Yttrium-90-Labeled Anti-Glypican 3 Radioimmunotherapy Halts Tumor Growth in an Orthotopic Xenograft Model of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the second most lethal malignancy globally and is increasing in incidence in the United States. Unfortunately, there are few effective systemic treatment options, particularly for disseminated disease. Glypican-3 (GPC3) is a proteoglycan cell surface receptor overexpressed in most HCCs and provides a unique target for molecular therapies. We have previously demonstrated that PET imaging using a 89Zr-conjugated monoclonal anti-GPC3 antibody (αGPC3) can bind to minute tumors and allow imaging with high sensitivity and specificity in an orthotopic xenograft mouse model of HCC and that serum alpha-fetoprotein (AFP) levels are highly correlated with tumor size in this model. In the present study, we conjugated 90Y, a high-energy beta-particle-emitting radionuclide, to our αGPC3 antibody to develop a novel antibody-directed radiotherapeutic approach for HCC. Luciferase-expressing HepG2 human hepatoblastoma cells were orthotopically implanted in the livers of athymic nude mice, and tumor establishment was verified at 6 weeks after implantation by bioluminescent imaging and serum AFP concentration. Tumor burden by bioluminescence and serum AFP concentration was highly correlated in our model. Yttrium-90 was conjugated to αGPC3 using the chelating agent 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and injected via the tail vein into the experimental mice at a dose of 200 μCi/mouse or 300 μCi/mouse. Control mice received DOTA-αGPC3 without radionuclide. At 30 days after a single dose of the radioimmunotherapy agent, mean serum AFP levels in control animals increased dramatically, while animals treated with 200 μCi only experienced a minor increase, indicating cessation of tumor growth, and animals treated with 300 μCi experienced a reduction in serum AFP concentration, indicating tumor shrinkage. Mean tumor-bearing liver weight in control animals was also significantly greater than that in animals that received either dose of 90Y-αGPC3. These results were achieved without significant toxicity as measured by body condition scoring and body weight. The results of this preclinical pilot demonstrate that GPC3 can be used as a target for radioimmunotherapy in an orthotopic mouse model of HCC and may be a target of clinical significance, particularly for disseminated HCC.

A preclinical model of CD38-pretargeted radioimmunotherapy for plasma cell malignancies

The vast majority of patients with plasma cell neoplasms die of progressive disease despite high response rates to novel agents. Malignant plasma cells are very radiosensitive, but the potential role of radioimmunotherapy (RIT) in the management of plasmacytomas and multiple myeloma has undergone only limited evaluation. Furthermore, CD38 has not been explored as a RIT target despite its uniform high expression on malignant plasma cells. In this report, both conventional RIT (directly radiolabeled antibody) and streptavidin-biotin pretargeted RIT (PRIT) directed against the CD38 antigen were assessed as approaches to deliver radiation doses sufficient for multiple myeloma cell eradication. PRIT demonstrated biodistributions that were markedly superior to conventional RIT. Tumor-to-blood ratios as high as 638:1 were seen 24 hours after PRIT, whereas ratios never exceeded 1:1 with conventional RIT. (90)Yttrium absorbed dose estimates demonstrated excellent target-to-normal organ ratios (6:1 for the kidney, lung, liver; 10:1 for the whole body). Objective remissions were observed within 7 days in 100% of the mice treated with doses ranging from 800 to 1,200 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin, including 100% complete remissions (no detectable tumor in treated mice compared with tumors that were 2,982% ± 2,834% of initial tumor volume in control animals) by day 23. Furthermore, 100% of animals bearing NCI-H929 multiple myeloma tumor xenografts treated with 800 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin achieved long-term myeloma-free survival (>70 days) compared with none (0%) of the control animals.

A comparative evaluation of conventional and pretargeted radioimmunotherapy of CD20-expressing lymphoma xenografts

Radioimmunotherapy with anti-CD20 monoclonal antibodies is a promising new treatment approach for patients with relapsed B-cell lymphomas. However, the majority of patients treated with conventional radiolabeled anti-CD20 antibodies eventually have a relapse because the low tumor-to-blood and tumor-to-normal organ ratios of absorbed radioactivity limit the dose that can be safely administered without hematopoietic stem cell support. This study assessed the ability of a streptavidin-biotin "pretargeting" approach to improve the biodistribution of radioactivity in mice bearing Ramos lymphoma xenografts. A pretargeted streptavidin-conjugated anti-CD20 1F5 antibody was infused, followed 24 hours later by a biotinylated N-acetylgalactosamine-containing "clearing agent" and finally 3 hours later by (111)In-labeled DOTA-biotin. Tumor-to-blood ratios were 3:1 or more with pretargeting, compared with 0.5:1 or less with conventional (111)In-1F5. Tumor-to-normal organ ratios of absorbed radioactivity up to 56:1 were observed with pretargeting, but were 6:1 or less with conventional (111)In-1F5. Therapy experiments demonstrated that 400 microCi (14.8 MBq) or more of conventional (90)Y-1F5 was required to obtain major tumor responses, but this dose was associated with lethal toxicity in 100% of mice. In marked contrast, up to 800 microCi (29.6 MBq) (90)Y-DOTA-biotin could be safely administered by the pretargeting approach with only minor toxicity, and 89% of the mice were cured. These data suggest that anti-CD20 pretargeting shows great promise for improving current therapeutic options for B-cell lymphomas and warrants further preclinical and clinical testing.

Biotin reagents for antibody pretargeting. 5. Additional studies of biotin conjugate design to provide biotinidase stability

An investigation was conducted in which the stabilities of four structurally different biotin derivatives were assessed with regard to biotinamide bond hydrolysis by the enzyme biotinidase. The biotin derivatives studied contained an extra methylene in the valeric acid chain of biotin (i.e., homobiotin), or contained conjugated amino acids having hydroxymethylene, carboxylate, or acetate functionalities on a methylene alpha to the biotinamide bond. The biotinidase hydrolysis assay was conducted on biotin derivatives that were radioiodinated at high specific activity, and then subjected to diluted human serum at 37 degrees C for 2 h. After incubation, assessment of biotinamide bond hydrolysis by biotinidase was readily achieved by measuring the percentage of radioactivity that did not bind with avidin. As controls, an unsubstituted biotin derivative which is rapidly cleaved by biotinidase and an N-methyl-substituted biotin derivative which is stable to biotinidase cleavage were included in the study. The results indicate that increasing the distance from the biotin ring structure to the biotinamide bond by one methylene only decreases the rate of biotinidase cleavage, but does not block it. The data obtained also indicate that placing a hydroxymethylene, carboxylate, or acetate alpha to the biotinamide bond is effective in blocking the biotinamide hydrolysis reaction. These data, in combination with data previously obtained, which indicate that biotin derivatives containing hydroxymethylene or carboxylate moieties retain the slow dissociation rate of biotin from avidin and streptavidin [Wilbur, D. S., et al. (2000) Bioconjugate Chem. 11, 569-583], strongly support incorporation of these structural features into biotin derivatives being used for in vivo targeting applications.

Biotin reagents for antibody pretargeting. 4. Selection of biotin conjugates for in vivo application based on their dissociation rate from avidin and streptavidin

An investigation was conducted to determine the affect of structural variation of biotin conjugates on their dissociation rates from Av and SAv. This information was sought to help identify optimal biotin derivatives for in vivo applications. Fifteen biotin derivatives were conjugated with a cyanocobalamin (CN-Cbl) derivative for evaluation of their "relative" dissociation rates by size exclusion HPLC analysis. Two biotin-CN-Cbl conjugates, one containing unaltered biotin and the other containing iminobiotin, were prepared as reference compounds for comparison purposes. The first structural variations studied involved modification of the biotinamide bond with a N-methyl moiety (i.e., sarcosine conjugate), lengthening the valeric acid side chain by a methylene unit (i.e., homobiotin), and replacing the biotinamide bond with thiourea bonds in two conjugates. The rate of dissociation of the biotin-CN-Cbl derivative from Av and SAv was significantly increased for biotin derivatives containing those structural features. Nine additional biotin conjugates were obtained by coupling amino acids or functional group protected amino acids to the biotin moiety. In the conjugates, the biotin moiety and biotinamide bond were not altered, but substituents of various sizes were introduced alpha to the biotinamide bond. The results obtained from HPLC analyses indicated that the rate of dissociation from Av or SAv was not affected by small substituents alpha to the biotinamide (e.g., methyl, hydroxymethyl, and carboxylate groups), but was significantly increased when larger functional groups were present. On the basis of the results obtained, it appears that biotin conjugates which retain an unmodified biotin moiety and have a linker molecule conjugated to it that has a small functional group (e.g., hydroxymethylene or carboxylate) alpha to the biotinamide bond are excellent candidates for in vivo applications. These structural features are obtained in the biotin amino acid conjugates: biotin-serine, biotin-aspartate, biotin-lysine, and biotin-cysteine. Importantly, these biotin derivatives can be readily conjugated with other molecules for specific in vivo applications. In our studies, these derivatives will be used in the design of new biotin conjugates to carry radionuclides for cancer therapy using the pretargeting approach.

Evaluation of biotin-dye conjugates for use in an HPLC assay to assess relative binding of biotin derivatives with avidin and streptavidin

In this investigation, studies were conducted to determine if size exclusion HPLC could be used to assess relative association rates (on-rates) and dissociation rates (off-rates) of biotin derivatives from avidin (Av) and streptavidin (SAv). For easy detection and quantification of biotin derivatives, molecules that can be detected by UV absorbance were conjugated to biotin. Concern that conjugation of the chromophoric moieties (dyes) might affect biotin binding with Av and SAv or might interact with the HPLC column led to evaluation of 10 biotin-dye conjugates. The dyes conjugated with biotin included dansyl, cyanocobalamin (CN-Cbl), coumarin 343, Lissamine-rhodamine, fluorescein, Cascade Blue, Lucifer Yellow, Oregon Green, tetramethylrhodamine, and Alexa Fluor 594. The biotin-dye conjugates were initially evaluated to determine their peak characteristics on two different size exclusion HPLC columns. Measurement of the percent of biotin-dye conjugate bound with Av in the presence of an equal quantity of biotin provided an association rate relative to biotin. All of the biotin-dyes tested had association rates within a factor of 3x (slower) that of biotin. The relative dissociation rate of biotin-dye conjugates was assessed by challenging the biotin conjugate bound to Av or SAv with a large excess of biotin. All of the initial biotin-dye conjugates tested bound Av and SAv tightly resulting in very slow dissociation rates. From the biotin-dye conjugates studied, biotin-CN-Cbl, 6b, was selected as the best conjugate for the HPLC assay. To test the HPLC assay, an iminobiotin-CN-Cbl conjugate, 13a, and a biotin-sarcosine-CN-Cbl conjugate, 13b, were synthesized. The fact that the iminobiotin does not bind with Av at physiological pH was easily detected in the size exclusion HPLC assay. The biotin-sarcosine-CN-Cbl conjugate was expected to have a more rapid dissociation rate than the other biotin-dye conjugates. This was confirmed in that HPLC assay. Although 13b bound tightly with Av in the absence of added biotin, it was completely released within 1 h when challenged by an excess of biotin. A slower dissociation of 13b was noted with SAv. The results obtained indicate that CN-Cbl conjugates of biotin derivatives can be used to determine relative on-rates and off-rates of biotin derivatives with Av and SAv. The studies also demonstrated that the biotin-CN-Cbl conjugate, 6b, can be used as a reference compound to compare on-rates and off-rates of nonchromophoric biotin derivatives.

Development of new biotin/streptavidin reagents for pretargeting

The high affinity of biotin for streptavidin has made this pair of molecules very useful for in vivo applications. To optimize reagents for one potential in vivo application, antibody-based pretargeting of cancer, we have prepared a number of new biotin and streptavidin derivatives. The derivatives developed include new radiolabeled biotin reagents, new protein biotinylation reagents, and new biotin multimers for cross-linking and/or polymerization of streptavidin. We have also modified streptavidin by site-directed mutation and chemical modification to improve its in vivo characteristics, and have developed new reagents for cross-linking antibody fragments with streptavidin. A brief overview of these new reagents is provided.

Radioiodination of cyanocobalamin conjugates containing hydrophilic linkers: preparation of a radioiodinated cyanocobalamin monomer and two dimers, and assessment of their binding with transcobalamin II

This report describes an investigation aimed at preparation of radioiodinated cyanocobalamin (CN-Cbl) monomers and dimers with improved water solubility and decreased nonspecific binding. In the investigation, synthesis and radioiodination reactions of one monomeric and two dimeric CN-Cbl derivatives were conducted. The initial step in the synthesis of the CN-Cbl derivatives was mild acid hydrolysis of CN-Cbl, 1, followed by separation of the resultant corrin ring b-, d-, and e-monocarboxylate isomers. The investigation was limited to preparation of conjugates of CN-Cbl-e-carboxylate, 2, as earlier studies had shown binding of that isomer with recombinant human transcobalamin II (rhTCII) was similar to CN-Cbl. In a second synthetic step, the hydrophilic linker moiety, 4,7,10-trioxa-1,13-tridecandiamine, 3, was conjugated with 2 to form the adduct, 4. The synthesis of a monomeric CN-Cbl derivative, 6a, which can be used for radioiodination, was accomplished by reaction of 4 with p-tri-n-butylstannylbenzoate tetrafluorophenyl (TFP) ester, 5a. Two CN-Cbl dimers containing the arylstannane radioiodination moiety were also synthesized. The first dimer, 8a, was synthesized by cross-linking 4 with a stannylbenzoyl-aminoisophthalate di-TFP ester, 7a. The second dimer, 11a, was synthesized by reacting benzene tricarboxylate tri-TFP ester, 10, in a stepwise manner with 1 equiv of the adduct of 5a and 3 (forming 9a), followed by 2 equiv of 4. Iodobenzoate HPLC standards, 6b, 8b, and 11b, used in the radioiodination studies, were prepared in a manner similar to that of the stannylbenzoate derivatives. Radioiodinations were performed by reacting 6a, 8a, or 11a with N-chlorosuccinimide and Na[(125)I]I in methanol under neutral conditions. Radiochemical yields of 17-42% were obtained. Evaluation of the binding properties of radiolabeled CN-Cbl conjugates with rhTCII showed that the dimer of CN-Cbl, 11b, bound more avidly than the monomer, 6b, and that the binding affinity of the dimer is essentially equivalent to that of unmodified CN-Cbl. Incubation of radioiodinated monomer, [(125)I]6b, and dimer, [(125)I]11b, with rhTCII followed by size-exclusion chromatographic analysis provided data that the monomer bound one rhTCII molecule whereas two rhTCII molecules were bound to approximately 30% of the dimer.

Biotin reagents for antibody pretargeting. 3. Synthesis, radioiodination, and evaluation of biotinylated starburst dendrimers

We are investigating the hypothesis that biotin multimers can be used with streptavidin and monoclonal antibody conjugates in cancer pretargeting protocols to provide a method of increasing the amount of radioactivity bound on cancer cells in patients. As part of that investigation, a series of biotinylated Starburst dendrimers (BSBDs) have been prepared and evaluated in vitro and in vivo. In this study, a new biotinidase-stabilized, water-solubilizing biotinylation reagent was prepared and reacted with Starburst (PAMAM) dendrimers, generations 0, 1, 2, 3, and 4. The reaction conditions employed resulted in perbiotinylation of generation 0 (four biotin moieties conjugated), generation 1 (eight biotin moieties conjugated), generation 2 (16 biotin moieties conjugated), and generation 3 (32 biotin moieties conjugated). With generation 4, incomplete biotinylation was achieved resulting in the largest portion of that BSBD having 51 biotin moieties (of 64 possible) conjugated. The ability of each BSBD to cross-link streptavidin (SAv) was examined in an in vitro assay. In that assay, an assessment was made of the quantity of [125I]SAv bound with polystyrene-bound SAv after treatment with the synthesized BSBDs. All BSBDs cross-linked the polystyrene-bound SAv with [125I]SAv; however, the amount of [125I]SAv bound varied with the different BSBDs. Roughly 1 equiv of [125I]SAv was bound when Starburst dendrimers containing three or four biotin moieties (generation 0) were used. Two equivalents were bound with BSBD generation 1, and 4 equiv were bound with BSBDs generations 2, 3, and 4. To assess the distribution of BSBDs generations 0, 1, and 2 in mice (at 4 h postinjection), a method was developed for radioiodinating them using the NHS ester of p-[125I]iodobenzoate ([125I]PIB). It was found that the radioiodinated BSBDs had low blood concentrations (i.e., 0.13-0.20% ID/g) at the 4 h time point. In fact, most tissues examined had low concentrations of biotinylated dendrimers, except kidney and liver. Kidney had the highest concentration of [125I]-labeled BSBDs, and its concentration increased with increasing size and charge of dendrimer (e.g., 8-48% ID/g). On the basis of the increased radioactivity observed in the in vitro assay and the rapid clearance from blood in mice, additional in vivo studies with perbiotinylated Starburst dendrimer, generation 2, are planned.

Engineering the isoelectric point of a renal cell carcinoma targeting antibody greatly enhances scFv solubility

BACKGROUND

The murine A6H monoclonal antibody targets a cell surface antigen associated with renal cell carcinoma with high specificity and excellent biodistribution properties. Tumor to blood ratios of > 40:1 have been achieved in clinical studies.

OBJECTIVES

In order to generate an antibody engineering system that would allow the construction of improved derivatives for diagnostics and therapeutics, a single-chain Fv antibody (scFv) derived from A6H was constructed. The initial single-chain Fv, constructed with a cysteine residue and hexa-histidine sequence at the C-terminus, displayed a limited solubility of 100 microg/ml at pH 7.4. The low solubility and refolding yield of the original single-chain Fv required that a more soluble variant be designed and constructed.

STUDY DESIGN

We hypothesized that lowering the pI of the scFv antibody away from the physiological range would yield a more soluble antibody. A derivative was thus subsequently engineered with five glutamic acid residues followed by the cysteine and hexa-histidine residues. The cysteine was included to provide a conjugation site for future radiolabeling studies.

RESULTS

The redesigned A6H single-chain Fv has a predicted pI of 6.1, relative to 7.5 for the native scFv. The redesigned A6H scFv displayed a greatly enhanced solubility of > 15 mg/ml at pH 7.4. Both the original scFv and the redesigned single-chain Fv exhibited a strong tendency to form dimers and soluble high molecular weight aggregates. The monomer and disulfide bonded dimer were separated from the aggregates and complete cell binding isotherms were obtained, demonstrating that the purified A6H scFv retains much of the activity of the parent monoclonal.

CONCLUSION

The addition of glutamic acid to the C-terminus of poorly soluble scFv antibodies could provide a straightforward avenue for improving their solubility properties. The increased solubility of the A6H scFv allowed the purification of the monomeric and dimeric species from the soluble aggregated species.

Streptavidin in antibody pretargeting. 2. Evaluation Of methods for decreasing localization of streptavidin to kidney while retaining its tumor binding capacity

An investigation has been conducted to determine if the kidney localization of recombinant streptavidin can be decreased to improve its characteristics in pretargeting protocols. Three different methods of accomplishing this were evaluated. The first method, blocking kidney uptake with a preadministration of recombinant streptavidin in which biotin occupied all of the binding sites, was unsuccessful. In a second method, l-lysine administration was used to block kidney localization. This method worked well, decreasing the concentration to 29% of the unmodified amount at 8 h postinjection. However, this method suffered from a requirement for constant infusion of lysine during the period of observation. A third method, use of succinylated recombinant streptavidin, was found to be the best approach. Succinylation of streptavidin was readily accomplished with very good protein recovery. With the succinylated streptavidin, the kidney concentration was only 14% of that of nonmodified streptavidin at 4 h postinjection. While these results demonstrated that the concentration of streptavidin could be decreased in the kidney, it was important to assess whether the tumor colocalization of streptavidin with biotinylated antibody was affected under those conditions. As part of our continuing investigation of pretargeting, a new water-solubilized biotinidase-stabilized biotinylation reagent was prepared. Using that reagent in a pretargeting experiment, an equivalent quantity of succinylated recombinant streptavidin as biotinylated antibody Fab' was localized in a tumor xenograft model. In that experiment, the kidney concentration was decreased to less than 10% of that obtained with unmodified recombinant streptavidin at 24 h postinjection. The results of our investigation have demonstrated that succinylation of streptavidin improves its distribution characteristics for pretargeting applications. The fact that succinylated streptavidin has no specific tissue localization should allow its use as a carrier of radioactivity in "two-step" pretargeting protocols.

Streptavidin in antibody pretargeting. Comparison of a recombinant streptavidin with two streptavidin mutant proteins and two commercially available streptavidin proteins

In this investigation, a comparison of wild type recombinant streptavidin (r-SAv) with two genetically engineered mutant r-SAv proteins was undertaken. The investigation also included a comparison of the r-SAv with two streptavidin (SAv) proteins from commercial sources. In vitro characterization of the SAv proteins was conducted by HPLC, SDS-PAGE, IEF, and electrospray mass spectral analyses. All SAv proteins studied appeared to be a single species by size exclusion chromatography (HPLC) and SDS-PAGE analyses, but multiple species were noted in the IEF and MS analyses. In vivo comparisons of the SAv proteins were accomplished with dual isotope-labeled SAv in athymic mice. In an initial experiment, tissue localization of r-[131I]SAv directly radiolabeled using chloramine-T was compared with r-SAv radiolabeled with the N-hydroxysuccinimidyl p-iodobenzoate conjugate ([125I]-PIB), a radioiodination reagent that has been shown to result in iodine-labeled proteins which are stable to in vivo deiodination. The data obtained indicated that there is little difference in the distribution (except kidney localization) when r-SAv labeled by the two methods. Data obtained from comparison of r-[131I]SAv with a disulfide-stabilized r-SAv mutant (r-SAv-H127C), a C-terminal cysteine-containing r-SAv mutant (r-[125I]SAv-S139C), and two 125I-labeled SAv proteins obtained from commercial sources indicated that their distributions were quite similar, except the kidney concentrations were generally lower than that of r-[131I]SAv. On the basis of the similar distributions of the SAv proteins studied, it appears that the r-SAv mutants may be interchanged for the (wild type) r-SAv in pretargeting studies. Further, the similarity of distributions with two commercially available SAv proteins suggests that the results obtained in our studies and those of other groups may be directly compared (with consideration of animal model, sacrifice time, etc.).

Biotin reagents for antibody pretargeting. 2. Synthesis and in vitro evaluation of biotin dimers and trimers for cross-linking of streptavidin

Polymerization and/or cross-linking of recombinant streptavidin (r-SAv) with biotin derivatives containing two biotin moieties (biotin dimers) or three biotin moieties (biotin trimers) has been investigated as a model for reagents to be used to increase the amount of radioactivity on cancer cells in tumor pretargeting protocols. In the investigation, six biotin dimers and three biotin trimers were synthesized. Most biotin derivatives synthesized had ether containing linker molecules incorporated to improve their aqueous solubility. The synthesized biotin dimers contained linker moieties which provided distances (when fully extended) of 13-49 A between biotin carboxylate carbon atoms, and the biotin trimers contained linker moieties which provided distances of 31-53 A between any two biotin carboxylate atoms. All of the biotin derivatives were evaluated for their ability to polymerize r-SAv in solution. When the biotin derivatives were mixed with r-SAv, none of the biotin dimers caused polymerization, but all of the biotin trimers resulted in complete polymerization. Some of the biotin dimers did cross-link r-SAv (to form r-SAv dimers, trimers, etc.), but the percentage of cross-linking was low (< or = 40%). The length of the linker molecule was important in cross-linking of biotin dimers. While linkers which provided distances of 13 and 19 A between biotin carboxylate carbon atoms did not result in cross-linking, a linker which provided a 17 A distance resulted in a small (< or = 10%) amount of cross-linking. Also cross-linking was increased in biotin dimers with linkers which provided distances between biotin carboxylate carbon atoms of > or = 23 A. Cross-linking of streptavidin bound in polystyrene wells with biotin dimers and trimers was also examined. In those experiments, an excess of each biotin derivative was incubated at 37 degrees C for 10-30 min in polystyrene wells containing bound SAv. After the excess biotin derivative was rinsed from the wells, an excess of r-[125I]SAv was incubated for another 10-30 min. The amount of r-[125I]SAv bound after rinsing the excess from the wells was an indicator of the extent of cross-linking that occurred. The process of alternating additions of reagents was repeated four times to demonstrate that bound radioactivity could be increased with each addition of [125I]SAv. The results of cross-linking r-SAv in polystyrene wells paralleled results from cross-linking in solution.

Cobalamin analogues modulate the growth of leukemia cells in vitro

Analogues of cyanocobalamin (CN-Cbl), with functional groups attached to either the various propionamide groups of the corrin ring or to the ribose-nucleotide linker arm, have been evaluated in a cobalamin (Cbl)-dependent in vitro cell growth assay. In this bioassay, CN-Cbl supported, in a dose-dependent manner, the growth of the murine lymphoma BW5147 and the Cbl carrier protein, human apo-transcobalamin II, reduced the required concentration of Cbl by 100-1000-fold. Any chemical modification of Cbl decreased its ability to support cellular viability and proliferation, with several of the modifications abrogating activity completely. All of the Cbl analogues that promoted growth required the presence of apo-transcobalamin II for the optimal support of cell growth. Generally, Cbl analogues modified at the d-position of the corrin ring and, to a lesser degree, analogues modified at the b- position supported cell growth, whereas analogues with modifications at the e-position did not support cell growth. Mixing experiments demonstrated an inverse order of potency of Cbl analogues to inhibit cell growth. Thus, Cbl analogues with modifications at the e-position were potent inhibitors, whereas b-analogues exhibited only partial inhibitory activity at high molar excess, and d-analogues had no inhibitory activity at all. These results indicate that modifications at the e-position of Cbl abolish the ability of Cbl to support cell growth and generate potent inhibitors of Cbl-dependent cell growth.

Biotin reagents for antibody pretargeting. Synthesis, radioiodination, and in vitro evaluation of water soluble, biotinidase resistant biotin derivatives

As part of our development of antibody pretargeting for cancer therapy, an investigation has been conducted to examine the stability of water solubilized, radioiodinated biotin derivatives toward biotinidase degradation in mouse and human serum. Eight new biotin derivatives were synthesized to conduct the study. The biotin derivatives synthesized contained (1) the biotin moiety, (2) a water solubilizing linker moiety, (3) p-iodobenzoate or p-tri-n-butylstannylbenzoate moieties, and (4) in some of the compounds, N-methyl or alpha-methyl containing moieties were added to block biotinidase activity. The linker moiety, 4,7,10-trioxa-1,13-tridecanediamine, 5, was included in the biotin derivatives to improve their water solubility, and it also functioned as a 17 A spacer between the biotin and the benzoyl moieties. Four of the new biotin derivatives (12, 14, 22, and 23) contained a p-tri-n-butylstannylbenzoyl moiety as precursors which could be radioiodinated in the last synthetic step. The other four biotin derivatives (13, 15, 24, and 25) contained p-iodobenzoyl moieties and were used as HPLC reference standards. Initial studies involved radioiodination of 12 to yield [125I]13. Radioiodinated 13, which did not contain a moiety for blocking biotinidase activity, was found to be rapidly degraded in both mouse and human serum at 37 degrees C. Derivatives which were designed to be stable to biotinidase incorporated N-methyl and alpha-methyl moieties adjacent to the biotin carboxylate group. In one set of biotin derivatives (14 and 15), the N-methyl moiety was obtained by incorporating N,N-dimethyl-4,7,10-trioxa-1,13-tridecanediamine, 9, as a linker in the place of 5. In the second set of biotin derivatives (22 and 24), the N-methyl moiety was introduced by incorporating a sarcosine (N-methylglycine) moiety between biotin and 5. The radioiodinated N-methyl containing biotin derivatives [125I]15 and [125I]24 were found to be very stable to biotinidase degradation. An alpha-methyl group was obtained in a pair of biotin derivatives (23 and 25) by incorporating a 3-aminobutyric acid moiety between biotin and 5. The radioiodinated alpha-methyl containing derivative, [125I]25, was found to have an intermediate stability with regards to biotinidase degradation.

Synthesis of cobalamin dimers using isophthalate cross-linking of corrin ring carboxylates and evaluation of their binding to transcobalamin II

Several cobalamin (Cbl) dimers have been prepared for evaluation as potential antiproliferative agents in the treatment of AIDS-related lymphoma. The Cbl dimers were synthesized by cross-linking Cbl carboxylates, produced by acid hydrolysis of the b-, d-, and e-propionamide side chains of cyanocobalamin (CN-Cbl), through an isophthalate molecule. Linking molecules were used between the Cbl carboxylates and the isophthalate moiety. The linkers were incorporated to provide a distance between the two Cbl molecules such that the dimeric Cbls might bind two molecules of transcobalamin II (TCII), the Cbl transport protein in plasma. Initially, the linking moiety used was 1,12-diaminododecane, but the resulting dimers had low aqueous solubility. To improve the solubility of the dimers, 4,7,10-trioxa-1,13-tridecanediamine was employed as the linking moiety. This improved the water solubility of the dimers considerably, while retaining the distance between the Cbl molecules at 41-42 A (fully extended). To introduce additional substitution on Cbl dimers, 5-aminoisophthalic acid was used as the cross-linking reagent. p-Iodobenzoyl and p-(tri-n-butylstannyl)benzoyl conjugates of 5-aminoisophthalate were synthesized and used to prepare Cbl dimers. The stannylbenzoyl-conjugated Cbl dimers were prepared as precursors to be used in radioiodination reactions, and the iodobenzoyl-conjugated Cbl dimers were prepared as HPLC standards for the radioiodinated product. Attempts to iodinate/radioiodinate the stannylbenzoyl Cbl dimers were unsuccessful. Although an explanation for this is not readily apparent, the failure to react may be due to the lipophilicity of the linker used and the steric environment of the two Cbl moieties. A biotinylated derivative of 5-aminoisophthalate was also synthesized and used to prepare biotinylated-Cbl dimers. In a competitive rhTCII binding assay with [57Co]CN-Cbl, Cbl dimers containing the lipophilic diaminododecane linking moiety had decreased binding avidities compared to those of Cbl monomers substituted at the same corrin ring carboxylate. However, Cbl dimers containing the water-solubilizing trioxadiamine linker appeared to have avidities similar to those of the Cbl monomers.

Synthesis of 125I-labeled oligonucleotides from tributylstannylbenzamide conjugates

A rapid and efficient method for the synthesis of 125I-labeled oligodeoxynucleotides ([125I]ODNs) is described. The key intermediates are tributylstannylbenzamide-modified ODNs (Sn-ODNs). Reaction conditions are described for the preparation of 5'-modified Sn-ODNs. Treatment with NaI and chloramine T gave conversion to the desired I-ODN, which was easily isolated by reversed phase chromatography. Thermal denaturation (Tm) studies showed that hybridization properties were not disturbed by the 4-iodobenzamide modification. An [125I]ODN was prepared and characterized by hybridization to 32P-labeled DNA targets. Sequence specific cleavage of the target DNA strand by 125I was measured.

Antibody fragments in tumor pretargeting. Evaluation of biotinylated Fab' colocalization with recombinant streptavidin and avidin

An evaluation of the use of a biotinylated monoclonal antibody Fab' fragment in tumor pretargeting was conducted. As a model system, tumor colocalization of avidin or recombinant streptavidin (r-streptavidin) and the biotinylated Fab' fragment (Fab'-S-biotin) of A6H, an antirenal cell carcinoma antibody, was evaluated in athymic mice bearing human renal cell carcinoma xenografts. A new water soluble sulfhydryl reactive biotinylation reagent, N-(13-N-maleimdo-4, 7,10-trioxatridecanyl)-biotinamide, was synthesized and used for biotinylation of Fab'. A biodistribution of ChT-labeled A6H Fab'-S-biotin was conducted. Data from that distribution indicated that the Fab'-S-biotin localized well (i.e. 28% ID/g at 24 h) to human tumor xenografts in athymic mice. Subsequently, a biodistribution study involving pretargeting radioiodinated A6H Fab'-S-biotin to tumor xenografts, followed by administration of r-streptavidin at 4 or 20 h, was conducted. Specific colocalization of r-streptavidin to tumors containing the A6H Fab'-S-biotin was evident from the data obtained. In a similar biodistribution study, specific colocalization of avidin to tumors pretargeted with A6H Fab'-S-biotin was also observed. The avidin used in the study was radioiodinated with the N-hydroxysuccinimidyl ester of p-[125I]iodobenzoate ([125I]PIB-NHS). Very low concentrations (e.g. 0.35% ID/g) of avidin colocalized at the tumor. To further show that specific colocalization within the tumor xenografts had occurred with biotinylated A6H Fab', radioiodinated avidin and r-streptavidin were co-injected into athymic mice bearing tumor xenografts to obtain their distributions without having biotinylated Fab' present. At 20 h postinjection, only small differences in the blood and tumor concentrations of either protein were observed, indicating that the specific tumor colocalization seen in the previous two biodistributions must have been due to the presence of Fab'-S-biotin. Calculations were conducted to estimate how much r-streptavidin (as a molar ratio) was colocalized. From the data obtained it was estimated that 36-61% of the tumor-localized Fab'-S-biotin molecules were bound with r-streptavidin and 4-23% bound with avidin, under the conditions studied.

Synthesis and nca-radioiodination of arylstannyl-cobalamin conjugates. Evaluation of aryliodo-cobalamin conjugate binding to transcobalamin II and biodistribution in mice

A new method of preparing radiolabeled cobalamin derivatives has been developed. The method involves the use of cobalamin-tri-n-butylstannyl hippurate conjugates as intermediates to obtain radioiodinated cobalamin-iodohippurate conjugates. The arylstannyl functionality was used as an exchangeable group to obtain high specific activity radioiodinations and to circumvent some deleterious side reactions common to cobalamins under electrophilic iodination conditions. The first step in the synthesis of tri-n--butylstannyl hippurate conjugates was to obtain free carboxylate groups on the cobalamin moiety. This was accomplished by mild acid hydrolysis of the b-, d-, or e-propionamide side chains on the corrin ring, followed by careful separation of the isomeric products. The second step was to couple a linking molecule (diaminododecane) to the carboxylate. The final step was to conjugate p-tri-n-butylstannyl hippurate to the cobalamin-diaminododecane adduct. All three isomeric cobalamin-p-tri-n-butylstannyl hippurate conjugates were prepared, as were the corresponding cobalamin-p-iodohippurate conjugates (HPLC standards). Radioiodination reactions were conducted with N-chlorosuccinimide and Na[*I]I in Me OH using conditions previously developed for arylstannylations. However, unlike the previous reactions, a key factor in obtaining the desired radioiodinated cobalamins was that the reaction be conducted under neutral conditions. Isolated yields of 40-65% were obtained for all three cobalamin isomers. Specific activities of 10-33% theoretical were obtained for the radioiodinated cobalamins. Evaluation of competitive binding of (nonradioactive) cobalamin-iodohippurate conjugates with recombinant human transcobalamin II showed that the e-isomer bound nearly as well as [57Co]cyanocobalamin (50%), whereas the b-isomer had decreased binding (6%) and the d-isomer was significantly decreased in its binding (0.7%). Two biodistributions of the radioiodinated e-isomer were conducted in athymic mice. One biodistribution investigated tissue localization in mice bearing a renal cell carcinoma xenograft, and the other biodistribution investigated tissue localization when the radioiodinated cyanocobalamin was mixed with 1% BSA prior to injection. A comparison of the results of the two biodistributions and a discussion of how they relate to previous [57/60Co]cyanocobalamin biodistributions are provided.

Synthesis of cobalamin-biotin conjugates that vary in the position of cobalamin coupling. Evaluation of cobalamin derivative binding to transcobalamin II

Six cobalamin-biotin conjugates have been prepared. The cobalamin-biotin conjugates were prepared to evaluate the effect that the location of attachment had on the binding with transcobalamin II (TCII), the cobalamin binding protein in plasma, and to evaluate their potential use for in vitro and in vivo applications. This study focused only on the effect of binding with TCII. To decrease the possibility of steric problems in binding of the cobalamin conjugates with TCII, and biotin's binding with streptavidin or avidin, moieties of 11-18 atoms in length were used as linkers. Four biotin conjugates were prepared which were attached to the corrin ring of the cobalamin molecule (on b-, c-, d-, and e-side chains). One conjugate was attached to the 5'-OH of the ribose moiety, and another conjugate was attached at the cobalt metal (in place of the cyanide moiety of cyanocobalamin). Competitive binding studies were conducted where various amounts of the cobalamin-biotin conjugates and their precursor cobalamin derivatives competed with [57Co]cyanocobalamin for binding of recombinant human TCII (rhTCII). Evaluation of cobalamin derivatives which were conjugated at the 5'-OH of ribose or the cobalt metal center indicated that conjugation at either of these positions had little effect on binding with rhTCII. However, conjugates where the attachment was made on the corrin ring substituents had a large variation in binding with rhTCII. Conjugates on the e-propionamide side chain had little effect (relative affinity was equal to or decreased less than a factor of 3) on binding with rhTCII, conjugates of the b-isomer had decreased binding (relative affinity decreased less than a factor of 10), conjugates of the d-propionamide had further decreased binding (relative affinity decreased between 44 and 69 times), and conjugates on the c-acetamide group had poor binding to rhTCII (relative affinity decreased between 295 and 1160 times). The significance of the side chains on the corrin ring in providing specificity and high-affinity binding with rhTCII is discussed.

Fast neutron radiotherapy and boron neutron capture therapy: application to a human melanoma test system

Fast neutron radiotherapy has proven to be an effective form of treatment in a selected subset of tumors (salivary gland tumors, sarcomas, and locally-advanced prostate cancer), but has not proven to be more beneficial than conventional photon irradiation for the majority of tumor types upon which it has been tested. Normal tissue tolerance limits preclude simply further escalating the neutron dose. Boron neutron capture (BNC) provides a way of selectively augmenting the radiation dose to the tumor. This process is described, and cell culture and animal model data reviewed. An irradiation configuration was developed where an enhancement of 2.10(-3) for 1 microgram of 10B per gram of tissue was achieved. This is similar to the enhancement achievable in the center of a 20 x 20 cm field envisioned for future applications such as metastases in the brain. A boron concentration of 50 micrograms per gram of tumor tissue leads to a 10% increase in the delivered physical dose in this scenario. The first human test of BNC enhancement of a fast neutron radiotherapy beam using pharmacologically-acceptable doses of orally-administered, 10B-enriched, L-paraboronophenylalanine is reported. An enhancement of tumor response was demonstrated for a melanoma skin nodule test system. Boron levels achieved in blood, skin, and tumors are presented. Future research plans are discussed.

Synthesis and radioiodination of a nido-1,2-carboranyl derivative of 2-nitroimidazole

The synthesis of a nido-carboranyl congener of misonidazole, 1-(3'-nido-carboranyl-2'-hydroxy)propyl-2-nitroimidazole, has been carried out. Alternative methods of preparations were conducted to optimize the chemical yield, with a five step synthesis giving an overall yield (from 1,2-carborane) of 36%. A diastereomeric pair of nido-carboranyl compounds was obtained. The diastereomeric nido-carboranyl misonidazole congeners were (radio)iodinated to yield (> 90%) a mixture of diastereomeric compounds in which the iodine had bonded to a boron atom on the nido-carborane moiety. These compounds will be investigated for their application to boron neutron capture therapy (BNCT) and hypoxia imaging of cancer.

Monoclonal antibody Fab' fragment cross-linking using equilibrium transfer alkylation reagents. A strategy for site-specific conjugation of diagnostic and therapeutic agents with F(ab')2 fragments

An investigation was conducted to evaluate the feasibility of site-selective addition of diagnostic and therapeutic agents to monoclonal antibody F(ab')2 fragments through cross-linking of antibody Fab' fragments. In the investigation, trifunctional equilibrium transfer alkylation cross-link (ETAC) reagents, 4-[2,2-bis[(p-tolylsulfonyl)methyl]acetyl]benzoic acid, 1a, N-[4-[2,2bis[(p-tolylsulfonyl)methyl]acetyl]-benzoyl]-4- (tri-n-butylstannyl)phenethylamine, 3a, and N-[4-[2,2-bis[(p-tolylsulfonyl)methyl]acetyl]- benzoyl]-4-[125,131I]iodophenethylamine, 3b, were synthesized. The ETAC derivatives were reacted with Fab' fragments of an antirenal cell carcinoma antibody (A6H) produced from reduction of F(ab')2 using 1,4-dithiothreitol. Cross-linking of Fab' was obtained to yield a radioiodinated modified F(ab')2, [mF(ab')2], fragment. The cross-linking reaction produced mixed addition products, requiring the desired mF(ab')2 to be separated from radioiodinated Fab' by size exclusion HPLC. Tumor cell binding immunoreactivities varied (60-90%) for five isolated mF(ab')2 preparations but were consistent with other radiolabeled antibody preparations tested on the same day. In vitro stability testing indicated that the mF(ab')2 was reasonably stable toward loss of the ETAC cross-linking reagent, except under strongly basic conditions. Under reducing sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analyses, protein bands believed to be cross-linked heavy chain dimers were observed. Biodistribution of purified radioiodinated A6H mF(ab')2 was conducted in athymic mice bearing a renal cell carcinoma xenograft (TK-82). A nonmodified control A6H F(ab')2, radioiodinated as a p-[125,131I]-iodobenzoyl conjugate, was coinjected for comparison. The radioiodionated mF(ab')2 had a similar distribution to the radioiodinated control at 3.5, 19, and 43 h postinjection. In another study, the distribution of radioiodinated A6H Fab' was evaluated at 4 and 24 h to establish clearance and pharmacokinetics for comparison with the data obtained from the mF(ab')2. The biodistribution data indicated that A6H mF(ab')2 was quite different from that of A6H Fab'. The results from this preliminary study suggest that it may be possible to attach (large polymeric) diagnostic or therapeutic agents to monoclonal antibody F(ab')2 fragments through the use of ETAC reagents.

Boron neutron capture therapy: a mechanism for achieving a concomitant tumor boost in fast neutron radiotherapy

PURPOSE

For many years neutron radiation has been used to treat malignant disease both as fast neutron radiotherapy and as thermal neutron induced boron neutron capture therapy (BNCT). To date, these two approaches have been used independently of one another due to the large difference in neutron energies each employs. In this paper we discuss the potential application of BNCT to enhance the therapeutic effectiveness of a fast neutron radiotherapy beam.

METHODS AND MATERIALS

Measurements are presented for the thermal neutron component that is spontaneously developed as the University of Washington fast neutron radiotherapy beam penetrates a water phantom. The biological effect of this thermalized component on cells "tagged" with boron-10 (10B) is modeled mathematically and the expected change in cell survival calculated. The model is then extended to estimate the effect this enhanced cell killing would have for increased tumor control.

RESULTS

The basic predictions of the model on changes in cell survival are verified with in vitro measurements using the V-79 cell line. An additional factor of 10-100 in tumor cell killing appears achievable with currently available 10B carriers using our present neutron beam. A Poisson model is then used to estimate the change in tumor control this enhanced cell killing would produce in various clinical situations and the effect is sufficiently large so as to be clinically relevant. It is also demonstrated that the magnitude of the thermalized component can be increased by a factor of 2-3 with relatively simple changes in the beam generating conditions.

CONCLUSION

BNCT may provide a means of enhancing the therapeutic effectiveness of fast neutron radiotherapy in a wide variety of clinical situations and is an area of research that should be aggressively pursued.

Enhancement of fast neutron beams with boron neutron capture therapy. A mechanism for achieving a selective, concomitant tumor boost

Both fast neutron radiotherapy and boron neutron capture therapy (BNCT) have been utilized to treat malignant disease. Herein we discuss the potential of combining these treatments to enhance the effectiveness of fast neutron therapy through a concomitant BNCT boost. Using a fast neutron beam generated from a 50 MeV proton on beryllium reaction, we have determined that 0.1% of the beam per microgram of boron-10 per gram of tissue (microgram/g) can be deposited via BNCT. Our mathematical modeling predicts that BNCT enhancement of our beam will lead to an additional 1-2 logs of tumor cell kill for boron-10 concentrations of 30-50 micrograms/g. We have validated this via V-79 cell line in vitro measurements. A Poisson model estimation of how this additional cell kill will influence local tumor control, predicts that BNCT enhancement of fast neutron radiation will lead to a clinically significant improvement in outcome.

Preparation and evaluation of para-[211At]astatobenzoyl labeled anti-renal cell carcinoma antibody A6H F(ab')2. In vivo distribution comparison with para-[125I]iodobenzoyl labeled A6H F(ab')2

A preliminary investigation of an 211At labeled anti-renal cell carcinoma antibody fragment, A6H F(ab')2, was conducted. In the investigation, A6H F(ab')2 was labeled by conjugation with N-succinimidyl p-[211At]astatobenzoate, and the in vivo biodistribution was evaluated in athymic mice bearing TK-82 renal cell carcinoma xenografts. As a control, p-[125I]iodobenzoyl labeled A6H F(ab')2 was coinjected with the astatinated F(ab')2. The data obtained demonstrated that the two radiolabels (211At and 125I) had quite similar distributions, providing evidence that the 211At remained attached to the A6H F(ab')2 in vivo. Further, the astatinated antibody attained a 2:1 tumor-to-blood ratio, and greater than 35:1 tumor-to-muscle ratio, at 4h post-injection, suggesting that this antibody conjugate could be used to evaluate treatment of metastatic renal cell carcinoma in a mouse model.

Synthesis and radioiodination of N-Boc-p-(tri-n-butylstannyl)-L-phenylalanine tetrafluorophenyl ester: preparation of a radiolabeled phenylalanine derivative for peptide synthesis

An investigation to prepare a phenylalanine derivative which could be radioiodinated and used directly in peptide synthesis was conducted. N-Boc-p-(tri-n-butylstannyl)-L-phenylalanine tetrafluorophenyl ester was targeted and synthesized from N-Boc-p-iodo-L-phenylalanine. The requisite aryl stannylation reaction was found to be best conducted using the phenylalanine methyl ester. Thus, N-Boc-p-iodo-L-phenylalanine methyl ester was prepared and stannylated using bis(tributyltin) and tetrakis-(triphenylphosphine)palladium(0) in refluxing toluene to prepare N-Boc-p-(tri-n-butylstannyl)-L-phenylalanine methyl ester. Demethylation with aqueous base was accomplished without racemization to yield N-Boc-p-(tri-n-butylstannyl)-L-phenylalanine. Preparation of the targeted stannylphenylalanine tetrafluorophenyl ester was then accomplished using 2,3,5,6-tetrafluorophenol and 1,3-dicyclohexyl-carbodiimide in anhydrous THF. Iodination and radioiodination reactions of the targeted compound were conducted in MeOH/1% HOAc to yield 83-95% of the desired N-Boc-p-[*I]iodo-L-phenylalanine tetrafluorophenyl ester.

Radiohalogenation of proteins: an overview of radionuclides, labeling methods, and reagents for conjugate labeling

Direct labeling of proteins with radionuclides of iodine will continue to be the method of choice to answer questions addressed in many future studies. However, it seems likely that a increasing number of applications of radiohalogenated proteins will require, or benefit from, conjugate labeling. While many radiohalogen conjugates have been studied, a large proportion of them have only undergone preliminary studies to date, leaving a question of their overall utility. Phenolic conjugates give good radioiodination labeling yields, but mixtures of radiohalogenated products and problems with in vivo stability can be expected. This fact, along with the fact that phenolic compounds do not have a general application to radiohalogens, makes them less attractive than other alternatives. Radiohalogen labeling through the use of organometallic intermediates has proven to be facile, resulting in high yields of high specific activity labeled small-molecule conjugates. Although the choice of which organometallic intermediate to use may depend somewhat on the radionuclide employed, arylstannanes appear to have the most general applicability. Fluorine-18 labeling of small-molecule conjugates has been best accomplished by ipso aromatic nucleophilic substitution (exchange) reactions. Radiohalogenated small molecules have been prepared which can be conjugated with specific functional groups (e.g. amines, sulfhydryl groups, and carbohydrates) or conjugated nonspecifically with groups in the proximity of the conjugate when it is photolyzed. On the basis of previous studies, good conjugation yields (i.e. 60-90%) can be expected for reactions with specific groups, whereas low yields (i.e. 1-5%) can be expected for conjugations with reactive nitrenes and carbenes. However, recent developments in the chemistry of conjugates that produce nitrenes and carbenes will likely improve the radiolabeling yields. There have been too few comparative studies to readily assess which is the best approach to take when beginning a study involving radiohalogenation of a protein or peptide. However, it is clear that radiohalogenated conjugates of proteins can offer an advantage over direct labeling in that conjugates may be designed which provide some control over in vivo stability and secondary distribution of metabolites. Conjugates can be prepared which are designed to utilize in vivo biochemical processes to release a radiohalogenated small molecule from a tissue (i.e. kidney or liver) or retain the radioactivity at the target tissue (e.g. tumor). Aside from the designing of conjugates with linking molecules for desired biological effects, the ultimate future goal for the radiolabeling chemical should be to prepare protein conjugates which can be radiohalogenated in a single one-step procedure.

Preparation and in vivo evaluation of an N-(p-[125I]iodophenethyl)maleimide-antibody conjugate

Protein sulfhydryl reactive N-(4-[125I]iodophenethyl)maleimide (IPEM, 5) was obtained from N-[4-(tri-n-butylstannyl)phenethyl]maleimide in 59-100% radiochemical yield. Conjugation of 5 to NR-ML-05 Fab, a murine anti-melanoma antibody Fab fragment that had been previously reduced with dithiothreitol (DTT), was effected in an average of 85% yield. Results from in vitro chemical challenges and serum stability studies on the IPEM conjugate of NR-ML-05 Fab (6) indicated a stable covalent attachment of the radioiodine. A biodistribution study of the IPEM conjugate in tumor-bearing athymic nude mice showed lack of significant accumulation of radioiodine in the thyroid and stomach which was an indication of in vivo stability. The observed uptake in tumor was consistent with that obtained for Chloramine-T- or p-iodobenzoate-labeled NR-ML-05 Fab conjugates.

Astatine-211 labeling of an antimelanoma antibody and its Fab fragment using N-succinimidyl p-astatobenzoate: comparisons in vivo with the p-[125I]iodobenzoyl conjugate

Astatine-211 labeling of an antimelanoma antibody, NR-ML-05, and its Fab fragment with N-succinimidyl p-[211At]astatobenzoate (2a) has been described. Preparation of the astatinated intermediate 2a was accomplished by distilling astatine-211 from an irradiated bismuth target directly into a reaction mixture containing an organometallic compound, N-succinimidyl p-(tri-n-butylstannyl)benzoate (1), and an oxidant, N-chlorosuccinimide, in 5% HOAc/MeOH. Trapping of distilled astatine as 2a was found to be efficient, resulting in 70-90% yields based on the amount of astatine-211 in the reaction mixture. The dry distillation technique employed gave recoveries of astatine-211 which ranged from 20% to 75%. Conjugation of 2a to NR-ML-05 and its Fab fragment was accomplished in 40-60% yields. The [211At]astatobenzoyl-conjugated antibodies were found to be stable in vitro when challenged by strong denaturants and nucleophilic reagents. Coinjected dual-labeled studies of the 2a astatinated antibodies and the same antibodies labeled with N-succinimidyl p-[125I]iodobenzoate (2b) in athymic mice bearing the human tumor xenograft A375 Met/Mix demonstrated that both radiolabeled antibodies had equivalent tumor localization. Data from the dual-labeled biodistribution of the intact antibody suggests that the astatine is stably attached. Data from the dual-labeled Fab fragment suggests that a portion of the astatine label is released as astatide, either from the astatinated Fab or from a catabolite.

Radioiodinated iodobenzoyl conjugates of a monoclonal antibody Fab fragment. In vivo comparisons with chloramine-T-labeled Fab

A comparative investigation of the biodistributions of radioiodinated p- and m-iodobenzoyl conjugates of a monoclonal antibody Fab fragment, NR-LU-10 Fab, and the same antibody Fab fragment radioiodinated by the chloramine-T (ChT) method has been carried out in mice. Coinjected, dual-isotope studies in athymic mice with tumor xenografts have demonstrated that there are only minor differences in the in vivo distributions of the iodobenzoyl-labeled Fabs, except in the excretory organs, kidneys, and intestines, where major differences were observed. Similarly, coinjection of either the p-iodobenzoyl or m-iodobenzoyl conjugate of NR-LU-10 Fab with the Fab radioiodinated with ChT/radioiodide into BALB/c mice provided additional data that indicated that the two iodobenzoyl conjugates distributed similar in a number of selected tissues. The tissue-distribution differences of the regioisomeric iodobenzoyl conjugates in relation to the ChT-radioiodinated Fab were large for the stomach and neck, consistent with previous studies. The most notable difference between the two iodobenzoyl conjugates was the kidney activity, where the m-iodobenzoyl conjugate was similar to the directly labeled Fab, but the p-iodobenzoyl-conjugated Fab was higher by nearly a factor of 2.

Variables influencing tumor uptake of anti-melanoma monoclonal antibodies radioiodinated using para-iodobenzoyl (PIB) conjugate

Tumor uptake was examined with respect to antigen expression, time-dependent biodistribution, dose of Mab injected, tumor size, and tumor site (i.e., subcutaneous versus lung or liver metastases). NR-ML-05, 96.5, and P94 showed significantly greater uptake in subcutaneous tumors than CL207 and 5.1 (p less than 0.05). NR-ML-05 had a significantly higher tumor uptake at 24 hr (11.9 +/- 0.51) than at 72 hr (4.0 +/- 0.37) or 144 hr (2.7 +/- 0.84) after injection (p less than 0.001). The other four Mabs had similar tumor distribution at all three time points. The tumor uptake of four Mabs (96.5, P94, CL207. 5.1) differed with respect to in vitro versus in vivo binding to tumor, tumor type, dose of Mab, and tumor site (subcutaneous versus metastases). In contrast, NR-ML-05 demonstrated consistent uptake in tumors independent of the above parameters. These data suggest that certain host parameters can influence in vivo tumor targeting depending on characteristics of each Mab studied.

Radiolabeling of a monoclonal antibody with N-succinimidyl para-[77Br]bromobenzoate

A preliminary study involving the radiolabeling of an anti-melanoma antibody through conjugation of a radiobrominated small molecule, N-succinimidyl para-[77Br]bromobenzoate, has been accomplished in 22% overall yield when no-carrier-added radiobromine was employed. In vitro analyses demonstrated that the radiobrominated antibody was immunocompetent, retaining 80% of the unlabeled antibody immunoreactivity.

Evaluation of iodovinyl antibody conjugates: comparison with a p-iodobenzoyl conjugate and direct radioiodination

The preparations and conjugations of 2,3,5,6-tetrafluorophenyl 5-[125I/131I]iodo-4-pentenoate (7a) and 2,3,5,6-tetrafluorophenyl 3,3-dimethyl-5-[125I/131I]iodo-4-pentenoate (7b) to monoclonal antibodies are reported. Reagents 7a and 7b were prepared in high radiochemical yield by iododestannylation of their corresponding 5-tri-n-butylstannyl precursors. Radioiodinated antibody conjugates were prepared by reaction of 7a or 7b with the protein at basic pH. Evaluation of these conjugates by several in vitro procedures demonstrated that the radiolabel was attached to the antibody in a stable manner and that the conjugates maintained immunoreactivity. Comparative dual-isotope biodistribution studies of a monoclonal antibody Fab fragment conjugate of 7a and 7b with the same Fab fragment labeled with N-succinimidyl p-[131I]iodobenzoate (PIB, p-iodobenzoate, 2) or directly radioiodinated have been carried out in tumor-bearing nude mice. Coinjection of the Fab conjugate of 7a with the Fab conjugate of 2 demonstrated that the biodistributions were similar in most organs, except the neck tissue (thyroid-containing) and the stomach, which contained substantially increased levels of the 7a label. Coinjection of the Fab conjugate of 7a with the Fab fragment radioiodinated by using the chloramine-T method demonstrated that the biodistributions were remarkably similar, suggesting roughly equivalent in vivo deiodination of these labeled antibody fragments. Coinjection of the Fab conjugate of 7a with the Fab conjugate of 7b indicated that there was approximately a 2-fold reduction in the amount of in vivo deiodination of the 7b conjugate as compared to the 7a conjugate.

Biodistribution of p-iodobenzoyl (PIP) labeled antibodies in a murine lymphoma model

A monoclonal antibody against the murine T-cell antigen Thy 1.1 was radioiodinated using N-succinimidyl p-iodobenzoate (PIP) in an attempt to decrease deiodination of the labeled antibody. The biodistribution of the PIP labeled antibody was compared to Iodogen labeled antibody in Thy 1.1+ lymphoma bearing AKR/Cum mice, where the antibody was tumor specific, and AKR/J mice where the antibody reacted with both tumor and normal T-cells. PIP labeling resulted in decreased iodine concentrations in stomach and salivary gland as compared to Iodogen labeling. There was little difference in radioiodine concentrations between the two preparations in tumor, lymphoid tissues or other organs. These results suggest deiodination of intact antibody plays little role in the clearance of radioiodinated anti-Thy 1.1 antibody from tissues.

Development of a stable radioiodinating reagent to label monoclonal antibodies for radiotherapy of cancer

A method of radioiodinating monoclonal antibodies such that the labeled antibodies do not undergo in vivo deiodination has been studied. The method utilizes conjugation of succinimidyl para-iodobenzoate to the antibody. The iodobenzoate was radiolabeled by using an organometallic intermediate to facilitate the reaction. Thus, succinimidyl para-tri-n-butylstannylbenzoate was radiolabeled in 60-90% radiochemical yield and subsequently conjugated to the antibody in 80-90% yield. Animal biodistribution studies were carried out with two separate anti-melanoma antibodies (9.2.27 and NR-M1-05) labeled by this method, and examined in nude mice bearing human melanoma tumor xenografts. Very large differences in the localization of radioactivity were observed in the thyroids and stomachs of mice when the iodobenzoyl-labeled antibodies were compared with the same antibodies labeled using the chloramine-T method of radioiodination. Few other significant differences in the tissue distribution of the radioiodinated antibodies were seen.