Radiolabeled CCK/gastrin peptides for imaging and therapy of CCK2 receptor-expressing tumors

Cholecystokinin (CCK) receptors are overexpressed in numerous human cancers, like medullary thyroid carcinomas, small cell lung cancers and stromal ovarian cancers. The specific receptor-binding property of the endogenous ligands for these receptors can be exploited by labeling peptides with a radionuclide and using these as carriers to guide the radioactivity to the tissues that express the receptors. In this way, tumors can be visualized using positron emission tomography and single photon emission computed tomography imaging. A variety of radiolabeled CCK/gastrin-related peptides has been synthesized and characterized for imaging. All peptides have the C-terminal CCK receptor-binding tetrapeptide sequence Trp-Met-Asp-Phe-NH₂ in common or derivatives thereof. This review focuses on the development and application of radiolabeled CCK/gastrin peptides for radionuclide imaging and radionuclide therapy of tumors expressing CCK receptors. We discuss both preclinical studies as well as clinical studies with CCK and gastrin peptides.     

Treatment of high-grade glioma with radiolabeled peptides

The management of high-grade glioma (HGG) patients in clinical routine represents a challenging task. HGG has a poor prognosis because of early recurrence or therapy-refractory disease following first-line standard therapy, which includes a multidisciplinary approach involving radical surgical resection followed by external beam radiation therapy in combination with chemotherapy.Glioma cells are known to express specific receptors or glycoproteins on their surface which can be used as biological targets for treatment. The application of radiopharmaceuticals consisting of a targeting and an effector domain has led to the introduction of new treatment approaches, aiming at a tumor-specific treatment sparing normal brain tissue.One of these new modalities is the peptide receptor radionuclide therapy (PRRT). Peptides labeled with radioactive nuclides can bind directly to the tumor cells and deliver high doses of radioactivity directly to the tumor tissue.This article reviews the literature for PRRT in HGG.     

177Lu–DO3A–HSA–ZEGFR:1907: characterization as a potential radiopharmaceutical for radionuclide therapy of EGFR-expressing head and neck carcinomas

Epidermal growth factor receptor 1 (EGFR) is an attractive target for radionuclide therapy of head and neck carcinomas. Affibody molecules against EGFR (Z(EGFR)) show excellent tumor localizations in imaging studies. However, one major drawback is that radiometal-labeled Affibody molecules display extremely high uptakes in the radiosensitive kidneys which may impact their use as radiotherapeutic agents. The purpose of this study is to further explore whether radiometal-labeled human serum albumin (HSA)-Z(EFGR) bioconjugates display desirable profiles for the use in radionuclide therapy of EGFR-positive head and neck carcinomas. The Z(EFGR) analog, Ac-Cys-Z(EGFR:1907), was site-specifically conjugated with HSA. The resulting bioconjugate 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A)-HSA-Z(EGFR:1907) was then radiolabeled with either (64)Cu or (177)Lu and subjected to in vitro cell uptake and internalization studies using the human oral squamous carcinoma cell line SAS. Positron emission tomography (PET), single photon emission computed tomography (SPECT), and biodistribution studies were conducted using SAS-tumor-bearing mice. Cell studies revealed a high (8.43 ± 0.55 % at 4 h) and specific (0.95 ± 0.09 % at 4 h) uptake of (177)Lu-DO3A-HSA-Z(EGFR:1907) as determined by blocking with nonradioactive Z(EGFR:1907). The internalization of (177)Lu-DO3A-HSA-Z(EGFR:1907) was verified in vitro and found to be significantly higher than that of (177)Lu-labeled Z(EFGR) at 2-24 h of incubation. PET and SPECT studies showed good tumor imaging contrasts. The biodistribution of (177)Lu-DO3A-HSA-Z(EGFR:1907) in SAS-tumor-bearing mice displayed high tumor uptake (5.1 ± 0.44 % ID/g) and liver uptake (31.5 ± 7.66 % ID/g) and moderate kidney uptake (8.5 ± 1.08 % ID/g) at 72 h after injection. (177)Lu-DO3A-HSA-Z(EGFR:1907) shows promising in vivo profiles and may be a potential radiopharmaceutical for radionuclide therapy of EGFR-expressing head and neck carcinomas.     

Renal toxicity after radionuclide therapy

During the past 10 years, a variety of radiolabeled monoclonal antibodies, antibody fragments, and low-molecular- weight oncophilic peptides have been used to deliver radioactivity to target cells for therapeutic purposes. The high and persistent localization of several of these radiolabeled molecules in the kidneys raised concern about potential renal radiation toxicity compromising therapeutic effectiveness. In particular, radiolabeled peptides, such as yttrium-90-labeled synthetic somatostatin analogues, have initiated a discussion on the safety profiles of the various somatostatin derivatives in recent clinical trials. In general, the toxicity risk seems to depend on the characteristics of the oncophilic molecule, such as the molecular weight, electric charges and clearance pathways as well as the chemical and physical characteristics of the applied radionuclide. Encouraging results for the prevention of radiation-induced renal damage by radiolabeled peptides have been obtained by co-infusion of positively charged amino acids. The available literature on nephrotoxicity after radiolabeled peptide therapy is reviewed, and therapeutic options that have become available as a result of greater insights into putative pathogenic mechanisms are discussed.     

In vivo evaluation of cysteine-based chelators for attachment of 99mTc to tumor-targeting Affibody molecules

Affibody molecules present a new class of affinity proteins, which utilizes a scaffold based on a 58-amino acid domain derived from protein A. The small (7 kDa) Affibody molecule can be selected to bind to cell-surface targets with high affinity. An Affibody molecule (ZHER2:342) with a dissociation constant (Kd) of 22 pM for binding to the HER2 receptor has been reported earlier. Preclinical and pilot clinical studies have demonstrated the utility of radiolabeled ZHER2:342 in imaging of HER2-expressing tumors. The small size and cysteine-free structure of Affibody molecules enable complete peptide synthesis and direct incorporation of radionuclide chelators. The goal of this study was to evaluate if incorporation of the natural peptide sequences cysteine-diglycine (CGG) and cysteine-triglycine (CGGG) sequences would enable labeling of Affibody molecules with 99mTc. In a model monomeric form, the chelating sequences were incorporated by peptide synthesis. The HER2-binding affinity was 280 and 250 pM for CGG-ZHER2:342 and CGGG-ZHER2:342, respectively. Conjugates were directly labeled with 99mTc with 90% efficiency and preserved the capacity to bind specifically to HER2-expressing cells. The biodistribution in normal mice showed a rapid clearance from the blood and the majority of organs (except kidneys). In the mice bearing SKOV-3 xenografts, tumor uptake of 99mTc-CGG-ZHER2:342 was HER2-specific and a tumor-to-blood ratio of 9.2 was obtained at 6 h postinjection. Gamma-camera imaging with 99mTc-CGG-ZHER2:342 clearly visualized tumors at 6 h postinjection. The results show that the use of a cysteine-based chelator enables 99mTc-labeling of Affibody molecules for imaging.     

Phage display library derived peptides that bind to human tumor melanin as potential vehicles for targeted radionuclide therapy of metastatic melanoma

Metastatic melanoma remains an incurable disease, and there is a great need for novel therapeutic modalities. We have recently identified melanin as a target for radionuclide therapy of melanoma and demonstrated the feasibility of this approach using a 188-rhenium ( (188)Re)-radiolabeled melanin-binding decapeptide to fungal melanin known as 4B4. Although the results indicated that radiolabeled melanin-binding decapeptide had activity against melanoma, that peptide also manifested high kidney uptake and this might become a concern during clinical trials. We hypothesized that by identifying peptides with different amino acid composition against tumor melanin we might be able to decrease their kidney uptake. Using the Heptapeptide Ph.D.-7 Phage Display Library, we identified three heptapeptides that bind to human tumor melanin. These peptides were radiolabeled with (188)Re via HYNIC ligand, and their comprehensive biodistribution in A2058 human metastatic melanoma tumor-bearing nude mice was compared to that of (188)Re-4B4 decapeptide. While tumor uptake of heptapeptides was quite similar to that of (188)Re-4B4 decapeptide, there was dramatically less uptake in the kidneys at both 3 h (6% ID/g vs 38%) and 24 h (2% ID/g vs 15%) postinjection. Administration of one of the generated heptapeptides, (188)Re-HYNIC-AsnProAsnTrpGlyProArg, to A2058 human metastatic melanoma-bearing nude mice resulted in significant retardation of the tumor growth. Immunofluorescence showed that in spite of their relatively small size heptapeptides were not able to penetrate through the membranes of viable melanoma cells and bound only to extracellular melanin, which provides assurance that they will be safe to healthy melanin-containing tissues during radionuclide therapy. Thus, these heptapeptides appear to have potentially significant advantages for targeted therapy of melanoma relative to existing melanin-binding peptides.     

Gel purification of radiolabeled nucleic acids via phosphorimaging: Dip-N-Dot

RNA and DNA oligonucleotides radiolabeled with ³²P or ³³P often require gel electrophoresis to remove undesired side and/or degradation products. Common ways to visualize these molecules after electrophoresis are by ultraviolet (UV) shadowing, which necessarily reduces the specific activity of the oligonucleotide, and by autoradiography using film, which is cumbersome and increases the cost of generating the radiolabeled molecule. A more cost-effective method is to physically inject the gel with a “Dip-N-Dot” solution of dye and radionuclide after electrophoresis but prior to phosphorimaging. The gel can be overlaid on its computer-generated image, allowing the labeled molecules to be visualized quickly.     

Serotherapy of ovarian cancer

The development of monoclonal antibodies has permitted the identification of several ovarian-tumor-associated antigens which might serve as targets for serotherapy in vivo. With the exception of antibodies directed against growth factor receptors, unmodified monoclonal reagents must activate complement (C') components or bind effector cells to destroy tumor targets. Antibody-dependent cell-mediated cytotoxicity (ADCC) may be particularly important for eliminating tumor cells in vivo. A shortage of functionally active effector cells can limit the efficacy of serotherapy with heteroantisera or monoclonal reagents. The use of immunostimulants such as Corynebacterium parvum has increased the number and activity of effector cells for ADCC within the peritoneal compartment of mice and of patients with ovarian cancer. Intraperitoneal serotherapy can achieve direct contact between antibody and microscopic deposits of ovarian tumor cells which persist following cytoreductive operations and cytotoxic chemotherapy. Conjugation of monoclonal antibodies with radionuclides, drugs or toxins might increase the potency of serotherapy and circumvent the effector shortage. Clinical studies to date have evaluated radionuclide conjugates for imaging and for therapy. Patients with a small volume of disease have responded to treatment. Preclinical models suggest that drug and toxin conjugates might also prove active. Recent studies have demonstrated a synergistic interaction between different immunotoxins. Ovarian carcinoma is likely to be a valuable clinical model for evaluating immunoconjugates which react with epithelial tumor cells.     

Criteria for the selection of the most desirable radionuclide for radiolabeling monoclonal antibodies

Efficient labeling of monoclonal antibodies depends on a number of key factors, mostly related to the characteristics of the radionuclide itself and to the manner of its incorporation into the protein. Such factors include the physical half-life, the photon or particle energy of the radionuclide and its selective deposition of energy in tissues, the method of labeling used (covalent binding or chelation), and the effect that the chemical changes inherent in the labeling process may have on the properties of the protein or of its fragments. The major biological factor in determining the radionuclide of choice for labeling is the projected use of the labeled antibody. When the intended use is diagnostic, then what is required is high-photon density for achieving the high resolution needed for imaging, whereas therapeutic use requires radionuclides with high energy deposition at the target sites, i.e. β or α emitters. A further consideration is to be given to the mode of administration of the radiolabeled monoclonal antibody: determination of the radiopharmacokinetic parameters of compartmental models of biodistribution of the labeled monoclonal antibody and/or its fragments may also assist in selecting which radionuclide may be best to use for radiolabeling a given monoclonal antibody intended for either tumor diagnosis, prognosis and/or therapy.     

Multidisciplinary Approach including Receptor Radionuclide Therapy with 90Y-DOTATOC ([90Y-DOTA0,Tyr3]-OCTREOTIDE) and 177Lu-DOTATATE ([177Lu-DOTA0,Tyr3]-OCTREOTATE) in Ectopic Cushing Syndrome from Ametastatic Gastrinoma: A Promising Proposal

ObjectiveTo present a case of a young woman with Cushing syndrome caused by ectopic production of adrenocorticotropic hormone from a metastatic pancreatic gastrin-secreting endocrine carcinoma, who had a good response to combination peptide receptor radionuclide therapy.MethodsWe review the history, physical examination, laboratory investigations, and radiographic findings in this unusual patient. Moreover, the multimodal interventions are described and discussed.ResultsIn a 38-year-old woman with typical signs of cortisol excess, laboratory studies revealed diabetes mellitus, hypokalemia, and high levels of adrenocorticotropic hormone, plasma cortisol, and urinary cortisol. Abdominal computed tomography showed a 4-cm pancreatic mass and multiple metastatic lesions in the liver, and ectopic Cushing syndrome was diagnosed. Treatment consisted of surgical debulking of the tumor, ketoconazole, somatostatin analogues, chemoembolization of the liver metastatic lesions, and peptide receptor radionuclide therapy with the radiolabeled somatostatin analogues ⁹⁰Y-DOTATOC ([⁹⁰Y-DOTA⁰,Tyr³]-octreotide) and ¹⁷⁷Lu-DOTATATE ([¹⁷⁷Lu-DOTA⁰,Tyr³]-octreotate). The 5¹/2-year follow-up showed positive results, which included complete regression of all clinical and hormonal evidence of the tumor and substantial decrease in the size and number of hepatic metastatic lesions. The patient achieved and still maintains an optimal quality of life.ConclusionTo the best of our knowledge, this is the first report of a multidisciplinary approach including peptide receptor radionuclide therapy with ⁹⁰Y-DOTATOC and ¹⁷⁷Lu-DOTATATE, which proved to be effective in improving clinical outcome in a case of metastatic endocrine carcinoma of the pancreas in conjunction with ectopic Cushing syndrome. In this unusual case, the patient has one of the longest durations of survival in this setting described in the literature. (Endocr Pract. 2008; 14:213-218)     

Many bacterial species bind human IgD.

Forty-four bacterial strains belonging to 19 species were tested for their IgD-binding capacity by incubation with radiolabeled human IgD. A high binding of IgD to Neisseria catarrhalis and Hemophilus influenzae and a moderate binding of IgD to streptococci of the groups A, C, and G were found. Two strains of N. catarrhalis were tested for their ability to bind selectively the IgD in normal pooled serum and in three serum samples with IgD M components and were found to possess this property. Binding studies with radiolabeled IgD Fab and Fc fragments indicated that the binding mainly but not exclusively involves the CH1 region of the IgD molecule.     

Aromatic radiofluorination and biological evaluation of 2-aryl-6-[18F]fluorobenzothiazoles as a potential positron emission tomography imaging probe for β-amyloid plaques

To develop agents for radionuclide imaging Aβ plaques in vivo, we prepared three fluorine-substituted analogs of arylbenzothiazole class; compound 2 has a high affinity for Aβ (K(i)=5.5nM) and the specific binding to Aβ in fluorescent staining. In preparation for the synthesis of these arylbenzothiazole analogs in radiolabeled form as an Aβ plaques-specific positron emission tomography (PET) imaging probe, we investigated synthetic route suitable for its labeling with the short-lived PET radionuclide fluorine-18 (t(1/2)=110min) and diaryliodonium tosylate precursors (12, 13a-e and 14). 2-Aryl-6-[(18)F]fluorobenzothiazoles ([(18)F]1-3) were synthesized in efficiently short reaction times (40-60min) with high radiochemical yields (19-40%), purities (>95%) and specific activities (85-118GBq/μmol). Tissue distribution studies showed that high radioactivity of [(18)F]2 accumulated in the brain with rapid clearance in healthy mice. Radioactive metabolites were analyzed in brain samples of mice and corresponded to 81% of parent remained by 30min after a tail-vein injection. These results suggest that [(18)F]2 is a promising probe for evaluation of Aβ plaques imaging in brain using PET.     

Isolated soluble fractions from the murine B16 melanoma induce primary in vitro syngeneic antitumor responses

Summary This paper extends our previous studies, which documented our ability to isolate immunogenic entities from nonimmunogenic or weakly immunogenic tumors.B16 melanoma cells failed, in our in vitro experimental system, to induce anti-B16 cytotoxic responses in spleen cells derived from normal syngeneic C57BL/6 mice. The B16 melanoma cellular homogenate was fractionated on an Ultrogel AcA 34 column, and the various fractions were tested for their ability to induce anti-B16 cytotoxic responses under the same conditions as those used for intact B16, the nonimmungenic tumor cells. Certain fractions, some of them with relatively low protein concentrations, induced anti-B16 cytotoxic responses in spleen cells of normal C57BL/6 mice, whereas others, some of them with relatively high protein concentrations, failed to induce such responses. One fraction (Fr.), designated Fr. 5/6, was examined in detail. It was found that in normal syngeneic spleen cells this fraction induced effector cells that efficiently killed (at various E : T ratios) the relevant B16 target cells and RBL5 syngeneic tumor cells, but not the YAC allogeneic tumor cells or C57BL/6 lymphoblasts. Furthermore, an excess of unlabeled B16 cells most efficiently blocked the ability of these anti-B16 effector cells to kill radiolabeled B16 target cells. RBL5 tumor cells, YAC tumor cells, or C57BL/6 lymphoblasts failed to block these effector cells efficiently. A significant fraction of the effector cells induced with Fr. 5/6 was characterized as thymus-derived cells (Thy-1⁺, Thy-2⁺3⁺ cells). It was suggested that another fraction of the cellular population was natural killer cells, which cytolyzed the RBL5 target cells. Various theoretical and practical aspects of these findings are discussed.     

Influence of DOTA Chelator Position on Biodistribution and Targeting Properties of (111)In-Labeled Synthetic Anti-HER2 Affibody Molecules

Affibody molecules are a class of affinity proteins. Their small size (7 kDa) in combination with the high (subnanomolar) affinity for a number of cancer-associated molecular targets makes them suitable for molecular imaging. Earlier studies demonstrated that the selection of radionuclide and chelator may substantially influence the tumor-targeting properties of affibody molecules. Moreover, the placement of chelators for labeling of affibody molecules with (99m)Tc at different positions in affibody molecules influenced both blood clearance rate and uptake in healthy tissues. This introduces an opportunity to improve the contrast of affibody-mediated imaging. In this comparative study, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was conjugated to the synthetic affibody molecule Z(HER2:S1) at three different positions: DOTA-A1-Z(HER2:S1) (N-terminus), DOTA-K58-Z(HER2:S1) (C-terminus), and DOTA-K50-Z(HER2:S1) (middle of helix 3). The affinity for HER2 differed slightly among the variants and the K(D) values were determined to be 133 pM, 107 pM and 94 pM for DOTA-A1-Z(HER2:S1), DOTA-K50-Z(HER2:S1), and DOTA-K58-Z(HER2:S1), respectively. Z(HER2:S1)-K50-DOTA showed a slightly lower melting point (57 °C) compared to DOTA-A1-Z(HER2:S1) (64 °C) and DOTA-K58-Z(HER2:S1) (62 °C), but all variants showed good refolding properties after heat treatment. All conjugates were successfully labeled with (111)In resulting in a radiochemical yield of 99% with preserved binding capacity. In vitro specificity studies using SKOV-3 and LS174T cell lines showed that the binding of the radiolabeled compounds was HER2 receptor-mediated, which also was verified in vivo using BALB/C nu/nu mice with LS174T and Ramos lymphoma xenografts. The three conjugates all showed specific uptake in LS174T xenografts in nude mice, where DOTA-A1-Z(HER2:S1)and DOTA-K58-Z(HER2:S1) showed the highest uptake. Overall, DOTA-K58-Z(HER2:S1) provided the highest tumor-to-blood ratio, which is important for a high-contrast imaging. In conclusion, the positioning of the DOTA chelator influences the cellular processing and the biodistribution pattern of radiolabeled affibody molecules, creating preconditions for imaging optimization.     

A target structure for a series of human cloned natural killer cell lines is recognized by both anti-TNKtar and 4F2 monoclonal antibodies

It was shown recently that a surface antigen termed TNKtar was likely to serve as a target molecule for three distinct human NK clones expressing the same clonotypic determinant (termed NKTa) present on a 90 KD recognition structure. In the present studies, we investigated whether TNKtar and a previously described antigen termed 4F2 were related. Parallel immunoprecipitations from membrane lysates of the same cells showed that both anti-TNKtar and 4F2 Mab precipitate a heterodimeric structure which resolves as two bands of identical m.w. (40 and 80 KD) in SDS-PAGE analysis under reducing conditions. Sequential immunoprecipitations demonstrated that the two antibodies are directed at the same molecule. However, one antibody did not block subsequent binding of the other, and vice versa, suggesting that anti-TNKtar and 4F2 Mab are directed at two distinct epitopes of the molecule. Functionally, it was found that 4F2 Mab was able, as well as anti-TNKtar, to selectively block cytotoxic function of JT9 cloned cells. Furthermore, as reported previously for anti-TNKtar, 4F2 had no effect when additional NKTa-NK clones were used as effector cells in cytotoxicity assays. Finally, cold target inhibition assays were performed by using cold target cells precoated with either anti-TNKtar or 4F2 Mab. These experiments showed that preincubation of target cells with either antibody blocked their ability to compete with their radiolabeled counterpart. Such results further strengthen the hypothesis that the activation antigen recognized by both anti-TNKtar and 4F2 Mab serves as a specific target structure for NKTa+ NK active clones. We discuss the importance of previous data concerning the 4F2 molecule in light of this functional role, which had not been identified previously.     

Strategies for the labeling of halogen-substituted peroxisome proliferator-activated receptor gamma ligands: potential positron emission tomography and single photon emission computed tomography imaging agents

Well-known as an important regulator of lipid metabolism and adipocyte differentiation, the peroxisome proliferator-activated receptor gamma (PPARgamma) also has potential use as a target for antitumor therapy in certain cancers. To develop agents for radionuclide imaging PPARgamma in vivo, we synthesized fluorine, bromine, and iodine-substituted analogs (1-3) of a high-affinity benzophenone-tyrosine PPARgamma ligand; all three analogs retain very high affinity for the PPARgamma receptor. In preparation for the synthesis of these PPARgamma ligands in radiolabeled form, we have synthesized two types of precursors: (a) an aryltributylstannane (9), from which the bromine and iodine-substituted analogs (2 and 3) can readily be prepared by electrophilic destannylation, and (b) three diaryliodonium tosylate derivatives (12a-c), precursors for nucleophilic aromatic fluorination using fluoride ion. Conditions were developed whereby the thiophenyliodonium tosylate (12c) underwent nucleophilic aromatic substitution with fluoride ion, efficiently and in short reaction times, to produce the desired fluorine-substituted target compound 1. These reactions laid the groundwork for producing these three PPARgamma ligands in radiolabeled form; in addition, our use of diaryliodonium ion precursors for aromatic fluorination in this series provides an example that should encourage application of this approach for radiofluorination of more complicated radiopharmaceuticals.     

The use of radionuclides for tumor therapy

The successful use of radionuclides for tumor therapy depends to a major extent on the ability to achieve a high concentration of radioactivity in the tumor relative to other radiosensitive organs not involved by tumor, such as bone marrow, intestinal mucosa, liver, and kidneys. Techniques designed to achieve such differential localization of the radionuclides include the use of (1) radiopharmaceuticals that enter specific metabolic pathways unique to certain tumor types; (2) radiolabeled antibodies that attach to tumor-associated antigens present on tumor cell surfaces; (3) heterologous antibodies that attach to tumor-associated antigens present on tumor cell surfaces and which are then identified by radiolabeled antibodies directed against the species in which the original, unlabeled antibody was made; and (4) radiolabeled compounds injected regionally at the tumor site. Although both clinical and experimental evidence on the use of radionuclides for tumor therapy is encouraging in preliminary studies, extensive further research needs to be done in this area to insure the clinical efficacy of radionuclides for tumor therapy.     

A novel nicotinic acetylcholine receptor antagonist radioligand for PET studies

Using positron emission tomography (PET) with a specific and selective radioligand targeting nicotinic acetylcholine receptor (nAChR) would allow us to better understand various nAChR related CNS disorders. The use of radiolabeled nAChR antagonists would provide a much safer pharmacological profile, avoiding most peripheral side effects that might be generated from radiolabeled nAChR agonists even at the tracer level; thus, PET imaging with nAChR antagonists would facilitate clinical application. A potent and selective nAChR antagonist was labeled and characterized with PET in non-human primates. Its high brain uptake, high signal-to-noise ratio, and high specific binding strongly suggest a great potential to carry out imaging studies in humans. In addition, the use of a C-11 radiotracer would allow us to perform multiple PET studies in the same individual within a short time frame. The presence of an iodine atom in the molecule also allows the possibility to label with radioiodine for SPECT studies.     

Fluorine-substituted ligands for the peroxisome proliferator-activated receptor gamma (PPARgamma): potential imaging agents for metastatic tumors

The peroxisome proliferator-activated receptor gamma (PPARgamma), a primary regulator of lipid metabolism, is present in many tumor cell lines and animal tumor systems and, in some cases, can mediate effective antitumor therapy with potent synthetic ligands. In an approach to image tumors with positron-emission tomography (PET) based on their content of PPARgamma, we have synthesized two fluorine-substituted analogues of a high affinity ligand from the phenylpropanoic acid class. The analogue having the highest affinity for PPARgamma was labeled with the positron-emitting radionuclide fluorine-18. In tissue distribution studies in normal rats and in SCID mice bearing human breast tumor xenografts, this compound did not show evidence of receptor-mediated uptake. The prospects for using PPARgamma as a target for imaging tumors may be limited by the low receptor concentrations in tumors and by the pharmacokinetic behavior of this class of ligands, which appears to be more favorable for therapy than for imaging.     

Shared antigens between human malignant melanoma cells and Mycobacterium bovis (BCG).

This study was undertaken to investigate the antigenic relationships between human malignant melanoma cells and Mycobacterium bovis (BCG). Rabbits were immunized with sonicates of BCG or with malignant melanoma cells from different patients and the resulting antisera were tested for their capacity to bind radiolabeled soluble extracts prepared from BCG and melanoma cells. The binding of antibodies to radiolabeled antigens was studied by precipitation of radiolabeled antigen-antibody complexes by anti-rabbit immunoglobulin. Antibodies in sera from rabbits immunized with either BCG (anti-BCG) or melanoma cells (anti-melanoma) bound both the labeled BCG and melanoma antigens. Control antisera, from rabbits immunized with human acute or chronic lymphatic leukemia cells or with normal human spleen cells, did not bind significant amounts of radiolabeled BCG. Antibodies in sera from rabbits immunized with normal spleen cells bound small but significant amounts of radiolabeled melanoma antigens. Binding by anti-BCG and anti-melanoma to the radiolabeled antigens was studied before and after absorption of antisera with cells from human melanoma, leukemia, guinea pig hepatoma, and normal human spleen cells. Inhibition studies using unlabeled BCG extracts also were carried out. The absorption and inhibition studies confirmed that the binding reactions were specific and that antigens from five melanoma patients shared antigenic determinants with BCG.