Abstract CT269: A highly sensitive and specific PARylation assay confirms significant and durable target engagement by AZD5305 in patients
Authors: Lombardi Benedetta , Marco-Casanova Paola , Karapetsas Athanasios , Hornett Mya , Lukacs Edit , Sugibayashi Ko , Cosulich Sabina , Dowson Adam , Moorthy Ganesh , Brown Jessica , Yap Timothy A. , Schram Alison M. , Balmana Judith , Luen Stephen J. , Im Seock-Ah , Sharp Adam , Baird Richard , Harrington Elizabeth A. , Linardopoulos Spiros 
Published: April 2023

Abstract
Background AZD5305 is a selective PARP1 inhibitor (PARPi) generated for improved therapeutic index compared with non-selective PARPi. Measurement of PARylation inhibition has been used to determine the pharmacodynamics (PD) of PARPi in clinical trials, but the assays used were insensitive and non-specific. Here we report on a new, highly sensitive and specific PARylation assay as evaluated in the PETRA trial (NCT04644068).
Methods PETRA is a Phase 1/2 clinical study investigating AZD5305 alone or in combination with other anticancer agents in patients (pts) with advanced solid tumors. Peripheral blood mononuclear cells (PBMC) collected at multiple timepoints during treatment (cycle [C] 0 day [D] 1 pre-dose, 1, 4, 24 and 48 hours [hrs] post-dose, and C1D1 pre-dose) were lysed, subjected to an ex-vivo PARylation reaction, and quantified with the novel MSD PARylation assay.
Results PARylation data were available from 32 patients treated from 20 mg to 140 mg. AZD5305 significantly inhibited PARylation in PBMC with maximum PARylation inhibition occurring around time to plasma concentration maximum (Tmax, 1 to 4 hrs after dosing; Table). At longer timepoints, a majority of patients showed &gt;90% PARylation inhibition up to 48 hrs, 72 hrs, and even 168 hrs. Inhibition &gt;90% for up to 72 hrs was observed in 4/4 evaluable samples at 140 mg, while inhibition &gt;90% for an extended period (up to 168 hrs) was observed in 3/4 samples at 20 mg and 1/2 samples at 60 mg. These data demonstrate that AZD5305 causes significant PARylation inhibition at doses ≥20 mg which is consistent with in vitro findings.
Conclusions Significant and durable PARylation inhibition was demonstrated with the novel PARylation assay at all doses of AZD5305 in PBMC samples from the majority of pts tested.
Table. Residual PARylation levels by dose in the PETRA study 20 mg 60 mg Time post-dose 1 hour 4 hours 1 hour 4 hours Patients with &gt;90% PARylation inhibition, n (%) 9/15 (60.0) 10/15 (66.7) 11/13 (84.6) 10/13 (76.9) Median residual PARylation levels (residual PARylation), % 5.3 5.0 4.0 3.7
Citation Format: Benedetta Lombardi, Paola Marco-Casanova, Athanasios Karapetsas, Mya Hornett, Edit Lukacs, Ko Sugibayashi, Sabina Cosulich, Adam Dowson, Ganesh Moorthy, Jessica Brown, Timothy A. Yap, Alison M. Schram, Judith Balmana, Stephen J. Luen, Seock-Ah Im, Adam Sharp, Richard Baird, Elizabeth A. Harrington, Spiros Linardopoulos. A highly sensitive and specific PARylation assay confirms significant and durable target engagement by AZD5305 in patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT269.