DNA and RNA folds in transcription complex as evidenced by iodine-125 radioprobing

Abstract Folding of the DNA and RNA strands in an arrested T7 RNA polymerase (RNAP) transcription complex was studied by radioprobing, a novel method based on an analysis of the strand breaks produced by decay of the iodine-125 incorporated at the C5 position of cytosine. (125)I-labeled cytosines were incorporated into transcripts at different positions relative to the site of the arrest. The intensities of the DNA breaks inversely correlate with the distances from the (125)I decay site, so the radioprobing data provide information about the spatial RNA/DNA folding during transcription. We found that the yield of DNA strand breaks is significantly higher in the template than the non-template strand. This is consistent with local opening of the DNA duplex and formation of a hybrid between RNA and the template DNA strand. Our data demonstrate that the RNA-DNA hybrid has a nonuni form A-like structure. When the (125)I is incorporatedseven nucleotides from the active center of RNAP, the yield of strand breaks is substantially lower than if (125)I is positioned at the ends of the hybrid. Consequently, the DNA and RNA strands are located closer to each other at the ends of the hybrid and somehow separated in the middle. Surprisingly, the (125)I-induced breaks were detected in both DNA strands upstream from the transcription "bubble" indicating that DNA and RNA are closely associated outside the RNAP cleft. Thus, radioprobing data imply that the RNA/DNA fold in the complex with T7 RNAP is more complicated than had been anticipated by the existing models. Based on the present data, we suggest a sterically feasible model explaining how formation of the long RNA-DNA hybrid can result in the initiation-to-elongation switch in the T7 transcription complex. According to this model, the topological linkage between the DNA and RNA strands provides the necessary stability for the elongation complex, while permitting movement of the polymerase along the DNA duplex.

[Clinical mycoplasmosis outbreak due to Mycoplasma ovis in sheep from Shalta, Argentina. Clinical, microbiological and molecular diagnosis]

Mycoplasma ovis is an obligatory parasite of the erythrocytes from small ruminants (sheep, goat), wherein it causes chronic or acute anaemia. This agent shows worldwide distribution. However, its dispersion is still unknown in Argentina. This work describes an outbreak of mycoplasmosis occurred in January 2007 in a sheep flock from Rosario de la Frontera, Salta, Argentina. Adult sheep became ill with a mortality rate of 17.8%. All blood smears (n = 11) examined by Giemsa stain showed the presence of small basophile bodies characteristic of M. ovis infection, indicating a high prevalence of the infection in the flock. The molecular diagnosis (n = 9) confirmed the findings through the amplification of two fragments from the 16S rRNA gene. This is the third report of M. ovis in Argentina and the first one concomitant with clinical signs at flock level.

Microarray analysis of differentially expressed genes after exposure of normal human fibroblasts to ionizing radiation from an external source and from DNA-incorporated iodine-125 radionuclide

Exposure of cells to ionizing radiation (IR) produces changes in the expression level of a large number of genes. However, less is known of gene-expression changes caused by local radiation exposure from radionuclides within cells. We studied changes in the genome-wide gene expression induced by decay of 125I incorporated into DNA as [125I]-iododeoxyuridine (125I-IUdR) in normal IMR-90 human lung fibroblasts and compared them with the changes produced by external gamma-radiation delivered at high (HDR) or low (LDR) dose rate. We found that more than 2000 genes were consistently up- or down-regulated following HDR and LDR gamma-radiation. The profiles of differentially expressed genes following HDR and LDR shared about 64% (up) and 74% (down) genes in common, with many genes identified as radiation-responsive for the first time. In contrast, in all only 206 genes changed their expression level in the 125I-IUdR-treated cells, even though the total number of DNA double-strand breaks (DSB) produced by 125I-IUdR exceeded that produced by the gamma-radiation. With few exceptions, the expression levels of 125I-IUdR-responsive genes were also altered following gamma-irradiation. Therefore, nuclear DNA-localized decays of 125I produce 10 times fewer differentially expressed genes than whole-cell exposure to gamma-radiation of comparable dose. These results suggest that the effect of IR on the changes in global gene expression depends on the distribution of energy depositions within the cell. In contrast to cell survival, DNA DSB may not be the major factor modulating changes in gene expression following irradiation.

The issue of risk in complex adaptive systems: the case of low-dose radiation induced cancer

Living systems exist in hierarchical levels of biological organization, ascending from the basic atomic-molecular level, to the cellular level, the tissue-organ level, and the whole organism. All levels and elements at each level communicate with each other though intricate intra- and intercellular signaling through many specified molecular interactions. These regulate homeostasis between the system levels and their individual elements. The probability of a defined effect at the basic atomic-molecular level per impact increment of a toxic agent, such as ionizing radiation, at that level appears constant at low doses, even if the probability constant may change as a consequence of a previous exposure. Thus, at a given state of the system, the incidence of effect at the atomic-molecular level increases linearly with the number of impact increments in terms of energy deposition events. Primary effects may amplify to damage and there are immediate attempts at repairing the damage from an effect. Amplification and propagation of damage at, and from, the basic to higher levels of biological organization meets resistance, the degree of which per impact increment is not constant. It changes with the number of impact increments. This resistance encompasses both physico-chemical and biochemical reactions. The corresponding biochemical reactions express the physiological system's capacity to respond to perturbations of homeostasis at and between the various levels. Types and degrees of these responses depend on the system and the degree of homeostatic perturbation. At relatively mild to moderate degrees of perturbation, protective responses appear with a delay of hours and may last for months, shield also against endogenous non-radiogenic damage, and in doing so may prevail over radiogenic damage. With increasing degrees of homeostatic perturbation, damage eventually overwhelms adaptive protection. Thus, systems do not respond in a linear function of impact increments at the lowest level of biological organization. For assessing the probability of radiation damage per absorbed dose, i.e., risk, in complex adaptive systems, both damaging and protecting responses need attention, and to exclude one for the other is scientifically unjustified and misleading.

Dosimetry and risk from low- versus high-LET radiation of Auger events and the role of nuclide carriers

PURPOSE

To analyse the lethality to mammalian cells of (125)I-decays in DNA, in antipyrine in the whole cell and in oligodeoxynucleotides in the nucleus outside DNA as a function of Auger event-site and number.

MATERIALS AND METHODS

Auger events cause both low- and high-linear energy transfer energy depositions including charge neutralization at the daughter nuclide. Microdosimetry allows the expression of absorbed dose to a defined micromass and the number of such events at given sites. Published data were used to relate micromass dose and event number to the dose to reduce survival to 37% of the initial survival (D37).

RESULTS

The D37 of (125)I-decays in DNA was 0.1 Gy in terms of absorbed dose to the cell nucleus and about 30 in terms of average decays per nucleus or whole cell. The D37 of (125)I-decays in antipyrine was 1.5 Gy for absorbed dose to the cell nucleus, about 250 in terms of average decays per nucleus and about 2 x 10(3) for average decays per whole cell. (125)I-decays in oligodeoxynucleotides were much less toxic than (125)I-decays in antipyrine by a factor of about 25 in terms of average absorbed dose to the cell nucleus, by a factor or about 40 in terms of average decays per cell nucleus and by a factor of six in terms of average decays per whole cell.

CONCLUSION

The unexpected low toxicity of (125)I-decays in nuclear oligodeoxynucleotides outside the DNA in comparison with (125)I-decays in antipyrine in the nucleus or the whole cell demands further attention on the role of oligodeoxynucleotides in altering cellular radiation sensitivity.

Antisense radiotherapy: targeting full-size mdrl mRNA with 125I-labelled oligonucleotides

PURPOSE

Antisense radiotherapy is an approach based on the targeting of mRNA of specific genes by complementary oligonucleotide probes labelled with an Auger-electron-emitting radioisotope. Decay of the Auger emitter should specifically destroy the targeted mRNA while producing minimal damage to the rest of mRNA pool and the nuclear DNA. The feasibility of this approach was investigated by using full-length human multidrug-resistance gene (mdr1) mRNA as a target.

MATERIALS AND METHODS

Antisense oligonucleotides were labelled with [125I] I-dCTP by primer extension and annealed to target mRNA. Breaks in the target mRNA were analysed by denaturing polyacrylamide gel electriphoresis.

RESULTS

The efficiency of 125I-labelled antisense oligonucleotides in producing RNA strand breaks was tested on short synthetic RNA and DNA targets. The position and specificity of 125I-induced breaks in the full-length mRNA were then tested and compared with the cleavage of the target by RNase H. The distribution of the breaks in the longer mRNA is different from that in the short RNA targets, most likely due to a complex folding of RNA strands in the full-length mRNA.

CONCLUSIONS

The authors posit that 125I-labelled antisense probes could be useful not only for targeting mRNA, but also as probes for mRNA folding in vivo.

Physics must join with biology in better assessing risk from low-dose irradiation

This review summarises the complex response of mammalian cells and tissues to low doses of ionising radiation. This thesis encompasses induction of DNA damage, and adaptive protection against both renewed damage and against propagation of damage from the basic level of biological organisation to the clinical expression of detriment. The induction of DNA damage at low radiation doses apparently is proportional to absorbed dose at the physical/chemical level. However, any propagation of such damage to higher levels of biological organisation inherently follows a sigmoid function. Moreover, low-dose-induced inhibition of damage propagation is not linear, but instead follows a dose-effect function typical for adaptive protection, after an initial rapid rise it disappears at doses higher than approximately 0.1-0.2 Gy to cells. The particular biological response duality at low radiation doses precludes the validity of the linear-no-threshold hypothesis in the attempt to relate absorbed dose to cancer. In fact, theory and observation support not only a lower cancer incidence than expected from the linear-no-threshold hypothesis, but also a reduction of spontaneously occurring cancer, a hormetic response, in the healthy individual.

Antigene radiotherapy: targeted radiodamage with 125i-labeled triplex-forming oligonucleotides

Antigene radiotherapy is based upon damaging selected genes by a high dose of radiation from radionuclides delivered to this gene by a sequence-specific DNA-binding molecule. Here we describe our recent trials of antigene radiotherapy using the human mdr1 gene over-expressed in KB-V1 cells as a model. As a delivery molecule, we used a triplex-forming oligonucleotide (TFO) with a binding site in intron 14 of mdr1. This TFO was labeled with an Auger-electron-emitting radionuclide 125I. Decay of 125I releases a shower of low energy electrons that produce DNA strand breaks mostly within 10 bp from the decay site. Targeting in situ was assessed by restriction enzyme digestion of the DNA recovered from the TFO-treated cells followed by Southern hybridization with DNA probes flanking the target sequence. Double-strand breaks in the target sequence were detected in purified nuclei and digitonin-permeabilized cells, but not in the intact cells when TFO were delivered with liposomes. On the basis of these observations we hypothesized that there are cytoplasmic factors that bind such TFO and deliver them into the nucleus, but do not release them inside the nucleus, thus preventing TFO from binding their genomic targets. To test this hypothesis we (i) delivered TFO along with an excess of unlabeled oligonucleotide with an arbitrary sequence ("ballast") and (ii) conjugated TFO with a nuclear localization sequence peptide (NLS). We have found that TFO/NLS conjugates cleaved the target in a concentration-dependent manner regardless of the presence of the "ballast" oligonucleotide. In contrast, TFO without NLS cleaved the target only in the presence of an excess of the "ballast." These results may provide a new insight into the mechanism of intracellular transport of oligonucleotides.

Sequence-specific gene cleavage in intact mammalian cells by 125I-labeled triplex-forming oligonucleotides conjugated with nuclear localization signal peptide

Triplex-forming oligonucleotides (TFO) are designed to bind sequence specifically to their DNA targets without a significant disturbance of the double helix. They have been proposed to deliver DNA-reactive agents to specific DNA sequences for gene targeting applications. We suggested the use of 125I-labeled TFO for delivery of the energy of radioiodine decay to specific genes. This approach is called antigene radiotherapy. Here we demonstrate the ability of 125I-labeled TFO to produce sequence-specific breaks within a target in the human mdrl gene in cultured cells. TFO and TFO conjugated with a nuclear localization signal peptide (NLS) were delivered into cells using cationic liposomes. This was done either alone or in the presence of an excess of a "ballast" oligonucleotide with an unrelated sequence. In all cases, nuclear localization of TFO and survival of the cells after treatment has been confirmed. Breaks in the gene target were analyzed by restriction enzyme digestion of the DNA recovered from the TFO-treated cells followed by Southern hybridization with DNA probes flanking the target sequence. We have found that TFO/NLS conjugates cleave the target in a concentration-dependent manner regardless of the presence of the "ballast" oligonucleotide. In contrast, TFO without NLS cleaved the target only in the presence of an excess of the "ballast." We hypothesize that TFO and TFO/NLS are delivered into the nucleus by different pathways. These results provide a new insight into the mechanism of intracellular transport of oligonucleotides and open new avenues for improvement of the efficacy of antigene therapies.

Simulation of 125I induced DNA strand breaks in a CAP-DNA complex

The E. coli catabolite gene activator protein (CAP)-DNA complex with 125I located at the position of the H5 atom of the cytosine near the centre was incorporated into the PARTRAC track structure code. DNA strand breaks due to irradiation were calculated by track structure and radical attack simulations; strand breaks due to neutralisation of the highly charged 125Te ion were derived from a semi-empirical distribution. According to the calculations, the neutralisation effect dominates the strand breakage frequency at 2 bases away from the 125I decay site on both strands. The first breakage distribution counted from a 32P labelled end on the strand with 125I agreed well with experimental data, but on the opposite strand, the calculated distribution is more concentrated around the decay site and its yield is about 20% larger than the measured data.

A prospective study of 2-[18F] fluoro-2-deoxy-D-glucose/positron emission tomography scan, 99mTc-labeled arcitumomab (CEA-scan), and blind second-look laparotomy for detecting colon cancer recurrence in patients with increasing carcinoembryonic antigen levels

BACKGROUND

An increasing carcinoembryonic antigen (CEA) level in the absence of disease on imaging studies can present a diagnostic challenge. We evaluated 2-[18F] fluoro-2-deoxy-D-glucose and positron emission tomography (FDG-PET) scan and CEA scan before second-look laparotomy as a means of localizing recurrent colorectal cancer.

METHODS

Patients underwent computed tomography scan, bone scan, colonoscopy, and magnetic resonance imaging, and those without evidence of disease or resectable disease in the abdomen had FDG-PET and CEA scans. At second-look laparotomy, a surgeon blinded to the results of the FDG-PET and CEA scans performed an exploration and mapped findings. A second surgeon, with knowledge of the FDG-PET and CEA scans, then explored the patient; all lesions were biopsied or resected for pathology.

RESULTS

In 28 patients explored, disease was found at operation in 26 (94%). Ten had unresectable disease. FDG-PET scans predicted unresectable disease in 90% of patients. CEA scans failed to predict unresectable disease in any patient. In 16 patients found to have resectable disease or disease that could be treated with regional therapy, FDG-PET scan predicted this in 81% and CEA scan in 13%.

CONCLUSIONS

FDG-PET scan can predict those patients who would likely benefit from a laparotomy. If the FDG-PET scan indicates resectable disease, laparotomy can be considered. However, if the findings predict unresectable disease or the absence of disease, the patient should pursue systemic therapy or continued observation.

In vitro repair of complex unligatable oxidatively induced DNA double-strand breaks by human cell extracts

We describe a new assay for in vitro repair of oxidatively induced DNA double-strand breaks (DSBs) by HeLa cell nuclear extracts. The assay employs linear plasmid DNA containing DNA DSBs produced by the radiomimetic drug bleomycin. The bleomycin-induced DSB possesses a complex structure similar to that produced by oxidative processes and ionizing radiation. Bleomycin DSBs are composed of blunt ends or ends containing a single 5'-base overhang. Regardless of the 5'-end structure, all bleomycin-induced DSBs possess 3'-ends blocked by phosphoglycolate. Cellular extraction and initial end joining conditions for our assay were optimized with restriction enzyme-cleaved DNA to maximize ligation activity. Parameters affecting ligation such as temperature, time, ionic strength, ATP utilization and extract protein concentration were examined. Similar reactions were performed with the bleomycin-linearized substrate. In all cases, end-joined molecules ranging from dimers to higher molecular weight forms were produced and observed directly in agarose gels stained with Vistra Green and imaged with a FluorImager 595. This detection method is at least 50-fold more sensitive than ethidium bromide and permits detection of </=0.25 ng double-stranded DNA per band in post-electrophoretically stained agarose gels. Consequently, our end-joining reaction requires </=100 ng substrate DNA and >/=50% conversion of substrate to product is achieved with simple substrates such as restriction enzyme-cleaved DNA. Using our assay we have observed a 6-fold lower repair rate and a lag in reaction initiation for bleomycin-induced DSBs as compared to blunt-ended DNA. Also, end joining reaction conditions are DSB end group dependent. In particular, bleomycin-induced DSB repair is considerably more sensitive to inhibition by increased ionic strength than repair of blunt-ended DNA.

Strand breaks in whole plasmid dna produced by the decay of (125)I in a triplex-forming oligonucleotide

DNA strand breaks produced by the decay of (125)I positioned against a specific site in plasmid DNA via a triplex-forming oligonucleotide were studied both in the immediate vicinity of the site of the decay with a single nucleotide resolution and in the whole plasmid by measuring the percentages of supercoiled, open-circular and linear forms. The localized breaks are distributed within 10 bp in each direction from the decay site with maxima in both strands just opposite the (125)I-dC residue in the triplex-forming oligonucleotide. The distributions of breaks in the two DNA strands are almost symmetrical, in agreement with the geometry of the pyrimidine motif triplex. We found that about 25% of the double-strand breaks were located outside the 90-bp fragment containing the triplex-forming oligonucleotide binding sequence. The ratio of single- to double-strand breaks in the whole plasmid was 11 for bound triplex-forming oligonucleotide compared to 26 when the triplex-forming oligonucleotide was free in solution. The number of double-strand breaks per decay of (125)I was 0.46 for bound triplex-forming oligonucleotide and 0.17 for free triplex-forming oligonucleotide. Comparing the data on the localized damage and those for the whole plasmid, we concluded that, in addition to DNA breaks that are confined to a helical turn around the (125)I atom, the decay can produce breaks hundreds of base pairs away in the plasmid molecule. This linear plasmid molecule containing radiation-induced damage at a specific DNA site should be useful in studies of the molecular mechanisms of DNA repair.

Distribution of DNA strand breaks produced by iodine-123 and indium-111 in synthetic oligodeoxynucleotides

Antigene radiotherapy, a procedure based on delivery of short-range Auger-electron-emitting radioisotopes to target genes via sequence-specific triplex-forming oligonucleotides, has been successfully demonstrated in vitro using the well-studied radionuclide 125I. To proceed with in vivo trials, Auger electron emitters with shorter half-lives than 125I are required. Here we report a study of the efficiency and distribution of sequence-specific DNA strand breaks produced by decay of 123I and mIIn. 123I and 111In were introduced into triplex-and duplex-forming oligodeoxyribonucleotides (ODNs) through carbohydrate linkers of various lengths. Labeling with radioiodine was performed through tributylstannylbenzamide intermediates while 111In was attached via DTPA. The Auger-emitter-labeled ODNs were hybridized to a single-stranded DNA target, to form duplexes. After decay accumulation, the target DNA samples were assayed for strand breaks using a sequencing gel-electrophoresis technique. For the first time, we observed footprints of DNA strand breaks produced by 123I and 111In. Most of the breaks were located within 10 nucleotides from the decay site. The yield of strand breaks per decay varies; decay of 111In breaks DNA almost 10 times more effectively than decay of 123I. Both 123I and 111In are less effective in breaking DNA strands than 121I, which reflects the higher total energy of the Auger decay process of 125I.

Development of DNA-based radiopharmaceuticals carrying Auger-electron emitters for antigene radiotherapy

PURPOSE

Antigene radiotherapy (AR) is based on targeting localized radiodamage to specific sites in the genome by using sequence-specific triplex-forming oligonucleotides (TFO) to carry Auger-electron-emitters (A-Ettr) such as Iodine-125 (125I) to the target gene sequence. The radiodecay of an A-Ettr produces a cascade of low-energy electrons and creates a highly positively-charged daughter atom; delivered by a TFO, it should produce double-strand breaks (dsb) localized to the specific DNA target sequence. The result should be a "knock-out" of the targeted gene.

METHODS AND MATERIALS

As a model, we used the MDR1 gene amplified nearly 100 times in the human KB-V1 carcinoma cell line. Chemically modified TFO complementary to the polypurine/polypyrimidine region of the MDR1 gene were synthesized and radiolabeled with 125I-dCTP by the primer extension method. Purified plasmid and genomic DNA and extracted nuclei were treated with 125I-TFO and analyzed for sequence-specific cleavage by electrophoresis in agarose gel and Southern hybridization.

RESULTS

We created 125I-TFO that could effectively recognize, bind, and cleave the target sequence in plasmid and genomic DNA. We showed that these 125I-TFO in nanomolar concentrations were able to cleave the target MDR1 gene sequence in a natural environment, i.e., within the eucaryotic nucleus.

CONCLUSION

125I-TFO can effectively introduce sequence-specific dsb to a target within the MDR1 gene, both in purified DNA and inside intact nuclei. Chemically modified TFO conjugated with nuclear localization signal appear to be a promising delivery vehicle for future in vivo trials of AR.

Targeting the human mdr1 gene by 125I-labeled triplex-forming oligonucleotides

Antigene radiotherapy is our approach to targeting specific sites in the genome by combining the highly localized DNA damage produced by the decay of Auger electron emitters, such as 125I, with the sequence-specific action of triplex-forming oligonucleotides (TFO). As a model, we used the multidrug resistance gene (mdr1) overexpressed and amplified nearly 100 times in the human KB-V1 carcinoma cell line. Phosphodiester pyrrazolopyrimidine dG (PPG)-modified TFO complementary to the polypurine-polypyrimidine region of the mdr1 gene were synthesized and labeled with 125I-dCTP at the C5 position of two cytosines by the primer extension method. 125I-TFO were delivered into KB-V1 cells with several delivery systems. DNA from the 125I-TFO-treated cells was recovered and analyzed for sequence-specific cleavage in the mdr1 target by Southern hybridization. Experiments with plasmid DNA containing the mdr1 polypurine-polypyrimidine region and with purified genomic DNA confirmed the ability of the designed 125I-TFO to bind to and introduce double-strand breaks into the target sequence. We showed that 125I-TFO in nanomolar concentrations can recognize and cleave a target sequence in the mdr1 gene in situ, that is, within isolated nuclei and intact digitonin-permeabilized cells. Our results demonstrate the ability of 125I-TFO to target specific sequences in their natural environment, that is, within the eukaryotic nucleus. The nearly 100-fold amplification of the mdr1 gene in KB-V1 cells affords a very useful cell culture model for evaluation of methods to produce sequence-specific DNA double-strand breaks for gene-specific radiotherapy.

Development of DNA-based radiopharmaceuticals carrying Auger-electron emitters for anti-gene radiotherapy

Targeting of radiation damage to specific DNA sequences is the essence of antigene radiotherapy. This technique also provides a tool to study molecular mechanisms of DNA repair on a defined, single radiodamaged site. We achieved such sequence-specific radiodamage by combining the highly localized DNA damage produced by the decay of Auger-electron-emitters such as 125I with the sequence-specific action of triplex-forming oligonucleotides (TFO). TFO complementary to polypurine-polypyrimidine regions of human genes were synthesized and labeled with 125I-dCTP by the primer extension method. 125I-TFO were delivered into cells with several delivery systems. In addition, human enzymes capable of supporting DNA single-strand-break repair were isolated and assessed for their role in the repair of this lesion. Also, the mutagenicity and repairability of 125I-TFO-induced double strand breaks (DSB) were assessed by repair of a plasmid possessing a site-specific DSB lesion. Using plasmids containing target polypurine-polypyrimidine tracts, we obtained the fine structure of sequence-specific DNA breaks produced by decay of 125I with single-nucleotide resolution. We showed that the designed 125I-TFO in nanomolar concentrations could bind to and introduce double-strand breaks into the target sequences in situ, i.e., within isolated nuclei and intact digitonin-permeabilized cells. We also showed 125I-TFO-induced DSB to be highly mutagenic lesions resulting in a mutation frequency of nearly 80%, with deletions comprising the majority of mutations. The results obtained demonstrate the ability of 125I-TFO to target specific sequences in their natural environment--within eucaryotic nucleus. Repair of 125I-TFO-induced DNA damage should typically result in mutagenic gene inactivation.

Radioprobing of a RecA-three-stranded DNA complex with iodine 125: evidence for recognition of homology in the major groove of the target duplex

A fundamental problem in homologous recombination is how homology between DNAs is recognized. In all current models, a recombination protein loads onto a single strand of DNA and scans another duplex for homology. When homology is found, a synaptic complex is formed, leading to strand exchange and a heteroduplex. A novel technique based on strand cleavage by the Auger radiodecay of iodine 125, allows us to determine the distances between (125)I on the incoming strand and the target sugars of the duplex DNA strands in an Escherichia coli RecA protein-mediated synaptic complex. Analysis of these distances shows that the complex represents a post-strand exchange intermediate in which the heteroduplex is located in the center, while the outgoing strand forms a relatively wide helix intertwined with the heteroduplex and located in its minor groove. The structure implies that homology is recognized in the major groove of the duplex.

DNA cleavage by 111In-labeled oligodeoxyribonucleotides

We studied the fine structure of DNA damage produced by the decay of 111In incorporated into duplex and triplex DNA strands to evaluate the usefulness of this radionuclide for sequence-specific DNA cleavage.

METHODS

Oligodeoxyribonucleotides (ODNs) were prepared with 111In attached by diethylenetriaminepentaacetic acid (DTPA) at the 5' end or 3' end through a long chemical linker or to an internal nucleotide position through a short linker. Subsequent formation of DNA duplexes and triplexes was confirmed by gel electrophoresis. The 111In-induced breaks were assayed in denaturing polyacrylamide gel electrophoresis with a single-nucleotide resolution.

RESULTS

111In-labeled oligonucleotides of high specific activity (740-1554 TBq/mmol) were synthesized. The presence of the bulky 111In-DTPA group did not impede duplex or triplex formation. Localized DNA breaks were observed in all duplexes and triplexes formed. The majority of DNA breaks in duplex formations were located within +/- 10 nucleotides from the site of attachment of the 111In-bearing linker. The yield of DNA breaks per decay was 0.38 in a duplex with internally modified ODNs. This is nearly 2 times less than the yield of DNA breaks in the same duplex with 1251 attached through the same linker. The yield of DNA breaks in the pyrimidine and purine strands of DNA triplexes with 111In attached to the triplex-forming ODNs through the linkers of different length varied from 0.05 to 0.10. The distribution of DNA breaks was wider in comparison with the duplex experiment. The lower yields of breaks per 111In decay compared with 125I may be not only the result of lower deposited energy but also of the ionic repulsion of the negatively charged 111In-DTPA group from the DNA strands.

CONCLUSION

We have shown that decay of 111In produces highly localized DNA breaks. 111In introduced into triplex- and duplex-forming ODNs through hydrocarbon linkers produces sequence-specific DNA strand breaks with an efficiency nearly comparable with that of 1251. These findings are supportive of our proposed use of 111In-ODNs for gene-specific radiotherapy.

Pulmonary lymphangioleiomyomatosis: correlation of ventilation-perfusion scintigraphy, chest radiography, and CT with pulmonary function tests

PURPOSE

To determine the findings on ventilation-perfusion (V-P) scintigrams, computed tomographic (CT) scans, and chest radiographs and correlate them with pulmonary function test results in patients with lymphangioleiomyomatosis.

MATERIALS AND METHODS

V-P scintigraphy, chest radiography, conventional and thin-section CT, and pulmonary function tests were performed in 39 patients. The images were graded on a scale of 0 (normal) to 3 (severely abnormal).

RESULTS

Imaging abnormalities were found on 92% of ventilation scintigrams, 92% of perfusion scintigrams, 79% of chest radiographs, 100% of CT scans, and 100% of thin-section CT scans. On ventilation scintigrams, 28 (72%) patients demonstrated a speckling pattern. On CT scans, all patients had pulmonary cysts. Univariate analysis showed that extent of disease on chest radiographs and CT scans, cyst size, V-P abnormalities, and degree of speckling were inversely correlated with forced expiratory volume in one second (FEV(1)), diffusing capacity of lung for carbon monoxide, and the ratio of FEV(1) to forced vital capacity (FVC) (P <.01) but not with FVC and total lung capacity. Larger cyst size correlated with extent of disease at CT, but not significantly (P =.056).

CONCLUSION

Scintigraphic and radiologic abnormalities are seen in a majority of patients with lymphangioleiomyomatosis. On ventilation scintigrams, a frequently seen speckling pattern may be related to accumulation of radionuclide in pulmonary cysts-a hallmark of the disease at CT. Findings with each imaging modality correlate with certain pulmonary functions.

Gene targeted DNA double-strand break induction by (125)I-labeled triplex-forming oligonucleotides is highly mutagenic following repair in human cells

A parallel binding motif 16mer triplex-forming oligonucleotide (TFO) complementary to a polypurine-polypyrimidine target region near the 3'-end of the SupF gene of plasmid pSP189 was labeled with [5-(125)I]dCMP at position 15. Following triplex formation and decay accumulation, radiation-induced site-specific double-strand breaks (DSBs) were produced in the pSP189 SupF gene. Bulk damaged DNA and the isolated site-specific DSB-containing DNA were separately transfected into human WI38VA13 cells and allowed to repair prior to recovery and analysis of mutants. Bulk damaged DNA had a relatively low mutation frequency of 2.7 x 10(-3). In contrast, the isolated linear DNA containing site-specific DSBs had an unusually high mutation frequency of 7.9 x 10(-1). This was nearly 300-fold greater than that observed for the bulk damaged DNA mixture, and >1.5 x 10(4)-fold greater than background. The mutation spectra displayed a high proportion of deletion mutants targeted to the(125)I binding position within the SupF gene for both bulk damaged DNA and isolated linear DNA. Both spectra were characterized by complex mutations with mixtures of changes. However, mutations recovered from the linear site-specific DSB-containing DNA presented a much higher proportion of complex deletion mutations.

The stability of DNA triplexes inside cells as studied by iodine-125 radioprinting

We studied the stability of a DNA triplex resulting from the binding of a 38 nt long purine motif triplex-forming oligonucleotide (TFO) to a covalently closed plasmid containing a target sequence from the human HPRT gene. Our in vitro experiments showed that the triplex formed at plasmid and TFO concentrations as low as 10(-9)M. Once formed, the triplex was remarkably stable and could withstand 10 min incubation at 65 degrees C. We next delivered these TFO-plasmid complexes into cultured human cells. To monitor the TFO-plasmid complexes inside cells we applied a new technique that we call 'radioprinting'. Because the TFO was(125)I labeled, we could quantitatively monitor the triplexes by measuring(125)I-induced DNA strand breaks in the target plasmid sequence. We found that the triplexes remain stable inside the cells for at least 48 h. Based on these findings we propose using TFO for indirect labeling of intact plasmid DNA. As a demonstration, we show that the intracellular distribution of a fluorescein-labeled TFO was different when it was liposome-delivered into cultured human cells alone or in a complex with the plasmid. In the latter case, the fluorescence was detected in nearly all the cells while detection of the plasmid by use of a marker gene (beta-galactosidase) revealed expression of the gene in only half of the cells.

Glut-1 and hexokinase expression: relationship with 2-fluoro-2-deoxy-D-glucose uptake in A431 and T47D cells in culture

Uptake of 2-[18F]-2-deoxy-D-glucose (FDG) has been used as a marker of increased glucose metabolism to visualize, stage, and monitor progression of human cancers with positron emission tomography. Many human tumors have been shown to overexpress the Glut-1 glucose transport protein. The aim of this study is to define whether a quantitative relationship exists between the amount of Glut-1 expressed by cells and their ability to accumulate FDG. We characterized the expression of the known facilitative and sodium-dependent glucose transporter isoforms in six different cancer cell lines used in our laboratory (A431, MDA-MB-231, T47D, CaCo II, MCF7, and HepG2). A431 and T47D cells express, respectively, the highest and lowest amount of Glut-1 mRNA by Northern blot of all of the cells analyzed, and no other glucose transporter isoforms were detectable by this method in both cell lines. Both total and plasma membrane-associated Glut-1 protein levels were higher in A431 than in T47D cells, consistent with the higher Glut-1 mRNA levels. However, T47D cells accumulate FDG more rapidly than do A431 cells. 3-O-Methylglucose transport is higher in A431 cells. Although hexokinase I and II mRNA levels are higher in A431 cells than in T47D cells, the ability of mitochondrial preparations to phosphorylate FDG is higher in T47D cells. Our data indicate that in these cultured cells, FDG uptake correlates better with FDG phosphorylating activity of mitochondrial preparations rather than the level of expression of the Glut-1 or hexokinase I and II genes.

Targeting of transferrin receptors in nude mice bearing A431 and LS174T xenografts with [18F]holo-transferrin: permeability and receptor dependence

The goal of this study was to investigate whether 18F-labeled transferrin (Tf), which has a molecular weight (Mr) of approximately 79,000, binds to Tf receptor sites in tumors in a specific manner within the time frame commensurate with the half-life of 18F (109.7 min). We have previously shown that [18F]holo-Tf ([18F]Tf) maintains all properties of native Tf in vitro and that it can specifically target liver Tf receptor sites in vivo.

METHODS

The distribution of [18F]Tf, using [18F]albumin (Alb) or [14C]Alb as a control, was studied over a 6-h period in nude mice bearing LS174T and A431 xenografts of a high- and low-permeability tumor, respectively.

RESULTS

Measurements of Tf receptor concentration in the tumor extracts suggest similar binding capacities. In vivo, liver uptake values were higher for [18F]Tf than for both [18F]Alb and [14C]Alb throughout the study, indicating specific binding. In contrast, tumor Tf uptake values remained below those of the Alb tracers, and tumor-to-blood ratios of [18F]Tf in each xenograft increased in parallel with those of the Alb tracers. The permeabilities of [14C]Alb and [18F]Tf in LS174T were calculated to be 1.29+/-0.49 and 1.03+/-0.38 microL/min/g (mean +/- SD), respectively, whereas the permeabilities of the two tracers in A431 were 0.79+/-0.24 and 0.44+/-0.04 microL/min/g. Pharmacokinetic modeling of the data using these permeabilities and the high plasma and extracellular concentrations of endogenous Tf showed that the observed uptake values in the two xenografts are consistent with a non-receptor-mediated distribution. In the liver, the absence of permeability barriers yields specific [18F]Tf binding to receptors compared with the [14C]Alb control, within 5 min after injection.

CONCLUSION

Receptor-mediated accumulation of [18F]Tf in tumor xenografts is impaired by rate-determining permeability and competition from endogenous Tf and is not achieved in a time frame of 6 h.

Detecting the DNA kinks in a DNA-CRP complex in solution with iodine-125 radioprobing

Auger-electron-emitting radioisotopes such as 125I produce DNA strand breaks within nanometer range of the decay site. Here we analyze these breaks in order to study changes in DNA conformation upon binding with cyclic AMP receptor protein (CRP) in solution. The clear difference we found in break frequency in the CRP-DNA complex, as compared to the naked DNA duplex, correlates with the increased distances between the deoxyriboses and the radioiodine atom caused by the CRP-induced kink observed in the cocrystal. Thus, we demonstrate that 125I radioprobing can be used to study fine conformational changes of DNA within DNA-protein complexes.

Determination of human DNA polymerase utilization for the repair of a model ionizing radiation-induced DNA strand break lesion in a defined vector substrate

Human DNA polymerase and DNA ligase utilization for the repair of a major class of ionizing radiation-induced DNA lesion [DNA single-strand breaks containing 3'-phosphoglycolate (3'-PG)] was examined using a novel, chemically defined vector substrate containing a single, site-specific 3'-PG single-strand break lesion. In addition, the major human AP endonuclease, HAP1 (also known as APE1, APEX, Ref-1), was tested to determine if it was involved in initiating repair of 3'-PG-containing single-strand break lesions. DNA polymerase beta was found to be the primary polymerase responsible for nucleotide incorporation at the lesion site following excision of the 3'-PG blocking group. However, DNA polymerase delta/straightepsilon was also capable of nucleotide incorporation at the lesion site following 3'-PG excision. In addition, repair reactions catalyzed by DNA polymerase beta were found to be most effective in the presence of DNA ligase III, while those catalyzed by DNA polymerase delta/straightepsilon appeared to be more effective in the presence of DNA ligase I. Also, it was demonstrated that the repair initiating 3'-PG excision reaction was not dependent upon HAP1 activity, as judged by inhibition of HAP1 with neutralizing HAP1-specific polyclonal antibody.

What is new in nuclear medicine imaging?

Modern nuclear medicine tests can be of great help to today's surgical oncologists. The new techniques of positron emission tomography (PET) and single photon emission computed tomography (SPECT), and the availability of new radiopharmaceuticals for localization of tumors and assessment of organ function provide information vital to the management of oncology patients.

Phase I study of the pharmacokinetics of a radioimmunoconjugate, 90Y-T101, in patients with CD5-expressing leukemia and lymphoma

Ten patients with advanced or refractory CD5-expressing hematologic neoplasms [two with chronic lymphocytic leukemia and eight with cutaneous T-cell lymphoma (CTCL)] were treated in a Phase I study with the radioimmunoconjugate 90Y-T101, which targets CD5+ lymphocytes. Prior imaging studies using 111In-T101 demonstrated uptake in involved lymph nodes and skin in patients with CTCL, and Phase I studies with unmodified T101 demonstrated transient responses. In this study, patients were treated with 5 or 10 mCi of 90Y chelated to T101 via isothiocyanatobenzyl diethylenetriamine pentaacetic acid, along with tracer doses of 111In-T101 for imaging. The biodistribution of the radioimmunoconjugate was determined by measuring 90Y and 111In blood clearance, urine excretion, and accumulation in bone marrow and in involved skin lesions. The intravascular pharmacokinetics of 90Y were predicted by 111In-labeled T101. The greatest differences in biodistribution between 111In and 90Y were in the higher bone accumulation of 90Y and its lower urinary excretion. Imaging studies demonstrated targeting of skin lesions and involved lymph nodes in CTCL patients. The predominant toxicity was bone marrow suppression. Rapid antigenic modulation of CD5 on circulating T and B cells was observed. Recovery of T-cell populations occurred within 2-3 weeks; however, suppression of B-cell populations persisted after 5+ weeks. All CTCL patients developed human antimouse antibody after one cycle and thus were not retreated; one patient with chronic lymphocytic leukemia received a second cycle of therapy. Partial responses occurred in five patients, two with chronic lymphocytic leukemia and three with CTCL. The median response duration was 23 weeks. One CTCL patient who subsequently received electron beam irradiation to a residual lesion is disease-free after 6 years.

Radiotoxicity of iodine-125-labeled oligodeoxyribonucleotides in mammalian cells

We investigated the distribution, stability and radiotoxicity of 125I-oligodeoxyribonucleotides (125I-ODN) in human fibrosarcoma HT-1080 cells to study the radiotoxic effects of the Auger electron emitter 125I delivered to the cells by ODN.

METHODS

We delivered 125I-ODN into the cells via complexing with a liposomal delivery system. To assess the intracellular distribution and stability of 125I-ODN delivered by the liposomal delivery system, we used autoradiography, fluorescent and confocal microscopy and electrophoresis. To study the radiotoxicity of the unbound 125I-ODN, we used a clonogenic assay. The radiotoxicity of 125I-ODN delivered by the liposomal delivery system was compared with that of freely diffusible 125I-antipyrine, membrane-excluded 125I-bovine serum albumin and DNA incorporated 125I-deoxyuridine (125I-UdR).

RESULTS

Oligodeoxyribonucleotides accumulated in the cell nucleus within a few hours of incubation. On the basis of the number of decays at 37% survival, 125I-ODN are 2 times more radiotoxic than 125I-antipyrine, which is freely diffusible into cells, and 8 times more radiotoxic than 125I-bovine serum albumin, which remains outside cells. However, the radiotoxicity of unbound 125I-ODN is almost 3 orders of magnitude lower than that of DNA-incorporated 125I-UdR. The 125I-ODN are not significantly degraded by intracellular nucleases during the time of uptake incubation.

CONCLUSION

The dramatic difference in radiotoxicity between 125I-ODN and 125I-UdR confirms that, despite the nuclear localization, 125I-ODN are not bound to or incorporated within the genomic DNA. Our data demonstrate that the radiotoxicity of Auger electron emitters is determined by the radiation dose delivered to nuclear DNA, not necessarily to the nucleus. Therefore, relatively high intracellular concentrations of unbound 125I-ODN can be achieved without causing significant cell death.

Effect of radionuclide linker structure on DNA cleavage by 125I-labeled oligonucleotides

We studied the yield and distribution of DNA strand breaks produced by decay of 125I introduced into triplex-forming and duplex-forming oligodeoxyribonucleotide (ODNs) through linkers of various lengths. ODNs were prepared with 125I attached at the 5'-end with a long linker or to an internal nucleotide position with a short linker. The 125I-ODNs were hybridized to either a single-stranded target to form duplexes or to a double-stranded target to form triplexes. After decay accumulation, the duplex and triplex samples were assayed for strand breaks in a sequencing gel. The yield of strand breaks per decay was 0.34 for duplex with the 5'-modified ODN and 0.66 for duplex with internally modified ODN. The triplex samples with internal 125I have different yields of DNA breaks in the pyrimidine and purine strands, 0.16 and 0.37, respectively. The yield of DNA breaks in the pyrimidine strand of the triplex with the 5'-modified ODN is 0.46. The majority of breaks are located within 5 nucleotides from the decay site. The yield of strand cleavage per decay of 125I was nearly two-fold lower with the described linkers in comparison with the results obtained when 125I is directly attached to the C-5 position of cytosine. Nevertheless, the rapid iodination procedure reported here combined with the possibility of multiple incorporations of 125I on the linkers makes such 125I-ODNs promising agents for sequence-specific cleavage of DNA.

Radioprobing of DNA: distribution of DNA breaks produced by decay of 125I incorporated into a triplex-forming oligonucleotide correlates with geometry of the triplex

The distribution of breaks produced in both strands of a DNA duplex by the decay of 125I carried by a triplex-forming DNA oligonucleotide was studied at single nucleotide resolution. The 125I atom was located in the C5 position of a single cytosine residue of an oligonucleotide designed to form a triple helix with the target sequence duplex. The majority of the breaks (90%) are located within 10 bp around the decay site. The addition of the free radical scavenger DMSO produces an insignificant effect on the yield and distribution of the breaks. These results suggest that the majority of these breaks are produced by the direct action of radiation and are not mediated by diffusible free radicals. The frequency of breaks in the purine strand was two times higher that in the pyrimidine strand. This asymmetry in the yield of breaks correlates with the geometry of this type of triplex; the C5 of the cytosine in the third strand is closer to the sugar-phosphate backbone of the purine strand. Moreover, study of molecular models shows that the yield of breaks at individual bases correlates with distance from the 125I decay site. We suggest the possible use of 125I decay as a probe for the structure of nucleic acids and nucleoprotein complexes.

Diagnostic imaging in patients with paragangliomas. Computed tomography, magnetic resonance and MIBG scintigraphy comparison

OBJECTIVE

We compared iodine-131 metaiodobenzylguanidine (MIBG), computed tomography (CT) and magnetic resonance (MR) imaging studies in 40 patients with suspected or proven paragangliomas.

PATIENTS

21 patients were studied during the initial evaluation, while 19 were evaluated after surgery for paragangliomas. Furthermore, in 18 patients with benign (n = 10) or malignant (n = 8) tumors, MIBG uptake, MR signal intensity ratios (SIR) and CT parameters were analyzed.

RESULTS

In patients studied during the initial evaluation, CT and MR had a significant (p < 0.05) higher diagnostic sensitivity (100% for both) compared to MIBG (82%); conversely, MIBG showed a higher specificity (100%) compared to CT and MR (50% for both); the accuracy of MIBG (86%), CT and MR (91% for both) were similar. In patients evaluated after surgery, MIBG and MR had slightly better sensitivity (85% for both) compared to CT (77%). MIBG showed again better specificity (100%) than CT and MR (83% for both); the accuracy of MIBG, MR and CT were respectively 90%, 84% and 79%. While no significant differences were observed in MR SIR and CT findings between benign and malignant tumors, MIBG uptake was significantly higher in malignant compared to benign lesions (p < 0.03).

CONCLUSIONS

Our results suggest that CT and MR are particularly useful in the initial evaluation of patients with suspected paragangliomas. MIBG should be recommended during the post-surgical follow-up of such patients since recurrent, malignant or extra-adrenal disease frequently occur. Finally, while MR SIR and CT features are not able to distinguish malignant paragangliomas from benign tumors, MIBG uptake is higher in malignant lesions compared to benign tumors providing a diagnostic criterion to differentiate these lesions.

Radiolabeled peptides and other ligands for receptors overexpressed in tumor cells for imaging neoplasms

For better detection of neoplasms by scintigraphy, ligands that bind specifically to surface receptors overexpressed in tumor cells are being developed. These ligands must be labeled with the readily available radionuclides of iodine, [111In] or [99mTc], without blocking their interaction with the specific cell membrane receptors. The promising ligands include bioactive endogenous peptides or their analogues, inhibitors of glucose transport proteins, estrogen and sigma receptor ligands, growth factors, and cytokines.

Evaluation of human transferrin radiolabeled with N-succinimidyl 4-[fluorine-18](fluoromethyl) benzoate

Iron metabolism plays a key role in cell proliferation and survival in rapidly growing cancer cells. Uptake is mediated by the carrier protein transferrin. The increased need for iron has been used as a method to target tumors and there is well-documented evidence that certain tumors can be imaged with tracers such as 67Ga, that mimic transferrin-mediated iron uptake. To obtain a tracer that would be better able to quantitate transferrin kinetics and indirectly evaluate iron metabolism, we have labeled human transferrin with the positron emitter, 18F, with a one-step high-specific activity method developed in our laboratory.

METHODS

We measured the binding affinities of [18F]diferric (holo-) and iron-free (apo-) transferrin on two human cell lines. We also compared cellular uptake of [18F]holo-transferrin and [67Ga]citrate in various conditions, and washout of label incorporated into cells.

RESULTS

The binding affinity of [18F]holo-transferrin was found to be the same as that reported for [125I]holo-transferrin. In our hands there was no significant difference in binding affinity between diferric holo-transferrin and iron-free apo-transferrin. [18F]holo-transferrin uptake rapidly reaches a steady-state equilibrium between the intracellular and extracellular environment, while gallium accumulation linearly increases with time. [18F]holo-transferrin is rapidly recycled out of the cell with similar kinetics to those reported for [125I]holo-transferrin.

CONCLUSION

[18F]holo-transferrin displays the properties of native transferrin and appears suitable for quantitative evaluation of transferrin kinetics in vivo.

Transferrin dependence of Ga (NO3)3 inhibition of growth in human-derived small cell lung cancer cells

The effect of a combination of anti-transferrin receptor (TFR) antibody, 42/6, and Ga(NO3)3 on cell growth was examined in small cell lung cancer (SCLC) cell lines: classic, NCI-H209, NCI-H345, NCI-H510; and variant, NCI-H82 and NCI-N417. The role of TFR and transferrin (TF) in Ga(NO3)3 cellular uptake was also tested. Exogenous TF did not enhance the cytotoxicity of Ga. At > 3 micrograms/mL, Ga(NO3)3 inhibited growth in all cell lines in TF-supplemented or deficient media. At < 3 micrograms/mL, Ga stimulated growth for all cells but this effect was eliminated by TF or 42/6. Classic SCLC lines required 3-4-fold less exogenous gallium than variant lines to reduce cell number by 50%. The mean Ga uptake (ng/10(6) cells) in H345 and H209 cell lines was 4-5-fold compared to H82 and N417 uptake (P < 0.001). 42/6 reduced exogenous TF-stimulated growth. Antibody plus Ga(NO3)3 caused a slight further cell number decline in all cell lines in TF-supplemented or deficient media. These results suggest that the addition of 42/6 antibody treatment would not increase the effectiveness of Ga(NO3)3 in patients. Both exogenous and endogenous TF and TFR play an important role in Ga uptake in these cells.

Ultrastructure of selected struvite-containing urinary calculi from cats

OBJECTIVE

To elucidate the ultrastructural details of struvite-containing urinary calculi from cats.

DESIGN

Specimens studied were inclusive of the range of textures visible during preliminary analysis by use of a stereoscopic dissecting microscope. Textural types, which were used to infer crystal growth conditions, were differentiated with regard to crystal habit, crystal size, growth orientation, and primary porosity.

SAMPLE POPULATION

Thirty specimens were selected from a collection of approximately 1,600 feline urinary calculi: 20 of these were composed entirely of struvite, and 10 consisted of struvite and calcium phosphate (apatite).

PROCEDURE

Qualitative and quantitative analyses of specimens included use of plain and polarized light microscopy, x-ray diffractometry, scanning electron microscopy with backscattered electron imagery, x-ray fluorescence scans, and electron probe microanalysis.

RESULTS

Four textural types were recognized among struvite calculi, whereas 2 textural types of struvite-apatite calculi were described.

CONCLUSIONS

The presence of minute, well interconnected primary pores in struvite-containing urinary calculi from cats is an important feature, which may promote possible interaction of calculi with changes in urine composition.

CLINICAL RELEVANCE

Primary porosity, which can facilitate interaction between the calculus and changing urine composition, may explain the efficacy of dietary or medicinal manipulations to promote the dissolution of struvite-containing uroliths from this species.

Sequence-specific DNA breaks produced by triplex-directed decay of iodine-125

Triplex forming oligonucleotides (TFO) labeled with Auger emitters could be ideal vehicles to deliver radioactive-decay energy to specific DNA sequences, causing DNA breaks and, subsequently, inactivation of these sequences. To demonstrate this approach we labeled with 125I (two 125I per molecule on average) a purine-rich 38-mer which forms a stable triplex with a polypurine x polypyrimidine stretch in the human HPRT gene. Decay of 125I in the bound TFO was shown to cause sequence-specific double strand breaks (DSB) in the target HPRT sequence cloned into plasmid DNA. No sequence-specific breaks were observed if 125I-labeled TFO were not bound to the plasmid DNA. After 60 days of decay accumulation (one 125I half-life) approximately a quarter of all plasmid molecules contained sequence-specific DSB, corresponding to 0.3 site-specific DSB per decay. Sequencing gel analysis shows that the DNA breaks are distributed within a few bases of the maxima at those bases opposite to the positions of 125I in the TFO.

Targeting cancer micrometastases with monoclonal antibodies: a binding-site barrier

Monoclonal antibodies penetrate bulky tumors poorly after intravenous administration, in part because of specific binding to the target antigen. Experiments presented here demonstrate an analogous phenomenon in micrometastases; poor antibody penetration, attributable to a "binding-site barrier" phenomenon, can be seen in guinea pig micrometastases as small as 300 microns in diameter. Increasing the dose of antibody can partially overcome this limitation, but at a cost in specificity.

Quantitation of triple-helix formation using a photo-cross-linkable aryl azide/biotin/oligonucleotide conjugate

DNA triple-helix formation has potential applications in gene mapping and as the basis of "antigene" pharmaceuticals; however, the methods for quantitation of triple-helix formation are limited, especially for purine(purine-pyrimidine)-based triplexes. We present a novel method for detection and quantitation of triple-helix formation by triple-helix-forming oligonucleotides. The oligonucleotide is conjugated to a photoactivatable cross-linker, sulfosuccinimidyl 3-[[2-[6-(biotinamido)-2-(p-azidobenzamido)hexanamido] ethyl]dithio] propionate. After incubation with the target DNA, exposure to light labels the target with biotin. The labeled target can be quantified by a chemiluminescent assay. A 26-mer oligonucleotide previously reported to form a purine(purine-pyrimidine) triplex with the upstream region of the c-myc gene was studied and found to bind to its target with Kd of approximately 100 nM at 37 degrees C, 10 mM MgCl2, pH 7.5, consistent with previous reports. This new technique can be used under a variety of conditions and in kinetic experiments and may be extendible to use in living cells.

The cytotoxicity of decays of tritium and iodine-125 incorporated in DNA of mammalian cells. Implications for the low-LET dosimetry of incorporated nuclides

To quantify the toxicity of low-LET radiation from incorporated radionuclides, we have determined the toxicity of decays of [3H]dThd pulse-incorporated into CHO cells in early S phase, with the cells frozen for decay accumulation at 30, 120 or 360 min after the pulse. D37 values of 1500, 2000 and 2100 decays were found by colony formation assay, corresponding to average nuclear doses of 4.6 and 2.7 Gy at the 30- and 360-min times. As D37 for external irradiation (60Co, 2.2 Gy/min) under these conditions is approximately 18 Gy, these results confirm the inadequacy of the dosimetry used for external irradiation to predict the biological effects of the low-LET radiation from incorporated radionuclides. We also determined the toxicity of 125IdU administered as above, and have confirmed the previously reported finding that D37 falls dramatically from 165 decays at 30 min to 40 decays at 360 min. Using the data for tritium to estimate the effect of the dose of 125I low-LET radiation, we conclude that even at 30 min, most of the toxicity of the 125I decays is due to the high-LET portion of the 125I electron spectrum, not the low-LET portion as reported previously.

Kinetic analysis of muscarinic receptors in human brain and salivary gland in vivo

Previous studies in rats have suggested that the muscarinic acetylcholine receptor (mAChR) antagonist (S)-3-quinuclidinyl-(S)-4-[123I]iodobenzilate [(SS)-IQNB] may be useful for the in vivo evaluation of mAChRs in humans as a control for the higher-affinity compound (RR)-IQNB. We have directly tested this hypothesis and examined the distribution of mAChRs in brain regions and parotid glands of healthy human volunteers in vivo using (RR)- and (SS)-IQNB (relatively high- and low-affinity antagonists, respectively), planar imaging, and pharmacokinetic analysis. This is the first in vivo study of mAChRs in humans that has employed stereoisomeric ligands and metabolic analysis to determine specific receptor binding. We observed that (SS)-IQNB showed much faster clearance from blood than (RR)-IQNB and different metabolite profiles. Also, the transport kinetics of the enantiomers were different. The estimated binding potential (approximately Bmax/Kd) of (RR)-IQNB was highest in two cortical regions, intermediate in parotid gland, and lowest in cerebellum. The aggregate results show that in humans (SS)-IQNB does not act as an ideal general probe to measure the nonspecific IQNB distribution. However, (RR)-IQNB does appear to have value when used for studies of human brain mAChRs.

Sequence-specific DNA double-strand breaks induced by triplex forming 125I labeled oligonucleotides

A triplex-forming oligonucleotide (TFO) complementary to the polypurine-polypyrimidine region of the nef gene of the Human Immunodeficiency Virus (HIV) was labeled with 125I at the C5 position of a single deoxycytosine residue. Labeled TFO was incubated with a plasmid containing a fragment of the nef gene. Decay of 125I was found to cause double-strand breaks (DSB) within the nef gene upon triplex formation in a sequence specific manner. No DSB were detected after incubation at ionic conditions preventing triplex formation or when TFO was labeled with 32P instead of 125I. Mapping DSB sites with single base resolution showed that they are distributed within 10 bp of a maximum located exactly opposite the position of the [125I] IdC in the TFO. We estimate that on average the amount of DSB produced per decay is close to one.

Effect of metabolism on retention of indium-111-labeled monoclonal antibody in liver and blood

The effect of a chelator structure on the metabolic fate of the 111In-labeled monoclonal antibody (Mab) T101 was investigated in normal Balb/c mice to assess the importance of this chemical parameter in the reduction of the background radioactivity in blood and liver.

METHODS

Mab T101 was conjugated with either 2-(p-isothiocyanatobenzyl)-6-methyl-diethylaminetriaminepentaac etic acid (DTPA) (1B4M), 2-(p-isothiocyanatobenzyl) cyclohexyl-DTPA (CHX-B) or cyclic DTPA dianhydride (cDTPA) and then radiolabeled with 111In. Normal mice were injected intravenously with these 111In-labeled T101 conjugates and sacrificed in groups of five up to 5 days postinjection for comparative biodistribution studies and analyses of liver, blood and urine samples for radioindium products.

RESULTS

The biodistribution of 111In-1B4M-T101 and 111In-CHX-B-T101 were similar to each other but significantly different from that of 111In-cDTPA-T101, particularly in the blood and liver. Size-exclusion high-performance liquid chromatography indicated that the concentration of the intact 111In-immunoglobulin (Ig)G in liver decreased with similar rates for the three conjugates. Meanwhile, the concentration of a small DTPA-like metabolite in liver increased to a different peak value (4.6% ID/g for the cDTPA conjugate and 1.6% ID/g for the 1B4M and CHX-B conjugates) approximately at 24 hr and maintained a steady-state concentration up to 5 days.

CONCLUSION

The thiourea linkage between T101 and the 111In-labeled chelates and a higher complex stability and higher lipophilicity of 111In-1B4M and 111In-CHX-B appear to be responsible for lower liver and higher blood radioactivity for the 1B4M and CHX-B conjugates.

Application of high affinity binding concept to radiolabel avidin with Tc-99m labeled biotin and the effect of pI on biodistribution

In order to label avidin with Tc-99m, we took advantage of the high affinity binding of biotin to avidin; we radiolabeled a biotin derivative with Tc-99m and then bound this Tc-99m labeled biotin derivative to avidin. For our labeling approach, N epsilon-biotinyl-L-lysine (Biocytin) was reacted with the N-hydroxy-succinimide ester of benzoylmercaptoacetyltriglycine (Bz-MAG3). The resulting Bz-MAG3-Biocytin was labeled with Tc-99m using Tc-99m glucarate as a Tc-99m transchelating agent and mixed with avidin at a 1:1 molar ratio resulting in almost a quantitative labeling yield. Tc-99m-MAG3-Biocytin/Avidin was stable in serum at 37 degrees C with 97 and 95% of the total Tc-99m activity still bound to avidin at 2 and 24 h, respectively. The biodistribution of Tc-99m-MAG3-Biocytin/Avidin in normal Balb/c mice showed a high liver and kidney uptake with 56.6 and 28.9%, respectively at 10 min. We attempted to lower the liver and the kidney activities by reducing the isoelectric point (pI) of avidin by conjugating succinic acid moieties at lysine residues of avidin (pI 10). The kidney uptake decreased to 19.0, 3.1 and 1.7% when the pI of avidin was reduced to 7.0-9.3, 5.5-6.2 and 4.0-4.8, respectively. The lowering of the pI, however, did not change the liver activity appreciably.

Ultrastructure and mineral composition of urinary calculi from horses

Urinary calculi from 17 horses with urolithiasis were examined to study their mineral content and ultrastructure. Among the analytic methods used were X-ray diffractometry, scanning electron microscopy, and electron microprobe analysis. The calculi initially were observed by use of a stereoscopic dissecting microscope and generally were found to have nodular surfaces surrounding a banded or granular-to-chalky interior. Observation by scanning electron microscopy revealed an intricate pattern of irregularly concentric, fine bands and spherules. These had a round, finely banded, globular texture formed by precipitation of ultrafine-grained radiating crystals. The original pore spaces (ie, between spherules, between bands and spherules, or between crystal generations) could be observed as primary porosity. Precipitation and dissolution of these urinary calculi were observed to be spontaneous processes, which can occur simultaneously within an individual calculus. Another prominent feature of the ultrastructure was secondary porosity (spontaneous dissolution) which, in its incipient stages, appeared to be site-selective (ie, some bands appeared to be more susceptible to development of pinpoint porosity). Textures indicative of dissolution were observed not only on the calculus surface, but within the calculus interior as well. Areas that had more advanced stages of dissolution, resulting in increased secondary porosity, also were observed. All 17 samples of the study were found to be composed of calcium carbonate in the form of the mineral calcite, although minor quantities of 2 other polymorphs of calcium carbonate, minerals vaterite and aragonite, also were encountered. Vaterite was observed in 5 of the samples, whereas aragonite was found in 1 sample. Strontium and sulfur were observed as trace elements in 3 of the calculi, whereas magnesium was present in all calculi. Magnesium was observed to substitute for calcium within the calcite crystal lattice in larger quantities than those of strontium or sulfur. Magnesium K alpha X-ray dot maps generated by use of an electron microprobe analyzer indicated that the distribution pattern of magnesium appeared to closely follow layer-by-layer growth of the calculus. Magnesium distribution also appeared to be related to porosity development. In samples where preferential dissolution was observed, the more porous areas had higher magnesium content. Quantitative chemical analyses, using the electron microprobe analyzer, confirmed these observations. Association of the magnesium distribution pattern to the primary growth texture of the calculus indicated that magnesium content of the calculus varied during the formation process. This also indicated that changes in urine chemical analytes may be reflected in composition of the calculi formed.(ABSTRACT TRUNCATED AT 400 WORDS)

Streptavidin distribution in metastatic tumors pretargeted with a biotinylated monoclonal antibody: theoretical and experimental pharmacokinetics

We have developed a pharmacokinetic model for the analysis of a protocol that involves injection of a biotinylated monoclonal antibody followed at a later time by radiolabeled streptavidin. Three distinct physiological spaces are described: an avascular tumor nodule, the normal tissue surrounding the tumor, and the plasma. The model incorporates processes such as plasma kinetics, transcapillary transport, interstitial diffusion, binding reactions, and lymphatic clearances. We have modeled cases in which antigen turnover does not occur, in which antigen turnover does occur (24-h time constant), and in which circulating antibody is cleared from the plasma immediately prior to injection of streptavidin. We have calculated the spatial and temporal distributions of a tumor-specific antibody and of streptavidin in the tumor nodule using parameter values that simulate conditions of recent experiments on metastatic nodules in the guinea pig lung. The theoretical distribution of streptavidin in the tumor nodule shows an initial localization at the periphery that progresses to a fairly uniform distribution throughout the nodule, a temporal sequence that is very similar to experimental observation. This finding indicates that, in a tumor pretargeted with biotinylated antibody, streptavidin can encounter significant retardation in its penetration as a consequence of the high affinity interaction between these two species. Tumor:blood and tumor:lung ratios were calculated and compared to experimental results. In addition, the calculated tumor:blood ratios, tumor:lung ratios, and relative exposures were compared to values obtained from a model of one-step antibody delivery. The two-step protocol yielded an approximately 2- to 3-fold enhancement in these pharmacokinetic indices compared with the one-step method.

Two-step targeting of experimental lung metastases with biotinylated antibody and radiolabeled streptavidin

Two-step monoclonal antibody tumor targeting using an avidin-biotin system has unique characteristics because of the high-affinity binding (10(15) M-1) and the lower molecular weight ligands (avidin, streptavidin, or biotin) used as carriers of radioisotopes, toxins, or drugs. The distribution of radiolabeled streptavidin in a two-step targeting strategy was investigated in lung metastases of line 10 carcinoma in guinea pigs. The microdistribution of administered D3 monoclonal antibody and 125I-labeled streptavidin in metastatic nodules was examined by immunohistochemistry and autoradiography, and the uptake was quantitated. With monoclonal antibody pretargeting, streptavidin was found mainly at the periphery of metastatic nodules 1.5 h after injection; it had penetrated deeper at 4 h and was approaching homogeneity in many of the tumor nodules at 24 h. These results indicate that streptavidin can penetrate into metastatic nodules more rapidly than can the antibody. The concentration of streptavidin in metastatic nodules 4 h after injection was 5.6 times higher for the pretargeted group than for the nonpretargeted group, and the pretargeting index was 4.7. Although the absolute uptake of streptavidin had decreased between 4 and 24 h, the metastasis:blood ratio had increased from 1.2 to 2.4. When compared with the animals injected with 125I-labeled D3 antibody alone, the pretargeted group achieved higher tumor:blood and tumor:lung ratios and a higher localization index at early times after injection of the radiolabeled species.

A malignant melanoma imaging agent: synthesis, characterization, in vitro binding and biodistribution of iodine-125-(2-piperidinylaminoethyl)4-iodobenzamide

In order to develop improved radiopharmaceuticals for imaging malignant melanoma, we have synthesized and characterized 125I-and 131I-labeled (2-piperidinylaminoethyl)4-iodobenzamide (PAB). In vitro binding profiles of IPAB and N-(2-diethylaminoethyl)4-iodobenzamide (IDAB, a structurally related analog of IPAB) for a variety of neurotransmitter receptors suggested that both IPAB and IDAB possessed a high sigma-1 affinity and a low affinity for sigma-2 sites. In vitro homologous competition binding studies of [125I]PAB with human malignant melanoma cell A2058 showed that the tracer was bound to the cells with a high affinity (Ki = 6.0 nM) and that the binding was saturable. Biodistribution studies in nude mice implanted with human malignant melanoma xenografts showed good tumor uptake (3.87% ID/g at 1 hr, 2.91% ID/g at 6 hr and 1.02% ID/g at 24 hr) of [125I]PAB. High tumor-to-nontarget organ ratios were obtained at 24 hr postinjection. Tumor-to-blood, liver, muscle, lung, intestines, heart and brain ratios at 24 hr were 17.80, 3.88, 94.58, 14.29, 10.87, 37.07 and 90.01, respectively. Tumor imaging with [131I]PAB in a nude mice model xenografted with human malignant melanoma at 24 hr clearly delineated the tumor with very little activity in any other organ. These results demonstrate that sigma-1 receptors could be used as external markers for malignant melanoma.