Pak. J. Pharm. Sci., Vol.26, No.2, March 2013, pp.299-305 299 Novel indium-111 labeled gastrin peptide analogues (MG-CL1-4): Synthesis and quality control Syed Ali Raza Naqvi 1* , Zulfiqar Ali Khan 1 , Saeed Ahmad Nagra 2 , Muhammad Yar 3 , Tauqir A. Sherazi 4 , Sohail Anjum Shahzad 4 , Syed Qaiser Shah 5 , Nasir Mahmood 6 , Malik Muhammad Ishfaq 7 and Stephen John Mather 8* 1 Department of Chemistry, Government College University, Faisalabad, Pakistan 2 Institute of Chemistry, Quid-e-Azam Campus, University of the Punjab, Lahore, Pakistan 3 Interdisciplinary Research Center in Biomedical Materials, COMSATS Institute of Information Technology, Lahore, Pakistan, 4 Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad, Pakistan 5 Center for Nuclear and Molecular Studies, University of Peshawar, Peshawar, K.P.K. Pakistan 6 Department of Allied Health Sciences and Chemical Pathology, University of Health Sciences, Lahore, Pakistan 7 Pakistan Institute of Nuclear Science and Technology (PINSTECH) Nilore, Islamabad, Pakistan 8 Center for Molecular Oncology and Imaging, Institute of Cancer, Queen Mary University of London, Barts and the London School of Medicine and Dentistry, London, United Kingdom Abstract: Radiolabeled neuropeptides are widely investigated to diagnose and therapy of tumors. These peptides get internalization after binding with particular receptors at the surface of cells and finally move to lysosome. Internalization into tumor cells helps in mapping the infected site. Minigastrin peptide analogues (MG-CL1-4) were synthesised and labeled with 111-In radioisotope under different sets of conditions for imaging CCk-2 receptor bearing tumors. Different parameters such as temperature (80-100°C), pH (4-12), incubation time (5-30 minutes) and dilution effect were investigated to get the maximum labeling yield and stability. The results indicated that MG-CL1-4 is successfully labeled with indium-111 at pH 4.5 with heating at 98°C for 15 minute. At these conditions i.e. heating, pH and incubation minimum oxidized and maximum labeling yield, more than 94 %, was obtained. The labeling stability was studied by incubating the radiolabeled complex for predefined time points in PBSA and blood serum. Results show that more than 90% radiolabeled MG-CL1-4 remained intact. Keywords: Indium-111, minigastrin peptides, MG-CL, labeling and gastrin hormone INTRODUCTION The development of radiolabeled neuropeptides as a tool to target malignant tumors for either imaging or radiotherapeutic purposes is focus of interest in nuclear medicine research (Mather et al., 2007; Reubi et al., 1997; Sosabowski et al., 2007). Several receptor systems, for example, Somatostatin, VIP, CCk-B, CCk-A, Substance-P, Bombesin/GRP, Neurotensin etc., (Behr et al., 1999; Mather 2007; Naqvi et al., 2010) have been investigated for the development of peptide-based radiopharmaceuticals for imaging and treatment (Cornelio et al., 2007). Following the binding of neuropeptides to their respective receptors on the tumor cell surface, internalization into intracellular vesicles occurs as a result of endocytosis that makes the bases of imaging (Aylin et al., 2008). The unstable radioligand degrades before reaching the surface of tumor cells and subsequently into the lysosome. This behaviour causes different levels of radiotoxicity in blood stream, body tissues and in different organs due to non specific accumulation. Particularly, the unstable radioconjugates on moving toward tumor cells through blood stream may degrade due to its physiochemical conditions and by the action of enzymes present in the blood. Presence of stability correlation between in vitro and in vivo conditions leads to establish successful radio-pharmaceuticals (Ocak et al., 2011). Duncan et al (1987) introduced enzyme substrate sequence into the sequence of peptide in order to enhance the stability in the blood and release of radiopeptides into the cytoplasm (Duncan 1987). The target to non target ratio depends on the labeling stability against physiochemical and enzymatic action of blood. The gastrin peptide sequence -EAYGWMDF-NH 2 is generally felt to be the prerequisite requirement for high binding affinity for the receptors (Mather et al., 2007). We designed and synthesis the series of four minigastrin peptides having a cathepsin-labile site to reduce unexpected radioconjugate degradation. We introduced a cathepsin B proteolytic site, GFLG, (Lu et al., 2002) and then di-His after receptor-binding sequences, and, in order to generate a variety of different radiometabolites with, presumably different properties, subsequently substituted other amino acids for the histidine residues. The full list of peptides studied is shown in table 1. A DOTA chelator introduced for radiolabeling with 111-In and a dihistidine *Corresponding author: e-mail: a_naqvi313@yahoo.com