Synthesis and evaluation of novel bifunctional chelating agents based on 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid for radiolabeling proteins.

Detailed synthesis of the bifunctional chelating agents 2-methyl-6-(p-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (1B4M-DOTA) and 2-(p-isothiocyanatobenzyl)-5, 6-cyclohexano-1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetate (CHX-DOTA) are reported. These chelating agents were compared to 2-(p-isothiocyanatobenzyl)-1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetic acid (C-DOTA) and 1, 4, 7, 10-Tetraaza-N-(1-carboxy-3-(4-nitrophenyl)propyl)-N', N", N"'-tris(acetic acid) cyclododecane (PA-DOTA) as their (177)Lu radiolabeled conjugates with Herceptin. In vitro stability of the immunoconjugates radiolabeled with (177)Lu was assessed by serum stability studies. The in vivo stability of the radiolabeled immunoconjugates and their targeting characteristics were determined by biodistribution studies in LS-174T xenograft tumor-bearing mice. Relative radiolabeling rates and efficiencies were determined for all four immunoconjugates. Insertion of the 1B4M moiety into the DOTA backbone increases radiometal chelation rate and provides complex stability comparable to C-DOTA and PA-DOTA while the CHX-DOTA appears to not form as stable a (177)Lu complex while exhibiting a substantial increase in formation rate. The 1B4M-DOTAmay have potential for radioimmunotherapy applications.

Delivery of the alpha-emitting radioisotope bismuth-213 to solid tumors via single-chain Fv and diabody molecules.

Intravenously administered anti-tumor single-chain Fv (scFv) and diabody molecules exhibit rapid clearance kinetics and accumulation in tumors that express their cognate antigen. In an attempt to fit the rate of isotope decay to the timing of delivery and duration of tumor retention, anti-HER2/neu CHX-A" DTPA-C6.5K-A scFv and diabody conjugates were labeled with the alpha-particle emitter (213)Bi (t(1/2) = 47 min). Radioimmunotherapy studies employing 0.64, 0.35, or 0.15 microCi of (213)Bi-labeled C6.5K-A diabody or 1.1, 0.6, or 0. 3 microCi of (213)Bi-labeled C6.5K-A scFv were performed in nude mice bearing early, established SK-OV-3 tumors. Only the 0.3 microCi dose of (213)Bi-labeled C6.5K-A scFv resulted in both acceptable toxicity and a reduction in tumor growth rate. The specificity of the anti-tumor effects was determined by comparing the efficacy of treatment with 0.3 and 0.15 microCi doses of (213)Bi-labeled C6.5K-A scFv and (213)Bi-labeled NM3E2 (an irrelevant scFv) in nude mice bearing large established tumors. The 0.3 microCi dose of (213)Bi on both the C6.5K-A and NM3E2 scFvs resulted in similar anti-tumor effects (p = 0.46) indicating that antigen-specific targeting was not a factor. This suggests that the physical half-life of (213)Bi may be too brief to be effectively paired with systemically-administered diabody or scFv molecules.

Evaluation of 225Ac for vascular targeted radioimmunotherapy of lung tumors.

Several alpha particle emitting radioisotopes have been studied for use in radioimmunotherapy. Ac-225 has the potential advantages of a relatively long half life of 10 days, and a yield of 4 alpha emissions in its decay chain with a total energy release of approximately 28 MeV. A new, 12 coordination site chelating ligand, HEHA, has been chemically modified for coupling to targeting proteins without loss of chelating ability. HEHA was coupled with MAb 201B which binds to thrombomodulin and accumulates efficiently in murine lung. Ac-225 was bound to the HEHA-MAb 201B conjugate and injected into BALB/c mice bearing lung tumor colonies of EMT-6 mammary carcinoma. Biodistribution data at 1 and 4 h postinjection indicated that, as expected, 225Ac was delivered to lung efficiently (> 300% ID/g). The 225Ac was slowly released from the lung with an initial t1/2 = 49 h, and the released 225Ac accumulated in the liver. Injection of free HEHA was only partially successful in scavenging free 225Ac. In addition to the slow release of 225Ac from the chelate, data indicated that decay daughters of 225Ac were also released from the lung. Immediately after organ harvest, the level of 213Bi, the third alpha-decay daughter, was found to be deficient in the lungs and to be in excess in the kidney, relative to equilibrium values. Injected doses of 225Ac MAb 201B of 1.0 microCi, delivering a minimum calculated absorbed dose of about 6 Gy to the lungs, was effective in killing lung tumors, but also proved acutely radiotoxic. Animals treated with 1.0 microCi or more of the 225Ac radioconjugate died of a wasting syndrome within days with a dose dependent relationship. We conclude that the potential for 225Ac as a radioimmunotherapeutic agent is compromised not only by the slow release of 225Ac from the HEHA chelator, but most importantly by the radiotoxicity associated with decay daughter radioisotopes released from the target organ.

Synthesis, conjugation, and radiolabeling of a novel bifunctional chelating agent for (225)Ac radioimmunotherapy applications.

225Ac (t(1/2) = 10 days) is an alternative alpha-emitter that has been proposed for radioimmunotherapy (RIT) due to its many favorable properties, such as half-life and mode of decay. The factor limiting use of (225)Ac in RIT is the lack of an acceptably stable chelate for in vivo applications. Herein is described the first reported bifunctional chelate for (225)Ac that has been evaluated for stability for in vivo applications. The detailed synthesis of a bifunctional chelating agent 2-(4-isothiocyanatobenzyl)-1,4,7,10,13, 16-hexaazacyclohexadecane- 1,4,7,10,13,16-hexaacetic acid (HEHA-NCS) is reported. This ligand was conjugated to three monoclonal antibodies, CC49, T101, and BL-3 with chelate-to-protein ratios between 1.4 and 2. The three conjugates were radiolabeled with (225)Ac, and serum stability study of the [(225)Ac]BL-3-HEHA conjugate was performed.

Oligonucleotide conjugates of Eu(III) tetraazamacrocycles with pendent alcohol and amide groups promote sequence-specific RNA cleavage.

Eu(III) complexes of two neutral bifunctional tetraaaza macrocyclic ligands ¿1-[1-carboxamido-3-(4-nitrophenyl)propyl]-4,7,10-tris(2-hydroxyethyl)-1 ,4,7,10-tetraazacyclododecane and 2-(4-nitrobenzyl)-1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraaza cyclododecane¿ are prepared. Eu(III) complexes of the isothiocyanate derivatives of these macrocycles are treated with oligonucleotides containing 2'-O-propyl-amine linkers to form conjugates. Hydrolytic cleavage of an oligoribonucleotide is promoted by Eu(III) macrocyclic oligonucleotide conjugates containing complementary (antisense) sequences. Cleavage is not observed in the presence of Eu(III) conjugates containing scrambled sequences nor by free complex. Despite the fact that one of the free macrocyclic complexes is more reactive than the other, the extent of cleavage observed is similar for conjugates containing either Eu(III) macrocyclic complex.

Synthesis, characterization, and evaluation of a novel bifunctional chelating agent for the lead isotopes 203Pb and 212Pb.

Radioisotopes of Pb(II) have been of some interest in radioimmunotherapy and radioimmunoimaging (RII). However, the absence of a kinetically stable bifunctional chelating agent for Pb(II) has hampered its use for these applications. 203Pb (T(1/2) = 52.02 h) has application potential in RII, with a gamma-emission that is ideal for single photon emission computerized tomography, whereas 212Pb (T(1/2) = 10 h) is a source of highly cytotoxic alpha-particles via its decay to its 212Bi (T(1/2) = 60 min) daughter. The synthesis of the novel bifunctional chelating agent 2-(4-isothiocyanotobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra- (2-carbamoyl methyl)-cyclododecane (4-NCS-Bz-TCMC) is reported herein. The Pb[TCMC]2+ complex was less labile to metal ion release than Pb[DOTA]2- at pH 3.5 and below in isotopic exchange experiments. In addition to increased stability to Pb2+ ion release at low pH, the bifunctional TCMC ligand was found to have many other advantages over the bifunctional 1,4,7,10-tetraazacyclodocane-1,4,7,10-tetraacetic acid (DOTA) ligand. These include a shorter and more straightforward synthetic route, a more efficient conjugation reaction to a monoclonal antibody (mAb), with a higher chelate to protein ratio, a higher percent immuroreactivity, and a more efficient radiolabeling reaction of the mAb-ligand conjugate with 203Pb.

Labeling and distribution of linear peptides identified using in vivo phage display selection for tumors.

To develop targeting molecules to be used for vascular targeting of short half-lived alpha-emitters for radioimmunotherapy, linear peptide phage display libraries were selected in vivo for binding to IC-12 rat tracheal tumors growing in severe combined immune deficient mice. After three rounds of selection, 15 phage clones were analyzed for DNA sequence, and the deduced translation products of cDNA inserts were compared. Three consensus sequences were chosen from three separate experimental selection series and peptides of these sequences with added -gly-gly-tyr were obtained. Peptides were radiolabeled on tyrosine with (125)I and the biodistribution in tumor-bearing mice was determined. The radioiodinated peptides were stable in vitro and when injected in tumor-bearing mice approximately 3.0 %ID/g accumulated in the tumor; however, much of the (125)I was found in the gastrointestinal tract and thyroid, indicative of dehalogenation of the labeled peptide. Radiolabeling peptide 2 with N-succinimidyl-3-(125)I-iodobenzoate resulted in faster excretion, which in turn resulted in lower levels in tumor and other organs, especially thyroid and gastrointestinal tract. Peptide 2 was derivatized with the bifunctional isothiocyanates of cyclohexyl-B diethylenetriaminepentaacetic acid (DTPA) or CHX-A" DTPA by direct conjugation or with a hydroxylamine derivative of 1B4M-DTPA (2-(p-[O-(carboxamylmethyl)hydroxylamine]benzyl)-6-methyl-diethylenetriamine-N,N,N',N",N"-pentaacetic acid ) coupled at the N-terminus. The primary molecular species in the conjugated products were shown by mass spectrometry to have one DTPA per peptide. Peptide chelate conjugates were radiolabeled with (213)Bi and the products tested for biodistribution in tumor-bearing mice. The data show that chelation of (213)Bi to peptides was accomplished by both the direct method of DTPA attachment and by the method using the linker at the N-terminus. Only small amounts of peptide accumulated at tumor sites. We conclude that phage display is a powerful tool to select peptides with restricted binding specificity; however, the peptides isolated to date do not bind with high retention to tumor sites in vivo.

Eu(III) macrocyclic complexes promote cleavage of and bind to models for the 5'-cap of mRNA. Effect of pendent group and a second metal ion.

The interaction of three Eu(III) macrocyclic complexes Eu(THED)3+, Eu(ATHC)3+, and Eu(ATHC)3+, and Eu(S-THP)3+ with two 5'-cap model compounds, GpppG and m7GpppG is studied (THED = 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,-10-tetraazacyclododecane, ATHC = 1-(carbamoylmethyl)-4,7,10-tris(2-hydroxyethyl)-1,4,7,10- tetraazacyclododecane, S-THP = 1S,4S,7S,10S-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane). Laser-induced excitation luminescence spectroscopy is used to study the binding of Eu(S-THP)3+ to GpppG (K = 5.9 x 10(4) M-1) and to characterize the Eu(S-THP)-GpppG complex. Both Eu(THED)3+ and Eu(S-THP)3+ bind to m7GpppG as monitored by use of fluorescence spectroscopy with binding constants of 5.9 x 10(3) and 4.4 x 10(4) M-1, respectively. The kinetics of cleavage of GpppG by two macrocyclic complexes is studied. Cleavage of GpppG by Eu(THED)3+ is accelerated by 15-fold in the presence of an equivalent of Zn(NO3)2 at pH 7.3, 37 degrees C, suggesting that two metal ions accelerate the cap cleavage reaction. Eu(ATHC)3+ promotes cleavage of GpppG with a pseudo-first-order rate constant of 2.6 x 10(-5) s-1 at pH 7.3, 37 degrees C, and 0.30 mM complex.

Targeting strategies for cancer radiotherapy.

Novel strategies to increase the therapeutic ratio in clinical radioimmunotherapy studies are needed. Limitations to radioimmunotherapy include bone marrow suppression due to the long circulating half-life of radiolabeled monoclonal antibodies (mAbs) and heterogeneous tumor penetration of the high-molecular-weight mAb. An approach to overcome these problems is the use of genetically engineered mAbs. The engineered mAb discussed in this paper contains a deletion in the constant region of the mAb that increases its tumor penetration and blood clearance compared with the intact mAb. Radiolabeling of this mAb should lead to a similar radiation-absorbed dose to tumor compared with the intact mAb, but reduce the radiation absorbed dose to bone marrow. In addition, low or variable expression of tumor-associated target antigens or receptors may lead to low or heterogeneous tumor uptake of radiolabeled mAbs. This report also discusses a novel approach toward systemic radiotherapy that combines gene transfer techniques (to increase tumor receptor expression) with radiolabeled peptides that target the induced receptor. The radiolabeled peptides achieve good tumor uptake, rapid tumor penetration, and rapid blood clearance. A humanized construct of the CC49 (HuCC49) high-affinity anti-TAG-72 mAb, as well as a construct with the CH2 region deleted (HuCC49deltaCH2), were labeled with 131I and 177Lu. Biodistribution of the radiolabeled constructs was evaluated 24 h after regional i.p. injection in athymic nude mice bearing i.p. LS174T human colon cancer xenografts. The 131I-HuCC49deltaCH2 showed a median tumor uptake of 5.5% ID/g which was similar to that of 131I-HuCC49 at 5.2% ID/g. However, the median blood concentration of 131I-HuCC49deltaCH2 was 0.2% ID/g which was significantly lower than 0.8% ID/g for 1311-HuCC49. The uptake of the constructs in other normal tissues were similar. The 177Lu-HuCC49deltaCH2 showed a median tumor uptake of 9.4% ID/g, which was slightly higher than that of 177Lu-HuCC49 at 7.9% ID/g. The median blood concentration of 177Lu-HuCC49deltaCH2 was 0.2% ID/g, which was significantly lower than 0.4% ID/g for 177Lu-HuCC49. The uptake of the antibody constructs in other normal tissues were similar except for the kidney. The tumor:blood ratios of 177Lu-HuCC49 and 177Lu-HuCC49deltaCH2 were 19.4 and 60.2, respectively, at 24 h after injection. The purpose of the second aspect of the study was to determine the biodistribution of 64Cu-1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA)-octreotide in a human ovarian cancer model induced to express human somatostatin receptor subtype 2 (SSTr2) using gene transfer techniques as a prelude to future therapy studies. Mice bearing i.p. SKOV3.ip1 tumors transduced with an adenoviral vector encoding the cDNA for SSTr2 (AdSSTr2) and injected i.p. with 64Cu-TETA-octreotide showed a median uptake of 24.3% ID/g in tumor at 4 h postinjection compared with 4.9% ID/g at 18 h after injection. Also, tumor uptake of 64Cu-TETA-octreotide at 4 h was not significantly different when administered either 2 or 4 days after injection of AdSSTr2 (P = 0.076). 64Cu-TETA-octreotide should be useful for targeted radiotherapy against tumors that are genetically induced to express high levels of SSTr. These two novel targeting strategies show promise for improved cancer radioimmunotherapy.

Oligonucleotide-europium complex conjugate designed to cleave the 5' cap structure of the ICAM-1 transcript potentiates antisense activity in cells.

The 5' cap structure of mRNA is a N7 methylated guanosine residue that is linked by a 5'-5' triphosphate linkage to the 5'-terminus of cellular and viral RNAs synthesized by RNA polymerase II. This unique structure facilitates several processes of mRNA metabolism, including splicing, nucleocytoplasmic transport,initiation of translation, and degradation. Previous research has demonstrated that the lanthanide macrocycle complex, Eu(THED)3+, effectively cleaves the 5' cap structure of mRNA in solution by nucleophilic attack of the triphosphate linkage via the metal-activated hydroxyethyl group of the THED ligand. This report shows that attachment of a Eu(THED)3+analog to the 3'-terminus of an antisense oligonucleotide, which targets the 5'-terminus of the intercellular adhesion molecule 1 mRNA, potentiates the inhibitory activity of the antisense oligonucleotide in cytokine-treatedendothelial cells.

Spectrophotometric method for determination of bifunctional macrocyclic ligands in macrocyclic ligand-protein conjugates.

A simple spectrophotometric assay for determination of bifunctional polyazacarboxylate-macrocyclic ligands of different sizes that are conjugated to proteins has been developed for: 12-membered macrocycle DOTA (2-[4-nitrobenzyl]-1, 4, 7, 10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) and analogs, the 15-membered PEPA macrocycle (2-[4-nitrobenzyl]-1,4,7,10,13-pentaazacyclopentadecane-N,N',N'',N ''',N''''-pentaacetic acid), and the large 18-membered macrocycle HEHA (1,4,7,10,13,16-hexaazacyclooctadecane-N,N',N'',N''',N'''',N'''''- hexaacetic acid). The method is based on titration of the blue-colored 1:1 Pb(II)-Arsenazo II (AAIII) complex with the polyazacarboxylate macrocyclic ligand in the concentration range of 0-2.5 microM, wherein color change occurring upon transchelation of the Pb(II) from the AAIII to the polyazamacrocyclic ligand is monitored at 656 nm. The assay is performed at ambient temperature within 20 min without any interfering interaction between the protein and Pb(II)-AA(III) complex. Thus, this method also provides a ligand-to-protein ratio (L/P ratio) that reflects the effective number of ligands per protein molecule available to radiolabeling. The method is not suitable for 14-membered TETA macrocycle (2-[4-nitrobenzyl]-1, 4, 8, 11-tetraazacyclotetradecane N,N',N'',N'''-tetraacetic acid) because of low stability constant of Pb(II)-TETA complex. The method is rapid, simple and may be customized for other polyazacarboxylate macrocyclic ligands.

Improved synthesis of the bifunctional chelating agent 1,4,7,10-tetraaza-N-(1-carboxy-3-(4-nitrophenyl)propyl)-N',N'',N'''-tri s(acetic acid)cyclododecane (PA-DOTA).

A concise synthesis of the bifunctional chelating agent 1,4,7,10-tetraaza-N-(1-carboxy-3-(4-nitrophenyl)propyl)-N',N'',N'' '-tris(acetic acid)cyclododecane (PA-DOTA) is reported. Difficulties involving the production of partially alkylated products and their removal have been addressed and obviated. After the pure nitro form of PA-DOTA was obtained, conversion to the isothiocyanato form PA-DOTA (1, conjugation to HuCC49 and HuCC49deltaCH2 monoclonal antibodies was achieved. Subsequent radiolabeling with 177Lu was performed, demonstrating a useful bifunctional chelating agent suitable for clinical radioimmunotherapy applications.