Rapid blood clearance of injected mouse IgG2a in SCID mice

Engineering of a Fully Human Anti-MUC-16 Antibody and Evaluation as a PET Imaging Agent

Antibodies that recognize cancer biomarkers, such as MUC16, can be used as vehicles to deliver contrast agents (imaging) or cytotoxic payloads (therapy) to the site of tumors. MUC16 is overexpressed in 80% of epithelial ovarian cancer (EOC) and 65% of pancreatic ductal adenocarcinomas (PDAC), where effective ‘theranostic’ probes are much needed. This work aims to develop fully human antibodies against MUC16 and evaluate them as potential immuno-PET imaging probes for detecting ovarian and pancreatic cancers. We developed a fully human monoclonal antibody, M16Ab, against MUC16 using phage display. M16Ab was conjugated with p-SCN-Bn-DFO and radiolabeled with 89Zr. 89Zr-DFO-M16Ab was then evaluated for binding specificity and affinity using flow cytometry. In vivo evaluation of 89Zr-DFO-M16Ab was performed by microPET/CT imaging at different time points at 24−120 h post injection (p.i.) and ex vivo biodistribution studies in mice bearing MUC16-expressing OVCAR3, SKOV3 (ovarian) and SW1990 (pancreatic) xenografts. 89Zr-DFO-M16Ab bound specifically to MUC16-expressing cancer cells with an EC50 of 10nM. 89Zr-DFO-M16Ab was stable in serum and showed specific uptake and retention in tumor xenografts even after 120 h p.i. (microPET/CT) with tumor-to-blood ratios > 43 for the SW1990 xenograft. Specific tumor uptake was observed for SW1990/OVCAR3 xenografts but not in MUC16-negative SKOV3 xenografts. Pharmacokinetic study shows a relatively short distribution (t1/2α) and elimination half-life (t1/2ß) of 4.4 h and 99 h, respectively. In summary, 89Zr-DFO-M16Ab is an effective non-invasive imaging probe for ovarian and pancreatic cancers and shows promise for further development of theranostic radiopharmaceuticals.

A comparison of DFO and DFO* conjugated to trastuzumab-DM1 for complexing 89Zr - In vitro stability and in vivo microPET/CT imaging studies in NOD/SCID mice with HER2-positive SK-OV-3 human ovarian cancer xenografts

INTRODUCTION

Desferrioxamine (DFO) is conjugated to antibodies to chelate ⁸⁹Zr for PET, but DFO forms a hexadentate complex with Zr⁴⁺ that exhibits instability contributing to bone uptake of ⁸⁹Zr, while the cationic charge of the Zr⁴⁺-DFO complex may promote normal tissue uptake of the radioimmunoconjugates (RICs). DFO* is a novel chelator that forms a more stable octadentate and neutral complex with ⁸⁹Zr. Our aim was to compare the in vitro stability of [⁸⁹Zr]Zr-DFO*-human IgG (hIgG) and [⁸⁹Zr]Zr-DFO-hIgG RICs, and the in vivo PET imaging properties of the antibody-drug conjugate (ADC), trastuzumab-DM1 (T-DM1), labeled with ⁸⁹Zr by conjugation to DFO or DFO*.

METHODS

SCN-pPhe-DFO and SCN-pPhe-DFO* were reacted with hIgG at a 14.6-fold excess or with T-DM1 at a 4.1-fold or 10-fold excess, respectively, purified and labeled with ⁸⁹Zr. The number of DFO* introduced was determined by measuring the absorbance at 245/252 nm and the protein concentration was measured at 280 nm. The stability of [⁸⁹Zr]Zr-DFO*-hIgG was studied in vitro in human plasma, and by challenge with a 385-fold excess (0.1 mM) of DFO or EDTA. An inverse stability study was performed with [⁸⁹Zr]Zr-DFO-hIgG challenged with 0.1 mM DFO*. The HER2 binding affinity of [⁸⁹Zr]Zr-DFO*-T-DM1 was measured in a direct (saturation) binding assay using SK-BR-3 human breast cancer cells or SK-OV-3 human ovarian cancer cells. The biodistribution of [⁸⁹Zr]Zr-DFO*-T-DM1 and [⁸⁹Zr]Zr-DFO-T-DM1 were compared in non-tumor bearing Balb/c mice and in NOD/SCID mice with s.c. SK-OV-3 xenografts at 96 h post-intravenous injection (p.i.). MicroPET/CT images were obtained at 96 h p.i. of the RICs.

RESULTS

hIgG and T-DM1 were conjugated to 4.5-5.3 and 3.1 chelators (DFO or DFO*), respectively, and labeled with ⁸⁹Zr to a final radiochemical purity of 91-99%. [⁸⁹Zr]Zr-DFO*-hIgG was stable in vitro in human plasma or to challenge with 0.1 mM EDTA, but incubation with 0.1 mM DFO caused 26.0 ± 2.1% loss of ⁸⁹Zr after 5 days. In contrast, incubation of [⁸⁹Zr]Zr-DFO-hIgG with 0.1 mM DFO* resulted in 77.0 ± 3.9% loss of ⁸⁹Zr after 5 days. [⁸⁹Zr]Zr-DFO*-T-DM1 retained high affinity binding to HER2 on SK-BR-3 and SK-OV-3 cells with a K_d = 2.2 ± 0.3 nM and 1.9 ± 0.3 nM, respectively, and B_max = 3.4 ± 0.1 × 10⁵ and 1.1 ± 0.04 × 10⁵ receptors/cell, respectively. Biodistribution studies of [⁸⁹Zr]Zr-DFO-T-DM1 and [⁸⁹Zr]Zr-DFO*-T-DM1 in Balb/c and NOD/SCID mice revealed significantly lower uptake in bone, liver, kidneys, and spleen for [⁸⁹Zr]Zr-DFO*-T-DM1 than [⁸⁹Zr]Zr-DFO-T-DM1. Uptake of [⁸⁹Zr]Zr-DFO*-T-DM1 and [⁸⁹Zr]Zr-DFO-T-DM1 in SK-OV-3 tumors was moderate [5.0 ± 1.8% injected dose/g (%ID/g) and 6.3 ± 0.6%ID/g, respectively; P = 0.18]. Tumors were imaged with both RICs.

CONCLUSION

We conclude that DFO* conjugated to T-DM1 provides more stable complexation of ⁸⁹Zr and therefore, [⁸⁹Zr]Zr-DFO*-T-DM1 would be more useful than [⁸⁹Zr]Zr-DFO-T-DM1 to probe the delivery of T-DM1 to tumors by PET, which we previously found is correlated with response to treatment with T-DM1 in mouse tumor xenograft models.

ADVANCES IN KNOWLEDGE AND IMPLICATION FOR PATIENT CARE

This study is the first to directly compare the PET imaging properties of [⁸⁹Zr]Zr-DFO*-T-DM1 and [⁸⁹Zr]Zr-DFO-T-DM1 in a HER2-overexpressing tumor xenograft mouse model. Our results indicate that [⁸⁹Zr]Zr-DFO*-T-DM1 provides superior imaging properties due to the greater stability of the [⁸⁹Zr]Zr-DFO* than [⁸⁹Zr]Zr-DFO complex.

Panitumumab Modified with Metal-Chelating Polymers (MCP) Complexed to 111In and 177Lu-An EGFR-Targeted Theranostic for Pancreatic Cancer

A metal-chelating polymer (MCP) with a polyglutamide (PGlu) backbone presenting on average 13 DOTA (tetraazacyclododecane-1,4,7,10-tetraacetic acid) chelators for complexing ¹¹¹In or ¹⁷⁷Lu and 10 polyethylene glycol (PEG) chains to minimize liver and spleen uptake was conjugated to antiepidermal growth factor receptor (EGFR) monoclonal antibody (mAb), panitumumab. Because panitumumab-MCP may be dual-labeled with ¹¹¹In and ¹⁷⁷Lu for SPECT, or radioimmunotherapy (RIT) exploiting the Auger electrons or β-particle emissions, respectively, we propose that panitumumab-MCP could be a useful theranostic agent for EGFR-positive PnCa. Bioconjugation was achieved by reaction of a hydrazine nicotinamide (HyNIC) group on the MCP with an aryl aromatic aldehyde introduced into panitumumab by reaction with succinimidyl-4-formylbenzamide (S-4FB). The conjugation reaction was monitored by measurement of the chromophoric bis-aryl hydrazone bond formed (ε_350 nm = 24 500 M^-1 cm^-1) to achieve two MCPs/panitumumab. Labeling of panitumumab-MCP with ¹¹¹In or ¹⁷⁷Lu demonstrated that masses as small as 0.1 μg were labeled to >90% labeling efficiency (L.E.) and a specific activity (SA) of >70 MBq/μg. Panitumumab-DOTA incorporating two DOTA per mAb was labeled with ¹¹¹In or ¹⁷⁷Lu to a maximum SA of 65 MBq/μg and 46 MBq/μg, respectively. Panitumumab-MCP-¹⁷⁷Lu exhibited saturable binding to EGFR-overexpressing MDA-MB-468 human breast cancer cells. The K_d for binding of panitumumab-MCP-¹⁷⁷Lu to EGFR (2.2 ± 0.6 nmol/L) was not significantly different than panitumumab-DOTA-¹⁷⁷Lu (1.0 ± 0.4 nmol/L). ¹¹¹In and ¹⁷⁷Lu were stably complexed to panitumumab-MCP. Panitumumab-MCP-¹¹¹In exhibited similar whole body retention (55-60%) as panitumumab-DOTA-¹¹¹In in NOD-scid mice up to 72 h postinjection (p.i.) and equivalent excretion of radioactivity into the urine and feces. The uptake of panitumumab-MCP-¹¹¹In in most normal tissues in NOD-scid mice with EGFR-positive PANC-1 human pancreatic cancer (PnCa) xenografts at 72 h p.i. was not significantly different than panitumumab-DOTA-¹¹¹In, except for the liver which was 3-fold greater for panitumumab-MCP-¹¹¹In. Tumor uptake of panitumumab-MCP-¹¹¹In (6.9 ± 1.3%ID/g) was not significantly different than panitumumab-DOTA-¹¹In (6.6 ± 3.3%ID/g). Tumor uptake of panitumumab-MCP-¹¹¹In and panitumumab-DOTA-¹¹¹In were reduced by preadministration of excess panitumumab, suggesting EGFR-mediated uptake. Tumor uptake of nonspecific IgG-MCP (5.4 ± 0.3%ID/g) was unexpectedly similar to panitumumab-MCP-¹¹¹In. An increased hydrodynamic radius of IgG when conjugated to an MCP may encourage tumor uptake via the enhanced permeability and retention (EPR) effect. Tumor uptake of panitumumab-DOTA-¹¹¹In was 3.5-fold significantly higher than IgG-DOTA-¹¹¹In. PANC-1 tumors were imaged by microSPECT/CT at 72 h p.i. of panitumumab-MCP-¹¹¹In or panitumumab-DOTA-¹¹¹In. Tumors were not visualized with preadministration of excess panitumumab to block EGFR, or with nonspecific IgG radioimmunoconjugates. We conclude that linking panitumumab to an MCP enabled higher SA labeling with ¹¹¹In and ¹⁷⁷Lu than DOTA-conjugated panitumumab, with preserved EGFR binding in vitro and comparable tumor localization in vivo in mice with s.c. PANC-1 human PnCa xenografts. Normal tissue distribution was similar except for the liver which was higher for the polymer radioimmunoconjugates.

Development and preclinical studies of 64Cu-NOTA-pertuzumab F(ab')2 for imaging changes in tumor HER2 expression associated with response to trastuzumab by PET/CT

We previously reported that microSPECT/CT imaging with ¹¹¹In-labeled pertuzumab detected decreased HER2 expression in human breast cancer (BC) xenografts in athymic mice associated with response to treatment with trastuzumab (Herceptin). Our aim was to extend these results to PET/CT by constructing F(ab')₂ of pertuzumab modified with NOTA chelators for complexing ⁶⁴Cu. The effect of the administered mass (5-200 µg) of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ was studied in NOD/SCID mice engrafted with HER2-positive SK-OV-3 human ovarian cancer xenografts. Biodistribution studies were performed in non-tumor bearing Balb/c mice to predict radiation doses to normal organs in humans. Serial PET/CT imaging was conducted on mice engrafted with HER2-positive and trastuzumab-sensitive BT-474 or trastuzumab-insensitive SK-OV-3 xenografted mice treated with weekly doses of trastuzumab. There were no significant effects of the administered mass of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ on tumor or normal tissue uptake. The predicted total body dose in humans was 0.015 mSv/MBq, a 3.3-fold reduction compared to ¹¹¹In-labeled pertuzumab. MicroPET/CT images revealed specific tumor uptake of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ at 24 or 48 h post-injection in mice with SK-OV-3 tumors. Image analysis of mice treated with trastuzumab showed 2-fold reduced uptake of ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ in BT-474 tumors after 1 week of trastuzumab normalized to baseline, and 1.9-fold increased uptake in SK-OV-3 tumors after 3 weeks of trastuzumab, consistent with tumor response and resistance, respectively. We conclude that PET/CT imaging with ⁶⁴Cu-NOTA-pertuzumab F(ab')₂ detected changes in HER2 expression in response to trastuzumab while delivering a lower total body radiation dose compared to ¹¹¹In-labeled pertuzumab.

Subcutaneous bioavailability of therapeutic antibodies as a function of FcRn binding affinity in mice

The neonatal Fc receptor (FcRn) plays an important and well-known role in immunoglobulin G (IgG) catabolism; however, its role in the disposition of IgG after subcutaneous (SC) administration, including bioavailability, is relatively unknown. To examine the potential effect of FcRn on IgG SC bioavailability, we engineered three anti-amyloid β monoclonal antibody (mAb) reverse chimeric mouse IgG2a (mIgG2a) Fc variants (I253A.H435A, N434H and N434Y) with different binding affinities to mouse FcRn (mFcRn) and compared their SC bioavailability to that of the wild-type (WT) mAb in mice. Our results indicated that the SC bioavailability of mIgG2a was affected by mFcRn-binding affinity. Variant I253A.H435A, which did not bind to mFcRn at either pH 6.0 or pH 7.4, had the lowest bioavailability (41.8%). Variant N434Y, which had the greatest increase in binding affinity at both pH 6.0 and pH 7.4, had comparable bioavailability to the WT antibody (86.1% vs. 76.3%), whereas Variant N434H, which had modestly increased binding affinity at pH 6.0 to mFcRn and affinity comparable to the WT antibody at pH 7.4, had the highest bioavailability (94.7%). A semi-mechanism-based pharmacokinetic model, which described well the observed data with the WT antibody and variant I253A.H435A, is consistent with the hypothesis that the decreased bioavailability of variant I253A.H435A was due to loss of the FcRn-mediated protection from catabolism at the absorption site. Together, these data demonstrate that FcRn plays an important role in SC bioavailability of therapeutic IgG antibodies.

Effects of anti-VEGF on pharmacokinetics, biodistribution, and tumor penetration of trastuzumab in a preclinical breast cancer model

Both human epidermal growth factor receptor 2 (HER-2/neu) and VEGF overexpression correlate with aggressive phenotypes and decreased survival among breast cancer patients. Concordantly, the combination of trastuzumab (anti-HER2) with bevacizumab (anti-VEGF) has shown promising results in preclinical xenograft studies and in clinical trials. However, despite the known antiangiogenic mechanism of anti-VEGF antibodies, relatively little is known about their effects on the pharmacokinetics and tissue distribution of other antibodies. This study aimed to measure the disposition properties, with a particular emphasis on tumor uptake, of trastuzumab in the presence or absence of anti-VEGF. Radiolabeled trastuzumab was administered alone or in combination with an anti-VEGF antibody to mice bearing HER2-expressing KPL-4 breast cancer xenografts. Biodistribution, autoradiography, and single-photon emission computed tomography-X-ray computed tomography imaging all showed that anti-VEGF administration reduced accumulation of trastuzumab in tumors despite comparable blood exposures and similar distributions in most other tissues. A similar trend was also observed for an isotype-matched IgG with no affinity for HER2, showing reduced vascular permeability to macromolecules. Reduced tumor blood flow (P < 0.05) was observed following anti-VEGF treatment, with no significant differences in the other physiologic parameters measured despite immunohistochemical evidence of reduced vascular density. In conclusion, anti-VEGF preadministration decreased tumor uptake of trastuzumab, and this phenomenon was mechanistically attributed to reduced vascular permeability and blood perfusion. These findings may ultimately help inform dosing strategies to achieve improved clinical outcomes.

Auger electron radioimmunotherapeutic agent specific for the CD123+/CD131- phenotype of the leukemia stem cell population

Our aim was to construct and characterize (111)In-nuclear translocation sequence (NLS)-7G3, an Auger electron-emitting radioimmunotherapeutic agent that preferentially recognizes the expression of CD123 (interleukin-3 receptor [IL-3R] α-subchain) in the absence of CD131 (IL-3R β-subchain) displayed by leukemia stem cells.

METHODS

Monoclonal antibody 7G3 was modified with 13-mer peptides [CGYGPKKKRKVGG] harboring the NLS of SV-40 large T-antigen and with diethylenetriaminepentaacetic acid for labeling with (111)In. Immunoreactivity was evaluated in a competition radioligand binding assay and by flow cytometry. Nuclear localization of (111)In-NLS-7G3 was studied by cell fractionation in CD123(+)/CD131(-) acute myelogenous leukemia (AML)-3, -4, and -5 cells or in primary AML or normal leukocytes. Micro-SPECT was performed in nonobese diabetic (NOD)/severe combined immune deficient (SCID) mice engrafted subcutaneously with Raji-CD123 tumors or with disseminated AML-3 or -5 cells. The cytotoxicity of (111)In-NLS-7G3 on AML-5 cells was studied after 7 d in culture by trypan blue dye exclusion. DNA damage was assessed using the γ-H2AX assay.

RESULTS

NLS-7G3 exhibited preserved CD123 immunoreactivity (affinity, 4.6 nmol/L). Nuclear importation of (111)In-NLS-7G3 in AML-3, -4, or -5 cells was specific and significantly higher than unmodified (111)In-7G3 and was greater in primary AML cells than in normal leukocytes. Rapid elimination of (111)In-NLS-7G3 in NOD/SCID mice prevented imaging of subcutaneous Raji-CD123 tumors. This phenomenon was Fc-dependent and IgG(2a) isotype-specific and was overcome by the preadministration of excess IgG(2a) or using (111)In-NLS-7G3 F(ab')(2) fragments. AML-3 and -5 cells were engrafted into the bone marrow or spleen or at extramedullary sites in NOD/SCID mice. Micro-SPECT/CT with (111)In-NLS-7G3 F(ab')(2) showed splenic involvement, whereas foci of disease were seen in the spine or femur or at extramedullary sites in the brain and lymph nodes using (111)In-NLS-7G3 IgG(2a). The viability of AML-5 cells was reduced by exposure in vitro to (111)In-NLS-7G3; this reduction was associated with an increase in unrepaired DNA double-strand breaks.

CONCLUSION

(111)In-NLS-7G3 is a promising novel Auger electron-emitting radioimmunotherapeutic agent for AML aimed at the leukemia stem cell population. Micro-SPECT/CT was useful for visualizing the engraftment of leukemia in NOD/SCID mice.

Therapy of human carcinoma xenografts with antibodies to EGFr and HER-2 conjugated to radionuclides emitting low-energy electrons

PURPOSE

Low-energy electrons (10-50 keV) can be effective and specific cytotoxic agents when delivered to the cell surface by antibodies, because their path length in tissue is comparable to a cell diameter. In this study, we have begun to evaluate the therapeutic potential of antibodies (Abs) conjugated to (111)In against carcinoma xenografts in nude mice.

METHODS

Abs to EGFr or HER-2 were labeled with (111)In to a high specific activity of approximately 1.48 GBq/mg (40 mCi/mg). They were injected into nude mice 5-6 days after inoculation of human carcinoma cells, either A431 or SK-OV-3, and tumor growth was monitored. In preliminary in vitro experiments, we calculated the cumulative decays per cell, estimated the centigray dose delivered to the nucleus, and related this to the fraction surviving.

RESULTS

Abs to both antigens provided significant protection in nude mouse xenograft models (p values ranging from <0.05 to <0.001). Some mice appeared to be cured, but most had delayed tumor growth. The specificity of the effect was demonstrated by testing non-reactive Abs labeled in the same way. The radioactivity was required, because unconjugated Abs had no therapeutic effect. The maximum tolerated dose was required in order for therapy to be effective, but most of the treated mice had no significant weight loss or other overt signs of toxicity.

CONCLUSION

Abs labeled with nuclides emitting low-energy electrons, such as (111)In, can be effective therapeutic agents against microscopic s.c. tumors. This strategy should be considered for clinical applications.

Therapy of Small Subcutaneous B-Lymphoma Xenografts with Antibodies Conjugated to Radionuclides Emitting Low-Energy Electrons

Purpose: Radionuclides emitting low-energy electrons (Auger and conversion electrons of &lt;50 keV) are potentially useful for cancer therapy when conjugated to an antibody, because they can irradiate the cell to which they bind while producing relatively little irradiation of surrounding cells and tissues. We showed previously the ability of such antibody conjugates to treat micrometastatic, disseminated human B-lymphoma in a severe combined immunodeficient mouse model using an anti-CD74 antibody. In this study, we have evaluated the ability of such conjugates to treat s.c. tumors.

Experimental Design: Severe combined immunodeficient mice were injected s.c. with Raji, Daudi, or RL B-lymphoma human tumor cells. Antibodies to CD74, CD20, or HLA-DR were radiolabeled with 111In or 125I and injected i.v. at various times starting at day 5, and tumor growth was monitored. Controls included the testing of unlabeled antibodies, labeled nonreactive antibodies, and a combination of the two.

Results: Therapy of s.c. B-lymphoma was more difficult than therapy of tumor cells that had been injected i.v. Although large, macroscopic tumors were not effectively treated, therapy was effective on s.c. Daudi tumors on day 36 after injection of this slowly growing tumor, with an 111In anti-CD74 antibody given in two doses. An anti-CD20 antibody labeled with either 111In or 125I was able to effectively treat s.c. RL tumors when given as late as day 16 after tumor inoculation. The largest tumors that were effectively treated were macroscopic thin discs (&lt;2 mm in diameter) growing on the mesentery.

Conclusion: These results extend previous evidence that antibody conjugates with emitters of low-energy electrons can be effective therapeutic agents for micrometastatic cancer.