The PARP1 selective inhibitor saruparib (AZD5305) elicits potent and durable antitumor activity in patient-derived BRCA1/2-associated cancer models

Abstract

Background: Poly (ADP-ribose) polymerase 1 and 2 (PARP1/2) inhibitors (PARPi) are targeted therapies approved for homologous recombination repair (HRR)-deficient breast, ovarian, pancreatic, and prostate cancers. Since inhibition of PARP1 is sufficient to cause synthetic lethality in tumors with homologous recombination deficiency (HRD), PARP1 selective inhibitors such as saruparib (AZD5305) are being developed. It is expected that selective PARP1 inhibition leads to a safer profile that facilitates its combination with other DNA damage repair inhibitors. Here, we aimed to characterize the antitumor activity of AZD5305 in patient-derived preclinical models compared to the first-generation PARP1/2 inhibitor olaparib and to identify mechanisms of resistance.

Methods: Thirteen previously characterized patient-derived tumor xenograft (PDX) models from breast, ovarian, and pancreatic cancer patients harboring germline pathogenic alterations in BRCA1, BRCA2, or PALB2 were used to evaluate the efficacy of AZD5305 alone or in combination with carboplatin or an ataxia telangiectasia and Rad3 related (ATR) inhibitor (ceralasertib) and compared it to the first-generation PARPi olaparib. We performed DNA and RNA sequencing as well as protein-based assays to identify mechanisms of acquired resistance to either PARPi.

Results: AZD5305 showed superior antitumor activity than the first-generation PARPi in terms of preclinical complete response rate (75% vs. 37%). The median preclinical progression-free survival was significantly longer in the AZD5305-treated group compared to the olaparib-treated group (> 386 days vs. 90 days). Mechanistically, AZD5305 induced more replication stress and genomic instability than the PARP1/2 inhibitor olaparib in PARPi-sensitive tumors. All tumors at progression with either PARPi (39/39) showed increase of HRR functionality by RAD51 foci formation. The most prevalent resistance mechanisms identified were the acquisition of reversion mutations in BRCA1/BRCA2 and the accumulation of hypomorphic BRCA1. AZD5305 did not sensitize PDXs with acquired resistance to olaparib but elicited profound and durable responses when combined with carboplatin or ceralasertib in 3/6 and 5/5 models, respectively.

Conclusions: Collectively, these results show that the novel PARP1 selective inhibitor AZD5305 yields a potent antitumor response in PDX models with HRD and delays PARPi resistance alone or in combination with carboplatin or ceralasertib, which supports its use in the clinic as a new therapeutic option.

Keywords: Antitumor activity; BRCA1/2; Breast cancer; DNA damaging agent; HRD; Homologous recombination deficiency; PARP inhibitors; PARP1 selective; RAD51; Targeted therapy.

Fig. 1

Antitumor activity of AZD5305 and olaparib in PDXs. A Waterfall plot showing the antitumor activity of AZD5305 and olaparib in N = 13 PDX models, measured as percentage of tumor volume change compared with the tumor volume on day 1. Each bar indicates the mean and SEM of at least three biological replicates (open circles). The mean of control tumors (n ≥ 3) is represented with solid circles. + 20%, − 30%, and − 95% are marked by dotted lines to indicate the range of PD, SD, PR, and CR, respectively. The lower box summarizes the RAD51 score (%), the specific gene alteration harbored by each model and its allelic status, the cancer type, and previous treatment history with DDR drugs in the “Early”-stage vs. “Metastatic” disease setting. Rev, reversion mutation; DDR, DNA damage repair. p values, pairwise Wilcoxon test corrected for multiple testing. B Comparison of preclinical mRECIST criteria-based readouts to evaluate response to PARPi. pCRR, preclinical complete response rate; pORR, preclinical overall response rate; pCBR, preclinical benefit rate. Percentage of tumors, 95% confidence intervals (CI95%), forest plot, and p values (generalized linear mixed-effects model) are shown

Fig. 2

Analysis of response to AZD5305 and olaparib in PDXs. A Kaplan–Meier curve showing pPFS for AZD5305- and olaparib-treated individual tumors and B analysis of response and forest plot for pPFS (%) in PARPi-sensitive and resistant tumors (AZD5305, n = 63; olaparib, n = 71) and pTTP in PARPi-sensitive tumors (AZD5305, n = 45; olaparib, n = 50). p values, mixed-effects Cox model

Fig. 3

Mechanisms of resistance to AZD5305 and olaparib in PDXs. A Quantification by IF of the RAD51 score (%) in tumors that progressed under treatment with either PARPi, compared to tumors treated in the response phase (day 12) and untreated controls. p values are shown. Location of pathogenic (up) and reversion (down) mutations on the domain structure of B BRCA1 and C BRCA2. Each dot represents a patient. D Fraction of resistance mechanisms detected in individual tumors that had progressed to AZD5305 and olaparib in purple and green, respectively

Fig. 4

Biomarkers of DNA damage and replication stress in PDX models. Quantification of the percentage of tumor cells A in S/G2-phase (geminin-positive) with γ-H2AX nuclear foci, B in S/G2-phase (geminin-positive) with phospho-RPA nuclear foci, or C with at least one micronucleus in AZD5305-resistant (n = 6) and sensitive (n = 6) available PDX models, untreated or treated for 12 days with olaparib 100 mg/kg and AZD5305 1 mg/kg. Black lines indicate the biomarker mean of each group. p values (linear mixed-effects models) are indicated with significant values labeled in bold

Fig. 5

Antitumor activity of AZD5305 as single agent and in combination with other DNA damaging agents. Waterfall plots showing the antitumor activity of AZD5305 with A carboplatin as single agent and in combination in n = 6 PDX models and with B AZD6738 as single agent and in combination in n = 5 models, measured as percentage of tumor volume change, compared with the tumor volume on day 1. Each bar indicates the mean and SEM of at least three biological replicates (open circles). The mean of control tumors (n ≥ 3) is represented with solid circles. + 20%, − 30%, and − 95% are marked by dotted lines to indicate the range of PD, SD, PR, and CR, respectively. The lower box summarizes the RAD51 score (%), the specific gene alteration harbored by each model, the cancer type, and previous treatment history with DDR drugs in the "Early"-stage vs. "Metastatic" disease setting. Rev, reversion mutation; DDR, DNA damage repair. p values, pairwise Wilcoxon test corrected for multiple testing