Pak. J. Pharm. Sci., Vol.31, No.6(Suppl), November 2018, pp.2585-2589 2585 Enzymatic degradation study of 111 In-labeled minigastrin peptides using cathepsin B enzyme and AR42J cancer cell line for the development of neuroendocrine tumor imaging radiopharmaceuticals Syed Ali Raza Naqvi 1 *, Muhammad Yameen 2 , Zaib Hussain 3 *, Sadia Asim 4 , Muhammad Usman 1 , Naeem-Ul-Haq Khan 1 and Khizar Abbas 5 1 Department of Chemistry, Government College University, Faisalabad, Pakistan 2 Department of Biochemistry, Government College University, Faisalabad, Pakistan 3 Institute of Chemistry, University of the Punjab, Lahore, Pakistan 4 Department of Chemistry, Government College University for Women, Faisalabad, Pakistan 5 Faculty of Pharmacy, Bahauddin Zakariya University, Multan, Pakistan Abstract: Neuroendocrine tumors (NET) are the rare tumors which often impose graveyard threat. These tumors are characterized by the overexpression of various G-protein coupled receptors including cholecystokinin (CCK) receptors-1 and 2 (A or B). Minigastrin peptides are being investigated for theranostic purposes of CCK-2 receptor positive NET. The minigastrin analogue (APHO70) was modified by engineering enzyme susceptible tetrapeptide sequence into APHO70 peptide to reduce the random degradation by lysosome enzymes which pave the way to random trafficking in patient’s body and dipeptide addition at c-terminus. All the four modified minigastrin peptides (MG-CL1-4) were investigated for lysosome cathepsin B (catB) enzyme susceptibility and fate into AR42J cancer cell line. The indium-111 labeled MG-CL1-4 peptides were also studied for target (tumor) and non-target saccumulation by using tumor induced mice. The RP-HPLC analysis result showed nonspecific cleavage of standard 111 In-APH070 and 111 In-MGCL1 while specific cleavage was noted in case of 111 In-MGCL (2-4). The effect of specific and non-specific cleavage on biodistribution in tumor induced nude mice model indicates the promising accumulation of 111 In-MGCL2, 111 In-MGCL3, and 111 In-MGCL4 radiotracers while 111 In-MGCL1 showed less accumulation. 111 In-MGCL2 and 111 In-MGCL3 showed highest target-to-kidney ratio (T/K) i.e. 1.71 and 1.72, respectively whereas standard compound showed T/K 1.13. In conclusion, the two indium-111 labeled analogues i.e. 111 In-MGCL2 and 111 In-MGCL3 showed promising sensitivity for tumor and could be tested for further investigation to reach pre-clinical studies. Keywords: Enzyme; peptide cleavage; tumor imaging; gastrin peptides; neuroendocrine tumor. INTRODUCTION In recent years, molecular imaging is gaining an ample attention due to its promising diagnostic potential of deep seated diseases at molecular level (Uliel, et al., 2013). Whereas invasive radiological modalities such as x-rays, ultrasound, magnetic resonance imaging (MRI) and computed tomographic (CT) scan helps after certain morphological and entomological alteration in disease tissues. Molecular imaging works non-invasively through administration of a radionuclide labeled biologically active molecules such as antibody, peptides, or organic synthetic molecules followed by acquisition and quantification through single photon emission computed tomography (SPECT), or positron emission tomography (PET) (Rashid, et al., 2017). Radiolabeled regulatory peptides (RRP) have shown promising potential for tumor imaging and therapy due to its strong affinity for receptors which over express at the cancer cell surface – this gives the cancer therapy procedure called peptide receptor radionuclide therapy (PRRT) (Bodei, et al., 2017; Lozza, et al., 2013). The main advantages of this procedure lies in the rapid production of radiolabeled peptides, easy to administrate and cell endocytosis internalization, low immunogenicity and rapid clearance from non-target body organs (Dash, et al., 2015). Radiolabeled peptides (RPs) reaches into cell lysosome following the binding to the peptide receptors at the tumor cell surface (Rosenbaum, et al., 2009) where RPs are fragmented into labeled and non-labeled moieties. Non-specified fragmentation of RPs may produce daughter RPs which can accumulate non-specifically and consequently pose radiotoxicity. There in lysosome a significant number of enzymes of cathepsin family (cathepsin B, L, S, K, A, G, D, and E), work together or individually in reducing or acidic conditions (pH < 5). Out of different cathepsin enzymes, catB and L are widely distributed in specific cells which handle peptides and proteins to fragment into small sequences. The radiolabeled peptides also face the similar kind of fragmentation in lysosome (Naqvi, et al., 2010). The aim of this study was to investigate the effect of catB enzyme susceptible peptide sequence (-Gly-Phe-Leu-Gly-), which *Corresponding author: e-mail: draliraza@gcuf.edu.pk; drzh1972@hotmail.com