Transferrin promotion of 67Ga and 59Fe uptake by cultured mouse myeloma cells

Radiotracer 67Ga-citrate is used as a tumor-seeking agent in clinical imaging investigations although fundamental reasons for its high uptake in certain malignant lesions remain unexplained. The mechanism by which 67Ga becomes concentrated in tumor cells has been investigated by comparing 67Ga and 59Fe uptake by cultured mouse myeloma cells with particular reference to uptake stimulation by transferrin. Concentrations of human transferrin down to 2 microgram/ml greatly stimulated cellular uptake of both tracers, whereas bovine transferrin proved relatively inactive. The rates of stimulated uptake of both tracers were similar as was their high degree of retention by cells, but their quantitative dependencies on transferrin concentration showed characteristic differences. Pretreatment of human transferrin with saturating amounts of nonradioactive Fe3+ canceled its ability to promote 59Fe uptake, but it had little effect on its promotion of 67Ga uptake. Further increase in the amount of added Fe3+ did cause a progressive depression of 67Ga uptake, but this effect probably relates to the iron distribution in the whole-cell culture system including the fetal calf serum component of cell growth medium. The results suggest that 67Ga and 59Fe reveal different aspects of the interaction of transferrin with cells.     

Uptake of gallium-67 by human leukemic cells: demonstration of transferrin receptor-dependent and transferrin-independent mechanisms

We have studied the role of transferrin and the transferrin receptor in the uptake of 67Ga by the human leukemic cell line HL60. In the absence of transferrin, HL60 cells incorporated about 1% of the 67Ga dose over 6 h. The presence of transferrin increased cellular 67Ga uptake approximately 10-fold. Transferrin-mediated uptake of 67Ga was blocked by an anti-transferrin receptor monoclonal antibody, and decreases in the density of cellular transferrin receptors led to corresponding decreases in the transferrin-dependent uptake of 67Ga. Changes in the cellular ferritin content did not significantly influence the uptake of 67Ga by either transferrin-independent or transferrin-dependent pathways. Regardless of the mechanism of uptake, a significant amount of intracellular 67Ga was found to be associated with immunoprecipitable ferritin as well as with a free pool. This free intracellular 67Ga appeared to be kinetically active since cells released 67Ga back to the media over time. Our results demonstrate the existence of a dual mechanism for the cellular uptake of 67Ga and suggest that the preferential uptake of 67Ga by lymphomas is related to the high density of transferrin receptors known to be expressed by these tumors in vivo.     

Role of the acidic receptosome in the uptake and retention of 67Ga by human leukemic HL60 cells

The uptake of 67Ga by HL60 cells requires binding of 67Ga-transferrin (Tf) to cell surface Tf receptors. To further examine this process, we have studied early events in the cellular uptake of 67GaTf. Cell surface-bound 67GaTf and 59FeTf displayed similar kinetics during the first 10 min of uptake. Thereafter, approximately 10% of intracellular 67Ga was released by cells while 59Fe internalization continued to increase with time. In pulse-chase studies of 125I-Tf-67Ga uptake, internalized 125I-Tf, but not 67Ga, was chased out of cells by nonradioactive Tf-Ga. Exposure of cells to monensin, a carboxylic ionophore, during initial uptake decreased the internalization of both 125I-Tf and 67Ga. Exposure to monensin at a later time, after cells had incorporated 125I-Tf-67Ga or 59FeTf, caused an increase in the release of 67Ga and 59Fe with a decrease in the release of 125I-Tf. Ammonium chloride inhibited the internalization of both 67Ga and 59Fe. 67GaTf uptake by HL60 cells involves initial internalization into an acidic receptosome. This is followed by dissociation of 67Ga and Tf and subsequent trafficking of each to separate intracellular compartments. Disruption of this process by monensin results in the release of 67Ga from cells.     

67Ga and 59Fe uptakes by cultured human lymphoblasts and lymphocytes

Studies with spontaneously proliferating human lymphoblastoid cells have shown that cellular uptake of both 67Ga and 59Fe3+ (as citrates) become appreciably increased with small additions of purified human transferrin to the cultures. In many respects, their uptake responses were broadly similar to those previously reported for cultured mouse lymphoid tumor cells. Studies were also undertaken with cultured human lymphocytes, but transferrin promotion of tracer uptakes could not be detected for these cells. In addition, cellular uptakes for phytochemagglutinin-stimulated cultures were not significantly increased over their unstimulated controls.     

The transferrin receptor in hepatocyte nodules: binding properties, subcellular distribution and endocytosis

Transferrin binding was found to be around 60-fold higher inhepatocyte nodules compared to normal liver, with no apparentdifferences in binding affinity or molecular weight of bindingproteins, indicating that the increase in [¹²⁵I]trans-ferrinbinding was the result of an increased number of binding siteswith similar properties as in normal liver. The relative ‘induction’of transferrin receptors was most marked in the total membranefraction followed by membrane sub-fractions comprising endoplasmkreticulum, the Golgi complex and endocytic vesicles. Despitethe increased number of transferrin receptors, the in vivo endocytosisof transferrin, measured as uptake of [¹²⁵I]ferrotransferrin,was quantitatively similar in nodular cells compared to normalliver. The number of transferrin receptors in regenerating liver,48 h after partial hepatectomy, was increased 20-fold over normallevels, but binding affinity, receptor structure and kineticsof transferrin uptake were normal. A slower than normal rateof ⁵⁹Fe accumulation in hepatocyte nodules may suggest an alterationin the dissociation of iron from ferro-transferrin, therebysuggesting one mechanism relevant to the iron storage deficiencyin nodular cells.     

Effect of modulation of the transferrin receptor on gallium-67 uptake and cytotoxicity in lymphoma cell lines.

Gallium-67 is a radionuclide that accumulates in haematological malignancies and is used for diagnostic purposes. Uptake of 67Ga into the cell occurs via the transferrin receptor, which is differentially expressed during the various cell cycle phases. With the aim of selectively increasing 67Ga uptake, we studied whether the transferrin receptor (TfR) expression could be modulated in the U937 and U715 lymphoma cell lines by cytostatic drugs inducing cell cycle phase accumulation. We tested clinically relevant drugs such as 1-beta-D-arabinofuranosylcytosine (Ara-C), hydroxyurea and methotrexate. Cytotoxicity was determined by testing the clonogenic capacity of the lymphoma cell lines. All three drugs induced an increase in S-phase content, TfR expression and 67Ga uptake in U937 and U715 single cells. The combinations of drugs and 67Ga resulted in an additive effect on the clonogenic capacity. In U937 spheroids, cultured by the fibrin clot technique, we found an accumulation in the S-phase too as well as an increase of the transferrin receptor expression after Ara-C preincubation. As in single cells 67Ga uptake was increased without synergistic effects on the clonogenic capacity. In conclusion, priming with drugs induces increased transferrin receptor expression and 67Ga uptake. Inhibition of clonogenic capacity was additive rather than synergistic.     

Preclinical evaluation of a new bombesin analog for imaging of gastrin-releasing peptide receptors

Bombesin (BBN) is a peptide showing high affinity for the gastrin-releasing peptide receptor. Tumors such as prostate, small cell lung cancer, breast, gastric, and colon cancer are known to over express receptors to BBN and gastrin-releasing peptide (GRP). The goal of this study was to evaluate a new (67)Ga radiolabeled BBN analog based on the bifunctional chelating ligand DOTA (1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetic acid), which could be used as a tool for diagnosis of GRP receptor-positive tumors. DOTA-GABA-BBN (7-14) NH(2) was synthesized using a standard Fmoc strategy. Labeling with (67)Ga was performed at 95°C for 30 minutes in ammonium acetate buffer (pH?=?4.8). Radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was examined in the presence of human serum at 37°C up to 24 hours. The receptor-bound internalization and externalization rates were studied in GRP receptor expressing PC-3 cells. Biodistribution of radiopeptide was studied in nude mice bearing PC-3 tumor. Labeling yield of >90% was obtained corresponding to a specific activity of approximatrly 2.6?MBq/nmol. Peptide conjugate showed good stability in the presence of human serum. The radioligand showed a good and specific internalization into PC-3 cells (16.13%?±?0.71% at 4 hours). After 4 hours, a considerable amount of activity (52.42%?±?1.86%) was externalized. In animal biodistribution studies, a receptor-specific uptake of radioactivity was observed in GRP-receptor-positive organs. After 4 hours, the uptake in mouse tumor and pancreas was 1.30%?±?0.18% ID/g (percentage of injected dose per gram of tissue) and 1.21%?±?0.13% ID/g, respectively. These data show that [(67)Ga]-DOTA-GABA-BBN (7-14) NH? is a specific radioligand for GRP receptor positive tumors and is a suitable candidate for clinical studies.     

Improved tumor-to-organ ratios of a novel 67Ga-human epidermal growth factor radionuclide conjugate with preadministered antiepidermal growth factor receptor affibody molecules

The overexpression of the epidermal growth factor receptor (EGFR) in head and neck squamous cell carcinoma (HNSCC) is associated with poor prognosis. Targeted nuclear imaging of the EGFR expression could improve the diagnostics in patients with HNSCC. However, the high expression of EGFR in normal organs may conceal the tumor uptake and therefore limit the use. This study assesses the biodistribution of a novel human epidermal growth factor (hEGF) radionuclide conjugate after preinjection with anti-EGFR affibody molecules. hEGF was conjugated with p-SCN-Bn-NOTA and labeled with (67)Ga. The biodistribution of [(67)Ga]Ga-NOTA-Bn-NCS-hEGF in nude mice with EGFR-expressing xenografts was evaluated either alone or 45 minutes after preinjection with one of the anti-EGFR affibody molecules Z(EGFR:1907), (Z(EGFR:1907))(2), or (Z(EGFR:955))(2). The novel radioimmunoconjugate, [(67)Ga]Ga-NOTA-Bn-NCS-hEGF, demonstrated high stability in vitro and specific binding to hEGF in vitro and in vivo. Preinjection with anti-EGFR affibody molecules improved the tumor-to-organ ratio in the liver, salivary glands, and colon. Overall, the dimeric high-affinity affibody molecule (Z(EGFR:1907))(2) exhibited the best results. These findings show that preblocking with an anti-EGFR affibody molecule is a promising tool that could improve the outcome of radionuclide-based imaging of EGFR-expressing tumors.     

High-dose gallium-67 therapy in patients with relapsed acute leukaemia: a feasibility study.

Gallium-67 (67Ga) accumulates in malignant tissues via the transferrin receptor without need for a monoclonal antibody and emits cytotoxic low-energy electrons. In this study we investigated the feasibility, pharmacokinetics, toxicity and preliminary efficiency of high-dose 67Ga injected intravenously (i.v.) in patients with acute leukaemia not responding to conventional therapy. Twelve doses of 36-105 mCi of Gallium67 citrate were administered as a push injection to eight patients with resistant leukaemia in a pilot study. All five patients with acute myeloid leukaemia (AML) and three patients with acute lymphoblastic leukaemia (ALL) had resistant disease or resistant relapse. No (sub)acute toxicity was observed. Independent of the administered dose, whole-blood radioactivity levels 10 min after administration measured only 1.25 +/- 1.39 microCi ml-1, indicating a large volume of distribution. Urine excretion in the first 24 h ranged from 18% to 51.5% (median 29.5%) of the administered dose. Cellular uptake of 67Ga was less than in previous in vitro studies. Whole-body radiation dose was estimated to be 0.25 +/- 0.03 cGy mCi-1. Red marrow dose was estimated to be between 0.18 +/- 0.02 and 0.97 +/- 0.12 cGy mCi-1. One definite response was observed in an ALL patient with disappearance of skin lesions, normalisation of the enlarged spleen and profound leucopenia. Three other patients showed transient reductions in white blood cell counts without disappearance of blasts from the peripheral blood. We conclude that high-dose i.v. 67Ga can be safely administered but that the uptake of 67Ga in blast cells must increase to make 67Ga therapeutically useful in patients with relapsed leukaemia.     

Gallium-67 citrate uptake by cultured tumor cells, stimulated by serum transferrin.

The uptake of the tumor scanning agent 67-Ga citrate by cultured mouse tumor cells wqs greatly enhanced with small additons of certain sera to the culture medium. This stimulation of uptake was due to a macromolecular serum component, provisionally identified with transferrin. The response under optimum conditions was so great as to suggest that the 67-Ga uptake process depends entirely on the action of transferrin. The same mechanism probably operates in vivo and may help explain interesting features of the tracer's tissue distribution, particularly its tumor affinity.     

Development of drug resistance to gallium nitrate through modulation of cellular iron uptake

We have shown that transferrin-gallium (Tf-Ga) blocks DNA synthesis through inhibition of cellular iron incorporation and a diminution in the activity of the iron-dependent M2 subunit of ribonucleotide reductase. To examine the mechanisms of drug resistance to gallium, we developed a subline of HL60 cells (R cells) which is 29-fold more resistant to growth inhibition by gallium nitrate than the parent line (S cells). R cells displayed a 2.5-fold increase in transferrin (Tf) receptor expression, without a change in receptor affinity for Tf. The uptake and release of 67Ga were similar for both S and R cells. The uptake of 59Fe-Tf by S cells was inhibited by gallium nitrate over 24-48 h of incubation. In contrast, 59Fe-Tf uptake by R cells, although initially inhibited by gallium nitrate at 24 h, was no longer inhibited at 48 h of incubation. 59FeCl3 uptake by R cells was significantly greater than that of S cells, regardless of the time in culture. Despite the increase in 59Fe uptake by R cells, the ferritin content of these cells was lower than that of S cells. The ribonucleotide reductase electron spin resonance signal of R cells was comparable to that of S cells. R cells were not cross-resistant to Adriamycin, vincristine, cis-platinum or hydroxyurea. Resistance to gallium nitrate in this subline of HL60 cells results primarily from the ability of cells to overcome the gallium-induced block in iron incorporation. In addition, intracellular iron in R cells appears to traffic preferentially to a non-ferritin compartment.     

Inhibition of PDGF receptor signaling in tumor stroma enhances antitumor effect of chemotherapy

Lowering of tumor interstitial hypertension, which acts as a barrier for tumor transvascular transport, has been proposed as a general strategy to enhance tumor uptake and therapeutic effects of anticancer drugs. The tyrosine kinase platelet-derived growth factor (PDGF) beta-receptor is one mediator of tumor hypertension. The effects of PDGF antagonists on chemotherapy response were investigated in two tumor models that display PDGF receptor expression restricted to the tumor stroma, and in which PDGF antagonists relieve tumor hypertension. Inhibitory PDGF aptamers and the PDGF receptor tyrosine kinase inhibitor STI571 enhanced the antitumor effect of Taxol on s.c. KAT-4 tumors in SCID mice. Treatment with only PDGF antagonists had no effect on tumor growth. Taxol uptake in tumors was increased by treatment with PDGF antagonists. Cotreatment with PDGF antagonists and Taxol was not associated with antiangiogenic effects, and PDGF antagonists did not enhance the Taxol effect on in vitro growth of KAT-4 cells. STI571 also increased the antitumor effects of 5-fluorouracil on s.c. PROb tumors in syngeneic BDIX rats, without increasing the effect of 5-fluorouracil on cultured PROb cells. Expression of PDGF receptors in tumor stroma, as well as tumor hypertension, occurs in most common solid tumors. Therefore, our results have implications for treatment regimens for large patient groups and merit clinical testing. In conclusion, our study identifies inhibition of PDGF signaling in tumor stroma as a novel, possibly general strategy for enhancement of the therapeutic effects chemotherapy.     

Tumor-localizing and photosensitizing properties of the main components of hematoporphyrin derivative

Both 3H-labeled and unlabeled hematoporphyrin derivative (HPD) were analyzed by high-pressure liquid chromatography (HPLC) and gel permeation chromatography. Four main components were isolated by HPLC, and two were isolated by gel permeation chromatography. The tumor-localizing ability of each component was tested and compared to that of 67Ga and 3H2O by injection in mice bearing Lewis lung carcinoma. The photosensitizing abilities of the HPLC-separated components in vitro were also tested. Finally, porphyrin extracts of tumors from mice given HPD were analyzed by HPLC. The tumor-localizing ability of the components increased with decreasing polarity. While crude HPD localized in tumor tissue only to the same extent as did 3H2O, Component 7 was almost as effective as was 67Ga in localizing in the tumor. The cellular uptake of HPD components increased with decreasing polarity. In accordance with this, the low-polarity components were the most effective photosensitizers.     

Anemia in malignant tumors. Loss of circulating transferrin due to lysosomal storage in human and rat malignant tumors]

The cellular uptake of 67Ga-labelled transferrin was studied in the carcinoma of the human respiratory tract and in the Morris hepatoma 5123 C of the rat. In both types of tumors a positive tumor imaging by 67Ga-transferrin scintigraphy was evident. The intracellular distribution pattern of the radioactivity showed that the incorporated 67Ga-transferrin is accumulated within the lysosomes of the tumors. The uptake of 67Ga-transferrin by the tumor cells resulted in a faster disappearance of the tracer from the blood. The accelerated disappearance of 67Ga-transferrin from the blood showed a direct correlation to the mass of tumor cells. The loss of circulating 67Ga-transferrin from the blood showed a close parallelism to the grade of the anemia observed in the tumor bearing rats. We conclude from these findings that the uptake of transferrin into the tumor cells is one of the factors which are responsible for the anemia observed in malignant diseases.     

Tumor imaging using 201Tl chloride and 67Ga citrate in nucleomedical surveys]

Among nucleomedical examinations for cancer, tumor imaging using 201Tl chloride and 67Ga citrate can be easily performed at almost any hospital. However, the planar images usually obtained are not sufficient to detect small tumorous lesions. For more accurate detection, tomographic images particularly single photon emission CT (SPECT) is necessary. SPECT is also indispensable in cancer diagnosis. Whether 201Tl SPECT or 67Ga SPECT is chosen depends upon tumor localization, histological type, and the final purpose of the study. 201Tl accumulates in almost all tumors, but it is not suitable for detection of abdominal lesions, because there is much physiological accumulation in the small intestine and kidney. In contrast, 67Ga does not always accumulate in adenocarcinomas. 201Tl SPECT and 67Ga SPECT are more useful in the functional imaging of cancer than is morphological tumor diagnosis. Both methods are useful in monitoring treatment effectiveness, detecting recurrent lesions after surgery and radiotherapy, and predicting the grade of malignancy of the tumor. Tumor SPECT using 201Tl chloride and/or 67Ga citrate provides clinically useful information not obtained by morphological tumor diagnosis only.     

Factors influencing the toxicity of diethylaminoethylreserpine to tumor cells: studies with four transplantable tumors

The toxicity of 1-[2-(diethylamino)ethyl]reserpine (DL-152) has been measured for 4 transplantable mouse tumors. DL-152 was found to be toxic to cells of all the tumor models tested (KHT fibrosarcoma, RIF-1 fibrosarcoma, EMT-6 adenocarcinoma and Lewis lung carcinoma) when the drug was given by intraperitoneal injection to the tumor-bearing mouse and cell survival was measured by excision assay. For the KHT tumor, hypoxic cells were found to be more sensitive to the drug in vivo than were aerated cells, and a similar response to hypoxia was observed in vitro, suggesting that sensitization occurred at the cellular level. Neither EMT-6 nor RIF-1 tumors showed increased sensitivity to the drug when cells were exposed under hypoxic conditions in vivo or in vitro. However, when the response of aerated cells of the 3 tumors was compared, the relative sensitivities for tumors exposed in vivo did not show the same ranking as the results of in vitro toxicity assays. This difference in in vitro and in vivo response in the different tumor models did not appear to be related to pharmacokinetic factors since the maximum tissue concentration and the rate of clearance of the drug were similar for all the tumors studied.     

Brain tumor iron uptake measured with positron emission tomography and 52Fe-citrate

Summary Iron and transferrin are required for DNA synthesis and cell division. Cellular iron uptake is mediated by transferrin receptors. In order to investigate whether iron uptake in brain tumors is associated with their histological grade, we studied 24 patients (5 astrocytoma, 11 glioblastoma, 8 meningioma) using positron emission tomography and 12 Fe-citrate. Tracer uptake from blood into brain and tumor tissue was assessed 1. using multiple time graphical analysis yielding a measure for unidirectional net tracer uptake (Ki and 2.) testing a one- and two-tissue kinetic compartment model, where K1 denotes tracer uptake from blood into tissue, k2 efflux from tissue into plasma, and 0 specific tracer binding. In the plasma, ⁵²Fe was bound to a 80 kD protein (transferrin). Ki (in units of 10^–5/min) was higher in glioblastomas (Ki mean ± SD 13.6 ± 6.1) compared with astrocytomas (4.8 ± 3.5, Mann Whitney p = 0.015) and contralateral brain (2.2 ± 0.9, Mann Whitney p = 0.009). Highest values were found in meningiomas (no blood-brain barrier (BBB); Ki 33.4 ± 16.5, Mann Whitney p = 0.008 compared with glioblastomas). Among the compartment models, fitting with Kl1 and regional plasma volume explained the data best (one-tissue model), data fits were not significantly improved by addition of a k2 or k3 parameter. K1 and Ki values were significantly correlated (Spearman Rank, p = 0.0006). We conclude that ⁵²Fe accumulation in tumors is governed by tracer uptake at the BBB, and does not reflect number of transferrin receptors at the level of tumor cells.     

Neuroendocrine tumor targeting: study of novel gallium-labeled somatostatin radiopeptides in a rat pancreatic tumor model

Somatostatin analogs labeled with radionuclides are of considerable interest in the diagnosis and therapy of SSTR-expressing tumors, such as gastroenteropancreatic, small cell lung, breast and frequently nervous system tumors. In view of the favorable physical characteristics of the Ga isotopes (67)Ga and (68)Ga, enabling conventional tumor scintigraphy, PET and possibly internal radiotherapy, we focused on the development of a Ga-labeled somatostatin analog suitable for targeting SSTR-expressing tumors. For this purpose, 3 somatostatin analogs, OC, TOC and TATE were conjugated to the metal chelator DOTA and labeled with the radiometals (111)In, (90)Y and (67)Ga. They were then evaluated for their performance in the AR4-2J pancreatic tumor model by testing SSTR2-binding affinity, internalization/externalization in isolated cells and biodistribution in tumor-bearing nude mice. Surprisingly, we found that, compared to (111)In or (90)Y, labeling with (67)Ga considerably improved the biologic performance of the tested somatostatin analogs with respect to SSTR2 affinity and tissue distribution. (67)Ga-labeled DOTA-somatostatin analogs were rapidly excreted from nontarget tissues, leading to excellent tumor-to-nontarget tissue uptake ratios. Of interest for radiotherapeutic application, [(67)Ga]DOTATOC was strongly internalized by AR4-2J cells. Furthermore, our results suggest a link between the radioligand charge and its kidney retention. The excellent tumor selectivity of Ga-DOTA somatostatin analogs together with the different applications of Ga in nuclear oncology suggests that Ga-DOTA somatostatin analogs will become an important tool in the management of SSTR-positive tumors.     

New pansomatostatin ligands and their chelated versions: affinity profile, agonist activity, internalization, and tumor targeting.

PURPOSE: Somatostatin receptor (sst) targeting is an established method to image and treat sst-positive tumors. Particularly, neuroendocrine tumors express the receptor subtype 2 in high density, but sst1, sst3, sst4, and sst5 are also expressed to some extent in different human tumors. Currently used targeting peptides mainly have sst2 affinity. We aimed at developing (radio)peptides that bind with high affinity to all receptor subtypes. EXPERIMENTAL DESIGN: Carbocyclic octapeptides were coupled with macrocyclic chelators for radiometal labeling. Affinity, internalization, and agonist potencies were determined on sst1- to sst5-expressing cell lines. Biodistribution was determined on nude mice bearing HEK-sst2 or AR4-2J and HEK-sst3 tumors. RESULTS: High affinity to all receptor subtypes was found. Y(III)-KE88 showed agonistic properties at all five sst receptor subtypes as it inhibits forskolin-stimulated cyclic AMP production. Surprisingly, very low or even absent sst2 receptor internalization was found compared with currently clinically established octapeptides, whereas the sst3 internalization was very efficient. Biodistribution studies of [(111)In]KE88 and [(67)Ga]KE88/[(68)Ga]KE88 reflected the in vitro data. In nude mice with s.c. implanted sst2 (HEK-sst2, AR4-2J)-expressing and sst3 (HEK-sst3)-expressing tumors, high and persistent uptake was found in sst3-expressing tumors, whereas the uptake in the sst2-expressing tumors was lower and showed fast washout. The kidney uptake was high but blockable by coinjection of lysine. CONCLUSION: This peptide family shows pansomatostatin potency. As radiopeptides, they are the first to show a full pansomatostatin profile. Despite some drawback, they should be useful for imaging sst2-expressing tumors with short-lived radiometals, such as (68)Ga, at early time points and for sst3-expressing tumors at later time points with longer-lived radiometals, such as (64)Cu or (86)Y.     

Enhancement of tumor outgrowth by tumor-associated blocking factors.

Transplanted lines of 3-methylcholanthrene (MCA)-induced BALB/c sarcomas were used for experiments in which tumors were mechanically suspended in Waymouth's culture medium, the suspensions centrifuged and filtered and the remaining cell-free "tumor fluids" (TF) tested for activity in vitro and in vivo. TF were found to inhibit ("block") cell-mediated cytotoxicity to tumor target cells in vitro with specificity for each tumor tested and even at high dilutions (1:1,000-1:10,000). In vivo, TF enhances the outgrowth of subcutaneously transplanted tumors in non-immune syngeneic mice. This enhancement had both specific and non-specific components, with the non-specific component being most apparent. Enhancement was best demonstrated when the TF were inoculated together with the tumor inocula. In one of two experiments in which TF were given intraperitoneally, specific enhancement was seen, whereas in the other, inhibition of tumor outgrowth was observed.