Expanding the Chemical Space of Transforming Growth Factor-ß (TGFß) Receptor Type II Degraders with 3,4-Disubstituted Indole Derivatives.

The TGFß type II receptor (TßRII) is a central player in all TGFß signaling downstream events, has been linked to cancer progression, and thus, has emerged as an auspicious anti-TGFß strategy. Especially its targeted degradation presents an excellent goal for effective TGFß pathway inhibition. Here, cellular structure-activity relationship (SAR) data from the TßRII degrader chemotype 1 was successfully transformed into predictive ligand-based pharmacophore models that allowed scaffold hopping. Two distinct 3,4-disubstituted indoles were identified from virtual screening: tetrahydro-4-oxo-indole 2 and indole-3-acetate 3. Design, synthesis, and screening of focused amide libraries confirmed 2r and 3n as potent TGFß inhibitors. They were validated to fully recapitulate the ability of 1 to selectively degrade TßRII, without affecting TßRI. Consequently, 2r and 3n efficiently blocked endothelial-to-mesenchymal transition and cell migration in different cancer cell lines while not perturbing the microtubule network. Hence, 2 and 3 present novel TßRII degrader chemotypes that will (1) aid target deconvolution efforts and (2) accelerate proof-of-concept studies for small-molecule-driven TßRII degradation in vivo.

Isotopic Radiolabeling of Crizotinib with Fluorine-18 for In Vivo Pet Imaging.

Crizotinib is a tyrosine kinase inhibitor approved for the treatment of non-small-cell lung cancer, but it is inefficient on brain metastases. Crizotinib is a substrate of the P-glycoprotein, and non-invasive nuclear imaging can be used to assess the brain penetration of crizotinib. Positron emission tomography (PET) imaging using fluorine-18-labeled crizotinib would be a powerful tool for investigating new strategies to enhance the brain distribution of crizotinib. We have synthesized a spirocyclic hypervalent iodine precursor for the isotopic labeling of crizotinib in a 2.4% yield. Because crizotinib is an enantiomerically pure drug, a chiral separation was performed to afford the (R)-precursor. A two-step radiolabeling process was optimized and automated using the racemic precursor to afford [18F](R,S)-crizotinib in 15 ± 2 radiochemical yield and 103 ± 18 GBq/µmol molar activity. The same radiolabeling process was applied to the (R)-precursor to afford [18F](R)-crizotinib with comparable results. As a proof-of-concept, PET was realized in a single non-human primate to demonstrate the feasibility of [18F](R)-crizotinib in in vivo imaging. Whole-body PET highlighted the elimination routes of crizotinib with negligible penetration in the brain (SUVmean = 0.1). This proof-of-concept paves the way for further studies using [18F](R)-crizotinib to enhance its brain penetration depending on the P-glycoprotein function.

Chiral Chromatographic Isolation on Milligram Scale of the Human African Trypanosomiasis Treatment d- and l-Eflornithine.

Eflornithine is a recommended treatment against the otherwise fatal parasitic disease late stage human African trypanosomiasis (HAT), also known as Gambian sleeping sickness. It is administered repeatedly as a racemic mixture intravenously (IV) together with oral nifurtimox. Racemic eflornithine has been investigated in clinical trials for oral dosing. However, due to low systemic exposures at a maximum tolerated oral dose, the drug is continued to be administered IV. The eflornithine enantiomers, d- and l-eflornithine, have different affinities to the target enzyme ornithine decarboxylase, suggesting that the pharmacodynamics of the enantiomers may differ. The aim of this study was to develop a method for isolation of d- and l-eflornithine from a racemic mixture. Several chiral stationary phases (CSPs) were evaluated for enantioselectivity using supercritical fluid chromatography (SFC) or high-performance liquid chromatography (HPLC). None of the tested CSPs rendered separation of the enantiomers in SFC mode. Separation of the enantiomers with SFC on the CSP Chiralpak IG was only achieved on an analytical scale after derivatization with ortho-phthalaldehyde (OPA). This was the first reported enantioselective SFC method for an eflornithine derivate. However, due to poor stability, the eflornithine-OPA derivates degraded and no chemically pure enantiomers were obtained. The CSP that showed enantioselectivity in HPLC mode was Chirobiotic R, which resulted in a successful isolation on a semipreparative milligram scale. The isolated eflornithine enantiomers will be tested in nonclinical in vitro and in vivo studies to support and assess the feasibility of a future clinical program with an oral HAT treatment.

Unexpected carbonate salt formation during isolation of an enantiopure intermediate by supercritical fluid chromatography.

The enantiomers of a chiral building block to be used in pre-clinical manufacturing were separated using supercritical fluid chromatography (SFC). Despite an extensive evaluation of different columns and solvent combinations followed by a careful optimization of the chromatographic method, the preparative separation suffered from low throughput and high solvent consumption. Consequently, additional improvements were necessary. By utilizing stacked injections, the chromatographic run time was almost halved, and the high solvent consumption was reduced by recycling of the two mobile phase components, carbon dioxide and methanol. The carbon dioxide was reprocessed by the SFC instrument, whereas methanol was evaporated and recycled from the fractions collected. Hence, the originally inefficient separation method was turned into a more sustainable one, and the desired enantiopure intermediate was delivered to be used in the following synthesis of the selected candidate drug. Unfortunately, when the intermediate was used in the subsequent amide coupling, a surprisingly poor yield was obtained. This was caused by an unexpected formation of a stable carbonate salt of the intermediate under the chromatographic conditions used. By removal of the carbonate prior to the amide coupling reaction, the manufacturing campaign could be saved, and the candidate drug was successfully delivered in time.

Saccharin Aza Bioisosteres-Synthesis and Preclinical Property Comparisons.

Saccharin is a well-known scaffold in drug discovery. Herein, we report the synthesis and preclinical property comparisons of three bioisosteres of saccharin: aza-pseudosaccharins (cluster B), and two new types of aza-saccharins (clusters C and D). We demonstrate a convenient protocol to selectively synthesize products in cluster C or D when primary amines are used. Preclinical characterization of selected matched-pair products is reported. Through comparison of two diastereomers, we highlight how stereochemistry affects the preclinical properties. Given that saccharin-based derivatives are widely used in many chemistry fields, we foresee that structures exemplified by clusters C and D offer new opportunities for novel drug design, creating a chiral center on the sulfur atom and the option of substitution at two different nitrogens.

Strategy for large-scale isolation of enantiomers in drug discovery.

A strategy for large-scale chiral resolution is illustrated by the isolation of pure enantiomer from a 5kg batch. Results from supercritical fluid chromatography will be presented and compared with normal phase liquid chromatography. Solubility of the compound in the supercritical mobile phase was shown to be the limiting factor. To circumvent this, extraction injection was used but shown not to be efficient for this compound. Finally, a method for chiral resolution by crystallization was developed and applied to give diastereomeric salt with an enantiomeric excess of 99% at a 91% yield. Direct access to a diverse separation tool box will be shown to be essential for solving separation problems in the most cost and time efficient way.

Delivering the promise of SFC: a case study.

During the past years there has been a rapid development in supercritical fluid chromatography (SFC) instrumentation making it a highly efficient and robust technique. Although much is written about the advantages of SFC over liquid chromatography (LC), there are not many direct comparisons detailing the gain in purification throughput, the savings in solvent consumption and the reduced environmental impact for large-scale SFC applications. We will show that a research scale separation laboratory built to handle multigram amounts can be used for kilogram separations when moving from LC to SFC.