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Biochemistry |
Institut für Molekularbiologie und Biophysik, ETH Hönggerberg, CH-8093 Zürich, Switzerland [F. V., L. T., D. N.]; Dipartimento di Biologia Molecolare, Sezíone di Biochimica, Università di Siena, 53100 Siena, Italy [L. G.]; and Istituto Nazionale per la Ricerca sul Cancro/Centro di Biotecnologie Avanzate, 16132 Genova, Italy [L. Z.]
| ABSTRACT |
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| INTRODUCTION |
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The targeting of angiogenesis is attractive for a number of reasons. Because tumor growth and metastatic spread depend on angiogenesis, the selective delivery of toxic agents to new-forming blood vessels should offer a therapeutic benefit (2, 3, 4, 5, 6, 7, 8) . Unlike tumor cells, endothelial cells do not mutate, and resistance is, therefore, less likely to develop. It should be possible to use a single reagent, specific for angiogenesis, for the selective detection and/or ablation of different types of tumor. Furthermore, markers on new-forming blood vessels should be readily accessible to binding molecules (e.g., Abs) injected i.v. and should overcome the well-recognized problem of slow tumor penetration of targeting agents specific for markers on the membrane of tumor cells (9, 10, 11) .
Research into antiangiogenic therapeutic strategies has mainly focused on the search for inhibitory molecules (1, 2, 3) . Only few reports, with no quantitative biodistribution analyses, have indicated that it may indeed be possible to target angiogenesis in vivo (5 , 12) .
The ED-B domain of fibronectin, a sequence of 91 amino acids identical in mouse, rat, dog, and man, which is inserted by alternative splicing into the fibronectin molecule, specifically accumulates around neovascular structures (13, 14, 15, 16, 17) and could represent a target for molecular intervention. Indeed, we have recently shown with fluorescent techniques that anti-ED-B scFvs3 (12) that are injected in tumor-bearing mice selectively accumulate in tumoral blood vessels.
These experiments, performed using Ab fragments with affinities in the low nanomolar range, demonstrated that it is possible to selectively target tumoral angiogenesis in vivo. They also raised the issue of whether Abs against markers of angiogenesis with further enhancement in binding affinity and/or valence would show an improved localization in tumoral neovasculaturean important parameter for the development of diagnostic and therapeutic applications.
Here we present the results of biodistribution studies in tumor-bearing mice performed with two radiolabeled Abs directed against the same epitope of the ED-B domain of fibronectin and differing in affinity by 760-fold. We quantitatively investigated whether binding affinity influences tumor targeting in vivo when Abs are injected i.v. as purified monomers or homodimers.
| MATERIALS AND METHODS |
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The genes coding for scFv(E1) and scFv(L19) were subcloned into NcoI/NotI digested pDN5 (19) according to standard techniques (20) . The corresponding Ab fragments, carrying a myc tag epitope at the COOH terminus (21) , were expressed from TG1 Escherichia coli cells harboring the appropriate plasmids and were affinity-purified from bacterial supernatants using an ED-B-Sepharose column, as described previously (12) .
Native PAGE (19) and gel filtration analysis of purified scFvs showed that E1 and L19 consist of a monomeric and a dimeric fraction, which were quantitatively separated and recovered using a Superdex 75 HR 10/30 column (Pharmacia, Uppsala, Sweden). The stability of the purified monomer and dimer was checked by native gel electrophoresis with Homogenous 12.5% gels (Pharmacia) by using a Phast System (Pharmacia) as described previously (19) .
The stability of E1 and L19 in serum was evaluated by incubating the Ab in FCS at 37°C for different time periods (04 h) and analyzing the Ab reactivity in the resulting mixture by ELISA as described previously (14) , applying biotinylated ED-B onto streptavidin-coated ELISA plates (Boehringer, Mannheim, Germany).
Epitope Retention Assay.
The ability of L19 to compete for the binding of E1 to a common epitope on the ED-B domain of fibronectin was investigated by real-time interaction analysis with surface plasmon resonance detection, using a BIAcore 1000 instrument (Pharmacia), according to established protocols (12
, 22)
. Biotinylated ED-B (12)
was applied to streptavidin-coated microsensor chips until the resulting surface bound
1000 RUs of scFv fragments. Binding studies were performed using 1 µM purified scFv solutions in PBS (50 mM phophate buffer (pH 7.2)-100 mM NaCl), with a 5-µl/min flow. For studies of competitive binding to the same epitope of the ED-B domain, the higher affinity scFv(L19) was injected first until saturation of the microsensor chip was achieved, inasmuch as this Ab shows a flat dissociation profile.
Western blot analysis of Ab specificity was performed as described previously (17) .
Radioiodination.
Immediately after their chromatographic separation (see previous section), dimeric and monomeric forms of scFv(L19) were radiolabeled with 125I using the Iodogen method (Pierce, Rockford, IL). Briefly, 100200 µg of protein were combined with 100200 µCi of 125I, followed by separation from unincorporated iodine using a PD-10 disposable gel filtration column (Pharmacia). Ab immunoreactivity after labeling was evaluated by loading an aliquot of radiolabeled sample onto 200 µl of ED-B-Sepharose resin (capacity, >3 mg scFv/ml resin; Ref. 12
) on a pasteur pipette, followed by radioactive counting of the flow-through and eluate fractions. Immunoreactivity, defined as the ratio between the counts of the eluted protein and the sum of the counts of the eluted and flow-through fractions, was 81% for monomeric L19, 92% for dimeric L19, 90% for monomeric E1, and 91% for dimeric E1.
The purity of radiolabeled samples was analyzed by reducing SDS-PAGE using Homogenous 20 Phast gels and a Phast System (Pharmacia), followed by autoradiography with a Phosphorimager (Molecular Dynamics, Sunnyvale, CA). Native PAGE of the radiolabeled samples, performed using Homogenous 12.5 Phast gels, native buffer strips, and a Phast System (Pharmacia) followed by autoradiography (19) , was performed to ensure that monomeric and dimeric Ab samples retained their oligomeric state after radioiodination.
Biodistribution Studies.
Biodistribution studies were performed under a license (Tumor Targeting, Bewilligung 53/97) issued to D .N. by the Veterinäramt des Kantons Zürich. Nude mice (1014 week-old CD1 nude mice, females) were obtained from LTK (Zurich, Switzerland). F9 murine teratocarcinomas were implanted s.c. as described previously (12
, 17)
. When tumor size reached 100300 mg, 1 µg (1 µCi) of scFv in 150 µl of saline solution, radiolabeled on the same day, was injected i.v. Previous biodistribution experiments with anti-ED-B Abs and F9 teratocarcinoma had shown that the amount of scFv injected is well below the dose necessary to saturate the tumor antigen pool. In fact, at least 10 µg of scFv per gram of tumor can selectively localize in F9 teratocarcinoma (12)
. Furthermore, biodistribution experiments performed with two radiolabeled Ab fragments directed against irrelevant antigens resulted in tumor:blood ratios of <1 at 124 h after injection.
Mice were killed at 4 and 24 h after injection, and organs were weighed and radioactively counted. Three animals were used for each time point. Targeting results of representative organs are expressed as %ID of Ab/g of tissue. The mean error and SE for each group of data were calculated, and T:O ratios were determined. Significance levels were calculated using Students t test.
Immunohistochemical studies with cryostat sections of F9 murine teratocarcinoma were performed as described previously (17) .
| RESULTS |
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To assess how binding affinity influences tumor targeting properties, it is essential to examine Abs that recognize the same antigen and the same epitope with different affinity values (10)
. Previous experiments that were aimed at affinity-maturing recombinant Abs using phage display technology have shown that affinity mutants retain the epitope of the parental Ab (12
, 24, 25, 26, 27, 28)
. To confirm that E1 and L19 recognize the same epitope, we studied their binding to the ED-B domain of fibronectin covalently immobilized on a BIAcore microsensor chip (12
, 22)
by real-time interaction analysis with surface plasmon resonance detection. Fig. 1, a and b
, shows the BIAcore sensograms of scFv(L19) and scFv(E1) binding to ED-B. The two Abs did not bind to microsensor chips coated with irrelevant antigens (streptavidin, glutathione S-transferase, BSA, and hen egg lysozyme; data not shown). The different kinetic stability of the two Ab-antigen complexes can be observed in the portion of the sensogram during which the complexes are washed with a flow of buffered saline and are allowed to dissociate. We then saturated the ED-B-coated microsensor chip with four successive injections of L19 (amount of Ab bound, 1100 RUs) and tested whether E1 could still bind to the antigen at a comparable level of RUs (Fig. 1c)
. As expected, L19 inhibited the binding of E1 to the ED-B domain of fibronectin, which confirmed that the epitope recognized by the two Abs was retained during affinity maturation.
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Both E1 and L19 monomeric and dimeric fractions showed comparable reactivity in immunohistochemistry with cryostat sections of F9 teratocarcinoma performed as described previously (Ref. 14
; Fig. 4
), suggesting that the Ab preparations conserve the same specificity toward new-forming blood vessels. We had previously shown that Ab affinity does not affect performance in immunohistochemistry (14)
.
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Biodistribution studies at 4 h and 24 h were performed to examine the role of Ab affinity on the specificity and the degree of tumor retention. One µg of purified radioiodinated scFv (1 µCi) in either monomeric or dimeric form was administered to the mice by i.v. injection. The biodistribution results (Tables 1
and 2
) revealed a correlation between the affinity and the %ID of radioiodinated Ab/g delivered to the tumor.
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The higher affinity L19 Ab was significantly superior to E1 also in terms of T:O ratios (confidence levels ranged between 80% and 99.99% for all of the pairwise comparisons of T:O ratios at 4 h and 24 h). Comparisons of the results obtained with monomeric and dimeric preparations revealed that T:O ratios were not significantly influenced by the Ab valence. However, T:O values for L19 dimer at 24 h were consistently better than those of the other Abs analyzed because of the longer Ab retention in the tumor.
| DISCUSSION |
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Our tumor model was an aggressive murine F9 teratocarcinoma tumor s.c. grafted in mice. It is generally believed that syngeneic models allow a better evaluation of Ab specificity than do models in which a human tumor marker is present in a human tumor xenografted in mice but is otherwise absent in other murine tissues. The situation with the ED-B domain of fibronectin is particularly favorable; because its amino acid sequence is identical in mouse and man, the results of our biodistribution experiments in mice may to some extent be predictive of the performance of the Abs in humans.
High-affinity L19both monomer and dimertarget the tumoral neovasculature significantly better than their E1 counterparts (Tables 1
and 2
). Tumor:blood ratios were already greater than 2 at 4 h after administration, suggesting the potential usefulness of L19 for the immunoscintigraphic detection of angiogenesis in patientsnamely an application in which short-lived radionuclides such as 99mTc or 123I have to be used to minimize exposure to radiation. Moreover, tumor:kidney and tumor:liver ratios, which are often problematic for intact monoclonal Abs and for some Ab fragments (37)
, were extremely favorable. Additional advantages are that recombinant human Abs: (a) can be expressed in large amounts in bacteria (38
, 39)
and yeast (40)
; (b) are not immunogenic; and (c) are easy to validate for clinical applications (41)
.
L19 dimer accumulates in the tumor in significantly larger amounts than does L19 monomer, both at 4 h and 24 h, given its longer half-life in blood. This observation was somewhat surprising considering the extremely high affinity of monomeric L19 and the good accessibility of its antigen, which is localized prevalently around new-forming blood vessels (Fig. 4). We can postulate two different explanations for the fact that targeting of the ED-B domain of fibronectin in vivo is a relatively slow process: (a) microcirculation in new-forming blood vessels could be impaired, preventing the Abs from rapidly reaching their target; or (b) the process of crossing the endothelial layer, which precedes the Abs binding to the fibronectin isoform located in the abluminal site of the vessel, could be slow because of interstitial pressure (9) . In either case, it is interesting to notice that bivalence and slower clearance of the Ab appear to drive the targeting process, despite the increased Ab size.
The investigation of the role of Ab affinity on tumor targeting performance has been addressed experimentally in the past with binders directed against tumor markers located on the cell membrane (42) . Very recently, Adams et al.(10) reported the first biodistribution study performed with Ab affinity mutants directed against the same epitope of HER2/neu. This more rigorous experimental approach excludes the possibility that differences in targeting performance may be due to different degrees of accessibility of the epitopes within the tumor. Their findings that increased affinity leads to improved selective tumor delivery by monomeric scFvs are in good agreement with our results obtained here with Abs directed against a marker of angiogenesis.
The amount of L19 dimer selectively delivered to the tumor compares favorably with the tumor retention of other recombinant Ab fragments directed against cellular markers assayed in different tumor models in mice (for a review, see Refs. 43
, 10
, 11
, 34
, 37 , and references therein). Considering that tumoral neovasculature represents only a small percentage of the total tumor mass (Fig. 4)
, the %ID of L19 retained per gram of vasculature is extremely high and could have an impact in the diagnosis and therapy of tumors with high angiogenetic index. It will be interesting to evaluate the usefulness of L19 for the detection of angiogenesis in patients by immunoscintigraphic (44)
and IR fluorescence (12
, 45)
methodologies, as well as its performance as a selective vehicle for delivering toxic agents to tumor neovasculature (2, 3, 4, 5, 6, 7, 8
, 46)
. Work in this direction is in progress in our laboratories.
| Note Added in Proof |
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| FOOTNOTES |
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1 Supported by an internal Grant of the ETH Zürich (to D. N.), by the Stiftung zur Krebsbekämpfung (to D. N.), and by the Associazione Italiana Ricerca sul Cancro (to L. Z.). ![]()
2 To whom requests for reprints should be addressed, at Institut für Molekularbiologie und Biophysik, ETH Hönggerberg, CH-8093 Zürich, Switzerland. E-mail: dario{at}mol.biol.ethz.ch ![]()
3 The abbreviations used are: Ab, antibody; scFv, single-chain Fv Ab fragment; T:O, tumor:organ; %ID, percent injected dose; RU, (BIAcore) resonance unit. ![]()
Received 7/27/98. Accepted 11/11/98.
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