Imaging efficacy of a hepatocyte-selective polyiodinated triglyceride for contrast-enhanced computed tomography.

RATIONALE AND OBJECTIVES: DHOG-LE is an injectable polyiodinated triglyceride lipid emulsion providing contrast enhancement of the liver in CT. Studies were conducted to characterize the imaging efficacy of various DHOG-LE formulations as a function of both the administered iodine dose and the formulation composition. MATERIALS AND METHODS: Four DHOG-LE preparations consisting of either 10, 20, 25, or 30% (w/v) total lipid were administered to anesthetized female Sprague Dawley rats as single intravenous bolus doses of 50, 100, 150, and/or 300 mg I/kg (n = 3 to 6 rats/formulation and dose). A 25% triolein lipid emulsion prepared without iodine was administered as a vehicle control at the highest dose volume (n = 6). Liver enhancement was evaluated as a function of time (0 to 24 hours) after administration of contrast by analyzing regions of interest from sequential body scans. RESULTS: At all dose levels, liver enhancement was observed after injection of each DHOG-LE formulation. Regardless of formulation composition, similar enhancement of the liver was noted when administered at an equivalent iodine dose. Liver enhancement increased proportionately with increasing iodine dose. Mean peak intensities for 50, 100, 150, and 300 mg I/kg doses were 78 HU (42% above baseline), 101 HU (84% above baseline), 125 HU (127% above baseline), and 195 HU (255% above baseline), respectively. Liver time-intensity profiles exhibited rapid uptake, prolonged enhancement up to 3 hours, and complete clearance of the majority of the formulations tested by 24 hours. Time and duration of peak intensities were also directly related to iodine dose. CONCLUSIONS: In the animal model tested, DHOG-LE imaging efficacy was directly related to iodine dose and was independent of formulation composition. Thus, administration of DHOG-LE in highly concentrated lipid preparations minimized administered dose volume and resulted in appreciable liver enhancement, even at the lowest dose of 50 mg I/kg.

Polyiodinated triglyceride lipid emulsions for use as hepatoselective contrast agents in CT: effects of physicochemical properties on biodistribution and imaging profiles.

RATIONALE AND OBJECTIVES: A novel lipid emulsion (LE) was developed for hepatoselective delivery of a polyiodinated triglyceride (ITG) with potential for use in CT. This work assessed the effects of mean particle size, total administered dose, and formulation composition on the in vivo biodistribution and imaging profiles of the ITG-LE in rats. METHODS: The concentration of radioactivity derived from intravenously administered 125I-ITG-LE was determined as a function of time after injection. CT imaging studies of the abdomen evaluated the extent of hepatic enhancement after administration of ITG-LE. RESULTS: Mean emulsion particle diameter and total administered dose exerted the greatest effect on ITG-LE biodistribution profiles. In the optimal delivery scenario, >70% of the administered dose localized to the liver 30 minutes after injection. Liver enhancement profiles in CT imaging studies were consistent with biodistribution profiles. CONCLUSIONS: These results suggest that an appropriately formulated and administered dose of ITG-LE provides tissue-selective localization of contrast material for use in CT.

Radiolabeled cholesteryl iopanoate/acetylated low density lipoprotein as a potential probe for visualization of early atherosclerotic lesions in rabbits.

PURPOSE: Atherosclerosis is the underlying factor leading to such cardiovascular diseases (CVD) as stroke, aneurysm, and myocardial infarction. The early detection of atherosclerotic plaques is considered to be crucial for successful prevention and/or therapeutic and dietary intervention of CVD. Current diagnostic practice, on the other hand, can only detect the problem at an advanced stage. The purpose of this study was to examine the potential of using a radiolabeled cholesterol ester analog/acetylated low density lipoprotein (AcLDL) conjugate as a diagnostic agent for the early and non-invasive detection of atherosclerosis and for the monitoring of the effects of drug therapy. METHODS: Cholesteryl iopanoate (CI), a cholesterylester analog, was synthesized, radiolabeled, and incorporated into AcLDL. Early atherosclerotic lesions were induced in New Zealand White rabbits. 125[-CI/AcLDL was injected intravenously at 2 microCi/kg. Blood samples were taken at different time intervals after injection and clearance of the injected drug from blood was studied. The rabbits were sacrificed after 72 hours and the distribution of radioactivity in various organs was investigated. Aortae of both atherosclerotic lesion and control rabbits were removed for Sudan IV staining and autoradiography in order to confirm the formation of the atherosclerotic lesion and localization of radioactivity. RESULTS: The injected drug was found to be cleared from blood following a two compartment model. Radioactivity in the atherosclerotic aorta was found to be about 8 times higher than that in normal aorta, suggesting that the proposed diagnostic probe was selectively taken up by the atherosclerotic lesion. The autoradiography and staining confirmed that the localization of the proposed probe was superimposed with the atherosclerotic lesion site. CONCLUSIONS: The results suggested that incorporation of CI into AcLDL resulted in the selective localization of CI at the atherosclerotic plaque areas. CI/AcLDL labeled with appropriate radioisotope has the potential to be used as a probe for visualization of early atherosclerotic lesion using scintigraphy technology.

Lipid-based blood-pool CT imaging of the liver.

RATIONALE AND OBJECTIVES: We have recently developed an iodinated lipid-based contrast agent capable of residing in the blood pool for extended periods of time relative to conventional water-soluble contrast agents. The purpose of this study was to examine the effects of combining this new blood-pool agent (ITG-PEG) with a hepatocyte-selective agent (ITG-LE; Molecular Biosystems) for accurate CT detection of small (< 10 mm) VX2 tumors in rabbit liver. MATERIALS AND METHODS: Preliminary pharmacokinetic analyses were conducted in SD rats (12) by injection of either I-125-labeled ITG-PEG or I-125-labeled ITG-LE followed by subsequent blood collection and quantification of radioactivity. Preliminary CT studies were conducted in both normal (3) and tumor-bearing NZW rabbits (2). Tumor-bearing rabbits were laparotomized and VX2 cells injected directly into the hepatic parenchyma to produce a total of eight focal lesions (2-10 mm diameter). Animals underwent CT scanning 10 days later with multiple techniques including noncontrast and helical i.v. enhanced (600 mg I/kg iohexol), and then 24 hours later using both ITG-PEG and ITG-LE (200 mg I/kg). Tissue density measurements (HU) of liver, tumor, and blood (descending aorta) were acquired in each case for comparison. Tumor morphology was verified by gross pathologic inspection. RESULTS: Pharmacokinetic analysis in rats as well as CT studies in normal rabbits revealed that ITG-PEG remains in the blood-pool phase for more than 2 hours following i.v. administration. In fact, blood density in normal rabbit obtained with ITG-PEG was 95.1 HU +/- 5.8 at 120 minutes compared to 90.7 HU +/- 6.1 immediately after injection. Although liver enhancement was greater with iohexol (67 HU within 1 minute of injection), than for ITG-PEG/ITG-LE (32 HU, 60 minutes postinjection), liver to lesion ratios favored ITG-PEG/ITG-LE due to significant enhancement of tumor itself with iohexol (+40 HU). Tumor enhancement was minimal with ITG-PEG/ITG-LE. Lesions were subjectively much better defined with ITG-PEG/ITG-LE with sharper edge definition. CONCLUSION: In these animal models, a new iodinated lipid-based contrast agent composed of both blood pool and hepatocyte-selective components afforded favorable CT imaging results compared to a conventional urographic agent, albeit at one-third the total iodine dose.

Adrenal proteins bound by a reactive intermediate of mitotane.

PURPOSE: Mitotane (o,p'-DDD), is the only adrenolytic agent available for the treatment of adrenocortical carcinoma. Previous studies have shown that mitotane covalently binds to adrenal proteins following its metabolism in adrenocortical tissue to a reactive acyl chloride intermediate. It was the objective of this study to compare the electrophoresis separation patterns of such adducts following activation of mitotane by various adrenocortical sources. METHODS: With the use of a 125I-labeled analog of mitotane, 1-(2-chlorophenyl)-1-(4-iodophenyl)-2,2-dichloroethane, gel electrophoresis patterns were obtained for homogenates from bovine, canine and human adrenocortical preparations as well as from a human adrenal preparation. Western immunoblotting analysis was used to test the resulting patterns for adducts of cytochrome P-450scc and adrenodoxin. RESULTS: The electrophoresis separations were similar for all preparations, with bands at apparent molecular weights of 49.5 and 11.5 kDa being the most pronounced. Radiolabeling of the proteins of a human adrenal cancer cell line NCI H-295 was weak, but a band at 11.5 kDa was detected. Western immunoblotting analyses indicated that the band at 49.5 kDa corresponded in molecular weight to that of adrenal cytochrome P-450scc, but the band at 11.5 kDa did not correspond to adrenodoxin. CONCLUSIONS: The similarity of the results with canine and bovine adrenal preparations to that of human material offers useful systems for studying mitotane and its analogs. This should aid in understanding the mechanism of action of mitotane and in the design of compounds for the treatment of adrenocortical carcinoma.

Physicochemical characterization of a synthetic lipid emulsion for hepatocyte-selective delivery of lipophilic compounds: application to polyiodinated triglycerides as contrast agents for computed tomography.

A synthetic lipid emulsion (LE) has been developed with physicochemical properties that closely resemble those of a specific class of naturally-occurring lipoproteins known as chylomicron remnants. The formulation has the potential to serve as a hepatocyte-selective delivery system for any lipophilic or amphipathic compounds that can be associated with the internal lipid phase of the emulsion. In the present studies, a lipophilic polyiodinated triglyceride (ITG) was successfully incorporated into the delivery vehicle to form a stable chylomicron-remnant-like emulsion capable of localizing material to the liver following intravenous injection. The preferred ITG-LE formulation was shown to have a mean particle diameter of less than 200 nm and a particle size stability profile in excess of 12 months. The viscosity, pH, and osmolality of the formulation also appeared favorable for safe and convenient intravenous injection. The particle size profile, chemical properties, and high degree of incorporation of ITG into the emulsion suggest that the ITG-LE formulation holds substantial promise as a hepatocyte-selective imaging agent for computed tomography of the liver. Biodistribution, elimination, and computed tomography (CT) imaging results in animals corroborated the hepatocyte-selective nature of the ITG-LE formulation.

Biological disposition and imaging of a radioiodinated alkylphosphocholine in two rodent models of breast cancer.

UNLABELLED: Iodine-125-12-[m-iodophenyl]-dodecylphosphocholine (NM-324) has been shown to accumulate in a variety of animal tumor models. Moreover, preliminary pharmacokinetic studies with NM-324 are being conducted in cancer patients. The present study was undertaken to examine the potential application of NM-324 as a breast tumor-imaging agent. METHODS: Two animal models of breast cancer were utilized: namely, syngenic inbred Lewis female rats bearing the rat mammary tumor (RMT) and athymic mice with HT-39 human tumor xenografts. After i.v. administration of NM-324, the tissue distribution of radioactivity was determined at various time points. Gamma camera scintigrams were also acquired to confirm the biodistribution results. Macro- and microautoradiography were used to analyze cellular distribution of radioactivity in tumors. RESULTS: In the rat mammary tumor model, levels of radioactivity in the tumor reached a maximum at 24 hr after i.v. administration (1.65% ID/g, tumor-to-blood 6.4). These tumors could be visualized by gamma camera scintigraphy as early as 1 hour after administration. In the nude mouse model, levels of radioactivity in tumor reached a maximum at 48 hr after i.v. administration (4.96 %ID/g, tumor-to-blood 5.5). Tissues expected to interfere with the resolution of breast lesions such as fat, heart, lung and muscle displayed much lower concentrations of the radioactivity. Gamma camera scintigraphy confirmed the results observed from biodistribution experiments. Lipid extraction of the tumors and major organs in both animal models showed the sole presence of unchanged NM-324. Microautoradiographic analysis of slices of rat mammary and HT-39 tumors provided additional information regarding the intratumoral distribution of radioactivity. CONCLUSION: The ability of radioiodinated phospholipid analogs to accumulate in breast tumors reinforces the need for further investigation of this type of radiopharmaceutical as tumor imaging agents.

Formulation of polyiodinated triglyceride analogues in a chylomicron remnant-like liver-selective delivery vehicle.

PURPOSE: A formulation methodology for the incorporation of polyiodinated triglyceride (ITG) analogues into a protein-free chylomicron remnant-like emulsion was developed to provide a vehicle for the selective hepatic delivery of these agents for contrast-enhanced X-ray computed tomography (CECT). METHODS: Triglyceride emulsions (10% w/v) were prepared at various processing pressures, temperatures and times with a Microfluidizer 110-S using different emulsion component proportions to establish processing and compositional parameters in order to afford stable ITG emulsions (ITG-LE) approaching 200 nm mean diameter. RESULTS: Preliminary data indicated that with a formulation composed of 2.4% dioleoyl PC with a cholesterol:DOPC mole ratio of 0.4 emulsified at 14,700 psi, 35 degrees C for 10 min routinely afforded ITG-LE in the desired size range. The elimination of salt and amino acid from the bulk phase enhanced the stability of the ITG-LE. Incorporation of cholesterol into the monolayer was of critical importance in generating a stable emulsion near the targeted size, with a C:DOPC mole ratio of 0.4 producing a size minimum relative to higher or lower C:DOPC values. CONCLUSIONS: The ITG analogues can be readily incorporated into stable remnant-like emulsions of relatively uniform particle size. Combination of the unique ITG contrast agent with the remnant-like delivery vehicle demonstrates a high degree of hepatic selectivity in biodistribution studies and offers significant potential for selective hepatic CECT.

Computed tomography scanning of Morris hepatomas with liver-specific polyiodinated triglycerides.

RATIONALE AND OBJECTIVES: We compared the computed tomography (CT) scanning characteristics of a polyiodinated triglyceride analog with those of a urographic agent to distinguish Morris-7777 hepatoma (MH) cells from normal hepatocytes in rats. METHODS: Eighteen Buffalo rats were laparotomized and MH cells injected directly into the hepatic parenchyma or introduced via the portal vein to produce, respectively, focal or diffuse lesions in the liver. Baseline CT scans were obtained 21 days after implantation and prior to intravenous administration of either the polyiodinated triglyceride (45-100 mg I/kg) or the nonionic contrast agent, iohexol (560 mg I/kg). Images were obtained at 0-3 hr and 24 hr. Gross pathologic inspection was performed to validate the imaging results. RESULTS: Hepatomas were nearly isodense with normal liver parenchyma in many of the animals, rendering lesion detection difficult with no contrast agent. The bolus administration of iohexol improved lesion detection in many cases. Lesion conspicuity, however, was significantly improved with the polyiodinated triglyceride at less than one eighth the dose of iohexol. CONCLUSION: Because of their biochemical nature, polyiodinated triglyceride analogs are specifically cleared by the liver. Consequently, they offer several advantages over nonspecific urographic agents in their ability to enhance lesion conspicuity in this hepatoma model.

Synthesis and biological evaluation of radioiodinated phospholipid ether stereoisomers.

Radioiodinated phospholipid ethers have shown the remarkable ability to selectively accumulate in a variety of animal tumors as well as in human tumor xenografts. It has been suggested that this tumor avidity may arise as a consequence of metabolic differences between tumor and corresponding normal tissue. One such compound, 1-O-[12-(m-iodophenyl)dodecyl]-2-O-methyl-rac-glycero-3-phosphocholine (NM-294), contains a chiral center at the sn-2 position. The unnatural S- and natural R-enantiomers (4 and 5, respectively) of NM-294 were synthesized in order to provide further information on the mechanism(s) responsible for the tumor avidity of phospholipid ethers. In vitro cytotoxicity studies demonstrated a lack of stereospecificity. Biodistribution studies in rats bearing the Walker 256 tumor demonstrated the S- and R-isomers to have similar tissue uptake at 24 and 48 h after administration. Tumor-to-blood ratios at 24 h were 11.1 and 11.0 for the S- and R-isomers, respectively. In addition, gamma-camera scintigrams of tumor-bearing rats at various time points after iv administration of the S- and R-isomers did not show any qualitative differences in the distribution of radioactivity. Prior studies have shown that rac-NM-294 was not a substrate for phosphatidylcholine specific phospholipase C, but was a substrate for two forms of phospholipase D (PLD). Therefore, metabolism studies with 4 and 5 with various forms of PLD were performed. PLD from cabbage demonstrated a degree of stereoselectivity. In the presence of 1% ethanol, the R-isomer was metabolized to the greatest extent, followed by rac-NM-294 and the S-isomer. PLD isolated from Streptomyces chromofuscus failed to demonstrate any stereoselectivity. The results suggest that the mechanism(s) of retention of these compounds in tumors may not involve a highly stereoselective component.

Synthesis and biological evaluation of radioiodinated phospholipid ether analogs.

Previous work has shown that radioiodinated phospholipid ether analogs with the iodine-125 substituted on the meta position of the aromatic ring readily localized in a variety of animal tumors. In an effort to ascertain the importance of such meta substitution, three phospholipid ether analogs with the iodine-125 in the para position were synthesized for evaluation as potential tumor-localizing imaging agents. 12-(p-Iodophenyl)dodecyl phosphocholine, 1-O-[12-(p-iodophenyl)dodecyl]-1,3-propanediol-3-phosphocholine, and 1-O-[12-(p-iodophenyl)dodecyl]-2-O-methyl-3-rac-glycerophosphocholine were synthesized and labeled with iodine-125 via an isotope exchange procedure. Similar to previous results with the meta substituted analogs, tissue distribution studies with the three para analogs demonstrated tumor localization and retention of radioactivity at 24 h after i.v. injection. In all three cases, the para isomers showed greater tumor avidity than the meta isomers and clearance of the radiotracer from the tumor was much slower than the clearance from nontarget tissue. 12-(p-Iodophenyl)dodecyl phosphocholine afforded the greatest tumor-to-nontarget tissue ratio. For example, the tumor-to-blood and tumor-to-liver ratios at 24 h were 10.96 and 1.85, respectively. As a result of such selective tumor retention, it was possible to clearly delineate the tumor using gamma-camera scintigraphy.

Bovine adrenal cortex transformations of mitotane [1-(2-chlorophenyl)-1-(4-chlorophenyl)-2,2-dichloroethane; o,p'-DDD] and its p,p'- and m,p'-isomers.

The adrenalytic activity of mitotane (o,p'-DDD) has made it useful in the treatment of adrenocortical carcinoma and Cushing's syndrome. In support of a study to develop mitotane analogs as more effective therapeutic agents and as a basis for understanding the toxicity of related compounds in the adrenals, the biotransformations of o,p'-DDD in adrenocortical homogenate preparations have been studied and compared with those of its m,p'- and p,p'-isomers. Aliphatic oxidation to the corresponding acetic acid derivative, o,p'-, m,p'- or p,p'-DDA, was the major transformation for all the preparations. In the comparisons of the DDD isomers, the order of both DDA formation and apparent covalent binding was o,p'- > m,p'- > p,p'-DDD. There was also evidence for alpha-hydroxylation at the benzylic carbon with subsequent loss of water to form ethylene derivatives. This was a minor pathway for o,p'-DDD, but was the major pathway for the other two isomers. Thus, while the total yields of metabolites of o,p'- and m,p'-DDD were similar and at least twice that of the p,p'-isomer, their distribution of metabolites differed significantly. The effects of the three isomers on cell growth and cortisol production with the human adrenocortical carcinoma cell line, NCI H-295, followed the same order as their DDA formation and tissue binding. It is proposed that hydroxylation by the adrenal cortex at the beta-carbon leads to an adrenalytic effect, whereas hydroxylation at the alpha-carbon would represent an alternate deactivation pathway.

Polyiodinated triglyceride analogs as potential computed tomography imaging agents for the liver.

A series of glyceryl 2-oleoyl 1,3-bis[omega-(3-amino-2,4,6-triiodophenyl)] alkanoates was synthesized, radioiodinated with iodine-125, emulsified, and evaluated for their ability to selectively localize in the liver for potential use as hepatographic agents in computed tomography. All seven analogs displayed rapid liver uptake wherein between 65 and 78% of the injected dose accumulated in the liver by 30 min. Liver values ranged from 46 to 93% 3 h after injection which corresponded to liver to blood ratios ranging from 21 to 450. Moreover, subsequent elimination of radioactivity from the liver was nearly linear with respect to alkyl chain length. Analogs with longer alkyl chain length were eliminated from the liver more rapidly than their shorter chain counterparts. Because of their biochemical similarities to naturally occurring triglycerides, these novel analogs may prove useful not only for high-resolution anatomic imaging of focal liver lesions, but also for evaluating a variety of diffuse diseases known to affect hepatic function and biochemistry.

Metabolic activation and binding of mitotane in adrenal cortex homogenates.

Mitotane [1-(2-chlorophenyl)-1-(4-chlorophenyl)-2,2-dichloroethane, o,p'-DDD] is an adrenocorticolytic agent of value in the treatment of adrenocortical carcinoma and Cushing's syndrome. In support of a program to develop agents superior to mitotane, it is the purpose of this study to explore the relationship of the metabolism of mitotane to its binding to adrenal cortex tissue from several sources. The objective was to detect the mitotane moiety responsible for its covalent binding in various test systems. Studies were conducted with an 125l-labeled analog of mitotane, 1-(2-chlorophenyl)-1-(4-iodophenyl)-2,2-dichloroethane, prior to a comparison to results with lower specific activity [14C]mitotane. With dog adrenal cortical whole homogenates, the majority of covalent binding was to proteins with an additional one-sixth of the total bound radioactivity associated with a phospholipid fraction. No radioactivity was associated with DNA. The rank order of species in regard to metabolism and protein binding was bovine > dog > rat adrenal homogenates > human normal adrenal or tumor homogenates. The percentage of radioactivity recovered from the hydrolysates of those fractions was uniformly high. In addition, the only metabolite present in the hydrolysates corresponded to 1-(2-chlorophenyl)-1-(4-iodophenyl)acetic acid from the iodo analog of o,p'-DDD and the corresponding o,p'-dichlorodiphenylacetic acid (o,p'-DDA) from o,p'-DDD. Our results are consistent with an acyl chloride being the reactive intermediate formed from the dichloromethyl moiety of mitotane, which leads to both DDA metabolite formation and binding to adrenal cortical bionucleophiles.

alpha-Methylene and glycidic acid analogs of IPPA as potential myocardial imaging agents.

Using organozinc cross-coupling reactions, two radiolabeled analogs of 15-(p-iodophenyl) pentadecanoic acid (IPPA) have been designed and synthesized as potential scintigraphic imaging agents for the heart. Both 15-(4-iodophenyl)-tridecylglycidic acid and 15-(4-iodophenyl)-2-methylene pentadecanoic acid were synthesized and radioiodinated. In tissue biodistribution studies in rats, only the alpha-methylene derivative of IPPA displayed a consistently higher heart to blood ratio and a substantially lower degree of thyroid accumulation than did IPPA alone. With respect to a scintigraphic imaging efficacy, the alpha-methylene analog of IPPA and IPPA itself showed essentially equivalent cardiac imaging profiles in rabbits, with a slight extension in imaging time for the alpha-methylene analog of IPPA.

Biodistribution, metabolism, and excretion of radioiodinated phospholipid ether analogs in tumor-bearing rats.

The phospholipid ether analog, [125I]-1-O-[12-(m-iodophenyl)dodecyl]propanediol-3-phosphocholine (NM-295) was synthesized and evaluated for its ability to visualize tumors. Preliminary studies were performed in rats bearing the Walker 256 carcinosarcoma. Most of the radioactivity was cleared from the animals during the first 24 hours. However, the tumor showed a decreased rate of clearance of radioactivity when compared with non-target tissue. This difference in the clearance rate allowed for excellent images of the tumor at 24 hours. Scintigraphic images compared favorably with other radioiodinated phospholipid ether analogs such as [125I-rac-1-O-[12-(m-iodophenyl)dodecyl]-2-O-methylglycero-3- phosphocholine (NM-294) and [125I]-12-(m-iodophenyl)dodecyl phosphocholine (NM-324). In contrast with the latter two compounds, however, tissue distribution studies revealed that NM-295 cleared at a much faster rate from all tissues, including tumor. In addition, within 24 hours following administration of NM-295, over 70% of the radioactivity was excreted as compared to 50% and 20% for NM-294 and NM-324, respectively. The majority of excreted radioactivity appeared in the urine for all three compounds. Thin-layer chromatography of urine and fecal extracts showed the presence of metabolites only. In contrast, lipid extracts of either liver or tumor demonstrated only the presence of the parent compound. Therefore, these data suggest that in each case it was the parent phospholipid analog that was taken up and retained by the tissues, while the metabolic product(s) was cleared and excreted from the animal.

Selective localization of a radioiodinated phospholipid ether analog in human tumor xenografts.

Administration of [125I]-rac-1-O-[12-(m-iodophenyl)dodecyl-2-O-methylglycero-3- phosphocholine (NM-294) to athymic mice implanted with human tumors of several histologies, including adenocarcinoma of the ovary and colon, melanoma and small-cell carcinoma of the lung, resulted in excellent images of the tumors by gamma camera scintigraphy. Images of the tumor were obtained at 5 days or more postinjection, by which time nearly all background activity had cleared from the liver and gastrointestinal tract. Tumor-to-blood ratios at this time were quite high and ranged from approximately 8:1 (melanoma) to 30:1 (ovarian carcinoma), which is consistent with the scintigraphic images obtained in all human tumor models. Lipid extraction of the liver and tumor at 13 days postinjection showed that most of the radioactivity in these tissues remained associated with the parent compound, with only a small amount retained by the liver. Appropriately radioiodinated NM-294 has substantial potential as a tumor-avid radiopharmaceutical.