Preparation and evaluation of a 99mTc-labeled rhodamine-conjugated angiotensin peptide as a cardiac function imaging agent

Objectives Angiotensin II (Ang II) is an octapeptide that has been known to play an important role in cardiovascular function. In an attempt to develop a SPECT-based cardiac agent with enhanced targeting efficiency, we linked Ang II to rhodamine (Rh), a lipophilic cation (like 99mTc-MIBI), that specifically accumulates in the myocardium. The Rh conjugated Ang II was evaluated for its potential as a heart imaging agent.

Methods Rh-Lys-Gly-Cys-Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-CONH2 was prepared by solid-phase peptide synthesis following the Fmoc/HBTU chemistry. NHS-Rh was attached to the peptide via the free amino group of Lys residue by manual synthesis. Rh-Ang II conjugate was radiolabeled with Tc-99m by the stannous-tartrate exchange labeling method. In vitro stability was determined in human plasma and in excess cysteine and histidine. In vivo biodistribution/pharmacokinetics was performed in Balb/c mice and Sprague Dawley rats.

Results The bioconjugate radiolabeled efficiently with Tc-99m (>75%) as determined by HPLC. Tc-99m-Rh-Ang II exhibited stability in vitro as follows: His>Cys>plasma. In mice, the radioconjugate displayed efficient clearance from the blood and excreted mainly through the renal route with some elimination by the hepatobiliary pathway. The uptake in the heart was 1.79±0.51% ID/g as early as 30 min p.i. whereas, accumulation of radioactivity in the lungs, liver, stomach and kidneys varied between 1 to 10% ID/g). In rats, the hybrid conjugate showed better pharmacokinetic characteristics with low uptake in the major organs/tissues (<4.0% ID/g). The uptake in the heart, 1.70±0.42% ID/g, was found to be higher than in the blood and muscle resulting in good heart-to-blood and heart-to-muscle ratios.

Conclusions Our preliminary findings suggest that the use of hybrid conjugate appears to hold promise as a new and attractive approach for rapid and efficient diagnostic imaging of heart