma. Seven of 10 patient specimens available for immunohistochemical assessment of PSMA expression in tumor-associated vasculature demonstrated PSMA staining. No HAHA response was seen. Three patients of 27 with stable disease received re-treatment. Acceptable toxicity and excellent targeting of known sites of metastases were demonstrated in patients with multiple solid tumor types, highlighting a potential role for the anti-PSMA antibody J591 as a vascular-targeting agent.

Qualification and application of a surface plasmon resonance-based assay for monitoring potential HAHA responses induced after passive administration of a humanized anti Lewis-Y antibody

A sensitive, surface plasmon resonance (SPR)-based assay monitoring potential human–anti-human antibody (HAHA) reactions against the monoclonal antibody (mAb) IGN311 is presented. The latter is a fully humanized Lewis-Y carbohydrate specific mAb that is currently tested in a passive immune therapy approach in a clinical phase I trial. For the SPR experiments a BIACORE 3000 analyzer was used. The ligand IGN311 was covalently coupled to the carboxy-methylated dextran matrix of a CM5 research grade chip (BIACORE). In the course of a fully nested experimental design, a four parameter logistic equation was identified as appropriate calibration model ranging from 0.3 μg/mL (lower limit of quantitation, LLOQ) to 200 μg/mL (upper limit of quantitation, ULOQ) using an anti-idiotypic mAb (‘HAHA mimic’) as calibrator. The bias ranged from −2.4% to 5.5% and the intermediate precision expressed as 95% CI revealed values from 5.6% to 8.3%. Specificity was evaluated using six human serum matrices from healthy donors spiked with calibrator at the limit of quantitation (LOQ) with >80% of values being recovered with less than 25% relative error. The qualified assay was applied to monitor potentially induced HAHA reactivity in 11 patients from a clinical phase I trial with passively administered IGN311. Of the 11 patients, one high HAHA responder and several low responders were identified. Protein-G depletion experiments with human serum samples revealed that the observed response is predominantly caused by IgG binding to the ligand. The characteristics of these HAHA responses were all of the so-called ‘Type I’ which is defined by a peak response around day 15 that decreases from this point steadily suggesting that some kind of tolerance is established. Therefore, this type of HAHA response is regarded as non critical for the patient's safety.

A Phase I Trial of Humanized Monoclonal Antibody A33 in Patients with Colorectal Carcinoma: Biodistribution, Pharmacokinetics, and Quantitative Tumor Uptake

To determine the in vivo characteristics of huA33, a CDR-grafted humanized antibody against the A33 antigen, we have conducted an open-label, dose escalation, biopsy-based phase I trial of huA33 in patients with colorectal carcinoma. Patients with colorectal carcinoma were infused with [131I]huA33 (400 MBq: 10 mCi) and [125I]huA33 (40 MBq: 1 mCi) 1 week before surgery. There were four huA33 dose levels (0.25, 1.0, 5.0, and 10 mg/m2). Adverse events, pharmacokinetics, biodistribution, tumor biopsies, and immune responses to huA33 were evaluated. There were 12 patients entered into the trial (6 males and 6 females; age range, 39-66 years). No dose-limiting toxicity was observed. The biodistribution of huA33 showed excellent uptake of [131I]huA33 in metastatic colorectal carcinoma. Pharmacokinetic analysis showed no significant difference in terminal half-life (T1/2beta) between dose levels (mean +/- SD, 86.92 +/- 22.12 hours). Modeling of colon uptake of huA33 showed a T1/2 of elimination of 32.4 +/- 8.1 hours. Quantitative tumor uptake ranged from 2.1 x 10(-3) to 11.1 x 10(-3) %ID/g, and tumor/normal tissue and tumor/serum ratios reached as high as 16.3:1 and 4.5:1, respectively. Biosensor analysis detected low-level human anti-human antibody responses in four patients following huA33 infusion. huA33 shows selective and rapid localization to colorectal carcinoma in vivo and penetrates to the center of large necrotic tumors, and colon elimination half-life of huA33 is equivalent to basal colonocyte turnover. The excellent targeting characteristics of this humanized antibody indicate potential for the targeted therapy of metastatic colorectal cancer in future trials.

Phase I trial of yttrium-90—labeled anti—prostate-specific membrane antigen monoclonal antibody J591 for androgen-independent prostate cancer: Milowsky MI, Nanus DM, Kostakoglu L, Vallabhajosula S, Goldsmith SJ, Bander NH, Division of Hematology and Medical Oncology, Department of Medicine, Weill Medical College of Cornell University, New York, NY. J Clin Oncol 2004;22:2522–31

To determine the maximum-tolerated dose (MTD), toxicity, human antihuman antibody (HAHA) response, pharmacokinetics, organ dosimetry, targeting, and preliminary efficacy of yttrium-90–labeled anti-prostate-specific membrane antigen monoclonal antibody J591 (90Y-J591) in patients with androgen-independent prostate cancer (PC). Patients with androgen-independent PC and evidence of disease progression received indium-111-J591 for pharmacokinetic and biodistribution determinations followed 1 week later by 90Y-J591 at five dose levels: 5, 10, 15, 17.5, and 20 mCi/m2. Patients were eligible for up to three re-treatments if platelet and neutrophil recovery was satisfactory. Twenty-nine patients with androgen-independent PC received 90Y-J591, four of whom were re-treated. Dose limiting toxicity (DLT) was seen at 20 mCi/m2, with two patients experiencing thrombocytopenia with non-life-threatening bleeding episodes requiring platelet transfusions. The 17.5-mCi/m2 dose level was determined to be the MTD. No re-treated patients experienced DLT. Nonhematologic toxicity was not dose limiting. Targeting of known sites of bone and soft tissue metastases was seen in the majority of patients. No HAHA response was seen. Antitumor activity was seen, with two patients experiencing 85% and 70% declines in prostate-specific antigen (PSA) levels lasting 8 and 8.6 months, respectively, before returning to baseline. Both patients had objective measurable disease responses. An additional six patients (21%) experienced PSA stabilization. The recommended dose for 90Y-J591 is 17.5 mCi/m2. Acceptable toxicity, excellent targeting of known sites of PC metastases, and biologic activity in patients with androgen-independent PC warrant further investigation of 90Y-J591 in the treatment of patients with PC.

Pharmacokinetics and immunogenicity profiles for fully human monoclonal antibodies against soluble and membrane bound antigens in patients with psoriasis and melanoma

Pharmacokinetics (PK) and immunogenicity were among the factors limiting the successful application of murine monoclonal antibodies (mAB). These limitations led to development of technologies to limit/eliminate the xenogeneic component. Furthermore, direct binding of mAB to a membrane bound antigen (AG) with subsequent internalization of the AG-mAB complex could enhance the mAB clearance(CL) and immunogenicity. We examined PK and immunogenicity of fully human IgG2 antibodies, ABX-IL8 and ABX-MA1, generated using Xenomouse® technology, against non-internalizing/soluble (IL8) and membrane bound/internalizing (CD146, Muc18) AGs, in patients with psoriasis and melanoma, respectively. Following administration of single doses of 0.3, 1.0, and 3.0 mg/kg in psoriasis patients, pharmacokinetics of ABX-IL8 proved to be linear (CL: 2.8±0.6, 3.3±1.4, and 2.3±0.6 mL/day/kg). ABX-IL8 PK was time-independent following administration of multiple doses. ABX-IL8 PK in monkeys was linear over a wide dose range (CL:~7.0 mL/day/kg) and, using allometric scaling, provided reasonable estimates of human PK. In contrast, ABX-MA1 exhibited target-mediated PK in monkeys with a faster CL at lower doses (CL:23.0±7.0 & 7.0±0.4 mL/day/kg at 2 & 20 mg/kg). A similar non-linearity was observed in melanoma patients. No human anti-human antibody response was detected in any of the patients highlighting the lack of immunogenicity of these fully human mABs generated by Xenomouse® technology. Clinical Pharmacology & Therapeutics (2004) 75, P39–P39; doi: 10.1016/j.clpt.2003.11.148

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.

A Phase IB 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.