[18F]AZD2461, an Insight on Difference in PARP Binding Profiles for DNA Damage Response PET Imaging

Abstract

Background: Poly (ADP-ribose) polymerase (PARP) inhibitors are extensively studied and used as anti-cancer drugs, as single agents or in combination with other therapies. Most radiotracers developed to date have been chosen on the basis of strong PARP1-3 affinity. Herein, we propose to study AZD2461, a PARP inhibitor with lower affinity towards PARP3, and to investigate its potential for PARP targeting in vivo.

Methods: Using the Cu-mediated ¹⁸F-fluorodeboronation of a carefully designed radiolabelling precursor, we accessed the ¹⁸F-labelled isotopologue of the PARP inhibitor AZD2461. Cell uptake of [¹⁸F]AZD2461 in vitro was assessed in a range of pancreatic cell lines (PSN-1, PANC-1, CFPAC-1 and AsPC-1) to assess PARP expression and in vivo in xenograft-bearing mice. Blocking experiments were performed with both olaparib and AZD2461.

Results: [¹⁸F]AZD2461 was efficiently radiolabelled via both manual and automated procedures (9 % ± 3 % and 3 % ± 1 % activity yields non-decay corrected). [¹⁸F]AZD2461 was taken up in vivo in PARP1-expressing tumours, and the highest uptake was observed for PSN-1 cells (7.34 ± 1.16 %ID/g). In vitro blocking experiments showed a lesser ability of olaparib to reduce [¹⁸F]AZD2461 binding, indicating a difference in selectivity between olaparib and AZD2461.

Conclusion: Taken together, we show the importance of screening the PARP selectivity profile of radiolabelled PARP inhibitors for use as PET imaging agents.

Keywords: AZD2461; Cancer; Molecular imaging; PARP; PET.

Fig. 1.

a Structures and b PARP1, 2 and 3 inhibitory profiles of AZD2461 1, proto-deborylated side-product AZD2461-H 2 and olaparib 3. The PARP inhibitor Rucaparib was added as for comparison.

Fig. 2.

Synthesis of [¹⁸F]AZD2461 from SEM-protected boronate precursor 4 via Cu-mediated radiofluorination.

Fig. 3.

a Biodistribution in mice bearing PSN-1, CFPAC-1, AsPC-1 or PANC-1 tumour xenografts, at 1 h post-injection of [¹⁸F]AZD2461. b Tumour uptake and tumour/pancreas ratios in PSN-1, CFPAC-1, AsPC-1 and PANC-1 xenografts 1 h post-injection c Western blot probing for PARP-1 in PSN-1, PANC-1, CFPAC-1 and AsPC-1 cell lines. (**P < 0.01).

Fig. 4.

Blocking experiments performed in PSN-1 xenograft-bearing mice. Cold, unlabelled AZD2461 or olaparib (20 μg) was administered intravenously, 30 min prior to [¹⁸F]AZD2461. a Biodistribution, b tumour uptake.

Fig. 5.

In vitro uptake of [¹⁸F]AZD2461 and [¹⁸F]olaparib in PSN-1 cells, 30 min after addition of [¹⁸F]AZD2461. An excess of cold, unlabelled olaparib or AZD2461 was added (100 μM). (* P < 0.05, ** P < 0.01, **** P < 0.0001).