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Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.

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

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68Ga-Glutamate peptide-3 aminoethyl estradiol

68Ga-GAP-EDL
, PhD
National Center for Biotechnology Information, NLM, NIH, Bethesda, MD 20894

Created: ; Last Update: June 18, 2009.

Chemical name:68Ga-Glutamate peptide-3 aminoethyl estradiolImage GAPEDL.jpg
Abbreviated name:68Ga-GAP-EDL
Synonym:
Backbone:Estradiol
Target:Estrogen receptor
Mechanism:Receptor-ligand binding
Method of detection:Positron emission tomography (PET)
Source of signal:68Ga
Activation:Not required
Studies:
  • Checkbox In vitro
  • Checkbox Rodents
Structure of GAP-EDL. The exact location of 68Ga in the structure is unknown.

Background

[PubMed]

Approximately 10% of women of reproductive age are affected by endometriosis, a gynecological disorder (1). Painful periods, pelvic pain, and infertility are often associated with endometriosis. Although a variety of mechanisms, including the roles of different cytokines, growth factors, and hormones, have been proposed in the development of endometriosis, the exact etiology and epidemiology of this condition is unclear (1-3). Menstruation in the opposite direction with spillage of viable endometrial cells into the peritoneum is often observed in cycling women (4). Adhesion, proliferation, and spread of the shed endometrial cells to the peritoneum are believed to support the development of endometriosis (5-7). Endometriosis is usually detected by a surgical technique such as a laparoscopy. Performing such a procedure on patients with infertility is often complex, and it provides information only about the morphological and anatomical changes in the tissue (8). Moreover, endometriosis usually progresses or recurs after surgery, and any relief to the patient is temporary (9).

Because the prevalence and progression of endometriosis cannot be studied in humans without a laparoscopy, non-primate and primate animal models, including rabbits, have been used to investigate this disease (10-12). It has also been shown that estrogen receptors (ER) are overexpressed in the uterine endometrium and endometriotic lesions in endometriosis (4). In an effort to develop a non-invasive method to detect endometriosis, Takahashi et al. explored the use of estrogen, an ER ligand, to image endometriosis induced in rabbits (8). For this, estradiol (EDL) was conjugated to a glutamate peptide (GAP), which contained 5–10 glutamic acid residues, to facilitate the chelation of radioactive gallium (68Ga) to obtain 68Ga-GAP-EDL to image endometriosis.

Synthesis

[PubMed]

The synthesis of 68Ga-GAP-EDL was performed as described by Takahashi et al. (8, 13). To start, estrone was dissolved in anhydrous ethanol. Sodium ethoxide and bromoacetonitrile were added to the solution and the mixture was heated under reflux for 3 h. The ethanol was allowed to evaporate completely and the residue was dissolved in ethyl acetate. The solution was washed with water and the organic layer was filtered after drying over magnesium sulphate. The ethyl actetate was evaporated under reduced pressure and the remaining solid product, 3-acetonitrile estradiol, was washed with ether. The yield of this reaction was 75%. The solid product was dissolved in tetrahydrofuran, and lithium aluminum hydride was added to the mixture and stirred for 4 h. The solvent was evaporated to yield a solid that was dissolved in ethyl acetate. This solution was washed with water, dried over magnesium sulphate, and filtered. The solvent was evaporated to obtain 3-cyanomethyl estrone (EDL) with a yield of 92%. The structure of EDL was confirmed by nuclear magnetic resonance.

For the synthesis of GAP-EDL, the sodium salt of GAP was converted to the acid form by the addition of 2 N hydrochloric acid (HCl). This solution was dialyzed to remove all the acid and freeze-dried to obtain the acidic form of GAP. The GAP acid was dissolved in dimethylformamide (DMF), and dicyclohexyl carbodiimide and 4-N,N-dimethyl aminopyridine were added to it. This mixture was stirred at room temperature for 2 days. The DMF was evaporated under reduced pressure; 1 N sodium bicarbonate was added to it and extracted with chloroform. The aqueous solution was dialyzed, filtered, and freeze-dried to obtain GAP-EDL. The percent yield of this reaction was not provided in the publication. The GAP-EDL complex contained 7% (weight/weight) EDL as determined by ultraviolet spectroscopy.

For the synthesis of 68Ga-GAP-EDL, 68Ga was eluted from a 68Ge/68Ga generator with 0.1 N HCl, and the acidic solution was passed through an anion resin cartridge to trap the 68Ga. The cartridge was washed with 4 N HCl and air-dried. The 68Ga was eluted as 68Ga-chloride (68GaCl3) from the cartridge with water, and the pH was adjusted to 4–5 with sodium hydroxide and sodium acetate. GAP-EDL in acetate buffer was added to the 68GaCl3 solution, and the reaction was completed by warming at 37°C for 20 min. Radiochemical purity of the product was determined by instant thin-layer chromatography. The Rf values for GAP-EDL and 68GaCl3 were reported to be 0.1 and 0.9, respectively. The radiochemical purity of the radiotracer was >95%. Specific activity of the radiotracer was not provided in the publication (8).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

The uptake of 68Ga-GAP-EDL was studied in the 13762 rat adenocarcinoma cell line (8), and accumulation of the radiopharmaceutical in these cells was shown to increase with time. Addition of cold estrone (estradiol) inhibited the uptake in a dose-dependent manner. This indicated that the uptake of 68Ga-GAP-EDL in the cells was an ER-mediated process.

Animal Studies

Rodents

[PubMed]

No publications are currently available.

Other Non-Primate Mammals

[PubMed]

The uptake of 68Ga-GAP-EDL was investigated in rabbits with induced endometriosis (8). Endometrial tissues were implanted in the rabbits, and the implanted endometrial tissue was microscopically visible in the animals after 8 weeks. The uptake of 68Ga-GAP-EDL by the rabbits was investigated with and without the treatment of tamoxifen, a competitive inhibitor of estrogen for binding to the ER. A cross-sectional review of the coronal and sagittal images taken by positron emission tomography showed that the endometrial implant tissue could be differentiated from the skeletal, hepatobiliary, and urinary uptake of the label. Prior treatment of the animals with tamoxifen resulted in the inhibition of label uptake by the endometrial tissue. This demonstrated that the uptake of 68Ga-GAP-EDL in the endometrial implants was ER-mediated. From this observation, the investigators suggested that 68Ga-GAP-EDL can be used as a non-invasive imaging agent for the detection and monitoring of endometriosis and other diseases associated with the overexpression of ER, such as breast cancer .

Non-Human Primates

[PubMed]

No publications are currently available.

Human Studies

[PubMed]

No publications are currently available.

References

1.
Lebovic D.I., Mueller M.D., Taylor R.N. Immunobiology of endometriosis. Fertil Steril. 2001;75(1):1–10. [PubMed: 11163805]
2.
Cramer, D.W. and S.A. MissmerThe epidemiology of endometriosis. Ann N Y Acad Sci, 2002955: p. 11-22; discussion 34-6, 396-406. [PubMed: 11949940]
3.
Hever A., Roth R.B., Hevezi P., Marin M.E., Acosta J.A., Acosta H., Rojas J., Herrera R., Grigoriadis D., White E., Conlon P.J., Maki R.A., Zlotnik A. Human endometriosis is associated with plasma cells and overexpression of B lymphocyte stimulator. Proc Natl Acad Sci U S A. 2007;104(30):12451–6. [PMC free article: PMC1941489] [PubMed: 17640886]
4.
Leyendecker G., Herbertz M., Kunz G., Mall G. Endometriosis results from the dislocation of basal endometrium. Hum Reprod. 2002;17(10):2725–36. [PubMed: 12351554]
5.
Halme J., Hammond M.G., Hulka J.F., Raj S.G., Talbert L.M. Retrograde menstruation in healthy women and in patients with endometriosis. Obstet Gynecol. 1984;64(2):151–4. [PubMed: 6234483]
6.
Kruitwagen R.F., Poels L.G., Willemsen W.N., de Ronde I.J., Jap P.H., Rolland R. Endometrial epithelial cells in peritoneal fluid during the early follicular phase. Fertil Steril. 1991;55(2):297–303. [PubMed: 1991528]
7.
Evers J.L. The defense against endometriosis. Fertil Steril. 1996;66(3):351–3. [PubMed: 8751728]
8.
Takahashi N., Yang D.J., Kurihara H., Borne A., Kohanim S., Oh C.S., Mawlawi O., Kim E.E. Functional Imaging of Estrogen Receptors with Radiolabeled-GAP-EDL in Rabbit Endometriosis Model. Acad Radiol. 2007;14(9):1050–7. [PubMed: 17707312]
9.
DeCherney A.H. Endometriosis: recurrence and retreatment. Clin Ther. 1992;14(6):766–72. [PubMed: 1286484]
10.
D'Hooghe T.M., Bambra C.S., Raeymaekers B.M., De Jonge I., Lauweryns J.M., Koninckx P.R. Intrapelvic injection of menstrual endometrium causes endometriosis in baboons (Papio cynocephalus and Papio anubis). Am J Obstet Gynecol. 1995;173(1):125–34. [PubMed: 7631669]
11.
Schenken R.S., Asch R.H. Surgical induction of endometriosis in the rabbit: effects on fertility and concentrations of peritoneal fluid prostaglandins. Fertil Steril. 1980;34(6):581–7. [PubMed: 7450077]
12.
Steinleitner A., Lambert H., Suarez M., Serpa N., Roy S. Immunomodulation in the treatment of endometriosis-associated subfertility: use of pentoxifylline to reverse the inhibition of fertilization by surgically induced endometriosis in a rodent model. Fertil Steril. 1991;56(5):975–9. [PubMed: 1936333]
13.
Takahashi N., Yang D.J., Kohanim S., Oh C.S., Yu D.F., Azhdarinia A., Kurihara H., Zhang X., Chang J.Y., Kim E.E. Targeted functional imaging of estrogen receptors with 99mTc-GAP-EDL. Eur J Nucl Med Mol Imaging. 2007;34(3):354–62. [PubMed: 17021817]

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