Pre-labelling versus direct labelling of anthrax proteins for imaging of matrix metalloproteinases activity using DOTA-GA

Imaging of Fibroblast Activation Protein in Cancer Xenografts Using Novel (4-Quinolinoyl)-glycyl-2-cyanopyrrolidine-Based Small Molecules

Fibroblast activation protein (FAP) has become a favored target for imaging and therapy of malignancy. We have synthesized and characterized two new (4-quinolinoyl)-glycyl-2-cyanopyrrolidine-based small molecules for imaging of FAP, QCP01 and [111In]QCP02, using optical and single-photon computed tomography/CT, respectively. Binding of imaging agents to FAP was assessed in six human cancer cell lines of different cancer types: glioblastoma (U87), melanoma (SKMEL24), prostate (PC3), NSCLC (NCIH2228), colorectal carcinoma (HCT116), and lung squamous cell carcinoma (NCIH226). Mouse xenograft models were developed with FAP-positive U87 and FAP-negative PC3 cells to test pharmacokinetics and binding specificity in vivo. QCP01 and [111In]QCP02 demonstrated nanomolar inhibition of FAP at Ki values of 1.26 and 16.20 nM, respectively. Both were selective for FAP over DPP-IV, a related serine protease. Both enabled imaging of FAP-expressing tumors specifically in vivo. [111In]QCP02 showed high uptake at 18.2 percent injected dose per gram in the U87 tumor at 30 min post-administration.

Pre-labelling versus direct labelling of anthrax proteins for imaging of matrix metalloproteinases activity using DOTA-GA

INTRODUCTION

Increased activity of matrix metalloproteases (MMPs) is associated with reduced survival in several cancer subtypes. Aiming to produce an MMP tumour cell-selective cytotoxin, we genetically modified both components of the AB-type lethal toxin from Bacillus anthracis. Component A, Protective Antigen (PA-WT), was re-engineered to form an oligomeric pore in cell membranes only when cleaved by MMPs (PA-L1). The pore-translocation domain (LFn - N-terminal, 30 kDa) of the Lethal Factor (LF), component B, was fused to the catalytic domain of Pseudomonas exotoxin-A to increase its cytotoxic effect when delivered to cancerous cells. Here, we develop radiolabelled forms of LFn for MMP activity imaging by SPECT using the LFn/PA-L1 system.

METHODS

DOTA-GA-maleimide was conjugated to LFn to allow radiolabelling with 111In via two different routes: (1) LFn was conjugated with maleimide-DOTA-GA under mild conditions, and then radiolabelled in acidic conditions at 95°C, or (2) 111In was coordinated to maleimide-DOTA-GA first and then conjugated via maleimide chemistry to LFn. Circular Dichroism Spectroscopy of LFn was performed to evaluate changes in its secondary structure. Cell uptake assays using the differently labelled forms of [111In]In-DOTA-GA-LFn in the presence or not of PA-WT or PA-L1 were performed.

RESULTS

LFn was successfully radiolabelled by either strategy. Comparison of the secondary structure content of LFn exposed to 37°C or 95°C, showed a loss of alpha helix content at higher temperatures. Cell uptake of both forms of [111In]In-DOTA-GA-LFn, labelled directly or indirectly, was significantly higher in MMP-positive cells, in the presence of PA-L1, compared to controls. Notably, despite being exposed to high temperatures, uptake of directly labelled [111In]In-DOTA-GA-LFndir was higher than indirectly labelled [111In]In-DOTA-GA-LFnindir.

CONCLUSIONS

111In-radiolabelling of LFn results in a functional molecule that targets MMP-activity in cells when combined with PA-L1. [111In]In-LFn/PA-L1 is a promising MMP activity imaging agent for SPECT imaging.