[99mTc]Tc-PentixaTec

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.

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.

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.

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.

Similar to [18F]-FDG, [99mTc]Tc-1-thio-D-glucose ([99mTc]Tc-TG) also binds to GLUT receptors. The aim of this Phase I study was to evaluate the safety, biodistribution and dosimetry of [99mTc]Tc-TG. Twelve lymphoma patients were injected with 729 ± 102 MBq [99mTc]Tc-TG. Whole-body planar imaging was performed in 10 patients at 2, 4, 6 and 24 h after injection. In all 12 patients, SPECT/CT (at 2 h) and SPECT (at 4 and 6 h) imaging was performed. Vital signs and possible side effects were monitored during imaging and up to 7 days after injection. [99mTc]Tc-TG injections were well-tolerated and no side effects or alterations in blood and urine analyses data were observed. The highest absorbed dose was in the kidneys and urinary bladder wall, followed by the adrenals, prostate, bone marrow, lungs, myocardium, ovaries, uterus, liver and gall bladder wall. [99mTc]Tc-TG SPECT/CT revealed foci of high activity uptake in the lymph nodes of all nine patients with known nodal lesions. Extranodal lesions were detected in all nine cases. In one patient, a lesion in the humerus head, which was not detected by CT, was visualized using [99mTc]Tc-TG. Potentially, [99mTc]Tc-TG can be considered as an additional diagnostic method for imaging GLUT receptors in lymphoma patients.