Hyafil F, Pelisek J, Laitinen I, Schottelius M, Mohring M, Döring Y, Van der Vorst E, Kallmayer M, Steiger K, Poschenrieder A, Notni J, Fischer J, Baumgartner C, Rischpler C, Nekolla S, Weber C, Eckstein HH, Wester HJ, Schwaiger M

27.10.2016 [Original Artikel]

68Ga-pentixafor is a radiotracer for positron emission tomography (PET) that binds with nanomolar affinity to CXCR4. The CXCR4 receptor is expressed at the surface of inflammatory cells. The objective of the study was to analyze the ability of radiolabeled pentixafor to detect CXCR4 expression on inflammatory cells present in atherosclerotic plaques of an experimental rabbit model. Atherosclerotic plaques were induced by endothelial abrasion of the right carotid artery and abdominal aorta of 7 rabbits fed an atherogenic diet. Five non injured rabbits fed a chow diet were used as controls. Rabbits were imaged on a PET-MRI system after injection of 15 MBq/kg of 68Ga-pentixafor. Vascular signal was quantified as tissue to background ratio (TBR). Biodistribution and autoradiographic studies were performed 1 hour after injection of 7.5 MBq/kg of 125I-pentixafor. In addition, blocking studies were performed in two atherosclerotic rabbits with pre-injection of the CXCR4 inhibitor AMD3100. Tracer uptake was quantified on arterial cryosections using autoradiography and compared to CXCR4 and RAM-11 (macrophage) expression on adjacent histological sections. One hour after injection of 68Ga-pentixafor, strong signals were detected in vivo with PET-MRI in atherosclerotic plaques of the abdominal aorta and right carotid artery as compared to normal control arteries (average TBR = 1.95 ± 0.51 vs. 1.22 ± 0.25 and TBR = 1.24 ± 0.38 vs. 0.96 ± 0.37, respectively; P < 0.05 for both). Blocking studies with pre-injection of a CXCR4 inhibitor reduced 125I-pentixafor uptake in atherosclerotic plaques by ~ 40 %. 125I-pentixafor uptake in the vessel wall on autoradiographies was located in macrophage-rich regions of atherosclerotic plaques and correlated with the intensity of CXCR4 expression on corresponding cryosections (r2 = 0.61; P < 0.05). 68Ga-pentixafor allows for the noninvasive detection of CXCR4 expression in the vessel wall with PET and emerges as a potential alternative to FDG for the assessment of macrophage infiltration in atherosclerotic plaques.

Poschenrieder A, Schottelius M, Schwaiger M, Wester HJ

21.09.2016 [Original Artikel]

Due to its overexpression in a variety of tumor types, the chemokine receptor 4 (CXCR4) represents a highly relevant diagnostic and therapeutic target in nuclear oncology. Recently, [(68)Ga]pentixafor has emerged as an excellent imaging agent for positron emission tomography (PET) of CXCR4 expression in vivo. In this study, the corresponding [(68)Ga]-1,4,7-triazacyclononane-triacetic acid (NOTA) analog was preclinically evaluated and compared to the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) parent compound [(68)Ga]pentixafor. NOTA-pentixafor was synthesized by combining solid and solution-phase peptide synthesis. The CXCR4 receptor affinities of [(68)Ga]pentixafor and [(68)Ga]NOTA-pentixafor were determined in competitive binding assays using the leukemic CXCR4-expressing Jurkat T-cell line and [(125)I]FC131 as the radioligand. Internalization and cell efflux assays were performed using CXCR4-transfected Chem-1 cells. Small-animal PET and biodistribution studies were carried out using Daudi-tumor bearing SCID mice.  [(68)Ga]NOTA-pentixafor showed a 1.4-fold improved affinity towards CXCR4 (IC50). However, internalization efficiency into CXCR4(+)-Chem-1 cells was substantially decreased compared to [(68)Ga]pentixafor. Accordingly, small-animal PET imaging and biodistribution studies revealed a 9.5-fold decreased uptake of [(68)Ga]NOTA-pentixafor in Daudi lymphoma xenografts (1.7 ± 0.4 % vs 16.2 ± 3.8 % ID/g at 90 min p.i.) and higher levels of non-specific accumulation, primarily in the excretory organs such as the liver, intestines, and kidneys (2.3 ± 0.9 % vs 2.0 ± 0.3 % ID/g, 1.9 ± 0.8 % vs 0.7 ± 0.2 % ID/g, and 2.7 ± 1.1 % vs 1.7 ± 0.9 % ID/g, respectively). Despite enhanced CXCR4-affinity in vitro, the [(68)Ga]NOTA-analog of pentixafor showed reduced CXCR4 targeting efficiency in vivo. In combination with enhanced background accumulation, this resulted in significantly inferior PET imaging contrast, and thus, [(68)Ga]NOTA-pentixafor offers no advantages over [(68)Ga]pentixafor.

Poschenrieder A, Osl T, Schottelius M, Hoffmann F, Wirtz M, Schwaiger M, Wester H-J

12.06.2016 [Original Artikel]

In vivo quantification of CXCR4 expression using [68Ga]pentixafor for positron emission tomography (PET) imaging has gained significant clinical interest as CXCR4 plays a fundamental role in oncology and possesses potential prognostic value when overexpressed. To combine the excellent CXCR4-targeting properties of pentixafor-based tracers with the favorable radionuclide properties of 18F for high-resolution PET imaging, we developed an Al18F-labeled 1,4,7-triazacyclononane-triacetic acid (NOTA) analog of pentixather. Al18F-labeling of NOTA-pentixather was performed in aqueous dimethyl sulfoxide (DMSO) at pH = 4 (105°C, 15 minutes). CXCR4 affinities were determined in competitive binding assays, and both biodistribution and small-animal PET studies were performed in Daudi lymphoma-bearing mice. Under non-optimized conditions, [18F]AlF-NOTA-pentixather was obtained in radiochemical yields of 45.5% ± 13.3% and specific activities of up to 24.8 GBq/ µmol. Compared with [natGa]pentixafor, [natF]AlF-NOTA-pentixather showed 1.4-fold higher CXCR4 affinity. [18F]AlF-NOTA-pentixather displayed high and CXCR4-specific in vivo uptake in Daudi xenografts (13.9% ± 0.8% injected dose per gram [ID/g] at 1 hour post injection [p.i.]). Because of its enhanced lipophilicity (logP 1.4), [18F]AlF-NOTA-pentixather showed increased accumulation in the gall bladder and intestines. However, tumor/background ratios of 7.0 ± 1.2, 2.0 ± 0.3, 2.2 ± 0.4, 16.5 ± 6.5, and 29.2 ± 4 for blood, liver, small intestine, gut, and muscle, respectively, allowed for high-contrast visualization of Daudi tumors using PET (1 hour p.i.). The relatively straightforward radiosynthesis and efficient CXCR4 targeting of [18F]AlF-NOTA-pentixather demonstrate the successful implementation of 18F-complexation chemistry and pentixather-based CXCR4 targeting. Upon pharmacokinetic optimization, this class of tracers holds great promise for future application in humans.

Herhaus P, Habringer S, Philipp-Abbrederis K, Vag T, Gerngross C, Schottelius M, Slotta-Huspenina J, Steiger K, Altmann T, Weißer T, Steidle S, Schick M, Jacobs L, Slawska J, Müller-Thomas C, Verbeek M, Subklewe M, Peschel C, Wester HJ, Schwaiger M, Götze K, Keller U

12.05.2016 [Original Artikel]

Acute myeloid leukemia originates from leukemia-initiating cells that reside in the protective bone marrow niche. CXCR4/CXCL12 interaction is crucially involved in recruitment and retention of leukemia-initiating cells within this niche. Various drugs targeting this pathway have entered clinical trials. To evaluate CXCR4 imaging in acute myeloid leukemia, we first tested CXCR4 expression in patient-derived primary blasts. Flow cytometry revealed that high blast counts in patients with acute myeloid leukemia correlate with high CXCR4 expression. The wide range of CXCR4 surface expression in patients was reflected in cell lines of acute myeloid leukemia. Next, we evaluated the CXCR4-specific peptide Pentixafor by positron emission tomography imaging in mice harboring CXCR4 positive and CXCR4 negative leukemia xenografts, and in 10 patients with active disease. [(68)Ga]Pentixafor-positron emission tomography showed specific measurable disease in murine CXCR4 positive xenografts, but not when CXCR4 was knocked out with CRISPR/Cas9 gene editing. Five of 10 patients showed tracer uptake correlating well with leukemia infiltration assessed by magnetic resonance imaging. The mean maximal standard uptake value was significantly higher in visually CXCR4 positive patients compared to CXCR4 negative patients. In summary, in vivo molecular CXCR4 imaging by means of positron emission tomography is feasible in acute myeloid leukemia. These data provide a framework for future diagnostic and theranostic approaches targeting the CXCR4/CXCL12-defined leukemia-initiating cell niche.

Poschenrieder A, Schottelius M, Schwaiger M, Kessler H, Wester HJ

26.04.2016 [Original Artikel]

The overexpression of the chemokine receptor 4 (CXCR4) in different epithelial, mesenchymal, and hematopoietic cancers makes CXCR4 an attractive diagnostic and therapeutic target. However, targeting the CXCR4 receptor with small cyclic pentapeptide-based radiopharmaceuticals remains challenging because minor structural modifications within the ligand-linker-chelate structure often significantly affect the receptor affinity. Based on the excellent in vivo properties of CXCR4-directed pentapeptide [(68)Ga]pentixafor (cyclo(-D-Tyr-N-Me-D-Orn(AMB-DOTA)-L-Arg-L-2-Nal-Gly-)), this study aims to broaden the spectrum of applicable (radio)metal-labeled pentixafor analogs. Cyclic pentapeptides, based on the pentixafor scaffold, were synthesized by a combined solid- and solution-phase peptide synthesis. The CXCR4 receptor affinities of the cold reference compounds were determined in competitive binding assays using CXCR4-expressing Jurkat T - cell leukemia cells and [(125)I]FC131 as the radioligand. Metalated pentixafor derivatives with cyclic and acyclic chelators were synthesized by solid-phase peptide synthesis and evaluated in vitro. The resulting CXCR4 affinities (IC50) were highly dependent on the chelator and metal used. Two pentapeptides, Ga-NOTA and Bi-DOTA conjugates, offer an improved affinity compared to [(68)Ga]pentixafor. Based on the pentapeptide [(68)Ga]pentixafor, a broad range of metal-labeled analogs were investigated. The affinities of the new compounds were found to be strongly dependent on both the chelator and the metal used. Bi-labeled pentixafor showed high receptor affinity and seems to be a promising ligand for further preclinical evaluation and future α-emitter-based endoradiotherapy.

Lapa C, Lückerath K, Kleinlein I, Monoranu CM, Linsenmann T, Kessler AF, Rudelius M, Kropf S, Buck AK, Ernestus RI, Wester HJ, Löhr M, Herrmann K

25.01.2016 [Original Artikel]

Chemokine receptor-4 (CXCR4) has been reported to be overexpressed in glioblastoma (GBM) and to be associated with poor survival. This study investigated the feasibility of non-invasive CXCR4-directed imaging with positron emission tomography/computed tomography (PET/CT) using the radiolabelled chemokine receptor ligand (68)Ga-Pentixafor. 15 patients with clinical suspicion on primary or recurrent glioblastoma (13 primary, 2 recurrent tumors) underwent (68)Ga-Pentixafor-PET/CT for assessment of CXCR4 expression prior to surgery. O-(2-(18)F-fluoroethyl)-L-tyrosine ((18)F-FET) PET/CT images were available in 11/15 cases and were compared visually and semi-quantitatively (SUVmax, SUVmean). Tumor-to-background ratios (TBR) were calculated for both PET probes. (68)Ga-Pentixafor-PET/CT results were also compared to histological CXCR4 expression on neuronavigated surgical samples. (68)Ga-Pentixafor-PET/CT was visually positive in 13/15 cases with SUVmean and SUVmax of 3.0±1.5 and 3.9±2.0 respectively. Respective values for (18)F-FET were 4.4±2.0 (SUVmean) and 5.3±2.3 (SUVmax). TBR for SUVmean and SUVmax were higher for (68)Ga-Pentixafor than for (18)F-FET (SUVmean 154.0±90.7 vs. 4.1±1.3; SUVmax 70.3±44.0 and 3.8±1.2, p<0.01), respectively. Histological analysis confirmed CXCR4 expression in tumor areas with high (68)Ga-Pentixafor uptake; regions of the same tumor without apparent (68)Ga-Pentixafor uptake showed no or low receptor expression. In this pilot study, (68)Ga-Pentixafor retention has been observed in the vast majority of glioblastoma lesions and served as readout for non-invasive determination of CXCR4 expression. Given the paramount importance of the CXCR4/SDF-1 axis in tumor biology, (68)Ga-Pentixafor-PET/CT might prove a useful tool for sensitive, non-invasive in-vivo quantification of CXCR4 as well as selection of patients who might benefit from CXCR4-directed therapy.

Lapa C, Lückerath K, Rudelius M, Schmid JS, Schoene A, Schirbel A, Samnick S, Pelzer T, Buck AK, Kropf S, Wester HJ, Herrmann K

25.01.2016 [Original Artikel]

Chemokine receptor CXCR4 is a key factor for tumor growth and metastasis in several types of human cancer. This study investigated the feasibility of CXCR4-directed imaging of small cell lung cancer (SCLC) with positron emission tomography/computed tomography (PET/CT) using the radiolabelled chemokine ligand [68Ga]Pentixafor. 10 patients with primarily diagnosed (n=3) or pre-treated (n=7) SCLC (n=9) or large cell neuroendocrine carcinoma of the lung (LCNEC, n=1) underwent [68Ga]Pentixafor-PET/CT. 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG, n=6) and/or somatostatin receptor (SSTR)-directed PET/CT with [68Ga]DOTATOC (n=5) and immunohistochemistry (n=10) served as standards of reference. CXCR4-PET was positive in 8/10 patients and revealed more lesions with significantly higher tumor-to-background ratios than SSTR-PET. Two patients who were positive on [18F]FDG-PET were missed by CXCR4-PET, in the remainder [68Ga]Pentixafor detected an equal (n=2) or higher (n=2) number of lesions. CXCR4 expression of tumor lesions could be confirmed by immunohistochemistry. Non-invasive imaging of CXCR4 expression in SCLC is feasible. [68Ga]Pentixafor as a novel PET tracer might serve as readout for confirmation of CXCR4 expression as prerequisite for potential CXCR4-directed treatment including receptor-radio(drug)peptide therapy.

Vag T, Gerngross C, Herhaus P, Eiber M, Philipp-Abbrederis K, Graner FP, Ettl J, Keller U, Wester HJ, Schwaiger M

14.01.2016 [Original Artikel]

CXCR4 is a chemokine receptor that is overexpressed in various human cancers and is involved in tumor metastasis. The aim of this proof-of-concept study was to evaluate a novel CXCR4-targeted PET probe in patients with solid cancers with reported in vitro evidence of CXCR4 overexpression and to estimate its potential diagnostic value. Twenty-one patients with histologically proven pancreatic cancer, laryngeal cancer, non-small cell lung cancer, prostate cancer, melanoma, breast cancer, hepatocellular carcinoma, glioblastoma, sarcoma, or cancer of unknown primary underwent PET imaging using the novel CXCR4 nuclear probe (68)Ga-pentixafor. The SUVmax of the liver, spleen, and bone marrow was measured to determine physiologic tracer distribution. For evaluation of tracer accumulation in solid cancers, SUVmax and tumor-to-background (T/B) ratios were determined in a total of 43 malignant lesions, including 8 primary tumors, 3 locally recurrent tumors, and 32 metastases. When available, the SUVmax of malignant lesions was compared with the corresponding SUVmax measured in routine (18)F-FDG PET. Moderate tracer accumulation was detectable in the liver, bone marrow, and spleen, with a mean SUVmax of 3.1, 3.7, and 5.6, respectively. By visual interpretation criteria, 9 of 11 primary and locally recurrent tumors were detectable, exhibiting a mean SUVmax of 4.7 (range, 2.1-10.9) and a mean T/B ratio of 2.9. Twenty of 32 evaluated metastases were visually detectable (mean SUVmax, 4.5 [range, 3.2-13.8]; mean T/B ratio, 2.8). The highest signal was detected in a patient with non-small cell lung cancer (SUVmax, 10.9; T/B ratio, 8.4) and a patient with cancer of unknown primary (SUVmax, 13.8; T/B ratio, 8.1). Compared with (18)F-FDG PET, which was additionally performed in 10 patients, (68)Ga-pentixafor PET had a lower SUVmax in all measured malignant lesions. On the basis of these first observations in a small and heterogeneous patient cohort, the in vitro CXCR4 expression profile of solid cancers and metastases described in the previous literature does not seem to sufficiently depict the in vivo distribution revealed by CXCR4-targeted PET. Moreover, the detectability of solid cancers seems to be generally lower for (68)Ga-pentixafor than for (18)F-FDG PET.