Potential tumor- or organ-imaging agents. 27. Polyiodinated 1,3-disubstituted and 1,2,3-trisubstituted triacylglycerols.

A series of glyceryl 1,3-bis- and 1,2,3-tris[omega-(3-amino-2,4,6-triiodophenyl)alkanoates] were synthesized, radioiodinated with iodine-125, and evaluated for their ability to selectively localize in the liver for potential use as hepatographic imaging agents. Of the nine target compounds synthesized and evaluated in rats, glyceryl 1,2,3-tris[3-(3-amino-2,4,6-triiodophenyl)propionate] (5b) displayed rapid and sustained liver specificity. This agent was found to accumulate in the liver in concentrations of 60, 75, and 86% of the administered dose at 5 min, 30 min, and 24 h, respectively. Moreover, the 24-h liver-to-blood ratio of 235 justifies further studies in higher animal species.

Potential tumor- or organ-imaging agents. 26. Polyiodinated 2-substituted triacylglycerols as hepatographic agents.

A series of omega-(3-amino-2,4,6-triiodophenyl)alkanoic acids and the corresponding 1,3-dipalmitoylglycerol 2-[omega-(3-amino-2,4,6-triiodophenyl)alkanoates] were synthesized, radioiodinated with iodine-125, and evaluated for their ability to selectively localize in the liver for potential use as hepatographic imaging agents. Acid analogues 1d and 1e afforded relatively high levels of radioactivity in the liver (45 and 49% injected dose) 5 min after intravenous administration to rats. These acids displayed a marked propensity to become bound to plasma albumin. In contrast, triacylglycerol analogues 10a and 10c did not become immediately associated with plasma albumin but instead rapidly became associated with plasma lipoproteins and showed a different tissue distribution profile than free acids 1a and 1c. Although long-chain triacylglycerol analogues 10d and 10e exhibited some capacity to accumulate in the liver at 5 and 30 min, respectively, analysis of the plasma revealed significant in vivo ester hydrolysis. It would thus appear that liver radioactivity following administration of 10d and 10e was due to uptake of the free acid and not the intact triacylglycerol. Triacylglycerol analogues 10a and 10c, on the other hand, were taken up intact and showed liver accumulations of 25 and 35% of the administered dose at 30 min.

Potential tumor- or organ-imaging agents. 23. Sterol esters of iopanoic acid.

A series of sterol esters of iopanoic acid was synthesized and evaluated for their potential to selectively localize in liver and steroid-secreting tissues for possible application in either computed tomography or nuclear medicine imaging. Unlike free iopanoic acid (1), which was rapidly cleared following intravenous administration to rats, cholesteryl iopanoate (2) was found to accumulate in liver, adrenal cortex, and ovary. At 24 h, the ovary was found to contain the highest concentration of 2. The ability of 2 to accumulate in the above tissues was attributed to its resistance to hydrolysis. Pregnenolone iopanoate (3) and dehydroepiandrosterone iopanoate (4), on the other hand, were shown to reach unusually high concentrations in the adrenal cortex within 0.5 h of administration but declined to much lower levels by 24 h. Lipid extraction of tissues showed 3 and 4 to be susceptible to in vivo hydrolysis, which undoubtedly was a major factor in their clearance from adrenal tissue.

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.

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.

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.

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.

Biodistribution of a new lipid-soluble CT contrast agent. Evaluation of cholesteryl iopanoate in the rabbit.

Cholesteryl iopanoate, the prototype for a new class of lipid-soluble, site-specific contrast agents, has undergone preliminary evaluation for organic-specific uptake. This report describes the tissue distribution profile of this radioiodinated sterol ester in the rabbit as a function of time following intravenous injection of tracer doses. Selective accumulation of this agent in the liver and adrenal was observed relative to blood and other tissues. Maximum contrast concentration was achieved between 24 and 48 hours postinjection. Analysis of lipid extra of liver, adrenal, and plasma indicated that the compound was relatively resistant to hydrolysis and dehalogenation. The hepatic selectivity and retention of cholesteryl iopanoate support further study of this or related agents for site-specific, contrast-enhanced computed tomography.

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.

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.

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.

Patient specific treatment planning for systemically administered radiopharmaceuticals using PET/CT and Monte Carlo simulation.

The efficacy of systemically administered radiopharmaceuticals depends on the physiological path of the targeting molecule and the physical characteristics of the attached radionuclide. NM404 is a candidate for patient specific dosimetry because it can be used concurrently for both diagnosis and therapy. Radiolabeling NM404 with [(124)I] affords the possibility of performing noninvasive PET imaging while [(131)I] allows for radiotherapy. Patient specific dosimetry for radiation treatment planning for NM404 uses serial PET/CT data and Monte Carlo. [(124)I]NM404 PET helps to determine the organ at risk by which the maximum injected activity of [(131)I]NM404 will depend. The subsequent work uses a software interface (SCMS) to convert patient PET/CT data of a liver metastasis into a Monte Carlo environment for radiation transport analysis. Thereby, the dosimetry within the liver and tumor during both diagnostic and therapeutic procedures was determined. The results showed that per MBq injected of [(124)I] and [(131)I], the tumor receives an average of 1.2 and 1.5mGy, respectively, while the liver receives 0.031 and 0.022mGy, respectively.

Low-attenuation oral GI contrast agents in abdominal-pelvic computed tomography.

BACKGROUND: We designed and evaluated a low-attenuation oral contrast agent for abdominal-pelvic computed tomography (CT). METHODS: In vitro studies, were performed initially to evaluate the imaging characteristics of multiple solutions. These studies resulted in two solutions being compared with the presently accepted oral CT agents of dilute iodinated contrast and water. Ninety-eight consecutive subjects already scheduled for routine outpatient abdominal-pelvic CT were enrolled. Subjects were randomized to water (n = 30), fiber solution (n = 32), polyethylene glycol (PEG; n = 11), or dilute iodinated solution (DI; n = 25). Examinations were then evaluated for gastric distention, small bowel distention, small bowel wall visualization, and colonic transit. A questionnaire was given to the study subjects for feedback concerning taste and potential side effects from these agents. RESULTS: PEG tended to provide better bowel distention, wall visualization, and colonic transit compared with water, fiber solution, and DI. Areas of statistical significance included: (1) average bowel diameter in the left upper quadrant for water was 17.50 mm, whereas that for PEG was 21.88 mm (p < 0.05); (2) average bowel diameter in the pelvis for water was 14.79 mm, that for fiber was 15.67 mm, and that for PEG was 18.48 mm (p < 0.05); (3) wall visualization was better with PEG than with fiber (p < 0.05); (4) successful transit of contrast to the colon occurred in every subject who received PEG compared with only 20% of those received water and 39% of those who received fiber (p < 0.05). Similar trends for the superiority of PEG over DI were noted, although many of these did not reach statistical significance. CONCLUSION: PEG solution has imaging characteristics related to bowel wall visualization, luminal distention, and colonic transit that make it an effective oral agent for abdominal pelvic CT examination.

Radioiodinated Cholesteryl lopanoate as a Potential Probe for the in Vivo Visualization of Atherosclerotic Lesions in Animals.

Radioiodinated cholesteryl iopanoate, a nonhydrolyzable cholesteryl ester probe, showed increased uptake into atherosclerotic aortas of cholesterol-fed rabbits in comparison with normal rabbits. Auto-radiography of the aortas showed the radioactivity to be concentrated in areas of visible atherosclerotic involvement. Lipid extraction and thin-layer chromatography of this tissue as well as liver, adrenal, and plasma confirmed the resistance of this probe to hydrolysis. These findings suggest that (125)I-cholesteryl iopanoate may prove useful for noninvasively monitoring atherosclerosis in intact laboratory animals.

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.

Esters of iopanoic acid as liver-specific CT contrast agents: biodistribution and CT evaluation.

The synthesis and preliminary biodistribution data for a series of sterol-like esters of iopanoic acid having potential value as liver-specific CT contrast agents are described. Structural modification of the sterol portion of the iopanoate ester afforded a group of compounds that displayed tissue specificity similar to cholesteryl iopanoate, the prototype ester of this series, but were rapidly cleared from the target tissues after hydrolysis. From the biodistribution data, the most promising of these agents, pregnenolone iopanoate (PI), was evaluated by CT in rabbits receiving a radiologic dose equivalent to 30 mg I/kg. The hepatic parenchyma was enhanced within 2 h of infusion to a maximal level of 31 HU above precontrast values. Hepatic CT attenuation returned to normal within 24 h. However, CT performed after PI infusion into Vx2 tumor-bearing rabbits failed to provide superior images compared with those acquired following bolus administration of urographic contrast.

Potential organ or tumor imaging agents. 32. A triglyceride ester of p-iodophenyl pentadecanoic acid as a potential hepatic imaging agent.

A triglyceride analog, glycerol-2-palmitoyl-1,3-di-15-(p-iodophenyl)pentadecanoate (DPPG) was synthesized and radiolabeled for evaluation as a potential functional liver scintigraphic agent. Uptake of DPPG was compared in normal, diabetic, tumor-bearing and heparin pretreated rats, revealing differences in uptake and clearance of radioactivity, correlating with hepatic lipase activity of these groups. Similar results were observed by gamma-camera scintigraphy. Comparing the uptake of DPPG with that of its fatty acid component, 15-(p-iodophenyl)pentadecanoic acid (IPPA), revealed that the peak uptake of IPPA in the liver was about half that of DPPG. Based upon these findings, DPPG warrants further study as a hepatic radiodiagnostic agent.