Jürgen Grünberg

Site‐Specific and Stoichiometric Modification of Antibodies by Bacterial Transglutaminase

The therapeutic efficacy of antibodies can be substantially enhanced by conjugation of cytotoxic compounds such as chemotherapeutics and particle-emitting radionuclides. Intuitively one would assume that the therapeutic index would improve as the number of cytotoxic entities conjugated to the antibody increases. However, recent studies on auristatin–antibody conjugates in mice have demonstrated that a drug/antibody molar ratio of 4:1 results in optimal efficacy and in vivo tolerability. Unfortunately, conventional chemical strategies for protein modification are difficult to control and give rise to heterogeneous populations of immunoconjugates with variable stoichiometries, each of which has its own in vivo characteristics. The introduction of artificial, bio-orthogonal groups for site-specific and stoichiometric protein modification offers a potential solution to this problem. Such strategies are en vogue but are often laborious and still risk product heterogeneity. Transglutaminases (TGs, E.C. 2.3.2.13) catalyze acyltransfer reactions between the g-carboxamide group of glutamine (a side chain, which is otherwise chemically inert under physiological conditions) and the primary e-amino group of lysine, to form catabolically stable isopeptide bonds (Figure 1a). Most TGs are promiscuous with respect to the lysine substrate and accept even simple 5-aminopentyl groups as lysine surrogates. The criteria for a glutamine residue to be recognized by the enzyme, however, are muchmore stringent: it should be both located in a flexible region of the protein and flanked by specific amino acids. Given this inherent selectivity, we hypothesized that TG would be an alternative for the site-specific and stoichiometric functionalization of antibodies. For this study we used bacterial transglutaminase (BTG) because it is robust, inexpensive, and easy to handle. Our group is interested in radioimmunoconjugates for diagnostic and therapeutic applications, where low off-target accumulation of radioactivity is crucial. Earlier studies performed with radiolabeled monoclonal antibodies (mAbs) demonstrated that high numbers of metal chelators adversely affect the biological behavior of radioimmunoconjugates. Therefore, we tested the features of BTG for the preparation of immunoconjugates that are functionalized with different metal chelators and radiolabeled with different diagnostic and therapeutic radionuclides. Deferoxamine (DF, 1), an antidote for metal poisoning, has recently been identified as a suitable chelator for radionuclides such as Ga and Zr. During the course of our studies we recognized that without further derivatization deferoxamine is already a potent BTG substrate. Furthermore, the metal chelating system 4-(1,4,8,11tetraazacyclotetradec-1-yl)methyl benzoic acid (CPTA, 2) was derivatized with a 1,5-diaminopentane (cadaverine) spacer (Figure 1b; see the Supporting Information for details on the synthesis). To probe the scope of the new strategy we investigated other (model) substrates, which are of potential Figure 1. a) TG-mediated modification of Gln (Q) with a substrate containing lysine or a lysine surrogate. b) Substrates used in this study.

Targeting of renal carcinoma with 67/64Cu-labeled anti-L1-CAM antibody chCE7: selection of copper ligands and PET imaging.

In order to optimize radiocopper labeling of anti-L1-CAM antibody chCE7, five bifunctional copper chelators were synthesized and characterized (CPTA-N-hydoxysuccinimide, DO3A-L-p-isothiocyanato-phenylalanine, DOTA-PA-L-p-isocyanato-phenylalanine, DOTA-glycyl-L-p-isocyanato-phenylalanine and DOTA-triglycyl-L-p-isocyanato-phenylalanine). Substitution with more than 11 chelators per antibody molecule was found to influence immunoreactivity and biodistributions of (67)Cu-MAb chCE7 significantly. CPTA-labeled antibody achieved the best tumor to normal tissue ratios when biodistributions of the different (67)Cu-chCE7 conjugates were assessed in tumor-bearing mice. High resolution PET imaging with (64)Cu-CPTA-labeled MAb chCE7 showed uptake in lymph nodes and heterogeneous distribution in tumor xenografts.

The low-energy β− and electron emitter 161Tb as an alternative to 177Lu for targeted radionuclide therapy

INTRODUCTION The low-energy β(-) emitter (161)Tb is very similar to (177)Lu with respect to half-life, beta energy and chemical properties. However, (161)Tb also emits a significant amount of conversion and Auger electrons. Greater therapeutic effect can therefore be expected in comparison to (177)Lu. It also emits low-energy photons that are useful for gamma camera imaging. METHODS The (160)Gd(n,γ)(161)Gd→(161)Tb production route was used to produce (161)Tb by neutron irradiation of massive (160)Gd targets (up to 40 mg) in nuclear reactors. A semiautomated procedure based on cation exchange chromatography was developed and applied to isolate no carrier added (n.c.a.) (161)Tb from the bulk of the (160)Gd target and from its stable decay product (161)Dy. (161)Tb was used for radiolabeling DOTA-Tyr3-octreotate; the radiolabeling profile was compared to the commercially available n.c.a. (177)Lu. A (161)Tb Derenzo phantom was imaged using a small-animal single-photon emission computed tomography camera. RESULTS Up to 15 GBq of (161)Tb was produced by long-term irradiation of Gd targets. Using a cation exchange resin, we obtained 80%-90% of the available (161)Tb with high specific activity, radionuclide and chemical purity and in quantities sufficient for therapeutic applications. The (161)Tb obtained was of the quality required to prepare (161)Tb-DOTA-Tyr3-octreotate. CONCLUSIONS We were able to produce (161)Tb in n.c.a. form by irradiating highly enriched (160)Gd targets; it can be obtained in the quantity and quality required for the preparation of (161)Tb-labeled therapeutic agents.

The soluble form of the cancer-associated L1 cell adhesion molecule is a pro-angiogenic factor.

A soluble form of the L1 cell adhesion molecule (sL1) is released from various tumor cells and can be found in serum and ascites fluid of uterine and ovarian carcinoma patients. sL1 is a ligand for several Arg-Gly-Asp (RGD)-binding integrins and can be deposited in the extracellular matrix. In this study we describe a novel function of this physiologically relevant form of L1 as a pro-angiogenic factor. We demonstrated that the anti-L1 monoclonal antibody (mAb) chCE7 binds near or to the sixth Ig-like domain of human L1 which contains a single RGD sequence. mAb chCE7 inhibited the RGD-dependent adhesion of ovarian carcinoma cells to sL1 and reversed the sL1-induced proliferation, matrigel invasion and tube formation of bovine aortic endothelial (BAE) cells. A combination of sL1 with vascular endothelial growth factor-A (VEGF-A(165)), which is an important angiogenic inducer in tumors, strongly potentiated VEGF receptor-2 tyrosine phosphorylation in BAE cells. Chick chorioallantoic membrane (CAM) assays revealed the pro-angiogenic potency of sL1 in vivo which could be abolished by chCE7. These results indicate an important role of released L1 in tumor angiogenesis and represent a novel function of antibody chCE7 in tumor therapy.

L1-CAM-targeted antibody therapy and 177Lu-radioimmunotherapy of disseminated ovarian cancer

The L1‐cell adhesion molecule (L1‐CAM) is highly expressed in various cancer types including ovarian carcinoma but is absent from most normal tissue. A chimeric monoclonal antibody, chCE7, specifically binds to human L1‐CAM and exhibits anti‐proliferative effects on L1‐CAM‐expressing tumor cells. The goal of this study was to evaluate the efficacy of a novel 177Lu‐chCE7 radioimmunotherapeutic agent and to compare it to a treatment protocol with unlabeled, growth‐inhibiting chCE7 in a mouse xenograft model of disseminated ovarian cancer. chCE7agl, an aglycosylated IgG1 variant with improved pharmacokinetics, was conjugated with 1,4,7,10‐tetraazacyclododecane‐N‐N′‐N′‐N‴‐tetraacetic acid (DOTA) and labeled with the low‐energy β‐emitter 177Lu. Tumor growth and survival were assessed after a single i.v. dose of 8 MBq (60 μg) radioimmunoconjugate in nude mice bearing either subcutaneous or intraperitoneal SKOV3.ip1 human ovarian cancer tumors. Therapeutic efficacy was compared with three times weekly i.p. administration of 10 mg/kg unconjugated chCE7. In vivo analysis of 177Lu‐chCE7agl biodistribution demonstrated high and specific accumulation of radioactivity at the tumor site with maximal tumor uptake of up to 48.0 ± 8.1% ID/g at 168 h postinjection. A single treatment with 177Lu‐DOTA‐chCE7agl caused significant retardation of tumor growth and prolonged median survival from 33 to 71 days, while administration of a nontargeted 177Lu‐immunoconjugate had no beneficial effect. Three times weekly i.p. application of unlabeled chCE7 10 mg/kg similarly increased survival from 44 to 72 days. We conclude that a single dose of 177Lu‐DOTA‐chCE7agl is as effective as repeated administration of nonradioactive chCE7 for treatment of small intraperitoneal tumors expressing L1‐CAM.

DOTA-Functionalized Polylysine: A High Number of DOTA Chelates Positively Influences the Biodistribution of Enzymatic Conjugated Anti-Tumor Antibody chCE7agl

Site-specific enzymatic reactions with microbial transglutaminase (mTGase) lead to a homogenous species of immunoconjugates with a defined ligand/antibody ratio. In the present study, we have investigated the influence of different numbers of 1,4,7,10-tetraazacyclododecane-N-N′-N′′-N′′′-tetraacetic acid (DOTA) chelats coupled to a decalysine backbone on the in vivo behavior of the chimeric monoclonal anti-L1CAM antibody chCE7agl. The enzymatic conjugation of (DOTA)1-decalysine, (DOTA)3-decalysine or (DOTA)5-decalysine to the antibody heavy chain (via Gln295/297) gave rise to immunoconjugates containing two, six or ten DOTA moieties respectively. Radiolabeling of the immunoconjugates with 177Lu yielded specific activities of approximately 70 MBq/mg, 400 MBq/mg and 700 MBq/mg with increasing numbers of DOTA chelates. Biodistribution experiments in SKOV3ip human ovarian cancer cell xenografts demonstrated a high and specific accumulation of radioactivity at the tumor site for all antibody derivatives with a maximal tumor accumulation of 43.6±4.3% ID/g at 24 h for chCE7agl-[(DOTA)-decalysine]2, 30.6±12.0% ID/g at 24 h for chCE7agl-[(DOTA)3-decalysine]2 and 49.9±3.1% ID/g at 48 h for chCE7agl-[(DOTA)5-decalysine)]2. The rapid elimination from the blood of chCE7agl-[(DOTA)-decalysine]2 (1.0±0.1% ID/g at 24 h) is associated with a high liver accumulation (23.2±4.6% ID/g at 24 h). This behavior changed depending on the numbers of DOTA moieties coupled to the decalysine peptide with a slower blood clearance (5.1±1.0 (DOTA)3 versus 11.7±1.4% ID/g (DOTA)5, p<0.005 at 24 h) and lower radioactivity levels in the liver (21.4±3.4 (DOTA)3 versus 5.8±0.7 (DOTA)5, p<0.005 at 24 h). We conclude that the site-specific and stoichiometric uniform conjugation of the highly DOTA-substituted decalysine ((DOTA)5-decalysine) to an anti-tumor antibody leads to the formation of immunoconjugates with high specific activity and excellent in vivo behavior and is a valuable option for radioimmunotherapy and potentially antibody-drug conjugates (ADCs).

Future prospects for SPECT imaging using the radiolanthanide terbium-155 — production and preclinical evaluation in tumor-bearing mice

INTRODUCTION We assessed the suitability of the radiolanthanide (155)Tb (t1/2=5.32 days, Eγ=87 keV (32%), 105keV (25%)) in combination with variable tumor targeted biomolecules using preclinical SPECT imaging. METHODS (155)Tb was produced at ISOLDE (CERN, Geneva, Switzerland) by high-energy (~1.4 GeV) proton irradiation of a tantalum target followed by ionization and on-line mass separation. (155)Tb was separated from isobar and pseudo-isobar impurities by cation exchange chromatography. Four tumor targeting molecules - a somatostatin analog (DOTATATE), a minigastrin analog (MD), a folate derivative (cm09) and an anti-L1-CAM antibody (chCE7) - were radiolabeled with (155)Tb. Imaging studies were performed in nude mice bearing AR42J, cholecystokinin-2 receptor expressing A431, KB, IGROV-1 and SKOV-3ip tumor xenografts using a dedicated small-animal SPECT/CT scanner. RESULTS The total yield of the two-step separation process of (155)Tb was 86%. (155)Tb was obtained in a physiological l-lactate solution suitable for direct labeling processes. The (155)Tb-labeled tumor targeted biomolecules were obtained at a reasonable specific activity and high purity (>95%). (155)Tb gave high quality, high resolution tomographic images. SPECT/CT experiments allowed excellent visualization of AR42J and CCK-2 receptor-expressing A431 tumors xenografts in mice after injection of (155)Tb-DOTATATE and (155)Tb-MD, respectively. The relatively long physical half-life of (155)Tb matched in particular the biological half-lives of (155)Tb-cm09 and (155)Tb-DTPA-chCE7 allowing SPECT imaging of KB tumors, IGROV-1 and SKOV-3ip tumors even several days after administration. CONCLUSIONS The radiolanthanide (155)Tb may be of particular interest for low-dose SPECT prior to therapy with a therapeutic match such as the β(-)-emitting radiolanthanides (177)Lu, (161)Tb, (166)Ho, and the pseudo-radiolanthanide (90)Y.

Paclitaxel improved anti-L1CAM lutetium-177 radioimmunotherapy in an ovarian cancer xenograft model

BackgroundTodays standard treatment of advanced-stage ovarian cancer, including surgery followed by a paclitaxel-platinum-based chemotherapy, is limited in efficacy. Recently, we could show that radioimmunotherapy (RIT) with 177Lu-labelled anti-L1 cell adhesion molecule (L1CAM) monoclonal antibody chCE7 is effective in ovarian cancer therapy. We investigated if the efficacy of anti-L1CAM RIT can be further improved by its combination with paclitaxel (PTX).MethodsIn vitro cell viability and cell cycle arrest of human ovarian cancer cells were assessed upon different treatment conditions. For therapy studies, nude mice (n = 8) were injected subcutaneously with IGROV1 human ovarian carcinoma cells and received a single dose of 6 MBq 177Lu-DOTA-chCE7 alone or in combination with 600 μg PTX (31.6 mg/kg). Tumour growth delay and survival were determined. To investigate whether PTX can influence the tumour uptake of the radioimmunoconjugates (RICs), a biodistribution study (n = 4) and SPECT/CT images were acquired 120 h post injections of 2 MBq 177Lu-DOTA-chCE7 alone or in combination with 600 μg PTX.ResultsLu-DOTA-chCE7 in combination with PTX revealed a significantly decreased cell viability of ovarian carcinoma cells in vitro and was effective in a synergistic manner (combination index < 1). PTX increased the RIT efficacy by arresting cells in the radiosensitive G2/M phase of the cell cycle 24 h post treatment start. In vivo combination therapy including 177Lu-DOTA-chCE7 and PTX resulted in a significantly prolonged overall survival (55 days vs. 18 days/PTX and 29 days/RIT), without weight loss and/or signs of toxicity. Biodistribution studies revealed no significant difference in tumour uptakes of 177Lu-DOTA-chCE7 72 h post injection regardless of an additional PTX administration.ConclusionsCombination of anti-L1CAM 177Lu-RIT with PTX is a more effective therapy resulting in a prolonged overall survival of human ovarian carcinoma-bearing nude mice compared with either monotherapy. The combination is promising for future clinical applications.

Radiopharmaceuticals: From molecular imaging to targeted radionuclide therapy

The research and development of smart radiodrugs is the goal of the Center of Radiopharmaceutical Science of ETH, PSI, and USZ. Positron Emission Tomography (PET) allows the non-invasive visualization of biochemical processes within the body. Radiolabeled PET-tracers allow the study of neurophysiological diseases like Alzheimer, Parkinsons disease or the imaging of metastatic tumors. PET-techniques are nowadays an important part of routine nuclear medicine diagnosis. Tumor-cell targeting biomolecules (e.g. antibodies or peptides) coupled to therapeutic radionuclides can sterilize the malignant cells while sparing healthy tissue. This so-called targeted radionuclide therapy has made tremendous progress in the recent years and the first approved radiotherapeutics are available for clinical use.

Imaging of renal carcinoma xenografts with 64Cu-labelled anti-L1-CAM antibody chCE7

High-resolution PET imaging, with the dedicated small animal PET tomograph quad-HIDAC (Oxford Positron Systems, UK), of Caki-2 renal cell carcinoma xenografts with 64Cu-CPTA-labelled mAb chCE7. A nude mouse (26 g) with Caki-2 renal cell carcinoma tumours on the right and left sides was injected with 21 MBq (27 μg) 64Cu-CPTAchCE7 and data were acquired for 70 min at 26.5 h post injection. The reconstructed images represent a series of coronal slices (0.3 mm thickness) from ventral (1) to dorsal (12) (T, tumour; L, lymph node). Strong uptake is observed in two subcutaneous tumour xenografts on the back of the mouse (Table 1, and T in coronal sections 7–12). Lower levels of activity are also demonstrated in other organs: the liver and the spleen (sections 1–6), as well as lymph nodes (submandibular and parotid lymph nodes: sections 1 and 2; suprascapular and supraclavicular lymph nodes: sections 3–5; and deep inguinal lymph nodes: sections 6–8). The renal cell tumours do not metastasise, and as the chCE7 antibody does not react with mouse L1-CAM, the accumulation is not due to specific target cell binding. Accumulation of radioactivity in the lymph nodes is higher than in the liver and spleen and is likely to be due to radiocopper labelled metabolites accumulating in cells of the reticuloendothelial system present in lymph nodes.