Synchronizing chemistry, quantum mechanics and radioactivity in a revolutionary renewed atom model. Part 1: the elements where Z is 1-10 (H, He, Li, Be, B, C, N, O, F, Ne).

The alliance between the reigning quantum mechanical atom model and chemistry still is a difficult one when it comes to an adequate explanation for e.g. the covalent bond, inversion, chirality, or hydrogen bonds. Overruling Rutherford's extrapolation from gold to hydrogen, an atom model is described that provides improved answers to these phenomena while the hybridization principle and the covalent bond are re-defined by giving neutrons a much more prominent role than they have in the reigning quantum mechanical model. It is postulated that a neutron is not just there to assist the strong force in surpassing the repulsive coulombic forces between the protons in the nucleus, but the neutron is the modus operandi of molecular geometry, and as such plays a part in chemical reactivity, bond length and bond strength.

Comparison of the octadentate bifunctional chelator DFO*-pPhe-NCS and the clinically used hexadentate bifunctional chelator DFO-pPhe-NCS for 89Zr-immuno-PET.

PURPOSE: All clinical 89Zr-immuno-PET studies are currently performed with the chelator desferrioxamine (DFO). This chelator provides hexadentate coordination to zirconium, leaving two coordination sites available for coordination with, e.g., water molecules, which are relatively labile ligands. The unsaturated coordination of DFO to zirconium has been suggested to result in impaired stability of the complex in vivo and consequently in unwanted bone uptake of 89Zr. Aiming at clinical improvements, we report here on a bifunctional isothiocyanate variant of the octadentate chelator DFO* and the in vitro and in vivo comparison of its 89Zr-DFO*-mAb complex with 89Zr-DFO-mAb. METHODS: The bifunctional chelator DFO*-pPhe-NCS was prepared from previously reported DFO* and p-phenylenediisothiocyanate. Subsequently, trastuzumab was conjugated with either DFO*-pPhe-NCS or commercial DFO-pPhe-NCS and radiolabeled with Zr-89 according to published procedures. In vitro stability experiments were carried out in saline, a histidine/sucrose buffer, and blood serum. The in vivo performance of the chelators was compared in N87 tumor-bearing mice by biodistribution studies and PET imaging. RESULTS: In 0.9 % NaCl 89Zr-DFO*-trastuzumab was more stable than 89Zr-DFO-trastuzumab; after 72 h incubation at 2-8 °C 95 % and 58 % intact tracer were left, respectively, while in a histidine-sucrose buffer no difference was observed, both products were ≥ 92 % intact. In vivo uptake at 144 h post injection (p.i.) in tumors, blood, and most normal organs was similar for both conjugates, except for skin, liver, spleen, ileum, and bone. Tumor uptake was 32.59 ± 11.95 and 29.06 ± 8.66 % ID/g for 89Zr-DFO*-trastuzumab and 89Zr-DFO-trastuzumab, respectively. The bone uptake was significantly lower for 89Zr-DFO*-trastuzumab compared to 89Zr-DFO-trastuzumab. At 144 h p.i. for 89Zr-DFO*-trastuzumab and 89Zr-DFO-trastuzumab, the uptake in sternum was 0.92 ± 0.16 and 3.33 ± 0.32 % ID/g, in femur 0.78 ± 0.11 and 3.85, ± 0.80 and in knee 1.38 ± 0.23 and 8.20 ± 2.94 % ID/g, respectively. The uptake in bone decreased from 24 h to 144 h p.i. about two fold for the DFO* conjugate, while it increased about two fold for the DFO conjugate. CONCLUSIONS: Zr-DFO*-trastuzumab showed superior in vitro stability and in vivo performance when compared to 89Zr-DFO-trastuzumab. This makes the new octadentate DFO* chelator a candidate successor of DFO for future clinical 89Zr-immuno-PET.

Development of novel ADCs: conjugation of tubulysin analogues to trastuzumab monitored by dual radiolabeling.

Tubulysins are highly toxic tubulin-targeting agents with a narrow therapeutic window that are interesting for application in antibody-drug conjugates (ADC). For full control over drug-antibody ratio (DAR) and the effect thereof on pharmacokinetics and tumor targeting, a dual-labeling approach was developed, wherein the drug, tubulysin variants, and the antibody, the anti-HER2 monoclonal antibody (mAb) trastuzumab, are radiolabeled. (131)I-radioiodination of two synthetic tubulysin A analogues, the less potent TUB-OH (IC50 > 100 nmol/L) and the potent TUB-OMOM (IC50, ~1 nmol/L), and their direct covalent conjugation to (89)Zr-trastuzumab were established. Radioiodination of tubulysins was 92% to 98% efficient and conversion to N-hydroxysuccinimide (NHS) esters more than 99%; esters were isolated in an overall yield of 68% ± 5% with radiochemical purity of more than 99.5%. Conjugation of (131)I-tubulysin-NHS esters to (89)Zr-trastuzumab was 45% to 55% efficient, resulting in ADCs with 96% to 98% radiochemical purity after size-exclusion chromatography. ADCs were evaluated for their tumor-targeting potential and antitumor effects in nude mice with tumors that were sensitive or resistant to trastuzumab, using ado-trastuzumab emtansine as a reference. ADCs appeared stable in vivo. An average DAR of 2 and 4 conferred pharmacokinetics and tumor-targeting behavior similar to parental trastuzumab. Efficacy studies using single-dose TUB-OMOM-trastuzumab (DAR 4) showed dose-dependent antitumor effects, including complete tumor eradications in trastuzumab-sensitive tumors in vivo. TUB-OMOM-trastuzumab (60 mg/kg) displayed efficacy similar to ado-trastuzumab emtansine (15 mg/kg) yet more effective than trastuzumab. Our findings illustrate the potential of synthetic tubulysins in ADCs for cancer treatment.

Inert coupling of IRDye800CW and zirconium-89 to monoclonal antibodies for single- or dual-mode fluorescence and PET imaging.

IRDye800CW and zirconium-89 ((89)Zr) have very attractive properties for optical imaging and positron emission tomography (PET) imaging, respectively. Here we describe a procedure for dual labeling of mAbs with IRDye800CW and (89)Zr in a current good manufacturing practice (cGMP)-compliant way. IRDye800CW and (89)Zr are coupled inertly, without impairment of immunoreactivity and pharmacokinetics of the mAb. Organ and whole-body distribution of the final product can be assessed by optical and PET imaging, respectively. For this purpose, a minimal amount of the chelate N-succinyldesferrioxamine (N-sucDf) is first conjugated to the mAb. Next, N-sucDf-mAb is conjugated with IRDye800CW, after which the N-sucDf-mAb-IRDye800CW is labeled with (89)Zr. After each of these three steps, the product is purified by gel filtration. The sequence of this process avoids unnecessary radiation exposure to personnel and takes about 5 h. The process can be scaled up by the production of large batches of premodified mAbs that can be dispensed and stored until they are labeled with (89)Zr.

89Zr-PET radiochemistry in the development and application of therapeutic monoclonal antibodies and other biologicals.

Positron emission tomography with (89)Zr can be used to follow the behaviour of therapeutic monoclonal antibodies (mAbs) and other biologicals in vivo. The favourable radiophysical characteristics of (89)Zr allow multiple days PET scanning after injection. For the coupling of (89)Zr to proteins six desferrioxamine (DFO)-based bifunctional chelators have been described, five of which forming stable complexes in vivo. Of the methods that give stable complexes three are based on random lysine modification of mAbs and two on site-specific engineering. Up to now only two methods, random lysine modification with N-suc-DFO or DFO-Bz-NCS, have been used in clinical studies. In this review firstly aspects of the physicochemical properties and production of (89)Zr are emphasized as well as important items that have to be taken into account for current good manufacturing practice (cGMP) compliant production of (89)Zr-labeled proteins. Next, the different DFO-based conjugation strategies will be discussed with respect to synthesis, and their (pre)clinical evaluation particularly in the field of oncology.

Nanocolloidal albumin-IRDye 800CW: a near-infrared fluorescent tracer with optimal retention in the sentinel lymph node.

PURPOSE: At present, the only approved fluorescent tracer for clinical near-infrared fluorescence-guided sentinel node (SN) detection is indocyanine green (ICG), but the use of this tracer is limited due to its poor retention in the SN resulting in the detection of higher tier nodes. We describe the development and characterization of a next-generation fluorescent tracer, nanocolloidal albumin-IRDye 800CW that has optimal properties for clinical SN detection. METHODS: The fluorescent dye IRDye 800CW was covalently coupled to colloidal human serum albumin (HSA) particles present in the labelling kit Nanocoll in a manner compliant with current Good Manufacturing Practice. Characterization of nanocolloidal albumin-IRDye 800CW included determination of conjugation efficiency, purity, stability and particle size. Quantum yield was determined in serum and compared to that of ICG. For in vivo evaluation a lymphogenic metastatic tumour model in rabbits was used. Fluorescence imaging was performed directly after peritumoral injection of nanocolloidal albumin-IRDye 800CW or the reference ICG/HSA (i.e. ICG mixed with HSA), and was repeated after 24 h, after which fluorescent lymph nodes were excised. RESULTS: Conjugation of IRDye 800CW to nanocolloidal albumin was always about 50% efficient and resulted in a stable and pure product without affecting the particle size of the nanocolloidal albumin. The quantum yield of nanocolloidal albumin-IRDye 800CW was similar to that of ICG. In vivo evaluation revealed noninvasive detection of the SN within 5 min of injection of either nanocolloidal albumin-IRDye 800CW or ICG/HSA. No decrease in the fluorescence signal from SN was observed 24 h after injection of the nanocolloidal albumin-IRDye 800CW, while a strong decrease or complete disappearance of the fluorescence signal was seen 24 h after injection of ICG/HSA. Fluorescence-guided SN biopsy was very easy. CONCLUSION: Nanocolloidal albumin-IRDye 800CW is a promising fluorescent tracer with optimal kinetic features for SN detection.

89Zr-nanocolloidal albumin-based PET/CT lymphoscintigraphy for sentinel node detection in head and neck cancer: preclinical results.

UNLABELLED: Identifying sentinel nodes near the primary tumor remains a problem in, for example, head and neck cancer because of the limited resolution of current lymphoscintigraphic imaging when using (99m)Tc-nanocolloidal albumin. This study describes the development and evaluation of a nanocolloidal albumin-based tracer specifically dedicated for high-resolution PET detection. METHODS: (89)Zr was coupled to nanocolloidal albumin via the bifunctional chelate p-isothiocyanatobenzyldesferrioxamine B. Quality control tests, including particle size measurements, and in vivo biodistribution and imaging experiments in a rabbit lymphogenic metastasis model were performed. RESULTS: Coupling of (89)Zr to nanocolloidal albumin appeared to be efficient, resulting in a stable product with a radiochemical purity greater than 95%, without affecting the particle size. PET showed distinguished uptake of (89)Zr-nanocolloidal albumin in the sentinel nodes, with visualization of lymphatic vessels, and with a biodistribution comparable to (99m)Tc-nanocolloidal albumin. CONCLUSION: (89)Zr-nanocolloidal albumin is a promising tracer for sentinel node detection by PET.

Synthesis of phosphine and antibody-azide probes for in vivo Staudinger ligation in a pretargeted imaging and therapy approach.

The application of intact monoclonal antibodies (mAbs) as targeting agents in nuclear imaging and radioimmunotherapy is hampered by the slow pharmacokinetics of these molecules. Pretargeting with mAbs could be beneficial to reduce the radiation burden to the patient, while using the excellent targeting capacity of the mAbs. In this study, we evaluated the applicability of the Staudinger ligation as pretargeting strategy using an antibody-azide conjugate as tumor-targeting molecule in combination with a small phosphine-containing imaging/therapeutic probe. Up to 8 triazide molecules were attached to the antibody without seriously affecting its immunoreactivity, pharmacokinetics, and tumor uptake in tumor bearing nude mice. In addition, two (89)Zr- and (67/68)Ga-labeled desferrioxamine (DFO)-phosphines, a (177)Lu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-phosphine and a (123)I-cubyl phosphine probe were synthesized and characterized for their pharmacokinetic behavior in nude mice. With respect to the phosphine probes, blood levels at 30 min after injection were <5% injected dose per gram tissue, indicating rapid blood clearance. In vitro Staudinger ligation of 3.33 µM antibody-azide conjugate with 1 equiv of radiolabeled phosphine, relative to the azide, in aqueous solution resulted in 20-25% efficiency after 2 h. The presence of 37% human serum resulted in a reduced ligation efficiency (reduction max. 30% at 2 h), while the phosphines were still >80% intact. No in vivo Staudinger ligation was observed in a mouse model after injection of 500 µg antibody-azide, followed by 68 µg DFO-phosphine at t = 2 h, and evaluation in blood at t = 7 h. To explain negative results in mice, Staudinger ligation was performed in vitro in mouse serum. Under these conditions, a side product with the phosphine was formed and ligation efficiency was severely reduced. It is concluded that in vivo application of the Staudinger ligation in a pretargeting approach in mice is not feasible, since this ligation reaction is not bioorthogonal and efficient enough. Slow reaction kinetics will also severely restrict the applicability of Staudinger ligation in humans.

Facile labelling of an anti-epidermal growth factor receptor Nanobody with 68Ga via a novel bifunctional desferal chelate for immuno-PET.

PURPOSE: The ∼15 kDa variable domains of camelid heavy-chain-only antibodies (called Nanobodies®) have the flexibility to be formatted as monovalent, monospecific, multivalent or multispecific single chain proteins with either fast or slow pharmacokinetics. We report the evaluation of the fast kinetic anti-epidermal growth factor receptor (EGFR) Nanobody 7D12, labelled with (68)Ga via the novel bifunctional chelate (BFC) p-isothiocyanatobenzyl-desferrioxamine (Df-Bz-NCS). Df-Bz-NCS has recently been introduced as the chelate of choice for (89)Zr immuno-positron emission tomography (PET). METHODS: Nanobody 7D12 was premodified with Df-Bz-NCS at pH 9. Radiolabelling with purified (68)Ga was performed at pH 5.0-6.5 for 5 min at room temperature. For in vitro stability measurements in storage buffer (0.25 M NaOAc with 5 mg ml(-1) gentisic acid, pH 5.5) at 4°C or in human serum at 37°C, a mixture of (67)Ga and (68)Ga was used. Biodistribution and immuno-PET studies of (68)Ga-Df-Bz-NCS-7D12 were performed in nude mice bearing A431 xenografts using (89)Zr-Df-Bz-NCS-7D12 as the reference conjugate. RESULTS: The Df-Bz-NCS chelate was conjugated to Nanobody 7D12 with a chelate to Nanobody molar substitution ratio of 0.2:1. The overall (68)Ga radiochemical yield was 55-70% (not corrected for decay); specific activity was 100-500 MBq/mg. Radiochemical purity of the conjugate was >96%, while the integrity and immunoreactivity were preserved. (68/67)Ga-Df-Bz-NCS-7D12 was stable in storage buffer as well as in human serum during a 5-h incubation period (<2% radioactivity loss). In biodistribution studies the (68)Ga-labelled Nanobody 7D12 showed high uptake in A431 tumours (ranging from 6.1 ± 1.3 to 7.2 ± 1.5%ID/g at 1-3 h after injection) and high tumour to blood ratios, which increased from 8.2 to 14.4 and 25.7 at 1, 2 and 3 h after injection, respectively. High uptake was also observed in the kidneys. Biodistribution was similar to that of the reference conjugate (89)Zr-Df-Bz-NCS-7D12. Tumours were clearly visualized in a PET imaging study. CONCLUSION: Via a rapid procedure under mild conditions a (68)Ga-Nanobody was obtained that exhibited high tumour uptake and tumour to normal tissue ratios in nude mice bearing A431 xenografts. Fast kinetic (68)Ga-Nanobody conjugates can be promising tools for tumour detection and imaging of target expression.

Conjugation and radiolabeling of monoclonal antibodies with zirconium-89 for PET imaging using the bifunctional chelate p-isothiocyanatobenzyl-desferrioxamine.

The positron emitter zirconium-89 ((89)Zr) has very attractive properties for positron emission tomography (PET) imaging of intact monoclonal antibodies (mAbs) using immuno-PET. This protocol describes the step-by-step procedure for the facile radiolabeling of mAbs or other proteins with (89)Zr using p-isothiocyanatobenzyl-desferrioxamine (Df-Bz-NCS). First, Df-Bz-NCS is coupled to the lysine-NH(2) groups of a mAb at pH 9.0 (pre-modification), followed by purification using gel filtration. Next, the pre-modified mAb is labeled at room temperature by the addition of [(89)Zr]Zr-oxalic acid solution followed by purification using gel filtration. The entire process of pre-modification, radiolabeling and purification steps will take about 2.5 h.

p-Isothiocyanatobenzyl-desferrioxamine: a new bifunctional chelate for facile radiolabeling of monoclonal antibodies with zirconium-89 for immuno-PET imaging.

PURPOSE: Immuno-PET is an emerging imaging tool for the selection of high potential antibodies (mAbs) for imaging and therapy. The positron emitter zirconium-89 ((89)Zr) has attractive characteristics for immuno-PET with intact mAbs. Previously, we have described a multi-step procedure for stable coupling of (89)Zr to mAbs via the bifunctional chelate (BFC) tetrafluorophenol-N-succinyldesferal (TFP-N-sucDf). To enable widespread use of (89)Zr-immuno-PET, we now introduce the novel BFC p-isothiocyanatobenzyl-desferrioxamine B (Df-Bz-NCS) and compare its performance in (89)Zr-immuno-PET with the reference BFC TFP-N-sucDf. METHODS: Three mAbs were premodified with Df-Bz-NCS and labeled with (89)Zr at different pHs to assess the reaction kinetics and robustness of the radiolabeling. Stability of both (89)Zr-Df-Bz-NCS- and (89)Zr-N-sucDf-conjugates was evaluated in different buffers and human serum. Comparative biodistribution and PET studies in tumor-bearing mice were undertaken. RESULTS: The selected conjugation conditions resulted in a chelate:mAb substitution ratio of about 1.5:1. Under optimal radiolabeling conditions (pH between 6.8-7.2), the radiochemical yield was >85% after 60 min incubation at room temperature, resulting in radioimmunoconjugates with preserved integrity and immunoreactivity. The new radioimmunoconjugate was very stable in serum for up to 7 days at 37 degrees C, with <5% (89)Zr release, and was equally stable compared to the reference conjugate when stored in the appropriate buffer at 4 degrees C. In biodistribution and imaging experiments, the novel and the reference radioimmunoconjugates showed high and similar accumulation in tumors in nude mice. CONCLUSIONS: The novel Df-Bz-NCS BFC allows efficient and easy preparation of optimally performing (89)Zr-labeled mAbs, facilitating further exploration of (89)Zr-immuno-PET as an imaging tool.

(124)I-L19-SIP for immuno-PET imaging of tumour vasculature and guidance of (131)I-L19-SIP radioimmunotherapy.

PURPOSE: The human monoclonal antibody (MAb) fragment L19-SIP is directed against extra domain B (ED-B) of fibronectin, a marker of tumour angiogenesis. A clinical radioimmunotherapy (RIT) trial with (131)I-L19-SIP was recently started. In the present study, after GMP production of (124)I and efficient production of (124)I-L19-SIP, we aimed to demonstrate the suitability of (124)I-L19-SIP immuno-PET for imaging of angiogenesis at early-stage tumour development and as a scouting procedure prior to clinical (131)I-L19-SIP RIT. METHODS: (124)I was produced in a GMP compliant way via (124)Te(p,n)(124)I reaction and using a TERIMO module for radioiodine separation. L19-SIP was radioiodinated by using a modified version of the IODO-GEN method. The biodistribution of coinjected (124)I- and (131)I-L19-SIP was compared in FaDu xenograft-bearing nude mice, while (124)I PET images were obtained from mice with tumours of <50 to approximately 700 mm(3). RESULTS: (124)I was produced highly pure with an average yield of 15.4 +/- 0.5 MBq/microAh, while separation yield was approximately 90% efficient with <0.5% loss of TeO(2). Overall labelling efficiency, radiochemical purity and immunoreactive fraction were for (124)I-L19-SIP: approximately 80 , 99.9 and >90%, respectively. Tumour uptake was 7.3 +/- 2.1, 10.8 +/- 1.5, 7.8 +/- 1.4, 5.3 +/- 0.6 and 3.1 +/- 0.4%ID/g at 3, 6, 24, 48 and 72 h p.i., resulting in increased tumour to blood ratios ranging from 6.0 at 24 h to 45.9 at 72 h p.i.. Fully concordant labelling and biodistribution results were obtained with (124)I- and (131)I-L19-SIP. Immuno-PET with (124)I-L19-SIP using a high-resolution research tomograph PET scanner revealed clear delineation of the tumours as small as 50 mm(3) and no adverse uptake in other organs. CONCLUSIONS: (124)I-MAb conjugates for clinical immuno-PET can be efficiently produced. Immuno-PET with (124)I-L19-SIP appeared qualified for sensitive imaging of tumour neovasculature and for predicting (131)I-L19-SIP biodistribution.

Quantitative PET imaging of Met-expressing human cancer xenografts with 89Zr-labelled monoclonal antibody DN30.

PURPOSE: Targeting the c-Met receptor with monoclonal antibodies (MAbs) is an appealing approach for cancer diagnosis and treatment because this receptor plays a prominent role in tumour invasion and metastasis. Positron emission tomography (PET) might be a powerful tool for guidance of therapy with anti-Met MAbs like the recently described MAb DN30 because it allows accurate quantitative imaging of tumour targeting (immuno-PET). We considered the potential of PET with either (89)Zr-labelled (residualising radionuclide) or (124)I-labelled (non-residualising radionuclide) DN30 for imaging of Met-expressing tumours. MATERIALS AND METHODS: The biodistribution of co-injected (89)Zr-DN30 and iodine-labelled DN30 was compared in nude mice bearing either the human gastric cancer line GLT-16 (high Met expression) or the head-and-neck cancer line FaDu (low Met expression). PET images were acquired in both xenograft models up to 4 days post-injection (p.i.) and used for quantification of tumour uptake. RESULTS: Biodistribution studies in GTL-16-tumour-bearing mice revealed that (89)Zr-DN30 achieved much higher tumour uptake levels than iodine-labelled DN30 (e.g. 19.6%ID/g vs 5.3%ID/g, 5 days p.i.), while blood levels were similar, indicating internalisation of DN30. Therefore, (89)Zr-DN30 was selected for PET imaging of GLT-16-bearing mice. Tumours as small as 11 mg were readily visualised with immuno-PET. A distinctive lower (89)Zr uptake was observed in FaDu compared to GTL-16 xenografts (e.g. 7.8%ID/g vs 18.1%ID/g, 3 days p.i.). Nevertheless, FaDu xenografts were also clearly visualised with (89)Zr-DN30 immuno-PET. An excellent correlation was found between PET-image-derived (89)Zr tumour uptake and ex-vivo-assessed (89)Zr tumour uptake (R(2)=0.98). CONCLUSIONS: The long-lived positron emitter (89)Zr seems attractive for PET-guided development of therapeutic anti-c-Met MAbs.

Immuno-PET: a navigator in monoclonal antibody development and applications.

Monoclonal antibodies (mAbs) have been approved for use as diagnostics and therapeutics in a broad range of medical indications, but especially in oncology. In addition, hundreds of new mAbs, engineered mAb fragments, and nontraditional antibody-like scaffolds directed against either validated or novel tumor targets are under development. Immuno-positron emission tomography (PET), the tracking and quantification of mAbs with PET in vivo, is an exciting novel option to improve diagnostic imaging and to guide mAb-based therapy. In this review, recent technical advances leading to a jump ahead in mAb imaging are discussed. The availability of proper positron emitters, sophisticated radiochemistry, and advanced PET and PET-computed tomography scanners is crucial in these developments. Immuno-PET might play an important future role in cancer staging, in the improvement and tailoring of therapy with existing mAbs, and in the efficient development of novel mAbs. An overview of the preclinical and first clinical immuno-PET studies is provided.

Preparation and evaluation of (89)Zr-Zevalin for monitoring of (90)Y-Zevalin biodistribution with positron emission tomography.

PURPOSE: To evaluate whether (89)Zr can be used as a PET surrogate label for quantification of (90)Y-ibritumomab tiuxetan ((90)Y-Zevalin) biodistribution and dosimetry before myeloablative radioimmunotherapy. METHODS: Zevalin was labelled with (89)Zr by introducing N-succinyldesferal (N-sucDf) as a second chelate. For comparison of the in vitro stability of (89)Zr-Zevalin and (88)Y-Zevalin (as a substitute for (90)Y), samples were incubated in human serum at 37 degrees C up to 6 days. Biodistribution of (89)Zr-Zevalin and (88)Y-Zevalin was assessed at 24, 48, 72 and 144 h p.i. by co-injection in nude mice bearing the non-Hodgkin's lymphoma (NHL) xenograft line Ramos. The clinical performance of (89)Zr-Zevalin-PET was evaluated via a pilot imaging study in a patient with NHL, who had undergone [(18)F]FDG-PET 2 weeks previously. RESULTS: Modification of Zevalin with N-sucDf resulted in an N-sucDf-to-antibody molar ratio of 0.83+/-0.04. After radiolabelling and purification, the radiochemical purity and immunoreactivity of (89)Zr-Zevalin always exceeded 95% and 80%, respectively. (89)Zr-Zevalin showed the same stability in serum as (88)Y-Zevalin, with a radiochemical purity >95% during a period of 6 days. The co-injected (89)Zr-Zevalin and (88)Y-Zevalin conjugates showed a very similar biodistribution, except for liver and bone accumulation at 72 and 144 h p.i., which was significantly higher for (89)Zr than for (88)Y. PET images obtained after injection of (89)Zr-Zevalin showed clear targeting of all known tumour lesions. CONCLUSION: (89)Zr-Zevalin and (88)Y-Zevalin showed a very similar biodistribution in mice, implying that (89)Zr-Zevalin-PET might be well suited for prediction of (90)Y-Zevalin biodistribution in a myeloablative setting.

(89)Zr as a PET surrogate radioisotope for scouting biodistribution of the therapeutic radiometals (90)Y and (177)Lu in tumor-bearing nude mice after coupling to the internalizing antibody cetuximab.

UNLABELLED: Immuno-PET as a scouting procedure before radioimmunotherapy (RIT) aims at confirming tumor targeting and accurately estimating radiation dose delivery to both tumor and normal tissues and might therefore be of value for selection of patient candidates for RIT. A prerequisite for this approach is that PET radioimmunoconjugates and RIT radioimmunoconjugates must show a similar biodistribution. In the present study, we evaluated the potential of the long-lived positron emitter (89)Zr to predict biodistribution of the residualizing therapeutic radiometals (88)Y (as a substitute for (90)Y) and (177)Lu when labeled to the monoclonal antibody (mAb) cetuximab via different types of chelates. Cetuximab was selected as a model mAb because it abundantly internalizes after binding to the epidermal growth factor receptor. METHODS: Cetuximab was labeled with (89)Zr using succinylated desferrioxamine B (N-sucDf). The chelates p-benzyl isothiocyanate-1,4,7,10-tetraazacyclododecane-1,4,7, 10-tetraacetic acid (p-SCN-Bz-DOTA) and p-isothiocyanatobenzyl diethylenetriaminepentaacetic acid (p-SCN-Bz-DTPA) were both used for radiolabeling with (88)Y and (177)Lu. For measurement of the in vitro stability of each of the 5 radioimmunoconjugates, samples were incubated in freshly prepared human serum at 37 degrees C up to 16 d. Biodistribution was assessed at 24, 48, 72, and 144 h after intraperitoneal coinjection of the PET and RIT conjugates in nude mice bearing the squamous cell carcinoma xenograft line A431. RESULTS: Cetuximab premodification with N-sucDf, p-SCN-Bz-DOTA, or p-SCN-Bz-DTPA resulted in chelate-to-mAb molar ratios of about 1. After radiolabeling and purification, the radiochemical purity and immunoreactive fraction of the conjugates always exceeded 97% and 93%, respectively. All conjugates were stable in serum, showing a radioactivity release of less than 5% until day 7. From day 7 until day 16, an enhanced release was observed for the (89)Zr-N-sucDf, (88)Y-p-SCN-Bz-DTPA, and (177)Lu-p-SCN-Bz-DTPA conjugates. The coinjected PET and RIT conjugates showed similar biodistributions, except for the thighbone and sternum. For example, the (89)Zr-N-sucDf conjugate showed a 2.0-2.5 times higher radioactivity accretion in the thighbone than did the RIT conjugates at 72 h after injection. CONCLUSION: In view of the advantages of PET over SPECT, (89)Zr-immuno-PET is a promising modality for in vivo scouting of (90)Y- and (177)Lu-labeled mAbs, although care should be taken when estimating bone marrow doses.

The promise of immuno-PET in radioimmunotherapy.

Immuno-PET as a quantitative imaging procedure before or concomitant with radioimmunotherapy is an attractive option to improve confirmation of tumor targeting and especially assessment of radiation dose delivery to both tumor and normal tissues. General information about PET, PET systems, and quantification is provided in this review. The requirements for an appropriate positron emitter and characteristics of the most attractive candidate emitters for immuno-PET are discussed. An overview of preclinical and clinical immuno-PET studies reported in the literature is provided.

High-quality 124I-labelled monoclonal antibodies for use as PET scouting agents prior to 131I-radioimmunotherapy.

PURPOSE: Monoclonal antibodies (MAbs) labelled with 124I are an attractive option for quantitative imaging with positron emission tomography (PET) in a scouting procedure prior to 131I-radioimmunotherapy (131I-RIT). In this study, three important items in the labelling of MAbs with 124I were introduced to obtain optimal and reproducible product quality: restoration of radiation-induced inorganic deterioration of the starting 124I solution, radiation protection during and after 124I labelling, and synchronisation of the I/MAb molar ratio. METHODS: A new method was applied, using an NaIO3/NaI carrier mix, realising in one step >90% restoration of deteriorated 124I into the iodide form and chemical control over the I/MAb molar ratio. Chimeric MAb (cMAb) U36 and the murine MAbs 425 and E48 were labelled with 124I using the so-called Iodogen-coated MAb method, as this method provides optimal quality conjugates under challenging radiation conditions. As a standardising condition, NaIO3/NaI carrier mix was added at a stoichiometric I/MAb molar ratio of 0.9. For comparison, MAbs were labelled with 131I and with a mixture of 124I, 123I, 126I and 130I. RESULTS: Labelling with 124I in this setting resulted in overall yields of >70%, a radiochemical purity of >95%, and preservation of MAb integrity and immunoreactivity, including at the patient dose level (85 MBq). No significant quality differences were observed when compared with 131I products, while the iodine isotope mixture gave exactly the same labelling efficiency for each of the isotopes, excluding a different chemical reactivity of 124I-iodide. The scouting performance of 124I-cMAb U36 labelled at the patient dose level was evaluated in biodistribution studies upon co-injection with 131I-labelled cMAb U36, and by PET imaging in nude mice bearing the head and neck cancer xenograft line HNX-OE. 124I-cMAb and 131I-cMAb U36 labelled with a synchronised I/MAb molar ratio gave fully concordant tissue uptake values. Selective tumour uptake was confirmed with immuno-PET, revealing visualisation of 15 out of 15 tumours. CONCLUSION: These results pave the way for renewed evaluation of the potential of 124I-immuno-PET for clinical applications.

Quantitative 89Zr immuno-PET for in vivo scouting of 90Y-labeled monoclonal antibodies in xenograft-bearing nude mice.

UNLABELLED: Immuno-PET as a scouting procedure before radioimmunotherapy (RIT) aims at the confirmation of tumor targeting and the accurate estimation of radiation dose delivery to both tumor and normal tissues. Immuno-PET with (89)Zr-labeled monoclonal antibodies (mAbs) and (90)Y-mAb RIT might form such a valuable combination. In this study, the biodistribution of (89)Zr-labeled and (88)Y-labeled mAb ((88)Y as substitute for (90)Y) was compared and the quantitative imaging performance of (89)Zr immuno-PET was evaluated. METHODS: Chimeric mAb (cmAb) U36, directed against an antigen preferentially expressed in head and neck cancer, was labeled with (89)Zr using the bifunctional chelate N-succinyldesferrioxamine B (N-sucDf) and with (88)Y using the bifunctional chelate p-isothiocyanatobenzyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (p-SCN-Bz-DOTA). The radioimmunoconjugates were coinjected in xenograft-bearing nude mice, and biodistribution was determined at 3, 24, 48, 72, and 144 h after injection. (89)Zr was evaluated and compared with (18)F in phantom studies to determine linearity, resolution, and recovery coefficients, using a high-resolution research tomograph PET scanner. The potential of PET to quantify cmAb U36-N-sucDf-(89)Zr was evaluated by relating image-derived tumor uptake data (noninvasive method) to (89)Zr uptake data derived from excised tumors (invasive method). RESULTS: (89)Zr-N-sucDf-labeled and (88)Y-p-SCN-Bz-DOTA-labeled cmAb U36 showed a highly similar biodistribution, except for sternum and thigh bone at later time points (72 and 144 h after injection). Small differences were found in kidney and liver. Imaging performance of (89)Zr approximates that of (18)F, whereas millimeter-sized (19-154 mg) tumors were visualized in xenograft-bearing mice after injection of cmAb U36-N-sucDf-(89)Zr. After correction for partial-volume effects, an excellent correlation was found between image-derived (89)Zr tumor radioactivity and gamma-counter (89)Zr values of excised tumors (R(2) = 0.79). CONCLUSION: The similar biodistribution and the favorable imaging characteristics make (89)Zr a promising candidate for use as a positron-emitting surrogate for (90)Y.

Long-lived positron emitters zirconium-89 and iodine-124 for scouting of therapeutic radioimmunoconjugates with PET.

Antibody-PET imaging might be of value for the selection of radioimmunotherapy (RIT) candidates to confirm tumor targeting and to estimate radiation doses to tumor and normal tissues. One of the requirements to be set for such a scouting procedure is that the biodistributions of the diagnostic and therapeutic radioimmunoconjugates should be similar. In the present study we evaluated the potential of the positron emitters zirconium-89 ((89)Zr) and iodine-124 ((124)I) for this approach, as these radionuclides have a relatively long half-life that matches with the kinetics of MAbs in vivo (t(1/2) 3.27 and 4.18 days, respectively). After radiolabeling of the head and neck squamous cell carcinoma (HNSCC)-selective chimeric antibody (cMAb) U36, the biodistribution of two diagnostic (cMAb U36-N-sucDf-(89)Zr and cMAb U36-(124)I) and three therapeutic radioimmunoconjugates (cMAb U36-p-SCN-Bz-DOTA-(88)Y-with (88)Y being substitute for (90)Y, cMAb U36-(131)I, and cMAb U36-MAG3-(186)Re) was assessed in mice with HNSCC-xenografts, at 24, 48, and 72 hours after injection. Two patterns of biodistribution were observed, one pattern matching for (89)Zr- and (88)Y-labeled cMAb U36 and one pattern matching for (124)I-, (131)I-, and (186)Re-cMAb U36. The most remarkable differences between both patterns were observed for uptake in tumor and liver. Tumor uptake levels were 23.2 +/- 0.5 and 24.1 +/- 0.7%ID/g for the (89)Zr- and (88)Y-cMAb U36 and 16.0 +/- 0.8, 15.7 +/- 0.79 and 17.1 +/- 1.6%ID/g for (124)I-, (131)I-, and (186)Re-cMAb U36-conjugates, respectively, at 72 hours after injection. For liver these values were 6.9 +/- 0.8 ((89)Zr), 6.2 +/- 0.8 ((88)Y), 1.7 +/- 0.1 ((124)I), 1.6 +/- 0.1 ((131)I), and 2.3 +/- 0.1 ((186)Re), respectively. These preliminary data justify the further development of antibody-PET with (89)Zr-labeled MAbs for scouting of therapeutic doses of (90)Y-labeled MAbs. In such approach (124)I-labeled MAbs are most suitable for scouting of (131)I- and (186)Re-labeled MAbs.