Graduation Date

Spring 5-6-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Pharmaceutical Sciences

First Advisor

Jered Garrison

Abstract

The neurotensin receptor 1 (NTR1) is overexpressed in many cancers, due to its role as a growth pathway. These NTR1-positive cancers include pancreatic, colon, prostate and breast cancers. In the radiopharmaceutical field, the overexpression of NTR1 in cancer has prompted the development of NTR1-targeted diagnostics and therapeutics. The neurotensin (NT) peptide exhibits low nanomolar affinity for NTR1 and has been the paradigm for NTR1-targeted agents. Since the 1980’s, radiolabeled NT analogs have been developed and evaluated for targeting NTR1-positive cancers. Since native NT is rapidly degraded in vivo by a variety of peptidases, a tremendous amount of effort has been put forth to design stabilized NT analogs with increased in vivo efficacy. To further enhance NTR1-targeted agents for diagnosis and therapy, our work has focused on strategies to increase the binding affinity, stability and optimize the pharmacokinetic profile of NTR1-targeted radiopharmaceuticals. In this dissertation, our work includes: 1) the investigation of the structure-activity relationship of the spacer groups in the NTR1-targeted agent design; 2) the evaluation of the effect charge distribution has on NTR1 binding and the biodistribution profile and 3) the utilization of peptidase inhibitors to extend the activity of NTR1-targeted agents.

It is well known that 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and other radiometal chelators, can negatively impact the receptor binding of targeted peptides. Utilizing DOTA and a known metabolically stabilized NTR1-targeted peptide ([(N-α-Me)Arg8,Dmt11,Tle12]NT(6-13)), we evaluated a series of spacer groups to examine what steric impact, if any, DOTA had on NTR1-targeted peptides. We observed that the binding affinity was negatively affected by the direct conjugation (i.e., absence of spacer) with DOTA. However, the optimal binding activity can be restored with the inclusion of β-Ala or longer spacer groups. Our following studies investigated the impact the Lys6 charge had on NTR1 binding and the overall biodistribution profile. It was observed that translation of this amino acid further away from the peptide influenced the receptor binding, internalization and kidney retention profile of the NTR1-targeted peptide. Lastly, we examined if peptidase inhibitors, such as phosphoramidon (PA), would improve the targeting efficacy of NTR1-targeted agents. We observed that PA limited the degradation of the peptide and resulted in an increased NTR1-positive tumor uptake. Based on these findings, we increased our understanding of the structure-activity relationships and in vivo degradation of NTR1-targeted agents. Utilizing this knowledge, we plan on optimizing the design of future NTR1-targeted agents for diagnostic imaging and radiotherapy.

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