Synthesis and characterisation of DOTA-kisspeptin-10 as a potential gallium-68/lutetium-177 pan-tumour radiopharmaceutical

Abstract

Kisspeptin (KISS1) and its cognate receptor (KISS1R) are implicated in the progression of various cancers. A gallium-68 labelled kisspeptin-10 (KP10), the minimal biologically active structure, has potential as a pan-tumour radiopharmaceutical for the detection of cancers. Furthermore, a lutetium-177 labelled KP10 could find therapeutic application in treating oncological diseases. DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) was attached to the NH2-terminus of KP10 as we posited from our previous publications that this modification would not impair biological activity. Here, we showed that the biological activity, as monitored by stimulation of inositol phosphate accumulation in HEK293 transfected with the KISS1R gene, was indeed similar for KP10 and DOTA-KP10. The optimisation of radiolabelling with gallium-68 and lutetium-177 is described. Stability in serum, plasma and whole blood was also investigated. Pharmacokinetics and biodistribution were established with micro-PET/CT (positron emission tomography/computerised tomography) and ex vivo measurements. Dynamic studies with micro-PET/CT demonstrated that background clearance for the radiopharmaceutical was rapid with a blood half-life of 18 ± 3 min. DOTA-KP10 demonstrated preserved functionality at KISS1R and good blood clearance. These results lay the foundation for the further development of DOTA-KP10 analogues that have high binding affinity along with proteolytic resistance.

Keywords: antimetastatic; peptide receptor radionuclide therapy (PRRT); pharmacokinetics; positron emission tomography (PET); tumorigenesis.

FIGURE 1

The potential roles of KISS/KISS1R expression in different tumour types (drawn with a licensed version of BioRender.com)., , , ,

FIGURE 2

Structure of DOTA‐KP10. M = gallium‐68 or lutetium‐177.

FIGURE 3

Stimulation of inositol phosphates accumulation in HEK293. cells expressing KISS1R by the DOTA‐KP10 ligand and native KP10. Data points were fitted by sigmoidal dose–response curves and are presented as a percentage of the maximal response of KP10 (set at 100%). Data are expressed as mean and SD from five independent assays. KP10 R ² = 0.9419.

FIGURE 4

The effects of various parameters on the radiolabelling efficiency of gallium‐68 and compound molarity of lutetium‐177 with DOTA‐KP10. (A) The effect of compound molarity. (B) The effect of pH on ⁶⁸Ga‐complexation. (C) The effects of temperature and incubation time on ⁶⁸Ga‐product yield. (D) The effect of compound molarity on ¹⁷⁷Lu‐labelling.

FIGURE 5

Radiochromatogram of a radiolabelling performed in optimal reaction conditions resulting in a radiochemical purity of more than 95%. Note the retention time of [⁶⁸Ga]Ga‐DOTA‐KP10 at 8.2 min.

FIGURE 6

Example chromatograms of [⁶⁸Ga]Ga‐DOTA‐KP10 demonstrating change in radiochemical purity: (A) control incubated without blood; (B) degradation at 5 min incubation with plasma; (C) degradation at 5 min incubation with serum; (D) degradation at 5 min incubation with whole blood.

FIGURE 7

Dynamic micro‐PET/CT imaging in different coronal cross sections (A) showing a gradual decrease in blood pool activity over 34 min; the image‐guided (CT based region of interest of the myocardium, N = 4). SUV analysis was performed to draw the time‐activity curve (B) to allow for calculation of the physiological half‐life of [⁶⁸Ga]Ga‐DOTA‐KP10 (R ² = 0.895). CT, computerised tomography; PET, positron emission tomography; SUV, standardized uptake value.

FIGURE 8

An example of a full body image (prone) in maximum intensity projection (right panel) from a 120‐min delayed micro‐PET/CT acquisition (right) with cross‐sectional micro‐PET/CT image slices (left panel) demonstrating noticeable radioactivity in (A) liver, (B) right kidney, (C) left kidney and small intestine. CT, computerised tomography; PET, positron emission tomography.

FIGURE 9

[⁶⁸Ga]Ga‐DOTA‐KP10 biodistribution derived from ex vivo gamma counting of dissected organs at 150 min postinjection. Results are presented as % injected dose per gram. Data are expressed as mean and SD (N = 5). On the right graph (A), major organs are indicated, and on the left graph, (B) excretory organs are provided. Note the low uptake by the brain.