A Receptor-Binding Optical Imaging Agent for the Estimation of Hepatic Receptor Concentration
David R. Vera, Ph.D.1, Robert F. Mattrey, M.D.1, Carl K. Hoh, M.D.1, Laura McIntosh, Ph.D.2
1 U.C. San Diego, USA
2 ART Advanced Research Technologies Inc.
Excerpt from Program and Abstracts -- The Second Annual Meeting of the Society for Molecular Imaging (August 15-18, 2003)
Optically labeled receptor-binding agents offer high observational sensitivity, as well as the ability to measure receptor density in target tissues. Our objective was to demonstrate receptor-binding properties of an optically labeled receptor-binding diagnostic agent, Cy5.5-DTPA-galactosyl-dextran.
The optical reporter Cy5.5 was covalently coupled to DTPA-galactosyl-dextran or DTPA-dextran. DMSO was added to the N-succinimidyl-ester of the dye (0.15mg). The Cy5.5 solution was added dropwise to a DMSO solution of DTPA-galactosyl-dextran or DTPA-dextran (90 nmole dextran/ml). After one hour the product was purified (G25) to yield approximately 8 Cy5.5s per dextran T70.
Male BALBc mice (10 mg/kg ketamine, 1 mg/kg acepromizin IP) were imaged in the SAMI imaging system (ART Advanced Research Technologies Inc.). Cy5.5-DTPA-dextran or Cy5.5-DTPA-galactosyl-dextran was administered (24 nmole/kg) via the tail vein. With the animals supine on a heated scan plate, a rectangular area covering the heart and liver was selected using a top-viewing digital camera. The rectangle was raster scanned in 2 mm steps using appropriate excitation/emission wavelengths at 22.7 uwatts in 25 sec. Scans were repeated continuously for 30 mins. ROIs over the heart and liver were used to generate curves of fluorescence intensity.
The receptor-bonding agent, Cy5.5-DTPA-galactosyl-dextran, displayed hepatic accumulation similar to [99mTc]DPTA-galactosyl-dextran. At 2.5 minutes a typical liver ROI contained 4,500 kcpm and peaked within 20 minutes at 10,000 kcpm. When compared to the radiolabel receptor-binding agent, Cy5.5-DTPA-galactosyl-dextran displayed similar pharmacokinetic properties with an approximately 100-fold sensitivity -- typical nuclear count rates using a pinhole collimator were 100 kcpm. The time-intensity curves for Cy5.5-DTPA-dextran did not demonstrate hepatic uptake.
This study suggests three potential roles for optical imaging for receptor-binding diagnostic agents. First is the use of optical reporters during the initial development, when receptor density is unknown. Second, the increased sensitivity should permit the implementation of tomography. Third is the estimation of target receptor concentration via kinetic modeling.
R.F. Mattrey, None.