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Molecular Imaging and Contrast Agent Database (MICAD) [Internet].

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[99mTc](CO)3N-(pyridin-2-yl-methyl)-N[2-(4-sulfamoylphenyl)-ethyl]aminoethyl acetate

, PhD
National Center for Biotechnology Information, NLM, NIH, Bethesda, MD 20894

Created: ; Last Update: October 28, 2010.

Chemical name:[99mTc](CO)3N-(pyridin-2-yl-methyl)-N[2-(4-sulfamoylphenyl)-ethyl]aminoethyl acetateimage 99309642 in the ncbi pubchem database
Abbreviated name:[99mTc]5
Agent Category:Compound
Target:Carbonic anhydrase IX
Target Category:Enzyme
Method of detection:Single-photon emission computed tomography (SPECT); gamma planar imaging
Source of signal / contrast:99mTc
  • Checkbox In vitro
  • Checkbox Rodents
Click on the above structure of [99mTc]5 for additional information in PubChem.



Hypoxic tumors are often resistant to radio- and chemotherapy, have a high metastatic potential, and predict a poor outcome for the cancer patient (1). Although several techniques (invasive and noninvasive) are available for the detection of hypoxia in tumors, these methods are not completely reliable because they yield variable diagnoses, have functional limitations, show incomplete penetration of tumors, or do not detect hypoxia in all tumor types (2, 3). A common feature of most solid cancerous tumor types is the presence of hypoxic conditions (2) and the overexpression of carbonic anhydrase IX (CA IX; exists in several isomeric forms), a transmembrane cell-surface metallo-enzyme that has been shown to acidify the pH of tumor cells to ~6.5 compared to pH ~7.4 for normal cells (4, 5). The induction of CA IX through the hypoxia-inducible factor-I transcription factor pathway and its role in tumorigenesis has been described in detail by Guler et al. (5). In view of its role in the development and progression of neoplastic tumors, oncologists and researchers are pursuing CA IX inhibitors for the detection or therapy of different cancer types. As a result, several small molecule inhibitors of CA are available, and among these compounds some sulfonamide derivatives have shown high selectivity for CA IX (6). It has also been shown that not only overexpression but also activation of the enzyme facilitates the accumulation of sulfonamides in a hypoxic tumor (7). Therefore, noninvasive imaging of CA IX would be an excellent technique to detect tumor hypoxia to develop a suitable treatment regimen for cancer patients. To achieve this, [99mTc](CO)3N-(pyridin-2-yl-methyl)-N[2-(4-sulfamoylphenyl)-ethyl]aminoethyl acetate ([99mTc]5) was synthesized and evaluated for the detection of CA IX (7). Biodistribution of the tracer was studied in mice bearing HT-29 cell xenograft tumors because HT-29 cells express CA IX.

Other Sources of Information

Hypoxia-related chapters in MICAD

Clinical trials on CA IX inhibitors

Human CA IX in Entrez Gene (Gene ID 768)

Protein and mRNA sequence of human CA IX

Human CA IX in Online Mendelian Inheritance in Man (OMIM) database

Hypoxia response in National Cancer Institute-Nature Pathways Interaction Database

Crystal structure of the catalytic domain of the human CA IX



The synthesis of [99mTc]5 has been described by Akurathi et al. (7). The radiochemical yield, purity, and specific activity of the tracer were not reported.

In Vitro Studies: Testing in Cells and Tissues


To study the affinity of [99mTc]5 for the I, II, IX, and XII isozymes of CA under in vitro conditions, a rhenium analog of the compound (Re-4) was used (7). Re-4 was reported to have Ki values (nM) of 3,440, 50, 58, and 45.1 for human CA isozymes I, II, IX, and XII, respectively. The selectivity of Re-4 (ratio of the Ki of CA IX to the Ki of XII) was determined to be 0.86.

Incubation of HT-29 cells in the presence of Re-4 under hypoxic conditions was reported to acidify the extracellular environment of the cells (from pH 7.4 to pH 6.9) (7). In addition, the hypoxic cells were shown to express approximately four-fold higher amounts of CA IX compared to the normal cells as determined with Western blot analysis. Using the same technique the investigators showed that a similar overexpression of the enzyme was observed in tumor cells analyzed under ex vivo conditions.

The distribution coefficient of [99mTc]5 was determined with the shake flask and back extraction methods using a 1-octanol/phosphate buffer mixture (pH 7.4) as described by Akurathi et al. (7). The log Doctanol/buffer values for the tracer were reported to be 0.05 ± 0.00 and 0.06 ± 0.02, respectively, indicating that the radiochemical was moderately lipophilic.

Animal Studies



Akurathi et al. studied the biodistribution of [99mTc]5 in anesthetized (2% isoflurane and 98% O2) NMRI-nu (nu/nu) mice bearing HT-29 cell xenograft tumors (7). The animals (n = 3 per time point) were euthanized at 0.5, 1, 2, and 4 h post-injection (p.i.), and the major organs were collected to determine the amount of accumulated radioactivity. The results were expressed as percent injected dose per gram tissue (% ID/g). The kidneys had the highest accumulation of the tracer with 36.0 ± 2.4% ID/g at 1 h p.i., which decreased to 17.8 ± 2.5% ID/g at 4 h p.i., followed by the liver (16.7 ± 2.8% ID/g and 3.1 ± 0.8% ID/g at 1 h and 4 h p.i., respectively) and the intestines (6.0 ± 0.3% ID/g and 17.6 ± 0.2% ID/g at 1 h and 4 h p.i., respectively). The tumors showed a negligible amount of radioactivity (0.1 ± 0.0% ID/g at 1 h p.i.), which was washed out by 4 h p.i. However, the tumor/blood ratio was reported to increase from 0.7 ± 0.2 at 1 h p.i. to 1.0 ± 0.1 at 4 h p.i.

Blocking studies were performed in mice (n = 3 animals per group) pretreated with unlabeled 4-(2-aminoethyl)benzenesulfonamide (a precursor of [99mTc]5; 10 mg/kg body weight administered intraperitoneally) and injected with the radiochemical 1 h later (7). Control animals were not treated with the unlabeled precursor. The animals were euthanized 1 h after the [99mTc]5 injection, and the highest radioactivity was detected in the liver (35.68 ± 1.91% ID/g compared to 43.10 ± 2.86% ID/g for the control mice), followed by the intestines (35.54 ± 4.0% ID/g compared to 36.52 ± 3.40% ID/g for the control mice) and kidneys (17.67 ± 1.86% ID/g compared to 17.35 ± 3.73% ID/g for the control mice). The amount of radioactivity accumulated in the tumors was not reported.

Possible reasons for almost no accumulation of [99mTc]5 in the tumors have been described elsewhere (7). On the basis of the results obtained from the different studies, the investigators concluded that [99mTc]5 was not a suitable tracer for the detection of tumors expressing CA IX (7).

Other Non-Primate Mammals


No publications are currently available.

Non-Human Primates


No publications are currently available.

Human Studies


No publications are currently available.

Supplemental Information


No information is currently available.


Michalski, M.H. and X. Chen, Molecular imaging in cancer treatment. Eur J Nucl Med Mol Imaging, 2010. [PMC free article: PMC3022114] [PubMed: 20661557]
Carlin S., Khan N., Ku T., Longo V.A., Larson S.M., Smith-Jones P.M. Molecular targeting of carbonic anhydrase IX in mice with hypoxic HT29 colorectal tumor xenografts. PLoS One. 2010;5(5):e10857. [PMC free article: PMC2877709] [PubMed: 20523727]
Mees G., Dierckx R., Vangestel C., Van de Wiele C. Molecular imaging of hypoxia with radiolabelled agents. Eur J Nucl Med Mol Imaging. 2009;36(10):1674–86. [PMC free article: PMC2758191] [PubMed: 19565239]
De Simone G., Supuran C.T. Carbonic anhydrase IX: Biochemical and crystallographic characterization of a novel antitumor target. Biochim Biophys Acta. 2010;1804(2):404–9. [PubMed: 19679200]
Guler O.O., De Simone G., Supuran C.T. Drug design studies of the novel antitumor targets carbonic anhydrase IX and XII. Curr Med Chem. 2010;17(15):1516–26. [PubMed: 20166929]
Winum J.Y., Scozzafava A., Montero J.L., Supuran C.T. Inhibition of carbonic anhydrase IX: a new strategy against cancer. Anticancer Agents Med Chem. 2009;9(6):693–702. [PubMed: 19601749]
Akurathi V., Dubois L., Lieuwes N.G., Chitneni S.K., Cleynhens B.J., Vullo D., Supuran C.T., Verbruggen A.M., Lambin P., Bormans G.M. Synthesis and biological evaluation of a 99mTc-labelled sulfonamide conjugate for in vivo visualization of carbonic anhydrase IX expression in tumor hypoxia. Nucl Med Biol. 2010;37(5):557–64. [PubMed: 20610160]


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