Synthesis and Biological Evaluation of 99mTc-DTPA-bis(His) as a Potential Probe for Tumor Imaging with SPECT

Technetium-99m chelator-free radiolabeling of specific glutamine tumor imaging nanoprobe: in vitro and in vivo evaluations

Introduction

Nowadays, molecular imaging radiopharmaceuticals', nanoparticles', and/or small-molecule biomarkers' applications are increasing rapidly worldwide. Thus, researchers focus on providing the novel, safe, and cost-effective ones.

Materials and methods

In the present experiment, technetium-99m (^99mTc)-labeled PEG-citrate dendrimer-G₂ conjugated with glutamine (nanoconjugate) was designed and assessed as a novel tumor imaging probe both in vitro and in vivo. Nanoconjugate was synthesized and the synthesis was confirmed by Fourier transform infrared, proton nuclear magnetic resonance, liquid chromatography-mass spectrometry, dynamic light scattering, and static light scattering techniques. The toxicity was assessed by XTT and apoptosis and necrosis methods.

Results

Radiochemical purity indicates that the anionic dendrimer has a very high potential to complex formation with ^99mTc and is also very stable in the human serum in different times. Results from the imaging procedures showed potential ability of nanoconjugates to detect tumor site.

Conclusion

Suitable features of the anionic dendrimer show that it is a promising agent to improve nanoradiopharmaceuticals.

Ultrasound molecular imaging of arterial thrombi with novel microbubbles modified by cyclic RGD in vitro and in vivo

Despite immense potential, ultrasound molecular imaging (UMI) of arterial thrombi remains very challenging because the high-shear arterial flow limits binding of site-targeted microbubbles to the thrombi. The linear Arg-Gly-Asp (RGD) peptides have been successfully applied to evaluate venous, atrial, and arteriolar thrombi, but have thus far failed in the detection of arterial thrombi. Cyclic RGD (Arg-Gly-Asp-D-Phe-Cys) is a cyclic conformation of linear RGD peptides, which has much higher binding-affinity and selectivity for binding to the glycoprotein (GP) IIb/IIIa receptor than its linear counterpart and thus is likely to be an optimal targeted molecular probe for ultrasound molecular imaging of arterial thrombi. In this study, we sought to assess the feasibility of a novel microbubble conjugated with cyclic RGD (Mb-cyclic RGD) in UMI of arterial thrombi in vitro and in vivo. As expected, Mb-cyclic RGD had greater GP IIb/IIIa-targeted binding capability in all shear stress conditions. In addition, the shear stress at half-maximal detachment of Mb-cyclic RGD was 5.7-fold higher than that of microbubbles with nonspecific peptide (Mb-CON) (p<0.05). Mb-cyclic RGD enhanced the echogenicity of the platelet-rich thrombus in vitro whereas Mb-CON did not produce enhancement. In the in vivo setting, optimal signal enhancement of the abdominal aortic thrombus was displayed with Mb-cyclic RGD in all cases. Mean video intensity of the abdominal aortic thrombi with Mb-cyclic RGD was 3.2-fold higher than that with Mb-CON (p<0.05). The novel Mb-cyclic RGD facilitated excellent visualisation of arterial thrombi using UMI and showed great promise for clinical applications.

Magnetic resonance imaging of osteosarcoma using a bis(alendronate)-based bone-targeted contrast agent

Magnetic resonance (MR) is currently used for diagnosis of osteosarcoma but not well even though contrast agents are administered. Here, we report a novel bone-targeted MR imaging contrast agent, Gd₂-diethylenetriaminepentaacetate-bis(alendronate) (Gd₂-DTPA-BA) for the diagnosis of osteosarcoma. It is the conjugate of a bone cell-seeking molecule (i.e., alendronate) and an MR imaging contrast agent (i.e., Gd-DTPA). Its physicochemical parameters were measured, including pK_a, complex constant, and T₁ relaxivity. Its bone cell-seeking ability was evaluated by measuring its adsorption on hydroxyapatite. Hemolysis was investigated. MR imaging and biodistribution of Gd₂-DTPA-BA and Gd-DTPA were studied on healthy and osteosarcoma-bearing nude mice. Gd₂-DTPA-BA showed high adsorption on hydroxyapatite, the high MR relaxivity (r₁) of 7.613mM^-1s^-1 (2.6 folds of Gd-DTPA), and no hemolysis. The MR contrast effect of Gd₂-DTPA-BA was much higher than that of Gd-DTPA after intravenous injection to the mice. More importantly, the MR imaging of osteosarcoma was significantly improved by Gd₂-DTPA-BA. The signal intensity of Gd₂-DTPA-BA reached 120.3% at 50min, equal to three folds of Gd-DTPA. The bone targeting index (bone/blood) of Gd₂-DTPA-BA in the osteosarcoma-bearing mice was very high to 130 at 180min. Furthermore, the contrast enhancement could also be found in the lung due to metastasis of osteosarcoma. Gd₂-DTPA-BA plays a promising role in the diagnoses of osteosacomas, including the primary bone tumors and metastases.

Synthesis, in vitro and in vivo small-animal SPECT evaluation of novel technetium labeled bile acid analogues to study (altered) hepatic transporter function

INTRODUCTION

Hepatobiliary transport mechanisms are crucial for the excretion of substrate toxic compounds. Drugs can inhibit these transporters, which can lead to drug-drug interactions causing toxicity. Therefore, it is important to assess this early during the development of new drug candidates. The aim of the current study is the (radio)synthesis, in vitro and in vivo evaluation of a technetium labeled chenodeoxycholic and cholic acid analogue: [(99m)Tc]-DTPA-CDCA and [(99m)]Tc-DTPA-CA, respectively, as biomarker for disturbed transporter functionality.

METHODS

[99mTc]-DTPA-CDCA([(99m)Tc]-3a) and [99mTc]-DTPA-CA ([(99m)Tc]-3b) were synthesized and evaluated in vitro and in vivo. Uptake of both tracers was investigated in NTCP, OCT1, OATP1B1, OATP1B3 transfected cell lines. Km and Vmax values were determined and compared to [(99m)Tc]-mebrofenin ([(99m)Tc]-MEB). Efflux was investigated by means of CTRL, MRP2 and BSEP transfected inside-out vesicles. Metabolite analysis was performed using pooled human liver S9. Wild type (n=3) and rifampicin treated (n=3) mice were intravenously injected with 37MBq of tracer. After dynamic small-animal SPECT and short CT acquisitions, time-activity curves of heart, liver, gallbladder and intestines were obtained.

RESULTS

We demonstrated that OATP1B1 and OATP1B3 are the involved uptake transporters of both compounds. Both tracers show a higher affinity compared to [(99m)Tc]-MEB, but are in a similar range as endogenous bile acids for OATP1B1 and OATP1B3. [(99m)Tc]-3a shows higher affinities compared to [(99m)Tc]-3b. Vmax values were lower compared to [(99m)Tc]-MEB, but in the same range as endogenous bile acids. MRP2 was identified as efflux transporter. Less than 7% of both radiotracers was metabolized in the liver. In vitro results were confirmed by in vivo results. Uptake in the liver and efflux to gallbladder + intestines and urinary bladder of both tracers was observed. Transport was inhibited by rifampicin.

CONCLUSION

The involved transporters were identified; both tracers are taken up in the hepatocytes by OATP1B1 andOATP1B3 with Km and Vmax values in the same range as endogenous bile acids and are secreted into bile canaliculi via MRP2. Dynamic small-animal SPECT imaging can be a useful noninvasive method of visualizing and quantifying hepatobiliary transporter functionality and disturbances thereof in vivo, which could predict drug pharmacokinetics.

Comparative evaluation of glutamate-sensitive radiopharmaceuticals: Technetium-99m-glutamic acid and technetium-99m-diethylenetriaminepentaacetic acid-bis(glutamate) conjugate for tumor imaging

Single-photon emission computed tomography has become a significant imaging modality with huge potential to visualize and provide information of anatomic dysfunctions that are predictive of future diseases. This imaging tool is complimented by radiopharmaceuticals/radiosubstrates that help in imaging specific physiological aspects of the human body. The present study was undertaken to explore the utility of technetium-99m (⁹⁹(m)Tc)-labeled glutamate conjugates for tumor scintigraphy. As part of our efforts to further utilize the application of chelating agents, glutamic acid was conjugated with a multidentate ligand, diethylenetriaminepentaacetic acid (DTPA). The DTPA-glutamate conjugate [DTPA-bis(Glu)] was well characterized by IR, NMR, and mass spectroscopy. The biological activity of glutamic acid was compared with its DTPA conjugate by radiocomplexation with ⁹⁹(m)Tc (labeling efficiency ≥98%). In vivo studies of both the radiolabeled complexes ⁹⁹(m)Tc-Glu and ⁹⁹(m)Tc-DTPA-bis(Glu) were then carried out, followed by gamma scintigraphy in New Zealand albino rabbits. Improved serum stability of ⁹⁹(m)Tc-labeled DTPA conjugate indicated that ⁹⁹(m)Tc remained bound to the conjugate up to 24 hours. Blood clearance showed a relatively slow washout of the DTPA conjugate when compared with the labeled glutamate. Biodistribution characteristics of the conjugate in Balb/c mice revealed that DTPA conjugation of glutamic acid favors less accumulation in the liver and bone and rapid renal clearance. Tumor scintigraphy in mice showed increasing tumor accumulation, stable up to 4 hours. These preliminary studies show that ⁹⁹(m)Tc-DTPA-bis(Glu) can be a useful radiopharmaceutical for diagnostic applications in single-photon emission computed tomography imaging.

Kinetics of formation for lanthanide (III) complexes of DTPA-(Me-Trp)2 used as imaging agent

Diethlenetriamine-N,N,N'N''N''-pentaacetic acid (DTPA)-bis (amide) analogs have been synthesized and evaluated as a potential biomedical imaging agents. Imaging and biodistribution studies were performed in mice that showed a significant accumulation of DTPA analogs in brain. The stability and protonation constants of the complexes formed between the ligand [DTPA-(Me-Trp)(2)] and Gd(3+), Eu(3+), and Cu(2+) have been determined by pH potentiometry (Gd(3+), Eu(3+)) and spectrophotometry (Cu(2+)) at 25 °C and at constant ionic strength maintained by 0.10 M KCl. The kinetic inertness of Gd [DTPA-(Me-Trp)(2)] was characterized by the rates of exchange reactions with Zn(2+) and Eu(3+). In the Eu(3+) exchange, a second-order [H(+)] dependence was found for the pseudo-first-order rate constant [k(0) = (4.5 ± 1.2) × 10(-6)/s; k(1) = 0.58 ± 0.1 /M/s, k(2) = (6.6 ± 0.2) × 10(4) /M(2)/s, k(3) = (4.8 ± 0.8) × 10(-4) /M/s]. In the Eu(3+) exchange, at pH <5.0, the rate decreases with increasing concentration of the exchanging ion. At physiological pH, the kinetic inertness of [DTPA-(Me-Trp)(2)] is more inert than GdDTPA(2-), the most commonly used MRI contrast agent (t(1/2) = 127 h). High kinetic stability is an important requirement for the Gd complexes used as contrast enhancement agents in magnetic resonance imaging.