Cyclotron production of high specific activity ⁵⁵Co and in vivo evaluation of the stability of ⁵⁵Co metal-chelate-peptide complexes

Abstract

This work describes the production of high-specific activity ⁵⁵Co and the evaluation of the stability of ⁵⁵Co-metal-chelate-peptide complexes in vivo. ⁵⁵Co was produced via the ⁵⁸Ni(p,α)⁵⁵Co reaction and purified using anion exchange chromatography with an average recovery of 92% and an average specific activity of 1.96GBq/µmol. ⁵⁵Co-DO3A and ⁵⁵Co-NO2A peptide complexes were radiolabelled at 3.7MBq/µg and injected into HCT-116 tumor xenografted mice. PET imaging and biodistribution studies were performed at 24 and 48 hours post injection and compared with that of ⁵⁵CoCl₂. Both ⁵⁵Co-metal-chelate complexes demonstrated good in vivo stability by reducing the radiotracers’ uptake in the liver by 6-fold at 24 with ~1% ID/g and at 48 hours with ~0.5% ID/g, and reducing uptake in the heart by 4-fold at 24 hours with ~0.7% ID/g and 7-fold at 48 hours with ~0.35% ID/g. These results support the use of ⁵⁵Co as a promising new radiotracer for Positron Emission Tomography (PET) imaging of cancer and other diseases.

Introduction

Positron emission tomography (PET) is a common imaging modality in nuclear medicine. Clinical interest in positron emitting metals has increased due to their longer half-lives, which are more suitable for radiolabeling macromolecules like antibodies, peptides, and nanoparticles over traditional PET isotopes such as ¹⁸F, ¹¹C, and ¹⁵O (1, 2). Currently the most common radiometals used in PET imaging are ⁶⁴Cu, ⁶⁸Ga, ⁸⁹Zr, and ⁸⁶Y with ⁶⁴Cu, ⁶⁸Ga, and ⁸⁹Zr being used in clinical trials (3–5). The chemistry of each metal is different and chelates need to be optimized for the radiometal of interest that will provide stable metal-chelate complexes in vivo.

⁵⁵Co is another isotope of interest for PET imaging. It has a half-life of 17.5 hr, a positron branching ratio of 77%, and an average positron energy of 570 keV; qualities that make it well suited for imaging with peptides, small molecules, and antibodies (6–11). ⁵⁵CoCl₂ has previously been used clinically to image ischemia in stroke patients (12–15) and in late-onset epileptic seizures (16) due to its ability to mimic calcium uptake. However, it is important to study the preclinical pharmacological properties of this isotope when it will be incorporated into targeting ligands as imaging agents to probe other diseases. Thus, we investigated the biodistribution of free ⁵⁵CoCl₂ along with the stability of ⁵⁵Co-chelate-peptide complexes in vivo.

⁵⁵Co can be produced via several nuclear reactions such as ⁵⁸Ni(p,α)⁵⁵Co (17–20), ⁵⁶Fe(p,2n)⁵⁵Co (21, 22), and ⁵⁴Fe(d,n)⁵⁵Co (23, 24). The ⁵⁴Fe(d,n)⁵⁵Co has the highest measured cross section at low energies (8, 25), however ⁵⁴Fe has a low natural abundance and thus this target may be cost-prohibitive for routine production in most laboratories. The ⁵⁶Fe(p,2n)⁵⁵Co reaction also creates undesirable ⁵⁶Co (via ⁵⁶Fe(p,n)⁵⁶Co), a positron emitting isotope with a half-life of 77d which is chemically inseparable from ⁵⁵Co. Additionally, the decay scheme for this isotope yields many high energy photons. The proton reaction on ⁵⁸Ni has a higher cross section at lower energies than the proton reaction on ⁵⁶Fe (19, 20, 22) making it more desirable for low-energy (15 MeV) cyclotrons. This method also produces a small amount of an inseparable side product, ⁵⁷Co (t_1/2=271.8d) at higher energies with a Q-value of 8.17 MeV. ⁵⁷Co decays 100% by electron capture and has a low-energy gamma ray of 122 keV. In addition to ⁵⁷Co, this method also produces another side product ⁵⁷Ni (t_1/2=35.6h), which can be used as a way to monitor the separation of ⁵⁵Co from the starting nickel material by tracking the characteristic gamma rays via gamma spectroscopy.

The production of ⁵⁵Co using ^natNi, and ⁵⁸Ni have been previously reported (11, 18, 26, 27). The specific activity of ⁵⁵Co has been investigated using both ion chromatography of productions using ⁵⁸Ni (26) and via DOTA and NOTA titration of productions using ^natNi (27). High effective specific activity (ESA) of radiometals is important as metal contaminants have negative impacts on radiolabelling (28, 29). For best radiolabelling results using ⁵⁵Co, ESA measurements must be performed and productions optimized so that high-specific activity material can be obtained.

In this work, we discuss the production of ⁵⁵Co via the ⁵⁸Ni(p,α)⁵⁵Co reaction and report its ESA using both 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) titrations and Ion Chromatography (28). We also report the in vivo biodistribution ⁵⁵CoCl₂ and compare it with its ⁵⁵Co-chelate complexes linked to a peptide up to 48 hrs. The macrocyclic chelates, DOTA and 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), were chosen because they meet the coordination chemistry required to bind ⁵⁵Co. DOTA and NOTA are commonly attached to peptides via one of their carboxylic acid arms, resulting in DO3A- and NO2A-peptide conjugates, respectively (30, 31). As a model system, we applied ⁵⁵Co to a peptide ligand, L19K-FDNB, which has been shown to have a long blood clearance time (32). This property makes this peptide an optimal system for studying the stability of ⁵⁵Co-chelate complexes as ⁵⁵Co-DO3A-L19K-FDNB and ⁵⁵Co-NO2A-L19K-FDNB in vivo at time points up to 48 hrs. The biodistribution data for ⁵⁵CoCl₂, ⁵⁵Co-NO2A-L19K-FDNB, and ⁵⁵Co-DO3A-L19K-FDNB were compared at 24 and 48 hrs post injection to establish the in vivo stability of ⁵⁵Co chelated with NO2A- and DO3A-peptide conjugates in tumor-bearing mice.

Materials and Methods

Materials

Trace metals grade reagents were purchased from Sigma-Aldrich (USA) and used without purification and Milli-Q deionized water (18 MΩ cm⁻¹) was used for all dilutions unless stated otherwise. All glassware and vials were acid washed in 8M HNO₃ for 24 hrs prior to use. DOTA was purchased from Macrocyclics (USA). Two versions of the peptide L19K were synthesized by CPC Scientific consisting of the sequence DO3A- or NO2A-PEG_4-GGNECDIARMWEWECFERK-CONH₂, with a Cys-Cys disulfide bridge and polyethylene glycol (PEG) as a spacer between peptide and chelate. ⁵⁸Ni was purchased from Isoflex (USA) with 99.48% isotopic enrichment.

Targetry and Irradiation

45–55 mg of ⁵⁸Ni in powder form was plated onto a gold disk by electrodeposition as previously described by both McCarthy et al. (33) and Szelecsenyi et al. (34) The electroplating cell was 9 cm in height with an inner diameter of 1.8 cm. The bottom of the cell consisted of a teflon base that connected to the gold disk exposing a 5 mm circle. A graphite rod was used as the cathode and stirred the solution slowly as a voltage of 2.5 V was applied for ~12 hrs. The current remained between 8–20 mA throughout the process. Targets were irradiated on a 15 MeV cyclotron (CS-15) for 20–60 µAhrs and were able to withstand currents up to 30µA. Targets were allowed to sit for two hours prior to processing to allow short-lived contaminants to decay.

Purification

For processing, targets were placed in 10 mL of 9 M HCl and heated with reflux for approximately one hour in order to dissolve the nickel from the gold disk. Once the solution cooled it was placed in a 1 cm × 10 cm glass column (Biorad, USA) with 2.5 g AG1-X8 resin (Biorad, USA). In order to determine separation conditions, the eluate along with 10–40 mL of 9 M HCl were collected followed by another 10 mL of 0.5 M HCl to elute the ⁵⁵Co. Fractions of 1 mL were collected and analyzed using an High Purity Germanium (HPGe) detector (Canberra, USA) and the final ⁵⁵Co fractions were evaporated to dryness and reconstituted with 20 µL Milli-Q water. ⁵⁵Co productions were analyzed using Ion Chromatography (28) for transition metal contamination.

Effective Specific Activity

DOTA titrations were performed in order to determine the effective specific activity of ⁵⁵Co productions and the method was adapted from the TETA (1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid) titration method reported previously by McCarthy et al. (33) 1.2 MBq (5 µL) of ⁵⁵Co was added to eight different amounts of DOTA in ammonium acetate pH 5.5 ranging from 4.3 × 10⁻⁴ µmol to 6.3 × 10⁻² µmol. Final volume was brought to 50 µL using 0.5 M ammonium acetate buffer pH 5.5. The solutions were placed in an agitating incubator at 37 °C for 30 min. Solutions were cooled to room temperature and then centrifuged. A 1 µL aliquot from each DOTA concentration along with 1 µL of unbound ⁵⁵Co, for use as a control, were spotted separately onto a silica plate for Thin Layer Chromatography (TLC) using a 1:1 mixture of 10% w/v ammonium acetate and methanol as the eluent. Plates were analyzed using a Radio TLC Plate Reader (Bioscan, USA) and analyzed for the percent ⁵⁵Co incorporated into DOTA. Data was plotted as the molar concentration of DOTA vs. % ⁵⁵Co incorporation. The curve was fit using a sigmoid plot fit program in Prism (Graphpad, USA). The EC50 value, concentration of ⁵⁵Co that bound to 50% of the DOTA molecules, was determined from this fit. The ESA was calculated as two times the EC50 value.

Animal Models

All animal care was performed as stated in the Guide for Care and Use for Laboratory Animals by the National Institutes of Health under a protocol approved by the Animal Studies Committee at Washington University in St. Louis. Female athymic Nu/Nu mice (National Cancer Institute, USA) aged 6–9 weeks were anesthetized with a ketamine/xylazine cocktail (VEDCO, USA). 100 µL of approximately 2 × 10⁷ cell/mL HCT-116 colon cancer cells suspended in saline were subcutaneously injected into the shoulder. Tumors were allowed to grow for two weeks before imaging and biodistribution studies.

Small-Animal PET/CT imaging

Prior to imaging, animals were anesthetized with 2% isoflurane. 100 µL of 74 kBq/µL ⁵⁵CoCl₂ in saline was injected into HCT-116 tumor bearing mice (n=4) via tail vein injection and imaged using an Inveon MicroPET/CT scanner (Siemens, USA) at 2, 24, and 48 hrs post injection. Static PET images were acquired for 20 minutes. PET data was reconstructed using standard methods with the maximum a posteriori probability (MAP) algorithm and co-registered with CT (computed tomography) using image display software (Inveon Research Workplace Workstation, Siemens, USA). Volumes of Interest (VOI) were drawn using CT anatomical guidelines.

Biodistributions

⁵⁵Co-NO2A-L19K-FDNB and ⁵⁵Co-DO3A-L19K-FDNB were prepared and radiolabelled similar to the ⁶⁴Cu analogues described by Marquez et al. (32) and with a final specific activity of 3.7 MBq/µg. 100 µL of a). 74 kBq/µL ⁵⁵CoCl₂, b). 37 kBq/µL ⁵⁵Co-NO2A-FDNB, or c). 37 kBq/µL ⁵⁵Co-DO3A-FDNB in saline were injected into HCT-116 tumor bearing mice. For each agent, (n=3) mice were sacrificed at 24 and 48 hrs post injection followed by removal of blood, lung, liver, spleen, kidney, muscle, fat, heart, brain, bone, tumor, stomach, small intestine (sm int), upper large intestine (u lg int), and lower large intestine (l lg int). Each organ was weighed and measured for radioactivity using a gamma counter. The radioactivity was background subtracted, decay corrected to the time of injection and reported as % ID/g (percent injected dose/g tissue).

Statistical Analysis

All data were analyzed using Graphpad Prism version 6 (Graphpad, USA) and reported as mean ± standard deviation (SD). P values were calculated using one-way ANOVA to compare more than two groups with one variable. P values with 95% confidence interval (P < 0.05) were considered significant.

Results

Production and Purification of High Specific Activity ⁵⁵Co

⁵⁸Ni was plated onto a gold disk with an average efficiency of 95 ± 3 % and thickness of ~300µm. Irradiations produced an average of 6 ± 1 MBq ⁵⁵Co/µAh which is consistent with yields predicted by Kaufman (20) and about 30% lower than yields predicted by Reimer et al. (19). ⁵⁷Ni and ⁵⁷Co were co-produced at rates of 16 ± 2 kBq/µAh and 11 ± 2 kBq/µAh respectively and were approximately 25% and 20% lower than yields predicted by Reimer et al. (19) respectively.

Due to the co-production of ⁵⁷Ni, separation could be analyzed by measuring the characteristic gamma rays of ⁵⁷Ni (E₁=1.378 MeV, I₁=81.7% and E₂=0.127 MeV, I₂=16.7%) and ⁵⁵Co (E₁=0.931 MeV, I₁=75%; E₂=0.477 MeV, I₂=20.2% and E₃=1.409 MeV, I₃=16.9%) in each fraction using an HPGe detector. Elution profiles for ⁵⁷Ni and ⁵⁵Co are shown in Figure 1. ⁵⁷Co contamination was determined by analyzing its characteristic gamma ray 0.122 MeV (85.6%) in each fraction. Washing the column with an additional 10–40 mL 9 M HCl removed nickel without significant loss of ⁵⁵Co. The average recovery of ⁵⁵Co with a 40 mL 9 M HCl column wash was 92 ± 3 %.

Figure 1.

Concentration of ⁵⁵Co and ⁵⁷Ni in each fraction, as measured using HPGe detection of the characteristic gamma rays, showing good separation.

ESA measured via DOTA titration was found to be 259 MBq/µmol DOTA when washing the column with 10 mL 9 M HCl. Increasing the column wash to 40 mL resulted in an increased ESA of 1.96 GBq/µmol DOTA (Figure 2). Ion Chromatography measured nickel concentrations to be 94.6 µmol/MBq and 757 nmol/MBq for 10 mL and 40 mL column washes respectively. The only radioactive impurity found in the final ⁵⁵Co fraction was ⁵⁷Co at 0.2 % of the total ⁵⁵Co activity.

Figure 2.

⁵⁵Co-DOTA titration curves demonstrating a seven-fold increase in ESA (259 MBq/µmol to 1.96 GBq/µmol) when washing the column with an additional 40 mL 9 M HCl acid versus a 10 mL column wash.

⁵⁵CoCl₂ Small-Animal PET/CT Imaging and Biodistribution

As an emerging radioisotope applicable in oncologic PET imaging, very little data existed about the in vivo stability of different ⁵⁵Co-chelate complexes. HCT-116 tumor xenografts were imaged (Figure 3A) and post PET biodistribution studies (Table 1) were performed at 2, 24, and 48 hrs p.i. (post injection) to investigate the distribution of free ⁵⁵CoCl₂ in this model. Interestingly, free ⁵⁵CoCl₂ was observed in the tumor at each of these time points. Tumor-to-blood ratios at 2 and 48 hrs were 0.6 ± 0.1 and 1.9 ± 0.4 respectively (P=0.006) exhibiting a 3-fold increase due to fast blood clearance and relatively slow tumor wash out (Figure 3B and Table 1). High uptake in the heart at 2 hrs p.i. could be due to the potential of Co²⁺ to mimic calcium influx (12, 15). Clearance of free ⁵⁵CoCl₂ occurred through the liver and kidney as indicated by the high uptake values at 2 hrs p.i. followed by about 2-fold consecutive decrease in ⁵⁵CoCl₂ uptake at 24 and 48 hrs p.i. (Table 1).

Figure 3.

24 and 48 hr PET images of ⁵⁵CoCl₂ showing uptake in the tumor and clearance through the liver, kidney, and intestines (A). Tumor-to-blood ratio exhibits a 3-fold increase from 2–48 hrs (B).

Table 1.

Biodistribution data for A) ⁵⁵CoCl₂ 2, 24, and 48 hrs p. i., B) ⁵⁵Co-NO2A-L19K-FDNB 24 and 48 hrs p. i., and C) ⁵⁵Co-DO3A-L19K-FDNB 24 and 48 hrs p. i.

	2 hr (%ID/g ± SD)	24 hr (%ID/g ± SD)			48 hr (%ID/g ± SD)
Organ	A	A	B	C	A	B	C
Blood	4.5 ± 0.6	1.40 ± 0.04	1.57 ± 0.09	1.4 ± 0.2	0.46 ± 0.05	0.63 ± 0.09	0.68 ± 0.04
Lung	4.0 ± 0.3	1.9 ± 0.1	0.9 ± 0.1	0.8 ± 0.1	0.98 ± 0.09	0.46 ± 0.06	0.56 ± 0.03
Liver	15.7 ± 0.4	6.2 ± 0.8	1.0 ± 0.1	1.1 ± 0.2	2.7 ± 0.7	0.44 ± 0.03	0.7 ± 0.1
Spleen	1.9 ± 0.4	0.82 ± 0.05	0.6 ± 0.1	0.56 ± 0.08	0.46 ± 0.04	0.37 ± 0.04	0.35 ± 0.03
Kidney	10.5 ± 1.4	4.4 ± 0.4	12.6 ± 4.1	16.0 ± 3.8	1.9 ± 0.2	2.3 ± 1.0	5.8 ± 1.2
Muscle	0.6 ± 0.1	0.37 ± 0.08	0.31 ± 0.04	0.32 ± 0.09	0.17 ± 0.03	0.15 ± 0.02	0.19 ± 0.04
Fat	1.5 ± 0.9	0.7 ± 0.4	0.8 ± 0.4	0.4 ± 0.1	0.23 ± 0.05	0.3 ± 0.1	0.30 ± 0.03
Heart	4.8 ± 0.4	2.8 ± 0.7	0.7 ± 0.1	0.63 ± 0.06	1.4 ± 0.2	0.33 ± 0.04	0.39 ± 0.03
Brain	0.31 ± 0.02	0.17 ± 0.03	0.08 ± 0.06	0.05 ± 0.01	0.11 ± 0.01	0.02 ± 0.01	0.02 ± 0.01
Bone	1.3 ± 0.2	0.8 ± 0.1	0.28 ± 0.04	0.30 ± 0.04	0.5 ± 0.1	0.14 ± 0.02	0.26 ± 0.05
Tumor	2.8 ± 0.6	1.6 ± 0.2	0.9 ± 0.1	1.1 ± 0.2	0.86 ± 0.08	0.49 ± 0.09	0.71 ± 0.03
Stomach	1.1 ± 0.1	1.0 ± 0.2	0.14 ± 0.05	0.22 ± 0.03	0.5 ± 0.1	0.09 ± 0.04	0.12 ± 0.03
Sm int	3.6 ± 0.6	1.1 ± 0.2	0.26 ± 0.01	0.31 ± 0.04	0.50 ± 0.04	0.14 ± 0.04	0.19 ± 0.03
U lg int	4.3 ± 0.8	1.5 ± 0.1	0.9 ± 0.1	0.8 ± 0.1	0.7 ± 0.1	0.46 ± 0.06	0.56 ± 0.03
L lg int	1.9 ± 1.0	2.2 ± 0.3	1.0 ± 0.1	1.1 ± 0.2	1.2 ± 0.2	0.44 ± 0.03	0.7 ± 0.1

Stability of ⁵⁵Co-Chelate Complexes

The stability of radiometal-chelate combinations is crucial when designing new PET imaging probes. Complexes that are not stable in vivo can lead to the radiometal decomplexing from the chelate and accumulating in different organs throughout the body, increasing background signal and dose to non-target organs. The longer blood clearance associated with the L19K-FDNB peptide which we chose as our model system to investigate ⁵⁵Co, allows for the potential decomplexation of ⁵⁵Co-chelate complexes to be measured at clinically relevant time points in order to evaluate the in vivo stability of these complexes. It has previously been established that changing the radiometal on peptides can have a drastic effect on the affinity of the peptide to its receptor (35). Using ⁵⁵Co to radiolabel the NO2A- or DO3A-peptides dramatically decreased the affinity for their tumor-associated target, VEGF, (32) as shown by their reduced tumor uptake compared with the ⁶⁴Cu-labeled analog, which was radiolabelled at the same specific activity (supplementary Figure S1).

The 24 and 48 hr biodistribution studies of ⁵⁵Co-NO2A-L19K-FDNB and ⁵⁵Co-DO3A-L19K-FDNB were compared with free ⁵⁵CoCl₂ (Table 1). These data show that both NO2A and DO3A chelates radiolabelled with ⁵⁵Co exhibit good in vivo stability, as shown by their low uptake in the liver, lung, heart, bone, stomach, and small intestine compared with the high uptake of free ⁵⁵CoCl₂ in these organs. The difference in uptake is most notable in the liver, where the ⁵⁵Co-labelled peptides had a 6-fold lower uptake than free ⁵⁵CoCl₂ at 24 and 48 hrs. ⁵⁵Co-labelled peptides had a 4-fold lower uptake than free ⁵⁵CoCl₂ in the heart at 24 hrs and a 7-fold lower uptake at 48 hrs. While the affinity of this particular peptide was negatively affected by radiolabelling with ⁵⁵Co, this study shows that DOTA and NOTA-derived chelates form stable complexes with ⁵⁵Co in vivo and may be used to investigate other probes which are insensitive to changes in radiometals.

Discussion

The ⁵⁸Ni(p,α)⁵⁵Co reaction is an effective route for producing high specific activity ⁵⁵Co. Previously this method was reported using a copper disk (18) as opposed to gold, however using copper as the target backing material could have a negative effect on the effective specific activity as copper has a high affinity to both NOTA and DOTA chelators and would compete with ⁵⁵Co for binding. The downside to this reaction is that it has a lower cross section when compared with other radiometals such as ⁶⁴Cu and ⁸⁹Zr, which may limit the availability of this isotope to facilities that have solid target cyclotron capabilities.

The biodistribution of ⁵⁵CoCl₂ is comparable to that previously observed for ⁶⁴CuCl₂, as both metals are divalent cations and may have similar interactions with transport proteins in vivo (36–40). The tumor uptake of ⁵⁵Co is interesting and warrants further investigation. It is possible that its uptake is due to the overexpression of calcium ion channels often found in cancer cells (41). Several studies have shown the uptake of ⁵⁵CoCl₂ in ischemic cells and this is believed to be due to ⁵⁵Co partially mirroring calcium influx (12, 15).

The in vivo stability of the NOTA and DOTA analogs, NO2A and DO3A, complexed with ⁵⁵Co provides the foundation for developing ⁵⁵Co labelled peptides, antibodies, nanoparticles and small molecules. ⁵⁵Co-NOTA and ⁵⁵Co-DOTA complexes have significantly lower liver uptake when compared to ⁶⁴Cu-NOTA and ⁶⁴Cu-DOTA complexes (Figure 4) (7, 42) which could be beneficial in cases where reduced background signal is desired i.e. liver metastases. The lower liver uptake observed with the ⁵⁵Co complexes implies that the transchelation problem that exists with ⁶⁴Cu (42–44) is greatly reduced with the use of ⁵⁵Co. This reduction in transchelation for the cobalt complexes in agreement with the transfer half-life of 800 hours for Co from Co(DOTATOC) in human blood serum measured by Heppler et al. (7). Additionally the high positron branching ratio (4 times that of ⁶⁴Cu and 3 times that of ⁸⁹Zr) leads to similar images with a lower amount of radioactivity administered. One drawback, however, is the higher dose to non-target tissue from the additional gamma rays present from the decay of ⁵⁵Co.

Figure 4.

Comparison of the 24 hr (A) and 48 hr (B) biodistributions of ⁵⁵CoCl₂ with ⁵⁵Co-NO2A-FDNB and ⁵⁵Co-DO3A-FDNB exhibiting the in vivo stability of ⁵⁵Co-NO2A and ⁵⁵Co-DO3A complexes.

Since the affinity of some peptides is dependent on the attached radiometal it would be interesting to examine this effect on different peptide models. The anti-VEGF peptide used in this work exhibited lower tumor uptake than previously measured with ⁶⁴Cu (Supplementary Figure S1), however in work done by Heppler et al. (7) ⁵⁵Co-DOTATOC showed higher affinity for the somatostatin type two receptor than any other radiometals measured. These different studies imply that ⁵⁵Co may also coordinate with the peptide probe concurrently with the chelate in order to elicit a change in the peptide’s affinity for its target. Therefore, determining the structure-activity relationship of these peptide-chelate-metal complexes would be significant for designing superior PET imaging probes.

Conclusions

⁵⁵Co can be made with high specific activity via the ⁵⁸Ni(p,α)⁵⁵Co reaction. The in vivo stability of ⁵⁵Co labeled NOTA and DOTA derivatives make this radioisotope a promising isotope for many different applications. Future work should compare the affinity of ⁵⁵Co labeled peptides with other radiometals to optimize the best metal-chelate-peptide combination for the application.