Enhanced Delivery of SN-38 to Human Tumor Xenografts with an Anti-Trop-2-SN-38 Antibody Conjugate (Sacituzumab Govitecan)

PURPOSE

This study examined the delivery of SN-38 to Trop-2-expressing tumors and assessed the constitutive products in the serum, liver, and small intestine in nude mice bearing human tumor xenografts (Capan-1 or NCI-N87) given a single injection of irinotecan (40 mg/kg; ∼ 0.8 mg/mouse, containing ∼ 460 μg SN-38 equivalents) or sacituzumab govitecan (IMMU-132), an antibody-drug conjugate composed of a humanized anti-Trop-2 IgG coupled site specifically with an average of 7.6 molecules of SN-38.

EXPERIMENTAL DESIGN

At select times, tissues were extracted and concentrations of the products measured by reversed-phase high-performance liquid chromatography (HPLC).

RESULTS

In serum, >98% irinotecan cleared within 5 minutes; peak levels of SN-38 and SN-38G (glucuronidated SN-38) were detected in equal amounts at this time, and no longer detected after 6 to 8 hours. IMMU-132 was detected in the serum over 3 days, and at each interval, ≥ 95% of total SN-38 was bound to the antibody. Intact IMMU-132 cleared with a half-life of 14 hours, which closely reflected the in vitro rate of SN-38 released from the conjugate in mouse serum (i.e., 17.5 hours), whereas the IgG portion of the conjugate cleared with a half-life of 67.1 hours. In vitro and in vivo studies disclosed IgG-bound SN-38 was protected from glucuronidation. Area under the curve (AUC) analysis indicated that IMMU-132 delivers 20-fold to as much as 136-fold more SN-38 to tumors than irinotecan, with tumor:blood ratios favoring IMMU-132 by 20- to 40-fold. Intestinal concentrations of SN-38/SN-38G also were 9-fold lower with IMMU-132.

CONCLUSIONS

These studies confirm a superior SN-38 tumor delivery by IMMU-132 compared with irinotecan.

Sacituzumab Govitecan (IMMU-132), an Anti-Trop-2/SN-38 Antibody-Drug Conjugate: Characterization and Efficacy in Pancreatic, Gastric, and Other Cancers

Sacituzumab govitecan (IMMU-132) is an antibody-drug conjugate (ADC) made from a humanized anti-Trop-2 monoclonal antibody (hRS7) conjugated with the active metabolite of irinotecan, SN-38. In addition to its further characterization, as the clinical utility of IMMU-132 expands to an ever-widening range of Trop-2-expressing solid tumor types, its efficacy in new disease models needs to be explored in a nonclinical setting. Unlike most ADCs that use ultratoxic drugs and stable linkers, IMMU-132 uses a moderately toxic drug with a moderately stable carbonate bond between SN-38 and the linker. Flow cytometry and immunohistochemistry disclosed that Trop-2 is expressed in a wide range of tumor types, including gastric, pancreatic, triple-negative breast (TNBC), colonic, prostate, and lung. While cell-binding experiments reveal no significant differences between IMMU-132 and parental hRS7 antibody, surface plasmon resonance analysis using a Trop-2 CM5 chip shows a significant binding advantage for IMMU-132 over hRS7. The conjugate retained binding to the neonatal receptor, but it lost greater than 60% of the antibody-dependent cell-mediated cytotoxicity activity compared to that of hRS7. Exposure of tumor cells to either free SN-38 or IMMU-132 demonstrated the same signaling pathways, with pJNK1/2 and p21(WAF1/Cip1) upregulation followed by cleavage of caspases 9, 7, and 3, ultimately leading to poly-ADP-ribose polymerase cleavage and double-stranded DNA breaks. Pharmacokinetics of the intact ADC in mice reveals a mean residence time (MRT) of 15.4 h, while the carrier hRS7 antibody cleared at a similar rate as that of the unconjugated antibody (MRT ∼ 300 h). IMMU-132 treatment of mice bearing human gastric cancer xenografts (17.5 mg/kg; twice weekly × 4 weeks) resulted in significant antitumor effects compared to that of mice treated with a nonspecific control. Clinically relevant dosing schemes of IMMU-132 administered either every other week, weekly, or twice weekly in mice bearing human pancreatic or gastric cancer xenografts demonstrate similar, significant antitumor effects in both models. Current Phase I/II clinical trials ( ClinicalTrials.gov , NCT01631552) confirm anticancer activity of IMMU-132 in cancers expressing Trop-2, including gastric and pancreatic cancer patients.

First-in-Human Trial of a Novel Anti-Trop-2 Antibody-SN-38 Conjugate, Sacituzumab Govitecan, for the Treatment of Diverse Metastatic Solid Tumors

PURPOSE

Sacituzumab govitecan (IMMU-132) is an antibody-drug conjugate (ADC) targeting Trop-2, a surface glycoprotein expressed on many epithelial tumors, for delivery of SN-38, the active metabolite of irinotecan. This phase I trial evaluated this ADC as a potential therapeutic for pretreated patients with a variety of metastatic solid cancers.

EXPERIMENTAL DESIGN

Sacituzumab govitecan was administered on days 1 and 8 of 21-day cycles, with cycles repeated until dose-limiting toxicity or progression. Dose escalation followed a standard 3 + 3 scheme with 4 planned dose levels and dose delay or reduction allowed.

RESULTS

Twenty-five patients (52-60 years old, 3 median prior chemotherapy regimens) were treated at dose levels of 8 (n = 7), 10 (n = 6), 12 (n = 9), and 18 (n = 3) mg/kg. Neutropenia was dose limiting, with 12 mg/kg the maximum tolerated dose for cycle 1, but too toxic with repeated cycles. Lower doses were acceptable for extended treatment with no treatment-related grade 4 toxicities and grade 3 toxicities limited to fatigue (n = 3), neutropenia (n = 2), diarrhea (n = 1), and leukopenia (n = 1). Using CT-based RECIST 1.1, two patients achieved partial responses (triple-negative breast cancer, colon cancer) and 16 others had stable disease as best response. Twelve patients maintained disease control with continued treatment for 16 to 36 weeks; 6 survived 15 to 20+ months. No preselection of patients based on tumor Trop-2 expression was done.

CONCLUSIONS

Sacituzumab govitecan had acceptable toxicity and encouraging therapeutic activity in patients with difficult-to-treat cancers. The 8 and 10 mg/kg doses were selected for phase II studies.

Abstract P5-01-01: Pretargeted immuno-PET with an anti-carcinoembryonic antigen (CEA) bispecific antibody (BsMAb) and a 68Ga-labeled hapten-peptide compared to conventional imaging and FDG-PET in metastatic breast cancer patients (BC): First results

Objectives: Different imaging methods are available to detect BC recurrence, but several new noninvasive antibody imaging methods targeting membranous IGF-1R expression1, HER2/neu2 or CD1383 have been tested in BC pre-clinical trials. Today, a new generation anti-CEA x anti-HSG humanized trivalent TF2 BsMAb and 68Ga-IMP288 HSG peptide is available with good features for immuno-PET in preclinical studies4. This study aimed to compare the sensitivity of anti-CEA immuno-PET/CT using pretargeted 68Ga-IMP288 to morphological imaging and FDG-PET/CT in metastatic BC patients.

Methods: Ten patients, enrolled in an optimization immuno-PET study underwent whole-body immuno-PET/CT recorded 1h and 2h after injection of 150 MBq of 68Ga-IMP288 pretargeted by 120 nmol of TF2 injected 24h to 30h before, in addition to thoracic-abdominal-pelvic CT and FDG-PET/CT. Bone (n=5) and brain MRI (n=2) were also performed in some cases to confirm abnormalities detected by other modalities. The gold standard was determined by follow-up and a lesion detected by at least 2 imaging modalities was considered as positive.

Results: Median CA15-3 was 264.1 kUI/L (31.6 to 2448) and median CEA was 48.25 µg/L (9.5 to 1359.0). A total of 537 lesions were detected by immuno-PET/CT, 247 by CT, 160 by bone MRI, and 428 by FDG-PET/CT. To date, 524 lesions were confirmed as pathologic by the gold standard: 17 in nodes, 1 in lung, 83 in liver, 418 in bone, 1 in skin, and 4 in brain. Overall sensitivity of immuno-PET was 92.8%, with 100% sensitivity for bone, liver, skin, and brain, 92.8% for nodes, and 28.6% for lung. Overall sensitivity of CT and FDG-PET/CT were 74% and 95.2%, respectively. CT and FDG-PET/CT had 54.5 and 100% sensitivity for nodes, 90 and 100 % for liver, 100 and 85.7% for lung, and 68% and 94% for bone, respectively. Bone MRI had 92.3% sensitivity. Brain lesions were only detected by immuno-PET/CT and confirmed by MRI. Median tumor SUVpeak on immuno-PET at 1h and FDG-PET/CT were 5.05 (3.52 to 24.55) and 3.3 (0.47-10.95), respectively. A 2h, median tumor SUVpeak on immuno-PET was 5.23 (3.09-34.27), 5/9 patients showing an increased tumor uptake between 1 and 2h, with no lesion being detected only at 2h.

Conclusion: These results demonstrate the high accuracy of anti-CEA pretargeted immuno-PET/CT for staging BC patients, especially for bone, liver and brain evaluation. Immuno-PET allowed detection of bone lesions in eras not explored by MRI.

1. Heskamp S. J Nucl Med 2010.

2. Dijkers ECF. J Nucl Med 2009.

3. Rousseau C. EJNMMI Res. 2011.4. Schoffelen, Mol Cancer Ther, 2010.

Citation Format: Mario Campone, Aurore Rauscher, Alain Faivre-Chauvet, Thomas Carlier, Ludovic Ferrer, Pierre Baumgartner, David M Goldenberg, Robert M Sharkey, Jacques Barbet, Françoise Kraeber-Bodéré, Caroline Rousseau. Pretargeted immuno-PET with an anti-carcinoembryonic antigen (CEA) bispecific antibody (BsMAb) and a 68Ga-labeled hapten-peptide compared to conventional imaging and FDG-PET in metastatic breast cancer patients (BC): First results [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-01-01.

A pretargeting system for tumor PET imaging and radioimmunotherapy

Labeled antibodies, as well as their fragments and antibody-derived recombinant constructs, have long been proposed as general vectors to target radionuclides to tumor lesions for imaging and therapy. They have indeed shown promise in both imaging and therapeutic applications, but they have not fulfilled the original expectations of achieving sufficient image contrast for tumor detection or sufficient radiation dose delivered to tumors for therapy. Pretargeting was originally developed for tumor immunoscintigraphy. It was assumed that directly-radiolabled antibodies could be replaced by an unlabeled immunoconjugate capable of binding both a tumor-specific antigen and a small molecular weight molecule. The small molecular weight molecule would carry the radioactive payload and would be injected after the bispecific immunoconjugate. It has been demonstrated that this approach does allow for both antibody-specific recognition and fast clearance of the radioactive molecule, thus resulting in improved tumor-to-normal tissue contrast ratios. It was subsequently shown that pretargeting also held promise for tumor therapy, translating improved tumor-to-normal tissue contrast ratios into more specific delivery of absorbed radiation doses. Many technical approaches have been proposed to implement pretargeting, and two have been extensively documented. One is based on the avidin-biotin system, and the other on bispecific antibodies binding a tumor-specific antigen and a hapten. Both have been studied in preclinical models, as well as in several clinical studies, and have shown improved targeting efficiency. This article reviews the historical and recent preclinical and clinical advances in the use of bispecific-antibody-based pretargeting for radioimmunodetection and radioimmunotherapy of cancer. The results of recent evaluation of pretargeting in PET imaging also are discussed.

Tumor and red bone marrow dosimetry: comparison of methods for prospective treatment planning in pretargeted radioimmunotherapy

Background

Red bone marrow (RBM) toxicity is dose-limiting in (pretargeted) radioimmunotherapy (RIT). Previous blood-based and two-dimensional (2D) image-based methods have failed to show a clear dose-response relationship. We developed a three-dimensional (3D) image-based RBM dosimetry approach using the Monte Carlo-based 3D radiobiological dosimetry (3D-RD) software and determined its additional value for predicting RBM toxicity.

Methods

RBM doses were calculated for 13 colorectal cancer patients after pretargeted RIT with the two-step administration of an anti-CEA × anti-HSG bispecific monoclonal antibody and a ¹⁷⁷Lu-labeled di-HSG-peptide. 3D-RD RBM dosimetry was based on the lumbar vertebrae, delineated on single photon emission computed tomography (SPECT) scans acquired directly, 3, 24, and 72 h after ¹⁷⁷Lu administration. RBM doses were correlated to hematologic effects, according to NCI-CTC v3 and compared with conventional 2D cranium-based and blood-based dosimetry results. Tumor doses were calculated with 3D-RD, which has not been possible with 2D dosimetry. Tumor-to-RBM dose ratios were calculated and compared for ¹⁷⁷Lu-based pretargeted RIT and simulated pretargeted RIT with ⁹⁰Y.

Results

3D-RD RBM doses of all seven patients who developed thrombocytopenia were higher (range 0.43 to 0.97 Gy) than that of the six patients without thrombocytopenia (range 0.12 to 0.39 Gy), except in one patient (0.47 Gy) without thrombocytopenia but with grade 2 leucopenia. Blood and 2D image-based RBM doses for patients with grade 1 to 2 thrombocytopenia were in the same range as in patients without thrombocytopenia (0.14 to 0.29 and 0.11 to 0.26 Gy, respectively). Blood-based RBM doses for two grade 3 to 4 patients were higher (0.66 and 0.51 Gy, respectively) than the others, and the cranium-based dose of only the grade 4 patient was higher (0.34 Gy). Tumor-to-RBM dose ratios would increase by 25% on average when treating with ⁹⁰Y instead of ¹⁷⁷Lu.

Conclusions

3D dosimetry identifies patients at risk of developing any grade of RBM toxicity more accurately than blood- or 2D image-based methods. It has the added value to enable calculation of tumor-to-RBM dose ratios.

IMMU-130, a unique antibody-drug conjugate (ADC) of SN-38 targeting CEACAM5 antigen: Preclinical basis for clinical activity in metastatic colorectal cancer (mCRC)

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Background: IMMU-130 is a CEACAM5-targeted ADC, labetuzumab-SN-38, with the drug being the active form of the topoisomerase I inhibitor, CPT-11, and substituted at 7-8 moles/mole of IgG. This agent is in phase I/II clinical trials in patients with relapsed mCRC (NCT01270698; NCT01605318). Methods: Therapy experiments were conducted in female athymic nude mice (n=5-10 per group), 4-6 weeks of age, bearing s.c. LS174T human colon carcinoma xenografts of ~0.2 cm3 size, or 2 weeks after lung metastases were generated by i.v. injection of GW-39 human colon carcinoma cells. Untreated controls, including a non-targeting ADC, were included. Biodistribution was examined in the s.c model using a single 12.5-mg/kg dose of the ADC or unconjugated labetuzumab, each spiked with 111In-labeled substrate. Tolerability studies were conducted in white New Zealand rabbits. Results: In the metastatic model, fractionated dosing of a total of 50 mg/kg of ADC showed that 2 × the schedules of twice-weekly × 2 weeks with 1 week off and once-weekly × 2 weeks with 1 week off doubled the median survival vs. untreated mice, and were better than a 25-mg/kg dose given on days 1 and 15 (p < 0.0474; log-rank). Pre-dosing with as much as twice the dose of labetuzumab as the ADC dose in the metastatic model did not affect median survival (p > 0.15). Therapy experiments in the s.c. model revealed that IMMU-130, with 50% of drug released in ~20 h, was superior to the conjugate with an ultrastable linker, that the ADC was better than an MTD of 5FU/leucovorin chemotherapy (p < 0.0001), and that the ADC could be combined with bevacizumab for improved efficacy (p < 0.031). Significantly better efficacy for the specific ADC vs. nonspecific ADC was observed. Pharmacokinetics in mice indicated ~25% longer half-life for the unconjugated MAb vs. ADC, but with minimal impact on tumor uptake. A tolerability study in rabbits showed the NOAEL to be the human equivalent dose of 40-60 mg/kg, given as two doses. Conclusions: Preclinical data show an excellent therapeutic window for IMMU-130, which appears to be translated into the clinical experience thus far. The potential for combination therapy is also indicated.

Phase I/II trial of IMMU-132 (isactuzumab govitecan), an anti-Trop-2-SN-38 antibody drug conjugate (ADC): Results in patients with metastatic gastrointestinal (GI) cancers

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Background: Trop-2 is a tumor-associated glycoprotein highly expressed in many epithelial cancers. Elevated expression has been linked to more aggressive disease and a poorer prognosis. IMMU-132 is an ADC comprising a humanized mAb binding to Trop-2 and conjugated to SN-38 (drug: Ab ratio = 7.6), the active metabolite of irinotecan. After finding potent activity in human tumor xenografts, a phase I/II trial was undertaken (NCT01631552). Methods: Pts with relapsed/refractory metastatic cancers were enrolled, starting at a dose of 8 mg/kg given on days 1 and 8 of a 3-week cycle. Dose levels of 8 and 10 mg/kg were chosen for phase II (N=47). Results: Sixty pts with advanced GI cancers were enrolled in phase I/II. Neutropenia was the principal dose-limiting toxicity, with fatigue, diarrhea, nausea, and vomiting as other reported toxicities. In the phase II pts with median prior therapies of 3 (range 1-7), the following moderate/severe drug-related toxicities occurred: neutropenia (Gr 3, 18.8%; Gr 4, 10.4%); fatigue (Gr 3, 14.6%; Gr 4, 0%), anemia (Gr 3, 10.4%; Gr 4, 0%); diarrhea (Gr 3, 4.2%; Gr 4, 0%). Of 29 CRC pts (10 mg/kg [N=9], 8 mg/kg [N=20]), 1 had a PR and 14 had SD as the best response by RECIST, with a time to progression (TTP) of 11.5+ months (mo) for the PR (65% shrinkage) and a median of 4.8+ mo for the SD pts (5 ongoing). This is a disease control rate (DCR) of 51.7%. Thirteen CRC pts had KRAS mutations, 7 with SD (median TTP = 4.4+ mo [range, 2.8-7.8 mo; 3 ongoing]). Of 15 pts with pancreatic ca, 8 had SD as best RECIST response (median TTP = 3.4 mo); DCR=53.3%. Among 11 pts with esophageal ca, 7 had CT assessments with 1 PR (TTP, 6.9+ mo), and 4 SD (TTP, 4.0+, 5.0, 6.0, and 6.9 mo) as best response; DCR=62.5%. Of 5 gastric ca pts, only 3 had post-baseline CT assessments, all with SD (1 with 19% target lesion reduction and an ongoing TTP of 6.7+ mo). Conclusions: IMMU-132 is a novel anti-cancer therapeutic, conjugating a topoisomerase I inhibitor to an internalizing, cancer-selective mAb. This ADC can be given safely and repeatedly over many months to heavily pretreated patients. Encouraging activity in pts with several metastatic gastrointestinal tumors has been observed. Clinical trial information: NCT01631552.

Pretargeted dual-modality immuno-SPECT and near-infrared fluorescence imaging for image-guided surgery of prostate cancer

Radical removal of malignant lesions may be improved using tumor-targeted dual-modality probes that contain both a radiotracer and a fluorescent label to allow for enhanced intraoperative delineation of tumor resection margins. Because pretargeting strategies yield high signal-to-background ratios, we evaluated the feasibility of a pretargeting strategy for intraoperative imaging in prostate cancer using an anti-TROP-2 x anti-HSG bispecific antibody (TF12) in conjunction with the dual-labeled diHSG peptide (RDC018) equipped with both a DOTA chelate for radiolabeling purposes and a fluorophore (IRdye800CW) to allow near-infrared optical imaging. Nude mice implanted s.c. with TROP-2-expressing PC3 human prostate tumor cells or with PC3 metastases in the scapular and suprarenal region were injected i.v. with 1 mg of TF12 and, after 16 hours of tumor accumulation and blood clearance, were subsequently injected with 10 MBq, 0.2 nmol/mouse of either (111)In-RDC018 or (111)In-IMP288 as a control. Two hours after injection, both microSPECT/CT and fluorescence images were acquired, both before and after resection of the tumor nodules. After image acquisition, the biodistribution of (111)In-RDC018 and (111)In-IMP288 was determined and tumors were analyzed immunohistochemically. The biodistribution of the dual-label RDC018 showed specific accumulation in the TROP-2-expressing PC3 tumors (12.4 ± 3.7% ID/g at 2 hours postinjection), comparable with (111)In-IMP288 (9.1 ± 2.8% ID/g at 2 hours postinjection). MicroSPECT/CT and near-infrared fluorescence (NIRF) imaging confirmed this TROP-2-specific uptake of the dual-label (111)In-RDC018 in both the s.c. and metastatic growing tumor model. In addition, PC3 metastases could be visualized preoperatively with SPECT/CT and could subsequently be resected by image-guided surgery using intraoperative NIRF imaging, showing the preclinical feasibility of pretargeted dual-modality imaging approach in prostate cancer.

Pretargeted Radioimmunotherapy of Prostate Cancer with an Anti-TROP-2×Anti-HSG Bispecific Antibody and a (177)Lu-Labeled Peptide

TROP-2 is a pancarcinoma marker that is expressed at high levels in many epithelial cancers, including prostate cancer (PC). The trivalent bispecific antibody TF12 (anti-TROP2 × anti-HSG [histamine-succinyl-glycine]) has shown to effectively target PC. In this study, the efficacy of pretargeted radioimmunotherapy (PRIT) with multiple cycles of TF12 and (177)Lu-labeled diHSG-peptide (IMP288) in mice with s.c. PC3 tumors was investigated and compared with that of conventional RIT with (177)Lu-labeled anti-TROP-2 mAb hRS7.

METHODS

The potential of one, two, and three cycles of PRIT using the TF12 pretargeted (177)Lu-IMP288 (41 MBq per cycle) was determined in mice with s.c. PC3 tumors, and compared with the efficacy and toxicity of RIT with (177)Lu-hRS7 dosed at the maximum tolerated dose (11 MBq).

RESULTS

PRIT of two and three cycles showed significantly higher median survival (> 150 days) compared with PRIT of one cycle of TF12 and (177)Lu-IMP288 (111 days, p < 0.001) or the controls (76 days, p < 0.0001). All mice treated with the mAb (177)Lu-hRS7 survived at the end of the experiment (150 days), compared with 80% in the mice that were treated with three cycles of PRIT and 70% in the group that received two cycles of PRIT. Clinically significant hematologic toxicity was found only in the groups that received either three cycles of PRIT (p < 0.0009) or RIT (p < 0.0001).

CONCLUSIONS

TROP-2-expressing PC can be targeted efficiently with TF12 and radiolabeled IMP288. (177)Lu-IMP288 accumulated rapidly in the tumors. PRIT of multiple cycles inhibited the growth of s.c. PC3 tumors. Clinically relevant hematological toxicity was observed in the group that received three cycles of PRIT; however, conventional RIT with the parent mAb (177)Lu-hRS7 was at least as effective with similar toxicity.

Anti-CD22 90Y-epratuzumab tetraxetan combined with anti-CD20 veltuzumab: a phase I study in patients with relapsed/refractory, aggressive non-Hodgkin lymphoma

A lingering criticism of radioimmunotherapy in non-Hodgkin lymphoma is the use of cold anti-CD20 antibody along with the radiolabeled anti-CD20 antibody. We instead combined radioimmunotherapy with immunotherapy targeting different B-cell antigens. We evaluated the anti-CD22 (90)Y-epratuzumab tetraxetan with the anti-CD20 veltuzumab in patients with aggressive lymphoma in whom at least one prior standard treatment had failed, but who had not undergone stem cell transplantation. Eighteen patients (median age 73 years, median of 3 prior treatments) received 200 mg/m(2) veltuzumab once-weekly for 4 weeks, with (90)Y-epratuzumab tetraxetan at planned doses in weeks 3 and 4, and (111)In-epratuzumab tetraxetan in week 2 for imaging and dosimetry. Veltuzumab effectively lowered levels of B cells in the blood prior to the radioimmunotherapy doses. No significant immunogenicity or change in pharmacokinetics of either agent occurred in combination. (111)In imaging showed tumor targeting with acceptable radiation dosimetry to normal organs. For (90)Y-epratuzumab tetraxetan, transient myelosuppression was dose-limiting with 6 mCi/m(2) (222 MBq/m(2)) × 2 being the maximal tolerated dose. Of 17 assessable patients, nine (53%) had objective responses according to the 2007 revised treatment response criteria, including three (18%) complete responses (2 relapsing after 11 and 13 months, 1 continuing to be clinically disease-free at 19 months), and six (35%) partial responses (1 relapsing after 14 months, 5 at 3 - 7 months). Responses occurred in patients with different lymphoma histologies, treated at different (90)Y dose levels, and with a predicted risk of poor outcome, most importantly including five of the six patients treated with the maximal tolerated dose (2 of whom achieved durable complete responses). In conclusion, the combination of (90)Y-epratuzumab tetraxetan and veltuzumab was well-tolerated with encouraging therapeutic activity in this difficult-to-treat population.