DESIGN, SYNTHESIS, AND EVALUATION OF MOLECULAR IMAGING PROBES FOR CATHEPSIN B

Abstract

Cathepsin B is a cysteine protease that is overexpressed in cancers that are likely to metastasize. Molecular imaging can be used to non-invassively detect cathepsin B in vivo in order to characterize cancer progression and treatment response. This thesis describes the development of four different strategies to develop cathepsin B targeted probes. First, an SAR strategy was used to synthesize and test a number of AOMK inhibitors that incorporated iodine. A high affinity (Ki = 180 nM) lead 2.23a was converted to its radioactive analogue [125I]2.23a by optimizing radiolabelling procedures to maximize radiochemical yields (>26%) and radiochemical purities (> 95%). In vivo evaluation showed low tumour uptake (0.05% at 23 h p.i.) due to high in vivo deiodination (thyroid = 20% ID/g at 23 h p.i.). Next, AOMK inhibitors with greater stability were developed using the [ReCO3]+ core either via a direct linkage or a dendrimer platform. The binding affinities of these probes were lower than 2.23a (Ki > 350 nM). A pretargeted approach was then developed that could be used to image cathepsin B in vitro and in vivo. Third, an affinity label was developed using the AOMK warhead linked to TCO giving 4.5 (Ki = 190 nM). A cell assay was developed and intracellular targeting of cathepsin B was observed, something that has not been observed previously with a pretargeted agent in nuclear medicine. Of the five tetrazine based bioorthogonal pairs tested with 4.5, three were sufficient for this approach. The final approach was to use a cathepsin B substrate to link a radiopharmaceutical such as an iodobenzamide used for melanoma imaging. Synthetic strategies were explored to develop the radioactive and non-radioactive analogues. Challenges were encountered during the development of these probes due to stability, which require further optimization.