Next Generation of SiFAlin-Based TATE Derivatives for PET Imaging of SSTR-Positive Tumors: Influence of Molecular Design on In Vitro SSTR Binding and In Vivo Pharmacokinetics.

The Silicon-Fluoride-Acceptor (SiFA)-(18)F-labeling strategy has been shown before to enable the straightforward and efficient (18)F-labeling of complex biologically active substances such as proteins and peptides. Especially in the case of peptides, the radiolabeling proceeds kit-like in short reaction times and without the need of complex product workup. SiFA-derivatized, (18)F-labeled Tyr(3)-octreotate (TATE) derivatives demonstrated, besides strong somatostatin receptor (SSTR) binding, favorable in vivo pharmacokinetics as well as excellent tumor visualization by PET imaging. In this study, we intended to determine the influence of the underlying molecular design and used molecular scaffolds of SiFAlin-TATE derivatives on SSTR binding as well as on the in vivo pharmacokinetics of the resulting (18)F-labeled peptides. For this purpose, new SiFAlin-(Asp)n-PEG1-TATE analogs (where n = 1-4) were synthesized, efficiently radiolabeled with (18)F in a kit-like manner and obtained in radiochemical yields of 70-80%, radiochemical purities of ≥97%, and nonoptimized specific activities of 20.1-45.2 GBq/µmol within 20-25 min starting from 0.7-1.5 GBq of (18)F. In the following, the radiotracer's lipophilicities and stabilities in human serum were determined. Furthermore, the SSTR-specific binding affinities were evaluated by a competitive displacement assay on SSTR-positive AR42J cells. The obtained in vitro results support the assumption that aspartic acids are able to considerably increase the radiotracer's hydrophilicity and that their number does not affect the SSTR binding potential of the TATE derivatives. The most promising tracer (18)F-SiFAlin-Asp3-PEG1-TATE [(18)F]6 (LogD = -1.23 ± 0.03, IC50 = 20.7 ± 2.5 nM) was further evaluated in vivo in AR42J tumor-bearing nude mice via PET/CT imaging against the clinical gold standard (68)Ga-DOTATATE as well as the previously developed SiFAlin-TATE derivative [(18)F]3. The results of these evaluations showed that [(18)F]6-although showing very similar chemical and in vitro properties to [(18)F]3-exhibits not only a slowed renal clearance compared to [(18)F]3, but also a higher absolute tumor uptake compared to (68)Ga-DOTATATE, and furthermore enables excellent tumor visualization with high image resolution. These results emphasize the importance of systematic study of the influence of molecular design and applied structure elements of peptidic radiotracers, as these may considerably influence in vivo pharmacokinetics while not affecting other parameters such as radiochemistry, lipophilicity, serum stability, or receptor binding potential.

Predictive value of zero calcium score and low-end percentiles for the presence of significant coronary artery stenosis in stable patients with suspected coronary artery disease.

PURPOSE: To prospectively investigate the predictive value of a zero calcium score (CS) value as well as age- and sex-adjusted low-end CS percentiles for the presence of significant coronary artery stenosis in stable patients with suspected coronary artery disease (CAD). MATERIALS AND METHODS: In total, 87 consecutive stable patients with suspected CAD were prospectively enrolled in this study (33 women; 66 ± 10 years). All patients underwent non-enhanced CT for calcium scoring (CSCT) and contrast-enhanced coronary CT angiography (cCTA). Invasive coronary angiography (ICA) served as the reference standard in all patients. Diagnostic performance for the presence of significant stenosis (≥ 50% diameter) was calculated separately for CS in comparison to cCTA and ICA. RESULTS: ICA identified significant stenosis in 56/87 patients (64%). The mean CS was 571 ± 599. On a per patient based analysis, the sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for patients with a zero CS were 98.5%, 18.2%, 78.0% and 80.0%, respectively, compared to cCTA and 100%, 16.1%, 68.3% and 100%, respectively, compared to ICA. Low-end age- and sex-adjusted percentiles derived from asymptomatic Caucasian populations showed results comparable to a CS of zero. CONCLUSION: The prevalence of significant coronary artery stenosis is low in stable patients with suspected CAD and a CS of zero but also in patients below certain low-end age- and sex-adjusted percentile ranks. Thus, CS should be used as a gatekeeper prior to further diagnostic procedures in these patients. A CS value below certain age- and sex-adjusted percentile ranks seems to be of identical diagnostic value to a CS of zero in stable patients.

Multimodal mapping of human skin.

BACKGROUND: The combination of coherent anti-Stokes Raman scattering (CARS), second harmonic generation (SHG) and two-photon excited fluorescence (TPEF) imaging--referred to as multimodal imaging--provides complementary contrast based on molecular vibrations, the structure of various tissue components and endogenous fluorophores, respectively. OBJECTIVES: To present a comprehensive overview of the appearance of human skin in multimodal imaging. METHODS: Multimodal imaging of unstained skin cross-sections of 32 individuals was performed using a laser scanning microscope and picosecond laser pulse for excitation. RESULTS: The epidermis, dermis and subcutis are distinguishable in all three applied modalities, but are unveiled best in multimodal images. While the subcutis is dominated by the CARS signal, predominately SHG and the secondary TPEF signal detect the dermis. In contrast, no SHG signal is detected in the epidermis, whereas CARS and TPEF show equal contributions. Additionally, the appearance of the major skin appendages is described, i.e. the hair follicle, sebaceous and sweat glands, and blood vessels belonging to the vascular system. All four investigated functional units show a characteristic morphochemistry in TPEF and CARS, allowing identification of further subunits, e.g. the major components of the hair follicle, while the SHG signal delineates the localization of the functional units. CONCLUSIONS: Multimodal imaging is a powerful tool to investigate human skin by providing high contrast based on the molecular constitution. It is therefore suggested that multimodal imaging has a high potential in application to dermatological research and clinical diagnostics of various skin alterations.