Validation of 99mTc-labeled “4+1” fatty acids for myocardial metabolism and flow imaging: Part 1: myocardial extraction and biodistribution

Abstract

Introduction: C-13, F-18 and I-123 fatty acids (FA) are used for myocardial imaging. Recently, our group showed that [Tc-99m]-labeled "4+1" FA are extracted into the rat and guinea pig myocardium. The present study evaluates determinants of myocardial uptake and whole body biodistribution of these FA derivatives. Methods: Studies were performed with isolated perfused hearts of Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) with a FAT/CD36 deficiency, as well as with heart type FA binding protein knockout mice (H-FABP)(-/-) and H-FABP(+/+). Eight 4+1-Tc-99m-FA were applied for 3 min followed by 1-min washout. A mathematical model was used to analyze FA dynamics and binding to proteins. Whole-body distribution was studied in rats with and without Tween 80. In vitro fractionation studies with [Tc-99m]-FA assessed red blood cell uptake as well as association with plasma lipoproteins very low-density lipoprotein (VLDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Results: Myocardial extraction was 19.0-33.0% of the infused dose in isolated WKY and 15.2-26.4% in SHR hearts. However, H-FABP(-/-) showed a marked reduction of tracer extraction [2.8+/-0.6%ID (percent injected dose) vs. 17+/-2%ID P<001]. Uptake in red blood cells (<1.2% ID) and incorporation into lipoproteins were negligible. Incubation of Tc-99m-FA with albumin reduced ventricular extraction (P<.001) into the range of established iodinated FA tracers. polyoxyethylene(20) sorbitan monooleate improved the heart-to-liver ratio in the biodistribution studies. Conclusions: Myocardial uptake of [Tc-99m]-FA 4+1 derivatives is dependent on H-FABP. These substances may therefore provide a new tool to specifically assess regional myocardial changes of H-FABP. (C) 2009 Elsevier Inc. All rights reserved.