MicroPET and Autoradiographic Imaging of Breast Cancer α_v-Integrin Expression Using ¹⁸F- and ⁶⁴Cu-Labeled RGD Peptide

Abstract

Cell adhesion molecules alphavbeta3 and alphavbeta5 play a pivotal role in tumor angiogenesis and metastasis. Antiangiogenic therapy by using small peptide antagonists of alphav-integrins slows tumor growth and prevents tumor spread. The ability to visualize and quantify integrin expression will enable selection of appropriate patients for clinical trials, following determination of treatment efficacy and development of new potent drugs. We have previously labeled cyclic RGD peptide c(RGDyK) with 125I and 18F and applied the radiotracers to both subcutaneous and orthotopic brain tumor models. Here we conjugated c(RGDyK) with 1,4,7,10-tetraaza-1,4,7,10-tetradodecane-N,N',N' ',N' "-tetraacetic acid (DOTA) and labeled the DOTA-RGD conjugate with 64Cu (t1/2) = 12.8 h, 19% beta+) in high radiochemical purity and specific activity. The tumor targeting ability and in vivo kinetics of 64Cu-DOTA-RGD was compared with [18F]FB-RGD and 125I-RGD in orthotopic MDA-MB-435 breast cancer model. All three radiotracers revealed fast blood clearance and high tumor-to-blood and tumor-to-muscle ratios. 125I-RGD had higher tumor uptake than the corresponding 18F and 64Cu analogues. [18F]FB-RGD indicated a fast tumor washout rate and an unfavorable hepatobiliary excretion pathway, resulting in significant activity accumulation in gallbladder and intestines. 64Cu-DOTA-RGD had prolonged tumor retention (1.44 +/- 0.09 %ID/g at 4 h postinjection) and persistent uptake in the liver. All three tracers revealed receptor specific tumor accumulation which were illustrated by effective blocking via coinjection with a blocking dose of c(RGDyK). Static microPET imaging and whole-body autoradiography showed strong contrast from the contralateral background. In conclusion, overall molecular charge and characteristics of radiolabels have profound effects on tumor accumulation and in vivo kinetics of radiolabeled RGD peptide. Further modification of the RGD peptide and optimization of the tracer for prolonged tumor uptake and improved in vivo kinetics are being explored.

[ 64 Cu-NOTA-8-Aoc-BBN(7-14)NH 2 ] targeting vector for positron-emission tomography imaging of gastrin-releasing peptide receptor-expressing tissues

“…Studies show that the size of the parent macrocycle has a significant effect on the in vivo stability of 64 Cu conjugates (28). The more compact, neutral 64 Cu-NOTA complex of [ 64 Cu-NOTA-8-Aoc-BBN (7)(8)(9)(10)(11)(12)(13)(14)NH 2 ] appears to overcome demetallation and uptake of tracer by hepatobiliary proteins and accumulation and retention of conjugate in renal tissue in vivo, producing microPET images that are clearly superior to other conjugates described herein (23)(24)(25)(26)(27)(28)(29). Furthermore, ease of ligand synthesis, conjugation protocols, and radiolabeling techniques satisfies nearly all of the inherent require-ments for production of site-directed radiopharmaceuticals of this type.…”

“…However, ligands of this general type still suffer from difficult synthetic protocols and renal accumulation and retention of 64 Cu radionuclide (27)(28)(29). Thus, there is some impetus to improve the in vivo kinetic stability of 64 Cu-macrocyclic bioconjugates to reduce accumulation in collateral tissues such as liver (22)(23)(24)(25)(26). Furthermore, reduction of uptake in normal kidney would do much to improve the inherent renal toxicities of many peptide-based therapeutic agents (27)(28)(29).…”

“…Receptor-specific peptide conjugates containing the chelating agents DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and TETA (1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid) have shown some promise for production of Cu-64-labeled targeting vectors (22)(23)(24)(25)(26). However, 64 Cu 2ϩ complexes of these specific chelating ligands are only moderately stable under in vivo conditions, resulting in demetallation and subsequent accumulation in nontarget tissues such as liver.…”

Near-Infrared Fluorescent RGD Peptides for Optical Imaging of Integrin αvβ3 Expression in Living Mice

“…We recently radiolabeled DOTA-conjugated RGD monomer, dimer, and tetramer with 64 Cu and compared their tumor targeting efficacy and in vivo kinetics (33,37,46). Tetrameric RGD peptide tracer had almost twice as much tumor uptake than the corresponding dimer.…”

αvβ3 Integrin‐targeting radionuclide therapy and imaging with monomeric RGD peptide

“…Similar results with regard to other peptides have been reported. 18,22,23 The expression of a v b 3 integrin in microvessels in rat liver and lung as well as in osteoclast and osteoblast has been identified. [24][25][26][27] This fact suggests that there might be an expression of a v b 3 integrin in microvessels in normal tissues as well as in normal cells.…”