Well-Designed Bone-Seeking Radiolabeled Compounds for Diagnosis and Therapy of Bone Metastases

Ogawa, Kazuma; Ishizaki, Atsushi

doi:https://doi.org/10.1155/2015/676053

BioMed Research International

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Review Article | Open Access

Volume 2015 | Article ID 676053 | https://doi.org/10.1155/2015/676053

Well-Designed Bone-Seeking Radiolabeled Compounds for Diagnosis and Therapy of Bone Metastases

Kazuma Ogawa¹and Atsushi Ishizaki¹

Academic Editor: Bishnuhari Paudyal

Received24 Jul 2014

Revised04 Oct 2014

Accepted08 Oct 2014

Published14 May 2015

Abstract

Bone-seeking radiopharmaceuticals are frequently used as diagnostic agents in nuclear medicine, because they can detect bone disorders before anatomical changes occur. Furthermore, their effectiveness in the palliation of metastatic bone cancer pain has been demonstrated in the clinical setting. With the aim of developing superior bone-seeking radiopharmaceuticals, many compounds have been designed, prepared, and evaluated. Here, several well-designed bone-seeking compounds used for diagnostic and therapeutic use, having the concept of radiometal complexes conjugated to carrier molecules to bone, are reviewed.

1. Introduction

For many years, -bisphosphonate complexes, -methylenediphosphonate (-MDP, Figure 1(a)) and -hydroxymethylenediphosphonate (-HMDP, Figure 1(b)), have been clinically used for nuclear medical imaging of metastatic bone cancer [1–4], because their high sensitivity can detect bone metastases before occurrence of anatomical changes. Bone metastases are characterized as osteolytic, osteosclerotic, or mixed type of osteolytic and osteosclerotic; namely, osteolytic or osteosclerotic changes occur in lesion sites of bone metastases. These anatomical changes could cause pathologic fractures and severe pain.

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It has been known that strontium (Sr) acts as calcium mimic and accumulates in high osteoblastic activity lesions since strontium is one of the alkaline earth metals [5]. ⁸⁹Sr has a physical half-life of 50.5 days and emits beta particles with a maximum energy of 1.46 MeV (Table 1). Strontium-89 chloride (⁸⁹SrCl₂, Metastron) was the first radiopharmaceutical approved for the palliation of metastatic bone pain by the US Food and Drug Administration (FDA). ⁸⁹SrCl₂ for the palliation of metastatic bone pain for breast cancer patients and prostate cancer patients showed a pain relief rate of 57–92%. These studies are summarized in reviews [6–9].

Samarium-153 (¹⁵³Sm) has a physical half-life of 46.3 hours and emits beta particles with a maximum energy of 0.81 MeV (20%), 0.71 MeV (49%), and 0.64 MeV (30%) and a 28% abundance of gamma rays with energy of 103 keV (Table 1). ¹⁵³Sm-ethylenediaminetetramethylene phosphonic acid (EDTMP, Quadramet) is a complex of ¹⁵³Sm and EDTMP (Figure 1(c)), which has high affinity for bone mineral. ¹⁵³Sm-EDTMP was approved and has been widely used in the United States for palliation of metastatic bone pain. The biodistribution of ¹⁵³Sm-EDTMP is similar to that of bone scintigraphic agents such as -MDP (methylene diphosphonate) [10]. Accordingly, it was reported that the dosimetry of ¹⁵³Sm-EDTMP could be predicted using -MDP bone scintigraphy [11]. ¹⁵³Sm-EDTMP showed a pain relief rate in 62–84% of patients with metastatic bone pain. These studies are also summarized in reviews [6–8]. Meanwhile, in a study of comparison between the effects of the ⁸⁹SrCl₂ and ¹⁵³Sm-EDTMP to patients with bone metastases, there was no statistical difference in response rates [12].

Zoledronic acid (Zometa), which is a bisphosphonate compound, has been widely used for the prevention of skeletal complications. Lam et al. combined zoledronic acid and ¹⁵³Sm-EDTMP to treat hormone-refractory prostate cancer patients [13]. It was concluded that zoledronic acid treatment does not influence ¹⁵³Sm-EDTMP skeletal uptake and combined treatment is feasible and safe.

The therapeutic bone-seeking radiopharmaceutical radium-223 chloride (²²³RaCl₂) was approved by FDA and European Medicines Agency (EMA) in 2013 based on data from a phase III randomized trial (the Alpharadin in Symptomatic Prostate Cancer Patients: ALSYMPCA). Surprisingly, ²²³RaCl₂ significantly improved overall survival in patients with castration-resistant prostate cancer with bone metastases in the ALSYMPCA study [14, 15]. In addition, because it is the first radiopharmaceutical emitting alpha particles approved for clinical use, ²²³RaCl₂ is currently attracting much attention.

-MDP, -HMDP, ⁸⁹SrCl₂, ¹⁵³Sm-EDTMP, and ²²³RaCl₂ are milestones in the development of bone-seeking radiopharmaceuticals for clinical use (Table 2). Although developing superior bone-seeking compounds is difficult, we reviewed the promising well-designed bone-seeking compounds for diagnosis and therapy of bone metastases in basic research.

2. -Complex-Conjugated Bisphosphonate Compounds Designed to Overcome Drawbacks of -MDP and -HMDP Complexes

Although -MDP and -HMDP are considered to be the gold standards for bone scintigraphy agents, they have not yet been optimized from a chemical and pharmaceutical perspective, because these complexes are not well-defined single-chemical species but are mixtures of short-chain and long-chain oligomers [16]. Moreover, the phosphonate groups in -MDP and -HMDP are used both as ligands for coordination and as carriers to hydroxyapatite (HA) in bone [17], which may decrease the inherent affinity of MDP and HMDP for bone. To overcome these drawbacks, a more logical design strategy has been proposed on the basis of the conjugation of a stable radiometal complex to a carrier molecule to bone. This drug design allows the ligand and carrier function to work independently and effectively.

In 2002, Verbeke et al. described -L,L-ethylenedicysteine (EC) complex, a renal tracer agent known to have rapid renal excretion, conjugated to bisphosphonate (-EC-AMDP, Figure 2(a)) [18]. -EC-AMDP showed faster blood clearance and a higher bone/blood ratio compared with -MDP in animal experiments.

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In 2006, we reported -mercaptoacetylglycylglycylglycine- (MAG3-) conjugated bisphosphonate (-MAG3-HBP, Figure 2(b)) and -6-hydrazinonicotinic acid (HYNIC) with tricine and 3-acetylpyridine as coligands conjugated to bisphosphonate (-HYNIC-HBP, Figure 2(c)) [19]. In in vitro HA binding experiments, the binding rates of -MAG3-HBP and -HYNIC-HBP to HA were higher than those of -HMDP. In a biodistribution study in rats, -MAG3-HBP and -HYNIC-HBP showed higher accumulation in bone compared with -HMDP reflecting the in vitro findings. The blood clearance of -MAG3-HBP was delayed because of the high rate of protein binding in blood and the bone/blood ratio of -MAG3-HBP was lower than that of -HMDP. In contrast, the blood clearance of -HYNIC-HBP was as rapid as that of -HMDP and its bone/blood ratio was higher.

Liu et al. reported findings on -HYNIC-conjugated bisphosphonate (-HYNIC-AMDP, Figure 2(d)) in 2011 [20]. The authors found that -HYNIC-AMDP had a higher bone uptake and higher bone/blood and bone/muscle ratios at an early time point after injection as compared with -MDP. In that study, -HYNIC-AMDP showed favorable biodistribution as a bone-seeking agent, but the bone accumulation of -MDP, a bone scintigraphy agent as a control, appeared to be too low. Two tricine molecules are used as coligands in -HYNIC-AMDP. However, as it has been reported, the complex [](HYNIC)(tricine)₂ is not stable and exists in multiple forms, and the pharmacokinetics could be affected by the exchange reaction between tricine and protein in the plasma and tissues [21–23]. The pharmacokinetics of -HYNIC-AMDP may be improved by exchanging one tricine molecule for another molecule, such as one of the pyridine derivatives.

Palma et al. described -tricarbonyl complex, which is anchored by a pyrazolyl- (Pz-) containing ligand, conjugated to bisphosphonate compounds ([(CO)₃(PzNN-BP)], [(CO)₃(PzNN-ALN)], and [(CO)₃(PzNN-PAM)], Figures 3(a)–3(c)) [24, 25]. The structures of these technetium complexes were confirmed by reversed phase (RP) HPLC analyses. The identical retention time as the corresponding nonradioactive rhenium (Re) complexes revealed the structural analogy. Although [(CO)₃(PzNN-BP)] showed moderate bone uptake, the uptake was lower than that of -MDP. In contrast, the bone accumulation of [(CO)₃(PzNN-ALN)] and [(CO)₃(PzNN-PAM)] was high and comparable to that of -MDP. The bone/blood and bone/muscle ratios of [(CO)₃(PzNN-ALN)] and [(CO)₃(PzNN-PAM)] were higher than that of -MDP at 4 hours after injection because of their fast clearance. The difference in bone accumulation among -tricarbonyl complex-conjugated bisphosphonate compounds could be derived from the introduction of a hydroxyl group at the central carbon of the bisphosphonate because bisphosphonates containing the hydroxyl group have been reported to have higher affinity for bone minerals [26–28].

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In 2009, De Rosales et al. described -tricarbonyl complex-conjugated bisphosphonate that has the structure similar to that of [(CO)₃(PzNN-ALN)] but with dipicolylamine (DPA) was used as a chelation site ((CO)₃-DPA-alendronate, Figure 3(d)) [29]. In vitro study showed that (CO)₃-DPA-alendronate had higher affinity for HA than -MDP. In animal experiments, (CO)₃-DPA-alendronate showed high uptake in bone, comparable to -MDP.

As mentioned above, certain -complex-conjugated bisphosphonate compounds have shown favorable biodistribution as bone imaging agents and higher bone/blood ratios compared with those of -MDP or -HMDP. Consequently, these results suggest that the strategy of developing stable -complex-conjugated bisphosphonates is promising.

3. Radiogallium-Complex-Conjugated Bisphosphonate Compounds as Bone Imaging Agents for Positron Emission Tomography (PET)

⁶⁸Ga is a practical and interesting radionuclide for clinical PET because of its radiophysical properties, particularly as a ⁶⁸Ge/⁶⁸Ga generator-produced radionuclide has a half-life () of 68 minutes (Table 1) [30]. It does not require an on-site cyclotron and can be eluted on demand. Indeed, in principle, the long half-life of the parent nuclide ⁶⁸Ge ( days) provides a generator with a long lifespan.

The above-mentioned drug design concept, which is a stable complex-conjugated bisphosphonate, could also be applicable to gallium complexes. With the aim of developing a superior bone imaging PET tracer, some types of radiogallium complex-conjugated bisphosphonate compounds have been reported.

In 2010, Fellner et al. reported a human study of ⁶⁸Ga-DOTA-conjugated bisphosphonate (⁶⁸Ga-BPAMD, Figure 4(a)), containing 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) as a ligand for gallium [31]. ⁶⁸Ga-BPAMD showed high uptake in osteoblastic metastases lesions in a patient with prostate cancer (Figure 5). The maximal standardized uptake value () was 77.1 and 62.1 in the 10th thoracic and L2 vertebra for ⁶⁸Ga-BPAMD compared with respective values of 39.1 and 39.2 for ¹⁸F-fluoride, which is a typical bone imaging agent for PET (Table 2), respectively. In 2012, Fellner et al. reported the findings of basic experiments on ⁶⁸Ga-BPAMD using μ-PET with bone metastasis rat model [32]. ⁶⁸Ga-BPAMD highly accumulated in metastatic bone lesions compared with healthy bone in the same animal (contrast factor = ). The same research group further reported ⁶⁸Ga-DOTA-conjugated bisphosphonate derivatives, ⁶⁸Ga-BPAPD and ⁶⁸Ga-BPPED (Figures 4(b) and 4(c)) in 2013 [33]. The phosphinate-conjugated bisphosphonate ⁶⁸Ga-BPPED showed higher accumulation in bone compared with ⁶⁸Ga-BPAMD and ⁶⁸Ga-BPAPD, amide-conjugated bisphosphonates. The presence of phosphinate may contribute to an additional binding to HA, leading to higher accumulation in bone.

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Figure 5

⁶⁸Ga-BPAMD was injected intravenously into a patient with extensive bone metastases of prostate cancer. ⁶⁸Ga-BPAMD [maximum intensity projection (MIP) 50 min after injection (p.i.), 462 MBq] revealed intense accumulation in multiple osteoblastic lesions in the central skeleton, ribs, and proximal extremities: (a) = coronal PET, (b) = sagittal PET/CT. For comparison, (c) shows ¹⁸F-fluoride PET (sagittal, MIP 90 min p.i., 270 MBq). With kind permission from Springer Science+Business Media: Eur J Nucl Med Mol Imaging, PET/CT imaging of osteoblastic bone metastases with ⁶⁸Ga-bisphosphonates: first human study, 37, 2010, 834, Fellner et al.

In 2011, we also reported ⁶⁷Ga-DOTA complex-conjugated bisphosphonate (⁶⁷Ga-DOTA-Bn-SCN-HBP, Figure 4(d)) [34]. Although the aim was to develop a superior ⁶⁸Ga-labeled bone imaging agent for PET, in the initial basic studies ⁶⁷Ga was used because of its longer half-life. In biodistribution experiments in normal mice, ⁶⁷Ga-DOTA-Bn-SCN-HBP rapidly and highly accumulated in bone but was rarely observed in tissues other than bone. As a result, the bone/blood ratio of ⁶⁷Ga-DOTA-Bn-SCN-HBP was comparable to that of -HMDP, which is a gold standard for a bone scintigraphy agent.

Furthermore, in 2011, Suzuki et al. reported ⁶⁸Ga-NOTA-conjugated bisphosphonate, containing 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) as a ligand for gallium (⁶⁸Ga-NOTA-BP, Figure 4(e)) [35]. In biodistribution experiments using Wistar rats, ⁶⁸Ga-NOTA-BP showed faster clearance and a higher bone/blood ratio than -MDP and ¹⁸F-fluoride. Moreover, in PET study using a mouse model of bone metastasis, ⁶⁸Ga-NOTA-BP showed high accumulation of radioactivity in osteolytic lesions in the tibia.

These results suggest that the drug design concept of radio gallium complex-conjugated bisphosphonate could be useful for the development of ⁶⁸Ga PET imaging agents for bone disorders such as bone metastases.

4. Re-Complex-Conjugated Bisphosphonate for Palliation of Bone Metastases

Gamma ray emitter radionuclide and positron emitter radionuclide-labeled bone-seeking agents are used for the diagnosis of bone metastases. A prominent symptom caused by bone metastases is pain, which has a significant impact on the patients’ quality of life. Bone-seeking agents labeled with high-energy beta particle emitter radionuclides and alpha particle emitter radionuclides are used for palliation of pain caused by bone metastases. Rhenium, which has similar chemical properties to technetium, because they are members of family VIIA of the periodic table, has two useful radionuclides, ¹⁸⁶Re and ¹⁸⁸Re, which are useful for radionuclide therapy [36]. Both rhenium radionuclides emit not only beta particles for therapy but also gamma rays, which are suitable for diagnoses: ¹⁸⁶Re ( days, MeV, keV) and ¹⁸⁸Re ( hours, MeV, keV) (Table 1). In addition, ¹⁸⁸Re has a further advantage for clinical use because it is obtained from an in-house alumina-based ¹⁸⁸W/¹⁸⁸Re generator, similar to a generator [37].

When considering the use of rhenium in bone-seeking agents for palliation, similar to -MDP and -HMDP, it is known that rhenium coordinates with some bisphosphonate derivatives. ^186/188Re-1-hydroxyethylidene-1,1-diphosphonate (^186/188Re-HEDP, Figure 1(d)), which has high affinity for bone, has been prepared and used for clinical research [9, 38, 39]. Although the chemical properties of rhenium are similar to those of technetium as mentioned above, rhenium is more easily oxidized than technetium [40], and it has been reported that ¹⁸⁶Re-HEDP is not as stable as -bisphosphonate complexes [41]. Some studies reported that ¹⁸⁶Re-HEDP showed unexpected gastric uptake in patients with bone metastases [42, 43]. This may be derived from the accumulation of free rhenium (perrhenate: ) in the stomach due to the instability of ¹⁸⁶Re-HEDP [40, 44]. Moreover, as with -MDP and -HMDP, the phosphonate groups in ^186/188Re-HEDP are used as both ligands for coordination and as carrier to HA in bone, which may decrease the inherent affinity of HEDP for bone.

To overcome these problems, designing a stable ^186/188Re-complex-conjugated bisphosphonate would be useful. Therefore, we studied ¹⁸⁶Re-monoaminemonoamidedithiol- (MAMA-) and ¹⁸⁶Re-mercaptoacetylglycylglycylglycine- (MAG3-) conjugated bisphosphonate compounds (¹⁸⁶Re-MAMA-BP, ¹⁸⁶Re-MAMA-HBP, and ¹⁸⁶Re-MAG3-HBP: Figures 6(a)–6(c)) and reported their findings in 2004, 2006, and 2005, respectively [28, 45, 46]. In an in vitro stability experiment in buffer solution, the Re-complex-conjugated bisphosphonate compounds, ¹⁸⁶Re-MAMA-HBP, ¹⁸⁶Re-MAMA-BP, and ¹⁸⁶Re-MAG3-HBP, were more stable than ¹⁸⁶Re-HEDP. In the biodistribution experiments performed in normal mice, ¹⁸⁶Re-MAMA-HBP, ¹⁸⁶Re-MAMA-BP, and ¹⁸⁶Re-MAG3-HBP showed lower accumulation of radioactivity in stomach compared with ¹⁸⁶Re-HEDP. This result indicates that the drug design of Re-complex-conjugated bisphosphonates enabled better stability in vitro and in vivo. Of ¹⁸⁶Re-complex-conjugated bisphosphonate compounds, ¹⁸⁶Re-MAG3-HBP showed the most favorable biodistribution characteristics as bone-seeking radiopharmaceuticals such as high and selective bone accumulation, based on high hydrophilicity (log value: ) and the introduction of a hydroxyl group to the central carbon of the bisphosphonate structure. Previous studies suggested that the hydroxyl group affects affinity for bone minerals [26, 27]. We evaluated the therapeutic potential of ¹⁸⁶Re-MAG3-HBP for the palliation of metastatic bone pain using an animal model of bone metastasis [47]. The planar image of ¹⁸⁶Re-MAG3-HBP showed high accumulation of radioactivity in bone metastasis lesion. ¹⁸⁶Re-MAG3-HBP was more effective for palliation and was compared with ¹⁸⁶Re-HEDP using the hind paw withdrawal response to stimulation with von Frey filaments. Moreover, although ¹⁸⁶Re-HEDP did not affect tumor growth, ¹⁸⁶Re-MAG3-HBP significantly inhibited tumor growth.

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In 2007, Uehara et al. reported [¹⁸⁶Re]CpTR-Gly-APD (Figure 6(d)), which is a tricarbonyl [¹⁸⁶Re][(cyclopentadienylcarbonyl amino)-acetic acid] rhenium complex ([¹⁸⁶Re]CpTR-Gly)-conjugated bisphosphonate [48]. [¹⁸⁶Re]CpTR-Gly-APD showed characteristics superior to those of ¹⁸⁶Re-HEDP, such as higher stability in plasma, a higher binding rate for HA, higher bone accumulation, and lower plasma protein binding. When [¹⁸⁶Re]CpTR-Gly-APD with HEDP (9.0 mg/kg) was administered to mice, the accumulation of radioactivity in bone significantly decreased and the blood clearance was delayed. Therefore, the authors concluded that the specific activity of ¹⁸⁶Re-labeled bisphosphonate compounds is very important to bone accumulation and blood clearance.

In the -complex-conjugated bisphosphonate session, (CO)₃-DPA-alendronate has been discussed above. Using the same ligand, in 2010, De Rosales et al. reported ¹⁸⁸Re(CO)₃-DPA-alendronate (Figure 6(e)) [49]. ¹⁸⁸Re(CO)₃-DPA-alendronate can easily be synthesized with high specific activities and high yields (≥96%). ¹⁸⁸Re(CO)₃-DPA-alendronate showed higher stability in vitro compared with ¹⁸⁸Re-HEDP, which oxidized to ¹⁸⁸ (up to 75%) when placed in PBS for 48 hours at 37°C. In in vivo imaging, ¹⁸⁸Re(CO)₃-DPA-alendronate showed superior biodistribution of radioactivity than ¹⁸⁸Re-HEDP; that is, ¹⁸⁸Re(CO)₃-DPA-alendronate highly accumulated in metabolically active bone, such as the joints with low soft-tissue uptake.

These results indicate that the concept of the stable ¹⁸⁶Re-complex-conjugated bisphosphonates could be more useful and that novel ¹⁸⁶Re-complex-conjugated bisphosphonate complexes could be attractive candidates as palliative agents in metastatic bone pain.

5. Aspartic Acid Peptides as Carriers of Radionuclides to Bone

Several major noncollagenous bone proteins, such as osteopontin and bone sialoprotein, have repeating sequences of acidic amino acids (Asp or Glu) in their structures, offering potential HA binding sites [50–52]. It has been reported that polyglutamic and polyaspartic acids have a high affinity for HA and could be used as carriers for drug delivery to bone [53–55].

In 2013, Yanagi et al. reported -complex-conjugated aspartic acid (Asp) peptides [56]. They selected EC as a ligand to prepare stable technetium complexes, conjugated with one or two penta D-Asp peptides [-EC-(D-Asp)₅ or -EC-[(D-Asp)₅]₂, Figures 7(a) and 7(b)]. The HA binding of -EC-[(D-Asp)₅]₂ was higher than that of -EC-(D-Asp)₅. -EC-[(D-Asp)₅]₂ showed significantly lower accumulation in normal bone of mice compared with -MDP. In contrast, when compared with -MDP, -EC-[(D-Asp)₅]₂ showed the same degree of accumulation in a osteogenic lesion of tumor-bearing rat models. Thus, the uptake ratio of osteogenic lesion to normal bone (osteogenic lesion/normal bone) of -EC-[(D-Asp)₅]₂ after injection was higher than that of -MDP. The authors supposed that the higher osteogenic lesion/normal bone ratio derived forms the higher molecular size, which was determined by permeability through a membrane filter (10 kDa), of -EC-[(D-Asp)₅]₂ compared with that of -MDP and -EC-(D-Asp)₅.

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In 2013, we reported ⁶⁷Ga-DOTA-conjugated L-Asp peptides (⁶⁷Ga-DOTA-(Asp)_n, Figure 7(c)), which had varying peptide lengths (, 5, 8, 11, or 14) [57]. Binding affinity to HA of ⁶⁷Ga-DOTA-(Asp)_n increased with an increase in the length of the aspartate peptide. The HA binding of ⁶⁷Ga-DOTA-conjugated bisphosphonate, ⁶⁷Ga-DOTA-Bn-SCN-HBP, was inhibited by lower concentrations of alendronate, one of bisphosphonate compounds, compared with ⁶⁷Ga-DOTA-(Asp)₁₄. In biodistribution experiments of normal mice, ⁶⁷Ga-DOTA-(Asp)₈, ⁶⁷Ga-DOTA-(Asp)₁₁, and ⁶⁷Ga-DOTA-(Asp)₁₄ selectively and highly accumulated in bone (, , and % ID/g, resp.). Although the bone accumulation of ⁶⁷Ga-DOTA-(Asp)_n was lower than that of ⁶⁷Ga-DOTA-Bn-SCN-HBP, the blood clearance of ⁶⁷Ga-DOTA-(Asp)_n was more rapid. Accordingly, the bone/blood ratios of ⁶⁷Ga-DOTA-(Asp)₁₁ and ⁶⁷Ga-DOTA-(Asp)₁₄ were comparable to that of ⁶⁷Ga-DOTA-Bn-SCN-HBP. Moreover, the inhibition of radioactive bone accumulation by alendronate was greater after injection of ⁶⁷Ga-DOTA-Bn-SCN-HBP than that of ⁶⁷Ga-DOTA-(Asp)₁₄.

These results indicate that not only bisphosphonate molecules but also acidic amino acid peptide sequences could be useful as carriers of radionuclides to bone metastases lesions. Moreover, radiometal complex-conjugated acidic amino acid peptides may provide slightly different information from radiometal complex-conjugated bisphosphonates.

6. Carbon-11 Labeled Cathepsin K Inhibitors

Cathepsin K is a member of the papain family of cysteine peptidases with a primary physiological function of cleavage of type I and type II collagen [58]. The enzyme is highly expressed in activated osteoclasts, and a change in the number of the osteoclast is related to bone diseases such as osteoporosis [59]. Therefore, it could be useful to determine the changes in osteoclast numbers in such disease states by imaging cathepsin K. Because many inhibitors of cathepsin K have been synthesized and evaluated both in vitro and in vivo, their derivatives may be candidates as mother compounds for cathepsin K imaging agents. The possibility of targeting cathepsin K in in vivo imaging, using a near-infrared reporter probe, was confirmed in a previous report [60].

Rodnick et al. reported carbon-11-labeled cathepsin K inhibitors with high affinity as cathepsin K imaging agents, 2-cyano-4-(cyclohexylamino)-N-(4-[¹¹C]methoxyphenethyl)-pyrimidine-5-carboxamide ([¹¹C]1, Figure 8(a)) and 2-cyano-N-(4-[¹¹C]methoxyphenethyl)-4-(neopentylamino) pyrimidine-5-carboxamide ([¹¹C]2, Figure 8(b)) in 2014 [61]. Nonradioactive counterparts of [¹¹C]1 and [¹¹C]2 were reported in 2007 [62]. In that study, because the pyrimidine core structure docked into the cathepsin K active site, many types of derivatives based on a pyrimidine scaffold were synthesized and evaluated as cathepsin K inhibitors. Among them, the nonradioactive counterparts showed greater affinity and selectivity for cathepsin K. For inhibition of cathepsin K, cathepsin L, and cathepsin S, IC₅₀ values of compound 1 were 0.022, 0.17, and 0.7, and those of compound 2 were <0.003, 1.2, and 0.9, respectively. [¹¹C]1 and [¹¹C]2 were radiosynthesized by standard reaction conditions used for alkylation reactions with [¹¹C]methyl iodide. In vivoμ-PET imaging experiments showed that [¹¹C]1 and [¹¹C]2 inhibitors have a higher uptake in actively growing bone regions, such as distal ulnar, carpal, distal and proximal humeral, distal femur, and proximal tibia, than in nontarget regions such as muscle. The uptake in specific bone regions was based on specific binding to cathepsin K because the uptake was inhibited by pre- or coinjection of an excess amount of ligands. These results indicated that radiolabeled cathepsin K inhibitors could have potential as in vivo imaging agents to determine a change in the number of osteoclasts.

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7. Dual-Modality Single Photon Emission Computed Tomography/Near-Infrared (SPECT/NIR) Fluorescent Probe

Recently, multimodality molecular imaging combining several imaging techniques has attracted much attention in basic scientific and clinical research. Nuclear medical imaging can detect deep tissues in the body with high sensitivity, but there are some problems such as relatively poor spatial resolution [63].

Optical imaging is a relatively new imaging modality that offers real-time and nonradioactive and high-resolution imaging of fluorophores in lesion sites, but it is difficult to detect a deep tissue using this technique [64]. Fluorescence imaging with near-infrared (NIR, 700–900 nm wavelength) light reveals relatively low tissue absorption. IRDye78 is a heptamethine indocyanine-type NIR fluorophore with peak absorption at 771 nm and peak excitation emission at 806 nm. Pam78 (Figure 9(a)), a IRDye78-conjugated pamidronate (one of the bisphosphonate derivatives), has been reported as a NIR fluorescence imaging probe targeted to HA [65]. HA is considered to be a good marker for some diseases because calcification (HA deposition) occurs during the processes of cancer and atherosclerosis.

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Bhushan et al. reported the trifunctional diagnostic agent Pam-Tc/Re-800 (Figure 9(b)) in 2008 [66]. Pam-Tc/Re-800 possesses a radiometal complex as a nuclear imaging probe, a fluorescent site as a fluorescence imaging probe, and bisphosphonate having high affinity for HA as a carrier to bone in a molecule. In an in vitro experiment, Pam-Tc/Re-800 showed specific and selective binding to HA. In the fluorescence imaging of microcalcified breast cancer rat model, Pam-Re-800 detected breast cancer microcalcifications. In SPECT/CT imaging, Pam-Tc-800 showed not only accumulation in normal bone but also highly sensitive detection of breast cancer microcalcifications. In biodistribution experiments, the total body clearance of Pam-Tc-800 at 4 hours after injection was comparable to that of -MDP. Moreover, Pam-Tc-800 showed a higher uptake in bone and tumor than -MDP. These results indicated that the novel trifunctional agent could provide simultaneous imaging by SPECT and NIR fluorescence. Dual-modality imaging may compensate for the drawbacks of the other modalities.

8. Summary

In this paper, several well-designed bone-seeking compounds were reviewed. They are chemically well-characterized and different from -MDP and -HMDP. Some demonstrated superior biodistribution characteristics. The mechanism by which all the compounds (except the carbon-11 labeled cathepsin K inhibitors) accumulate in bone is derived due to a high affinity for HA. We estimate that ⁶⁸Ga-DOTA-conjugated bisphosphonate compounds, such as ⁶⁸Ga-BPAMD and ⁶⁸Ga-DOTA-Bn-SCN-HBP, are the most promising diagnostic agents for bone metastases because they show superior biodistribution characteristic and ⁶⁸Ga is a useful PET radionuclide in clinical. Moreover, as DOTA ligand could form a complex with not only ^67/68Ga but also ¹⁷⁷Lu and ⁹⁰Y, the palliation therapy is applicable using the same ligand. Namely, this system is “theranostics”, which is a combination of diagnosis and therapy.

Thus, the information from imaging data and the type of bone metastasis susceptible to treatment should be similar to those for existing bone-seeking radiopharmaceuticals. We hope that novel bone-seeking compounds that possess a different accumulation mechanism will be developed in the near future.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

References

G. Subramanian, J. G. McAfee, R. J. Blair, F. A. Kallfelz, and F. D. Thomas, “Technetium 99m methylene diphosphonate: a superior agent for skeletal imaging: comparison with other technetium complexes,” Journal of Nuclear Medicine, vol. 16, no. 8, pp. 744–755, 1975.
View at: Google Scholar
P. A. Domstad, J. J. Coupal, and E. E. Kim, “^99mTc-hydroxymethane diphosphonate: a new bone imaging agent with a low tin content,” Radiology, vol. 136, no. 1, pp. 209–211, 1980.
View at: Publisher Site | Google Scholar
C. Love, A. S. Din, M. B. Tomas, T. P. Kalapparambath, and C. J. Palestro, “Radionuclide bone imaging: an illustrative review,” Radiographics, vol. 23, no. 2, pp. 341–358, 2003.
View at: Publisher Site | Google Scholar
C. Mari, A. Catafau, and I. Carrio, “Bone scintigraphy and metabolic disorders,” Quarterly Journal of Nuclear Medicine, vol. 43, no. 3, pp. 259–267, 1999.
View at: Google Scholar
J. Davis, N. D. Cook, and R. J. Pither, “Biologic mechanisms of ⁸⁹SrCl₂ incorporation into type I collagen during bone mineralization,” Journal of Nuclear Medicine, vol. 41, no. 1, pp. 183–188, 2000.
View at: Google Scholar
F. M. Paes and A. N. Serafini, “Systemic metabolic radiopharmaceutical therapy in the treatment of metastatic bone pain,” Seminars in Nuclear Medicine, vol. 40, no. 2, pp. 89–104, 2010.
View at: Publisher Site | Google Scholar
F. M. Paes, V. Ernani, P. Hosein, and A. N. Serafini, “Radiopharmaceuticals: when and how to use them to treat metastatic bone pain,” Journal of Supportive Oncology, vol. 9, no. 6, pp. 197–205, 2011.
View at: Publisher Site | Google Scholar
K. Liepe and J. Kotzerke, “Internal radiotherapy of painful bone metastases,” Methods, vol. 55, no. 3, pp. 258–270, 2011.
View at: Publisher Site | Google Scholar
K. Ogawa and K. Washiyama, “Bone target radiotracers for palliative therapy of bone metastases,” Current Medicinal Chemistry, vol. 19, no. 20, pp. 3290–3300, 2012.
View at: Publisher Site | Google Scholar
J. F. Eary, C. Collins, M. Stabin et al., “Samarium-153-EDTMP biodistribution and dosimetry estimation,” Journal of Nuclear Medicine, vol. 34, no. 7, pp. 1031–1036, 1993.
View at: Google Scholar
L. Bianchi, A. Baroli, L. Marzoli, C. Verusio, C. Chiesa, and L. Pozzi, “Prospective dosimetry with ^99mTc-MDP in metabolic radiotherapy of bone metastases with ¹⁵³Sm-EDTMP,” European Journal of Nuclear Medicine and Molecular Imaging, vol. 36, no. 1, pp. 122–129, 2009.
View at: Publisher Site | Google Scholar
G. J. Dickie and D. Macfarlane, “Strontium and samarium therapy for bone metastases from prostate carcinoma,” Australasian Radiology, vol. 43, no. 4, pp. 476–479, 1999.
View at: Publisher Site | Google Scholar
M. G. E. H. Lam, A. Dahmane, W. H. M. Stevens, P. P. Van Rijk, J. M. H. De Klerk, and B. A. Zonnenberg, “Combined use of zoledronic acid and ¹⁵³Sm-EDTMP in hormone-refractory prostate cancer patients with bone metastases,” European Journal of Nuclear Medicine and Molecular Imaging, vol. 35, no. 4, pp. 756–765, 2008.
View at: Publisher Site | Google Scholar
C. Parker, S. Nilsson D Heinrich, S. I. Helle et al., “Alpha emitter radium-223 and survival in metastatic prostate cancer,” The New England Journal of Medicine, vol. 369, no. 3, pp. 213–223, 2013.
View at: Publisher Site | Google Scholar
O. Sartor, R. Coleman, S. Nilsson et al., “Effect of radium-223 dichloride on symptomatic skeletal events in patients with castration-resistant prostate cancer and bone metastases: results from a phase 3, double-blind, randomised trial,” The Lancet Oncology, vol. 15, no. 7, pp. 738–746, 2014.
View at: Publisher Site | Google Scholar
S. J. P. Tanabe Zodda, E. Deutsch, and W. R. Heineman, “Effect of pH on the formation of Tc(NaBH₄)-MDP radiopharmaceutical analogues,” International Journal of Applied Radiation and Isotopes, vol. 34, no. 12, pp. 1577–1584, 1983.
View at: Publisher Site | Google Scholar
K. Libson, E. Deutsch, and B. L. Barnett, “Structural characterization of a ⁹⁹Tc-diphosphonate complex. Implications for the chemistry of ^99mTc skeletal imaging agents,” Journal of the American Chemical Society, vol. 102, no. 7, pp. 2476–2478, 1980.
View at: Publisher Site | Google Scholar
K. Verbeke, J. Rozenski, B. Cleynhens et al., “Development of a conjugate of ^99mTc-EC with aminomethylenediphosphonate in the search for a bone tracer with fast clearance from soft tissue,” Bioconjugate Chemistry, vol. 13, no. 1, pp. 16–22, 2002.
View at: Publisher Site | Google Scholar
K. Ogawa, T. Mukai, Y. Inoue, M. Ono, and H. Saji, “Development of a novel ^99mTc-chelate-conjugated bisphosphonate with high affinity for bone as a bone scintigraphic agent,” Journal of Nuclear Medicine, vol. 47, no. 12, pp. 2042–2047, 2006.
View at: Google Scholar
L. Liu, G. Zhong, Y. Wei, M. Zhang, and X. Wang, “Synthesis and biological evaluation of a novel ^99mTc complex of HYNIC-conjugated aminomethylenediphosphonate as a potential bone imaging agent,” Journal of Radioanalytical and Nuclear Chemistry, vol. 288, no. 2, pp. 467–473, 2011.
View at: Publisher Site | Google Scholar
S. Liu, D. S. Edwards, R. J. Looby et al., “Labeling a hydrazino nicotinamide-modified cyclic IIb/IIIa receptor antagonist with ^99mTc using aminocarboxylates as coligands,” Bioconjugate Chemistry, vol. 7, no. 1, pp. 63–71, 1996.
View at: Publisher Site | Google Scholar
M. Ono, Y. Arano, T. Uehara et al., “Intracellular metabolic fate of radioactivity after injection of technetium-99m-labeled hydrazino nicotinamide derivatized proteins,” Bioconjugate Chemistry, vol. 10, no. 3, pp. 386–394, 1999.
View at: Publisher Site | Google Scholar
M. Ono, Y. Arano, T. Mukai et al., “Plasma protein binding of ^99mTc-labeled hydrazino nicotinamide derivatized polypeptides and peptides,” Nuclear Medicine and Biology, vol. 28, no. 2, pp. 155–164, 2001.
View at: Publisher Site | Google Scholar
E. Palma, B. L. Oliveira, J. D. G. Correia, L. Gano, L. Maria, and I. C. Santos, “A new bisphosphonate-containing ^99mTc(I) tricarbonyl complex potentially useful as bone-seeking agent: synthesis and biological evaluation,” Journal of Biological Inorganic Chemistry, vol. 12, no. 5, pp. 667–679, 2007.
View at: Publisher Site | Google Scholar
E. Palma, J. D. G. Correia, B. L. Oliveira, L. Gano, and I. C. Santos, “^99mTc(CO)₃-labeled pamidronate and alendronate for bone imaging,” Dalton Transactions, vol. 40, no. 12, pp. 2787–2796, 2011.
View at: Publisher Site | Google Scholar
E. Van Beek, M. Hoekstra, M. van de Ruit, C. Löwik, and S. Papapoulos, “Structural requirements for bisphosphonate actions in vitro,” Journal of Bone and Mineral Research, vol. 9, no. 12, pp. 1875–1882, 1994.
View at: Publisher Site | Google Scholar
J. L. Meyer and G. H. Nancollas, “The influence of multidentate organic phosphonates on the crystal growth of hydroxyapatite,” Calcified Tissue Research, vol. 13, no. 1, pp. 295–303, 1973.
View at: Publisher Site | Google Scholar
K. Ogawa, T. Mukai, Y. Arano et al., “Rhemium-186-monoaminemonoamidedithiol-conjugated bisphosphonate derivatives for bone pain palliation,” Nuclear Medicine and Biology, vol. 33, no. 4, pp. 513–520, 2006.
View at: Publisher Site | Google Scholar
R. T. M. De Rosales, C. Finucane, S. J. Mather, and P. J. Blower, “Bifunctional bisphosphonate complexes for the diagnosis and therapy of bone metastases,” Chemical Communications, no. 32, pp. 4847–4849, 2009.
View at: Publisher Site | Google Scholar
K. P. Zhernosekov, D. V. Filosofov, R. P. Baum et al., “Processing of generator-produced ⁶⁸Ga for medical application,” Journal of Nuclear Medicine, vol. 48, no. 10, pp. 1741–1748, 2007.
View at: Publisher Site | Google Scholar
M. Fellner, R. P. Baum, V. Kubíček et al., “PET/CT imaging of osteoblastic bone metastases with ⁶⁸Ga-bisphosphonates: first human study,” European Journal of Nuclear Medicine and Molecular Imaging, vol. 37, no. 4, article 834, 2010.
View at: Publisher Site | Google Scholar
M. Fellner, B. Biesalski, N. Bausbacher et al., “⁶⁸Ga-BPAMD: PET-imaging of bone metastases with a generator based positron emitter,” Nuclear Medicine and Biology, vol. 39, no. 7, pp. 993–999, 2012.
View at: Publisher Site | Google Scholar
Ø. S. Bruland, S. Nilsson, D. R. Fisher, and R. H. Larsen, “High-linear energy transfer irradiation targeted to skeletal metastases by the α-emitter ²²³Ra: adjuvant or alternative to conventional modalities?” Clinical Cancer Research, vol. 12, no. 20, part 2, pp. 6250s–6257s, 2006.
View at: Publisher Site | Google Scholar
K. Ogawa, K. Takai, H. Kanbara et al., “Preparation and evaluation of a radiogallium complex-conjugated bisphosphonate as a bone scintigraphy agent,” Nuclear Medicine and Biology, vol. 38, no. 5, pp. 631–636, 2011.
View at: Publisher Site | Google Scholar
K. Suzuki, M. Satake, J. Suwada et al., “Synthesis and evaluation of a novel ⁶⁸Ga-chelate-conjugated bisphosphonate as a bone-seeking agent for PET imaging,” Nuclear Medicine and Biology, vol. 38, no. 7, pp. 1011–1018, 2011.
View at: Publisher Site | Google Scholar
G. Ferro-Flores and C. Arteaga de Murphy, “Pharmacokinetics and dosimetry of ¹⁸⁸Re-pharmaceuticals,” Advanced Drug Delivery Reviews, vol. 60, no. 12, pp. 1389–1401, 2008.
View at: Publisher Site | Google Scholar
F. F. Knapp Jr., A. L. Beets, S. Guhlke et al., “Availability of rhenium-188 from the alumina-based tungsten-188/Rhenium-188 generator for preparation of rhenium-188-labeled radiopharmaceuticals for cancer treatment,” Anticancer Research, vol. 17, no. 3, pp. 1783–1795, 1997.
View at: Google Scholar
K. Liepe, R. Runge, and J. Kotzerke, “The benefit of bone-seeking radiopharmaceuticals in the treatment of metastatic bone pain,” Journal of Cancer Research and Clinical Oncology, vol. 131, no. 1, pp. 60–66, 2005.
View at: Publisher Site | Google Scholar
I. G. Finlay, M. D. Mason, and M. Shelley, “Radioisotopes for the palliation of metastatic bone cancer: a systematic review,” The Lancet Oncology, vol. 6, no. 6, pp. 392–400, 2005.
View at: Publisher Site | Google Scholar
E. Deutsch, K. Libson, J. L. Vanderheyden, A. R. Ketring, and H. R. Maxon, “The chemistry of rhenium and technetium as related to the use of isotopes of these elements in therapeutic and diagnostic nuclear medicine,” Nuclear Medicine and Biology, vol. 13, no. 4, pp. 465–477, 1986.
View at: Google Scholar
Y. Arano, M. Ono, K. Wakisaka et al., “Synthesis and biodistribution studies of ¹⁸⁶Re complex of 1-hydroxyethylidene-1,1-diphosphonate for treatment of painful osseous metastases,” Radioisotopes, vol. 44, no. 8, pp. 514–522, 1995.
View at: Google Scholar
F. De Winter, B. Brans, C. Van De Wiele, and R. A. Dierckx, “Visualization of the stomach on rhenium-186 HEDP imaging after therapy for metastasized prostate carcinoma,” Clinical Nuclear Medicine, vol. 24, no. 11, pp. 898–899, 1999.
View at: Publisher Site | Google Scholar
G. S. Limouris, S. K. Shukla, A. Condi-Paphiti et al., “Palliative therapy using rhenium-186-HEDP in painful breast osseous metastases,” Anticancer Research, vol. 17, no. 3B, pp. 1767–1772, 1997.
View at: Google Scholar
W.-Y. Lin, J.-F. Hsieh, S.-C. Tsai, T.-C. Yen, S. J. Wang, and F. F. Knapp Jr., “A comprehensive study on the blockage of thyroid and gastric uptakes of ¹⁸⁸Re-perrhenate in endovascular irradiation using liquid-filled balloon to prevent restenosis,” Nuclear Medicine and Biology, vol. 27, no. 1, pp. 83–87, 2000.
View at: Publisher Site | Google Scholar
K. Ogawa, T. Mukai, Y. Arano et al., “Design of a radiopharmaceutical for the palliation of painful bone metastases: rhenium-186-labeled bisphosphonate derivative,” Journal of Labelled Compounds and Radiopharmaceuticals, vol. 47, no. 11, pp. 753–761, 2004.
View at: Publisher Site | Google Scholar
K. Ogawa, T. Mukai, Y. Arano et al., “Development of a rhenium-186-labeled MAG3-conjugated bisphosphonate for the palliation of metastatic bone pain based on the concept of bifunctional radiopharmaceuticals,” Bioconjugate Chemistry, vol. 16, no. 4, pp. 751–757, 2005.
View at: Publisher Site | Google Scholar
K. Ogawa, T. Mukai, D. Asano et al., “Therapeutic effects of a ¹⁸⁶Re-complex-conjugated bisphosphonate for the palliation of metastatic bone pain in an animal model,” Journal of Nuclear Medicine, vol. 48, no. 1, pp. 122–127, 2007.
View at: Google Scholar
T. Uehara, Z. L. Jin, K. Ogawa et al., “Assessment of ¹⁸⁶Re chelate-conjugated bisphosphonate for the development of new radiopharmaceuticals for bones,” Nuclear Medicine and Biology, vol. 34, no. 1, pp. 79–87, 2007.
View at: Publisher Site | Google Scholar
R. T. M. de Rosales, C. Finucane, J. Foster, S. J. Mather, and P. J. Blower, “¹⁸⁸Re(CO)₃-dipicolylamine-alendronate: a new bisphosphonate conjugate for the radiotherapy of bone metastases,” Bioconjugate Chemistry, vol. 21, no. 5, pp. 811–815, 2010.
View at: Publisher Site | Google Scholar
W. T. Butler, “The nature and significance of osteopontin,” Connective Tissue Research, vol. 23, no. 2-3, pp. 123–136, 1989.
View at: Publisher Site | Google Scholar
A. Oldberg, A. Franzen, and D. Heinegard, “The primary structure of a cell-binding bone sialoprotein,” The Journal of Biological Chemistry, vol. 263, no. 36, pp. 19430–19432, 1988.
View at: Google Scholar
A. Oldberg, A. Franzen, and D. Heinegard, “Cloning and sequence analysis of rat bone sialoprotein (osteopontin) cDNA reveals an Arg-Gly-Asp cell-binding sequence,” Proceedings of the National Academy of Sciences of the United States of America, vol. 83, no. 23, pp. 8819–8823, 1986.
View at: Publisher Site | Google Scholar
S. Kasugai, R. Fujisawa, Y. Waki, K.-I. Miyamoto, and K. Ohya, “Selective drug delivery system to bone: Small peptide (Asp)₆ conjugation,” Journal of Bone and Mineral Research, vol. 15, no. 5, pp. 936–943, 2000.
View at: Google Scholar
K. Yokogawa, K. Miya, T. Sekido et al., “Selective delivery of estradiol to bone by aspartic acid oligopeptide and its effects on ovariectomized mice,” Endocrinology, vol. 142, no. 3, pp. 1228–1233, 2001.
View at: Publisher Site | Google Scholar
D. Wang, S. Miller, M. Sima, P. Kopečková, and J. Kopeček, “Synthesis and evaluation of water-soluble polymeric bone-targeted drug delivery systems,” Bioconjugate Chemistry, vol. 14, no. 5, pp. 853–859, 2003.
View at: Publisher Site | Google Scholar
M. Yanagi, T. Uehara, Y. Uchida et al., “Chemical design of ^99mTc-labeled probes for targeting osteogenic bone region,” Bioconjugate Chemistry, vol. 24, no. 7, pp. 1248–1255, 2013.
View at: Publisher Site | Google Scholar
K. Ogawa, A. Ishizaki, K. Takai et al., “Development of novel radiogallium-labeled bone imaging agents using oligo-aspartic acid peptides as carriers,” PLoS ONE, vol. 8, no. 12, Article ID e84335, 2013.
View at: Publisher Site | Google Scholar
M. Novinec and B. Lenarčic, “Cathepsin K: a unique collagenolytic cysteine peptidase,” Biological Chemistry, vol. 394, no. 9, pp. 1163–1179, 2013.
View at: Publisher Site | Google Scholar
L. Troeberg and H. Nagase, “Proteases involved in cartilage matrix degradation in osteoarthritis,” Biochimica et Biophysica Acta: Proteins and Proteomics, vol. 1824, no. 1, pp. 133–145, 2012.
View at: Publisher Site | Google Scholar
K. M. Kozloff, L. Quinti, S. Patntirapong et al., “Non-invasive optical detection of cathepsin K-mediated fluorescence reveals osteoclast activity in vitro and in vivo,” Bone, vol. 44, no. 2, pp. 190–198, 2009.
View at: Publisher Site | Google Scholar
M. E. Rodnick, X. Shao, K. M. Kozloff, P. J. H. Scott, and M. R. Kilbourn, “Carbon-11 labeled cathepsin K inhibitors: syntheses and preliminary in vivo evaluation,” Nuclear Medicine and Biology, vol. 41, no. 5, pp. 384–389, 2014.
View at: Publisher Site | Google Scholar
E. Altmann, R. Aichholz, C. Betschart et al., “2-Cyano-pyrimidines: a new chemotype for inhibitors of the cysteine protease cathepsin K,” Journal of Medicinal Chemistry, vol. 50, no. 4, pp. 591–594, 2007.
View at: Publisher Site | Google Scholar
R. Weissleder and U. Mahmood, “Molecular imaging,” Radiology, vol. 219, no. 2, pp. 316–333, 2001.
View at: Publisher Site | Google Scholar
V. Ntziachristos, C. Bremer, and R. Weissleder, “Fluorescence imaging with near-infrared light: new technological advances that enable in vivo molecular imaging,” European Radiology, vol. 13, no. 1, pp. 195–208, 2003.
View at: Google Scholar
A. Zaheer, R. E. Lenkinski, A. Mahmood, A. G. Jones, L. C. Cantley, and J. V. Frangioni, “In vivo near-infrared fluorescence imaging of osteoblastic activity,” Nature Biotechnology, vol. 19, no. 12, pp. 1148–1154, 2001.
View at: Publisher Site | Google Scholar
K. R. Bhushan, P. Misra, F. Liu, S. Mathur, R. E. Lenkinski, and J. V. Frangioni, “Detection of breast cancer microcalcifications using a dual-modality SPECT/ NIR fluorescent probe,” Journal of the American Chemical Society, vol. 130, no. 52, pp. 17648–17649, 2008.
View at: Publisher Site | Google Scholar
N. Pandit-Taskar, S. M. Larson, and J. A. Carrasquillo, “Bone-seeking radiopharmaceuticals for treatment of osseous metastases, part 1: alpha therapy with ²²³Ra-dichloride,” Journal of Nuclear Medicine, vol. 55, no. 2, pp. 268–274, 2014.
View at: Publisher Site | Google Scholar

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Copyright © 2015 Kazuma Ogawa and Atsushi Ishizaki. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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