Characteristics of Conventional and Microwave Sintered Iron Ore Preform.

In this study, compacted hematite (Fe2O3) preforms were made and sintered at various temperatures, such as 1250 °C and 1300 °C, using both conventional and microwave sintering methods. The density, porosity, microhardness, cold crushing strength, microphotographs, and X-ray diffraction (XRD) analysis of the sintered preforms were used to evaluate the performance of the two sintering methods. It was found that microwave sintered preforms possessed lesser porosity and higher density than conventionally sintered preforms owing to uniform heating of the powdered ore in microwave sintering method. Furthermore, it was also observed that microwave sintered preforms exhibited relatively higher cold crushing strength and hardness than conventionally sintered preforms. Thus, the overall results revealed that microwave sintering yielded better properties considered in the present study.

Regional myocardial substrate uptake in hypertensive rats: a quantitative autoradiographic measurement.

Severe hypertension causes global and regional changes in myocardial perfusion and substrate utilization. Regional perfusion and fatty acid utilization were evaluated by dual-tracer autoradiography in normotensive and hypertensive rats of the Dahl strain. The regional distributions of perfusion and fatty acid utilization were homogeneous in normotensive rats. Severe hypertension was associated with a homogeneous pattern of regional perfusion, but fatty acid utilization was focally decreased in the free wall of the left ventricle. The decrease in fatty acid uptake was associated with a concomitant increase in glucose utilization. These findings suggest that severe hypertension is associated with uniform myocardial perfusion and focal alterations in the substrates used for the performance of myocardial work.

Occurrence and distribution of hexachlorocyclohexane isomers in vegetation samples from a contaminated area.

The occurrence and distribution of four major hexachlorocyclohexane (HCH) isomers (alpha-, beta-, gamma- and delta-) were studied in vegetation samples of a highly contaminated area close to a small-scale industrial belt in Lucknow (North India). Eight species of plants were collected at different points of the contaminated area and different parts of the plants were separated in order to study the difference in uptake and accumulation. The samples were extracted by matrix solid-phase dispersion (MSPD) extraction and finally determined by a gas-chromatograph equipped with (63)Ni electron capture detector (ECD). HCH isomers were present in almost all samples and the concentration of total HCH in the plant sample analyzed varied between 13 and 44 mg kg(-1), being the main isomer of beta-HCH (8-22 mg kg(-1)). Lindane (gamma-HCH) was present in all samples (1-9 mg kg(-1)). Solanum torvum Sw., and Erianthus munja shows the highest and lowest capacity for accumulation of HCH, respectively with a significant difference at p<0.01 level. The highest concentration of HCH residue in root samples indicates the most likely mechanism of HCH accumulation in these plants was sorption of soil HCH on roots. Solanum torvum Sw., and Withania somnifera (L.) Dunal could accumulate considerable levels of HCH isomers (44 and 34 mg kg(-1), respectively). The results reflect the importance of plants in monitoring purposes and their potential for phytoremediation of HCH contaminated soils.

Production of actinomycin-D by the mutant of a new isolate of Streptomyces sindenensis.

An actinomycin-D producing strain was isolated from soil and characterized as Streptomyces sindenensis. The culture was subjected to UV irradiation and a mutant with 400% higher actinomycin-D production was isolated (400 mg/l(-1) as compared to 80 mg/l(-1) produced by the parent). Production medium was optimized and antibiotic yield with the mutant was enhanced to 850 mg/l(-1) which is 963% higher as compared with the parent.

Efficacy and toxicity of 67Cu-2IT-BAT-Lym-1 radioimmunoconjugate in mice implanted with human Burkitt's lymphoma (Raji).

Radioimmunotherapy has shown promising results for treatment of radiosensitive malignancies such as lymphoma. Positive responses have been reported in patients with non-Hodgkin's lymphoma treated with 131I-radiolabeled Lym-1, a mouse anti-lymphoma monoclonal antibody. In this study, the efficacy of 67Cu-radiolabeled Lym-1 was examined. Nude mice bearing human Burkitt's lymphoma (Raji) tumors (20-524 mm3) were treated with 12.4, 14.8, 18.5, and 23.3 MBq of 67Cu-2IT-BAT-Lym-1. Tumor size was measured to assess efficacy, and mouse weight, blood counts, and mortality were monitored to assess toxicity. In mice treated with 12.4, 14.8, and 18.5 MBq of 67Cu-2IT-BAT-Lym-1, 50% (9 of 18), 42% (5 of 12), and 50% (3 of 6) of tumors achieved remission or cure; 33% of tumors were cured overall; and significant regrowth delay was observed. The 23.3 MBq dose group did not yield meaningful efficacy data because of high mortality. In control groups receiving 14.8 and 18.5 MBq of the isotype-matched nonspecific monoclonal antibody radioimmunoconjugate, 67Cu-2IT-BAT-L6, 0% (0 of 15) and 17% (2 of 12) of tumors achieved a response; hence, targeted delivery of radiation was the dominant antitumor mechanism of 67Cu-2IT-BAT-Lym-1. LD50/30 for mice treated with 67Cu-2IT-BAT-Lym-1 and -L6 were 21.6 and 20.6 MBq, respectively. In conclusion, 67Cu-2IT-BAT-Lym-1 provided a therapeutic and frequently curative dose of radiation to tumored mice with modest toxicity.

Treatment of metastatic bone pain with tin-117m Stannic diethylenetriaminepentaacetic acid: a phase I/II clinical study.

The physical characteristics of Sn-117m combined with the biodistribution of the compound tin-117m (Stannic, 4+) diethylenetriaminepentaacetic acid (Sn-117m DTPA) suggest that it should be an excellent agent for the palliation of pain from bony metastases. Prior work has established the dosimetry and the safety for the material in human beings. The presence of low-energy conversion electrons should result in the relative sparing of the bone marrow while delivering a high radiation dose to sites of bony metastatic disease. Forty-seven patients with painful bone metastases from various malignancies were treated with Sn-117m DTPA. The patients were assigned to five different dose levels ranging from 2.64 to 10.58 MBq (71-286 microCi) per kg of body weight. Follow-up included review of pain diaries, performance scores, analgesic requirements, blood chemistries, and hematological assessment. Three patients received a second treatment. There was an overall response rate for relief of pain of 75% (range, 60-83%) in the 40 treatments that could be evaluated. No correlation was apparent in this limited series between response rate and the five dose levels used. The relief was complete in 12 patients (30%). The time to onset of pain relief was 19 +/- 15 days with doses < or = 5.29 MBq/kg and 5 +/- 3 days with doses > or = 6.61 MBq/kg. Myelotoxicity was minimal, with only one patient having a marginal grade 3 WBC toxicity. On the basis of our data, Sn-117m DTPA should be an effective and safe radiopharmaceutical for palliation of painful bony metastases. A large-scale trial is warranted to evaluate it in comparison to other similar agents.

Synthesis and biological properties of 4-amino- and 4-bromo-4-norpyridoxol. New approaches for the modification of the 4 position of vitamin B6.

4-Amino-4-norpyridoxol, a new key intermediate for the modification of the 4 position of vitamin B6, has been obtained by an unusual photochemical rearrangement of pyridoxal oxime. It has also been synthesized starting from 3,-alpha5-O-dibenzylpyridoxal, which was converted to 3,alpha5-O-dibenzylpyridoxamide. The latter, on Hoffman reaction, gave the desired 3,5-blocked 4-amino derivative. Several derivatives of this analogue have been prepared, and its existence in the amino tautomeric form has been established by NMR spectroscopy. A modified Sandmeyer reaction on 4-amino-4-norpyridoxol gave the 4-bromo analogue, which was found to be moderately active as an inhibitor of mouse mammary adenocarcinoma cells grown in cell culture, whereas the 4-amino analogue was not active at 10(-4)M. Other analogues containing electron-withdrawing and electron-donating substituents in the 4 position of pyridoxine were also tested.

Pancreas accumulation of radioiodinated HIPDM in mice and rats.

High pancreatic affinity for 131I-labeled HIPDM was observed in mice and rats. Although the brain uptake of [131I]HIPDM is very fast, the pancreatic uptake is rather slow. The pancreas to liver ratios (per gram) were 5.08 +/- 0.52 in mice and 5.15 +/- 0.65 in rats at 2 hr and 7.05 +/- 0.53 in mice and 8.06 +/- 1.14 in rats at 5 hr after administration. These ratios are higher than those obtained with routinely used pancreatic agent [75Se]selenomethionine. An increase in liver uptake and decrease in pancreatic uptake was observed at higher dose of carrier HIPDM, which resulted in lower pancreas to liver ratios. HIPDM is a new type of compound which shows predilection for pancreas. Our results suggest that [123I]HIPDM might be a useful agent for pancreas imaging.

Immunoreactivity and biodistribution of indium-111-labeled monoclonal antibody to a human high molecular weight-melanoma associated antigen.

The anti-human, high molecular weight-melanoma associated antigen (HMW-MAA) MoAb 225.28S was chelated with 111In and then tested for its in vitro reactivity with cultured human melanoma cells and for its biodistribution in human melanoma bearing nude mice. In vitro studies showed that the radiolabeled antibody reacted specifically with cultured melanoma cells. However, binding of DTPA to the monoclonal antibody reduced its titer with cultured melanoma cells from 1:1024 to 1:512. Further labeling of the DTPA-antibody conjugate with 111In caused an additional reduction of its titer to 1:128. Injection of the radiolabeled monoclonal antibody into nude mice resulted in the accumulation of significantly (p less than 0.001) higher radioactivity in melanoma tissue than in nude mice injected with either [111In] chloride or 111In-labeled antibody to human acid phosphatase. The specificity of the distribution of the radiolabeled antibody in nude mice also was indicated by its poor localization in lesions other than melanoma (e.g., human prostate carcinoma and chronic abscess). The localization of antibody in liver and kidney was also high, although lower than that achieved in tumor. These results indicate that 111In-labeled monoclonal antibodies to human tumor associated antigens may be useful for localizing malignant lesions. However, there is a need to improve labeling and/or purification of antibody in order to decrease renal and hepatic activity.

Indium-111-labeled LDL: a potential agent for imaging atherosclerotic disease and lipoprotein biodistribution.

Radiolabeling of low-density lipoprotein (LDL) and external imaging with a gamma camera would offer a means of taking advantage of the metabolic activity of developing atherosclerotic lesions in order to noninvasively detect and determine the extent of atherosclerotic cardiovascular disease. Indium-111-(111In) labeled LDL was prepared and its purity demonstrated by agarose electrophoresis and ultracentrifugation. In vitro studies with cultured human fibroblasts demonstrated significant inhibition of iodine-125-(125I) LDL binding to LDL receptors by 111In-LDL, although this was less than the inhibition produced by unlabeled LDL. Adrenal gland uptake of 111In-LDL by hypercholesterolemic rabbits was reduced by 86% compared to the level of uptake observed in normal rabbits. These results were compatible with downregulation of adrenal LDL receptors in the hypercholesterolemic rabbits. Uptake of 111In-LDL in the atherosclerotic proximal aorta of hypercholesterolemic rabbits was 2.5 times higher than in normal rabbits. These results suggest that 111In-LDL has the potential to be a useful agent for external imaging of atherosclerotic lesions and lipoprotein biodistribution.

Marrow-sparing effects of 117mSn(4+)diethylenetriaminepentaacetic acid for radionuclide therapy of bone cancer.

UNLABELLED: Several bone-seeking radionuclides (32P, 89Sr, 186Re, and 153Sm) have been used to treat bone pain. The limiting factor in this modality is marrow toxicity. Our hypothesis is that marrow toxicity can be reduced while maintaining therapeutic efficacy using radionuclides that emit short-range beta particles or conversion electrons (CEs). A recent study on 47 patients using the short-range CE emitter 117mSn(4+)diethylenetriaminepentaacetic acid (117mSn(4+)DTPA) supports this hypothesis. The hypothesis is now tested using 117mSn(4+)DTPA in a mouse femur model. METHODS: The survival of granulocyte-macrophage colony-forming cells (GM-CFCs) in femoral marrow is used as a biologic dosimeter for bone marrow. The dosimeter is calibrated by irradiating mice with exponentially decreasing dose rates of 137Cs gamma-rays with a dose-rate decrease half-time, Td, equal to the effective clearance half-time of 117mSn(4+)DTPA from the femur (222 h). When Td = 222 h, the mean absorbed dose required to achieve a survival fraction of 37% is 151 cGy. After calibration, 117mSn(4+)DTPA is administered and GM-CFC survival is determined as a function of injected activity. These data are used to experimentally determine the mean absorbed dose to the femoral marrow per unit injected activity. The kinetics of radioactivity in the marrow, muscle, and femoral bone are also determined. Finally, a theoretic dosimetry model of the mouse femur is used, and the absorbed doses to the femoral marrow and bone are calculated. RESULTS: The experimental mean absorbed dose to the femoral marrow per unit injected activity of 117mSn(4+)DTPA is 0.043 cGy/kBq. The theoretic mean absorbed dose to the femoral bone per unit injected activity is 1.07 cGy/kBq. If these data are compared with those obtained previously for 32P-orthophosphate, the radiochemical 117mSn(4+)DTPA yields up to an 8-fold therapeutic advantage over the energetic beta emitter 32P. CONCLUSION: The CE emitter 117mSn offers a large dosimetric advantage over energetic beta-particle emitters for alleviating bone pain, and possibly for other therapeutic applications, while minimizing marrow toxicity.

Comparative dual-tracer studies of carbon-14 tryptophan and iodine-131 HIPDM in animal models of pancreatic diseases.

Our previous studies have shown that a significant amount of the diamine derivative 131I-N,N,N'-trimethyl-N'-(2-hydroxy-3-methyl-5-iodobenzyl)-1,3- propanediamine (HIPDM) is taken up and retained by the normal pancreas. Therefore, we studied the uptake of [131I]HIPDM in various pathophysiological models in mice (chronic alcoholism, diabetes with beta-cell atrophy and obesity with beta-cell hypertrophy) and compared to 14C-L-Tryptophan (TRY) distribution in order to determine the factors influencing their pancreatic uptake. In normal animals, the pancreas uptake of TRY was generally higher than HIPDM. In diabetes, the relative concentration of both compounds was higher over the controls; however, in obesity, TRY showed lower accumulation than in controls while HIPDM showed no significant difference. Chronic ethanol (20%) ingestion increased TRY uptake in the pancreas compared to controls (36.88 +/- 3.21 vs. 30.03 +/- 4.17% ID/g; p less than 0.01) after 5 wk study period, but it decreased by 10 wk (22.36 +/- 0.95% ID/g; p less than 0.005). There were no significant changes in [131I]HIPDM distribution in alcoholics as compared to the controls. Radioiodinated HIPDM has potential advantages over [11C]TRY for pancreatic imaging since conventional imaging techniques can be employed. Our data, however, suggest that 11C-L-TRY is a more sensitive indicator of various pancreatic disorders.

Tin-117m(4+)-DTPA for palliation of pain from osseous metastases: a pilot study.

UNLABELLED: The physical and biological attributes of 117mSn(4+)-DTPA indicate that it should be an effective agent for palliative therapy of painful bony metastatic disease. The aim of this study was to evaluate whether or not this agent could effectively reduce pain while sparing the hemopoietic marrow from adverse effects. METHODS: Fifteen patients (10 males and 5 females) with painful bony metastases from various primary cancers were included in the study. Seven patients received 1.22 to 3.11 MBq/kg of 117mSn intravenously (Group 1) and eight patients received 4.85 to 5.77 MBq/kg (Group 2). All but one were treated as outpatients and followed for a minimum of 2 mo. RESULTS: In the first group, pain relief was non-assessable in four patients because of death or additional treatment of soft-tissue disease by another modality. One patient had no relief of pain, one had complete relief of pain and one had transient relief of pain. No myelotoxicity was observed. For Group 2, three patients achieved complete relief of pain, two good relief, two partial relief and one began to experience pain relief when he suffered a pathological fracture 2 mo post-treatment. None of these patients had myelotoxicity. CONCLUSION: Tin-117m(4+)-DTPA can reduce pain from metastatic disease to bone without inducing adverse reactions related to bone marrow. Further studies are needed to assess tolerance levels for the bone marrow and to evaluate response rates and duration of effect.

Ruthenium-97 hepatobiliary agents for delayed studies of the biliary tract. I. ru-97 PIPIDA: concise communication.

Failure of early diagnosis of biliary atresia results in the development of cirrhosis and death. Commonly used hepatobiliary agents are not ideal for follow-up studies because of their unfavorable physical properties or short half-life. The excellent physical properties of Ru-97 should overcome these limitations. Therefore, Ru-97 PIPIDA (N,alpha-(p-isopropyl acetanilide) iminoacetic acid) is being investigated as a potential hepatobiliary agent that would allow an improved diagnosis of the disease. Ruthenium-97 PIPIDA and Tc-99m PIPIDA showed similar blood clearance rates in dogs. Ru-97 PIPIDA scintigrams in dogs showed early uptake in liver and gallbladder and slow excretion through the gastrointestinal tract. Biodistribution studies were performed in normal rats and rats with biliary obstruction. The findings suggest that Ru-97 PIPIDA should be useful for delayed studies (1-3 days) of the biliary tract.

Potential of palladium-109-labeled antimelanoma monoclonal antibody for tumor therapy.

Palladium-109, a beta-emitting radionuclide, was chelated to the monoclonal antibody 225.28S to the high molecular weight antigen associated with human melanoma. The radiolabeled antibody maintained its specific in vitro reactivity with cultured human melanoma cells. Injection of the radiolabeled monoclonal antibody into nude mice bearing human melanoma resulted in significant accumulation of the radiolabel in the tumors: 19% injected dose/g; 38:1 and 61:1 tumor-to-blood ratios at 24 and 48 hr, respectively. The localization of the radiolabeled antibody in liver and kidney also was high, but appreciably lower than that achieved in tumor. These results suggest that Pd-109-labeled monoclonal antibody to tumor-associated antigens may have potential applications in tumor immunotherapy.

Dual tracer autoradiographic study of beta-methyl-(1-14C) heptadecanoic acid and 15-p-(131I)-iodophenyl-beta-methylpentadecanoic acid in normotensive and hypertensive rats.

The myocardial distribution of 15-p-[131I]iodophenyl-3-(R,S)-methylpentadecanoic acid (BMPDA) and 1[14C]-3-(R,S)-methylheptadecanoic acid (BMHDA) was compared in normotensive and hypertensive rats using quantitative dual tracer autoradiographic techniques. The myocardial distribution of carbon-14 [14C] BMHDA and iodine-131 [131I] BMPDA was nearly homogeneous in the normotensive rats, while both tracers showed similar, though very heterogeneous, distribution in hypertensive hearts with decreased uptake in the endocardial region. Our data demonstrate that myocardial distribution of [131I]BMPDA was essentially the same as [14C]BMHDA, and thus single photon emission computed tomographic imaging with 123I-labeled BMPDA could be useful for the detection of regional changes of myocardial fatty acid uptake in patients with prolonged and severe hypertension.

Tin-117m(4+)DTPA: pharmacokinetics and imaging characteristics in patients with metastatic bone pain.

UNLABELLED: Biokinetics and imaging characteristics of 117mSn(4+)DTPA have been studied in patients with metastatic bone pain. METHODS: Seventeen patients with bone pain due to metastasis were given three dose levels: 180 microCi/kg (6.66 MBq/kg), 229 microCi/kg (8.47 MBq/kg) and 285 microCi/kg (10.55 MBq/kg) body weight. Periodic blood and daily urine samples were collected for 14 days to measure percent injected activity retained in blood and that excreted in urine. Simultaneous anterior and posterior view whole-body images were obtained under identical scan settings at 1, 3.5 and 24 hr and on Days 3 and 7 and between 4-6 and 8-10 wk postinjection. The total body retention was calculated using the geometric mean counts. RESULTS: After intravenous injection, the total body clearance of 117mSn(4+)DTPA shows two components: a soft-tissue component and a bone component. The soft-tissue component accounts for 22.4% of the dose and consists of four subcomponents with an average biologic clearance half-time of 1.45 days (range 0.1-3.2 days). The bone component accounting for the remaining 77.6% of the dose shows no biologic clearance. A mean 22.4% of the dose is excreted in urine in 14 days; 11.4% within 24 hr. The uptake pattern appears similar to that of 99mTc-MDP. Peak uptake is observed in normal bone by 24 hr and metastatic lesions by 3-7 days. Pain palliation was observed with all three doses levels. CONCLUSION: Among the four potential bone pain palliation radionuclides, 117mSn(4+)DTPA demonstrates the highest bone uptake and retention. Some biokinetic and radionuclidic features of 117mSn(4+)DTPA are similar to other agents, but many features are different and unique and may make it an ideal bone pain palliation agent. Double-blind comparative studies are needed to determine its exact role in bone pain palliation.

67Cu-2IT-BAT-Lym-1 pharmacokinetics, radiation dosimetry, toxicity and tumor regression in patients with lymphoma.

UNLABELLED: Lym-1, a monoclonal antibody that preferentially targets malignant lymphocytes, has induced therapeutic responses and prolonged survival in patients with non-Hodgkin's lymphoma when labeled with 1311. Radiometal-labeled antibodies provide higher tumor radiation doses than corresponding 1311 antibodies. 67Cu has an exceptional combination of properties desirable for radioimmunotherapy, including gamma and beta emissions for imaging and therapy, respectively, a biocompatible half-time and absence of pathways contributing to myelotoxicity. The radioimmunoconjugate, 67Cu-21T-BAT-Lym-1, has been shown to be efficacious in nude mice bearing human Burkitt's lymphoma (Raji) xenografts. Based on these results, a clinical study of the pharmacokinetics and dosimetry of 67Cu-21T-BAT-Lym-1 in patients with lymphoma was initiated. METHODS: Eleven patients with advanced stage 3 or 4 lymphoma were given a preload dose of unmodified Lym-1, then an imaging dose of 126-533 MBq (3.4-14.4 mCi) 67Cu-21T-BAT-Lym-1. Total Lym-1 ranged from 25 to 70 mg dependent on the specific activity of the radioimmunoconjugate and was infused at a rate of 0.5-1 mg/min. Imaging, physical examination, including caliper measurement of superficial tumors, and analysis of blood, urine and fecal samples were performed for a period of 6-13 d after infusion to assess pharmacokinetics, radiation dosimetry, toxicity and tumor regression. RESULTS: In 7 patients, in whom superficial tumors had been accurately measured, tumors regressed from 18% to 75% (mean 48%) within several days of 67Cu-21T-BAT-Lym-1 infusion. The uptake and biological half-time of 67Cu-21T-BAT-Lym-1 in tumors were greater than those of normal tissues, except the mean liver half-time exceeded the mean tumor half-time. The mean tumor-to-marrow radiation ratio was 32:1, tumor-to-total body was 24:1 and tumor-to-liver was 1.5:1. Images were of very good quality; tumors and normal organs were readily identified. Mild and transient Lym-1 toxicity occurred in 6 patients; 1 patient developed a human antimouse antibody. There were no significant changes in blood counts or serum chemistries indicative of radiation toxicity. CONCLUSION: Because of the long residence time of 67Cu-21T-BAT-Lym-1 in tumors, high therapeutic ratios were achieved and, remarkably, numerous tumor regressions were observed after imaging doses. The results indicate considerable therapeutic potential for 67Cu-21T-BAT-Lym-1.

Ruthenium-97 DTPA: a new radiopharmaceutical for cisternography.

Ruthenium-97 DTPA (diethylenetriamine penta-acetic acid) was evaluated for its possible use as a cerebrospinal fluid imaging agent. Ru-97 has favorable physical properties that are highly suitable for imaging: decay by electron capture; gamma energy = 216 keV, 85%; T 1/2 = 2.9 days. Dogs were injected with 0.4 mCi Ru-97 DTPA or In-111 DTPA into the cisterna magna. The movement of the agents was monitored with a camera interfaced to a computer, or with a dual-probe system placed over the head and urinary bladder. In addition, blood and urine samples were collected at fixed intervals for 6 hr. High-quality images were obtained up to 48 hr after injection. The results show that the kinetics and excretion of Ru-97 DTPA are similar to those of In-111 DTPA. Radiation dose for identical activities is twice as high for In-111, in part because of greater abundance of the low-energy electron emission of In-111.

Is there life after technetium: what is the potential for developing new broad-based radionuclides?

The use of radionuclides for medical and for a multitude of other basic research applications has continued to grow at a very rapid pace. Procedures, based on their use as radiotracers for nuclear medicine imaging and for radiotherapy of cancer and other pathology, have become firmly established as important clinical modalities. It is estimated that on an annual basis in the United States alone, radionuclides are used medically in over 13 million imaging procedures, in over 100 million laboratory tests, and in an ever increasing number (> 100,000) for therapeutic administrations. One out of every four hospital patients undergoes a procedure that involves the use of radionuclides. Diagnostic imaging methods using planar/single-photon emission computed tomography and positron-emission tomography (PET) imaging, as well as the measurement of in vivo organ function, physiology, or biochemistry, have become indispensable tools in patient workup and management. More than 80% of all imaging studies (mostly anatomic) currently use technetium-99m (99mTc), because it has turned out to be the ideal isotope from various considerations. However, over the past few years, nuclear medicine has experienced a slow but steady evolution towards functional studies, quantitative PET imaging, and novel therapeutic approaches. New radionuclides are required for these applications, and their development has attracted considerable interest. This article reviews the current status and future prospects for the development of many new potential isotopes. Practical issues, such as the feasibility of large-scale production and wide-spread availability in a continuous reliable fashion, are addressed. To date, the data are not sufficient to answer the question as to whether any of these radionuclides (or their applications, for that matter) will eventually assume as broad-based a role as that of 99mTc. Nonetheless, there are a number of promising radionuclides that could assume an important place in the future practice of nuclear medicine.