Substituted 7-Amino-5-thio-thiazolo[4,5-d]pyrimidines as Potent and Selective Antagonists of the Fractalkine Receptor (CX₃CR1)

Abstract

We have developed two parallel series, A and B, of CX3CR1 antagonists for the treatment of multiple sclerosis. By modifying the substituents on the 7-amino-5-thio-thiazolo[4,5-d]pyrimidine core structure, we were able to achieve compounds with high selectivity for CX3CR1 over the closely related CXCR2 receptor. The structure-activity relationships showed that a leucinol moiety attached to the core-structure in the 7-position together with α-methyl branched benzyl derivatives in the 5-position displayed promising affinity, and selectivity as well as physicochemical properties, as exemplified by compounds 18a and 24h. We show the preparation of the first potent and selective orally available CX3CR1 antagonists.

Synthesis and preliminary biological evaluation of [11C]methyl (2-amino-5-(benzylthio)thiazolo[4,5-d]pyrimidin-7-yl)-d-leucinate for the fractalkine receptor (CX3CR1)

“…We were unable to reproduce the reported result. 6 However, the result of the competitive binding assay of ligand 5 was consistent with the result of radioligand [ 11 C]5 depletion experiment. Thus, we can conclude compound 5 is not a potent and selective antagonist of CX3CR1.…”

“…4,5 Methyl (2-amino-5-(benzylthio)thiazolo [4,5-d]pyrimidin-7-yl)-Dleucinate (5) recently developed by AstraZeneca is a potent and selective CX3CR1 antagonist with Ki 8.3 and 1940 nM for CX3CR1 and CXCR2, respectively, and selectivity index (SI) 230. 6 CX3CR1 has also become a promising target for molecular imaging of CX3CR1-mediated diseases and image-guided therapy using positron emission tomography (PET) modality. However, radionuclides including carbon-11 and fluorine-18 labeled CX3CR1 antagonists are still not reported.…”

“…Here we report the synthesis and preliminary biological evaluation of The reference standard 5 and its desmethylated acid precursor 2-amino-5-(benzylthio)thiazolo [4,5-d]pyrimidin-7-yl)-D-leucine (6) were synthesized as depicted in Scheme 1, according to the literature method with modifications. 6,8 The alkylation of commercially available starting material 2-amino-6-hydroxy-2-mercaptopyrimidine with benzyl bromide in 1 M NaOH gave compound 1 in 92% yield, which was collected by filtration and was sufficiently pure to be used in next step without further purification. Compound 1 was reacted with potassium thiocyanate, pyridine, and bromine in N,N-dimethylformamide (DMF) to provide intermediate 2 in 83% yield, which was followed by condensation at elevated temperature to afford the thiazole 3 in 95% yield.…”

“…In this reaction, the organic base N,N-dimethylanline was removed, consequently the reaction process and workup procedure were simplified, and the yield was increased from 60% to 91%. 6 The chloro group of compound 4 was subsequently displaced in a nucleophilic aromatic substitution reaction with excess methyl D-leucinate in the solvent anhydrous CH3CN or N-methylpyrrolidone and N,N-diisopropylethylamine (DIPEA) as a catalyst to give the standard compound 5 in only 11% yield. The poor yield is due to that it is difficult to displace aromatic chloro with methyl D-leucinate.…”

“…Then the competitive binding assay of ligand 5 was conducted following the literature method. 6 The assays were incubated at 25 °C for 2 h, and the results are shown in Figure 2. [ 125 I]Fractalkine was used as the radioligand, and fractalkine and buffer were used as a positive control and a negative control, respectively.…”