Highly efficient syntheses of [methyl-11C]thymidine and its analogue 4′-[methyl-11C]thiothymidine as nucleoside PET probes for cancer cell proliferation by Pd0-mediated rapid C-[11C]methylation

Abstract

Pd⁰-mediated rapid couplings of CH₃I (and then [¹¹C]CH₃I) with excess 5-tributylstannyl-2′-deoxyuridine and -4′-thio-2′-deoxyuridine were investigated for the syntheses of [methyl-¹¹C]thymidine and its stable analogue, 4′-[methyl-¹¹C]thiothymidine as PET probes for cancer diagnosis. The previously reported conditions were attempted using Pd₂(dba)₃/P(o-CH₃C₆H₄)₃ (1 : 4 in molar ratio) at 130 °C for 5 min in DMF, giving desired products only in 32 and 30% yields. Therefore, we adapted the current reaction conditions developed in our laboratory for heteroaromatic compounds. The reaction using CH₃I/stannane/Pd₂(dba)₃/P(o-CH₃C₆H₄)₃/CuCl/K₂CO₃ (1 : 25 : 1 : 32 : 2 : 5) at 80 °C gave thymidine in 85% yield. Whereas, CH₃I/stannane/Pd₂(dba)₃/P(o-CH₃C₆H₄)₃/CuBr/CsF (1 : 25 : 1 : 32 : 2 : 5) including another CuBr/CsF system promoted the reaction at a milder temperature (60 °C), giving thymidine in 100% yield. Chemo-response of thiothymidine-precursor was different from thymidine system. Thus, the above optimized conditions including CuBr/CsF system gave 4′-thiothymidine only in 40% yield. The reaction using 5-fold amount of CuBr/CsF at 80 °C gave much higher yield (83%), but unexpectedly, the reaction was accompanied by a considerable amount of undesired destannylated product. Such destannylation was greatly suppressed by changing to a CuCl/K₂CO₃ system using CH₃I/stannane/Pd₂(dba)₃/P(o-CH₃C₆H₄)₃/CuCl/K₂CO₃ (1 : 25 : 1 : 32 : 2 : 5) at 80 °C, giving the 4′-thiothymidine in 98% yield. The each optimized conditions were successfully applied to the syntheses of the corresponding PET probes in 87 and 93% HPLC analytical yields. [¹¹C]Compounds were isolated by preparative HPLC after the reaction conducted under slightly improved conditions, exhibiting sufficient radioactivity of 3.7–3.8 GBq and specific radioactivity of 89–200 GBq µmol⁻¹ with radiochemical purity of ≥99.5% for animal and human PET studies.