Correlation between molar activity, injection mass and uptake of the PARP targeting radiotracer [18F]olaparib in mouse models of glioma

Abstract

Purpose: Radiopharmaceuticals targeting poly(ADP-ribose) polymerase (PARP) have emerged as promising agents for cancer diagnosis and therapy. PARP enzymes are expressed in both cancerous and normal tissue. Hence, the injected mass, molar activity and potential pharmacological effects are important considerations for the use of radiolabelled PARP inhibitors for diagnostic and radionuclide therapeutic applications. Here, we performed a systematic evaluation by varying the molar activity of [¹⁸F]olaparib and the injected mass of [^TotalF]olaparib to investigate the effects on tumour and normal tissue uptake in two subcutaneous human glioblastoma xenograft models.

Methods: [¹⁸F]Olaparib uptake was evaluated in the human glioblastoma models: in vitro on U251MG and U87MG cell lines, and in vivo on tumour xenograft-bearing mice, after administration of [^TotalF]olaparib (varying injected mass: 0.04-8.0 µg, and molar activity: 1-320 GBq/μmol).

Results: Selective uptake of [¹⁸F]olaparib was demonstrated in both models. Tumour uptake was found to be dependent on the injected mass of [^TotalF]olaparib (µg) but not the molar activity. An injected mass of 1 μg resulted in the highest tumour uptake (up to 6.9 ± 1.3%ID/g), independent of the molar activity. In comparison, both the lower and higher injected masses of [^TotalF]olaparib resulted in lower relative tumour uptake (%ID/g; P < 0.05). Ex vivo analysis of U87MG xenograft sections showed that the heterogeneity in [¹⁸F]olaparib intratumoural uptake correlated with PARP1 expression. Substantial upregulation of PARP1-3 expression was observed after administration of [^TotalF]olaparib (> 0.5 µg).

Conclusion: Our findings show that the injected mass of [^TotalF]olaparib has significant effects on tumour uptake. Moderate injected masses of PARP inhibitor-derived radiopharmaceuticals may lead to improved relative tumour uptake and tumour-to-background ratio for cancer diagnosis and radionuclide therapy.

Keywords: Glioblastoma; Olaparib; PARP; PET; [18F]olaparib.

Fig. 1

A Western blotting for PARP1-3 expression in U251MG and U87MG lysates. B Chemical structure of [¹⁸F]olaparib, and its uptake in U251MG and U87MG cells ([^TotalF]olaparib final concentration: 0.1 μM, molar activity = 11.8 GBq/μmol). C Blocking of [¹⁸F]olaparib uptake in U251MG and U87MG cells by unlabelled olaparib or talazoparib. D Mean fluorescence intensity (MFI) for PARP1-3 of U87MG cells treated with unlabelled olaparib (0–1 μM), assessed by flow cytometric analysis. n = 3 independent experiments. Representative histogram frequency is presented in Additional file 1: Fig. S1. Asterisks indicate levels of significance: *P < 0.05

Fig. 2

A Biodistribution in selected tissues in U251MG or U87MG xenograft-bearing mice, 120 min after i.v. injection of [¹⁸F]olaparib (0.28–0.31 MBq, molar activity = 1.9 GBq/μmol) (n = 3/group), with or without an excess of unlabelled olaparib (20 μg). Further data are presented in Additional file 1: Table S1 and Figure S2. B Autoradiography of U87MG tumour sections showing ¹⁸F localisation, and immunohistochemical staining of adjacent U87MG tumour sections showing PARP1, 2 and 3 expression. Asterisks indicate levels of significance: ***P < 0.001; ****P < 0.0001

Fig. 3

A Biodistribution of [¹⁸F]olaparib in selected tissues in U251MG (top) or U87MG (bottom) xenografts bearing-mice, 120 min after i.v. administration of [^TotalF]olaparib (0.04–8.0 μg, [¹⁸F]olaparib: 0.28–13.89 MBq) with various molar activities (1–320 GBq/μmol), (n = 3/group). Full biodistribution data are presented in Additional file 1: Tables S2–S3. B Tumour uptake of [¹⁸F]olaparib at various injected masses (μg) and molar activities (also see Additional file 1: Table S4). Asterisks indicate levels of significance: *P < 0.05; **P < 0.01

Fig. 4

A Immunohistochemistry staining for PARP1, 2, or 3 in U87MG xenograft tumour sections harvested from animals 120 min after i.v. administration of varying masses (μg) of [^TotalF]olaparib or after irradiation (10 Gy). B Semi-quantification of PARP staining. Each data point represents 3 different analysed fields randomly selected on the tumour section (also see Additional file 1: Tables S5, S6)