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Purification and Analysis of Folate Dye ConjugatesAnisha Hari GanjiGovernors State University

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Purification and Analysis of Folate Dye Conjugates

A Project

Submitted

To

Governors State University

By

Anisha Hari Ganji

In Partial Fulfillment of the

Requirements for the Degree

Of

Masters in Science

May, 2011

Governors State University

University Park, Illinois

08 Fall

2

Dedicated to

Mom and Dad

3

ACKNOWLEDGEMENTS

Foremost, I would like to thank my project advisor, Dr Walter Henne for being extremely helpful

and encouraging. He has offered me great support and guided me patiently throughout the

project. It was fun working under him.

I also wish to thank my project committee members, Dr Patty Fu and Prof Kent for their

assistance throughout my project work period. I am grateful to our program secretary, Nancy for

helping me in all the ways possible.

I would like to acknowledge Dr Shailendra Kumar for his advising me and helping me in

completing my Master’s program. Finally, I am thankful to Governors State University for

providing financial and laboratory support for completing my project.

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Table of Contents

Abstract: ........................................................................................................................................ 5

Introduction ................................................................................................................................... 5 Folate: ......................................................................................................................................... 5 Mechanism of folic acid intake and role of folate receptors: ................................................. 7 Stability of folate:....................................................................................................................... 8 Use of folate as a targeting vehicle: .......................................................................................... 8

Discovery of a method that exploits folate receptor endocytosis:............................................. 9

99mTc-EC20 - A folate-derivatized radiopharmaceutical: (A SPECT tracer) ....................... 10

Folate conjugated shell cross-linked nanoparticles (SCK’s): ................................................. 11

111In-DTPA folate: ....................................................................................................................... 12

Conclusion: .................................................................................................................................. 13

References:................................................................................................................................... 14 Figure-1: Structure of folic acid ............................................................................................. 17 Figure 2:.................................................................................................................................... 18 Figure 3:.................................................................................................................................... 18 Figure 4:.................................................................................................................................... 19 Figure-5: ................................................................................................................................... 19 Figure 6 ..................................................................................................................................... 20 Figure 7 ..................................................................................................................................... 20 Figure-8: Uptake of folate conjugates .................................................................................... 21 Figure 9 ..................................................................................................................................... 21 Figure 10 ................................................................................................................................... 22 Figure 11 ................................................................................................................................... 22

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Abstract:

Folate receptors play an important role in drug delivery systems as they are known to

increase the potency and reduce toxicity of many cancer therapies. Folate receptors are present in

abundance on cancerous tissues more so than on non-cancerous tissues. Folate binds to a variety

of drugs that include chemotherapeutic agents, liposomes, radio-imaging agents,

immunotherapeutic drugs and the like. Folate has been researched and used over the past decade

as it is inexpensive and binds to the folate receptors with high affinity when compared to

monoclonal antibodies1. Nuclear imaging by SPECT and PET has interested many researchers.

Other processes include MRI and optical imaging. 99mTc-EC20 is one of the most promising

radiopharmaceutical used in the imaging of tumor cells. It is safe and when a clinical trial was

performed on 155 patients, 68% of them showed 99mTc-EC20 uptake by their tumors2. This has

also been used for Atherosclerosis which is used for imaging activated macrophages. 64Cu has

also been used in producing radiopharmaceuticals where is has been radiolabelled with TETA-

SCK and TETA-SCK-folate conjugates and administered intravenously to get accumulation in

solid tumors. The research showed that uptake of these conjugates was tumor size dependent and

the non-folate conjugates were seen more in the blood than the folate conjugates.

Introduction

Folate: Folate is a general term used to describe the derivatives of pteroic acid that possess vitamin

properties in the human body. Folate is reduced metabolite tetrahydrofolate (THF) which acts as

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a carrier of one-carbon units in various biochemical reactions. Folic acid has commonly used

names such as Folate, Vitamin B Complex, and Pteroyl L – glutamic acid. Others include Acide

folique, vit b9, tertrahydrofolate, dihydrofolate, folacin, acido folico, 5’–methyl tetra

hydrofolate, L-methylfolate3.

Folic acid is a water soluble vitamin B. Naturally available folic acid is found in leafy

vegetables like lettuce, spinach, broccoli, and in fruits like bananas, water melons and lemons,

beetroot, okra, mushrooms, yeasts, meat like beef liver and kidney. Synthetic folic acid is an

orange-yellow, tasteless, microcrystalline powder. It darkens when heated above 250˚C which is

followed by charring. Folic acid is comprised of a bicyclic pterin moiety attached by a methylene

bridge to p-amino benzoic acid which is attached through α- peptide bond to a single molecule of

L- glutamic acid.

Deficiency of folate leads to inadequate DNA replication and impaired cell division,

especially in the hemopoietic tissue of the bone marrow and epithelial cells of the GIT. Folic

acid is used for folic acid deficiency i.e. treating the low blood levels of folic acids. This is good

for anemic conditions, inability of the bowel to absorb nutrients, kidney dialysis, ulcerative

colitis, and liver diseases. Folic acid is very important for pregnant women as they have an

increased metabolic demand for folate. Folate intake helps in preventing miscarriage and neural

tube defects. Under federal law enacted in 1988, folic acid must be added in bread, cereal, bakery

items etc. Folate is considered to have low acute and chronic toxicity for humans. The dose

ranges from 250 to 1000 mcg. Unless directed by a health care professional, more than 400 mcg

is not to be taken by adults. Folic acid may cause side effects like itching, skin rash, difficulty in

breathing, redness etc3-10.

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Mechanism of folic acid intake and role of folate receptors: Folates are lipophobic in nature and play an important role in many biochemical processes in

the body like purine synthesis, conversion of serine to glycine, production of precursors for DNA

synthesis and its repair, and thymide synthesis. Folates traverse minimally through biological

membrane by simple diffusion Folate needed for these processes are acquired by two

mechanisms.

1) A transmembrane transporter called Reduced folate carrier (RFC). It is a protein with

12 membrane-spanning domains which transports reduced folates like N5 –

methytetrahydrofolate (MTF). RFC is present in tissues of the border-membrane of

the colon, duodenum, ileum and jejunum. It is also present in hepatocytes, choroid

plexus and retinal epithelium (RPE) of the eye. RFC is also found in serum and has

much lower affinity to folic acid (See Figure 8).

2) The other folate uptake system uses the GPI (Glycosylphatidylinositol) anchored

protein known as Folate receptor (FR) which transports the folates through fluid-

phase endocytosis. High affinity is needed for the receptor to bind to its ligands. A kd

of approx. 0.4nmol/L is needed for folic acid. Although there are three isoforms for

FR, only FR-α is physiologically relevant. These three isoforms are found in normal

differential epithelial cells like those found in the placenta, kidney and lungs11-15.

Vitamin folic acid is used as a molecular Trojan horse for selectively delivering the probes

that are attached to the cancer cells. The radioactive probe is linked to folic acid through

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glutamate moiety, whereas pteroate moiety is used for FR binding on the cancer cells as shown

in figure 6.

Stability of folate:

As folate is water soluble, it is often lost when blanching vegetables. This loss can be due to

both thermal degradation and leaching. Steam blanching and microwave blanching cause less

folate destruction because of the absence of leaching by water. Polyglutamyl folates, on the

other hand, are large electronegative molecules which have to be broken down to monoglutamate

forms in order to be absorbed into the cells. The proteases in saliva, gastric juices and pancreatic

secretions are not able to convert the Polyglutamyl folates to monoglutamate forms. However, it

can be readily converted by two folate conjugase which are found in jejunal tissue fractions,

brush-border exopeptidase and in intracellular endopeptidase. The small intestine fully absorbs

the monoglutamyl folate and absorption is greater in jejunum than in ileum. When the food is

ingested in the body the vitamin travels in the plasma as monoglutamyl folate in the

concentration of 10 to 35 nM. It is carried forward in plasma as free folate or loosely bound to

plasma proteins like albumin. Ten to 20% of monoglutamyl folate is taken by the liver from the

portal blood in the first pass, and the remaining is taken by the extrahepatic tissues. The liver

plays a major role in maintaining folate hemostasis where it stores up to 50 % of the total folate

present in the body16-19.

Use of folate as a targeting vehicle: The folate receptor which is a high folate binding protein is overexpressed in a wide variety

of human tumor cells as contrasted to normal cells. It is present in high concentrations in cancers

of the ovary, mammalian gland, colon, lung, throat, nose, prostrate and other organs. As folate is

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needed to maintain healthy chromosomes and to perform cell division, the normal cells take in

only the purest form of the molecule and reject the rest. The tumor cells on the other hand need

folate in abundant quantities so that cell division can be stimulated. Hence, the folate receptors

can gorge them in abundance20.

The carboxyl group on the folate molecule is linked through a covalent bond to other

molecules and the folate binds to the molecules more tightly than typical monoclonal antibodies.

Because of the chemical structure, folates are more stable and are simple to use. The main

advantage of using folate as a vehicle, is its size. It is almost 0.3% the weight of an antibody and

has a molecular weight of 441. Folates are highly tumor specific and also conveniently available.

Discovery of a method that exploits folate receptor endocytosis:

Folate-receptor-mediated drug targeting process was first discovered accidentally. Mark

Horn, a former graduate student, was researching receptor-mediated endocytosis in the plant

kingdom. For the internalization of the elicitors, he linked them to biotin and followed their

endocytosis with fluorescent streptavidin. Receptor-mediated endocytosis took place in plant

cells with biotinylated elicitors. Also when biotin was linked to animal proteins the same

observation was made. Other research tested whether vitamins too could be exploited to mediate

the delivery of macromolecules into animal cells. Biotin-mediated internalization was

demonstrated in some animal cells and further studies revealed that even riboflavin transported

proteins into a few types of animal cells. Christopher Leamon, a former graduate student,

observed that folic acid can deliver tethered proteins into cultured animal cells.

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In 1991, Philip S.Low and Christopher P.Leamon from Purdue University discovered that

many cells have an ability to internalize folate-conjugated macromolecules by a folate-receptor

mediated mechanism. Proteins like IgG, serum albumin, folate conjugated ribonuclease and

horseradish peroxidase were treated to enter KB cells. The researchers observed that folate

proteins which were fluorescent labeled were seen to bind and enter the KB cells, whereas

proteins that were not bound to folate did not enter the cells (Figure 2). Radiolabelled proteins

that were conjugated with folate were difficult to be removed from the cells completely by acid-

wash saline, whereas controls that lacked folate were easily removed by simple washing with

PBS. The data obtained from the experiments revealed that folate-conjugates are internalized in a

non-destructive manner and macromolecules that were bound to folates remained in an active

state. The data also supported the idea that folate internalization into KB cells follows a

nonlysozomal pathway1.

99mTc-EC20 - A folate-derivatized radiopharmaceutical: (A SPECT tracer)

Atherosclerosis is a leading cause of death in many countries. The word is obtained from the

Greek words athere which means gruel and skleros which means hard. It is a type of

arteriosclerosis, where plaque deposits are formed in the interior of the arteries. Many radio

imaging methods have been employed to reduce the risk of cardiovascular events by early

detection. Few of those methods include 18F-FDG which measures the sites with increased

metabolic activity, 99mTc-labeled anti-lectin like oxidized low density lipoprotein receptor 1

antibody, 99mTc-annexin A5 which exposes regions of apoptosis and thrombosis. In a research

conducted by Phillip.S.Low and associates from Purdue University, used 99mTc-EC20 – a folate

targeted chelate of 99mTc, which was used for the imaging of activated macrophages for

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atherosclerosis. ApoE -/- mice were raised on both normal and high fat diets. By maintaining the

mice at high fatty western diet, atherosclerosis was accelerated. Uptake of 99mTc-EC20 was diet

dependent as the mice maintained on a western diet showed more accumulation of 99mTc-EC20.

When the mice on the western diet were compared to the mice fed on a low fat diet, the signal

intensity of 99mTc-EC20 in aortocardiac region was increased upto 70%. It was also observed that

uptake of 99mTc-EC20 was FR-mediated when the mice on western diet were pre-injected with

100- fold excess of free folic acid ( See Figure 3).

18F-FDG has been used mostly for imaging atherosclerosis. It accumulates in metabolizing

cells, even those regions which are not linked with atherosclerosis. 99mTc-EC20 on the other

hand, focuses on sites that are abundant in activated macrophages. This helps in reducing

misinterpretation of results21.

Folate conjugated shell cross-linked nanoparticles (SCK’s):

Nanoparticles used for drug delivery and imaging of tumors have been a good challenge.

64Cu has been used widely for the production of radiopharmaceuticals because of its radioactive

decay with β+ and β- emission. Professors and students from the Washington University of

Medicine and University of Texas Southwestern Medical Center conducted a research on 64Cu.

TETA-SCK and TETA-SCK-folate were radiolabeled with 64Cu and administered intravenously

to obtain accumulation in solid tumors and were driven by the EPR effect. SCK’s can be

functionalized with different variety of ligands; prodrugs to have a broader targeting approach.

Female mice with KB tumor cell xenografts were used in the investigation. The mice were kept

on diet depleted of folate for three weeks before and also during the experiments to achieve

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plasma folate levels in a range that is common for humans. Excess folic acid was administered to

the mice first one day before and again one hour before the experiment was conducted in order to

evaluate the effects of competitive block of FR on SCK tumor uptake. Folated SCK’s were

absorbed more by the liver than were the non-folated ones, and because of this, the blood

retention property was seen more by the non-folated conjugates.

It was already known that the folate-conjugated nanoparticles and liposomes do not have

proper selectivity for solid tumors. This is due to passive extravasation from the permeable

tumor blood vessels which is known as EPR effect. This research has not only shown similar

results but also suggested the FR-specific interaction of folate-SCK’s in small tumors. Hence,

there is a great deal of variability in tumor uptake depending on the size of the tumor;

furthermore, radio labeled SKC’s can be used for imaging and treating tumors at early stages22,23.

111In-DTPA folate:

The folate receptors are overexpressed in neoplastic regions like the ovary, cervical, breast,

renal and others. The folate- binding protein is overexpressed in ovaries. Studies show that this

receptor system is targeted in vivo and in vitro by folate-chelate conjugates, 111In-DTPA- folate

being one of the main conjugates. In figure 5, 111In-DTPA-folate whole body scintigraphic

images of a healthy volunteer and an ovarian cancer patient are shown. It is observed that the

conjugate is accumulated in the malignant tissues and kidneys only. 111In-DTPA- folate has been

used widely in the clinical imaging of tumors as it has less clearing time from FR- negative

tissues. In a study to evaluate the ability of 111In-DTPA-folate, four groups of mice were used for

the study. Three out of four groups received 111In-DTPA-folate conjugate and one group that

received the conjugate also received an IV dose of folic acid. For competitive displacement of

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111In-DTPA-folate radiotracer, one group of mice receiving the conjugate also received a chase

dose of folic acid 3 hours later. The fourth group received unconjugated 111In-DTPA, and the

fifth group received 111In-citrate. 111In-DTPA-folate was significantly concentrated in KB tumor

cells with tumor uptake of 1.0% ± 0.5% of the injected dose (3.1% ± 0.6% ID/g) at four hours

post injection (Figure 11). The results demonstrated that 111In-DTPA-folate radiopharmaceutical

may be useful in humans for noninvasive imaging of folate receptors-positive tumors. The mice

bearing the folate receptor-positive tumor KB cells, kidneys, tumors are the only areas that

retained receptor targeted 111In radiolabel. Figure 5 represents the scintigraphic images of a

healthy volunteer and a patient with ovary cancer2,24,25.

Conclusion:

As folate has a wide range of advantages that include its cost, tumor specificity, and size, it is

of great interest to researchers. Folate receptors, which are abundantly found on cancer cells will

help in developing drugs and also for imaging purposes for detecting tumors. 111In-DTPA-folate

has less clearing time from FR- negative tissues. Hence, it is widely used in clinical imaging of

tumors and is in clinical trials. Radiolabeled SCK’s can be used for imaging and treating tumors

at early stages. For imaging Atherosclerosis, 99mTc-EC20 focuses on sites that are abundant in

activation macrophages unlike 18F-FDG which also gets accumulated in cells not linked with

Atherosclerosis. Hence, misinterpretation of results are reduced. 99mTc-EC20 is a powerful

radiotracer and gives clear images of the tumors.

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References:

1. Low PS, Henne WA, Doorneweerd DD. Discovery and development of folic-acid-based

receptor targeting for imaging and therapy of cancer and inflammatory diseases. Acc

Chem Res. Jan 2008;41(1):120-129.

2. Muller C, Schibli R. Folic acid conjugates for nuclear imaging of folate receptor-positive

cancer. J Nucl Med. Jan 2011;52(1):1-4.

3. Dietrich M, Brown CJ, Block G. The effect of folate fortification of cereal-grain products

on blood folate status, dietary folate intake, and dietary folate sources among adult non-

supplement users in the United States. J Am Coll Nutr. Aug 2005;24(4):266-274.

4. Doherty RF, Beecher GR. A method for the analysis of natural and synthetic folate in

foods. J Agric Food Chem. Jan 15 2003;51(2):354-361.

5. Lu Y, Xu LC, Parker N, et al. Preclinical pharmacokinetics, tissue distribution, and

antitumor activity of a folate-hapten conjugate-targeted immunotherapy in hapten-

immunized mice. Mol Cancer Ther. Dec 2006;5(12):3258-3267.

6. McKillop DJ, McNulty H, Scott JM, et al. The rate of intestinal absorption of natural

food folates is not related to the extent of folate conjugation. Am J Clin Nutr. Jul

2006;84(1):167-173.

7. Moccio DM, Sirotnak FM, Samuels LL, et al. Similar specificity of membrane transport

for folate analogues and their metabolites by murine and human tumor cells: a clinically

directed laboratory study. Cancer Res. Jan 1984;44(1):352-357.

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8. Mosley BS, Cleves MA, Siega-Riz AM, et al. Neural tube defects and maternal folate

intake among pregnancies conceived after folic acid fortification in the United States. Am

J Epidemiol. Jan 1 2009;169(1):9-17.

9. Scott JM, Weir DG, Molloy A, McPartlin J, Daly L, Kirke P. Folic acid metabolism and

mechanisms of neural tube defects. Ciba Found Symp. 1994;181:180-187; discussion

187-191.

10. Tamura T, Picciano MF. Folate and human reproduction. Am J Clin Nutr. May

2006;83(5):993-1016.

11. Antony AC. Folate receptors. Annu Rev Nutr. 1996;16:501-521.

12. Bosson G. Reduced folate carrier: biochemistry and molecular biology of the normal and

methotrexate-resistant cell. Br J Biomed Sci. 2003;60(2):117-129.

13. Brzezinska A, Winska P, Balinska M. Cellular aspects of folate and antifolate membrane

transport. Acta Biochim Pol. 2000;47(3):735-749.

14. Corona G, Giannini F, Fabris M, Toffoli G, Boiocchi M. Role of folate receptor and

reduced folate carrier in the transport of 5-methyltetrahydrofolic acid in human ovarian

carcinoma cells. Int J Cancer. Jan 5 1998;75(1):125-133.

15. Villanueva JA, Devlin AM, Halsted CH. Reduced folate carrier: tissue distribution and

effects of chronic ethanol intake in the micropig. Alcohol Clin Exp Res. Mar

2001;25(3):415-420.

16. Berg MJ. The importance of folic acid. J Gend Specif Med. May-Jun 1999;2(3):24-28.

17. Gregory JF, 3rd. Bioavailability of folate. Eur J Clin Nutr. Jan 1997;51 Suppl 1:S54-59.

18. Iyer R, Tomar SK. Folate: a functional food constituent. J Food Sci. Nov-Dec

2009;74(9):R114-122.

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19. McKillop DJ, Pentieva K, Daly D, et al. The effect of different cooking methods on

folate retention in various foods that are amongst the major contributors to folate intake

in the UK diet. Br J Nutr. Dec 2002;88(6):681-688.

20. Low PS, Kularatne SA. Folate-targeted therapeutic and imaging agents for cancer. Curr

Opin Chem Biol. Jun 2009;13(3):256-262.

21. Ayala-Lopez W, Xia W, Varghese B, Low PS. Imaging of atherosclerosis in apoliprotein

e knockout mice: targeting of a folate-conjugated radiopharmaceutical to activated

macrophages. J Nucl Med. May 2010;51(5):768-774.

22. Rossin R, Pan D, Qi K, et al. 64Cu-labeled folate-conjugated shell cross-linked

nanoparticles for tumor imaging and radiotherapy: synthesis, radiolabeling, and biologic

evaluation. J Nucl Med. Jul 2005;46(7):1210-1218.

23. Kularatne SA, Low PS. Targeting of nanoparticles: folate receptor. Methods Mol Biol.

2010;624:249-265.

24. Mathias CJ, Wang S, Waters DJ, Turek JJ, Low PS, Green MA. Indium-111-DTPA-

folate as a potential folate-receptor-targeted radiopharmaceutical. J Nucl Med. Sep

1998;39(9):1579-1585.

25. Ke CY, Mathias CJ, Green MA. Folate-receptor-targeted radionuclide imaging agents.

Adv Drug Deliv Rev. Apr 29 2004;56(8):1143-1160.

26. George F.M Ball . (2006). Vitamins in Foods. Florida: Taylor and Francis Group.

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List of Figures:

Figure-1: Structure of folic acid

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Figure 2: Uptake of fluorescent folate-labeled proteins by KB cells. A,C,E,G represent Phase-contrast micrographs of KB cells incubated with bovine IgG-FITC-folate for 5,30,60,360 min respectively. B,D,F and H are their corresponding fluorescent micrographs. I and K are phase- contrast micrographs showing KB cells that were incubated for 5 and 360 min respectively with bovine IgG labeled with FITC to the same extent but lacking conjugated folate. J and L are their corresponding fluorescent micrographs. M is confocal image of a single KB cell incubated for 30 min with RNase A- RITC-folate. (Leamon, Low, Proc Natl Acad Sci U S A, July 1 1991)

Figure 3: 99mTc-EC20 targets aortas of apoE2/2 mice and can be used as imaging agent for atherosclerosis. ApoE2/2 mice were fed either normal or Western diet for 25 wk and then

injected intraperitoneally with either 99mTc-EC20 or 99mTc-EC20 plus 100-fold excess free folic

acid. Radioimages were obtained on Kodak Imaging Station (A) (n 5 10), and regions of interest was quantitatively analyzed using instrument software (B) (n 5 10). Mice were then euthanized, and excised aortas were analyzed for radioactivity by g-counting (C; n 5 5). During imaging, 5-mm lead shields were used to cover abdomens to avoid interference from 99mTc-EC20 uptake in kidneys and bladder. Data in C are presented as mean +/- SD. *P< 0.05. FA= folic acid. (Wilfredo Ayala-L´opez, Wei Xia, Bindu Varghese and Philip S. Low, J Nucl Med 2010)14.

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Figure 4: Formation of 99mTc-EC20 or 99mTc-EC53

Figure-5: 111In-DTPA-folate whole body scintigraphic images of a healthy volunteer and an ovarian cancer patient. Uptake in the cancer patient is seen in both the malignant tissue and kidneys, whereas only kidney uptake is observed in the healthy individual. Image reproduced with permission from Endocyte, Inc.1

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Figure 6: Principle of FR-targeted nuclear imaging. (Muller, Schibli, Journal of Nuclear Medicine, Vol 52, No1, Jan 2011)2

Figure 7: a) Folate receptor on normal cells b) Folate receptors on malignant cells.

Figure-8: Uptake of folate conjugatesNo.1, Jan 2008)

Figure 9: SPECT/CT images of female mice bearing human ovarian IGROVa) After injecting 111ln-DTPAb) Injected in combination with predosed

(Muller, Schibli, Journal of Nuclear Medicine. Vol. 52, No 1, Jan 2011)

conjugates (Low, Henne, Accounts of Chemical Research. Vol. 41,

SPECT/CT images of female mice bearing human ovarian IGROV-1 xenografts DTPA-folate alone 4 hrs later

Injected in combination with predosed pemetrexed. (Muller, Schibli, Journal of Nuclear Medicine. Vol. 52, No 1, Jan 2011)

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(Low, Henne, Accounts of Chemical Research. Vol. 41,

1 xenografts

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Figure 10: Structure of 111In-DTPA-folate conjugate.

Figure 11: Tumor uptake of 111In-DTPA folate 111In-DTPA and 111In-citrate mice with folate-receptor-positive KB cell tumors. Each column represents data from one animal. The mean tumor uptake (+/- sd) is shown above the data for each group of animals. (Mathias, Wang, Waters, Turek, Low, Green, The journal of Nuclear Medicine, Vol 39, No 9, Sep 1998).

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