Positron-Emission Tomographic Imaging of a Fluorine 18-Radiolabeled Poly(ADP-Ribose) Polymerase 1 Inhibitor Monitors the Therapeutic Efficacy of Talazoparib in SCLC Patient-Derived Xenografts

Abstract

Introduction: Inhibitors of poly-(ADP)-ribose polymerase (PARP) are promising therapeutics for SCLC. We tested whether PARP inhibitor (PARPi) target engagement as measured by a fluorine 18-radiolabeled PARPi ([¹⁸F]PARPi) has the potential to predict drug efficacy in vivo.

Methods: Tumor growth inhibition during daily talazoparib treatment was evaluated in mice engrafted with SCLC patient-derived xenografts to evaluate talazoparib efficacy at multiple doses. Mice were intravenously injected with [¹⁸F]PARPi radiotracer at multiple timepoints after single doses of oral talazoparib to quantitatively assess the extent to which talazoparib could reduce tumor radiotracer uptake and positron-emission tomographic (PET)/computer tomographic activity. Tumors were harvested and tumor poly-(ADP) ribose level was measured by enzyme-linked immunosorbent assay.

Results: A dose range of talazoparib with differential therapeutic efficacy was established, with significant delay in time to reach 1000 mm³ for tumors treated with 0.3 mg/kg (p = 0.02) but not 0.1 mg/kg talazoparib. On PET/computed tomography with [¹⁸F]PARPi, reduction in [¹⁸F]PARPi uptake after talazoparib dosing was consistent with talazoparib clearance, with reduction in PET activity attenuating over 24 hours. Talazoparib target engagement, measured by maximum tumor PET uptake, increased in a dose-dependent manner (3.9% versus 2.1% injected dose/g for 0.1 and 0.3 mg/kg at 3 hours post-talazoparib, p = 0.003) and correlated with PARP enzymatic activity among individual tumors as measured by total tumor poly-(ADP) ribose (p = 0.04, R = 0.62 at 1 hour post-talazoparib).

Conclusions: PET imaging using [¹⁸F]PARPi has the potential to be a powerful tool in treatment monitoring by assessing PARPi target engagement in real-time.

Keywords: Drug target engagement; Poly(ADP-ribose) polymerase 1 inhibitors; Positron-emission tomography; SCLC; Talazoparib.

Figure 1:

Doses of 0.1 and 0.3 mg/kg talazoparib differ in efficacy as a single agent and in combination with radiation. A) Tumor growth curves and Kaplan-Meier plot of SCRX-Lu149 PDX treated with 20 doses (on Monday-Friday) of vehicle, 0.1 mg/kg, or 0.3 mg/kg talazoparib by oral gavage. Each line represents one mouse. A Kaplan-Meier event was defined as tumor volume reaching 1000 mm³. B) Tumor growth curves and Kaplan-Meier plots as above with the addition of four daily doses of 2 Gy radiation applied on days 2–5. Radiation was delivered 3 hours after talazoparib administration.

Figure 2:

[¹⁸F]PARPi PET/CT Experimental Design. A) Molecular structures of olaparib, talazoparib, and an ¹⁸F-labeled PARP inhibitor ([¹⁸F]PARPi) are demonstrated. The [¹⁸F]PARPi is formed by conjugating a 4-[¹⁸F]fluorobenzoic acid group to a scaffold of the small molecule olaparib. B) When [¹⁸F]PARPi is present without the competitive binding of talazoparib, it binds to the PARP1 enzyme, allowing visualization of the high PARP expressing SCLC PDX SCRX-Lu149. Tumor is marked with a “T” on PET images. Tumors are implanted in the right shoulder to prevent signal interference from the gut, as the [¹⁸F]PARPi radiotracer is eliminated by the hepatobiliary system and subsequently has increased activity in the intestines. C) Pre-treatment with 0.2 mg/kg talazoparib by oral gavage competitively blocks the binding of [¹⁸F]PARPi and results in lower activity. D) An experimental timeline demonstrates the method used for talazoparib pre-treatment and PET scanning. Talazoparib was orally dosed at 0.1, 0.2, or 0.3 mg/kg at various time points prior to intravenous injection of [¹⁸F]PARPi. Two hours after radiotracer injection, PET/CT imaging was performed, and mouse dissection was performed immediately after imaging.

Figure 3:

[¹⁸F]PARPi models talazoparib pharmacokinetics at a single dose. A) A dose of talazoparib 0.2 mg/kg was delivered by oral gavage at varying time intervals prior to radiotracer injection and PET/CT imaging. Maximum intensity projection images taken from mice treated are displayed, with subcutaneous SCRX-Lu149 PDX tumors denoted with “T”. B) Maximum and C) mean PET activities in each tumor were quantified, with the greatest reduction in PET activity at 1 to 3 hours post-talazoparib oral gavage. Each dot represents one mouse/PDX tumor. D) γ-counts of the tumors harvested immediately after imaging show similar trend in radiotracer activity.

Figure 4:

[¹⁸F]PARPi discriminates between high and low doses of talazoparib and correlates with PARP inhibitor activity. A) Doses of 0.1 mg/kg and 0.3 mg/kg talazoparib were administered to mice by oral gavage between 1 and 24 hours prior to [¹⁸F]PARPi radiotracer injection and PET/CT imaging. Maximum intensity projection images of mice are displayed, with SCRX-Lu149 PDX tumors denoted with a “T”. B) Maximum and C) mean PET activities in each tumor were quantified at each time point and dose. Each dot represents one mouse/PDX tumor for subpanels B-G. Significant differences in both mean and maximum PET activity between 0.1 mg/kg and 0.3 mg/kg were noted at the 3 hour time point. D) γ-counts of the tumors, harvested immediately after PET/CT imaging, show similar results as PET/CT imaging. E) Samples of tumors from the imaged mice were flash frozen immediately after PET/CT imaging, followed by quantification of PAR polymers, the product of the PARP enzyme, by ELISA. F) Maximum and G) mean PET signals are plotted against tumor PAR levels as quantified by ELISA for untreated control mice and mice treated with talazoparib at the one hour time point. Maximum PET signal correlated with tumor PAR and mean PET signal trended towards correlation (p values and Pearson’s correlation coefficient (R) displayed on graph). Lines of best fit are displayed with 95% confidence intervals (dotted lines).