[99mTc]Tc-PentixaTec: development, extensive pre-clinical evaluation, and first human experience.

PURPOSE: The clinical success non-invasive imaging of CXCR4 expression using [68 Ga]Ga-PentixaFor-PET warrants an expansion of the targeting concept towards conventional scintigraphy/SPECT with their lower cost and general availability. To this aim, we developed and comparatively evaluated a series of 99mTc-labeled cyclic pentapeptides based on the PentixaFor scaffold. METHODS: Six mas3-conjugated CPCR4 analogs with different 4-aminobenzoic acid (Abz)-D-Ala-D-Arg-aa3 linkers (L1-L6) as well as the corresponding HYNIC- and N4-analogs of L6-CPCR4 were synthesized via standard SPPS. Competitive binding studies (IC50 and IC50inv) were carried out using Jurkat T cell lymphoma cells and [125I]FC-131 as radioligand. Internalization kinetics were investigated using hCXCR4-overexpressing Chem-1 cells. Biodistribution studies and small animal SPECT/CT imaging (1 h p.i.) were carried out using Jurkat xenograft bearing CB17/SCID mice. Based on the preclinical results, [99mTc]Tc-N4-L6-CPCR4 ([99mTc]Tc-PentixaTec) was selected for an early translation to the human setting. Five patients with hematologic malignancies underwent [99mTc]Tc-N4-L6-CPCR4 SPECT/planar imaging with individual dosimetry. RESULTS: Of the six mas3-conjugated peptides, mas3-L6-CPCR4 (mas3-dap-r-a-Abz-CPCR4) showed the highest CXCR4 affinity (IC50 = 5.0 ± 1.3 nM). Conjugation with N4 (N4-L6-CPCR4) further improved hCXCR4 affinity to 0.6 ± 0.1 nM. [99mTc]Tc-N4-L6-CPCR4 also showed the most efficient internalization (97% of total cellular activity at 2 h) and the highest tumor accumulation (8.6 ± 1.3% iD/g, 1 h p.i.) of the compounds investigated. Therefore, [99mTc]Tc-N4-L6-CPCR4 (termed [99mTc]Tc-PentixaTec) was selected for first-in-human application. [99mTc]Tc-PentixaTec was well tolerated, exhibits a favorable biodistribution and dosimetry profile (2.1-3.4 mSv per 500 MBq) and excellent tumor/background ratios in SPECT and planar imaging. CONCLUSION: The successive optimization of the amino acid composition of the linker structure and the N-terminal 99mTc-labeling strategies (mas3 vs HYNIC vs N4) has provided [99mTc]Tc-PentixaTec as a novel, highly promising CXCR4-targeted SPECT agent for clinical application. With its excellent CXCR4 affinity, efficient internalization, high uptake in CXCR4-expressing tissues, suitable clearance/biodistribution characteristics, and favorable human dosimetry, it holds great potential for further clinical use.

C-X-C Motif Chemokine Receptor 4-Directed Scintigraphy Using [99mTc]Tc-Pentixatec in Primary Aldosteronism: A Proof-of-Concept Study.

C-X-C motif chemokine receptor 4 (CXCR4)-directed imaging has gained clinical interest in aiding clinical diagnostics in primary aldosteronism (PA). We retrospectively evaluated the feasibility of CXCR4-directed scintigraphy using the novel CXCR-4 ligand [99mTc]Tc-pentixatec in patients with PA. Methods: Six patients (mean age ± SD, 49 ± 15 y) underwent CXCR4-directed scintigraphy (including planar imaging and SPECT/CT) 30, 120, and 240 min after injection of 435 ± 50 MBq of [99mTc]Tc-pentixatec. Adrenal CXCR4 expression was analyzed by calculating lesion-to-contralateral ratios (LCRs). Imaging results were correlated to clinical information. Histopathology and clinical follow-up served as the standard of reference. Results: Three subjects showed lateralization of adrenal tracer accumulation, with a mean maximum lesion-to-contralateral ratio of 1.65 (range, 1.52-1.70), which correlated with morphologic findings on CT. One individual underwent adrenalectomy and presented with complete biochemical and clinical remission at follow-up. Histopathologic workup confirmed unilateral aldosterone-producing adenoma. Conclusion: [99mTc]Tc-pentixatec scintigraphy with SPECT in patients with PA is feasible and might offer a valuable alternative to CXCR4-directed imaging with [68Ga]Ga-pentixafor PET.