International Journal of Molecular Sciences Review Challenges on Cyclic Nucleotide Phosphodiesterases Imaging with Positron Emission Tomography: Novel Radioligands and (Pre-)Clinical Insights since 2016 Susann Schröder 1,2,* , Matthias Scheunemann 2, Barbara Wenzel 2 and Peter Brust 2 1 Department of Research and Development, ROTOP Pharmaka Ltd., 01328 Dresden, Germany 2 Department of Neuroradiopharmaceuticals, Institute of Radiopharmaceutical Cancer Research, Research Site Leipzig, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), 04318 Leipzig, Germany; [email protected] (M.S.); [email protected] (B.W.); [email protected] (P.B.) * Correspondence: [email protected]; Tel.: +49-341-234-179-4631 Abstract: Cyclic nucleotide phosphodiesterases (PDEs) represent one of the key targets in the research field of intracellular signaling related to the second messenger molecules cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP). Hence, non-invasive imaging of this enzyme class by positron emission tomography (PET) using appropriate isoform- selective PDE radioligands is gaining importance. This methodology enables the in vivo diagnosis and staging of numerous diseases associated with altered PDE density or activity in the periphery and the central nervous system as well as the translational evaluation of novel PDE inhibitors as therapeutics. In this follow-up review, we summarize the efforts in the development of novel PDE radioligands and highlight (pre-)clinical insights from PET studies using already known PDE Citation: Schröder, S.; Scheunemann, radioligands since 2016. M.; Wenzel, B.; Brust, P. Challenges on Cyclic Nucleotide Keywords: positron emission tomography; cyclic nucleotide phosphodiesterases; PDE inhibitors; Phosphodiesterases Imaging with PDE radioligands; radiochemistry; imaging; recent (pre-)clinical insights Positron Emission Tomography: Novel Radioligands and (Pre-)Clinical Insights since 2016. Int. J. Mol. Sci. 2021, 22, 3832. https:// doi.org/10.3390/ijms22083832 1. Introduction This follow-up of our first review in 2016 [1] aims to report on (I) novel radioligands Academic Editor: Fabio Naro for imaging of cyclic nucleotide phosphodiesterases (PDEs) with positron emission to- mography (PET) and (II) recent (pre-)clinical insights from PET studies using already Received: 26 February 2021 known PDE radioligands. The biological and pathophysiological background of PDEs has Accepted: 31 March 2021 previously been summarized [1] and thus, this review will be focused only on the current Published: 7 April 2021 radiotracer development as well as the latest research findings. Briefly, PDEs are a class of intracellular enzymes that are expressed throughout the body. They are encoded by Publisher’s Note: MDPI stays neutral 21 genes and divided into 11 families that are subdivided into various subfamilies with with regard to jurisdictional claims in different isoforms. Their central role is to hydrolyze the second messenger molecules, cyclic published maps and institutional affil- adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP) and, iations. therefore, to regulate the intracellular levels as well as the signaling cascades of these cyclic nucleotides. Several PDE inhibitors have been developed as therapeutics for treatment of various diseases like neurological, cardiovascular, immune or inflammatory disorders, cancer, and metabolism. However, the link between altered PDE expression and/or activity Copyright: © 2021 by the authors. and pathophysiological effects often remains unclear. Hence, in vivo imaging and quan- Licensee MDPI, Basel, Switzerland. tification of PDEs with appropriate radioligands for PET is commended as an important This article is an open access article research and translational tool in related (pre-)clinical investigations. distributed under the terms and To the best of our knowledge, there is still a lack of PET radioligands for the PDE conditions of the Creative Commons families 3, 6, 8, 9, and 11. Herein, we will review novel radioligands as well as current Attribution (CC BY) license (https:// results from (pre-)clinical PET studies related to the PDEs 1, 2A, 4, 5, 7 and 10A. creativecommons.org/licenses/by/ 4.0/). Int. J. Mol. Sci. 2021, 22, 3832. https://doi.org/10.3390/ijms22083832 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 45 Int. J. Mol. Sci. 2021, 22, 3832 2 of 44 2. PDE1 Radioligands 2. PDE1PDE1 Radioligands is one of dual-substrate specific PDEs that hydrolyses both cyclic nucleotides cAMP and cGMP. The special characteristic of the PDE1 family is the calcium/calmodulin- PDE1 is one of dual-substrate specific PDEs that hydrolyses both cyclic nucleotides dependentcAMP and cGMP.regulation The of special the enzyme characteristic activity. of Hence, the PDE1 PDE1 family is suggested is the calcium/calmodulin- as an integration pointdependent for intracellular regulation calcium of the enzyme and cyclic activity. nucleotide Hence, sign PDE1aling is suggestedcascades [2] as. anThe integration PDE1 en- zymepoint is for encoded intracellular by the calcium three gene ands PDE1A cyclic nucleotide, PDE1B and signaling PDE1C, cascadeswhich are [2 all]. Theexpressed PDE1 inenzyme the brain is encoded where PDE1B by the threerepresents genes thePDE1A most, PDE1B abundantand PDE1PDE1C isoform., which In are the all periphery, expressed PDE1Ain the brain is predominant where PDE1B in the represents kidneys, the while most PDE1C abundant is highly PDE1 expressed isoform. In in the the periphery, heart [3]. ItPDE1A is presumed, is predominant that PDE1 in-specific the kidneys, inhibitors while might PDE1C be suitable is highly for expressed the treatment in the of heart various [3]. neuropsychiatricIt is presumed, that and PDE1-specific neurodegenerative inhibitors diseases, might like be attention suitable fordeficit the hyperactivity treatment of vari-dis- order,ous neuropsychiatric depression, Parkinson and neurodegenerative’s disease and Huntington´s diseases, like disease attention [4–7 deficit]. hyperactivity disorder,The first depression, 11C- or 18 Parkinson’sF-labeled radioligan disease andds for Huntington’s PET imaging disease of PDE1 [4– have7]. been claimed in a patentThe first in 112011C- or [8]18 F-labeledas already radioligands reviewed by for Andrés PET imaging et al. [9] of. PDE1However, have beenit seems claimed that furtherin a patent publications in 2011 [8 ]regarding as already subsequent reviewed by biological Andrés etinvestigations al. [9]. However, of these it seems PDE1 that radi- fur- oligandsther publications are up to regarding now not subsequentavailable. At biological the international investigations symposium of these PDE1of functional radioligands neu- roreceptorare up to now mapping not available. of the living At the brain international (NRM) insymposium 2018, Kealey of et functional al. [10] reported neuroreceptor on the preliminarymapping of thein livingvivo evaluation brain (NRM) of in the 2018, novel Kealey PDE1 et al.-specific [10] reported radioligand on the preliminary(±)-[11C]PF- 04822163in vivo evaluation ([11C]1, see of Scheme the novel 1). Out PDE1-specific of a series of radioligand quinazoline (±-based)-[11C] PDE1PF-04822163-selective([ inhib-11C]1, itorsees Scheme developed1). Out by ofHumphrey a series of et quinazoline-based al. [11], compound PDE1-selective 1 proved to be inhibitors the most potent developed can- didateby Humphrey (Scheme et 1) al. and [11 was], compound thus selected1 proved for 11C to-labeling. be the most Starting potent from candidate the corresponding (Scheme1) 11 phenoland was precursor, thus selected [11C] for1 hasC-labeling. been synthesized Starting fromby O the-methylation corresponding using phenol [11C]CH precursor,3I [10] 11 11 (Scheme[ C]1 has 1). been synthesized by O-methylation using [ C]CH3I[10] (Scheme1). Scheme 1. Molecular structure, half maximal inhibitory concentrations ( (ICIC50 valuesvalues)) for for the the PDE1 PDE1 isoforms isoforms and and radiosyn- radiosyn- 11 thesis and of ([ C])C])11 [10,11][10,11] (RCY (RCY = = radiochemical radiochemical yield; yield; A Amm == molar molar activity; activity; EOS EOS = =end end of of synthesis). synthesis). PETPET imaging studies in rats showed high initialinitial brain uptake of [1111C]C]11 with a peak standardizedstandardized uptake uptake value value (SUV) (SUV) of of 7 7 at 1 min post injection (p.i.), followed by a fast 11 washout [10] [10].. In In the the target region striatum, only slightlyslightly higherhigher accumulationaccumulation ofof [[11C]1 comparedcompared to to the reference region cerebellum was observed, indicating high non-specificnon-specific binding.binding. M Metaboliteetabolite analysis analysis revealed revealed low low inin vivo vivo stabilitystability with with 45% and 30% of intact 11 [[11C]C]11 inin plasma plasma at at 5 5 min min and and 10 10 min min p.i., p.i., respectively, respectively, while while a a single single polar polar radiometabolite radiometabolite 11 was detected detected [10] [10].. Keal Kealeyey et et al. al. suggested suggested that that the the fast fast degradation degradation of [ of11C] [ 1 C]causes1 causes the rapidthe rapid brain brain clearance, clearance, but further but further studies studies to investiga to investigatete the radioligand the radioligand kinetics kinetics and spec- and ificityspecificity of binding of binding are arerequired required [10] [10. Overall,]. Overall, the the short short brain brain retention