**8.2 Receptor-based products**

99Tcm based receptor radiopharmaceuticals are not yet a clinical reality. The only successful case is that of 99Tcm - neogalactosyl glycoalbumin (NGA) for a bound receptor named hepatocyte binding protein (HBP) receptor, that binds galactose end glyco proteins. Tc-NGA would be useful for staging certain liver diseases (since HBP is implicated in liver malignancy) and for monitoring response to therapy [13]. The concept of BCA to attach receptor specific molecules with 99Tcm has been extensively investigated, but with limited success. Arduous chemical studies followed by receptor binding experiments have revealed poor specificity in most cases, like [Tc(V)O(DADS)-Progestin],[Tc(ra)(CO)(diethyldithiocarbamato-Spiperone)3], [Tc^-BATO-QNB] & [Tc(V)O(DADS)-QNB]. It appears that in all cases the complexation with technetium severely alters the bioactivity and precludes receptor binding. Attempts to overcome steric effects by increasing the distance between the essential functional groups have not been much successful. The important aspects to be reckoned with are molecular weight & size, lipophilicity changes, stereochemical effects and non-specific binding; two approaches called tridentate-monodentate (3 + 1) scheme (the former for facile chelation with Tc and the latter for presenting the receptor avid moiety) and pendant scheme have been pursued [13]. The novel concept of molecular mimics, i.e. Tc-chelate simulating a regular ring structure in a native receptor binding molecule, especially in a steroid (e.g. progesterone)/drug (e.g. morphine), is being pursued to target receptor sites for imaging using 99Tcm. Radiolabeled peptides have been recognized as the most likely successful candidates for imaging receptors (covered in another article in this Volume), based on the promising experience with mini labeled octreotide. The earlier stated problems in disguising and presenting technetium to the receptors persist, but scope for optimism is seen in this approach. In view of the importance of receptor imaging capability in health and disease, research efforts are continuing in more than one way [13, 14].
