Evaluation of a ^99mTc-Labeled Cyclic RGD Tetramer for Noninvasive Imaging Integrin α_vβ₃-Positive Breast Cancer

Abstract

Integrin alphavbeta3 plays a critical role in tumor angiogenesis and metastasis. Radiolabeled RGD peptides that are integrin alphavbeta3-specific are very useful for noninvasive imaging of integrin expression in rapidly growing and metastatic tumors. In this study, we determined the binding affinity of E{E[c(RGDfK)]2}2 (tetramer) and its 6-hydrazinonicotinamide conjugate (HYNIC-tetramer) against the binding of 125I-echistatin to the integrin alphavbeta3-positive MDA-MB-435 breast cancer cells. The athymic nude mice bearing MDA-MB-435 xenografts were used to evaluate the potential of ternary ligand complex [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3' '-trisulfonate) as a new radiotracer for imaging breast cancer integrin alphavbeta3 expression by single photon emission computed tomography (SPECT). It was found that the binding affinity of tetramer (IC50 = 51 +/- 11 nM) was slightly higher than that of its dimeric analogue (IC50 = 78 +/- 27 nM) and is comparable to that of the HYNIC-tetramer conjugate (IC50 = 55 +/- 11 nM) within the experimental error. Biodistribution data showed that [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] had a rapid blood clearance (4.61 +/- 0.81 %ID/g at 5 min postinjection (p.i.) and 0.56 +/- 0.12 %ID/g at 120 min p.i.) and was excreted mainly via the renal route. [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] had high tumor uptake with a long tumor retention (5.60 +/- 0.87 %ID/g and 7.30 +/- 1.32 %ID/g at 5 and 120 min p.i., respectively). The integrin alphavbeta3-specificity was demonstrated by co-injection of excess E[c(RGDfK)]2, which resulted in a significant reduction in tumor uptake of the radiotracer. The metabolic stability of [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] was determined by analyzing urine and feces samples from the tumor-bearing mice at 120 min p.i. In the urine, about 20% of [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] remained intact while only approximately 15% metabolized species was detected in feces. SPECT images displayed significant radiotracer localization in tumor with good contrast as early as 1 h p.i. The high tumor uptake and fast renal excretion make [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] a promising radiotracer for noninvasive imaging of the integrin alphavbeta3-positive tumors by SPECT.

Coligand effects on the solution stability, biodistribution and metabolism of the 99mTc-labeled cyclic RGDfK tetramer

“…Pentapeptide c(RGDfK) was purchased from Peptides International, Inc. (Louisville, Kentucky). HYNIC-E{E[c(RGDfK)] 2 } 2 (HYNIC-tetramer) and [ 99m Tc(HYNICtetramer)(tricine)(TPPTS)] were prepared according to procedures described in our previous report [30]. Na 99m TcO 4 was obtained from a commercial DuPont Pharma 99 Mo/ 99m Tc generator, N. Billerica, MA.…”

“…Animals were monitored on the gamma camera (PhoGama large field-of-view Anger camera and NucLearMac computer system). Sequential anterior images were collected for 5 min at the specified time (5,15,30,45,60,90 and 120 min) using the 256 × 256 image matrix while animals remained under anesthesia. Upon completion of the experiment, the tumor-bearing mice were sacrificed by sodium pentobarbital overdose (150 -200 mg/kg).…”

“…We and others have been using multimeric cyclic RGD peptides to develop the integrin α v β 3 -targeted radiotracers to image rapidly growing and metastatic tumors in several tumor-bearing animal models [27][28][29][30][31][32][33][34][35][36][37][38][39]. The RGD peptides serve as the targeting biomolecules to carry radionuclide (e.g.…”

“…Previously, we reported the evaluation of [ 99m Tc(HYNIC-tetramer)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3"-trisulfonate) as a new radiotracer for tumor imaging [30]. We found that [ 99m Tc(HYNIC-tetramer)(tricine)(TPPTS)] had a high tumor uptake with long tumor retention (5.60±0.87 %ID/g and 7.30±1.32 %ID/g at 5 and 120 min postinjection (p.i.…”

“…The athymic nude mice bearing MDA-MB-435 human breast cancer xenografts were used to evaluate their biological properties. Results from these studies will allow us to compare their biodistribution characteristics (tumor uptake, T/B ratios, excretion kinetics and metabolic stability) with those of [ 99m Tc(HYNIC-tetramer)(tricine)(TPPTS)] [30].…”