Facile synthesis of ¹⁸F-labeled Lapatinib for imaging with positron emission tomography

Objectives: Lapatinib, a potent EGFR (epidermal growth factor receptor) and HER2 (human epidermal growth factor receptor 2) tyrosine kinase inhibitor, is used in the clinic to treat advanced and metastatic HER2-positive breast cancers. ¹¹C-Lapatinib has been successful used in the clinic to detect brain metastases in patients with HER2-overexpressing breast cancer. However, the short half-life of ¹¹C (20.4 min) and the reported 4-step synthesis of ¹¹C-Lapatinib hinder its routine clinical application. Successful preparation of ¹⁸F-Lapatinib has also been reported, but the reported synthesis required 5 reaction steps. Such multi-step complicated approach remains a challenge for routine preparation of ¹⁸F-Lapatinib for clinical studies. In this study, we investigated if the synthesis of ¹⁸F-Lapatinib could be simplified using a pinacolyl arylboronate precursor and the recently reported copper-mediated ¹⁸F-fluorination reaction.

Methods: The ¹⁸F-Lapatinib labeling precursor was prepared in 3 steps: removal of the 3-fluorobenzyl group by refluxing Lapatinib in trifluoroacetic acid, protection of the secondary amino group with Boc, and coupling of 4-​(bromomethyl)​benzeneboronic acid pinacol ester to the phenol oxygen. ¹⁸F-Lapatinib was prepared in 2 steps: copper-mediated nucleophilic ¹⁸F-fluorination followed by N-Boc-deprotection with trifluoroacetic acid. The ¹⁸F-fluorination reaction conditions were: heating the pinacolyl arylboronate precursor (4 μmol), Cu(Otf)₂ catalyst (20 μmol) and pyridine (500 ppm) in N,N-dimethylformamide (0.7 mL) for 20 min at 110 °C.

Results: To prepare the precursor, our first attempt was protecting the secondary amino group of Lapatinib with Boc followed by removing the 3-fluorobenzyl group via Pd-catalyzed hydrogenation. N-Boc protected Lapatinib was obtained in 90% yield, but the subsequent hydrogenation did not proceed as expected even after 2-day incubation. Alternatively, de-3-fluorobenzylation was conducted by refluxing Lapatinib in trifluoroacetic acid for 23 h, followed by N-Boc protection. The intermediate, N-Boc protected de-3-fluorobenzylated Lapatinib was obtained in 71% yield over 2 steps. The final pinacolyl arylboronate precursor was obtained in 76% yield by coupling the intermediate with 4-​(bromomethyl)​benzeneboronic acid pinacol ester. The copper-mediated ¹⁸F-fluorination reaction using the optimized literature conditions provided the ¹⁸F-labeled intermediate in 24 ± 4% (n = 2) conversion yield. Subsequent addition of trifluoroacetic acid (0.3 mL) to the reaction mixture and heating at 75 °C for 10 min for N-Boc deprotection led to massive decomposition of the ¹⁸F-labeled intermediate. The decomposition was subspected to be catalyzed by the Lewis acid catalyst Cu(Otf)₂. Therefore, the ¹⁸F-fluorinated intermediate was purified by C18 Sep-Pak cartridge first to remove Cu(Otf)₂, and subsequently eluted off with a mixture of acetonitrile (0.5 mL) and trifluoroacetic acid (1 mL). Heating this reaction mixture at 75 °C for 15 min followed by HPLC purification provided the desired ¹⁸F-Lapatinib in 0.4 ± 0.1% (n = 2) decay-corrected radiochemical yield.

Conclusions: Despite low radiochemical yield, successful preparation of ¹⁸F-Lapatinib was achieved in 2 steps using the copper-mediated nucleophilic ¹⁸F-fluorination reaction. Currently, we are exploiting the use of a base-labile N-protecting group, so the ¹⁸F-fluorination and deprotection reactions can be carried out in one pot. In addition, the use of reported mixture of n-butanol and N,N-dimethylacetamide as the reaction solvent to improve radiochemical yield of the first-step copper-mediated ¹⁸F-fluorination reaction is currently underway.