Comparison Study of Two Differently Clicked 18F-Folates-Lipophilicity Plays a Key Role.

Kathrin Kettenbach,Hanno Schieferstein,Stefanie Pektor,Tobias Ross,Matthias Miederer,Laura Reffert,Raphael Eckert,Frank Rösch

doi:10.3390/ph11010030

Abstract

Within the last decade, several folate-based radiopharmaceuticals for Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) have been evaluated; however, there is still a lack of suitable 18F-folates for clinical PET imaging. Herein, we report the synthesis and evaluation of two novel 18F-folates employing strain-promoted and copper-catalyzed click chemistry. Furthermore, the influence of both click-methods on lipophilicity and pharmacokinetics of the 18F-folates was investigated. 18F-Ala-folate and 18F-DBCO-folate were both stable in human serum albumin. In vitro studies proved their high affinity to the folate receptor (FR). The lipophilic character of the strain-promoted clicked 18F-DBCO-folate (logD = 0.6) contributed to a higher non-specific binding in cell internalization studies. In the following in vivo PET imaging studies, FR-positive tumors could not be visualized in a maximum intensity projection images. Compared with 18F-DBCO-folate, 18F-Ala-folate (logD = −1.4), synthesized by the copper-catalyzed click reaction, exhibited reduced lipophilicity, and as a result an improved in vivo performance and a clear-cut visualization of FR-positive tumors. In view of high radiochemical yield, radiochemical purity and favorable pharmacokinetics, 18F-Ala-folate is expected to be a promising candidate for FR-PET imaging.

Highlights

Folic acid is an essential nutrient for de novo DNA synthesis [1,2]
In the following in vivo Positron Emission Tomography (PET) imaging studies, folate receptor (FR)-positive tumors could not be visualized in a maximum intensity projection images
In view of high radiochemical yield, radiochemical purity and favorable pharmacokinetics, 18F-Ala-folate is expected to be a promising candidate for FR-PET imaging

Summary

Introduction

Folic acid (vitamin B9) is an essential nutrient for de novo DNA synthesis [1,2]. For naturally occurring folic acid as well as its derivatives the generic term folate is used. To ensure sufficient folate supply different transport mechanisms are available. Circulating folates in the blood stream are carried via transporters across the cellular membrane: the anionic reduced folate carrier (RFC) transports tetrahydrofolate (THF) and the proton-coupled folate transporter (PCFT) various reduced folates [3,4]. Oxidized folates are transferred via the folate receptor (FR)-mediated endocytosis into cells. The FR has a high affinity for folic acid (Kd of 1 nM) and is overexpressed on a variety of highly proliferating cancer cells [5,6]. In healthy tissues the FR expression is strictly limited to a few sites such as the kidneys (proximal tubule), choroid plexus, lung, salivary glands and the placenta, making it an ideal oncological target for imaging and therapy [5,7,8]

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