Tissue distribution of radioiodinated FAUC113: assessment of a pyrazolo(1,5-a) pyridine based dopamine D4 receptor radioligand candidate

Synthesis and biological evaluation of PET tracers designed for imaging of calcium activated potassium channel 3.1 (KCa3.1) channels in vivo

Expression of the Ca²⁺ activated potassium channel 3.1 (K_Ca3.1) channel (also known as the Gàrdos channel) is dysregulated in many tumor entities and has predictive power with respect to patient survival. Therefore, a positron emission tomography (PET) tracer targeting this ion channel could serve as a potential diagnostic tool by imaging the K_Ca3.1 channel in vivo. It was envisaged to synthesize [¹⁸F]senicapoc ([¹⁸F]1) since senicapoc (1) shows high affinity and excellent selectivity towards the K_Ca3.1 channels. Because problems occurred during ¹⁸F-fluorination, the [¹⁸F]fluoroethoxy senicapoc derivative [¹⁸F]28 was synthesized to generate an alternative PET tracer targeting the K_Ca3.1 channel. Inhibition of the K_Ca3.1 channel by 28 was confirmed by patch clamp experiments. In vitro stability in mouse and human serum was shown for 28. Furthermore, biodistribution experiments in wild type mice were performed. Since [¹⁸F]fluoride was detected in vivo after application of [¹⁸F]28, an in vitro metabolism study was conducted. A potential degradation route of fluoroethoxy derivatives in vivo was found which in general should be taken into account when designing new PET tracers for different targets with a [¹⁸F]fluoroethoxy moiety as well as when using the popular prosthetic group [¹⁸F]fluoroethyl tosylate for the alkylation of phenols.

Expression of the Ca²⁺ activated potassium channel 3.1 (K_Ca3.1) channel (also known as the Gàrdos channel) is dysregulated in many tumor entities and has predictive power with respect to patient survival.

Synthesis and Pharmacological Evaluation of Fluorinated Quinoxaline-Based κ-Opioid Receptor (KOR) Agonists Designed for PET Studies

κ-Opioid receptors (KORs) play a predominant role in pain alleviation, itching skin diseases, depression and neurodegenerative disorders such as multiple sclerosis. Therefore, imaging of KOR by a fluorinated PET tracer was envisaged. Two strategies were followed to introduce a F atom into the very potent class of cis,trans-configured perhydroquinoxalines. Whereas the synthesis of fluoroethyltriazole 2 has already been reported, fluoropyrrolidines 14 (1-[2-(3,4-dichlorophenyl)acetyl]-8-[(R)-3-fluoropyrrolidin-1-yl]-perhydroquinoxalines) were prepared by SN2 substitution of a cyclic sulfuric acid derivative with hydroxypyrrolidine and subsequent transformation of the OH moiety into a F substituent. Fluoropyrrolidines 14 showed similar low-nanomolar KOR affinity and selectivity to the corresponding pyrrolidines, but the corresponding alcohols were slightly less active. In the cAMP and β-arrestin assay, 14b (proton at the 4-position) exhibited similar KOR agonistic activity as U-50,488. The fluoro derivatives 14b and 14c (CO2CH3 at the 4-position) revealed KOR-mediated anti-inflammatory activity as CD11c and the IFN-γ production were reduced significantly in mouse and human dendritic cells. Compounds 14b and 14-c also displayed anti-inflammatory and immunomodulatory activity in mouse and human T cells. The PET tracer [18F]-2 was prepared by 1,3-dipolar cycloaddition. In vivo, [18F]-2 did not label KOR due to very fast elimination kinetics. Nucleophilic substitution of a mesylate precursor provided [18F]-14c. Unfortunately, defluorination of [18F]-14c occurred in vivo, which was analyzed in detail by in vitro studies.

Synthesis, radiosynthesis, in vitro and first in vivo evaluation of a new matrix metalloproteinase inhibitor based on γ-fluorinated α-sulfonylaminohydroxamic acid

Background

To study MMP activity in vivo in disease, several radiolabeled MMP inhibitors functioning as radiotracers have been evaluated by means of SPECT and PET. Unfortunately, most of them suffer from metabolic instability, mainly hepatobiliary clearance and insufficient target binding. The introduction of a fluorine atom into MMPIs could contribute to target binding, enhance the metabolic stability and might shift the clearance towards more renal elimination. Recently developed α-sulfonylaminohydroxamic acid based γ-fluorinated inhibitors of MMP-2 and -9 provide promising fluorine interactions with the enzyme active site and high MMP inhibition potencies. The aim of this study is the (radio)synthesis of a γ-fluorinated MMP-2 and -9 inhibitor to evaluate its potential as a radiotracer to image MMP activity in vivo.

Results

Two new fluorine-containing, enantiomerically pure inhibitors for MMP-2 and -9 were synthesized in a six step sequence. Both enantiomers exhibited equal inhibition potencies in the low nanomolar and subnanomolar range. LogD value indicated better water solubility compared to the CGS 25966 based analog. The most potent inhibitor was successfully radiofluorinated. In vivo biodistribution in wild type mice revealed predominantly hepatobiliary clearance. Two major radioactive metabolites were found in different organs. Defluorination of the radiotracer was not observed.

Conclusion

(Radio)synthesis of a CGS based γ-fluorinated MMP inhibitor was successfully accomplished. The (S)-enantiomer, which normally shows no biological activity, also exhibited high MMP inhibition potencies, which may be attributed to additional interactions of fluorine with enzyme’s active site. Despite higher hydrophilicity no significant differences in the clearance characteristics compared to non-fluorinated MMPIs was observed. Metabolically stabilizing effect of the fluorine was not monitored in vivo in wild type mice.

Electronic supplementary material

The online version of this article (10.1186/s41181-018-0045-0) contains supplementary material, which is available to authorized users.