99mTc-Labeled mercaptoacetylglycylglycylglycine conjugated to hydroxy-bisphosphonate


Chopra A.

Publication Details



In vitro Rodents



Bisphosphonates (BPs) or nitrogen-containing bisphosphonates (NBPs) are often used for the management of pain palliation and disorders related to skeletal tissue, including those arising from cancer metastases, because these compounds exhibit a very high affinity for hydroxyapatite (HA), a component of the bone matrix (1). The NBPs tend to accumulate in osteoclasts at areas of increased bone metabolism by inhibiting the farnesyl diphosphate synthase, an important enzyme of the mevalonate pathway in the cell (2). Several BPs and NBPs are available commercially for clinical use to treat different bone disorders, and there are ongoing clinical trials approved by the United States Food and Drug Administration to evaluate these compounds for the treatment of various bone ailments. In addition, BPs are often labeled with 99mTc or 186/188Re and used for the imaging and treatment of bone metastases. However, these compounds have limitations primarily because they either exist as a mixture of anionic compounds with varying properties (e.g., [99mTc]-labeled methylene-diphosphonate (MDP) or [99mTc]-labeled hydroxymethylene-diphosphonate (HMDP)) or are rapidly degraded (e.g., [186/188Re]MDP) under in vivo conditions, resulting in a reduced uptake at targeted bone areas and an increased accumulation in non-target soft tissue (3). The 99mTc- or 186/188Re-labeled BPs were suggested to have these limitations because the compounds possess dual activities: one phosphonate group of the BP molecule acts as a radionuclide chelator, and the other phosphonate group binds to the target(s). Therefore, due to close proximity of the two groups, one activity may be interfering with the other (3).

To circumvent problems associated with the radiolabeled BPs, investigators developed NBPs with two independent activities such that the nitrogen-containing part would only chelate the radiotracer and the BP part would target the bone (4). Ogawa et al. synthesized and labeled two NBPs, mercaptoacetylglycylglycylglycine-hydroxy-bisphosphonate (MAG3-HBP) and 6-hydrazinopyridine-3-carboxylic acid-hydroxy-bisphosphonate (HYNIC-HBP), with 99mTc to obtain [99mTc]MAG3-HBP and [99mTc]HYNIC-HBP (4). The two imaging agents were valuated for binding to hydroxyapatite (HA) under in vitro conditions and for biodistribution in normal rats (4). This chapter presents the studies performed and the results obtained with [99mTc]MAG3-HBP. The studies and the results obtained with [99mTc]HYNIC-HBP are presented in a separate chapter in MICAD (www.micad.nih.gov) (5).

Other sources of information

Protein and mRNA sequence of human farnesyl diphosphate synthase

Gene information regarding human farnesyl diphosphate synthase (GeneID: 2224)

Farnesyl diphosphate synthase in Online Mendelian Inheritance in Man (OMIM)

Structure of farnesyl diphosphate synthase complexed with a bisphosphonate

Farnesyl diphosphate synthase in Kyoto Encyclopedia of Genes and Genomes (KEGG) Pathways

Related chapters in MICAD



The synthesis, characterization, and 99mTc labeling of MAG3-HBP were described by Ogawa et al. (4). The labeled product had a radiochemical yield of 73% and a purity of >95% as determined with reverse-phase high-performance liquid chromatography. The specific activity of the radiolabeled compound was not reported.

The production, radiochemical purity, yield, stability, and specific activity of [99mTc]HMDP, used for comparison in some of the studies, were not reported (4).

As determined in serum isolated from rats injected with the respective radiotracers, 88.7 ± 2.7% of [99mTc]MAG3-HBP was reported to bind to serum proteins compared with 97.7 ± 0.2% for [99mTc]HYNIC-HBP. The serum binding of [99mTc]HMDP was not reported.

In Vitro Studies: Testing in Cells and Tissues


An in vitro assay was used to compare the HA binding of 99mTc-labeled MAG3-HBP, HYNIC-HBP, and HMDP, respectively (4). Among these labeled compounds, both [99mTc]MAG3-HBP and [99mTc]HYNIC-HBP bound to a similar level with HA, and the binding was significantly higher (P < 0.05) than that exhibited by [99mTc]HMDP (for details see Figure 2 in Ogawa et al. (4)).

Animal Studies



Ogawa et al. investigated the biodistribution of [99mTc]MAG3-HBP, [99mTc]HYNIC-HBP, and [99mTc]HMDP in normal rats (4). The rats were given an intravenous injection of the respective radiotracers (n = 4–6 animals/time point for each radiotracer), and the animals were euthanized at predetermined time points up to 60 min after treatment. Organs of interest were removed from the euthanized animals and counted for accumulated radioactivity, and the data were presented as percent injected dose per gram tissue (%ID/g). The amount of radioactivity in blood from [99mTc]MAG3-HBP, [99mTc]HYNIC-HBP, and [99mTc]HMDP decreased from 0.98 ± 0.09% ID/g, 0.56 ± 0.05% ID/g, and 0.61 ± 0.03% ID/g at 5 min to 0.07 ± 0.01% ID/g, 0.03 ± 0.01% ID/g, and 0.04 ± 0.01% ID/g at 60 min after injection, respectively. A similar pattern of decreased radioactivity in the kidneys was reported with the three tracers, indicating that the label was eliminated from the animals through the urinary route. The femur (the only bone reported to have been removed from the rats) showed [99mTc]MAG3-HBP, [99mTc]HYNIC-HBP, and [99mTc]HMDP radioactivity uptake values of 1.80 ± 0.24% ID/g, 1.90 ± 0.21% ID/g, and 1.52 ± 0.14% ID/g at 5 min, respectively, which increased to 4.23 ± 0.25% ID/g, 3.96 ± 0.36% ID/g, and 3.13 ± 0.51% ID/g at 60 min after injection, respectively. The femur/blood ratios at 60 min were 59.7 ± 10.41, 123 ± 18.30, and 93.40 ± 19.81 for [99mTc]MAG3-HBP, [99mTc]HYNIC-HBP, and [99mTc]HMDP, respectively, which indicated that [99mTc]HYNIC-HBP was superior to [99mTc]MAG3-HBP and [99mTc]HMDP for binding to the bone.

On the basis of the above observations, the investigators concluded that [99mTc]HYNIC-HBP is a superior bone targeting agent compared with [99mTc]MAG3-HBP and has potential as a bone scintigraphy agent (4).

Other Non-Primate Mammals


No references are currently available.

Non-Human Primates


No references are currently available.

Human Studies


No references are currently available.

Supplemental Information


No information is currently available.


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Chopra, A., [99mTc]Labeled 6-hydrazinopyridine-3-carboxylic acid conjugated to hydroxy-bisphosphonate. Molecular Imaging and Contrast agent Database (MICAD) [database online]. National Library of Medicine, NCBI, Bethesda, MD, USA. Available from www​.micad.nih.gov, 2004 -to current.