Molecular modification of a recombinant anti-CD3epsilon-directed immunotoxin by inducing terminal cysteine bridging enhances anti-GVHD efficacy and reduces organ toxicity in a lethal murine model

Immunotoxin (IT) therapy shows potential for selectively eliminating GVHD-causing T cells in vivo, but the field has been hampered by toxicity. Previously, we showed that a genetically engineered IT consisting of a single-chain protein, including the anti-CD3sFv spliced to a portion of diphtheria-toxin (DT(390)) has anti-GVHD effects, but pronounced organ toxicity common to this class of agent. A recombinant DT(390) anti-CD3sFv protein previously shown to have anti-GVHD activity was modified to reduce its filtration into kidney by genetically inserting a cysteine residue downstream of the sFv moiety at the c-terminus of the protein. This modification produced an intermolecular disulfide bridge, resulting in a bivalent, rather than a monovalent IT, termed SS2, that selectively inhibited T-cell proliferation in vitro. Although monomer and SS2 were similar in in vitro activity, SS2 had a superior therapeutic index in vivo with at least 8-fold more being tolerated with reduced kidney toxicity. Most importantly, in a lethal model of GVHD, 40 microg SS2 given for 1 day, protected 100% of the mice from lethal GVHD for 3 months, whereas the maximum tolerated dose (MTD) of monomer protected only 33%. To our knowledge, this is the first time disulfide bonded ITs have been created in this way and this simple molecular modification may address several problems in the IT field because it (1) markedly increased efficacy curing mice of GVHD after a single daily treatment, (2) markedly decreased organ toxicity, (3) increased the tolerated dosage, and (4) created a therapeutic window where none existed before.

Fractionated radiation therapy in combination with adenoviral delivery of the cytosine deaminase gene and 5-fluorocytosine enhances cytotoxic and antitumor effects in human colorectal and cholangiocarcinoma models

Radiosensitization of human gastrointestinal tumors by 5-fluorouracil (5-FU) has been studied in vitro and clinically in human cancer therapy trials. The bacterial enzyme cytosine deaminase (CD) converts the nontoxic prodrug 5-fluorocytosine (5-FC) into 5-FU. Human colon cancer cells stably expressing CD have been shown by other investigators to be sensitized to radiation following treatment with 5-FC. We previously used an adenoviral vector under control of the cytomegalovirus promoter (AdCMVCD) encoding the CD gene in combination with 5-FC and a single fraction of radiation exposure to enhance cytotoxicity to human cholangiocarcinoma cells in vitro and in vivo. The purpose of this study was to determine whether AdCMVCD infection and 5-FC with multiple fraction low-dose radiotherapy results in enhanced cytotoxicity. In the present study, we utilized AdCMVCD and 5-FC with single fraction radiotherapy to demonstrate enhanced cytotoxicity to WiDr human colon carcinoma cells in vitro. Additionally, we tested this gene therapy/prodrug treatment strategy employing a fractionated radiation dosing schema in animal models of WiDr colon carcinoma and SK-ChA-1 cholangiocarcinoma. A prolonged WiDr tumor regrowth delay was obtained with AdCMVCD infection in combination with systemic delivery of 5-FC and fractionated external beam radiation therapy compared with control animals treated without radiation, without 5-FC, or without AdCMVCD. The results of treatment with AdCMVCD + 5-FC + radiation therapy to cholangiocarcinoma xenografts were equivalent to those obtained with systemic 5-FU administration + radiation. Thus, the use of AdCMVCD can be effectively combined with clinically relevant 5-FC and radiation administration schemes to achieve enhanced tumor cell killing and increased control of established tumors of human gastrointestinal malignancies.

Noninvasive monitoring of gene transfer using a reporter receptor imaged with a high-affinity peptide radiolabeled with 99mTc or 188Re

Gene therapy protocols require better modalities to monitor the location and level of transferred gene expression. One potential in vivo mechanism to assess gene expression would be to image the binding of a radiolabeled peptide to a reporter receptor that is expressed in targeted tissues. This concept was tested in a tumor model using a replication-incompetent adenoviral vector encoding the human type 2 somatostatin receptor (Ad5-CMVhSSTr2). Expression of the hSSTr2 reporter was imaged using a radiolabeled, somatostatin-avid peptide (P829).

METHODS

Bilateral subcutaneous A427 tumor xenografts were established on the flanks of athymic nude mice. These human-origin, non-small cell lung tumors are normally negative for hSSTr2 expression. One tumor was injected directly with Ad5-CMVhSSTr2, whereas the second tumor was injected directly with a control Ad5 vector. The mice were injected intravenously 48 h later with P829 peptide that was radiolabeled to high specific activity with 99mTc (half-life, 6 h) or 188Re (half-life, 17 h). Tumors were frozen and evaluated for somatostatin receptor expression using fluorescein-labeled somatostatin.

RESULTS

The accumulation of radiolabeled P829 in hSSTr2-expressing tumors was easily visualized by gamma camera imaging 3 h after injection. Imaging region of interest analyses and biodistribution studies confirmed a 5- to 10-fold greater accumulation of both radiolabeled P829 peptides in the Ad5-CMVhSSTr2-injected tumors versus control tumors injected with control Ad5 vectors. Ad5-CMVhSSTr2-injected tumors accumulated 2.5-3.8 percentage injected dose per gram 3 h after injection. Only Ad5-CMVhSSTr2-injected tumors expressed somatostatin receptors, as determined by immunohistochemistry.

CONCLUSION

These studies show the feasibility of imaging a 99mTc-labeled peptide's binding to a reporter receptor after in vivo gene transfer to tumor cells. The 188Re-labeled peptide worked equally well for this imaging approach and offers the additional advantage of energetic beta decay with potential therapeutic efficacy. 99mTc and 188Re are generator produced, an advantage for widespread availability and low cost, and both radioisotopes can be imaged with existing, high-resolution modalities. There is great potential for using 99mTc-labeled peptides for imaging gene transfer with the hSSTr2 reporter receptor, especially when the reporter correlates with the expression of therapeutic genes that can be included simultaneously in the gene therapy vector.

A robust summarize-regress procedure for tissue-specific pharmacokinetics

The destructive sampling necessary to obtain tissue-specific pharmacokinetics of radiolabeled ligands does not permit true time-dependent models to be estimated. To adjust for this, small groups of animals are sacrificed at varying time points and the percent injected dose per gram (PID) of radiolabeled ligands is summarized for each tissue of interest. Usually the average PID at each time point is modeled to obtain estimates of pharmacokinetic parameters for each tissue when utilizing summarize-regress procedures. In this paper, it is recommended that the median should be the summary measure at each time point to be modeled in these circumstances. The discrepancy that exists between modeling the average and median PID is demonstrated by an actual experiment. Finally, simulation studies are performed and the results are compared when using these two summary measures.

Experimental radioimmunotherapy

Experimental radioimmunotherapy (RIT) studies in animal models have contributed significantly to the design of clinical RIT protocols, although the results have not always been directly translated. Reviewed in this article are current areas of active research in experimental RIT to increase the therapeutic ratio that are likely to have a significant impact on the design of future clinical studies. Approaches for increasing the therapeutic efficacy of RIT include the development of new targeting molecules (genetically engineered monoclonal antibodies, antibody fragments, single-chain antibodies, diabodies and minibodies, fusion toxins, or peptides); improved labeling chemistry; novel radionuclide use and fractionation; locoregional administration; pretargeting; use of biological response modifiers or gene transfer techniques to increase target receptor expression; bone marrow transplantation; and combined modality therapy with external-beam radiation therapy, chemotherapy, or gene therapy. Further research with these new experimental approaches in preclinical animal models is necessary to contribute to advances in the treatment of cancer patients using radiolabeled antibodies and peptides.

Combined modality therapy of A431 human epidermoid cancer using anti-EGFr antibody C225 and radiation

BACKGROUND

Monoclonal antibodies (mAb) to epidermal growth factor receptor (EGFr) inhibit tumor cell proliferation and enhance cytotoxicity of chemotherapeutic agents. The purpose of this study was to investigate the interaction of the anti-EGFr antibody C225 combined with radiotherapy (RT) on EGFr expressing A431 human epidermoid cancer cells.

METHODS

Cell proliferation, apoptosis, EGFr expression and phosphorylation, and clonogenic survival were assayed in vitro. A431 tumor growth inhibition and immunohistochemistry analysis of EGFr expression and apoptosis were assessed in vivo.

RESULTS

C225 plus RT produced greater inhibition of A431 cell proliferation than C225 or RT alone which was corroborated by enhanced apoptosis. Similar clonogenic survival occurred following the addition of C225 to RT, although colonies were smaller in the presence of C225. C225 produced inhibition of EGF-induced phosphorylation of EGFr without concurrent down-regulation of surface receptor, which was not altered by RT. Combined treatment of mice bearing tumors demonstrated enhancement of complete regressions, reduction in time to tumor size doubling, and prolongation of survival. Significant apoptosis occurred in xenograft tumors treated with C225 with or without RT.

CONCLUSIONS

These data demonstrate an interaction between C225 and RT. C225-mediated apoptosis and inhibition of EGFr phosphorylation may be critical in the interaction. Studies to define the precise influence of combined modality treatment on the EGFr signal transduction cascade need to be pursued. The combination of growth factor receptor antibodies and RT has potential application in clinical oncology.

Paclitaxel derivatives for targeted therapy of cancer: toward the development of smart taxanes

The pharmacologic efficacy of the promising antitumor agent paclitaxel (Taxol) may be potentially enhanced through derivatization of the drug to a water-soluble tumor-recognizing conjugate. This work reports the design and synthesis of the first tumor-directed derivative of paclitaxel. A 7-amino acid synthetic peptide, BBN[7-13], which binds to the cell surface bombesin/gastrin-releasing peptide (BBN/GRP) receptor, was conjugated to the paclitaxel-2'-hydroxy function by a heterobifunctional poly(ethylene glycol) linker. The resulting conjugate, designated PTXPEGBBN[7-13], was soluble to the upper limit of tested concentrations (250 mg/mL). The conjugate completely retained the receptor binding properties of the attached peptide as compared with those of the unconjugated BBN[7-13]. In experiments with NCI-H1299 human nonsmall cell lung cancer cells, the cytotoxicity of the PTXPEGBBN[7-13] conjugate at a 15 nM dose was enhanced by a factor of 17.3 for 24 h and 10 for 96 h exposure times, relative to paclitaxel. The IC(50) of the conjugate, tested against the same cell line, was lower than the free drug by a factor of 2.5 for both 24 h and 96 h exposures. These results describe, for the first time, the design and synthesis of a soluble tumor-directed paclitaxel prodrug which may establish a new mode for the utilization of this drug in cancer therapy.

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.

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.

Comparison of multiple bolus and continuous injections of 131I-labeled CC49 for therapy in a colon cancer xenograft model

One of the problems in achieving cures with radioimmunotherapy is that hematological toxicity limits the quantity of radiolabeled monoclonal antibody (MAb) that can be administered. The MAb CC49 binds with high affinity to the TAG-72 antigen expressed in many human adenocarcinomas. We investigated tumor growth inhibition, survival, and tumor and bone marrow dosimetry after multiple bolus injections or continuous infusion of 131I-labeled CC49 MAb in a human colon cancer xenograft model to determine which method of administration results in the highest therapeutic ratio. Groups of athymic nude mice bearing established s.c. LS174T human colon cancer xenografts received three i.p. bolus injections (3X) of 131I-labeled CC49 (3X, days 0, 3, and 7) or were implanted i.p. with mini-osmotic pumps delivering 131I-labeled CC49 over 7 days. The total radionuclide doses administered were broken down into low-dose (< or = 450 microCi), medium-dose (450-800 microCi), and high-dose (> 800 microCi) groups. At the medium-dose level, the bolus-therapy animals did not have a significantly longer survival time but did have a significantly longer time-to-tumor doubling than the pump-therapy animals. The median survival for medium-dose bolus and pump therapy was 157 and 105 days, respectively, and the median time-to-tumor doubling was at least 114 and 77 days, respectively. At the low-dose level, the bolus-therapy animals had a significantly longer survival time but not a significantly longer time-to-tumor doubling than the pump-therapy animals. The median survival for low-dose bolus and pump therapy was 95.5 and 59 days, respectively, and the median time-to-tumor doubling was 73 and 38 days, respectively. The high-bolus dose was toxic. A comparison of the overall survival rate of pump therapy versus bolus therapy, excluding high-dose, resulted in the bolus-therapy animals having a longer survival time and a longer time-to-tumor doubling than the pump-therapy animals. Serial section autoradiography was used to reconstruct tumor activity density distributions over time. Average dose values calculated from total uptake data for 900 microCi administered activity yielded 158 Gy (3X) and 141 Gy (pump). Average three-dimensional doses using the radial histograms to calculate the absorbed fractions were 139 Gy and 123 Gy, respectively. This calculation includes energy loss external to the tumor. With cell proliferation parameters set to single fraction 60Co recurrence results, the effective dose (D(eff)) for local control was 11 Gy and 9 Gy, respectively. Three bolus injections resulted in a more uniform dose rate over a longer period, resulting in a calculated 19% improvement in D(eff) compared with pump administration. Dose to bone marrow was calculated assuming an activity concentration in bone marrow of 0.24 times the concentration in blood and an absorbed fraction of 0.63. For the 900-microCi 131I-labeled CC49 injected activity, pump administration resulted in an 80% higher calculated D(eff) to bone marrow compared with 3X bolus injection. These results demonstrate that 3X bolus injections were clearly superior to pump administration in terms of survival, tumor growth inhibition, tumor absorbed dose, and bone marrow dose.

Biodistribution study of 188Re-labeled trisuccin-HuCC49 and trisuccin-HuCC49deltaCh2 conjugates in athymic nude mice bearing intraperitoneal colon cancer xenografts

The trihydroxamate bifunctional chelating agent (BCA), trisuccin, has been shown to be a potential ligand for radiolabeling of monoclonal antibodies (MAbs) with rhenium radioisotopes, through an indirect postconjugation approach. The use of this trihydroxamate BCA made it possible to prepare stable BCA-MAb conjugates in pure form that could be radiolabeled with carrier-free 188Re. The anti-TAG-72 murine MAb, CC49, and its humanized derivatives are promising agents in the treatment of a number of malignancies with the CH2 domain-deleted MAb (HuCC49deltaCH2), which is of particular interest due to its rapid blood clearance. The biodistribution of 188Re-labeled conjugates of trisuccin with both humanized CC49 (HuCC49) and HuCC49deltaCH2 in athymic nude mice implanted i.p. with LS174T human colon carcinoma was studied. Trisuccin-MAb conjugates were synthesized at different BCA:MAb ratios by the 6-oxoheptanoic acid method using trisuccin hydrazide. The conjugates were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy for the number of incorporated trisuccin molecules. The conjugates were radiolabeled with carrier-free, generator-produced 188Re and purified by gel filtration on Sephadex G-25. Labeling yields and homogeneity of the labeled conjugates were analyzed by high-pressure liquid chromatography and instant TLC. Athymic nude mice were injected i.p. with LS174T human colon carcinoma cells, 7 days prior to injection of the labeled antibodies. 188Re-labeled MAbs were injected i.p., and the mice were sacrificed 24 h postinjection. Matrix-assisted laser desorption/ionization time-of-flight analyses showed stable incorporation of trisuccin into each MAb, with the measured ligand:MAb values positively correlating with the theoretical ratios. Labeling of the conjugates with 188Re proceeded with high yields, producing homogeneous 188Re-MAbs with good stabilities as shown by instant TLC and biodistribution analyses. Biodistribution of the radiolabeled MAbs at 24 h after injection showed median tumor uptake values of 23.5%ID/g and 17.6%ID/g for the 188Re-HuCC49deltaCH2 and 188Re-HuCC49, respectively. The blood clearance of the domain-deleted MAb was faster than that of the intact antibody. The blood values at 24 h after injection were 0.7%ID/g for 188Re-HuCC49deltaCH2 and 3.2%ID/g for 188Re-HuCC49. The results indicate that trisuccin is a promising agent for postconjugation labeling of antibodies with 188Re. Additionally, these results illustrate the potential of 188Re-HuCC49deltaCH2 in radioimmunodiagnosis and radioimmunotherapy of cancer.

Specific membrane receptor gene expression targeted with radiolabeled peptide employing the erbB-2 and DF3 promoter elements in adenoviral vectors

Radioimmunotherapy is limited by a variety of factors, including poor tumor penetration of monoclonal antibodies and low levels of intratumoral antigen expression. To address these limitations, a gene therapy strategy was devised to genetically induce tumor cells to express enhanced levels of membrane receptors with high affinity for a radiolabeled peptide. We designated this approach as genetic radioisotope targeting strategy. To this end, an adenoviral vector (AdCMVGRPr) encoding the murine gastrin-releasing peptide receptor (GRPr) was used to achieve a high level of binding of radiolabeled bombesin (BBN). To achieve genetic induction of membrane GRPr specifically to tumor cells, we constructed two adenoviral vectors encoding the GRPr gene under the control of the tumor-specific regulatory elements, DF3 (AdDF3GRPr) or erbB-2 (AderbGRPr). We investigated the binding of [125I]BBN to the GRPr following infection with AdDF3GRPr and AderbGRPr in a panel of human breast, pancreatic, and cholangiocarcinoma tumor cell lines. [125I]BBN binding and GRPr expression increased with increasing multiplicities of infection of AdCMVGRPr in all of the cell lines tested. Breast cancer cell lines expressing erbB-2 showed significant GRPr expression using AderbGRPr. A similar result was observed in breast and cholangiocarcinoma cells infected with AdDF3GRPr expressing MUC1 as detected by immunohistochemistry but was not seen in the pancreatic cell lines tested. Thus, adenoviral vectors with tissue-specific promoter elements can be used to achieve a selective expression of membrane receptors that can be targeted with a radiolabeled peptide. The use of such a transcriptional targeting approach may restrict gene expression to tumors and limit the radiation dose deposited in normal tissues in vivo.

In vivo localization of [(111)In]-DTPA-D-Phe1-octreotide to human ovarian tumor xenografts induced to express the somatostatin receptor subtype 2 using an adenoviral vector

Adenoviral vectors, encoding genes for cell surface antigens or receptors, have been used to induce their high level expression on tumor cells in vitro and in vivo. These induced antigens and receptors can then be targeted with radiolabeled antibodies or peptides for potential radiotherapeutic applications. The purpose of this study was to determine a dosing schema of an adenoviral vector encoding the human somatostatin receptor subtype 2 (AdCMVhSSTr2) for achieving the highest tumor localization of [(111)In]-DTPA-D-Phe1-octreotide, which binds to this receptor, in a human ovarian cancer model as a prelude to future therapy studies. AdCMVhSSTr2 was produced and used to induce hSSTr2 on A427 human nonsmall cell lung cancer cells and on SKOV3.ipl human ovarian cancer cells in vitro, as demonstrated by competitive binding assays using [125I]-Tyr1-somatostatin and [(111)In]-DTPA-D-Phe1-octreotide. Mice bearing i.p. SKOV3.ip1 tumors administered 1 x 10(9) plaque-forming units of AdCMVhSSTr2 i.p. 5 days after tumor cell inoculation, followed by an i.p. injection of [(111)In]-DTPA-D-Phe1-octreotide 2 days later, showed a range of 15.3-60.4% median injected dose/gram (ID/g) in tumor at 4 h after injection compared with 3.5% ID/g when [125I]-Tyr1-somatostatin was administered and 0.3% ID/g when the negative control peptide [125I]-mIP-bombesin was administered. Mice administered a control adenoviral vector encoding the gastrin-releasing peptide receptor did not have tumor localization of [(111)In]-DTPA-D-Phe1-octreotide (<1.6% ID/g), demonstrating specificity of [(111)In]-DTPA-D-Phe1-octreotide for the AdCMVhSSTr2 induced tumor cells. In another set of experiments, the tumor localization of [(111)In]-DTPA-D-Phe1-octreotide was not different 1, 2, or 4 days after AdCMVhSSTr2 injection (31.8, 37.7, and 40.7% ID/g, respectively; P = 0.88), indicating that multiple injections of radiolabeled peptide can be administered with equivalent uptake over a 4-day period. [(111)In]-DTPA-D-Phe1-octreotide tumor localization in animals administered AdCMVhSSTr2 on consecutive days or 2 days apart was 22.4% ID/g and 53.2% ID/g, respectively (P = 0.009) when [(111)In]-DTPA-D-Phe1-octreotide was given 1 day after the second AdCMVhSSTr2 injection. There was no difference in [(111)In]-DTPA-D-Phe1-octreotide localization after a single AdCMVhSSTr2 injection (40.7% ID/g) or two injections of AdCMVhSSTr2 given 1 (45.9% ID/g) or 2 (53.2% ID/g) days apart, where [(111)In]-DTPA-D-Phe1-octreotide was given in each case 4 days after the first AdCMVhSSTr2 injection (P = 0.65). Therefore, two AdCMVhSSTr2 injections did not increase [(111)In]-DTPA-D-Phe1-octreotide tumor localization compared with one injection, which eliminates concerns about an immune response to a second dose of AdCMVhSSTr2. This will be the basis for a therapeutic protocol with multiple administrations of an octreotide analogue labeled with a therapeutic radioisotope.

Further studies on the protein conjugation of hydroxamic acid bifunctional chelating agents: group-specific conjugation at two different loci

A procedure utilizing an activated ester approach for conjugation of unprotected hydroxamic acids to antibodies and peptides was recently reported. Here, an alternative method with advantages over the activated ester strategy is described. This protocol utilizes the hydrazone formation between a hydrazide derivative of the trihydroxamate ligand trisuccin and either a ketone derivative of antibody or the aldehyde groups, generated by oxidation of the carbohydrate residues. Thus, the trisuccin carboxylic acid (1) was derivatized with tert-butyl carbazate to the hydrazide 2, and the protecting groups were removed by catalytic hydrogenation and acidolysis with TFA to afford the hydroxamate hydrazide 4. Conjugation of 4 to monoclonal antibody CC49 was effected by two approaches: attachment through the amine (e.g., lysine) residues of the antibody or oxidation of the carbohydrate residues. The extent of conjugations were monitored by MALDI, through evaluation of the increases in molecular weights of the conjugates compared to the unconjugated antibody. The first approach utilizing a ketone linker (6-oxoheptanoic acid, OHA) which served as a hydrazide anchor, is being introduced in this report as a new technique for conjugation of hydrazide derivatives to proteins. The OHA approach proved to be a superior strategy over the aldehyde approach in the ease of the procedure and yield of protein recovery. It also had the advantage of yielding more control in adjusting the ligand-to-protein ratio and was therefore selected for protocol optimization. All conjugates resulting from both approaches were radiolabeled with 125I and screened for their immunoreactivity. Furthermore, the conjugates prepared through the optimized OHA protocol were radiolabeled with both 99mTc and 125I for which the radiolabeling yields and immunoreactivities are reported.

Differential susceptibility of primary and established human glioma cells to adenovirus infection: targeting via the epidermal growth factor receptor achieves fiber receptor-independent gene transfer

Adenovirus (Ad) vectors are promising for gene therapy of glioma due to their ability to achieve efficient gene transfer upon intratumoral administration. Yet in this context, Ad mediates widespread gene transfer to both tumor and surrounding parenchyma. Ad entry is dependent upon the expression of fiber receptors, such as coxsackie/adenovirus receptor, and alpha(v) integrins on the target cells for binding and internalization, respectively. We hypothesized that the susceptibility of human gliomas to Ad would likely be heterogeneous due to variable expression of these receptors. It was found that established human glioma cell lines exhibited differential susceptibility to Ad-mediated gene transfer, which correlated directly with the level of radiolabeled Ad binding and with the expression of coxsackie/adenovirus receptor but not with the expression of alpha(v) integrins. To circumvent the lack of fiber receptors and to target Ad gene transfer specifically to tumor cells, we used a bispecific antibody conjugate to ablate Ad binding to fiber receptors and retarget binding to the epidermal growth factor receptor (EGFR), a tumor-associated marker negligibly expressed in normal, mitotically quiescent neural tissues. The results demonstrate that EGFR-targeted Ad gene transfer was EGFR specific and independent of fiber-fiber receptor interactions. Furthermore, EGFR targeting significantly enhanced Ad gene delivery to 7 of 12 established glioma cell lines and to 6 of 8 cultured primary gliomas. Interestingly, EGFR-targeted Ad gene transfer did not correlate with EGFR expression across cell lines, suggesting the importance of other factors. This study establishes that fiber receptor expression limits the utility of Ad vectors for gene transfer to glioma cells and suggests that targeting Ad via EGFR may prove valuable for tumor-specific gene transfer to high-grade gliomas. These findings have key relevance in the context of Ad vector-based approaches for glioma gene therapy.

Radiosensitization mediated by a transfected anti-erbB-2 single-chain antibody in vitro and in vivo

PURPOSE

The erbB-2 receptor is overexpressed in several human cancers, including ovarian, prostate, and breast. We have developed plasmid and adenoviral vectors expressing an anti-erbB-2 single chain antibody (sFv), directed to the endoplasmic reticulum (ER) of target cells, that is cytotoxic to tumor cells overexpressing erbB-2 through induction of apoptosis. The anti-erbB-2 sFv also sensitizes erbB-2 overexpressing cells to the cytotoxic effects of cisplatin. On this basis, we hypothesized that human ovarian cancer cells expressing anti-erbB-2 sFv with downregulated erbB-2 product, p185erbB-2, also would be sensitized to ionizing radiation. Therefore, we designed experiments to test the ability of the anti-erbB-2 sFv to radiosensitize human ovarian cancer cells in vitro and in vivo.

METHODS AND MATERIALS

To test our hypothesis, we established subcutaneous (s.c.) tumors in the flanks of nude mice with SKOV3.ip1 human ovarian cancer cells and SKOV3 cells stably expressing the ER directed anti-erbB-2 sFv (SKOV3/pGT21). The tumors were treated with 10 Gy 60Co, or received no radiation. We then determined the regression rate, delay in regrowth, and time to tumor doubling of the tumors treated with radiation in the transfected group and controls. In addition, SKOV3.ip1 and SKOV3/pGT21 tumors were dissected from the irradiated animals and assayed for differences in p185erbB-2 expression at 12 weeks after irradiation by immunohistochemistry. Further, in vitro clonogenic survival assays were performed on the parental SKOV3.ip1 and SKOV3/pGT21 cell lines.

RESULTS

A statistical analysis of the combined data was done for two in vivo experiments. The analysis of the combined data showed that animals with irradiated tumor SKOV3/pGT21 had a significantly higher regression rate (p = 0.0055), longer delay in regrowth (p = 0.0001) and time to tumor doubling (p = 0.0004), than those animals with tumor SKOV3.ip1 that received radiation. We observed a similar significant effect for the same parameters in the unirradiated tumor SKOV3/pGT21 compared to unirradiated tumor SKOV3.ip1. Immunohistochemical analysis of the SKOV3/pGT21 tumor cells demonstrated focal accumulation of p185erbB-2 in scattered clumps of cells and less p185erbB-2 membrane expression than cells of SKOV3.ip1 tumors. However, SKOV3.ip1 and SKOV3/pGT21 cells had similar in vitro sensitivity to radiation.

CONCLUSIONS

These data support the hypothesis that tumors with reduced p185erbB-2 expression mediated by the anti-erbB-2 sFv are rendered more susceptible in vivo to the cytotoxic effects of ionizing radiation than tumors that maintain their normal expression of p185erbB-2. However, a similar effect was not observed with the same tumor cells in vitro. Thus, as has been described by others (1, 2), in vitro and in vivo results do not always correlate. Therefore, appropriate assays to assess clinical relevance need to be determined for each particular system studied.

A mouse bone marrow dosimetry model

Bone marrow is the primary dose-limiting organ in radioimmunotherapy. Athymic nude mouse models are used to guide radioimmunotherapy in humans. In the mouse, the dimensions of the marrow are comparable to the mean range of the beta particles for a wide variety of beta-emitting radionuclides, so local beta energy deposition cannot be assumed.

METHODS

We have developed a computer simulation model in which slab, spherical and cylindrical geometries of the bone marrow of the mouse were incorporated. The energy deposition within the marrow was estimated using beta dose point kernels for several beta-emitting radionuclides.

RESULTS

The calculated percentages of energy deposited in the mouse marrow using the full geometry were 46%, 24% and 10% for 131I-, 186Re- and 90Y-radiolabeled antibodies, respectively. Assuming a concentration of activity in the marrow of 0.36 times the blood activity concentration, the percentages of energy deposition in the marrow from marrow and whole-body sources were 61%, 40% and 29% for 131I, 186Re and 90Y, respectively.

CONCLUSION

This work shows that, even for the lower mean beta energy-emitting radionuclide, such as 131I, accurate computation of the mouse bone marrow dose involves including both the energy loss from beta decays within the marrow and dose contributions from tissue surrounding the marrow.

Combined cytosine deaminase expression, 5-fluorocytosine exposure, and radiotherapy increases cytotoxicity to cholangiocarcinoma cells

Cholangiocarcinoma is a malignancy that is resistant to current therapy. We applied the toxin gene therapy strategy of cytosine deaminase conversion of the nontoxic producing 5-fluorocytosine to 5-fluorouracil combined with radiotherapy to cholangiocarcinoma. The transduction efficiency of SK-ChA-1 cholangiocarcinoma cells was determined by fluorescence-activated cell-sorting analysis following infection with recombinant adenovirus AdCMVLacZ, which encodes thc gene for Beta-galactosidase. To evaluate cytosine deaminase-mediated conversion of 5-fluorocytosine to 5-fluorouracil and subsequent cytotoxicity, SK-ChA-1 cells were infected with the recombinant adenovirus AdCMVCD, which encodes cytosine deaminase, and exposed to 5-fluorocytosine for 6 to 8 days. Additive cytotoxicity of radiation therapy was evaluated by cobalt-60 exposure following AdCMVCD infection and 5-fluorocytosine treatment. SK-ChA-1 cells were transduced (98.4%) by AdCMVLacZ at 100 plaque-forming units per cell. Following infection with AdCMVCD and exposure to 5 to 100 microgram/ml of 5-fluorocytosine, 20% to 64% of SK-ChA-1 cells were killed. A combination of radiation and cytosine deaminase/5-fluorocytosine therapy resulted in enhanced cell killing (83.5% to 91.5%). Cholangiocarcinoma cells were transduced by recombinant adenoviral vectors and were killed by cytosine deaminase-mediated production of 5-fluorouracil. Enhanced cytotoxicity was seen with the addition of external beam radiation. These results provide a foundation for multimodality therapy for human cholangiocarcinoma that combines gene therapy technology with radiation therapy.

Tumor localization of a radiolabeled bombesin analogue in mice bearing human ovarian tumors induced to express the gastrin-releasing peptide receptor by an adenoviral vector

BACKGROUND

The adenoviral vector, AdCMVGRPr, has been used to induce the expression of the murine gastrin-releasing peptide receptor (GRPr) both in vitro and in vivo. A bombesin analogue ([125I]-mIP-bombesin) has been shown to bind with high affinity to GRPr and to localize to intraperitoneal (i.p.) ovarian tumors 2 days after induction of GRPr in an athymic nude mouse model. The present study was conducted to determine the level of localization of [(125/131)I]-mIP-bombesin in the tumors at 2, 4, and 7 days after AdCMVGRPr administration and to determine the feasibility of giving multiple doses of [131I]-mIP-bombesin for therapy.

METHODS

Human ovarian cancer cells (SKOV3.ip1) were infected in vitro with AdCMVGRPr and were assayed for receptor expression at 2, 4, and 7 days after infection by using a radiolabeled bombesin-binding assay. Biodistribution studies utilized athymic nude mice inoculated i.p. with SKOV3.ip1 cells. The tumors were induced to express GRPr with an i.p. injection of AdCMVGRPr followed by administration of [125I]-mIP-bombesin 2 days later (AdCMVLacZ or saline was used for negative controls). In addition, the tumor localization of [125I]-mIP-bombesin was determined 4 and 7 days after AdCMVGRPr administration. The tumor localization of [131I]-mIP-bombesin was compared with [125I]-mIP-bombesin in this in vivo model.

RESULTS

SKOV3.ip1 cells infected with AdCMVGRPr resulted in 80.3 +/- 5.9% binding of [125I]-Tyr4-bombesin at 2 days after infection, which decreased to 46.8 +/- 0.4% at 4 days and to 17.7 +/- 0.1% at 7 days. The biodistribution study showed that the tumor localization (14.9 +/- 8.2% injected dose/gram; ID/g) of [125I]-mIP-bombesin 2 days after administration of AdCMVGRPr was significantly greater than its localization in other organs (P < 0.003) and was significantly greater than in AcCMVLacZ- and saline-treated mice (P < 0.003). Injections of [125I]-mIP-bombesin at 4 and 7 days after a single AdCMVGRPr administration showed tumor localization of 4.5 +/- 3.0% ID/g at Day 4 and 3.9 +/- 3.5% ID/g at Day 7. The decreased localization at longer times after AdCMVGRPr infection correlated with in vitro results. The tumor uptake of [125I]-mIP-bombesin was comparable to the uptake of [131I]-mIP-bombesin (21.2 +/- 8.3% ID/g versus 15.4 +/- 5.6% ID/g, respectively), as was the normal tissue biodistribution.

CONCLUSIONS

The expression of GRPr in human ovarian cancer cells can be accomplished both in vitro and in vivo by using AdCMVGRPr, with the in vivo tumor localization of [125I]-mIP-bombesin being significantly greater than in control animals. The tumor localization of [125I]-mIP-bombesin and [131I]-mIP-bombesin at 2 days after AdCMVGRPr was comparable in a mouse model of human ovarian carcinoma. Injections of [125I]-mIP-bombesin at Days 4 and 7 after AdCMVGRPr infection resulted in tumor localization of [125I]-mIP-bombesin but at a level lower than 2 days. Thus, the total amount of radioactivity delivered to the tumor should be increased by multiple injections of [131I]-mIP-bombesin, which would be required for a therapeutic effect.

Experimental tumor targeting with radiolabeled ligands

BACKGROUND

Approaches have been developed in animal models to increase the localization of radiolabeled ligands (monoclonal antibodies and peptides) in tumors, to reduce their uptake in normal tissues, and to thus improve the tumor/normal tissue uptake ratios so that higher and more frequent doses of radionuclide could be used for radioimmunotherapy.

METHODS

These approaches to increase the localization of radiolabeled ligands in tumors involve the following three general strategies: (1) modifying ligands or radiolabeling techniques, (2) increasing blood and normal tissue clearance of radiolabeled ligands, and (3) modifying tumor delivery, tumor antigen, or receptor expression or increasing tumor vascular permeability or blood flow.

RESULTS

The use of such animal models permits the assessment of a wide range of ligands, radiolabeling conditions, and the efficacy of administration methods before their initial use in clinical trials. The prospects for the use of radiolabeled ligands in cancer detection and therapy are promising because of their specificity for binding to receptors on tumor cells or tumor endothelial cells.

CONCLUSIONS

Methods that increase the localization of radiolabeled ligands in solid tumors while reducing uptake in normal tissues will be required so that a sufficient radiation absorbed dose can be delivered for potentially curative treatment of radioresistant tumors in clinical radioimmunotherapy trials.

Synthesis of bombesin analogues for radiolabeling with rhenium-188

BACKGROUND

Gastrin-releasing peptide receptors (GRPR) are overexpressed in small cell lung carcinoma and some other human cancers. Small molecule peptides with antagonistic activities toward these receptors are potential radiotherapeutic agents.

METHODS

A 7-amino acid analogue of bombesin (BBN) was synthesized through solid-phase techniques. The peptide was conjugated to trisuccin prior to cleavage from the resin. The conjugate was hydrogenated to remove the hydroxamate-protecting benzyl groups followed by purification through reversed-phase high performance liquid chromatography (RP-HPLC). Rhenium-188 (188Re)-labeling of the trisuccin-peptide conjugate was performed by a SnCl2-reduced radioisotope and the labeled product was purified by RP-HPLC. The labeled conjugate was incubated with BNR-11 (3T3 mouse fibroblast cells stably transfected with murine GRPR) and PC-3 human prostate carcinoma GRPR positive cells. The nonradioactive peptide analogue was used as a competitive inhibitor and 125I-[Tyr4]-BBN was used as a positive control.

RESULTS

Solid-phase and solution phase synthesis afforded the conjugates of the hydroxamate ligand trisuccin with the 7-amino acid BBN analogue. The molecules differed by either a direct attachment of the trisuccin to the peptide (TrisBBN) or connection through a 6-carbon linker (TrisC6BBN). The overall yield for each synthesis was approximately 20%. Both conjugates showed the correct molecular weights on mass spectroscopy. Radiolabeling of the conjugates with 188Re were performed in > or = 90% yield. Cell-binding assays performed with BNR-11 (TrisBBN and TrisC6BBN) and PC-3 (TrisBBN) cell lines resulted in positive binding.

CONCLUSIONS

The synthesis and radiolabeling of Tris-BBN conjugates with 188Re were shown to be feasible. The yields of chemical syntheses and radiolabeling and positive binding of the radiolabeled conjugates to GRPR-positive tumor cells reveal promise in the use of these molecules for cancer imaging and therapy. More work is needed and is in progress to optimize the cell-binding properties.

Dosimetric comparison of bolus and continuous injections of CC49 monoclonal antibody in a colon cancer xenograft model

BACKGROUND

Improved understanding of dose and effective dose calculations may contribute to the optimization of fractionated radioimmunotherapy.

METHODS

Comparison three-dimensional tumor dosimetry was performed on athymic nude mice bearing established LS174T human colon carcinoma xenografts. Mice were given bolus intraperitoneal injections of 300 microCi 131I-labeled CC49 monoclonal antibody once (Day 0) or three times (Days 0, 3, and 7) or continuous intraperitoneal infusion with miniosmotic pumps over 7 days. Serial section autoradiography was used to reconstruct tumor activity density distributions for Days 3, 4, 7, 10, and 11 (single injection); Days 3, 4, 7, 8, and 11 (3 injections); and Days 4, 7, 10, and 13 (pump). At least three tumors were reconstructed at each time point. Uptakes in blood and tumor were measured up to 14 days (single injection), 11 days (3 injections), or 16 days (pump) after injection.

RESULTS

Average dose values calculated from total activity uptake data only (assuming no energy loss external to the tumor) yielded 102 Gy (single injection), 158 Gy (three injections), and 47 Gy (pump). Average doses using three-dimensional dose calculations were 88 Gy, 139 Gy, and 40 Gy, respectively. The nonuniformity of dose deposition affects treatment outcome, because cell loss is an exponential function of dose. Using the linear quadratic model with fractional cell survival to define an effective dose, D(eff) were calculated to be 20 Gy, 23 Gy, and 14 Gy, respectively. Cell proliferation affects outcome for variable dose-rate treatments. With cell proliferation parameters set to reproduce single-fraction 60Co recurrence results, D(eff) (for local control endpoint) were 8.9 Gy, 12.8 Gy, and 3.9 Gy, respectively. Three bolus injections compared with a single bolus injection were relatively less efficient in tumor uptake. However, three bolus injections resulted in a more uniform dose rate over a longer period, resulting in a 50% improvement in D(eff). The slower dose delivery for pump infusion resulted in a significantly lower D(eff), although dose-rate distributions were more uniform compared with the single bolus injection.

CONCLUSIONS

Improvement in dose-rate nonuniformities was observed for fractionated and continuous radiolabeled monoclonal antibody injections. Fractionated injections produced superior dosimetric results compared with single bolus or continuous injections.

Synthesis, rhenium-188 labeling and biodistribution studies of a phenolic ester derivative of trisuccin

Our previous results indicated that the trihydroxamate ligand, trisuccin, was a promising bifunctional chelating agent (BCA) for radiometal labeling of monoclonal antibodies with rhenium and technetium. An interest was developed to evaluate structural modifications of this compound from both synthetic and biological points of view. In this report we describe the synthesis of an esterified trisuccin (referred to as trisester), and conjugation of this new derivative to MAb CC49, radiolabeling of this conjugate with rhenium-188 (188Re), and biodistribution of the labeled conjugate in athymic nude mice. Thus, trisuccin (1) was esterified with benzyl 4-hydroxybenzoate in a DCC/DMAP reaction followed by removal of all benzyl protecting groups with catalytic hydrogenation. The resulting product was conjugated to CC49 by the active ester technique, through formation of the 2-nitrophenyl ester 6, and the conjugate was radiolabeled with generator-produced 188Re. The lead molecule trisuccin 1 was also conjugated to CC49 through the active ester 5 and the conjugate was radiolabeled by the same procedure to serve as the control conjugate. Biodistributions of the labeled conjugates were studied in athymic nude mice, transplanted s.c. with LS174T human colon cancer xenografts. Although an increase in the radiolabeling yield was observed for the esterified ligand-CC49 conjugate, as well as some increase in its immunoreactivity, as compared to those for the parent trisuccin molecule, there were no significant differences in their biodistribution. This new compound therefore may be useful in improving the conjugation and radiolabeling chemistries of this trihydroxamate ligand system.

Use of a novel cross-linking method to modify adenovirus tropism

Recombinant adenovirus (Ad) vectors can accomplish efficient in vivo gene transfer and thus are important in the context of a variety of gene therapy approaches. The cellular receptor for the Ad fiber knob is prevalent on a number of normal tissues which undermines the targeting of Ad to specific tumor cells. Therefore, the ablation of native Ad tropism and the introduction of novel Ad tropism are both necessary to target Ad vectors specifically to tumors. In this study, we have developed a flexible method for cross-linking the Fab fragment of a neutralizing anti-knob monoclonal antibody (1D6.14) to a cell receptor ligand. The cross-linking moieties are complementary low molecular weight recognition units, similar in concept to the avidin-biotin system. For proof of concept, we cross-linked 1D6.14 Fab to the basic fibroblast growth factor (FGF2). The Fab and FGF2 conjugates were synthesized and characterized both structurally and functionally. The conjugates were then complexed with an adenovirus vector carrying firefly luciferase (AdCMVLuc) and the resulting complex used to show infection of a number of tumor cell lines expressing FGF receptors. This cross-linking system should provide a rapid and convenient method of conjugating various ligands to the Fab fragment for targeting Ad vectors to different types of tumors.